Dwarf galaxies, the most abundant type of galaxy in groups and clusters, may hold the key to understanding galaxy evolution. In standard hierarchical structure formation scenarios, they are the building blocks of more massive galaxies. Conversely, the environment - e.g., the presence or absence of more massive galaxies in the vicinity - surrounding dwarf galaxies is expected to affect their evolution (e.g., Mayer et al. 2001). Furthermore, the intrinsic properties of a dwarf galaxy - e.g., its mass, density, gas content - are likely to influence the histories of star formation and chemical enrichment. However, the relative importance of external and internal properties and their actual effect on dwarf galaxy evolution remains unknown. The most detailed studies of dwarf galaxy properties have been carried out in the Local Group, but the limited sample size and the wide range of properties prevent us from obtaining a unified picture.
To arrive at a better understanding of what drives dwarf galaxy
evolution we are carrying out a comprehensive multi-wavelength
study of dwarf galaxies in the Local Volume (
5 Mpc).
The Local Volume contains several nearby galaxy groups as well as
galaxies in the relative isolation of the field.
After the conclusion of an all-sky survey to detect dwarf galaxy
candidates in the Local Volume
(Karachentsev et al. 2000, and references therein)
we are carrying out ground-based imaging to derive integrated
photometry and structural parameters (e.g., Makarova et al. 2002).
These efforts are complemented by two snapshot programs with the Hubble
Space Telescope's (HST) Wide Field and Planetary Camera 2 (WFPC2)
totalling 200 orbits to study stellar content, recent star formation
histories, distances, and relative positions within galaxy groups
(e.g., Karachentsev et al. 2002a,b,c).
Radio observations (e.g., Huchtmeier et al. 2000a,b) reveal the
H I content and radial velocity of the dwarfs, provided
that they contain detectable amounts of H I.
For a description of the project as a whole, see Grebel et al. (2000).
Since our HST data enable us to measure relative distances within galaxy groups, we are in the unique position of being able to assign membership probabilities to individual dwarf galaxies and to consider them based on their three-dimensional location within a group. Our ground-based imaging in combination with distances yields information on the luminous mass of dwarf galaxies. Compiling a database that, in addition, contains accurate information on chemical abundances will allow us to consider dwarf galaxies within their evolutionary context in groups and in the field. The knowledge of relative distances and intrinsic properties is a major step towards resolving what governs dwarf galaxy evolution.
The data presented here are part of our continuing efforts to obtain abundances for dwarf irregular galaxies (dIs) in the Local Volume and are based on the first two observing runs dedicated to this multi-year project. However, not all targets have HST-based distances. At the time of writing, about 150 out of the 200 dwarf galaxy candidates in the Local Volume have been observed, as we await the completion of the HST snapshot survey. Galaxies were selected according to the following criteria: (a) galaxies were visible at the epoch of observations, and (b) galaxies were sufficiently luminous to be observable with a 1.5-m class telescope. Hence, the present study consists primarily of results for individual galaxies and of the discussion for these galaxies in the metallicity-luminosity relationship. The solar value of the oxygen abundance of 12+log(O/H) = 8.87 (Grevesse et al. 1996) is adopted for the present work to facilitate comparisons with earlier studies. However, recent work has indicated that the solar value may in fact be smaller by about 0.1 to 0.2 dex (Allende Prieto et al. 2001; Holweger 2001).
The outline of this paper is as follows. A brief description of the observed galaxies is presented in Sect. 2. Observations and reductions of the data are presented in Sect. 3. Measurements and analysis are discussed in Sect. 4, and nebular abundances are presented in Sect. 5. A discussion of individual galaxies and of the metallicity-luminosity relation is provided in Sect. 6. A summary is given in Sect. 7.
Copyright ESO 2003