Summary:
The photometric repeatability of galaxies seen toward the
Abell 3558 galaxy cluster (z=0.048; Glat = 31 deg) demonstrate that 2MASS
satisfactorily achieves the
Level-1 photometric science requirements. The most accurate measurements are
made with the 7" radius circular apertures, which are the optimum
mags for color information for the fainter (i.e., smaller) galaxies.
The most versatile measurements are the isophotal mags, whose
repeatiblity RMS also satisfies the science requirements.
The isophotal mags, however, underrepresent the "total" flux of a
source by ~10-20%. The Kron mags, by design, are a better measure
of the total flux (but still systematically underrepresent the
"total" flux by ~8%), although the RMS scatter is considerably larger
than the isophotal mags. We recommend that the elliptical isophotal mags
be used as the "default" mag for most purposes, while the "total" mags
be used as the measure of the total flux coming from the source.o
Derived from a large repeated-scan data set, the photometric repeatability of the primary 2MASS apertures is presented. The apertures include the circular 7" radius, 20 mag arcsec-2 isophotal, Kron and total (extrapolated) versions. See Large Aperture Photometry: Profile Fitting, Isophotal & Total Mags.
A number of repeat scans of Abell 3558 were acquired during the month of April 2000 to complement the set taken in March of 1998. Scans compiled:
Key Points: Internal completeness and reliability satisfy the Level-1 Science requirements
Key Points: SNR=10 Thresholds: J=15.1, H=14.1, Ks=13.5; note the slight overestimation of the estimated uncertainty for K-band (one culpret: stellar confusion noise)
The galaxy is assumed to be symmetric and elliptically-shaped (again, reasonable assumptions for most resolved objects). We also assume that the shape of the galaxy is preserved from low to high radii -- a crude approximation that is needed for robustness and processing speed. The projected shape of a galaxy is determined at the 3-sigma isophote. The basic algorithm is detailed in Jarrett et al. 2000. An improved method for crowded fields has been deployed for the final 2MASS data processing; described in Improved Ellipse Fitting and Isophotal Photometry in Crowded Fields. The resultant axis ratio and position angle represent the galaxy shape.
Key Points:
SNR=10 Photometric Thresholds: J=14.8, H=14.0, Ks=13.4;
The uncertainty in the isophotal radii is typically 5-10%;
The axis ratio RMS is ~5%
Key Points:
SNR=10 Thresholds: J=14.9, H=13.8, Ks=13.0;
The RMS scatter is larger for the V3 version -- the reason
being that the Kron apertures are now allowed to grow
larger (closer to the total radius, hence increasing the
scatter).
Key Points: The Kron V2 (older) version is systematically 0.05 mags brighter than the isophotal mags, with a (incorrect) trend in brightness. The Kron V3 (improved) version is systematically 0.05 - 0.15 mags brighter than the isophotal mags, with no systematic trends in brightness. I.e, the systematic trends are greatly reduced in the newer (V3) version compared to V2.
Key Points:
SNR=10 Thresholds: J=14.5, H=13.0, Ks=12.5;
The formal (estimated) uncertainties in the photometry (denoted with the
purple dashed line) overestimate the true uncertainty
at the faint end (J > 14th mag, K > 12th mag). Overall the
repeatability in the "total" mags looks excellent.
Key Points: The isophotal mags are systematically 0.1 to 0.3 mags fainter than the total mags, with a more typical value ~0.2 mag. Hence, we conclude that the isophotal mags systematically underepresent the total flux by about 20%. The large scatter is probably due to intrinsic properties of the galaxies (e.g., morphology). Note the mag trend seen in the J-band.
Key Points: The Kron mags are systematically 0.05 to 0.2 mags fainter than the total mags, with a more typical value ~0.08 mag. Hence, we conclude that the Kron mags systematically underepresent the total flux by about 10%. Note the mag trend seen in J.
[Last Updated: 2002 Jul 15; by Tom Jarrett]
