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Register a new userThe ALMA Spectroscopic Survey in the HUDF: Multi-band constraints on line luminosity functions and the cosmic density of molecular gas
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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 $zsim 4$. 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, $rho_{rm H2}$, shows a redshift evolution with an increase from high redshift up to $zsim1.5$ 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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We use the results from the ALMA large program ASPECS, the spectroscopic survey in the Hubble Ultra Deep Field (HUDF), to constrain CO luminosity functions of galaxies and the resulting redshift evolution of $rho$(H$_2$). The broad frequency range covered enables us to identify CO emission lines of different rotational transitions in the HUDF at $z>1$. We find strong evidence that the CO luminosity function evolves with redshift, with the knee of the CO luminosity function decreasing in luminosity by an order of magnitude from $sim$2 to the local universe. Based on Schechter fits, we estimate that our observations recover the majority (up to $sim$90%, depending on the assumptions on the faint end) of the total cosmic CO luminosity at $z$=1.0-3.1. After correcting for CO excitation, and adopting a Galactic CO-to-H$_2$ conversion factor, we constrain the evolution of the cosmic molecular gas density $rho$(H$_2$): this cosmic gas density peaks at $zsim1.5$ and drops by factor of $6.5_{-1.4}^{+1.8}$ to the value measured locally. The observed evolution in $rho$(H$_2$) therefore closely matches the evolution of the cosmic star formation rate density $rho_{rm SFR}$. We verify the robustness of our result with respect to assumptions on source inclusion and/or CO excitation. As the cosmic star formation history can be expressed as the product of the star formation efficiency and the cosmic density of molecular gas, the similar evolution of $rho$(H$_2$) and $rho_{rm SFR}$ leaves only little room for a significant evolution of the average star formation efficiency in galaxies since $zsim 3$ (85% of cosmic history).
In this paper we use ASPECS, the ALMA Spectroscopic Survey in the {em Hubble} Ultra Deep Field (UDF) in band 3 and band 6, to place blind constraints on the CO luminosity function and the evolution of the cosmic molecular gas density as a function of redshift up to $zsim 4.5$. This study is based on galaxies that have been solely selected through their CO emission and not through any other property. In all of the redshift bins the ASPECS measurements reach the predicted `knee of the CO luminosity function (around $5times10^{9}$ K km/s pc$^2$). We find clear evidence of an evolution in the CO luminosity function with respect to $zsim 0$, with more CO luminous galaxies present at $zsim 2$. The observed galaxies at $zsim 2$ also appear more gas-rich than predicted by recent semi-analytical models. The comoving cosmic molecular gas density within galaxies as a function of redshift shows a factor 3-10 drop from $z sim 2$ to $z sim 0$ (with significant error bars), and possibly a decline at $z>3$. This trend is similar to the observed evolution of the cosmic star formation rate density. The latter therefore appears to be at least partly driven by the increased availability of molecular gas reservoirs at the peak of cosmic star formation ($zsim2$).
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, $rho_{rm dust}(M_ast>M,z)$ and $rho_{rm gas}(M_ast>M,z)$. At all redshifts, $rho_{rm dust}(M_ast>M,z)$ and $rho_{rm gas}(M_ast>M,z)$ 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 $zsim0.4$ and $zsim3$), 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 $zsim2$, 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 $zsim0$, a large fraction ($sim90%$) of dust formed in galaxies across cosmic time has been destroyed or ejected to the intergalactic medium.
We discuss the nature and physical properties of gas-mass selected galaxies in the ALMA spectroscopic survey (ASPECS) of the Hubble Ultra Deep Field (HUDF). We capitalize on the deep optical integral-field spectroscopy from the MUSE HUDF Survey and multi-wavelength data to uniquely associate all 16 line-emitters, detected in the ALMA data without preselection, with rotational transitions of carbon monoxide (CO). We identify ten as CO(2-1) at $1 < z < 2$, five as CO(3-2) at $2 < z < 3$ and one as CO(4-3) at $z = 3.6$. Using the MUSE data as a prior, we identify two additional CO(2-1)-emitters, increasing the total sample size to 18. We infer metallicities consistent with (super-)solar for the CO-detected galaxies at $z le 1.5$, motivating our choice of a Galactic conversion factor between CO luminosity and molecular gas mass for these galaxies. Using deep Chandra imaging of the HUDF, we determine an X-ray AGN fraction of 20% and 60% among the CO-emitters at $z sim 1.4$ and $z sim 2.6$, respectively. Being a CO-flux limited survey, ASPECS-LP detects molecular gas in galaxies on, above and below the main sequence (MS) at $z sim 1.4$. For stellar masses $ge 10^{10} (10^{10.5})$ M$_{odot}$, we detect about 40% (50%) of all galaxies in the HUDF at $1 < z < 2$ ($2 < z < 3$). The combination of ALMA and MUSE integral-field spectroscopy thus enables an unprecedented view on MS galaxies during the peak of galaxy formation.
One of the last missing pieces in the puzzle of galaxy formation and evolution through cosmic history is a detailed picture of the role of the cold gas supply in the star-formation process. Cold gas is the fuel for star formation, and thus regulates the buildup of stellar mass, both through the amount of material present through a galaxys gas mass fraction, and through the efficiency at which it is converted to stars. Over the last decade, important progress has been made in understanding the relative importance of these two factors along with the role of feedback, and the first measurements of the volume density of cold gas out to redshift 4, (the cold gas history of the Universe) has been obtained. To match the precision of measurements of the star formation and black-hole accretion histories over the coming decades, a two orders of magnitude improvement in molecular line survey speeds is required compared to what is possible with current facilities. Possible pathways towards such large gains include significant upgrades to current facilities like ALMA by 2030 (and beyond), and eventually the construction of a new generation of radio-to-millimeter wavelength facilities, such as the next generation Very Large Array (ngVLA) concept.
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