This paper is about "green pea" galaxies, which are so-called because they're small, highly concentrated and even look green because of their strong spectral line features. Green is a very rare colour in extragalactic astronomy, and indeed for stars in general. The blackbody curve over which they emit means that they emit so much light across the whole spectrum that green is always washed out by the blue and the red. Only when you get something that's not a blackbody, something emitting over a very narrow wavelength range, do you get anything green. And that's rare.
"Green peas" tend to be more distant, but there are also closer "blueberries", which you'll have guessed are much the same apart from the colour. Whether they have similar spectral line emission to green peas is not clear.
Anyway, the authors here try and figure out how the HI (atomic hydrogen) gas content of the green pea galaxies varies with their stellar content. A perfectly sensible goal given how strange these objects are; perhaps this could shed light on star formation in extreme objects. But I have to say I don't like it very much.
Like some other papers I've been reading lately, this one contains no new observations but uses entirely archival data. But unfortunately this one is more typical of its class, not actually finding anything new or at most a marginal, incremental difference. They have a sample of 19 HI detections (and 21 upper limits, which were observed but not detected), though they have to exclude a couple because of issues with the optical and/or UV data. Based on the details they go into, they seem to have re-processed the HI data, though it isn't at all clear why – was there some issue with the earlier analyses ? If so, they don't mention it.
Figure 2 is by far the most interesting. This plots the now-classic "main sequence" of galaxies showing how their star formation rate scales with stellar mass, in a nice, tight correlation. Green pea galaxies also seem to correlate but with a totally different, much steeper relation : they have far higher star formation rates than their stellar masses predict. This holds true for both the HI detections and non-detections. Unfortunately the upper limits of the non-detections are all over the place, so it's not possible to say if there's any real difference between those with and those without gas. And the main trend is not a new result.
What they instead concentrate on is finding other relations to the optical data. They say their sample is offset from a previous relation between the MHI/M* relation as a function of NUV (near UV) - r magnitude (basically a colour measurement*), but... well it might be, but to me that earlier relation, for normal galaxies, itself looks like a dodgy fit. So I don't think too much can be said here.
* One minor but quite interesting point they make is that using this UV-optical is better than using two optical wavebands for colour, since the green pea star light is much more UV-dominated than in normal galaxies.
One thing they do show which looks convincing, but I'm not sure if it's a new relation, is that green peas have excessively high gas fractions as a function of just about any parameter. New or not that's nice, but even here their plot could easily have been so much clearer. Then they try and plot gas fraction offset (measured – expected) as a function of various parameters, say there's a trend, but as far as I can tell there just isn't. This is one of those cases where if someone shows you a weak trend and you say, "I've seen worse claims"... this is one of those worse claims.
Then they try and find what sort of scaling relation does the best job of predicting the HI mass of green peas. As far as I can tell, they seem to have taken the standard practise of throwing everything against a wall and seeing what sticks a little too far. They come up with hideous relations involving different colours, stellar masses, star formation rates and surface brightness levels that to me has no obvious physical significance at all. Frustratingly, they don't discuss this.
And that I do find strange and annoying. It's reminiscent of p-hacking, where if you search for correlations using complicated enough relations and large enough data sets, you're bound to find something. Was there any prior reason to suspect this torturous relation had some physical significance ? If so then it's interesting ! If not, if, as I suspect, it's just an empirical fit, then it's just a statistical artifact. I'm not saying that's the case, only that they needed to be a lot clearer about what this is supposed to mean, physically, before I take any further interest in it.
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