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Extremely red quasars (ERQs) are an interesting sample of quasars in theBaryon Oscillation Spectroscopic Sample (BOSS) in the redshift range of $2.0 -3.4$ and have extreme red colours of $i-W3\ge4.6$. Core ERQs have strong CIVemission lines with rest equivalent width of $\ge100$\AA. Many core ERQs alsohave CIV line profiles with peculiar boxy shapes which distinguish them fromnormal blue quasars. We show, using a combination of kernel density estimationand local outlier factor analyses on a space of the $i-W3$ colour, CIV restequivalent width and line kurtosis, that core ERQs likely represent a  separate population rather than a smooth transition between normal  blue quasars and the quasars in the tail of the colour-REW distribution. Weapply our analyses to find new criteria for selecting ERQs in this 3D parameterspace. Our final selection produces $133$ quasars, which are \emph{three} timesmore likely to have a visually verified CIV broad absorption line feature thanthe previous core ERQ sample. We further show that our newly selected sampleare extreme objects in the intersection of the WISE AGN catalogue with theMILLIQUAS quasar catalogue in the colour-colour space of ($W1-W2$, $W2-W3$).This paper validates an improved selection method for red quasars which can beapplied to future datasets such as the quasar catalogue from the Dark EnergySpectroscopic Instrument (DESI).

Improved selection of extremely red quasars with boxy CIV lines in BOSS