5.7.4. Accretion-driven star formation
What is the duration of these cooling flows in clusters? Both optical and X-ray clusters have been observed at moderately high redshifts, and the gas distributions in clusters are relaxed and smooth; both suggest that the intracluster medium has been present in clusters for a significant fraction of the Hubble time. As described above, the cooling times at the centers of these flows are less than the Hubble time, which suggests that the flows have persisted for a significant fraction of the cluster lifetime. Thus the total mass of gas that has cooled and been accreted by the central galaxy in the cluster should be
![]() | (5.111) |
This assumes that the accreted mass is not ejected from the cluster center.
Although this is a small fraction of the total gas mass in a cluster
( 1014
M
), it
is comparable to the mass in luminous material in a very large
(cD) galaxy. It is important to understand where this gas goes after it
cools.
Many possibilities can be ruled out. First, the gas could remain gaseous but
cooler. However, the observations of the X-ray and optical line emission
give upper limits on the total amount of ionized gas well below the total
accreted
mass. Observations of the 21 cm hydrogen line from central galaxies in
clusters with cooling flows (Section 3.7)
give upper limits on the total amount of neutral
atomic hydrogen, typically MHI
109
M
(Burns et al.,
1981a;
Valentijn and
Giovanelli, 1982).
Because of the instrumental sensitivity, current observations do
not rule out the possibility that the accreted gas could be molecular
hydrogen, although it seems likely that the large amounts of molecular gas
required would lead to an efficient conversion into stars. As mentioned
above, many of the accreting galaxies are radio sources
(Burns et al.,
1981b),
and this may indicate that some of the accreted gas flows into the
galactic nucleus and is used to power the central engine of the radio
source. However, these sources require only
10-2
M
/ yr
of gas to provide their observed radio power
(Burns et al.,
1981b),
so it is likely that the
fraction of the gas accreted by the central engine is small. Moreover,
there could not be as much as 1012
M
in the
nucleus of these galaxies,
given their observed stellar velocity dispersions
(Sarazin and O'Connell,
1983).
Generally, conversion of gas into stars is quite efficient within
galaxies, and this conversion could provide both a stable reservoir for
the accreted gas and a partial explanation for the existence of central
dominant galaxies.
Burns et al.
(1981a)
argued that star formation cannot be occurring at a high rate in these
galaxies, because they would then contain more neutral hydrogen than is
observed (see above). However,
Fabian et al.
(1982b)
and Sarazin and
O'Connell (1983)
showed that the cooling times for neutral hydrogen were short enough
that all the accreted gas could be cooling through this temperature
range and
forming stars. Within the disk of our own galaxy, stars are formed with
a very wide range of masses extending up to
100
M
(Salpeter, 1955).
Stars more massive than
10
M
produce Type II supernovae when they die,
and the rate of these supernovae would probably heat the accreted gas
sufficiently to prevent the formation of cooling flows
(Wirth et al.,
1983).
Moreover, if the spectrum of stellar masses formed from the cooling gas
(the 'initial mass function') were similar to that in our own galaxy, the
central galaxies in the accretion flows would be considerably bluer and
brighter than they are observed to be
(Fabian et al.,
1982b,
1984a;
Sarazin and O'Connell,
1983).
Burns et al.
(1981a)
gave similar arguments and
concluded that star formation cannot be the ultimate reservoir for the
cooling gas.
However, there is really no reason why the initial mass function for
star formation in these cooling flows should be the same as that in the
disk of our galaxy. If the forming stars had low masses
1
M
, these
stars would not be very different from the stars found in typical elliptical
galaxies
(Cowie and Binney,
1977).
Since star formation is very poorly
understood and there is no successful quantitative theory for this
process, one cannot calculate the initial mass function directly. However,
Fabian et al.
(1982b) and
Sarazin and O'Connell
(1983)
have given a simple plausibility
argument as to why the initial mass function for star formation in cooling
flows might be limited to low mass stars; a similar argument for elliptical
galaxies in general was given by
Jura (1977).
It is assumed that stars form eventually from the thermally unstable
clouds of gas that are seen as optical
filaments. Star formation is assumed to start when these clouds become
gravitationally unstable and can no longer support themselves against
their own
gravity and the pressure of the surrounding medium. Clouds become
gravitationally unstable when their mass exceeds the Jeans' mass, which
for a spherical, static, nonmagnetic isothermal cloud of temperature
T immersed in a low density medium of pressure P is given by
(Spitzer, 1978)
![]() | (5.112) |
Once a cloud starts to collapse, the pressure within the cloud will
increase and
the Jeans' mass may be reduced; this can cause the cloud to fragment and
result
in lower mass stars being formed. It is difficult to produce stars more
massive
than MJ, however, because before a suitably massive
cloud could be assembled,
it would become unstable and collapse. Thus it is possible that the
Jeans' mass
may provide an upper limit on the mass of the largest stars that
form. In the
disk of our galaxy, the interstellar medium typically has a pressure of
P 2 ×
103k cm-3 K, and equation (5.112) gives
MJ
50
M
. In the
cooling flows in clusters, the pressures derived from models for the X-ray
emission or determined directly from the optical line emitting filaments are
103-4 times larger (P
106-7
k cm-3 K), and thus the Jeans' mass is
MJ
1M
. Thus
it is possible that only low mass stars are formed from the
cooling gas in clusters. (A similar argument was given earlier for low
mass star formation in normal elliptical galaxies by
Jura (1977).)
In Fabian et al.
(1982)
and Sarazin and
O'Connell (1983),
this conclusion
is shown to be unaffected by the temperature dependence in equation
(5.112).
Fabian et al. (1982b) also point out that star formation in cooling flows may be different than in the disk of our galaxy because the star forming regions in these flows are unlikely to contain dust grains. In star forming regions in our galaxy, most of the refractory heavy elements are in the form of solid dust grains, and these grains absorb starlight, emit infrared radiation, and act as a heat source for the gas. Dust grains are destroyed in high temperature gas. Since the gas in cooling flows is initially very hot, any grains would have evaporated (Cowie and Binney, 1977; Fabian et al., 1982b), and it is very difficult for grains to form in low density gas, even if it is cool. Thus it is unlikely that grains will be present in the cooling flows, even in the coolest, star forming clouds. The lack of grains probably lowers the gas temperature, which also tends to reduce MJ. Further, any attempt to estimate the star forming rate in these galaxies from the infrared emission of dust (Wirth et al., 1983) is likely to greatly underestimate the real rate.
Sarazin and O'Connell
(1983)
have calculated the expected colors and optical
spectra of central galaxies assuming that their stellar populations are
a mixture
of a normal giant elliptical population with a continuously forming
population
due to accretion-driven star formation. A variety of values for the
upper mass
limit and the shape of the initial mass function were used. They found
that the accreting galaxies should have spectra and colors measurably
different from those of nonaccreting giant ellipticals (color differences of
typically
(U - V)
-0.3 mag). They also
found that with accretion
rates of typically
100
M
/ yr,
the entire stellar population of the
central galaxies in many clusters could be due to accretion-driven star
formation.
Valentijn (1983)
has attempted to measure the colors of the stellar
populations in 7 cD galaxies and their spatial variations by surface
photometry in
two colors (B and V). He finds very large color gradients within these
galaxies of typically
(B - V)
0.4 mag, with the
galaxy centers being extremely red.
Valentijn argues that these gradients are the result of accretion-driven
star formation, and that as the pressure increases inwards in the
cooling flow, the
Jeans' mass is lowered (equation 5.112) and the stellar population becomes
redder. One problem is that the innermost regions of these galaxies are
so red
that the required stellar population would have a very small mass-to-light
ratio and could not provide enough light (for the observed accretion
rates) to account for the observed galaxy luminosity. The color gradients
observed by Valentijn are very large (larger than Sarazin and O'Connell
predicted), and it is very important that they be confirmed by further
observations. Valentijn's photometry appears to disagree, in some cases,
with that of other observers
(Hoessel, 1980;
Malumuth and Kirshner,
1985).
Color gradients might also result from dust extinction, abundance gradients
in an old stellar population, or mergers of galaxies having differing
colors. A more
direct way to detect a stellar population due to accretion is to observe
absorption features due to that population in the spectrum of the cD galaxy.
The galaxy NGC1275 in the Perseus cluster
(Figure 20;
Section 4.5.2)
is particularly interesting in this regard. Its stellar surface brightness
distribution is similar to that of a typical giant elliptical galaxy
(Oemler, 1976).
However, the stellar population is very blue, and has an A-star absorption
spectrum (Kent and
Sargent, 1979),
whereas typical giant elliptical galaxies are dominated by K stars.
Sarazin and O'Connell
(1983)
show that the
colors and spectrum of this galaxy can be understood if the luminous
portion of the galaxy is entirely due to accretion-driven star formation
at a rate of
300
/ yr
as given by the X-ray observations, and the upper
mass cutoff of the stars formed is 2.8
M
. (By
contrast,
Wirth et al.
(1983)
present a model for NGC1275 in which the initial mass function
for star formation is similar to that in our galaxy, and very massive O
stars are formed. However, in this model the star formation rate is more
than an order of magnitude less than the observed rate of accretion onto
NGC1275.)
One concern with NGC1275 is the presence of the foreground spiral galaxy.
Hu et al. (1983)
have suggested that this galaxy is colliding with the cooling flow
and that this collision powers the optical line emission. It is also
possible that this collision might affect the rate and initial mass
function of star formation in the galaxy. It is thus very important to
observe the spectra of other accreting
central dominant galaxies, and see if a younger stellar population can
be detected in them. Recently,
O'Connell et al.
(1987)
obtained spectra for the inner
regions of a number of accreting galaxies. The cD in A1795, which has
a very large accretion rate
400
M
/ yr
(Table 4), has an F-star
stellar spectrum, consistent with the entire galaxy being the result of
accretion-driven star formation with an upper mass cutoff of about 1.5
M
.
Central dominant cluster galaxies appear, in many cases, to have a very
large number of globular star clusters (spherical clusters of
105-6 stars) associated with them
(Harris et al.,
1983b).
Fabian et al.
(1984b)
have suggested that these
globular clusters might be produced by accretion-driven star formation. They
note that the mass of a globular cluster is similar to the Jeans' mass of
gas in the cooling flows at a temperature of 104 K, the
temperature at
which thermally unstable clouds are repressurized. However,
Fall and Rees (1985)
showed that the cooling time was much shorter than the
free-fall time for Jeans' unstable clumps at this temperature,
preventing the gravitational instability of the gas. The cooling time
depends on the
abundance of heavy elements, which may have been considerably lower
when galaxies first formed (Section 5.10).
Thus Fall and Rees argue that
globular clusters formed out of cooling flows during the formation of
galaxies.