2. Observations and data reduction

The data (Table 1 (click here)) were acquired on three nights (February 27 to March 1, 1995) using the infrared camera IRAC2 installed on the 2.2-meter telescope at the ESO's La Silla Observatory. This camera is equipped with a Hg:Cd:Te NICMOS3 array of pixels. The detector scale was chosen to be 0.52 arcsec/pixel corresponding to the field of view of about arcminutes. The seeing on the first night was 1.2'' (FWHM) for all the three filters; during the second and the third nights it got reduced to 0.9'' and 1.0'' in the H-band (observations in bands K and J were carried out during the first night only).

  

Galaxy Type D₂₅ d Nuclear activity,

(RC3) (s) ('') [Mpc] nucl. rings (nr), nucl. spirals (ns)

N 613 SB(rs)bc 380 17.9 Sey^a, ns^d

N 1079 RSAB(rs)0/a 208 17.1

N 1187 SB(r)c 330 16.7

N 1255 SAB(rs)bc 250 20.3

N 1302 RSB(r)0/a 233 20.5 nr^d

N 1353 SB(rs)b 203 18.4

N 1365 SB(s)b 673 19.4 Sey 1/H II^a,b, ns^d

N 1398 R'SB(r)ab 425 16.5

N 1433 R'SB(r)ab 387 11.1 nr^d

N 1512 SB(r)a 535 11.1 nr^d

N 1518 SB(s)dm 181

N 1640 SB(r)b 158 19.2

N 1744 SB(s)d 488 7.4

N 1784 SB(r)c 239 29.2

N 1792 SA(rs)bc 315 13.2

N 1808 RSAB(rs)a 387 10.4 H II^a,b, nr'^d

N 1832 SB(r)bc 154 24.0

N 2217 RSB(rs)0+ 268 19.0

N 2442 SAB(s)bc 330 15.5

N 2525 SB(s)c 173 19.3

N 2811 SB(rs)a 151 31.6

N 2911 SA(s)0 244 42.3 Sey 3^a

N 2935 R'SAB(s)b 218 28.3 nr^d

N 2997 SAB(rs)c 535 11.9 nr^d

N 3166 SAB(rs)0/a 287 17.4

N 3346 SB(rs)cd 173 17.1

N 3368 SAB(rs)ab 455 12.2

N 3384 SB(rs)0- 330 10.1 ?^a

N 3393 R'SB(rs)a 131 47.6 Sey 2^a

N 3593 SA(s)0/a 315 8.7 nr^d

N 3637 RSB(r)0/a 95 23.6

N 3673 SB(rs)b 218 23.9

N 3885 SA(s)0/a 144 22.1

N 3887 SB(r)bc 199 14.8

N 4050 SB(r)ab 185 23.6

N 4106 SB(s)0+ 97 26.8

N 4178 SB(rs)dm 308

N 4192 SAB(s)ab 586 Sey 3^a

N 4212 SAc 190

N 4216 SAB(s)b 488

N 4267 SB(s)0- 194 14.8

N 4424 SB(s)a 218

N 4438 SA(s)0/a 511 Sey 3^a

N 4442 SB(s)0 274 7.7

N 4454 RSB(r)0/a 120 30.3

N 4461 SB(s)0+ 213 26.4

N 4501 SA(rs)b 415 31.2 Sey 2^a

N 4503 SB0- 213 18.7

N 4519 SB(rs)d 190 16.7

N 4546 SB(s)0- 199 13.7

N 4612 SB(r)0+ 239 24.8

N 4665 SB(s)0/a 228 10.6

N 4684 SB(r)0+ 173 20.8

N 4689 SA(rs)bc 256 22.4

N 4694 SB0 190 16.4 H II^a

N 4731 SB(s)cd 396 19.5

N 4781 SB(rs)d 208 16.1

N 4856 SB(s)0/a 256 17.0

N 4900 SB(rs)c 134 13.1

N 4902 SB(r)b 181 35.2

N 4984 RSAB(rs)0+ 165 15.2 ?^c, nr^d

N 5101 RSB(rs)0/a 322 23.2

N 5236 SAB(s)c 773 H II^b, nr^d

N 5427 SA(s)c 169 35.3 Sey 2^a, nr^d

N 5566 SB(r)ab 396 20.8

N 5643 SAB(rs)c 274 13.7 Sey 2^a

N 5701 RSB(rs)0/a 256 20.9

N 6753 RSA(r)b 147 39.3 nr^d

N 6782 RSAB(r)a 131 48.5 nr^d

N 6810 SA(s)ab 190 23.5

E437-67 R'SB(r)ab 123 39.4 nr^d

I 1953 SB(rs)d 165 22.9

Typically (but not always; see Table 1 (click here)), four object frames were obtained for a galaxy in one band: exposure length for filters H, J and K was respectively 50 s (achieved by 5 elementary integrations of 10 s each, in order to avoid the detector saturation), 30 s ( s) and 50 s ( s), resulting in the total integration time of 200 s, 120 s and 200 s. To reduce the contamination by defective pixels (less than 1%), the telescope pointing was shifted by a few arcseconds for every object frame.

Since the sky in the near-IR varies on the timescale of the total integration time, a sky frame (of the same exposure length as for an object frame) was taken after each object frame: the typical observing sequence was thus OBJECT-SKY-O-S-O-S-O-S. The sky frames were offset from a galaxy by a few arcminutes. Dark current frames of all relevant exposure times were prepared as well.

The data was reduced by means of the ESO MIDAS package. First, from each object frame the subsequent sky frame was subtracted (no dark subtraction was needed here because of equal exposure lengths). The resulting images were divided by the flatfield (normalized to unity) to eliminate the variation in the pixel-to-pixel response (about 10%); the flatfield frame was constructed for each galaxy separately by median combining of dark-subtracted sky frames. In turn, the sky-subtracted and flatfielded images were aligned and averaged into one frame that was cleaned from remaining bad pixels (bi-linear interpolation) and intervening stars (bi-quadratic interpolation).

Table 1 (click here) summarizes basic information about observed objects: Column (1) Galaxy identification (N = NGC, E = ESO, I = IC), Column (2) Type according to RC3 (de Vaucouleurs et al. 1993), Column (3) Exposure time in filter H; four galaxies were observed also in K: NGC 1433 ( s), 3346 (), 3887 (), 5236 (), and five in J: NGC 1433, 3384, 3593, 3887, 5236 ( s all), Column (4) 25 B-mag/ isophotal diameter (from the Lyon-Meudon Extragalactic Database (LEDA), Paturel et al. 1989), Column (5) kinematical distance corrected for the Virgocentric inflow, H₀= 75 km/s/Mpc (from LEDA), Column (6) Nuclear activity, rings, spirals: from (a) Véron-Cetty & Véron (1996), (b) Telesco et al. (1993), (c) Devereux (1989), (d) Buta & Crocker (1993).

2.1. Calibration

To calibrate images, three standard infrared stars were observed each night. The rms error in the determination of the photometric zero points was 0.03 mag for all three filters on the first night. The zero points for the second and third night in band H were consistent to within the error with that for the first night and all the three were averaged to give the single zero point. The airmass correction was applied using the mean atmospheric extinction coefficients for the observing site: a_H=0.06, a_J=0.08, a_K = 0.11 mag/airmass; the airmass falls between 1 and 2.03 for our observations.

To test the photometric reliability, we have compared the results of our calibration to published photometry. In the H band, our sample has nine galaxies in common (NGC 1302, 1398, 1433, 1808, 2217, 5566, 5701, 6753 and 6810) with the aperture photometry of Griersmith et al. (1982). We have simulated the apertures of diameter 22'', 33'' and 56'' on our frames and found a mean magnitude difference, , of , and . The H-band aperture photometry of 10 other galaxies of our survey (NGC 3166, 3885, 3887, 4212, 4273, 4501, 4781, 4900, 4902 and 4984) was done by Devereux (1989): our magnitudes for his 9.3'' aperture differ by . Both comparisons given above could indicate a systematic offset of our calibration by 0.1-0.2 mag, however this number is within the errors quoted in the referenced papers. Another galaxy (NGC 2997) was measured by Forbes et al. (1992): in this case and + 0.07 for the 6'' and 12'' apertures. Finally, the surface photometry of Héraudeau et al. (1996) has one common object with us, NGC 6810, for which we find along the 60'' major-axis profile.

2.2. Ellipse fitting

To follow the isophotal twist, we have used the ellipse fitting algorithm FIT/ELL3 (in the MIDAS context SURFPHOT), developed by Bender & Möllenhoff (1987) for the study of the isophotal twist in elliptical galaxies (cf. Sect. 3). To parametrize the bars and double bars we use terms and quantities introduced by W95 to whom we refer the reader for details: typically, for a nearly face-on galaxy (projection effects are discussed in Sect. 3) with two bars, the ellipticity (e=1-b/a, where a and b are the ellipse semi-major and semi-minor axes) first grows to a first local maximum (at ) corresponding to the secondary (i.e. inner) bar, then falls to a minimum before climbing again to a primary bar maximum, (at ), after which it decreases towards the ellipticity of the disk, (see Fig. 1 (click here) in W95). We define the sizes of the bars by and . Position angles (measured from the North counterclockwise) of the bars and the disk are denoted , and . When the changes along a bar, we define and to be the at and , respectively. In agreement with W95 and E96, we shall classify the bar isophotes as twisted whenever the variation of the along a bar exceeds .

In the appendix, we present for individual galaxies the PAs and ellipticities plotted against the semi-major axis of the fitted ellipses which is scaled logarithmically in order to better see inner regions. We do not comment on any feature inside a=3'' since the ellipse fitting on artificial bars of known shapes proved not to be reliable there due to the seeing and small number of pixels. However, we show the profiles down to a=1'' since they often display a continuity below a=3'' and might provide a reference for eventual future observations with higher resolution. The unreliable region, a<3'', is separated by a vertical dash-dot line in plots.