On the left is an image of the entire galaxy that includes a portion of the much fainter halo far outside its bright disc 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 disc (bottom blue box). Each frame contains a bright globular cluster of stars, of which there are many in the Sombrero’s halo.
The new data 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 contain metal-poor stars, however, raising the question of why they were there but not in the broader halo. Typically, stars migrate away from their parent clusters as they age.
A possible explanation 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 disc and halo show no signs of such a huge disruption.