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Register a new userNeutral hydrogen in galactic fountains
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C. M. Booth
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Simulations of an isolated Milky Way-like galaxy, in which supernovae power a galactic fountain, reproduce the observed velocity and 21cm brightness statistics of galactic neutral hydrogen (HI). The simulated galaxy consists of a thin HI disk, similar in extent and brightness to that observed in the Milky Way, and extra-planar neutral gas at a range of velocities due to the galactic fountain. Mock observations of the neutral gas resemble the HI flux measurements from the Leiden-Argentine-Bonn (LAB) HI, survey, including a high-velocity tail which matches well with observations of high-velocity clouds. The simulated high-velocity clouds are typically found close to the galactic disk, with a typical line-of-sight distance of 13kpc from observers on the solar circle. The fountain efficiently cycles matter from the centre of the galaxy to its outskirts at a rate of around 0.5 M_sun/yr
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The ejection of the gas out of the disk in late-type galaxies is related to star formation and is due mainly to Type II supernovae. In this paper we studied in detail the development of the Galactic fountains in order to understand their dynamical evolution and their influence in the redistribution of the freshly delivered metals over the disk. To this aim, we performed a number of 3D hydrodynamical radiative cooling simulations of the gas in the Milky Way where the whole Galaxy structure, the Galactic differential rotation and the supernovae explosions generated by a single OB association are considered. A typical fountain powered by 100 Type II supernovae may eject material up to $sim 2$ kpc which than collapses back mostly in form of dense, cold clouds and filaments. The majority of the gas lifted up by the fountains falls back on the disk remaining within a radial distance $Delta R=0.5$ kpc from the place where the fountain originated. This localized circulation of disk gas does not influence the radial chemical gradients on large scale, as required by the chemical models of the Milky Way which reproduce the metallicity distribution without invoking large fluxes of metals. Simulations of multiple fountains fuelled by Type II supernovae of different OB associations will be presented in a companion paper.
We present new upper limits on the neutral hydrogen (HI) content within the stellar half-light ellipses of 15 Galactic dwarf spheroidal galaxies (dSphs), derived from pointed observations with the Green Bank Telescope (GBT) as well as Arecibo L-band Fast ALFA (ALFALFA) survey and Galactic All-Sky Survey (GASS) data. All of the limits Mlim are more stringent than previously reported values, and those from the GBT improve upon contraints in the literature by a median factor of 23. Normalizing by V-band luminosity Lv and dynamical mass Mdyn, we find Mlim/Lv ~ 10^{-3} Mo/Lo and Mlim/Mdyn ~ 5 x 10^{-5}, irrespective of location in the Galactic halo. Comparing these relative HI contents to those of the Local Group and nearby neighbor dwarfs compiled by McConnachie, we find that the Galactic dSphs are extremely gas-poor. Our HI upper limits therefore provide the clearest picture yet of the environmental dependence of the HI content in Local Volume dwarfs. If ram pressure stripping explains the dearth of HI in these systems, then orbits in a relatively massive Milky Way are favored for the outer halo dSph Leo I, while Leo II and Canes Venatici I have had a pericentric passage in the past. For Draco and Ursa Minor, the interstellar medium mass that should accumulate through stellar mass loss in between pericentric passages exceeds Mlim by a factor of ~30. In Ursa Minor, this implies that either this material is not in the atomic phase, or that another mechanism clears the recycled gas on shorter timescales.
We present preliminary results from a study of the neutral hydrogen (HI) properties of an X-ray selected sample of nearby loose galaxy groups. This forms part of a multi-wavelength investigation (X-ray, optical and radio) of the formation and evolution of galaxies within a group environment. Some initial findings of an ATNF Parkes Multibeam wide-area neutral hydrogen imaging survey of 17 nearby galaxy groups include two new, potentially isolated clouds of HI in the NGC 1052 and NGC 5044 groups and significant amounts of HI within the group virial radii of groups NGC 3557 and IC 1459 - two groups with complex X-ray structures that suggest they may still be in the act of virialisation. Here we present ATCA high-resolution synthesis-imaging follow-up observations of the distribution and kinematics of HI in these four groups.
This review summarizes recent studies of the cold neutral hydrogen gas associated with galaxies probed via the HI 21-cm absorption line. HI 21-cm absorption against background radio-loud quasars is a powerful tool to study the neutral gas distribution and kinematics in foreground galaxies from kilo-parsec to parsec scales. At low redshifts (z<0.4), it has been used to characterize the distribution of high column density neutral gas around galaxies and study the connection of this gas with the galaxys optical properties. The neutral gas around galaxies has been found to be patchy in distribution, with variations in optical depth observed at both kilo-parsec and parsec scales. At high redshifts (z>0.5), HI 21-cm absorption has been used to study the neutral gas in metal or Lyman-alpha absorption-selected galaxies. It has been found to be closely linked with the metal and dust content of the gas. Trends of various properties like incidence, spin temperature and velocity width of HI 21-cm absorption with redshift have been studied, which imply evolution of cold gas properties in galaxies with cosmic time. Upcoming large blind surveys of HI 21-cm absorption with next generation radio telescopes are expected to determine accurately the redshift evolution of the number density of HI 21-cm absorbers per unit redshift and hence understand what drives the global star formation rate density evolution.
Here I briefly highlight our studies of the gas content, kinematics and star formation in nearby dwarf galaxies (D < 10 Mpc) based on the `Local Volume HI Survey (LVHIS, Koribalski et al. 2018), which was conducted with the Australia Telescope Compact Array (ATCA). The LVHIS sample consists of nearly 100 galaxies, including new discoveries, spanning a large diversity in size, shape, mass and degree of peculiarity. The hydrogen properties of dwarf galaxies in two nearby groups, Sculptor and CenA / M83, are analysed and compared with many rather isolated dwarf galaxies. Around 10% of LVHIS galaxies are transitional or mixed-type dwarf galaxies (dIrr/dSph), the formation of which is explored. - I also provide a brief update on WALLABY Early Science, where we focus on studying the HI properties of galaxies as a function of environment. WALLABY (Dec < +30 degr, z < 0.26) is conducted with the Australian SKA Pathfinder (ASKAP), a 6-km diameter array of 36 x 12-m dishes, each equipped with wide-field (30 sq degr) Chequerboard Phased Array Feeds.
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