Sombrero Galaxy’s Halo Hints At A Turbulent Past

From new data gathered by NASA’s Hubble Space Telescope, scientists have been able to look beyond the smooth, settled “brim” of the Sombrero galaxy, and into its wilder past. The surprising observations of metal-rich stars in the galaxy’s halo, usually only found in a galaxy’s disk, indicates that its current state may possibly be the result of major galaxy mergers. Although it’s tidy appearance is not the usual messy scene found after recent cosmic collisions.

Located in the constellation Virgo, 28 million light-years away, the near edge-on M104 galaxy gained its “sombrero” nickname due to its prominent central bulge, encircled by thick, spiraling dust lanes. Famed not just for its beauty, the unique morphology of the galaxy has also sparked the curiosity of astronomers. Never quite fitting the traditional definitions of a spiral nor an elliptical galaxy, observations of the “hybrid” Sombrero offer up an insight into how such galaxies may have formed and assembled.

"The Sombrero has always been a bit of a weird galaxy, which is what makes it so interesting," Paul Goudfrooij of the Space Telescope Science Institute (STScI), Baltimore, said in a statement. "Hubble's metallicity measurements (the abundance of heavy elements in the stars) are another indication that the Sombrero has a lot to teach us about galaxy assembly and evolution."

Heavier elements are formed through the stellar “lifecycle” process. Therefore, the longer a galaxy has contained stars generating these elements, the more “metal-rich” new stars forming in those environments will be. Typically, these younger high-metallicity stars are found in the dense stellar populations in the main disk of the galaxy.

Out in the galaxy’s halo, older metal-poor stars are expected to be found. Generated early on in a galaxy’s history, when there were less heavy elements, these stars had been thrown out into the halo from a larger companion star going supernova. Globular clusters, containing old metal-poor stars, also contribute to the stellar population in the halo, when they migrate out of the disk and into the halo.

However, researchers report in a paper published in The Astrophysical Journal the expected star metallicity distribution was not observed in the Sombrero’s halo. "The absence of metal-poor stars was a big surprise," said Goudfrooij, "and the abundance of metal-rich stars only added to the mystery."

To try and solve the mystery, the team used a computer program to simulate events to discover the origin of the unexpected metallicity measurements. Defying expectations once again, the team found that the unperturbed Sombrero had likely undergone major accretion, or merger events billions of years ago. Colliding with one or more similarly massive galaxies that are rich in higher-metallicity stars could account for the measurements in Sombrero.

Although these massive merger events happen elsewhere in the universe, they normally have a more distorted appearance for billions of years, unlike the smooth disk of Sombrero. Our Milky Way galaxy is no stranger to mergers as well, however, the small satellite galaxies that it swallowed up in so-called “minor” accretions, are not on the same scale as Sombrero’s suspected past interactions.

"Hubble's observations of the Sombrero's halo are turning our generally accepted understanding of galaxy makeup and metallicity on its head," co-investigator Roger Cohen of STScI, said. Follow up observations of other “hybrid” galaxies will help determine how this unique group of galaxies formed and assembled over time. Future observatories will also continue to investigate Sombrero’s unexpected properties.