In our attempt to help answer the difficult question of whether spiral galaxies are opaque or not, we have initiated a program to study the distribution of stars and dust in the nearby, well resolved galaxies.
In the first paper of this series (Xilouris et al. 1997, hereafter Paper I), a detailed modelling of the late-type spiral galaxy UGC 2048 was done, using observations in the B, V, I bands and the radiative transfer model described in Kylafis & Bahcall (1987, hereafter KB87). The main conclusion from that study was that UGC 2048 would be optically thin throughout its surface, if it were to be seen face-on. In the present work, we continue our study by modelling the well-known spiral galaxy NGC 891, not only in the B, V, I optical bands, but also in the J and K near infrared bands.
Several studies dealing with two-dimensional decomposition of this galaxy have been reported (e.g., van der Kruit & Searle 1981; Bahcall & Kylafis 1985; Shaw & Gilmore 1989). In these studies, photometric parameters are derived by fitting various stellar models to the surface brightness of the galaxy in parts away from the dust lane. All of these studies suggest that NGC 891 could be fitted by a two-stellar-component model consisting either of a thin disk (with exponential or sech2 z type functions) and an law profile, or a thin disk and a thick disk, the second component being responsible for the distribution of population II stars. When the dust is included however, things become more complicated and a 3D radiative transfer model is needed. KB87 used such a model to interpret F-band data. Two disks, exponential in the radial direction and sech2 z in the vertical direction were used to describe the stellar and dust distributions and a fit to the surface brightness of the galaxy was done in several parts of the galaxy. The work that will be presented in this paper is complementary to that of KB87 in the sense that the model is extended to the five bands mentioned above and also a more complete model, including a bulge component, is used for a global fit to the surface brightness of the galaxy.
In Sect. 2 we describe the observations and the data reduction, in Sect. 3 we present the model that we have used, in Sect. 4 we fit the model to the real data, in Sect. 5 we draw our conclusions and a brief summary is presented in Sect. 6.
© European Southern Observatory (ESO) 1998
Online publication: March 3, 1998