5. Surface photometry and central structure of NGC 7217
5.1. The central region of NGC 7217
From our red exposure with the MPFS we have derived the surface distributions of the emission-line intensities of H and [NII ]6583, as well as of their ratio (see Fig. 8). NGC 7217 is a LINER, so the nitrogen emission is the strongest in its center; its distribution (Fig. 8, top) is concentrated towards the center, but still well resolved. The inner isophotes of the nitrogen emission seem to be elongated approximately north-south, i.e. orthogonally to the global line of nodes, but close to the dynamical major axis orientation found in the previous section for the circumnuclear ionized gas. The surface distribution of the H emission (Fig. 8, middle) differs from that of [NII ]6583: it demonstrates an emission ring with the radius of 11", as previously found by Pogge (1989). Besides the ring, a central concentration also aligned along the north-south direction is present. The [NII ]-to-H ratio (Fig. 8, bottom) varies significantly over a tiny area with a radius of 11": it is 0.5 in the H-ring, slightly above 1 at the inner edge of the H-ring, 4 in the nucleus and 5-10 at the edge of the central emission concentration. We can easily understand the [NII ]-to-H ratios in the ring and in the nucleus, as they are typical for a star formation site and for LINER excitation, respectively, but the high [NII ]-to-H ratio at the inner edge of the ring and at the outer edge of the central concentration needs further explanation. Buta et al. (1995) considered the rings in NGC 7217 in detail and noted that the H-ring coincides with the continuum (B-band) ring, but inside of both there is a dust ring. Similar shifts between the stellar (gaseous) and dust features are observed in spiral arms and large-scale bars, and there they are attributed to shocks. Our high [NII ]-to-H ratios in the center of NGC 7217 could be explained if the circumnuclear gaseous ring has a radial velocity component toward the center and if the central ionized gas disk is inclined with respect to the plane populated by surrounding gas: then shocks would be arisen in the required locations.
In our previous work (Zasov & Sil'chenko 1997) we reported a turn of the isophote major axis and an increase of ellipticity inside using HST WFPC2 data obtained through the F547M filter (the pivot wavelength is 5487 Å). But dust, which is obviously present in the center of NGC 7217, might distort the true shape of the brightness (mass) distribution. Now we add NICMOS data at 1.1 µm and 1.6 µm which are much less affected by dust. A comparison of the NICMOS and WFPC2 data (Fig. 9) shows their consistensy: indeed, the isophote major axis turns by and the ellipticity increases in the very center of the galaxy. The central gravitational potential shape in NGC 7217 may be an ellipsoid with a polar orientation of the projected largest axis, or an axisymmetric ellipsoid with the line of nodes orthogonal to the global plane of the galaxy. The latter suggestion is consistent with the alignment of the dynamical major axis of the circumnuclear ionized gas: this coincidence proves the circular rotation of the gas in the polar plane. The radius of the polar gaseous disk, some 3" (250 pc), is larger than the visible extent of the polar orientation of the photometric major axis, but as we see in the end of this section, the polar stellar structure is more extended than it can be deduced from the analysis of the integrated surface brightness alone - a decomposition of global structural components is necessary. We would only note that the orientation of the dynamical major axis of the stars within 3" from the nucleus implies a prolate form of the central stellar structure.
5.2. The whole galaxy
Photometric analysis of the global structure of NGC 7217 has a long history. Boroson (1981)tried to decompose the major axis brightness profile in the B-band, but unsuccessfully, because multiple blue rings made the B-band profile very irregular. Kent (1986) analysed a 2D image in the r-band, more regular than that in B, and decomposed it into a de Vaucouleurs' bulge and an exponential disk, with a 1:3 luminosity ratio. According to this decomposition, the exponential disk becomes the dominant component at . However, there are other, quite different variants of decomposition. Buta et al. (1995) obtained a de Vaucouleurs' bulge which dominates over the full radius range, with a total bulge-to-disk luminosity ratio of 2.3-2.4. A similar result was obtained by Baggett et al. (1998). Our analysis of the brightness profile of NGC 7217 (Zasov & Sil'chenko 1997) revealed the existence of two equally good decomposition variants: a de Vaucouleurs' bulge plus an exponential disk, with the bulge dominating over the full radius range, and a King's bulge plus an exponential disk, with the bulge being fainter than the disk in the radius range of 20" -180". The scalelengths of the fitted disks are very similar, 42" and 35", respectively, but their central brightness is of course very different. Moreover, even if one considers the central part of the brightness profile only, where the bulge predominance is beyond doubt, the effective radius of the de Vaucouleurs' law diminishes when approaching the center. Obviously, the surface brightness profile of NGC 7217 is too complex to be decomposed in unique way.
Since up to now there is no convergence in the published results of the decomposition of the brightness profile of NGC 7217, we would like to propose one more variant. For this purpose we have used photometric data from the ING Archive to calculate a brightness profile in the I-band. We have converted it into the standard Cousins system by comparison with the central I measurements from Sanchez-Portal et al. (2000). We then applied a procedure of recurrent fitting, the results of which are given in Table 3 and Fig. 10. First, the parameters are determined for the outermost parts of the exponential disk, the model map of this exponential disk is subtracted from the original map, in the residual map the position angle of the major axis and the mean ellipticity are measured, an azimuthally-averaged profile of the residual brightness is calculated, and all the steps above are repeated again. Fig. 10 presents the stages of this recurrent fitting for NGC 7217. Curiously, at every step the outer parts of the profiles look exponential. The innermost of the three exponential profiles can be traced as close to the nucleus as . The scalelength of the third disk is or 0.3 kpc - about 10% from the scalelength of the first disk. Such a ratio is typical for a galaxy consisting of an exponential disk and an exponential bulge (Courteau et al. 1996), though an exponential bulge in a Sab galaxy is a quite unusual phenomenon (Andredakis et al. 1995, de Jong 1996, Moriondo et al. 1998). However this fact allows us to treat NGC 7217 as a galaxy with an exponential bulge and two exponential disks. The final residual map obtained by subtracting the three model exponential-disk images from the original I-band image of the galaxy (Fig. 11) is quite clear. One can see the stellar rings at and , and these rings look asymmetrical: their southern halves are brighter. For a face-on disk galaxy, such as NGC 7217, it may mean either that the dust tori are thick, or that the dust (gas) disk is inclined with respect to the stellar disk harboring the rings, or, even more generally, that the dust and stellar rings have different inclinations because both can be inclined to the stellar disk. And finally, in the very center of the residual brightness map we see a feature which we expected to find: an oval with a major semiaxis of 4" -5" elongated in the north-south direction. This is just what we discussed at the end of the previous section: the stellar substructure corresponding to the area of maximum stellar velocity dispersion (Fig. 3, right) and of the enhanced (Fig. 1, right).
Table 3. Exponential parameters of the NGC 7217 image fits
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001