arXiv subjects

Maxime Trebitsch

Publications and source records attributed to Maxime Trebitsch.

At least 37 records · Page 2Linked to original sources

arXiv2025

A Glimpse of the New Redshift Frontier Through Abell S1063

We report the discovery of two galaxy candidates at redshifts between $15.7<z<16.4$ in JWST observations from the GLIMPSE survey. These robust sources were identified using a combination of Lyman-break selection and photometric redshift estimates. The ultra-deep NIRCam imaging from GLIMPSE, combined with the strong gravitational lensing of the Abell S1063 galaxy cluster, allows us to probe an intrinsically fainter population (down to $M_{\rm UV} =-17.0$ mag) than previously achievable. These galaxies have absolute magnitudes ranging from $M_{\rm UV} = -17.0$ to $-17.2$ mag, with blue ($β\simeq -2.87$) UV continuum slopes, consistent with young, dust-free stellar populations. The number density of these objects, log$_{\rm 10}$($ϕ$/[Mpc$^{-3}$ mag$^{-1}$])=$-3.47^{+0.13}_{-0.10}$ at $M_{\rm UV}=-17$ is in clear tension with pre-JWST theoretical predictions, extending the over-abundance of galaxies from $z\sim10$ to $z\sim 17$. These results, together with the scarcity of brighter galaxies in other public surveys, suggest a steep decline in the bright-end of the UV luminosity function at $z \sim 16$, implying efficient star formation and possibly a close connection to the halo mass function at these redshifts. Testing a variety of star formation histories suggests that these sources are plausible progenitors of the unusually UV-bright galaxies that JWST now routinely uncovers at $z = 10-14$. Overall, our results indicate that the luminosity distribution of the earliest star-forming galaxies could be shifting towards fainter luminosities, implying that future surveys of cosmic dawn will need to explore this faint luminosity regime.

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arXiv2025

JWST ASPIRE: How Did Galaxies Complete Reionization? Evidence for Excess IGM Transmission around ${\rm [O\,{\scriptstyle III}]}$ Emitters during Reionization

The spatial correlation between galaxies and the Ly$α$ forest provides insights into how galaxies reionized the Universe. Here, we present initial results on the spatial cross-correlation between [OIII] emitters and Ly$α$ forest at 5.4 6.5 QSO fields. Using data from five QSO fields, we find $2σ$ evidence for excess Ly$α$ forest transmission at ~20-40 cMpc around [OIII] emitters at z=5.86, indicating that [OIII] emitters reside within a highly ionized IGM. At smaller scales, the Ly$α$ forest is preferentially absorbed, suggesting gas overdensities around [OIII] emitters. Comparing with models including THESAN simulations, we interpret the observed cross-correlation as evidence for significant large-scale fluctuations of the IGM and the late end of reionization at z<6, characterized by ionized bubbles over 50 cMpc around [OIII] emitters. The required UV background necessitates an unseen population of faint galaxies around the [OIII] emitters. Furthermore, we find that the number of observed [OIII] emitters near individual transmission spikes is insufficient to sustain reionization in their surroundings, even assuming all [OIII] emitters harbour AGN with 100 % LyC escape fractions. Despite broad agreement, a careful analysis of ASPIRE and THESAN, using the observed host halo mass from the clustering of [OIII] emitters, suggests that the simulations underpredict the observed excess IGM transmission around [OIII] emitters, challenging our model of reionization. Potential solutions include larger ionized bubbles at z<6, more enhanced large-scale UV background or temperature fluctuations of the IGM, and possibly a patchy early onset of reionization at z>10. Current observational errors are dominated by cosmic variance, meaning future analyses of more QSO fields from JWST will improve the results.

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arXiv2025

A Quasar-Anchored Protocluster at z=6.6 in the ASPIRE Survey: I. Properties of [OIII] Emitters in a 10 Mpc Overdensity Structure

ASPIRE (A SPectroscopic survey of bIased halos in the Reionization Era) is a quasar legacy survey primarily using \textit{JWST} to target a sample of 25 $z>6$ quasars with NIRCam slitless spectroscopy and imaging. The first study in this series found evidence of a strong overdensity of galaxies around J0305$-$3150, a luminous quasar at $z=6.61$, within a single NIRCam pointing obtained in JWST Cycle 1. Here, we present the first results of a JWST Cycle 2 mosaic that covers 35 arcmin$^2$ with NIRCam imaging/WFSS of the same field to investigate the spatial extent of the putative protocluster. The F356W grism data targets [OIII]+H$β$ at $5.3 10$ cMpc.

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arXiv2025

A Quasar-Anchored Protocluster at z=6.6 in the ASPIRE Survey: II. An Environmental Analysis of Galaxy Properties in an Overdense Structure

We present paper II comprising a 35 arcmin$^2$ \textit{JWST}/NIRCam imaging and wide-field slitless spectroscopy mosaic centered on J0305$-$3150, a luminous quasar at $z=6.61$. The F356W grism data reveals 124 [OIII]+H$β$ emitters at $5.3<z<7$, 53 of which constitute a protocluster spanning (10 cMpc)$^2$ across $6.5<z<6.8$. We find no evidence of any broad-line AGN in individual galaxies or stacking, reporting a median H$β$ FWHM of 585 $\pm$ 152 km s$^{-1}$; however, the mass-excitation diagram and ``little red dot" color and compactness criteria suggest that there are a few AGN candidates on the outskirts of the protocluster. We fit the spectral energy distributions (SEDs) of the [OIII] emitters with \texttt{Prospector} and \texttt{Bagpipes}, and find that none of the SED-derived properties (stellar mass, age, or star formation rate) correlates with proximity to the quasar. While there is no correlation between galaxy age and local galaxy density, we find modest correlations between local galaxy density with increasing stellar mass, decreasing 10-to-100 Myr star formation rate ratios and decreasing nebular line equivalent widths. We further find that the protocluster galaxies are consistent with being more massive, older, and hosting higher star formation rates than the field sample at the 3$σ$ level, distributed in a filamentary structure which supports inside-out formation of the protocluster. There is modest evidence that galaxy evolution proceeds differently as a function of the density of local environment within protoclusters during the epoch of reionization, and the central quasar has little effect on the galaxy properties of the surrounding structure.

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arXiv2024

Astraeus X: Indications of a top-heavy initial mass function in highly star-forming galaxies from JWST observations at z>10

The James Webb Space Telescope (JWST) has uncovered an abundance of $z>10$ galaxies bright in the ultraviolet (UV) that has challenged traditional theoretical models at high redshifts. Recently, various new models have emerged to address this discrepancy by refining their description of star formation. Here we investigate whether modifications to the stellar initial mass function (IMF) alone can reproduce the $z>10$ UV luminosity functions (UV LFs) when the star formation rate is used as a proxy for the fraction of massive stars. We incorporate an Evolving IMF into the {\sc astraeus} galaxy evolution and reionisation simulation framework, which becomes increasingly top-heavy as the gas density in a galaxy rises above a given threshold. Our implementation accounts for the IMF's effects on supernova (SN) feedback, metal enrichment, and the UV and ionising emissivities. For this Evolving IMF model, we find that (i) the maximum UV luminosity enhancement is twice as large in massive galaxies ($ΔM_\mathrm{UV}\simeq2.6$) than those where star formation is strongly limited by SN feedback ($ΔM_\mathrm{UV}\simeq1.3$); (ii) it successfully reproduces the observed UV LFs at $z=5-15$; (iii) galaxies with top-heavy IMFs exhibit the highest star formation rates, driven by their location in local density peaks, which facilitates higher gas accretion rates; (iv) the $1σ$ variances in the UV luminosity are only slightly higher compared to when assuming a Salpeter IMF, but the $2σ$ variances are significantly increased by a factor of $1.4-2$ boosting the abundance of UV-bright galaxies at $z>10$; (v) reionisation begins earlier with more extended large ionised regions and fewer smaller ones during its initial stages, though these differences diminish at lower redshifts, leading to a similar end of reionisation at $z\simeq5.6$.}

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arXiv2024

A Bayesian approach to the halo galaxy - supermassive black hole connection through cosmic time

We study the co-evolution of dark matter halos, galaxies and supermassive black holes using an empirical galaxy evolution model from $z=0$ -- $10$. We demonstrate that by connecting dark matter structure evolution with simple empirical prescriptions for baryonic processes, we are able to faithfully reproduce key observations in the relation between galaxies and their supermassive black holes. By assuming a physically-motivated, direct relationship between the galaxy and supermassive black hole properties to the mass of their host halo, we construct expressions for the galaxy stellar mass function, galaxy UV luminosity function, active black hole mass function and quasar bolometric luminosity function. We calibrate the baryonic prescriptions using a fully Bayesian approach in order to reproduce observed population statistics. The obtained parametrizations are then used to study the relation between galaxy and black hole properties, as well as their evolution with redshift. The galaxy stellar mass -- UV luminosity relation, black hole mass -- stellar mass relation, black hole mass -- AGN luminosity relation, and redshift evolution of these quantities obtained from the model are qualitatively consistent with observations. Based on these results, we present upper limits on the expected number of sources for $z=5$ up to $z=15$ for scheduled JWST and \textit{Euclid} surveys, thus showcasing that empirical models can offer qualitative as well as quantitative prediction in a fast, easy and flexible manner that complements more computationally expensive approaches.

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arXiv2024

Wandering and escaping: recoiling massive black holes in cosmological simulations

After a merger of two massive black holes (MBHs), the remnant receives a gravitational wave (GW) recoil kick that can have a strong effect on its future evolution. The magnitude of the kick ($v_\mathrm{recoil}$) depends on the mass ratio and the alignment of the spins and orbital angular momenta, therefore on the previous evolution of the MBHs. We investigate the cosmic effect of GW recoil by running for the first time a high-resolution cosmological simulation including GW recoil that depends on the MBH spins (evolved through accretion and mergers), masses and dynamics computed self-consistently. We also run a twin simulation without GW recoil. The simulations are run down to $z=4.4$. We find that GW recoil reduces the growth of merger remnants, and can have a significant effect on the MBH-galaxy correlations and the merger rate. We find large recoil kicks across all galaxy masses in the simulation, up to a few $10^{11}\,\rm M_\odot$. The effect of recoil can be significant even if the MBHs are embedded in a rotationally supported gaseous structure. We investigate the dynamics of recoiling MBHs and find that MBHs remain in the centre of the host galaxy for low $v_\mathrm{recoil}/v_\mathrm{esc}$ and escape rapidly for high $v_\mathrm{recoil}/v_\mathrm{esc}$. Only if $v_\mathrm{recoil}$ is comparable to $v_\mathrm{esc}$ the MBHs escape the central region of the galaxy but might remain as wandering MBHs until the end of the simulation. Recoiling MBHs are a significant fraction of the wandering MBH population. Although the dynamics of recoiling MBHs may be complex, some retain their initial radial orbits but are difficult to discern from other wandering MBHs on radial orbits. Others scatter with the halo substructure or circularise in the asymmetric potential. Our work highlights the importance of including GW recoil in cosmological simulation models.

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arXiv2024

A Simulated Galaxy Laboratory: Exploring the Observational Effects on UV Spectral Absorption Line Measurements

Ultraviolet absorption line spectroscopy is a sensitive diagnostic for the properties of interstellar and circumgalactic gas. Down-the-barrel observations, where the absorption is measured against the galaxy itself, are commonly used to study feedback from galactic outflows and to make predictions about the leakage of HI ionizing photons into the intergalactic medium. Nonetheless, the interpretation of these observations is challenging and observational compromises are often made in terms of signal-to-noise, spectral resolution, or the use of stacking analyses. In this paper, we present a novel quantitative assessment of UV absorption line measurement techniques by using mock observations of a hydrodynamical simulation. We use a simulated galaxy to create 22,500 spectra in the commonly used SiII lines while also modeling the signal-to-noise and spectral resolution of recent rest-frame UV galaxy surveys at both high and low redshifts. We show that the residual flux of absorption features is easily overestimated for single line measurements and for stacked spectra. Additionally, we explore the robustness of the partial covering model for estimating column densities from spectra and find under-predictions on average of 1.25 dex. We show that the under-prediction is likely caused by high-column-density sight-lines that are optically-thick to dust making them invisible in UV spectra.

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arXiv2024

The Low-Redshift Lyman Continuum Survey: The Roles of Stellar Feedback and ISM Geometry in LyC Escape

One of the fundamental questions of cosmology is the origin and mechanism(s) responsible for the reionization of the Universe beyond $z\sim6$. To address this question, many studies over the past decade have focused on local ($z\sim0.3$) galaxies which leak ionizing radiation (Lyman continuum or LyC). However, line-of-sight effects and data quality have prohibited deeper insight into the nature of LyC escape. To circumvent these limitations, we analyze stacks of a consolidated sample of {\it HST}/COS observations of the LyC in 89 galaxies at $z\sim0.3$. From fitting of the continuum, we obtain information about the underlying stellar populations and neutral ISM geometry. We find that most LyC non-detections are not leaking appreciable LyC ($f_{esc}^{\rm LyC}<1$\%) but also that exceptional cases point to spatial variations in the LyC escape fraction $f_{esc}^{\rm LyC}$. Stellar populations younger than 3 Myr lead to an increase in ionizing feedback, which in turn increases the isotropy of LyC escape. Moreover, mechanical feedback from supernovae in 8-10 Myr stellar populations is important for anisotropic gas distributions needed for LyC escape. While mechanical feedback is necessary for any LyC escape, high $f_{esc}^{\rm LyC}$ ($>5$\%) also requires a confluence of young stars and ionizing feedback. A two-stage burst of star formation could facilitate this optimal LyC escape scenario.

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arXiv2024

Multivariate Predictors of LyC Escape I: A Survival Analysis of the Low-redshift Lyman Continuum Survey

To understand how galaxies reionized the universe, we must determine how the escape fraction of Lyman Continuum (LyC) photons (fesc) depends on galaxy properties. Using the z~0.3 Low-redshift Lyman Continuum Survey (LzLCS), we develop and analyze new multivariate predictors of fesc. These predictions use the Cox proportional hazards model, a survival analysis technique that incorporates both detections and upper limits. Our best model predicts the LzLCS fesc detections with a root-mean-square (RMS) scatter of 0.31 dex, better than single-variable correlations. According to ranking techniques, the most important predictors of fesc are the equivalent width (EW) of Lyman-series absorption lines and the UV dust attenuation, which track line-of-sight absorption due to HI and dust. The HI absorption EW is uniquely crucial for predicting fesc for the strongest LyC emitters, which show properties similar to weaker LyC emitters and whose high fesc may therefore result from favorable orientation. In the absence of HI information, star formation rate surface density ($Σ_{\rm SFR}$) and [O III]/[O II] ratio are the most predictive variables and highlight the connection between feedback and fesc. We generate a model suitable for z>6, which uses only the UV slope, $Σ_{\rm SFR}$, and [O III]/[O II]. We find that $Σ_{\rm SFR}$ is more important in predicting fesc at higher stellar masses, whereas [O III]/[O II] plays a greater role at lower masses. We also analyze predictions for other parameters, such as the ionizing-to-non ionizing flux ratio and Ly=alpha escape fraction. These multivariate models represent a promising tool for predicting fesc at high redshift.

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arXiv2024

Multivariate Predictors of LyC Escape II: Predicting LyC Escape Fractions for High-Redshift Galaxies

JWST is uncovering the properties of ever increasing numbers of galaxies at z>6, during the epoch of reionization. Connecting these observed populations to the process of reionization requires understanding how efficiently they produce Lyman continuum (LyC) photons and what fraction (fesc) of these photons escape into the intergalactic medium. By applying the Cox proportional hazards model, a survival analysis technique, to the Low-redshift Lyman Continuum Survey (LzLCS), we develop new, empirical, multivariate predictions for fesc. The models developed from the LzLCS reproduce the observed fesc for z~3 samples, which suggests that LyC emitters may share similar properties at low and high redshift. Our best-performing models for the z~3 galaxies include information about dust attenuation, ionization, and/or morphology. We then apply these models to z$\gtrsim$6 galaxies. For large photometric samples, we find a median predicted fesc=0.047-0.14. For smaller spectroscopic samples, which may include stronger emission line galaxies, we find that $\geq$33% of the galaxies have fesc >0.2, and we identify several candidate extreme leakers with fesc $\geq$0.5. The current samples show no strong trend between predicted fesc and UV magnitude, but limited spectroscopic information makes this result uncertain. Multivariate predictions can give significantly different results from single variable predictions, and the predicted fesc for high-redshift galaxies can differ significantly depending on whether star formation rate surface density or radius is used as a measure of galaxy morphology. We provide all parameters necessary to predict fesc for additional samples of high-redshift galaxies using these models.

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arXiv2024

A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE): Broad-line AGN at $z=4-5$ revealed by JWST/NIRCam WFSS

Low-luminosity AGNs with low-mass black holes (BHs) in the early universe are fundamental to understanding the BH growth and their co-evolution with the host galaxies. Utilizing JWST NIRCam Wide Field Slitless Spectroscopy (WFSS), we perform a systematic search for broad-line ${\rm Hα}$ emitters (BHAEs) at $z\approx 4-5$ in 25 fields of the ASPIRE (A SPectroscopic survey of biased halos In the Reionization Era) project, covering a total area of 275 arcmin$^2$. We identify 16 BHAEs with FWHM of the broad components spanning from $\sim$ 1000 km s$^{-1}$ to 3000 km s$^{-1}$. Assuming the broad linewidths arise due to Doppler broadening around BHs, the implied BH masses range from $10^7$ to $10^{8}~M_\odot$, with broad ${\rm Hα}$-converted bolometric luminosity of $10^{44.5}-10^{45.5}$ erg s$^{-1}$ and Eddington ratios of $0.07-0.47$. The spatially extended structure of the F200W stacked image may trace the stellar light from the host galaxies. The ${\rm Hα}$ luminosity function indicates an increasing AGN fraction towards the higher ${\rm Hα}$ luminosities. We find possible evidence for clustering of BHAEs: two sources are at the same redshift with a projected separation of 519 kpc; one BHAE appears as a composite system residing in an overdense region with three close companion ${\rm Hα}$ emitters. Three BHAEs exhibit blueshifted absorption troughs indicative of the presence of high-column-density gas. We find the broad-line and photometrically selected BHAE samples exhibit different distributions in the optical continuum slopes, which can be attributed to their different selection methods. The ASPIRE broad-line ${\rm Hα}$ sample provides a good database for future studies of faint AGN populations at high redshift.

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arXiv2024

Modelling LAEs in the Epoch of Reionization with OBELISK

Lya emitters (LAEs) are particularly useful objects for the study of the Epoch of Reionization. Lya profiles can be used to estimate the amount of ionizing photons that are able to escape the galaxies, and therefore to understand what objects contributed to reionization. However, Lya is a resonant line and its complex radiative transfer effects make the interpretation of the line challenging and require the use of appropriate radiative transfer methods for anything but the simplest gas distributions. With this work we aim to study the properties of simulated LAEs, and the robustness of these inferred properties under a change in the dust model. We also explore the Lyman Continuum (LyC) escape fraction of these galaxies and compare our results with observationally calibrated methods to infer this quantity from the Lya spectrum. We use the radiative transfer code RASCAS to perform synthetic observations of 13 flux-selected galaxies from the Obelisk simulation at redshift z = 6, towards the end of the Epoch of Reionization. Each galaxy was observed in Lya, ionizing and non-ionizing continuum from 48 different viewing angles. We show that the Lya profiles emitted from a galaxy present large variations with a change in viewing angle and that the relation between peak separation and Lya escape fraction is not as strong as previously found, as we find lines of sight with both low peak separation and low escape fraction, due to their dust content. We also show that the properties of the Lya line are reasonably robust under a change in dust model. Lastly, we compare the LyC escape fractions we derive from the simulation to three observationally calibrated methods of inferring this quantity. We determine that none of these relations reproduce the scatter that we find in our sample, and that high escape fraction lines of sight have both low peak separation and low dust extinction in the UV.

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arXiv2024

Low-redshift Lyman Continuum Survey: Radio continuum properties of low-$z$ Lyman continuum emitters

Sources that leak Lyman-continuum (LyC) photons and lead to the reionisation of the universe are intensely studied using multiple observing facilities. Recently, the Low-redshift LyC Survey (LzLCS) has found the first large sample of LyC emitting galaxies at low redshift ($z\sim 0.3$) with the Hubble Space Telescope/Cosmic Origins Spectrograph. The LzLCS sample contains a robust estimate of the LyC escape fraction ($f_\mathrm{esc}^\mathrm{LyC}$) for 66 galaxies spanning a wide range of $f_\mathrm{esc}^\mathrm{LyC}$. Here we, for the first time, aim to study the radio continuum (RC) properties of LzLCS sources and their dependence on $f_\mathrm{esc}^\mathrm{LyC}$. We present Karl G. Jansky Very Large Array RC observations at C (4-8 GHz), S (2-4 GHz) and L (1-2 GHz) bands for a sub-sample of the LzLCS sources. The radio spectral index ($α^{\mathrm{3GHz}}_\mathrm{6GHz}$) spans a wide range from being flat ( $\geq -0.1$) to very steep ($\leq -1.0$). The strongest leakers in our sample show flat $α^{\mathrm{3GHz}}_\mathrm{6GHz}$, weak leakers have $α^{\mathrm{3GHz}}_\mathrm{6GHz}$ close to normal star-forming galaxies, and non-leakers are characterized by steep $α^{\mathrm{3GHz}}_\mathrm{6GHz}$. We argue that a combination of young ages, free-free absorption, and a flat cosmic-ray energy spectrum can altogether lead to a flat $α^{\mathrm{3GHz}}_\mathrm{6GHz}$ for strong leakers. Non-leakers are characterized by steep spectra which can arise due to break/cutoff at high frequencies. Such a cutoff in the spectrum can arise in a single injection model of CRs characteristic of galaxies which have recently stopped star formation. Such a relation between $α^{\mathrm{3GHz}}_\mathrm{6GHz}$ and $f_\mathrm{esc}^\mathrm{LyC}$ hints at the interesting role of supernovae, CRs, and magnetic fields in facilitating the escape ( and/or the lack) of LyC photons. (Abridged)

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arXiv2024

A quasar-galaxy merger at $z\sim 6.2$: rapid host growth via accretion of two massive satellite galaxies

We present JWST/NIRSpec Integral Field Spectroscopy in the rest-frame optical bands of the system PJ308-21, a quasar at $z=6.2342$ caught as its host galaxy interacts with companion galaxies. We detect spatially extended emission of several emission lines (H$α$, H$β$, [OIII], [NII], [SII], HeII), which we use to study the properties of the ionized phase of the interstellar medium: the source and hardness of the photoionizing radiation field, metallicity, dust reddening, electron density and temperature, and star formation. We also marginally detect continuum starlight emission associated with the companion sources. We find that at least two independent satellite galaxies are part of the system. While the quasar host appears highly enriched and obscured, with AGN-like photoionization conditions, the western companion shows minimal dust extinction, low metallicity ($Z\sim0.4$ Z$_\odot$), and star-formation driven photoionization. The eastern companion shows higher extinction and metallicity ($Z\sim0.8$ Z$_\odot$) compared to the western companion, and it is at least partially photoionized by the nearby quasar. We do not find any indication of AGN in the companion sources. Our study shows that while the quasar host galaxy is already very massive ($M_{\rm dyn}>10^{11}$ M$_\odot$), it is still rapidly building up by accreting two relatively massive ($M_{\rm star}\sim 10^{10}$ M$_\odot$) companion sources. This dataset showcases the power of JWST in exposing the build-up of massive galaxies in the first Gyr of the Universe.

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arXiv2024

Galaxies with grains: unraveling dust evolution and extinction curves with hydrodynamical simulations

We introduce a model for dust evolution in the RAMSES code for simulations of galaxies with a resolved multiphase interstellar medium. Dust is modelled as a fluid transported with the gas component, and is decomposed into two sizes, 5 nm and 0.1 $μ\rm m$, and two chemical compositions for carbonaceous and silicate grains. Using a suite of isolated disc simulations with different masses and metallicities, the simulations can explore the role of these processes in shaping the key properties of dust in galaxies. The simulated Milky Way analogue reproduces the dust-to-metal mass ratio (DTM), depletion factors, size distribution and extinction curves of the Milky Way. Galaxies with lower metallicities reproduce the observed decrease in the DTM with metallicity at around a few 0.1 $\rm Z_\odot$. This break in the DTM corresponds to a galactic gas metallicity threshold that marks the transition from an ejecta-dominated to an accretion-dominated grain growth, and that is different for silicate and carbonaceous grains, with $\simeq$ 0.1 $\rm Z_\odot$ and $\simeq$ 0.5 $\rm Z_\odot$ respectively. This leads to more Magellanic Cloud-like extinction curves, i.e. with steeper slopes in the ultraviolet and a weaker bump feature at 217.5 nm, in galaxies with lower masses and lower metallicities. Steeper slopes in these galaxies are caused by the combination of the higher efficiency of gas accretion by silicate relative to carbonaceous grains and by the low rates of coagulation that preserves the amount of small silicate grains. Weak bumps are due to the overall inefficient accretion growth of carbonaceous dust at low metallicity, whose growth is mostly supported by the release of large grains in SN ejecta. We also show that the formation of CO molecules is a key component to limit the ability of carbonaceous dust to grow, in particular in low-metallicity gas-rich galaxies.

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arXiv2024

A Census of Photometrically Selected Little Red Dots at 4 < z < 9 in JWST Blank Fields

Observations with the James Webb Space Telescope (JWST) have uncovered numerous faint active galactic nuclei (AGN) at $z\sim5$ and beyond. These objects are key to our understanding of the formation of supermassive black holes (SMBHs), their co-evolution with host galaxies, as well as the role of AGN in cosmic reionization. Using photometric colors and size measurements, we perform a search for compact red objects in an array of blank deep JWST/NIRCam fields totaling $\sim640$ arcmin$^{2}$. Our careful selection yields 260 reddened AGN candidates at $4<z_{\rm phot}<9$, dominated by a point-source like central component ($\langle r_{\rm eff} \rangle <130$ pc) and displaying a dichotomy in their rest-frame colors (blue UV and red optical slopes). Quasar model fitting reveals our objects to be moderately dust extincted ($A_{\rm V}\sim1.6$), which is reflected in their inferred bolometric luminosities of $L_{\rm bol}$ = 10$^{44-47}$ erg/s, and fainter UV magnitudes $M_{\rm UV} \simeq$ $-17$ to $-22$. Thanks to the large areas explored, we extend the existing dusty AGN luminosity functions to both fainter and brighter magnitudes, estimating their number densities to be $\times100$ higher than for UV-selected quasars of similar magnitudes. At the same time they constitute only a small fraction of all UV-selected galaxies at similar redshifts, but this percentage rises to $\sim$10\% for $M_{UV}\sim -22$ at $z\sim7$. Finally, assuming a conservative case of accretion at the Eddington rate, we place a lower limit on the SMBH mass function at $z\sim5$, finding it to be consistent with both theory and previous JWST observations.

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arXiv2024

The impact of an evolving stellar initial mass function on early galaxies and reionisation

Observations with JWST have revealed an unexpected high abundance of bright z>10 galaxy candidates. We explore whether a stellar initial mass function (IMF) that becomes increasingly top-heavy towards higher redshifts and lower gas-phase metallicities results in a higher abundance of bright objects in the early universe and how it affects the evolution of galaxy properties compared to a constant IMF. We incorporate such an evolving IMF into the Astraeus framework that couples galaxy evolution and reionisation in the first billion years. Our implementation accounts for the IMF dependence of supernova feedback, metal enrichment, ionising and ultraviolet radiation emission. We conduct two simulations: one with a Salpeter IMF and one with the evolving IMF. Compared to a constant Salpeter IMF, we find that (i) the higher abundance of massive stars in the evolving IMF results in more light per unit stellar mass, a slower build-up of stellar mass and lower stellar-to-halo mass ratio; (ii) due to the self-similar growth of the underlying dark matter halos, the evolving IMF's star formation main sequence hardly deviates from that of the Salpeter IMF; (iii) the evolving IMF's stellar mass-metallicity relation shifts to higher metallicities while its halo mass-metallicity relation remains unchanged; (iv) the evolving IMF's median dust-to-metal mass ratio is lower due to its stronger SN feedback; (v) the evolving IMF requires lower values of the escape fraction of ionising photons and exhibits a flatter median relation and smaller scatter between the ionising photons emerging from galaxies and the halo mass. Yet, the topology of the ionised regions hardly changes compared to the Salpeter IMF. These results suggest that a top-heavier IMF alone is unlikely to explain the higher abundance of bright z>10 sources, since the lower mass-to-light ratio is counteracted by the stronger stellar feedback.

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