Figure 1: Pseudo-color images from HSC observation
which contains M81, M82, and NGC 3077. Diameter of the FOV is 1.5
degrees. Bottom-left: close-up of M81. Bottom-center: further close-up
of M81 showing the spiral arm. Bottom-right: color composite of the
images used for the analysis. Click to enlarge each frame. (Credit:
NAOJ/HSC Project)
Movie: Neighborhood of the spiral galaxy M81
Credit: NAOJ/HSC Project)
Cosmological archaeological studies such as this one help astronomers
refine their understanding of galaxy formation and evolution. The
currently favored cosmological galaxy models are based on the idea of
hierarchical structure formation: that structures in the universe such
as galaxies develop from small "overdensities" to become large-scale
objects. For example, the Milky Way and M81 first formed as part of a
local over-density in the primordial matter distribution – that is, the
earliest accumulations of matter in the young universe. They grew over
time via the agglomeration of numerous smaller building blocks, some of
which may have survived later mergers to become present-day dwarf
satellite galaxies. Establishing the presence and nature of these
satellites, and determining the large-scale structure and stellar
content of halos in spiral galaxies, is essential to understand and
explain the physics of hierarchical galaxy assembly.
Over the last decade, astronomers doing large photometric surveys
(that is, measuring the light intensities of celestial objects) have
found a number of new satellite galaxies, stellar streams, and
over-densities around the Milky Way and the Andromeda galaxies. The
detailed properties of stars in these systems are studied to reconstruct
the stellar contents of galaxies in the early stage, which is called
"Galactic Archeology" or "near-field cosmology". For the Galactic
Archeology study, it is necessary to resolve individual stars in a
galaxy, and observe across a good fraction of the galaxy's radius.
Until now, the outskirts of the Milky Way and Andromeda are the only
places that have been surveyed to sufficiently faint depths to enable
detailed tests of hierarchical galaxy assembly process across wide
scales.
The observing team started the M81 archeology study by using Hyper
Suprime-Cam (HSC). M81, also known as Bode's Galaxy, is located at a
distance of 11.7 million light-years, and is one of the nearest massive
spiral galaxies similar to the Milky Way. The super-wide field of view
of the HSC allowed the team to observe out to a projected radius of a
half-million light-years from the center of M81. The field includes 18
known member galaxies of the M81 group in only seven pointings. The
camera's high sensitivity enabled the team to observe vast numbers of
old red giant branch (RGB) stars as well as young main-sequence (MS)
stars, red supergiants, and asymptotic giant-branch stars at the
distance of M81.
The left panel in Figure 2
shows the spatial distribution of young MS stars and core
helium-burning stars, which are color-coded according to their i-band
luminosity. Bright stars are mainly located in the inner disk of M81,
while most of the young stars in outlying concentrations are fainter
than i=24 mag and have similar luminosity distributions as that of the
stellar stream between M81 and NGC 3077. They are between 30-160 million
years old. The study indicates the ages of stars in these tidal
features are synchronized to each other, and that these systems were
produced by recent tidal interactions between M81, M82 and NGC 3077.
Figure 2: Young main-sequence (MS) stars and red-giant
branch (RGB) stars around M81, M82, and NGC 3077. Left: yellow is
brighter stars, and blue is fainter stars. Right: color-coded for the
metallicity, namely yellow is metal rich, blue is metal poor. Solid line
shows the R25 radius of the galaxy measured in the visible light.
(Credit: NAOJ)
The distribution of RGB stars in the right panel of Figure 2
shows that the extended stellar halos of the three main galaxies
overlap each other, and that the outer regions of M82 and NGC 3077 are
highly perturbed. This is likely a consequence of the recent
gravitational encounter.
The color of each point in the figure is a rough proxy for
metallicity. The RGB stars in M82's outer halo have significantly bluer
colors, showing that they are more metal-poor than those in M81, the NGC
3077 halos and the inner halo of M82. The satellite galaxies, KDG 61,
BK5N, and IKN cannot be seen in the maps of young stars, but appear as
over-densities of old populations in the right panel.
This implies they
are not the product of the recent interaction between M81, M82 and NGC
3077.
The science team for this study consists of astronomers at Shanghai
Astronomical Observatory, National Astronomical Observatory of Japan,
Hiroshima University, University of Edinburgh, and University of
Cambridge. Their first results from the M81 study with Suprime-Cam on
Subaru Telescope were released in March 2010 at: (http://www.subarutelescope.org/Pressrelease/2010/03/18/index.html).
Team member Dr. Sakurako Okamoto (Shanghai Astronomical Observatory)
commented on this program: "Our deep panoramic view of the M81 group
demonstrates that the complexity long known to be present in neutral
hydrogen (HI) is equally matched in the low surface brightness stellar
component. Together with the Galactic Archeology study based on the HSC
wide-field survey of the Subaru Strategic Program, we hope to establish
the presence and nature of satellite galaxies, and determine the
large-scale structure and stellar content of halos of spiral galaxies in
general".
The team members are grateful to the entire staff at Subaru Telescope
and the HSC team. They acknowledge the importance of Maunakea within
the indigenous Hawaiian community.
The research paper titled "A Hyper Suprime-Cam View of the
Interacting Galaxies of the M81 Group" will be published in the
Astrophysical Journal Letters. This work was supported by the grants of
CAS (XDB09010100), NSFC (11333003), and JSPS (Grant-in-Aid for Young
Scientists B, 26800103).
Members of the research team:
- Sakurako Okamoto: Shanghai Astronomical Observatory, China
- Nobuo Arimoto: Subaru Telescope, National Astronomical Observatory of Japan/SOKENDAI (The Graduate University for Advanced Stuties), Japan
- Yoshihiko Yamada: Subaru Telescope, National Astronomical Observatory of Japan
- Yosuke Utsumi: Hiroshima Astrophysical Science Center, Hiroshima University, Japan
- Annette Ferguson: Institute for Astronomy, University of Edinburgh, Royal Observatory, UK
- Edouard Bernard: Institute for Astronomy, University of Edinburgh, Royal Observatory, UK
- Mike Irwin: Institute of Astronomy, University of Cambridge, UK
Source: Subaru Telescope
