Friday, November 13, 2015

Discovery of an Active Galactic Nucleus Driven Molecular Outflow in the Local Early-type Galaxy NGC 1266

lenticular (left) and elliptical (right) galaxy
The two forms of early-type galaxies, a lenticular/S0 (left) and an elliptical (left)
Since at least Edwin Hubble's time, the astronomy community has treated early-type (that is, elliptical and lenticular/S0 galaxies) as "red and dead". Thought to no longer show signs of star formation, I joined the ATLAS3D project to study these sorts of galaxies in-depth, which showed that this notion, of "red and dead" galaxies was wrong. In particular, the work done on the molecular gas by Prof. Lisa Young.

After searching CO(1-0) (known to be a good tracer of H2, which is where all the cold gas mass is located), over 20% of early-type galaxies had molecular gas reservoirs. Not only did this study look at the presence and mass within these galaxies, but it picked out a very unusual galaxy, NGC 1266.

NGC1266 CO2-1 spectrum
CO(2-1) spectrum of NGC1266 taken using the IRAM 30m
Before the ATLAS3D work, NGC 1266 looked like a regular lenticular galaxy from the original ground-based observations that were taken of it, but the molecular gas was not normal for early-type galaxies, and it was not even normal for star-forming spiral galaxies. The molecular gas looked more similar to what is seen in outflows from very young stars, with spectral wings that are broader than can be explained by the rotation of the gas in the galaxy.  In fact, there is gas emission (2% of the total thought to be outflowing) with such high velocities that the molecular gas will be escaping the galaxy. In order to fit this line profile, we needed to use a two component model: one that represented the central, non-outflowing gas, and the other representing the high-velocity outflowing gas. These fits showed that about 4e9 M⊙ of gas sit in the very center of the galaxy, and at least 3e7 M⊙ of molecular gas are outflowing.
Imaging of the CO emission overlaid atop Hα

But what force is capable of driving molecular gas out of the system? And what type of physical phenomenon can control that force? In order to answer this question, we turned to the imaging data to figure this out. First, we used two interferometers: the Combined Array for Research in Millimeter Astronomy (CARMA) and the Submillimeter Array (SMA). By looking at the extremes of the velocities seen in the CO emission, we were able to get size details about the molecular gas. With these, we could derive energies and timescales associated with the outflow. This was able to tell us that star formation was unable to drive the extreme velocities (≈200 km/s) and the large mass fluxes (≈13 M⊙) associated with the outflow. If it was not star formation, there was one other possibility. It was possible that an accreting supermassive black hole in the center of NGC 1266 is the driver of the outflow.

How does one go about proving the presence of an active galactic nucleus (AGN)? We turned to the X-rays. Although not always associated with an AGN, often times they are. We were able to get time from the Chandra X-ray telescope to look at the X-ray details of NGC 1266. This hinted strongly at the presence of an AGN (and shocks too!), with hard X-rays originating at the same location as a radio source, and sitting underneath the densest part of the non-outflowing molecular gas. The AGN energetics also proved to be up to the challenge of driving the molecular gas out of the galaxy. In this paper, we even suspected that given the rate of the outflow, all of the starforming fuel will be depleted from the system in less than 100 million years.

NGC1266 X-ray emission
NGC1266 X-ray (blue), radio (green) and Hα (red) emission
This galaxy was one of the first galaxies found to host AGN-driven molecular outflows, and was one of the most surprising. Unlike Markarian 231, which is our nearest-by quasar, there was no obviously powerful black hole to drive out the gas. And its outward appearance was hiding the monster beneath.

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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