4.3 Half-Light "Effective" Aperture & Concentration Index
The de Vaucouleurs "effective" aperture measures the galaxy half-light, as given by the "total" flux. For the total flux, we adopt the surface brightness profile extrapolation method (Section 4.2). Using the elliptical shape of the galaxy, we integrate in small annular steps starting from the center (r>5 arcsec) until we reach the integrated half-light point. We then interpolate across the surface brightness profile to arrive at a more precise half-light radius. We report the half-light radius for each band, and the corresponding half-light mean surface brightness (in mag/arcsec2 units). For small galaxies, the half-light radius is susceptible to circularizing effects from the PSF, but is generally not a concern for the Large Galaxy Atlas. Hence, we do not correct for PSF effects.
The concentration index characterizes the nuclear-to-bulge concentration of the galaxy. The index corresponds to the ratio of the 3/4 light-radius to the 1/4 light-radius (using the de Vaucouleurs convention). These radial points are derived in a similar fashion as to the half-light radius. . (Note: the 2MASS XSC also does not correct for "seeing" effects; the half-light radius and concentration index are consequently rendered unreliable for small galaxies).