The Milky Way as an Outlier

How ‘normal’ is the Milky Way? It’s an interesting question because as we look out into a visible universe filled with perhaps 100 billion galaxies, we base many of our models for their behavior on what we know of our own. That this may not be the best way to proceed is brought home by a much smaller study, the comparison between our Solar System and what we’ve been finding around other stars. Finding Solar System analogs has proven surprisingly difficult, although older models assumed outer gas giants and inner rocky worlds as a common pattern.

Thus I am keeping an eye on a survey called Satellites Around Galactic Analogs (SAGA), which is looking into galaxies with smaller satellite galaxies. We’re only in the early days of this survey, with eight galaxies now examined in a new paper from lead author Marla Geha (Yale University). But the goal is 100 galaxies, with 25 of these studied within the next two years.

Image: A three-color optical image of a Milky Way sibling. Credit: Sloan Digital Sky Survey.

Even now, however, the results are intriguing. It turns out that the satellite galaxies of the Milky Way are far more sedate than those in other galactic systems comparable in luminosity and environment. It’s not uncommon for ‘sibling’ galaxy satellites to be producing new stars, but the Milky Way’s satellites are generally inert. Like our Solar System, our galaxy too may have its quirks.

“We use the Milky Way and its surroundings to study absolutely everything,” said Geha, “Hundreds of studies come out every year about dark matter, cosmology, star formation, and galaxy formation, using the Milky Way as a guide. But it’s possible that the Milky Way is an outlier.”

Like the study of exoplanet atmospheres we looked at yesterday, comparative surveys like these are essential for placing what we see around us in a much broader, if not universal context. Thus far SAGA has generated complete spectroscopic coverage within 300 kpc, counting eight Milky Way analogs. The process of choosing ‘analogs’ is detailed and painstakingly recounted in the paper, but the gist of it is that the team looks at a galaxy’s K-band infrared luminosity as a proxy for stellar mass and considers a host of factors related to the galaxy’s halo and its large-scale environment including other nearby galaxies.

Thus far, SAGA has uncovered 25 new satellite galaxies, 14 of which meet the survey’s formal criteria, plus an additional 11 that remain incompletely surveyed. Given that the Sloan Digital Sky Survey had already found 13 satellites among these galaxies, we thus far have 27 satellites around 8 Milky Way analog galaxies that have been subjected to exhaustive analysis.

As to the Milky Way itself, we continue to find what the paper considers ‘faint satellites’ as large-area imaging surveys continue, but the number of bright satellites has remained fixed since the discovery of the Sagittarius dwarf spheroidal galaxy about twenty years ago. Geha and team consider the catalog of bright Milky Way satellites to be largely complete.

The SAGA survey is in its early days, but it is striking that 26 out of the 27 satellite galaxies considered are actively forming stars, unlike both the Milky Way and M31. As the paper notes:

The above results suggest that the satellite population of the Milky Way may not be representative of satellite populations in the larger Universe. Expanding the number of Milky Way analog galaxies with known satellites is required to use these objects as meaningful probes of both cosmology and galaxy formation.

And this is also interesting:

We have characterized complete satellite luminosity functions for 8 Milky Way analog hosts. We find a wide distribution in the number of satellites, from 1 to 9, in the luminosity range for which there are five satellites around the Milky Way. We see no statistically significant correlations between satellite number and host properties, although any correlation would be hard to detect robustly with our small sample size of hosts.

Bear in mind as the SAGA Survey continues that until now, we have based most of our information about satellite galaxies on what we see right here in the Milky Way and in M31. We’re now developing the larger picture that can help us place galaxy formation in context. Finding that even the galaxy we live in is not typical would fit the pattern of recent exoplanet discoveries in suggesting that galaxy as well as planet formation is a deeply stochastic process.

The paper is Geha et al., “The SAGA Survey: I. Satellite Galaxy Populations Around Eight Milky Way Analogs,” accepted at the Astrophysical Journal (preprint).