arXiv subjects

Ian Smail

Publications and source records attributed to Ian Smail.

At least 55 records · Page 3Linked to original sources

arXiv2021

An extreme case of galaxy and cluster co-evolution at $z$=0.7

We report the discovery of eMACSJ0252.4$-$2100 (eMACSJ0252), a massive and highly evolved galaxy cluster at $z=0.703$. Our analysis of Hubble Space Telescope imaging and VLT/MUSE and Keck/DEIMOS spectroscopy of the system finds a high velocity dispersion of 1020$^{+180}_{-190}$ km s$^{-1}$ and a high (if tentative) X-ray luminosity of $(1.2\pm 0.4)\times10^{45}$ erg s$^{-1}$ (0.1$-$2.4 keV). As extreme is the system's brightest cluster galaxy, a giant cD galaxy that forms stars at a rate of between 85 and 300 M$_\odot$ yr$^{-1}$ and features an extended halo of diffuse [OII] emission, as well as evidence of dust. Its most remarkable properties, however, are an exceptionally high ellipticity and a radially symmetric flow of gas in the surrounding intracluster medium, potential direct kinematic evidence of a cooling flow. A strong-lensing analysis, anchored by two multiple-image systems with spectroscopic redshifts, finds the best lens model to consist of a single cluster-scale halo with a total mass of $(1.9\pm0.1)\times 10^{14}$ M$_\odot$ within 250 kpc of the cluster core and, again, an extraordinarily high ellipticity of $e=0.8$. Although further, in-depth studies across the electromagnetic spectrum (especially in the X-ray regime) are needed to conclusively determine the dynamical state of the system, the properties established so far suggest that eMACSJ0252 must have already been highly evolved well before $z\sim 1$, making it a prime target to constrain the physical mechanisms and history of the co-evolution or dark-matter halos and baryons in the era of cluster formation.

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arXiv2021

Extensive Lensing Survey of Optical and Near-Infrared Dark Objects (El Sonido): HST H-Faint Galaxies behind 101 Lensing Clusters

We present a Spitzer/IRAC survey of H-faint ($H_{160} \gtrsim 26.4$, $<5σ$) sources in 101 lensing cluster fields. Across a CANDELS/Wide-like survey area of $\sim$648 arcmin$^2$ (effectively $\sim$221 arcmin$^2$ in the source plane), we have securely discovered 53 sources in the IRAC Channel-2 band (CH2, 4.5 $\mathrm{μm}$; median CH2$=22.46\pm0.11$ AB mag) that lack robust HST/WFC3-IR F160W counterparts. The most remarkable source in our sample, namely ES-009 in the field of Abell 2813, is the brightest H-faint galaxy at 4.5 $\mathrm{μm}$ known so far ($\mathrm{CH2}=20.48\pm0.03$ AB mag). We show that the H-faint sources in our sample are massive (median $M_\mathrm{star} = 10^{10.3\pm 0.3}$ $M_\odot$), star-forming (median star formation rate $=100_{-40}^{+60}$ $M_\odot$yr$^{-1}$) and dust-obscured ($A_V=2.6\pm0.3$) galaxies around a median photometric redshift of $z=3.9\pm0.4$. The stellar continua of 14 H-faint galaxies can be resolved in the CH2 band, suggesting a median circularized effective radius ($R_\mathrm{e,circ}$; lensing corrected) of $1.9\pm0.2$ kpc and $<1.5$ kpc for the resolved and whole samples, respectively. This is consistent with the sizes of massive unobscured galaxies at $z\sim4$, indicating that H-faint galaxies represent the dusty tail of the distribution of a wider galaxy population. Comparing with the ALMA dust continuum sizes of similar galaxies reported previously, we conclude that the heavy dust obscuration in H-faint galaxies is related to the compactness of both stellar and dust continua ($R_\mathrm{e,circ}\sim 1$ kpc). These H-faint galaxies make up $16_{-7}^{+13}$% of the galaxies in the stellar mass range of $10^{10}-10^{11.2}$ $M_\odot$ at $z=3\sim5$, contributing to $8_{-4}^{+8}$% of the cosmic star formation rate density in this epoch and likely tracing the early phase of massive galaxy formation.

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arXiv2021

Radio spectral properties of star-forming galaxies in the MIGHTEE-COSMOS field and their impact on the far-infrared-radio correlation

We study the radio spectral properties of 2,094 star-forming galaxies (SFGs) by combining our early science data from the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey with VLA, GMRT radio data, and rich ancillary data in the COSMOS field. These SFGs are selected at VLA 3GHz, and their flux densities from MeerKAT 1.3GHz and GMRT 325MHz imaging data are extracted using the "super-deblending" technique. The median radio spectral index is $α_{\rm 1.3GHz}^{\rm 3GHz}=-0.80\pm0.01$ without significant variation across the rest-frame frequencies ~1.3-10GHz, indicating radio spectra dominated by synchrotron radiation. On average, the radio spectrum at observer-frame 1.3-3GHz slightly steepens with increasing stellar mass with a linear fitted slope of $β=-0.08\pm0.01$, which could be explained by age-related synchrotron losses. Due to the sensitivity of GMRT 325MHz data, we apply a further flux density cut at 3GHz ($S_{\rm 3GHz}\ge50\,μ$Jy) and obtain a sample of 166 SFGs with measured flux densities at 325MHz, 1.3GHz, and 3GHz. On average, the radio spectrum of SFGs flattens at low frequency with the median spectral indices of $α^{\rm 1.3GHz}_{\rm 325MHz}=-0.59^{+0.02}_{-0.03}$ and $α^{\rm 3.0GHz}_{\rm 1.3GHz}=-0.74^{+0.01}_{-0.02}$. At low frequency, our stacking analyses show that the radio spectrum also slightly steepens with increasing stellar mass. By comparing the far-infrared-radio correlations of SFGs based on different radio spectral indices, we find that adopting $α_{\rm 1.3GHz}^{\rm 3GHz}$ for $k$-corrections will significantly underestimate the infrared-to-radio luminosity ratio ($q_{\rm IR}$) for >17% of the SFGs with measured flux density at the three radio frequencies in our sample, because their radio spectra are significantly flatter at low frequency (0.33-1.3GHz).

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arXiv2021

ALMA Observations of Lyman-alpha Blob 1: Multiple major-mergers and widely distributed interstellar media

We present observations of a giant Lyman-alpha blob in the SSA22 proto-cluster at z=3.1, SSA22-LAB1, taken with the Atacama Large Millimeter/submillimeter Array (ALMA). Dust continuum, along with [C II]158um, and CO(4-3) line emission have been detected in LAB1, showing complex morphology and kinematics across a ~100 kpc central region. Seven galaxies at z=3.0987-3.1016 in the surroundings are identified in [C II] and dust continuum emission, with two of them potential companions or tidal structures associated with the most massive galaxies. Spatially resolved [C II] and infrared luminosity ratios for the widely distributed media (L[C II]/LIR~0.01-0.001) suggest that the observed extended interstellar media are likely to have originated from star-formation activity and the contribution from shocked gas is probably not dominant. LAB1 is found to harbour a total molecular gas mass Mmol=(8.7+/-2.0)e+10 Msun, concentrated in the core region of the Ly-alpha-emitting area. While (primarily obscured) star-formation activity in the LAB1 core is one of the most plausible power sources for the Ly-alpha emission, multiple major-mergers found in the core may also play a role in making LAB1 exceptionally bright and extended in Ly-alpha as a result of cooling radiation induced by gravitational interactions.

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arXiv2021

ALMA 1.3 mm Survey of Lensed Submillimeter Galaxies (SMGs) Selected by Herschel: Discovery of Spatially Extended SMGs and Implications

We present an ALMA 1.3 mm (Band 6) continuum survey of lensed submillimeter galaxies (SMGs) at $z=1.0\sim3.2$ with an angular resolution of $\sim0.2$". These galaxies were uncovered by the Herschel Lensing Survey (HLS), and feature exceptionally bright far-infrared continuum emission ($S_\mathrm{peak} \gtrsim 90$ mJy) owing to their lensing magnification. We detect 29 sources in 20 fields of massive galaxy clusters with ALMA. Using both the Spitzer/IRAC (3.6/4.5 $\mathrm{μm}$) and ALMA data, we have successfully modeled the surface brightness profiles of 26 sources in the rest-frame near- and far-infrared. Similar to previous studies, we find the median dust-to-stellar continuum size ratio to be small ($R_\mathrm{e,dust}/R_\mathrm{e,star} = 0.38\pm0.14$) for the observed SMGs, indicating that star formation is centrally concentrated. This is, however, not the case for two spatially extended main-sequence SMGs with a low surface brightness at 1.3 mm ($\lesssim 0.1$ mJy arcsec$^{-2}$), in which the star formation is distributed over the entire galaxy ($R_\mathrm{e,dust}/R_\mathrm{e,star}>1$). As a whole, our SMG sample shows a tight anti-correlation between ($R_\mathrm{e,dust}/R_\mathrm{e,star}$) and far-infrared surface brightness ($Σ_\mathrm{IR}$) over a factor of $\simeq$ 1000 in $Σ_\mathrm{IR}$. This indicates that SMGs with less vigorous star formation (i.e., lower $Σ_\mathrm{IR}$) lack central starburst and are likely to retain a broader spatial distribution of star formation over the whole galaxies (i.e., larger $R_\mathrm{e,dust}/R_\mathrm{e,star}$). The same trend can be reproduced with cosmological simulations as a result of central starburst and potentially subsequent "inside-out" quenching, which likely accounts for the emergence of compact quiescent galaxies at $z\sim2$.

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arXiv2021

ALMA Lensing Cluster Survey: Bright [CII] 158 $μ$m Lines from a Multiply Imaged Sub-$L^{\star}$ Galaxy at $z=6.0719$

We present bright [CII] 158 $μ$m line detections from a strongly magnified and multiply-imaged ($μ\sim20-160$) sub-$L^{*}$ ($M_{\rm UV}$ = $-19.75^{+0.55}_{-0.44}$) Lyman-break galaxy (LBG) at $z=6.0719\pm0.0004$ from the ALMA Lensing Cluster Survey (ALCS). Emission lines are identified at 268.7 GHz at $\geq$ 8$σ$ exactly at positions of two multiple images of the LBG behind the massive galaxy cluster RXCJ0600$-$2007. Our lens models, updated with the latest spectroscopy from VLT/MUSE, indicate that a sub region of the LBG crosses the caustic and is lensed into a long ($\sim6''$) arc with a local magnification of $μ\sim 160$, for which the [CII] line is also significantly detected. The source-plane reconstruction resolves the interstellar medium (ISM) structure, showing that the [CII] line is co-spatial with the rest-frame UV continuum at the scale of $\sim$300 pc. The [CII] line properties suggest that the LBG is a rotation-dominated system whose velocity gradient explains a slight difference of redshifts between the whole LBG and its sub region. The star formation rate (SFR)-$L_{\rm [CII]}$ relations from the sub to the whole regions of the LBG are consistent with those of local galaxies. We evaluate the lower limit of the faint-end of the [CII] luminosity function at $z=6$, and find that it is consistent with predictions from semi analytical models and from the local SFR-$L_{\rm [CII]}$ relation with a SFR function at $z=6$. These results imply that the local SFR-$L_{\rm [CII]}$ relation is universal for a wide range of scales including the spatially resolved ISM, the whole region of galaxy, and the cosmic scale, even in the epoch of reionization.

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arXiv2020

Dust, gas, and metal content in star-forming galaxies at $z\sim3.3$ revealed with ALMA and Near-IR spectroscopy

We conducted sub-millimeter observations with the Atacama Large Millimeter/sub-millimeter Array (ALMA) of star-forming galaxies at $z\sim3.3$, whose gas-phase metallicities have been previously measured. We investigate the dust and gas contents of the galaxies at $z\sim3.3$ and study how galaxies are interacting with their circumgalactic/intergalactic medium at this epoch by probing their gas mass fractions and gas-phase metallicities. Single-band dust continuum emission tracing dust mass and the relation between the gas-phase metallicity and gas-to-dust mass ratio are used to estimate the gas masses. The estimated gas mass fractions and depletion timescales are $f_{\rm gas}=$ 0.20-0.75 and $t_{\rm dep}=$ 0.09-1.55 Gyr, respectively. Although the galaxies appear to tightly distribute around the star-forming main sequence at $z\sim3.3$, both quantities show a wider spread at a fixed stellar mass than expected from the scaling relation, suggesting a large diversity of fundamental gas properties among star-forming galaxies apparently on the main sequence. Comparing gas mass fraction and gas-phase metallicity between the star-forming galaxies at $z\sim3.3$ and at lower redshifts, star-forming galaxies at $z\sim3.3$ appear to be more metal-poor than local galaxies with similar gas mass fractions. Using the gas regulator model to interpret this offset, we find that it can be explained by a higher mass-loading factor, suggesting that the mass-loading factor in outflows increases at earlier cosmic times.

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arXiv2020

The Evolution of Gas-Phase Metallicity and Resolved Abundances in Star-forming Galaxies at $z \approx0.6-1.8$

We present an analysis of the chemical abundance properties of $\approx$650 star-forming galaxies at $z \approx0.6-1.8$. Using integral-field observations from the $K$-band Multi-Object Spectrograph (KMOS), we quantify the [NII]/H$α$ emission-line ratio, a proxy for the gas-phase Oxygen abundance within the interstellar medium. We define the stellar mass-metallicity relation at $z \approx0.6-1.0$ and $z \approx1.2-1.8$ and analyse the correlation between the scatter in the relation and fundamental galaxy properties (e.g. H$α$ star-formation rate, H$α$ specific star-formation rate, rotation dominance, stellar continuum half-light radius and Hubble-type morphology). We find that for a given stellar mass, more highly star-forming, larger and irregular galaxies have lower gas-phase metallicities, which may be attributable to their lower surface mass densities and the higher gas fractions of irregular systems. We measure the radial dependence of gas-phase metallicity in the galaxies, establishing a median, beam smearing-corrected, metallicity gradient of $ ΔZ/ ΔR=0.002 \pm0.004$ dex kpc$^{-1}$, indicating on average there is no significant dependence on radius. The metallicity gradient of a galaxy is independent of its rest-frame optical morphology, whilst correlating with its stellar mass and specific star-formation rate, in agreement with an inside-out model of galaxy evolution, as well as its rotation dominance. We quantify the evolution of metallicity gradients, comparing the distribution of $ΔZ/ ΔR$ in our sample with numerical simulations and observations at $z \approx0-3$. Galaxies in our sample exhibit flatter metallicity gradients than local star-forming galaxies, in agreement with numerical models in which stellar feedback plays a crucial role redistributing metals.

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arXiv2020

FIR-luminous [CII] emitters in the ALMA-SCUBA-2 COSMOS survey (AS2COSMOS): The nature of submillimeter galaxies in a 10 comoving Mpc-scale structure at z~4.6

We report the discovery of a 10 comoving Mpc-scale structure traced by massive submillimeter galaxies (SMGs) at z~4.6. These galaxies are selected from an emission line search of ALMA Band 7 observations targeting 184 luminous submillimeter sources ($S_{850μ{\rm m}}\geq$ 6.2 mJy) across 1.6 degrees$^2$ in the COSMOS field. We identify four [CII] emitting SMGs and two probable [CII] emitting SMG candidates at z=4.60-4.64 with velocity-integrated signal-to-noise ratio of SNR>8. Four of the six emitters are near-infrared blank SMGs. After excluding one SMG whose emission line is falling at the edge of the spectral window, all galaxies show clear velocity gradients along the major axes that are consistent with rotating gas disks. The estimated rotation velocities of the disks are 330-550 km s$^{-1}$ and the inferred host dark-matter halo masses are ~2-8 $\times$ 10$^{12}$M$_{\odot}$. From their estimated halo masses and [CII] luminosity function, we suggest that these galaxies have a high (50-100%) duty cycle and high (~0.1) baryon conversion efficiency (SFR relative to baryon accretion rate), and that they contribute $\simeq$2% to the total star-formation rate density at z=4.6. These SMGs are concentrated within just 0.3% of the full survey volume, suggesting they are strongly clustered. The extent of this structure and the individual halo masses suggest that these SMGs will likely evolve into members of a ~10$^{15}$M$_{\odot}$ cluster at z=0. This survey reveals synchronized dusty starburst in massive halos at z>4, which could be driven by mergers or fed by smooth gas accretion.

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arXiv2020

A kpc-scale resolved study of unobscured and obscured star-formation activity in normal galaxies at z = 1.5 and 2.2 from ALMA and HiZELS

We present Atacama Large Millimeter/Submillimeter Array (ALMA) continuum observations of a sample of nine star-forming galaxies at redshifts 1.47 and 2.23 selected from the High-$z$ Emission Line Survey (HiZELS). Four galaxies in our sample are detected at high significance by ALMA at a resolution of 0.25'' at rest-frame 355 $μ$m. Together with the previously observed H$α$ emission, from adaptive optics-assisted integral-field-unit spectroscopy (0.15'' resolution), and F606W and F140W imaging from the Hubble Space Telescope (0.2'' resolution), we study the star-formation activity, stellar and dust mass in these high-redshift galaxies at $\sim$kpc-scale resolution. We find that ALMA detection rates are higher for more massive galaxies ($M_*>10^{10.5}$ M$_\odot$) and higher [N {\sc ii}]/H$α$ ratios ($>0.25$, a proxy for gas-phase metallicity). The dust extends out to a radius of 8 kpc, with a smooth structure, even for those galaxies presenting clumpy H$α$ morphologies. The half-light radii ($R_{\rm dust}$) derived for the detected galaxies are of the order $\sim$4.5 kpc, more than twice the size of submillimetre-selected galaxies at a similar redshift. Our global star-formation rate estimates -- from far-IR and extinction-corrected H$α$ luminosities -- are in good agreement. However, the different morphologies of the different phases of the interstellar medium suggest complex extinction properties of the high-redshift normal galaxies.

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arXiv2020

An ALMA survey of the SCUBA-2 CLS UDS field: Physical properties of 707 Sub-millimetre Galaxies

We analyse the physical properties of a large, homogeneously selected sample of ALMA-located sub-mm galaxies (SMGs). This survey, AS2UDS, identified 707 SMGs across the ~1 sq.deg. field, including ~17 per cent, which are undetected at $K$>~25.7 mag. We interpret their ultraviolet-to-radio data using MAGPHYS and determine a median redshift of z=2.61+-0.08 (1$σ$ range of z=1.8-3.4) with just ~6 per cent at z>4. Our survey provides a sample of massive dusty galaxies at z>~1, with median dust and stellar masses of $M_d$=(6.8+-0.3)x10$^{8}$M$_\odot$ (thus, gas masses of ~10$^{11}$M$_\odot$) and $M_\ast=$(1.26+-0.05)x10$^{11}$M$_\odot$. We find no evolution in dust temperature at a constant far-infrared luminosity across z~1.5-4. The gas mass function of our sample increases to z~2-3 and then declines at z>3. The space density and masses of SMGs suggest that almost all galaxies with $M_\ast$>~3x10$^{11}$M$_\odot$ have passed through an SMG-like phase. The redshift distribution is well fit by a model combining evolution of the gas fraction in halos with the growth of halo mass past a threshold of $M_h$~6x10$^{12}$M$_\odot$, thus SMGs may represent the highly efficient collapse of gas-rich massive halos. We show that SMGs are broadly consistent with simple homologous systems in the far-infrared, consistent with a centrally illuminated starburst. Our study provides strong support for an evolutionary link between the active, gas-rich SMG population at z>1 and the formation of massive, bulge-dominated galaxies across the history of the Universe.

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arXiv2020

The ALMA Spectroscopic Survey Large Program: The Infrared Excess of z=1.5-10 UV-selected Galaxies and the Implied High-Redshift Star Formation History

We make use of sensitive (9.3 microJy/beam RMS) 1.2mm-continuum observations from the ASPECS ALMA large program of the Hubble Ultra Deep Field (HUDF) to probe dust-enshrouded star formation from 1362 Lyman-break galaxies spanning the redshift range z=1.5-10 (to ~7-28 Msolar/yr at 4 sigma over the entire range). We find that the fraction of ALMA-detected galaxies in our z=1.5-10 samples increases steeply with stellar mass, with the detection fraction rising from 0% at 10^9 Msolar to 85(-18)(+9)% at >10^{10} Msolar. Moreover, stacking all 1253 low-mass (<10^{9.25} Msolar) galaxies over the ASPECS footprint, we find a mean continuum flux of -0.1+/-0.4 microJy/beam, implying a hard upper limit on the obscured SFR of <0.6 Msolar/yr (4 sigma) in a typical low-mass galaxy. The correlation between the infrared excess IRX of UV-selected galaxies (L(IR)/L(UV)) and the UV-continuum slope is also seen in our ASPECS data and shows consistency with a Calzetti-like relation at >10^{9.5} M_{solar} and a SMC-like relation at lower masses. Using stellar-mass and beta measurements for z~2 galaxies over CANDELS, we derive a new empirical relation between beta and stellar mass and then use this correlation to show that our IRX-beta and IRX-stellar mass relations are consistent with each other. We then use these constraints to express the infrared excess as a bivariate function of beta and stellar mass. Finally, we present updated estimates of star-formation rate density determinations at z>3, leveraging current improvements in the measured infrared excess and recent probes of ultra-luminous far-IR galaxies at z>2.

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arXiv2020

An ALMA survey of the S2CLS UDS field: Optically invisible submillimetre galaxies

We analyse a robust sample of 30 near-infrared-faint (K>25.3, 5 sigma) submillimetre galaxies selected across a 0.96 deg^2 field, to investigate their properties and the cause of their lack of detectable optical/near-infrared emission. Our analysis exploits precise identifications based on ALMA 870um continuum imaging, combined with the very deep near-infrared imaging from the UKIDSS-UDS survey. We estimate that K>25.3 submillimetre galaxies represent 15+/-2 per cent of the total population brighter than S870=3.6mJy, with an expected surface density of ~450/deg^2 above S870>1mJy. As such they pose a source of contamination in surveys for both high-redshift "quiescent" galaxies and very-high-redshift Lyman-break galaxies. We show that these K-faint submillimetre galaxies are simply the tail of the broader submillimetre population, with comparable dust and stellar masses to K<25.3 mag submillimetre galaxies, but lying at significantly higher redshifts (z=3.44+/-0.06 versus z=2.36+/-0.11) and having higher dust attenuation (Av=5.2+/-0.3 versus Av=2.9+/-0.1). We investigate the origin of the strong dust attenuation and find indications that these K-faint galaxies have smaller dust continuum sizes than the K<25.3 galaxies, as measured by ALMA, which suggests their high attenuation is related to their compact sizes. We find a correlation of dust attenuation with star-formation rate surface density (Sigma_SFR), with the K-faint submillimetre galaxies representing the higher-Sigma_SFR and highest-Av galaxies. The concentrated, intense star-formation activity in these systems is likely to be associated with the formation of spheroids in compact galaxies at high redshifts, but as a result of their high obscuration these are completely missed in UV, optical and even near-infrared surveys.

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arXiv2020

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: The nature of the faintest dusty star-forming galaxies

We present a characterization of the physical properties of a sample of 35 securely-detected, dusty galaxies in the deep ALMA 1.2-mm image obtained as part of the ALMA Spectroscopic Survey in the {\it Hubble} Ultra Deep Field (ASPECS) Large Program. This sample is complemented by 26 additional sources identified via an optical/infrared source positional prior. Using their well-characterized spectral energy distributions, we derive median stellar masses and star formation rates (SFR) of $4.8\times10^{10}~M_\odot$ and 30 $M_\odot$ yr$^{-1}$, and interquartile ranges of $(2.4-11.7)\times10^{10}~M_\odot$ and $20-50~M_\odot$ yr$^{-1}$, respectively. We derive a median spectroscopic redshift of 1.8 with an interquartile range $1.1-2.6$, significantly lower than submillimeter galaxies detected in shallower, wide-field surveys. We find that 59\%$\pm$13\%, 6\%$\pm$4\%, and 34\%$\pm$9\% of our sources are within, above and below $\pm0.4$ dex from the SFR-stellar mass relation or main-sequence (MS), respectively. The ASPECS galaxies closely follow the SFR-molecular gas mass relation and other previously established scaling relations, confirming a factor of five increase of the gas-to-stellar mass ratio from $z=0.5$ to $z=2.5$ and a mild evolution of the gas depletion timescales with a typical value of 0.7 Gyr at $z=1-3$. ASPECS galaxies located significantly below the MS, a poorly exploited parameter space, have low gas-to-stellar-mass ratios of $\sim0.1-0.2$ and long depletion timescales $>1$ Gyr. Galaxies along the MS dominate the cosmic density of molecular gas at all redshifts. Systems above the main sequence have an increasing contribution to the total gas reservoirs from $z<1$ to $z=2.5$, while the opposite is found for galaxies below the MS.

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arXiv2020

The Evolution of the Baryons Associated with Galaxies Averaged over Cosmic Time and Space

We combine the recent determination of the evolution of the cosmic density of molecular gas (H_2) using deep, volumetric surveys, with previous estimates of the cosmic density of stellar mass, star formation rate and atomic gas (HI), to constrain the evolution of baryons associated with galaxies averaged over cosmic time and space. The cosmic HI and H_2 densities are roughly equal at z~1.5. The H_2 density then decreases by a factor 6^{+3}_{-2} to today's value, whereas the HI density stays approximately constant. The stellar mass density is increasing continuously with time and surpasses that of the total gas density (HI and H_2) at redshift z~1.5. The growth in stellar mass cannot be accounted for by the decrease in cosmic H_2 density, necessitating significant accretion of additional gas onto galaxies. With the new H_2 constraints, we postulate and put observational constraints on a two step gas accretion process: (i) a net infall of ionized gas from the intergalactic/circumgalactic medium to refuel the extended HI reservoirs, and (ii) a net inflow of HI and subsequent conversion to H_2 in the galaxy centers. Both the infall and inflow rate densities have decreased by almost an order of magnitude since z~2. Assuming that the current trends continue, the cosmic molecular gas density will further decrease by about a factor of two over the next 5 Gyr, the stellar mass will increase by approximately 10%, and cosmic star formation activity will decline steadily toward zero, as the gas infall and accretion shut down.

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arXiv2020

The ALMA Spectroscopic Survey in the HUDF: Multi-band constraints on line luminosity functions and the cosmic density of molecular gas

We present a CO and atomic fine-structure line luminosity function analysis using the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS). ASPECS consists of two spatially-overlapping mosaics that cover the entire ALMA 3mm and 1.2mm bands. We combine the results of a line candidate search of the 1.2mm data cube with those previously obtained from the 3mm cube. Our analysis shows that $\sim$80% of the line flux observed at 3mm arises from CO(2-1) or CO(3-2) emitters at $z$=1-3 (`cosmic noon'). At 1.2mm, more than half of the line flux arises from intermediate-J CO transitions ($J_{\rm up}$=3-6); $\sim12$% from neutral carbon lines; and $< 1$% from singly-ionized carbon, [CII]. This implies that future [CII] intensity mapping surveys in the epoch of reionization will need to account for a highly significant CO foreground. The CO luminosity functions probed at 1.2mm show a decrease in the number density at a given line luminosity (in units of $L'$) at increasing $J_{\rm up}$ and redshift. Comparisons between the CO luminosity functions for different CO transitions at a fixed redshift reveal sub-thermal conditions on average in galaxies up to $z\sim 4$. In addition, the comparison of the CO luminosity functions for the same transition at different redshifts reveals that the evolution is not driven by excitation. The cosmic density of molecular gas in galaxies, $ρ_{\rm H2}$, shows a redshift evolution with an increase from high redshift up to $z\sim1.5$ followed by a factor $\sim 6$ drop down to the present day. This is in qualitative agreement with the evolution of the cosmic star-formation rate density, suggesting that the molecular gas depletion time is approximately constant with redshift, after averaging over the star-forming galaxy population.

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arXiv2020

A Comparison of the Stellar, CO and Dust-Continuum Emission from Three, Star-Forming HUDF Galaxies at $z\sim 2$

We compare the extent of the dust, molecular gas and stars in three star-forming galaxies, at $z= 1.4, 1.6$ and $2.7$, selected from the Hubble Ultra Deep Field based on their bright CO and dust-continuum emission as well as their large rest-frame optical sizes. The galaxies have high stellar masses, $\mathrm{M}_*>10^{11}\mathrm{M}_\odot$, and reside on, or slightly below, the main sequence of star-forming galaxies at their respective redshifts. We probe the dust and molecular gas using subarcsecond Atacama Large Millimeter/submillimeter Array observations of the 1.3 mm continuum and CO line emission, respectively, and probe the stellar distribution using \emph{Hubble Space Telescope} observations at 1.6 \textmu m. We find that for all three galaxies the CO emission appears $\gtrsim 30\%$ more compact than the stellar emission. For the $z= 1.4$ and $2.7$ galaxies, the dust emission is also more compact, by $\gtrsim 50\%$, than the stellar emission, whereas for the $z=1.6$ galaxy, the dust and stellar emission have similar spatial extents. This similar spatial extent is consistent with observations of local disk galaxies. However, most high redshift observations show more compact dust emission, likely due to the ubiquity of central starbursts at high redshift and the limited sensitivity of many of these observations. Using the CO emission line, we also investigate the kinematics of the cold interstellar medium in the galaxies, and find that all three have kinematics consistent with a rotation-dominated disk.

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arXiv2020

An ALMA/NOEMA survey of the molecular gas properties of high-redshift star-forming galaxies

We present a survey of the molecular gas in 61 submillimetre galaxies (SMGs) selected from 870$μ$m continuum surveys of the COSMOS, UDS and ECDFS fields, using the Atacama Large Millimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA). 46 $^{12}$CO ($J=$2-5) emission lines are detected in 45 of the targets at $z=$1.2-4.8, with redshifts indicating that those which are submillimetre bright and undetected/faint in the optical/near-infrared typically lie at higher redshifts, with a gradient of $Δz/ΔS_{870}=$0.11$\pm$0.04mJy$^{-1}$. We also supplement our data with literature sources to construct a statistical CO spectral line energy distribution and find the $^{12}$CO line luminosities in SMGs peak at $J_{\rm up}\sim$6, consistent with the Cosmic Eyelash, among similar studies. Our SMGs lie mostly on or just above the main sequence, displaying a decrease in their gas depletion timescales $t_{\rm dep} = M_{\rm gas}/{\rm SFR}$ with redshift in the range $z\sim$1-5 and a median of 200$\pm$50Myr at $z\sim$2.8. This coincides with an increase in molecular gas fraction $μ_{\rm gas} = M_{\rm gas}/M_\ast$ across the same redshift range. Finally we demonstrate that the $M_{\rm baryon}$-$σ$ distribution of our SMGs is consistent with that followed by early-type galaxies in the Coma cluster, providing strong support to the suggestion that SMGs are progenitors of massive local spheroidal galaxies. On the basis of this we suggest that the SMG populations above and below an 870-$μ$m flux limit of $S_{870}\sim$5mJy may correspond to the division between slow- and fast-rotators seen in local early-type galaxies.

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