A Lyα EMITTER WITH AN EXTREMELY LARGE REST-FRAME EQUIVALENT WIDTH OF ∼900 Å AT z = 6.5: A CANDIDATE POPULATION III-DOMINATED GALAXY?

We have identified a very interesting Lyα emitter (LAE), whose Lyα emission line has an extremely large observed equivalent width of EW0 = 436+422− 149 Å, which corresponds to an extraordinarily large intrinsic rest-frame equivalent width of EWint0 = 872+844− 298 Å after the average intergalactic absorption correction. The object was spectroscopically confirmed to be a real LAE by its apparent asymmetric Lyα line profile detected at z = 6.538. The continuum emission of the object was definitely detected in our deep z′-band image; thus, its EW0 was reliably determined. Follow-up deep near-infrared spectroscopy revealed emission lines of neither He ii λ1640 as an apparent signature of Population III (Pop III) nor C iv λ1549 as proof of an active nucleus. No detection of the short-lived He ii λ1640 line is not necessarily inconsistent with the interpretation that the underlying stellar population of the object is dominated by Pop III. We found that the observed extremely large EW0 of the Lyα emission and the upper limit on the EW0 of the He ii λ1640 emission can be explained by population synthesis models favoring a very young age less than 2–4 Myr and massive metal-poor (Z < 10−5) or even metal-free stars. The observed large EW0 of Lyα is insufficiently explained by Population I/II synthesis models with Z ⩾ 10−3. However, we cannot conclusively rule out the possibility that this object is composed of a normal stellar population with a clumpy dust distribution, which could enhance the Lyα EW0, though its significance is still unclear.