5 Description of the new dwarf galaxy candidates

5.1 Photometric parameters

Photometric parameters were determined by use of the "growth curve'', F(r), (see BBP98). The total apparent magnitude m is given by

$$m = - 2.5 \,\log_{10} F(\infty),$$ (6)

and the average surface brightness within the effective radius $r_{\rm eff}$ is

$$<\mu>_{\rm eff}\ =\ m + 5\,\log_{10} \left( r_{\rm eff} \right) + 2.5\,\log_{10} \left( 2 \pi \right),$$ (7)

where $r_{\rm eff}$ is in arcsec and $\mu$ in mag/ $\Box \hbox {$^{\prime \prime }$ }$.

The intensity profiles of dwarf galaxies are well fitted by an exponential law, $I(r) = I_0\, \mbox{e}^{-\alpha r}$, (Binggeli & Cameron 1993) with scale-lengths $l = 1/\alpha$. In terms of the surface brightness, the exponential model is expressed by

$$\mu^{\rm ex} (r) = \mu^{\rm ex}(0) + 1.086\, \alpha r,$$ (8)

where $\mu(0)^{\rm ex}$ is the surface brightness extrapolated towards the centre. The upper index "ex'' refers to the exponential fit. According to the exponential model, the apparent magnitude is given by

$$m^{\rm ex} = \mu^{\rm ex}(0) + 5\,\log_{10}\,\alpha - 2.5\,\log_{10} (2\pi),$$ (9)

and the corresponding colour gradient is

$$\left[ \frac{{\rm d}(B-R)}{{\rm d}r} \right]^{\rm ex} = 1.086\,(\alpha_{B} - \alpha_{R}).$$ (10)

Before estimating photometric parameters, the images were corrected for the sky background, the contamination by foreground stars, and Galactic absorption adopting $A_{B} = 0.08\,\mbox{mag}$ and $A_{R} = 0.05\,\mbox{mag}$(BBP98).

The results are listed in Tables2 and 3 for the six new candidates and for some of the known galaxies. The systems F8D1, UGC5423, and UGC5455 were not considered here because they are located close to the edge of our survey field, where the background is quite inhomogenous. The systems Ho IX, A952+69, and the "garland'' were not analysed because of their irregular structure which is not well fitted by the exponential model.

   

Table 2: Global photometric properties in B and R for the six new LSB dwarf galaxy candidates and for known LSB dwarfs in the suvey field. (The radii r are given in arcsec, where the numbers in the index refer to the corresponding isophote in mag/ $\Box \hbox {$^{\prime \prime }$ }$)

name	B	B-R	$r_{B, \rm eff}$	$r_{R, \rm eff}$	$\langle \mu_{B}\rangle_{\rm eff}$	$\langle \mu_{R}\rangle_{\rm eff}$	rB,25	rB,26	rB,27	rR,25	rR,26	rR,27
cand1	17.2	1.3	26.3	24.9	26.3	24.9	-	10.2	25.9	17.4	32.9	45.9
cand2	17.6	1.0	10.4	9.7	24.7	23.5	8.3	15.3	19.9	14.0	19.6	23.8
cand3	16.6	0.2	11.6	11.3	23.9	23.6	13.2	19.1	28.7	14.8	21.4	30.3
cand4	17.4	-	8.5	-	24.1	-	8.1	13.7	18.2	-	-	-
cand5	17.6	-	8.7	-	24.3	-	8.5	13.5	19.9	-	-	-
cand6	18.8	-	5.2	-	24.4	-	4.7	7.3	10.0	-	-	-
BK1N	16.9	-	17.3	-	25.1	-	9.8	19.6	25.5	-	-	-
BK2N	18.0	0.9	12.6	11.0	25.5	24.3	4.9	9.9	17.1	10.7	17.6	23.6
BK3N	18.6	0.5	8.0	9.2	25.1	25.0	3.6	9.6	11.4	6.9	10.0	14.5
BK5N	17.5	-	19.6	-	26.0	-	-	11.6	25.3	-	-	-
KAR61	15.3	1.5	47.8	61.6	25.7	24.8	11.4	35.1	64.2	39.5	79.8	116.1
KAR64	15.4	-	38.5	-	25.3	-	16.3	34.0	54.3	-	-	-
KAR96	15.8	-	46.9	-	26.2	-	-	18.5	58.6	-	-	-

   

Table 3: Parameters from the fit of the exponential model for the objects from Table2. The upper index ex refers to the exponential model, l_B and l_R are the scale-lengths of the B and R images in kpc

name	$B^{\rm ex}$	$\mu_{B}^{\rm ex}(0)$	$\mu_R^{\rm ex}(0)$	lB	lR	$B^{\rm ex} - B$	$R^{\rm ex} - R$	$\left[\frac{{\rm d}(B-R)}{{\rm d}r} \right]^{\rm ex}$	$B-R^{\rm ex}$
cand1	17.0	25.4	23.9	0.32	0.30	-0.19	-0.08	-0.005	1.2
cand2	17.5	23.4	22.3	0.10	0.09	-0.06	-0.02	-0.017	0.9
cand3	16.6	23.0	22.6	0.13	0.12	-0.03	-0.03	-0.014	0.3
cand4	17.6	23.4	-	0.12	-	+0.15	-	-	-
cand5	17.6	23.3	-	0.09	-	-0.01	-	-	-
cand6	18.9	23.2	-	0.05	-	+0.07	-	-	-
BK1N	16.8	24.2	-	0.20	-	-0.03	-	-	-
BK2N	17.9	24.4	23.1	0.13	0.11	-0.06	-0.07	-0.028	0.9
BK3N	-	-	-	-	-	-	-	-	-
BK5N	17.5	24.8	-	0.20	-	+0.02	-	-	-
KAR61	15.2	24.5	23.7	0.50	0.67	-0.08	-0.02	+0.008	1.4
KAR64	15.6	24.0	-	0.34	-	+0.18	-	-	-
KAR96	15.3	25.4	-	0.74	-	-0.53	-	-	-

Figure 6: Images and B surface brightness profiles for the dwarf galaxy candidates 1, 2, and 3. The dotted lines represent the exponential fits. The horizontal lines indicate 1$^\prime$

Figure 7: As Fig.6, but for the dwarf galaxy candidates 4, 5, and 6

Figure 8: As Fig.6, but for the R band

Figure 9: B band surface brightness profiles for the dwarf galaxies BK1N, BK5N, KAR64, and KAR96. The dotted lines represent the exponential fits

Figure 10: Surface brightness profiles in B (left) and R (right) for the dwarf galaxies BK2N, BK3N and KAR61. The dotted lines represent the exponential fits

Figure 11: (B-R) colour profiles of the dwarf galaxy candidates 1, 2, 3 and of the dwarf galaxies KAR61, BK2N and BK3N. The dotted lines result from the exponential fits of the B and R images

The style of Table2 was adopted from BBP98. Thus, our results of the photographic photometry can be directly compared with their data from CCD photometry. For the 6 dwarf galaxies BK1N, BK2N, BK3N, BK5N, Kar61, and Kar64 contained both in our survey and in the list of BBP98, we find the following mean differences (here - BBP98): $\langle \Delta B \rangle = -0.13\pm0.23,\, \langle \Delta\langle \mu_{B}\rangle_{\rm eff}\rangle =$ $-0.11 \pm0.20\,\mbox{mag}/\Box\hbox{$^{\prime\prime}$ }, \langle \Delta \mu_{B}^{\rm ex}(0) \rangle = -0.16\pm0.27\,\mbox{mag}/$ $\Box\hbox{$^{\prime\prime}$ }, \langle \Delta r_{27}/r_{27} \rangle = -0.06\pm0.05,\, \langle \Delta l_{B}/l_{B} \rangle = 0.11\pm0.21$. For comparison, BBP98 estimate a total magnitude error of the order of 0.2mag due to uncertainties of the zero-point determination and of the correction for sky backround. A similar error is expected for our results. Moreover, there is a further source of uncertainties in our data due to the uncertainty of the extrapolation of the B band calibration curve towards faintest magnitudes. With respect to these error estimates, the agreement between our results and those from BBP98 is remarkably good.

5.2 Images and radial surface brightness profiles

In Figs.6 and 7, we present B images of the six new dwarf galaxy candidates along with the corresponding radial surface brightness profiles and the exponential fits. CCD images in the R band and in H$\alpha$+[N II] are available for the dwarf candidates 1, 2, and 3. The remaining three candidates were discovered, unfortunately, after the CCD observation campaigns. The R images are shown in Fig.8, again in combination with the surface brightness profiles. The (B-R) colour profiles are shown in Fig.11.

The H$\alpha$ images were carefully corrected for continuum emission by subtracting the R band images normalized in such a way that most of the stars disappear on the difference images. There is no indication for significant H$\alpha$ emission found for the three galaxies. Therefore, the H$\alpha$ images are not shown.

In Figs.9 and 10, we present also radial B surface brightness profiles for 7 previously known galaxies. (For images of these systems, see BBP98.) Three of these galaxies were also observed in R (Fig.10). In general, there is good agreement with the profiles shown by BBP98. Note also, that the exponential model provides a good fit to the radial profiles, with the only exception of BK3N. Colour profiles for three of these galaxies are given in Fig.11.

5.3 Individual descriptions of the new LSB dwarf galaxy candidates

Cand1 is perhaps the candidate with the highest M81 group membership probability. Moreover, if it belongs to the group, it is one of the members with the lowest surface brightness known so far (a lower surface brightness was measured only for F8D1, Caldwell et al. 1998). Cand1 appears rather structureless. Unfortunately, the image analysis is handicapped by both a bright foreground star close to and a fainter foreground star projected onto the galaxy. The global properties of cand1 are reminiscent of BK5N, but are in fact between BK5N and F8D1. The radial surface brightness profiles are well fitted by the exponential model with a relative large scale-length of about 0.3kpc and without any indication for a steepening of the profile towards the centre.

Cand2 is a small object about 10$^\prime$ SE of M82. The direct images indicate a nearly spherical outer LSB component and a small nucleus. The surface brightness profiles are steeper than exponential, as it is common for the faintest dE's (Caldwell & Bothun 1987). In the R image, a faint arc south of the nucleus is indicated which may be an artefact.

Cand3 is classified as an Irregular, mainly due to its extended, non-concentric core structure. With B-R = 0.2, this galaxy is exceptionally blue for M81 dwarfs ( $\langle B-R \rangle = 1.23 \pm 0.35$ for the objects studied by BBP98). Note, however, that we have not found evidence for H$\alpha$emission. Thus, cand3 may be a perturbed background galaxy. This interpretation is supported by the fact that two other, slightly perturbed spiral galaxies are located in the vicinity of cand3.

Cand4 is an irregularly shaped object located in the outskirts of the peculiar Sc galaxy NGC2976. Thus, it is tempting to speculate that cand4 may be a satellite of NGC2976, and that the irregularity is due to tidal distortion from this larger galaxy.

Cand5 is similar to cand2, except for the more irregular structure. The profile is well fitted by the exponential model.

Cand6 is the smallest, and therefore the faintest, of the objects in Table1. With respect to size, this object is close to the limit of the survey. The B surface brightness profile is perfectly fitted by an exponential.