Thermal Sunyaev-Zel'dovich Measurements of Locally Bright Galaxies with ACT DR6: Radio Source Contamination and Excess Compton-y Signal

Abstract

The Planck collaboration found a remarkable power-law relation between stellar mass and the thermal Sunyaev-Zeldovich (tSZ) signal for the Locally Bright Galaxy (LBG) sample, spanning over a decade in stellar mass. We re-examine this measurement using the Atacama Cosmology Telescope (ACT) DR6 component-separated Compton-$y$ maps, which provide lower noise and higher angular resolution than Planck, on a footprint spanning one-third of the sky. We recover a consistent power-law scaling between the cylindrical Compton-$y$ signal and stellar mass. Additionally, we identify residual contamination in the tSZ signal from radio sources at the few percent-level, which has not been considered previously. In parallel, we identify a factor-of-two excess in the Compton-$y$ signal in LBGs hosting co-spatial radio sources relative to those without, at fixed stellar mass. This excess persists to radii of at least 6 arcminutes, suggesting a halo-scale effect, and is recovered in the original Planck results when the same radio source subselection is applied. We consider two physical explanations: a systematic difference in halo mass at fixed stellar mass, or thermal energy injected into the circumgalactic medium by Active Galactic Nuclei, although we cannot currently distinguish between the two. This result has direct implications for tSZ cross-correlation measurements more broadly, and necessitates careful characterization of the radio source fraction in galaxy samples in future analyses.