Dead Galaxies

The New Scientist has an article on "dead galaxies", galaxies which have had no star formation for several billion years.

"Now, using NASA's Spitzer telescope, which trails behind the Earth in the coldness of space, astronomers have determined the galaxies are red because they are dead - no stars appear to have formed for 1.5 billion years. That arrested development happened early in the history of the universe - their distance means Spitzer is viewing them just 2 billion to 3 billion years after the big bang. "We think galaxies form over tens of billions of years," says lead researcher Ivo Labbé, an astronomer at the Carnegie Observatories in Pasadena, California, US. For example, he notes the 13 billion-year-old Milky Way is still forming stars today. "Surprisingly, we found galaxies that are fully formed and dead when the universe was only one-fifth its present age."

Their first theory is that supermassive black holes made the galaxies very active and heated the gas and dust in the galaxies so much that it flowed away, leaving the rest of the stars to sit and age alone. OK, that sounds plausible. I wonder what happened to all the dark matter. You have to model systems where the blast from the black holes is sufficient to drive gas away from the combined gravity of the dark and visible matter. The mass of the galaxy thus falls slightly, and some of the dark matter also drifts away. (It isn't blown away: the whole point of dark matter is that it doesn't interact easily with the ordinary forces such as electromagnetism.) After such a long time I don't think you could see the gas as any sort of obvious object any more: it should have mixed with intergalactic clouds. The lost dark matter would probably be in more of a thick ring--hard to see even if you could readily detect dark matter. (Assuming spiral galaxies--not stated in the article)

I wonder if one could compare the velocity profiles in these dark galaxies with those in more dynamic ones to see if you can model dark matter self-interaction? Probably not, since the distribution of star formation as a function of radius would have been skewed by the force of the blasts from the central black hole.

Interesting. . .