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A Dual Narrowband Survey for Hα Emitters at z=2.2: Demonstration of the Technique and Constraints on the Hα Luminosity Function

We present first results from a narrowband imaging program for intermediate redshift emission-line galaxies using the newly commissioned FourStar infrared camera at the 6.5m Magellan telescope. To enable prompt identification of Hα emitters, a pair of custom 1% filters, which sample low-airglow atmospheric windows at 1.19 μm and 2.10 μm, is used to detect both Hα and [OII]λ3727 emission from the same redshift volume at z=2.2. Initial observations are taken over a 130 arcmin^2 area in the CANDELS-COSMOS field. The exquisite image quality resulting from the combination of the instrument, telescope, and standard site conditions (~0.55" FWHM) allows the 1.19 μm and 2.10 μm data to probe 3σ emission-line depths down to 1.0e-17 erg/s/cm^2 and 1.2e-17 erg/s/cm^2 respectively, in less than 10 hours of integration time in each narrowband. For Hα at z=0.8 and z=2.2, these fluxes correspond to observed star formation rates of ~0.3 and ~4 Msun/yr respectively. We find 122 sources with a 1.19 μm excess, and 136 with a 2.10 μm excess, 41 of which show an excess in both bands. The dual narrowband technique, as implemented here, is estimated to identify about >80% of z=2.2 Hα emitters in the narrowband excess population. With the most secure such sample obtained to-date, we compute constraints on the faint-end slope of the z=2.2 Hα luminosity function. These "narrow-deep" FourStar observations have been obtained as part of the larger NewHα Survey, which will combine the data with "wide-shallow" imaging through a similar narrowband filter pair with NEWFIRM at the KPNO/CTIO 4m telescopes, to enable study of both luminous (but rare) and faint emission-line galaxies in the intermediate redshift universe. [Abridged]