In Figure we plot , , and versus . For the HydraI sample, the following trends are seen: increases with , is independent of , and therefore increases with . The Coma sample is compatible with the same pattern, although the trends for and are somewhat less clear. But still is highly correlated with . It should be recalled, that the part of the Coma sample that has data is not magnitude limited.
did not directly enter the calculation of and . However, depends on as (Eq. ). The Vazdekis et al. models can be approximated by (J97; Eq. ). Measurement errors in can therefore cause a slope in the - diagrams of . However, since the measurement errors in are small compared to the range in , the effect should be small. Further, is not affected since the effect cancels out.
For the combined HydraI+Coma sample (N=83),
the correlation between
is very significant,
a Spearman rank order test gives
A fit to the HydraI+Coma sample
with the sum of the absolute residuals in
gives the relation
In Figure we plot , , and versus . The relations are more noisy than for . Still, for the combined HydraI+Coma sample, the correlation between and is significant at the 2 sigma level, we find %. It seems that the most massive galaxies ( , or ) have a smaller variation in , , and than galaxies with lower mass, but this could be due to the small number of objects. Also galaxies with high velocity dispersion (say ) have a smaller scatter than galaxies with lower velocity dispersion, but the division is not as pronounced as for the mass.
What are the implications of
Worthey, Faber, & Gonzáles (1992)
was larger than zero in giant ellipticals,
reached 0.2-0.3 dex for the
average strongest-lined galaxies.
They reached this conclusion by
comparing data with models in the
These authors discussed the following possible explanations for
Magnesium and iron are preferably produced in supernovae (SNe)
of type II and Ia, respectively.
a change in the fraction (SN II)/(SN Ia)
will give a change in
The following three scenarios can give
1. Different time scales for star formation. The progenitor stars of type II SNe are more massive and short-lived than those of type Ia SNe. Therefore, if star formation is fast, a large fraction of the total amount of gas available for star formation will have been processed by type II SNe and locked up in long-lived stars before the first generation of type Ia SNe after 1 Gyr will enrich the interstellar medium (ISM) with iron. At this point, only a small fraction of gas is left for new long-lived stars to be formed out of this iron enriched material.
2. A variable IMF slope. A smaller IMF slope (i.e. a more flat IMF) results in the formation of more massive stars, which give rise to more type II SNe.
3. Selective loss mechanisms. If for some reason a galactic wind of some sort would retain magnesium with greater efficiency than iron, then it is also possible to get . In the standard picture of SN-driven winds, the outcome is , in contradiction to the observations.
Any viable theory for star formation in elliptical galaxies should be able to explain not only per se, but also the correlation between and velocity dispersion (or mass).
Also the ages are correlated with and , see Fig. and . For the combined HydraI+Coma sample, Spearman rank order tests give 0.01% for vs. , 0.04% for vs. , 0.01% for vs. , and 0.01% for vs. . Also for the ages we find that the scatter is smaller for the most massive galaxies ( , or ).
In Fig. we plot , , , , and versus total absolute magnitude in Gunn r, . Shown on the figure is the line . JF94 found this line to demarcate two classes of E and S0 galaxies. The E and S0 galaxies fainter than this limit were best fitted by a model with 10% of the galaxies being diskless, and 90% of the galaxies being drawn from a uniform distribution of relative disk luminosity . The E (no S0 galaxies found!) brighter than this limit were all diskless.
It is seen from Fig. that also for the five quantities studied here, there is a striking difference in properties for galaxies fainter and brighter than approximately . The brighter galaxies show a smaller scatter than the fainter galaxies. The brighter galaxies have an old stellar population, with a bit above average, a bit below average, and thus somewhat above average.
Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997
Bo Milvang-Jensen (firstname.lastname@example.org)