T. Jarrett, IPAC
(990420)
Comparing cataloged galaxies (via NED) with the 2MASS XSC provides a check on completeness and the catalog generation process (i.e., potential problems with softwared used to this end). This work complements that of John Huchra in which he is looking at the Zwicky galaxies absent from the XSC.
Generating a "seed" NED catalog is not all that easy (given the very restriced and archaic NED interface). Instead, I have used the "seed" catalog that GALWORKS pipeline processor used to characterized and extract extended sources. This catalog was generated using many (many!) NED queries with a size selection -- the NED galaxy must be at least 30" in size to pass into the GALWORKS "seed" catalog.
Hence, this catalog is complete in the sense that it is a size selected catalog (not to mention that NED itself is anything but complete; caveat emptor). I extracted all NED galaxies that are located in the XSC release scan areas -- I compiled all of the "bright galaxy" lists from DATE/SCAN directories relevant to the XSC. These "bright galaxy" list, by the way, *are* stored online and can be easily grabbed (ask me for details).
For the 2MASS XSC, I grabbed the list using the DB on 990420 (the catalog was generated on april 19th). This catalog represents the latest greatest version, close to (if not already) release quality.
Procedure: using NED catalog, compare 2MASS XSC catalog and find NED galaxies with no associated 2MASS match. The resulting list is composed of some 800 NED galaxies that do not have an XSC match (or about 30% of the total).
The next step is to compare this list with the 2MASS extended source database. The database contains many more sources than the XSC (mostly faint sources) and thus provides a means to validate the nature of the sources. The resulting list is composed of some 250 sources (see below). Consequently, over 500 NED galaxies are in neither the XSC or the database. Why is that? Two primary reasons: (1) the NED source has a very poor coordinate position, so in fact the position corresponds to blank sky, and (2) the source is too faint for 2MASS to detect (e.g., LSB galaxies). A less common (but definitely present) case includes the omission of the source due to bright star proximal masking, including their associated artifacts, such as diffraction spikes. This case represents a real hit on the completeness of the 2MASS survey. In principle, we lose something like 5% (TBD) of the sky due to bright stars.
NED Galaxies Omitted from the 2MASS XSC
Approximately 250 of the size-selected NED galaxies are not to be found in the XSC (but are in the database). The overwhelming reason is that the galaxy is too faint (see table below). The less likely (but present) reason is that the position of the putative NED galaxy sits (in fact) on a bright star. The table below list the 250 galaxies. I looked at those few cases (about a dozen) where the "galaxy" was brighter than the catalog mag threshold limits. Nearly all turned out to be bogus sources (stars or artifacts from bright stars). Notes are tacked onto the end of the table rows. For the source flux, see the columns entitled " j_m_7 h_m_7 k_m_7 ".
2MASS database vs. NED matches
NED vs. 2MASS Matches
The optimal way to do this experiment is to get a fresh and complete list from NED (covering the release) area. Then do the comparison with 2MASS. Unfortunately, the way NED is set up, it is very difficult (time consuming and tedious) toward this end.