Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994mnras.266..614v&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 266, NO. 3/FEB1, P. 614, 1994
Astronomy and Astrophysics
Astronomy
36
Scientific paper
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 V/V_max_ technique. The results allow a comparison between results from various inclination tests that use different photometric parameters and sample selection criteria. For Sb-Sc galaxies, isophotal diameters do not increase with inclination in complete samples, but do so in incomplete samples. This result explains part of the diversity of 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 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 scaleheight of the same order as that of the stars and a scalelength larger than that of the stars. The IRAS 60-micron data are not really mapping this component.
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