5.3.2 Absorption corrections
Frequency dependent (selective) and frequency independent (neutral) absorption - atmospheric absorption corrected for - can be present in:
Selective absorption in our Galaxy is well observed; it is strongly dependent on galactic latitude. Absorption in galaxies depends on morphological type and galaxy orientation. Intergalactic absorption - general or in localized clouds (Rudnicki 1988) - is still a matter of debate.
Suggestions for absorption go back to the interpretation of the earliest data, summarized in the NGC and Index catalogues. Charlier (1922) comments:
``A remarkable property of the image is that the nebulae seem to be piled up in clouds (as also the stars in the Milky Way). Such a clouding of the nebulae may be a real phenomenon, but it may also be an accidental effect caused by dark matter in the space or declared by condensation of the observations in singular points of the sky.''
The effects of galactic absorption were well understood by Hubble (1934), who presented the law of absorption essentially in its presently used form:
``Systematic variations in longitude are appreciable only in the lower latitudes, where obscuration appears to be conspicuously greater in the direction of the galactic center than in the opposite direction. There is a definite variation with latitude, which from the poles to about = 15° is represented by the cosecant formula
log Nm = C - 0.15 cosec
indicating an obscuration of 0.m5 from pole to pole, with no appreciable difference between the two hemispheres.''
Stebbins and Whitford (1937) found evidence of absorption in extragalactic nebulae from their extensive measurements of magnitudes and colours:
``The colors of the first three types E, Sa, Sb indicate an excess . . ., possibly because of selective absorption within the nebulae. There is no indication of selective absorption in internebular space.''
Intergalactic absorption was claimed by Wirtz (1924):
``While all computations of the correlation coefficient for the nebular characteristics, surface brightness and diameter, give very low absolute values, uncertain in each individual case, one finds, however, from different material always the same negative sign, i.e. the intensity [surface brightness] increases with increasing apparent diameter. If one takes the apparent diameter as a measure of distance, one can interpret this effect as resulting from a general cosmic absorption. If one positions the nebulae, distant Milky Way systems, at distances which correspond to the parallaxes of K. Lundmark . . . one can determine the amount of absorption. One finds it, with the help of the regression line relating magnitude to the logarithm of the large axis, to be extremely small, of the order of 10-5 magnitudes per 1000 parsec.''