Astron. Astrophys. 317, 36-42 (1997)

2. Observations and data reductions

2.1. H I data

Neutral hydrogen VLA observations of NGC 1300, originally presented by England (1989), have been re-reduced by JvM. The reduction method presented by JvM takes into account the difference in beam-size between the dirty and clean beam. Thereby the restored H I flux is found to converge well with single-dish values. Details concerning the original observations are given by England (1989). We present here the H I total column density map of NGC 1300 in Fig. 1, and the corresponding velocity field in Fig. 2. The overlaid H I contour corresponds to the noise level and, in contrast to England (1989), H I is detected in the entire bar region. The resolution of the H I data is 23:0006 22:0038 (beam FWHM).

Fig. 1. The H I total column density map of NGC 1300. The unit on the axes is arcseconds offset from the optical nucleus. The overlaid H I contour corresponds to the noise level. The angular interval used in the wedge analysis (see Sect. 3.1.2) is marked by the white lines centred on the line of nodes (black line), as derived in Sect. 3.1.1

Fig. 2. The H I velocity field for NGC 1300. The contour interval is 20 km s , and the zero velocity level is marked by the thick contour. The extent of the bar is indicated by the straight line

Fig. 3. The measured optical velocity points drawn on top of an optical B -band image of NGC 1300. The units on the axes are arcseconds. The reference points for the three optical spectra presented in Sect. 2.2, are marked with black crosses

Table 1. General data for NGC 1300

2.2. Optical data

The optical emission line slit observations presented by PH, are here complemented with three more slits observed by us in August 1985 with the Boller & Chivens CCD-equipped intermediate dispersion long slit spectrograph at the ESO 3.6 m telescope.

Data for our long slit observations are listed in Table 2 and 3. The columns of Table 2 contain: spectral region ("Blue" referring to the region covering H and [O III ] 5007 and "Red" to the region with [N II ] 6548,6583, H and [S II ] 6716,6731), pixel size in microns, ESO grating number, slit width in arcseconds, dispersion after calibration in Å/pixel, and scale along the slit in 00/pixel.

Table 2. Spectral parameters

Table 3. Identifications and slit positions for CCD spectra

The properties listed in Table 3 are: The second column contains the identification for each slit, found in the label of the panels in Fig. 7. The third column refers to the offset, in arcseconds, of the slit from the position of the optical nucleus. The position angle of the slits, measured from North through East, is listed in the fourth column.

Fig. 4. The variation of the parameters as a function of radius. a Inclination angle i, b position angle , c systemic velocity , d x - and e y - coordinates offset from the position of the optical nucleus

Our spectra were bias-subtracted, corrected for the mean dark current value and flat-fielded using averaged flat field frames. The wavelength calibration and a rectification in the dispersion direction was performed with MIDAS using He Ar spectra exposed before and after the object frames. Third degree polynomials were fitted to the He Ar lines and used to rebin the corresponding set of scan lines in the object spectra. The calibration procedure was designed and carried out by Maria Santos-Lleó (1986). The calibration was performed over sets of 9 scan-lines. Position, width, and intensity (not flux calibrated) of the spectral emission lines were measured by fitting them with a Gaussian profile.

The H line was, unfortunately, contaminated by a sky-line (atmospheric H O, P7 311, 6597.038Å), which had to be compensated for. We measured the contaminated H profile row by row. The total emission in the profile consists of the sky-line emission + H emission. Low intensity regions were found to correlate with a constant radial velocity, indicating a dominating sky-line. Assuming the contribution from H to be negligible in these regions, we extracted an average sky-line which was subtracted from the entire spectrum.

To deduce the zero point of position along the slits, we cross-correlated the measured continuum level in the spectra with a trace across H and R images observed by us using the Danish 1.5 m telescope on La Silla in Nov. 1993.

The velocities of the red spectra were found to have a zero-point offset (RED1: +22 km s, RED2: -12 km s ) when compared to the optical velocities in the data from PH. This may have been caused by differences in the light-path between the on- source data and the He Ar calibration lamps. We adjusted the zero-point in our spectra to fit the previous observations by PH.

Fig. 3 shows the position of the measured velocity points for the 16 optical spectra superposed on a B -band image of NGC 1300. Evidently the optical velocity coverage in the bar region is limited to a few points crossing the dust lanes, and a few points close to the end of the bar.

© European Southern Observatory (ESO) 1997

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