Images of 127 SAGA satellites. Figure 8 of Mao et al. (2021).

So Many Satellites

We identify 127 satellites around 36 SAGA hosts. The total number of satellites (Mr < −12.3) per host ranges from 0 to 9. This number modestly correlates with host galaxy luminosity, and more strongly with the brightest satellite luminosity.

Satellite luminosity functions of 36 SAGA systems. Figure 12 of Mao et al. (2021).

Luminosity Functions

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.

Quenched fraction of SAGA satellites as a function of stellar mass. Figure 11 of Mao et al. (2021).

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.

Distribution of the number of satellites per host, compared with theoretical predictions. Figure 16 of Mao et al. (2021).

Comparision with Theory

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 fit to the Milky Way satellite population.

Demostration of the friends-of-friends sky catalog matching code. Figure 2 of  Mao et al. (2021).

Software & Tools

We developed a friends-of-friends sky catalog matching code, which is publicly available as a standalone Python package. An image list web app based on Legacy Surveys Viewer is also available to facilitate visual target inspection.

SAGA Data Sets

Our Stage II data are available as CSV files: host table, satellite table; or as Google Sheets. You can access these data easily with Python and astropy:

from astropy.table import Table, join
saga_hosts = Table.read('https://sagasurvey.org/data/saga_stage2_hosts.csv')
saga_sats = Table.read('https://sagasurvey.org/data/saga_stage2_sats.csv')
saga_joined = join(saga_sats, saga_hosts, 'INTERNAL_HOSTID', 'left',
                   uniq_col_name='{table_name}{col_name}',
                   table_names=['', 'HOST_'])

If you are interested in other SAGA data that we have not yet made public, please contact SAGA members. If you use SAGA data in your research, please cite Geha et al. (2017, ApJ) and Mao et al. (2021, ApJ), and acknowledge the SAGA Survey:

This research made use of data from the SAGA Survey (sagasurvey.org). The SAGA Survey is a spectroscopic survey with data obtained from the Anglo-Australian Telescope, the MMT Observatory, and the Hale Telescope at Palomar Observatory. The SAGA Survey made use of public imaging data from the Sloan Digital Sky Survey (SDSS), the DESI Legacy Imaging Surveys, and the Dark Energy Survey, and also public redshift catalogs from SDSS, GAMA, WiggleZ, 2dF, OzDES, 6dF, 2dFLenS, and LCRS. The SAGA Survey was supported was supported by NSF collaborative grants AST-1517148 and AST-1517422 and by Heising–Simons Foundation grant 2019-1402.

SAGA Team Members

SAGA Science Goals

Put our Milky Way in Context

Our Milky Way Galaxy is the most well-studied galaxy in the Universe. A particularly informative component of the Milky Way is its system of satellite dwarf galaxies. To interpret the Milky Way satellite system in a cosmological context, it is critical to study a representative sample of systems analogous to the Milky Way. The SAGA Survey aims to determine dwarf galaxy satellite systems around 100 Milky Way analogs down to the brightness of the Leo I galaxy (Mr < −12.3).

Learn about the Galaxy-Halo Connection

Dwarf galaxies provide a key test of cosmological models on small scales. At present most of our information comes only from the satellite dwarf galaxies around the Milky Way and Andromeda galaxies. The SAGA Survey will provide new information about the connection between galaxies and their dark matter halos on the smallest scales.

SAGA Stage I Results

Read Stage I paper (Geha+ 2017) and Stage II paper (Mao+ 2021) on arXiv

In SAGA Stage I, we completed spectroscopic coverage within 300 kpc of 8 Milky Way analogs. We discovered 16 satellite galaxies, in addition to 13 satellites already known in the Sloan Digital Sky Survey, for a total of 29 satellites.

SAGA Stage I Data Sets

SAGA Stage I measured spectroscopic redshifts for 17,344 unique objects, including 15,734 galaxies and 1,610 stars. These spectra can be downloaded as a single FITS file (36.3 MB). Stage II satellite data are also available.

SAGA in Press

"Is the Milky Way an ‘outlier’ galaxy? Studying its ‘siblings’ for clues" - Yale News
"Could we be living in a misfit galaxy?" - Syfy
"Is the Milky Way a normal galaxy?" - EarthSky

Acknowledgements

The SAGA Survey (sagasurvey.org) is a spectroscopic survey with data obtained from the Anglo-Australian Telescope, the MMT Observatory, and the Hale Telescope at Palomar Observatory. The SAGA Survey made use of public imaging data from the Sloan Digital Sky Survey (SDSS), the DESI Legacy Imaging Surveys, and the Dark Energy Survey, and also public redshift catalogs from SDSS, GAMA, WiggleZ, 2dF, OzDES, 6dF, 2dFLenS, and LCRS. The SAGA Survey was supported was supported by NSF collaborative grants AST-1517148 and AST-1517422 and by Heising–Simons Foundation grant 2019-1402.

(If you use SAGA data in your research, please cite Geha et al. (2017, ApJ) and Mao et al. (2021, ApJ), and include the above paragraph in acknowledgement.)


The SAGA team thanks Emily Sandford for suggesting the acronym "SAGA".

The background image on this website shows SAGA satellites, whose 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. We thank Dustin Lang for building the Legacy Surveys Viewer.

Saga References

As part of the SAGA Survey, we have nicknamed each Milky Way analog system after a true saga.