Searching for large-scale structures around high-redshift radio galaxies with Herschel

This paper presents the first results of a far-infrared search for protocluster-associated galaxy overdensities using the Spectral and Photometric Imaging REciever ( SPIRE) instrument on-board the Herschel Space Observatory. Large (similar to 400 arcmin(2)) fields surrounding 26 powerful high-redshift radio galaxies ( 2.0 < z < 4.1; L-500MHz > 10(28.5) WHz(-1)) are mapped at 250, 350 and 500 mu m to give a unique wide-field sample. On average, the fields have a higher than expected, compared to blank fields, surface density of 500 mu m sources within 6 comoving Mpc of the radio galaxy. The analysis is then restricted to potential protocluster members only, which are identified using a far-infrared colour selection; this reveals significant overdensities of galaxies in two fields, neither of which are previously known protoclusters. The probability of finding two overdensities of this size by chance, given the number of fields observed, is 5 x 10(-4). Overdensities here exist around radio galaxies with L-500MHz greater than or similar to 10(29) WHz(-1) and z < 3. The radial extent of the average far-infrared overdensity is found to be similar to 6 comoving Mpc. Comparison with predictions from numerical simulations shows that the overdensities are consistent with having masses > 10(14)M(circle dot). However, the large uncertainty in the redshift estimation means that it is possible that these far-infrared overdensities consist of several structures across the redshift range searched.