5 Discussion

 The surface brightness profiles shown in Fig. 3 are quite typical for dwarf galaxies, being more or less straight lines, i.e. exponentials, in a large range of intermediate radii, with deviations from this in the innermost and the outermost parts, i.e. at small and very large radii; compare, e.g., with the profiles of M81 group dwarfs in Paper I, and of Virgo cluster dwarfs in Binggeli & Cameron (1993). The inner deviation from the exponential can be a luminosity cusp, which is common among dwarf ellipticals, especially nucleated ones. This feature is well seen in the only dwarf elliptical of our sample, UGC8882. Late-type dwarf galaxies (Sd, Sm, Im), on the other hand, tend to exhibit a central luminosity deficit with respect to the best-fitting exponential (see again Fig. 3). In most cases this deficit is simply caused by the irregular structure of the star-forming region which is confined to the central part of a galaxy. The peak surface brightness of a star-forming galaxy can be far off the center as referred to the faint outer isophotes. If the profile is centered on the faint outer isophotes (which in our opinion is the only way a profile makes sense, as it should, ideally, refer to center of the mass and not the luminosity distrubution), then the consequence is obviously an apparent luminosity deficit in the central part. This is dramatically demonstrated by NGC5474, but also HoIV (see Figs. 2 and 3). It simply means that the innermost part of the mean radial profile of a dwarf irregular (and sometimes even a late spiral) should not be taken at face value. This is a principal limitation of one-dimensional "surface'' photometry of non-symmetric galaxies.

Some of our galaxies show a luminosity excess above a pure exponential in the surface brightness profiles at large radii (Fig. 3), most strongly in NGC5238, but also in DDO168, DDO169, DDO183, NGC5477, DDO190, and DDO194. NGC5229 also shows an excess in one colour but in this case our photometric method is clearly not well suited, due to the galaxy being almost edge-on and warped. On average this excess sets in at a surface brightness of $\mu \mathrel{\hbox{\rlap{\lower.55ex \hbox {$\sim$}} \kern-.3em \raise.4ex \hbo... ...$} \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi{\hbox{$^{\prime\prime}$}}$ in B and $\mu \mathrel{\hbox{\rlap{\lower.55ex \hbox {$\sim$}} \kern-.3em \raise.4ex \hbo... ...$} \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi{\hbox{$^{\prime\prime}$}}$ in R. This trend was not observed with our M81 group dwarfs (Paper I), nor was it found in Virgo cluster irregulars, see Fig. 9 in Binggeli & Cameron (1993), whose radial profiles were followed to equally faint levels (most other photometric studies do not go faint enough, which renders a comparison difficult).

A possible reason one could think of for this feature is the fact that in the present photometry the elliptical shape of the running aperture was fixed for a given galaxy. That ellipse was determined at approximately $25 \,\mathrm{mag}/\ifmmode\hbox{\rlap{$\sqcap$}$\sqcup$}\else{\unskip\nobreak\h... ...$} \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi{\hbox{$^{\prime\prime}$}}$ (cf. Sect. 3). Suppose that the outermost part of a galaxy, at a surface brightness level well below $25 \,\mathrm{mag}/\ifmmode\hbox{\rlap{$\sqcap$}$\sqcup$}\else{\unskip\nobreak\h... ...$} \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi{\hbox{$^{\prime\prime}$}}$,is more spherical than the inner part, or that there is strong isophotal twisting: then the mean surface brightness profile derived from apertures of fixed ellipticity will become flatter at large radii, i.e. will show an excess of the kind discussed here. Galaxies such as DDO168, DDO169 and NGC5238 show such a behaviour, but at very low levels. For instance DDO168 has an apparent ellipticity of $\epsilon = 0.60$ and a position angle of 58 degrees for an ellipse fit at $< \mu_B \gt\ = 25 \, \mathrm{mag}/\ifmmode\hbox{\rlap{$\sqcap$}$\sqcup$}\else{\... ...$} \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi{\hbox{$^{\prime\prime}$}}$ and at $< \mu_B \gt\ = 27 \, \mathrm{mag}/\ifmmode\hbox{\rlap{$\sqcap$}$\sqcup$}\else{\... ...$} \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi{\hbox{$^{\prime\prime}$}}$ these values are resp. 0.42 and 62. For DDO169 these values are resp. 0.68, 44, 0.70 and 49 and for NGC5238 these values are resp. 0.32, 87, 0.33 and 88. In the last case $\epsilon$then rises to 0.41 in the outermost regions, with unchanged PA. It seems unlikely that this is the reason for the excess.

A possible physical explanation of the observed break in the surface brightness profiles - well-known from the photometry of disk galaxies - is of course the presence of two distinct galaxian components (like bulge and disk). The existence, in dwarf irregulars, of an underlying population of old stars that stretches over a large characteristic scale length, with a luminous, more concentrated, young population on top of it, is indeed highly expected. It has recently been shown that Local Group dwarf irregulars possess extended old halos (Minniti & Zijlstra 1996). With this regard it is also interesting that the observed break in the surface brightness profiles of our dwarfs seems to be well correlated with a flattening of the corresponding B-R colour profiles (see Fig. 4). The colour of the excess light is rather red, with the onset of the flatter part at a B-R value of $\sim 1.2 \pm 0.1 \,\mathrm{mag}$.

So, have we detected an underlying old halo population on purely photometric grounds? Unfortunaletly, this cannot be claimed (yet). The deviations from the exponential luminosity and colour profiles discussed here are mostly well within the error envelopes, as indicated in Figs. 3 and 4. Hence the significance for a single case is quite weak. However, we note again that the error envelopes are rather conservative (also see the note on DDO168 below). The tendency seen in so many galaxies is certainly very suggestive and a follow-up of this question by means of multicolour photometry of very high accuracy and sensitivity, in order to definitively prove the reality of the phenomenon, seems very desirable.