Astron. Astrophys. 364, 479-490 (2000)

2. Observations and data reduction

In 1998 we have undertaken two-dimensional spectroscopy of NGC 7217 with the Multi-Pupil Fiber Spectrograph (MPFS) of the 6m telescope of the Special Astrophysical Observatory (Nizhnij Arkhyz, Russia). Two spectral ranges were exposed: a blue-green one, 4250-5600 Å, and a red one, 5650-7000 Å. The detailed parameters of the spectral observations are given in Table 2. A grating of 1200 grooves per mm was used which provided a reciprocal dispersion of 1.3 Å per pixel and a spectral resolution of 3 Å. The seeing of FWHM= was estimated from a stellar exposure.

Table 2. Spectral observations of NGC 7217

These spectral observations have been made with the new variant of the panoramic spectrophotometer which became operational at the prime focus of the 6m telescope in the end of 1997. With respect to the previous variants of MPFS (Afanasiev et al. 1990, Afanasiev et al. 1996), the field of view is now increased and the common spectral range is larger due to the use of fibers: they transmit light from square elements of the galaxy image to the slit of the spectrograph (256 fibers) together with the sky background taken away from the galaxy itself (6 fibers). The size of one spatial element is . At the exit of the spectrograph a CCD registers all 262 spectra simultaneously. The primary reduction of the data is made within IDL. After bias subtracting, flatfielding, and one-dimensional spectra extraction from the CCD frame, we linearize and analyse each spectrum individually. The one-element spectral characteristics, such as fluxes in continuum or in emission lines, redshift, and absorption-line indices are then combined into two-dimensional arrays corresponding to the galactic region under consideration with the help of software developed earlier in the Special Astrophysical Observatory (Vlasyuk 1993) and with our own programs. To calculate absorption-line indices and their errors we have used also the program of Dr. Vazdekis. As a result, we obtain two-dimensional surface brightness distributions, velocity fields, and maps of stellar population characteristics. In the blue-green spectral range, we measure the absorption-line indices H, Mgb, , Fe5270, and Fe5335 in the popular Lick system (Worthey et al. 1994); to check the consistency of our measurements with the model indices calculated in this system (Worthey 1994), we also observed stars from their list (Worthey et al. 1994). Besides that, we use our blue-green spectra to derive a stellar velocity field in the center of NGC 7217 by cross-correlating elementary galactic spectra with the spectrum of a K-giant star - the brighter component of the visual binary STF 2788. In the red spectral range we have measured baricentric positions of the emission line [NII ]6583, which is the strongest in the center of NGC 7217, to derive a velocity field of the ionized gas. We have estimated the best accuracy of our velocity measurements as 10 km s^-1 from the night-sky line [OI]6300 analysis. For the absorption-line index accuracy, we have made estimates using the method of Cardiel et al. (1998): the typical error of the indices varies for the EW-like indices from 0.15 Å in the nucleus to 0.5 Å in the individual elements at the edges of the area investigated, and from 0.004 to 0.01 for . To keep a constant level of accuracy along the radius, we summed the spectra in concentric rings centered on the nucleus and studied the radial dependencies of the absorption-line indices by comparing them to the synthetic models of old stellar populations of Worthey (1994) and Tantalo et al. (1998). We estimate the mean accuracy of our azimuthally-averaged indices as 0.1 Å.

Also, we have taken several long-slit spectra of NGC 7217, which have been obtained at the William Herschel Telescope on La Palma with the ISIS, from the ING Archive. The details of exposures are also given in Table 2. These CCD frames have been reduced with the software of Dr. Valeri Vlasyuk (Vlasyuk 1993).

The photometric data involved in our analysis are taken from the ING and HST Archives. The broad-band I image of NGC 7217 has been obtained on June 1st, 1998, at the 1m Jacobus Kapteyn Telescope on La Palma. The exposure times were 10 min, 10 min, and 5 min, but only the first of the exposures was well guided; only this is analysed in this work. The seeing quality is estimated from neighbouring star measurements as . The central part of the galaxy has been also observed by the Hubble Space Telescope. The earlier observations with WFPC2 were made on June 10, 1994, through the filter F547M, with an exposure time of 5 min (Principal Investigator: W. Sargent, Program ID: 5419). Later, it was observed with the NICMOS2 through the filters F110W and F160W during 128 sec each on August 17, 1997 (Principal Investigator: M. Stiavelli, Program ID: 7331). The spatial resolution was for WFPC2 observations and for the NICMOS observations. We have derived morphological characteristics of the surface brightness distribution in NGC 7217 by analysing these images. The program FITELL of Dr. Vlasyuk has been used for tracing the isophote major axis position angle and ellipticity along the radius, and 2D image decomposition was performed with the software FVIZ and IMAR (Vlasyuk 1993) as well as with our own programs.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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