Galaxies in the epoch of reionization: Investigating the high-redshift galaxy population through simulations and observations

UNIVERSITATIS ACTA UPSALIENSIS

UPPSALA

Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 2036

Galaxies in the epoch of reionization

Investigating the high-redshift galaxy population through simulations and observations

CHRISTIAN BINGGELI

ISSN 1651-6214

ISBN 978-91-513-1196-8

Dissertation presented at Uppsala University to be publicly examined in Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, Thursday, 10 June 2021 at 13:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Stephen Wilkins (University of Sussex).

Online defence: https://uu-se.zoom.us/j/68040940351 Abstract

Binggeli, C. 2021. Galaxies in the epoch of reionization. Investigating the high-redshift galaxy population through simulations and observations. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 2036. 87 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-1196-8.

The cosmic reionization is the last major gas phase transition in cosmic history, yet it remains poorly understood. Current constraints indicate that early star-forming galaxies drove the reionization process through producing and releasing large numbers of ionizing photons into the intergalactic medium. However, our understanding of the ionizing escape fraction (f

esc

) and the general properties of high-redshift galaxies is still limited.

In this thesis, simulated galaxies and observations are used to investigate epoch-of- reionization galaxies and to explore methods that can aid future investigations of such objects.

Using simulations, we have shown that it may be possible to constrain f

esc

in epoch-of- reionization galaxies using quite simple diagnostics that should be observable with the upcoming James Webb Space Telescope (JWST). We also show that variations in star formation activity larger than those predicted in our simulations may lead to a possible degeneracy with high f

esc

. However, auxiliary information obtained with the JWST may allow us to disentangle variations in the star formation activity from high f

esc

.

We compare galaxies from several simulations to the recently spectroscopically confirmed z=9.1096 galaxy MACS1149-JD1. We find that none of the simulations are able to reproduce the large Balmer break observed in MACS1149-JD1, and argue that unless it represents an outlier in the high-redshift galaxy population, this may indicate that the simulations fail to capture some key physics. Finally, we present ALMA observations of the z=7.6637 galaxy z7_GSD_3811. This object remains undetected in several commonly detected FIR emission lines and FIR dust emission. Using SED-fitting and by comparing our observations to models and low-redshift observations, we show that our non-detections could indicate that the object is poor in metals and dust.

Our findings could help future observers to further constrain the nature of high-redshift galaxies and their role in reionization.

Keywords: galaxies: high-redshift, galaxies: ISM, galaxies: evolution, reionization, Lyman continuum

Christian Binggeli, Department of Physics and Astronomy, Observational Astronomy, 516, Uppsala University, SE-751 20 Uppsala, Sweden.

© Christian Binggeli 2021 ISSN 1651-6214

ISBN 978-91-513-1196-8

urn:nbn:se:uu:diva-440032 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-440032)

Till Linnéa

List of papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I Zackrisson, E., Binggeli, C., Finlator, K.; Gnedin, N. Y., Paardekooper, J.-P., Shimizu, I., Inoue, A. K., Jensen, H., Micheva, G., Khochfar, S.

and Dalla Vecchia, C. (2017)

The spectral evolution of the first galaxies. III.

Simulated James Webb Space Telescope spectra of reionization-epoch galaxies with Lyman-continuum leakage

The Astrophysical Journal 836, 78

II Binggeli, C., Zackrisson, E., Pelckmans, K., Cubo, R., Jensen, H. and Shimizu, I. (2018)

Lyman continuum leakage versus quenching with the James Webb Space Telescope: The spectral signatures of quenched star formation activity in reionization-epoch galaxies

Monthly Notices of the Royal Astronomical Society 479, 368-376 III Binggeli, C., Zackrisson E., Ma, X., Inoue A. K., Vikaeus, A.,

Hashimoto, T., Mawatari, K., Shimizu, I. and Ceverino D. (2019) Balmer breaks in simulated galaxies at z>6

Monthly Notices of the Royal Astronomical Society 489, 3827-3835 IV Binggeli, C., Inoue, A. K., Hashimoto, T, Toribio, M. C., Zackrisson,

E., Ramstedt, S., Mawatari, K., Harikane, Y., Matsuo, H., Okamoto, T., Ota, K., Shimizu, I., Tamura, Y., Taniguchi, Y. and Umehata, H. (2021) A puzzling non-detection of [O III ] and [C II ] from a z ≈ 7.7 galaxy observed with ALMA

Astronomy & Astrophysics 646, A26

Reprints were made with permission from the publishers.

Errata

I Zackrisson, E., Binggeli, C., Finlator, K.; Gnedin, N. Y., Paardekooper, J.-P., Shimizu, I., Inoue, A. K., Jensen, H., Micheva, G., Khochfar, S.

and Dalla Vecchia, C. (2021)

Erratum: The spectral evolution of the first galaxies. III.

Simulated James Webb Space Telescope spectra of reionization-epoch galaxies with Lyman-continuum leakage

The Astrophysical Journal 908, 116

II Binggeli, C., Zackrisson, E., Pelckmans, K., Cubo, R., Jensen, H. and Shimizu, I. (2020)

Erratum: Lyman continuum leakage versus quenching with the James Webb Space Telescope: The spectral signatures of quenched star forma- tion activity in reionization-epoch galaxies

Monthly Notices of the Royal Astronomical Society 496, 1766-1767

List of papers not included in the thesis

The following are publications to which I have contributed as author, but which are not included in the thesis.

1 Sugahara, Y., Inoue, A. K., Hashimoto, T. et al. (2021)

Big Three Dragons: A [N II ] 122 μm Constraint and New Dust- continuum Detection of A z = 7.15 Bright Lyman Break Galaxy with ALMA

Submitted to The Astrophysical Journal

2 Vikaeus, A., Zackrisson, E. and Binggeli, C., (2020)

The impact of star formation sampling effects on the spectra of lensed z > 6 galaxies detectable with JWST

Monthly Notices of the Royal Astronomical Society 492, 1706-1712 3 Zackrisson, E., Majumdar, S., Mondal, R. et al. (2020)

Bubble mapping with the Square Kilometer Array-I. Detecting galaxies with Euclid, JWST, WFIRST and ELT within ionized bubbles in the in- tergalactic medium at z> 6

Monthly Notices of the Royal Astronomical Society 493, 855-870 4 Giri, S. K., Zackrisson, E., Binggeli, C., Pelckmans, K. and

Cubo, R. (2020)

Identifying reionization-epoch galaxies with extreme levels of Lyman continuum leakage in James Webb Space Telescope surveys

Monthly Notices of the Royal Astro nomical Society 491 5277-5286 5 Tamura, Y., Mawatari, K., Hashimoto, T. et al. (2019)

Detection of the Far-infrared [O III] and Dust Emission in a Galaxy at Redshift 8.312: Early Metal Enrichment in the Heart of the

Reionization Era

The Astrophysical Journal 874, 27

6 Jensen, H., Zackrisson, E., Pelckmans, K., Binggeli, C.., Ausmees, K.

and Lundholm, U., (2016)

A Machine-learning Approach to Measuring the Escape of Ionizing Ra- diation from Galaxies in the Reionization Epoch

The Astrophysical Journal 827, 5

Contents

1 Introduction

. . . .

13

2 Hydrogen and Lyman continuum radiation

. . .

15

3 Cosmic reionization

. . . .

17

3.1 Constraints on reionization

. . . .

18

3.1.1 Constraints from quasars

. . . .

18

3.1.2 Constraints from the cosmic microwave background

^{. . .}

20

3.1.3 Constraints from Lyman- α emitters

. . . .

22

3.2 Driving sources of cosmic reionization

. . .

23

4 Properties of high-redshift star-forming galaxies

. . .

25

4.1 Dust properties

. . .

25

4.2 Star formation

. . . .

29

4.3 Far-infrared observations and the interstellar medium

. . .

31

5 The Lyman continuum escape fraction

. . .

36

5.1 Mechanisms of Lyman continuum leakage

. . .

37

5.2 Observations of leaking Lyman continuum

. . .

38

5.3 Indirect methods for constraining the escape fraction

. . .

39

5.3.1 Emission-line constraints on the escape fraction

. . . .

39

5.3.2 Absorption-line constraints on the escape fraction

. . .

41

5.3.3 Escape fractions from ionized proximity-zones and bubbles

. . .

41

5.3.4 Zackrisson et al. 2013: Nebular emission features

. . .

42

6 Synthetic spectra of high-redshift galaxies

. . . .

46

6.1 Population synthesis and model galaxy spectra

. . .

46

6.2 Simulated galaxies

. . .

47

6.3 Spectra of simulated galaxies

. . .

49

7 Radio interferometry and interferometric imaging

. . .

53

8 Summary of papers

. . . .

57

8.1 Paper I

. . .

57

8.2 Paper II

. . . .

61

8.3 Paper III

. . .

64

8.4 Paper IV

. . .

67

9 Summary and outlook

. . .

71

10 Contributions to included papers

. . . .

73

11 Svensk sammanfattning

. . . .

75

12 Acknowledgements

. . .

79

References

. . . .

81

1. Introduction

This thesis is devoted to studying galaxies during a period known as the epoch of reionization (EoR). Cosmic reionization represents one of the major gas phase transitions that have occurred throughout the history of Universe. As will be discussed in this thesis, reionization is an important yet poorly under- stood chapter in cosmic history.

The phase transition just prior to reionization was the recombination, during which the first neutral atoms formed, leading to a pre-galactic medium that consisted of neutral hydrogen, helium and trace amounts of lithium. During reionization, the process was reversed as hydrogen-ionizing photons from the first astrophysical sources were emitted into the neutral intergalactic medium (IGM). While helium also underwent a reionization, this thesis focuses on the reionization of hydrogen. The term ‘reionization’ will thus be used in the context of hydrogen reionization, unless otherwise stated.

Over the years, observational studies with increasingly advanced telescopes covering a wide wavelength range have been aimed at understanding the reion- ization and its drivers. Meanwhile, on the theory side, cosmological simula- tions run on super-computers have provided the astronomy community with increasingly detailed models of the process (see reviews by e.g. Fan et al., 2006a; Stark, 2016; Dayal & Ferrara, 2018).

Through these efforts, some facets of the reionization process, in particu- lar its timing and duration, have been fairly well constrained. Over the years, the emerging picture has become one of a reionization driven mainly by high- redshift star-forming galaxies. These galaxies thus were not only the progen- itors of the present-day galaxy population, but by driving reionization, they also affected the presence and distribution of neutral hydrogen in the IGM. As such, they likely had an impact on the subsequent formation of stars. How- ever, our picture of early star-forming objects is still incomplete. One part of the picture that yet has to be filled in relates to the fraction of ionizing photons released from these galaxies into the IGM (the escape fraction; f esc ), and the role this played in the reionzation process. The main obstacle in determining the escape fraction of EoR galaxies is that we cannot observe the leaking LyC directly. The reason for this is that the LyC photons that leak from these galax- ies are consumed in the neutral IGM during reionization, and therefore never reach us (Inoue & Iwata, 2008).

One way to circumvent this obstacle is to observe ionizing emission from

so-called low-redshift analogues in order to understand the physics behind

the leakage of ionizing photons and link these to other parts of the electro-

magnetic spectrum. Another way is to use models and simulations to predict

observable features that can be linked to leakage of ionizing photons. Both of these avenues have their limitations in the sense that it is unclear to what degree low-redshift analogues or simulated galaxies are actually representa- tive of the galaxies that likely drove reionization. Thus, understanding the fundamental properties of the high-redshift galaxy population, such as their star formation histories and dust properties is a piece of the puzzle of trying to form a complete picture of the evolution of the Universe. In coming years, the James Webb Space Telescope (JWST) will allow astronomers to study high- redshift galaxies at electromagnetic wavelengths hitherto unavailable for such distant and faint objects. In order to get the most out of the data the JWST will produce, it is crucial to have well-calibrated models that allow us to interpret the observations.

The focus of this work is the high-redshift galaxies that likely drove the

cosmic reionization, their properties and how we can use models in order to

understand future observations. First, I discuss a method for indirectly as-

sessing the escape fraction of ionizing photons from EoR galaxies. I test this

method on spectra of simulated high-redshift galaxies with varying star for-

mation histories, internal metallicity distributions and escape fractions. I also

discuss problems with this approach related to possible variations in the star

formation history of high-redshift galaxies. Related to this, I use cosmological

simulations in order to understand recent observational results of the galaxy

MACS1149-JD1, a redshift z ∼ 9.1 galaxy suggested to have experienced large

variations in its star formation activity. I discuss to what extent contemporary

simulations are able to reproduce these observations. Finally, I present ALMA

observations of a z ∼ 7.7 star-forming galaxy (z7_GSD_3811) and constrain

properties of the object using the observations.

2. Hydrogen and Lyman continuum radiation

During reionization, the neutral hydrogen first formed during recombination (Peebles, 1968) was reionized by highly energetic photons emitted from early astrophysical sources. In order to understand the reionization and the evolution of the IGM, we need to start with the most abundant element in the Universe, i.e. hydrogen.

The ionization potential of the hydrogen atom is 13.6 eV. This means that ultraviolet (UV) photons with energies above 13.6 eV (photons with wave- lengths shorter than 912 Å) are able to eject the single electron from the hy- drogen atom and ionize it. This energy limit, above which photons will be able to ionize hydrogen, is called the Lyman limit, and radiation with energies above the Lyman limit is called Lyman-continuum (LyC) radiation (Draine, 2011). The reionization process thus requires astrophysical sources that are able to efficiently produce LyC in order to ionize the IGM. Several types of astrophysical objects produce LyC photons, but in the context of this thesis, the most important sources are stars in early star-forming galaxies.

The stars that most efficiently produce LyC photons are hot and massive stars. The large ionizing flux from these stars can ionize the surrounding hydrogen (and other elements) in the interstellar medium (ISM) to form so- called H II regions. In such ionized regions, it is likely that the free electrons encounter another proton, and recombine back into neutral hydrogen. When this recombination occurs to the ground state of hydrogen directly, a new LyC photon is emitted. This will then likely be absorbed by another hydrogen atom in the surrounding ISM, leading to additional ionizations. If the amount of neutral hydrogen in the surrounding ISM is small enough, a fraction of the produced LyC photons can escape the ISM without encountering a hydrogen atom. If the recombination occurs to an excited state of hydrogen, a number of non-ionizing photons will be emitted as the electron cascades down through the discrete energy levels of hydrogen on its way to the ground state. The tran- sition between an excited state of hydrogen and the ground state will produce a UV photon in the Lyman series. For example, the transition between the first excited state (2p) to the ground state (1s) leads to the emission of a Lyman- α (Ly α) photon, with a wavelength ≈ 1216 Å. The transitions from the second and third and higher excited states to the ground state are called Lyman- β, Lyman- γ and so on. Similarly, transitions from higher excited states to the first excited state of hydrogen lead to emission of photons in the Balmer series (mainly optical photons, usually denoted H α, Hβ, Hγ and so on).

Since H II regions form mainly around hot, massive and thus short-lived

stars, the Lyman and Balmer emission lines trace the recent star formation

(e.g. Kennicutt, 1998; Kennicutt & Evans, 2012). While the transport of Ly α

through the IGM becomes complicated due to its resonant nature (Hayes et al.,

2010), the Balmer lines are excellent probes of star formation when the escape

fraction is low. With the upcoming JWST, we should be able to observe UV

and optical emission lines during the very earliest stages of galaxy formation

and evolution. In paper I and paper II, we discuss a method for determining

the amount of LyC radiation that escapes from high-redshift galaxies using

Balmer lines in combination with the UV slopes (see section 4.1) of these

objects.

3. Cosmic reionization

The current standard model of Big Bang cosmology is the Lambda cold dark matter ( ΛCDM) model. A widely accepted extension to ΛCDM is cosmolog- ical inflation. According to this model, the Universe started in an extremely hot and dense state. Within the first fraction of a second, it is thought to have undergone a rapid expansion known as inflation (Guth, 1981), in which the volume of the Universe increased by a factor of ∼ 10 ⁷⁸ (Guth & Kaiser, 2005). It is thought that this rapid expansion led to the homogeneous and isotropic distribution of matter which is reflected in large-scale galaxy surveys and observations of the cosmic microwave background radiation (CMBR). Af- ter inflation followed a period of more modest expansion and cooling. During this period, the Big Bang nucleosynthesis led to the formation of the first com- posite nuclei (Alpher et al., 1948).

However, the Universe was still too hot for neutral atoms to form, and so it remained optically thick to photons, since these would scatter off free, charged particles in the hot, dense and ionized gas. At z ∼ 1100, the Universe had cooled to a temperature that allowed the first neutral atoms to form in the event known as recombination (Peebles, 1968). As a result of recombination, the photons could decouple from the baryonic matter and flow freely through the transparent Universe (see e.g. Barkana & Loeb, 2001). The light emitted at recombination is observed today as a nearly uniform background in all direc- tions on the sky, and is known as the cosmic microwave background radiation (CMBR).

While the post-recombination Universe allowed photons to travel freely, no sources of light had yet formed. Instead, the Universe was filled with a neutral gas. The formation of the first stars signified the end of this period known as the Dark Ages (see Rees, 1998), and the beginning of the cosmic dawn.

The cosmic dawn culminated with the cosmic reionization, when the the first stars and galaxies produced LyC at a rate which could not be matched by the recombination rate in the IGM and thereby started to ionize the surrounding gas. Models suggest that in the early parts of reionization, only the gas in close proximity of the objects became ionized, leading to ‘bubbles’ of ionized gas.

As a result of variations both in the gas density and the distribution of ioniz- ing sources, models predict that reionization was patchy and inhomogeneous, with large variations in the neutral gas fraction (e.g. Furlanetto et al., 2004;

Iliev et al., 2006). Over time, and as the number of ionizing sources increased,

smaller ionized regions merged, leaving ever smaller patches and filaments of

neutral gas that were slowly etched away over time. It has been argued that

this patchy nature of reionization is reflected in observations of high-redshift quasars which have showed that the neutral fraction of the IGM varies signif- icantly between different lines of sight (e.g. Fan et al., 2006b; Willott et al., 2007; Becker et al., 2015; Barnett et al., 2017; Bosman et al., 2018; Eilers et al., 2018). An illustration showing the evolution of the Universe from the Big Bang to the current day is shown in figure 3.1.

While this thesis focuses on the study of the galaxies present during reion- ization, there are also ways to study the IGM in the early Universe more directly. The ‘spin-flip’ 21 cm spectral line of neutral hydrogen provides a unique way to study the neutral hydrogen and its distribution in the pre- reionization and reionization era. This line, which has a wavelength of 21 cm, arises due to the difference in the energy of the two hyperfine levels in the ground state of hydrogen (Pritchard & Loeb, 2012). Due to cosmological redshift, the line is measured at meter wavelengths at the present day. Re- cently, the EDGES (Experiment to Detect the Global EoR Signature) team claimed to have detected the signal at z ∼ 17, and argued that the their mea- surement is consistent with the signal induced by early stars (Bowman et al., 2018). Other ongoing and planned experiments aimed at measuring the global 21 cm signal, such as the Large-Aperture Experiment to Detect the Dark Ages (LEDA), Radio Experiment for the Analysis of Cosmic Hydrogen (REACH), Shaped Antennas to measure the background RAdio Spectrum (SARAS3) and Probing Radio Intensity at high-Z from Marion (PRI ^Z M), will likely be able to confirm this detection and provide additional constraints on the globally averaged signal. In addition, experiments such as the Low-Frequency Array (LOFAR), the Hydrogen Epoch of Reionization Array (HERA) and the future Square Kilometre Array (SKA) may be able to measure spatial fluctuations in the 21 cm signal.

3.1 Constraints on reionization

In order to understand the driving mechanism behind the cosmic reionization, it is crucial to constrain the timing and duration of the process. Over the years, several different observational probes have been able to provide us with constraints on the evolution of the IGM in the EoR. In the following three sections, I will give an overview of current constraints and describe some of the ways in which these have been derived.

3.1.1 Constraints from quasars

Quasars are an extremely bright type of active galactic nuclei (AGN). Their

high brightness allows us to observe and use them as background sources to

probe the IGM at very large distances. This makes them an extremely useful

tool when trying to constrain the timing of cosmic reionization. One strong

INFL A TION

REDSHIFT

^{z = 0}

z ~ 30 - 20 z ~ 1100

RECOMBINATION FIRST STARS

REIONIZATION NOW BIG BANG

z ~ 10 - 6

Figure 3.1. An illustration showing the evolution of the Universe, from the Big Bang and inflation to the left, and the present-day Universe to the right (z = 0). Note that cosmic time increases toward the right and the redshift increases toward the left. Some important events in the cosmic history are highlighted at the top of the figure: recom- bination (z ∼ 1100), the cosmic dawn and formation of the first stars (z ∼ 30 – 20;

Bromm, 2013), reionization (z ∼ 10 – 6, see 3.1), and the Universe today (z = 0).

piece of evidence for the timing of the cosmic reionization and the increas- ing fraction of neutral gas in the IGM at higher redshifts comes from the Ly α forest and Gunn-Peterson trough in the spectra of high redshift quasars. The Ly α forest is the result of clouds of intervening neutral gas along the line of sight toward a distant quasar. As the clouds will absorb Ly α at wavelengths corresponding to their redshift, this will produce a set of absorption features on the blue side of the redshifted quasar Ly α emission line. The collection of such absorption features is known as the Ly α forest (see Rauch, 1998). While the optical depth of intervening gas varies between different lines of sight, the number of absorption lines making up the Ly α forest increases at higher red- shifts (see figure 3.2). This occurs due to the increasing number of intervening neutral gas clouds as we approach reionization. At redshifts ∼ 6 we start to observe complete absorption on the blue side of the of the quasar Ly α line.

This feature, originally predicted by Gunn & Peterson (1965), is known as the Gunn-Peterson trough, and since complete absorption at these wavelengths occurs for relatively small neutral fractions, it is seen as an indication that the cosmic reionization was mostly done at z ∼ 6.

The presence of complete absorption troughs can be used to constrain the

neutral fraction in an inhomogenously ionized IGM. Patches of ionized gas

will lead to transmission ‘spikes’ in the absorption troughs, with dark gaps,

created by the presence of significant amounts of neutral hydrogen, in be-

tween. The number and length of these dark gaps provide information that can be used to constrain the neutral fraction of the IGM (e.g. Songaila & Cowie, 2002; Fan et al., 2006b; Mesinger, 2010). With a related method, McGreer et al. (2015) recently showed that the fraction of pixels showing no flux in high-redshift quasar spectra are consistent with a neutral fraction ≤ 6% at z = 5.9, which is also consistent with a reionization process that was mostly done at z ∼ 6.

If the IGM neutral fraction is large, the absorption in the Gunn-Peterson trough will have a long tail, or wing, extending toward longer wavelengths.

The shape of this feature, known as the Gunn-Peterson damping wing, will depend on the neutral fraction of hydrogen around a target quasar, and can thus be used to probe the neutral fraction using high-redshift quasars (Miralda- Escudé, 1998). This method becomes complex mainly due to the risk of find- ing dense clumps of neutral hydrogen in an otherwise ionized IGM. Such a clump in front of a targeted quasar would give a similar signature as a sub- stantially neutral IGM. Nevertheless, this method has been used to constrain the neutral fraction of the IGM during reionization using both quasars and gamma-ray bursts (GRBs; e.g. Totani et al., 2006; Mortlock et al., 2011; Totani et al., 2016; Greig et al., 2017; Bañados et al., 2018; Davies et al., 2018; Greig et al., 2019; Wang et al., 2020). While there are still significant differences between the neutral fractions derived by different authors, even for the same targets, the overarching picture is consistent with a reionization process which was still ongoing at z ∼ 7 and which was mostly done at z ∼ 6.

It is, however, important to note, that there are still open questions regard- ing the timing of reionization inferred from quasar spectra. While quasar con- straints have been argued to indicate a reionization process which was mostly finished around z ∼ 6, scenarios in which large patches of neutral hydrogen ex- ist all the way to z ∼ 5.5, and where reionization finishes at z = 5.3, have been argued to better explain the large variation and evolution in Gunn-Peterson troughs at z  5.5 (Kulkarni et al., 2019a).

3.1.2 Constraints from the cosmic microwave background

Another powerful probe of the timing of cosmic reionization is the CMBR.

Since the CMB photons emitted at recombination may scatter off free elec- trons as they travel though space, they are sensitive to the density of free elec- trons in the IGM. An imprint left by this scattering is a damping of small-scale temperature anisotropies in the CMBR. Free electrons generated during reion- ization will lead to a larger optical depth of CMBR photons, through which the timing of the reionization process can be constrained (see e.g. Haiman &

Knox, 1999). For example, an early reionization scenario leads to a larger

optical depth compared to a late reionization scenario, and thus will also lead

to a larger damping of small-scale anisotropies. In addition, the scattering of

Wavelength (Å)

Flux (arb. units)

2400 2450 2500 2550 2600 2650 2700 2750 2800 2850

4800 4900 5000 5100 5200 5300 5400 5500 5600

7800

7600 8000 8200 8400 8600 8800

012301230123

PG1634+706 z=1.337 1422+231 z=3.635

1030+0524 z=6.290

Figure 3.2. The Ly α forest in spectra of quasars observed at z = 1.337 (top), z = 3.635 (middle) and z = 6.290 (bottom). The spectra are shown at observed wavelengths.

The Ly α forest lines, increasing with redshift can be seen on the blue (left) side of the redshifted quasar Ly α emission line, shown to the to the right in the spectra (at

≈ 2840 Å in the top panel). Image credit: Robert Carswell. Retrieved March 1, 2021 from Bob Carswell’s webpage, https://www.ast.cam.ac.uk/~rfc/zevol6e.

jpg. Adapted with permission.

CMB photons on free electrons at reionization affects the polarization in the CMBR. By combining measurements of the polarization with measurements of the temperature anisotropies, the accuracy of optical depth measurements and thus constraints on reionization can be substantially improved (Zaldar- riaga et al., 1997; Eisenstein et al., 1999).

The most recent measurements of CMB anisotropies, including both po-

larization and temperature maps, indicate a midpoint of reionization of z _re =

7 .7 ± 0.7 (Planck Collaboration et al., 2020). Taking into account the effect

of photon scattering off electrons with a bulk velocity (the Kinetic Sunyaev-

Zeldovich effect) allows for a better determination of the duration of reioniza-

tion (Planck Collaboration et al., 2016). This does, however, require a model

of the patchy reionization process. This was first done by Zahn et al. (2012),

who found a value of Δz < 4 (95% confidence limit) using data from the South

Pole Telescope (SPT). Here, Δz is defined as the redshift duration over which

the Universe goes from 80% neutral to 1% neutral. More recently, Planck Col-

laboration et al. (2016) found a value of Δz < 2.5 (95% confidence limit, with

the same Δz definition as Zahn et al., 2012), using Planck data in combination

with data from the Atacama Cosmology Telescope and the SPT. Their results

also indicate that the Universe was less than 10% neutral at z  10.

3.1.3 Constraints from Lyman- α emitters

A third way in which we can constrain the timing of cosmic reionization is through observations of Ly α emitters (LAE) at high redshifts. Since Lyα pho- tons get absorbed by neutral hydrogen, two consequences arise from cosmic reionization. Firstly, we expect to observe a decrease in the number of LAE with increasing redshift at the EoR. Secondly, a patchy reionization process leads to large spatial variations in the neutral fraction, and creates large ion- ized regions through which Ly α is able to escape. Thus, patchy reionization should leave an imprint in the clustering of Ly α-emitting sources.

Observations of Lyman-break galaxies (LBGs; high-redshift galaxies iden- tified via photometry using the rapid decline in flux at 912 Å due to absorption of LyC in the IGM) have shown that the fraction of these that also exhibit Ly α emission declines at z  6 (Pentericci et al., 2014; Schenker et al., 2014; Fu- rusawa et al., 2016). Additional indications of an increased neutral fraction at z  6 come from the Lyα luminosity function. The Lyα luminosity function gives the space density of LAE as a function of Ly α luminosity. Similarly, the UV luminosity function gives the space density of UV-emitting galaxies per UV luminosity or magnitude. Several studies have found a redshift evolution of the Ly α luminosity function at z  6 (Ouchi et al., 2010; Kashikawa et al., 2011; Konno et al., 2014; Zheng et al., 2017; Ota et al., 2017; Konno et al., 2018). This evolution is not matched by a corresponding evolution in the UV luminosity function, indicating that Ly α becomes rarer at these redshifts. Both the decline in the fraction of LBGs which exhibit Ly α and the evolution of the Ly α luminosity function is consistent with an IGM which is increasingly neu- tral at z  6. One has to keep in mind that the possible evolution of galaxy properties may also lead to this effect. However, galaxies at lower redshift seem to follow the opposite trend, with stronger Ly α at increasing redshift, possibly due to the lower dust content (Hayes et al., 2011) and/or lower neu- tral hydrogen covering fractions (Jones et al., 2013) as we move toward earlier times.

Several studies have used the clustering of LAE in combination with reion- ization models to constrain the neutral fraction (Ouchi et al., 2010; Sobacchi &

Mesinger, 2015; Hutter et al., 2015; Ouchi et al., 2018). Recent measurements

from Ouchi et al. (2018), using a sample of 2000 LAE at z ∼ 6−7, found good

consistency with several other constraints on reionization, indicating a neutral

fraction of 0 .15 ± 15 at z ∼ 6.6. Slightly higher neutral fractions are inferred

from the evolution of Ly α equivalent widths in Lyman-break galaxies at a

similar redshift. For example, Mason et al. (2018); Hoag et al. (2019) found

neutral fractions of 0.59 ⁰ _−0.15 ^.11 at z ∼ 7 and 0.88 ⁰ _−0.10 ^.05 at z = 7.6 ± 0.6.

3.2 Driving sources of cosmic reionization

There is still very much an ongoing debate regarding the details of reionization and its driving sources, ranging from star-forming galaxies to quasars to ex- otic sources of ionizing radiation. With the help of the constraints discussed in the previous sections, it is possible to infer which astrophysical sources likely drove the cosmic reionization. Over the years, backed by a large number of observational and theoretical studies, star-forming galaxies have become the main candidate for driving reionization (e.g. Faucher-Giguère et al., 2008; Ra- zoumov & Sommer-Larsen, 2010; Becker & Bolton, 2013; Robertson et al., 2015; Kulkarni et al., 2019b; Dayal et al., 2020). However, a galaxy-driven reionization hinges on the amount of LyC that escapes from EoR galaxies, which is still highly uncertain. A number of analyzes have shown that current constraints on reionization seem consistent with a purely galaxy-driven reion- ization if galaxies have average escape fractions somewhere around 10 – 30%

(Finkelstein et al., 2012a; Sun & Furlanetto, 2016; Mitra et al., 2018; Naidu et al., 2020). There are, however, also studies that present scenarios where star-forming galaxies are able to drive the cosmic reionization with a substan- tially lower average escape fraction ( < 5%; see e.g. Finkelstein et al., 2019).

As discussed in section 5.2, there are now a substantial number of detections of leaking LyC from galaxies up to redshift ∼ 4, but whether galaxies in the EoR generally exhibit escape fractions f _esc  10% is still somewhat unclear.

There are also other open questions regarding the role of star-forming galax- ies in reionization. For example, the shape of the UV luminosity function at the faint end, and therefore the contribution of objects too faint to be observed at present, is still uncertain (e.g. Stark, 2016). This is especially interesting considering the large number of theoretical studies that highlight the contribu- tion of low mass galaxies to the total ionizing budget (Razoumov & Sommer- Larsen, 2010; Ferrara & Loeb, 2013; Wise et al., 2014; Paardekooper et al., 2015; Liu et al., 2016; Dayal et al., 2017, 2020). In addition to low-mass galaxies being more numerous than high-mass galaxies, simulations also indi- cate that the more shallow potential wells in low-mass objects may facilitate high escape fractions (Razoumov & Sommer-Larsen, 2010; Ferrara & Loeb, 2013; Wise et al., 2014; Paardekooper et al., 2015). In contrast, Sharma et al.

(2016); Naidu et al. (2020) found that bright objects that are within current

detection limits release the bulk of the LyC photons required for the cosmic

reionization. Their result showed that the escape fraction may follow the op-

posite trend, with higher escape fractions in bright, massive objects. A reion-

ization process primarily driven by massive galaxies seems, however, to be at

odds with recent constraints from the Low Frequency Array (LOFAR; Mondal

et al., 2020). There are also uncertainties regarding the properties of the high-

redshift galaxy population in general. The escape fraction of ionizing photons

and/or ionizing production efficiency may increase at higher redshifts, possi-

bly complicating inferences made using observations at low redshifts (see e.g.

Inoue et al., 2006; Becker & Bolton, 2013; Stark et al., 2015; Bouwens et al., 2016).

There is also an ongoing debate regarding the role of quasars and other types of AGN in the cosmic reionization. These objects are not only efficient pro- ducers of ionizing radiation, but some evidence also suggests that they exhibit high ionizing escape fractions ( f _esc  50%, (Cristiani et al., 2016; Grazian et al., 2018; Romano et al., 2019)). Often, the escape fraction of quasars is assumed to be 100%. However, there is still some uncertainty in these values, with some observations pointing toward lower escape fractions (Cowie et al., 2009; Micheva et al., 2017). One of the main arguments against an AGN- driven reionization is simply that the space density of these objects decreases at z  3 (e.g. Fan et al., 2004; Masters et al., 2012), which would make them too rare during EoR to produce the bulk of the ionizing photons.

A recent detection of a faint population of AGN at z ∼ 4 − 6 by (Giallongo et al., 2015) re-sparked this debate, and a number of recent studies have found that AGN may have significantly contributed to reionization (Boutsia et al., 2018; Giallongo et al., 2019; Grazian et al., 2020). However, other studies have been unable to verify these results, and the larger part of the literature still points toward a subdominant contribution to the reionization photon budget by AGN (Cowie et al., 2009; Willott et al., 2010; Kashikawa et al., 2015;

Ricci et al., 2017; Onoue et al., 2017; McGreer et al., 2018; Parsa et al., 2018;

Kulkarni et al., 2019b; Cowie et al., 2020; Shin et al., 2020; Shen et al., 2020;

Kim et al., 2020). This scenario is also consistent with a number of recent theoretical studies which favor a galaxy-driven reionization process (e.g. Qin et al., 2017; Mitra et al., 2018; Hassan et al., 2018; Puchwein et al., 2019).

While most of the discussion regarding the sources of reionization has fo-

cused on star-forming galaxies and AGN, it is likely that other sources, such

as X-ray binaries and microquasars, also contributed to reionization. Whether

this contribution was substantial, or only marginal is, however, still unclear

(Tanaka et al., 2012; Madau & Fragos, 2017; Douna et al., 2018).

4. Properties of high-redshift star-forming galaxies

The key to forming a complete picture of the cosmic reionization lies in un- derstanding the galaxies present in the high-redshift Universe. Part of the work toward this understanding has come down to pushing the sensitivity of spectroscopic observations in order to spectroscopically confirm galaxies at increasingly high redshifts. Such observations have provided successful spec- troscopic detections all the way up to z ∼ 11 ¹ , albeit with steeply decreasing numbers at the highest redshifts. In addition, a large number of both observa- tional and theoretical studies have aimed to provide a more detailed account of the properties of high-redshift galaxies in general. In the following sections, some of these properties will be discussed.

In figure 4.1 an example of a model high-redshift galaxy spectrum is shown.

Some of the spectral features that have been used to constrain properties of the high-redshift galaxy population, and that are discussed in the following sec- tions are indicated in this figure. Also shown in this figure are the wavelength coverages of some of the telescopes/instruments that have been extensively used to study these objects, such as the Hubble space telescope (HST), the Spitzer space telescope, the Very Large Telescope (VLT), the Keck observa- tory and the ALMA observatory. I also indicate the wavelength coverage of the upcoming JWST, which will provide photometric and spectroscopic ca- pabilities covering a large wavelength range, from 0.6 μm to 28 μm with a significantly larger light-collecting area than current space-based facilities.

4.1 Dust properties

In addition to dark matter, stars and gas, most galaxies contain some amount of interstellar dust. This dust resides in the ISM in the form of submicron-sized grains consisting of compounds of metals ² such as carbon, oxygen, magne- sium, silicon and iron (Draine, 2011). Since the elements that are used to build dust grains are metals, the dust content of galaxies is expected to fol- low the metal content of these objects. In addition to affecting the physics inside galaxies, dust has a strong impact on the light that we observe through

1

GN-z11 is currently the most distant galaxy which has been spectroscopically confirmed (Oesch et al., 2016; Jiang et al., 2020)

2

Following common astronomical usage, all elements heavier than He are referred to as metals.

2

1

0

-1

-2

-3

z=7

Wavelength(Å)

10⁴ 10⁵ 10⁶ 10⁷

JWST ALMA (band 4-10)

HST

Spitzer/IRAC

[OIII] 88 m

[CII] 158 m

H

[OII] 3727 Å H

[OIII] 4959, 5007 Å

Rest-frame UV

Continuum Rest-frame FIR dust-continuum

VLT & Keck

log(flux/mJy)

Figure 4.1. Synthetic spectrum from rest-frame UV to FIR wavelengths of a high- redshift galaxy (z=7). Some spectral features that are discussed in the text (such as the UV slope and Balmer emission lines) are indicated in the figure. Also indicated are the wavelength coverages of the upcoming JWST and a selection of facilities/instruments that have been extensively used to study high-redshift galaxies. The wavelength axis is shown in the observer frame, while wavelengths of individual lines are given in the rest-frame. The spectrum was generated using the BAGPIPES code (Carnall et al., 2018).

effects of scattering and absorption. An incomplete understanding of the dust properties of high-redshift galaxies may make other observations and their in- terpretations prone to errors.

Since dust extinguishes light more effectively at short wavelengths, inter- stellar dust reddens the spectra of background stars in galaxies. The strength of this effect at different wavelengths is commonly described by a so-called extinction or attenuation law. Often, the terms attenuation law and extinction law are used interchangeably. However, there is a fundamental difference in that an extinction law technically only considers absorption and scattering of light out of the line of sight, while attenuation considers absorption and scat- tering of light both out of and into the line of sight. Examples of extinction laws are the Milky way, Large magellanic cloud (LMC) and Small magellanic cloud (SMC) laws (e.g. Pei, 1992), which differ due to the grain size and com- position of the dust. On the other hand, the Calzetti law (Calzetti et al., 2000) is an attenuation law derived using observations of nearby starburst galaxies.

The reddening will affect not only the continuum of observed galaxies but

also line emission and the ratios of lines at different wavelengths. Since the

relative ratios of certain emission lines are well constrained from theory (for

example, the Balmer lines of hydrogen), the observed line ratios can be used

to correct the spectrum of an observed galaxy for dust. In the absence of lines

that can be used for this correction, a feature which can also be used to con- strain the dust content in star-forming galaxies is the spectral slope of the UV continuum (see Rest-frame UV continuum in figure 4.1). Since the intrinsic UV slope (in the absence of dust-reddening) mainly depends on the nature of the most massive and hot stars, it is also sensitive to the SFH and metallicity (and to some degree the escape fraction of ionizing photons; Wilkins et al., 2016). Translating measured UV slopes to dust attenuation thus requires some assumption regarding the intrinsic UV slope, either from simulations and mod- els (e.g. Bouwens et al., 2014; Wilkins et al., 2016), or from empirical relations (e.g. Meurer et al., 1999).

Usually, the UV slope is parametrized with a power law f _λ ∝ λ ^β with a slope β (e.g. Calzetti et al., 1994), and slopes where β < −2 are referred to as

‘blue’ while slopes with β > −2 are referred to as ‘red’. This is since a UV continuum with β = −2 is flat when expressed in units of Jy, and thus has zero color in the AB magnitude system. For galaxies at high redshift, the UV slope is typically determined through HST imaging, sometimes in combination with ground-based imaging, using, for example, the connection:

β = − m 1 − m 2

2 .5log(λ c ¹ /λ c ² ) − 2 (4.1) Where m 1 , m 2 are the measured magnitudes, and λ c ¹ , λ c ² are the central wavelengths of the two imaging bands, respectively (Ono et al., 2010). It is important to note that this can lead to large uncertainties in the derived UV slopes, since if the available filters used to derive the slope are relatively close in wavelength, the short leverage means that small differences in the fluxes can lead to large differences in the UV slope.

Observations of large samples of galaxies at z  4 have highlighted two important features in the evolution of the UV slope. Firstly, there seems to be a correlation between UV slope and luminosity, such that galaxies with low luminosities exhibit the bluest UV slopes (Rogers et al., 2014; Bouwens et al., 2014). This correlation is also seen in theoretical studies (e.g. Dayal &

Ferrara, 2012; Mancini et al., 2016). Secondly, there seems to be an evolu- tion toward bluer UV slopes at higher redshifts (e.g. Finkelstein et al., 2012b;

Bouwens et al., 2014). This redshift evolution is, however, relatively mild,

and even at z ∼ 10, observed UV slopes seem to be consistent with, at least,

some amount of dust (Wilkins et al., 2016). Nevertheless, the redshift and

luminosity evolution of the UV slope have been interpreted as an effect of

chemical maturity, and that low luminosity and high-redshift objects contain

less metals and dust, and thus represent less evolved populations. This is also

reflected in several theoretical studies of the UV luminosity function at the

highest redshifts, where the amount of dust required to make observations

and theory fit is lower than in the low-redshift Universe (e.g. Hutter et al.,

2014; Khakhaleva-Li & Gnedin, 2016). Note however, that some simulations

find good agreement with UV luminosity functions while simultaneously pre-

dicting significant amounts of dust already at z ∼ 8, especially at high stellar masses (e.g. Wilkins et al., 2018). As discussed in section 3.1.3, a gradually lower dust content with increasing redshift has also been suggested as an ex- planation for the increasing Ly α emission between z ∼ 0 and z ∼ 6.

As light is absorbed by dust at optical and UV wavelengths, the dust is heated. This heat is then radiated away in the infrared part of the spectrum.

In addition to providing an excellent tool for spectroscopic redshift determi- nation and probing ISM properties, ALMA has been used as a powerful tool to study this thermal emission in EoR galaxies. Many observations of the FIR dust emission of galaxies at z  6 have only led to upper limits (see e.g.

Matthee et al., 2019, for a compilation). This is consistent with the blue UV slopes observed by HST and seems to indicate that high-redshift star-forming galaxies in general contain less dust than nearby star-forming galaxies, and have FIR/UV ratios more consistent with local metal-poor dwarf galaxies (e.g.

Maiolino et al., 2015; Matthee et al., 2019). However, a number of objects with significant dust emission also have been detected, indicating some scatter in the dust content of EoR galaxies (e.g. Watson et al., 2015; Laporte et al., 2017; Bowler et al., 2018; Tamura et al., 2019; Hashimoto et al., 2019). The interpretation of these results in terms of actual dust masses depends on as- sumptions regarding the dust temperature (as well as dust grain properties) in these objects (e.g. Behrens et al., 2018) which, while there are studies indicat- ing higher dust temperatures at high-redshifts (Bakx et al., 2020, e.g.), are still uncertain.

In addition, FIR observations in combination with UV observations provide a way to constrain the shape of the dust reddening law which best represents high-redshift galaxies trough the relation between the infrared excess (IRX) and the UV slope β (IRX-β relation). The shape of the reddening law at inter- mediate and high redshifts is still strongly debated, with some studies claiming consistency between observations and steep SMC-like relation (or possibly, an even steeper relation; Smit et al., 2018; Bouwens et al., 2016). Other studies find better consistency with the Meurer et al. (1999, also commonly called the Calzetti IRX-β relation) relation for local starburst galaxies (Koprowski et al., 2018; Hashimoto et al., 2019). Using a large sample of galaxies at z ∼ 4 − 6 from the ALPINE ALMA survey, Fudamoto et al. (2020) derive dust laws at z ∼ 4.5 and z ∼ 5.5 which are steeper than the SMC law. The translation between FIR dust continuum observations to IR luminosities often, however, relies on assumptions both regarding dust temperatures and grain properties.

It is important to note that the inferred IRX- β relations are therefore also sen- sitive to such assumptions (e.g. Behrens et al., 2018).

An interesting complication is the distribution of dust in galaxies. While

dust-corrections of observations are often done assuming a so-called dust-

screen, i.e. that dust affects all stars equally, we expect dust to be located

preferentially around young stars (Charlot & Fall, 2000). This can occur since

the birth-clouds of stellar clusters are dense, and are eventually dispersed by

supernovae and stellar winds as stars evolve. The timescale of this effect is expected to be relatively short, and birth-clouds seem to generally disperse within ∼ 10 Myr (e.g. Charlot & Fall, 2000; Chevance et al., 2020) once mas- sive stars start to form. Recently, Katz et al. (2019b) argued that differential obscuration, with high dust attenuation for stars with ages up to ∼ 50 Myr, may be a possible explanation for the large Balmer breaks observed in certain high-redshift objects.

4.2 Star formation

As discussed briefly in chapter 2, the stars responsible for producing the bulk of the ionizing photons in a galaxy are the young and massive stars. These do not only give rise to the emission lines we observe in the optical, but also to the majority of the rest-frame UV flux, and therefore, these features can be used to estimate the recent star formation activity in galaxies.

A commonly used and reliable way to estimate the SFR at lower redshifts is through the use of the H α line. However, observing this line in galaxies during EoR is not possible at present due to the lack of observational facilities with spectroscopic capabilities and sufficient sensitivity at relevant wavelengths, something which the JWST will remedy in the near future. The Ly α line also traces SFR and is observable at high redshifts, however, as discussed in chapter 2 and section 3.1.3, the usage of Ly α as an SFR tracer is hampered not only by the complex transport of Ly α through the ISM (Hayes et al., 2010), but also due to absorption in the IGM.

The most common way to infer star formation rates for high-redshift galax- ies is through their rest-frame UV continuum around 1500 Å. This relies on some assumptions regarding the relative distribution of stars of different masses (the initial mass function; IMF) and the timescales of variations in the SFR, which are assumed to be long (  50 – 100 Myr), (Kennicutt & Evans, 2012; Madau & Dickinson, 2014). Since the temperature of the stars, and thus the hardness of the spectrum is affected by the metallicity of the stars, the above-mentioned calibrations also have a slight dependence on metallicity. In addition, since UV photons are efficiently absorbed by dust, UV SFRs only trace the star formation which is not obscured by dust. In order to trace the obscured star formation, one can probe the IR SFR by, for example, observing the FIR dust emission of high-redshift galaxies using ALMA (see section 4.3 and paper IV).

Using such measurements of the SFRs in galaxies over a wide redshift range, it has been possible to show that the comoving ³ star formation rate

3

Comoving distances factor out the cosmic expansion, and thus do not change with the expan-

sion of space. This is in contrast to the proper distance, which will change as the Universe

expands. Comoving distances are often denoted by a lowercase ‘c’ in front of the unit, for

example cMpc

⁻³

.

density (SFRD; M _ yr ⁻¹ cMpc ⁻³ ) was increasing with time at high redshifts, peaked at around z ∼ 2 and declined at z  2 (Madau & Dickinson, 2014). Ad- ditionally, observations have shown that the specific star formation rate (star formation rate over stellar mass; sSFR, usually expressed in units of Gyr ⁻¹ ) increases with increasing redshift (Madau & Dickinson, 2014; Salmon et al., 2015). The emerging picture of these results is that the Universe was much more active earlier in its evolution.

Another way to try to understand the star formation activity at high redshifts is through simulations. Generally, simulations suggest that the SFR in galaxies was increasing over time during the EoR (e.g. Jaacks et al., 2012; Shimizu et al., 2014; Kimm et al., 2015; Katz et al., 2019b). There is, however, a significant difference between the variations in the SFR over time in different simulations. Some simulations suggest that star formation rates were quite smoothly increasing over time (Finlator et al., 2011). Generally, galaxies with higher masses tend to experience smoother star formation histories (SFHs;

the SFR as a function of time), owing to their deeper gravitational potential wells. This means that feedback, such as stellar winds or supernova explosions will be less effective at dispersing and removing the gas available for star- formation. Furthermore, as massive galaxies are likely to contain a larger number of regions which are able to host star formation, galaxy-wide SFRs smooth out and become less stochastic in these objects (Hopkins et al., 2014;

Kimm et al., 2015; Yajima et al., 2017). Many simulations produce large variations in the SFHs of EoR galaxies, especially in low mass galaxies (e.g.

Trebitsch et al., 2017), but also in galaxies with stellar masses up to a couple of 10 ⁸ M _ (Kimm & Cen, 2014; Kimm et al., 2015; Ma et al., 2015; Yajima et al., 2017; Ma et al., 2018). In paper II we showed that such fluctuations can lead to severe mis-interpretations of the spectral energy distributions (SEDs) of EoR galaxies if ignored.

On the observational side, recent observations of the z = 9.1096 galaxy MACS1149-JD1 indicate a strong Balmer break in the spectrum of this galaxy (Hashimoto et al., 2018). Since this break is strongest in A type stars, it be- comes more prominent as the more massive O and B type stars evolve off the main sequence and die. It is therefore associated with an aged population of stars (Wiklind et al., 2008). An old stellar population would not be unusual if observed in the low-redshift universe, but at z ∼ 9.1 it could have inter- esting implications for when the first stars formed in these types of objects.

Hashimoto et al. (2018) suggest that the strong break is due to large varia- tions in the star formation activity, and that MACS1149-JD1 has undergone a long passive period before a second burst of star formation occurred. In pa- per III, we showed that several different simulations are unable to reproduce a Balmer break of the size observed in MACS1149-JD1, even when account- ing for different assumptions on the dust reddening, escape fraction or IMF.

Similar features have been observed in other non-spectroscopically confirmed

high-redshift galaxies. While these are more uncertain, this could indicate

that SFHs were significantly more bursty than found in current simulations.

However, Katz et al. (2019b) have argued that dense ISM conditions, with dust preferentially located around younger stars could also explain the strong Balmer break in MACS1149-JD1 without requiring large variations in the star formation activity. In addition, recent results by Stefanon et al. (2021) seem to indicate the Balmer breaks such as the one observed in MACS1149-JD1 are not very common.

4.3 Far-infrared observations and the interstellar medium

In the high-redshift universe, optical lines commonly used to determine spec- troscopic redshifts and derive galaxy properties, such as H α, shift into wave- lengths unavailable to current observational facilities with spectroscopic capa- bilities. Nonetheless, at the other end of the spectrum, ALMA has provided the high-redshift community with a way to probe EoR galaxies through observa- tions of rest-frame FIR emission lines and dust-emission, since these redshift into sub-millimeter/millimeter wavelengths. As shown in figure 4.1, there are several emission lines that fall within the wavelength span of ALMA. To date, the two most commonly targeted lines are the [C II ] 158 μm and [O ^III ] 88 μm lines.

Due to the difference in ionization energies of the species responsible for the formation of these lines, they arise in different parts of the ISM. While part of the [C II ] emission is produced in H II regions, it is mainly generated in photo-dissociation regions (PDRs). PDRs are regions of gas in the ISM in which the chemical structure is strongly influenced by UV photons with en- ergies lower than that required to ionize hydrogen. In these regions, the UV flux is sufficiently hard to photo-dissociate molecules and photo-ionize atomic species with ionization potentials lower than that of hydrogen, but not suffi- ciently hard to ionize hydrogen. Thus, these are regions where hydrogen is mostly neutral or in molecular form. The [O III ] line, on the other hand, origi- nates in highly ionized gas in H II regions (e.g. Abel et al., 2005; Nagao et al., 2011; Vallini et al., 2015). The low energy required to excite the states respon- sible for the lines means that they are easily excited through collisions, and therefore provide important cooling in their respective environments. Their forbidden and long-lived nature, however, also means that they are sensitive to collisional de-excitation, making the lines sensitive to the density of the ISM (Draine, 2011).

Many early attempts to detect the [C II ] line from high-redshift galaxies

led to non-detections (Kanekar et al., 2013; Ouchi et al., 2013; Ota et al.,

2014; Maiolino et al., 2015; Schaerer et al., 2015). Several of the upper limits

derived for the emission line from these objects placed them below the [C II ]-

SFR relation derived for local galaxies by De Looze et al. (2014). Since then,

several [C II ] detections have been made in EoR galaxies, although a number of studies have continued to report non-detections (see compilations by e.g.

Matthee et al., 2019; Harikane et al., 2020). While the [O III ] line has been targeted in fewer high-redshift galaxies, it has been shown to be a reliable line for spectroscopic redshift confirmation, with a high detection rate all the way up to z ∼ 9.1 (Inoue et al., 2016; Laporte et al., 2017; Carniani et al., 2017;

Hashimoto et al., 2018; Tamura et al., 2019; Hashimoto et al., 2019; Harikane et al., 2020).

Over the years, the fact that a part of the high-redshift population exhibits relatively weak [C II ] emission has led to a discussion on whether high-redshift galaxies follow a different [C II ]-SFR relation than local galaxies. In a recent study of a compilation of galaxies at z ∼ 6 – 9, Harikane et al. (2020) derived a [C II ]-SFR relation for high-redshift galaxies that is significantly steeper than the local relation by De Looze et al. (2014, see figure 4.2).

On the other hand, other studies have found that if one accounts for galaxy sub-components and proper association of [C II ] and UV components, high- redshift galaxies follow a [C II ]-SFR relation which is consistent with the lo- cal one, albeit with significantly larger scatter (Carniani et al., 2018). Using a similar compilation as the one by Harikane et al. (2020), Matthee et al. (2019) argued that by using a consistent way to derive SFRs and [C II ] upper lim- its, one can get better agreement between high-redshift galaxies and the local relation at high SFRs. However, they also found a possible deviation at low SFRs.

Furthermore, using a large sample of z ∼ 4 − 6 galaxies from the recent ALPINE ALMA survey in combination with the compilation of Matthee et al.

(2019) and other high-redshift observations, Schaerer et al. (2020) found a re- lation with only a slightly steeper slope and lower normalization than the local relation (see figure 4.2). Similar results are found in a recent study by Carniani et al. (2020), where the authors have re-analyzed several high-redshift [C II ] datasets while compensating for surface brightness dimming due to extended [C II ] emission. Carniani et al. (2020), however, have also highlighted that the dispersion in the high-redshift relation is significantly higher than that ob- served in local H II and Starburst galaxies, which indicates a broader range in ISM properties. There are still uncertainties in the derived relations, partly due to the number of [C II ] undetected galaxies at high redshifts (especially with low SFRs), and partly due to assumptions used to derive SFRs in these objects, both from the UV and IR. A clear consensus on the [C II ]-SFR relation at high redshifts has yet to be reached.

Since the abundance of C ⁺ in the ISM scales with the gas metallicity, the

weak [C II ] seen in some high-redhift galaxies has been argued be an effect

of a low metallicity (e.g. Pentericci et al., 2016; Knudsen et al., 2016; Bradaˇc

et al., 2017; Matthee et al., 2017; Harikane et al., 2018). Using cosmological

simulations coupled with a UV radiative transfer code and a PDR modeling

code, Vallini et al. (2015) showed that a low gas metallicity could, in princi-

SFR

_UV+IR

(M
yr

^-1

) L

[C II]

(L )

Figure 4.2. The [C II ] 158 μm luminosity plotted versus the (UV+IR) SFR. The dark circles show z > 6 literature sample used in paper IV and the new upper-limit reported there. The local relation by De Looze et al. (2014) and its dispersion is shown by the yellow line and shaded region. The dashed and dash-dotted line shows the relations from the ALPINE ALMA survey, derived using 3σ and 6σ upper limits for non- detections, respectively. The dotted line shows the relation by Harikane et al. (2020).

ple, explain the low [C II ] observed in some high-redshift galaxies. They also highlight that strong stellar feedback could lead to weak [C II ] through the destruction of molecular clouds. However, using models calculated with the

CLOUDY photoioniztion code, Harikane et al. (2020) in a later study argued that the gas metallicity does not significantly affect the [C II ] luminosity. Even so, Harikane et al. (2020) highlight a secondary effect of a lower metallic- ity (specifically the stellar metallicity), which is a harder radiation field. This harder radiation may penetrate further into, and ionize a larger part of, the ISM, and therefore also regulate the [C II ] luminosity. Such a scenario could lead to lower [C II ] line strengths through the destruction of PDRs and growth of the highly ionized regions in which the [O III ] 88 μm emission line forms, and thus to stronger [O III ] emission and therefore, high [O III ]-to-[C II ] ratios.

Of course, the [O III ] line is also sensitive to the gas metallicity. Inoue et al.

(2014) have argued that their models best reproduce observations when the

[O III ] luminosity increases with increasing metallicity at Z  0.2 Z _ due to

an increased abundance of oxygen in the ISM, but turns over, and starts de-

creasing with increasing metallicity at Z  0.2 Z _ , possibly due to the change

in the hardness of the stellar radiation field. Thus, the [C II ] and [O III ] lines

are likely to some degree regulated by the combined effect of stellar and gas

metallicity.

The scenario of a hard radiation field driven by low stellar metallicity (or a young stellar population, see below) would seem consistent with a number of recent studies that have found high [O III ]-to-[C II ] ratios in high-redshift ob- jects (Inoue et al., 2016; Hashimoto et al., 2019; Harikane et al., 2020; Bakx et al., 2020). Recently, Harikane et al. (2020) found that the [O III ]-to-[C II ] ratios at high redshifts are systematically higher than those of local galax- ies. Recent results from Carniani et al. (2020), who reported marginal de- tections in several previously [C II ] un-detected objects by considering more extended [C II ] emission, point toward [O III ]-to-[C II ] ratios that are still high, but closer to those observed in local galaxies.

Alternatively, a low covering fraction of PDRs due to feedback could lead to low [C II ] and high [O III ]-to-[C II ] ratios (Harikane et al., 2020). Both the scenario of a hard radiation field and lower PDR covering fractions could, in principle, facilitate the escape of LyC, since they would lead to lower cover- ing fractions of neutral gas (Inoue et al., 2016; Harikane et al., 2020). Overall, both of these scenarios are also consistent with the apparent anti-correlation between [C II ] and Ly α observed by Carniani et al. (2018); Harikane et al.

(2018); Matthee et al. (2019); Harikane et al. (2020), since a lower neutral cov- ering fraction also would facilitate the escape of Ly α. Furthermore, the Lyα line becomes stronger with the harder radiation produced by low-metallicity stars (Raiter et al., 2010). On the other hand, Schaerer et al. (2020) are not able to reproduce the [C II ]-Ly α anti-correlation previously mentioned and ar- gued that this is due to their larger dataset and a more consistent way to derive SFRs.

It is important to highlight that there are still other mechanisms that could affect these lines. For example, a low C/O abundance ratio could also lead to lower [C II ] emission and higher [O III ]-to-[C II ] ratio (Harikane et al., 2018, 2020; Arata et al., 2020). Since oxygen is formed mainly in core-collapse supernovae, while carbon has a contribution from type Ia supernovae as well as AGB stars, carbon forms on slower time-scales (Maiolino & Mannucci, 2019). In young and un-enriched stellar populations, the C/O abundance ratio is expected to be lower, and [O III ]-to-[C II ] ratios are expected to be higher (Harikane et al., 2020; Arata et al., 2020). The density of the ISM also af- fects how far ionizing photons will penetrate, and how often collisions that can potentially de-excite [C II ] and [O III ] occur, and will also affect the line strengths and ratio (Ferrara et al., 2019; Harikane et al., 2020). Since the ratio of these lines is sensitive to the ionizing flux from the nearby stars and pos- sible feedback, the lines are also affected by a bursty star-formation history.

High [O III ]-to-[C II ] line ratios and [C II ] deficiencies would, for example, be

expected for objects that are currently undergoing a burst, and have large pop-

ulations of young stars (Pallottini et al., 2019; Katz et al., 2019a; Ferrara et al.,

2019). Furthermore, the [C II ] line forms in relatively cool regions, which

means that heating from the CMBR and decreased contrast compared to the