Empirical Modeling of the Redshift Evolution of the [NII]/H$\alpha$-ratio for Galaxy Redshift Surveys

TL;DR

This paper provides an empirical model for the [NII]/Hα flux ratio evolution with redshift and stellar mass, improving the accuracy of galaxy emission line measurements and forecasts for future surveys like Euclid and WFIRST.

Contribution

It introduces a new empirical parameterization of the [NII]/Hα ratio as a function of stellar mass and redshift, enhancing modeling and deblending in galaxy surveys.

Findings

Large variation in [NII]/Hα ratio at fixed redshift due to stellar mass dependence.

Constant contamination assumptions can significantly overestimate Hα luminosities.

Impacts on predictions of Hα emitters for Euclid and WFIRST.

Abstract

We present an empirical parameterization of the [NII]/H$\alpha$ flux ratio as a function of stellar mass and redshift valid at 0 < z < 2.7 and 8.5 < log(M) < 11.0. This description can easily be applied to (i) simulations for modeling [NII]$\lambda6584$ line emission, (ii) deblend [NII] and H$\alpha$ in current low-resolution grism and narrow-band observations to derive intrinsic H$\alpha$ fluxes, and (iii) to reliably forecast the number counts of H$\alpha$ emission-line galaxies for future surveys, such as those planned for Euclid and the Wide Field Infrared Survey Telescope (WFIRST). Our model combines the evolution of the locus on the Baldwin, Phillips & Terlevich (BPT) diagram measured in spectroscopic data out to z = 2.5 with the strong dependence of [NII]/H$\alpha$ on stellar mass and [OIII]/H$\beta$ observed in local galaxy samples. We find large variations in the [NII]/H$\alpha$ flux ratio at a fixed redshift due to its dependency on stellar mass; hence, the assumption of a constant [NII] flux contamination fraction can lead to a significant under- or overestimate of H$\alpha$ luminosities. Specifically, measurements of the intrinsic H$\alpha$ luminosity function derived from current low-resolution grism spectroscopy assuming a constant 29% contamination of [NII] can be overestimated by factors of ~8 at log(L) > 43.0 for galaxies at redshifts z = 1.5. This has implications for the prediction of H$\alpha$ emitters for Euclid and WFIRST. We also study the impact of blended H$\alpha$ and [NII] on the accuracy of measured spectroscopic redshifts.