The structure and chemical composition of the Carina dwarf
spheroidal galaxy is studied, using COSMOS measures of deep UKST and
AAT plates. A (B-V,V) colour magnitude diagram is constructed, and
such parameters as this galaxy's heliocentric distance (91±9 kpc),
giant branch metallicity parameters (ΔV₁.₄= 3.0±0.3, S = 5.0±1.1,
(B-V)og = 0.74±0.06) and Mironov Index (0.09±0.03) estimated.
The low luminosity counterpart of the extended giant branch
found by Mould et al (1982) in this galaxy has been identified and
separated from the first ascent giant stars. This latter branch is
shown to be possibly intrinsically broad, implying a mean metallicity
[Fe/H] = -1.8 with a possible real spread of about 0.26. Any
broadness has been calculated to be far too large to be explained
solely in terms of age e.g. by a continuous star formation history
since the Carina dwarf was formed. The results are consistent however
with e.g. (a) the dwarf spheroidals originating in a much more
massive external system (e.g. the Magellanic Clouds) where metal
enrichment occurred, and later being tidally 'ripped' out of this
system with their presently estimated masses or (b) if the dwarf
spheroidals evolved as isolated systems, then in the case of the
Carina dwarf, it must have lost more than 70% of its original mass.
Core (9.7'+0.8') and tidal (38'±10' ) radii as well as the
orientation position angle (72+2°) and mean ellipticity of the Carina
dwarf's isopleths (0.31±0.03) have been calculated, the latter result
showing that there exists very little variation of the ellipticity of
the isopleths with radius. These values, together with this galaxy's
mass of (0.3,6.1,17.5)x10⁵ M₈ (calculated by the Hodge (1971) method
of assuming that globular cluster and dwarf spheroidal galaxy
luminosity functions are similar) have been used to show that this
galaxy is interacting strongly with the Galaxy's gravitational field.
It is surmised that this could be one mechanism whereby the Carina
dwarf has lost mass (as required by case (b) above), after a number
of perigalactic passages.
More evidence, complementing that of Mould and Aaronson (1983),
that a substantial fraction of the Carina dwarf's membership is
younger than 15 Gyr has been found. This comes from (a) the
comparison of its luminosity function to that of M3 over a
substantial range of absolute magnitude, (b) the identification of
another carbon star in this galaxy and confirmation of the previously
suspected six by optical spectra, (c) the lack of variables and (d)
isochrone fits over both the giant and main sequence branches taken
together (for the first time).
The absolute magnitude of the Carina dwarf is estimated to be
Mᵥ= -9.6 with a range from -8.8 to -10.8, making it one of the least
luminous objects in the Local Group. Its central surface density and
brightness are so low ( Þ = (5± 4)x10⁻³ M₈)/pc³, b(B) = 25.3±1.0
magnitudes/arcsec²) that it would be barely detectable on IIIaJ UKST
plates at five times the distance of M31.
Spectra of carbon stars, using the RGO spectrograph with the
IPCS on the AAT have been used to question whether dwarf spheroidals
contain large quantities of non-luminous matter. The major 'evidence'
for this latter point comes from velocity dispersion measures using
carbon star spectra in some of the other dwarf spheroidals. It is
shown here, under the assumption of negligible variability of these
stars and using only the internal errors on the carbon stars'
velocities, that the velocity dispersion of the Carina dwarf is
10.4±3.0 kms⁻¹ implying a large M/L ratio for this galaxy.
The Galactocentric velocity of the Carina dwarf derived using
these spectra suggests that instead of being one of the Galaxy's
fastest moving satellites as previously thought (Cannon, Niss and
Norgaard-Nielsen 1981), it is practically stationary (15±5kms⁻),
removing some of the previous evidence for a Galactic heavy halo.