- Their paper completed the submission process before publication of the other one
- They submitted later but both the authors and referee were unaware of the other paper
- They're doing a Donald Trump, like when he declared that impeachment required both high crimes and misdemeanours. Perhaps they deem the other galaxies to be at high redshift, so it's okay that this one is only at intermediate redshift.
I also found their introduction to oddly imply that mergers between galaxies are not an effect of environment, but that's nick-picking. Otherwise it's nice.
They note that the effects of ram pressure stripping, the primary way of forming long one-sided tails, are still unclear. Given long enough and it will strip all the gas in a galaxy, quenching its star formation. But what happens during that process is potentially much more complicated : it might immediately reduce star formation by lowering the gas density, but it might also trigger star formation by compressing the gas (at least at the point of collision with the external gas). So a galaxy could potentially have its star formation reduced in some areas but temporarily increased in others.
Even more complex and controversial is what happens to the gas that gets stripped. Molecular gas is so dense that it probably doesn't get stripped directly, though some people think it might be possible in extreme circumstances. A few galaxies - not many mind - seem to have stellar tails as well as gaseous ones. It's unclear if these star-forming wakes result from direct stripping of molecular gas or if the molecular gas forms in the tail from the stripped atomic gas. And goodness only knows what happens to the stars formed in the tails after all the hullabaloo is over.
The galaxy they study here is a particularly dramatic case, forming stars as much as five times faster than other galaxies this massive. It's also a very massive galaxy, making material harder to strip. Yet it shows very clear tails and tentacles extending quite neatly and continuously from its disc - it is, as they say, a textbook jellyfish galaxy. What seems quite clear in this case is that the star-forming regions they see in the ultra-violet (which traces hot young stars) aren't just found in the tentacles, suggesting that ram pressure can indeed trigger star formation within the disc as well as in the stripped wake. Of course it's hard to be sure, since we don't know what the galaxy was like before stripping began, but the visual is awfully convincing.
Some of their other points are less clear. I don't say they're wrong, only that it's not obvious to me how they reach certain conclusions. They say, for example, that the stripped gas lags behind in velocity compared to the disc gas, but it doesn't look like that to me in the figure. They also say that if they account for this lag then the galaxy is rotating exactly as expected, but again I don't see how they actually do this. They also try a simple simulation to work out the galaxy's trajectory, finding that it's consistent with infall along a filament from another nearby cluster, but their description seems unnecessarily geared towards experts in orbital dynamics.
It's still a nice paper. Perhaps in a few years we'll have statistically significant samples of such objects, and then things will get a lot more interesting.
Jellyfish: Resolving the kinematics of extreme ram-pressure stripping at $z\sim0.3$
We present and discuss results from the first spatially resolved kinematic study of ram-pressure stripping of a massive late-type galaxy at intermediate redshifts. Our target, the spectacular "jellyfish" galaxy A1758N\_JFG1, was previously identified as a fast-moving member of the equal-mass merger A1758N ($z=0.28$) with a star-formation rate of 48 M$_\odot$ yr$^{-1}$, far above the galaxy main sequence.
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