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

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 ${10}^{10.5}<M_{\star ,\mathrm{h}\mathrm{o}\mathrm{s}\mathrm{t}}<{10}^{10.9}{M}_{\odot }$ 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 $\text{textsc}iii$] emission, enabling a nearly complete sample of star-forming, Magellanic Clouds-like satellites with $M_{\star ,\mathrm{s}\mathrm{a}\mathrm{t}}\gtrsim {10}^8{M}_{\odot }$. We find that $\sim 80\mathrm{\%}$ of hosts have 0-3 massive satellites, and $13\pm 4\mathrm{\%}$ 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 ${\log }_{10}[M_{\star ,\mathrm{s}\mathrm{a}\mathrm{t}}\mathrm{/}{M}_{\odot }]\gtrsim 10$. 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.