  We investigate Extremely Red Objects (EROs) using near- and mid-infrared
observations in five passbands (3.6 to 24 micron) obtained from the Spitzer
Space Telescope, and deep ground-based R and K imaging. The great sensitivity
of the IRAC camera allows us to detect 64 EROs 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 17% of
all galaxies detected by IRAC at 3.6 or 4.5 micron to be EROs. These
percentages rise to about 40% at 5.8 micron, and about 60% at 8.0 micron. We
utilize the spectral bump at 1.6 micron to divide the EROs into broad redshift
slices using only near-infrared colors (2.2/3.6/4.5 micron). We conclude that
two-thirds of all EROs lie at redshift z > 1.3. Detections at 24 micron 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.
