Isophotal diameter of galaxies: logd25

logd25 is the decimal logarithm of the length the projected major axis of a galaxy at the isophotal level 25 mag/arcsec2 in the B-band (this is the diameter of the galaxy if it is a disk).

Many papers were devoted to the study of diameters and specially to the reduction to the standard system defined by the isophote at the brightness of 25 B mag/arcsec2. The conclusion of these studies was published by Paturel et al. (1991); see also Paturel et al. (1997)

The diameters are expressed in log of 0.1 arcmin, logd25, according to the convention of Second Reference Catalog (de Vaucouleurs et al. 1976). For instance a diameter of 10 arcmin will be given as logd25=2.00.

The shape of the galaxy at the same isophotal level (B=25 mag/arcsec 2) is defined in the parameter logr25.

The diameter corrected for galactic extinction, logdc is derived from logd25.

HyperLeda Questions: