The SAGA Survey. III. A Census of 101 Satellite Systems around Milky Way-mass Galaxies (2404.14498v3)

Abstract: We present Data Release 3 (DR3) of the Satellites Around Galactic Analogs (SAGA) Survey, a spectroscopic survey characterizing satellite galaxies around Milky Way (MW)-mass galaxies. The SAGA Survey DR3 includes 378 satellites identified across 101 MW-mass systems in the distance range of 25-40.75 Mpc, and an accompanying redshift catalog of background galaxies (including about 46,000 taken by SAGA) in the SAGA footprint of 84.7 sq. deg. The number of confirmed satellites per system ranges from zero to 13, in the stellar mass range of 10^6 to 10^10 solar masses. Based on a detailed completeness model, this sample accounts for 94% of the true satellite population down to a stellar mass of 10^7.5 solar masses. We find that the mass of the most massive satellite in SAGA systems is the strongest predictor of satellite abundance; one-third of the SAGA systems contain LMC-mass satellites, and they tend to have more satellites than the MW. The SAGA satellite radial distribution is less concentrated than the MW's, and the SAGA quenched fraction below 10^8.5 solar masses is lower than the MW's, but in both cases, the MW is within 1 sigma of SAGA system-to-system scatter. SAGA satellites do not exhibit a clear corotating signal as has been suggested in the MW/M31 satellite systems. Although the MW differs in many respects from the typical SAGA system, these differences can be reconciled if the MW is an older, slightly less massive host with a recently accreted LMC/SMC system.

• The paper introduces SAGA DR3, a comprehensive census of 101 satellite systems around Milky Way-mass galaxies that reveals diverse luminosity and stellar mass functions.
• It employs multi-band imaging and spectroscopic data to analyze satellite quenching and radial distributions, highlighting a lower quenched fraction than in the Local Group.
• The study identifies a strong correlation between the most massive satellite and host properties, offering key insights for refining galaxy formation models.

Overview of the SAGA Survey: A Census of 101 Satellite Systems

The "SAGA Survey: A Census of 101 Satellite Systems around Milky Way-mass Galaxies" presents an extensive analysis of satellite galaxies orbiting Milky Way-like systems. Conducted by a collaborative effort from institutions such as the University of Utah, Yale University, and Stanford University, this paper marks the third data release (DR3) of the SAGA Survey, providing valuable insights into the satellite population properties across different galactic environments.

Key Objectives and Methodology

The primary goal of the SAGA (Satellites Around Galactic Analogs) Survey is to build a comprehensive characterization of satellite galaxies around over one hundred Milky Way-mass galaxies. By targeting systems at distances ranging from 25 to 40.75 Mpc, the survey aims to understand satellite dynamics, luminosity functions, and the effect of galactic environment on satellite quenching and morphological evolution. This involves detailed multi-band imaging from the DESI Legacy Surveys combined with spectroscopic data primarily gathered through the 2dF and Hectospec instruments.

Significant Findings

• Satellite Luminosity and Stellar Mass Functions: The survey identifies a noticeable variation in the satellite luminosity and stellar mass functions across different systems, indicating significant intrinsic diversity within the satellite populations. The stellar mass function follows a power law, revealing a prevalence of low-mass satellites, consistent with theoretical predictions within the ΛCDM framework. However, the paper indicates a potential deviation in the relative abundance of quenched satellites in the SAGA systems compared to the Local Group.
• Satellite Quenching: It was noted that SAGA systems tend to have a lower fraction of quenched satellites compared to the Local Group, which may suggest variations in infall times or orbital dynamics affecting star formation. The quenched fraction increases noticeably below a stellar mass of $10^{8.5} M_{\odot}$, revealing a break in the continuity of satellite quenching.
• Radial Distribution and Co-rotating Pairs: The SAGA satellites exhibit a less concentrated radial distribution compared to the Milky Way, attributed potentially to different dynamical histories or environment interactions. The paper did not find significant evidence for the existence of co-rotating satellite pairs within the surveyed systems, which contrasts with findings in the Local Group.
• Correlation with Host and Satellite Properties: A robust correlation is established between the mass of the most massive satellite and the overall number of satellites within a system. This correlation positions the most massive satellites as indicators of host halo properties, which could inform future models correlating dark matter halo characteristics with visible galactic components.

Implications and Future Directions

The SAGA Survey significantly enriches the catalog of known satellite systems and provides a vital observational dataset to test and refine galaxy formation models. These findings challenge existing paradigms about satellite quenching and radial positioning, necessitating a re-evaluation of how mass accretion and host integrations impact satellite morphology and quenched fractions.

Further work will extend towards refining theoretical frameworks like the UniverseMachine to better understand satellite system evolution by incorporating empirical data from SAGA DR3. Additionally, ongoing and planned follow-up studies aim to investigate gas kinematics and metallicity distributions within these satellites to explore environmental quenching mechanisms further.

Overall, this work lays an essential groundwork for future cosmological simulations and observational strategies aimed at uncovering the mechanisms governing satellite galaxy formation and evolution within the broader context of the universe's hierarchical structure.