Most of our information on E-galaxies comes from colours and spectral
features of integrated light interpreted with the aid of population
synthesis models based on
the theory of stellar evolution and a spectral library. A classical
result is the correlation between the Lick Mg2 index and
central velocity dispersion
(Bender 1992).
For an old population, Mg2 should be a good measure of the
overall heavy-element abundance Z, dominated by oxygen and other
-elements,
because Mg itself is one of these and Mg and Si supply 2/3 of the free
electrons providing H- opacity in red-giant
atmospheres. However, age is a complication
and the correlation with iron is more problematic (cf.
Figure 5). At face value,
based on single stellar population (SSP) models by
Worthey (1994) and by
Buzzoni (1995),
the nuclear Z or Mg abundance increases with depth of the potential
well, whereas that of iron does not: the Mg/Fe dilemma. According to
theoretical simulations by
Thomas, Greggio & Bender
(1999)
and Thomas & Kauffmann
(1999),
the expectation would be that star formation goes on for longer in the
bigger E-galaxies, making their weighted-mean age smaller and Mg/Fe
smaller rather than larger.
|
Figure 5. Plot of an iron feature against Mg2. Filled circles and squares represent the nuclear regions (central 5 arcsec) of elliptical galaxies, while the sloping lines show the mean trend with galactocentric distance in each one. Triangles show model predictions for ages of 9 (solid) and 18 Gyr (open), based on SSP models that fit features in globular clusters assuming [Mg/Fe] = 0. A young model with [Fe/H] = 0 fits the nucleus of M 32 quite well, and the predicted trends with metallicity run roughly parallel to several of the observational lines, but the trend among nuclei is not fitted at all. After Worthey, Faber & Gonzalez (1992). Courtesy Guy Worthey. |
There is also a "G-dwarf" problem, at least for nuclei, in the sense that SSP models fit the UV spectra better than those incorporating simple models of galactic chemical evolution (Bressan, Chiosi & Fagotto 1994; Worthey, Dorman & Jones 1996; Greggio 1997). One suggestion has been that the nuclei are pre-enriched with processed infalling material during a rapid clumpy collapse (Greggio 1997). The "concentration model" of Lynden-Bell (1975) may also be relevant to this situation, but according to Worthey, Dorman & Jones this is not just a nuclear problem.
Some notable results emerge from the recent study by Jørgensen (1999) of spectral features of galaxies in the Coma cluster. She confirms the existence of an age-metallicity relation as envisaged in the numerical simulations of Thomas & Kauffmann (1999), both for iron and magnesium, consistent with the view that galaxies with deep enough potential wells to hold on to their gas for longer reach higher metallicities. At any age, the galaxies with the highest velocity dispersions have the highest metallicity judged from magnesium, but for iron quite anomalously the opposite is the case, which makes one wonder about the calibration. Finally, Mg/Fe is independent of age and increases with velocity dispersion, which is hard to explain on the basis of the orthodox view of the unaided effects of a time lag for SNIa. Thomas (1999) has suggested that galactic nuclei may be affected by sporadic starbursts with a flat IMF.
The question of the IMF, or at least the yield, is also raised by the supply of iron and other elements to the X-ray gas in rich clusters of galaxies. Adapting an argument due to Renzini et al. (1993), we can start from the empirical finding of Arnaud et al. (1992) that the total mass of iron in the gas is proportional to the total optical luminosity of the E and S0 galaxies in the cluster according to
| (1) |
whence if
M* / L*
10 solar units, then
|
where the overall true yield
(2)
has been obtained by simply dividing the mass of iron by the
mass of the stars; since the iron: oxygen ratio is about solar, the same
result would have been obtained if we had considered oxygen instead of
iron. This yield, however, is very high in comparison with values of
Z
or
slightly less that come up in studies of chemical evolution in the solar
neighbourhood (e.g.
Pagel & Tautvaisiene
1995),
raising the question of whether such a high
value is actually universal and the lesser yields found in other
contexts just a consequence of mass loss from the systems. If so, it
would be sufficient to
enrich the intergalactic medium to the 1/3 of solar value postulated by
Mushotsky & Loewenstein
(1997).
2 Defined as the mass of newly synthesised and ejected heavy elements from a generation of stars divided by the mass remaining in long-lived stars and compact remnants (Searle & Sargent 1972). Back.