It is probably no exaggeration to claim that the most important
outstanding question in the study of the formation and evolution of
dwarf galaxies is the origin of the remarkable diversity of star
formation histories observed among the dwarf Ellipticals (a class that
includes the so-called dwarf Spheriodals) of the Local Group. Prior to
the first hints of a discrepancy in the early 1980s, the paradigm that
dEs galaxies consist entirely of old stars was universally accepted,
and amongst those unfamiliar with developments in the field, this view
is still surprisingly prevalent. Yet it is now abundantly clear that
the simple picture of dEs as consisting of entirely old (age > 10 Gyr)
stars is no longer valid. For example, among the Milky Way's dE
companions we see systems with dominant old populations, systems with
minor intermediate-age (age ~ 2 to 10 Gyr) components, and systems
where the intermediate-age component dominates the old stars. Even in
these latter systems there is further variety; in Carina the on-going
star formation occurred in a number of discrete episodes while in
Fornax it was approximately continuous. This variety of star formation
histories is not restricted to the Galaxy's companions. Recent
observations with HST/WFPC2 have shown that M31's dEs have also had
extended epochs of star formation.
Despite the large amount of observational data available for Local
Group dEs, there is currently no explanation for the diversity of star
formation histories, only hints. For example, among the Galaxy's
companions there is a tendency for the systems with stronger
intermediate-age components to lie at larger Galactocentric
distances. This also appears to be the case for M31's companions
where, at least among the lower luminosity dE companions, it is the
more distant system And II that has a definite intermediate-age
population; systems closer to M31 lack such stars. These results
suggest that proximity to a "parent galaxy" influences the evolution
of dwarf galaxies. Indeed, recent theoretical simulations have shown
how a dwarf irregular on an initial "plunging" orbit in the Milky Way
halo can be converted into a dE satellite. At the same time it is
notable that, with one exception, all the isolated dwarfs in the Local
Group are not dEs; they show either recent or on-going star
formation. The one local exception to this hypothesis that "parent
galaxies" nurture initial gas-rich dwarf companions into dE galaxies,
is the isolated dE Tucana. Despite its lack of association with any
large galaxy, it nevertheless possesses a dominant old stellar
population and there are no signs of any intermediate-age
component. The existence of this system demonstrates that proximity to
a large galaxy cannot be the only factor governing the evolution of
dwarf galaxies.
To make progress in understanding the processes that govern dwarf
galaxy evolution, we need to study systems beyond the Local
Group. Such studies can be targeted at dEs that occupy a variety of
environments, thereby allowing us to more readily distinguish between
intrinsic properties and "parent galaxy" influence. In particular, we
need to target a sizeable fraction of the dEs within our "Local
Volume", the sphere of radius ~10 Mpc centered on the Local Group,
seeking to establish what fraction of these systems show
intermediate-age populations. This volume includes the relatively
loose Sculptor Group, the more compact Cen A group, and a variety of
other galaxy aggregations such as the loose association of galaxies
that contain the Circinus galaxy at a distance of 6-7 Mpc. The
observational signature of an intermediate-age population is the
presence of upper-AGB stars, i.e., stars with sufficient mass to
evolve on the AGB to luminosities above that of the red giant branch
tip. For such stars there is a good correlation between the luminosity
of the brightest upper-AGB star and the age of the intermediate-age
component. The observations are best done at near-infrared wavelengths
since these pass bands cover the wavelengths where the majority of the
flux is emitted. Bolometric corrections are therefore small and well
defined and the amplitude of variability is much reduced relative to
the optical. Single epoch J and K band measurements then
suffice for a determination of the bolometric magnitudes (two pass
bands are required as the bolometric correction to the K
magnitude is a function of J-K color). Although current
near-infrared imagers can reach upper-AGB stars in the nearer of the
dEs within the Local Volume using relatively long integration times,
given the likelihood of a diversity of star formation histories, a
sizeable sample of dwarfs in a variety of environments needs to be
studied if underlying trends are to be revealed. GSAOI is the ideal
instrument with which to carry out this program.
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