4.1.1 Chemical abundances of dwarf irregular galaxies
Dwarf irregular galaxies are in general metal-poor. Nebular abundances in Local Group dIs range between roughly one third of solar (e.g. LMC) and ~ 1/40 of solar. Abundances of dIs are usually derived from H II regions. For some dIs there exists also metallicity estimates of [Fe/H] using the same technique as generally used for dEs (Colour-Magnitude diagrams).
We present in Table 1 some basic properties for the most metal-poor (oxygen abundance less than or equal to 1/10 of the solar value) Local Group dIs. Some dIs (Leo A, SagDIG and Sextans A) do indeed have oxygen abundances comparable to the most metal-poor blue compact galaxies. If available, we present also [Fe/H] values based on stellar photometry, absolute B-magnitudes and the logarithm of the estimated total (dynamical) mass expressed in solar masses.
|Galaxy name||12 + log(O/H)||[Fe/H]||MB||log()|
Only for a few dIs have abundances of individual stars been determined. For the Magellanic Clouds, Pagel (1993) concludes that the agreement between stellar and gaseous heavy element abundances is about as good as can be expected in view of the uncertainties. However, for NGC 6822, metal lines of early type stars are considerably weaker than expected from the measured nebular oxygen abundance (Massey et al. 1995). If there is a real discrepancy, this is worrying since one argues that ISM oxygen abundances can be obtained with high accuracy. Direct spectroscopic observations of individual stars in more dIs are much needed.
Skillman et al. (1989) confirmed in a seminal paper a strong correlation between absolute magnitude and metallicity for local dIs in the sense that the fainter systems have lower metallicities. This was hinted already in some earlier papers: e.g. Lequeux et al. (1979), Kinman and Davidson (1981). The existence of a relation between metallicity and luminosity constitutes a fundamental observation that we need to understand to address the chemical evolution of dwarf galaxies. The metallicity-luminosity relation for dIs and other galaxies will be further addressed in Sect. 7.1. More recent work in this area include that of Richer and McCall (1995) and Hidalgo-Gámez and Olofsson (1998). The comparison of dIs and dEs is not straightforward, as will be discussed in next section, since different chemical elements are sampled in general.
The best studied dIs reside in the Local Group, but plenty of observations of relatively nearby dIs outside the Local Group can be found in the literature (e.g. Skillman et al. 1989). The use of H II regions for determining oxygen abundances makes even relatively distant dIs accessible for chemical investigation of their ISM. Dwarf irregulars in the M81 group have been studied by Skillman et al. (1994) and Miller & Hodge (1996), and dIs in the Sculptor group by Miller (1996). Abundances are similar to Local Group dIs and follow the same metallicity-luminosity relationship (Skillman et al. 1989, Skillman 1998). A sample of quiescent dwarfs (dIs/LSBGs) studied by van Zee et al. (1997b, c) shows again typical Local Group abundances and adheres to the metallicity-luminosity relation; see also the section on LSBGs (4.3.1). The most metal-poor dIs are included in Table 3.
Vilchez (1995) presented a spectrophotometric study of star forming dwarf galaxies (dIs and BCDs) in different environments, from low density to the core of the Virgo cluster. For a given luminosity, the high density environment dwarfs are systematically overabundant (~ 0.5 dex) with respect to dwarfs in low density environments and the Skillman (1989) relation. However, metallicities especially for dwarfs in Virgo are uncertain and the observed over-metallicity may be spurious. Later work by Lee et al. (1998) finds O-abundances of 12 + log(O/H) = 8.0 to 8.3 for absolute magnitudes ~ -15 > MB > -16 indicating that the systematic overabundance of Virgo dIs with respect to local ones is small or non-existent. Some cluster dwarfs seem to be anomalously metal rich (perhaps related to the ``tidal dwarfs'', see below), while others appear to be rather normal (Vilchez 1999; private communication). It now appears that dwarf galaxies can be spawned in the process of violent galaxy interactions. These ``tidal dwarfs'' will be discussed separately in Sect. 4.5.
Carbon abundances have been determined for a few dIs by Garnett et al. (1995) and Kobulnicky and Skillman (1998), although most galaxies in these investigations are rather BCGs, revealing a strong positive correlation between [O/H] and [C/O]. Nitrogen abundances exist for more dIs (e.g. Garnett 1990), but rarely for the most metal-poor Local Group dIs. The typical [N/O] values for metal-poor dIs are of the same order as those of BCGs. The [C/O] and [N/O] nebular abundance ratios will be further discussed in the BCG section.