Extremely red objects in the Lockman Hole

We investigate extremely red objects (EROs) using near- and mid-infrared observations in five passbands (3.6 to 24 mum) obtained from the Spitzer Space Telescope, and deep ground-based R and K imaging. The great sensitivity of the Infrared Array Camera (IRAC) camera allows us to detect 64 EROs ( a surface density of 2.90 +/- 0.36 arcmin(-2); [3.6](AB) < 23.7) in only 12 minutes of IRAC exposure time, by means of an R - [3.6] color cut (analogous to the traditional red R - K cut). A pure infrared K - [3.6] red cut detects a somewhat different population and may be more effective at selecting z > 1.3 EROs. We find similar to17% of all galaxies detected by IRAC at 3.6 or 4.5 mum to be EROs. These percentages rise to about 40% at 5.8 mum, and about 60% at 8.0 mum. We utilize the spectral bump at 1.6 mum to divide the EROs into broad redshift slices using only near-infrared colors (2.2/3.6/4.5 mum). We conclude that two-thirds of all EROs lie at redshift z > 1.3. Detections at 24 mum imply that at least 11% of 0.6 < z < 1.3 EROs and at least 22% of z > 1.3 EROs are dusty star-forming galaxies.