Tuesday, December 1, 2015

Suppression of Star Formation in NGC 1266 Part 2

This paper has broken up into 2 parts. Here is part 2 (part 1 is here).

In the previous blog, I talked about how we set out to determine the star formation rate and the molecular gas properties in NGC 1266, as well as how the two related to one another. Through a thorough investigation to pin together the most reliable tracers of the underlying gas mass and star formation, we showed that star formation is taking place with a inefficiency of at least a factor of 50. Given the density of the gas in this region, this suppression is surprising. We crossed our t’s and dotted our i’s in working through our observations, so a factor of 50 is unlikely to come from misassumptions made with regards to the star formation or gas mass measurement.

That leaves us to find a physical explanation for this suppression. The rate at which the gas in a galaxy forms stars is about balance. Stars form at places that gravitational collapse wins against forces such as radiation and turbulence. The fact that 1% star formation efficiency (with scatter) is seen in most objects tells us that this energy balance is about the same in most galaxies. In NGC 1266, the most likely cause of the star formation suppression is that this balance has changed. Something additional is fighting against the gravitational collapse. So we set out to use the observations we had in-hand to find the culprit of the suppression, thus we turned to energy balance. In this case, we can depend on a rich history of theoretical work, but in this case, the Toomre criterion is where we turn for an idea. The Toomre Q parameter describes the balance between gravitational collapse and forces outward, such as rotation and turbulence.

Toomre Q parameter
The numerator in the Toomre Q equation deals with energies that balance against gravitation, in this case, the internal dispersion of the gas (σ, a.k.a. the random motions) and the rotation of the disk the gas is in (κ, the epicyclic frequency). Both of these provide extra support against gravity, so the higher either σ or κ is, the more support there will be against gravity. The denominator in the equation deals with gravity, G is the gravitational constant, and Σ is the surface density, that is, the amount of material within an area, the higher Σ is, the more gravity there is to balance out. So, we figured out how much energy we needed in the numerator to balance the denominator. The epicyclic frequency κ was measured in the discovery paper, we got the surface density Σ in this paper, and σ was reported in Pellegrini et al. (2013) based on a fit to the higher CO transitions from Herschel. These told us that Q < 0.4, and thus should have been highly unstable. We then looked directly at the observations of the dense gas, which did not have any obvious ordered motion, and so we wondered if we missed some of the kinetic energy.

A look at the dense gas in NGC 1266. Not only are the line wings visible (in the lower left panel) in the dense gas, but the it is clear from the observations that the gas is stirred up (right) and is not rotating the way the stars are (upper left)
So we tried fitting using pure energy balance, and calculated the radial velocity needed instead to balance the mass of the central molecular gas. When we did this, we found that about 100 km/s was needed, which fit our picture! It was very possible that there was enough injected turbulence in this system to explain the suppression of star formation!

Radial velocity needed for stabilization
The suppression of star formation serves other bigger purposes too. For one, if we look back at when we think the triggering event was, we needed to be able to explain how the molecular gas in the system could have lasted for so long without becoming stars. If the AGN is able to suppress star formation on such long timescales, we have a natural explanation. The gas is being preserved. In fact, it is likely being two-fold preserved.

First, the mass outflow rate is not the same as the mass depletion rate. Removing the gas entirely from the system would mean needing to impart enough energy that the gas exceeds the escape velocity. Most of the gas does not meet this criterion, and will instead rain back into the galaxy, depositing its energy back into the disk as turbulence. This allows for a much longer gas survival time. The star formation suppression also acts to extend the lifetime of the gas in the galaxy, so you are able to explain why the gas has remained for so long. So the question is how often does this happen? NGC 1266 was hiding in plain sight for a long time before we truly understood it, and it is beginning to look like molecular outflows are universal in galaxies where molecular gas sits near the supermassive black hole. It is possible that this sort of event is how the M-sigma relation is regulated. But for now, NGC 1266 is a first example that may make up a class of galaxies, which we are just on the cusp of being able to study properly.

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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