A Dual Narrowband Survey for H\alpha\ Emitters at z=2.2: Demonstration   of the Technique and Constraints on the H\alpha\ 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 H\alpha\ emitters, a pair of custom 1% filters, which sample low-airglow atmospheric windows at 1.19 \mu m and 2.10 \mu m, is used to detect both H\alpha\ and [OII]\lambda 3727 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 \mu m and 2.10 \mu m data to probe 3\sigma\ 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 H\alpha\ 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 \mu m excess, and 136 with a 2.10 \mu m 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 H\alpha\ 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 H\alpha\ luminosity function. These "narrow-deep" FourStar observations have been obtained as part of the larger NewH\alpha\ 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] 
Janice C. Lee (STScI), C. Ly (STScI), L. Spitler (Swinburne U.), I.   Labbe (Leiden U.), S. Salim (Indiana U.), S.E. Persson...
astro-ph.CO