Friday, May 26, 2017

The ATLAS3D Project - X. On the origin of the molecular and ionized gas in early-type galaxies

(this paper was led by Tim Davis as first-author. I am second author of this paper.)
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?
One formidable way to study these questions is to grab a sample of already-transformed galaxies, and study them in such detail that the clues from their past lives (and deaths) are observable. That is what the ATLAS3D survey set out to do. Last time, I talked about the imaging of the molecular gas in ATLAS3D galaxies. This time, we can talk more about its interpretation.

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?

Aligned stars and gas (left) could be from an internal origin. The gas misaligned with the stars (right) shown as either polar (90° misaligned) or counter-rotating (180° misaligned) and must be from an external origin, like an accretion event or a minor merger.
Because of this new IFU data, we can actually measure this, by investigating whether the gas and the stars are aligned (that is, if they are rotating along the same axis or not).  The above figure takes a rotating galaxy, with rotation of the stars and the gas shown. If the gas is not aligned with the stars, then it has to come from an outside event, like the accretion of gas or a merger with a small companion.

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 reason that there are mostly dead galaxies in clusters is simple: it is because they are not able to acquire any new gas, because they are "bathing" in the hot intracluster medium, disallowing new accretion. This paper's results actually do support this quite well: that gas is aligned in Virgo likely means that the gas came from some internal process. Field galaxies on the other hand have many more opportunities to accrete new gas.

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.

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