3. A magnesium distinct nucleus and a Fe-rich circumnuclear disk
To study properties of the stellar populations in the central regions of galaxies, we use metal- and hydrogen-line indices confined to a rather narrow green spectral range, namely, H, Mgb, Fe5270, and Fe5335. In the case of NGC 7217 there are some problems with these indices, however. First of all, rather intense H emission is observed in the center of the galaxy, except in the narrow radial range of . Therefore one can expect that the absorption-line index H, which is a good indicator of stellar population age, will be strongly contaminated by the emission almost everywhere and therefore cannot be used. Secondly, in the nucleus itself a noticeable emission line [NI ]5200 is seen, and as Goudfrooij & Emsellem (1996) noted, this emission causes an overestimation of the absorption-line index Mgb because it falls into the continuum band of this index. Therefore we are forced to use an index which has a broader continuum base than Mgb, but is more dependent on the correct calibration of the global spectrum shape. To calculate , we use galactic spectra calibrated into absolute fluxes.
Fig. 1 shows the isocontours of two-dimensional distributions of the Lick absorption-line indices, (left) and (Fe5270+Fe5335)/2 (right) in the central 16" of NGC 7217. The magnesium index is strongly peaked near the nucleus; it decreases rather symmetrically with radius, and the shape of outer contours is not too different from the shape of continuum isophotes. However, in the very center an elongated structure seems to appear at the limit of the spatial resolution; it is a bit shifted to the east from the center of the brightness distribution. The distribution of the iron index is much more complicated. In the patchy pattern, one can distinguish an elongated structure of enhanced (Fe5270+Fe5335)/2, also shifted to the east from the nucleus and much more extended than the analogous structure in the map: it can be traced up to . We shall refer to it as the `Fe-rich disk'; in the next sections we will argue that it can be indeed related to the circumnuclear stellar disk. However, let us note here that the structure with the enhanced metal absorption lines is elongated in the south-north direction whereas the global line of nodes is oriented approximately east-west (see Table 1).
Lick absorption-line indices have been well calibrated with respect to the integrated (luminosity-averaged) properties of stellar populations in numerous works using evolutionary synthesis techniques. Our conclusions derived below are based on the models of Worthey (1994) and Tantalo et al. (1998) for single-age single-metallicity stellar populations. Diagnostic diagrams of index vs. index are presented in Fig. 2. The diagram vs. (left) is being considered by many specialists in chemical evolution as a tool to limit the duration of the main star formation episode (see, e.g., Matteucci 1994). When Worthey et al. (1992) noted for the first time a magnesium overabundance in most bright elliptical galaxies, this phenomenon was soon treated as a natural consequence of the short duration of their star forming epoch, less than 1 Gyr. The nature of this effect is deduced from the theoretical prediction that magnesium has to be produced mainly by SNeII, which explode earlier than the bulk of the iron-producing SNeIa from the same stellar generation; if the star formation process stops between these two moments, the stars would have a higher magnesium-to-iron ratio than the solar, which corresponds to continuous star formation. The star formation histories in the centers of early-type disk galaxies are less clear than those in ellipticals, and observational results on their Mg/Fe ratios are also contradictory. Our statistics (Sil'chenko 1993) showed evidence for a solar Mg/Fe ratio in the centers of almost all disk galaxies, from Sc to S0, in both bulge- and disk-dominated objects. Jablonka et al. (1996) reported a strong magnesium overabundance, up to [Mg/Fe], in bulges brighter than , including our target galaxy NGC 7217. Our data in Fig. 2 (left) differs from those of Jablonka et al.: here, the nucleus of NGC 7217 lies near the model sequences of Worthey (1994) implying a solar Mg/Fe ratio or even a slight iron overabundance. This result is due to a smaller estimate, 0.25 instead of 0.28 in Jablonka et al. (1996), but mainly to a higher Fe5335 value, 3.0 Å instead of 2.4 Å. Curiously, the estimate of Fe5270 made by Jablonka et al. (1996) is quite consistent with ours. We cannot explain the cause of the agreement in Fe5270 and the disagreement in Fe5335 but we would like to note that in model calculations (e.g. see Worthey 1994) the values of Fe5270 and Fe5335 obtained are almost equal to each other, as is the case in our data.
The points related to the bulge of NGC 7217 are dispersed around the model sequences with a solar magnesium-to-iron ratio, and none of them fall into the area occupied by ellipticals of similar luminosity (), whose mean locus is plotted using the data from Trager et al. (1998) (a shaded horizontal band in Fig. 2, left). The azimuthally-averaged measurements for the bulge also lie along the sequences for [Mg/Fe]=0. But the most interesting behaviour is demonstrated by the points taken within the Fe-rich circumnuclear disk. In this rather bright, central part of the galaxy, the accuracy of the arcsecond element indices, of, say, , is better than 0.3 Å. The deviations of the extreme points of the Fe-rich disk at from the model sequence [Mg/Fe]=0 are at the 3 level, and certainly due to iron overabundance. This is a rather puzzling observational phenomenon common for some types of irregular galaxies (e.g. the LMC), and it is usually treated as evidence for a bursty character of the star formation (Gilmore & Wyse 1991, Marconi et al. 1994). When approaching the center of the disk, the Mg/Fe ratio rises and near the nucleus it becomes close to solar. The total metallicity of the circumnuclear disk is higher than that of the bulge. Obviously, the Fe-rich disk is a secondary formation product. The star formation within it had to be strongly inhomogeneous, with an exotic mechanism to result in iron overabundance at its outer edge: a kind of transient star forming circumnuclear ring.
As we mentioned earlier, the age diagnostics in the center of NGC 7217 is complicated by the noticeable Balmer emission contaminating the Lick index H. So in the diagram H vs. (Fig. 2, right) the majority of the observational points trace only upper limits for the age of the stellar population. But in the narrow gap between the LINER nucleus and the star forming ring (see Sect. 5), at the H emission is absent and we can use the H absorption-line index to estimate a luminosity-weighted age of the bulge stellar population. From Fig. 2 (right) one can see that the bulge of NGC 7217 is rather old, at least 10 Gyr. The ages of the nucleus and of the Fe-rich circumnuclear stellar disk cannot be determined exactly.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001