and Cosmology

3.9 Population Synthesis
137
Fig. 3.49. a) Evolution of colors between 0 ≤ t ≤ 17 × 10 9 yr
for a stellar population with star-formation rate given by
(3.69), for five different values of the characteristic time-scale
τ (τ =∞ is the limiting case for a constant star-formation
rate) –Galactic center see solid curves. The typical colors for
four different morphological types of galaxies are plotted. For
each τ, the evolution begins at the lower left, i.e., as a blue
population in both color indices. In the case of constant star
formation, the population never becomes redder than Irr’s; to
• A simple model of star-formation history reproduces
the colors of today’s galaxies fairly well.
• (Most of) the stars in elliptical and S0 galaxies are
old – the earlier the Hubble type, the older the stellar
population.
• Detailed models of population synthesis provide
information about the star-formation history, and
predictions by the models can be compared with
observations of galaxies at high redshift (and thus
smaller age).
We will frequently refer to results from population
synthesis in the following chapters. For example, we
will use them to interpret the colors of galaxies at
high redshifts and the different spatial distributions of
early-type and late-type galaxies (see Chap. 6). Also,
we will present a method of estimating the redshift of
galaxies from their broad-band colors (photometric redshifts).
As a special case of this method, we will discuss
the efficient selection of galaxies at very high redshift
(Lyman-break galaxies, LBGs, see Chap. 9). Because
the color and luminosity of a galaxy are changing even
achieve redder colors, τ has to be smaller. The dashed line
connects points of t = 10 10 yr on the different curves. Here,
a Salpeter IMF and Solar metallicity was assumed. The shift
in color obtained by doubling the metallicity is indicated by
an arrow, as well as that due to an extinction coefficient of
E(B − V ) = 0.1; both effects will make galaxies appear redder.
b) The dependence of colors and M/L on the metallicity
of the population
when no star formation is taking place, tracing back
such a passive evolution allows us to distinguish this
passive aging process from episodes of star formation
and other processes.
3.9.7 The Spectra of Galaxies
At the end of this section we shall consider the typical
spectra of different galaxy types. They are displayed
for six galaxies of different Hubble types in Fig. 3.50.
To make it easier to compare them, they are all plotted
in a single diagram where the logarithmic flux scale
is arbitrarily normalized (since this normalization does
not affect the shape of the spectra).
It is easy to recognize the general trends in these spectra:
the later the Hubble type, (1) the bluer the overall
spectral distribution, (2) the stronger the emission lines,
(3) the weaker the absorption lines, and (4) the smaller
the 4000-Å break in the spectra. From the above discussion,
we would also expect these trends if the Hubble
sequence is considered an ordering of galaxy types