Very attentive readers may remember a paper from 2022 claiming, with considerable and extensive justification, to have detected a new class of galaxian object : the ram pressure dwarf. These are similar to the much more well-known tidal dwarf galaxies, which form when gravitational encounters remove so much gas from galaxies that the stripped material condenses into a brand new object. Ram pressure dwarfs would be essentially similar, but result from ram pressure stripping instead of tidal encounters. A small but increasing number of objects in Virgo seem to fit the bill for this quite nicely, as they don't match the scaling relations for normal galaxies very well at all.
This makes today's paper, from 2024, a little late to the party. Here the authors are also claiming to have discovered a new class of object, which they call a, err... ram pressure dwarf. From simulations.
I can't very well report this one without putting my sarcastic hat on. So you discovered the same type of object but two years later and only in a simulation eh ? I see. And you didn't cite the earlier papers either ? Oh.
And I also have to point out an extremely blatant "note to self" that clearly got left in accidentally. On the very first page :
Among the ∼60 ram-pressure-stripped galaxies belonging to this sample, ionized gas powered by star formation has been detected (R: you can get ionized gas that is not a result of star formation as well, so maybe you could say how they have provided detailed information about the properties of the ionized gas, its dynamics, and star formation in the tails instead) in the tentacles.
No, that's not even the preprint. That's the full, final, published journal article !
Okay, that one made me giggle, and I sympathise. Actually I once couldn't be bothered to finish looking up the details of a reference so I put down "whatever the page numbers are" as a placeholder... but the typesetter fortunately picked up on this !
What does somewhat concern me at a (slightly) more serious level, though, is that this got through the publication process. Did the referee not notice this ? I seem to get picked up on routinely for the most minor points which frankly amount to no more than petty bitching, so it does feel a bit unfair when others aren't apparently having to endure the same level of scrutiny.
Right, sarcastic hat off. In a way, that this paper is a) late and b) only using simulations is advantageous. It seems that objects detected initially in observational data have been verified by theoretical studies fully independently of the original discoveries. That gives stronger confirmation that ram pressure dwarfs are indeed really a thing.
Mind you, I think everyone has long suspected in the back of their minds that ram pressure dwarfs could form. After all, why not ? If you remove enough gas, it stands to reason that sometimes part of it could become gravitationally self-bound. But it's only recently that we've had actual evidence that they exist, so having theoretical confirmation that they can form is important. That puts the interpretation of the observational data on much stronger footing.
Anyway, what the authors do here is to search one of the large, all-singing, all-dancing simulations for candidates where this would be likely. They begin by looking for so-called jellyfish galaxies, in which ram pressure is particularly strong so that the stripped gas forms distinct "tentacle" structures. They whittle down their sample to ensure they have no recent interactions with other galaxies, so that the gas loss should be purely due to ram pressure and not tidal encounters. Of the three galaxies in their sample which meet this criteria, they look for stellar and gaseous overdensities within their sample and find one good ram pressure dwarf candidate, which they present here.
By no means does this mean that such objects are rare. Their criteria for sample selection is deliberately strict so they can be extremely confident of what they've found. Quite likely there are many other candidates lurking in the data which they didn't find only because they had recent encounters with other galaxies, which would mean they weren't "purely" resulting from ram pressure. I use the quotes because determining which factor was mainly responsible for the gas loss can be extremely tricky. And simulation resolution limits mean there could be plenty of smaller candidates in there. The bottom line is that they've got only one candidate because they demand the quality of that candidate be truly outstanding, not because they're so rare as to be totally insignificant.
And that candidate does appear to be really excellent and irrefutable. It's a clear condensation of stars and gas at the end of the tentacle that survives for about a gigayear, with no sign of any tidal encounters being responsible for the gas stripping. It's got a total stellar mass of about ten million solar masses, about ten times as much gas, and no dark matter – the gas and stars are bound together by their own gravity alone. The only weird thing about it is the metallicity, which is extraordinarily large, but this appears to be an artifact of the simulations and doesn't indicate any fundamental problem.
In terms of the observational candidates, this one is similar in size but at least a hundred times more massive. Objects that small would, unfortunately, be simply unresolvable in the simulations because it doesn't have nearly enough particles. But this is consistent with this object being just the tip of a much more numerous iceberg of similar but smaller features. Dedicated higher resolution simulations might be able to make better comparisons with the observations, until someone finds a massive ram pressure dwarf in observational data.
I don't especially like this paper. It contains the phrase "it is important to note" no less than four times, it says "as mentioned previously" in relation to things never before mentioned, it describes the wrong panels in the figures, and it has many one-sentence "paragraphs" that make it feel like a BBC News article if the writer was unusually technically competent. But all of these quibbles are absolutely irrelevant to the science presented, which so far as I can tell is perfectly sound. As to the broader question of whether ram pressure dwarfs form a significant component of the galaxy population, and indeed how they manage to survive without dark matter in the hostile environment of a cluster... that will have to await further studies.
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