2 The data

 

 

Table 1: Journal of observations.

Field	$\alpha_{(2000)}$	$\delta_{(2000)}$	Date	Telescope &	Exposure	Filter	Airmass	Seeing	Epoch
#	(h min s)	($^\circ$ $\hbox{$^\prime$ }$ $\hbox{$^{\prime\prime}$ }$)	dd/mm/yyyy	Instrument	(s)			$\hbox{$^{\prime\prime}$ }$
1	13 26 23.5	-42 52 00	12/07/1999	Antu+FORS1	2$\times$900	U	1.54	0.52
1	13 26 23.5	-42 52 00	12/07/1999	Antu+FORS1	2$\times$900	V	1.77	0.54
1	13 26 23.5	-42 52 00	08/04/1999	Antu+ISAAC	10$\times$36$\times$10	$K_{\rm s}$	1.15	0.41	A
1	13 26 23.5	-42 52 00	07/05/1999	Antu+ISAAC	10$\times$36$\times$10	$K_{\rm s}$	1.05	0.40	B
1	13 26 23.5	-42 52 00	28/05/1999	Antu+ISAAC	10$\times$36$\times$10	$K_{\rm s}$	1.14	0.51	C
1	13 26 23.5	-42 52 00	15/04/2000	Antu+ISAAC	65$\times$6$\times$10	$K_{\rm s}$	1.09	0.35	D
1	13 26 23.5	-42 52 00	10/05/2000	Antu+ISAAC	65$\times$6$\times$10	$K_{\rm s}$	1.22	0.43	E
1	13 26 23.5	-42 52 00	07/06/2000	Antu+ISAAC	65$\times$6$\times$10	$K_{\rm s}$	1.13	0.60	F
1	13 26 23.5	-42 52 00	08/07/2000	Antu+ISAAC	65$\times$6$\times$10	$K_{\rm s}$	1.15	0.31	G
2	13 25 24.0	-43 10 00	11/07/1999	Antu+FORS1	2$\times$900	U	1.28	0.53
2	13 25 24.0	-43 10 00	11/07/1999	Antu+FORS1	2$\times$900	V	1.20	0.44
2	13 25 24.0	-43 10 00	08/04/1999	Antu+ISAAC	10$\times$36$\times$10	$K_{\rm s}$	1.05	0.36	A
2	13 25 24.0	-43 10 00	07/05/1999	Antu+ISAAC	10$\times$36$\times$10	$K_{\rm s}$	1.10	0.39	B
2	13 25 24.0	-43 10 00	29/05/1999	Antu+ISAAC	10$\times$36$\times$10	$K_{\rm s}$	1.06	0.44	C
2	13 25 24.0	-43 10 00	15/04/2000	Antu+ISAAC	65$\times$6$\times$10	$K_{\rm s}$	1.09	0.40	D
2	13 25 24.0	-43 10 00	12/05/2000	Antu+ISAAC	51$\times$6$\times$10	$K_{\rm s}$	1.22	0.51	Eb
2	13 25 24.0	-43 10 00	19/05/2000	Antu+ISAAC	20$\times$6$\times$10	$K_{\rm s}$	1.22	0.44	Ec
2	13 25 24.0	-43 10 00	07/06/2000	Antu+ISAAC	41$\times$6$\times$10	$K_{\rm s}$	1.13	0.57	Fa
2	13 25 24.0	-43 10 00	09/06/2000	Antu+ISAAC	30$\times$6$\times$10	$K_{\rm s}$	1.13	0.40	Fb
2	13 25 24.0	-43 10 00	08/07/2000	Antu+ISAAC	65$\times$6$\times$10	$K_{\rm s}$	1.15	0.40	G
2	13 25 24.0	-43 09 04	20/02/2000	NTT+SOFI	45$\times$10$\times$6	$K_{\rm s}$	1.04	0.55
2	13 25 24.3	-43 09 58	27/06/1998	HST+NIC3	256	F222M

Figure 1: Combined U- and V-band image of Field 1, taken at the VLT (UT1+FORS1). The field of view is $6\hbox {$.\mkern -4mu^\prime $ }8 \times 6\hbox {$.\mkern -4mu^\prime $ }8$. North is at the top and east to the left.

Figure 2: Combined U- and V-band image of Field 2, taken at the VLT (UT1+FORS1). The field of view is $6\hbox {$.\mkern -4mu^\prime $ }8 \times 6\hbox {$.\mkern -4mu^\prime $ }8$. North is at the top and east to the left.

The observations were carried out with the ESO Very Large Telescope (VLT) at Paranal Observatory, Chile. They consist of optical (Bessel U- and V-band) and near-IR ($K_{\rm s}$-band) images. We observed two fields in the halo of NGC 5128. Field 1 ( $\alpha_{2000}=13^{\rm h}26^{\rm m}23.5^{\rm s}$, $\delta_{2000}=-42^{\circ}52\hbox{$^\prime$ }00\hbox{$^{\prime\prime}$ }$; Fig. 1) was centered on the prominent N-E shell, $\sim$14 $\hbox{$^\prime$ }$ away from the center of the galaxy. Field 2 ( $\alpha_{2000}=13^{\rm h}25^{\rm m}26^{\rm s}$, $\delta_{2000}=-43^{\circ}10\hbox{$^\prime$ }00\hbox{$^{\prime\prime}$ }$; Fig. 2) was chosen to overlap with the HST field of Soria et al. (1996) and lies at a distance of $\sim$ $9\hbox{$^\prime$ }$ from the center of the galaxy. The optical data were obtained on 1999 July 11 and 12, while the $K_{\rm s}$-band images cover several epochs and will be used to search for long-period variable stars. All of the observations were secured in the service mode. Calibration data, bias, dark and flat-field images as well as photometric standards from Landolt (in optical; 1992) and Persson et al. (IR; 1998) catalogues, were supplied by the ESO calibration plan. The observations are summarized in Table 1.

2.1 Optical observations and data reduction

Two 15-min exposures in both U and V were acquired for each of the two fields in NGC 5128 using FORS1 (FOcal Reducer and low dispersion Spectrograph) on VLT Unit Telescope 1 (Antu). The FORS1 detector is a 2048$\times$2048 Tektronix CCD, thinned and anti-reflection coated. The pixel size is 24$\times$24 $\mu$m. The field of view is $6\hbox {$.\mkern -4mu^\prime $ }8 \times 6\hbox {$.\mkern -4mu^\prime $ }8$ and the scale is $0\hbox{$.\!\!^{\prime\prime}$ }2/$pixel.

For service observations in direct-imaging mode, the FORS1 CCD is read out in four-port read-out mode. We used the ESO pipeline reductions which are based on MIDAS package specially developed to reduce images with 4 different amplifiers. With it the overscan for each amplifier was subtracted individually and after the subtraction of bias the images were corrected for the flat-field.

2.2 Near-IR observations and data reduction

For the near-IR $K_{\rm s}$-band observations we used ISAAC, also on Antu. In this wavelength domain (0.9-2.5 $\mu$m) the detector is a 1024$\times$1024 Hawaii Rockwell array. The field of view is $2\hbox{$.\mkern-4mu^\prime$ }5 \times 2\hbox{$.\mkern-4mu^\prime$ }5$ and the scale is $0\hbox{$.\!\!^{\prime\prime}$ }147/$pixel. A series of 10-s exposures were taken at each epoch, usually in groups of six, with the number of repeats depending on weather conditions and any technical problems. The total exposure time for each epoch is given in Table 1.

An additional $K_{\rm s}$-band observation of Field 2, with a total on-target exposure time of 45 min, was acquired with the SOFI instrument at the ESO 3.5-m New Technology Telescope (NTT) at La Silla Observatory, Chile. The field of view of SOFI is $4\hbox{$.\mkern-4mu^\prime$ }94 \times 4\hbox{$.\mkern-4mu^\prime$ }94$ and the scale is $0\hbox{$.\!\!^{\prime\prime}$ }292/$pixel.

We also obtained observations with the NIC3 array on HST, using the F222M filter, which is similar to (although narrower than) the standard K filter. The NIC3 field of view is $51\hbox{$.\!\!^{\prime\prime}$ }2 \times 51\hbox{$.\!\!^{\prime\prime}$ }2$, much smaller than the one of ISAAC.

The standard procedure in reducing IR data consists of (i) dark subtraction, (ii) flat-field correction, (iii) sky subtraction, (iv) registering and combining the images. We did not use the ESO pipeline reduction product. Good sky subtraction in a crowded field like that of a galactic halo is particularly important. For that step we used the DIMSUM package (Stanford et al. 1995) within IRAF. In DIMSUM the sky subtraction is made in two passes. In the first, a median sky is computed for each image from the six frames that are closest in time. The shifts between the sky-subtracted frames are then computed and all the images stacked together using a rejection algorithm to remove cosmic rays. An object mask is computed for the coadded image and then shifted back in order to create object masks for the individual frames. In the second pass, the sky subtraction is made using the object masks to avoid overestimation of the sky level. These masks are also used to check that the bright object cores were not removed as cosmic rays in the previous pass. After the mask-pass sky subtraction, all frames belonging to a single epoch are registered with imalign and combined with imcombine task in IRAF.