Elliptical galaxies

Elliptical galaxies have, as their name suggests, simple elliptical appearances. Some examples of ellipticals are shown in Fig. 17.2. The ellipticals are classified according to their degree of eccentricity. The ones that look spherical (zero eccentricity) are called E0, and the most eccentric are called E7.

The most common type of elliptical galaxies are called dwarf ellipticals, since they are also the smallest. Their sizes are typically a few kilopar-secs and their masses are a few million solar masses. More spectacular are the giant ellipticals, with extents up to 100 kpc and masses of about 1012M0, with some with masses up to a factor of ten higher.

The gas content of ellipticals is low. Studies of HI, using the 21 cm line emission, as well as IRAS observations of weak emission from their dust, suggest that the mass of the interstellar medium may be up to about 1% of the mass of the stars that we see. The low gas content rules out the possibility that ellipticals eventually flatten to form spirals. The continuing process of star formation in a galaxy depletes its supply of interstellar matter,



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