Extremely red quasars in BOSS

Red quasars are candidate young objects in an early transition stage of massive galaxy evolution. Our team recently discovered a population of extremely red quasars (ERQs) in the Baryon Oscillation Spectroscopic Survey (BOSS) that has a suite of peculiar emission-line properties including large rest equivalent widths (REWs), unusual 'wingless' line profiles, large N-V/L-y alpha, N-V/C (IV), Si (IV)/C (IV) and other flux ratios, and very broad and blueshifted [O (III)]lambda 5007. Here we present a new catalogue of C-IV and N-V emission-line data for 216 188 BOSS quasars to characterize the ERQ line properties further. We show that they depend sharply on UV-to-mid-IR colour, secondarily on REW(C (IV)), and not at all on luminosity or the Baldwin Effect. We identify a 'core' sample of 97 ERQs with nearly uniform peculiar properties selected via i-W3 >= 4.6 (AB) and REW(C (IV)) >= 100 angstrom at redshifts 2.0-3.4. A broader search finds 235 more red quasars with similar unusual characteristics. The core ERQs have median luminosity log L(ergs s(-1)) similar to 47.1, sky density 0.010 deg(-2), surprisingly flat/blue UV spectra given their red UV-to-mid-IR colours, and common outflow signatures including BALs or BAL-like features and large C (IV) emission-line blueshifts. Their SEDs and line properties are inconsistent with normal quasars behind a dust reddening screen. We argue that the core ERQs are a unique obscured quasar population with extreme physical conditions related to powerful outflows across the line-forming regions. Patchy obscuration by small dusty clouds could produce the observed UV extinctions without substantial UV reddening.