arXiv: Spilker et al. 2016b
I’m really excited to share the results from this paper! Up until now, my research has basically exclusively focused on dusty, star-forming galaxies discovered by the South Pole Telescope. This project is my first branch out into new territory, and I’m very grateful to the band of trusty collaborators who were willing to get dragged along on this adventure! This was an idea that started entirely because I really wanted to use the VLA for something, and didn’t much care what. An idea and a couple conversations later, and we were in business!
In this paper, I wanted to understand a bit more about a specific population of galaxies. For about a decade, we’ve known that early quiescent galaxies (at z~2 or so) are physically smaller than quiescent galaxies in the nearby universe despite having similar masses – they were more dense by about a factor of 100! Recently, various groups have identified galaxies which might be the immediate progenitors of these early quiescent galaxies. The progenitors have similar masses and structures as the quiescent galaxies, but appear to still be forming new stars at roughly normal rates.
The question my collaborators and I wanted to answer here was: is there a difference in the gas contents between these probable quiescent progenitors and normal galaxies at this epoch? To do this, we looked at three of these objects with the VLA, trying to detect emission from the carbon monoxide CO(1-0) line, which is a well-known tracer of molecular hydrogen gas in galaxies. At this time in the history of the universe, normal galaxies tend to have quite a lot of gas – about half of their total mass is in the form of gas – which means they can form a substantial amount of stars later in their history. If the progenitor population of galaxies has significantly different amounts of gas compared to most galaxies, that could provide some hints as to how the process of star formation quenching occurs.
In fact, that’s exactly what we saw! Despite trying pretty hard, we only detected one of the three galaxies we looked at in CO(1-0). This means that the galaxies we were looking at do indeed have much less gas than we expected, and much less compared to basically every other galaxy at this point in the history of the universe! This might mean that the way (or at least one of the ways) that galaxies stop forming stars is pretty simple – they just run out of the raw material they need!
This also means that (contrary to my initially-uninformed personal prejudices) these galaxies *do* look like they’re plausibly going to quench star formation very quickly. If they continue to form stars at their current rates and don’t accrete any more gas, they will completely run out of gas in less than 100 million years, similar to some of the most highly star-forming objects in the universe! In the figure at right, the arrow depicts what happens if we assume a different conversion between CO and gas mass – either way, these objects will probably stop forming new stars on very short timescales.
This was a really fun and (in my opinion) interesting result to come about from a project that started basically on a whim! I’m currently pursuing a wide array of followup observations to try to figure out more and different aspects of how this population of galaxies evolves, so stay tuned!