GALEX Team, CalTech, NASA
Larger ultraviolet image.
This ultraviolet image highlights
a 150,000-ly-wide ring of young
and hot, blue stars that surrounds
Andromeda's central bulge (more
from APOD and GALEX).
On October 18, 2006, astronomers using NASA's Spitzer Space Telescope announced the discovery of two dust rings (or "holes") in Andromeda's dust disk using infrared light that provide evidence of an ancient head-on collision with neighboring dwarf galaxy along its polar axis Messier 32 (M32) some 210 million years ago. Computer simulations support the hypothesis that the passage of the much smaller galaxy created violent waves of gravitational interactions that left rings of gas and dust propagating outward from the site of the impact. Since Andromeda is much more massive than M32, the larger galaxy was not substantially disrupted, but M32 lost more than half its initial mass in the course of the collision (more).
Pauline Barmby, CfA, JPL, NASA -- larger infrared image
Holes in Andromeda's disk may be from an ancient collision with satellite galaxy M32 (more).
Active Galactic Nucleus
STScI, NASA
Larger image.
The central 30 light-years of Andromeda
contains two galactic nuclei, which suggests
that the great spiral consumed a major
galactic companion whose substance has
been mostly merged except for its central
core (more from APOD and STScI).
In the 1990s, astronomers using the Hubble Space Telescope found that Andromeda has a nucleus with a double structure. The "nuclear hot-spots" are located close together, considering that the galaxy's spiral disk has been estimated to be anywhere from 150,000 to more than 200,000 ly across while the observed central area measures only around 30 ly wide. Subsequent ground-based observations led some astronomers to speculate that two galactic nuclei do indeed exist, are moving with respect to each other, and that one nucleus is slowly disrupting the other through tidal forces. As a result, some astronomers believed that that one nucleus may be the remains a smaller satellite galaxy that was "eaten" by Andromeda (Corbin et al, 2001; Gerssen et al, 1995; and Lauer et al, 1993). In 2005, astronomers using the Hubble Space Telecope announced that the two two bright blobs are actually composed of an elliptical ring of older red stars and a smaller, brighter, and denser disk of young blue stars of around 200 million years old around the galaxy's central black hole (NASA press release).
Michael Garcia, Stephen Murray,
Palomar Sky Survey
Larger image.
The black square in the center
of Andromeda's spiral disk has
been observed with x-rays to
reveal a supermassive black
hole, as as well as smaller
ones (more from CXC).
Andromeda's core has a supermassive central black hole of around 140 million Solar-masses (latest NASA press release). Recent observations with the Chandra X-Ray Observatory also reveal numerous other bright X-ray sources, most of which are probably due to binary systems where a star is feeding gas into a neutron star and black hole. A very cool X-ray source has been identified about 10 light years south of the galactic center. A second, hotter X-ray source was found to be at a position consistent with the position of the super-massive black hole.
Garcia et al, 2001;
T. Brown et al, 2001;
CXC, SAO, NASA
Larger x-ray image.
The blue dot is an unusually "cool"
million degree X-ray source that
lies just below an black hole
(yellow) that may be X-ray bright
from matter swirling toward a
supermassive black hole of 30
million Solar-masses (more
from CXC).
Andromeda's satellite (or "companion") galaxies include M32 and M110, two bright dwarf elliptical galaxies that are the brightest of a swarm of smaller companions. By late 1999, however, at least 10 satellite galaxies of Andromeda were known, including NGC 185 (which was discovered by William Herschel), and NGC 147 (discovered by Heinrich Ludwig d'Arrest, 1822-1875) as well as the very faint dwarf systems And I, And II, And III, possibly And IV (which may be a cluster or a remote background galaxy), And V, And VI (also called the Pegasus dwarf), and And VII (also the Cassiopeia dwarf).
Satellite galaxy M32 may be interacting to distort the disk structure of Andromeda itself, whose spiral arms of neutral hydrogen are displaced from those consisted of stars by around 4,000 light-years and so cannot be continuously followed in the area closest to its smaller neighbor. Computer simulations have shown that such disturbances can be modelled by assuming a recent close encounter with a small companion of the mass of M32, which also suggest M32 has lost many stars from such an encounter to be spread out in Andromeda's halo.
-
Using the Hubble Space Telescope, astronomers had previously announced in 2003 that they had obtained the deepest visible-light image ever taken of the sky to resolve approximately 300,000 stars in Andromeda's luminous halo. By capturing both faint dwarf stars and bright giant stars, astronomers were able to estimate the age of many members of Andromeda's halo population by analyzing color and brightness distributions. Describing an initial hypothesis subsequently contested in January 2007, the astronomers indicated that they had found many stars spanning a wide range of ages from six to 13 billion years old, which is much wider than that of the stellar population of the Milky Way's halo where 11- to 13-billion-year-old, metal-poor stars reside. (More discussion and close-up images from Alan M. MacRobert at Sky and Telescope).
STScI, NASA
Larger image.
Although Andromeda's luminous halo
was thought to include many younger
stars around six to 13 billion years
old in 2003, new observations of
old red giant, halo stars up to
500,000 light-years away from
Andromeda's core were announced
in 2007 (2003; and 2007 findings).
On January 7, 2007, astronomers announced finding old low-metallicity, red giant stars up to some 500,000 light-years from Andromeda's center which suggests that the galaxy is up to five times larger than originally thought, so that its luminous halo may actually overlap with that of the Milky Way. The new finding also suggests that previous observers mis-identified relatively metal-rich red giants in Andromeda's galactic bulge as halo stars. Based on observations of the Milky Way and other galaxies, the metallicity of stars farther from the galactic center should fall with distance from the core (more).
Ann Feild, STScI, NASA
Larger image.
Although a 2003 study of 300,000 stars
in Andromeda's halo indicated that their
age range was wider than those found in
the Milky Way's (more from STScI and
APOD), this observation has been revised
by subsequent observations that suggest
that Andromeda's luminosity may be five
times larger than originally thought (more).
In addition, a giant stream of metal-rich stars was recently detected in Andromeda's halo (Ibata et al, 2001). The presence of younger stars in Andromeda's halo may the result of a more violent phase of the galaxy past involving mergers with smaller satellite galaxies. Furthermore, numerical simulations of the movements of Andromeda and the Milky Way suggest that the two big spiral galaxies themselves may eventually collide and merge within five to 10 billion years.
-
Old but Bright Globular Cluster
A small and compact satellite of Andromeda, G1 is the brightest globular star cluster in the Local Group. Also known as Mayall II, G1 contains at least 300,000 old stars. Despite its globular appearance, however, G1 may actualy be the stripped down core of a dwarf spheroidal galaxy (like SagDEG, the Milky Way's satellite) that has been "shredded" by its larger host. G1 may have at least 10 to 18 million Solar-masses, at least twice the mass of Omega Centauri, the Milky Way's largest globular (Meylan et al, 1998). It is located around 130,000 to 170,000 ly from Andromeda's nucleus.
Michael Rich, Kenneth Mighell,
James D. Neill, Wendy Freedman,
Columbia University, Carnegie
Observatories, STScI, NASA
Larger image.
One of Andromeda's more compact satellites
is G1, the brightest globular star cluster in
the Local Group (more from STScI and APOD).
G1 appears to be nearly as old as the oldest of the roughly 250 known globulars in the Milky Way Galaxy and so probably was formed shortly after the birth of the first stars at the beginning of the universe. Unlike many other globulars, it has a "rather high mean metallicity of [Fe/H] = --0.95, somewhat similar to 47 Tucanae" which may be the result of self enrichment during an early phase of cluster evolution (Meylan et al, 2001). Recently, some astronomers detected a 20,000 Solar-mass black hole in G1's core (more from STScI and Gebhardt et al, 2002).
-
Eva Grebel,
Andreas Koch,
University of Basel,
NOAO/AURA/NSF,
Keck Observatory
Larger illustration.
Many of Andromeda's
satellite galaxies
are located within
a plane pendicular
to its disk (more).
Similar polar planes containing contain many of the Milky Way's companion galaxies were found around three decades ago by William Kunkel and Donald Lynden-Bell. One hypothesis is that such satellite galaxies are tiny left-overs from the break-up of a more massive galaxy which has since been swallowed by their host but still move within the orbital plane of their predecessor, as galactic mergers are believed to be a main mechanism of galactic growth. A second possibility is that the observed alignment with the poles of spiral galaxies' disks traces the otherwise invisible distribution of non-luminous, so-called dark matter around these massive galaxies. Finally, it is also possible that the observed orientation along a plane is a consequence of the infall of satellites along dark matter filaments, as cosmological models predict density fluctuations or matter concentrations which would attract neighboring clumps and continued growth that lead to streams of dark and luminous matter along filamentary features of the so-called cosmic web. Small galaxies forming in dark matter streams could end up in preferred sheets determined by their infall direction toward massive galaxies. Indeed, the Andromeda satellite plane points to the nearby spiral galaxy M33 as well as to the M81 group of galaxies (more).
Other Information
More information and images of Andromeda may be available at NASA and IPAC's Extragalactic Database.
Up-to-date technical summaries on Andromeda may be available at: NASA's ADS Abstract Service for the Astrophysics Data System; the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access; and the NSF-funded, arXiv.org Physics e-Print archive's search interface.
In Greek mythology, Andromeda was rescued from Cetus, the Whale, by Perseus who also married her. This constellation is most easily seen in Autumn for observers in the Northern Hemisphere, but may be visible from June through February. For more information on stars and other objects in Constellation Andromeda and an illustration, go to Christine Kronberg's Andromeda. For another illustration, see David Haworth's Andromeda.
http://www.solstation.com/x-objects/andromeda.htm
No comments:
Post a Comment