No Overdensity of Lyman-Alpha Emitting Galaxies around a Quasar at z ̃ 5.7

Abstract

Bright quasars, observed when the universe was less than one billion years old (z > 5.5), are known to host massive black holes (̃10<SUP>9</SUP> M<SUB>☉</SUB>) and are thought to reside in the center of massive dark matter overdensities. In this picture, overdensities of galaxies are expected around high-redshift quasars. However, observations based on the detection of Lyman-break galaxies (LBGs) around these quasars do not offer a clear picture: this may be due to the uncertain redshift constraints of LBGs, which are solely selected through broadband filters. To circumvent such uncertainties, we here perform a search for Lyman-alpha emitting galaxies (LAEs) in the field of the quasar PSO J215.1512-16.0417 at z ̃ 5.73, through narrowband deep imaging with FORS2 at the Very Large Telescope. We study an area of 37 arcmin<SUP>2</SUP>, I.e., ̃206 comoving Mpc<SUP>2</SUP> at the redshift of the quasar. We find no evidence of an overdensity of LAEs in the quasar field with respect to blank-field studies. Possible explanations for these findings may be that our survey volume is too small, or that the strong ionizing radiation from the quasar hinders galaxy formation in its immediate proximity. Another possibility is that these quasars are not situated in the dense environments predicted by some simulations.