Messier 104, a lenticular galaxy located 28 million light-years away in the constellation Virgo, now has a new chapter in its story — an extended halo of metal-rich stars with barely a sign of the expected metal-poor stars that have been observed in the halos of other galaxies.
This Hubble image shows Messier 104. Image credit: NASA / ESA / Hubble Heritage Team / STScI / AURA.
“Also known as the Sombrero galaxy, Messier 104 has always been a bit of a weird galaxy, which is what makes it so interesting,” said Dr. Paul Goudfrooij, an astronomer with the Space Telescope Science Institute and a member of the team who observed the galaxy using the NASA/ESA Hubble Space Telescope.
“Hubble’s metallicity measurements (i.e., the abundance of heavy elements in the stars) are another indication that Messier 104 has a lot to teach us about galaxy assembly and evolution,” he added.
“The observations of the Sombrero’s halo are turning our generally accepted understanding of galaxy makeup and metallicity on its head,” said Dr. Roger Cohen, also from the Space Telescope Science Institute.
The astronomers, puzzling over the new data from Hubble, turned to sophisticated computer models to suggest explanations for the perplexing inversion of conventional galactic theory.
The results suggest the equally surprising possibility of major mergers in the galaxy’s past, though Messier 104’s majestic structure bears no evidence of recent disruption.
“The absence of metal-poor stars was a big surprise, and the abundance of metal-rich stars only added to the mystery,” Dr. Goudfrooij said.
On the left is an image of Messier 104 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 galaxy’s halo. Messier 104’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. A possible explanation for Messier 104’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. Image credit: NASA / Digitized Sky Survey / P. Goudfrooij, STScI / Hubble Heritage Team / STScI / AURA.
In a galaxy’s halo astronomers expect to find earlier generations of stars with less heavy elements, called metals, as compared to the crowded stellar cities in the main disk of a galaxy.
Elements are created through the stellar lifecycle process, and the longer a galaxy has had stars going through this cycle, the more element-rich the gas and the higher-metallicity the stars that form from that gas.
These younger, high-metallicity stars are typically found in the main disk of the galaxy where the stellar population is denser — or so goes the conventional wisdom.
Complicating the facts is the presence of many old, metal-poor globular clusters of stars. These older, metal-poor stars are expected to eventually move out of their clusters and become part of the general stellar halo, but that process seems to have been inefficient in Messier 104.
The astronomers compared their results with recent computer simulations to see what could be the origin of such unexpected metallicity measurements in the galaxy’s halo.
The results also defied expectations, indicating that the unperturbed Messier 104 had undergone major accretion, or merger, events billions of years ago.
Unlike our Milky Way Galaxy, which is thought to have swallowed up many small satellite galaxies in so-called ‘minor’ accretions over billions of years, a major accretion is the merger of two or more similarly massive galaxies that are rich in later-generation, higher-metallicity stars.
The satellite galaxies only contained low-metallicity stars that were largely hydrogen and helium from the Big Bang.
Heavier elements had to be cooked up in stellar interiors through nucleosynthesis and incorporated into later-generation stars.
This process was rather ineffective in dwarf galaxies such as those around our Milky Way, and more effective in larger, more evolved galaxies.
The results for Messier 104 are surprising because its smooth disk shows no signs of disruption.
By comparison, numerous interacting galaxies, like the iconic Antennae galaxies, get their name from the distorted appearance of their spiral arms due to the tidal forces of their interaction.
Mergers of similarly massive galaxies typically coalesce into large, smooth elliptical galaxies with extended halos — a process that takes billions of years. But Messier 104 has never quite fit the traditional definition of either a spiral or an elliptical galaxy. It is somewhere in between — a hybrid.
The study was published in the Astrophysical Journal.
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Roger E. Cohen et al. 2020. The Strikingly Metal-rich Halo of the Sombrero Galaxy. ApJ 890, 52; doi: 10.3847/1538-4357/ab64e9
This article is based on text provided by the National Aeronautics and Space Administration.
