The SAGA Survey

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Key Findings of the SAGA Survey

The SAGA Survey aims to advance our understanding of the universe through the detailed study of low-mass satellite galaxies around systems similar to the Milky Way. These galaxies provide critical insights into galaxy formation and evolution and to how our Milky Way fits in to the broader universe.
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An image of a Milky Way-like galaxy and its system of satellite galaxies. The SAGA survey identified six small satellite galaxies in orbit around this Milky Way analog.
Credit: Yasmeen Asali (Yale), with images from the DESI Legacy Survey Sky Viewer

Satellites are smaller galaxies in both mass and size which orbit around larger, host galaxies. Just as smaller satellites orbit the Earth, these satellite galaxies can be captured by the gravitational pull of massive host galaxies and fall into the host's dark matter halo.

The SAGA Survey has identified several hundred satellite galaxies orbiting Milky Way-mass host galaxies, constructing a detailed picture of satellite populations across 101 systems similar to our own galaxy. Our data reveal many things about the characteristics of these systems! We find that environmental factors play a crucial role in shaping the life cycles of dwarf galaxies. For example, galaxies closer to their host are more likely to have their star formation suppressed or quenched. This supports theories that suggest environmental influences are significant in determining the evolutionary path of galaxies.

Additionally, we find a correlation between the most massive satellites and the total number of satellites in these systems -- systems that have more massive satellites similar to the LMC, the largest satellite of our Milky Way, tend to have more satellites overall. This correlation indicates that the way low-mass galaxies infall into the gravitational wells of more massive hosts as these systems form may depend on more than just the host galaxy mass.

Can you spot the satellites?

Among the galaxies shown below, two are satellites and the rest are background galaxies. Can you guess which ones they are?

SAGA Guessing Game

Our Milky Way in Context

A detailed view of the Milky Way
The Milky Way Galaxy. Credit: NASA/ESA

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. However, the Milky Way itself represents just one instance of what a Milky Way system may look like in the universe. As a result, it is critical to study a representative sample of systems analogous to the Milky Way to understand how these systems form and evolve. Having identified complete satellite populations for 101 Milky-Way mass hosts, the SAGA Survey is uniquely suited to address the question of whether the Milky Way system we live in is typical!

The SAGA Survey results indicate that the Milky Way’s satellite population is a unique combination of older satellites that have ceased star formation and newer, active ones, the LMC and SMC, that only recently fell into the Milky Way's dark matter halo.

Our findings suggest that the Milky Way's satellite system, while typical in many ways, differs from the average SAGA Milky Way-like system in terms of the star forming properties of it's satellites. Most of the satellites of the Milky Way have ceased forming stars, unlike many of the SAGA systems where we find large numbers of star forming satellites. However, the number of Milky Way satellites is consistent with the distribution of SAGA systems. These results indicate that the Milky Way's satellite population is a combination of two groups: a collection of older, lower-mass satellites and a recently acquired pair of massive, star-forming satellites (the LMC and SMC).

Exploring the Galaxy-Halo Connection

Simulation Image
Dark matter halos and cosmic web in a simulated universe. Credit: Ralf Kaehler/SLAC National Accelerator Laboratory

According to our current understanding of cosmology, all galaxies form in regions of space where there is an overdensity of dark matter, referred to as a dark matter halo. These dark matter halos will grow and evolve alongside the galaxies they contain, and this galaxy-halo connection can provide key constraints on cosmological models. Dwarf galaxies in particular provide a crucial test of how cosmological models perform at small scales; however, at present, most of our constraining observational information comes only from the satellite dwarf galaxies around the Milky Way and Andromeda galaxies.

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