Observations of Elliptical Galaxies. II
Astronomy 626: Spring 1995
Correlations between the global parameters of elliptical galaxies
lead to a variety of useful relationships, the most important being
the fundamental plane. Correlations between core parameters
imply that ellipticals are similar to the bulges of disk galaxies,
but that both are different from dwarf spheroidal galaxies.
Color & Line-Strength Gradients
- As a rule, E galaxies become redder toward their
centers; a factor of 10 decrease in radius typically
produces an ~0.03 change in b-V and a
~0.10 change in u-V (KD89). The bulges of
disk galaxies show even stronger gradients (Wirth & Shaw
- Absorption features in the integrated spectra of E galaxies
also show significant gradients with radius: Metal lines
become stronger with decreasing radius, while H-beta lines may
get stronger or weaker. Line-strength gradients in
bright galaxies are smaller than those in faint galaxies
but there are large variations from galaxy to galaxy (Gorgas
& Efstathiou 1987, Franx et al. 1989).
- Color and line-strength gradients are both consistent with the
interpretation that metalicity is the primary factor
varying with radius (Faber 1977). The logarithmic slope of the
metal abundance is estimated to be about -0.2 for a
typical elliptical (KD89).
- Spectral synthesis models indicate that the stars at any
given radius span a range in metal abundance (e.g.
Pickles 1987). This is consistent with the ~2 dex
spread in metalicity seen in the bulge of the Milky Way
(Whitford & Rich 1983).
- Evidence for a spread in ages may be present in the deep
Balmer lines seen some systems and in indications of blue
light above & beyond what one expects for a purely old
population (Pickles 1987).
Global Parameter Correlations
- Integrated colors of E galaxies show systematic trends with
luminosity: Brighter galaxies are redder by ~0.10 in
u-V per magnitude (Mihalas & Binney 1981, Chap.
5-4). As within individual galaxies, it appears that here too
the changes in color may be largely explained by changes in
- The luminosities of E galaxies are highly correlated with
their velocity dispersions; this is the Faber-Jackson
relation, generally expressed as a power law,
(1) L ~ sigma ,
where L is the galaxy luminosity, sigma is
the central line-of-sight velocity dispersion, and
the index n is about 4, but shows
significant variations from sample to sample (KD89).
- A significant correlation is also seen between the effective
(or half-light) radius, R_e, and the surface
brightness at that radius, I_e, of the form (KD89)
(2) R_e ~ I_e .
The Fundamental Plane of Elliptical Galaxies
Core Parameters & Galaxy Families
Due date: 2/2/95
6. Suppose that all galaxies are actually extremely thin disks,
aligned randomly to our line of sight. What distribution of axial
ratios would we expect to see in this case?
7. Using Eq. 4, the definition of the mass-to-light ratio
(M/L), and the assumption that all elliptical galaxies have
similar forms, derive Eq. 5.
8. How could you use the correlation between central surface
brightness I_0 and core radius r_c (see Eq. 7) as a
tool for measuring distances? What kind of accuracy could you
achieve, assuming all the scatter shown in Fig. 6 of K87 is
- de Zeeuw, T. (ed) 1987. Structure and Dynamics of
- Faber, S.M. 1977. in The Evolution of Galaxies and Stellar
Populations, ed. B.M. Tinsley & R.B. Larson, p. 157.
- Franx, M. et al. 1989. A. J. 98, 538.
- Gorgas, J. & Efstathiou, G. 1987. in de Zeeuw 1987, p. 189.
- Kormendy, J. 1985. Ap. J. 295, 73.
- Kormendy, J. 1987. in de Zeeuw 1987, p. 17 (K87).
- Kormendy, J. & Djorgovski, S. 1989. Ann. Rev. Astr.
Ap. 27, 235 (KD89).
- Mihalas, D. & Binney, J.J. 1981, Galactic Astronomy.
- Pickles, A. 1987. in de Zeeuw 1987, p. 203.
- Whitford, A.E. & Rich, R.M. 1983. Ap. J. 274,
- Wirth, A. & Shaw, R. 1983. A. J. 88, 171.
Joshua E. Barnes
Last modified: January 24, 1995