5.2. Tidal Effects Rather than Dark Matter?

Instead of a smooth surface density profile that one might expect from a relaxed population, Ursa Minor shows statistically significant stellar density variations (Kleyna et al. 1998). Fornax's four ancient globular clusters are located at distances larger than the galaxy's core radius. Dynamical friction should have lead to orbital decay in only a few Gyr (much less time than the globular clusters' lifetimes) and have turned Fornax into a nucleated dSph. Simulations by Oh, Lin, & Richer (2000) suggest that the best mechanism to have prevented this evolution is significant mass loss through Galactic tidal perturbation and the resulting decrease in the satellite galaxy's gravitational potential, which may have increased the clusters' orbital semimajor axes and efficiently counteracted the spiralling-in through dynamical friction. The detection of a possible extended population of extratidal stars around the dSph Carina might imply that this galaxy has now been reduced to a mere 1% of its initial mass (Majewski et al. 2000). If such significant tidal disruption is indeed real and widespread then the present-day stellar content of nearby dSphs cannot easily be used to derive evolutionary histories over a Hubble time. Furthermore, extended extratidal stars are not expected if the galaxy is dark-matter dominated (Moore 1996).

Additional indications in favor of the impact of galactic tides come from the structural parameters of dSphs (Irwin & Hatzidimitriou 1995), which seem to imply tidal disruption in several cases. Furthermore, substantial tidal disruption by the Milky Way is evidenced by the Magellanic Clouds and Magellanic stream, by the Sagittarius dSph, and by Galactic globular clusters (Gnedin & Ostriker 1997; Grillmair et al. 1995; Leon, Meylan, & Combes 2000). Tracer features include gaseous and stellar tidal tails (Putman et al. 1998; Majewski et al. 1999; Odenkirchen et al. 2000). The conversion of velocity dispersions into M/L ratios and dark matter fractions assumes virial equilibrium, a condition that is violated in the case of severe tidal disruption.

Tidal heating due to resonant orbital coupling between the time-dependent Galactic gravitational field and the internal oscillation time scales of dSphs (Kuhn & Miller 1989; Kuhn 1993; Kuhn, Smith, & Hawley 1996) may inflate the dSphs' velocity dispersions, but see Pryor (1996) for arguments against the efficiency of this mechanism.

As shown by Piatek & Pryor (1995) tides can, but need not inflate the global M/L/ratio to values. Indeed, in a galaxy suffering tidal disruption the velocity dispersion can be sustained at its virial equilibrium value, and the central density is maintained even after substantial mass loss (Oh, Lin, & Aarseth 1995). Pryor (1996) noted that a velocity gradient across a galaxy that is larger than the velocity dispersion is the clearest signature of tidal disruption, but such a gradient is not obvious in the Galactic dSphs.

Kroupa (1997) and Klessen & Kroupa (1998) proposed that stellar tidal tails may look like dSphs when seen along the line of sight, an orientation that follows naturally from their N-body simulations. The ordered motions in the tidal remnants would appear as increased velocity dispersion since they occur along the line of sight. These models can roughly reproduce the observed correlations between central surface brightness, absolute magnitude, and M/L. The predicted line-of-sight extension of the dSphs can be tested, in principle, through accurate measurements of the apparent width of their HBs. The predicted high orbital eccentricity, a consequence of the required radial orbits in the model, could be checked through accurate proper motion measurements with astrometric satellite missions such as SIM and GAIA. The tidal remnants may be leftovers from earlier mergers as suggested by the observation that the Galactic dSph galaxies appear to be located near at least two polar planes or great circles (the Magellanic Stream and the Fornax-Leo-Sculptor Stream; e.g., Kunkel & Demers 1996; Kunkel 1979; Lynden-Bell 1982; Majewski 1994). Such tidal remnants would not likely contain dark matter. The observed ages and abundances of galaxies potentially associated with ``streams'' constrain the time at which the break-up of a more massive parent could have occurred. This event must have happened very early on when the parent had not yet experienced significant enrichment. Siegel & Majewski (2000) suggest that galaxies potentially belonging to a stream may have originated from a common -2.3 dex progenitor and subsequently followed their own evolution.

The impact of Galactic tides remains a valid alternative to large amounts of dark matter in nearby dSphs. The determination of velocity dispersions of distant or even isolated dSphs, which are unlikely to be subject to tidal effects, is an important test of whether high M/L ratios in dSphs are largely caused by environmental effects (see, e.g., the discussion in Bellazzini, Fusi Pecci, & Ferraro 1996). Stellar velocity dispersions indicative of high M/L were found in the most distant (~ 270 kpc) potential Milky Way dSph companion Leo I (Mateo et al. 1998) and in the outlying (~ 280 kpc) M31 transition-type satellite LGS 3 (Cook et al. 1999), but measurements of truly isolated Local Group dSphs such as Tucana and Cetus are still lacking.