2 Observations and data reduction

We selected 14 early-type spirals with observed rotation curves (Rubin et al. 1985; Fillmore et al. 1986), and with diameter $\leq$ 8 arcmin so that they could be imaged within our field of view. The sample is listed in Table 1, objects with anomalously slowly rising rotation curves are marked with an asterisk. Morphological type and coordinates are taken from NED. Three galaxies reside in the Virgo Cluster: NGC 4419, 4450 and 4698 (Binggeli et al. 1985). The remaining 11 are members of loose groups (Huchra & Geller 1982; Geller & Huchra 1983; Rubin et al. 1985; Garcia 1993). Distances were computed according to the infall model in the Third Reference Catalog of Bright Galaxies (RC3, de Vaucouleurs et al. 1991), and assuming an H₀ = 50kms^-1Mpc^-1. For each group or cluster its mean redshift was used; the adopted distance scale implies a Virgo distance modulus of 31.9 mag. The position angle (Col. 6) is obtained at the outer isophotes, measured East from North; the optical radius and the B-band magnitude (Cols. 7, 8) are from the RC3; Col. 9 reports the bands in which each galaxy has been observed.

  

Table 1: The galaxy sample

Images were acquired at the Gornergrat Infrared Telescope (TIRGO, 1.5 m, f/20) in the J (1.2 $\mu$m) and K (2.2$\mu$m) bands, using the Arcetri NIR camera ARNICA (Lisi et al. 1996), equipped with a NICMOS 3 detector. The field of view of the camera is 4$\times$4 arcmin with a plate scale of 0.97 arcsec pixel^-1. Galaxies were observed by spending half of the total integration time ($\sim 15$ min) on source, and half on the sky, alternating sky and source frames. The seeing FWHM was typically 2'', and sky magnitudes were roughly 15.5 and 12.5mag arcsec^-2in J and K, respectively. Flat fields were obtained from sky frames; the final images are typically flat to $0.3-0.5\%$ in the J band, and to 0.1% or better in the K band, giving (1$\sigma$) limiting magnitudes of 21.5 and 20.5mag arcsec^-2in J and K respectively. All image reduction was performed with IRAF and the STSDAS packages. Standard stars were selected from the UKIRT Faint Standard List (Casali & Hawarden 1992). Zero points were derived for each night of observations after correcting for atmospheric extinction using mean TIRGO values (0.11 and 0.07 mag airmass^-1 in J and K respectively; Hunt et al. 1987). Images were then corrected for extinction within our Galaxy and for redshift. Galactic extinction was determined according to the Burstein-Heiles values given in RC3, using the extinction curve given in Cardelli et al. (1989); K-corrections for redshift were calculated from the coefficients given by Frogel et al. (1978).

Color images were made by combining the J- and K-band magnitude images after first registering them to a fraction of a pixel. When the seeing differed significantly between the two bands, the higher-resolution image was degraded to the level of the lower resolution one. The K-band images and J-K color images are shown in the top panels of Fig. 1.

For five of the sample galaxies we can compare the K-band major-axis profiles with low-resolution (28'' aperture) profiles obtained with a single-element photometer (Giovanardi & Hunt 1996); the data are shown as filled circles in Fig. 1. As can be seen, the agreement is quite good in spite of the pointing uncertainties in the photometer profiles.

  

Figure 1: K-band images, J-K color images, parametric, and non-parametric decompositions. The upper left panel shows the K-band map; contours range from 21 to 14mag arcsec-2in unit steps. Images are oriented with North up and East left, and offsets are given in arcsec. The upper right panel shows the J-K color map, with the color table ranging from 0.5 (black) to 1.3 (white). The lower left panels show the results of the parametric decomposition with the K-band elliptically-averaged radial profiles shown as filled triangles, and the major-axis cut shown as a solid line. The parametric bulge and disk are shown as dot-dashed and dashed lines, respectively, and their sum as a dotted line. 28'' aperture profiles (Giovanardi & Hunt 1996) are shown as large filled circles, if available. The remaining lower left panels show the elliptically-averaged profiles in J-K, and r-K with the r-band taken from Kent (1988), if available. Also shown in the J-K plots are the fitted color profile (dotted line), and the fitted colors of bulge (dot-dashed line) and disk (dashed line). The lower right panel shows the results of the non-parametric decompositions, with the same coding as for the parametric decompositions