Studying diffuse, warm gas in the outskirts of 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.