Abstract / Description of output
The physical properties inferred from the spectral energy distributions
(SEDs) of z > 3 galaxies have been influential in shaping our
understanding of early galaxy formation and the role galaxies may play
in cosmic reionization. Of particular importance is the stellar mass
density at early times, which represents the integral of earlier star
formation. An important puzzle arising from the measurements so far
reported is that the specific star formation rates (sSFRs) evolve far
less rapidly than expected in most theoretical models. Yet the
observations underpinning these results remain very uncertain, owing in
part to the possible contamination of rest-optical broadband light from
strong nebular emission lines. To quantify the contribution of nebular
emission to broadband fluxes, we investigate the SEDs of 92
spectroscopically confirmed galaxies in the redshift range 3.8 <z
<5.0 chosen because the Hα line lies within the Spitzer/IRAC
3.6 μm filter. We demonstrate that the 3.6 μm flux is
systematically in excess of that expected from stellar continuum alone,
which we derive by fitting the SED with population synthesis models. No
such excess is seen in a control sample of spectroscopically confirmed
galaxies with 3.1 <z <3.6 in which there is no nebular
contamination in the IRAC filters. From the distribution of our 3.6
μm flux excesses, we derive an Hα equivalent width distribution
and consider the implications for both the derived stellar masses and
the sSFR evolution. The mean rest-frame Hα equivalent width we
infer at 3.8 <z <5.0 (270 Å) indicates that nebular
emission contributes at least 30% of the 3.6 μm flux and, by
implication, nebular emission is likely to have a much greater impact
for galaxies with z ~= 6-7 where both warm IRAC filters are
contaminated. Via our empirically derived equivalent width distribution,
we correct the available stellar mass densities and show that the sSFR
evolves more rapidly at z > 4 than previously thought, supporting up
to a 5× increase between z ~= 2 and 7. Such a trend is much closer
to theoretical expectations. Given our findings, we discuss the
prospects for verifying quantitatively the nebular emission line
strengths prior to the launch of the James Webb Space Telescope.
Original language | English |
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Pages (from-to) | 129 |
Journal | Astrophysical Journal |
Volume | 763 |
Publication status | Published - 1 Feb 2013 |
Keywords / Materials (for Non-textual outputs)
- galaxies: evolution
- galaxies: formation
- galaxies: high-redshift
- galaxies: starburst
- surveys
- ultraviolet: galaxies