2 Photometry of southern field dwarfs

2.1 Sample and imaging

Figure 1: B-band CCD images. Image size: $3\farcm3 \times 3\farcm3$, except for ESO 115-G021 and ESO 154-G023, for which the size is $6\farcm6 \times 6\farcm6$. North is up and east to the left.

Figure 1: continued.

Figure 2: B-band CCD image of the newly discovered galaxy NGC 2784 DW1, together with PGC 166099, the actual target, and NGC 2784. Image size: $9\farcm9 \times 9\farcm9$. North is up and east to the left.

The two main sources of the present photometric sample of dwarf galaxies are the catalogue of nearby galaxies by Schmidt & Boller (1992) and the list of galaxies by Karachentsev et al. (1999). Both include galaxies with a distance limit of roughly 10 Mpc. We selected 25, mostly field dwarf candidates in the southern sky from these catalogues. Since some of the dwarfs are rather close companions of giant galaxies, they are not as isolated as might be associated with the definition of field galaxy. However, all but one galaxy do not belong to one of the known groups in the volume in question and are therefore defined as field dwarfs. Most of the candidates are late type galaxies. A list of the objects selected and observed along with some basic data is given in Table 1. The columns of Table 1 are as follows:
Columns 2 and 3: identification of the observed galaxy;
Cols. 4 and 5: their 2000.0 epoch coordinates (from NED);
Col. 6: morphological type in the classification system of Sandage & Binggeli (1984);
Col. 7: galaxy semi-major (R₂₅) and semi-minor axis (r₂₅) in arcsecs and at the level of $25\; {\rm mag}/\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hf... ...{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\arcsec$; used as ellipse fit parameters;
Col. 8: position angle at the level of the $25\; {\rm mag}/\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hf... ...{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\arcsec$ isophote, measured from north to east;
Col. 9: total B-band magnitude, corrected for galactic extinction (cf. Table 2);
Cols. 10 and 11: heliocentric radial velocity in km s^-1 (from the NED) and distance in Mpc (mostly from Karachentsev et al. 1999 and Huchtmeier et al. 2000; see also Sect. 3.1);
Col. 12: absolute B-band magnitude, based on the data given in Cols. 9 and 11.

 

Table 1: Basic data of the observed dwarf galaxies.

No.	Ident. 1	Ident. 2	RA	Dec	Type	$R_{25}\times r_{25}$	PA	B	$V_{\rm hel}$	Dist.	MB
(1)	$\;\;\;\;$(2)	$\;\;\;\;$(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)
1.	ESO 410-G005	KK 3	00 15 31.13	-32 10 55.46	dE	37.0$\times$25.4	46	14.84	...	1.9	-11.55
2.	ESO 473-G024	PGC 1920	00 31 23.06	-22 46 02.30	Im	28.0$\times$12.8	38	15.96	541	8.7	-13.74
3.	ESO 115-G021	PGC 9962	02 37 48.10	-61 20 18.00	Sm	148.5$\times$22.2	45	13.23	513	4.8	-15.18
4.	ESO 154-G023	PGC 11139	02 56 50.38	-54 34 17.10	Sm	197.0$\times$42.4	42	12.62	578	5.9	-16.23
5.	NGC 1311	ESO 200-G007	03 20 07.37	-52 11 06.68	Sm	93.6$\times$27.8	41	13.09	571	5.7	-15.69
6.	IC 1959	ESO 200-G039	03 33 11.80	-50 24 38.28	Sdm	76.5$\times$18.3	-29	13.21:	640	6.7	-15.92:
7.	IC 2038	ESO 157-G001	04 08 54.10	-55 59 31.20	Sd	52.4$\times$15.3	-27	14.93:	712	7.6	-14.47:
8.	NGC 1800	ESO 422-G030	05 06 24.07	-31 57 10.90	Sm/BCD	60.6$\times$34.3	-70	13.01	803	7.1	-16.25
9.	AM 0521-343	KK45	05 23 23.40	-34 34 30.00	Im	23.3$\times$15.6	-65	15.74:	...	9.9	-14.24
10.	ESO 555-G028	PGC 18370	06 04 27.92	-19 37 20.70	Im	31.7$\times$11.8	-26	16.01:	882	6.4	-13.02:
11.	ESO 489-G056	PGC 19041	06 26 16.98	-26 15 56.20	Im	22.9$\times$16.5	24	15.42	495	3.7	-12.42
12.	ESO 490-G017	PGC 19337	06 37 56.60	-25 59 58.70	Im	51.9$\times$40.5	-26	13.67:	499	7.0	-15.55:
13.	ESO 308-G022	PGC 19382	06 39 33.08	-40 43 18.50	Im	18.8$\times$17.0	-20	15.67	821	7.5	-13.71
14.	PGC 20125	AM 0704-582	07 05 17.40	-58 31 14.00	Im	27.3$\times$19.1	-6	14.44	554	3.8	-13.46
15.	ESO 558-G011	PGC 20171	07 06 56.84	-22 02 26.10	Im	72.0$\times$51.4	44	12.83:	737	7.1	-16.43:
16.	ESO 059-G001	PGC 21199	07 31 18.20	-68 11 16.80	Im	71.8$\times$51.0	-19	13.35	528	3.7	-14.49
17.	ESO 006-G001	PGC 23344	08 19 23.26	-85 08 41.10	Im	37.5$\times$31.7	9	14.30	738	7.0	-14.92
18.	UGCA 148	DDO 56	09 09 46.54	-23 00 33.00	Im	37.9$\times$27.1	64	14.91	725	6.3	-14.09
19.	NGC 2784 DW1	...	09 12 18.50:	-24 12 41.00:	dE,N:	17.2$\times$9.0	-89	16.38:	...	...	...
20.	PGC 166099	KK 73	09 12 29.20	-24 14 28.00	dE,N	23.5$\times$19.3	-60	15.50	...	6.0	-13.39
21.	UGCA 153	ESO 564-G030	09 13 12.08	-19 24 31.00	Sm	42.0$\times$17.6	-43	15.02	765	7.0	-14.20
22.	NGC 2915	ESO 037-G003	09 26 11.49	-76 37 35.60	Sm/BCD	89.1$\times$50.3	-50	12.01	468	5.3	-16.61
23.	UGCA 193	PGC 29086	10 02 36.00	-06 00 49.00	Sdm	92.8$\times$10.3	17	14.67	662	9.2	-15.15
24.	UGCA 200	PGC 29299	10 05 35.20	-07 44 44.00	dE,N	23.1$\times$13.8	-31	16.16:	...	9.2	-13.66:
25.	NGC 3115 DW1	PGC 29300	10 05 41.59	-07 58 53.50	dE,N	54.3$\times$49.2	2	13.38	698	9.2	-16.44

Distances are available from the catalogues of Karachentsev et al. (1999) and Huchtmeier et al. (2000) for all but one of the observed galaxies. These distance estimates are based on the luminosity of the brightest blue stars, assumed group membership, or heliocentric radial velocity. Our galaxies are spread over almost the whole selected distance interval. NGC 2784 DW1 is a newly detected galaxy, therefore we do not have an immediate clue regarding its distance, but given its morphology (dE, N), its size and the proximity to NGC 2784 it is very likely a satellite of NGC 2784. The mean absolute magnitude of M_B = -14.67 is rather bright, which is due to the fact that about a third of our galaxies have distances >7 Mpc. The brightest dwarf is NGC 2915, which is one of the two Blue Compact Dwarf galaxies in our sample. Most of the observed galaxies are classified as Im or Sm; only five are dwarf ellipticals, all being satellites of giant galaxies. ESO 410-G005 (dE or dSph), the nearest and faintest galaxy of our sample, is a possible member of the Sculptor group (see Karachentsev et al. 2000). A CCD gallery of our sample galaxies is shown in Figs. 1 and 2. The observations have been carried out with the 1.54-m Danish Telescope at the European Southern Observatory on La Silla, Chile, between November 23, 2000 and November 27, 2000. We used the DFOSC $2048 \times 2048$ CCD camera. We took three 10 min exposures in B and three 5 min exposures in R, except ESO 410-G005, for which only one 30 and one 15 min exposure in B and R, respectively, has been taken. The field of view is $13 \farcm 3 \times 13 \farcm 3$ with a resolution of $0 \farcs 39$ per pixel. The gain was set to $0.77\rm e^-$per ADU, and the CCD was read out with a readout noise of $3.11\rm e^-$. Seeing was between $0 \farcs 8$ and $2 \farcs 0$.

2.2 Photometric procedures

The reduction of the frames and the measurements of the photometric parameters have been performed within ESO's image processing package MIDAS and follows the procedures of Papers IV and VI. The three raw images have first been combined, bias-subtracted and then flat-fielded. The flat-field frames were obtained by taking the median of several twilight flats, which we took before and after the observing runs. The background was determined by fitting a tilted plane to average intensities of frame areas unaffected by bright stars or the object itself. The same procedure was repeated to check the remaining gradient of the background. Remaining gradients of up to $0.5\%$ can be regarded as small enough for our purpose, which we achived after one substraction.

For the calibration we used standard stars from AU. Landolt (1992). The fields with the standard stars were imaged before and after each observing block of our targets i.e. six to seven times per night. In all four nights we could observe under photometric conditions, therefore we could combine all standard stars of one night for the calibration.

The galaxy frames were then cleaned from disturbing foreground stars or background galaxies. Only the objects, for which the membership to one of these classes was obvious, were removed. Due to the similarity of certain bright, star forming regions in the irregular galaxies to foreground stars, some of these might have remained on the frames by mistake. With the available resolution, such confusions can not be avoided. However, the measured parameters can not be wrong by much, as any bright foreground stars, which really would affect the results, are easily cognizable. That the removal of bright foreground stars is important indeed was shown in Paper VI, Sect. 4.4.

After this "cleaning'' we used the ellipse fitting routine FIT/ELL3 to fit an ellipse to the isophote with the surface brightness of $\sim$25 mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\arcsec$. The center, the ellipticity and the position angle of the major axis of this ellipse were then used to integrate the galaxy light in elliptic apertures of growing equivalent radius $r=\sqrt{ab}$, where a and b are the major and minor axis of the corresponding ellipse, respectively. Plotting the obtained intensities against equivalent radius yields the growth curve (integrated light profile). From this curve all model-free parameters can be derived (see below). The behaviour of this curve at larger radii also submits another possibility to check the flatness and level of the background. A perfectly flat background with intensity zero would show up in a perfectly asymptotic growth curve, whereas remaining gradients or deviants from zero cause an always rising or suddenly decreasing curve. A slightly positive (negative) background can now be corrected by subtracting (adding) a constant to all pixel intensities in the frame. These corrections should be small compared to the substracted background intensity. In general our flat fielding and background substraction procedures worked very well, only the images taken very close to the galactic plane, which were crowded by foreground stars and where a determination of the background was difficult, suffered from remaining gradients. However, these gradients only affect the faintest parts of the galaxies and do not strongly influence the global parameters considered in this work.

2.3 Photometric parameters and radial profiles

All model-free photometric parameters can be determined on the basis of the growth-curve. Its asymptotic value yields the total intensity of the galaxy, $I_{\rm T}$, which is related to the total apparent magnitude, m, through:

$$m = -2.5~\log(I_{\rm T}) + c.$$ (1)

c is a constant derived from the calibration. m translates to B or R depending on the colour band. The effective radius can be read off at half of the total growth curve intensity. Since we integrated the galaxy light in elliptical apertures, a radius refers always to an equivalent radius, if not stated separately. Together with the total apparent magnitude this yields the effective surface brightness by

$$\langle\mu\rangle_{\rm eff}~[{\rm mag}/\ifmmode\hbox{\rlap{$\... ...0\endgraf}\fi\arcsec]=m+5~\log(r_{\rm eff}[\arcsec]) +1.995.$$ (2)

The global photometric parameters of the observed galaxies are listed in Table 2 (left part), where all magnitudes, surface brightnesses, and colours listed are corrected for galactic extinction based on Schlegel et al. (1998). The columns of Table 2 represent: Col. 2: name of the galaxy.

Col. 3: total apparent magnitude in the B band.

Col. 4: total B-R colour index.

Col. 5: galactic absorption in B using the extinction maps of Schlegel et al. (1998). Cols. 6 and 7: effective radius in B and R, respectively, in arcseconds. Cols. 8 and 9: effective surface brightnesses in B and R, respectively, in mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\arcsec$. The surface brightness profiles, obtained by differentiating the growth curve with respect to equivalent radius, are shown in Fig. 3 with a resolution or bin size of $0 \farcs 39$, which corresponds also to the pixel size. The profiles are traced down to the radius where the growth curve becomes flat.

 

Table 2: Global photometric properties and model parameters of the observed dwarf galaxies.

No.	Galaxy	B	B-R	AB	$r^B_{\rm eff}$	$r^R_{\rm eff}$	$\mu^B_{\rm eff}$	$\mu^R_{\rm eff}$	$\mu^B_{0,\exp}$	$\mu^R_{0,\exp}$	$1/\alpha_B$	$1/\alpha_R$	$\Delta m_B$	$\Delta m_R$
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)	(13)	(14)	(15)
1.	ESO 410-G005	14.84	1.02	0.06	24.20	25.55	23.75	22.85	22.67	21.55	14.56	14.52	0.02	-0.07
2.	ESO 473-G024	15.96	0.61	0.08	16.93	17.54	24.10	23.56	22.00	21.72	7.32	8.16	-0.28	-0.18
3.	ESO 115-G021	13.23	0.91	0.11	33.19	34.81	22.83	22.02	21.63	20.87	19.35	20.34	-0.03	0.00
4.	ESO 154-G023	12.62	0.82	0.07	52.98	54.49	23.23	22.48	20.10	19.75	18.66	21.28	-0.87	-0.68
5.	NGC 1311	13.09	0.89	0.09	22.00	24.00	21.80	21.10	21.74	20.59	17.50	16.79	0.44	0.27
6.	IC 1959	13.21:	0.88:	0.05	20.08:	20.35:	21.72:	20.87:	19.25	19.01	8.54	9.88	-0.61	-0.29
7.	IC 2038	14.93:	1.09:	0.05	14.10:	14.07:	22.67:	21.58:	21.63	20.57	8.68	8.62	0.01	0.06
8.	NGC 1800	13.01	0.98	0.06	14.40	16.44	20.80	20.10	21.64	20.53	15.01	15.51	0.75	0.55
9.	AM 0521-343	15.74:	0.79:	0.12	11.70:	12.07:	23.07:	22.35:	21.60	20.95	6.12	6.57	-0.07	-0.08
10.	ESO 555-G028	16.01:	0.87:	0.38	19.19:	20.51:	24.42:	23.69:	23.43	21.89	12.00	9.87	0.03	-0.22
11.	ESO 489-G056	15.42	0.68	0.28	11.55	12.15	22.73	22.16	23.01	21.82	9.59	8.86	0.68	0.35
12.	ESO 490-G017	13.67:	0.83:	0.34	22.83:	25.56:	22.46:	21.87:	21.30	20.57	13.28	14.41	0.03	0.00
13.	ESO 308-G022	15.67	0.80	0.38	25.79	23.30	24.72	23.70	23.81	22.78	15.93	14.32	0.13	0.13
14.	PGC 20125	14.44	0.74	0.51	53.32	57.34	25.07	24.49	23.75	21.97	30.67	22.51	-0.12	-0.49
15.	ESO 558-G011	12.83:	1.01:	1.60	27.10:	30.53:	21.99:	21.24:	22.08	20.64	21.42	20.49	0.60	0.27
16.	ESO 059-G001	13.35	0.96	0.63	43.47	44.24	23.53	22.61	22.20	21.17	25.21	24.86	-0.15	-0.15
17.	ESO 006-G001	14.30	1.24	0.83	20.05	23.50	22.80	21.91	22.12	21.16	13.66	16.03	0.15	0.08
18.	UGCA 148	14.91	1.03	0.72	20.01	19.91	23.41	22.37	21.40	20.35	9.47	9.40	-0.39	-0.39
19.	NGC 2784 DW1	16.38:	1.12:	0.89	20.67:	19.85:	24.95:	23.74:	23.68	22.53	12.56	12.36	-0.19	-0.18
20.	PGC 166099	15.50	1.16	0.85	19.52	20.10	23.95	22.85	23.09	21.96	12.84	13.38	0.05	-0.01
21.	UGCA 153	15.02	0.94	0.38	30.47	31.32	24.43	23.55	23.32	22.34	17.94	18.58	0.03	-0.08
22.	NGC 2915	12.01	0.91	1.18	17.20	21.91	20.18	19.80	21.26	20.22	19.60	20.37	0.79	0.58
23.	UGCA 193	14.67	0.90	0.16	16.65	17.71	22.77	22.01	21.98	21.35	10.96	12.32	0.11	0.13
24.	UGCA 200	16.16:	1.38:	0.20	17.99:	19.55:	24.43:	23.23:	23.19	22.26	10.59	12.52	-0.09	0.00
25.	NGC 3115 DW1	13.38	1.38	0.23	29.98	28.88	22.76	21.30	22.26	20.72	21.36	20.01	0.24	0.22

Beside the model-free parameters described above there are some convenient model-dependent parameters as well, namely central surface brightness, scale length, and goodness-of-fit parameter. The derived values for the individual galaxies are collected in Table 2 (right part) where the columns read as follows (again after correction for galactic extinction): Cols. 10 and 11: extrapolated central surface brightness according to Eq. (3) in $B~[{\rm mag}/\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil... ...$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\arcsec]$ and $R~[{\rm mag}/\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil... ...$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi\arcsec]$, respectively. Cols. 12 and 13: exponential scale length in arcseconds $[\arcsec]$, in B and R, respectively. Cols. 14 and 15: difference between the total magnitude as derived from the exponential model and the true total magnitude in B and R, respectively.

The meaning of these parameters and the reason for their introduction is straightforward. Since the sample considered here consists mainly of dwarf irregular galaxies, most of the resulting profiles look rather noisy, above all in the central parts. At larger radii the profiles become more and more straight lines, which corresponds to an exponential behaviour of the intensity profile. The outer profile might represent the older, underlying stellar disk of the galaxy and therefore provides a physically meaningful parameter when fitted by an exponential function (de Vaucouleurs 1959; Binggeli & Cameron 1993). In the surface brightness (magnitude) representation this model is of the form:

$$\mu(r) = \mu_0+1.086 \alpha r.$$ (3)

The extrapolated central surface brightness $\mu_0$ and the exponential scale length $1/\alpha$ are the two free parameters of the exponential fit. The fitting range must be defined for each galaxy individually. For most of the galaxies the choice of this interval was quite obvious, for three cases however the best fitting region was hard to find (i.e. ESO 308-G022, PGC 20125, UGCA 153). In these cases we used rather large radius ranges for the fits to compensate for the fluctuations. As mentioned, the extrapolated central surface brightness and the scale length of the best-fitting exponential for the individual galaxies are listed in Table 2, while the corresponding profiles are plotted as solid lines in Fig. 3 along with the observed profiles. Note that scientists who decide to include the central portions of their profiles for the exponential fitting as well may get rather different scale lengths and central surface brightnesses.

The deviation of the observed profile from a pure exponential law is expressed by the difference $\Delta m$ between the total magnitude of an exponential intensity law, given by

$$m_{\exp} = \mu^{\exp}_0 + 5~\log(\alpha) - 1.995,$$ (4)

and the actual measured total magnitude. This readily explains the difference $\Delta m$ to be a measure for the goodness of a fit by means of an exponential intensity profile.

Figure 3: Radial surface brightness profiles of the observed dwarf galaxies in B (bottom curves) and R (upper curves). The solid lines represent the exponential fits, as described in the text.

Figure 4: Radial B-R colour profiles.

Before we will address questions of accuracy in more detail in the next section, a note on colour and colour profiles. The B-R colours of our sample galaxies lie between 0.61 and 1.38. As expected the 20 late-type dwarfs in the sample are very different in colour compared to the five dwarf ellipticals. The mean colour for the late-types is $\langle B-R \rangle_{\rm L}=0.90$ whereas for the dwarf ellipticals we get $\langle B-R \rangle_{\rm E}=1.21$. The "bluest'' dE is ESO 410-G005 with B-R=1.02, for which the classification as dE might indeed be doubtful (see Notes on individual galaxies). Among the late-type dwarfs two are rather red: IC 2038 with B-R=1.09, which is a disk galaxy seen almost edge-on and therefore reddened by the internal dust absorption, and ESO 006-G001 with B-R=1.24. The latter galaxy has a rather peculiar morphology. Its central part is dominated by bright, knotty features, which, as a whole, shows even a spiral structure. This "late-type part'' of the galaxy is surrounded by a spherical, dwarf elliptical-like component, which makes the classification rather difficult. For the galaxies lying close to the zone of avoidance (e.g. ESO 558-G011), the accuracy of the determined colour can not be very high, as any slight uncertainty in the correction with respect to colour will produce a large error in the "true'' colour index.

B-R colour profiles are shown in Fig. 4. Due to the active or recent star formation, in the inner parts, late-type dwarfs normaly show a stellar population gradient, e.g. younger and bluer stars are centrally concentrated and older and redder stars dominate the outer parts of the galaxy. Therefore, their colour indices are expected to increase with galactocentric radius. However, only eight galaxies among the late-type dwarfs show such a positive colour gradient, another eight examples have a more or less flat colour profile and four actually have a negative gradient. As the scatter in some of these profiles is rather large the sign of their gradients is not obvious. Finally we remain with two clear outliers: UGCA 148 is classified as Im, but might be a transition type between Im and dE, because, aside from a couple of bright knots on the outer parts, the galaxy looks like a dwarf elliptical. UGCA 153 has clearly two spiral arms and can therefore probably be regarded as a disturbed (dwarf) spiral galaxy, hence the colour gradient makes sense.

2.4 Accuracy

The accuracy of the global photometric parameters depends of course on the correctness of the individual reduction steps and these, in turn, on the quality of the data. Apart from this the calibration represents another source of error on the global parameters. Our four observation nights have been calibrated individually by means of several standard stars. Since the whole observation could be carried out under good photometric and seeing conditions, we can not only check the stability of the calibration of each night, but also compare the results of the entire run. The scatter of the obtained zero points is quite low and yields a $1\sigma$ error of 0.03 mag. Another contribution to the error comes from the determination of the extinction, which was also measured each night. The $1\sigma$ error in this case is 0.02 mag. To check the accuracy of the flat-fielding and the background substraction, we fitted a plane to the background of the already reduced image and measured the remaining gradient (see Sect. 2.2). The mean of these gradients is $0.07\%$, which is so low that it can be neglected. However, small fluctuations in the background can not be discovered with this method. Hints of such deviations are provided by the corrections, which have to be applied to the background in order to get a perfectly asymptotic growth curve. Only in rare cases the first integration of the galaxy light, after the standard reduction steps, yielded a perfectly assymptotic growth curve. Therefore, we normally had to correct the background by a constant, which in general was small compared to the substracted background intensity. Considering these corrections, we are on the safe side by assigning an uncertainty of 0.05 mag to this effect, which gives, together with the error from the calibration, a total error of 0.1 mag.

Figure 5: Comparison of the total apparent magnitude in B (not corrected for galactic extinction) from this work with values from the literature.

However, our perspective view of the objects is independent of these technical aspects of the observations, but the accuracy of the results for a galaxy beeing partly covered in projection by another galaxy or by a swarm of forground stars can not be as high as for a "isolated'' object. This fact often leads to considerable differences of photometric parameters in the literature. The determined values, which are uncertain in this sense are flagged with a colon in Tables 1 and 2 and they have not been taken into account in the comparison with the parameters provided in other works shown in Fig. 5. The sources are: (1) Karachentsev et al. (1999), solid circles, who provide own results and parameters from the last version (LEDA) of the Principal Galaxy Catalogue (Paturel et al. 1992), (2) RC3 data (third reference catalogue of bright galaxies, de Vaucouleurs et al. 1991), open circles and (3) data from HYPERCAT, asteriks. We consider a deviation of $\leq$0.2 mag as a good agreement. What concernes the brightest galaxies our results are in agreement with at least two other sources. However, there are a few huge differences among the fainter objects. For PGC 20125 the comparison to (1) yields $\Delta B_{\rm T}=0.75$ mag. This galaxy is very diffuse and extended and therefore difficult to measure. The deviation might be caused partly by differencies in removing forground stars. The same holds for UGCA 153 with $\Delta B_{\rm T}=0.41$ mag to (1) and $\Delta B_{\rm T}=0.54$ mag to (2). The difference of $\Delta B_{\rm T}=0.55$ mag to (1) for UGCA 193 is not clear. The agreement with the actual value in LEDA is much better, $\Delta B_{\rm T}=0.25$ mag, supporting our result. For UGCA 148 (DDO 56) the three other sources agree very well ((1): $B_{\rm T}=15.36$ mag, (2): $B_{\rm T}=15.32$ mag, (3): $B_{\rm T}=15.42$ mag), whereas our result ( $B_{\rm T}=15.63$ mag) is quite deviant, which we can not explain. Altogether, our results are in good agreement with earlier studies.

2.5 Notes on individual galaxies

ESO 410-G005: the nearest galaxy of our sample and a probable member of the Sculptor Group (Côté et al. 1997). Since some stars are resolved and therefore must be very bright, this galaxy is probably a transition type between dwarf irregulars and dwarf ellipticals. This is also supported by the colour-magnitude diagram in Karachentsev et al. (2000), derived with the Hubble Space Telescope.
ESO 473-G024: confirmed member of the Sculptor Group (Côté et al. 1997). A contour plot in the J-band is shown in Bergvall et al. (1999).
ESO 115-G021: member of the Flat Galaxy Catalogue (Karachentsev et al. 1999). It has a bulge-like feature slightly offset towards the upper left and is therefore very likely a disk galaxy seen edge-on.
ESO 154-G023: again a rather flat, but bright galaxy. The exceptional shape of the surface brightness profile is caused by the remarkable bright star forming region near the upper left edge of the galaxy.
NGC 1311: disk galaxy viewed almost edge-on. No signs of spiral arms, but obviously with a bulge. The colour profile shows a blue gradient in the central part and a reddening in the outer parts.
IC 1959: irregular disk galaxy without bulge. Again a blue colour gradient in the inner parts, more extended than NGC 1311 and probably caused by internal extinction due to dust, and a reddening towards larger radii.
IC 2038: since Karachentsev et al. (1999) do not provide a distance for this galaxy, we can only rely on its radial velocity, which is $v_{\rm hel}=712 {\rm ~km~s}^{-1}$. However, as to its morphology and angular size the distance can not be much larger than 10 Mpc. The galaxy in the lower left is IC 2039, classified as S0. Ferguson & Sandage (1990) include it, together with IC 2038, in the Dorando Group at a distance of $\sim$20 Mpc, despite its radial velocity of $250 {\rm ~km~s}^{-1}$. In fact, the correctness of this velocity is somewhat doubtful. If it would be confirmed, the distance of IC 2039 ought to be much smaller and we would deal with a M32-like galaxy.
NGC 1800: this galaxy is undergoing a strong starburst, which is reflected in the bright bar. The B-R colour map in Fig. 6 shows that the bluest, and hence strongest, star forming region is not located at the center of the galaxy, but completly offset at the end of the bar. This starburst region is also showing up in the colour gradient, which suddenly decreases at $r \sim 12 \arcsec$. Marlowe et al. (1999) include this galaxy in their sample of blue amorphous galaxies and examine its taxonomy and starburst properties.

Figure 6: B-R colour map of NGC 1800. Shown is the interval 0.3 (black) $\leq B-R \leq 1.7$ (white). The image size is $1\farcm65 \times 1\farcm65$. North is up and east to the left.

AM 0521-343: peculier shape and many bright starforming regions asymetrically distributed.
ESO 555-G028: rather diffuse and faint galaxy with a bar and an oscillating colour gradient.
ESO 489-G056: the fuzzy object at the upper left of the galaxy is a background spiral. The galaxy has a peculier shape, the starforming regions being concentrated in the lower part. Kunth & Östlin (2000) include it in their list of the most metal-poor galaxies.
ESO 490-G017: a bright starforming region is visible in the lower left of the galaxy, another such region might be covered by the bright star on the right side of the galaxy. The two bright objects near the center are foreground stars.
ESO 308-G022: dwarf irregular galaxy with only a few bright regions and an extended and diffuse outer part.
PGC 20125: Very peculier shape. The galaxy is extended and has almost no compact region.
ESO 558-G011: this galaxy is located close to the galactic plane and therefore crowded with foreground stars. The colour profile is v-shaped: the galaxy becomes bluer from the center to the starforming regions and then reddens again towards the edge.
ESO 059-G001: a dwarf irregular galaxy with rudiments of spiral arms.
ESO 006-G001: this galaxy shows a considerable reddening in its colour profile. In fact, the envelope is very similar to a dwarf elliptical and the surface brightness profile is almost perfectly exponential in this part. The center is dominated by two engulfed chains of bright knots. A transition type between dwarf irregular and dwarf elliptical?
UGCA 148: the galaxy has a decreasing colour gradient (see Sect. 2.3). With the exception of the brighter regions at the right edge, the galaxy is very diffuse and uniform. It is probably a transition type between Im and dE. This is also supported by recent radio observations by Simpson et al. (2000), who find only a small amount of HI gas.
NGC 2784 DW1 and PGC 166099: in the field of our target object PGC 166099 we discovered another very faint galaxy, which we call NGC 2784 DW1. Figure 2 shows a B-band CCD image of the two dwarf galaxies together with the Sa galaxy NGC 2784. Neither the NED nor the LEDA data bases list any object at this position. In view of the size and the visiual appearance of NGC 2784 DW1 on our image we suggest that it is another companion of NGC 2784. Two bright stars lie in front of the galaxy, covering a substantial part of its image. We consider the fainter object near the center as the nucleus of the galaxy and classify it therefore as dE,N. Adopting the distance of NGC 2784 we get an absolute magnitude of M_B=-12.40, which is rather faint for a dE,N. PGC 166099 (KK 73) appears in the list of Karachentsev et al. (1999) and is clearly a dE,N. Its distance has not been measured directly, but it is also considered to be a satellite of NGC 2784.
UGCA 153: since the galaxy is quite isolated the striking feature on the upper part is most likely some kind of spiral arm rather than a tidal tail. There is also a rudiment of another arm on the opposite side. The colour gradient also supports the suggestion that this object might be a (disturbed) dwarf spiral (see Sect. 2.3).
NGC 2915: this is the brightest galaxy in the La Silla sample with M_B=-16.61. There are many contributions in the literature for this galaxy, which is commonly classified as a blue compact dwarf, but with a very extended HI disk, which even shows some traces of spiral structure (see Bureau et al. 1999). An optical study is presented in Meurer et al. (1994). Like for NGC 1800 the colour gradient indicates that the brightest star forming region is offset.
UGCA 193: a disk galaxy seen perfectly edge-on. The measured colour of B-R=0.90 might be an overestimate due to some internal extinction. The galaxy is also a member of the Flat Galaxy Catalogue (Karachentsev et al. 1999). Haynes et al. (1998) present a 21 cm HI line profile, which clearly shows that the galaxy is rotating.
UGCA 200: this dE,N is a faint companion of the S0 galaxy NGC 3115. Not surprisingly for a dwarf elliptical it is very red, B-R=1.38, and has a rising colour gradient.
NGC 3115 DW1: this galaxy, another companion of NGC 3115, is a typical dE,N, very bright and very red. An analysis of its Globular Cluster system, based on HST observations, is presented in Puzia et al. (2000). They also include an image from the Digitized Sky Survey, which shows the relative position of the dwarf and NGC 3115. Also visible, but not marked, is UGCA 200.