Halo of the Sombrero Galaxy

The majestic Sombrero galaxy (M104) is a bright enigma, a hybrid that conforms to neither a spiral nor elliptical structure. It's also alone in the sky, not embedded inside a cluster of other galaxies. How did it get this way? Astronomers used NASA's Hubble Space Telescope to investigate the history of this unusual galaxy, resolving tens of thousands of individual stars in the galaxy's vast halo, and measuring their metallicity — the presence of heavy elements — which can provide forensic clues to the galaxy's origin.

On the left is an image of the entire galaxy that includes a portion of the much fainter halo far outside its bright disk and bulge. Hubble photographed two regions in the halo (one of which is shown by the white box). The images on the right zoom in to show the level of detail Hubble captured. The orange box, a small subset of Hubble's view, contains myriad halo stars. The stellar population increases in density closer to the galaxy's disk (bottom blue box). Each frame contains a bright globular cluster of stars, of which there are many in the Sombrero's halo.

The data gathered by Hubble surprised scientists, upending expectations set by the halos of other massive galaxies. The Sombrero's halo contained more metal-rich stars than expected, but even stranger was the near-absence of old, metal-poor stars typically found in the halos of massive galaxies. Many of the globular clusters, however, contain metal-poor stars — why were they there, but not in the broader halo? Typically, stars migrate away from their parent clusters as they age. A possible explanation for the Sombrero's perplexing features is that it is the product of the merger of massive galaxies billions of years ago, even though the smooth appearance of the galaxy's disk and halo show no signs of such a huge disruption.