Observations of hydroxyl in early-type galaxies

(this paper was led by James McBride as first-author. I am second author of this paper.)

This paper is quite special to me, because it was a fishing expedition that seemed unlikely to detect anything, and then nature and the universe surprised us.

Hydroxyl (OH) is a molecule that is often found in very dense regions, and is one of the lines that has been found to mase. Masers were actually the progenitor to lasers, which we are all now extremely familiar with (thanks to Charlie Townes for their discovery! He won the Nobel prize for it.)

Masing of an OH molecule (adapted from the figure here)

MASER stands for a Microwave Amplification by Stimulated Emission of Radiation, in which some
molecule gets pumped to a high excited level which decays to a sub-level, then to hit the ground state will decay by a MASER transition. It strongly amplifies a very particular transition in the molecule, leading to unphysical conditions (if the masing was not occurring). OH is one of those molecules that mases.

Originally, many thought that to get this line, the perfect conditions needed to be present. There needed to be enough energy pumping the molecule that it could have the maser transition, and the molecule was thought to be present in very dense gas. After the maser lines were found (both in the Galaxy as well as in bright, dense, prolifically star-forming extragalactic sources), absorption was also found against strong radio continuum sources. But the sources that these transitions were found in were almost always interacting galaxies with powerful star formation, or potentially starbursting galaxies. No one ever thought to look for this emission or absorption in "red and dead" galaxies, because why bother?

OH absorption against the radio continuum of NGC5866, one of the detected ATLAS3D galaxies (adapted from McBride et al. 2015)

In case the refrain of this section of the blog, and this paper have not resonated yet, it is always important to look, even when something seems very unlikely, because that is where you discover and learn things. We searched 12 dense gas rich ATLAS3D early-type galaxies for OH masing and emission, and despite the fact that these galaxies do not look like the "typical" OH detected objects, we detected 4 of them (3 new detections, including NGC5866 pictured above, and NGC1266). These are early-type galaxies. They are not strong, prolifically star-forming. They should not be detected. So why were they?

Detected OH masers and absorbers, compared to our newly detected OH absorbers (adapted from McBride et al. 2015)

First, the OH observations had started to be focused on infrared-bright (read that as: prolifically star-forming). But, if you looked, it was clear that the dense gas in our systems were in the same part of the diagram as many other OH detections. So it's not surprising that we don't see masing (since there is not enough incident radiation to "pump"), the gas conditions allowed for the formation of OH.

Optical image of NGC5866 from the Hubble Space Telescope.
Image credit: Hubble Heritage

There are a couple of other reasons too. First off, all of these galaxies had strong radio cores (Kristina Nyland wrote a good paper on that) for the OH to absorb against. The galaxies had dense gas. But there was something else. Take NGC5866 for instance, picture just below. Notice anything about it? The thing is completely edge-on, meaning that we are staring through the entire disk of the galaxy (and in this case, also a bar seen in molecular gas). Turns out that 3/4 of the galaxies that were detected had similarly edge-on geometries. It meant there was more gas available to absorb the radio continuum. NGC1266, the non-edge on system has extremely dense molecular gas very near to the radio continuum source (blogged about here). So, as long as you have dense gas along the line of sight that can absorb a bright radio continuum source, you can see OH!

When you think about it, this makes a lot of sense. But it was still a lot of fun discovering the unexpected, and getting to change the conventional wisdom. James wrote a clean and clear paper on this, and I highly recommend it as a read, especially if you want to understand more about the wonderful and interesting world of OH.

The official published version can be found on NASA ADS.

The arXiv pre-print version (prepared by James) can be found here.