Song Huang and Alexie Leauthaud tell us about their new galaxy cluster finder, which uses the stellar mass in the outer region of a galaxy as a method to determine the mass of the galaxy’s cluster.

It feels a bit like magic (to me) that the stars in individual galaxies can be used to weigh the mass of the whole cluster, but like other mass proxies one can devise a scaling relationship between the proxy and the mass – and then the proof is in the empirical pudding.

Concerning that pudding, Song and Alexie’s new proxy has a comparably small scatter in its scaling relation i.e. when compared to the most common proxy for optical surveys, the richness (i.e. the number of galaxies of a certain type in the cluster).

The most massive clusters found by this method are also more easily modelled than the most massive clusters found via richness, i.e. there seem to be smaller systematic effects that need to be taken account.

So, whatever magic is going on, it’s working!

Overall, Song and Alexie think a holistic approach to weighing galaxy clusters is the best way to go, using richness and stellar mass estimates, as well as observables from surveys at other wavelengths like X-Ray temperature and the SZ effect on CMB photons that are scattered by hot gas inside clusters.

It looks like an exciting, multiwavelength, future for galaxy cluster cosmology, which should further open up a very non-linear distance scale to precision tests of the growth of structure, modified gravity and the primordial density perturbations.

Song: https://dr-guangtou.github.io/
Alexie: https://alexie.sites.ucsc.edu/
The paper: https://arxiv.org/abs/2109.02646