SPECTROSCOPIC STUDY OF THE HST/ACS PEARS EMISSION-LINE GALAXIES

We present spectroscopy of 76 emission-line galaxies (ELGs) in Chandra Deep Field South taken with the LDSS3 spectrograph on the Magellan Telescope. These galaxies are selected because they have emission lines with the Advanced Camera for Surveys (ACS) grism data in the Hubble Space Telescope Probing Evolution and Reionization Spectroscopically (PEARS) grism Survey. The ACS grism spectra cover the wavelength range 6000-9700 angstrom and most PEARS grism redshifts are based on a single emission line + photometric redshifts from broadband colors; the Magellan spectra cover a wavelength range from 4000 angstrom to 9000 angstrom and provide a check on redshifts derived from PEARS data. We find an accuracy of sigma(z) = 0.006 for the ACS grism redshifts with only one catastrophic outlier. We probe for active galactic nuclei (AGNs) in our sample via several different methods. In total, we find 7 AGNs and AGN candidates out of 76 galaxies. Two AGNs are identified from the X-ray full-band luminosity, L-X-ray,L- FB > 10(43) erg s(-1), the line widths, and the power-law continuum spectra. Two unobscured faint AGN candidates are identified from the X-ray full-band luminosity L-X-ray,L-FB similar to 10(41) erg s(-1), the hardness ratio and the column density, and the emission-line and X-ray derived SFRs. Two candidates are classified based on the line ratio of [N II]lambda 6584/H alpha versus [O III]lambda 5007/H beta(BPT diagram), which are between the empirical and theoretical demarcation curves, i.e., the transition region from star-forming galaxies to AGNs. One AGN candidate is identified from the high-ionization emission line He II angstrom 4686.