6. Dust mass
The mass of dust associated with the near-infrared emission can be only roughly estimated, because it depends on the (unknown) grain sizes and composition. Assuming graphite grains and following Barvainis (1987), the infrared spectral luminosity of an individual graphite grain is given by:
where a is the grain radius, is the absorption efficiency of the grains, and is the Planck function for a grain temperature . Following Barvainis (1987), we take a=0.05 µm and =0.058. With =900 K, we find .
In NGC 7469 the K band flux is =2.2 10-25 ergs s- 1 cm-2 Hz-1, at a distance D=66 Mpc. We derive the number of hot (=900 K) grains: n=7.58 1046. Taking a grain density =2.26 g cm-3, we get: M(hot dust) 0.05 . This result is comparable to the 0.02 mass of hot dust found in another Seyfert 1 nucleus, Fairall 9 (Clavel et al., 1989), but substantially larger than in other AGN: 2.5 10 in NGC 3783 (Glass, 1992), 7 10 in NGC 1566 (Baribaud et al., 1992) and 5 10 in NGC 4593 (Santos-Lléo et al., 1995).
© European Southern Observatory (ESO) 1998
Online publication: July 27, 1998