Halpha Equivalent Widths from the 3D-HST survey: evolution with redshift and dependence on stellar mass

TL;DR

This study analyzes the evolution of Halpha equivalent widths across redshifts 0.8 to 2.2, revealing a consistent decline over time and a strong correlation with redshift, independent of stellar mass, confirming galaxy evolution models.

Contribution

First to combine 3D-HST data with ground surveys for a comprehensive analysis of EW(Halpha) evolution from z=0 to 2.2, highlighting its dependence on redshift and mass.

Findings

EW(Halpha) decreases towards the present epoch.

EW(Halpha) is lower in higher-mass galaxies at all redshifts.

EW(Halpha) scales approximately as (1+z)^{1.8}.

Abstract

We investigate the evolution of the Halpha equivalent width, EW(Halpha), with redshift and its dependence on stellar mass, taking advantage of the first data from the 3D-HST survey, a large spectroscopic Treasury program with the Hubble Space Telescope WFC3. Combining our Halpha measurements of 854 galaxies at 0.8<z<1.5 with those of ground based surveys at lower and higher redshift, we can consistently determine the evolution of the EW(Halpha) distribution from z=0 to z=2.2. We find that at all masses the characteristic EW(Halpha) is decreasing towards the present epoch, and that at each redshift the EW(Halpha) is lower for high-mass galaxies. We measure a slope of EW(Halpha) ~ (1+z)^(1.8) with little mass dependence. Qualitatively, this measurement is a model-independent confirmation of the evolution of star forming galaxies with redshift. A quantitative conversion of EW(Halpha) to sSFR is very model dependent, because of differential reddening corrections between the continuum SED and the Balmer lines. The observed EW(Halpha) can be reproduced with a simple model in which the SFR for galaxies rises to the epoch of z~2.5 and then decreases with time to z = 0. The model implies that the EW(Halpha) rises to 400 A at z=8. The sSFR evolves faster than EW(Halpha), as the mass-to-light ratio also evolves with redshift. In this context, we find that the sSFR evolves as (1+z)^(3.2), nearly independent of mass, consistent with previous reddening insensitive estimates. We confirm previous results that the observed slope of the sSFR-z relation is steeper than the one predicted by models, but models and observations agree in finding little mass dependence.