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Register a new userAn ALMA/NOEMA survey of the molecular gas properties of high-redshift star-forming galaxies
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We present a survey of the molecular gas in 61 submillimetre galaxies (SMGs) selected from 870$mu$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 $Delta z/Delta S_{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 $zsim$1-5 and a median of 200$pm$50Myr at $zsim$2.8. This coincides with an increase in molecular gas fraction $mu_{rm gas} = M_{rm gas}/M_ast$ across the same redshift range. Finally we demonstrate that the $M_{rm baryon}$-$sigma$ 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-$mu$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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Lyman break analogues (LBAs) are a population of star-forming galaxies at low redshift (z ~ 0.2) selected in the ultraviolet (UV). These objects present higher star formation rates and lower dust extinction than other galaxies with similar masses and luminosities in the local universe. In this work we present results from a survey with the Combined Array for Research in Millimetre-wave Astronomy (CARMA) to detect CO(1-0) emission in LBAs, in order to analyse the properties of the molecular gas in these galaxies. Our results show that LBAs follow the same Schmidt-Kennicutt law as local galaxies. On the other hand, they have higher gas fractions (up to 66%) and faster gas depletion time-scales (below 1 Gyr). These characteristics render these objects more akin to high-redshift star-forming galaxies. We conclude that LBAs are a great nearby laboratory for studying the cold interstellar medium in low-metallicity, UV-luminous compact star-forming galaxies.
We present the Evolution of molecular Gas in Normal Galaxies (EGNoG) survey, an observational study of molecular gas in 31 star-forming galaxies from z=0.05 to z=0.5, with stellar masses of (4-30)x10^10 M_Sun and star formation rates of 4-100 M_Sun yr^-1. This survey probes a relatively un-observed redshift range in which the molecular gas content of galaxies is expected to have evolved significantly. To trace the molecular gas in the EGNoG galaxies, we observe the CO(1-0) and CO(3-2) rotational lines using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). We detect 24 of 31 galaxies and present resolved maps of 10 galaxies in the lower redshift portion of the survey. We use a bimodal prescription for the CO to molecular gas conversion factor, based on specific star formation rate, and compare the EGNoG galaxies to a large sample of galaxies assembled from the literature. We find an average molecular gas depletion time of 0.76 pm 0.54 Gyr for normal galaxies and 0.06 pm 0.04 Gyr for starburst galaxies. We calculate an average molecular gas fraction of 7-20% at the intermediate redshifts probed by the EGNoG survey. By expressing the molecular gas fraction in terms of the specific star formation rate and molecular gas depletion time (using typical values), we also calculate the expected evolution of the molecular gas fraction with redshift. The predicted behavior agrees well with the significant evolution observed from z~2.5 to today.
We compare the relations among various integrated characteristics of ~25,000 low-redshift (z<1.0) compact star-forming galaxies (CSFGs) from Data Release 16 (DR16) of the Sloan Digital Sky Survey (SDSS) and of high-redshift (z>1.5) star-forming galaxies (SFGs) with respect to oxygen abundances, stellar masses M*, far-UV absolute magnitudes M(FUV), star-formation rates SFR and specific star-formation rates sSFR, Lyman-continuum photon production efficiencies (xi_ion), UV continuum slopes beta, [OIII]5007/[OII]3727 and [NeIII]3868/[OII]3727 ratios, and emission-line equivalent widths EW([OII]3727), EW([OIII]5007), and EW(Halpha). We find that the relations for low-z CSFGs with high equivalent widths of the Hbeta emission line, EW(Hbeta)>100A, and high-z SFGs are very similar, implying close physical properties in these two categories of galaxies. Thus, CSFGs are likely excellent proxies for the SFGs in the high-z Universe. They also extend to galaxies with lower stellar masses, down to ~10^6 Msun, and to absolute FUV magnitudes as faint as -14 mag. Thanks to their proximity, CSFGs can be studied in much greater detail than distant SFGs. Therefore, the relations between the integrated characteristics of the large sample of CSFGs studied here can prove very useful for our understanding of high-z dwarf galaxies in future observations with large ground-based and space telescopes.
We study the molecular gas properties of high-$z$ galaxies observed in the ALMA Spectroscopic Survey (ASPECS) that targets a $sim1$ arcmin$^2$ region in the Hubble Ultra Deep Field (UDF), a blind survey of CO emission (tracing molecular gas) in the 3mm and 1mm bands. Of a total of 1302 galaxies in the field, 56 have spectroscopic redshifts and correspondingly well-defined physical properties. Among these, 11 have infrared luminosities $L_{rm{}IR}>10^{11}$ L$_odot$, i.e. a detection in CO emission was expected. Out these, 7 are detected at various significance in CO, and 4 are undetected in CO emission. In the CO-detected sources, we find CO excitation conditions that are lower than typically found in starburst/SMG/QSO environments. We use the CO luminosities (including limits for non-detections) to derive molecular gas masses. We discuss our findings in context of previous molecular gas observations at high redshift (star-formation law, gas depletion times, gas fractions): The CO-detected galaxies in the UDF tend to reside on the low-$L_{rm{}IR}$ envelope of the scatter in the $L_{rm{}IR}-L_{rm{}CO}$ relation, but exceptions exist. For the CO-detected sources, we find an average depletion time of $sim$ 1 Gyr, with significant scatter. The average molecular-to-stellar mass ratio ($M_{rm{}H2}$/$M_*$) is consistent with earlier measurements of main sequence galaxies at these redshifts, and again shows large variations among sources. In some cases, we also measure dust continuum emission. On average, the dust-based estimates of the molecular gas are a factor $sim$2-5$times$ smaller than those based on CO. Accounting for detections as well as non-detections, we find large diversity in the molecular gas properties of the high-redshift galaxies covered by ASPECS.
We present the detection of CO(5-4) with S/N> 7 - 13 and a lower CO transition with S/N > 3 (CO(4-3) for 4 galaxies, and CO(3-2) for one) with ALMA in band 3 and 4 in five main sequence star-forming galaxies with stellar masses 3-6x10^10 M/M_sun at 3 < z < 3.5. We find a good correlation between the total far-infrared luminosity LFIR and the luminosity of the CO(5-4) transition LCO(5-4), where LCO(5-4) increases with SFR, indicating that CO(5-4) is a good tracer of the obscured SFR in these galaxies. The two galaxies that lie closer to the star-forming main sequence have CO SLED slopes that are comparable to other star-forming populations, such as local SMGs and BzK star-forming galaxies; the three objects with higher specific star formation rates (sSFR) have far steeper CO SLEDs, which possibly indicates a more concentrated episode of star formation. By exploiting the CO SLED slopes to extrapolate the luminosity of the CO(1-0) transition, and using a classical conversion factor for main sequence galaxies of alpha_CO = 3.8 M_sun(K km s^-1 pc^-2)^-1, we find that these galaxies are very gas rich, with molecular gas fractions between 60 and 80%, and quite long depletion times, between 0.2 and 1 Gyr. Finally, we obtain dynamical masses that are comparable with the sum of stellar and gas mass (at least for four out of five galaxies), allowing us to put a first constraint on the alpha_CO parameter for main sequence galaxies at an unprecedented redshift.
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