3 The CCD data

3.1 Description of the observations

Figure 3: Positions of the 610 galaxies in our CCD catalogue, with the observed fields superimposed. The size of each field is 6.4$\times$6.4 arcmin2. Positions are drawn relatively to the centre defined in the text

The observations were performed with the Danish 1.5 m telescope at ESO La Silla during 2 nights on November 2 and 3, 1994. A sketch of the observed fields with the positions of the galaxies appearing in the CCD catalogue is displayed in Fig. 3. The central field was centered on the coordinates of the cluster center, defined both by the position of the cD and by the centroid of the X-ray emission (Pislar 1998): 04$^{\rm h}$33$^{\rm m}$37.9$^{\rm s}$, $-13^\circ 15'47''$ (equinox 2000.0). There was almost no overlap between the various fields (only a few arcseconds). Johnson V and Rfilters were used. Exposure times were 10 mn for all fields; 1 mn exposures were also taken for a number of fields with bright stars in order to avoid saturation. The detector was CCD #28 with 1024²pixels of 24 $\mu$m, giving a sampling on the sky of 0.377''/pixel, and a size of 6.4$\times$6.4 arcmin² for each field. The seeing was poor: 2.4'' the first night and 1.1'' the second night.

3.2 Data reduction

Corrections for bias and flat-field were performed in the usual way with the IRAF software. Only flat fields obtained on the sky at twilight and dawn were used; dome flat fields were discarded because they showed too much structure.

Each field was reduced separately. The photometric calibration took into account a zero point correction, the airmass (AM) and the color index (V-R). For the first night, the best fits to obtain the calibrated magnitudes $V_{{\rm cal}}$ and $R_{{\rm cal}}$ from the measured magnitudes $V_{\rm m}$and $R_{\rm m}$ were obtained for the following parameters:

$$V_{{\rm cal}} = V_{\rm m} - 2.2 - 0.47 \ AM + 0.025 (V-R)_{{\rm cal}},$$

$$R_{{\rm cal}} = R_{\rm m} - 2.07 - 0.51 \ AM + 0.0046 (V-R)_{{\rm cal}},\rm\ with$$

$$(V-R)_{{\rm cal}} = -0.14 + 0.04 \ AM + 1.02 (V-R)_{{\rm m}} .$$

For the second night, the corresponding relations were:

$$V_{{\rm cal}} = V_{\rm m} - 2.711 - 0.075 \ AM + 0.009 (V-R)_{{\rm cal}},$$

$$R_{{\rm cal}} = R_{\rm m} - 2.677 - 0.071 \ AM + 0.072 (V-R)_{{\rm cal}},\ \rm with$$

$$(V-R)_{{\rm cal}} = -0.034 - 0.004 \ AM + 0.941 (V-R)_{{\rm m}} .$$

The error bars on the various calibration parameters are the following: V band, first night: $-2.2 \pm 0.1$, $-0.47 \pm 0.08$, $0.025 \pm 0.025$; R band, first night: $-2.07 \pm 0.15$, $-0.51 \pm 0.11$, $0.0046 \pm 0.037$; V band, second night: $-2.711 \pm 0.004$, $-0.075 \pm 0.004$, $0.009 \pm 0.003$; R band, second night: $-2.677 \pm 0.002$, $-0.071 \pm 0.002$, $0.072 \pm 0.002$. Note that the color term for the first night in the R band is in fact undefined; however, we have kept it for the sake of coherence between all calibrations. It also appears that both the tranformation and extinction terms for this first night strongly differ from those for the second night. For the first night, the range in standards and in air-mass is smaller, which may explain the discrepancy, together with perhaps poorer photometric conditions.

Since the exposure times were the same in V and R, a number of galaxies were detected in R but not in V. For these objects, a photometric calibration was performed without using a color term. The corresponding relations were: $R_{{\rm cal}} = R_{\rm m} - 2.06 - 0.51 \ AM$ for the first night and $R_{{\rm cal}} = R_{\rm m} - 2.70 - 0.03 \ AM$ for the second night. The corresponding error bars on these parameters are: $-2.06 \pm 0.14$, $-0.51 \pm 0.11$, $-2.70 \pm 0.04$, $-0.028 \pm 0.028$.

Objects were automatically detected using the DAOPHOT/ DAOFIND tasks of IRAF. This task first performs a convolution with a Gaussian with characteristics set according to the seeing in each frame (FWHM of the star-like profiles in the image) as well as the CCD readout noise and gain. Objects are then identified as the peaks of the convolved image which are higher than a given threshold above the local sky background (chosen as approximately equal to 4 times the rms of the mean sky level on the image). A list of detected objects is thus produced and interactively corrected on the displayed image so as to discard spurious objects, add undetected ones (usually close to the CCD edges) by modifying the detection parameters and dispose of false detections.

We used the package developed by Le Fèvre (Le Fèvre et al. 1986) to obtain for each field a catalogue with the (x,y) galaxy positions, isophotal radii, excentricities, major axis, position angles, and V and R magnitudes within the 26.5 isophote. To perform a star-galaxy classification based on the compactness parameter described in Le Fèvre et al. (1986), we measured the required information for each object with dedicated software we developed. Very bright stars are saturated and deviate significantly from the Gaussian-like PSF involved in the computation of the classification parameter. They are therefore classified as non stellar objects with this criterion and had to be eliminated manually. The rms accuracy on these CCD magnitudes is about 0.1 magnitude, and their errors are in all cases smaller than 0.2 magnitude.

The astrometry of this CCD catalogue is accurate to about 2.0 arcsec as verified from the average mutual angular distance between CCD and MAMA equatorial coordinates for the galaxies included in both catalogues.

Figure 4: Histogram of the R magnitudes of the 610 galaxies in the CCD catalogue

The histogram of the R magnitudes in the CCD catalogue is displayed in Fig. 4. The turnover value of this histogram is located around $R\simeq 22.5-23$, suggesting that our catalogue is complete up to $R\sim 22$.

The histogram of the (V-R) colour is plotted in Fig. 5.

Figure 5: Distribution of the (V-R) colour as a function of R for the 239 galaxies detected in the V band in our CCD catalogue

3.3 Transformation laws between the photometric systems

By identifying galaxies in our CCD catalogue with objects in our photographic plate catalogue, we derived the following calibration relations between our photographic plate $b_{\rm J}$ magnitudes and our R CCD magnitudes: first night: $R_{\rm CCD} = b_{\rm J} - 0.28$, rms = 0.05 (5 galaxies); second night: $R_{\rm CCD} = b_{\rm J} - 0.30$, rms = 0.01 (9 galaxies); mean value: $R_{\rm CCD} = b_{\rm J} - 0.28 \pm 0.01$, rms = 0.01 (10 galaxies). We did not include any colour term, because it did not make the fit any better. The difference between the observed R band CCD magnitude $R_{\rm CCD}$ and the R magnitude calculated from $b_{\rm J}$ with the above formula is plotted in Fig. 6 as a function of the CCD Rmagnitude. This difference is small and does not appear to increase with magnitude.

3.4 The CCD catalogue

The CCD photometric data for the galaxies in the field of Abell 496 are given in Table 2. The meaning of the columns is the following:
(1) Running number;
(2) to (4) right ascension (equinox 2000.0);
(5) to (7) declination (equinox 2000.0);
(8) 26.5 magnitude isophotal radius in arcseconds;
(9) excentricity e defined as $\sqrt{1-({b\over a})^2}$, where a and b are the major and minor axes respectively;
(10) position angle of the major axis (in degrees from North to East);
(11) and (12) V and R magnitudes;
(13) and (14) X and Y positions in arcseconds relative to the X-ray centre.