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

5.3. Baryon Fraction

The ratio of baryonic to total mass in groups and clusters can provide interesting constraints on cosmological models (e.g. Walker et al 1991, White et al 1993). The two known baryonic components in groups are the galaxies and the hot gas. The total mass in galaxies can be estimated by measuring the total galaxy light and assuming an appropriate mass-to-light ratio for each galaxy based on its morphological type. While ideally the luminosity function of each group should be used to measure the total light, generally most authors have included only the contribution of the most luminous galaxies. Fortunately, these galaxies account for nearly all the light in the group. The mass-to-light ratios of X-ray groups are generally in the range M / L_B ~ 120-200 h₁₀₀ M / L (Mulchaey et al 1996a), which is comparable to the mass-to-light ratios found in rich clusters. However, these estimates are made out to the radius of X-ray detection, so the values out to the virial radius could be larger. Assuming standard mass-to-light ratios for ellipticals and spirals, the mass in galaxies in X-ray groups is typically in the range 3 × 10¹¹-2 × 10¹² h^-1₁₀₀ M (Figure 9).

Figure 9. Distribution of galaxy mass for the sample of groups used in Figure 8.

The mass in the intragroup medium can be estimated from the model fit to the surface brightness profile. The gas-mass estimates depend both on the radius out to which X-rays are detected (Henriksen & Mamon 1994) and on the spectral properties assumed (for example, the gas metallicity; Pildis et al 1995). For these reasons, different authors have derived significantly different gas masses for the same systems (cf Mulchaey et al 1996a). For most groups, the gas mass is in the range ~ 2 × 10¹⁰-10¹² h₁₀₀^-5/2 M (Figure 10). This is somewhat less than or comparable to the mass in galaxies. Note, however, that the gas mass is much more strongly dependent on H_o, and for more realistic (i.e. lower) values of H_o, the gas mass can be somewhat higher than the galaxy mass. The observed gas mass-to-stellar mass ratio tends to decrease as the temperature of the system decreases. This trend extends from rich clusters to individual elliptical galaxies. David et al (1995) estimate that the gas-to-total mass fraction is approximately 2% in ellipticals, 10% in groups and 20-30% in rich clusters. However, the hot gas in groups is detected to a much smaller fraction of the virial radius than in rich clusters, so comparisons made at the current level of X-ray detection may not accurately reflect the global gas fractions (Loewenstein 2000). In fact, much of the intragroup gas probably lies beyond the current X-ray detection limits, and on more global scales, groups may not be gas-poor compared to clusters. Consequently, the total gas masses of groups may be severely underestimated by ROSAT observations. On scales of the virial radius, the intragroup medium is likely the dominant baryonic component in these systems. In fact, Fukugita et al (1998) estimated that diffuse gas in groups is the dominant baryon component in the nearby universe. A fundamental assumption in Fukugita et al's calculation is that all groups contain an intragroup medium and that the absence of X-ray detections in many groups is primarily a result of lower virial temperature rather than the absence of plasma. Regardless of whether this assumption is valid or not, it is now clear that intragroup gas is an important baryonic constituent of the local universe.

Figure 10. Distribution of intragroup medium mass for the sample of groups used in Figure 8.

Adding up the baryons in galaxies and intragroup gas and comparing to the total mass, one finds that the known baryonic components typically account for only 10-20% of the total mass that is derived using the X-ray data (Figure 11; Mulchaey et al 1993; Ponman & Bertram 1993; David et al 1994; Pildis et al 1995; David et al 1995; Doe et al 1995; Davis et al 1995, 1996; Mulchaey et al 1996a; Pedersen et al 1997). This provides some of the strongest evidence to date that small groups of galaxies are dominated by dark matter. The ratio of mass in observed baryonic components to total mass (i.e. the "baryon fraction") in general is smaller in groups than in rich clusters (David et al 1995, David 1997). However, the lower observed baryon fractions of groups may largely reflect the fact that much of the hot gas occurs beyond the radius of current X-ray detection. Even if the observed baryon fractions of groups are representative of the global values, the baryon fractions in X-ray groups are still too high to be consistent with the low baryon fractions required for = 1 and standard big bang nucleosynthesis (cf White et al 1993).

Figure 11. Distribution of total observed baryonic mass to total group mass for the sample of groups used in Figure 8. The low "baryonic fractions" derived for groups indicate that these systems are dominated by dark matter.