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Clumpy dust disks

Effects of Clumping on the Observed Properties of Dusty Galaxies

Simone Bianchi, Andrea Ferrara, Jonathan I. Davies, Paul B. Alton, MNRAS, 311, 601.

Abstract: We present Monte Carlo radiative transfer simulations for spiral galaxies modelled as a stellar disk and a two-phase clumpy dust distribution. We divide the volume occupied by the dust into a three-dimensional grid and assign each cell a clump or smooth medium status. Cell dimension, clump dust mass and spatial distribution are derived from the observed properties of Giant Molecular Clouds and molecular gas in the Galaxy. We produce models for several values of the optical depth and fraction of the interstellar medium residing in clumps. As a general result, clumpy models are less opaque than the corresponding homogeneous models. For the adopted parameters, the increase in the fraction of energy that escapes the disk is moderate, resulting in surface brightness profiles that are less than one magnitude brighter than those of the homogeneous models. The effects of clumping are larger for edge-on views of the disk. This is in contrast with previous preliminary results for clumping in the literature. We show how differences arise from the different parametrisation and clump distribution adopted. We also consider models in which a fraction of the stellar radiation is emitted within the clumps. In this case, galaxies are less transparent than in the case when only dust is clumped. The opacity can be even higher than in the homogeneous case, depending on the fraction of embedded stellar emission. We point out the implications of the results for the determination of the opacity and dust mass of spiral galaxies. A postscript version of the paper is available.

Attenuation: this file contains the attenuation (i.e. the ratio between the observed flux and the intrinsic, unextinguished total one) as a function of optical depth tauV, inclination i, fraction of gas in clumps f_c and fraction of embedded emission f_emb. 8 values of i are available (Bianchi, Ferrara & Giovanardi, 1996, ApJ, 465, 127). The file is organised as follows: 
                f_emb                                              tauV ->          
           0.00  0.15  0.50                               0.1  0.5  1.0  2.0 5.0 10.0
      0.25   X     .     .                          20     .    .    .    .   .    . 
 f_c  0.50   .     .     .                          37     .    .    .    .   .    . 
      0.75   .     .     .                        i 48     .  Attenuation .   .    . 
                                         X =        58     .    .    .    .   .    . 
                                                  | 66     .    .    .    .   .    . 
      where each X is a subset                    | 75     .    .    .   values    . 
      for fixed f_c and f_emb                    \/ 82     .    .    .    .   .    . 
                                                    90     .    .    .    .   .    .


The attenuation file for the homogeneous case can be found here here. The attenuation values are compatible with those of a S??_ME04 B/T=0 model in the Atlas of Monte Carlo Models of Dust Extinction in Galaxies for Cosmological Applications (Ferrara, Bianchi, Cimatti, Giovanardi 1999), the small differences arising from the different choice of the inclination bands, much narrower in that work.

Major and Minor axis profiles: Clumpy models

In the following table, links are given to files that contains minor and major axis profiles for all the 8 inclination explored, for the optical depth tauV as given by the link name and for f_c and f_emb as derivable from the table format. Each file contains, in the first column, the projected distance from the center of the galaxy (in stellar radial scalelengths). Columns 2-9 contain the minor axis profiles for each inclination (negative values of the projected distance correspond to position along the z-axis on the part of the galaxy model closer to the observer). Columns 10-17 contain the values for the major axis, that are given for positive values of the distance only, because of the models symmetry. Full resolution data are given: therefore there are 201 values along the minor axis and 101 along the major. Major axis profiles in the paper has been obtained after smoothing with a 10 pixel box. Profiles can be plotted by using a simple SM macro available here.
f_emb 
0.00  0.15  0.50 
0.25  0.1, 0.5, 1.0, 2.0, 5.0, 10. 0.1, 0.5, 1.0, 2.0, 5.0, 10. 0.1, 0.5, 1.0, 2.0, 5.0, 10.
f_c  0.50  0.1, 0.5, 1.0, 2.0, 5.0, 10. 0.1, 0.5, 1.0, 2.0, 5.0, 10. 0.1, 0.5, 1.0, 2.0, 5.0, 10.
0.75  0.1, 0.5, 1.0, 2.0, 5.0, 10. 0.1, 0.5, 1.0, 2.0, 5.0, 10. 0.1, 0.5, 1.0, 2.0, 5.0, 10.

Major and Minor axis profiles: Homogeneous models

We have also produced several homogeneous models, to compare to the clumpy models. In the following table the first line gives links to homogeneous models with the same range of optical depths as for the clumpy models. Homogeneous and clumpy models with the same tauV have the same dust mass. The transparent model is provided as well. The rest of the table gives links to homogeneous models with the same optical depth as the smooth medium in each clumpy model characterised by f_c and tauV (see paper, Section 2 and Table 1). The files are organised as for the major and minor axis profiles of clumpy models and can be read by the same SM macro.
tauV
0.0 0.1 0.5 1.0 2.0 5.0 10.
0.25  0.075 0.375 0.75 1.5 3.75 7.5
f_c 0.50  0.05 0.25 0.5 1.0 2.5 5.0
0.75  0.025 0.125 0.25 0.5 1.25 2.5
All the files in a single gzipped tar file (size 416KB)
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