Galaxies: Born Blue, Red when Dead, Fat Die First

I just had the strangest realization. Stepping into the Galaxy Evolution session of oral presentations I got to listen to the AstroPixie present her research. Like me, she is a UT person (I got my PhD, she is getting her PhD). I knew that. What I didn’t know is we both have/had one of the same dissertation advisors: Gary Hill (I also worked with John Kormendy. What a strange small field it is.

HST / Hubble Heritage TeamGalaxies began to form just a few billion years after the Big Bang. As we learned yesterday, it now looks like this formation took too different paths, with giant spiral galaxies forming out of mass gas + dust collapse, that had the stars and the full fledged giant galaxy all forming at once. at the same time, in other less dense areas, smaller systems also formed dwarf and often irregular galaxies. In their earliest days, these fresh born systems shown blue, with the light of sort lived O and B giant stars dominating the total light emitted by the galaxies. Over time each of these types of systems had different evolutionary patterns, until today we find ourselves living in a universe populated by red elliptical galaxies devoid of blue O and B stars. Red and dead, these giant systems often live in large groups, clusters and super clusters of galaxies. Side by side on the sky, but not in the universe, we find blue spiral galaxies like our own. These star forming systems are often isolated, in small groups, or on the edges of galaxy clusters (where they have yet to interact with other systems).

Understanding stellar evolution requires one to separate out many variables: size of galaxy, environment of galaxy, and when in time we are observing the galaxy (objects that are farther away we see as they appeared in the past because it takes time for the light to reach us).

In Amanda Bauer’s nice talk she discussed how mass alone effects galaxy evolution by observing a set of field galaxies – isolated galaxies that do not belong to groups (like the one we are in) or larger systems (clusters, like the Abell systems you often see in the news). She found “a clear separation between disk-dominated (sersic n > 2.5) and bulge-dominated galaxies, such that disky galaxies have higher specific star formation rates and lower stellar masses at all redshifts.” This means that galaxies like the sombreo galaxy (shown above, image credit NASA / Hubble Heritage Team) have within the past several years all tended to look and behave the same way : They are actively forming stars, but they haven’t come anywhere close to using up their gas and dust, and remain rich in materials available for future star formation. At the same time, she notes the highest mass objects in her sample have the lowest specific star formation rates.

If you want to live some where alive, live somewhere small. The fat die young in galaxy populations.

The specific star formation rate is an interesting analytical way of looking at things I hadn’t previously used. It looks at the rate of star formation as a function of a galaxies stellar mass. I’m going to have to look into this more and write more later. For now, here’s a paper.

2 Comments

  1. Nick Cross January 11, 2008 at 10:20 am #

    I think disk-dominated galaxies to have sersic n 2.5.

  2. astropixie January 12, 2008 at 2:24 pm #

    nice to finally meet you, pamela!

Leave a Reply