T. Jarrett, IPAC
(980519)
Combining the low stellar source density scans from RTB nights 971110n, 971116n, 971205n, 980114n & 980125n, over 180 sq. degrees of sky are covered, containing some 7500 galaxies. The following plots show the results for the combined data. The object of this exercise is to derive score, mag and color criteria to clean the database of false galaxy detections (typically double stars). These criteria can be applied in a post-processing filter mechanism (e.g., using the database).
The extended source results for the individual RTB nights have been fully analyzed to gauge the performance of GALWORKS. For a summary of other RTB nights, see
The area normalized galaxy number counts for the individual and
combined distributions
are discussed in
J, H and Ks Galaxy Counts .
Star - Galaxy Discrimination Parameters
Notes:
galaxies == filled white circles
stars == small red triangles
doubles == large red triangles
triples == blue crosses
artifacts == red crosses
unknowns == yellow dots
Extended Source Colors
Notes: colors computed using fixed circular radius=7" apertures; galaxies are denoted by the white filled circles; double stars by the red triangles; mean colors are represented K-corrections for spirals are represented by the magenta line and triangle points -- each point is 0.1 in redshift -- and ellipticals by the grey line and square points; the main sequence (dwarf and giant branches) are shown in green.
Yellow dotted lines represent "redshift" zones; they are roughly parallel to the "giant" star branch, and roughly perpendicular to the K-correction track.
The blue dashed line represents a color-color criterion (color break) between galaxies and false sources (double stars mostly). TChester has investigated this line in more depth by looking at the point sources in the database. See Distinguishing Galaxies from Point Sources Using Point Source Color-Color Plots .
Color Score
Using the "color score" defined by TChester in the above memo; namely,
Position Angle and Axil Ratio
9 < Ks < 13
(dark blue=J, green=H, red=Ks; cyan = super J+H+K)
9 < Ks < 12
12 < Ks < 13
13 < Ks < 13.5
13.5 < Ks < 14
!! From the preceding plots, it can be seen that
the positional angle of the 3-sigma isophotal ellipse
fit is preferentially aligned along ~-45 degrees. This
is the now (or soon to be) famous 2MASS position angle
bias. See
Source of Biases In Position Angle Determinations For 2MASS
Galaxies
Examples of Incorrectly Derived P.A.
The following images show various examples of galaxies that appear to have positional angles significantly different from that of the optical. We believe the 2MASS PSF is the primary culpret. See above note.
PSF From 971116n
derived diagnostic PSF ellip-parameter values at 10% peak isophot:
J: b/a= 0.770 , angle= 55 deg
H: b/a= 0.800 , angle= 0 deg
K: b/a= 0.770 , angle= 35 deg
note: position angle reference frame: 0 deg = west
(to convert to astronomical p.a., add 90 deg)
derived diagnostic PSF ellip-parameter values at 10% peak isophot:
J: b/a= 0.700 , angle= 55 deg
H: b/a= 0.850 , angle= 45 deg
K: b/a= 0.750 , angle= 40 deg
note: position angle reference frame: 0 deg = west
(to convert to astronomical p.a., add 90 deg)
