4.1 Implications of the Correlation of Mgas / Mstellar with Tgas
Although the correlation between Mgas / Mstellar and Tgas is based on only a few systems and requires further confirmation, we briefly explore the possible implications of the correlation.
4.1.1 Efficiency of Galaxy Formation
The ratio of the gas mass to the stellar mass, Mgas / Mstellar, shown in Figure 8, can be related to the efficiency of star formation. We assume a scenario in which the luminous matter (stars and the ICM) form the bulk of the baryonic material and the remainder of the virial mass is in the form of hot or cold dark matter. Then, as long as groups and clusters are closed systems which do not lose their intracluster material, the efficiency of galaxy formation, the conversion of baryons from gas to stars in galaxies, can be written as
(2) |
where Mlum = Mstellar + Mgas, or equivalently as
(3) |
(assuming the expelled gas from galaxies is small and can be neglected). Thus by measuring Mgas / Mstellar, we can study the efficiency of star formation in systems ranging from groups to rich clusters. Our analysis shows that the star (and galaxy) formation efficiency ranges from 50% for groups to as little as 15% for rich clusters. If all the ICM in groups is gas ejected from galaxies, and we use this injection rate for all clusters, then the galaxy formation efficiency would be 100% for groups but lower for rich clusters (as low as 17% for Mgas / Mstellar = 6 to as high as 50% for Mgas / Mstellar = 3). Although the amount of luminous material (gas+stars) remains relatively constant for all clusters (Blumenthal et al. 1984), the efficiency of galaxy formation decreases as one moves to richer systems. In other words although the richest systems obviously produced more galaxies, their efficiency of galaxy formation was lower.
Interpreting the ratio of gas mass to stellar mass as a measure of galaxy formation efficiency requires that clusters be "closed" systems, that is, no material may be added or lost. The gas in the ICM is enriched both during an early phase of massive star supernovae (Type II) and continuing through the present with primarily Type I supernovae and mass loss from older stars. Since the gravitational potential of poor clusters is sufficient to bind the enriched material ejected in supernova winds driven from the galaxies, none of the material in the ICM should be lost from these systems. Furthermore, based on the computed enrichment rates, extensive amounts of matter could not have been expelled by the galaxies and entirely lost from poor and rich clusters if their ICM's are to have significant heavy-element abundances. Therefore the change in the ratio of gas mass to stellar mass with cluster richness cannot be explained by a loss of hot intracluster material from the groups and poor clusters. The relative constancy over rich and poor clusters of the fraction of the cluster virial mass made up by luminous material (stars and gas) also supports the notion of a "closed" system.
4.1.2 Correlation of Iron Abundance with Tgas
As described above (see also Jones and Forman 1990 and David et al. 1989a) the ratio of the gas to stellar mass, Mgas / Mstellar, increases from unity in groups and poor clusters with low temperatures (~ 2 keV) to values of 3-6 in systems with high gas temperatures (6-10 keV). This correlation, combined with an understanding of the production of heavy elements, predicts a correlation of heavy element abundance with gas temperature.
The groups which are luminous X-ray sources are dense systems and have stellar populations comparable to rich clusters (Morgan, Kayser, and White 1975). Also, the correlation of galaxy population with local density (Dressler 1980, and Postman and Geller 1984) supports the similarity of the galaxy populations in the groups and clusters. Therefore, the production of heavy elements should be directly proportional to the stellar light, or equivalently stellar mass, since comparable populations will have similar mass-to-light ratios. Thus, the larger the ratio of gas mass to stellar mass, the more dilute the stellar products like iron. Since Mgas / Mstellar increases with increasing Tgas, we predict that hotter clusters (those with larger Mgas / Mstellar) will have lower iron abundances than cooler clusters. This prediction assumes that the clusters and groups are closed systems, i.e. no gas is expelled or accreted.
Figure 9 shows quantitative predictions for the
correlation of iron abundance with
gas temperature. The two solid curves are derived by taking a simple
parameterization
for the dependence of Mgas /
Mstellar on Tgas and assuming that
enriched material is
expelled from galaxies only during an early wind phase during which Type
II supernovae can readily drive a galactic wind (see
David, Forman, and
Jones, 1989b).
The two curves
assume different initial mass functions (the upper curve has a power law
exponent = 2
and the lower curve has =
2.5). Note that an amount of enriched material equal to
that expelled in the wind is produced by stellar evolution and could be
liberated by
ram pressure stripping. The present estimates of supernova yields can
explain the
observed heavy element abundances in the intracluster gas as
Figure 9 shows. The
ejected gas is extremely enriched and is diluted to the observed values
by mixing with the predominantly primordial component of the
intracluster medium.
Figure 9. The iron abundance (as a fraction
of the solar value) is plotted against gas temperature. The data are
taken from
Henriksen (1985),
Hughes et
al. (1988) and
Arnaud et al. (1987).
The smooth curves are predictions based on a parameterization
of the relation between Mgas / Mstellar and
Tgas as well as a model for the evolution of
stars with two different initial mass functions.
The assumption that groups and clusters are closed systems (i.e.
gas is not expelled
or accreted in significant quantities) can be tested by observing
clusters with progressively
lower temperatures. If ejection becomes important below some temperature,
Tcrit, then one would observe an increasing heavy
element abundance from the hottest
clusters down to those with temperatures equal to
Tcrit. Below Tcrit, the winds would
serve to expel enriched material and the abundance would decline (or
remain constant) as the gas temperature decreases further.
The present measurements of iron abundances are too inaccurate to verify
the above model or test possibilities for the origin of the ICM.
Mushotzky (1984)
and Henriksen (1985)
summarize present results. For rich clusters, Henriksen reports a possible
correlation of decreasing iron abundance with increasing gas
temperature, as predicted, but
the data are not sufficiently precise to yield quantitative
results. Those observations
were for only quite luminous clusters (Lx > 2 x
1044 ergs sec-1) while in general we
expect the abundances to be highest in the low luminosity clusters.
Hughes et al. (1988)
have measured a precise iron abundance of 22% of the solar value for the
rich Coma cluster. To adequately test for differences in
abundances, it is particularly important
to obtain comparable measurements for low temperature (high galaxy formation
efficiency) systems. By determining accurate values of the heavy-element
abundances of
the ICM in both poor and rich clusters, one could better investigate the
properties of
the IMF (e.g., exponent), the efficiency of galaxy formation, and
the origin and
enrichment of the ICM. A precise determination of the heavy element
abundance of the
intracluster medium for a sample of clusters ranging from groups to rich
clusters has
implications for the amount of material in the ICM that must be
primordial. In particular,
determining a high solar abundance for the ICM in Morgan groups, as suggested
by the arguments above, would confirm that the origin of most of the hot
gas in rich clusters must be primordial.
4.1.3 The Energy Content of the ICM
The changing ratio of gas mass to stellar mass also will affect the energy (or
temperature) of the ICM. By measuring the surface brightness profiles
and independently
by measuring the ratio of the velocity dispersion to the gas
temperature, one can
estimate the energy per unit mass of the galaxies compared to that of
the gas. From the
surface brightness profiles, this value for rich clusters is generally ~
2/3. The values
calculated from the measured velocity dispersions and gas temperatures
have a wider range (but see
Flanagan (1988)
who suggests a resolution for the Perseus discrepancy).
By comparison to rich clusters, the surface brightness profiles for hot
gas around single
dominant cluster galaxies such as M87 and the cD groups such as AWM7
yield a value ~ 1/2
(Kriss, Cioffi, and
Canizares 1983;
their parameter =
-3). This implies
that the groups and individual central galaxies have more energy per
unit mass in gas
compared to the constituent galaxies than do rich clusters. For the
groups and poor
clusters where the stellar mass is comparable to the gas mass, there may
be significant
heating of the ICM by the ejected material which may account for the observed
difference between the groups and the clusters.