4. Effective and total near-infrared magnitudes
4.1. Apparent magnitudes and colors
Effective and total near-infrared magnitudes have been computed for all the galaxies with known effective aperture by fitting the growth curves introduced in the previous section to the observed data , where is the uncertainty on the magnitude . The asymptotic magnitude depends primarily on the shape of the growth curve at large aperture. To avoid problems with small apertures suffering from seeing or off-centering, or which are contaminated by nuclear emission or a central starburst, we remove all the observations at from the fit.
The procedure to compute the total and effective NIR magnitudes , and the corresponding uncertainties , due to the uncertainties in the aperture magnitudes is detailed in Appendix B.
The global uncertainty on taking also into account the uncertainties in and is finally computed from
and the same for .
In many cases, the photometric type is unknown and we estimate it from the morphological type T with the following relations derived by cross-correlating Hypercat and the RC3:
We then simply assume .
The uncertainty on the colors , where , is computed as
However, this is clearly an overestimate because J, H and K data have usually been observed simultaneously and their errors are presumably highly positively correlated.
When B is one of the band, e.g. , the partial derivatives are unknown. We then estimate as
This will usually be a slight overestimate since the partial derivatives of B and should have the same sign, but most of the uncertainty comes actually from the NIR data themselves.
4.2. Absolute magnitudes
The absolute magnitude is computed from the apparent magnitude as
where is the velocity of a galaxy derived from the redshift z and corrected for the movement of the Sun in the restframe of the Virgo cluster according to the equations (18), (19), (20), (31) and (32) from Paturel et al. (1997). We assume . The uncertainty on the absolute magnitude is computed from
where is the typical peculiar velocity of galaxies in the Las Campanas Redshift Survey (Lin et al. 1996). At low redshift, peculiar velocities perturb the redshift-distance relation and increase the uncertainty on the absolute magnitude. All the absolute magnitudes of the galaxies with have been discarded in the following. has been computed according to the formulae (1) and (2) of the Appendix A of Paturel et al. (1997), taking into account both the internal uncertainty on the values of the redshift and the discrepancies between them.
Covariances between the colors and the absolute magnitudes have also been computed.
The complete catalog, containing both the optical and the NIR magnitudes, will be published in a forthcoming paper and will also be available in electronic form.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999