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Register a new userThe ALMA Spectroscopic Survey in the HUDF: Constraining the Molecular Content at $log{(M_*/M_odot)} sim 9.5$ with CO stacking of MUSE detected $zsim1.5$ Galaxies
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We report molecular gas mass estimates obtained from a stacking analysis of CO line emission in the ALMA Spectroscopic Survey (ASPECS) using the spectroscopic redshifts from the optical integral field spectroscopic survey by the Multi Unit Spectroscopic Explorer (MUSE) of the {it Hubble} Ultra Deep Field (HUDF). Stacking was performed on subsets of the sample of galaxies classified by their stellar mass and position relative to the main-sequence relation (on, above, below). Among all the CO emission lines, from cotwoone to CO(6-5), with redshifts accessible via the ASPECS Band~3 and the MUSE data, cotwoone provides the strongest constraints on the molecular gas content. We detect cotwoone emission in galaxies down to stellar masses of $log{(M_*/M_odot)}=10.0$. Below this stellar mass, we present a new constraint on the molecular gas content of $zsim1.5$ main-sequence galaxies by stacking based on the MUSE detections. We find that the molecular gas mass of main-sequence galaxies continuously decreases with stellar mass down to $log{(M_*/M_odot)}approx9.0$. Assuming a metallicity-based CO--to--$rm H_2$ conversion factor, the molecular gas-to-stellar mass ratio from $log{(M_*/M_odot)}sim9.0$ to $sim10.0$ does not seem to decrease as fast as for $log{(M_*/M_odot)}>10.0$, which is in line with simulations and studies at lower redshift. The inferred molecular gas density $rho{rm (H_2)}=(0.49pm0.09)times10^8,{rm M_odot,Mpc^{-3}}$ of MUSE-selected galaxies at $zsim1.5$ is comparable with the one derived in the HUDF with a different CO selection. Using the MUSE data we recover most of the CO emission in our deep ALMA observations through stacking, demonstrating the synergy between volumetric surveys obtained at different wavebands.
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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).
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.
We investigate the CO excitation and interstellar medium (ISM) conditions in a cold gas mass-selected sample of 22 star-forming galaxies at $z=0.46-3.60$, observed as part of the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS). Combined with VLA follow-up observations, we detect a total of 34 CO $J rightarrow J-1$ transitions with $J=1$ up to $8$ (and an additional 21 upper limits, up to $J=10$) and six [C I] ${^3P}_1 rightarrow {^3P}_0$ and ${^3P}_2 rightarrow {^3P}_1$ transitions (and 12 upper limits). The CO(2-1) and CO(3-2)-selected galaxies, at $z=1.2$ and $2.5$, respectively, exhibit a range in excitation in their mid-$J=4,5$ and high-$J=7,8$ lines, on average lower than ($L_{rm IR}$-brighter) BzK-color- and submillimeter-selected galaxies at similar redshifts. The former implies that a warm ISM component is not necessarily prevalent in gas mass-selected galaxies at $z=1.2$. We use stacking and Large Velocity Gradient models to measure and predict the average CO ladders at $z<2$ and $zgeq2$, finding $r_{21}=0.75 pm 0.11$ and $r_{31}=0.77 pm 0.14$, respectively. From the models, we infer that the galaxies at $zgeq2$ have intrinsically higher excitation than those at $z<2$. This fits a picture in which the global excitation is driven by an increase in the star formation rate surface density of galaxies with redshift. We derive a neutral atomic carbon abundance of $(1.9 pm 0.4) times 10^{-5}$, comparable to the Milky Way and main-sequence galaxies at similar redshifts, and fairly high densities ($geq 10^4$ cm$^{-3}$), consistent with the low-$J$ CO excitation. Our results imply a decrease in the cosmic molecular gas mass density at $zgeq2$ compared to previous ASPECS measurements.
We present a power spectrum analysis of the ALMA Spectroscopic Survey Large Program (ASPECS LP) data from 84 to 115 GHz. These data predominantly probe small-scale fluctuations ($k=10$-$100$ h Mpc$^{-1}$) in the aggregate CO emission in galaxies at $1 lesssim z lesssim 4$. We place an integral constraint on CO luminosity functions (LFs) in this redshift range via a direct measurement of their second moments in the three-dimensional (3D) auto-power spectrum, finding a total CO shot noise power $P_{textrm{CO,CO}}(k_{textrm{CO(2-1)}}) leq 1.9times10^2$ $mu$K$^2$ (Mpc h$^{-1}$)$^3$. This upper limit ($3sigma$) is consistent with the observed ASPECS CO LFs in Decarli et al. 2019, but rules out a large space in the range of $P_{textrm{CO,CO}}(k_{textrm{CO(2-1)}})$ inferred from these LFs, which we attribute primarily to large uncertainties in the normalization $Phi_*$ and knee $L_*$ of the Schechter-form CO LFs at $z > 2$. Also, through power spectrum analyses of ASPECS LP data with 415 positions from galaxies with available optical spectroscopic redshifts, we find that contributions to the observed mean CO intensity and shot noise power of MUSE galaxies are largely accounted for by ASPECS blind detections, though there are $sim20$% contributions to the CO(2-1) mean intensity due to sources previously undetected in the blind line search. Finally, we sum the fluxes from individual blind CO detections to yield a lower limit on the mean CO surface brightness at 99 GHz of $langle T_{textrm{CO}} rangle = 0.55pm0.02$ $mu$K, which we estimate represents $68$-$80$% of the total CO surface brightness at this frequency.
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.
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