T. Jarrett, IPAC
(970822)
A proposed form of galaxy photometry is that developed by Kron (1980, ApJS, 43, 305), in which the first image moment is used to determine the circular / elliptical aperture from which the flux of the galaxy is integrated. See also Koo (1986, ApJ, 311, 651) for additional discussion of the Kron technique. The DENIS project is using Kron magnitudes as their primary flux measurement for galaxies (cf. Bertin & Arnouts, AASupp, 117, 393). 2MASS should adopt a similar strategy to foster compatibility between the two projects.
Kron (1980) claims that you capture most of the galaxy flux (>90%) if you use an aperture size twice the radius of the first image moment radius. Although not intuitive, he shows evidence that this aperture is applicable (re: robust) to different galaxy (morph) types, including spirals and ellipticals. DENIS claims that better than 94% of a galaxy's flux is captured if one uses a factor of 2.5 times the radius of the first image moment radius.
The following gif images show some sample galaxies with circular apertures overlayed on the images. The apertures correspond to the Kron radius (twice the image moment radius) and the 20 mag per sq.arcsec isophotal radius (circular).
Note: the image moment is computed by summing the
product of the radius * pixel intensity, and normalizing
the sum by the total flux; i.e., the usual method to
compute the intensity weighted first moment. The sum
includes all pixels within a radius (max radius corresponds
to an isophotal value equal to
one sigma of the sky noise)
from the central
core of the galaxy.
Green Circle == 20 mag/arcsec**2 isophot
Red Circle == Kron radius
Green Circle == 20 mag/arcsec**2 isophot
Red Circle == Kron radius
Green Circle == 20 mag/arcsec**2 isophot
Red Circle == Kron radius
J band, left panel; K band, right panel.
Note the isophotal substitution for masked pixels due to a nearby
bright source.
Green Circle == 20 mag/arcsec**2 isophot
Red Circle == Kron radius
Green Circle == 20 mag/arcsec**2 isophot
Red Circle == Kron radius
K band, upper panel; J band, lower panel.
Green Circle == 20 mag/arcsec**2 isophot
Red Circle == Kron radius
J band, left panel; K band, right panel.
The following images show some Hercules cluster galaxies with the Kron aperture marked in the color "purple". The 20 mag per sq.arcsec isophotal aperture is marked in the color "blue". Here the Kron aperture corresponds to 2 * r1, the first moment radius.
The following images show some Hercules cluster galaxies with the Kron aperture marked in the color "purple". Here the Kron aperture corresponds to 2.5 * r1, the first moment radius, which is the prescription that DENIS uses.
