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CASU analysis of WFCAM commissioning data - IV Photometric Properties

Document number: VDF-TRE-IOA-00008-0008

22th March 2005

This document describes the photometric technical assesment carried out in the WFCAM 20041108 data using colour-magnitude and colour-colour diagrams.

For a description of the data and processing see the documents linked at the bottom of this page.

The figure below shows the layout of the field (made by all objects detected), each stack (or pointing) in different colour. The field observed is centred at ra=158.27614, dec=55.150715 (big red cross in the figure) and consist of four stacks (~0.8 sq. deg).

Colour-magnitude diagram

The first image below shows the J vs. J-K and H vs. J-H colour-magnitude diagrams for 2MASS sources in the Lockman Hole area in cyan. Sources detected by 2MASS inside the area observed with WFCAM are marked in blue. Second figure shows the same diagram for sources detected with WFCAM, with objects classified as point like in blue and those classified as extended in red. J-K~1 is frequently used to separate stars from galaxies (for those extragalactic astronomers which do not want contamination from stars). Red objects J-K>2 are generally consistent with redshift z>2 objects.

Colour-colour diagrams

The figure below shows the J-H vs H-K colour-colour diagram with some galaxy models overlaid (Elliptical galaxy, Seyfert 2, M82 starburst galaxy, Spiral galaxy, Arp220). All models start at the lower left at z=0; first triangle in each model is z=1, second triangle z=2 and third triangle z=3 (click on the image to get a large version).
It is still to see what the objects in the lower right area of the diagrams are. Those mostly disappear when using a K<17 only sample.

Galaxy models have been computed using 2MASS filters and converted to the WFCAM system using the relations in the first comissioning document. These are:
J_wfcam  =  J_2mass - 0.10*(J_2mass - H_2mass)
H_wfcam = H_2mass + 0.15*(J_2mass - H_2mass)
K_wfcam = K_2mass - 0.05*(J_2mass - K_2mass)

K-band galaxy counts

K-band galaxy counts shown in figure below (blue dots) look sensible. Star galaxy separation done using a simple colour-cut in J-K. For comparison the K-band counts from the CADIS sourvey are also shown as black triangles.

Cumulative counts

Cumulative counts shown below for the four filters YJHK. Extended objects in blue, point like in red.

Created by Eduardo Gonzalez-Solares
Last modified 2005-03-22 13:02