MadSci Network: Astronomy

Re: Can new galaxies form?

Date: Wed Jan 24 06:45:40 2001
Posted By: Meghan Gray, Grad student, Astronomy, Cambridge University
Area of science: Astronomy
ID: 979277371.As

Hi Michael,

Thanks for a great question, and I'm very sorry for the delay in replying! You're correct -- the formation of galaxies as fundamental units of structure is indeed a feature of the earlier universe. In the current picture of galaxy formation, we know from observations of the microwave background that the very early universe (ie just after the Big Bang) was very smooth. Despite that, there must have been some initial perturbations, or small regions where the density was slightly greater than average. By a runaway process these fluctuations would grow even more dense due to the gravitational instabilities and so formed the seeds for galaxy formation.

From the time of the Big Bang to the present age of the universe, there has been plenty of time for galaxy-sized gas masses to cool, collapse, and form stars. For any uncollapsed haloes of that size to exist today would be unlikely, as it would require some mechanism to have supported them against the inevitable gravitational collapse and resulting bursts of star formation.

Looking for the 'epoch of galaxy formation' is a very hot topic in astronomy today. There are conflicting views as to whether all galaxies formed at very high redshift and have been evolving since, or if formation took place over a wide redshift interval. Key to this debate is the search for 'primordial' or 'primeval' galaxies: the proto-galaxies that presumably represent galaxies in their infancy, as they formed their first generations of stars. With technical advances in instrumentation, we are now able to see to higher and higher redshift (e.g. the Hubble Deep Field) and to observe at infrared wavelengths to see emission from young stars that would ordinarily be obscured at optical wavelengths by dust.

So at present we do know (from observations of quasars or by looking at the star formation history of the universe) that galaxies did tend to be more active in the past, and that the rate of star formation has declined to the present day (see, for example, this paper on galaxies at high redshift). That is not to say, however, that we have reached some sort of state of equilibrium, and nothing more is happening! Galaxies such as our own continue to evolve (and to form stars) and continue to merge with their neighbours (as we will do in a few billion years when the Milky Way collides with the Andromeda galaxy next door) and to swallow up smaller satellite galaxies. So in that sense new galaxies are forming, but not in the first generation sense that you mentioned.

Here are a couple of other references you might like to look at:

Lecture notes on galaxy formation

A review article on primeval galaxies

Hope this answers some of your questions!


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