In collaboration with researchers from the USA, MPA scientists have
mounted a series of ambitious experiments that use a combination of
quasar absorption-line spectra, neutral hydrogen line data, and
state-of-the-art cosmological hydrodynamical simulations to probe the
interface between galaxies and their surrounding gaseous environment.
The researchers found that galaxies with gas-rich disks are embedded
within gas-rich halos and that the gas in these halos is distributed
smoothly and relatively isotropically.
Galaxies need gas to fuel star formation; how galaxies acquire gas is
therefore central to our understanding of galaxy evolution. In the
standard paradigm, galaxies grow primarily through the accretion of gas
that flows from the Inter-Galactic Medium (IGM), through the dark matter
halo, and eventually settling onto the disk of the galaxy. Galaxies
like or own Milky Way need a continuous supply of gas to fuel star
formation, but little is known about the way in which gas cools and
condenses into the disk due to difficulties in observationally mapping
the disk/halo interface.
Bright quasars at large distances from the observer act as cosmic
light beacons. As the light from distant quasars travels through the
Universe, it encounters gas clouds containing mainly hydrogen. These
clouds absorb and scatter ultraviolet photons, leading to characteristic
dips (or absorption lines) in the spectrum of the quasar, the so-called
"Lyman α forest". By choosing quasars that happen to be positioned in
such a way that their light will pass within a short distance (a few
hundred kiloparsec) of a foreground galaxy, we are able to probe the gas
in the so-called "circum-galactic medium" surrounding these systems.
Two large programmes to investigate the circum-galactic medium around
nearby galaxies have now received a total allocation of 200 orbits of
observation time with the Hubble Space Telescope (HST). The first of
these, COS-GASS, used the Cosmic Origins Spectrograph (COS) on board HST
to probe neutral hydrogen around nearby galaxies out to the outer
radius of their surrounding dark matter halos.
The COS-GASS programme found a highly significant correlation (at
99.5% confidence) between the strength of the Lyman α absorption lines,
which are tracing neutral hydrogen in the surrounding halo, with the
ratio of gas mass to stellar mass within the disk. This means that
galaxies with gas-rich disks are embedded within gas-rich halos.
The Lyman α signature was detected in nearly every quasar spectrum
and the average strength of the Lyman α lines decreased gradually as a
function of distance from the galaxy. Finally, the strength of the
Lyman α lines seems to be independent of the orientation of the disk.
This means that the gas in the surrounding gas halos is distributed
smoothly and relatively isotropically.
The quasar spectra obtained as part of the COS-GASS programme mainly
probed sightlines well outside the disk of the galaxy. In 2015, the
follow-on, large programme COS-DISK was approved to probe gas at the
interface between disk and circum-galactic medium. While reduction,
processing and analysis of the HST data is being carried out at Johns
Hopkins University in Baltimore, MPA scientists are closely involved in
using state-of-the-art cosmological hydro-dynamical simulations to
interpret the observational data.
Most of the work so far has focused on the Illustris simulations. The
simulation includes thousands of galaxies with masses in the range of
the galaxies in the COS-GASS and COS-DISK samples, making it ideal for
studying how the disk, circum-galactic medium and disk/halo interface
properties vary as a function of the stellar mass of the galaxy,
morphological type, star formation rate, and gas mass fraction.
Connection between the Circumgalactic Medium and the Interstellar Medium of Galaxies: Results from the COS-GASS Survey
ApJ, 813, 46B, 2015
Source , DOI
2. Nelson et al.
The illustris simulation: Public data release
A&C, 13, 12N, 2015
Source , DOI
3. Kauffmann, Borthakur & Nelson
The morphology and kinematics of neutral hydrogen in the vicinity of z = 0 galaxies with Milky Way masses - a study with the Illustris simulation
MNRAS, 462, 3751K, 2016
Source , DOI
