(Originally published by KIPAC)
Around the Milky Way Galaxy orbit nearly two dozen known small satellite galaxies that have wide a range of sizes, all of which are much smaller than the Milky Way itself. Like miniature replicas of their big cousin, these 'dwarf' satellite galaxies are systems of stars where the total mass and gravitational behavior is dominated by a concentration of unseen dark matter. Theoretical simulations on computers and models of dark matter structure formation indicate that these satellite galaxies should indeed form, but with a hitch.
Given what we know about dark matter as inputs, these models predict that there should be dozens more faint satellite galaxies around the Milky Way than are observed. What happened to these faint dwarfs? In spite of the mismatch between the theory and data on satellite galaxies, there had always been a relatively easy way out of this dilemma. It relied on our Milky Way not being typical amongst galaxies of its kind. Previous work by KIPAC scientists has determined that the Milky Way is in fact anomalous in having two relatively large satellites such as the Large and Small Magellanic Clouds. But is it also anomalous in having a small number of fainter satellites?
KIPAC postdoc Louie Strigari and Professor Risa Wechsler set out to test this by cataloging the abundance of dwarf satellites around other galaxies. They use data from relatively nearby galaxies in the Sloan Digital Sky Survey Data Release 8, searching the images of galaxies for nearby, much smaller ones. This procedure is complicated because in a telescope image the three-dimensional Universe is projected onto a two-dimensional surface, so any given apparently small or faint point near a relatively nearby galaxy could be a dwarf satellite galaxy or it could me a more distant, larger galaxy. The best way to determine whether two objects that appear nearby in an image are actually nearby to each other is to see if they have the same redshift, indicating that they are a similar distance from us.
Strigari and Wechsler used statistical techniques combining several redshift estimation methods. Spectroscopy can measure redshifts directly, but is only available for a limited number of the brightest objects, which would lead to a very biased sample. Photometric redshift estimation techniques infer the objects' redshifts from their broadband fluxes, but are prone to errors and uncertainty. In combining objects with spectroscopic redshifts and different photometric redshift estimation measures, they were able to achieve a more robust determination of the typical numbers of satellite galaxies.
It turns out that in this regard we're not special after all. In terms of its faintest population of satellite galaxies, the Milky Way is not too different than the rest of the galaxies like it in the Universe. Strigari and Wechsler determine that the mean number of dwarf galaxies per large galaxy is consistent with the number observed around our Milky Way. This means that the interplay between observations and theoretical models of dark matter will have to continue, as our current understanding of the expected dark matter distribution in the cosmos cannot explain the observed data on dwarf satellites. One fascinating possibility is that the more abundant dark matter satellites predicted by simulations have in fact formed, but the light from the first, luminous stars pushed away the gas that could potentially form future stars, leaving dark 'ghost' satellites. Further observations will be needed to explore this and other scenarios.