|The (modern) Hubble Tuning Fork. Image credit: Space Telescope Science Institute|
As the last blog talked about, we start with the Hubble tuning fork (Edwin Hubble published this in 1936, which is pretty amazing given how nearly it gets it right.) Since we figured out that galaxies can be broken down simply into morphological types: the older "red and dead" elliptical and lenticulars versus the young star-forming blue spirals, the question of how a galaxy dies and transforms from the star-forming spiral into a red and dead galaxy is really important. How will the Milky Way do it? Do we know every way this can happen? How do we even study that?
A revolution to observing properties of galaxies came in the form of integral field spectroscopy and integral field units (IFUs for short). These were instruments that could take many spectra in a footprint, mapping the entire spatial extent of a galaxy. (The CALIFA survey logo on the left demonstrates what that footprint looks like.)
IFU observations can directly trace stellar rotation by getting a spatially resolved spectral map of the galaxy, detecting absorption lines that are predominantly found in the atmospheres of stars, and measuring the average velocity in that particular spaxel (think: pixel that you are getting a spectrum from.) Combining that with the kinematics of the gas, and you can compare the two, asking the fundamental question: are the stars and gas linked, or was the gas acquired from an external source?
This paper explored the origin of gas in the ATLAS3D early-type galaxies, discussed in the previous blog. But instead of just looking at how many galaxies of each type were observed, it also looked at what sorts of environments those galaxies were in. It's been well known for a while that galaxies that are found in clusters are often "red and dead", as compared to the field. This is known as the morphology-density relation. So this paper looked at whether the alignment of gas was different in Virgo, the canonical high density region surveyed by ATLAS3D, and the field. While the detection rates of molecular gas in these two populations was the same, the alignment of their gas was not.
|The alignment of gas compared to stars in Virgo cluster ATLAS3D early-type galaxies vs. those in the field. There are a lot more galaxies in the field with misaligned gas. Adapted from Davis el al. 2011|
The quandary lies in why the detection rates are the same, despite this confirmation that the origin of the gas has to be different? It could be that there is something that is different about the remaining gas in the galaxies in Virgo as compared to the ones in the field, which is a thread I will pick up in the next blogged paper. Stay tuned!
The official published version can be found on NASA ADS.
The arXiv pre-print version (prepared by Tim) can be found here.