HST WFC3 Early Release Science: Emission–Line Galaxies from IR Grism Observations

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We present grism spectra of emission–line galaxies (ELGs) from 0.6–1.6 microns from the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). These new infrared grism data augment previous optical Advanced Camera for Surveys G800L (0.6–0.95 micron) grism data in GOODS–South, extending the wavelength covereage well past the G800L red cutoff. The ERS grism field was observed at a depth of 2 orbits per grism, yielding spectra of hundreds of faint objects, a subset of which are presented here. ELGs are studied via the H , [O iii ], and [O ii] emission lines detected in the redshift ranges 0.2.z.1.6, 1.2.z.2.4 and 2.0.z.3.6 respectively in the G102 (0.8–1.1 microns; R210) and G141 (1.1–1.6 microns; R130) grisms. The higher spectral resolution afforded by the WFC3 grisms also reveals emission lines not detectable with the G800L grism (e.g., [S ii] and [S iii] lines). From these relatively shallow observations, line luminosities, star–formation rates, and grism spectroscopic redshifts are determined for a total of 25 ELGs to mAB(F098M)25 mag. The faintest source in our sample—with a strong but unidentified emission–line—is mAB(F098M)=26.9 mag. We also detect the expected trend of lower specific star formation rates for the highest mass galaxies in the sample, indicative of downsizing and discovered previously from large surveys. These results demonstrate the remarkable efficiency and capability of the WFC3 NIR grisms for measuring galaxy properties to faint magnitudes.


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