The SAGA Survey III

The SAGA Survey is a galaxy redshift survey that characterizes satellite systems around Milky Way-mass galaxies at 25 to 40 Mpc, filling in in the regime where we can survey hundreds of MW-mass satellite systems and still obtain a sizable satellite population per system.

We have surveyed 101 Milky Way-mass systems, obtaining spectra for hundreds of objects per square degree within 300 kpc (in projection) of each host galaxy, covering about 85 square degrees of sky. We identify Milky Way-mass hosts using a combination of stellar mass, halo mass, and mild isolation criteria (that still allow Local Group like systems).

Among the 101 systems we have identified 378 satellites, the majority of which have their first redshift measurements by SAGA. This figure shows the satellite galaxy images grouped by satellite systems. Within each system the satellite images are sorted from bright to faint. The number of confirmed satellites per system ranges from 0 to 13. Four of the 101 systems have the same level of survey completeness but do not have any confirmed satellites.

We can now construct the satellite stellar mass functions for 101 Milky Way analogs, providing a clear picture of the system-to-system scatter in the satellite populations. In the mass range the SAGA Survey focuses on, the number of satellites our Milky Way has is average when compared to the SAGA systems. However, most SAGA systems that have a massive satellite (like the LMC) host more satellites than the Milky Way has!

With 101 satellite systems, we can explore what host or system properties correlate most strongly with the satellite abundance. We found the mass of the most massive satellite has the strongest correlation (more to the right) with satellite abundance in the SAGA systems. This correlation is even stronger than the correlation between halo mass and satellite abundance (yes, it sounds surprising, but is actually expected from simulations!)

Among SAGA satellite systems, we find no strong evidence for co-rotating planes of satellites (which, if they exist, should appear as an excess above the horizontal band in this figure). There is, however, a small excess of close satellite pairs (near the right). We also found the satellites in close pairs tend to have a small velocity difference.

The quenched fraction of SAGA satellites increases with decreasing stellar mass, as one would expect. The SAGA satellite quenched fraction is lower than that of the Local Group (except for at LMC-mass scale), but by only about 1σ when considering the system-to-system scatter. In Paper IV (Geha+ 2024) we further explore the star-forming properties of SAGA satellites and correlations between quenched fraction and host galaxy properties.

The satellite abundance observed in SAGA systems is consistent with theoretical predictions based on Lambda-Cold Dark Matter simulations augmented by a galaxy-halo connection model. The model used here, UM-SAGA, is an extension to UniverseMachine and attempts to capture environmental quenching with halo assembly histories. Read more about UM-SAGA and its predictions in Paper V (Wang+ 2024).

In addition to all these cool things that we learn from the SAGA satellite systems, the SAGA Survey obtained about 46,000 redshifts for galaxies that are within the SAGA footprint but not associated with those satellite systems. These redshifts are for galaxies that are faint and low-redshift, and hence can enable a wide range of studies, such as J.F.Wu+ 2022 which uses this sample to train machine learning methods to identify other low-redshift galaxies with photometric data only, and Kado-Fong+ 2024 which uses the SAGA background sample to constrain the mass loading factor.