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Register a new userThe EGNoG Survey: Molecular Gas in Intermediate-Redshift Star-Forming Galaxies
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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.
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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$5mJy may correspond to the division between slow- and fast-rotators seen in local early-type galaxies.
A series of gravitational instabilities in a circumnuclear gas disk (CND) are required to trigger gas transport to a central supermassive black hole (SMBH) and ignite Active Galactic Nuclei (AGNs). A test of this scenario is to investigate whether an enhanced molecular gas mass surface density ($Sigma_{rm mol}$) is found in the CND-scale of quasars relative to a comparison sample of inactive galaxies. Here we performed sub-kpc resolution CO(2-1) observations with ALMA of four low-redshift ($z sim 0.06$), luminous ($sim 10^{45}$ erg s$^{-1}$) quasars with each matched to a different star-forming galaxy, having similar redshift, stellar mass, and star-formation rate. We detected CO(2-1) emission from all quasars, which show diverse morphologies. Contrary to expectations, $Sigma_{rm mol}$ of the quasar sample, computed from the CO(2-1) luminosity, tends to be smaller than the comparison sample at $r < 500$ pc; there is no systematic enhancement of $Sigma_{rm mol}$ in our quasars. We discuss four possible scenarios that would explain the lower molecular gas content (or CO(2-1) luminosity as an actual observable) at the CND-scale of quasars, i.e., AGN-driven outflows, gas-rich minor mergers, time-delay between the onsets of a starburst-phase and a quasar-phase, and X-ray-dominated region (XDR) effects on the gas chemical abundance and excitation. While not extensively discussed in the literature, XDR effects can have an impact on molecular mass measurements particularly in the vicinity of luminous quasar nuclei; therefore higher resolution molecular gas observations, which are now viable using ALMA, need to be considered.
High signal-to-noise, representative spectra of star-forming galaxies at z~2, obtained via stacking, reveal a high-velocity component underneath the narrow H-alpha and [NII] emission lines. When modeled as a single Gaussian, this broad component has FWHM > 1500 km/s; when modeled as broad wings on the H-alpha and [NII] features, it has FWHM > 500 km/s. This feature is preferentially found in the more massive and more rapidly star-forming systems, which also tend to be older and larger galaxies. We interpret this emission as evidence of either powerful starburst-driven galactic winds or active supermassive black holes. If galactic winds are responsible for the broad emission, the observed luminosity and velocity of this gas imply mass outflow rates comparable to the star formation rate. On the other hand, if the broad line regions of active black holes account for the broad feature, the corresponding black holes masses are estimated to be an order of magnitude lower than those predicted by local scaling relations, suggesting a delayed assembly of supermassive black holes with respect to their host bulges.
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.
[abridged] We present interferometric CO observations of twelve z~2 submillimetre-faint, star-forming radio galaxies (SFRGs) which are thought to be ultraluminous infrared galaxies (ULIRGs) possibly dominated by warmer dust (T_dust ~> 40 K) than submillimetre galaxies (SMGs) of similar luminosities. Four other CO-observed SFRGs are included from the literature, and all observations are taken at the Plateau de Bure Interferometer (PdBI) in the compact configuration. Ten of the sixteen SFRGs observed in CO (63%) are detected at >4sigma with a mean inferred molecular gas mass of ~2*10^10 M_sun. SFRGs trend slightly above the local ULIRG L_FIR-L_CO relation. Since SFRGs are about two times fainter in radio luminosity but exhibit similar CO luminosities to SMGs, this suggests SFRGs are slightly more efficient star formers than SMGs at the same redshifts. SFRGs also have a narrow mean CO line width, 320+-80km/s. SFRGs bridge the gap between properties of very luminous >5*10^12 L_sun SMGs and those of local ULIRGs and are consistent with intermediate stage major mergers. We suspect that more moderate-luminosity SMGs, not yet surveyed in CO, would show similar molecular gas properties to SFRGs. The AGN fraction of SFRGs is consistent with SMGs and is estimated to be 0.3+-0.1, suggesting that SFRGs are observed near the peak phase of star formation activity and not in a later, post-SMG enhanced AGN phase. This CO survey of SFRGs serves as a pilot project for the much more extensive survey of Herschel and SCUBA-2 selected sources which only partially overlap with SMGs. Better constraints on CO properties of a diverse high-z ULIRG population are needed from ALMA to determine the evolutionary origin of extreme starbursts, and what role ULIRGs serve in catalyzing the formation of massive stellar systems in the early Universe.
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