Lyman-alpha and CIII] Emission in z=7-9 Galaxies: Accelerated Reionization Around Luminous Star Forming Systems? (1606.01304v1)

Abstract: We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z~7-9 selected to have intense optical line emission by Roberts-Borsani et al. (2016). Previous follow-up has revealed Lyman-alpha in two of the four galaxies. Our new MOSFIRE observations confirm that Lyman-alpha is present in the entire sample. We detect Lyman-alpha emission in COS-zs7-1, confirming its redshift as z=7.154, and we detect Lyman-alpha in EGS-zs8-2 at z=7.477, verifying a tentative detection presented in an earlier study. The ubiquity of Lyman-alpha in this sample is puzzling given that the IGM is likely significantly neutral over 7<z\<9. To investigate this result in more detail, we have initiated a campaign to target UV metal emission in the four Lyman-alpha emitters as a probe of both the radiation field and the velocity offset of Lyman-alpha. Here we present the detection of intense CIII] emission in EGS-zs8-1, a galaxy from this sample previously shown to have Lyman-alpha at z=7.73. Photoionization models indicate that an intense radiation field and low metallicity are required to reproduce the intense CIII] and optical line emission. We argue that this extreme radiation field is likely to affect the local environment, increasing the transmission of Lyman-alpha through the galaxy. Moreover, the centroid of CIII] indicates that Lyman-alpha is redshifted from the systemic value by 340 km/s. This velocity offset is larger than that seen in less luminous systems, providing an additional explanation for the transmission of Lyman-alpha emission through the IGM. Since the transmission is further enhanced by the likelihood that such systems are also situated in the densest regions with the largest ionized bubbles, the visibility of Lyman-alpha at z\>7 is expected to be strongly luminosity-dependent, with the most effective transmission occurring in systems with intense star formation.

• The paper presents spectroscopic evidence of persistent Lyα emission in all four luminous galaxies at z=7–9 despite a mostly neutral intergalactic medium.
• It reports a significant Lyα velocity offset of 340 km s⁻¹ and strong CIII] equivalent widths, indicating an intense radiation field and low metallicity.
• The findings suggest that bright, star-forming galaxies in dense cosmic regions may drive early and accelerated reionization.

Lyα and CIII] Emission in z=7-9 Galaxies: Accelerated Reionization Around Luminous Star Forming Systems?

This paper presents an analysis of spectroscopic observations of four luminous galaxies at redshifts between approximately 7 and 9, utilizing the Keck/MOSFIRE spectrograph. The sample was chosen based on intense optical line emissions, which led to prior detection of Lyα emission in two of these galaxies. The observations confirm the presence of Lyα emission in all four galaxies, despite the expectation of a significantly neutral intergalactic medium (IGM) during this epoch.

The authors detect Lyα emission in COS-zs7-1, confirming its redshift at z=7.154, and verify the tentative Lyα emission in EGS-zs8-2 at z=7.477. To further understand the puzzling Lyα prevalence, the paper initiates a campaign to detect UV metal line emission, focusing on EGS-zs8-1, where a notable equivalent width of the [CIII], CIII] λλ1907,1909 doublet is measured. Utilizing photoionization models, the authors deduce an intense radiation field (log10 ξ*ion ≈ 25.6) and a moderately low metallicity (0.11 Z⊙), which are essential to reproducing both CIII] and the intense optical line emissions.

A significant finding is the velocity offset of Lyα, redshifted by 340 km s^-1 from the systemic value. This offset, larger than those seen in less luminous systems, suggests a mechanism for enhanced Lyα transmission through the IGM. The authors propose that these luminous galaxies are likely positioned in dense cosmic regions with accelerated evolution and larger ionized bubbles, suggesting that the visibility of Lyα at z>7 is heavily dependent on luminosity and star formation intensity.

The results have substantial implications. Practically, they suggest that Lyα transmission may be more efficient in bright, star-forming galaxies located in cosmic overdensities, therefore offering insights into strategic targets for reionization studies. Theoretically, if the described environmental conditions and radiation fields are common among early massive galaxies, the paper suggests a potential bias in current models of cosmic reionization by enhancing Lyα transmissivity far earlier than traditionally expected.

Future directions focus on further characterization of velocity offsets and expanding the spectroscopic sample to validate the suggested correlations between luminosity, local environment, and Lyα visibility. These findings underscore the need for a reassessment of reionization models, taking into account the etheroscopic conditions of galaxies and the complex interplay within dense regions of the early universe.