The SAGA Survey II

The SAGA Survey is a galaxy redshift survey that will measure distance for satellite galaxies around 100 Milky Way-like galaxies at 25 to 40 Mpc, characterizing these satellite system down to r = 20.75.

For each central galaxy, we survey its neighboring sky area with a radius of 300 kpc (about half degree), which corresponds to the virial radius of its dark matter halo.

The main challenge of the SAGA Survey is the sheer number of background galaxies (yellow). Our Stage I effort only used a conservative color cut, which resulted in a large number of training data (blue) and enabled us to develop a much more efficient target selection strategy (green) based on color and surface brightness. We can now efficient select very low-redshift galaxy candidates (z < 0.015; see histogram) and obtain redshifts to identify satellites (purple). We continue to obtain redshifts for galaxies outside our primary targets in a discovery mode.

In our Stage II result, we have completed the census (high spectroscopic coverage for primary targets) for 36 Milky Way-like systems. Plot shows the spatial distribution of the satellites in each system. The total number of satellites (M_r < −12.3) per system ranges from 0 to 9.

Among these 36 systems, we have identified 127 satellites, majority of which did not redshift measurement before SAGA. Plot shows the images of all 127 satellites, grouped by the system. Within each system the satellite images are sorted from bright to faint. The red triangle indicates quenched (quiescent) satellites.

For the first time, dozens of complete satellite luminosity functions down to M_r = −12.3 of Milky Way analogs have been measured. The satellite luminosity function of the Milky Way is consistent with being drawn from the same distribution as the SAGA systems. Satellite number correlates strongly with the brightest satellite luminosity.

The quenched fraction of SAGA satellites increases with decreasing stellar mass: 2/50 bright satellites are quenched (M_r < −16, similar to LMC/SMC) while 16/55 faint satellites (−16 < M_r < −12.3) are quenched; this quenched fraction is lower than that in the Local Group!

Among SAGA satellite systems, we find no evidence for co-rotating planes of satellites (if exist, should appear as an excess in the plot). There exists a slight hint of satellite groups (nearby satellites sharing similar velocities). However, current number of satellites is still too small to be conclusive.

SAGA-measured total satellite number, satellite luminosity functions, and radial distributions are all largely consistent with predictions based on a ΛCDM + galaxy-halo connection model (developed by Ethan Nadler) fit to the Milky Way satellite population.

We have more than more than quadrupled the sample size thanks to improved software infrastructures, deep photometric imaging catalogs, and target selection strategy. As of today, we have more than 80 SAGA systems completed! We also apply our efficient target selection methods to finding very low-redshift galaxies with spare fibers of the S⁵ and DESI surveys.