Annu. Rev. Astron. Astrophys. 2000. 38: 289-335 Copyright © 2000 by Annual Reviews. All rights reserved

5.8. Fossil Groups

Because of their relatively low velocity dispersions and high galaxy densities, groups of galaxies provide ideal sites for galaxy-galaxy mergers. Numerical simulations suggest that the luminous galaxies in a group will eventually merge to form a single elliptical galaxy (Barnes 1989, Governato et al 1991, Bode et al 1993, Athanassoula et al 1997). The merging timescales for the brightest group members (M M^*) are typically a few tenths of a Hubble time for an X-ray detected group (Zabludoff & Mulchaey 1998). Therefore, by the present day some groups have likely merged into giant ellipticals. Outside of the high-density core, the cooling time for the intragroup medium is longer than a Hubble time; thus, while the luminous galaxies in some groups have had enough time to merge into a single object, the large-scale X-ray halo of the original groups should remain intact. This means that a merged group might appear today as an isolated elliptical galaxy with a group-like X-ray halo (Ponman & Bertram 1993).

Using the ROSAT All-Sky Survey data, Ponman et al (1994) found the first such "fossil" group candidate. The RXJ1340.6+4018 system has an X-ray luminosity comparable to a group, but ~ 70% of the optical light comes from a single elliptical galaxy (Jones et al 2000). The galaxy luminosity function of RXJ1340.6+4018 indicates a deficit of galaxies at approximately M^*. The luminosity of the central galaxy is consistent with it being the merger product of the missing M^* galaxies. Jones et al (2000) have studied the central galaxy in detail and find no evidence for spectral features implying recent star formation, which indicates the last major merger occurred at least several gigayears ago.

Several other fossil group candidates are now known. Mulchaey & Zabludoff (1999) discovered a large X-ray emitting halo around the optically-selected isolated elliptical NGC 1132. Although the NGC 1132 system contains no other luminous galaxies, there is evidence for an extensive dwarf galaxy population clustered around the central galaxy. The dwarfs in NGC 1132 are comparable in number and distribution to the dwarfs found in X-ray groups (Zabludoff & Mulchaey 1998). The existence of a clustered dwarf population in fossil groups is not surprising because the galaxy-galaxy merger and dynamical friction timescales for faint galaxies in groups are significantly longer than the timescales for the luminous galaxies (Zabludoff & Mulchaey 1998). Hence, the dwarf galaxy population, like the X-ray halo, will remain long after the central elliptical has formed.

Vikhlinin et al (1999) have found four potential fossil groups in their large-area ROSAT survey of extended X-ray sources. (Their sample includes RXJ1340.6+4018 and two X-ray sources detected in earlier Einstein surveys but not previously recognized as potential group remnants.) Given the large surface area they covered in their survey, Vikhlinin et al were able to estimate the spatial density of X-ray fossil groups for the first time and found that these objects represent ~ 20% of all clusters and groups with an X-ray luminosity greater than 5 × 10⁴² h₁₀₀^-2 ergs s^-1. The number density of fossil groups is comparable to the number density of field ellipticals, so most, if not all, luminous field ellipticals may be the product of merged X-ray groups.

Although the X-ray and optical properties of some luminous, isolated elliptical galaxies are consistent with the merged group interpretation, another possibility is that these systems may have simply formed with a deficit of luminous galaxies (Mulchaey & Zabludoff 1999). Distinguishing between these two scenarios will be difficult, if not impossible. Regardless, these objects are massive enough and found in large enough numbers that they are cosmologically important. Vikhlinin et al (1999) estimated that the contribution of fossil groups to the mass density of the universe is comparable to the contribution of massive clusters. These objects are also an important reminder that galaxies are not always good tracers of mass and large-scale structure: optical group catalogs would miss these large mass concentrations.