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

Abstract

We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z 7 − 9 selected to have intense rest-frame optical line emission by Roberts-Borsani et al. (2016). Previous spectroscopic follow-up has revealed Lyα emission in two of the four galaxies. Our new MOSFIRE observations confirm that Lyα is present in the entire sample. We detect Lyα emission in the galaxy COS-zs7-1, confirming its redshift as z Lyα = 7.154, and we detect Lyα in EGS-zs8-2 at z Lyα = 7.477, verifying a tentative detection presented in an earlier study. The ubiquity of Lyα emission in this unique photometric sample is puzzling given that the IGM is expected to be significantly neutral over 7 < z < 9. To investigate this surprising result in more detail, we have initiated a campaign to target UV metal line emission in the four Lyα emitters as a probe of both the ionizing radiation field and the velocity offset of Lyα at early times. Here we present the detection of very large equivalent width [CIII], CIII] λλ1907,1909Å emission in EGS-zs8-1 (W CIII],0 = 22 ± 2Å), a galaxy from this sample previously shown to have Lyα emission at z = 7.73. Photoionization models indicate that an intense radiation field (log 10 ξ * ion [erg −1 Hz] 25.6) and moderately low metallicity (0.11 Z ) are required to reproduce the CIII] line emission and intense optical line emission implied by the broadband SED. We argue that this extreme radiation field is likely to affect the local environment, increasing the transmission of Lyα through the galaxy. Moreover, the centroid of CIII] emission indicates that Lyα is redshifted from the systemic value by 340 km sec −1 . This velocity offset is larger than that seen in less luminous systems and provides an additional explanation for the transmission of Lyα emission through the intergalactic medium. Since the transmission is further enhanced by the likelihood that such systems are also situated in the densest regions with accelerated evolution and the largest ionized bubbles, the visibility of Lyα at z > 7 is expected to be strongly luminosity-dependent, with the most effective transmission occurring in systems with intense star formation.

ALMA [C ii] 158 μm Detection of a Redshift 7 Lensed Galaxy behind RX J1347.1−1145*

“…Pentericci et al (2016) reported velocity shifts of ∼100 km s −1 between [C II] and Lyα for their three most significant detections. For a higher-luminosity galaxy, Stark et al (2017) reported an offset between CIII] and Lyα of ∼340 km s −1 . Given the small velocity offset of RX J1347:1206 (and similar other low-luminosity galaxies), it seems that at z∼7, Lyα is much closer to systemic velocity than is the case for low-z LAEs at similar UV-continuum luminosities.…”

Extended ionised and clumpy gas in a normal galaxy at z = 7.1 revealed by ALMA

“…Velocity shifts between optical/UV lines, and in particular relative to Lyα, have been found and investigated by several authors (e.g. Pettini et al 2002;Shapley et al 2003;Gallerani et al 2016;Stark et al 2017;Williams et al 2014;Erb et al 2014). In the case of Lyα IGM absorption can cause the centroid of Lyα to be artificially redshifted (i.e.…”

HST Imaging of the Brightest z ∼ 8–9 Galaxies from UltraVISTA: The Extreme Bright End of the UV Luminosity Function

“…At z 8 that wavelength range contains C III] 1907, 1909 ll Å. The resulting equivalent width would be in excess of ∼500 Å, physically unlikely (see e.g., Stark et al 2017 who found EW 22 0~Å for C III] at z 7 ). Even assuming that C III] is dominating the flux in the UltraVISTA H band, similar EW for CIV would be required to explain the flux in the K S band, together with a very red SED red-ward of K S to match the fluxes in the 3.6 μm and 4.5 μm bands.…”