X-ray Observations of Groups

Most galaxies in the local universe, including our own Galaxy, belong to small groups. Even though groups are the most common environments of galaxies, we know surprisingly little about these systems, especially compared to rich clusters of galaxies. This is because group studies are hampered by small number statistics: a typical poor group contains only a few bright galaxies. Some of the outstanding questions regarding these systems include: 1) Are groups bound physical systems or simply chance superpositions of galaxies along the line of sight? 2) If groups are bound, why have more of them not merged to form a single galaxy, given their high galaxy densities and short crossing times? 3) What is the typical mass of a group and how do groups contribute to the mass density of the universe? 4) How is the evolution of galaxies different in groups than in rich clusters? and 5) How have groups evolved in time?

The recent discovery that many groups are X-ray emitters has provided considerable new insight into these important systems. ROSAT imaging studies indicate that the X-ray emission in groups is often extending well-beyond the apparent optical extent of the group. This X-ray emission is believed to be bremstralung emission from a very low density, highly ionized gas. I like to refer to this extended gas component as the intragroup medium in analogy to the X-ray emitting intracluster medium found in rich clusters of galaxies. The X-ray data can be used to derive many of the most important properties of the gas: 1) the global gas temperature; 2) the projected temperature as a function of radius; 3) the projected gas density as a function of radius and 4) the \lq\lq metallicity\rq\rq \ of the gas, derived from the strengths of emission lines of abundant elements like iron, oxygen, silicon and sulfur. With these observables and minor assumptions about the state of the hot gas, we can probe the dynamics of groups and the star formation history of the group galaxies.

Perhaps the most fundamental quantity of a group is its total mass. X-ray observations allow us to obtain an estimate for this quantity that is independent of other methods. Since the sound crossing times in groups is short compared to a Hubble time, the intragroup medium should be in hydrostatic equilibrium. When the condition of hydrostatic equilibrium is met, the total mass interior to any particular radius is a simple function of the gas temperature and gas density. As these quantities can be measured directly from the X-ray observations, the mass of the system can be calculated. In the last few years, Richard Mushotzky (NASA/GSFC), Dave Davis (MIT), David Burstein (ASU) and I have applied the hydrostatic equilibrium technique to approximately 40 poor groups. The typical group mass is approximately one-fifth the mass of a cluster like Virgo. However, because the number density of X-ray detected groups is approximately five to ten times higher than the number density of clusters like Virgo, the contribution of X-ray detected groups to the total mass density of the universe is comparable to that of rich clusters, confirming the comological significance of these poorer systems.

X-ray observations of groups are also allowing us to address the question of the origin of the intragroup medium. In particular, metallicity measurements may help distinguish between a mostly primordial origin for the gas and gas reprocessed through stars in the galaxies. In clusters, it is thought that metals in the intracluster medium were produced in elliptical galaxies since the mass in metals correlates well with the total optical light in elliptical galaxies; i.e. a fixed amount of starlight appears to contaminate a fixed amount of intracluster gas. Similarly, in groups it also seems likely that the primary polluters are elliptical galaxies. We find that the metallicity of the intragroup medium varies significantly from group to group with some systems being very metal poor (10-20% solar), while others are enriched (50-60% solar) compared to clusters. Furthermore, there is evidence of a correlation between the metallicity of the gas and the temperature of the gas: the gas metallicity increases as one moves from the coolest, low mass systems to the hotter, high mass systems. There is also a strong correlation between the temperature of the intragroup medium and the early-type fraction of the groups: the cool, low mass groups contain mostly spirals, while the hotter groups contain mostly ellipticals. Thus, the correlation between temperature and metallicity may simply reflect the fact that the hotter groups contain more elliptical galaxies.

Although elliptical galaxies may be responsible for the enrichment of the hot gas in both groups and clusters, there are hints that the nucleosynthetic histories of galaxies in these two types of systems are very different. For example, the abundance ratio of silicon to iron is approximately twice the solar value in rich clusters, implying that most of the enrichment of the intracluster medium was by Type II supernovae. In contrast, the silicon to iron ratio is groups is solar, suggesting Type II supernovae were less prevalent in groups.

Significant gains in our understanding of the hot gas in groups can be expected in the next few years with the launch of three new powerful X-ray telescopes: AXAF, XMM and ASTRO-E. Together these instruments will allow the spatial distribution of the gas and its chemical composition to be determined in great detail.

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