Weekly Science Update
Wednesday, December 19, 2007
Markarian Galaxies
- Markarian galaxies were discovered and cataloged by the Russian astronomer Biniamin Markarian and his colleagues starting in the early 1960's. These galaxies are unusual in that they appear very blue, with many of them being dominated by a bright nucleus. These nuclei are now thought to contain massive black holes around which X-rays and jets of particles are being emitted; the main galaxies themselves are often the sites of dramatic star formation. In short, Markarian galaxies are vigorously active places, but no one is quite sure why, or in what critical ways they differ from our own quiescent Milky Way.

The active star formation in Markarian galaxies makes them prime objects for infrared studies because the copious dust in these regions emits strongly in the infrared. The dust is embedded in molecular gas, and that too can be probed at infrared wavelengths. Two SAO astronomers, Howard Smith and Matt Ashby, together Eduardo Gonzalez-Alfonso (a visitor to SAO) and three of their colleagues, have observed and analyzed the water and OH molecules (and other molecular species) in the bright galaxy Markarian (Mrk) 231 using the Infrared Space Observatory. Mrk 231 is the brightest galaxy in our part of the universe. The OH molecule is a particularly powerful probe because it has several strong, bright infrared features that are sensitive to the local environment; in fact, because the OH molecule is very abundant, its lines are the strongest infrared molecular lines seen in many galaxies.

In a paper soon to appear in the Astrophysical Journal, the scientists modeled the precise details of the lines of OH, and H2O in Mrk 231. The analysis shows that the active nucleus, not star forming clouds, is responsible for about two-thirds of all of the luminosity of the galaxy. The calculations also find two regions of activity. The first is a small (only a few hundred light-years in diameter), relatively warm cloud (or clouds) of dense material next to the nucleus that is producing stars (their formation may, however, have been stimulated by emission from the nucleus). In addition, about a thousand light-years from the nucleus is a large, colder complex of material which is actively producing new stars. By using molecular emission to unravel the small internal structures of this bright galaxy, the scientists have figured out where most of the energy arises and how much of it comes from star formation activity. As a result, they also are able quantify why the normal processes ongoing in the Milky Way are so different from the dramatic ones in Mrk 231.


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