Clustering of red galaxies around the z=1.53 quasar 3C270.1
  In the paradigm of hierarchical galaxy formation, luminous radio galaxies
mark mass assembly peaks that should contain clusters of galaxies. Observations
of the z=1.53 quasar 3C270.1 with the Spitzer Space Telescope at 3.6-24 micron
and with the 6.5-m MMT in the z'- and Y-bands allow detection of potential
cluster members via photometric redshifts. Compared with nearby control fields,
there is an excess of 11 extremely red objects (EROs) at 1.33 < z_phot < 1.73,
consistent with a proto-cluster around the quasar. The spectral energy
distributions (SEDs) of 3/4 of the EROs are better fitted with passive
elliptical galaxies than withdust-reddened starbursts, and of four sources
well-detected on an archival HST snapshot image, all have undisturbed
morphologies. However, one ERO, not covered by the HST image, is a double
source with 0.8" separation on the z' image and a marginal (2sigma) 24 micron
detection indicating a dust-enshrouded starburst. The EROs are more luminous
than L* (H = -23.6 AB mag at z=1.5).
