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Title: A study of the Carina dwarf spheroidal galaxy
Author: Godwin, Philip John
ISNI:       0000 0001 3501 5171
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1986
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Astrophysics