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We have identified 335 galaxy cluster and group candidates, 106 of which areat z > 1, using a 4.5 um selected sample of objects from a 7.25 deg^2 region inthe Spitzer Infrared Array Camera (IRAC) Shallow Survey. Clusters wereidentified as 3-dimensional overdensities using a wavelet algorithm, based onphotometric redshift probability distributions derived from IRAC and NOAO DeepWide-Field Survey data. We estimate only ~10% of the detections are spurious.To date 12 of the z > 1 candidates have been confirmed spectroscopically, atredshifts from 1.06 to 1.41. Velocity dispersions of ~750 km/s for two of theseargue for total cluster masses well above 10^14 M_sun, as does the massestimated from the rest frame near infrared stellar luminosity. Although notselected to contain a red sequence, some evidence for red sequences is presentin the spectroscopically confirmed clusters, and brighter galaxies aresystematically redder than the mean galaxy color in clusters at all redshifts.The mean I - [3.6] color for cluster galaxies up to z ~ 1 is well matched by apassively evolving model in which stars are formed in a 0.1 Gyr burst startingat redshift z_f = 3. At z > 1, a wider range of formation histories is needed,but higher formation redshifts (i.e. z_f > 3) are favored for most clusters.

Clusters of Galaxies in the First Half of the Universe from the IRAC Shallow Survey