5.5 Integrated Flux

The apparent integrated flux is measured with four different apertures, defined in Section 4; see also Table 2. The standard measure comes from the Ks-band isophotal aperture, corresponding to roughly 1-sigma of the rms background noise, or 20 mag arcsec-2. The distribution of isophotal aperture magnitudes is shown in Figure 10. The integrated fluxes range from 1 to 8 mags (M31, no surprise, being the brightest galaxy), with a statistical mode near 6th mag.

The "total" integrated flux is typically 0.1 to 0.2 mag brighter than the isophotal mag, as shown for all three NIR bands in Figure 11. Early-type galaxies show the smallest differences, typically 5-10%, with the most resolved object of all, globular cluster NGC104, showing virtually no difference between total and isophotal fluxes. Of note, the other highly resolved object, M31 (type Sb), also indicates that most of the NIR light is captured by the isophotal aperture. The Andromeda result should be viewed with caution, however, as we are not fully confident that the total flux of M31 has been captured due to the extreme angular extent of the galaxy and the associated difficulty with removing the IR background. On the whole, the largest excursions occur for low surface brightness, late-type and peculiar galaxies. We surmise that, for these galaxies, a considerable fraction of the NIR light (>20%) is completely lost in the background noise of 2MASS images. In addition to extrapolation of the surface brightness distribution ("total" aperture), NIR light can be recovered using an optimized Kron aperture (4.2). Figures 12 and 13 show the performance of Kron apertures in comparison to the isophotal aperture and the "total" aperture. The Kron aperture flux is typically within 10% of the "total" aperture flux, which is not a surprising result given that they are close cousins, rtot being a common parameter to both measures.


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