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3. Photometric properties of NGC 6181

A monochromatic image was constructed from the Perot-Fabry observational data in order to see the morphology of the inner part of NGC 6181. The Htex2html_wrap_inline1611 map of the central part of NGC 6181 (Fig. 1 (click here)) reveals two bright central sources none of which coincides with the center of the isophotes in the continuum. Two faint tails of Htex2html_wrap_inline1613 emission embrace the central continuum source which is located in the area of very weak emission. Bearing in mind the absence of radio emission from the NGC 6181 nucleus, one may conclude that the nucleus of this galaxy is very quiescent.

The direct images obtained with the 1m telescope were used first of all to find the precise position of the center of the galaxy in the continuum. It was determined with respect to five nearby stars. Location of the center was compared with the Htex2html_wrap_inline1615 distribution and with the dynamical center position (see the next section). In addition we tried to derive some surface brightness distribution characteristics. Fig. 2 (click here) demonstrates rather smooth image in I, with a weak bar-like disk elongation in the inner part, and a more clumpy image in B; prominent spiral arms extend up to the outermost radii in all passbands, confirming the grand-design classification of arms made by Elmegreen & Elmegreen (1984).

Figure 4: Radial distributions of azimuthally averaged surface brightness in the B and V passbands. The straight lines represent the disk exponential laws fitted to the radius range of 27tex2html_wrap1717 to 46tex2html_wrap1719

Figure 4 (click here) presents B and V azimuthally averaged radial surface brightness profiles assuming PA (line of nodes)=tex2html_wrap_inline1633 and inclination tex2html_wrap_inline1635 in accordance with the velocity field analysis (see below). It shows that this galaxy possesses a very compact bulge which does not affect light distributions beyond the radius 7tex2html_wrap1721. In the range tex2html_wrap_inline1639 a brightness excess is noticeble over the simple exponential law extrapolated from the outer parts; this excess is reproduced in all four filters being the largest (0.15 mag) in the B passband. It seems that the radius of 25tex2html_wrap1727 is a boundary between two disk subsystems. The disk scale measured in the range 25tex2html_wrap1729-45tex2html_wrap1731 for all four passbands slightly decreases from blue to red (Fig. 5 (click here)) being in general accordance with earlier results of Elmegreen & Elmegreen (1984) and Roth (1994).

Figure 5: Disk scale variation with spectral range. Previous published data are also plotted as comparison

Figure 6: Color radial profiles averaged over azimuth in the galaxy plane. The rms error of a single point is less than 0.05 mag

The azimuthally averaged color tends to be bluer up to tex2html_wrap_inline1649 and then some reddening occurs, but the slopes of these trends are quite different for different colors (Figs. 6 (click here) and 7 (click here)). In Fig. 7 (click here) we try to compare radial color variations in NGC 6181 with models for old stellar populations showing pure metallicity trend (Worthey 1994) and with a mean observational sequence of galactic colors (Buta & Williams 1995), which is known to be defined mainly by different present-time star formation rate. Color excesses expected due to interstellar reddening in the Galaxy are also shown. The reddening in B-V at tex2html_wrap_inline1653 has an azimuthally non-homogeneous character: it is a distinct, very red spot to the west from the nucleus obviously related to a local dust concentration; the optical characteristics of the dust may be unusual because the spot is absent in V-R and V-I colors. The other color variations seem to be rather azimuthally homogeneous. Comparison of the observed and the expected color trends shows that between the radii 7tex2html_wrap1741 and 25tex2html_wrap1743, i.e. in the inner disk distinguished by some brightness excess, the observed color variations may be satisfactorily explained by variations of star formation intensity, because the point grid is roughly parallel to the observational sequence of integrated galactic colors. However the observed color variations in the outer disk are more complicated and rather unusual - especially for tex2html_wrap_inline1663, where the reddening of B-V and V-R occurs under the constant V-I. This looks quite inexplicable in the frames of simple effects which influence the color.

Figure 7: (V-R, B-V) a) and (V-I, B-V) b) diagrams for the radial color variations in NGC 6181. Points are plotted through one arcsecond step. Estimates of V-R and V-I have systematic shift (see the text)

Figure 8: Variations of the orientation of the photometric major axis along the radius. For tex2html_wrap_inline1683 measurements taken for all passbands were averaged, beyond this radius only I isophotes were used. The long-dashed line indicates the orientation of the outermost isophotes according to our measurement of the SKYVIEW picture, the short-dashed line shows the kinematical line of nodes

The isophote form analysis (pure-ellipse fitting) was carried out to check a possible deviation from axial symmetry. The ellipticity between tex2html_wrap_inline1687 and tex2html_wrap_inline1689 from the center gradually increases from 0.10 (bulge) to 0.35 - a behavior which is quite normal for a galaxy whose inclination is about tex2html_wrap_inline1691. The radial dependence of tex2html_wrap_inline1693 is presented in Fig. 8 (click here). We see an unambiguous turn of isophotes in the very center of NGC 6181. Measurements in all passbands show that at the radius of tex2html_wrap_inline1695 the position angle of the major axis is tex2html_wrap_inline1697 with an uncertainty less than tex2html_wrap_inline1699, which differs by tex2html_wrap_inline1701 from the orientation of the outermost isophotes (tex2html_wrap_inline1703, Nilson 1973; tex2html_wrap_inline1705, our measurement of the SKYVIEW isophote at the tex2html_wrap_inline1707). In the radius range tex2html_wrap_inline1709 isophotes are also twisted by tex2html_wrap_inline1711, but in the opposite sense with respect to the innermost region. Only beyond tex2html_wrap_inline1713, where the radial brightness distribution follows a pure exponential law, the isophote major axis becomes aligned with the line of nodes.

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