Prof Mihos has a new Java applet called PopSynth that uses models of how galaxy properties such as color and luminosity change as they age, to see what we can infer about the galaxy's star formation history. We are the beta testers.
You can access the applet here. Set metallicity Z=0.02 (which is solar metallicity) and set starburst strength b=0 (or the applet doesnt work). You may need to fiddle with the axis ranges to get all the plot on the page at once.
(a) Work out how the age distribution of a galaxy's stars changes with different star formation history. "Single burst" has all the star formation happen instantaneously at t=0. "Constant" has a constant star formation rate from t=0 to t=15 Gyr. "Declining" means that the star formation rate varies as e^(-t/tau), with tau of 2 or 5 Gyr. Assume that "Rising" is an exponential rise with tau = 5 Gyr. Plot the age distribution at t=13.5 Gyr of the galaxy's stars for each option.
(b) You can find the Mv and B-V color of an A0 and K0 star in the table below. How much brighter is an A0 star than a K0 star?
Using the age distributions derived above, and assuming that the main sequence lifetimes are 10^7 and greater than 2x10^10 years respectively, work out what proportion of the stars from the single burst and constant, histories will be A0 and K0 stars at t=13.5 Gyr (make B and F stars honorary A stars, G and M stars honorary K stars). Make a rough estimate of the B-V color of each of these two star formation histories at 13.5 Gyr.
(c) Now, use the applet to explore more fully how a galaxy's light output (luminosity and color) changes as it ages, for different star formation histories. Plot (and print out) the B-V color against time and the V magnitude against time for the single burst and constant histories. Use your results from (b) above to explain these plots.
(d) Now look at the data for elliptical galaxies ("RC3 Elliptical" -- the RC3 is a catalog of galaxy properties for nearby galaxies, so at the present time, t=13.5 Gyr) and for late type spirals ("RC3 Sc/d"). On the plots of B-V color vs time, you only see a band for the range of colors; also plot U-B vs B-V color so you can see individual galaxy datapoints and get a feeling for how the data are distributed in that band. Using the results from (c) and the color distributions, discuss what you can infer about the star formation histories of elliptical and late type spiral galaxies from these plots.