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Register a new userThe Atomic Hydrogen Content of Galaxies as a function of Group-Centric Radius
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We apply a spectral stacking technique to Westerbork Synthesis Radio Telescope observations to measure the neutral atomic hydrogen content (HI) of nearby galaxies in and around galaxy groups at $z < 0.11$. Our sample includes 577 optically-selected galaxies (120 isolated galaxies and 457 satellites) covering stellar masses between 10$^{10}$ and 10$^{11.5}$ M$_{odot}$, cross-matched with Yangs group catalogue, with angular and redshift positions from the Sloan Digital Sky Survey. We find that the satellites in the centres of groups have lower HI masses at fixed stellar mass and morphology (characterised by the inverse concentration index) relative to those at larger radii. These trends persist for satellites in both high-mass ($M_{rm halo} > 10^{13.5}h^{-1}$M$_{odot}$) and low-mass ($M_{rm halo} leqslant 10^{13.5}h^{-1}$M$_{odot}$) groups, but disappear if we only consider group members in low local density ($Sigma <$ 5 gal/Mpc$^{-2}$) environments. Similar trends are found for the specific star formation rate. Interestingly, we find that the radial trends of decreasing HI mass with decreasing group-centric radius extend beyond the group virial radius, as isolated galaxies close to larger groups lack HI compared with those located more than $sim$3.0 $R_{180}$ away from the center of their nearest group. We also measure these trends in the late-type subsample and obtain similar results. Our results suggest that the HI reservoir of galaxies can be affected before galaxies become group satellites, indicating the existence of pre-processing in the infalling isolated galaxies.
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We use spectral stacking to measure the contribution of galaxies of different masses and in different hierarchies to the cosmic atomic hydrogen (HI) mass density in the local Universe. Our sample includes 1793 galaxies at $z < 0.11$ observed with the Westerbork Synthesis Radio Telescope, for which Sloan Digital Sky Survey spectroscopy and hierarchy information are also available. We find a cosmic HI mass density of $Omega_{rm HI} = (3.99 pm 0.54)times 10^{-4} h_{70}^{-1}$ at $langle zrangle = 0.065$. For the central and satellite galaxies, we obtain $Omega_{rm HI}$ of $(3.51 pm 0.49)times 10^{-4} h_{70}^{-1}$ and $(0.90 pm 0.16)times 10^{-4} h_{70}^{-1}$, respectively. We show that galaxies above and below stellar masses of $sim$10$^{9.3}$ M$_{odot}$ contribute in roughly equal measure to the global value of $Omega_{rm HI}$. While consistent with estimates based on targeted HI surveys, our results are in tension with previous theoretical work. We show that these differences are, at least partly, due to the empirical recipe used to set the partition between atomic and molecular hydrogen in semi-analytical models. Moreover, comparing our measurements with the cosmological semi-analytic models of galaxy formation {sc Shark} and GALFORM reveals gradual stripping of gas via ram pressure works better to fully reproduce the properties of satellite galaxies in our sample, than strangulation. Our findings highlight the power of this approach in constraining theoretical models, and confirm the non-negligible contribution of massive galaxies to the HI mass budget of the local Universe.
We present recalibrations of the GALFORM semi-analytical model of galaxy formation in a new N-body simulation with the Planck cosmology. The Planck Millennium simulation uses more than 128 billion particles to resolve the matter distribution in a cube of $800$ Mpc on a side, which contains more than 77 million dark matter haloes with mass greater than $2.12 times 10^{9} h^{-1} {rm M_{odot}}$ at the present day. Only minor changes to a very small number of model parameters are required in the recalibration. We present predictions for the atomic hydrogen content (HI) of dark matter halos, which is a key input into the calculation of the HI intensity mapping signal expected from the large-scale structure of the Universe. We find that the HI mass $-$ halo mass relation displays a clear break at the halo mass above which AGN heating suppresses gas cooling, $approx 3 times 10^{11} h^{-1} M_{rm odot}$. Below this halo mass, the HI content of haloes is dominated by the central galaxy; above this mass it is the combined HI content of satellites that prevails. We find that the HI mass - halo mass relation changes little with redshift up to $z=3$. The bias of HI sources shows a scale dependence that gets more pronounced with increasing redshift.
The gas content of the complete compilation of Local Group dwarf galaxies (119 within 2 Mpc) is presented using HI survey data. Within the virial radius of the Milky Way (224 kpc here), 53 of 55 dwarf galaxies are devoid of gas to limits of M$_{rm HI}<10^4$ M$_odot$. Within the virial radius of M31 (266 kpc), 27 of 30 dwarf galaxies are devoid of gas (with limits typically $<10^5$ M$_odot$). Beyond the virial radii of the Milky Way and M31, the majority of the dwarf galaxies have detected HI gas and have HI masses higher than the limits. When the relationship between gas content and distance is investigated using a Local Group virial radius, more of the non-detected dwarf galaxies are within this radius (85$pm1$ of the 93 non-detected dwarf galaxies) than within the virial radii of the Milky Way and M31. Using the Gaia proper motion measurements available for 38 dwarf galaxies, the minimum gas density required to completely strip them of gas is calculated. Halo densities between $10^{-5}$ and $5 times 10^{-4}$ cm$^{-3}$ are typically required for instantaneous stripping at perigalacticon. When compared to halo density with radius expectations from simulations and observations, 80% of the dwarf galaxies with proper motions are consistent with being stripped by ram pressure at Milky Way pericenter. The results suggest a diffuse gaseous galactic halo medium is important in quenching dwarf galaxies, and that a Local Group medium also potentially plays a role.
We present interferometric observations of HI in nine slow rotator early-type galaxies of the Atlas3D sample. With these data, we now have sensitive HI searches in 34 of the 36 slow rotators. The aggregate detection rate is 32% $pm$ 8%, consistent with previous work; however, we find two detections with extremely high HI masses, whose gas kinematics are substantially different from what was previously known about HI in slow rotators. These two cases (NGC 1222 and NGC 4191) broaden the known diversity of HI properties in slow rotators. NGC 1222 is a merger remnant with prolate-like rotation and, if it is indeed prolate in shape, an equatorial gas disc; NGC 4191 has two counterrotating stellar discs and an unusually large HI disc. We comment on the implications of this disc for the formation of $2sigma$ galaxies. In general, the HI detection rate, the incidence of relaxed HI discs, and the HI/stellar mass ratios of slow rotators are indistinguishable from those of fast rotators. These broad similarities suggest that the HI we are detecting now is unrelated to the galaxies formation processes and was often acquired after their stars were mostly in place. We also discuss the HI nondetections; some of these galaxies that are undetected in HI or CO are detected in other tracers (e.g. FIR fine structure lines and dust). The question of whether there is cold gas in massive galaxies scoured nuclear cores still needs work. Finally, we discuss an unusual isolated HI cloud with a surprisingly faint (undetected) optical counterpart.
We use observations made with the Giant Metrewave Radio Telescope (GMRT) to probe the neutral hydrogen (HI) gas content of field galaxies in the VIMOS VLT Deep Survey (VVDS) 14h field at $z approx 0.32$. Because the HI emission from individual galaxies is too faint to detect at this redshift, we use an HI spectral stacking technique using the known optical positions and redshifts of the 165 galaxies in our sample to co-add their HI spectra and thus obtain the average HI mass of the galaxies. Stacked HI measurements of 165 galaxies show that 95 per cent of the neutral gas is found in blue, star-forming galaxies. Among these galaxies, those having lower stellar mass are more gas-rich than more massive ones. We apply a volume correction to our HI measurement to evaluate the HI gas density at $z approx 0.32$ as $Omega_{HI}=(0.50pm0.18) times 10^{-3}$ in units of the cosmic critical density. This value is in good agreement with previous results at z < 0.4, suggesting no evolution in the neutral hydrogen gas density over the last $sim 4$ Gyr. However the $z approx 0.32$ gas density is lower than that at $z sim 5$ by at least a factor of two.
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