Tracing the gas around DESI galaxies with SZ and X-ray

Tue, 09 Jul 2024 00:00:00 +0000

On large scales the cosmic matter distribution is relatively well understood, from observations and simulations - it is largely governed by the laws of gravity. On smaller scales - inside clusters and groups of galaxies - a rich interplay of gravity and feedback processes driven by black holes and stars significantly complicate the picture. These processes are crucial for our understanding of galaxy formation, but they also bias weak-lensing measurements that don’t account for the redistribution of matter.

What we have shown so far

One way to probe the distribution of matter inside these objects is through the Sunyaev–Zel’dovich (SZ) effect - a tiny CMB temperature shift produced when CMB photons scatter off electrons in galaxy clusters and groups. In a paper led by Boryana Hadzhiyska together with the ACT and DESI teams (incl. myself; , Phys. Rev. D 2024) we used ACT CMB data to observe the kinematic Sunyaev–Zel’dovich (kSZ) effect - scattering of electrons with bulk velocities - around galaxies identified by DESI. We showed that the feedback processes appear to be stronger than expected from numerical simulations.

This analysis had one critical caveat; we didn’t know the mass of the objects we were looking at. We calibrated group masses by looking at how they gravitationally lense the CMB. In keeping with my focus on ensuring this type of analysis is robust, I contributed important systematics checks, demonstrating that CMB lensing measurements could be used to constrain the mass of the galaxies we had mapped without bias (Hadzhiyska et al. , Phys. Rev. D 2025).

What we are working on now

The kSZ (and the related thermal SZ - scattering off thermal electrons) are not the only way to constrain the gas distribution. Another popular method is observations of the X-ray emission from these objects.

We are currently mapping the X-ray emission around the same objects and plan to analyse it jointly with kSZ and tSZ data, unveiling the new details about the formation of cosmic structure on the smallest scales.

DESI × CMB lensing

Fri, 05 Jul 2024 00:00:00 +0000

— the Dark Energy Spectroscopic Instrument — is mapping tens of millions of galaxies and quasars to produce the most detailed redshift survey of the universe to date. Cross-correlating that map with CMB lensing measurements gives us a clean view of how cosmic structure has grown across cosmic time.

I lead the development and deployment of the measurement and analysis pipeline for the DESI DR2 CMB lensing cross-correlation analyses. This is data engineering at scale: galaxy catalogs spanning tens of millions of objects across six distinct samples, CMB lensing maps covering tens of thousands of square degrees, distributed computing pipelines that thread careful systematics control from raw data through to cosmological constraints.

What’s next

The same machinery extends naturally to DESI’s full 5-year dataset and future CMB lensing maps from the Simons Observatory. The pipeline is built to be reusable across galaxy samples, lensing experiments, and parameter spaces.

DESI |

Tracing the gas around DESI galaxies with SZ and X-ray

What we have shown so far

What we are working on now

DESI × CMB lensing

What’s next