and Cosmology

9.6 Galaxy Formation and Evolution
9.6.4 Cosmic Downsizing
The hierarchical model of structure formation predicts
that smaller-mass objects are formed first, with more
massive systems forming later in the cosmic evolution.
As discussed before, there is ample evidence for this
to be the case; e.g., galaxies are in place early in the
cosmic history, whereas clusters are abundant only at
redshifts z 1. However, looking more closely into the
issue, apparent contradictions are discovered. For example,
the most massive galaxies in the local Universe,
the massive ellipticals, contain the oldest population of
stars, although their formation should have occurred
later than those of less massive galaxies. In turn, most
of the star formation in the local Universe seems to
be associated with low- or intermediate-mass galaxies,
whereas the most massive ones are passively evolving.
Now turning to high redshift: for z ∼ 3, the bulk of star
formation seems to occur in the LBGs, which, according
to their clustering properties (see Sect. 9.1.1), are
associated with high-mass halos. The study of passively
evolving EROs indicates that massive old galaxies were
in place as early as z ∼ 2, hence they must have formed
very early in the cosmic history. The phenomenon that
massive galaxies form their stars in the high-redshift
Universe, whereas most of the current star formation
occurs in galaxies of lower mass, has been termed
“downsizing”.
This downsizing can be studied in more detail using
redshift surveys of galaxies. The observed line
width of the galaxies yields a measure of the characteristic
velocity and thus the mass of the galaxies
(and their halos). Such studies have been carried out
in the local Universe, showing that local galaxies
have a bimodal distribution in color (see Sect. 3.7.2),
which in turn is related to a bimodal distribution in
the specific star-formation rate. Extending such studies
to higher redshifts, by spectroscopic surveys at
fainter magnitudes, we can study whether this bimodal
distribution changes over time. In fact, such
studies reveal that the characteristic mass separating
the star-forming galaxies from the passive ones
evolves with redshift, such that this dividing mass
increases with z. For example, this characteristic
mass decreased by a factor of ∼ 5 between z = 1.4
and z = 0.4. Hence, the mass scale above which most
galaxies are passively evolving decreases over time,
restricting star formation to increasingly lower-mass
galaxies.
Studies of the fundamental plane for field ellipticals
at higher redshift also point to a similar conclusion.
Whereas the massive ellipticals at z ∼ 0.7 lie on the
fundamental plane of local galaxies when passive evolution
of their stellar population is taken into account,
lower-mass ellipticals at these redshifts have a smaller
mass-to-light ratio, indicating a younger stellar population.
Also here, the more massive galaxies seem to be
older than less massive ones.
Another problem with which models of galaxy formation
are faced is the absence of very massive galaxies
today. The luminosity function of galaxies is described
reasonably well by a Schechter luminosity function, i.e.,
there is a luminosity scale L ∗ above which the number
density of galaxies decreases exponentially. Assuming
plausible mass-to-light ratios, this limiting luminosity
translates into a halo mass which is considerably lower
than the mass scale M ∗ (z = 0) above which the abundance
of dark matter halos is exponentially cut-off. In
fact, the shape of the mass spectrum of dark matter halos
is quite different from that of the stellar mass (or luminosity)
spectrum of galaxies. Why, then, is there some
kind of maximum luminosity (or stellar mass) for galaxies?
It has been suggested that the value of L ∗ is related
to the ability of gas in a dark matter halo to cool; if the
mass is too high, the corresponding virial temperature
of the gas is large and the gas density low, so that the
cooling times are too large to make gas cooling, and thus
star formation, efficient. With a relatively high cosmic
baryon density of Ω b = 0.045, however, this argument
fails to provide a valid quantitative explanation.
The clue to the solution of these problems may come
from the absence of cooling flows in galaxy clusters.
As we saw in Sect. 6.3.3, the gas density in the inner
regions of clusters is large enough for the gas to cool
in much less than a Hubble time. However, in spite of
this fact, the gas seems to be unable to cool, for otherwise
the cool gas would be observable by means of
intense line radiation. This situation resembles that of
the massive galaxies: if they were already in place at
high redshifts, why has additional gas in their halos
(visible, e.g., through its X-ray emission, and expected
in structure formation models to accrete onto the host
halo) not cooled and formed stars? The solution for
this problem in galaxy clusters was the hypothesis that
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