** updated ** (970829)
The results presented in this memo describe the photometric repeatibility of the five Herc scans. Preliminary results revealed several galaxies with very poor photometric repeatibility. These 'outlier' points turned out to be either (1) galaxies in close proximity to large extended galaxies in the Herc core, (2) galaxies in close proximity to other galaxies, or (3) galaxies contaminated by star in their disk or in close proximity. The plots below color-code the type of contamination: none, bright galaxies, nearby galaxies or stars.
A new photometric measure, Kron Aperture photometry, is introduced at the bottom of this page. This measure is proposed as a substitute to the adaptive aperture measurement. The repeatibility using Kron apertures is very good, thus providing a more robust measurement of the quasi-total flux of a galaxy.
Galaxy photometry is not a straight forward process. A multitude of apertures (some fixed, some adaptive) may be used to integrate the flux of an extended src . Thus, the uncertainties in the flux measurement are not only from the background noise (sky noise) but also from uncertainties in the shape and size of the integrating aperture itself. This memo presents the repeatibility results for a number of key photometric measures, including fixed circular apertures (radius = 10), both circular and elliptically shaped versions of: isophotal (20 mag per sq. arcsec), petrosian, and adaptive apertures. We must also keep in mind what the level-spec target:
Fixed Circular Aperture, Radius = 10
white filled circles: individual points
green triangles: mean of the binned
distribution
green error bars: one-sigma
standard deviation in the distribution.
orange diamonds :
predicted distribution uncertainty based on
poisson statistics
dark blue filled circles: galaxies with probable contamination
from nearby large galaxy(s).
light blue filled circles: galaxies with contamination
from nearby galaxies(s).
magenta filled circles: galaxies with contamination
from nearby stars (typically).
Based on what we would predict for the scatter in the photometry (see orange diamonds) the results (green triangles) are consistent with background limited photometry (i.e., the errors are dominated by the sky noise and at J, the read noise).
Isophotal Photometry
The 20 mag per sq. arcsec isophot is fit with both a circular and elliptical aperture. Note that the 20 mag per sq. arcsec isophot corresponds to about 1-sigma of the sky background at K, 1.8-sigma at H, and 3.6-sigma at J; thus, it represents most of the flux at K, and a fraction at H and J.
Circular Aperture
The large scatter associated with the brighter galaxies is due to the fact that the galaxies are bigger than the maximum window (about 70 pix) within which GALWORKS operates. The other very deviant point in the isophotal radii and the isophotal photometry are due to nearby source contamination. Nearly all of the outliers turn out to be contaminated galaxies (blue filled circles).
The 10% limits in the photometric repeatibility are about J = 15, H = 14.4, and K = 12.9. Thus the level-1 specification (H band) is achieved in this photometric measure.
Elliptical Aperture
Again, nearly all of the outliers turn out to be contaminated galaxies (blue filled circles). The 10% limits in the photometric repeatibility are about J = 15.1, H = 14.5, and K = 13.0. Thus the level-1 specification is achieved in this photometric measure.
Petrosian Photometry
We define the petrosian radius to be the point where the ratio of the mean brightness in an aperture to the mean isophotal brightness becomes 5.0. GALWORKS determines this point by first fitting a modified exponential to the isophotal profile, then performing the ratio of mean surface brightness to the fitted isophotal brightness, and finally linearly interpolating to arrive at the petrosian radius. The reader may like to look at some single-channel detailed analysis of petrosian photometry to become familiar with this method; c.f. Circular and Fixed Elliptical Apertures: The Petrosian and Isophotal Photometry
Circular Aperture
Elliptical Aperture
The scatter in the petrosian radii is larger than that of the isophotal case, and correspondingly the photometry repeatibility is much worse than that of the isophotal case. This is to be expected given that the petrosian radius requires one additional non-trivial step compared to simple isophotal photometry.
Adaptive Aperture Photometry -- Total Integrated Flux
An adaptive aperture is used to capture as much flux of the object as possible, which, for the fainter galaxies (and spirals), represents the total flux. The basic method is given in Adaptive Aperture Photometry
Circular Aperture
The orange diamond symbols denote the predicted distribution uncertainty based on poisson statistics (including sky noise, source noise, and read noise).
Elliptical Aperture
The scatter is large for the brightest galaxies because, again, these galaxies are larger than the GALWORKS window. The other deviant points are contaminated galaxies. Since the total flux requires a very large aperture (about twice as big as the isophotal aperture), the measure is at the mercy of background sky noise and src contamination.
The elliptical apertures provide a more precise measurement compared to the circular case.
Kron Aperture Photometry
Kron apertures are derived from the galaxy' first image moment radius. See Kron Apertures for more information. Here we precisely mean that the aperture radius is given by 2.5 * r1, where r1 is the first moment radius. Both circular and elliptical versions are derived. The Kron flux represents a quasi-total flux (>90-96% flux is captured).
As in turns out, the Kron radius is about the same size as the 20 mag per sq. arcsec isophote radius for the H and K bands (thus, corresponding to about 1-sigma above the sky background level ). For the J band, the Kron radius is larger than the 20 mag per sq. arcsec isophote since the background brightness is much lower at J. The relationship between the Kron apertures and the isophotal apertures can be seen here:
