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Astron. Astrophys. 330, 412-418 (1998)

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2. Observations and data reduction

2D spectroscopy of the central parts of the polar-ring galaxies NGC 2685 and IC 1689 has been made by using the Multi-Pupil Field Spectrograph (MPFS, Afanasiev et al. 1990) of the 6m telescope of the Special Astrophysical Observatory (Nizhnij Arkhyz, Russia). The log of the observations is given in Table 1.


[TABLE]

Table 1. 2D spectroscopy of NGC 2685 and IC 1689


MPFS allows us to obtain simultaneously from 95 to 160 spectra from an extended area of the investigated galaxy. A set of enlargers projecting an object onto a rectangular array of microlenses provides a varying scale; we have used the [FORMULA] enlarger with the scale of about 1.3" per lens. The entire field of view is limited by the detector size; the detectors used and the fields of view obtained are listed in Table 1.

We have exposed the galaxies in two spectral ranges, named green and red. The green spectral range, 4700-5400 Å, contains a lot of stellar absorption lines including [FORMULA], MgI [FORMULA], FeI [FORMULA] and others. It was used to study stellar rotation fields by a cross-correlation method. Bright components of the binary stars ADS 15470, STF 2788, and ES 560 have been taken as templates. A typical spectral resolution in this spectral range was 5-6 Å, a stellar velocity accuracy was about of 20 km/s. The green exposures were also used to study absorption-line strength radial profiles. For this purpose blank sky areas were exposed separately; sky-subtracted spectra were co-added in concentric rings centered onto the galactic nuclei to keep a constant level of signal-to-noise ratio along the radius. An accuracy of absorption-line equivalent widths is estimated as 0.3 Å for spectra registered with IPCS and 0.1 Å for spectra registered with CCD. Frames exposed in the red spectral range (6450-6750 Å) contain emission lines [FORMULA] and [NII] [FORMULA] and are used to obtain velocity fields of the ionized gas. The spectral resolution is higher than in the green and reaches 2 Å for the observations of NGC 2685 in October 1994. An accuracy of gas velocities is better than 30 km/s. The data reduction - bias subtraction, flatfielding, cosmic ray hit removing, extraction of one-dimensional spectra, wavelength calibration, surface brightness and velocity mapping - have been done by using the software developed in the Special Astrophysical Observatory (Vlasyuk 1993).

[FIGURE] Fig. 1a and b. The isovelocities of stars (left) and ionized gas (right) in the center of NGC 2685. The plus marks the photometric center of the galaxy. North is up, east is to the left. Gray-scaled continuum image is given to show an orientation of the galaxy isophotes.

[FIGURE] Fig. 2a and b. The isovelocities of stars (left) and ionized gas (right) in the center of IC 1689. The plus marks the photometric center of the galaxy. The plots are turned by 44o the north being in the upper right corner, to show better a location of the polar ring. Gray-scaled continuum image is given to show an orientation of the galaxy isophotes.

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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998
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