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We present a sample of 68 low-z Mg II low-ionization broad absorption-line (loBAL) quasars. The sample is uniformly selected from the Sloan Digital Sky Survey Data Release 5 according to the following criteria: (1) redshift 0.4 < z ≤ 0.8, (2) median spectral S/N > 7 pixel −1 , and (3) Mg II absorption-line width ∆v c ≥ 1600 km s −1 . The last criterion is a trade-off between the completeness and consistency with respect to the canonical definition of BAL quasars that have the 'balnicity index' BI > 0 in C IV BAL. We adopted such a criterion to ensure that ∼ 90% of our sample are classical BAL quasars and the completeness is ∼ 80%, based on extensive tests using high-z quasar samples with measurements of both C IV and Mg II BALs. We found (1) Mg II BAL is more frequently detected in quasars with narrower Hβ emission-line, weaker [O III] emission-line, stronger optical Fe II multiplets and higher luminosity. In term of fundamental physical parameters of a black hole accretion system, loBAL fraction is significantly higher in quasars with a higher Eddington ratio than those with a lower Eddington ratio. The fraction is not dependent on the black hole mass in the range concerned. The overall fraction distribution is broad, suggesting a large range of covering factor of the absorption material. (2) [O III]-weak loBAL quasars averagely show undetected [Ne V] emission line and a very small line ratio of [Ne V] to [O III]. However, the line ratio in non-BAL quasars, which is much larger than that in [O III]weak loBAL quasars, is independent of the strength of the [O III] line. (3) loBAL and non-loBAL quasars have similar colors in near-infrared to optical band but different colors in ultraviolet. (4) Quasars with Mg II absorption lines of intermediate width are indistinguishable from the non-loBAL quasars in optical emission line properties but their colors are similar to loBAL quasars, redder than non-BAL quasars. We also discuss the implication of these results.

Neutral Helium multiplets, He I* λλ3189, 3889, 10830 are very useful diagnostics to the geometry and physical conditions of the absorbing gas in quasars. So far only a handful of He I* detections have been reported. Using a newly developed method, we detected He I*λ3889 absorption line in 101 sources of a well-defined sample of 285 Mg II BAL quasars selected from SDSS DR5. This has increased the number of He I* BAL quasars by more than one order of magnitude. We further detected He I*λ3189 in 50% (52/101) quasars in the sample. The detection fraction of He I* BALs in Mg II BAL quasars is ∼35% as a whole, and increases dramatically with increasing spectral signal-to-noise ratios, from ∼18% at S/N ≤ 10 to ∼93% at S/N ≥ 35. This suggests that He I* BALs could be detected in most Mg II LoBAL quasars, provided spectra S/N is high enough. Such a surprisingly high He I* BAL fraction is actually predicted from photo-ionization calculations based on a simple BAL model. The result indicates that He I* absorption lines can be used to search for BAL quasars at low-z, which cannot be identified by ground-based optical spectroscopic survey with commonly seen UV absorption lines. Using He I* λ3889, we discovered 19 BAL quasars at z < 0.3 from available SDSS spectral database. The fraction of He I* BAL quasars is similar to that of LoBAL objects.

We report the detection of a strong Milky Way-type 2175Å extinction bump at z = 2.1166 in the quasar spectrum towards SDSS J121143.42+083349.7 from the Sloan Digital Sky Survey (SDSS) Data Release 10. We conduct follow up observations with the Echelle Spectrograph and Imager (ESI) onboard the Keck-II telescope and the Ultraviolet and Visual Echelle Spectrograph (UVES) on the VLT. This 2175Å absorber is remarkable in that we simultaneously detect neutral carbon (C i), neutral chlorine (Cl i), and carbon monoxide (CO). It also qualifies as a damped Lyman alpha system. The J1211+0833 absorber is found to be metal-rich and has a dust depletion pattern resembling that of the Milky Way disk clouds. We use the column densities of the C i fine structure states and the C ii/C i ratio (under the assumption of ionization equilibrium) to derive the temperature and volume density in the absorbing gas. A Cloudy photoionization model is constructed, which utilizes additional atoms/ions to constrain the physical conditions. The inferred physical conditions are consistent with a canonical cold (T ∼ 100 K) neutral medium with a high density (n(H i) ∼ 100 cm −3 ) and a slightly higher pressure than the local interstellar medium. Given the simultaneous presence of C i, CO, and the 2175Å bump, combined with the high metallicity, high dust depletion level and overall low ionization state of the gas, the absorber towards J1211+0833 supports the scenario that the presence of the bump requires an evolved stellar population.

In this Letter, the properties of the extended radio emission form Sloan Digital Sky Survey Data Release 7 quasars with 0.4 < z < 0.8 is studied. This low-redshift sample is useful since any corresponding FIRST radio observations are sensitive enough to detect extended flux in even the weakest Fanaroff-Riley II (FR II) radio sources. In the sample, 2.7% of the sources have detectable extended emission on larger than galactic scales (>20-30 kpc). The frequency of quasars with FR II level extended radio emission is ≈2.3% and >0.4% of quasars have FR I level extended radio emission. The lower limit simply reflects the flux density limit of the survey. The distribution of the long-term time-averaged jet powers of these quasars, Q, has a broad peak ∼3 × 10 44 erg s −1 that turns over below 10 44 erg s −1 and sources above 10 45 erg s −1 are extremely rare. It is found that the correlation between the bolometric (total thermal) luminosity of the accretion flow, L bol , and Q is not strong. The correlation of Q with narrow line luminosity is stronger than the correlation with broad line luminosity and the continuum luminosity. It is therefore concluded that previous interpretations of correlations of Q with narrow line strengths in radio galaxies as a direct correlation of jet power and accretion power have been overstated. It is explained why this interpretation mistakenly overlooks the sizeable fraction of sources with weak accretion luminosity and powerful jets discovered by Ogle et al.