Lyα and C iii] emission in z = 7–9 Galaxies: accelerated reionization around luminous star-forming systems?

Abstract

We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z similar or equal to 7-9 selected to have intense optical line emission by Roberts-Borsani et al. Previous follow-up has revealed Ly alpha in two of the four galaxies. Our new MOSFIRE observations confirm that Lya is present in the entire sample. We detect Lya emission in the galaxy COS-zs7-1, confirming its redshift as z.(Ly alpha) = 7.154, and we detect Lya in EGS-zs8-2 at z(Ly alpha) = 7.477, verifying an earlier tentative detection. The ubiquity of Lya in this sample is puzzling given that the IGM is expected to be significantly neutral over 7 < z < 9. To investigate this result in more detail, we have initiated a campaign to target UV metal lines in the four Lya emitters as a probe of both the ionizing field and the Lya velocity offset at early times. Here we present the detection of C III] emission in the z = 7.73 galaxy EGS-zs8-1, requiring an intense radiation field and moderately low metallicity. We argue that the radiation field is likely to affect the local environment, increasing the transmission of Lya through the galaxy. Moreover, the centroid of C III] indicates that Lya is redshifted by 340 km s (1). This velocity offset is larger than that seen in less luminous systems, providing an explanation for the transmission of Lya emission through the IGM. Since the transmission is further enhanced by the likelihood that such systems are also situated in large ionized bubbles, the visibility of Lya at z > 7 is expected to be strongly luminosity-dependent, with transmission accelerated in systems with intense star formation.