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Subsections

2 Observations and data reduction

2.1 The sample

The galaxies presented in this paper have been selected from the sample of elliptical and lenticular galaxies whose ionized gas distribution has been studied by Macchetto et al. (1996); the dust content of IC 5105, NGC 636 and NGC 1600 was studied by Ferrari et al. (1999). We selected those galaxies which were compatible with the observation epoch. The selected galaxies are brighter than $B_{\rm T}=13.0$ mag and their morphological types are from The Revised Shapley-Ames Catalog of Bright Galaxies (Sandage & Tammann 1987). Table 1 presents morphological types, apparent $B_{\rm T}$ magnitudes, B extinction and distances (scaled to H0=65 km s-1 Mpc-1) of the 12 selected objects.

2.2 Observations

The observations were made during 3 nights at the Las Campanas Observatory, Chile, with the infrared camera C40IRC (NICMOS3 HgCdTe detector, $256\times256$ pixels) attached to the 1 m Swope telescope. This instrumental arrange covers, on the sky, an area of $2.5\times2.5$ arcmin2, with a scale of 0.6 arcsec/pixel. We used the broadband filters J, H and $K_{\rm s}$, centered on 1.24, 1.65 and 2.16 $\mu$m, with 0.22, 0.30 and 0.33 $\mu$m passbands. The H filter is equivalent to the H filter in the CIT/CTIO system; filters J and $K_{\rm s}$ are variations of standard J and K filters. The J filter is less sensitive to variations in atmospheric water vapor transmission, while $K_{\rm s}$ does not suffer from background thermal radiation as much as the K filter. For the specific characteristics of these filters, see Persson et al. (1998), who also give infrared photometric data for several stars, allowing us to derive a transformation between systems K and $K_{\rm s}$; from a least-square fit, we found $K_{\rm s}=1.018~K-0.162$.

For all galaxies, we obtained a set of 32 exposures of 30, 30 and 25 s for J, Hand $K_{\rm s}$ respectively, except for NGC 720, NGC 7562 and NGC 7796, where we got only 16 exposures. Images were dithered each 4 exposures. Seeing conditions were fair, varying from 1.05 $^{\prime\prime}$ to 1.83 $^{\prime\prime}$, and all nights were photometric. Standard stars from Persson et al. (1998) were also observed, taking 10 exposures of 10 s, with the same dithering conditions used for the galaxies.

2.3 Data reduction

The IRAF reduction package was used to conduct all reduction steps. The images were dark-subtracted and flat-fielded. For each galaxy, the images were median-combined in order to produce its corresponding sky frame. Bad pixels were corrected using CCDMASK and FIXPIX tasks. The final image for each galaxy was obtained combining the sky-subtracted, bad-pixel corrected individual images.

A very pronounced effect in the H images is the formation of fringes. The use of twilight sky as a flat-field, with short exposures ($\sim$6000 ADU), minimizes this effect in flat-field frames; therefore this correction must be done only in object frames. This correction is made as we calculate the median frame (sky) in each pixel; as the fringing is fixed along the detector, the subtraction of the median extracts this pattern from the science frames.

2.4 Calibration

Photometric calibrations were made using the photometric standards of Persson et al. (1998). IRAF task PHOT was used to perform aperture photometry, giving the instrumental magnitudes of these stars. The adopted photometric transformation to standard magnitudes was

\begin{displaymath}m_{\lambda}^{\rm cal}=C_\lambda+m_{\lambda}^{\rm inst}-k_{\lambda}X,
\end{displaymath} (1)

where $C_\lambda$ is the zero-point constant for $\lambda$ filter, $k_\lambda$ is the atmospheric extinction coefficient and X is the airmass. Like Galaz (2000), we did not use color terms in these equations, since these corrections are of the same order as the instrumental errors. A least-square method was applied to determine the calibration constants $C_\lambda$ and $k_\lambda$ for each filter.

The estimated calibration errors are 0.011 mag for J, 0.060 mag for H and 0.059 mag for $K_{\rm s}$. While H and $K_{\rm s}$ errors are of the same order of those expected from photoelectric photometry (Ducati et al. 2001), the J band shows lower values.


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