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Title: Star formation history and evolution of the nearest gas-rich dwarf galaxies
Author: Grossi, Marco
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2004
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Neutral hydrogen surveys in the last decade have revealed a new class of galaxies where the gas component can be dominant compared to the stellar one (Mhi/Lb < 1). These objects may be either young (< 1 Gyr) or in a way "retarded", having evolved at a very slow rate without efficiently converting their large amount of gas into stars within a Hubble time. Here we analyse three such galaxies in the Centaurus A group at a distance of about 4.5 Mpc (discovered with the HIPASS survey), which have no analogues in the Local Group (Banks et al. 1999). From their optical morphology they appear to be dwarf spheroidals and low surface brightness, yet they are gas-rich (Mhi/Lb > 1) with gas-mass-to-stellar light ratios larger than typical dwarf irregular galaxies. These systems should be favoured hosts for starburst, yet a faint star formation region has been detected in only one object. Are these galaxies truly young Or rather, what inhibits the conversion of gas into stars slowing down their evolution Is there a connection with the environment they are evolving in the attempt to answer some of the above questions we have analysed data in Hi, Ha, V and / bands (taken with the Hubble Space Telescope) and the optical spectrum of the Hll region of one dwarf (HIPASS J1337-39). In particular we have constructed /, (V - I) Colour Magnitude Diagrams (CMDs) and we have compared the data-sets with theoretical models, using isochrones of various metallicities (Bertelli et al. 1994) and simulated CMDs (Harris & Zaritsky 2001). All three galaxies have well determined Red Giant Branches (RGBs) which put them in the Centaurus A group at distances between 4.5 and 5 Mpc. The well populated RGBs suggest that these systems cannot . be younger than 2 Gyr. The evidence of asymptotic giant branch (AGB) stars in two out of three galaxies suggest that they contain populations with ages of up to 10 Gyr. Older Horizontal Branch (HB) and RR Lyrae stars would be two magnitudes fainter than the photometric limit reached with the HST at this distance, therefore we cannot infer the presence of an old (> 10 Gyr) stellar population. The age-metallicity degeneracy has been broken in one case (HIPASS J1337-39) where we measured a low metal abundance (Z 1/30 solar) from the analysis of the Hll region. The remaining two galaxies do also show properties which are at least consistent with similarly low metallicities. From the comparison of the observed and model CMD's we infer that all three galaxies are gas-rich because their Star Formation Rates have been very low during their long lives (< 10--3 A/ yr_1). We argue that in such systems, star formation (SF) may have been sporadic and local, although one object (HIPASS J1321-31) has a peculiar red plume in its CMD suggesting that most of its stars were formed in a "miniburst" 300--500 Myr ago. Neither their low Hi surface densities, nor their low metal content favour high star formation rates or starbursts, although we can not exclude connections with the local environment. Two out of three galaxies seem to be located in the outskirts of the group, being at about or less than 1 Mpc from the more massive galaxies of the group. The low metal abundances may be the consequence of the ejection of enriched material via supernovae-driven winds during the SF episodes, but it is likely they were not violent enough to blow away the majority of gas (Mac Low k Ferrara 1999, Ferrara & Tolstoy 2000). The question of why there are no similar dwarf galaxies in the Local Group remains open and the study of similar objects in the nearby groups is necessary to widen the sample of galaxies with such properties. In the Appendix A we present the first part of a still ongoing project with the aim of studying the stellar population of "retarded" galaxies at the other end of the mass scale---spirals with Mhi > 10io A/ and Mhi/Lb 1---The optical, near IR and 21cm observations are briefly presented and discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available