DWARF ELLIPTICAL GALAXIES
These small, gas-poor galaxies (dE's ; usually with MV > - 18) have very low surface brightness, and are found (for the most part) in galaxy clusters or near large galaxies. Many of the brighter ones also possess a bright nucleus (and classified dE,N). The name `dwarf elliptical' is actually a bit of a misnomer, as this class of LSB object is not really a fainter version of a true elliptical galaxy, but structurally different. A typical dE has a mass of about a billion solar masses, or about 1/1000 that of a typical giant galaxy. They contain very little (or no) gas, which makes them different from dwarf irregular galaxies.
There are 3 relatively bright dE's in the Local Group: NGC 147, 185 and NGC 205 (pictured above right), all companions to the large spiral galaxy M31 (the Andromeda Galaxy). There are hundreds of similar galaxies in the relatively nearby Virgo cluster of galaxies; even more exist in the large Coma Cluster. These small galaxies are of interest to astronomers as it is possible that they (or really, their progenitors) are the `fragments' from which the larger galaxies formed (in the `bottom-up'scenario of galaxy formation).
In addition to the brighter dE galaxies, there also exists much less luminous examples, usually called `dwarf spheroidals' (dSphs). There are almost 20 such objects in the Local Group of galaxies alone, all with masses of about 10-100 million solar masses. 9 of them are actually companions to our own Milky Way galaxy, and 9 more are companions to M31. Although these are the smallest galaxies known, they often have very different star formation histories -- a surprising result as these objects were not expected to be able to hold onto any of the gas needed for stars to form. dSphs also have lower surface brightnesses than dE's. An example (below) is the faint VCC 1311A, which is likely a dSph in the Virgo cluster (from Durrell 1997, AJ, 113, 531).
An excellent review article about dwarf elliptical galaxies has been done by Ferguson & Binggeli (1994), A&ARev 6, 67.
TIDAL DWARF GALAXIES
TO BE ADDED!!!
