O V] λλ1213.8,1218.3 emission from extended nebulae around quasars: contamination of Ly α and a new diagnostic for AGN activity in Ly α emitters

We investigate the potential for the emission lines O V] λλ1213.8,1218.3 and He II λ1215.1 to contaminate flux measurements of Ly α λ1215.7 in the extended nebulae of quasars. We have computed a grid of photoionization models with a substantial range in the slope of the ionizing power law (-1.5 < α < -0.5), gas metallicity (0.01 < Z/Z_⊙ < 3.0), gas density (1 < n_H < 10⁴ cm^-3), and ionization parameter (10^-5 < U < 1.0). We find the contribution from He II λ1215.1 to be negligible, i.e. <0.1 of Ly α flux, across our entire model grid. The contribution from O V] λλ1213.8,1218.3 is generally negligible when U is low (≲10^-3) and/or when the gas metallicity is low (Z/Z_⊙ ≲ 0.1). However, at higher values of U and Z we find that O V] can significantly contaminate Ly α, in some circumstances accounting for more than half the total flux of the Ly α + He II + O V] blend. We also provide means to estimate the fluxes of O V] λλ1213.8,1218.3 and He II λ1215.1 by extrapolating from other lines. We estimate the fluxes of O V] and He II for a sample of 107 Type 2 active galaxies at z > 2, and find evidence for significant (≥10 per cent) contamination of Ly α fluxes in the majority of cases (84 per cent). We also discuss prospects for using O V] λλ1213.8,1218.3 as a diagnostic for the presence of AGN activity in high-z Ly α emitters, and caution that the presence of significant O V] emission could impact the apparent kinematics of Ly α, potentially mimicking the presence of high-velocity gas outflows.