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

5.10. The Local Group

Finally, it is interesting to consider the implications X-ray observations of other groups have for our own Local Group. The idea that the Local Group might contain a hot intragroup medium dates back to the work of Kahn & Woltjer (1959). The X-ray detection of other groups has led to renewed interest in this idea. Suto et al (1996) proposed that a hot halo around the Local Group with a temperature of ~ 1 keV and column density N_H ~ 10²¹ cm^-2 could explain the observed excess in the X-ray background below 2 keV. The X-ray halo would also generate temperature anisotropies in the microwave background via the Sunyaev-Zeldovich effect. There is no evidence for such anisotropies in the COBE MDR maps, however (Banday & Górski 1996). Furthermore, the gas temperature and column density assumed by Suto et al (1996) are probably overestimated, given the ROSAT observations of other groups (Pildis & McGaugh 1996). In fact, the strong trend for spiral-only groups not to be X-ray detected suggests that the Local Group is unlikely to produce appreciable amounts of X-ray emission (Pildis & McGaugh 1996, Mulchaey et al 1996b).

Although the Local Group is probably not X-ray bright, a significant gas component may exist at cooler temperatures (Mulchaey et al 1996b, Fields et al 1997). Given the expected virial temperature of the Local Group (~ 0.2 keV), the detection of this gas in emission would be exceedingly difficult. However, an enriched collisionally ionized gas at these temperatures is expected to produce prominent absorption features in the far-UV region. The strongest features result from lithium-like ions O VI, Ne VIII, Mg X and Si XII (Verner et al 1994). Lines of sight to hot stars in the Magellanic Clouds are known to show O VI absorption features, but it is not clear whether this gas is associated with intragroup gas or gas in our own Galaxy. There may be other ways to infer the presence of warm gas in the Local Group. Wang & McCray (1993) found evidence in the soft X-ray background for a thermal component with temperature ~ 0.2 keV, which could be due to a warm intragroup medium in the Local Group (see, however, Sidher et al 1999, who argue that the X-ray halo of the Galaxy dominates). Maloney & Bland-Hawthorn (1999) have recently considered the ionizing flux produced by warm intragroup gas and find that it is unlikely to dominate over the cosmic background or the ultraviolet background produced by the luminous members of the Local Group. Still, encounters between the intragroup gas and the Magellanic Stream may be responsible for the strong H emission detected by Weiner & Williams (1996).

The existence of an intragroup medium in the Local Group may also be relevant to the H I high velocity clouds (HVCs; for a review see Wakker & van Woerden 1997). Recently, Blitz et al (1999) revived the idea that many of the HVCs may be dark-matter dominated structures falling onto the Local Group. In this scenario, some of the HVCs collide near the center of the Local Group and produce a warm intragroup medium. If the Blitz et al (1999) scenario is correct, one would expect to find similar H I clouds in other nearby groups. Blitz et al (1999) suggested that several HVC analogs have indeed been found. However, Zwaan & Briggs (2000) completed a H I strip survey of the extragalactic sky with Arecibo and detected no objects resembling the HVCs in other groups. The failure of the Arecibo survey to detect H I does not necessarily rule out the Blitz et al (1999) model. One possibility is that the groups in the Zwaan & Briggs (2000) survey contain an X-ray-emitting intragroup gas and that the H I clouds do not survive this hostile environment. Unfortunately, the X-ray properties of the Zwaan & Briggs (2000) groups are currently unknown. The conclusions of Zwaan & Briggs (2000) are also sensitive to the masses assumed for the H I clouds. Braun & Burton (2000) argued for a lower HVC H I mass and concluded that the sensitivity and coverage of Zwaan & Briggs's (2000) survey was not sufficient to detect analogs of the HVCs in other groups. A more serious problem may be the number statistics of moderate redshift Mg II and Lyman limit absorbers, which appear to be inconsistent with a Local Group origin for the HVCs (Charlton et al 2000). Regardless, it is clear that future H I surveys of X-ray detected and X-ray-non-detected groups could provide important insight into the relationship between hot and cold gas in galaxy groups.