Friday, July 7, 2017

After the Interaction: an Efficiently Star-forming Molecular Disk in NGC5195

Most of the blogs that have already been written here discuss the unexpected. Gas in "red and dead" galaxies or molecular outflow hosts from an unlikely source. This paper and this object are different. We expected the object to behave weirdly and found instead that it seemed much more ordinary than one would think given its nature. The object I blog about here is NGC5195, or M51b, the smaller companion to the Whirlpool Galaxy.

M51a and M51b, courtesy of this site. This also shows what amateur astronomers contribute to scientific progress - look at those tidal tails!
NGC5195 is the ellipsoidal companion to the Whirlpool Galaxy. This interaction is the reason that M51 looks the way it does (being the canonical example of a "grand design" spiral), and is thought to be a 3:1 interaction (with NGC5195 being about 3x less massive than M51a). When we look at the violence of this interaction, we can make a lot of predictions about what would be going on in that smaller galaxy. Perhaps there is a strong AGN, because all of the gas has been driven into the middle. Perhaps there is no gas left. Perhaps the gas that is there is forming stars prolifically, or maybe it is suppressed by turbulence. This is what we set out to investigate.

NGC5195 as seen in the infrared by Spitzer (red traces a star-forming material: polycyclic aromatic hydrocarbons, or PAHs) and in CO, where the velocity field is also plotted (adapted from Alatalo et al. 2016).

We used CARMA to look at multiple molecular lines within NGC5195, to study the nature and behavior of its molecular gas. There is a slight complication though: it is so close to M51 (which is forming plenty of stars), that we need to be careful when assigning the molecular gas to the galaxy. It was believed that the gas in NGC5195, having been disturbed by the interaction that took place with M51 would have unsettled the gas, and possibly left enough turbulence that it might not be forming stars efficiently. This is what we set out to test.

The first thing to point out is that the rotation inside of NGC5195 was regular. That is, rotation is dominating the gas, meaning that the gas is not turbulence dominated. That has implications for the star formation (without turbulence, what is fighting against gravity?) too, which was the next thing we checked.

We took the Herschel 70μm data, which is a good tracer of star formation (via the cold dust, see this Calzetti paper for more details), and we one-to-one mapped it with the CARMA CO data. The Herschel map gave us the star formation surface density (that is, how many stars are forming per area on the source, usually kiloparsec^2) and the CARMA maps gave us the gas surface density. You can then divide the gas surface density by the star formation surface density, and get something called the depletion time. That is, how long going at the rate each parcel of gas will take to completely form into stars. What we found was that overall, it would take about 4 billion years. This is pretty close to average for star-forming galaxies, and completely consistent with star formation rates of early-type galaxies. This means that the star formation in NGC5195 is pretty much efficient, so there is not additional turbulence that the gravitation of the gas has to fight against.

So, in spite of this galaxy having undergone a significant interaction (there are also signs of young stars in the galaxy, which formed about a billion years ago), the gas has already settled back down and is forming stars normally. It makes you wonder, how significant are these extraordinary events to a galaxy? Because at least in NGC5195, it did not take long to revert to the default, with regularly rotating gas and normal, efficient star formation.

The official published version can be found on NASA ADS.
To get a PDF version made by me, you can download it here.

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