Quasars probing intermediate-redshift star-forming galaxies

Abstract

We present a sample of 46 [O III]-emitting galaxies at z < 0.8 detected in the fibre spectra of quasars from the Sloan Digital Sky Survey, Data Release 7 (SDSS-DR7) through an automatic search procedure. We also detect [O II] and Hβemission lines from most of these galaxies in the SDSS spectra. We study both the emission and absorption properties of a subsample of 17 galaxies in the redshift range z= 0.4-0.7, where Mg ii lines are covered by the SDSS spectra. The measured lower limits on the star formation rates of these galaxies are in the range 0.2-20 M_⊙ yr^-1. The emission-line luminosities and (O/H) metallicities from R23 measured in this sample are similar to what is found in normal galaxies at these redshifts. Thus, this constitutes a unique sample of intermediate-redshift star-forming galaxies where we can study the quasi-stellar object (QSO) absorber-galaxy connection. Strong Mg ii (W_λ2796≿ 1 Å) as well as Mg i absorption lines are detected in the QSO spectra at the redshift of most of these galaxies. Strong Fe ii (W_λ2600 > 1Å) absorption lines are also generally detected whenever the appropriate wavelength ranges are covered. This suggests that most of these systems could be bona fide damped Lyman αsystems. We investigate various possible relations between the Mg ii rest equivalent widths and the emission-line properties. We find a possible (2σ) correlation between the emission-line metallicity of the galaxies and the Mg ii rest equivalent width of the absorbers [log(O/H) + 12 = 0.1W_λ2796+ 8.27], which could be a consequence of an underlying mass-metallicity relation. However, [O iii]-selected Mg ii systems represent only a minor fraction of the strong Mg ii absorbers. We find this cannot be attributed to biases related either to the spectral signal-to-noise ratio or to the brightness of the QSOs. We measure the average observed fluxes (collected into the SDSS fibre) of the [O ii] and [O iii] lines associated to Mg ii-selected systems through stacking technique. We find that the average luminosities of emission lines are higher for systems with larger W_λ2796. The stacked luminosities are found to be below the typical detection limit in individual spectra, indicating that faint galaxies can contribute appreciably to the observed population of strong Mg ii absorbers at intermediate redshifts. We also present long-slit spectroscopic observations of SDSS J113108+202151, the most luminous line-emitting galaxy in our z ≥ 0.4 sample. Surprisingly, we find that the line-emitting region does not coincide with the nearby extended bright galaxy with consistent photometric redshift seen in the SDSS image.