Everyone: my second paper is out as a preprint!! This was such a cool project, and Quinn (the first author) is great to work with. And yep, it's on dark matter!! Here's the preprint, and a thread on the general idea of the paper: https://arxiv.org/abs/2011.10627 

It's pretty neat: central galaxies in clusters that act as gravitational lenses are the main mass that's bending spacetime, but when they have smaller dwarf galaxies around them that are close enough and massive enough to them, those can perturb the lens

(fittingly, we call them "perturbers"), and we can see these perturbations in the data. This paper is about a lensing galaxy like the one I described above, with a perturber. Thing is: you can't see it in visible light AT ALL! It's also way too dense in the center.

So, is it a dark matter halo w/no stars? Not necessarily, it's far away which reduces its brightness from our perspective. But the fact that we can't see it does place a limit on how much light it can emit. The question: can this subhalo be produced with Cold Dark Matter (CDM)?

Here's where I come in: simulated toy universes!! I used the Illustris TNG simulations, both DM + stars and DM only (stars and gas experience different interactions than dark matter, so the galaxies in DM + stars (and gas) simulations evolve differently than those in DM only.

I found all subhalos that are similar to this one in both toy universes. I found that, when looking at all the criteria, no subhalos look like this one. The dark-matter-only one was to test if CDM alone can explain it, and that had no matches either (in fact it was worse!).

So this one is very hard to explain with CDM (unless we're missing something) and may need to try other models (ahem, SIDM anyone?! )

This project is special because it's the one that I truly learned how to code. Here's to the next paper