A Ly-alpha Emitter with an Extremely Large Rest-frame Equivalent Width
  of ~900A at z=6.5: A Candidate of Population III-dominated Galaxy?
  We have identified a very interesting Ly-alpha emitter, whose Ly-alpha
emission line has an extremely large observed equivalent width of
EW_0=436^{+422}_{-149}A, which corresponds to an extraordinarily large
intrinsic rest-frame equivalent width of EW_0^{int}=872^{+844}_{-298}A after
the average intergalactic absorption correction. The object was
spectroscopically confirmed to be a real Ly-alpha emitter by its apparent
asymmetric Ly-alpha line profile detected at z=6.538. The continuum emission of
the object was definitely detected in our deep z'-band image; thus, its EW_0
was reliably determined. Follow-up deep near-infrared spectroscopy revealed
emission lines of neither He II lambda1640 as an apparent signature of
Population III, nor C IV lambda1549 as a proof of active nucleus. No detection
of short-lived He II lambda1640 line is not necessarily inconsistent with the
interpretation that the underlying stellar population of the object is
dominated by Population III. We found that the observed extremely large EW_0 of
the Ly-alpha emission and the upper limit on the EW_0 of the He II lambda1640
emission can be explained by population synthesis models favoring a very young
age less than 2-4Myr and massive metal-poor (Z<10^{-5}) or even metal-free
stars. The observed large EW_0 of Ly-alpha is hardly 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 Ly-alpha EW_0, though
its significance is still unclear.
