The degree to which there was "early assembly" of galaxies (stars formed early on in the universe) vs. "late assembly" of galaxies (e.g. stars formed at many times, up to and including today) still seems to cause controversy.
Large Bulges and
Large elliptical galaxies formed early on, but smaller galaxies and the disk components of spiral galaxies seem to still be growing due to considerable ongoing star formation.
Some evidence for later (and ongoing) galaxy formation is:
- gas being funnelled into the centres of spiral galaxies by their bar components, leading to the gradual formation of small bulges
- evidence (not conclusive) for the accretion of cold gas in the outermost parts of spiral galaxies, leading to star formation and the build-up of the disk component in the outer parts of a galaxy. This gas may come from intergalactic space.
- Massive gaseous halos around dwarf galaxies that contain enough gas to keep small galaxies growing and forming stars for a long time into the future.
- merger and accretion of galaxies, e.g. obvious cases of ongoing galaxy merger, plus less obvious cases where the halos of galaxies like M31 and the Milky Way contain stars that originated in dwarf galaxies that were cannibalized.
- some elliptical galaxies are surrounded by "incipient disks" of gas, which may eventually form disks of stars around the elliptical component. The gas is at very low density, so more sensitive HI surveys may conceivably find that this phenomenon is common.
cheers,
Bad galaxy man
Added in edit:
The current Star Formation Rate (e.g. in solar masses per annum) in galaxies is, on average, much lower than it is in distant galaxies seen at large look-back times. Thus, several textbooks give the impression that the universe is today nearing the end of ongoing star formation (with the galaxies gradually dimming due to the death of the existing stars). However, results such as
Williams et al., 2009, ApJ, 695, L15 ((
http://iopscience.iop.org/1538-4357/695/1/L15)) , which implies that the outer disk of M33 is much younger than its inner disk, suggest that the outer parts of spiral galaxies are able to capture gas from external sources and therefore continue building up their stellar mass by means of star formation that continues for billions of years into the future. So perhaps we are much further from the dreaded "cold dark" era when the universe is completely dead!
Williams et al suggest that the rules of galaxy evolution may well be very distinct for relatively low mass galaxies like the LMC and M33. The type of gradual galaxy growth they infer for M33 is known in the trade as "inside-out disk formation"
