ANDROMEDA GALAXY RENE A. M. WALTERBOS The Andromeda galaxy is the largest member of the local group of galaxies. At a distance of 700 kpc or 2.3 million light-years, it is also the closest spiral galaxy to our own (a kiloparsec is 3260 light-years). It is often referred to as M31 or Messier 31, because it is number 31 in the famous catalog of Messier (1730-1817). The Andromeda galaxy has played a major role in developing our understanding of galactic structure and evolution. In the 1920s, Edmund Hubble discovered Cepheid variable stars in M31, which established its extragalactic nature, because the distance derived for the Cepheids from the period-luminosity relation placed M31 well beyond the bounds of our Milky Way. This discovery proved that galaxies other than our own exist. In 1943, Walter Baade was able to resolve the central part of M31 into thousands of red stars. These were clearly different from the bluer, more luminous stars that can be seen in the spiral arms. His observations established the concept of two different stellar populations, types 1 and 2. Type 1 stars are young and luminous and are currently still forming in most galaxies. Type 2 stars, on the other hand, are old and much fainter. The discovery of two stellar populations provided important input for models of stellar evolution. Progress in observational techniques has been rapid and today catalogs exist of the distributions of HII regions, OB associations, globular clusters, open clusters, planetary nebulae, novae, and other objects in M31. In addition, detailed observations exist of the various phases of he interstellar medium: atomic and molecular hydrogen gas, dust, regions of the various phases of the interstellar medium atomic and molecular hydrogen gas, dust, regions of ionized gas(HII regions and supernova remnants), and relativistic electrons and the magnetic field. Both space-based and ground-based telescopes operating over the entire wavelength range from radio to x-ray have been used in these observations. STRUCTURE As in most spiral galaxies, the stellar distribution in M31 has two major components, a thin disk with superposed on it the spiral arms, and a spheroidal component which is strongly concentrated to the center and is flattened with an axial ratio of about 0.6. The light distribution of the disk falls off more slowly like an exponential function with a scale length of about 5 kpc. Table 1 gives a summary of the properties of M31 and of our galaxy. Figure 1 shows an optical image of M31. The light from the stellar disk is bluer at larger distances from the center. This can be caused by a decrease in the average metallicity of the disk stars with radius, or by an increase in the fraction of relatively younger stars with radius. At about 18 kpc from the center, the stellar disk seems to bend out of its principal plane, a phenomenon known as a "warping." A similar warp occurs in the gas distribution in M31, as is apparent from radio observations of atomic hydrogen. The warp in the gas disk occurs at slightly larger distances from the center. Tables 1. Properties of Andromeda and Milky Way Galaxies PARAMETER ANDROMEDA MILKY WAY Distance to center 690 kpc 8 kpc Hubble type Sb I-II Sbc II Total luminosity (in solar units) 3x1>10x10 L* 1.8x10x10 L* Luminosity of spheroidal component 7.7x10x9 L* 2.8 x 10x9 L* Velocity dispersion of stars in bulge 155 kms-1 130kms-1 Luminosity of disk component 2.4x10x10 L* 1.6 x 10x10 L* Optical diameter at brightness of 25 mag arcsec-2 30kpc 25kpc Scale length of disk 5-6 kpc 4-5 kpc Number of globular clusters 300-400 130-160 Mass of atomic hydrogen gas (in solar units) 3.9 x 10x9 M* 4x10x9 M* Atomic gas disk extends to at least: 25-30 kpc radius 20-25 kpc radius Infrared luminosity emitted by dust 2.6 x 10x9 L* 12 x 10x9 L* An as-yet unsolved problem concerns the shape of the spiral arm pattern of M31. Spiral arm segments show up clearly in various tracers of young objects, such as interstellar gas and dust, regions of ionized gas, O and B stars, and open clusters. However, analyses of the distribution of these tracers do not result in a unique picture of the spiral arm structure. Both a one-armed leading spiral and a two-armed trailing spiral structure have been proposed. The problem is that the small angle between our line of sight and the principal plane of M31 makes it difficult to disentangle the spiral arms. Furthermore, the arms may be distorted by the gravitational interaction with the close elliptical companion, M32. In addition to these visible components, M31, like other spiral galaxies, has an invisible halo of unknown matter. The dark halo is inferred from the rotational speed of gas that orbits the galaxy. The velocity of the gas stays constant at 230 km-1s out to at least 35 kpc from the Center. The amount of mass that is required to explain this is much larger than can be accounted for by the visible matter the discrepancy is at least a factor 2, possibly as much as a factor 10. STELLAR POPULATIONS In Baade's picture, the spheroidal component contains old stars with low abundances of metals (elements heavier than helium), whereas the disk contains younger stars with relatively high metal abundances. The metal abundance increases with time, because the material is continuously enriched by the processes of stellar evolution. Therefore, young stars will be more metal rich than old stars. Recent studies show that the distribution of stellar populations in M31 is more complicated. Stars in the outer parts of the spheroid indeed have metal abundances, about 10 times lower than our sun, but there is considerable spread. The central part of the spheroid contains metal-rich stars, and the spheroidal component may consist of a metal-poor halo and a central stellar bulge that has stars of varying metal abundances, some of them more metal rich than our sun. The disk contains stars in a range of ages and metal abundances. Observations with the Hubble space telescope will aid considerably in unraveling the stellar populations in M31 and other galaxies, since with this instrument much fainter stars can be detected and much more crowded regions close to the center can be resolved than with ground-based observations. The Andromeda galaxy has more globular clusters than the Milky Way, which is possibly related to its larger stellar spheroid. Some 300 have been identified; the true number may be even larger. The cluster system in M31 resembles the galactic cluster system in many ways, but some differences exist. In particular, detailed spectroscopic studies indicate differences in the stellar populations of some clusters that were unexpected and are not yet fully understood. In addition, M31 has some blue compact clusters hat must be much younger than the canonical globular cluster that is as old as the universe. INTERSTELLAR MEDIUM AND STAR FORMATION All tracers of star-forming regions and the interstellar medium are concentrated in an annulus between 8-12 kpc from the center. The annular structure results from projection of various segments of the complicated spiral arm pattern, and is probably not a true torus. Figure 2 shows the distribution of atomic hydrogen gas and infrared emission from dust. The amount of atomic hydrogen gas in M31 is comparable to that in our galaxy. However, molecular gas, as inferred from observations of the CO molecule, has lower surface densities than in our galaxy. Observations of regions of ionized gas and far-infrared emission from dust indicate that the overall level of star formation is also down from our galaxy by a factor of 5-10. In spite of this relatively low level of activity, detailed observations of the massive O and B stars in the spiral arms can and have been done. The massive stars appear to be similar to those in our galaxy, although there are some indications that stars above 60 M* may be absent in M31. The atomic hydrogen distribution is characterized by numerous holes and shell-like structures. These result from stellar winds from young stars and supernova explosions that deposit energy in the interstellar medium. Information on the magnetic field structure in M31 has been obtained from radio observations of the synchrotron radiation that is emitted by fast-moving electrons spiraling around the field lines. The magnetic field is oriented along the spiral arms with a large degree of ordering. Some 30 supernova remnants have been discovered in M31 from optical images taken in various emission lines. Most of them are found in the spiral arms which indicates that they result from massive stars that exploded. About half of these have recently also been detected at radio wavelengths. The most recent supernova explosion occurred in 1885 close to the center of M31. This was not a massive star that exploded, but a supernova originating in a close double-star system. THE CENTRAL REGION The central region of the Andromeda galaxy differs in several respects from that of our galaxy. The most conspicuous difference concerns the lack of interstellar matter in the inner kiloparsecs in M31. No atomic or molecular hydrogen has been detected here, nor is there any evidence for ongoing star formation. This is very different from the situation in our galaxy, where active star formation is taking place at the center. Optical images do show small dust patches in the inner region; this dust is also detected in far-infrared images obtained with the IRAS satellite. Furthermore, ionized gas has been found but the mass involved is small, perhaps a few thousand solar masses. The structure of the ionized gas is intriguing, however. It shows spiral arms in a plane that is tilted from the principal plane of M31's disk. The size of this spiral pattern is about 1 kpc. It is also visible in radio continuum images. The gas presumably originates from mass loss by old stars. The hot central stars of planetary nebulae and collisions between gas clouds are responsible for ionizing the gas. The overall level of radio continuum emission from the central region of M31 is 10 times weaker than that from the central region of our galaxy. The emission is nonthermal in nature. Furthermore, there is no evidence for a compact, unresolved radio source in the center of M31, contrary to our galaxy. High-resolution optical imagery of M31 done from a balloon-borne telescope in the mid-1970s shows a distinct nuclear component in the inner 1 or 2 pc of the light profile. Recent findings provide evidence for the presence of a black hole at the center with a mass of 10 or 100 million solar masses. The evidence comes from high-resolution spectroscopy of the center, which shows a large rotational velocity and velocity dispersion very close, at about 1 to 2 pc, from the nucleus. The presence of a black hole is usually postulated in galaxies with active nuclei; the M31 observations provide strong evidence that normal galaxies may also have black holes at their centers. With the upcoming new generation of space-and ground-based instruments, M31 will once again be one of the prime targets for observations. Much remains to be learned, and much will be learned in the next decade. Additional Reading Beck, R. and Wielebinski, R. (1981). Radio waves from M31 and M33. Sky and Telescope 61 495. Hodge, P.W.(1981). Atlas of the Andromeda Galaxy. University of Washington Press, Seattle. Hodge, P.W.(1981). The Andromeda galaxy. Scientific American 244 (No. 1)92. See also Galaxies, Infrared Emission; Galaxies, local Group.