An article published in “The Astrophysical Journal” reports a study on the Sombrero Galaxy and in particular on its halo. A team of researchers used the Hubble space telescope, finding a surprising abundance of stars rich in heavy elements, and computer simulations to try to understand the origin of this galaxy difficult to catalog because it’s a hybrid between the elliptical and the spiral shapes. The conclusion is that its growth occurred from the merging of two or perhaps even more massive galaxies in a chaotic process.
Between 29 and 31 million light years away, the Sombrero Galaxy, also known by catalog names among which M104 and NGC 4594 are the most common, is well known to astronomers since it was discovered in 1781 by the French astronomer Pierre Méchain. The ring of dust that surrounds the nucleus of this galaxy is its most visible structure but, despite the many observations made over time with increasingly sophisticated instruments, astronomers struggled to offer a theory that explains the strange shape that determined its nickname. This new research offers a solution to this mystery.
The image (NASA, Digital Sky Survey, P. Goudfrooij (STScI) and The Hubble Heritage Team (STScI/AURA)) shows on the left the whole Sombrero Galaxy and on the right some zooms: in the white box a region of the halo, in the orange box a small part of the Hubble view with halo stars, and in the blue box a densely populated area of stars close to the galactic disk.
The observations of the Sombrero Galaxy with the Hubble Space Telescope brought a surprise because in its halo astronomers expected to find old generations of stars poor in metals – which means heavy elements – compared to its main disk, where generally younger stars are found that are richer in metals. Instead, metal-poor stars are absent while metal-rich ones are present in abundance. There’s also the presence of many globular clusters where there are metal-poor old stars. They’re supposed to slowly move out of those clusters to become part of the halo but this process seems inefficient in the Sombrero Galaxy.
The researchers performed computer simulations to try to understand the reasons for that high metallicity, which means the abundance of metals, in the Sombrero Galaxy’s halo. The surprising scenario that matches the data obtained indicates that the strange characteristics observed are due to big mergers, which occurred perhaps a few billion years ago. The Milky Way probably grew over time swallowing dwarf galaxies, but the Sombrero Galaxy probably went through the merger of galaxies with similar masses rich in high-metallicity stars.
This result is surprising because the Sombrero Galaxy’s shape is strange but it’s regular while the galaxies that went through important merger processes have distorted shapes even after a long time. Mergers of galaxies with similar masses usually lead to a large elliptical galaxy with an extended halo but the Sombrero Galaxy has a shape that looks like a hybrid between an elliptical and a spiral shape.
The researchers hope to carry out further research on the surprising characteristics of the Sombrero Galaxy with the space telescopes that will enter service in the coming years: the Wide Field Infrared Survey Telescope (WFIRST) and the James Webb. This galaxy’s shape makes it particularly recognizable, but astronomers are realizing that it’s an intriguing object of study for a number of reasons.
