Case Studies

** updated ** (970910)

Nine Hercules galaxies were chosen for case study. All are bright (K < 12.5) and most are spirals -- thus providing a nice sample to compare GALWORKS ellipse fitting routines and photometry with those obtained using the IRAF-STSDAS tasks. This purpose of this exercise is to test the integrity and robustness of the GALWORKS routines on bright and highly inclined spirals.

IMAGES

The JHK mags correspond to the 20 mag per sq.arcsec isophotal value (elliptical), and the axis ratios correspond to that measured for the "super" JHK coadd.

ID# 3587 ; J = 13.1, K = 12.1; (b/a) = 0.58

ID# 3685 ; J = 12.8, K = 11.6; (b/a) = 0.34

ID# 3735 ; J = 12.2, K = 11.1; (b/a) = 0.64

ID# 3745 ; J = 12.3, K = 11.0; (b/a) = 0.24

ID# 3826 ; J = 13.1, K = 11.7; (b/a) = 0.84

ID# 5384 ; J = 12.5, K = 11.3; (b/a) = 0.60

ID# 6287 ; J = 11.6, K = 10.5; (b/a) = 0.52

ID# 6756 ; J = 13.7, K = 12.5; (b/a) = 0.42

ID# 9159 ; J = 11.9, K = 10.7; (b/a) = 0.92

NED search by position Look it up in NED !!

Photometry: STSDAS vs. GALWORKS

Fixed Apertures

Fixed Circular Aperture Photometry

Circular ISOPHOTAL Aperture Photometry -- fixed using GALWORKS radii

Elliptical ISOPHOTAL Aperture Photometry -- fixed using GALWORKS radii

Variable Apertures

Variable Elliptical Aperture Photometry: Ellip Fit
Ellipse Aperture(s) fit to the 3-sigma contour (per band).

Variable Elliptical Aperture Photometry: J band
Aperture: 20 mag per sq. arcsec isophote

Variable Elliptical Aperture Photometry: J band
Aperture: 21 mag per sq. arcsec isophote

Variable Elliptical Aperture Photometry: H band
Aperture: 20 mag per sq. arcsec isophote

Variable Elliptical Aperture Photometry: H band
Aperture: 21 mag per sq. arcsec isophote

Variable Elliptical Aperture Photometry: K band
Aperture: 20 mag per sq. arcsec isophote

Variable Elliptical Aperture Photometry: K band
Aperture: 21 mag per sq. arcsec isophote

Summary

Photometry on nine relatively bright Herc galaxies were performed with the IRAF-STSDAS routines (e.g., "ellipse"), the results were compared with those obtained with GALWORKS.

Fixed aperture photometry -- in which both the radial and azimuthal components of the aperture are held fixed -- showed no appreciable difference between the GALWORKS and STSDAS. The differences were less than 1%, primarily due to the use of fractional pixels in GALWORKS, but not in STSDAS.

Variable aperture photometry -- in which we let STSDAS determine the elliptical fit parameters for its isophotal photometry -- showed that both STSDAS and GALWORKS arrive at very similar elliptial aperture shapes as well as the resultant photometry derived from these apertures.

axis ratio: the difference in the axis ratio as determined at the 3-sigma isophote was typically less than 0.05, particularly at J where we have the most sensitivity.
position angle: the difference in p.a. as determined at the 3-sigma isophote was only a few degrees for the inclined galaxies (b/a < 0.8) and around 10 degrees for the rounder galaxies.
semi-major axis: the difference in r(semi) as determined at the 20 mag/arcsec**2 isophote was less than 1 to 3 pixels at all three bands.
Photometry: using the elliptical aperture fit to the 20 mag/arcsec**2 isophote, the difference in mag ranged from 0 to 5% at J, 0 to 2% at H and 0 to 3% at K, all well within the formal errors (due to background noise). The mag differences were due to the slight differences in the apertures -- comprising the axis ratio, position angle and final semi-major axis.
At the 21 mag/arcsec**2 isophote, the difference in mag was typically only 1% more than that derived for the 20 mag/arcsec**2 isophote -- again, well within the formal errors.

We conclude that GALWORKS is performing photometry as designed and that, at least for brighter galaxies, K < 13 mag, both fixed and isophotal photometry are robust measurements for a diverse set of extended sources. These conclusions refer to the internal consistency of the galaxy photometry. External checks have not been conducted at this time.