arXiv subjects

J. A. Hodge

Publications and source records attributed to J. A. Hodge.

At least 37 records · Page 2Linked to original sources

arXiv2021

Measurements of the dust properties in z~1-3 SMGs with ALMA

We present Atacama Large Millimetre Array (ALMA) 2mm continuum observations of a complete and unbiased sample of 99 870micron-selected sub-millimeter galaxies (SMGs) in the Extended Chandra Deep Field South (ALESS). Our observations of each SMG reach average sensitivities of 53 microJy/beam. We measure the flux densities for 70 sources, for which we obtain a typical 870micron-to-2mm flux ratio of 14 +/- 5. We do not find a redshift dependence of this flux ratio, which would be expected if the dust emission properties of our SMGs were the same at all redshifts. By combining our ALMA measurements with existing Herschel/SPIRE observations, we construct a (biased) subset of 27 galaxies for which the cool dust emission is sufficiently well sampled to obtain precise constraints on their dust properties using simple isothermal models. Thanks to our new 2mm observations, the dust emissivity index is well-constrained and robust against different dust opacity assumptions. The median dust emissivity index of our SMGs is $β\simeq1.9\pm0.4$, consistent with the emissivity index of dust in the Milky Way and other local and high-redshift galaxies, as well as classical dust grain model predictions. We also find a negative correlation between the dust temperature and $β$, similar to low-redshift observational and theoretical studies. Our results indicate that $β\simeq2$ in high-redshift dusty star-forming galaxies, implying little evolution in dust grain properties between our SMGs and local dusty galaxy samples, and suggesting these high-mass and high-metallicity galaxies have dust reservoirs driven by grain growth in their ISM.

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arXiv2021

Ultra-faint [CII] emission in a redshift = 2 gravitationally-lensed metal-poor dwarf galaxy

Extreme emission-line galaxies (EELGs) at redshift z=1-2 provide a unique view of metal-poor, starburst sources that are the likely drivers of the cosmic reionization at z$\geq6$. However, the molecular gas reservoirs of EELGs - the fuel for their intense star-formation - remain beyond the reach of current facilities. We present ALMA [CII] and PdBI CO(2-1) observations of a z=1.8, strongly lensed EELG SL2S 0217, a bright Lyman-$α$ emitter with a metallicity 0.05 $Z_\odot$. We obtain a tentative (3-4$σ$) detection of the [CII] line and set an upper limit on the [CII]/SFR ratio of $\leq1\times10^6$ $L_\odot$/($M_\odot$ yr$^{-1}$), based on the synthesized images and visibility-plane analysis. The CO(2-1) emission is not detected. Photoionization modelling indicates that up to 80% of the [CII] emission originates from neutral or molecular gas, although we can not rule out that the gas is fully ionized. The very faint [CII] emission is in line with both nearby metal-poor dwarfs and high-redshift Lyman-$α$ emitters, and predictions from hydrodynamical simulations. However, the [CII] line is 30$\times$ fainter than predicted by the De Looze et al. [CII]-SFR relation for local dwarfs, illustrating the danger of extrapolating locally-calibrated relations to high-redshift, metal-poor galaxies.

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arXiv2020

COLDz: Deep 34 GHz Continuum Observations and Free-free Emission in High-redshift Star-forming Galaxies

The high-frequency radio sky has historically remained largely unexplored due to the typical faintness of sources in this regime, and the modest survey speed compared to observations at lower frequencies. However, high-frequency radio surveys present an invaluable tracer of high-redshift star-formation, as they directly target the faint radio free-free emission. We present deep continuum observations at 34 GHz in the COSMOS and GOODS-North fields from the Karl G. Jansky Very Large Array (VLA), as part of the COLDz survey. The deep COSMOS mosaic spans $\sim10\text{arcmin}^2$ down to $σ=1.3μ\text{Jy beam}^{-1}$, while the wider GOODS-N observations cover $\sim50\text{arcmin}^2$ to $σ=5.3μ\text{Jy beam}^{-1}$. We present the deepest 34 GHz radio number counts to date, with five and thirteen continuum detections in COSMOS and GOODS-N, respectively. Nine galaxies show 34 GHz continuum emission that is originating from star-formation, although for two sources this is likely due to thermal emission from dust. Utilizing deep ancillary radio data at 1.4, 3, 5 and 10 GHz, we decompose the spectra of the remaining seven star-forming galaxies into their synchrotron and thermal free-free components, finding typical thermal fractions and synchrotron spectral indices comparable to those observed in local star-forming galaxies. Using calibrations from the literature, we determine free-free star-formation rates (SFRs), and show that these are in agreement with SFRs from spectral energy distribution fitting and the far-infrared/radio correlation. Our observations place strong direct constraints on the high-frequency radio emission in typical galaxies at high-redshift, and provide some of the first insight in what is set to become a key area of study with future radio facilities as the Square Kilometer Array Phase 1 and next-generation VLA.

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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

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

A Multi-wavelength Analysis of the Faint Radio Sky (COSMOS-XS): the Nature of the Ultra-faint Radio Population

Ultra-deep radio surveys are an invaluable probe of dust-obscured star formation, but require a clear understanding of the relative contribution from radio AGN to be used to their fullest potential. We study the composition of the $μ$Jy radio population detected in the Karl G. Jansky Very Large Array COSMOS-XS survey based on a sample of 1540 sources detected at 3 GHz over an area of $\sim350\text{arcmin}^2$. This ultra-deep survey consists of a single pointing in the well-studied COSMOS field at both 3 and 10 GHz and reaches RMS-sensitivities of $0.53$ and $0.41μ$Jy beam$^{-1}$, respectively. We find multi-wavelength counterparts for $97\%$ of radio sources, based on a combination of near-UV/optical to sub-mm data, and through a stacking analysis at optical/near-infrared wavelengths we further show that the sources lacking such counterparts are likely to be high-redshift in nature (typical $z\sim4-5$). Utilizing the multi-wavelength data over COSMOS, we identify AGN through a variety of diagnostics and find these to make up $23.2\pm1.3\%$ of our sample, with the remainder constituting uncontaminated star-forming galaxies. However, more than half of the AGN exhibit radio emission consistent with originating from star-formation, with only $8.8\pm0.8\%$ of radio sources showing a clear excess in radio luminosity. At flux densities of $\sim30μ$Jy at 3 GHz, the fraction of star-formation powered sources reaches $\sim90\%$, and this fraction is consistent with unity at even lower flux densities. Overall, our findings imply that ultra-deep radio surveys such as COSMOS-XS constitute a highly effective means of obtaining clean samples of star-formation powered radio sources.

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arXiv2020

An Ultra-deep Multi-band VLA Survey of the Faint Radio Sky (COSMOS-XS): Source Catalog and Number Counts

We present ultra-deep, matched-resolution Karl G. Jansky Very Large Array (VLA) observations at 10 and $3$ GHz in the COSMOS field: the COSMOS-XS survey. The final 10 and $3$ GHz images cover $\sim16\rm{arcmin}^{2}$ and $\sim180\rm{arcmin}^{2}$ and reach median rms values of $0.41μ\rm{Jy\,beam}^{-1}$ and $0.53μ\rm{Jy\,beam}^{-1}$, respectively. Both images have an angular resolution of $\sim 2.0''$. To fully account for the spectral shape and resolution variations across the broad bands, we image all data with a multi-scale, multi-frequency synthesis algorithm. We present source catalogs for the 10 and $3$ GHz image with 91 and 1498 sources, respectively, above a peak brightness threshold of $5σ$. We present source counts with completeness corrections included that are computed via Monte Carlo simulations. Our corrected radio counts at $3$ GHz with direct detections down to $\sim2.8μ$Jy are consistent within the uncertainties with other results at 3 and 1.4 GHz, but extend to fainter flux densities than previous direct detections. The ultra-faint $3$ GHz number counts are found to exceed the counts predicted by the semi-empirical radio sky simulations developed in the framework of the SKA Simulated Skies project, consistent with previous P(D) analyses. Our measured source counts suggest a steeper luminosity function evolution for these faint star-forming sources. The semi-empirical Tiered Radio Extragalactic Continuum Simulation (T-RECS) predicts this steeper evolution and is in better agreement with our results. The $10$ GHz radio number counts also agree with the counts predicted by the T-RECS simulation within the expected variations from cosmic variance. In summary, the multi-band, matched-resolution COSMOS-XS survey in the well-studied COSMOS field provides a high-resolution view of the ultra-faint radio sky that can help guide next generation radio facilities.

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arXiv2020

An ALMA Survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS Field: The Far-infrared/Radio correlation for High-redshift Dusty Star-forming Galaxies

We study the radio properties of 706 sub-millimeter galaxies (SMGs) selected at 870$μ$m with the Atacama Large Millimeter Array from the SCUBA-2 Cosmology Legacy Survey map of the Ultra Deep Survey field. We detect 273 SMGs at $>4σ$ in deep Karl G. Jansky Very Large Array 1.4 GHz observations, of which a subset of 45 SMGs are additionally detected in 610 MHz Giant Metre-Wave Radio Telescope imaging. We quantify the far-infrared/radio correlation through parameter $q_\text{IR}$, defined as the logarithmic ratio of the far-infrared and radio luminosity, and include the radio-undetected SMGs through a stacking analysis. We determine a median $q_\text{IR} = 2.20\pm0.03$ for the full sample, independent of redshift, which places these $z\sim2.5$ dusty star-forming galaxies $0.44\pm0.04$ dex below the local correlation for both normal star-forming galaxies and local ultra-luminous infrared galaxies (ULIRGs). Both the lack of redshift-evolution and the offset from the local correlation are likely the result of the different physical conditions in high-redshift starburst galaxies, compared to local star-forming sources. We explain the offset through a combination of strong magnetic fields ($B\gtrsim0.2$mG), high interstellar medium (ISM) densities and additional radio emission generated by secondary cosmic rays. While local ULIRGs are likely to have similar magnetic field strengths, we find that their compactness, in combination with a higher ISM density compared to SMGs, naturally explains why local and high-redshift dusty star-forming galaxies follow a different far-infrared/radio correlation. Overall, our findings paint SMGs as a homogeneous population of galaxies, as illustrated by their tight and non-evolving far-infrared/radio correlation.

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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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arXiv2020

Full of Orions: a 200-pc mapping of the interstellar medium in the redshift-3 lensed dusty star-forming galaxy SDP.81

We present a sub-kpc resolved study of the interstellar medium properties in SDP.81, a z=3.042 strongly gravitationally lensed dusty star-forming galaxy, based on high-resolution, multi-band ALMA observations of the FIR continuum, CO ladder and the [CII] line. Using a visibility-plane lens modelling code, we achieve a median source-plane resolution of ~200 pc. We use photon-dominated region (PDR) models to infer the physical conditions - far-UV field strength, density, and PDR surface temperature - of the star-forming gas on 200-pc scales, finding a FUV field strength of ~10^3-10^4 G0, gas density of ~10^5 cm^-3 and cloud surface temperatures up to 1500 K, similar to those in the Orion Trapezium region. The [CII] emission is significantly more extended than that FIR continuum: ~50 per cent of [CII] emission arises outside the FIR-bright region. The resolved [CII]/FIR ratio varies by almost 2 dex across the source, down to ~2x10^-4 in the star-forming clumps. The observed [CII]/FIR deficit trend is consistent with thermal saturation of the C+ fine-structure level occupancy at high gas temperatures. We make the source-plane reconstructions of all emission lines and continuum data publicly available.

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arXiv2019

First detection of the [OI] 63-um emission from a redshift 6 dusty galaxy

We report a ground-based detection of the [OI] 63-um line in a z=6.027 gravitationally lensed dusty star-forming galaxy (DSFG) G09.83808 using the APEX SEPIA 660 receiver, the first unambiguous detection of the [OI]63 line beyond redshift 3, and the first obtained from the ground. The [OI]63 line is robustly detected at 22$\pm$5 Jy km s$^{-1}$, corresponding to an intrinsic (de-lensed) luminosity of $(5.4\pm1.3)\times10^{9}$ L$_\odot$. With the [OI]63/[CII] luminosity ratio of 4, the [OI]63 line is the main coolant of the neutral gas in this galaxy, in agreement with model predictions. The high [OI]63 luminosity compensates for the pronounced [CII] deficit ([CII]/FIR$\simeq4\times10^{-4}$). Using photon-dominated region models, we derive a source-averaged gas density $n=10^{4.0}$ cm$^{-3}$, and far-UV field strength $G=10^4 G_0$, comparable to the z=2-4 DSFG population. If G09.83808 represents a typical high-redshift DSFG, the [OI]63 line from z=6 non-lensed DSFGs should be routinely detectable in ALMA Band 9 observations with $\sim$15 min on-source, opening a new window to study the properties of the earliest DSFGs.

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arXiv2019

Revealing the Stellar Mass and Dust Distributions of Submillimeter Galaxies at Redshift 2

We combine high-resolution ALMA and HST/CANDELS observations of 20 submillimeter galaxies (SMGs) predominantly from the AS2UDS survey at z~2 with bright rest-frame optical counterparts (Ks < 22.9) to investigate the resolved structural properties of their dust and stellar components. We derive two-dimensional stellar-mass distributions that are inferred from spatial mass-to-light ratio (M/L) corrections based on rest-frame optical colors. Due to the high central column densities of dust in our SMGs, our mass distributions likely represent a lower limit to the true central mass density. The centroid positions between the inferred stellar-mass and the dust distributions agree within 1.1 kpc, indicating an overall good spatial agreement between the two components. The majority of our sources exhibit compact dust configurations relative to the stellar component (with a median ratio of effective radii Re,dust/Re,Mstar = 0.6). This ratio does not change with specific star-formation rate (sSFR) over the factor of 30 spanned by our targets, sampling the locus of "normal" main sequence galaxies up to the starburst regime, log(sSFR/sSFRMS) > 0.5. Our results imply that massive SMGs are experiencing centrally enhanced star formation unlike typical spiral galaxies in the local Universe. The sizes and stellar densities of our SMGs are in agreement with those of the passive population at z=1.5, consistent with these systems being the descendants of z~2 SMGs.

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arXiv2019

ALMA reveals potential evidence for spiral arms, bars, and rings in high-redshift submillimeter galaxies

We present sub-kpc-scale mapping of the 870 $μ$m ALMA continuum emission in six luminous ($L_{\rm IR}~\sim~5~\times10^{12}$ L$_{\odot}$) submillimeter galaxies (SMGs) from the ALESS survey of the Extended Chandra Deep Field South. Our high-fidelity 0.07$''$-resolution imaging ($\sim$500 pc) reveals robust evidence for structures with deconvolved sizes of $\lesssim$0.5-1 kpc embedded within (dominant) exponential dust disks. The large-scale morphologies of the structures within some of the galaxies show clear curvature and/or clump-like structures bracketing elongated nuclear emission, suggestive of bars, star-forming rings, and spiral arms. In this interpretation, the ratio of the `ring' and `bar' radii (1.9$\pm$0.3) agrees with that measured for such features in local galaxies. These potential spiral/ring/bar structures would be consistent with the idea of tidal disturbances, with their detailed properties implying flat inner rotation curves and Toomre-unstable disks (Q<1). The inferred one-dimensional velocity dispersions ($σ_{\rm r}\lesssim$ 70-160 km s$^{-1}$) are marginally consistent with the limits implied if the sizes of the largest structures are comparable to the Jeans length. We create maps of the star formation rate density ($Σ_{\rm SFR}$) on $\sim$500 pc scales and show that the SMGs are able to sustain a given (galaxy-averaged) $Σ_{\rm SFR}$ over much larger physical scales than local (ultra-)luminous infrared galaxies. However, on 500 pc scales, they do not exceed the Eddington limit set by radiation pressure on dust. If confirmed by kinematics, the potential presence of non-axisymmetric structures would provide a means for net angular momentum loss and efficient star formation, helping to explain the very high star formation rates measured in SMGs.

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arXiv2019

An ALMA survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS field: Source catalogue and properties

We present the catalogue and properties of sources in AS2UDS, an 870-$μ$m continuum survey with the Atacama Large Millimetre/sub-millimetre Array (ALMA) of 716 single-dish sub-millimetre sources detected in the UKIDSS/UDS field by the SCUBA-2 Cosmology Legacy Survey. In our sensitive ALMA follow-up observations we detect 708 sub-millimetre galaxies (SMGs) at $>$\,4.3$σ$ significance across the $\sim$\,1-degree diameter field. We combine our precise ALMA positions with the extensive multi-wavelength coverage in the UDS field to fit the spectral energy distributions of our SMGs to derive a median redshift of $z_{\rm phot}=$\,2.61$\pm$0.09. This large sample reveals a statistically significant trend of increasing sub-millimetre flux with redshift suggestive of galaxy downsizing. 101 ALMA maps do not show a $>$\,4.3$σ$ SMG, but we demonstrate from stacking {\it Herschel} SPIRE observations at these positions, that the vast majority of these blank maps correspond to real single-dish sub-millimetre sources. We further show that these blank maps contain an excess of galaxies at $z_{\rm phot}=$\,1.5--4 compared to random fields, similar to the redshift range of the ALMA-detected SMGs. In addition, we combine X-ray and mid-infrared active galaxy nuclei activity (AGN) indicators to yield a likely range for the AGN fraction of 8--28\,\% in our sample. Finally, we compare the redshifts of this population of high-redshift, strongly star-forming galaxies with the inferred formation redshifts of massive, passive galaxies being found out to $z\sim$\,2, finding reasonable agreement -- in support of an evolutionary connection between these two classes of massive galaxy.

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arXiv2019

Strong FUV fields drive the [CII]/FIR deficit in z~3 dusty, star-forming galaxies

We present 0.15-arcsec (1 kpc) resolution ALMA observations of the [CII] 157.74 um line and rest-frame 160-um continuum emission in two z~3 dusty, star-forming galaxies - ALESS 49.1 and ALESS 57.1, combined with resolved CO(3-2) observations. In both sources, the [CII] surface brightness distribution is dominated by a compact core $\leq$1 kpc in radius, a factor of 2-3 smaller than the extent of the CO(3-2) emission. In ALESS 49.1, we find an additional extended (8-kpc radius), low surface-brightness [CII] component. Based on an analysis of mock ALMA observations, the [CII] and 160-um continuum surface brightness distributions are inconsistent with a single-Gaussian surface brightness distribution with the same size as the CO(3-2) emission. The [CII] rotation curves flatten at $\simeq$2 kpc radius, suggesting the kinematics of the central regions are dominated by a baryonic disc. Both galaxies exhibit a strong [CII]/FIR deficit on 1-kpc scales, with FIR-surface-brightness to [CII]/FIR slope steeper than in local star-forming galaxies. A comparison of the [CII]/CO(3-2) observations with PDR models suggests a strong FUV radiation field ($G_0\sim10^4$) and high gas density ($n\mathrm{(H)}\sim10^4-10^5$ cm$^{-3}$) in the central regions of ALESS 49.1 and 57.1. The most direct interpretation of the pronounced [CII]/FIR deficit is a thermal saturation of the C+ fine-structure levels at temperatures $\geq$500 K, driven by the strong FUV field.

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arXiv2018

Science with an ngVLA. Cold gas in High-z Galaxies: CO as redshift beacon

The goal of this science case is to address the use of a ngVLA as a CO redshift machine for dust-obscured high-redshift galaxies which lack of clear counterparts at other wavelengths. Thanks to its unprecedentedly large simultaneous bandwidth and sensitivity, the ngVLA will be able to detect low--J CO transitions at virtually any $z>1$. In particular, at $z>4.76$ two CO transitions will be covered in a single frequency setting, thus ensuring unambiguous line identification. The ngVLA capabilities fill in a redshift range where other approaches (e.g., photometric redshifts, search for optical/radio counterparts, etc) typically fail due to the combination of intrinsically faint emission and increasing luminosity distance. This will allow us to explore the formation of massive galaxies in the early cosmic times.

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arXiv2018

Science with an ngVLA. Cold gas in High-z Galaxies: The molecular gas budget

The goal of this science case is to accurately pin down the molecular gas content of high redshift galaxies. By targeting the CO ground transition, we circumvent uncertainties related to CO excitation. The ngVLA can observe the CO(1-0) line at virtually any $z>1.5$, thus exposing the evolution of gaseous reservoirs from the earliest epochs down to the peak of the cosmic history of star formation. The order-of-magnitude improvement in the number of CO detections with respect to state-of-the-art observational campaigns will provide a unique insight on the evolution of galaxies through cosmic time.

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arXiv2018

Science with an ngVLA. Cold gas in High-z Galaxies: The dense ISM

The goal of this science case is to study physical conditions of the interstellar medium (ISM) in distant galaxies. In particular, its densest component is associated with the inner cores of clouds -- this is where star formation takes place. Carbon monoxide is usually used to trace molecular gas emission; however, its transitions are practically opaque, thus preventing astronomers from piercing through the clouds, into the deepest layers that are most intimately connected with the formation of stars. Other dense gas tracers are required, although they are typically too faint and/or at too low frequencies to be effectively observed in high redshift galaxies. The ngVLA will offer for the first time the sensitivity at radio frequencies that is needed to target [CI]$_{1-0}$ (at $z>5$), as well as the ground transitions of dense gas tracers of the ISM such as HCN, HNC, HCO+ (at various redshifts $z>1$), beyond the tip of the iceberg of the hyper-luminous sources that could be studied up to now. These new tools will critically contribute to our understanding of the intimate interplay between gas clouds and star formation in different environments and cosmic epochs.

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