Dr Kilian Walsh

One of the great pleasures of my job is being involved in the bestowal of PhDs! Today, Kilian Walsh (NYU) defended his PhD, which he did with Jeremy Tinker (NYU). The thesis was about the connections between galaxies and their halos. As my loyal reader knows, I find it amazing that this description of the world works at all, but it works incredibly well.

One of the puzzling results from Walsh's work is that although halos themselves have detailed properties that depend on how, where, and when they assembled their mass, the properties of the galaxies that they contain don't seem to depend on any halo property except the mass itself! So the halos have (say) spin parameters that depend on assembly time, the galaxies don't seem to have properties that depend on halo spin parameter! Or if they do, it's pretty subtle. This subject is called halo assembly bias and galaxy assembly bias; there is plenty of the former and none of the latter. Odd.

Of course the tools used for this are blunt tools, because we don't get to see the halos! But Walsh's work has been about sharpening those tools. (I could joke that he sharpens them from extremely blunt to very blunt!) For example, he figured out how to use the void probability function in combination with clustering to put stronger constraints on halo occupation models.

Congratulations Dr. Walsh!