arXiv subjects

Mark T. Sargent

Publications and source records attributed to Mark T. Sargent.

At least 19 recordsLinked to original sources

arXiv2026

Tracing cosmic star formation history through radio continuum spectral energy distribution and non-thermal emission

As a tracer of massive star formation unaffected by dust, the radio continuum emission provides a unique window into the formation of the first stars and galaxies in the Universe. Recent observations show that the integrated rest-frame mid-radio (~1-10 GHz) luminosity of galaxies serves as one of the most robust tracers of the star formation rate (SFR). These studies further demonstrate that the synchrotron spectral index and the shape of the radio spectral energy distribution (SED) evolves with redshift as a consequence of the cosmic evolution of star formation activity. These findings underscore the importance of deep multi-band radio continuum observations in calibrating the SFR of early galaxies and understanding the astrophysical processes governing their assembly and evolution over cosmic time. This chapter presents recent progress in radio SFR calibrations for star-forming galaxies (SFGs) and reviews radio-continuum studies of the cosmic star formation history (SFH). We highlight the transformative potential of SKA AA4, whose broad frequency coverage and high sensitivity will enable well-constrained radio SEDs for SFGs across a wide redshift range.

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arXiv2026

Advancing Astrophysics with the SKA II

Advancing Astrophysics with the SKA II (AASKAII), written by our science community, outlines the transformative scientific advances that will be enabled by the SKA telescopes. In the decade since the publication of the previous edition, telescope designs have matured, construction has commenced, and the SKA Organisation has evolved into the SKA Observatory (SKAO). At the same time, observations from SKA precursor and pathfinder telescopes have provided new insights into longstanding scientific challenges while revealing entirely new phenomena. Published in advance of the first science verification campaign for the SKA Observatory, this volume looks ahead to the coming decades of discovery and innovation in radio astronomy. AASKAII spans the broad range of scientific research enabled by the SKA telescopes, SKA-Mid and SKA-Low. The contributions are organised into six thematic categories according to their scientific focus. The opening section presents overview chapters from the SKA Science Working Groups, around which our community is organised. Each overview provides the broader context that connects the contributions in this volume to the key scientific questions being pursued by their respective communities.

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arXiv2026

Unveiling the roles of thermal and nonthermal processes in the ISM & IGM structure formation and evolution of galaxies with SKAO

Investigating the thermal and nonthermal processes in the interstellar medium (ISM) and intergalactic medium (IGM) is vital to understanding the evolution of galaxies over cosmic time. Resolved observations with SKA pathfinders show that the nonthermal processes, in which magnetic fields and cosmic rays are involved, can decelerate the formation of massive stars in strongly magnetized regions in nearby galaxies. They can also contribute to the onset of winds and outflows in galaxies. The effects of these processes are stronger at higher redshifts as a result of star formation activities. The SKA Observatory will allow a major breakthrough by mapping the thermal and nonthermal processes in distant universe galaxies, shedding light on the role of the ISM and IGM in the evolution of galaxies. We demonstrate this by simulating the radio continuum and HI emission from local galaxies back to high redshifts. Our simulations show that the AA4 surveys will make it possible to trace the thermal and nonthermal processes of the ISM in galaxies that are analogs to M51 and NGC6946, traced in continuum beyond cosmic noon (z=2-3) and the gas content traced by HI beyond z=1. Both simulations and precursor observations indicate the importance of nonthermal feedback at cosmic noon.

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arXiv2026

Cm-wavelength Studies of Molecular Gas and Star Formation at High Redshift with the SKA

The Square Kilometre Array will be a revolutionary instrument for the study of gas in the distant Universe. At frequencies below ~50 GHz, observations of redshifted emission from low-J transitions of CO, HCN, HCO+, and HNC, etc. provide insight into the kinematics and mass budget of the cold, dense star-forming gas in galaxies. Over the past decade, sensitive imaging using ALMA has detected and resolved the redshifted high-J molecular CO line emission and far-infrared fine structure lines in samples of galaxies over a wide redshift range, shedding light on active star-formation processes at the early epoch of galaxy evolution. In recent years, increasing numbers of young galaxies at high redshift are discovered by JWST, which significantly improved our knowledge of different galaxy populations across cosmic time. In this updated chapter of the SKA science book, we would like to highlight the importance of studies of the low-J molecular lines in high-z galaxies using SKA toward high frequencies, discussing the request of frequency coverage beyond 15 GHz and emphasizing its crucial role in exploring the cold molecular gas content in the young galaxy populations in the early universe and investigating the regions of active-star formation using molecular CO and various dense gas tracers.

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arXiv2026

Opening new parameter space windows on galaxy/AGN co-evolution with SKA radio continuum surveys

In this chapter we provide an overview of the science enabled by the SKAO, focusing on galaxy/AGN co-evolution studies. In particular we discuss a number of radio continuum `reference' surveys with the SKAO, highlighting the role they can play in advancing this research field with respect to the pre-SKAO era. Alongside well-explored scenarios for wedding cake-like, tiered extragalactic surveys at specific frequencies, we also address the scope for complementary efforts to obtain deep multi-frequency imaging over parts of (an) extragalactic field(s). In addition to providing key information on the physical properties of the emitting sources, such multi-frequency imaging will make important contributions to the calibration of observables from surveys with sparser radio spectral coverage. In this context, we explore possible pathways that can fully exploit the SKAO from initial (AA*) to baseline capabilities (AA4). Finally, we highlight observational synergies with other major facilities -- for wide field and targeted follow-up science -- that will be operational in the 2030s, and for which joint coverage of extragalactic fields will generate significant legacy value

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arXiv2025

Galaxies as stochastic systems: why the next breakthrough in galaxy evolution requires one hundred million spectra

Each galaxy is observed only once along its life, making galaxy evolution fundamentally an inverse statistical problem: time-dependent physics must be inferred from ensembles of single-epoch snapshots. To move beyond descriptive scaling relations toward physical regulation mechanisms of star formation, quenching, chemical enrichment and black hole growth, galaxies must be treated as realizations of a stochastic process whose hyper-parameters (e.g., correlation timescales, burstiness, duty cycles) are inferred hierarchically. This demands both depth and scale: continuum S/N sufficient for absorption-line ages and chemistry, and samples far larger than those in SDSS, DESI, 4MOST or MOONS, which provide either depth or size but not both across $0<z<3$. Once the relevant axes of mass, redshift, environment, structure and evolutionary phase are populated, the requirement naturally rises from $10^7$ to $\sim10^8$ galaxies. This is the regime where stochastic hyper-parameters can be well constrained and where comparisons to simulations and cosmological forward models become limited by theory rather than observations. We outline the science enabled by such a programme and the corresponding requirements for a future ESO wide-field spectroscopic facility capable of delivering tens to hundreds of millions of rest-UV-optical spectra over $0\lesssim z\lesssim3$.

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arXiv2025

The molecular gas content throughout the low-z merger sequence

Exploiting IRAM 30m CO spectroscopy, we find that SDSS post-merger galaxies display gas fractions and depletion times enhanced by 25-50%, a mildly higher CO excitation, and standard molecular-to-atomic gas ratios, compared to non-interacting galaxies with similar redshift, stellar mass ($M_{\star}$) and star-formation rate (SFR). To place these results in context, we compile further samples of interacting or starbursting galaxies, from pre-coalescence kinematic pairs to post-starbursts, carefully homogenising gas mass, $M_{\star}$ and SFR measurements in the process. We explore systematics by duplicating our analysis for different SFR and $M_{\star}$ estimators, finding good qualitative agreement in general. Gas fractions and depletion times are enhanced in interacting pairs, albeit by less than for post-mergers. Among all samples studied, gas fraction and depletion time enhancements appear largest in young (a few 100 Myr) post-starbursts. While there is only partial overlap between post-mergers and post-starbursts, this suggests that molecular gas reservoirs are boosted throughout most stages of galaxy interactions, plausibly due to torque-driven inflows of halo gas and gas compression. The gas fraction and depletion time offsets of mergers and post-starbursts anti-correlate with distance from the galaxy main sequence $Δ({\rm MS})$, evidencing the role of SFE in driving the high SFRs of the strongest starbursts. Post-starbursts display the steepest dependency of gas fraction and SFE-offsets on $Δ({\rm MS})$, with an evolving normalisation that reflects gas reservoir depletion over time. Our multi-sample analysis paints a coherent picture of the starburst-merger connection throughout the low-z merger sequence. It reconciles contradictory literature findings by highlighting that gas fraction enhancements and SFE variations both play their part in merger-driven star formation.

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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

Cosmic evolution of radio-excess AGNs in quiescent and star-forming galaxies across $0 < z < 4$

Recent deep and wide radio surveys extend the studies for radio-excess active galactic nuclei (radio-AGNs) to lower luminosities and higher redshifts, providing new insights into the abundance and physical origin of radio-AGNs. Here we focus on the cosmic evolution, physical properties and AGN-host galaxy connections of radio-AGNs selected from a sample of ~ 500,000 galaxies at 0 < z < 4 in GOODS-N, GOODS-S, and COSMOS fields. Combining deep radio data with multi-band, de-blended far-infrared (FIR) and sub-millimeter data, we identify 1162 radio-AGNs through radio excess relative to the FIR-radio relation. We study the cosmic evolution of 1.4 GHz radio luminosity functions (RLFs) for star-forming galaxies (SFGs) and radio-AGNs, which are well described by a pure luminosity evolution of $L_*\propto (1+z)^{-0.31z+3.41}$ and a pure density evolution of $Φ_*\propto (1+z)^{-0.80z+2.88}$, respectively. We derive the turnover luminosity above which the number density of radio-AGNs surpasses that of SFGs. This crossover luminosity increases as increasing redshift, from $10^{22.9}$ W Hz$^{-1}$ at z ~ 0 to $10^{25.2}$ W Hz$^{-1}$ at z ~ 4. At full redshift range (0 < z < 4), we further derive the probability ($p_{radio}$) of SFGs and quiescent galaxies (QGs) hosting a radio-AGN as a function of stellar mass ($M_*$), radio luminosity ($L_R$), and redshift (z), which yields $p_{radio}\propto (1+z)^{3.54}M_*^{1.02}L_R^{-0.90}$ for SFGs, and $p_{radio}\propto (1+z)^{2.38}M_*^{1.39}L_R^{-0.60}$ for QGs, respectively. It indicates that radio-AGNs in QGs prefer to reside in more massive galaxies with larger $L_R$ than those in SFGs, and radio-AGN fraction increases towards higher redshift in both SFGs and QGs with a more rapid increase in SFGs. Further, we find that the radio-AGN fraction depends on accretion states of BHs and redshift in SFGs, while in QGs it also depends on BH (or galaxy) mass.

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arXiv2023

Looking ahead to the sky with the Square Kilometre Array: simulating flux densities & resolved radio morphologies of $0<z<2.5$ star-forming galaxies

SKA-MID surveys will be the first in the radio domain to achieve clearly sub-arcsecond resolution at high sensitivity over large areas, opening new science applications for galaxy evolution. To investigate the potential of these surveys, we create simulated SKA-MID images of a $\sim$0.04 deg$^{2}$ region of GOODS-North, constructed using multi-band HST imaging of 1723 real galaxies containing significant substructure at $0<z<2.5$. We create images at the proposed depths of the band 2 wide, deep and ultradeep reference surveys (RMS = 1.0 $μ$Jy, 0.2 $μ$Jy and 0.05 $μ$Jy over 1000 deg$^{2}$, 10-30 deg$^{2}$ and 1 deg$^{2}$ respectively), using the telescope response of SKA-MID at 0.6" resolution. We quantify the star-formation rate - stellar mass space the surveys will probe, and asses to which stellar masses they will be complete. We measure galaxy flux density, half-light radius ($R_{50}$), concentration, Gini (distribution of flux), second-order moment of the brightest pixels ($M_{20}$) and asymmetry before and after simulation with the SKA response, to perform input-output tests as a function of depth, separating the effects of convolution and noise. We find that the recovery of Gini and asymmetry is more dependent on survey depth than for $R_{50}$, concentration and $M_{20}$. We also assess the relative ranking of parameters before and after observation with SKA-MID. $R_{50}$ best retains its ranking, whilst asymmetries are poorly recovered. We confirm that the wide tier will be suited to the study of highly star-forming galaxies across different environments, whilst the ultradeep tier will enable detailed morphological analysis to lower SFRs.

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arXiv2023

"Dust Giant": Extended and Clumpy Star-Formation in a Massive Dusty Galaxy at $z=1.38$

We present NOEMA CO (2-1) line and ALMA 870 $μ$m continuum observations of a main-sequence galaxy at $z=1.38$. The galaxy was initially selected as a "gas-giant", based on the gas mass derived from sub-mm continuum (log$(M_{\rm gas}/M_{\odot})=11.20\pm0.20$), however the gas mass derived from CO (2-1) luminosity brings down the gas mass to a value consistent with typical star-forming galaxies at that redshift (log$(M_{\rm gas}/M_{\odot})=10.84\pm0.03$). Despite that the dust-to-stellar mass ratio remains elevated above the scaling relations by a factor of 5. We explore the potential physical picture and consider an underestimated stellar mass and optically thick dust as possible causes. Based on the updated gas-to-stellar mass ratio we rule out the former, and while the latter can contribute to the dust mass overestimate it is still not sufficient to explain the observed physical picture. Instead, possible explanations include enhanced HI reservoirs, CO-dark H$_2$ gas, an unusually high metallicity, or the presence of an optically dark, dusty contaminant. Using the ALMA data at 870 $μ$m coupled with $HST$/ACS imaging, we find extended morphology in dust continuum and clumpy star-formation in rest-frame UV in this galaxy, and a tentative $\sim 10$ kpc dusty arm is found bridging the galaxy center and a clump in F814W image. The galaxy shows levels of dust obscuration similar to the so-called $HST$-dark galaxies at higher redshifts, and would fall into the optically faint/dark $JWST$ color-color selection at $z>2$. It is therefore possible that our object could serve as low-$z$ analog of the $HST$-dark populations. This galaxy serves as a caveat to the gas masses based on the continuum alone, with a larger sample required to unveil the full picture.

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arXiv2023

The hidden side of cosmic star formation at z > 3: Bridging optically-dark and Lyman break galaxies with GOODS-ALMA

Our current understanding of the cosmic star formation history at z>3 is primarily based on UV-selected galaxies (i.e., LBGs). Recent studies of H-dropouts have revealed that we may be missing a large proportion of star formation that is taking place in massive galaxies at z>3. In this work, we extend the H-dropout criterion to lower masses to select optically dark/faint galaxies (OFGs), in order to complete the census between LBGs and H-dropouts. Our criterion (H> 26.5 mag & [4.5] < 25 mag) combined with a de-blending technique is designed to select not only extremely dust-obscured massive galaxies but also normal star-forming galaxies. In total, we identified 27 OFGs at z_phot > 3 (z_med=4.1) in the GOODS-ALMA field, covering a wide distribution of stellar masses with log($M_{\star}$/$M_{\odot}$) = 9.4-11.1. We find that up to 75% of the OFGs with log($M_{\star}$/$M_{\odot}$) = 9.5-10.5 were neglected by previous LBGs and H-dropout selection techniques. After performing stacking analyses, the OFGs exhibit shorter gas depletion timescales, slightly lower gas fractions, and lower dust temperatures than typical star-forming galaxies. Their SFR_tot (SFR_ IR+SFR_UV) is much larger than SFR_UVcorr (corrected for dust extinction), with SFR_tot/SFR_UVcorr = $8\pm1$, suggesting the presence of hidden dust regions in the OFGs that absorb all UV photons. The average dust size measured by a circular Gaussian model fit is R_e(1.13 mm)=1.01$\pm$0.05 kpc. We find that the cosmic SFRD at z>3 contributed by massive OFGs is at least two orders of magnitude higher than the one contributed by equivalently massive LBGs. Finally, we calculate the combined contribution of OFGs and LBGs to the cosmic SFRD at z=4-5 to be 4 $\times$ 10$^{-2}$ $M_{\odot}$ yr$^{-1}$Mpc$^{-3}$, which is about 0.15 dex (43%) higher than the SFRD derived from UV-selected samples alone at the same redshift.

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arXiv2022

Lightweight HI source finding for next generation radio surveys

Future deep HI surveys will be essential for understanding the nature of galaxies and the content of the Universe. However, the large volume of these data will require distributed and automated processing techniques. We introduce LiSA, a set of python modules for the denoising, detection and characterization of HI sources in 3D spectral data. LiSA was developed and tested on the Square Kilometer Array Science Data Challenge 2 dataset, and contains modules and pipelines for easy domain decomposition and parallel execution. LiSA contains algorithms for 2D-1D wavelet denoising using the starlet transform and flexible source finding using null-hypothesis testing. These algorithms are lightweight and portable, needing only a few user-defined parameters reflecting the resolution of the data. LiSA also includes two convolutional neural networks developed to analyse data cubes which separate HI sources from artifacts and predict the HI source properties. All of these components are designed to be as modular as possible, allowing users to mix and match different components to create their ideal pipeline. We demonstrate the performance of the different components of LiSA on the SDC2 dataset, which is able to find 95% of HI sources with SNR > 3 and accurately predict their properties.

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arXiv2021

COLDz: Probing Cosmic Star Formation With Radio Free-free Emission

Radio free-free emission is considered to be one of the most reliable tracers of star formation in galaxies. However, as it constitutes the faintest part of the radio spectrum -- being roughly an order of magnitude less luminous than radio synchrotron emission at the GHz frequencies typically targeted in radio surveys -- the usage of free-free emission as a star formation rate tracer has mostly remained limited to the local Universe. Here we perform a multi-frequency radio stacking analysis using deep Karl G. Jansky Very Large Array observations at 1.4, 3, 5, 10 and 34 GHz in the COSMOS and GOODS-North fields to probe free-free emission in typical galaxies at the peak of cosmic star formation. We find that $z \sim 0.5 - 3$ star-forming galaxies exhibit radio emission at rest-frame frequencies of $\sim 65 - 90$ GHz that is $\sim 1.5 - 2\times$ fainter than would be expected from a simple combination of free-free and synchrotron emission, as in the prototypical starburst galaxy M82. We interpret this as a deficit in high-frequency synchrotron emission, while the level of free-free emission is as expected from M82. We additionally provide the first constraints on the cosmic star formation history using free-free emission at $0.5 \lesssim z \lesssim 3$, which are in good agreement with more established tracers at high redshift. In the future, deep multi-frequency radio surveys will be crucial in order to accurately determine the shape of the radio spectrum of faint star-forming galaxies, and to further establish radio free-free emission as a tracer of high-redshift star formation.

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arXiv2021

Sub-millimetre compactness as a critical dimension to understand the Main Sequence of star-forming galaxies

We study the interstellar medium (ISM) properties as a function of the molecular gas size for 77 infrared-selected galaxies at $z \sim 1.3$. Molecular gas sizes are measured on ALMA images that combine CO(2-1), CO(5-4) and underlying continuum observations, and include CO(4-3), CO(7-6)+[CI]($^3 P_2-^3P_1$), [CI]($^3 P_1-^3P_0$) observations for a subset of the sample. The $\gtrsim 46 \%$ of our galaxies have a compact molecular gas reservoir, and lie below the optical disks mass-size relation. Compact galaxies on and above the main sequence have higher CO excitation and star formation efficiency than galaxies with extended molecular gas reservoirs, as traced by CO(5-4)/CO(2-1) and CO(2-1)/$L_{\rm IR, SF}$ ratios. Average CO+[CI] spectral line energy distributions indicate higher excitation in compacts relative to extended sources. Using CO(2-1) and dust masses as molecular gas mass tracers, and conversion factors tailored to their ISM conditions, we measure lower gas fractions in compact main-sequence galaxies compared to extended sources. We suggest that the sub-millimetre compactness, defined as the ratio between the molecular gas and the stellar size, is an unavoidable information to be used with the main sequence offset to describe the ISM properties of galaxies, at least above $M_{\star} \geqslant 10^{10.6}$ M$_{\odot}$, where our observations fully probe the main sequence scatter. Our results are consistent with mergers driving the gas in the nuclear regions, enhancing the CO excitation and star formation efficiency. Compact main-sequence galaxies are consistent with being an early post-starburst population following a merger-driven starburst episode, stressing the important role of mergers in the evolution of massive galaxies.

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arXiv2021

The non-linear infrared-radio correlation of low-z galaxies: implications for redshift evolution, a new radio SFR recipe, and how to minimize selection bias

The infrared-radio correlation (IRRC) underpins many commonly used radio luminosity-star formation rate (SFR) calibrations. In preparation for the new generation of radio surveys we revisit the IRRC of low-$z$ galaxies by (a) drawing on the best currently available IR and 1.4 GHz radio photometry, plus ancillary data over the widest possible area, and (b) carefully assessing potential systematics. We compile a catalogue of $\sim$9,500 z $<$ 0.2 galaxies and derive their 1.4 GHz radio ($L_{\mathrm{1.4}}$), total IR, and monochromatic IR luminosities in up to seven bands, allowing us to parameterize the wavelength-dependence of monochromatic IRRCs from 22-500 $μ$m. For the first time for low-$z$ samples, we quantify how poorly matched IR and radio survey depths bias measured median IR/radio ratios, $\overline{q}_{\mathrm{TIR}}$, and discuss the level of biasing expected for low-z IRRC studies in ASKAP/MeerKAT fields. For our subset of $\sim$2,000 high-confidence star-forming galaxies we find a median $\overline{q}_{\mathrm{TIR}}$ of 2.54 (scatter: 0.17 dex). We show that $\overline{q}_{\mathrm{TIR}}$ correlates with $L_{\mathrm{1.4}}$, implying a non-linear IRRC with slope 1.11$\pm$0.01. Our new $L_{\mathrm{1.4}}$-SFR calibration, which incorporates this non-linearity, reproduces SFRs from panchromatic SED fits substantially better than previous IRRC-based recipes. Finally, we match the evolutionary slope of recently measured $\overline{q}_{\mathrm{TIR}}$-redshift trends without having to invoke redshift evolution of the IRRC. In this framework, the redshift evolution of $\overline{q}_{\mathrm{TIR}}$ reported at GHz frequencies in the literature is the consequence of a partial, redshift-dependent sampling of a non-linear IRRC obeyed by low-$z$ {\it and} distant galaxies.

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arXiv2020

The ALMA Spectroscopic Survey in the HUDF: The Cosmic Dust and Gas Mass Densities in Galaxies up to $z\sim3$

Using the deepest 1.2 mm continuum map to date in the Hubble Ultra Deep Field obtained as part of the ALMA Spectroscopic Survey (ASPECS) large program, we measure the cosmic density of dust and implied gas (H$_{2}+$H I) mass in galaxies as a function of look-back time. We do so by stacking the contribution from all $H$-band selected galaxies above a given stellar mass in distinct redshift bins, $ρ_{\rm dust}(M_\ast>M,z)$ and $ρ_{\rm gas}(M_\ast>M,z)$. At all redshifts, $ρ_{\rm dust}(M_\ast>M,z)$ and $ρ_{\rm gas}(M_\ast>M,z)$ grow rapidly as $M$ decreases down to $10^{10}\,M_\odot$, but this growth slows down towards lower stellar masses. This flattening implies that at our stellar mass-completeness limits ($10^8\,M_\odot$ and $10^{8.9}\,M_\odot$ at $z\sim0.4$ and $z\sim3$), both quantities converge towards the total cosmic dust and gas mass densities in galaxies. The cosmic dust and gas mass densities increase at early cosmic time, peak around $z\sim2$, and decrease by a factor $\sim4$ and 7, compared to the density of dust and molecular gas in the local universe, respectively. The contribution of quiescent galaxies -- i.e., with little on-going star-formation -- to the cosmic dust and gas mass densities is minor ($\lesssim10\%$). The redshift evolution of the cosmic gas mass density resembles that of the star-formation rate density, as previously found by CO-based measurements. This confirms that galaxies have relatively constant star-formation efficiencies (within a factor $\sim2$) across cosmic time. Our results also imply that by $z\sim0$, a large fraction ($\sim90\%$) of dust formed in galaxies across cosmic time has been destroyed or ejected to the intergalactic medium.

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arXiv2019

Morphological evolution and galactic sizes in the L-Galaxies SA model

In this work we update the L-Galaxies semi-analytic model (SAM) to better follow the physical processes responsible for the growth of bulges via disc instabilities (leading to pseudo-bulges) and mergers (leading to classical bulges). We address the former by considering the contribution of both stellar and gaseous discs in the stability of the galaxy, and we update the latter by including dissipation of energy in gas-rich mergers. Furthermore, we introduce angular momentum losses during cooling and find that an accurate match to the observed correlation between stellar disc scale length and mass at z ~ 0.0 requires that the gas loses 20% of its initial specific angular momentum to the corresponding dark matter halo during the formation of the cold gas disc. We reproduce the observed trends between the stellar mass and specific angular momentum for both disc- and bulge-dominated galaxies, with the former rotating faster than the latter of the same mass. We conclude that a two-component instability recipe provides a morphologically diverse galaxy sample which matches the observed fractional breakdown of galaxies into different morphological types. This recipe also enables us to obtain an excellent fit to the morphology-mass relation and stellar mass function of different galactic types. Finally, we find that energy dissipation during mergers reduces the merger remnant sizes and allows us to match the observed mass-size relation for bulge-dominated systems.

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