Classification of SWIRE Sources using Density Estimation in Colour Space.

We estimate the Probability Density Functions (PDFs) describing the N-dimensional mid-infrared colour (and optical photometric redshift) distributions of the galaxy populations in the Validation Field of the Spitzer Wide-area Infrared Extragalactic Legacy survey (SWIRE) located in the Lockman Hole. For sources with detections in all four of the IRAC bands, we identify Gaussian distributions, using 3 colours and an independently derived photometric redshift (zph). We then classify the sources by assigning them to the distribution to which they are most likely to belong. We find two of our distributions contain sources of low redshift ( ≈0.2), the third distribution contains sources of intermediate redshift ( ≈0.9), and our fourth distribution contains sources of high redshift ( ≈1.6).