Annu. Rev. Astron. Astrophys. 2003. 41: 191-239 Copyright © 2003 by Annual Reviews. All rights reserved

3. BASIC PROPERTIES OF ELLIPTICAL GALAXIES AND THEIR HOT GAS

If elliptical galaxies were perfectly homologous stellar systems with identical stellar populations, then the "central" velocity dispersion _o, stellar mass M and half-light (effective) radius R_e would be related by the virial theorem, _o² = (M / R_e) = (L_V / R_e)(M / L_V) with constant . Instead, non-homology and/or stellar population variations conspire to place elliptical galaxies on a nearby fundamental plane _o² (L_V / R_e)[R_e^0.22 _o^0.49], implying that (M / L_V) R_e^0.22 _o^0.49 M^0.24 R_e^-0.02 L_V^0.32 R_e^-0.03 (Dressler et al. 1987; Djorgovski & Davis 1987). The width of the fundamental plane is remarkably small (Renzini & Ciotti 1993). In projection the fundamental plane indicates that the binding energy per unit mass decreases with stellar (or galactic) mass, _o² M^0.32 (Faber et al. 1997), i.e., hot interstellar gas is less bound in low mass ellipticals. This regularity in the global properties of elliptical galaxies is useful in interpreting the X-ray emission from the hot interstellar gas they contain.

The internal structure is also quite uniform among massive E galaxies. For those with de Vaucouleurs (r^1/4) stellar profiles the stellar distribution is completely determined by the optical half-light radius R_e and the total stellar mass M_*t. Provided the dark halos consist of cold, non-interacting particles, the halo density profile is determined by its virial mass, M_dh, the mass enclosing a cosmic overdensity of ~ 100 relative to the critical density _c = 3H₀² / 8 G in a flat CDM universe with _M = 0.3 (Eke, Navarro, & Frenk 1998; Bullock et al. 2000). If elliptical galaxies formed by mergers in galaxy groups, as generally believed, then the hot gas they contain must be understood in this evolutionary context. The r^1/4 stellar density profile in ellipticals and the high incidence ( 50 percent) of counter-rotating or kinematically independent stellar cores are natural consequences of hierarchical mergers (Hernquist & Barnes 1991; Bender 1996). While many massive E galaxies currently reside in the dense cores of rich galaxy clusters, the orbital velocities of cluster member galaxies are too high for efficient mergers.

In spite of these regularities, giant elliptical galaxies come in two flavors depending on their mass or optical luminosity. Low luminosity ellipticals have power law central stellar density profiles, disky isophotes, and oblate symmetry consistent with their moderate rotation. Ellipticals of high luminosity have flatter stellar cores within a break radius r_b (typically a few percent of R_e), boxy isophotes, and have aspherical structures caused by anisotropic stellar velocities, not by rotation which is generally small (Kormendy & Bender 1996; Faber et al. 1997; Lauer et al. 1998). Nevertheless, many of the most luminous E0 and E1 galaxies are thought to be intrinsically spherical to a reasonable approximation (Merritt & Tremblay 1996) and these galaxies are among the most luminous in X-rays. The transition from power-law to core ellipticals occurs gradually over -20 M_V - 22 where both types coexist on the same fundamental plane. Finally, optical spectra of ellipticals often show evidence of a (sub)population of younger stars, particularly in ellipticals with power-law profiles (e.g. Trager et al. 2000; Terlevich & Forbes 2002).

The X-ray luminosities L_x of elliptical galaxies correlate with L_B in a manner that also exhibits a transition with increasing mass (Canizares et al. 1987; Donnelly et al. 1990; White & Sarazin 1991; Fabbiano, Kim & Trinchieri 1992; Eskridge, Fabbiano, & Kim 1995a, b, c; Davis & White 1996; Brown & Bregman 1998; Beuing et al. 1999). For elliptical galaxies of low luminosity (i.e. L_B L_{B, crit} = 3 × 10⁹ L_B, ) O'Sullivan et al. (2001) find that the bolometric X-ray and optical luminosities are approximately proportional, L_x L_B. The X-ray emission from these galaxies is apparently dominated by low-mass X-ray binary stars with a different (typically harder) spectrum than that of the interstellar gas (Brown & Bregman 2001). The brightest stellar X-ray sources can be individually resolved with Chandra (e.g. Sarazin, Irwin & Bregman 2000; Blanton, Sarazin, & Irwin 2001a). However, the X-ray emission from more luminous ellipticals (L_B L_{B, crit}) varies approximately as L_x L_B² (O'Sullivan et al. 2001), clearly indicating a non-stellar origin, i.e., the hot gas. As shown in Figure 1, the scatter about this correlation is enormous, L_x varies by almost two orders of magnitude for galaxies with similar L_B.

Figure 1. A plot of the bolometric X-ray luminosity and B-band optical luminosity for elliptical galaxies (RC3 type T - 4) from the compilation of O'Sullivan et al. (2001). X-ray detections are shown with filled circles and upper limits with open triangles. The dashed line is an approximate locus of the total luminosity Lx,* LB of stellar and other discrete sources also from O'Sullivan et al.

The large scatter in the L_x L_B² correlation has received much attention but an explanation in terms of some specific environmental or intrinsic property of the galaxies has been elusive (e.g. Eskridge, Fabbiano, & Kim 1995a, b, c). White and Sarazin (1991) and Henriksen & Cousineau (1999) find that ellipticals having other massive galaxies nearby have systematically lower L_x / L_B while Brown & Bregman (2000) find a positive correlation with L_x / L_B and the local density of galaxies. Pellegrini (1999) presented evidence that ellipticals with power law profiles have much smaller range in L_x / L_B, as might be expected if a larger fraction of L_x in these galaxies has a stellar origin (Irwin & Sarazin 1998). Power law ellipticals tend to be non-central in groups/clusters and therefore tidally subordinate with some exceptions. More luminous group-centered, group-dominant ellipticals typically have much larger L_x / L_B, enhanced by an additional contribution of circumgalactic or intragroup hot gas (Helsdon et al. 2001; Matsushita 2001). The possible influence of galactic rotation and (oblate) flattening on L_x / L_B have been explored with limited success (Nulsen et al. 1984; Kley & Mathews 1995; Brighenti & Mathews 1996; Pellegrini, Held & Ciotti 1997; D'Ercole & Ciotti 1998). While there is some evidence that L_x / L_B is several times lower in flattened and rotating ellipticals, this cannot explain the much larger scatter observed. In the evolutionary scheme of Ciotti et al. (1991) the scatter results from a transition from early Type Ia driven winds to cooling inflows, but this scenario (discussed below) produces too much iron in the hot gas (Loewenstein & Mathews 1991). Mathews and Brighenti (1998) showed that ellipticals with larger L_x also have more extended hot gas, L_x / L_B (R_ex / R_e)^0.60±0.20, where R_ex is the half-brightness radius for the X-ray image which can extend out to ~ 10R_e or beyond. This correlation (see also Fukugita & Peebles 1999 and Matsushita 2001) may result from the tidal competition for diffuse baryonic gas among elliptical-dominated galaxy groups as they formed with different degrees of relative isolation. Ram pressure stripping must also influence L_x / L_B for E galaxies in richer clusters (e.g. Toniazzo & Schindler 2001).