The nature of Hβ plus [O III] and [O II] emitters to z ∼ 5 with HiZELS: stellar mass functions and the evolution of EWs

We investigate the properties of similar to 7000 narrow-band selected galaxies with strong H beta+[O III] and [O II] nebular emission lines from the High-z Emission-Line Survey between z similar to 0.8 and 5.0. Our sample covers a wide range in stellar mass (M-stellar similar to 10(7.5-12.0) M-circle dot), rest-frame equivalent widths (EWrest similar to 10-10(5) angstrom), and line luminosities (L-line similar to 10(40.5-43.2) erg s(-1)). We measure the H beta+[O III]-selected stellar mass functions out to z similar to 3.5 and find that both M-star and phi(star) increases with cosmic time. The [O II]-selected stellar mass functions show a constant M-star approximate to 10(11.6) M-circle dot and a strong, increasing evolution with cosmic time in phi(star) in line with Ha studies. We also investigate the evolution of the EWrest as a function of redshift with a fixed mass range (10(9.5-10.0) M-circle dot) and find an increasing trend best represented by (1+z)(3.81 +/- 0.14) and (1 + z)(2.72 +/- 0.19) up to z similar to 2 and similar to 3 for H beta+[O III] and [O II] emitters, respectively. This is the first time that the EWrest evolution has been directly measured for beta+[O III] and [O II] emitters up to these redshifts. There is evidence for a slower evolution for z > 2 in the H beta+[O III] EWrest and a decreasing trend for z > 3 in the [OII] EWrest evolution, which would imply low [OII] EW at the highest redshifts and higher [O III]/[O II] line ratios. This suggests that the ionization parameter at higher redshift may be significantly higher than the local Universe. Our results set the stage for future near-IR space-based spectroscopic surveys to test our extrapolated predictions and also produce z > 5 measurements to constrain the high-z end of the EWrest and [O III]/[O II] evolution.