Friday, June 23, 2017

The ATLAS3D Project - XIV. The extent and kinematics of the molecular gas in early-type galaxies.

(this paper was led by Tim Davis as first-author. I am second author of this paper.)

We continue our exploration of the molecular gas properties of early-type galaxies, which I have already blogged about here, here, and here. In this case, we look again at a paper that Tim Davis wrote, in particular, about the extent of molecular gas in our ATLAS3D early-type galaxies.

We start by thinking about the "default" galaxies: spiral, late-type, star-forming galaxies are the ones that have been studied most often in molecular gas. This makes sense, because the galaxies that are forming stars are also the ones with the most prevalent molecular gas, and were the ones assumed to have molecular gas. As we've already discussed, early-type galaxies until very recently were assumed to be "red and dead". Some hints that that might not be true existed, but ATLAS3D was the first group to put a quantifiable value on just how many still had cold gas, albeit usually at small molecular gas fractions. This though explains why it is really just now that we are getting around to trying to understand how molecular gas behaves in early-type galaxies, and how that compares to the "default."
The stars (underlying photo) and molecular gas (blue) in the Whirlpool Galaxy. Image credit: PAWS Team/IRAM/NASA HST/ T. A. Rector, this site.
Molecular gas tends to inhabit certain regions of a galaxy.  In late-type spirals, this is often found in the nucleus and along the spiral arms (as is seen in the Whirlpool Galaxy above). If we zoomed out a bit more, the gas would not be in such beautifully pronounced clumps, but its relationship to the galaxy and the stars could still be seen. Some of the best work was done by the Berkeley-Illinois-Maryland Array Survey of Nearby Galaxies (BIMA-SONG), which mapped dozens of nearby spiral galaxies with the precursor to CARMA, BIMA. One of the first results was looking at how far out the molecular gas in these galaxies traversed, authored by Michael Regan. The main result seemed to be that overall, the gas traced the starlight in these galaxies fairly well.

In spirals, this is hardly a surprise. The very stars in the spiral are often forming out of that molecular gas, so their connection makes sense. What we set out to do was find out if the same was true for early-type galaxies. The first thing that we noticed though, was that the overall extent (that is, how far out in physical units the gas is found) of the molecular gas in the early-types was smaller than in the late-types of BIMA-SONG. But again, that was in absolute terms. What about when you look at the gas relative to the stars?

The extent of the molecular gas (traced by CO) compared to the extent of the stars in ATLAS3D galaxies (top, red) and BIMA-SONG spirals (bottom, purple). The extents match fairly well (adapted from Davis et al. 2013)
In that, the story is different. The extents compared to the stars match quite well. So the molecular gas does not look different in early-types than in late-types, when we take into account the nature of the stars in both. There are a few possible reasons for this. In spirals, the gas is forming stars, which add to the stellar component of the galaxy. Gravitational torques are also acting on the gas. And we posited another cause: that some of the gas is recycled from the stars, resulting in the extents being related.
The extent of the molecular gas (traced by CO) in both Virgo and field ATLAS3D early-type galaxies. Here we see the extents are different (adapted from Davis et al. 2013)
Despite the fact that the kind of galaxy that the gas is in does not appear to have much impact on the extent, there is something that does factor in. The environment. Like the molecular gas alignment, and the CO isotope ratios, the extent of the molecular gas also depends on environment, with the Virgo galaxies having the most compact molecular gas distributions compared to the stars. And again, some of our favorite explanations could do this. Ram pressure does not necessarily just act on the neutral gas (as we discussed last time), and could preferentially evaporate the less dense clouds farther out. Additionally, in Virgo, where none of the gas can be externally acquired, the internal stellar mass loss could be even more important, leading to a tighter coupling between stars and gas, explaining the extent.

One thing is clear though, it is time to get better observations of galaxies of all types to begin to understand molecular gas: its fate, its relationships, and its origin. Especially in galaxy clusters (like Virgo).

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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