A Coherent Study of Emission Lines from Broad-Band Photometry: Specific Star-Formation Rates and [OIII]/H{\beta} Ratio at 3 < z < 6

TL;DR

This study models emission lines and star-formation rates of galaxies from redshift 3 to 6, revealing rapid galaxy growth and evolving emission line ratios, with implications for understanding early galaxy evolution.

Contribution

It provides a consistent model linking galaxy properties, emission lines, and star-formation rates at high redshift without stellar mass fitting, and explores the evolution of emission line ratios.

Findings

Hα equivalent width increases with redshift, flattening above z~2.5.

Specific star-formation rate scales as (1+z)^2.4 below z~2.2 and (1+z)^1.5 at higher z.

[OIII]/Hβ ratio increases with redshift, correlating with sSFR.

Abstract

We measure the H{\alpha} and [OIII] emission line properties as well as specific star-formation rates (sSFR) of spectroscopically confirmed 3<z<6 galaxies in COSMOS from their observed colors vs. redshift evolution. Our model describes consistently the ensemble of galaxies including intrinsic properties (age, metallicity, star-formation history), dust-attenuation, and optical emission lines. We forward-model the measured H{\alpha} equivalent-widths (EW) to obtain the sSFR out to z~6 without stellar mass fitting. We find a strongly increasing rest-frame H{\alpha} EW that is flattening off above z~2.5 with average EWs of 300-600A at z~6. The sSFR is increasing proportional to (1+z)^2.4 at z<2.2 and (1+z)^1.5 at higher redshifts, indicative of a fast mass build-up in high-z galaxies within e-folding times of 100-200Myr at z~6. The redshift evolution at z>3 cannot be fully explained in a picture of cold accretion driven growth. We find a progressively increasing [OIII]{\lambda}5007/H{\beta} ratio out to z~6, consistent with the ratios in local galaxies selected by increasing H{\alpha} EW (i.e., sSFR). This demonstrates the potential of using "local high-z analogs" to investigate the spectroscopic properties and relations of galaxies in the re-ionization epoch.