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Register a new userH_2 molecular gas absorption-selected systems trace CO molecular gas-rich galaxy overdensities
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Anne Klitsch
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Absorption-selected galaxies offer an effective way to study low-mass galaxies at high redshift. However, the physical properties of the underlying galaxy population remains uncertain. In particular, the multiphase circum-galactic medium is thought to hold key information on gas flows into and out of galaxies that are vital for galaxy evolution models. Here we present ALMA observations of CO molecular gas in host galaxies of H_2-bearing absorbers. In our sample of six absorbers we detect molecular gas-rich galaxies in five absorber fields although we did not target high-metallicity (>50 per cent solar) systems for which previous studies reported the highest detection rate. Surprisingly, we find that the majority of the absorbers are associated with multiple galaxies rather than single haloes. Together with the large impact parameters these results suggest that the H_2-bearing gas seen in absorption is not part of an extended disk, but resides in dense gas pockets in the circum-galactic and intra-group medium.
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We have used the Atacama Large Millimeter/submillimeter Array (ALMA) to carry out a search for CO (3$-$2) or (4$-$3) emission from the fields of 12 high-metallicity ([M/H]~$geq -0.72$,dex) damped Lyman-$alpha$ absorbers (DLAs) at $z approx 1.7-2.6$. We detected CO emission from galaxies in the fields of five DLAs (two of which have been reported earlier), obtaining high molecular gas masses, $rm M_{mol} approx (1.3 - 20.7) times (alpha_{rm CO}/4.36) times 10^{10} ; M_odot$. The impact parameters of the CO emitters to the QSO sightline lie in the range $b approx 5.6-100$~kpc, with the three new CO detections having $b lesssim 15$~kpc. The highest CO line luminosities and inferred molecular gas masses are associated with the highest-metallicity DLAs, with [M/H]~$gtrsim -0.3$,dex. The high inferred molecular gas masses may be explained by a combination of a stellar mass-metallicity relation and a high molecular gas-to-stars mass ratio in high-redshift galaxies; the DLA galaxies identified by our CO searches have properties consistent with those of emission-selected samples. None of the DLA galaxies detected in CO emission were identified in earlier optical or near-IR searches and vice-versa; DLA galaxies earlier identified in optical/near-IR searches were not detected in CO emission. The high ALMA CO and C[{sc ii}]~158$mu$m detection rate in high-$z$, high-metallicity DLA galaxies has revolutionized the field, allowing the identification of dusty, massive galaxies associated with high-$z$ DLAs. The H{sc i}-absorption criterion identifying DLAs selects the entire high-$z$ galaxy population, including dusty and UV-bright galaxies, in a wide range of environments.
We present the detection analysis of a diffuse molecular cloud at z$_{abs}$=2.4636 towards the quasar SDSS J1513+0352(z$_{em},simeq$ 2.68) observed with the X-shooter spectrograph(VLT). We measure very high column densities of atomic and molecular hydrogen, with log N(HI,H$_2$)$simeq$21.8,21.3. This is the highest H$_2$ column density ever measured in an intervening damped Lyman-alpha system but we do not detect CO, implying log N(CO)/N(H$_2$) < -7.8, which could be due to a low metallicity of the cloud. From the metal absorption lines, we derive the metallicity to be Z $simeq$ 0.15 Z$_{odot}$ and determine the amount of dust by measuring the induced extinction of the background quasar light, A$_V$ $simeq$ 0.4. We also detect Ly-$alpha$ emission at the same redshift, with a centroid located at a most probable impact parameter of only $rho,simeq$ 1.4 kpc. We argue that the line of sight is therefore likely passing through the ISM of a galaxy as opposed to the CGM. The relation between the surface density of gas and that of star formation seems to follow the global empirical relation derived in the nearby Universe although our constraints on the star formation rate and on the galaxy extent remain too loose to be conclusive. We study the transition from atomic to molecular hydrogen using a theoretical description based on the microphysics of molecular hydrogen. We use the derived chemical properties of the cloud and physical conditions (T$_k,simeq$90 K and n$simeq$250 cm$^{-3}$ derived through the excitation of H$_2$ rotational levels and neutral carbon fine structure transitions to constrain the fundamental parameters that govern this transition. By comparing the theoretical and observed HI column densities, we are able to bring an independent constraint on the incident UV flux, which we find to be in agreement with that estimated from the observed star formation rate.
We have obtained 12CO(1--0) data of the nearby barred spiral galaxy M83 from Atacama Large Millimeter/submillimeter Array and Nobeyama 45m observations. By combining these two data sets, the total CO flux has been recovered, and a high angular resolution (2 corresponding to ~40 pc at the distance of M83) has been achieved. The field of view is 3 corresponding to ~3.4 kpc and covers the galactic center, bar, and spiral arm regions. In order to investigate how these galactic structures affect gas properties, we have created a probability distribution function (PDF) of the CO integrated intensity (I_CO), peak temperature, and velocity dispersion for a region with each structure. We find that the I_CO PDF for the bar shows a bright-end tail while that for the arm does not. Since the star formation efficiency is lower in the bar, this difference in PDF shape is contrary to the trend in Milky Way studies where the bright-end tail is found for star-forming molecular clouds. While the peak temperature PDFs are similar for bar and arm regions, velocity dispersion in bar is systematically larger than in arm. This large velocity dispersion is likely a major cause of the bright-end tail and of suppressed star formation. We also investigate an effect of stellar feedback to PDF profiles and find that the different I_CO PDFs between bar and arm regions cannot be explained by the feedback effect, at least at the current spatial scale.
Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be under-abundant and/or sub-thermally excited. We performed one of the most sensitive (down to $mathrm{tau_{rms}^{CO} sim 0.002}$ and $mathrm{tau_{rms}^{HCO^+} sim 0.0008}$) sub-millimeter molecular absorption line observations towards 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to $mathrm{A_v< 0.32,mag}$ and hcop was down to $mathrm{A_v < 0.2,mag}$, where atomic gas and dark molecular gas (DMG) starts to dominate. Multiple transitions measured in absorption toward 3C454.3 allow for a direct determination of excitation temperatures $mathrm{T_{ex}}$ of 4.1,K and 2.7,K, for CO and for hcop, respectively, which are close to the cosmic microwave background (CMB) and provide explanation for their being undercounted in emission surveys. A stronger linear correlation was found between $mathrm{N_{HCO^+}}$ and $mathrm{N_{H_2}}$ (Pearson correlation coefficient P $sim$ 0.93) than that of $mathrm{N_{CO}}$ and $mathrm{N_{H_2}}$ (P $sim$ 0.33), suggesting hcop being a better tracer of H$_2$ than CO in diffuse gas. The derived CO-to-h2 conversion factor (the CO X-factor) of (14 $pm$ 3) $times$ 10$^{20}$ cm$^{-2}$ (K kms)$^{-1}$ is approximately 6 times larger than the average value found in the Milky Way.
Recent molecular line observations with ALMA and NOEMA in several Brightest Cluster Galaxies (BCG) have revealed the large-scale filamentary structure at the center of cool core clusters. These filaments extend over 20-100kpc, they are tightly correlated with ionized gas (H$alpha$, [NII]) emission, and have characteristic shapes: either radial and straight, or also showing a U-turn, like a horse-shoe structure. The kinematics is quite regular and laminar, and the derived infall time is much longer than the free-fall time. The filaments extend up to the radius where the cooling time becomes larger than the infall time. Filaments can be perturbed by the sloshing of the BCG in its cluster, and spectacular cooling wakes have been observed. Filaments tend to occur at the border of cavities driven in the X-ray gas by the AGN radio jets. Observations of cool core clusters support the thermal instability scenario, which accounts for the multiphase medium in the upper atmospheres of BCG, where the right balance between heating and cooling is reached, and a chaotic cold gas accretion occurs. Molecular filaments are also seen associated to ram-pressure stripped spiral galaxies in rich galaxy clusters, and in jet-induced star formation, suggesting a very efficient molecular cloud formation even in hostile cluster environments.
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