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So, What's the Deal with "S0" Galaxies?

Illustration Credit: NASA, ESA, and A. Feild (STScI)

Galaxies of type S0 (pronounced "s-zero")are distinguished from spiral galaxies, with both being flavors of disk galaxy. Edwin Hubble, the astronomer for which the Hubble telescope is named, postulated the existence of S0 galaxies as a transition class between ellipticals and two groups of spirals (barred and non-barred), but they were not clearly recognized until the 1950s.

For years, it was common to figure that cluster processes (ram pressure, galaxy interactions, tidal forces from the cluster core, etc.) played a special role in turning one-time spirals into S0s. This was based on the plain statistical fact that galaxy colors and types change with redshift in rich clusters, with S0s appearing almost as fast as spirals vanish. Showing how these galaxy structures in clusters change with cosmic time has been a major contribution of Hubble imaging.

There is a good case for multiple roads leading to S0s, with the common feature being shutdown of star formation in a spiral galaxy. Some of them may simply be the spiral galaxies that formed stars most briskly in the early Universe, running out of gas - at least the cool gas that can form stars. NGC 5866 may be an example, with the dribble of star formation suggested by the dust lane plus deep infrared measurements from satellite telescopes, and it's lying in a sparse environment. It is distinctly not part of a cluster; the nearest bright neighbor is the edgewise spiral NGC 5907, lying 1.4 degrees away and also in Draco. That projects to 250 Kpc, so these may be as close to each other as 1/3 the distance from here to Andromeda.

It is interesting to note from the Hubble image that the dust lane does not extend out to the edge of the flat blue disk of stars, but falls short. Dust is evident in the disks of many S0 galaxies. Due to Hubble’s line of sight to the galaxy, it is unclear whether the dust extends into the nucleus of the galaxy or lies in an annulus or ring around that is swept clear of dust and gas near the nucleus. The common scenario of most S0 galaxies is that they have a restricted annular dust distribution. This occurs among virtually all well-attested S0s with dust lanes seen edge-on, that it serves as a secondary classification characteristic. It also fits with what is seen in S0 galaxies observed at a more favorable inclinations, where the dust is concentrated in an annulus within the disk of stars.

We see this well in NGC 5866, as the starlight disk extends well outside the dust absorption. This tells us that the dust and gas still in the galaxy and potentially available to form stars does not stretch nearly as far out in the disk as it did when most of these stars in the disk were formed. In recent years, several kinds of data have shown that all S0s are completely devoid of star formation; Halpha images often show a trickle in the form of H II regions, typically fainter than in normal spirals, and there is sometimes detectable H I emission from atomic hydrogen.

- Text supplied by Bill Keel (University of Alabama, Tuscaloosa)