The SAGA Survey

A spectroscopic galaxy survey aiming to characterize 100 satellite systems around galactic analogs

Yao-Yuan Mao / @yaoyuanmao
Survey Website / Full Paper

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Images of SAGA satellites (as website background)

Survey Design

Click on figures below to enlarge.

Putting the Milky Way in a cosmological context

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.

A survey that fully utilizes multi-object spectroscopy

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.

Efficient target selection

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.

Stage II Science Results

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Census of satellites in 36 Milky Way-like systems

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 (Mr < −12.3) per system ranges from 0 to 9.

127 satellites identified

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.

Satellite Luminosity Function

For the first time, dozens of complete satellite luminosity functions down to Mr = −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.

Satellite Quenched Fraction

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

Co-rotating Plane?

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.

Comparing with ΛCDM prediction

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.


Click on the table below to enlarge.

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.

SAGA Survey Stages

Stage I: Build complete sample of a few Milky Way analogs using simple & conservative cuts.
Stage II: Use data from Stage I to design an efficient targeting strategy.
Stage III: Efficiently measure satellite LF for 100 Milky Way analog to Mr = −12.3.

Public Data

The SAGA Survey Stage II data (hosts and satellites) are publicly available and can be accessed easily with Python and astropy.

Get in touch

Links. Survey Website / Stage I Paper (Geha+ 2017 ApJ) / Stage II Paper (Mao+ 2021 ApJ).

The SAGA Team. Marla Geha (Yale), Yao-Yuan Mao (Rutgers), Risa Wechsler (Stanford), Nitya Kallivayalil (UVa), Ethan Nadler (Stanford), Erik Tollerud (STScI), Ben Weiner (Arizona).

SAGA satellite images are taken from the Legacy Surveys Viewer and based on data from the Dark Energy Survey, DESI Legacy Imaging Surveys, and Sloan Digital Sky Survey.

SAGA Survey was supported by National Science Foundation and Heising-Simons Foundation. Y.-Y. Mao was supported by NASA through the NASA Hubble Fellowship Program.

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