Annu. Rev. Astron. Astrophys. 1998. 36:
435-506 Copyright © 1998 by Annual Reviews. All rights reserved |
The Local Group is a dangerous place for dwarf galaxies. NGC 205
and Sagittarius have wandered too close to their
dominant parents and
exhibit clear kinematic and structural signatures of tidal distortions
(Hodge 1973,
Bender et al 1991,
Pryor 1996,
Ibata et al 1997).
Irwin &
Hatzidimitriou (1995)
noted that many nearby dSph systems
show a strong correlation of tidal radius or ellipticity with
the strength of the external tidal field.
Bellazzini et al (1996)
have shown convincingly that the central surface brightness,
0,
of dSph galaxies obey a bivariate relation in
0,
Ltot, and RGC, where
RGC is the Galactocentric distance of the galaxy.
They show that Sagittarius in particular appears to be unbound, even
in its core
(Mateo et al 1995c,
but see Ibata et al 1992).
These disrupted dwarfs should produce relatively long-lived streams in
the halos of galaxies such as M31 and the Milky Way (1-2 Gyr;
P Harding, private communication).
Lynden-Bell & Lynden-Bell
(1995)
concluded that one possible stream can be traced out with the
Magellanic Stream
(Wakker & van
Woerden 1997),
Ursa Minor, Draco, and possibly Carina and Sculptor. A recent
determination of the proper motion of Sculptor
(Schweitzer et al
1995)
suggests that this
galaxy is not part of this putative (or any other proposed) stream.
There have been many intriguing claims of halo substructure in recent
years (e.g.
Arnold & Gilmore
1992,
Majewski 1992,
Côté et al
1993,
Kinman et al 1996)
that could possibly be remnants of disrupted dwarfs.
More recently,
Alcock et al (1997b),
Zaritsky & Lin
(1997) claimed to detect
a possible signature of a foreground galaxy or galaxy tidal remnant
towards the LMC.
Gallart (1998)
suggested instead that this new "galaxy" is in fact due to the
signature of known but subtle stellar evolutionary phases that are becoming
apparent in the large-scale photometric surveys carried out in the
LMC. This is
not the first time that a putative new galaxy has been detected directly
in front of a known Local Group dwarf:
Connolly (1985)
identified a number of
"foreground" RR Lyr stars toward the LMC that he concluded are members
based on their photometric properties but are nonmembers kinematically.
Saha et al (1986)
also identified some anomalously bright RR Lyr-like stars,
apparently in front of the Carina dSph galaxy, that could be either part of an
extended halo of the LMC or possibly associated with a foreground system. A
more natural explanation may be that these are instead anomalous Cepheids in
Carina itself
(Mateo et al 1995a).
In none of these cases is the true nature of all of
these "foreground" stars conclusively established, and in the case of
the LMC, it is not unreasonable to suppose that a tidal
tail is present
(Zaritsky & Lin 1997).
Nevertheless, it seems wise to treat claims of the existence of galaxies
or tidal features directly in front of known Local Group systems with
particular caution.
Mateo (1996),
Unavane et al (1996)
have discussed the possibility
that a large fraction of the Galactic halo has been constructed from
disrupted dSph systems. The latter considered Carina to be the
template of such a system, while
Mateo (1996)
compared the
properties of the ensemble of all Galactic dSph satellites
with the halo. Neither approach is strictly correct. Carina arguably has
the most unusual stellar population of any dSph system
(Section 6.3;
Figure 8); it is clearly
not an appropriate choice as a template for the halo. On the other hand,
present-day dSph systems are anomalous "survivors" that were able to
form stars over a longer period than systems that were destroyed. They
probably also follow orbits (relative to the Galaxy) that are quite
distinct from the orbits of the
galaxies that were consumed; thus, even an average of the stellar
populations of
all remaining dSph galaxies should not be expected to precisely match the
current halo population. Given these differences in approach, the
two studies nevertheless essentially agree: No more than 10% of the halo could
have Carina-like progenitors
(Preston et al
1994),
but more than 50%
of the halo could have formed from galaxies similar to the entire
ensemble of Galactic dSph systems (though see
van den Bergh 1994b).