6 Discussion

In this section we briefly discuss how our present field dwarf objects compare with those from our previous work. In the following we use the term "irregular'' as synonymous to "late-type dwarf'', subsuming the true irregulars of types Im and BCD, as well as Sm and Sd galaxies.

Figure 5: Extrapolated central surface brightness, $\mu _0$, versus absolute B magnitude for a sample of cluster dwarf irregulars (small dots) and Bremnes' (2001) sample of group irregulars (coded with different symbols for the different groups, as indicated) and field irregulars (filled squares). The field irregulars from the present study are added as filled circles. The vertical dotted lines indicate the range of completeness, -17 < MB < -13. The full and dashed lines are fits to the data as described in the text.

In Bremnes (2001) it is shown that the photometric parameters of dwarf galaxies significantly depend on the environment. In particular, dwarf galaxies in galaxy groups and the field, of either the elliptical or late type, tend to have higher surface brightness, of up to one magnitude, than their counterparts in clusters of galaxies. In Fig. 5, which is taken from Bremnes (2001), we illustrate this effect with late-type ("irregular'') dwarfs and add the field irregulars from the present paper. Shown is the relation between the central surface brightness from an exponential fit to the blue surface brightness profile, $\mu _0$, and the absolute blue magnitude, M_B, for a sample of cluster irregulars (small dots) and a sample of group and field irregulars as coded in the figure with different symbols. Bremnes (2001) has included 16 field galaxies that lie outside of any group (shown as filled squares). Our own field dwarfs are added as filled circles, where we have discarded the dwarf elliptical Kar49 and the S0 galaxy UGC2689, leaving us with 17 objects.

The two lines shown in the figure are of the form

$$\mu_0 = 0.50 M_B + b.$$ (6)

The full line is a least squares fit to the cluster irregulars (small dots) with $b_{\rm cl} = 30.18$, the broken line is a fit to Bremnes' (2001) sample of group and field irregulars with $b_{{\rm g}+{\rm f}} = 29.36$, which is equivalent to a surface brightness offset of $\Delta m = -0.82$ as compared to the cluster sample. This difference is highly significant, given the uncertainty in bof the (less numerous) cluster irregulars of 0.24 mag (see Bremnes 2001).

Within the data set of Bremnes (2001) the 16 field irregulars (filled squares) show a tendency to have even higher central surface brightness values in the mean than the group irregulars. For Bremnes' field irregulars alone one would get $b_{\rm f} = 28.99$, i.e.  $\Delta m = -0.37$ as compared to the group + field sample. Although not statistically significant, given the small number of field objects, such a trend of ever higher surface brightness for dwarf irregulars in ever lower density environments (from clusters to groups to the field) would certainly agree with the general interpretation of the effect by Bremnes (2001), viz. that a higher-density environment could have slightly enhanced the mean star formation rate over the past, which would lead to a lower star formation activity (surface brightness) at present. However, this more subtle trend from groups to the field is not confirmed by the present data. It is already apparent from Fig. 5 that our 17 field irregulars (filled circles), even if we neglect DDO97, have lower surface brightness in the mean than Bremnes' 16 field irregulars (filled squares). Fitting Eq. (6) to our irregulars would give b = 29.82 (29.68 without DDO97), which is even closer to the cluster line than the total group + field sample of Bremnes (2001). But again the number is to small for statistical significance. All field irregulars taken together would give b = 29.42, or $\Delta m = -0.76$ as compared to the cluster sample, while Bremnes' (2001) group + field sample gave $\Delta m = -0.82$.

In summary, we find no evidence for a systematic photometric difference between field and group dwarf irregulars, while there is strengthened evidence for such a difference between non-cluster and cluster dwarf irregulars.