Strong Far-Infrared Cooling Lines, Peculiar CO Kinematics, and Possible Star-formation Suppression in Hickson Compact Group 57

I just came upon an excellent blog in the AMIGA group about this paper, and wanted to advertise it!

If I needed to come up with a single sentence to describe this paper, it would be "deconstructing an interaction to understand its physics piece by piece." That sort of paper follows from the NGC5195 blog, but we are now moving to bigger galaxies and more complicated distributions.

First, what is a Hickson Compact Group? First, we zoom out and think about environments and densities of galaxies. Before, we focused mainly on isolated galaxies versus galaxies in clusters. As you can imagine, it is more of a spectrum. Galaxies can also be found in different sorts of groups, from loose groups to compact groups. In this case, we are going to look at a specific set of galaxies in a compact group configuration.

Here's the definition.

“By compact group, we mean a small, relatively isolated system of typically four or five galaxies in close proximity to one another." - Paul Hickson, 1997 ARA&A 35, 357

Seyfert's Sextet (HCG 79). Image Credit: Hubble Legacy Archive, NASA, ESA

Compact groups are low dispersion and high density. In clusters, the density of galaxies is high, but their velocities relative to one another is also high, so there are few opportunities to collide. Isolated galaxies may have low speeds, but are not dense enough to interact with their neighbors. Compact group environments encourage collisions, which in turn, encourage fast evolution of galaxies from blue star-forming spirals to red, quiescent ellipticals and lenticulars.

Hickson Compact Group evolutionary cycle shows both the way the group changes as well as the way that the galaxies within the group evolve. Image from Alatalo et al. 2015

This rapid evolution of both group and galaxies makes Hickson Compact Groups excellent environments to study galaxy evolution in. In fact, the mid-infrared colors of HCG galaxies are bimodal in a way that other galaxy populations are not. (In a future blog, I will talk more about the mid-IR colors of galaxies). The group I am in has studied this phenomenon, looking specifically at those few HCG galaxies that inhabit those intermediate colors and seeing if there was anything interesting about them. As you may have guessed, there was.

We used the spectrographic prowess of the Spitzer Space Telescope to look at the mid-infrared emission lines. In this case, you look at the molecular H2 lines. These mid-infrared lines require there to be warm or hot hydrogen. The excitation of this line can come from many things, including radiation from stars or an AGN, or shocks. In cases where radiation is the main culprit, the H2 lines will not be the only ones excited. PAH emission will also be excited. So, a simple test exists to figure out whether the excitation is likely from radiation (also the most common mechanism). If the ratio of the H2 luminosity to the PAH luminosity exceeds 4%, then you can be pretty sure that the excitation mechanism is not radiation, leaving it likely that it is due to shocks. Ogle et al 2007 coined these sorts of galaxies "Molecular Hydrogen Emitting Galaxies", or MoHEGs. When Cluver et al 2013 looked at a set of HCG galaxies, and found that the galaxies inhabiting the intermediate mid-IR colors were MoHEGs, meaning that shocks were quite prevalent in these transitioning group galaxies.

(Left:) HCG57 in optical (g,r,i) with CO(1-0) overlaid.  (Right:) HCG57 in infrared (3.6,4.5,8μm) with [C II] overlaid. Adapted from Alatalo et al. 2014

When trying to understand this population, it makes sense to look and think about exquisite case studies. In this case, we explored one of the MoHEGs, the HCG57 system. Both HCG57a (the disky galaxy in the middle of the above image) and HCG57d (the ringed galaxy toward the top) are MoHEGs, so we aimed CARMA and Herschel at them to try to understand what this meant for this system's gas. The [C II] emission from Herschel confirmed the suspicion that much of the gas is excited by shocks. In fact, the [C II] shows a bridge of gas connecting the two galaxies to one another. What does that mean?

(Left:) The Schmidt-Kennicutt law for different gas regions in HCGs 57a, showing that they, like NGC 1266 are quite far off of the "standard" relation, inefficient. (Right:) A position-velocity diagram of the CO of HCG 57a, separated by regions, showing multiple components, representing different physical states.
Adapted from Alatalo et al. 2014

For starters, the molecular gas in HCG 57a is extremely disturbed. When we change the way we look at the gas, this time in position-velocity space, three distinct components are seen. Regular rotation (like you see in most star-forming galaxies), a compact and high velocity component (possibly an outflow, possibly a bridge to HCG 57d), and a splash ring. The gas is clearly quite turbulent. And that shows when you look at its star formation properties. HCG 57a is off the Schmidt-Kennicutt relation, just like NGC 1266 was. And the disturbed position-velocity diagram can give us a good idea as to why - the gas is turbulent, making it harder for it to gravitationally collapse and form stars. This gives us a clue into the fate of gas in these interactions, namely, that it can stick around a lot longer than one might think, just from the turbulence slightly inhibiting star formation.

But the gas also answers another question for us. It helps us understand what happened in this system. The splash ring, not necessarily noticeable on first-glance at the optical data, says that there was a collision. The smaller, HCG 57d, plowed through HCG 57a a short while ago (think: 10s of millions of years), creating a splash ring that we can see in HCG 57d. The ring is still propagating through HCG 57a, which is why star formation is still suppressed, because the turbulence is still present. And that shock is seen brightly in the other tracers, like the [C II]. Overall, this system is the anatomy of a collision in a compact group. The collision drives a shock, which injects turbulence, and causes the gas to not form stars efficiently, allowing the gas to linger for longer than one would think. Once that shock has completed its propagation through the system, the star formation returns to normal (like in HCG 57d), and we await the next collision in this highly violent environment. Is HCG 57 similar to other systems in this class? We bring you that story in the next paper blog...

The official published version can be found on NASA ADS.

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