Astrobites: Galaxies May Be Eating Gas All the Time!

Spiral galaxies, like our own Milky Way , are continuously forming stars from molecular hydrogen stored in the disks. Without additional gas supply from outside of the disks, galaxies’s star-forming activities would be halted in a few billion years due to the shortage of molecular gas – so called the gas consumption problem. To alleviate the crisis, both theories and simulations suggest that there may exist large amount of gas surrounding the disks of galaxies, which could potentially be accreted onto the disks to form stars (see other solutions in Putman+2012). Astronomers intensively search the evidence of the existence of such gas in the vicinity of galaxies – the circumgalactic medium (CGM). The CGM is loosely defined as the gas confined within the virial radius of a galaxy; i.e., gas is gravitationally bound to the central galaxy. Thus far, observers have found that for galaxies that are as luminous as the Milky Way, their CGM could contain as much gas as the stellar mass of the disks (e.g., Werk+2014), serving as gas reservoirs that would keep fueling the disks and maintain the star-forming activities.

If the galaxies were able to accrete gas from their CGM to feed their disks’ star formation, one might expect some correlation between the properties of the CGM gas and that of the disk gas. The authors of this paper performed a statistical study on a sample of galaxies to search for such correlation. Specifically, they aimed to understand the difference/similarity between the neutral hydrogen (H $$\rm\scriptstyle I$$) in the CGM and that in the disks.

The Technique to Search for CGM Gas

The CGM gas of a galaxy usually has very low density, thus emission line from this gas is weak and extremely difficult to detect with current telescopes. However, if there is a bright target (usually quasi-stellar objects, or QSOs) in the background of the gas, the photons from the target would be blocked (absorbed) due to the gas. This blockage of photons would result in absorption lines at certain wavelengths of the target’s continuum. This technique is broadly used in search of gas in the CGM of galaxies.

In this paper, the authors applied this absorption-line technique to study the CGM gas in 45 nearby galaxies selected from the GASS sample. The galaxies have stellar mass ranging from 10$${}^{10.1}$$ to 10$${}^{11.1}$$ solar mass; the mass range was decided to include both galaxies that are forming stars and those that have ceased star formation. For each galaxy, they managed to observe a QSO that is within the virial radius ($$\sim$$250 kpc) of the galaxy. They estimated the line strength of the Lyman alpha (Ly$$\alpha$$) absorption line from the QSO spectrum, which provides an estimate of the amount of H $$\rm\scriptstyle I$$ in the CGM. The angular distance of each QSO to its central galaxy varies from 63 kpc to 231 kpc. Therefore, with a total number of 45 galaxy-QSO pairs, the authors were able to construct a relatively complete picture of H $$\rm\scriptstyle I$$ at different radii and locations of the CGM. On the other hand, they estimated the total mass of H $$\rm\scriptstyle I$$ contained in the disk by analyzing the H $$\rm\scriptstyle I$$ 21-cm emission from the disk of each galaxy.

Left (Fig 2 in Borthakur+2015): the X axis shows the distance – termed as $$\rho$$ – of the QSO to the central galaxy and the Y axis shows the line strength – termed W (equivalent width)– of the Ly$$\alpha$$ absorption. Blue dots represent galaxies that are actively forming stars while red dots show galaxies that are quiescent (not forming stars). Yellow and cyan diamonds are complementary samples from the COS-Halos survey. Right (Fig 6 in Borthakur+2015): the X axis shows the total mass of H $${\scriptstyle\rm I}$$ in the galaxies’ disks, and the Y axis shows the strength of the Ly$$\alpha$$ absorption due to H $${\scriptstyle\rm I}$$ in the galaxies’ CGM. Blue and red dots are the same as those in the left, green stars are the statistical-binned results within four different H $${\scriptstyle\rm I}$$ mass ranges.