The Opacities of Spiral Galaxies using Volume Representative Samples


MNRAS 266, 614  1994 
Edwin A. Valentijn
 
SRON, Space Research Groningen,
P.O. Box  800, 9700 AV Groningen, The Netherlands 

Abstract


The study of the opacities of spiral galaxies is focused here on the
optical depth in the Blue band in the regions of the discs between the
effective radius and the 26 mag arcsec^{-2} blue isophote.  A sample
of galaxies drawn from the ESO-LV catalogue is explored using
completeness tests which employ the Vmax technique.  The results allow
a comparison between results from various inclination tests that use
different photometric parameters and sample selection criteria.  For
Sb-c galaxies, isophotal diameters do not increase with inclination in
complete samples, but do so in incomplete samples.  This result explains
part of the diversity in the literature.  An additional analysis,
incorporating the effect of interlopers in a diameter-selected sample,
employs log N - log diameter distributions and provides new results on
the transparency in the outer regions for several central surface
brightness `families' of Sb, Sc and Sd galaxies.  These results and
earlier findings match well and suggest an average overall picture of
the optical depth at various locations in the discs of these galaxies. 
Sb galaxies and high surface brightness galaxies of later type (Sc, Sd)
are found to have low-C values (optically thick, high internal
extinctions) over the whole of their discs, while the fainter surface
brightness Sc and Sd galaxies are best modelled with a C=0.5-0.6 at
the effective radius (semi-transparent) and C=0.7-0.9 at the 26 mag
arcsec^{-2} isophote (nearly transparent). 

A relatively large optical depth and a corresponding low overall disc
C value at the outer radii are best understood in terms of an
obscuring component with a scale-height of the same order as that of the
stars and a scale-length larger than that of the stars.  The IRAS
60-\mum data are not really mapping this component.  

keywords: dust, extinction - galaxies: interstellar matter - galaxies:
spiral