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Register a new userCGM$^{2}$ I: The Extent of the Circumgalactic Medium Traced by Neutral Hydrogen
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We present initial results from the textit{COS and Gemini Mapping the Circumgalactic Medium} (mbox{CGMCGM} $equiv$ CGM$^{2}$) survey. The CGM$^{2}$ survey consists of 1689 galaxies, all with high-quality Gemini GMOS spectra, within 1 Mpc of twenty-two $z lesssim 1$ quasars, all with S/N$sim$10 {emph{HST/COS}} G130M$+$G160M spectra. For 572 of these galaxies having stellar masses $10^{7} M_{odot} < M_{star} < 10^{11} M_{odot}$ and $z lesssim 0.5$, we show that the ion{H}{1} covering fraction above a threshold of NHI$>10^{14} $cm$^{-2}$ is $gtrsim 0.5$ within 1.5 virial radii ($R_{rm vir} sim R_{200m}$). We examine the ion{H}{1} kinematics and find that the majority of absorption lies within $pm$ 250 km s$^{-1}$ of the galaxy systemic velocity. We examine ion{H}{1} covering fractions over a range of impact parameters to infer a characteristic size of the CGM, $R^{14}_{rm CGM}$, as a function of galaxy mass. $R^{14}_{rm CGM}$ is the impact parameter at which the probability of observing an absorber with NHI $>$ 10$^{14}$ cm$^{-2}$ is $>$ 50%. In this framework, the radial extent of the CGM of $M_{star} > 10^{9.9} M_{odot}$ galaxies is $R^{14}_{rm CGM} = 346^{+57}_{-53}$ kpc or $R^{14}_{rm CGM} simeq 1.2R_{rm vir}$. Intermediate-mass galaxies with $10^{9.2} < M_{star}/M_{odot} < 10^{9.9}$ have an extent of $R^{14}_{rm CGM} = 353^{+64}_{-50}$ kpc or $R^{14}_{rm CGM} simeq 2.4R_{rm vir}$. Low-mass galaxies, $M_{star} < 10^{9.2} M_{odot}$, show a smaller physical scale $R^{14}_{rm CGM} = 177_{-65}^{+70}$ kpc and extend to $R^{14}_{rm CGM} simeq 1.6R_{rm vir}$. Our analysis suggests that using $R_{rm vir}$ as a proxy for the characteristic radius of the CGM likely underestimates its extent.
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The formation and evolution of galaxies with low neutral atomic hydrogen (HI) masses, M$_{rm HI}$$<$10$^{8}h^{-2}$M$_{odot}$, are affected by host dark matter halo mass and photoionisation feedback from the UV background after the end of reionization. We study how the physical processes governing the formation of galaxies with low HI mass are imprinted on the distribution of neutral hydrogen in the Universe using the hierarchical galaxy formation model, GALFORM. We calculate the effect on the correlation function of changing the HI mass detection threshold at redshifts $0 le z le 0.5$. We parameterize the clustering as $xi(r)=(r/r_{0})^{-gamma}$ and we find that including galaxies with M$_{rm HI}$$<$10$^{8}h^{-2}$M$_{odot}$ increases the clustering amplitude $r_{0}$ and slope $gamma$ compared to samples of higher HI masses. This is due to these galaxies with low HI masses typically being hosted by haloes with masses greater than 10$^{12}{h}^{-1}$M$_{odot}$, and is in contrast to optically selected surveys for which the inclusion of faint, blue galaxies lowers the clustering amplitude. We show the HI mass function for different host dark matter halo masses and galaxy types (central or satellite) to interpret the values of $r_{0}$ and $gamma$ of the clustering of HI-selected galaxies. We also predict the contribution of low HI mass galaxies to the 21cm intensity mapping signal. We calculate that a dark matter halo mass resolution better than $sim$10$^{10}{h}^{-1}$M$_{odot}$ at redshifts higher than 0.5 is required in order to predict converged 21cm brightness temperature fluctuations.
We report ALMA observations of the neutral atomic carbon transitions [CI] and multiple CO lines in a sample of $sim30$ main sequence galaxies at $zsim1$, including novel information on [CI](2-1) and CO(7-6) for 7 of such normal objects. We complement our observations with a collection of $>200$ galaxies with coverage of similar transitions, spanning the $z=0-4$ redshift interval and a variety of ambient conditions from local to high-redshift starbursts. We find systematic variations in the [CI]/IR and [CI]/high-$J$ ($J=7$) CO luminosity ratios among the various samples. We interpret these differences as increased dense molecular gas fractions and star formation efficiencies in the strongest high-redshift starbursts with respect to normal main sequence galaxies. We further report constant $L_{rm [CI]2-1}$/$L_{rm [CI]1-0}$ ratios across the galaxy populations and redshifts, suggesting that gas temperatures $T_{rm exc}$ traced by [CI] do not strongly vary. We find only a mild correlation with $T_{rm dust}$ and that, generally, $T_{rm exc} lesssim T_{rm dust}$. We fit the line ratios with classical PDR models, retrieving consistently larger densities and intensities of the UV radiation fields in submm galaxies than in main sequence and local objects. However, these simple models fall short in representing the complexity of a multiphase interstellar medium and should be treated with caution. Finally, we compare our observations with the Santa Cruz semi-analytical model of galaxy evolution, recently extended to simulate submm emission. While we confirm the success in reproducing the CO lines, we find systematically larger [CI] luminosities at fixed IR luminosity than predicted theoretically. This highlights the necessity of improving our understanding of the mechanisms regulating the [CI] emission on galactic scales. We release our data compilation to the community.
Over the past decade, Lyman-alpha and metal line absorption observations have established the ubiquity of a gas-rich circumgalactic medium (CGM) around star-forming galaxies at z~0.2 potentially tracing half of the missing baryonic mass within galaxy halos. Unfortunately, these observations only provide a statistical measure of the gas in the CGM and do not constrain the spatial distribution and kinematics of the gas. Furthermore, we have limited sensitivity to Lyman-alpha at z~0 with existing instruments. As such, we remain ignorant of how this gas may flow from the CGM onto the disks of galaxies where it can fuel ongoing star-formation in the present day. Fortunately, 21-cm HI observations with radio telescopes can map HI emission providing both spatial and kinematic information for the CGM in galaxies at z=0. Observations with phased array feeds, radio cameras, on single-dish telescopes yield unmatched surface brightness sensitivity and survey speed. These observations can complete the census of HI in the CGM below N(HI)<10^17 cm^-2 and constrain how gas accretion is proceeding in the local universe, particularly when used in concert with UV absorption line data.
We have studied the small scale distribution of atomic hydrogen (HI) using 21-cm absorption spectra against multiple-component background radio continuum sources from the 21-SPONGE survey and the Millennium Arecibo Absorption Line Survey. We have found $>5sigma$ optical depth variations at a level of $sim0.03-0.5$ between 13 out of 14 adjacent sightlines separated by a few arcseconds to a few arcminutes, suggesting the presence of neutral structures on spatial scales from a few to thousands of AU (which we refer to as tiny scale atomic structure, TSAS). The optical depth variations are strongest in directions where the HI column density and the fraction of HI in the cold neutral medium (CNM) are highest, which tend to be at low Galactic latitudes. By measuring changes in the properties of Gaussian components fitted to the absorption spectra, we find that changes in both the peak optical depth and the linewidth of TSAS absorption features contribute to the observed optical depth variations, while changes in the central velocity do not appear to strongly impact the observed variations. Both thermal and turbulent motions contribute appreciably to the linewidths, but the turbulence does not appear strong enough to confine overpressured TSAS. In a majority of cases, the TSAS column densities are sufficiently high that these structures can radiatively cool fast enough to maintain thermal equilibrium with their surroundings, even if they are overpressured. We also find that a majority of TSAS is associated with the CNM. For TSAS in the direction of the Taurus molecular cloud and the local Leo cold cloud, we estimate densities over an order of magnitude higher than typical CNM densities.
Galaxies are surrounded by extended atmospheres, which are often called the circumgalactic medium (CGM) and are the least understood part of galactic ecosystems. The CGM serves as a reservoir of both diffuse, metal-poor gas accreted from the intergalactic medium, and metal-rich gas that is either ejected from galaxies by energetic feedback or stripped from infalling satellites. As such, the CGM is empirically multi-phased and complex in dynamics. Significant progress has been made in the past decade or so in observing the cosmic-ray/B-field, as well as various phases of the CGM. But basic questions remain to be answered. First, what are the energy, mass, and metal contents of the CGM? More specifically, how are they spatially distributed and partitioned in the different components? Moreover, how are they linked to properties of host galaxies and their global clustering and intergalactic medium environments? Lastly, what are the origin, state, and life-cycle of the CGM? This question explores the dynamics of the CGM. Here we illustrate how these questions may be addressed with multi-wavelength observations of the CGM.
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