Galaxies and the Universe - Evolution of galaxies -...

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Evolution of GalaxiesThere are several senses in which galaxies evolve with time, not all necessarily running at the same rates forvarious kinds of galaxy. We may distinguish for convenience evolution of stellar populations, chemicalabundances, and dynamics of a galaxy, remembering that in real galaxies these are all tied together at some level.I will cover the basic principles needed to understand modelling these processes, and observational approaches toeach one.The likelihood of observing galaxy evolution can be seen from comparing several relevant timescales. TheHubble time is in the range 1-2 × 1010years. This matches the main-sequence lifetime of solar-mass stars,making it many generations of more luminous massive stars. And dynamically, the rotation period on our part ofthe Milky Way is 0.25 Gyr, so the relaxation time for a massive galaxy to form structure should be several Gyr.Population evolutioncombines the history of star formation in a galaxy and stellar evolution of its constituents,giving changes in the HR diagram of a galaxy (generally changing from place to place in the galaxy) with time.To predict the population at some time, we thus need the SFR at all previous times and an understanding ofstellar evolution for all relevant masses; see the discussion underspectrum and population synthesis. Ellipticalgalaxies, or the bulges of spirals, are popular tests of synthesis models, since they are assumed to have had mostof their stars formed in a short period long ago, and undergone changes due only to stellar evolution since then(the condition ofpassive evolution). As a sample calculation, here's a series of models of a rapid burst of starformation, from the code by Rocca-Volmerange and Guiderdoni, sampled at approximately 1-Gyr intervals:One piece of software in particularly wide use is the GISSEL package by Bruzual and Charlot (see ApJ 405, 538,1993, with later improvements). One can also predict from such results the expected history of any particularcolor index or line strength. Color indices usually have K-corrections folded in, since we can seldon tailor filtersto cover wide redshift ranges; line indices have less of a problem since they are measured spectroscopically andare nearly monochromatic for all the galaxies in a sample. Still dealing with ellipticals, Hamilton (1985 ApJ 297,371) searched for evolution in an index of the spectral break at 4000 Å (the so-called H-K break) for redshifts up
to aboutz=0.9, finding at most marginal evidence of any evolution. He presents a figure that nicely connectslook-back time and the stellar evolution timescale (and shows why such work indirectly constrains H0,connecting a redshift-scaled time to the absolute timescales of stellar lifetimes), which I have gleefully cribbedbelow (courtesy of the AAS). Note that the redshift-time mapping is sensitive to cosmological parametersincluding both the Hubble constant H0and deceleration parameterq0, plus for good measure any nonzero valueof the cosmological constant Λ.

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Term
Fall
Professor
williamkeel
Tags
Galaxies, Big Bang, Redshift, Hubble s law

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