Active galactic nuclei (AGNs) that are buried under large masses of molecular gas can be very difficult to identify and detect. The main ways we had found AGNs in the past were from either their extreme luminosities (for instance, in quasars), where the luminosity of the point was so large it was very difficult to make the case that star formation alone was able to make enough light. AGNs also began to be identifiable based on the specific energy with which they ionized gas, being too hard (that is, containing too many ultraviolet and X-ray photons) to be from star formation, found through line ratios between two elements with different excitation potential. X-rays became the gold standard for finding AGNs, with strong X-rays being difficult to produce in any way other than in an accreting supermassive black hole. If there is not extremely dense gas between you (the viewer) and the AGN, these are very good ways to find these objects. Urry & Padovani showed that the angle over which we are viewing the accreting black hole can make all the difference on what we see, deemed the so-called “unification model.” But, what if the AGN is isotropically covered?
In the first paper blog on the NGC 1266 discovery, I talked a lot about the molecular gas. There is a lot of it, and it is dense. This means that we probably don’t have a preferential viewing angle into the black hole to really confirm that there is an AGN (despite the presence of some X-rays). This paper set out to solve this problem, because it turns out that radio can provide us an answer. Jim Condon was able to show that there is a natural limit to how bright a compact starburst can be in the radio, with a limit of around 10^5 Kelvin. So if you can prove that your source is bright enough and compact enough, you can make the case that the radio emission is not coming from a starburst. In our case, we knew that NGC 1266 contained radio emission. When the discovery paper was written, we did not have limits on the size, so we turned to the Very Long Baseline Array (VLBA). The VLBA is a unique sort of telescope, in that it combines the photons it collects in radio dishes that are separated across the continent, from Washington to Hawaii to New Hampshire to the Virgin Islands. Using this telescope, you get superb resolution on bright sources, and so you can prove an object is extremely compact.
We pointed the VLBA at NGC 1266, and found that we detected the bright radio source! When we derived the radio brightness temperature of the point source we found it to be over 10^7 Kelvin, 100 times larger than the starburst limit. This meant that it was very likely that in the center of NGC 1266, we could confirm the presence of an AGN. On top of that, the superb resolution also gave us an extremely accurate location of the AGN. We used this information to plot the exact location of the AGN both in comparison to the molecular gas, as well as in comparison to the newly acquired data from the Hubble Space Telescope. With these two measurements we could confirm that the AGN was sitting directly underneath the densest molecular gas (which we supposed was likely in the discovery paper). The Hubble Space Telescope image, taken in the B-band (for blue light, which comes from young stars, and is easier for dust to attenuate), we could see the vertex of the outflow in dust appear, with the AGN sitting at its origin. We now knew that there was an AGN sitting under an immense pile of molecular gas (in fact, if the gas estimates were correct, then this AGN was even Compton thick), and the outflow that we found originated at its position. Finally, we used our newest radio data (including new data from the Very Large Array), we updated the constraints on the mechanical power that could be coming from the AGN, further confirming that it was sufficient to drive the molecular outflow.
This paper broke the ambiguity of whether or not there was an AGN in the system, by finding it and placing it under a huge column of molecular gas, and showed that the AGN seemed to be the origin of the molecular outflow, also traced in dust by Hubble. Finally, it showed that the AGN radio jet power could drive the molecular outflow we discussed in the discovery paper. Not too bad for a continent-wide telescope and the most famous telescope that exists today.
The official published version can be found on NASA ADS.
To get a PDF version made by me, you can download it here.