An optical image of galaxy M82 with the ionized gas of hydrogen (Hα) shown in pink flowing out of the galaxy.
Image Credit: NASA, ESA, The Hubble Heritage Team, (STScI/AURA)
The diffuse gas around galaxies is hard to
detect, but shows properties which are quite different to the
star-forming gas inside a galaxy. Scientists at MPA have used
observations from the recent MaNGA survey to study how the ionized gas
changes with distance from the center of the galaxy. They have
demonstrated the usefulness of adding spectra from multiple galaxies in
order to analyze the gas in the outskirts of galaxies. Their study shows
that the brightness of the gas decreases, while its temperature
increases the further the gas is located from the center of the galaxy.
The differences between star-forming and circumgalactic gas also seem to
correlate with the star-formation rate and stellar mass of the
galaxies.
Understanding gas in and around
galaxies is crucial to understanding star formation. The gas within a
galaxy is the main ingredient for forming stars, and these stars, in
turn, enrich the gas with heavy elements, or “metals”. Continuous star
formation needs a constant supply of gas, and most likely this comes
from a reservoir of gas surrounding the galaxy in its outskirts, or
halo, called the circum-galactic medium (CGM). Additionally, enriched
gas flows out of the galaxies through supernova explosions, galactic
winds, active galactic nuclei, etc. (see Fig 1 for an example of gas
outflows). By studying the gas in the CGM and near the disk-halo
boundary we can better understand these regulatory processes, gas
properties and flows.
Gas in the halo is difficult to study
because it is very faint and diffuse. Cold neutral gas can be seen by
looking for neutral hydrogen (HI), and through HI surveys it is known
that most galaxies have large reservoirs of gas surrounding the
galaxies. Warm ionized gas with temperatures around 1000 K can be
detected with optical emission lines and in the outskirts of galaxies
this is called extra-planar, diffuse ionized gas (eDIG). Most previous
work has been done with long exposures of individual nearby galaxies,
including our own Milky Way.
With optical spectroscopy, only a few
handfuls of galaxies have been studied, as it is difficult to obtain
exposures deep enough to detect and analyze the diffuse gas. These
studies find that the eDIG has different properties compared to gas in
star-forming regions. Both the eDIG and star-forming gas are ionized
mostly by energy from massive OB stars. As these stars are located in
the disk of the galaxy, many of the differences arise because the eDIG
is farther away from the OB stars than the gas in star-forming regions.
Some other differences are not so easy to explain and vary from galaxy
to galaxy. In some galaxies an additional source of energy may be needed
to explain the properties of the eDIG, such as turbulence or shocks in
the gas, or hot evolved stars in the outskirts of galaxies.
With a new dataset from the survey
Mapping Nearby Galaxies at APO (MaNGA), which is part of the Sloan
Digital Sky Survey (SDSS) IV, a group of MPA scientists addressed these
differences and questions about the eDIG. As an Integral Field Unit
survey, MaNGA takes spectra at multiple spatial locations. The eDIG is
faint and diffuse and in Fig 2 we show an example for the MaNGA
observations of one particular galaxy. Adding multiple spectra taken at
similar locations from similar edge-on, late-type galaxies, we can study
the faint diffuse gas.
The first year of MaNGA data includes a
sample of 49 galaxies that are suitable for this study. We add the
spectra from these 49 galaxies from 7 different locations off the disk
of the galaxies to find how the eDIG varies with distance from the
center of the galaxy. Our analysis shows that the brightness of the eDIG
decreases logarithmically with distance and that most likely the
temperature of the gas increases with distance from the center of the
galaxies.
For a more detailed analysis, e.g. to
figure out which type of galaxies need an additional energy source and
what type of source, we to split the sample by different properties of
the galaxies, such as stellar mass or star formation. With the first
year of data we split the full sample in half and find that in galaxies
with a higher star formation rate, the eDIG is more similar to the
star-forming gas inside the galaxies compared to low star-forming
galaxies where the eDIG is markedly different. Moreover, galaxies with
higher stellar mass have a steeper temperature gradient compared to
those with lower stellar mass. In the future, with more data, we will be
able to split the sample even further to better understand these
questions.
Author:
Postdoc
Phone:
2215
Email: ymamay@mpa-garching.mpg.de
Original Publication
1. A. Jones, G. Kauffmann, R. D'Souza, D. Bizyaev, D. Law, L. Haffner, Y. Bahe, B. Andrews, M. Bershady, J. Brownstein, B. Cherinka, A.Diamond-Stanic, N. Drory, R. A. Riffel, S. F. Sanchez, D. Thomas, D. Wake, R. Yan, K. Zhang.
SDSS IV MaNGA: Deep observations of extra-planar, diffuse ionized gas around late-type galaxies from stacked IFU spectra
