Astron. Astrophys. 336, 823-828 (1998)

4. Flux measurements

Fluxes have been measured on the reduced images (not deconvolved). They have been measured simulating a circular aperture with a radius varying from 4 to 12 pixels (128 to 384 pc). In Table 2, we summarize these measurements, together with measurements in others wavebands. We note that the PAH line emission at 3.3µm contributes at most 6% of the flux in the L band.

Table 2. Photometric data for NGC 7469

The L and L' band apertures have been centered on the peak of the corresponding flux distributions and it was assumed that the peak of the K-band flux distribution is coincident with these peaks.

This assumption implies that the central source is the hotest component in the AGN of NGC 7469, heating the surrounding dust, and thus is the brightest source at 2.2, 3.5 and 3.8 µm .

Due to the adaptive optics system, the infrared camera field has a position fixed in regard to the centroid of the visible counterpart of the object observed. Observing a star, we determine the infrared image reference position, which corresponds to the visible reference position (for a star, the infrared and optical peaks are coincident). Any offset of the galaxy infrared peak relatively to the star infrared peak would correspond to a real offset between the galaxy infrared peak and its visible peak. This gives an indication for the positioning of the infrared with respect to visible sources, in the AGN.

For a type 1 AGN, models suggest that we observe a face-on nucleus: then, the torus center should correspond to both the optical and infrared emission peaks. We find that the NGC 7469 L (and L') band peak is offset by " north of the visible continuum peak. Down to the achieved spatial resolution (0.35") this shift is not significant. The coincidence between the visible and L band peaks in NGC 7469 hence indicates that the nucleus is indeed seen face-on, in contrary to the the case of NGC 1068, a type 2 AGN, where we find an offset between the infrared and the optical peaks of 0.3" (Marco et al., 1997) suggesting that the nucleus is viewed at a large inclination angle.

We have estimated the fraction of unresolved flux in the L (and L') band images, by normalizing the PSF to the same peak surface brightness as the galaxy and taking the ratio of the total normalized PSF flux to the total galaxy flux.

The flux in the unresolved core (r55 pc) accounts for at least 55% of the total flux, and therefore is about 1.2 times the flux of the extended source possibly related to the narrow line region (NLR) dust. Miles et al. (1996) give a value of 60% in the mid-IR ( 10µm) for the fraction of unresolved flux. The spatial resolution of their data is around 0.6", which prevents us to perform a direct comparison with the flux prediction at 10 µm from the unresolved dust component measured at 3.5 µm and 3.8 µm with an angular resolution of 0.4". However, we can check these two estimates in terms of their relative values: we predict that the unresolved (r0.4") dust component detected at 3.5 and 3.8 µm , with a temperature of 900 K, should radiate around mJy at 10 µm . This is indeed smaller than the observed 650 mJy emission measured by Miles et al. (1996) from their unresolved source (r0.65").

What is the nature of the unresolved core ? It could be related to a compact dust/molecular thick torus like in the unified models by Pier & Krolik (1992a, 1992b), Efstathiou & Rowan-Robinson (1995), Granato & Danese (1994), Granato et al. (1996, 1997), or result from emission by hot dust (T900 K) mixed with gas in the NLR/BLR interface, and shielded from the intense UV radiation field.

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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