Evolutionary Histories of Dwarf Galaxies in the Local Group

Abstract

The star formation histories of Local Group (LG) dwarf galaxies and more distant potential LG members are reviewed. Problems in defining the spatial extent of the LG and membership are briefly discussed. The morphological types found in the LG are presented, and it is suggested that we see continuous evolution from low-mass dwarf irregulars (dIrrs) to dwarf spheroidal galaxies (dSph) in the LG.Star formation histories for LG dwarfs and nearby LG candidates are compiled using population boxes. No two dwarfs, irrespective of morphological type, show the same evolutionary history, and all vary widely in ages of their subpopulations and in their enrichment history. The lack of gas in dSphs and certain dwarf ellipticals (dEs) is puzzling both with respect to their star formation histories and the expected mass loss from red giants, but a new photoionization scenario may reconcile these contradictions. Old populations, often spatially very extended, may be a common property of dwarf galaxies, though their fractions can be very small. Almost all types of dwarf galaxies studied in detail so far show spatial variations in ages and abundances such as radial age/metallicity gradients. The observed star formation histories impose constraints on merger and accretion scenarios.Properties of the Milky Way dwarf spheroidals are compared to the M 31 dSphs and discussed in the framework of the ram pressure/tidal stripping scenario. It is demonstrated that the newly discovered LG dwarfs follow the same relationship for central surface brightness, mean metallicity, and absolute magnitude as the other LG dwarfs.

The most metal-poor galaxies

“…Other metallicity indicators include the colour and morphology of the horizontal branch (HB) in the CMD (Grebel 1998), although this cannot be uniquely translated into metallicities due to poorly understood "second parameter effects", also known from the study of HBs in globular clusters. Intermediate in character between photometric and spectroscopic abundance determinations is the fraction of carbon (C) stars among the late type giants, and the number of carbon rich (WC) to nitrogen rich (WN) Wolf-Rayet (WR) stars.…”

“…It is not our intent here to give a general description of the Local Group dwarfs, recent and excellent reviews can be found in e.g. Mateo (1998, which also contains a compilation of very useful data) and Grebel (1998); see also the book "Stellar astrophysics for the Local Group" (eds. Aparicio et al 1998) and the proceedings of the recent IAU symposium "The Stellar Content of Local Group Galaxies" (eds.…”

“…The star formation history (SFH) of Local Group dIs has been reviewed recently by Grebel (1998), and Mateo (1998). Most dIs seem to have experienced more or less continuous star formation over a Hubble time.…”

“…Dwarf elliptical galaxies were long thought to be made up of exclusively old stars, but it has now become evident that many seem to have experienced several star formation episodes, and in many cases quite recently (Grebel 1998). This makes the distinction between dEs and dIs less clear.…”

Mergers and the outside-in formation of dwarf spheroidals

“…The latter, in particular, explains naturally why most dSphs in the Local Group (where they are best studied given their low luminosity) are at present satellites of the Milky Way or Andromeda galaxies (van den Bergh 1994;Grebel 1999;Mateo 2008). Tidal stirring, however, fails to explain the origin of isolated dSphs such as Cetus (or Tucana), whose large distance from any major galaxy excludes repeated tides as the main mechanism driving its morphology, or its transformation.…”