Spectroscopic detection of CIV in a galaxy at z=7.045: Implications for the ionizing spectra of reionization-era galaxies (1504.06881v1)

Abstract: We present Keck/MOSFIRE observations of UV metal lines in four bright gravitationally-lensed z~6-8 galaxies behind the cluster Abell 1703. The spectrum of A1703-zd6, a highly-magnified star forming galaxy with a Lyman-alpha redshift of z=7.045, reveals a confident detection of the nebular CIV emission line (unresolved with FWHM < 125 km/s). UV metal lines are not detected in the three other galaxies. At z~2-3, nebular CIV emission is observed in just 1% of UV-selected galaxies. The presence of strong CIV emission in one of the small sample of galaxies targeted in this paper may indicate hard ionizing spectra are more common at z~7. The total estimated equivalent width of the CIV doublet (38 A) and CIV/Lyman-alpha flux ratio (0.3) are comparable to measurements of narrow-lined AGNs. Photoionization models show that the nebular CIV line can also be reproduced by a young stellar population, with very hot metal poor stars dominating the photon flux responsible for triply ionizing carbon. Regardless of the origin of the CIV, we show that the ionizing spectrum of A1703-zd6 is different from that of typical galaxies at z~2, producing more H ionizing photons per unit 1500A luminosity and a larger flux density at 30-50 eV. If such extreme radiation fields are typical in UV-selected systems at z>7, it would indicate that reionization-era galaxies are more efficient ionizing agents than previously thought. Alternatively, we suggest that the small sample of Lyman-alpha emitters at z>7 may trace a rare population with intense radiation fields capable of ionizing their surrounding hydrogen distribution. Additional constraints on high ionization emission lines in galaxies with and without Lyman-alpha detections will help clarify whether hard ionizing spectra are common in the reionization era.

Spectroscopic Detection of C IVλ1548 in a Galaxy at z = 7.045: Implications for the Ionizing Spectra of Reionization-Era Galaxies

The paper by Stark et al. presents a compelling investigation into the spectroscopic characteristics of galaxies from the reionization era, particularly focusing on the detection of the nebular emission line C IVλ1548 in a galaxy at a redshift of z = 7.045. Utilizing the Keck/MOSFIRE instrument, the paper analyzes UV metal lines in several gravitationally-lensed galaxies behind the Abell 1703 cluster, offering significant insights into early cosmic environments and the properties of star-forming galaxies during this pivotal epoch.

Key Findings and Observations

One of the noteworthy outcomes is the confident detection of the C IVλ1548 line in the galaxy A1703-zd6, exhibiting a signal-to-noise ratio of 5.1. The equivalent width of the C IV doublet is measured to be approximately 38 Å, with the C IV/Lyα flux ratio found to be 0.3. These metrics align with those observed in narrow-lined active galactic nuclei (AGNs). Furthermore, this paper introduces the possibility that such emission could stem from intense radiation fields generated by young stellar populations comprising very hot, metal-poor stars.

Implications for Ionizing Spectra

The implications of these findings are substantial, suggesting a variance in the ionizing spectra of galaxies at different cosmic times. The ionizing photon production rate per unit 1500 Å luminosity in A1703-zd6 is noted to be notably higher than that typically seen in galaxies at lower redshifts (z ≈ 2), indicating a potentially greater contribution to cosmic reionization from these early galaxies than previously thought. This high production rate of ionizing photons supports theories that these galaxies can maintain a significant ionizing photon output necessary for reionizing the intergalactic medium during the late stages of reionization.

Future Directions

Further exploration in this field will be essential, particularly focusing on a broader sampling of UV-selected systems at comparable redshifts. Distinguishing whether such intense radiation fields are a common characteristic of reionization-era galaxies will refine our understanding of their role in early universe reionization. Additionally, examining high-redshift galaxies devoid of Lyα emissions could offer contrasting insights that could illuminate whether these spectral features are unique or prevalent across broader populations of early galaxies.

In conclusion, the work by Stark et al. exemplifies a rigorous approach to characterizing the spectroscopic properties of distant galaxies, paving the way for future studies that aim to decipher the complexities of early cosmic environments.