next previous
Up: A ring-like zone

2. Observations

The two-dimensional velocity field of NGC 6181 was obtained at the 6m telescope on September 24, 1993. The scanning Perot-Fabry interferometer was installed inside of the pupil plane of a focal reducer which was attached to the F/4 prime focus of the telescope. An intensified photon counting system (IPCS) tex2html_wrap_inline1423 was used as the detector. Instrument parameters are given in Table 2 (click here).

Table 2: Scanning Perot-Fabry observations parameters

The ``Image-interferometry method" was described in detail earlier (Boulesteix et al. 1983; Amram et al. 1991). Observational data (galaxy and wavelength calibration data) were converted into cubes of 32 images (tex2html_wrap_inline1453), with the linear scale being 0.70 arcsec/px and a spectral resolution of about 2-2.5 channels (tex2html_wrap_inline1455). A reduction of observational data (correction for phase shifting, subtraction of night-sky emission spectrum, construction of velocity map, a.s.o.) was done by using standard methods, and the Perot-Fabry reducing software ADHOC developed at Marseille Observatory (Boulesteix 1993) was used.

The continuum subtracted Htex2html_wrap_inline1457 image of NGC 6181 (Fig. 1 (click here)) reveals that the Htex2html_wrap_inline1459 emission is rather strong over the whole galactic disk. So the high quality of the two-dimensional velocity distribution obtained allows to get more detailed information about gas motions in the disk of this galaxy. A sub-cube (tex2html_wrap_inline1461), containing the main fraction of Htex2html_wrap_inline1463 emission from the galaxy, was extracted from a reduced data cube. This smaller cube was used for the analysis of the velocity field.

Figure 1: Monochromatic Htex2html_wrap_inline1465 emission map of NCC 6181. The cross marks the position of the nucleus

Table 3: Direct image observations

Direct images of NGC 6181 were obtained at the telescope Zeiss-1000 of SAO RAN. Eleven frames of the galaxy have been derived with a CCD camera through the B, V, R and I filters of Johnson's system (the log of the observations is given in Tab. 3 (click here)). The seeing quality ranged from 2.1tex2html_wrap1595 (R images) to 2.8tex2html_wrap1597 (V and I images).

Figure 2: The I and B images of NGC 6181. The frame is tex2html_wrap_inline1533, north is up and east is to the left

Gray-scaled sky-subtracted I and B images are presented in Fig. 2 (click here)a and 2b; flux-calibrated isophotes in B (tex2html_wrap_inline1541 and tex2html_wrap_inline1543 for the innermost and the outermost isophotes respectively with the step of tex2html_wrap_inline1545) are shown in Fig. 2c. The B and V frames were calibrated by using 14 aperture photoelectric measurements of NGC 6181 from Burstein et al. (1987). The range of aperture radii is from 16tex2html_wrap1605 to 48tex2html_wrap1607. Zero-point magnitudes are obtained with accuracy better than 0.01 mag, color terms are found to be negligible. The sky brightnesses are estimated as tex2html_wrap_inline1555 in V and tex2html_wrap_inline1559 in B; these values coincide with mean sky brightnesses measured fifteen years ago in the Special Astrophysical Observatory at tex2html_wrap_inline1563 by Neizvestny (1981). To check our BV calibration, we compared multi-aperture photoelectric data for NGC 6181 taken from the catalogue of Longo & Vaucouleurs (1983, 1985) (18 entities excluding old data of PET-54 and BIG-51) with values simulated for the same apertures from our CCD frames. In Fig. 3 (click here) one can see a rather good consistency with the photoelectric data, even for large apertures. The calibration of R and I frames was indirect and less precise because standard stars were not observed. Instead B-V colors for five faint stars in the field of the galaxy were measured, and adopting them to be dwarfs we ascribe them mean V-R and V-I colors in accordance with their spectral types (Straizys 1977). The formal accuracy of the calibration constants determined in such a way is 0.12 mag, but we admit a possible systematic shift of our R and I magnitudes by up to 0.3 mag.

Figure 3: Comparison of published V and B multi-aperture photoelectric data on NGC 6181 and present CCD observations

next previous
Up: A ring-like zone

Copyright EDP Sciences