2MASS V3 Processing

1. Turnover in JHK magnitude histograms

This test was designed to determine if the source extractions were deep enough (i.e. fainter than the Level 1 specs) even in the presence of airglow. I made histograms of the J, H, and K magnitudes for each scan in the OPS test database with 0.2 magnitude sized bins. I then determined the magnitude at which the differential magnitude histogram peaked. In practice, I divided each scan into 12 equal sections (30' wide). (In practice, I should have done this on a coadd by coadd basis.) Figure 1 shows the magnitude at which the histograms turnover as a function of the stellar surface density brighter than 13th magnitude. The dashed lines are the sensitivity limits in the Level 1 specs.
For each band, the histograms peak at a brighter magnitude with increasing surface density. This general trend is expected, but I have not had a chance to compute the expected confusion limits for comparison.
Figure 2 shows the trend of stellar density with galactic latitude. The dashed line in this figure show my eyeball estimate of the surface density at which the magnitude histograms peak at the Level 1 specs. In all three bands, this occurs at galactic latitudes of less than 10 degrees.
There are a few regions in all three bands in which the magnitudes peak at a brighter magnitude than expected from the observed surface density. The regions in H and K are all in the Orion Nebula cluster region, where it is well known that the magnitude distribution peaks at K=12 and declines toward fainter magnitudes. The J-band plot contains an additional 6 regions at high galactic latitude where the turnover is brighter than J=15.8. Four of these regions are associated with a saturated Read 1 source, one with the cluster NGC 6121, and one with the cluster M67.

**Figure 1: Peak magnitude vs. Surface Density**
J	H	K

**Figure 2: Surface Density vs Galactic Latitude**
J	H	K

2. H-K color maps in the Galactic Center

Figure 3 a-c shows images of the average J-K, J-H, and H-K color near the galactic center. (The upper right corner is missing in the plots due to an error I made in downloading the data from IRSA.) There are clear systematic variations of the H-K color that are aligned with the in-scan direction. These patches of apparently anomolous color occur for portions of a 2MASS tile on a size scale of an image coadd. Another region near the galactic center does not show these systematic variations, so the origin of the H-K offsets is not necessarily related to the high source density in the galactic center.
Figure 3 d-f shows the average color as a function of declination for a small portion of the image. The H-K offset can be as large as 0.05 magnitudes.
By blinking the gif images there may be a similar offset in J-K as well, but it is by no means obvious. I don't see anything wrong in J-H image, so the problem may just be with K-band. I am uncertain of this though.

**Figure 3: H-K color images near galactic center**
a) J-K	b) J-H	c) H-K



d) Declination cut for J-K	e) Declination cut for J-H	f) Declination cut for H-K



g) H-K vs Declination	h) H-K from V2 processing	i) H-K from V3 processing



j) H-K vs K mag

3. K vs Sigma K for sources near the Galactic Center

Figure 4 shows a plot of K vs sigma(K) for a small region near the Galactic Center. The interesting feature of this plot is the sources with a small Sigma(k) between K=10 and 12 mag. These are clearly below the minimum noise level for most of the sources. Here is ra/dec listing of a few of these sources:

#         ra|       dec|    j_m|    h_m|    k_m|j_msig|h_msig|k_msig|    h_k|
  269.285573 -34.122017  13.333  12.493  11.669  0.076  0.089  0.016   0.824
  269.316593 -34.042313  13.272  12.443  11.836  0.092  0.111  0.016   0.607
  269.317639 -34.041981  13.376  12.711  11.811  0.061  0.084  0.016   0.900
  269.249309 -34.391212  12.650  12.737  11.744  0.016  0.078  0.017   0.993
  269.301367 -34.254623  12.384  11.654  11.511  0.065  0.017  0.017   0.143
  269.336702 -34.768394  13.778  11.799  11.599  0.089  0.017  0.017   0.200
  269.342592 -33.993706  13.456  12.891  11.769  0.059  0.077  0.017   1.122
  269.344056 -35.080879  12.602  13.128  11.625  0.019  0.085  0.017   1.503
  269.349326 -34.094463  13.618  11.950  11.795  0.089  0.016  0.017   0.155
  269.345701 -34.624535  13.945  13.290  11.807  0.064  0.113  0.018   1.483
  269.260502 -34.730671  12.792  11.980  11.870  0.064  0.017  0.019   0.110
  269.252101 -34.184959  12.696  13.063  11.489  0.021  0.064  0.020   1.574

I made a detail search in the database around a few of these objects. In the handful of the objects I checked, the j_blend and h_blend was 2, and the k_blend flag was 1 and k_prg_flg was "upl". This means the source was deblended at J and H but not K. It is not clear to me though why this should lead to a low error estimate in K band.

Figure 4: K vs sigma(K) for sources near the galactic center