KECK SPECTROSCOPY OF 3 <z< 7 FAINT LYMAN BREAK GALAXIES: THE IMPORTANCE OF NEBULAR EMISSION IN UNDERSTANDING THE SPECIFIC STAR FORMATION RATE AND STELLAR MASS DENSITY

Abstract

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.

The Galaxy UV Luminosity Function Before the Epoch of Reionization

“…We find good agreement with observations (Pérez- González et al 2008;Stark et al 2013;Tomczak et al 2014;Grazian et al 2015;Song et al 2015) taking into account the scatter in observations at z < 2, and the uncertain but potentially significant contribution of massive quiescent galaxies at low redshift. (2015); Song et al (2015).…”

Dusty star-forming galaxies at high redshift

“…This is determined by observed rest-frame ultraviolet luminosity functions (e.g. Bouwens et al, 2006;Reddy et al, 2008;Oesch et al, 2010) at high redshift and the observed main sequence (Noeske et al, 2007b;Stark et al, 2013).…”

“…This however assumes that high-J CO transitions trace the same region as low-J CO lines, however this often cannot be the case as the integrated SMG star formation history would exceed the local baryon density . Similarly, potential discrepancies in the integrated cosmic star formation history, and evolution of the stellar mass function may imply either a bottom light or top heavy IMF at high-z, with the idea that such an IMF would cause inferred SFRs to decrease, and bring the two values into agreement (Hopkins & Beacom, 2006;Elsner et al, 2008;Pérez-González et al, 2008;Wilkins et al, 2008), though issues related to luminosity function integration as well as nebular line contamination in stellar mass estimates may relieve some of these tensions (Reddy & Steidel, 2009;Stark et al, 2013). van Dokkum (2008 suggested that the color evolution of early type galaxies at z ∼ 1, combined with their mass to light ratios may be well described by a bottom light IMF, though note in van Dokkum & Conroy (2012) that the same observations could be consistent with a Salpeter IMF.…”

The photometric properties of galaxies in the early Universe

“…At very high redshift (z ∼ 7 and above) the inclusion of nebular emission in SED templates can affect stellar mass estimates by up to 0.4 dex where strong lines intersect the relevant broad-band filters (e.g. Schaerer & de Barros 2009;Stark et al 2013;Wilkins et al 2013b). The choice of SPS model has also been demonstrated to affect physical properties inferred from observations and photometric properties predicted by galaxy formation simulations (e.g.…”