Star Formation Suppression in Compact Group Galaxies: a New Path to Quenching?

Another excellent blog on this paper by the AMIGA group, enjoy!

We've covered suppression, and we've now had an introduction to Hickson Compact groups. This all started with HCG 57a and d... and the idea that turbulence within some of these compact group systems has the potential to inhibit star formation in the molecular gas. Here we expand upon this first inkling that something interesting is going on in the molecular gas of these rapidly transforming galaxies by looking at a larger sample of them. 14 galaxies in 12 Hickson Compact groups to be exact.

The Hickson Compact groups observed in this CARMA study, 3-color g-r-i images are from PanSTARRS.

All of these HCGs were observed by Spitzer, though not all of them were MoHEGs. Many of them have elevated H2, but not all of them. Some of the galaxies are part of tight interactions amongst group members, and some are farther afield from the other group members. Overall, this is a set of galaxies that mostly share one property: they are in a group environment.

The CO(1-0) image of HCG 40c taken by CARMA, overlaid upon the PanSTARRS g-r-i image, plus the average velocity map from CARMA. Adapted from Alatalo et al. 2015

We aimed CARMA at this set of sources, which had already been detected in molecular gas by the IRAM 30m. The goal of this project was imaging the molecular gas. We had hopes of finding more systems like HCG 57, possibly testing the importance of turbulence in other systems, but up until this point, very few objects had been found to fall off of the Schmidt-Kennicutt relation. The CARMA images are beautiful, as evidenced by the image of the HCG 40 gas above.

The Schmidt-Kennicutt relation with the HCG galaxies plotted in red.
The HCGs fall below the relation. Adapted from Alatalo et al. 2015

These HCG systems were also studied in depth with Herschel, meaning we had full spectral energy distributions (SEDs) of each of these galaxies, and were able to determine their star formation rates accurately (fitting the far-infrared SED is one of the best ways to determine star formation rates...) Once we had star formation rates, and molecular gas masses, CARMA gave us the last piece of the puzzle by giving us the sizes of the star-forming molecular gas regions. So we put them onto the Schmidt-Kennicutt relation. Many of the systems we studied had normal efficiencies, but a lot more do not. In fact, the average suppression for all the HCGs is 10, meaning on average, the molecular gas in these HCG systems is forming 10x fewer stars than it should be. In the most extreme of the systems, the star formation suppressed by factors of 30-50! This is getting near NGC 1266...

The first question upon seeing this is why? The answer here seems to also be turbulence. Just as in NGC 1266, injecting turbulence into the molecular gas, and not allowing far-infrared cooling lines to return it to equilibrium, allowing for gravitational collapse into stars. We found that the amount of energy that is needed to balance the gravitation was attainable just from the shocks of the system, by comparing to the H2 luminosity. The energy injection timescales of galaxies in compact groups are longer than those in mergers, which may allow for a longer injection timescale, leading to this suppression (that we don't see in merging or interacting galaxies.)

The star formation suppression versus the colors of each HCG galaxy. The redder the molecular gas rich HCG galaxy, the higher the suppression in the gas. Adapted from Alatalo et al. 2015

We tried correlating the star formation suppression with different galaxy properties: [C II] luminosity, H2 luminosity, galaxy mass, molecular gas fraction, and finally galaxy color. Most properties did not correlate with the suppression, but galaxy color did (as did specific star formation rate). Our HCG systems have plenty of red massive galaxies, with large reservoirs of molecular gas, but that molecular gas is not forming stars. Does this mean that these galaxies stopped forming stars long ago despite their significant molecular reservoirs?  Well, in this limited set of special galaxies in special environments, we can say that suppressed star formation may be one of the drivers of galaxy evolution.

This has bigger implications for galaxy evolution in general too. Studies have been popping up showing that "dead" galaxies still contain molecular gas (including our own!), and some great work on post-starburst galaxies are showing that they also contain significant reservoirs of molecular gas, despite having quenched their star formation. This challenges the "standard model" for quenching galaxies - that gas must be expelled first and star formation ceases later. Inklings that this is not the only path popped up circumstantially, but this paper shows some of the first evidence that rendering molecular gas infertile could indeed be a way to quench star formation and transition a galaxy, without requiring its molecular reservoir to be rapidly expelled. This also means that "AGN feedback" is not strictly necessary, as the AGN was plugged in to remove the gas rapidly.

It stands to question whether this mode - that is, suppressing the star formation in the molecular gas - is one that is universalizable, or whether it is only going to be seen in the unusual environments like in shocked HCG systems, where the gravitational torques and chaotic motions of the group members provide a constant supply of new turbulence. But it is encouraging to see that we no longer always require an explosive feedback step to explain why galaxies transition and quench their star formation so rapidly.

The official published version can be found on NASA ADS.

To get a PDF version made by me, you can download it here.