Backlighting extended gas halos around luminous red galaxies: kinematic Sunyaev-Zel'dovich effect from DESI Y1 x ACT

TL;DR

This work reports a high-significance kSZ measurement around DESI Y1 LRGs by velocity-weighted CAP stacking on ACT DR6 maps, achieving $S/N \approx 10$ over ~4,000 deg$^2$ and revealing gas extending beyond the virial radius. The authors reconstruct LOS peculiar velocities from the galaxy density field and probe gas profiles across redshift, stellar mass, and luminosity, finding no strong redshift evolution but clear mass and luminosity dependence. They compare the observed profiles to hydrodynamical simulations (Illustris and IllustrisTNG), finding a preference for stronger feedback that drives gas farther out, though results depend on the amplitude modeling; the method also shows consistency with photometric kSZ measurements and yields robust detections for DESI Y1 BGS and ELG samples. Forecasts indicate $S/N \sim 20$ for DESI Y3 and up to $S/N \sim 50$ with LSST redMaGiC and upcoming CMB surveys, promising tighter constraints on baryonic physics and improved calibration of baryonic uncertainties in weak lensing.

Abstract

The gas density profile around galaxies, shaped by feedback and affecting the galaxy lensing signal, is imprinted on the cosmic microwave background (CMB) by the kinematic Sunyaev-Zel'dovich effect (kSZ). We precisely measure this effect ($S/N\approx 10$) via velocity stacking with more than 800,000 spectroscopically confirmed luminous red galaxies (LRG) from the Dark Energy Spectroscopic Instrument (DESI) Y1 survey, which overlap with the Atacama Cosmology Telescope (ACT) Data Release 6 temperature maps over $\geq$ 4,000 deg$^2$. We explore the kSZ dependence with various galaxy parameters and find no significant trend with redshift, but clear trends with stellar mass and absolute magnitude in $g$, $r$, and $z$ bands. Our analysis suggests that the gas extends beyond the dark matter halo (99.5\% confidence, i.e. PTE = 0.005). We find a tentative preference for hydrodynamical simulation models with stronger feedback that drives gas further out (Illustris $z=0.5$, PTE = 0.37) over weaker-feedback cases (IllustrisTNG $z=0.8$, PTE = 0.045), though with limited statistical significance. In all cases, a free multiplicative amplitude was fit to the simulated profiles, and further modeling work is required to firm up these conclusions. We find consistency between kSZ profiles around spectroscopic and photometric LRG, with comparable statistical power, thus increasing our confidence in the photometric analysis. Additionally, we present the first kSZ measurement around DESI Y1 bright galaxy sample (BGS) and emission-line galaxies (ELG), whose features match qualitative expectations. Finally, we forecast $S/N \sim 50$ for future stacked kSZ measurements using data from ACT, DESI Y3, and Rubin Observatory. These measurements will serve as an input for galaxy formation models and baryonic uncertainties in galaxy lensing.

Figures (21)

• Figure 1: ACT DR6 and DESI LRG Y1 footprints in equatorial coordinates. In gray, we show the full ACT footprint, while in blue, we show a section of the ACT DR6 survey, where the galactic plane has been masked out, following Planck Planck2020. The brightest extended sources and point sources are also masked out, as shown by the small holes visible in the blue area. In purple and violet, we show the North Galactic Cap (NGC) and South Galactic Cap (SGC) samples from DESI Y1, respectively. The SGC overlaps almost fully with the ACT DR6 map, with $\sim78\%$ of the SGC LRG galaxies lying within the ACT footprint, while the NGC overlap is $\sim46\%$. The total overlapping region between the two surveys corresponds to $\sim$4,300 deg$^2$.
• Figure 2: Redshift distribution of the DESI LRG Y1 galaxies overlapping the ACT map ($\sim 39\%$ of the total LRG sample). The galaxies were divided into four bins given their spectroscopic redshift, shown by the vertical lines. Additional summary information, including mean redshift and number of objects per bin, is included in Table \ref{['tab:result_reports']}.
• Figure 3: The binned stellar mass distribution of the DESI LRG Y1 galaxies overlapping the ACT DR6 map, as estimated by Zhou_2023, is provided. Additional summary information for the bins is available in Table \ref{['tab:result_reports']}.
• Figure 4: Absolute magnitude distribution of the DESI LRG galaxies for the photometric bands $g$ (in green), $r$ (in red), and $z$ (in gray). These were obtained by matching the target IDs of the spectroscopic galaxies with the DESI Imaging Legacy Survey Zhou_2023 and then converting the flux to absolute magnitude per band.
• Figure 5: Visualization of the reconstructed LOS peculiar velocities of the DESI LRG Y1 using the linear velocity reconstruction method presented in Sec. \ref{['sec:vel_rec']}. On top of the DESI sample, we include the LOS reconstructed velocities of the CMASS (constant mass) sample from the Baryon Oscillation Spectroscopic Survey Alam2017Schaan2021Ahn2014 (DESI's predecessor) in a semi-transparent overlay to show the increment in distance and total number of galaxies. Each dot represents a galaxy, and its color will be blue (red) if it moves towards (away from) the observer in the center. As in Fig. \ref{['fig:footprint']}, NGC and SGC stand for North and South Galactic Cap, respectively, and $z$ stands for redshift.