As an external check of the 2MASS extended source coordinate positions we cross-identify galaxies with catalogued galaxies within the NASA Extragalactic Database (NED). The 2MASS sample consists of extended sources in the First Incremental Data Release. We find ~13,000 matches for northern declination galaxies and ~20,000 matches for the south, representing about 15-20% of the total number of 2MASS extended sources. We restrict coordinate comparison to only NED galaxies with reported accurate coordinate positions (uncertainty major axis < 2´´), reducing the number of galaxy matches to a few thousand in total. These matches are almost always from high signal-to-noise ratio sources (SNR > 12). Our preliminary results pointed to a systematic difference between 2MASS and NED galaxies. Further investigation (given below) suggests that two or three major catalogs in NED are the culprit.
The 2MASS extended source processor determinates the position for a galaxy in two different ways: the J-band peak-pixel position and the intensity-weighted (centroid) using the "super" J+H+Ks combined image. The latter position is the most precise, based on repeatibility tests (and is consistent with the superior SNR and centroiding technique). The equatorial coordinates are then derived from the pixel coordinates using the position reconstructed parameters in the Atlas Image from which the galaxy is detected.
The extended source equatorial position may be different from the point-source position (which is derived assuming the source is a point source), but there should be no statistical systematic difference. This is confirmed in the following plot. There is no evident systematic in either dra (right ascension difference) or ddec (declination difference). The RMS for the comparision between the J-band peak-pixel position and the point source position is ~0.5´´, while the RMS for the "super" image centroid comparison is ~0.2´´. We will use the "super" image centroid position for the NED vs. 2MASS comparison. The 2MASS point source astrometry has been shown to be accurate to 0.1-0.2´´, based on comparison with the Tycho astrometric catalogs; hence, the extended source positions should have a comparable absolute astrometric accuracy (~0.3´´ RMS for high SNR sources).
In Figure 1 we show the 2MASS extended source positions vs. 2MASS point source position. The parameters dra and ddec refer to the equatorial position difference in arcseconds, with dra shown with a solid white line, and ddec with a green dotted line. The upper panel shows the extended source position based on the J-band peak-pixel position, while the lower panel shows the extended source position based on the "super" image centroid
We divide the 2MASS-NED matches between the northern and southern declinations to check for possible systematics in the 2MASS northern and southern data. Figures 2 and 3 show the results for 2MASS vs. NED, where we have not attempted to delineate the NED catalogs (except in that we have used only sources with good positions).
The northern galaxies exhibit a systematic in the declination difference of order -0.24´´, while the right ascension shows no systematic. For sourthern galaxies, there is both an RA and Dec systematic, -0.2´´ and -0.4´´, respectively. Since the NED sample consists of sources originating from different catalogs (and presumably, different astrometry) it is difficult to interpret these position systematics. Instead, we break down the NED sample into astrometrically uniform catalogs below.
|Figure 2||Figure 3|
In Figure 4 we show the 2MASS extended source positions vs. APMUKS positions. Here we see a clear dra, ddec systematic difference between 2MASS and APMUKS (The APM Galaxy survey by Maddox, Sutherland, Efstathiou & Loveday, 1990, MNRAS, 243, 692) of -0.4´´ and -0.6´´, respectively.
In Figure 5 we show the 2MASS extended source positions vs. NPM (north) positions. In Figure 6 we show the 2MASS extended source positions vs. NPM (south) positions. Here, both the northern and southern samples show a clear declination systematic of -0.4´´ between 2MASS and the NPM1G catalog (Lick Northern Proper Motion Program by Klemola, Jones, & Hanson 1987, AJ, 94, 501). There is no evident right ascension systematic difference for either northern or southern samples.
|Figure 5||Figure 6|
In Figure 7 we show the 2MASS extended source positions vs. NGP positions. Similar to the 2MASS extended source position vs. NPM comparison above, 2MASS and the NGP (Odewahn & Aldering 1995, AJ, 110, 2009) shows a systematic -0.4´´ in declination comparison, suggesting some astrometric connection between NGP and NPM1G. In fact, the NGP sample was astrometrically calibrated with NPM1G positons.
In Figure 8 we show the 2MASS extended source positions vs. Abell cluster galaxy positions. Comparing 2MASS to "Abell" identified galaxies, we see a systematic coordinate difference of -0.2´´ and -0.4´´ for dra and ddec, respectively. Since this sample is probably not astrometrically uniform, this comparison may not be all that enlightening.
In Figure 9 we show the 2MASS extended source positions vs. FIRST positions. There is no evident systematic difference in either the declination or right ascension comparison between 2MASS and FIRST (The FIRST Survey: Faint Images of the Radio Sky at Twenty Centimeters, Becker, White & Helfand, 1995, ApJ, 450, 559).
We conclude that even though NED is composed of galaxies from a wide variety of surveys and projects, most of the positions are derived from just a few astrometric catalogs, including the APMUKS, NPM1G and FIRST (see references above). Consequently, a straight comparison between 2MASS positions and NED positions can reveal systematics in either data set. We do, in fact, see a clear systematic in the declination axis between 2MASS and NPM1G (both northern and southern declinations), and a bias in both RA and Dec comparing 2MASS and APMUKS (southern galaxies). The most enlightening comparison comes from the FIRST sample of galaxies, claimed to be astrometrically accurate. Here we find no evidence for a bias in RA or Dec. Hence, we believe the 2MASS coordinate positions are solid (giving a stronger weight to the radio positions of FIRST). Note that this is consistent with the absolute accuracy of the 2MASS point source astrometry.
Our major findings are that there is no systematic difference between the 2MASS extended source positions and the 2MASS point source positions; the RMS scatter in this comparison is ~0.2´´, which implies that the 2MASS extended source astrometry should be accurate to ~0.3´´ in comparison to the TYCHO astrometric catalog; there is a small, -0.4´´, declination bias in the NPM1G galaxy position astrometry, compared to 2MASS (the NPG1G positions are systematically too small by 0.4"); there is a small bias in the APMUKS galaxy position astrometry, -0.4´´ and -0.6´´ in RA and Dec, respectively, compared to 2MASS (the APM positions are systematically too small by 0.4´´ and 0.6´´); there is no systematic difference between 2MASS and FIRST galaxy position astrometry; the RMS scatter between the two sets is ~0.3´´ to 0.4´´ for both the RA and Dec axis.
[Last Updated: 1999 October 12 by T. Jarrett. Modified 2000 Aug 30 by S. Van Dyk.]