Feb 21 2020
NASA’s Hubble Space Telescope has revealed a new, surprising data, suggesting that the settled and smooth “brim” of the disk of Sombrero galaxy disk may be hiding a chaotic past.
On the left is an image of the Sombrero galaxy (M104) 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. 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. 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. Image Credit: NASA/Digitized Sky Survey/P. Goudfrooij (STScI)/The Hubble Heritage Team (STScI/AURA).
The sensitivity and sharpness of the Hubble Space Telescope resolve an innumerable number of individual stars existing in the Sombrero’s large and extended halo—the area beyond a central part of a galaxy, usually composed of older stars.
These recent observations of the Sombrero galaxy are becoming traditional theory on its head and demonstrate just a small part of older and metal-poor stars existing in the halo. The observations also show an abundance of metal-rich stars that are unanticipated and are usually found only in the central bulge and in a galaxy’s disk.
One possible explanation is the previous mergers of the major galaxy, although the stately Sombrero galaxy does not display any sign of the messy proof of the latest merger of giant galaxies.
The Sombrero has always been a bit of a weird galaxy, which is what makes it so interesting. Hubble’s metallicity measurements (i.e., 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.
Paul Goudfrooij, Scientist, Space Telescope Science Institute
The Space Telescope Science Institute (STScI) is located in Baltimore, Maryland.
“Hubble’s observations of the Sombrero’s halo are turning our generally accepted understanding of galaxy makeup and metallicity on its head,” stated co-investigator Roger Cohen from STScI.
The Sombrero galaxy (M104) has long been a favorite of amateur skywatchers and astronomers for its curious structure and vivid beauty. Now, this galaxy has a new chapter in its unusual story—that is, a massive halo of metal-rich stars that have hardly a sign of the anticipated metal-poor stars—stars that have been viewed in the halos of other kinds of galaxies.
Intrigued over the data from Hubble Space Telescope, scientists turned to advanced computer models to propose explanations for the confusing inversion of traditional galactic theory.
Such outcomes indicate a similarly unexpected possibility of considerable mergers in the galaxy’s history, even though the regal structure of the Sombrero galaxy does not show any proof of the latest disruption. These extraordinary findings and potential explanations were published in the Astrophysical Journal.
The absence of metal-poor stars was a big surprise, and the abundance of metal-rich stars only added to the mystery.
Paul Goudfrooij, Scientist, Space Telescope Science Institute
In the halo of a galaxy, astronomers expect to identify previous generations of stars that have less heavy elements, known as metals, as compared to the packed stellar cities present in a galaxy’s main disk.
The stellar “lifecycle” process is responsible for creating elements. Within a galaxy, if stars go through this cycle for a longer time, the gas will be richer in elements and the stars that develop from that gas will have higher metallicity. Such high-metallicity and younger stars are often present in the galaxy’s main disk where the stellar population is thicker—or so goes the traditional wisdom.
But the presence of several older and metal-poor globular clusters of stars complicates the facts. Older and metal-poor stars like these are believed to ultimately shift away from their clusters and become part of the common stellar halo. But it seems that the process has not been efficient in the Sombrero galaxy.
The researchers then compared their outcomes with the latest computer simulations to check the nature of the origin of such unanticipated metallicity measurements within the halo of the galaxy.
The outcomes also disregarded expectations and indicated that the undisturbed Sombrero galaxy has experienced significant merger, or accretion, events billions of years ago.
The Milky Way galaxy is believed to have consumed many numbers of tiny satellite galaxies in what are known as “minor” mergers across billions of years. By contrast, a major accretion refers to the merger of two or more analogously giant galaxies that have an abundant number of later-generation, higher-metallicity stars.
Only low-metallicity stars were present in the satellite galaxies and these were mostly helium and hydrogen from the big bang. It was necessary to cook up heavier elements in stellar interiors via nucleosynthesis and integrate them into later-generation stars. This was a rather ineffective process in tiny galaxies like those around the Milky Way galaxy, but more effective in more evolved and larger galaxies.
The Sombrero results are unexpected because its smooth and settled disk does not show any sign of disruption. On the other hand, many interacting galaxies, such as the iconic Antennae galaxies, were named so from the twisted appearance of their spiral arms caused by the tidal forces of their interaction.
Usually, mergers of analogously giant galaxies coalesce into smooth and large elliptical galaxies that have extended halos. This process takes billions of years. However, the Sombrero galaxy has never quite fit the conventional definition of either an elliptical or a spiral galaxy and is somewhere in between—in other words, it is a hybrid.
For this specific project, the researchers selected the Sombrero galaxy mostly for its unusual morphology. The team wanted to determine how such kinds of “hybrid” galaxies could have emerged and assembled over time.
Subsequent research on halo metallicity distributions will be carried out with many other galaxies at distances analogous to that of the Sombrero galaxy. The researchers are looking forward to upcoming observatories pursuing the investigation into surprising properties of the Sombrero galaxy.
The Wide-Field Infrared Survey Telescope (WFIRST) has a field of view 100 times that of the Hubble Space Telescope. It will be able to capture a continuous image of the halo of the galaxy and, at the same time, pick up additional stars in infrared light.
In addition, the James Webb Space Telescope will also be useful for its resolution and deeper infrared sensitivity just like the Hubble Space Telescope.