Monday, December 7, 2015

Escape, Accretion, or Star Formation? The Competing Depleters of Gas in the Quasar Markarian 231

This paper came about because I had a set of beautiful unpublished data from CARMA that needed to be given its due credit, and I had noticed a bad habit of my astronomy community to call the detection of an AGN-driven molecular outflow a “special time in the life of that galaxy,” yet finding these objects was becoming ubiquitous. The extraordinary resolution of the CARMA images of this source tied into this point of view made it irresistible to write up - and do it by myself!

HST image of Mrk 231
Markarian 231 (Mrk 231) is not only our nearest-by quasar, it appears to be a supermassive black hole which we are staring down the throat of. Mrk 231 is also a prolific starburst, forming about 170 M⦿/yr of stars per year. It has a gargantuan amount of molecular gas all sitting very close to the AGN. This together makes Mrk 231 a very interesting source, but in 2010, Mrk 231 was also found to have a molecular outflow: the first found and published by Feruglio et al. The problem was that the outflow was so compact, it was very difficult to resolve where the gas was. They knew that the gas was found in wings (like in NGC 1266) but did not have the resolution to know where the gas was. Enter CARMA.

We used CARMA in one of its most extended arrays, that is, when the baseline between two antennas was as large as 1 kilometer, giving us spectacular resolution for the millimeter: 0.7”. We pointed CARMA at Mrk 231, detecting the CO(1-0) without a problem, as well as bright radio continuum in 3mm and carbon mono sulfide (CS). Using the CS, we confirmed the inferred masses from other studies for the dense gas, and we successfully detected the wings. More importantly though, we resolved the lobes! Now, armed with the wing emission as well as the actual size scale of the source (thus allowing us to infer a timescale), we were able to put good limits on the mass outflow rate, which we predicted to be 390 M⦿/yr. A bit smaller than the original work, but still capable of blowing away all of the molecular gas in the system rapidly. This is where the work from NGC 1266 kicked in. I wasn’t so sure that Mrk 231 was truly going to deplete its gas that fast. Evidence was mounting that more and more objects had molecular outflows, and the stellar populations within quasars (and Mrk 231 in particular) seemed to indicate that the quenching happened longer ago than one would infer from how quickly the gas supposedly depleted. So I turned to a different rate to determine depletion: the mass escape rate. 
HST image (grayscale) with the CARMA blueshifted and redshifted lobes overlaid.

In NGC 1266, only 2% of the molecular material is actually escaping the galaxy. That completely changed the picture of how long the molecular gas could sit in the center. I went ahead and calculated the same thing for Mrk 231 - how much mass was actually escaping the galaxy and thus depleting? A lot less than was estimated. By estimating the escape velocity in the center, Mrk 231 only had a depletion rate around 200 M⦿/yr, which was consistent with the star formation rate. This meant that the rapid <10 million year exhaustion of the gas extended to closer to 50 million years, which was much more consistent with what the stellar population in that system said it should be anyway.

The CARMA spectrum of the emission in Mrk 231, taken from the moment0 map (upper left corner)
 Finding molecular outflows in AGNs is still a new field. And in the discovery paper on NGC 1266 we made the same error, thinking that the time in the galaxy was special. A time where an AGN has a lot of molecular gas near it might be special, but it seems that nature has conspired to allow the gas to survive a lot longer than we might expect.
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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