A Large Spiral Galaxy
Wider and possibly brighter than our own Milky Way, the Andromeda Galaxy was once thought to be the dominant member of the Local Group of galaxies. Although it is Milky Way's nearest large galactic neighbor, this large spiral galaxy (type Sb with two arms) lies around 2.52 ± 0.14 million light-years (ly) from the Solar System (Ribas et al, 2005). It can be found in (0:40:27+40:40:12, J2000; and 0:42:44.3+41:16:9.4, ICRS 2000) Constellation Andromeda, the Chained Maiden. It is located northwest of Mu and Beta Andromedae (Mirach); west of Nu Andromedae; northeast of Theta and Sigma Andromedae; north of Pi, Delta, and Epsilon Andromedae; and south of Theta and Omega Cassiopeiae. Andromeda can be seen by Human eyes from Earth without a telescope as a "little cloud" (see Akira Fujii's photo to better relate the galaxy's location to the brightest stars of Constellation Andromeda).
© Jason Ware,
www.galaxyphoto.com
(Used with permission)
Larger image (more).
Andromeda has a bright
yellowish nucleus, dark
winding dustlanes, and
bluish spiral arms and
star clusters (more
from APOD).
Andromeda has a bright disk that is now believed to span as much as 228,000 ly in width (Chapman et al, 2005). In 2005, astronomers announced that Andromeda's disk actally extends far further out, so that the disk spans at least 260,000 light-years -- almost twice the size of the bright disk seen in photographs (Ibata et al, 2005). The outer disk emits nearly 10 percent of the galaxy's total light and may be comprised of metal-poor stars stripped from smaller galaxies that strayed too close. On January 7, 2007, a team of astronomers announced the discovery of low-metallicity, red giant stars up to some 500,000 light-years from Andromeda's core which suggests that the galaxy is much larger than originally thought, so that Andromeda's luminous halo may actually overlap with that of the Milky Way (BBC News -- more below).
WISE, UCLA,
CalTech, JPL, NASA
Larger and jumbo
infrared images.
In this 2010 infrared image,
dust heated by newborn,
massive stars are depicted
in yellow and red, which
trace out the spiral arms,
while mature stars are
colored blue (more).
In the venerable Star Names: Their Lore and Meaning, Richard Hinckley Allen noted that: "... the Great Nebula, the Queen of the Nebulae, ..., is said to have been known as far back as A.D. 905; was described by [Abd-al-Rahman] Al Sufi as the Little Cloud before 986; and appeared on a Dutch star-map of 1500." According to Robert Burnham, Jr. (1931-93): "The first hint of the true nature of the Andromeda Galaxy came late in 1923 when several [C]epheid variable stars were identified in the system [by Edwin Powell Hubble (1889-1953) who thus] ... definitively established the great spiral as an extra-galactic object ...."The galaxy is frequently referred to as M31 because it was the 31st object in the Messier Catalogue of diffuse objects that Charles Messier (1730-1817) found not to be comets. Subsequently, the "nebula" was also designated as NGC 224 by John Louis Emil Dreyer (1852-1926) in his New General Catalogue (NGC) of Nebulae and Clusters of Stars, which was first published in 1887 and later supplemented with Index Catalogue (IC) I in 1895 and IC II in 1907.
Bill Schoening, Vanessa Harvey,
REU program, NOAO/AURA/NSF
Larger red, green, and blue composite image.
Recent observations indicate that,
although the spiral disk of Andromeda
may be much larger than that of the
Milky Way, the galaxy appears to be
much less dense, with a smaller mass
counting its dark matter halo (box
view at page bottom -- more at NOAO).
Careful estimates of Andromeda's angular diameter obtained using 2-inch binoculars by Robert Jonckhere from 1952 to 1953 indicated that Andromeda's disk had a diameter of over 200,000 ly (assuming a distance of 2.9 million ly). Hence, Andromeda's spiral disk may as much as twice as large as the Milky Way's. Although Andromeda was long thought to be the most massive galaxy in the Local Group, recent data suggest that Andromeda's visible mass may total around 300 to 400 billion Solar-masses. This is considerably less than more recent estimates of the Milky Way's visible mass of as much as 600 billion or more Solar-masses, which suggests that the Milky Way may be much denser than Andromeda. These results apparently have been confirmed by recent estimates of the total halo masses of the two spirals that account for the gravitational effects of their invisible dark matter, which suggest that Andromeda has a total of around 700 billion to 1.2 trillion Solar-masses compared to 0.93 to 1.9 trillion or more for the Milky Way (more discussion from (Xue et al, 2008; Evans et al, 2000; and Evans and Wilkinson, 2000).
IRAS, IPAC, NASA
Larger infrared image.
Young stars are probably being born
in many dusty regions of Andromeda
that are bright in infrared wavelengths,
with many short-lived but massive, blue
stars in the more intense white and
yellow areas (more from IPAC).
The brightest star cloud in Andromeda has its own NGC number, NGC 206. One of the largest star-forming regions known in the Local Group of galaxies, Sir William Friedrich Wilhelm Herschel (1738-1822, portrait) noted it in his catalog as H V.36 on his discovery of the diffuse object on October 17, 1786. Located next to a dark nebula towards the southwestern, outer edge of Andromeda's spiral disk (another photo), the cloud's bright blue stars give an indication of the massive star cluster's youth (more from Astronomy Picture of the Day).
B.J. Mochejska (Warsaw University),
The DIRECT Project, FLWO, MDM
Larger blue, visual, and infrared,
composite image.
One of the largest clusters of young blue stars
in the Local Group of galaxies, NGC 206 is
located in one of Andromeda's dusty spiral arms
(more from APOD and CfA).
So far, only one supernova has been recorded in the Andromeda Galaxy, but it was the first to be detected outside the Milky Way. Known as Supernova 1885 for the year of its appearance, it has also been designated as S Andromedae. Ernst Albrecht Hartwig (1851-1923) observed it on August 20, 1885 at Dorpat Observatory in Estonia. While found independently by several other observers, only Hartwig realized its significance. The supernova reached a magnitude of six between August 17th and 20th but then faded to magnitude 16 by February 1890.
Philip Choi, Puragra Guhathakurta, UCSC, KPNO -- larger blue and infrared image.
Andromeda has an "extreme" warp in its outer spiral disk, possibly from interactions
with satellite galaxies (more from UCSC and UCOLICK), as well as debris trails
from past mergers with other galaxies (more).
Astronomers have been finding evidence of a warp in Andromeda's spiral disk for some time. The faint outer parts of a spiral galaxy are more susceptible to warping because they are less strongly bound by the gravitational and other forces that keep disk stars in a plane and are also more susceptible to the influence of neighboring galaxies. As a result, the outer regions of a rotating body of stars and gas can deviate from the plane of the disk, like an old record album exposed to too much heat. Such a warp tends to occur at the outer edges, while the inner part of the spiral disk continues to look reasonably flat. Andromeda's warp is especially pronounced on the northeast (left) side of its major axis. Such galactic warps are very difficult to demonstrate conclusively because the outer portions of a spiral disk are extremely faint compared to their bright central regions. However, the warp in Andromeda may be the most extreme case of a warped spiral galaxy found thus far. Possible causes of the warp include interactions between Andromeda and its smaller satellite galaxies (more discussion).
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).
On September 16, 2009, NASA's Swift Satellite Mission released the highest resolution ultra-violet (UV) image of Andromeda available. Covering an area some 200,000 light-years (ly) wide and 100,000 ly high, the image was compiled from 330 UV images made at wavelengths of 192.8, 224.6, and 260 nanometers. The result covers some 20,000 UV sources among the hottest and youngest stars and stellar remnants in Andromeda's spiral arms, densest clusters, and violent core around its central, supermassive black hole (more from NASA and Astronomy Picture of the Day).
Stefan Immler,
Erin Grand, Swift,
GSFC, NASA
Larger UV image.
This new high-resolution
UV image highlights the
hottest and youngest stars
and energetic stellar
remnants in Andromeda's
spiral arms, densest
clusters, and violent
core around its central
black hole (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.
Very Large Galactic 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).
Satellite Galaxies
On January 11, 2006, astronomers announced their discovery that many of Andromeda's faint companion galaxies lie within a thin sheet running perpendicular through the galaxy's Andromeda disk. Nine out of 14 low-mass satellites lying with 1.3 million light-years from Andromeda are found within this sheet, whose typical width is only 52,000 light years (about two percent of the distance between Milky Way and Andromeda). The sheet runs through Andromeda's core and is almost exactly aligned with its polar axis.
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).
McConnachie et al, 2009;
PAndAS
Larger composite image and
composite illustration
.
Andromeda has been growing
by pulling stars (as well as
gas) from smaller satellite
galaxies, such as bright
neighbor Triangulum (more).
On September 2, 2009, a team of astronomers (using the Canada-France Hawaii Telescope for the Pan-Andromeda Archaeological Survey or PAndAS) announced the discovery of stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of Andromeda (M31). They also found that the halo of the giant spiral galaxy has non-native stars and "coherent structures" (such as stellar streams or "tidal tails" and a gaseous as well as stellar warp in its spiral disk). M31's nearest bright neighbor, satellite galaxy Triangulum (M33), moreover, has a stellar structure (including a warped disk) suggestive of a recent encounter with M31 around 2.5 billion years ago, and model simulations indicate that the two galaxies will have an even more violent encounter in about 2 billion years). Their observations supports the hierarchical galaxy formation model through the apparent interactions of M31 and M33 in the continuing growth of galaxies in the modern era (more from PAndAS; UBC news release; Lisa Grossman, New Scientist, September 2, 2009; McConnachie et al, 2009; and Martin et al, 2009).
Courtesy of PAndAS
Larger illustration (more).
The two galaxies will have an
even more violent encounter
in about 2 billion years (more).
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