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.

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.

After the Interaction: an Efficiently Star-forming Molecular Disk in NGC5195

Most of the blogs that have already been written here discuss the unexpected. Gas in "red and dead" galaxies or molecular outflow hosts from an unlikely source. This paper and this object are different. We expected the object to behave weirdly and found instead that it seemed much more ordinary than one would think given its nature. The object I blog about here is NGC5195, or M51b, the smaller companion to the Whirlpool Galaxy.

M51a and M51b, courtesy of this site. This also shows what amateur astronomers contribute to scientific progress - look at those tidal tails!

NGC5195 is the ellipsoidal companion to the Whirlpool Galaxy. This interaction is the reason that M51 looks the way it does (being the canonical example of a "grand design" spiral), and is thought to be a 3:1 interaction (with NGC5195 being about 3x less massive than M51a). When we look at the violence of this interaction, we can make a lot of predictions about what would be going on in that smaller galaxy. Perhaps there is a strong AGN, because all of the gas has been driven into the middle. Perhaps there is no gas left. Perhaps the gas that is there is forming stars prolifically, or maybe it is suppressed by turbulence. This is what we set out to investigate.

NGC5195 as seen in the infrared by Spitzer (red traces a star-forming material: polycyclic aromatic hydrocarbons, or PAHs) and in CO, where the velocity field is also plotted (adapted from Alatalo et al. 2016).

We used CARMA to look at multiple molecular lines within NGC5195, to study the nature and behavior of its molecular gas. There is a slight complication though: it is so close to M51 (which is forming plenty of stars), that we need to be careful when assigning the molecular gas to the galaxy. It was believed that the gas in NGC5195, having been disturbed by the interaction that took place with M51 would have unsettled the gas, and possibly left enough turbulence that it might not be forming stars efficiently. This is what we set out to test.

The first thing to point out is that the rotation inside of NGC5195 was regular. That is, rotation is dominating the gas, meaning that the gas is not turbulence dominated. That has implications for the star formation (without turbulence, what is fighting against gravity?) too, which was the next thing we checked.

We took the Herschel 70μm data, which is a good tracer of star formation (via the cold dust, see this Calzetti paper for more details), and we one-to-one mapped it with the CARMA CO data. The Herschel map gave us the star formation surface density (that is, how many stars are forming per area on the source, usually kiloparsec^2) and the CARMA maps gave us the gas surface density. You can then divide the gas surface density by the star formation surface density, and get something called the depletion time. That is, how long going at the rate each parcel of gas will take to completely form into stars. What we found was that overall, it would take about 4 billion years. This is pretty close to average for star-forming galaxies, and completely consistent with star formation rates of early-type galaxies. This means that the star formation in NGC5195 is pretty much efficient, so there is not additional turbulence that the gravitation of the gas has to fight against.

So, in spite of this galaxy having undergone a significant interaction (there are also signs of young stars in the galaxy, which formed about a billion years ago), the gas has already settled back down and is forming stars normally. It makes you wonder, how significant are these extraordinary events to a galaxy? Because at least in NGC5195, it did not take long to revert to the default, with regularly rotating gas and normal, efficient star formation.

The official published version can be found on NASA ADS.

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

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.

The ATLAS3D Project - XIV. The extent and kinematics of the molecular gas in early-type galaxies.

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

We continue our exploration of the molecular gas properties of early-type galaxies, which I have already blogged about here, here, and here. In this case, we look again at a paper that Tim Davis wrote, in particular, about the extent of molecular gas in our ATLAS3D early-type galaxies.

We start by thinking about the "default" galaxies: spiral, late-type, star-forming galaxies are the ones that have been studied most often in molecular gas. This makes sense, because the galaxies that are forming stars are also the ones with the most prevalent molecular gas, and were the ones assumed to have molecular gas. As we've already discussed, early-type galaxies until very recently were assumed to be "red and dead". Some hints that that might not be true existed, but ATLAS3D was the first group to put a quantifiable value on just how many still had cold gas, albeit usually at small molecular gas fractions. This though explains why it is really just now that we are getting around to trying to understand how molecular gas behaves in early-type galaxies, and how that compares to the "default."

The stars (underlying photo) and molecular gas (blue) in the Whirlpool Galaxy. Image credit: PAWS Team/IRAM/NASA HST/ T. A. Rector, this site.

Molecular gas tends to inhabit certain regions of a galaxy.  In late-type spirals, this is often found in the nucleus and along the spiral arms (as is seen in the Whirlpool Galaxy above). If we zoomed out a bit more, the gas would not be in such beautifully pronounced clumps, but its relationship to the galaxy and the stars could still be seen. Some of the best work was done by the Berkeley-Illinois-Maryland Array Survey of Nearby Galaxies (BIMA-SONG), which mapped dozens of nearby spiral galaxies with the precursor to CARMA, BIMA. One of the first results was looking at how far out the molecular gas in these galaxies traversed, authored by Michael Regan. The main result seemed to be that overall, the gas traced the starlight in these galaxies fairly well.

In spirals, this is hardly a surprise. The very stars in the spiral are often forming out of that molecular gas, so their connection makes sense. What we set out to do was find out if the same was true for early-type galaxies. The first thing that we noticed though, was that the overall extent (that is, how far out in physical units the gas is found) of the molecular gas in the early-types was smaller than in the late-types of BIMA-SONG. But again, that was in absolute terms. What about when you look at the gas relative to the stars?

The extent of the molecular gas (traced by CO) compared to the extent of the stars in ATLAS3D galaxies (top, red) and BIMA-SONG spirals (bottom, purple). The extents match fairly well (adapted from Davis et al. 2013)

In that, the story is different. The extents compared to the stars match quite well. So the molecular gas does not look different in early-types than in late-types, when we take into account the nature of the stars in both. There are a few possible reasons for this. In spirals, the gas is forming stars, which add to the stellar component of the galaxy. Gravitational torques are also acting on the gas. And we posited another cause: that some of the gas is recycled from the stars, resulting in the extents being related.

The extent of the molecular gas (traced by CO) in both Virgo and field ATLAS3D early-type galaxies. Here we see the extents are different (adapted from Davis et al. 2013)

Despite the fact that the kind of galaxy that the gas is in does not appear to have much impact on the extent, there is something that does factor in. The environment. Like the molecular gas alignment, and the CO isotope ratios, the extent of the molecular gas also depends on environment, with the Virgo galaxies having the most compact molecular gas distributions compared to the stars. And again, some of our favorite explanations could do this. Ram pressure does not necessarily just act on the neutral gas (as we discussed last time), and could preferentially evaporate the less dense clouds farther out. Additionally, in Virgo, where none of the gas can be externally acquired, the internal stellar mass loss could be even more important, leading to a tighter coupling between stars and gas, explaining the extent.

One thing is clear though, it is time to get better observations of galaxies of all types to begin to understand molecular gas: its fate, its relationships, and its origin. Especially in galaxy clusters (like Virgo).

The official published version can be found on NASA ADS.

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

Evidence of boosted 13CO/12CO ratio in early-type galaxies in dense environments

Last blog we learned about the strange things happening to gas in dense environments, like the Virgo cluster. This blog, we are going to continue in that vein. But first, a refresher on why 13CO is interesting.

Cartoon version of a molecular cloud. Image credit: Katey Alatalo

If we had eyes that could look at a molecular cloud, we would see a ton of molecular hydrogen, and a smattering of other elements. Namely, molecular gas is nearly all molecular hydrogen, with the second most abundant molecule being carbon monoxide (CO). In fact, for every 10,000 hydrogen molecules, there is 1 CO molecule. So why is it that we use CO when we study cold gas in galaxies, rather than molecular hydrogen? Because of their dipole moments. As shown in the figure, H2 has two symmetric atoms that make it up, meaning that if it rotates, it does not release a dipole electric charge, meaning that it has very weak emission. CO on the other hand, is made out of asymmetric atoms (the 12 nucleon-carbon and the 16-nucleon oxygen), so it creates a large dipole moment and thus very strong emission.

There are some caveats of course. First off, in the clouds, both the H2 and the CO are usually optically thick. That means that we are not counting every single emitting photon. This normally would be a problem, but it was found that in molecular clouds, the total luminosity in CO was related to the total mass of the clouds, something called Larson's law. This means that in many cases, we are able to use the CO luminosity to trace the underlying mass of molecular gas. This might not be true in very low metallicity sources, which have not been enriched yet with carbon and oxygen, and this relation comes with a large set of uncertainties. One of the ways that we compensate for those uncertainties is by turning to optically thin molecules. The easiest one to observe is also a species of CO, but this time it is 13CO (that is a 13-nucleon carbon and a 16-nucleon oxygen). This isotope is about 70x less abundant than 12CO, and is thought to be optically thin. This means that it should provide a more faithful accounting of the total molecular mass, because we are able to count every emitting molecule. This also means the line is intrinsically weaker than the 12CO line. And for the paper I am blogging, it was the 13CO that we set out to detect.

We turned to the 17 galaxies from the ATLAS3D CARMA survey that we had simultaneously observed both the 12CO and the 13CO lines. The 13CO ratio can differ from source to source for a lot of reasons, so we were expecting a random scatter of sources. But the scatter was not random, not at all. Virgo early-type galaxies have higher 13CO/12CO ratios than field galaxies.

The distribution of 12CO/13CO ratios in early-type galaxies (ETGs) that shows that Virgo has elevated ratios compared to field galaxies (those not in clusters). Adapted from Alatalo et al. 2015.

What could this mean? There are plenty of ways we can do this, but here are a few that might also work in the context of Virgo galaxies having aligned molecular gas and equivalent detection rates to field early-type galaxies. First is midplane pressure. In Virgo, the galaxies are subjected to extra pressure from the intracluster medium (the hot gas that fills the space between the galaxies in the Virgo cluster), which could increase optical depth of the gas in Virgo ETGs. This though would not impact the detection rates. Second is enrichment. Different types and masses of stars create different carbon isotopes when they are undergoing fusion. This means that in galaxies that has gas that has been recycled over and over, the gas can become more enriched in one isotope than in another. Given the lack of primordial gas in Virgo galaxies, this could certainly have changed the abundance of 13C relative to 12C. Though again, is unable to explain the same detection rates. Finally, it could be preferential stripping of low mass molecular clouds (coined survival of the densest). Ram pressure stripping can strip away atomic and unbound gas, but perhaps it does not stop there, and also is able to remove the bound molecular gas in the smaller molecular clouds, leaving only the largest. This would mean that the 13CO would also become close to being optically thick, and is similar to the ratio seen in the largest molecular clouds in the Milky Way. Or, it could be none of the above, and we could be seeing a mass effect. The early-type galaxies in Virgo are the most massive, so this relation could simply say that massive galaxies retain their gas for longer.

The question of why Virgo galaxies appear to be special is not yet resolved. We don't know why the detection rates are the same (looking at more early-types in more clusters will help). We also don't know why the galaxies in Virgo seem to have more dense gas than their field galaxy counterparts. But high resolution observations on scales of the molecular clouds in Virgo (both spirals and ellipticals, and in the outskirts and internally), perhaps we can start to distinguish between possible mechanisms.

The official published version can be found on NASA ADS.
Get a PDF version made by me here.

The ATLAS3D Project - X. On the origin of the molecular and ionized gas in early-type galaxies

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

The (modern) Hubble Tuning Fork. Image credit: Space Telescope Science Institute

As the last blog talked about, we start with the Hubble tuning fork (Edwin Hubble published this in 1936, which is pretty amazing given how nearly it gets it right.) Since we figured out that galaxies can be broken down simply into morphological types: the older "red and dead" elliptical and lenticulars versus the young star-forming blue spirals, the question of how a galaxy dies and transforms from the star-forming spiral into a red and dead galaxy is really important. How will the Milky Way do it? Do we know every way this can happen? How do we even study that?

http://califa.caha.es/

One formidable way to study these questions is to grab a sample of already-transformed galaxies, and study them in such detail that the clues from their past lives (and deaths) are observable. That is what the ATLAS3D survey set out to do. Last time, I talked about the imaging of the molecular gas in ATLAS3D galaxies. This time, we can talk more about its interpretation.

A revolution to observing properties of galaxies came in the form of integral field spectroscopy and integral field units (IFUs for short). These were instruments that could take many spectra in a footprint, mapping the entire spatial extent of a galaxy. (The CALIFA survey logo on the left demonstrates what that footprint looks like.)

IFU observations can directly trace stellar rotation by getting a spatially resolved spectral map of the galaxy, detecting absorption lines that are predominantly found in the atmospheres of stars, and measuring the average velocity in that particular spaxel (think: pixel that you are getting a spectrum from.) Combining that with the kinematics of the gas, and you can compare the two, asking the fundamental question: are the stars and gas linked, or was the gas acquired from an external source?

Aligned stars and gas (left) could be from an internal origin. The gas misaligned with the stars (right) shown as either polar (90° misaligned) or counter-rotating (180° misaligned) and must be from an external origin, like an accretion event or a minor merger.

Because of this new IFU data, we can actually measure this, by investigating whether the gas and the stars are aligned (that is, if they are rotating along the same axis or not).  The above figure takes a rotating galaxy, with rotation of the stars and the gas shown. If the gas is not aligned with the stars, then it has to come from an outside event, like the accretion of gas or a merger with a small companion.

This paper explored the origin of gas in the ATLAS3D early-type galaxies, discussed in the previous blog. But instead of just looking at how many galaxies of each type were observed, it also looked at what sorts of environments those galaxies were in. It's been well known for a while that galaxies that are found in clusters are often "red and dead", as compared to the field. This is known as the morphology-density relation. So this paper looked at whether the alignment of gas was different in Virgo, the canonical high density region surveyed by ATLAS3D, and the field. While the detection rates of molecular gas in these two populations was the same, the alignment of their gas was not.

The alignment of gas compared to stars in Virgo cluster ATLAS3D early-type galaxies vs. those in the field. There are a lot more galaxies in the field with misaligned gas. Adapted from Davis el al. 2011

The reason that there are mostly dead galaxies in clusters is simple: it is because they are not able to acquire any new gas, because they are "bathing" in the hot intracluster medium, disallowing new accretion. This paper's results actually do support this quite well: that gas is aligned in Virgo likely means that the gas came from some internal process. Field galaxies on the other hand have many more opportunities to accrete new gas.

The quandary lies in why the detection rates are the same, despite this confirmation that the origin of the gas has to be different? It could be that there is something that is different about the remaining gas in the galaxies in Virgo as compared to the ones in the field, which is a thread I will pick up in the next blogged paper. Stay tuned!

The official published version can be found on NASA ADS.
The arXiv pre-print version (prepared by Tim) can be found here.

The ATLAS3D project - XVIII. CARMA CO imaging survey of early-type galaxies

Early-type and late-type galaxies from the Hubble tuning fork

It is time to change gears from AGN outflows and the different ways that they impact galaxy evolution to the important question of why do “red and dead” galaxies stay dead? In the first blog entry of this series, we talked about the fact that NGC 1266 was found as part of the Atlas3D survey, but now it is time to zoom out and look at the galaxies inside the Atlas3D survey at all 261 galaxies. This blog post takes a lot of the intro from my thesis: “Molecular gas in early-type galaxies” with advisor Carl Heiles.

It starts back with work done by Edwin Hubble, looking at the difference between galaxies, splitting them into early-type and late-type galaxies. Galaxies were also found to be bimodal in color, with blue galaxies and red galaxies. Most red galaxies are also early-type galaxies, and most blue galaxies are late-type galaxies. The Atlas3D project was designed to look at early-type (morphological classification) galaxies in-depth, acting as galactic archaeologists. In that, the team looked in-depth at 261 early-type galaxies, determining the motion of their stars, and taking a more careful, unbiased look at early-type galaxies as a population than had ever been done before. The conclusions most pertinent to the work I did was that over 20% of these objects had a reservoir of molecular gas, and many also had neutral gas (H I), so these dead galaxies often still had the remnants of star forming fuel. These objects never seemed to have large reservoirs of gas (compared to their mass of stars), but gas they had.

Molecular gas (shown in yellow) is overlaid on top of the starlight of the 30 CARMA Atlas3D galaxies

Understanding how dead galaxies could still have gas required deeper observations of the molecular gas, focusing on imaging rather than just detecting. At first, the Atlas3d team was getting a couple galaxies here and there, but by turning to a partnership with Berkeley, were able to command the magnificent power of CARMA. Instead of getting a handful of early-type galaxies each semester, CARMA created a longer-term survey, imaging 30 early-type galaxies, more than doubling the amount of these early-type galaxies that had been imaged up until this point. Our job was to take these images, investigate the extraordinary cases (like NGC 1266), and create download-quality data from the survey so others could use it. That task fell to me.

This paper was used to detail the data acquisition and reduction that took place, with a few little results. For instance, the molecular gas with signs of being morphologically disrupted tended to be bluer in general, suggesting that the gas in these systems was probably acquired from a minor merger, which then underwent a small burst of star formation. I then went on to take the molecular gas we thought was from these minor mergers and compared that to the predicted minor merger rate, finding that our minor mergers were consistent. But for the most part, the point of this paper was showcasing the exquisite data from CARMA. 

Mean velocity maps of the CARMA early-type galaxies.

The official published version can be found on NASA ADS.

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

Escape, Accretion, or Star Formation? The Competing Depleters of Gas in the Quasar Markarian 231

This paper came about because I had a set of beautiful unpublished data from CARMA that needed to be given its due credit, and I had noticed a bad habit of my astronomy community to call the detection of an AGN-driven molecular outflow a “special time in the life of that galaxy,” yet finding these objects was becoming ubiquitous. The extraordinary resolution of the CARMA images of this source tied into this point of view made it irresistible to write up - and do it by myself!

HST image of Mrk 231

Markarian 231 (Mrk 231) is not only our nearest-by quasar, it appears to be a supermassive black hole which we are staring down the throat of. Mrk 231 is also a prolific starburst, forming about 170 M⦿/yr of stars per year. It has a gargantuan amount of molecular gas all sitting very close to the AGN. This together makes Mrk 231 a very interesting source, but in 2010, Mrk 231 was also found to have a molecular outflow: the first found and published by Feruglio et al. The problem was that the outflow was so compact, it was very difficult to resolve where the gas was. They knew that the gas was found in wings (like in NGC 1266) but did not have the resolution to know where the gas was. Enter CARMA.

We used CARMA in one of its most extended arrays, that is, when the baseline between two antennas was as large as 1 kilometer, giving us spectacular resolution for the millimeter: 0.7”. We pointed CARMA at Mrk 231, detecting the CO(1-0) without a problem, as well as bright radio continuum in 3mm and carbon mono sulfide (CS). Using the CS, we confirmed the inferred masses from other studies for the dense gas, and we successfully detected the wings. More importantly though, we resolved the lobes! Now, armed with the wing emission as well as the actual size scale of the source (thus allowing us to infer a timescale), we were able to put good limits on the mass outflow rate, which we predicted to be 390 M⦿/yr. A bit smaller than the original work, but still capable of blowing away all of the molecular gas in the system rapidly. This is where the work from NGC 1266 kicked in. I wasn’t so sure that Mrk 231 was truly going to deplete its gas that fast. Evidence was mounting that more and more objects had molecular outflows, and the stellar populations within quasars (and Mrk 231 in particular) seemed to indicate that the quenching happened longer ago than one would infer from how quickly the gas supposedly depleted. So I turned to a different rate to determine depletion: the mass escape rate. 

HST image (grayscale) with the CARMA blueshifted and redshifted lobes overlaid.

In NGC 1266, only 2% of the molecular material is actually escaping the galaxy. That completely changed the picture of how long the molecular gas could sit in the center. I went ahead and calculated the same thing for Mrk 231 - how much mass was actually escaping the galaxy and thus depleting? A lot less than was estimated. By estimating the escape velocity in the center, Mrk 231 only had a depletion rate around 200 M⦿/yr, which was consistent with the star formation rate. This meant that the rapid <10 million year exhaustion of the gas extended to closer to 50 million years, which was much more consistent with what the stellar population in that system said it should be anyway.

The CARMA spectrum of the emission in Mrk 231, taken from the moment0 map (upper left corner)

 Finding molecular outflows in AGNs is still a new field. And in the discovery paper on NGC 1266 we made the same error, thinking that the time in the galaxy was special. A time where an AGN has a lot of molecular gas near it might be special, but it seems that nature has conspired to allow the gas to survive a lot longer than we might expect.

The official published version can be found on NASA ADS.

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

Suppression of Star Formation in NGC 1266 Part 2

This paper has broken up into 2 parts. Here is part 2 (part 1 is here).

In the previous blog, I talked about how we set out to determine the star formation rate and the molecular gas properties in NGC 1266, as well as how the two related to one another. Through a thorough investigation to pin together the most reliable tracers of the underlying gas mass and star formation, we showed that star formation is taking place with a inefficiency of at least a factor of 50. Given the density of the gas in this region, this suppression is surprising. We crossed our t’s and dotted our i’s in working through our observations, so a factor of 50 is unlikely to come from misassumptions made with regards to the star formation or gas mass measurement.

That leaves us to find a physical explanation for this suppression. The rate at which the gas in a galaxy forms stars is about balance. Stars form at places that gravitational collapse wins against forces such as radiation and turbulence. The fact that 1% star formation efficiency (with scatter) is seen in most objects tells us that this energy balance is about the same in most galaxies. In NGC 1266, the most likely cause of the star formation suppression is that this balance has changed. Something additional is fighting against the gravitational collapse. So we set out to use the observations we had in-hand to find the culprit of the suppression, thus we turned to energy balance. In this case, we can depend on a rich history of theoretical work, but in this case, the Toomre criterion is where we turn for an idea. The Toomre Q parameter describes the balance between gravitational collapse and forces outward, such as rotation and turbulence.

Toomre Q parameter

The numerator in the Toomre Q equation deals with energies that balance against gravitation, in this case, the internal dispersion of the gas (σ, a.k.a. the random motions) and the rotation of the disk the gas is in (κ, the epicyclic frequency). Both of these provide extra support against gravity, so the higher either σ or κ is, the more support there will be against gravity. The denominator in the equation deals with gravity, G is the gravitational constant, and Σ is the surface density, that is, the amount of material within an area, the higher Σ is, the more gravity there is to balance out. So, we figured out how much energy we needed in the numerator to balance the denominator. The epicyclic frequency κ was measured in the discovery paper, we got the surface density Σ in this paper, and σ was reported in Pellegrini et al. (2013) based on a fit to the higher CO transitions from Herschel. These told us that Q < 0.4, and thus should have been highly unstable. We then looked directly at the observations of the dense gas, which did not have any obvious ordered motion, and so we wondered if we missed some of the kinetic energy.

A look at the dense gas in NGC 1266. Not only are the line wings visible (in the lower left panel) in the dense gas, but the it is clear from the observations that the gas is stirred up (right) and is not rotating the way the stars are (upper left)

So we tried fitting using pure energy balance, and calculated the radial velocity needed instead to balance the mass of the central molecular gas. When we did this, we found that about 100 km/s was needed, which fit our picture! It was very possible that there was enough injected turbulence in this system to explain the suppression of star formation!

Radial velocity needed for stabilization

The suppression of star formation serves other bigger purposes too. For one, if we look back at when we think the triggering event was, we needed to be able to explain how the molecular gas in the system could have lasted for so long without becoming stars. If the AGN is able to suppress star formation on such long timescales, we have a natural explanation. The gas is being preserved. In fact, it is likely being two-fold preserved.

First, the mass outflow rate is not the same as the mass depletion rate. Removing the gas entirely from the system would mean needing to impart enough energy that the gas exceeds the escape velocity. Most of the gas does not meet this criterion, and will instead rain back into the galaxy, depositing its energy back into the disk as turbulence. This allows for a much longer gas survival time. The star formation suppression also acts to extend the lifetime of the gas in the galaxy, so you are able to explain why the gas has remained for so long. So the question is how often does this happen? NGC 1266 was hiding in plain sight for a long time before we truly understood it, and it is beginning to look like molecular outflows are universal in galaxies where molecular gas sits near the supermassive black hole. It is possible that this sort of event is how the M-sigma relation is regulated. But for now, NGC 1266 is a first example that may make up a class of galaxies, which we are just on the cusp of being able to study properly.

The official published version can be found on NASA ADS.

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