Slitless spectroscopy onboard space telescopes is a powerful tool to detect emission-line objects such as emission-line galaxies (ELGs) and quasars. In this work, we present a study of ELGs observed with slitless spectroscopy by the Hubble Space Telescope (HST) in a deep field of ∼44 arcmin2. This is one of the deepest HST fields with a wealth of imaging and spectral data. In particular, previous VLT/MUSE observations have covered this field and identified a large number of ELGs. We reduce the HST spectra using the latest pipeline with a forward modeling algorithm and construct a sample of ELGs. By comparing with the MUSE spectra, we characterize our ELG detection in the HST spectra, including the impact of the line flux, line width, signal-to-noise ratio, etc. We find that the morphological broadening may affect the detection of ELGs, such that more compact sources are easier to be detected in slitless spectra. We discuss its implications to future slitless spectroscopic surveys that will be carried out by the China Space Station Telescope (CSST) and find that the CSST slitless spectroscopy has a capability comparable to that of HST in terms of the detection of emission lines.

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