1.Introduction
Here are presented results of 20 & 15 scans observed
in 971012n and in 971015n respectively near the galactic plane. The field
lies between a galactic latitude of -15 and -5 degree. See the
field covered for a complete view of the observed path of sky
in equatorial coordinate and in
galactic coordinates . The area covered is about 14.5 squared
degree and 10.9 respectively. The plotted points represent galaxies detected
with 2MASS.
Following are some photometric and number
counts results.
2.The data
A. 971012n
The data consist of extended source candidates from
GALWORKS
each visually inspected to ascertain their true nature (real galaxy vs
point source or multiple source). The sample is 479 galaxies of size less
than 0.8' and between 12 and 16 J magnitude . The
field lies at low galactic latitude with appreciable stellar
number density: the maximum density defined as the log of the number of
stars brighter than K=14 per degree square in those scans is 3.4.
B. 971015n
The sample is 286 galaxies of size less than 0.8'
and between 12 and 16 J magnitude . The
field lies at lower galactic latitude with a more appreciable
stellar number density: the number density as defined above ranges between
3.4 and 3.6 for those scans.
Those stellar densities are in good agreement with the predictions . Such that having a star on top of a galaxy becomes likely. Indeed, visual inspection reveals a large fraction of the galaxies being contaminated by stars. The colors are likely to be affected as well.
3.Which magnitude to use?
971012n
The GALWORKS
algorithm gives us different kinds of magnitude measurement: with circular
or elliptical aperture, total or within a given surface brightness.
It is a good start to see which one is the best to use in this analysis.
For the purpose of color analysis, small fixed circular
apertures are sufficient since most galaxies are small and given that our
data has a high stellar contamination rate, a fixed circular aperture is
more robust than an adaptive elliptical aperture. Furthermore, those bring
additional errors due to the uncertainty in the ellipse fit parameters.
This is seen very clearly when we plot the 1 sigma uncertainty versus the
magnitude. This
shows the 1 sigma uncertainty versus magnitude in the three bands.
See the table below for the meaning of the notation. Note that the
scatter is slightly reduced using the
J band fiducial at fixed surface brightness with a elliptical aperture.
The next graph shows that it is even less using the J
band fiducial at fixed brightness with a circular aperture. The best
candidate is the fixed
radius with a circular aperture magnitude. This is good only for small
objects which is the case here. So,
in the next sections we shall use the fixed radius with a circular aperture.
The radius is 7 arcsec.
| Magnitude | Description |
| Jfe , Hfe , Kfe | Total using elliptical aperture ; fiducial = K |
| Jf21, Hf21, Kf21 | At isophote 21 mag per arcsec^2 with elliptical aperture ; fiducial = J |
| J0fe, H0fe, K0fe | At isophote 20 mag per arcsec^2 with elliptical aperture ; fiducial = K |
| Jfc, Hfc, Kfc | Total using circular aperture ; fiducial = K |
| Jf21c, Hf21c, Kf21c | At isophote 21 mag per arcsec^2 with circular aperture ; fiducial = J |
| J7, H7, K7 | Using a fixed circular aperture (radius =7 arcsec) |
4.Number counts and color-color mag results
A. 971012n
The number counts allows us to evaluate the completeness
of the sample. The number
counts plots suggests we are complete until J= 15.5, H=
14.5 and K=14 (upper limits). In order to see if there is a visible effect
from the galactic extinction, we plotted the
number count versus the galactic latitude for the J band.
The left plots are in normal scale while the right plots are in log scales.
There is no descernable loss in completeness going from glat = -14 to glat
= -10. Since J is expected to redden the most with the galactic latitude
(see the extinction table for details; the numbers come from the models
of Jarrett 1992) , we can conclude that we are still complete at those
latitude.
The J-K color is expected to redden with decreasing
galactic latitude (see the table below) due to increasing dust opacity.
However, the plot
J-K versus galactic latitude shows no such a dependence. This is because
the reddening effect (0.05 mag in delta(J-K)) is too small compared with
1 sigma uncertainty in the photometry.
In the color-color plane, J-K
versus H-K and J-H
versus H-K the reddening effects are barely discernable, less than
0.1 in H-K, comparable to the 1 sigma photometric error. The red circles
denote the high SNR points, delta_mag < 0.1). The effect the photometric
uncertainty can be seen comparing the black to red (high snr) points.
B.971015n
Here, the number
counts plots suggests we are complete until J= 15, H= 14.5
and K=13.5 (upper limits). Again, in order to see if there is a visible
effect from the galactic extinction, we plotted the
number count versus the galactic latitude.
The J-K color is expected to redden even more
as we get closer to the plane (see table below). Again, the plot
J-K versus galactic latitude (for both scans) shows no such
a dependence. To see this more carefully, we plotted the same data with
the following restrictions:
-with the higher snr galaxies.
This tends to lower the scatter due to uncertainties. Note that the threshold
applied onto the deltaJ-K were 0.21 and 0.14 for the 971012n and 971015n
scans respectively. This was done to prevent to remain with too fewer data.
-with a cut off in magnitude.
Since, we tend to select redder galaxies (see the tail going up in the
J-K vs J plots), we applied this cutoff to be sure of the completeness
of the sample.
What all this shows is the reddening of J-K
is still small even at those latitudes.
Expected Extinction and Reddening
| Glong & Glat | AV | AJ | AH | AK | AJ-AK | AJ-AH | |
| 185->192; -5->-7 | 1.16 | 0.32 | 0.19 | 0.12 | 0.202 | 0.13 | |
| 185->192; -7->-10 | 0.87 | 0.24 | 0.14 | 0.09 | 0.152 | 0.10 | |
| 185->192 ; -10->-12 | 0.698 | 0.191 | 0.115 | 0.070 | 0.122 | 0.076 | |
| 185->192 ; -12->-14 | 0.602 | 0.165 | 0.099 | 0.060 | 0.105 | 0.066 | |
| 185->192 ; -14->-16 | 0.531 | 0.146 | 0.087 | 0.053 | 0.092 | 0.059 |
5.Clusters detection in the field
971012n
The Jarrett
galaxy cluster algorithm finds three
clusters in the 15 deg^2 field. The most significant cluster is located
at ra=79.2, dec +17.2. There is no cluster in the data base in
that area.
Star - Galaxy Discrimination Parameters
The following plots show the various star-galaxy discrimination parameters applied to the 971015n scans (density = 3.5; glat = 5 to 10 deg).
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,
