Chris Peterson wrote:
Ann wrote:That spiral galaxy, IC 4040, is interesting. How can it have held on to its gas supply and managed to turn into such recent and widespread star formation, when all the other galaxies are either completely yellow or mostly very yellow?
As a rule, I don't think yellow, inactive galaxies have failed to "hold on" to their gas supply. These galaxies usually have plenty of hydrogen available for star formation. It's just too homogeneous. All you need is a bit of tidal interaction with another galaxy, a few nice supernovas to seed some dust, or the right kind of shock front along a spiral arm edge to get things rolling again. My understanding is that there's a lot of evidence that galaxies (especially those still the spiral stage) can cycle through periods of inactivity and periods of star formation.
Maybe you are right, Chris, but maybe not.
Take a look at this splendid picture by Adam Block of the Perseus Cluster of galaxies
. At top right you can see a pair of yellow galaxies involved in a "dry merger", a merger or interaction of two yellow galaxies that doesn't result in any significant star formation. Other yellow galaxies in the cluster are also very close to one another and almost certainly interacting, and they are surrounded by large, faint yellow halos. But we can see no sign of any significant star formation.
Also, in the widefield image of the Coma Cluster we see many galaxies that are very close to one another. Certainly they must be interacting, but we see no obvious signs of recent star formation in these galaxies.
I agree that elliptical galaxies contain a lot of gas. But they have lost their ability to pile up that gas in large, dense, cold molecular clouds. That is why they can't form new stars.
I believe that isolated small galaxies may undergo a burst of star formation and then "settle down" as dwarf ellipticals. But these small galaxies almost certainly don't contain a supermassive black hole. If they suddenly interact with another galaxy again, they may well undergo significant renewed star formation. There are signs that the Large Magellanic Cloud may have undergone "quiet" periods between bursts of star formation, judged by the age of the clusters in this galaxy. See "The Gaalxies of the Local Group" by Sidney van den Bergh, Cambridge Astrophysics Series 35, © Cambridge University Press 2000. See the chapter about The Large Magellanic Cloud, part 6.7, Evolutionary history of the Large Cloud.
Have you read that large galaxies containing supermassive black hole may still suddenly experience bursts of star formation? And have you read that "red and dead" galaxies in large clusters may suddenly start forming significant numbers of stars again? If so, where have you read it?