7 Photometric catalog

7.1 Compilation of the photometric catalog

To create the final photometric catalog we merged the individual catalogs of the objects detected in the co-added B filter image and in the co-added I filter image. We decided to use these two catalogs as a basis, since the images in these two filters correspond to the best seeing conditions and since most types of objects are expected to be detected in at least one of these two bands.

The merging of the I and B catalogs was carried out as follows: we first matched the positions of the detected objects and their corresponding images in the two filters. This was done by visual inspection of the entries of the objects on both frames. This procedure gave us a clear view of the success of our automatic detection procedure and allowed us to reject obviously false identifications. In order to avoid mis-matches in the final catalog, each entry in the B catalog was first assigned a corresponding entry in the I catalog and vice versa. A cross-match of the B versus I and I versus B entries allowed us to identify false matches, which were checked again until a perfect cross-match was derived.

The initial catalogs in B and I contained 7206 and 6900 entries, respectively. After the visual cross-matching, we deleted 15 objects from the B catalog and 8 objects from the I catalog. These were mostly objects close to the edges of the field. In a few cases, 2 objects separated by a few pixels (e.g. a merging pair of galaxies) were detected in the B band, whereas in the I band only one object in between the two B band objects was found (essentially at the center of the common envelope of both galaxies). In such cases the entry in the I band was deleted. This left us with 7191 entries in the B catalog and 6892 entries in the I catalog. Now we merged both catalogs to form the final photometric catalog. This catalog contains 8753 objects. 5327 out of the 8753 objects were detected in both filters (61%), whereas 1864 (21%) were detected in B only and 1562 (18%) were detected in I only. We emphasize here that a non-detection does not necessarily mean that the object is not present on the frame, it rather means that the object was not detected by SExtractor with the parameters set here.

Since SExtractor may use a different number of pixels to derive the total magnitudes in B and I, the colours of very extended objects computed from the total magnitudes are not reliable. Therefore the catalog also contains aperture magnitudes in UBgRIJKs. An aperture of $2\hbox{$^{\prime\prime}$ }$ was chosen in order to minimize the errors due to blending and since the faint objects usually have diameters of $\leq 2\hbox{$^{\prime\prime}$ }$. The aperture magnitudes were derived by first convolving all frames to the same seeing ( $1\hbox{$^{\prime\prime}$ }$ FWHM) and then performing aperture photometry on the positions of the objects detected in B and I in the convolved frames. For objects detected in B only, we used the aperture photometry based on the positions in the B catalog, whereas the aperture photometry based on the positions in the I catalog were used for the remaining objects (detection on both frames or I-only detections). Thus for many objects, which were initially not detected in either filter, useful photometric data could be given.

Finally, the galactic absorption towards the FORS Deep Field was estimated. We used the formulae 2 and 3 in Cardelli et al. (1989) and adopted E(B-V) = 0.018 (Burstein & Heiles 1982) and $A_V = 3.1 \times {E}({B}-{V})$ to calculate the extinction correction for each filter. The central wavelengths for each filter were taken from the ESO Web-page. We derived A_U/A_V = 1.555, A_B/A_V = 1.365, A_g/A_V = 1.105, A_R/A_V = 0.790, A_I/A_V = 0.631, A_J/A_V = 0.283 and A_Ks/A_V = 0.117resulting in A_U = 0.087 mag, A_B = 0.076 mag, A_g = 0.062 mag, A_R = 0.041 mag, A_I = 0.035 mag, A_J = 0.016 mag and A_Ks = 0.007 mag, respectively. The values for the extinction agree to $\leq$0.01 mag with those listed in the NED. The photometric catalog described below is not corrected for galactic extinction. However, the completeness limits as well as the number counts shown in Sect. 8 were derived with a galactic extinction correction.

7.2 Contents of the photometric catalog

The full catalog containing 8753 objects is available in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/49.

As an illustration of its content we list in Table 4 the entries 2630-2639.

For each object we report the following parameters:

ID: the identification number. The objects have been sorted first by right ascension (2000), followed by declination (2000). The identification numbers provide a cross-reference to the spectroscopic and other observations of the FDF (e.g. Noll et al., in prep.).

RA, Dec: the positions of the objects in the FDF for J2000.0. Their accuracy has been examined by comparing the positions of 31 well-isolated, evenly distributed objects on the I frame of the FDF, to those listed in the USNO catalog (Monet 1998). The mean difference in right ascension is $0\hbox{$.\!\!^{\prime\prime}$ }21\pm0\hbox{$.\!\!^{\prime\prime}$ }38$ and the mean difference in declination is $0\hbox{$.\!\!^{\prime\prime}$ }14\pm0\hbox{$.\!\!^{\prime\prime}$ }40$. Given a typical accuracy of $0\hbox{$.\!\!^{\prime\prime}$ }25$ for objects in the USNO catalog our positions have an accuracy of $\sim$ $0\hbox{$.\!\!^{\prime\prime}$ }5$.

${m}_{\rm BT}$, $\sigma_{\rm BT}$, ${m}_{\rm IT}$, $\sigma_{\rm IT}$: the total magnitudes (Vega-system) and associated mean errors of the detected sources in the B and I band images, respectively, as measured using the SExtractor routine mag_auto on the co-added and unconvolved frames. Mag_auto is an automatic aperture routine based on Kron's (1980) "first moment'' algorithm, which determines the sum of counts in an elliptical aperture. The semimajor axis of this aperture is defined by 2.5 times the first moments of the flux distribution within an ellipse roughly twice the isophotal radius, within a minimum semimajor axis of 3.5 pixels. This routine is intended to give the most precise estimate of "total magnitudes'', at least for galaxies, and takes into account the blending of nearby objects.

${m}_{\rm UBgRIJKs [2\hbox{$^{\prime\prime}$ }]}, \sigma_{UBgRIJKs}$: UBgRIJKs magnitudes (Vega-System) and associated errors within an aperture of $2\hbox{$^{\prime\prime}$ }$. They (and their errors) were measured on the co-added and convolved frames using SExtractor. The positions listed in the catalog were used for this procedure. An aperture of $2\hbox{$^{\prime\prime}$ }$ was chosen in order to minimize the errors due to blending. Moreover, the faint objects in the FDF usually have diameters of $\leq$2 $\hbox{$^{\prime\prime}$ }$. Choosing a larger aperture would result in larger photometric errors due to the sky background. For extended objects, the mean errors of the aperture magnitudes are generally smaller than for the total magnitudes, as the aperture photometry selected the regions of high surface brightness. The magnitudes were not corrected for blending. Blended objects can be identified from the column Flag1 (see below).

 

Table 4: Excerpt from the FDF object catalog. The entries with the IDs 2630-2639 are displayed as examples.

ID	RA (2000)	Dec (2000)	${m}_{\rm BT}$	$\sigma_{\rm BT}$	${m}_{\rm IT}$	$\sigma_{\rm IT}$	mU [ $2\hbox{$^{\prime\prime}$ }$]	$\sigma_{U}$	mB [ $2\hbox{$^{\prime\prime}$ }$]	$\sigma_{B}$	mg [ $2\hbox{$^{\prime\prime}$ }$]	$\sigma_{g}$	mR [ $2\hbox{$^{\prime\prime}$ }$]	$\sigma_{R}$	mI [ $2\hbox{$^{\prime\prime}$ }$]	$\sigma_{I}$
2630	1 5 57.28	-25 48 02.3	27.75	0.19	25.30	0.10	26.99	0.27	27.61	0.05	27.72	0.10	26.10	0.03	25.34	0.02
2631	1 5 57.29	-25 45 00.1			24.42	0.03					30.73	1.65	26.57	0.04	24.49	0.01
2632	1 5 57.29	-25 48 46.9	26.13	0.05	24.98	0.07	25.96	0.10	26.20	0.01	25.92	0.02	25.42	0.02	25.05	0.02
2633	1 5 57.30	-25 44 56.6	24.47	0.01	22.75	0.01	24.60	0.03	24.60	0.01	23.74	0.01	23.26	0.01	22.87	0.01
2634	1 5 57.30	-25 48 14.2	27.69	0.16					27.77	0.06	28.23	0.17	26.84	0.06	26.78	0.09
2635	1 5 57.31	-25 43 52.3			25.02	0.09	26.22	0.13	26.42	0.02	26.11	0.02	25.66	0.02	25.33	0.02
2636	1 5 57.31	-25 44 02.2	24.85	0.04	23.43	0.04	25.53	0.07	25.53	0.01	25.12	0.01	24.56	0.01	24.12	0.01
2637	1 5 57.31	-25 44 15.2	26.60	0.09	26.19	0.17	26.76	0.22	26.83	0.02	26.72	0.04	26.46	0.04	26.16	0.05
2638	1 5 57.31	-25 46 23.5	27.36	0.16	25.65	0.09	27.58	0.46	27.43	0.04	27.45	0.08	26.72	0.05	25.67	0.03
2639	1 5 57.31	-25 47 51.1	26.17	0.08	25.11	0.10	26.42	0.16	26.85	0.02	26.74	0.04	26.22	0.03	25.60	0.03

ID	mJ [ $2\hbox{$^{\prime\prime}$ }$]	$\sigma_{J}$	mKs [ $2\hbox{$^{\prime\prime}$ }$]	$\sigma_{Ks}$	FWHM [ $^{\prime\prime}$]	Elong	PA [$^\circ$]	Cstar	Flag1	Flag2	Flag3	weight_B	weight_I
2630			21.97	0.20	0.74	1.17	17.9	0.40	0			1.000	1.000
2631	21.36	0.01	20.35	0.03	0.52	1.02	111.7	0.98	0	Ionly	L star		1.000
2632	26.58	2.38	22.37	0.29	0.78	1.12	82.1	0.26	0			1.000	1.000
2633	22.09	0.03	20.91	0.06	0.53	1.04	36.2	0.98	0		QSO	1.000	1.000
2634					1.01	1.25	00.6	0.61	0	Bonly		1.000
2635	23.70	0.18			1.13	1.19	129.3	0.00	3	Ionly			0.984
2636	22.71	0.07	20.75	0.07	0.73	1.34	90.2	0.09	3			0.984	1.000
2637					1.07	1.87	76.9	0.40	0			1.000	1.000
2638					0.80	1.49	19.1	0.43	0			1.000	1.000
2639	24.02	0.23	22.96	0.50	1.34	1.16	21.6	0.01	2			1.000	1.000

The next four columns (FWHM, elongation, position angle, star-galaxy classification parameter) describe the morphology of the objects. Since the FWHM, elongation and position angle may have high errors and are sometimes unreliable for faint objects, this information is provided for objects brighter than our 50$\%$ completeness limit (27.69 in B, 26.37 in I) only. Moreover, we do not list these values for objects where SExtractor derived a ${\it FWHM} < 0\hbox{$.\!\!^{\prime\prime}$ }4$(FWHM is $0\hbox{$.\!\!^{\prime\prime}$ }53$ in co-added I band frame and $0\hbox{$.\!\!^{\prime\prime}$ }6$ in co-added B band frame). The information should also be treated with caution for brighter objects having a star-galaxy classification parameter >0.9.

FWHM: Full width at half maximum of the objects in arcsec as determined by SExtractor by a Gaussian fit to the core.

Elong: Elongation of the images. The elongation is defined as A/B, where A and B are given by the 2nd order moment of the light distribution along the major and minor axis, respectively.

PA: The position angle of the major axis, measured from North to East, with N-S = 0.

Cstar: Star-galaxy classification parameter returned by SExtractor based on the morphology of the objects on the image. A classification near 1.0 describes point like sources whereas a classification close to 0.0 describes extended sources.

Flag1: flags returned by SExtractor with the following notation:

1: object has neighbours bright and close enough to bias significantly mag_auto; 2: the object was originally blended with another one; 3: sum of 1 + 2; 4: at least one pixel of the object is saturated (or very close to saturation); 7: sum of 1 + 2 + 4; 8: the object is truncated (e.g. too close to the image boundary); 16: object aperture data are incomplete or corrupted; 17: sum of 1 + 16; 18: sum of 2 + 16; 19: sum of 1 + 2 + 16; 24: sum of 8 + 16.

Flag2: here we report if an object was detected on the B frame only ("Bonly''), on the I frame only ("Ionly''). If there is no entry, the object is detected by SExtractor on both frames.

Flag3: a preliminary classification of 35 point-like objects (QSOs, stars) from our spectroscopic survey (Noll et al., in prep.).

weight_B, weight_I: averaged weights of all pixels used to determine ${m}_{\rm BT}$ and ${m}_{\rm IT}$, respectively. They were derived from the combined weight maps which are described in Sect. 4. A weight of 1 means that all pixels used to derive the magnitude are fully exposed and not affected by bad areas. Most of the detections with low weights are close to the edges of the FDF where the total integration times are lower.