New observations of the large spiral galaxy M81, conducted with the Subaru Telescope’s Hyper Suprime-Cam, has allowed researchers to demonstrate hierarchical galaxy assembly processes in a nearby galaxy located outside of the Local Group for the first time.
The observations, which were part of a galactic archaeology study led by researchers from the Shanghai Astronomical Observatory and the National Astronomical Observatory of Japan, used the telescope’s prime-focus camera to observe M81 and two of its brightest neighbors, M82 and NGC3077, using deep, super-wide field images of the galaxies and their stars.
“Until now, the outskirts of Milky Way and Andromeda Galaxy are the only places which have been surveyed to sufficiently faint depths and wide areas to enable detailed tests of the hierarchal galaxy assembly process on these scales,” study author Dr. Sakurako Okamoto of the Shanghai Astronomical Observatory told redOrbit via email.
“With such a small sample, it is not possible to determine which behavior is representative or to discount the idea that one of these systems may be anomalous in their properties. Therefore, M81 is a prime target as the third case,” added Dr. Okamoto.
Young star, neutral hydrogen distributions linked
They discovered that the spatial distribution of the young stars in these galaxies closely followed that of their distribution of neutral hydrogen. The study will help astronomers to refine their understanding of galaxy formation and evolution, particularly those models based on the concept that galaxies develop from small “overdensities” and become larger-scaled structures.
Known as hierarchical galaxy assembly, this model has been applied to both the Milky Way and to the Andromeda Galaxy, demonstrating that these galaxies originally formed as part of a local over-density in the primordial matter distribution (the earliest matter accumulations known in the young universe). Over time, they grew through the collection of smaller building blocks, some of which might have survived later mergers to become present-day dwarf satellite galaxies.
According to Dr. Okamoto’s team, establishing the presence and nature of these satellite galaxies and determining the large-scale structure and stellar content of halos within them is necessary to better understand and explain the physics of hierarchical galaxy assembly. Over the past 10 years astronomers conducting large photometric surveys (measuring light intensity of celestial objects) have discovered several new satellite galaxies, over-densities, and stellar streams surrounding the Milky Way and Andromeda galaxies.
Researchers study the properties of stars in these systems to reconstruct the stellar contents found in early-stage galaxies, a field known as “galactic archeology” or “near-field cosmology”. In this type of research, it’s necessary to resolve individual stars in a galaxy across a sizable percentage of the galaxy’s radius, but until now, only the outskirts of the Milky Way and Andromeda have been the only areas analyzed deeply enough to allow for in-depth tests of the hierarchical galaxy assembly process across a vast scale, they added.
Demonstrating how galaxies cannibalize smaller neighbors
By using the Hyper Suprime-Cam (HSC) to study the archeology of M81, a galaxy located some 11.7 million light years away, the researchers were able to observe out to a projected radius of a half-million light-years from the center of the spiral galaxy. The sensitivity of the camera made it possible for the researchers to observe old red giant branch (RGB), young main-sequence (MS) stars, red supergiants, and asymptotic giant-branch stars at the distance of M81.
Their study revealed that the bright stars were primarily focused in the inner disk of M81 while most of the young stars in outlying concentrations were fainter and had luminosity distributions similar to those of the stellar stream between M81 and NGC 3077. The stars are between 30 and 160 million years old, and the research indicates that these systems were most likely produced by recent tidal interactions between M81, M82, and NGC 3077.
Furthermore, the astronomers found that the distribution of RGB stars revealed that the extended stellar halos of these three galaxies overlap one another, and that M82 and NGC 3077 have outer regions that are highly perturbed, likely due to the recent gravitational encounter. They also were able to discover that the RGB stars in the outer halo of M82 were bluer in color than those in M81 or the inner halo of M82, indicating that they contained less metal.
“Our results show in detailed that how galaxies capture and cannibalize their smaller neighbors and how large galaxies like the Milky Way have formed and evolved over time,” Dr. Okamoto told redOrbit. “People can use it to infer the hierarchal galaxy assembly at the nearby and distant universe, and trace how galaxies evolve.”
(Image credit: NAOJ/HSC Project)
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