The Merian Survey: A Statistical Census of Bright Satellites of Milky Way Analogs

Abstract

We present a statistical census of bright, star-forming satellite galaxies around Milky Way (MW) analogs using the first data release of the Merian Survey. Our sample consists of 393 MW analogs with stellar masses $10^{10.5} < M_{\star, \rm host} < 10^{10.9} M_\odot$ at redshifts $0.07 < z < 0.09$, all central galaxies of their own dark matter halos. Using photometric selection -- including magnitude, color, angular size, photometric redshift, and size-mass cuts -- we identify 793 satellite candidates around these 393 hosts. Our selection leverages two medium-band filters targeting H$\alpha$ and [O \textsc{iii}] emission, enabling a nearly complete sample of star-forming, Magellanic Clouds-like satellites with $M_{\star, \rm sat} \gtrsim 10^{8} M_\odot$. We find that $\sim80\%$ of hosts have 0-3 massive satellites, and $13\pm4\%$ have two satellites (similar to the MW). Satellite abundance correlates with total stellar mass, and we provide significantly improved statistics for the most massive satellites at $\log_{10}[M_{\star, \rm sat}/M_{\odot}] \gtrsim 10$. The completeness-corrected radial distribution is less centrally concentrated than an NFW profile. In contrast, the Milky Way satellites are more centrally concentrated than the 50\% richest Merian systems, but are broadly consistent with the 50\% most centrally concentrated Merian systems. Our results highlight the power of medium-band photometry for satellite identification and provide a key benchmark for studying satellite quenching, environmental effects, and hierarchical galaxy formation.