Andromeda Galaxy

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).

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 ).

: 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

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

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.

Andromeda has an “extreme” warp in its outer spiral disk, possibly from interactions with satellite galaxies

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 .

This ultraviolet image highlights a 150,000-ly-wide ring of young and hot, blue stars that surrounds Andromeda's central bulge

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.