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Astron. Astrophys. 329, L21-L24 (1998)

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3. Results

3.1. The polarized broad H [FORMULA]

The most striking result is the prominent emission feature at 6575 Å (observed [FORMULA]) in the P nuclear spectrum. The center of this feature is within 100 km/s (1/10 of the instrumental resolution) of the wavelength of the narrow H [FORMULA] (cf. O94), has a full width half maximum of 72 Å (3300 km/s) and is much broader than the unresolved H [FORMULA] +[NII] complex in the [FORMULA] spectrum which has FWHM=42 Å. The P nuclear spectrum does not show other emission/absorption features with the possible exception of H [FORMULA] which is marginally (2 [FORMULA]) detected at [FORMULA] 4870 Å. In particular, no significant variation of P and [FORMULA] is visible at the position of the prominent [OIII] and [SII] narrow lines, and this indicates that [NII] does not contribute to the broad H [FORMULA], but rather dilutes it (see also Sect. 4.2)

The spatial extent of the broad H [FORMULA] was estimated from a row-by-row polarimetric reduction of the 2D spectra. The resulting distribution along the slit is peaked on the nucleus, concentrated within [FORMULA] 3" and basically unresolved. Interestingly however, some broad H [FORMULA] emission seems to reappear [FORMULA] 8" SE of the nucleus, and this causes the marginal detection of broad H [FORMULA] in the SE P spectrum (cf. Fig. 2). This indicates that another BLR mirror may exist [FORMULA] 150 pc SE from the nucleus and in the direction of the dust lane, but the evidence is only marginal. Higher s/n spectra are required to verify this possibility.

3.2. Dilution of the scattered nuclear spectrum

The standard method to determine the intrinsic polarization properties of the nuclear spectrum is to estimate the contribution of stellar flux to the observed continuum, and correct for it assuming that the diluting stellar continuum is unpolarized. The stellar template normally used for this purpose is that of an old stellar system (e.g. M32), but this provides a very poor fit to the Circinus galaxy whose spectrum shows strong H [FORMULA] absorption and other features typical of B-A stars associated to a relatively young (circum)nuclear starburst whose presence is also demonstrated by other observational evidences (e.g. Oliva et al. 1995).

A much more accurate, and indeed the most natural stellar template is the NW spectrum which is intrinsically unpolarized and does not show any trace of the scattered nuclear component. A comparison between the nuclear and the NW spectra is shown in Fig. 3 where the most remarkable result is that the equivalent width of the stellar features is the same in both spectra, and the maximum contribution from a featureless continuum is only [FORMULA] 3% of the total observed flux. A straight correction for the stellar dilution is therefore impossible.

[FIGURE] Fig. 3. Comparison between the observed nuclear and NW spectra, the latter is scaled and reddened to match the continuum level and shape of the nuclear spectrum. The lower panel shows the EFOSC1 spectra discussed here while the upper panel is based on higher resolution EMMI observations which will be discussed elsewhere. Note that the narrow emission lines and interstellar Na-D absorption are relatively stronger in the nucleus, but the stellar absorption features have the same equivalent widths in both spectra which are virtually indistinguishable. This indicates that the scattered featureless nuclear continuum accounts for a very small fraction ([FORMULA] 3%) of the observed continuum.

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© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997