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 H \beta emission lines from most of these galaxies in the SDSS spectra .
We study both the emission and absorption properties of a sub-sample 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 _ { \odot } 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 QSO absorber - galaxy connection .
Strong Mg ii ( W _ { \lambda 2796 } \ga 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 _ { \lambda 2600 } > 1 Å ) 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- \alpha systems .
We investigate various possible relations between the Mg ii rest equivalent widths and the emission line properties .
We find a possible ( 2 \sigma ) correlation between the emission-line metallicity of the galaxies and the Mg ii rest equivalent width of the absorbers ( \log ( { O / H } ) +12 = 0.1 W _ { \lambda 2796 } +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 can not 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 lumiosities of emission lines are higher for systems with larger W _ { \lambda 2796 } .
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 \geq 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 .