2. The data

2.1. Sample

The sample galaxies were chosen according to the following selection criteria: (i) morphological type , (ii) magnitude and (iii) colour . Apparent ellipticities cover the whole range from face-on to edge-on view. The reason for using these criteria is to obtain a sample which is on the one hand appropriate for measurements of internal extinction and on the other hand representative for the photographic catalogue of spiral galaxies used by Cunow (1992) and Cunow & Wargau (1996) for statistical investigations of internal extinction.

The photographic catalogue is part of the Muenster Redshift Project MRSP (Schuecker et al. 1996a,b), which uses filmcopies of the ESO/SERC J- and R-surveys and low-dispersion objective prism plates. The internal extinction was measured for a magnitude-limited sample of spiral galaxies in the ESO/SERC field Nos. 351, 352, 411 and 412. In addition to J- and R-data, U- and I-plates of these fields were also used.

In order to allow comparison, the galaxies used in this work were selected to be members of the photographic sample. This restricts the magnitude range to and the positions to the ESO/SERC field nos. 351, 352, 411 and 412. In order to have profiles with better resolution and signal-to-noise ratio as well, galaxies with which fulfill selection criteria (i) and (iii) were chosen also.

The number of accurate redshifts available for galaxies in the magnitude range is small, so no selection according to redshift was made. For 11 of the selected galaxies, redshifts are found in the literature. It is planned to obtain redshifts for the other galaxies as well. The redshift information is important because the dust content of a spiral galaxy is closely connected to the luminosity; the brighter the galaxy, the more dust it contains.

For the galaxies with redshifts available, absolute B magnitudes were calculated using the apparent magnitudes given in Table 4 and a K-correction of k = 3z. A range of -21.2 < M_B < -18.9 with a mean of <M_B> = -20.2 is found. and q₀ = 0.5 are adopted throughout this paper.

The galaxies were not explicitly chosen according to bulge-to-disc ratio. However, the bulge-to-disc ratio is implicitely given by the morphological type. Since galaxies with have significant bulges, the presence of the bulge must be taken into account. The selected galaxies were checked for dust lanes. No dust lanes were found.

Table 1 gives a list of the sample galaxies. The parameters are taken from the ESO-LV catalogue (Lauberts & Valentijn 1989). Apparent ellipticity and position angle PA are obtained from ellipse fits to B octants. PA is measured from north over east. Redshifts are given in Table 2.

 

no. h m s T

339-0120 19 55 06 -40 57 00 4.0 13.76 1.06 0.18

463-0200 20 40 11 -30 02 00 3.0 13.45 1.17 0.64 152.8 (= IC 5039)

235-0080 20 48 39 -48 57 54 2.6 13.16 1.16 0.24 110.2 (= NGC 6970)

286-0630 21 06 32 -45 43 54 3.0 13.98 1.08 0.53

411-0170 00 45 29 -29 28 42 3.0 15.73 1.46 0.33

351-0180 00 51 25 -33 34 00 4.0 15.27 1.11 0.73

351-0220 00 52 45 -36 55 42 3.0 15.56 1.21 0.45 164.7

351-0240 00 54 50 -36 51 42 3.0 16.07 1.46 0.78 142.4

351-0250 00 56 01 -36 55 48 3.0 15.28 1.35 0.34 110.5

351-0310 00 58 39 -35 30 42 3.0 15.43 1.41 0.34 169.4

412-0210 01 12 09 -31 26 42 3.0 14.89 1.38 0.63

352-0400 01 16 44 -36 11 18 4.0 15.32 1.22 0.46 176.2

352-0540 01 19 13 -36 48 18 3.0 15.19 1.38 0.75 121.7

352-0720 01 22 56 -36 04 36 3.0 15.12 1.27 0.39 173.4

 

ESO-LV no. c z Reference

339-0120 15098 Huchra et al. (1995)

463-0200 12700 Bottinelli et al. (1990)

235-0080 15200 Sandage (1978)

286-0630 12668 Lawrence et al. (1994)

351-0180 15450 Lauberts & Valentijn (1989)

351-0250 10416 Maia et al. (1996)

351-0310 11605 Parker & Watson (1990)

412-0210 15514 Menzies et al. (1989)

352-0400 15130 Parker & Watson (1990)

352-0540 10421 Parker & Watson (1990)

352-0720 10563 Mathewson & Ford (1996)

2.2. Observations

The observations were carried out with the CCD camera of the 1.0 m telescope at the South African Astronomical Observatory (SAAO) in Sutherland, South Africa. The camera contains a Tek chip of pixels. The pixel size is which corresponds to a scale of 0.35 arcsec/pixel and to an image size of . The filters available are Johnson B and V and Cousins R and I.

The CCD images were obtained within 8 nights in September and October 1994. During 5.5 nights conditions were photometric, during the remaining 2.5 nights conditions were not photometric but still good enough for obtaining profiles with a reasonable signal-to-noise ratio. The seeing varied between and FWHM. ESO-LV 351-0180, 351-0220, 351-0240 and 351-0310 (B-image) are those galaxies which could only be observed in non-photometric conditions. The other galaxies were observed in photometric conditions.

For each CCD field B, V, R and I frames were taken. The exposure times were 900 s for each frame. For V, R and I, two frames were taken for each filter and for B four images were taken. This gives a total exposure time of 3600 s for B and 1800 s for V, R and I, respectively.

The CCD magnitudes are calibrated with the magnitudes of E-region stars given by Menzies et al. (1980). The magnitude and colour ranges of the chosen stars were 8.02 < V < 9.82 and 0.11 < (B-V) < 1.61, respectively. The standard stars were observed several times during a night with exposure times between 3 s and 40 s per frame. The data were used for determination of the transformation of the instrumental CCD magnitudes into the international BVRI system.

2.3. Data reduction

Bias and flat field corrections as well as interpolations over bad pixels were done by SAAO staff. For each CCD frame the bias was measured from the overscan region. For flat field division mean flat fields were used. The accuracy of the corrected image is determined from the differences in the sky background in different areas of the CCD field after flat fielding. It is about .

The dark current is negligible even for large integration times, so no dark correction was made. The long exposures show a number of cosmic ray events. These were removed from the images using the procedure described by Cunow (1993a). This method looks for "objects" with profiles which are steeper than those of the stars and replaces their pixels by the median of the surrounding area. The CCD images were checked for fringing. No fringing was found and no correction applied.

The final image for each galaxy and each filter was obtained by adding all frames for this galaxy and filter. The images were matched by using the positions of the stars. For each image the mode of an area undisturbed by stars was used for sky subtraction. Interactive checks of the background level assured that the correct sky value was subtracted. The uncertainty of the sky level due to gradients across the image area is about . For a few galaxies, stars are found in front of the galaxy. These star pixels were flagged and neglected during the profile measurement. Surface brightness, ellipticity and position angle profiles were obtained by using the ellipse fitting procedure of the surface photometry package within MIDAS. The profiles were determined for each filter separately.

Photometric calibration was done with the standard star measurements. The instrumental magnitudes were corrected for extinction effects by applying the mean extinction coefficients obtained for this site (Menzies 1992). The standard star measurements were used for the determination of the magnitude zero points and the transformation of the instrumental CCD magnitudes into the international BVRI system. The following transformation equations were applied:

X is the airmass, , , and are the instrumental magnitudes and B, V, R and I the magnitudes in the international system. For the standard stars, the comparison of the CCD magnitudes with the catalogue magnitudes shows a scatter of for B, for V, for R and for I. No colour term has been included in the transformation, because tests showed that no significant colour terms exist. According to these results, an accuracy of is expected for the surface brightness profiles of the galaxies. The surface photometry errors due to the uncertainties of flat field division and sky subtraction are obtained from the percentages determined above. They are given in Table 3.

 

23 0.04 22 0.03 21 0.01 21 0.02

24 0.12 23 0.06 22 0.03 22 0.04

25 0.40 24 0.19 23 0.06 23 0.12