Dwarf galaxies and all-sky HI surveys FTW \m/

I've been coming across a lot of work, in the optical and the radio domain to identify and understand dwarf/satellite galaxies, stellar tidal streams and neutral HI (HI is neutral Hydrogen and HII is singly ionized hydrogen i.e a proton) in the universe. This is one of many such efforts (a few of which I will write about later on) where the authors used the SKA pathfinder telescope BETA/ASKAP to observe the galaxy group IC 1459. The full paper can be found here.

Let me first tell you why it's a challenge to observe neutral HI in the local universe and why these results are so interesting, IMO. All galaxies form from HI (again, neutral Hydrogen) but once the galaxy/stars start(s) forming, neutral HI close to the sources of radiation get ionized into HII. Only HI in the outer reaches of the galaxy is left unionized. And given that it's at the edge of the galaxy, it's not nearly dense enough to emit strongly. Don't get me wrong, there is still a good amount of HI in-between the individual stars in the galaxy but the important thing is that it extends way wayyy further out into the galaxy than the stars do. It is therefore important to understand how far the HI extends and to understand how much HI is there overall. It is also important to understand HI in the context of galaxy groups/clusters because it will help us understand how galaxies grow; do they feed on the HI of smaller dwarf galaxies, are there lanes of stripped HI moving from a low-mass to a high-mass galaxy. I am getting images of a day when telescopes are good enough that we will be able to see the kind of HI filamental structure predicted by cosmological simulations. But I digress.

Coming back to the paper now, let me tell you that the telescope being used, BEAT/ASKAP, is a precursor to the SKA (Square Kilometer Array), an ambitious project to build a very large interferometer in Australia, a radio telescope that will surpass the abilities of any other on Earth (or in Space for that matter). As for the data analysis section, while I have very little experience working on radio astronomical data (but none of this kind), I do not myself understand exactly how data reduction is done. I understand the gist of it but I don't get the details. So, moving on the results section of the paper, having redetected ~9 dwarf galaxies in the group, the authors were also able to find 3 large clumps of HI near two galaxies, clumps that were previously thought to be part of the galaxy which are now spatially resolved as not. Two of the clumps, near a galaxy account for as much as 1/3rd of all of the HI in that galaxy. The authors also speculate that these blobs around galaxies are the denser regions in much larger, less-dense filaments, stripping HI from galaxies. Like I said, one day we will be able to see the filamental structure in galaxy clusters.

The results coming out of ASKAP/SKA are surely ones to follow over the next decade, results that might open up a whole new observational domain for us.

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