Messier 32

Elliptical Galaxy M32 (NGC 221), type E2, in Andromeda

A Satellite of the Andromeda Galaxy, M31

Discovered 1749 by Guillaume-Joseph-Hyacinthe-Jean-Baptiste Le Gentil de la Galaziere (Le Gentil).

Messier 32 (M32, NGC 221) is the small yet bright companion of the Great Andromeda Galaxy, M31, and as such a member of the Local Group of galaxies. It can be easily found when observing the Andromeda Galaxy, as it is situated 22 arc minutes exactly south of M31's central region, overlaid over the outskirts of the spiral arms. It appears as a remarkably bright round patch, slightly elongated at position angle 150-330 deg, and is easily visible in small telescopes. Its ellipticity is about E2, i.e. the smaller diameter, or axis, or its elliptically shaped image, projected along our line of sight, is about a fraction of 0.2, or 20 percent, shorter than its larger axis.

M32 is an elliptical dwarf of only about 3 billion solar masses, and a linear diameter of some 8,000 light years, very small compared to its giant spiral-shaped neighbor. Nevertheless and surprising for such a small galaxy, its nucleus is of comparable properties as that of M31: About 100 million solar masses, 5000 suns per cubic parsecs, are in rapid motion around a central supermassive object. Because of this nucleus, M32 is sometimes classified as cE2 instead of simply E2, e.g. by NED.

Near the center of this galaxy, the sky would be dominated by this object, and full with the members of this galaxy, while at the edges, only one hemisphere would be filled with them, the other showing only few outlying stars and the intergalactic space. Toward M31, this galaxy would give a fascinating view in the night sky of a virtual astronomer in the outskirts of M32.

M32 appears to us superimposed over the spiral arms of greater M31. Therefore, it is of interest if it lies before or behind the great galaxy's disk. Spectroscopic investigations have not shown any absorption which would be expected if its light had passed the interstellar matter in M31's disk, which suggests that M32 is closer to us than that portion of M31.

The radial velocity of M32 has been measured at 203 km/s (R. Brent Tully) or 205 +/- 8 km/s (NED) in approach in the heliocentric system, i.e., toward our Solar System; corrected for galactic rotation, M32 is currently about at rest (RV=0) w.r.t. the Milky Way's Galactic Center. Compared to M31, it is approaching about 100 km/s slower, and considering its closer distance, it is apporaching M31 at this velocity in the radial component.

M32 and the other bright companion of M31, M110, are the closest bright elliptical galaxies to us, therefore also the among best investigated. They were both first resolved into stars by Walter Baade in 1944 with the 100-inch Hooker telescope on Mt. Wilson when he also resolved the nucleus of M31 (Baade 1944). Baade recognized that their stars were mostly old population II stars, and about as bright (and thus at roughly the same distance) as M31, thus confirming their proximity to the large spiral galaxy. There are remarkable differences between these dwarf galaxies: While M32 is a typical generic elliptical, compact and of high surface brightness, M110 is much more loose, of lower surface brightness, and exposes peculiar structures; now, M110 is often classified as a dwarf spheroidal galaxy instead of elliptical. Remarkably, M32 has no globular clusters (again, in difference to M110 which has 8).

M32, like typical elliptical galaxies, is mostly made up of old stars, of which only the lower-mass, intrinsically fainter ones have survived to now; as usual in such old populations (e.g., also in globular clusters), the more massive stars have presumably ended their active, nuclear-burning lives long ago - they are now white dwarfs or neutron stars. However, spectra and color of this galaxy (M32 has an overall spectral type of G3 and color index B-V = +0.75) indicate that its stars have chemical abundances different from those in old globulars which are poor in heavy elements. Instead, there seems to be a population of stars richer in heavy elements, which are apparently much younger, only 2 or 3 billion years old, mixed between the old stars as minor contamination.

Between the stars of M32, some planetary nebulae have been found, but no clouds of interstellar matter, neither gas clouds nor dust lanes nor neutral hydrogen, nor any open clusters. Apparently, M32 is no more able to form any new stars, but consists of old stars, mixed up with some of intermediate age. According to investigations of multicolor data, this stellar population is much more similar to that of much larger elliptical than that of typical dwarfs of its size, which are typically of dwarf spheroidal type.

Novae occur in M32 occasionally. One recent nova was discovered in M32 on August 31, 1998 within the Lick Observatory Supernova Search Program by a team of astronomers from the University of California at Berkeley headed by E. Halderson (1998). This nova occurred about 28.5 arc seconds west and 44.7" south of the galaxy's nucleus and reached mag 16.5. Supernovae have not yet been observed in this galaxy.

As its stellar population, size of nucleus, and compactness indicate, M32 looks more like a much larger elliptical galaxy. Therefore, it seems possible that M32 was once much larger, but lost its outer stars, and also all globular clusters it may have had, in one or more past close encounters with the Andromeda Galaxy M31. These stars and clusters were absorbed by, or integrated in, and are now part of the halo of M31. That M32 has recently undergone a closer encounter with its larger neighbor is suggested because it apparently caused and left disturbances in the big galaxy's spiral pattern.

M32 was the first elliptical galaxy ever discovered, by Le Gentil on October 29, 1749. Charles Messier remarked in his description that he had first seen this object in 1757 (his first record of an observation of one of "his" objects), cataloged it on August 3, 1764, and included M32, together with M110, in his drawing of Andromeda's "Great Nebula". Halton Arp has included it as No. 168 in his Catalogue of Peculiar Galaxies.

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