Palomar-QUEST candidate selection for March 2004

Position matching

Data from 20040123 and 20040124 was positionally crossmatched using a 1.5" threshold; over the range, 120 < RA < 200.

Aperture magnitude correction

The magnitude distributions show a problem:

There are matched pairs where the psf magnitude on one night is 25; however, the aperture magnitude for this object is not necessarily 0.0.

For objects with a psf magnitude = 25 and a non-zero aperture magnitude, a corrected psf magnitude was estimated by adding the following median (psf mag - aper mag) correction (derived from psf mag range 16 - 18):
20040123: B = -0.074, I = -0.042, R = -0.011
20040124: B = -0.13, I = -0.068, R = -0.044
Magnitudes were then averaged for each object and average colours derived.

Colour-colour offsets

Only objects detected in the same column on each night were selected. A colour-colour offset was then applied on a per column basis so that the stellar locus for each column was correctly zero pointed. The offsets used for this data were:

Column	B-R	R-I
2	-1.55	0.15
3	-1.15	-0.15
4	-1.25	0.25
5	-1.30	0.20
8	-0.90	-0.10
9	-1.05	0.25
11	-1.25	0.30
13	-1.15	0.15
14	-2.35	0.25
16	-1.90	0.05
17	-1.35	0.25
18	-1.20	0.35
19	-1.25	0.00
20	-1.45	0.40
23	-1.60	0.40
24	-1.20	0.10
25	-1.50	0.65
26	-1.30	-1.00
27	-1.15	0.10

There were initially ~285000 objects.

Cloud

From both the number density of objects against RA and magnitude differences against RA, the effect of cloud can be seen and objects from these regions were excluded:

RA < 123
130 < RA < 132
136 < RA < 138
143 < RA < 152
156 < RA < 163
165 < RA < 166
171 < RA < 173
185 < RA < 186

This reduces the number of objects to ~202000.

Geometry cuts

These are regions where RA, Dec plots show an unusually high concentration of objects; objects in these were excluded:

124 < RA < 125, -1.5 < Dec < -1.4
132 < RA < 132.25, Dec > 0
133 < RA < 134, Dec > 1.8
135 < RA < 136, Dec > 1.5
138 < RA < 140, Dec > 1.65
142 < RA < 143
164 < RA < 165
182 < RA < 184, -1.75 < Dec < -1.4
184 < RA < 191.5, -1.85 < Dec < -1.5
184 < RA < 188, Dec > 1.8
187 < RA < 188, -1 < Dec < -0.85
187 < RA < 188.2, -0.65 < Dec < -0.5
190 < RA < 191, 0 < Dec < 0.2
192 < RA < 193, Dec > 2
-0.33 < Dec < -0.28

This reduces the number of objects to ~183000.

R dropouts

Objects with R = 25 (psf mag = 25 and aper mag = 0 on both nights) were selected as R band dropouts.

Colour-colour plots

Colour-colour plots for the objects in magnitude slices:

The squares are Quest objects corresponding to the positions of known z>4 quasars. The lines are the primary colour-colour cuts described next.

Primary colour-colour cuts

Objects in these regions were excluded from the candidate list:

B-R < 1.8, R-I < 1.9
R-I > 0.7 * (B-R - 1.8), 1.8 < B-R < 2.8
0.7 < R-I < 0.2 * R - 1.5

This reduces the number of objects to ~6400.

Secondary colour-colour cuts

Objects in these regions were then excluded from the candidate list:

0.9 / 1.2 * (B-R - 1.8) - 0.2 < R-I < 0.7, 1.8 < B-R < 3, 16 < R < 17
0.9 / 1.7 * (B-R - 1.8) - 0.2 < R-I < 0.7, 1.8 < B-R < 3.5, 17 < R < 18
0 < R-I < 0.5, 1.8 < B-R < 3, 18 < R < 19
0.5 * (B-R - 3) < R-I < 0.5, 3 < B-R < 4, 18 < R < 19
0.5 < R-I < 0.7, B-R > 1.8, 18 < R < 19
R > 19

This reduces the number of objects to ~2600.

Star-galaxy cuts

Objects with the parameters in their 20040124 data were removed:

max_pixel[3] / flux[3][3] > 0.09
psf_reduced_chi_2[3] > 2

This reduces the number of objects to ~980.

2MASS stamps

Combined JHK 2MASS stamps are examined for each remaining object; any object with a significant detection is excluded. This reduces the number of objects to ~460; of these, none had a radio detection.

Tertiary colour-colour cuts

The following cuts were made to the 183000 objects to give further candidates: For R < 16:

B-R < 0
R-I > 1
1.8 / 3 * B-R - 0.8 < R-I < 1, 0 < B-R < 3

For 16 < R < 17:

B-R < 0
R-I > 1.5
1 / 3 * B-R - 1 < R-I < 1.5, 0 < B-R < 3
1.5 * (B-R - 3) < R-I < 1.5, 3 < B-R < 4

For 17 < R < 18:

B-R < 0
R-I > 2
1.5 / 4 * B-R - 1 < R-I < 2, 0 < B-R < 4
1.5 * (B-R - 4) + 0.5 < R-I < 2, 4 < B-R < 5

For 18 < R < 19:

B-R < 0
R-I > 0.5
1.5 / 4 * B-R - 1 < R-I < 0.5, 0 < B-R < 4

Star-galaxy cuts were applied as before and omitting any objects previously identified as candidates produces another ~450 objects. 99 of these (B-R > 3, R < 17) were selected for 2MASS stamp checking which reduced the number of objects to .