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).

💡 Research Summary

The paper investigates whether a massive galaxy cluster or proto‑cluster exists around the high‑redshift quasar 3C 270.1 (z = 1.53), a key test of hierarchical galaxy formation models that predict luminous radio sources to sit at the peaks of mass assembly. Using the Spitzer Space Telescope, the authors obtained imaging in the IRAC bands (3.6, 4.5, 5.8, 8.0 µm) and MIPS 24 µm, complemented by deep optical–near‑infrared imaging from the 6.5‑m MMT in the z′ and Y filters. The combined dataset provides a broad wavelength coverage from the rest‑frame UV to the mid‑infrared, enabling robust photometric redshift estimates for faint galaxies in the quasar field.

Source extraction was performed with SExtractor on the combined images, yielding 215 objects detected at ≥5σ. Extremely red objects (EROs) were selected using colour cuts (z′–Y > 1.5 mag and