Discovery of a high-z protocluster with tunable filters: The case of 6C0140+326 at z= 4.4

Abstract

We present the first results obtained using a tunable narrow-band filter in the search for high-z protoclusters. Using the recently commissioned red tunable filter on the Gran Telescopio Canarias, we have searched for Lyα emitters in a 75arcmin2 field centred on the z= 4.413 radio galaxy 6C0140+326. With three different wavelength tunings, we find a total of 27 unique candidate Lyα emitters. The availability of three different wavelength tunings allows us to make estimates of the redshifts for each of the objects. It also allows us to separate a possible protocluster from a structure in the immediate foreground. This division shows that the foreground region contains significantly fewer Lyα emitters. Also, the spatial distribution of the objects in the protocluster field deviates from a random distribution at the 2.5σ level. The observed redshift distribution of the emitters is different from the expected distribution of a blank field at the ~3σ level, with the Lyα emitters concentrated near the radio galaxy at z > 4.38. The 6C0140+326 field is denser by a factor of 9 ± 5 than a blank field, and the number density of Lyα emitters close to the radio galaxy is similar to that of the z~ 4.1 protocluster around TN J1338-1942. We thus conclude that there is an overdensity of Lyα emitters around the radio galaxy 6C0140+326. This is one of few known overdensities at such a high redshift. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.