Observations of Elliptical Galaxies. I
Astronomy 626: Spring 1995
In the last 20 years our notions about E galaxies have changed
radically; these galaxies are much more complicated than they first
Folklore & Mythology
Traditionally, elliptical galaxies were seen as rather simple
systems. Their overall luminosity profiles were fit by the de
Vaucouleurs (1948) law, while the cores seen in surface photometry of
their central regions were frequently fit using King (1966) models.
In form, these galaxies were often assumed to be oblate spheroids,
flattened by rotation. The stars within them were viewed as belonging
to a single, ancient population analogous to the bulge and halo
populations of our galaxy; gas and dust were thought to be absent.
Finally, elliptical galaxies were considered to be dynamically
At one level or another, all of the above are incorrect.
- Seeing corrections are important; moreover, it's generally not
possible to make corrections without some assumptions about
the underlying luminosity distribution (K87).
- Few E galaxies actually have flat luminosity profiles at
small radii; instead, the profiles rise inward to the last
measured point (K87).
- Cores may exhibit unusual kinematics; for example, about a
quarter of all elliptical galaxies have cores which appear to
counterrotate with respect to the rest of the galaxy
- Although such `kinematically decoupled' cores are generally
not photometrically distinct, several E galaxies with
decoupled cores have features in their line-strength profiles
coincident with the kinematically decoupled regions (Bender
& Surma 1992).
- A few nearby E galaxies have nuclear star clusters
with densities much higher than the cores they reside in;
some of these nuclei may be rotating, disk-like systems (KD89).
- Projected axial ratios range from b/a = 1 to
~0.3, but not flatter (Schechter 1987).
- Apparent ellipticity is generally a function of projected
radius, with a wide range of profiles (Jedrzejewski 1987).
- Isophotal twists are common. Because it is highly unlikely
that intrinsically twisted galaxies could be
dynamically stable, such twists are generally interpreted as
evidence for triaxiality (K82).
- Elliptical galaxies are not elliptical; isophotes may
depart significantly from perfect ellipses. A Fourier
analysis of isophotal radius in polar coordinates implies that
most E galaxies are either `boxy' or `disky'; the amplitude
a(4) of the cos(4 theta) term is negative
for the former, and positive for the latter (KD89).
- The rotation velocities of bright E galaxies are much
too low to account for the flattenings we observe; fainter E
galaxies, however, rotate at about the rates implied by their
shapes (Davies 1987).
- E galaxies may exhibit minor-axis rotation; more generally,
the apparent rotation axis and the apparent minor axis may be
misaligned (Franx et al. 1991). While in most galaxies
these misalignments are modest, a few galaxies appear to rotate
primarily about their minor axes.
Shells & Other `Fine Structures'
- The surface brightnesses of E galaxies do not always decline
smoothly with radius. When a smooth luminosity profile is
subtracted from the actual surface brightness, `shells' or
`ripples', centered on the galaxy, are seen (Prieur 1990).
- The fraction of field E galaxies with shell-like features is
at least 17% and possibly more than 44% (KD89).
- The colors of shells indicate that they are composed of stars.
In many cases the shells are somewhat more blue than the
galaxies they occupy (KD89).
- Shell systems have a variety of morphologies; some galaxies
have shells transverse to the major axis and interleaved on
opposite sides of the center of the galaxy, while other
galaxies have shells distributed at all position angles
- Profile subtraction sometimes reveals other kinds of
structures in E galaxies, including embedded disks, linear
features or `jets' (not the jets seen in AGNs!),
`X-structures', etc. (Schweizer & Seitzer 1992).
Gas & Dust
- When examined with sufficient resolution, 25% to more than 40%
of E galaxies show features due to dust absorption (KD89).
- The dust lanes seen in E galaxies imply that the absorbing
material is distributed in rings or disks. Dust lanes may be
aligned with either the major or minor axes, or they may be
warped (Bertola 1987, Schweizer 1987, KD89).
- E galaxies contain modest amounts of cool and warm gas,
although not as much as is found in S galaxies. A few E
galaxies have extended disks of neutral hydrogen (Schweizer
- X-ray observations indicate that many ellipticals contain
10^9 to 10^10 solar masses of gas at
temperatures of ~10^7 K; this hot gas typically forms
a pressure-supported `atmosphere' around the galaxy (Schweizer
- Elliptical galaxies in rich galaxy clusters often exhibit
luminosity profiles which fall below a de Vaucouleurs law at
large radii. Such downturns are often attributed to tidal
truncation in the mean field of the cluster (K82).
- In contrast, E galaxies with close companions often have
luminosity profiles which rise above a de Vaucouleurs law at
large radii. These features may be plausibly blamed on tidal
- E galaxies in closely interacting systems sometimes exhibit
outer isophotes which are visibly egg-shaped and/or offset
with respect to the centers of their galaxies. Again, tidal
effects are strongly implicated (K82, Borne et al.
- On very deep exposures, some E galaxies are seen to have
`plumes' or `tails', while others (e.g. NGC 5128) show
rather irregular luminosity distributions. Tail-like
features may be signatures of major mergers involving one or
more dynamically cold disk galaxies (Schweizer 1987).
Color & Line-Strength Gradients; Global
Parameter Correlations; the Fundamental Plane of Elliptical Galaxies;
Core Parameters & Galaxy Families.
- Bender, R. & Surma, P. 1992, Astr. Ap. 229,
- Bertola, F. 1987. in de Zeeuw 1987, p. 135.
- Borne, K. et al. 1988. Ap. J. 333, 567.
- Davies, R.L. 1987. in de Zeeuw 1987, p. 63.
- de Zeeuw, T. (ed) 1987. Structure and Dynamics of
- Franx, M. et al. 1991. Ap. J. 383 112.
- Jedrzejewski, R.I. 1987. in de Zeeuw 1987, p. 37.
- King, I. 1966. A. J. 71, 64.
- Kormendy, J. 1982. in Morphology and Dynamics of
Galaxies, ed. L. Martinet & M. Mayor, p. 113 (K82).
- Kormendy, J. 1987. in de Zeeuw 1987, p. 17 (K87).
- Kormendy, J. & Djorgovski, S. 1989. Ann. Rev. Astr.
Ap. 27, 235 (KD89).
- Prieur, J.-L. 1990, in Wielen, R. 1990, p. 72.
- Schechter, P. 1987. in de Zeeuw 1987, p. 217.
- Schweizer, F. 1987. in de Zeeuw 1987, p. 109.
- Schweizer, F. & Seitzer, P. 1992, A. J., 104,
- Wielen, R. (ed) 1990. Dynamics and Interactions of
Joshua E. Barnes
Last modified: January 23, 1995