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

TL;DR

This work addresses how satellite populations around Milky Way analogs evolve within a $\Lambda$CDM framework by constructing a statistical census with the Merian Survey. It deploys two medium-band DECam filters centered on H$\alpha$ and [O III] to identify star-forming satellites around $\sim$400 MW-mass hosts in the narrow redshift window $0.07<z<0.09$, achieving photo-$z$ precision of $\sigma_{\Delta z/(1+z)} \approx 0.01$ and a completeness down to $M_{\star, {\rm sat}} \gtrsim 10^{8}\,M_\odot$. The study finds that about $80\%$ of hosts harbor $0$–$3$ massive satellites, with $13\pm4\%$ hosting two satellites, and reveals a positive correlation between satellite abundance and host stellar mass, particularly for satellites with $\log_{10}(M_{\star,\rm sat}/M_\odot) \gtrsim 10$. The completeness-corrected radial distribution is less centrally concentrated than an NFW profile, and the Milky Way system is more central than the 50% richest Merian systems but broadly aligns with the 50% most central Merian systems. Overall, the work demonstrates the efficacy of medium-band photometry for assembling large, distance-accurate satellite samples and provides a robust benchmark for studies of satellite quenching, environmental effects, and hierarchical galaxy formation.

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$α$ 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.

Figures (1)

• Figure 1: Color stamps from HSC are displayed for four representative systems in our sample, showcasing cases with 0, 1, 2, and 3 satellite candidates, arranged from the top left to the bottom right, respectively, together with the log stellar mass and spectroscopic redshift. Each panel features the main figure, which illustrates the host environment extending out to its virial radius, represented by the white dashed circle. To provide more detail, a zoomed-in view of the host galaxy is shown in the upper left corner of each panel. For systems with satellites, additional insets highlight the satellites, emphasizing their positions and characteristics within the host environment.