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 appeared.

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 unevolved.

At one level or another, all of the above are incorrect.

Cores

• 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 (KD89).
• 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).

Shapes

• 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).

Kinematics

• 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 (Prieur 1990).
• 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 1987).
• 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 1987).

Tidal Features

• 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 interactions (K82).
• 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. 1988).
• 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).

Next Time

Color & Line-Strength Gradients; Global Parameter Correlations; the Fundamental Plane of Elliptical Galaxies; Core Parameters & Galaxy Families.

References

• Bender, R. & Surma, P. 1992, Astr. Ap. 229, 441.
• 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 Elliptical Galaxies.
• 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, 1039.
• Wielen, R. (ed) 1990. Dynamics and Interactions of Galaxies.

Last modified: January 23, 1995