Even though my backlog of unread papers is a mile long, this one from today's arXiv was so pertinent I had to read it immediately.
You might remember that a few years ago I was co-author on a paper that re-examined an enormous gas cloud in the Virgo Cluster that looks a bit like a rhino. This was already known from ALFALFA observations, and with no clear parent galaxy but an enormous amount of gas, and no obviously-associated optical emission, it was undeniably weird. Most bits of atomic fluff floating around in galaxy clusters tend to be small, but this was was enormous : almost 200 kpc long and with an HI mass of over a billion times the mass of the Sun. Originally seen as a complex of HI clouds, our more sensitive observations showed that there was even more gas present and all the individual clouds, which originally looked like separate objects, are actually connected by fainter gas structures.
We showed that this made the already-suspicious candidate galaxies for the origin of the Complex even more implausible. To lose that much gas implies a very large galaxy indeed. Now galaxies can and do, of course, lose huge amounts of gas in clusters through ram pressure stripping : as they move through the cluster's own, much thinner gas, pressure builds up and eventually pushes gas right out of the galaxy's disc. But this process is also prone to dispersing and dissolving much of the gas, leaving it undetectable. This means that the gas we see today might only be a small part of that which was originally present, requiring an even more massive parent to supply the gargantuan amounts of gas needed.
We also suggested a new, but unlikely, possible parent : NGC 4522. This is one of the most famous examples of ram pressure stripping. It's got a nice clear gas tail, and even in the optical you can see signs of disturbances in the dust. Our observations showed a second gas tail, with different kinematics to that which was already known, lining up quite nicely with the Kent Complex (which Brain Kent prefers to call the boring name of the ALFALFA Complex 7). But the velocities of the galaxy and Complex are so different that it's difficult to believe the one could be the source of the other.
This latest paper changes the game. They find that there is actually some faint optical emission in one (small) part of the Complex, comparable in many ways to the putative ram pressure dwarfs the same authors proposed a couple of years ago. In those objects, it seems the stars form directly in the gas stripped due to ram pressure, forming stellar structures that might be gravitationally self-bound or might just be passing, transient features that will soon disperse. As with the others, the stars in this object are all young, with no evidence for a stellar population more than a few tens of megayears old. That's basically instantaneous for these sorts of features. It's worth also pointing out that this little star-forming region is very small in comparison with the rest of the Complex, maybe ~10" across compared to the 40' of the whole Complex (a factor of 240 difference in size !).
What they get with the new observations is not just the discovery of this faint patch of starlight, which by itself would be unconvincing : such little smudges aren't that uncommon, and it's just too small to be obviously related. No, they go much further than this, getting very nice resolved observations with Hubble but also a stellar velocity which is a perfect match for the Kent Complex. That makes the association about as secure as it's going to get. And they also measure the chemical composition (a.k.a. metallicity), showing that it's similar to the other potential ram pressure dwarfs but also allowing for comparisons to the other galaxies in the vicinity.
And they estimate, however roughly, the stellar mass. This is small, a few tens of thousands of times the mass of the Sun. Even only considering the particular clump of gas nearest the stars, the gas content is at least three thousand times higher than the stellar mas, which is extraordinary even in comparison to the other ram pressure dwarf candidates.
What does this mean for the origin of the complex ? Well, like our earlier paper, they conclude there aren't any fully convincing candidate galaxies, but they open the door a little. While we largely dismissed NGC 4522 because of its huge velocity difference with respect to the Complex (1,800 km/s, which is high even in a chaotic place like the Virgo Cluster), they're a bit more charitable. This extreme velocity with respect to the cluster means that it could have experienced incredibly strong ram pressure stripping and very recently too, recently enough that most of the cloud would remain intact and detectable. That would make NGC 4522 a plausible candidate because it wouldn't have needed to have had such a huge gas content, as not so much would have become undetectable yet.
It's also a good match in metallicity, though they're careful to point out that limited metallicity data for other galaxies in the vicinity makes this not such a strong diagnostic tool as we might like. There's also a large gap between NGC 4522 and the Complex, but this could simply imply two stripping episodes, and they say that simulations have shown that such features can indeed happen.
Of our preferred candidate, NGC 4445... well, "preferred" is too strong a word. It was really only the one we disliked the least. Anyway they raise the same objection that we do, that other observations show that NGC 4445 has a tail pointing in exactly the wrong direction. So again, two stripping episodes would be needed, with some weird geometry, but this one is kinematically closer to the complex, and could more easily account for enormous gas lost.
My immediate impression is that their arguments are quite persuasive : NGC 4522 could be the parent after all. We also objected because of the particular kinematics of the structure (one of the sub-clumps is at the wrong velocity), but without a dedicated numerical simulation, this is a weaker argument. The huge kinematic difference of the Complex and galaxy remains, as they say, problematic, especially given the rather low velocity dispersion in this part of the cluster, but it might not be fatal. Still, I wouldn't rule out NGC 4445 either.
What's really interesting about this object, I think, are three things besides the obvious how-did-the-damn thing-form in the first place. First, why is only part of it forming stars, and why has that part only started forming stars right now ? What's special about this particular section of the Complex – why aren't other bits of it forming stars as well ? What happened recently to trigger star formation ? Secondly, more generally, how does this relate to the other ram pressure dwarfs, given that it's so much more massively gas rich than all of the others ? Does it point to a similar origin or is this one a coincidence ? And thirdly, why is this structure so damnably complex ? Why does it have all these fiddly little details rather than being one big long stream ?
The most likely avenue of progress at this point would seem to be detailed, dedicated numerical simulations. More observational data might help of course, but I think if we could show that certain passages of galaxies in this part of the cluster would (or more likely, would not) produce even vaguely similar structures to this one, we'd be able to formulate a convincing argument for and against individual candidate galaxies. Not at all easy to do, but possible.
Regardless of its origin, it's a spectacular and fascinating object, and I'm gratified to see someone not only having properly read our previous work but also genuinely understanding it too. Were I there referee, this one would likely have gone through on a nod. In fact the other real question I'd have would be how it can be submitted as a letter when it's fifteen pages long and pretty fully fleshed out – may as well skip the letter phase and make it a full paper at this point.
No comments:
Post a Comment