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 (1208.3529v1)

Abstract: The physical properties inferred from the 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 (sSFR) 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 broad-band fluxes, we investigate the SEDs of 92 spectroscopically-confirmed galaxies in the redshift range 3.8<z\<5.0 chosen because the H-alpha line lies within the Spitzer/IRAC 3.6 um filter. We demonstrate that the 3.6 um flux is systematically in excess of that expected from stellar continuum, which we derive by fitting the SED with population synthesis models. No such excess is seen in a control sample at 3.1<z\<3.6 in which there is no nebular contamination in the IRAC filters. From the distribution of our 3.6 um flux excesses, we derive an H-alpha equivalent width (EW) distribution. The mean rest-frame H-alpha EW we infer at 3.8<z\<5.0 (270 A) indicates that nebular emission contributes at least 30% of the 3.6 um flux. Via our empirically-derived EW 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 5x 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 paper quantifies that nebular emission contributes around 30% of the 3.6 µm filter flux in galaxies at z≈3.8–5.0.
• The paper revises stellar mass estimates and indicates that the sSFR may increase up to 5 times from z≈2 to 7.
• The paper emphasizes that nebular emission contamination necessitates refined models to accurately interpret high-redshift galaxy properties.

Nebular Emission in the Study of High-Redshift Galaxies

The paper conducted by Stark et al. provides an in-depth exploration of the influence of nebular emission on the spectral energy distributions (SEDs) of high-redshift galaxies, specifically focusing on faint Lyman Break Galaxies (LBGs) within the redshift range $3 < z < 7$. This investigation emphasizes the critical role of nebular emission lines in interpreting galaxy properties from observational data, highlighting their effect on derived stellar masses and specific star formation rates (sSFR).

The paper begins by addressing the discrepancies observed between theoretical models and empirical data regarding the evolution of the sSFR at $z > 2$. Current challenges arise due to the potential nebular emission contamination of the broadband photometry from galaxies at these high redshifts. Specifically, strong nebular lines, such as Hα, can significantly influence the broadband fluxes, skewing results towards overestimated stellar masses and underestimated sSFRs.

To address this, the authors use a spectroscopically confirmed sample of 92 galaxies in the redshift range $3.8 < z < 5.0$, observing these through the Spitzer/IRAC 3.6$\mu$m filter. They demonstrate that this filter's flux systematically exceeds expectations from stellar continuum models alone. This observation suggests substantial contamination from nebular emission lines, notably Hα. By comparing these results with a control sample at lower redshifts, they derive a distribution of equivalent widths for Hα and estimate its contribution to the photometry.

Key Findings

1. Nebular Emission Contribution: The paper concludes that nebular emission contributes approximately 30% of the flux in the 3.6$\mu$m filter for galaxies in the range $3.8 < z < 5.0$. This significantly impacts the derived stellar masses and sSFRs, suggesting previous estimates need substantial revision to account for the nebular contribution.
2. Stellar Mass and sSFR Revisions: The authors correct stellar mass densities and propose a more dynamic evolution of the sSFR at $z > 4$ than previously acknowledged. They propose an increase up to 5× in the sSFR from $z \approx 2$ to 7, which aligns more consistently with theoretical predictions.
3. Nebular Emission's Broader Impact: The paper speculates on how nebular emission lines might further impact redshift $z \approx 6-7$ galaxies, where both Spitzer/IRAC filters are likely contaminated. These corrections could imply even more substantial adjustments to observed stellar mass functions and star formation rates.

Implications and Future Directions

This paper contributes fundamentally to our understanding of early galaxy formation and evolution. By quantifying the nebular emission line contamination, it recalibrates our interpretation of galaxy population data at high redshifts. Practically, this work has immediate implications for the computation of the baryonic mass budget and the star formation history of the universe, particularly during the epoch of reionization.

Looking forward, the authors postulate that future telescopes, particularly the James Webb Space Telescope (JWST), will be instrumental in providing more precise nebular line strengths through infrared spectroscopy. This will allow for even deeper understanding and more accurate corrections for high-redshift observations, facilitating a better match between observational results and theoretical models.

In conclusion, Stark et al. underscore the importance of considering nebular emissions in high-redshift studies, urging the astrophysical community to refine models and interpretative frameworks to better reflect these findings. As these insights augment the accuracy of galaxy evolution models, they pave the way for future research that could further bridge the gap between observation and theory.