High-redshift Galaxy Population Research Articles

High-redshift Merger Model for Low-frequency Gravitational Wave Background

In 2023, the Pulsar Timing Array Collaborations announced the discovery of a gravitational wave background (GWB), predominantly attributed to supermassive black hole binary (SMBHB) mergers. However, the detected GWB is several times stronger than the default value expected from galactic observations at low and moderate redshifts. Recent findings by the James Webb Space Telescope have unveiled a substantial number of massive, high-redshift galaxies, suggesting more massive SMBHB mergers at these early epochs. Motivated by these findings, we propose an “early merger” model that complements the standard merger statistics by incorporating these early, massive galaxies. We compare the early and standard “late merger” models, which assume peak merger rates in the local Universe, and match both merger models to the currently detected GWB. Our analysis shows that the early merger model has a significantly lower detection probability for single binaries and predicts a ∼30% likelihood that the first detectable single source will be highly redshifted and remarkably massive with rapid frequency evolution. In contrast, the late merger model predicts a nearly monochromatic first source at low redshift. The future confirmation of an enhanced population of massive high-redshift galaxies and the detection of fast-evolving binaries would strongly support the early merger model, offering significant insights into the evolution of galaxies and SMBHs.

First Detection of CO Isotopologues in a High-redshift Main-sequence Galaxy: Evidence of a Top-heavy Stellar Initial Mass Function

Recent observations and theories have presented a strong challenge to the universality of the stellar initial mass function (IMF) in extreme environments. A notable example has been found for starburst conditions, where evidence favors a top-heavy IMF, i.e., there is a bias toward massive stars compared to the IMF that is responsible for the stellar mass function and elemental abundances observed in the Milky Way. Local starburst galaxies have star formation rates similar to those in high-redshift main-sequence galaxies, which appear to dominate the stellar mass budget at early epochs. However, the IMF of high-redshift main-sequence galaxies is yet to be probed. Since 13CO and C18O isotopologues are sensitive to the IMF, we have observed these lines toward four strongly lensed high-redshift main-sequence galaxies using the Atacama Large Millimeter/submillimeter Array. Of our four targets, SDSS J0901+1814, at z ≈ 2.26, is seen clearly in 13CO and C18O, the first detection of CO isotopologues in the high-redshift main-sequence galaxy population. The observed 13C/18O ratio, 2.4 ± 0.8, is significantly lower than that of local main-sequence galaxies. We estimate the isotope ratio, oxygen abundance, and stellar mass using a series of chemical evolution models with varying star formation histories and IMFs. All models favor an IMF that is more top-heavy than that of the Milky Way. Thus, as with starburst galaxies, main-sequence galaxies in the high-redshift Universe have a greater fraction of massive stars than a Milky Way IMF would imply.

At the end of cosmic noon: Short gas depletion times in unobscured quasars at z ∼ 1

Unobscured quasars (QSOs) are predicted to be the final stage in the evolutionary sequence from gas–rich mergers to gas–depleted, quenched galaxies. Studies of this population, however, find a high incidence of far–infrared–luminous sources–suggesting significant dust-obscured star formation–but direct observations of the cold molecular gas fuelling this star formation are still necessary. We present a NOEMA study of CO(2–1) emission, tracing the cold molecular gas, in ten lensed z = 1 − 1.5 unobscured QSOs. We detected CO(2–1) in seven of our targets, four of which also show continuum emission (λrest = 1.3 mm). After subtracting the foreground galaxy contribution to the photometry, spectral energy distribution fitting yielded stellar masses of 109 − 11 M⊙, with star formation rates of 25−160 M⊙ yr−1 for the host galaxies. These QSOs have lower LCO′ than star–forming galaxies with the same LIR, and show depletion times spanning a large range (50−900 Myr), but with a median of just 90(αCO/4) Myr. We find molecular gas masses in the range ≤2−40 × 109(αCO/4) M⊙, which suggest gas fractions above ∼50% for most of the targets. Despite the presence of an unobscured QSO, the host galaxies are able to retain significant amounts of cold gas. However, with a median depletion time of ∼90 Myr, the intense burst of star formation taking place in these targets will quickly deplete their molecular gas reservoirs in the absence of gas replenishment, resulting in a quiescent host galaxy. The non–detected QSOs are three of the four radio–loud QSOs in the sample, and their properties indicate that they are likely already transitioning into quiescence. Recent cosmological simulations tend to overestimate the depletion times expected for these z ∼ 1 QSO–host galaxies, which is likely linked to their difficulty producing starbursts across the general high-redshift galaxy population.

Dark-ages reionization and galaxy formation simulation – XXI. Constraining the evolution of the ionizing escape fraction

ABSTRACT The fraction of ionizing photons that escape their host galaxies to ionize hydrogen in the intergalactic medium (IGM) is a critical parameter in analyses of the reionization era. In this paper, we use the meraxes semi-analytic galaxy formation model to infer the mean ionizing photon escape fraction and its dependence on galaxy properties through joint modelling of the observed high redshift galaxy population and existing constraints on the reionization history. Using a Bayesian framework, and under the assumption that escape fraction is primarily related to halo mass, we find that the joint constraints of the ultraviolet luminosity function, cosmic microwave background optical depth, and the Ly α forest require an escape fraction of $(18\pm 5)$ per cent for galaxies within haloes of M ≲ 109 M⊙ and $(5\pm 2)$ per cent for more massive haloes. In terms of galaxy properties, this transition in escape fraction occurs at stellar masses of M⋆ ∼ 107 M⊙, nearly independent of redshift. As a function of redshift, reionization is dominated by the smaller M⋆ ≲ 107 M⊙ galaxies with high escape fractions at z ≳ 6 and by the larger M⋆ ≳ 107 M⊙ galaxies with lower escape fractions at z ≲ 6. Galaxies with star formation rates of 10−2.5 M⊙yr−1 to 10−1.5 M⊙yr−1 provide the dominant source of ionizing photons throughout reionization. Our results are consistent with recent direct measurements of a $\sim 5~{{\ \rm per\ cent}}$ escape fraction from massive galaxies at the end of reionization and support the picture of low mass galaxies being the dominant sources of ionizing photons during reionization.

PRUSSIC

We present deep ALMA Band 3 observations of the HCN, HCO+, and HNC(4–3) emission in SDP.81, a well-studied z = 3.042, strongly lensed galaxy. These lines trace the high-density gas, which remains almost entirely unexplored in z ≥ 1 galaxies. Additionally, these dense-gas tracers are potentially powerful diagnostics of the mechanical heating of the interstellar medium. While the HCN(4–3) and HNC(4–3) lines are not detected, the HCO+(4–3) emission is clearly detected and resolved. This is the third detection of this line in a high-redshift star-forming galaxy. We find an unusually high HCO+/HCN intensity ratio of ≥2.2. Based on the modelling of the photodissociation region, the most likely explanation for the elevated HCO+/HCN ratio is that SDP.81 has low mechanical heating, making up less than 10% of the total energy budget, along with a sub-solar metallicity of Z ≈ 0.5 Z⊙. While such conditions might not be representative of the general population of high-redshift dusty galaxies, a lower-than-solar metallicity might significantly impact gas masses inferred from CO observations. In addition, we report the detection of CO(0–1) absorption from the foreground lensing galaxy and CO(1–0) emission from a massive companion to the lensing galaxy, approximately 50 kpc to the south-east.

The MillenniumTNG project: the galaxy population at z ≥ 8

ABSTRACT The early release science results from JWST have yielded an unexpected abundance of high-redshift luminous galaxies that seems to be in tension with current theories of galaxy formation. However, it is currently difficult to draw definitive conclusions form these results as the sources have not yet been spectroscopically confirmed. It is in any case important to establish baseline predictions from current state-of-the-art galaxy formation models that can be compared and contrasted with these new measurements. In this work, we use the new large-volume ($L_\mathrm{box}\sim 740 \, \mathrm{cMpc}$) hydrodynamic simulation of the MillenniumTNG project, suitably scaled to match results from higher resolution – smaller volume simulations, to make predictions for the high-redshift (z ≳ 8) galaxy population and compare them to recent JWST observations. We show that the simulated galaxy population is broadly consistent with observations until z ∼ 10. From z ≈ 10–12, the observations indicate a preference for a galaxy population that is largely dust-free, but is still consistent with the simulations. Beyond z ≳ 12, however, our simulation results underpredict the abundance of luminous galaxies and their star-formation rates by almost an order of magnitude. This indicates either an incomplete understanding of the new JWST data or a need for more sophisticated galaxy formation models that account for additional physical processes such as Population III stars, variable stellar initial mass functions, or even deviations from the standard ΛCDM model. We emphasize that any new process invoked to explain this tension should only significantly influence the galaxy population beyond z ≳ 10, while leaving the successful galaxy formation predictions of the fiducial model intact below this redshift.

Two modes of LyC escape from bursty star formation: implications for [C ii] deficits and the sources of reionization

ABSTRACT We use the SPHINX20 cosmological radiation hydrodynamics simulation to study how Lyman continuum (LyC) photons escape from galaxies and the observational signatures of this escape. We define two classes of LyC leaker: Bursty Leakers and Remnant Leakers, based on their star formation rates (SFRs) that are averaged over 10 Myr (SFR10) or 100 Myr (SFR100). Both have $f_{\rm esc}\gt 20{{\ \rm per\ cent}}$ and experienced an extreme burst of star formation, but Bursty Leakers have SFR10 > SFR100, while Remnant Leakers have SFR10 < SFR100. The maximum SFRs in these bursts were typically ∼100 times greater than the SFR of the galaxy prior to the burst, a rare 2σ outlier among the general high-redshift galaxy population. Bursty Leakers are qualitatively similar to ionization-bounded nebulae with holes, exhibiting high ionization parameters and typical H ii region gas densities. Remnant Leakers show properties of density-bounded nebulae, having normal ionization parameters but much lower H ii region densities. Both types of leaker exhibit [C ii]158μm deficits on the [C ii]–SFR100 relation, while only Bursty Leakers show deficits when SFR10 is used. We predict that [C ii] luminosity and SFR indicators such as H α and M$_{\rm 1500\mathring{\rm A}}$ can be combined to identify both types of LyC leaker and the mode by which photons are escaping. These predictions can be tested with [C ii] observations of known z = 3–4 LyC leakers. Finally, we show that leakers with $f_{\rm esc}\gt 20{{\ \rm per\ cent}}$ dominate the ionizing photon budget at z ≳ 7.5 but the contribution from galaxies with $f_{\rm esc}\lt 5{{\ \rm per\ cent}}$ becomes significant at the tail-end of reionization.

Shot noise and scatter in the star formation efficiency as a source of 21-cm fluctuations

ABSTRACT The 21-cm signal from cosmic dawn and the epoch of reionization probes the characteristics of the high redshift galaxy population. Many of the astrophysical properties of galaxies at high redshifts are currently unconstrained due to the lack of observations. This creates a vast space of possible astrophysical scenarios where the 21-cm signal needs to be modeled in order to plan for, and eventually fit, future observations. This is done with fast numerical methods which make simplifying approximations for the underlying physical processes. In this work, we quantify the effect of Poisson fluctuations and scatter in the star formation efficiency; while Poisson fluctuations are included in some works and not in others, scatter in the star formation efficiency is usually neglected, and all galaxies of a given mass are assumed to have the same properties. We show that both features can have a significant effect on the 21-cm power spectrum, most importantly in scenarios where the signal is dominated by massive galaxies. Scatter in the star formation efficiency does not simply enhance the effect of Poisson fluctuations; for example, we show that the power spectrum shape at cosmic dawn has a feature corresponding to the width of the galaxy brightness distribution. We also discuss some of the consequences for 21-cm imaging, and the signature of reduced correlation between the density and radiation fields.

MUSE observations of the blue compact dwarf galaxy Haro 14

Investigations of blue compact galaxies (BCGs) are essential to advancing our understanding of galaxy formation and evolution. BCGs are low-luminosity, low-metallicity, gas-rich objects that form stars at extremely high rates, meaning they are good analogs to the high-redshift star-forming galaxy population. Being low-mass starburst systems, they also constitute excellent laboratories in which to investigate the star formation process and the interplay between massive stars and their surroundings. This work presents results from integral field spectroscopic observations of the BCG Haro 14 taken with the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope in wide-field adaptive optics mode. The large MUSE field of view (1′×1′ = 3.8 × 3.8 kpc2 at the adopted distance of 13 Mpc) enables simultaneous observations of the central starburst and the low-surface-brightness host galaxy, which is a huge improvement with respect to previous integral field spectroscopy of BCGs. From these data we built galaxy maps in continuum and in the brightest emission lines. We also generated synthetic broad-band images in the VRI bands, from which we produced color index maps and surface brightness profiles. We detected numerous clumps spread throughout the galaxy, both in continuum and in emission lines, and produced a catalog with their position, size, and photometry. This analysis allowed us to study the morphology and stellar populations of Haro 14 in detail. The stellar distribution shows a pronounced asymmetry; the intensity peak in continuum is not centered with respect to the underlying stellar host but is displaced by about 500 pc southwest. At the position of the continuum peak we find a bright stellar cluster that with Mv = −12.18 appears as a strong super stellar cluster candidate. We also find a highly asymmetric, blue, but nonionizing stellar component that occupies almost the whole eastern part of the galaxy. We conclude that there are at least three different stellar populations in Haro 14: the current starburst of about 6 Myr; an intermediate-age component of between ten and several hundred million years; and a red and regular host of several gigayears. The pronounced lopsidedness in the continuum and also in the color maps, and the presence of numerous stellar clusters, are consistent with a scenario of mergers or interactions acting in Haro 14.

Cluster strong lensing with hierarchical inference

Context. Lensing by galaxy clusters is a versatile probe of cosmology and extragalactic astrophysics, but the accuracy of some of its predictions is limited by the simplified models adopted to reduce the (otherwise intractable) number of degrees of freedom. Aims. We aim to explore cluster lensing models in which the parameters of all cluster member galaxies are free to vary around some common scaling relations with non-zero scatter and deviate significantly from these relations if, and only if, the data require this. Methods. We devised a Bayesian hierarchical inference framework that enables the determination of all lensing parameters and the scaling relation hyperparameters, including intrinsic scatter, from lensing constraints and (if given) stellar kinematic measurements. We achieve this through BAYESLENS, a purpose-built wrapper around common parametric lensing codes that can sample the full posterior on parameters and hyperparameters; we release BAYESLENS with this paper. Results. We ran functional tests of our code against simple mock cluster lensing datasets with realistic uncertainties. The parameters and hyperparameters are recovered within their 68% credibility ranges and the positions of all the “observed” multiple images are accurately reproduced by the BAYELENS best-fit model, without over-fitting. Conclusions. We have shown that an accurate description of cluster member galaxies is attainable, despite a large number of degrees of freedom, through fast and tractable inference. This extends beyond the most updated cluster lensing models. The precise impact on studies of cosmography, galaxy evolution, and high-redshift galaxy populations can then be quantified on real galaxy clusters. While other sources of systematics exist and may be significant in real clusters, our results show that the contribution of intrinsic scatter in cluster member populations can now be controlled.

High-redshift radio galaxies: a potential new source of 21-cm fluctuations

ABSTRACT Radio sources are expected to have formed at high redshifts, producing an excess radiation background above the cosmic microwave background (CMB) at low frequencies. Their effect on the redshifted 21-cm signal of neutral hydrogen is usually neglected, as it is assumed that the associated background is small. Recently, an excess radio background has been proposed as a possible explanation for the unusually strong 21-cm signal reported by EDGES. As a result, the implications of a smooth and extremely strong excess radio background on both the sky-averaged 21-cm signal and its fluctuations have been considered. Here, we take into account the inhomogeneity of the radio background created by a population of high-redshift galaxies and show that it adds a new type of 21-cm fluctuations to the well-known contributions of density, velocity, Ly α coupling, heating, and reionization. We find that a population of high-redshift galaxies even with a moderately enhanced radio efficiency (unrelated to the EDGES result) can have a significant effect on the 21-cm power spectrum and global signal. For models that can explain the EDGES data, we show that the 21-cm power spectrum at z ∼ 17 is enhanced by up to two orders of magnitude compared to the CMB-only standard case, with a significantly modified shape and time evolution due to radio fluctuations. These fluctuations are within reach of upcoming radio interferometers. We also find that these models can be significantly constrained by current and future observations of radio sources.

The stellar populations of high-redshift dwarf galaxies

ABSTRACT We use high-resolution (≈10 pc), zoom-in simulations of a typical (stellar mass $M_\star \simeq 10^{10}\, {\rm M}_{\odot }$) Lyman Break Galaxy (LBG) at z ≃ 6 to investigate the stellar populations of its six dwarf galaxy satellites, whose stellar [gas] masses are in the range log (M⋆/M⊙) ≃ 6−9 [log (Mgas/M⊙) ≃ 4.3−7.75]. The properties and evolution of satellites show no dependence on the distance from the central massive LBG (<11.5 kpc). Instead, their star formation and chemical enrichment histories are tightly connected to their stellar (and sub-halo) mass. High-mass dwarf galaxies ($M_\star \gtrsim 5\times 10^8 \, {\rm M}_{\odot }$) experience a long history of star formation, characterized by many merger events. Lower mass systems go through a series of short star formation episodes, with no signs of mergers; their star formation activity starts relatively late (z ≈ 7), and it is rapidly quenched by internal stellar feedback. In spite of the different evolutionary patterns, all satellites show a spherical morphology, with ancient and more metal-poor stars located towards the inner regions. All six dwarf satellites experienced high star formation rate ($\rm \gt 5\, {\rm M}_{\odot }\, {\rm yr}^{-1}$) bursts, which can be detected by James Webb Space Telescope while targeting high-z LBGs.

High-redshift JWST predictions from IllustrisTNG: II. Galaxy line and continuum spectral indices and dust attenuation curves

ABSTRACT We present predictions for high redshift (z = 2−10) galaxy populations based on the IllustrisTNG simulation suite and a full Monte Carlo dust radiative transfer post-processing. Specifically, we discuss the H α and H β + $[\rm O \,{\small III}]$ luminosity functions up to z = 8. The predicted H β + $[\rm O \,{\small III}]$ luminosity functions are consistent with present observations at z ≲ 3 with ${\lesssim} 0.1\, {\rm dex}$ differences in luminosities. However, the predicted H α luminosity function is ${\sim }0.3\, {\rm dex}$ dimmer than the observed one at z ≃ 2. Furthermore, we explore continuum spectral indices, the Balmer break at 4000 Å; (D4000) and the UV continuum slope β. The median D4000 versus specific star formation rate relation predicted at z = 2 is in agreement with the local calibration despite a different distribution pattern of galaxies in this plane. In addition, we reproduce the observed AUV versus β relation and explore its dependence on galaxy stellar mass, providing an explanation for the observed complexity of this relation. We also find a deficiency in heavily attenuated, UV red galaxies in the simulations. Finally, we provide predictions for the dust attenuation curves of galaxies at z = 2−6 and investigate their dependence on galaxy colours and stellar masses. The attenuation curves are steeper in galaxies at higher redshifts, with bluer colours, or with lower stellar masses. We attribute these predicted trends to dust geometry. Overall, our results are consistent with present observations of high-redshift galaxies. Future James Webb Space Telecope observations will further test these predictions.

Shedding light on high-redshift galaxies with the 21cm signal

AbstractReionization represents an important epoch in the history in the Universe, when the first stars and galaxies gradually ionize the neutral hydrogen in the intergalactic medium (IGM). Understanding the nature of the ionizing sources, the associated ionization of the IGM, and its impact on subsequent structure formation and galaxy evolution by means of radiative feedback effects, represent key outstanding questions in current astrophysics. High-redshift galaxy observations and simulations have significantly extended our knowledge on the nature of high-redshift galaxies. However, essential properties such as the escape fraction of ionizing photons from galaxies into the IGM and their dependency on galactic properties remain essentially unknown, but determine significantly the distribution and time evolution of the ionized regions during reionization. Analyzing this ionization topology by means of the neutral hydrogen sensitive 21cm signal with radio interferometers such as SKA offers a complementary and unique opportunity to determine the nature of these first galaxies. I will show results from a self-consistent semi-numerical model of galaxy evolution and reionization, and discuss the potential of inferring galactic properties with the 21cm signal as well as the impact of reionization on the high-redshift galaxy population and its evolution.

C ii] 158 μm Emission from z ∼ 4 H i Absorption-selected Galaxies

Abstract We report on a search for the [C ii] 158 μm emission line from galaxies associated with four high-metallicity damped Lyα absorbers (DLAs) at z ∼ 4 using the Atacama Large Millimeter/submillimeter Array (ALMA). We detect [C ii] 158 μm emission from galaxies at the DLA redshift in three fields, with one field showing two [C ii] 158 μm emitters. Combined with previous results, we now have detected [C ii] 158 μm emission from five of six galaxies associated with targeted high-metallicity DLAs at z ∼ 4. The galaxies have relatively large impact parameters, ≈16–45 kpc, [C ii] 158 μm line luminosities of (0.36–30) × 108 L ⊙, and rest-frame far-infrared properties similar to those of luminous Lyman-break galaxies, with star formation rates of ≈7–110 M ⊙ yr−1. Comparing the absorption and emission line profiles yields a remarkable agreement between the line centroids, indicating that the DLA traces gas at velocities similar to that of the [C ii] 158 μm emission. This disfavors a scenario where the DLA arises from gas in a companion galaxy. These observations highlight ALMA’s unique ability to uncover a high-redshift galaxy population that has largely eluded detection for decades.

SCORCH. II. Radiation-hydrodynamic Simulations of Reionization with Varying Radiation Escape Fractions

Abstract In the Simulations and Constructions of the Reionization of Cosmic Hydrogen project, we present new radiation-hydrodynamic simulations with updated high-redshift galaxy populations and varying radiation escape fractions. The simulations are designed to have fixed Thomson optical depth τ ≈ 0.06, consistent with recent Planck observations, and similar midpoints of reionization 7.5 ≲ z ≲ 8.0, but with different ionization histories. The galaxy luminosity functions and ionizing photon production rates in our model are in good agreement with recent HST observations. Adopting a power-law form for the radiation escape fraction , we simulate the cases for a 8 = 0, 1, and 2 and find a 8 ≲ 2 in order to end reionization in the range of 5.5 ≲ z ≲ 6.5 to be consistent with Lyα forest observations. At fixed τ and as the power-law slope a 8 increases, the reionization process starts earlier but ends later with a longer duration Δz and the decreased redshift asymmetry Az. We find a range of durations 3.9 ≲ Δz ≲ 4.6 that is currently in tension with the upper limit Δz < 2.8 inferred from a recent joint analysis of Planck and South Pole Telescope observations.

The host galaxies of long gamma-ray bursts through cosmic time

As a result of their unprecedented luminosity, long [Formula: see text]-ray bursts (GRBs) and their afterglows pinpoint a population of high-redshift galaxies that is distinct and complementary to galaxies selected by deep sky surveys. In this way, GRBs facilitate a unique view into the high-redshift universe. The bright optical afterglow, for example, serves as a background light for absorption-line spectroscopy, and thus helps us to accurately characterize and understand the cosmochemical evolution of the interstellar matter, the properties of star-forming galaxies through cosmic time or dust extinction at redshifts up to [Formula: see text]. Similarly, detailed studies of the GRB explosion site, and the specific stellar population and galactic environment linked to the GRB, provide important constraints on the physical properties of the GRB progenitor like stellar mass, or on the interplay between metallicity and GRB formation. In this paper, we review recent results in both areas, focusing on novel results from extensive host observations of well-selected GRB samples.

The environment and host haloes of the brightest z ∼ 6 Lyman-break galaxies

By studying the large-scale structure of the bright high-redshift Lyman-break galaxy (LBG) population it is possible to gain an insight into the role of environment in galaxy formation physics in the early Universe. We measure the clustering of a sample of bright (-22.7<M_UV<-21.125) LBGs at z~6 and use a halo occupation distribution (HOD) model to measure their typical halo masses. We find that the clustering amplitude and corresponding HOD fits suggests that these sources are highly biased (b~8) objects in the densest regions of the high-redshift Universe. Coupled with the observed rapid evolution of the number density of these objects, our results suggest that the shape of high luminosity end of the luminosity function is related to feedback processes or dust obscuration in the early Universe - as opposed to a scenario where these sources are predominantly rare instances of the much more numerous M_UV ~ -19 population of galaxies caught in a particularly vigorous period of star formation. There is a slight tension between the number densities and clustering measurements, which we interpret this as a signal that a refinement of the model halo bias relation at high redshifts or the incorporation of quasi-linear effects may be needed for future attempts at modelling the clustering and number counts. Finally, the difference in number density between the fields (UltraVISTA has a surface density ~1.8 times greater than UDS) is shown to be consistent with the cosmic variance implied by the clustering measurements.

ETHOS - an effective theory of structure formation: predictions for the high-redshift Universe - abundance of galaxies and reionization.

We contrast predictions for the high-redshift galaxy population and reionization history between cold dark matter (CDM) and an alternative self-interacting dark matter model based on the recently developed ETHOS framework that alleviates the small-scale CDM challenges within the Local Group. We perform the highest resolution hydrodynamical cosmological simulations (a 36 Mpc3 volume with gas cell mass of ∼ 105 M⊙ and minimum gas softening of ∼ 180 pc) within ETHOS to date - plus a CDM counterpart - to quantify the abundance of galaxies at high redshift and their impact on reionization. We find that ETHOS predicts galaxies with higher ultraviolet (UV) luminosities than their CDM counterparts and a faster build-up of the faint end of the UV luminosity function. These effects, however, make the optical depth to reionization less sensitive to the power spectrum cut-off: the ETHOS model differs from the CDM τ value by only 10 per cent and is consistent with Planck limits if the effective escape fraction of UV photons is 0.1-0.5. We conclude that current observations of high-redshift luminosity functions cannot differentiate between ETHOS and CDM models, but deep James Webb Space Telescope surveys of strongly lensed, inherently faint galaxies have the potential to test non-CDM models that offer attractive solutions to CDM's Local Group problems.

The size evolution of star-forming and quenched galaxies in the IllustrisTNG simulation

We analyze scaling relations and evolution histories of galaxy sizes in TNG100, part of the IllustrisTNG simulation suite. Observational qualitative trends of size with stellar mass, star-formation rate and redshift are reproduced, and a quantitative comparison of projected r-band sizes at 0~<z<~2 shows agreement to much better than 0.25dex. We follow populations of z=0 galaxies with a range of masses backwards in time along their main progenitor branches, distinguishing between main-sequence and quenched galaxies. Our main findings are as follows. (i) At M_{*,z=0}>~10^{9.5}Msun, the evolution of the median main progenitor differs, with quenched galaxies hardly growing in median size before quenching, whereas main-sequence galaxies grow their median size continuously, thus opening a gap from the progenitors of quenched galaxies. This is partly because the main-sequence high-redshift progenitors of quenched z=0 galaxies are drawn from the lower end of the size distribution of the overall population of main-sequence high-redshift galaxies. (ii) Quenched galaxies with M_{*,z=0}>~10^{9.5}Msun experience a steep size growth on the size-mass plane after their quenching time, but with the exception of galaxies with M_{*,z=0}>~10^{11}Msun, the size growth after quenching is small in absolute terms, such that most of the size (and mass) growth of quenched galaxies (and its variation among them) occurs while they are still on the main-sequence. After they become quenched, the size growth rate of quenched galaxies as a function of time, as opposed to versus mass, is similar to that of main-sequence galaxies. Hence, the size gap is retained down to z=0.