Quasar identifications from the slitless spectra: a test from 3D-HST

ABSTRACT Slitless spectroscopy is a traditional method for selecting quasars. In this paper, we develop a procedure for selecting quasars (QSOs) using the 3D-HST G141 slitless spectra. We initially identify over 6000 sources with emission line width broader than typical emission line galaxies (ELGs) by analysing the 1D spectra. These ‘broad’ emission lines may originate from QSO’s broad lines ($\rm FWHM\ge 1200~\rm km\,s^{-1}$) or ELG’s convolved narrow lines ($\rm FWHM = 200\sim 300\,km\,s^{-1}$) with effective radii $\ge$0.3 arcsec (2.5 Kpc at z = 1). We then propose a criterion based on the reliability of the QSO component in the forward modeling results. Using the known QSOs, ELGs, and simulation data, we find that our criterion successfully selects about 90 per cent of known QSOs with H $\alpha$ or H $\beta$ line detection and point-like structures, with an ELG contamination rate of about 5 per cent. We apply this method to emission line sources without significant contamination and select 19 QSO candidates at redshift $z=0.12\!-\!1.56$. 12 of these candidates have Chandra X-ray detections. This sample covers a broader range of the rest-frame UV colours and has redder optical slopes compared to the SDSS QSOs, yet it is likely to be composed of normal QSOs rather than little red dots. Through spectral analysis, the estimated black hole masses of the sample are $10^{6.9}\!-\!10^{8.3} \,{\rm M}_{\odot }$. Our new candidates improve the completeness of the QSO sample at $z=0.8\!-\!1.6$ in the 3D-HST field. The proposed method will also be helpful for QSO selections via slitless spectroscopy in Euclid and the Chinese Space Station Telescope.