Variable Quasars with Narrow-line Absorption


Photo of Don York
Don York
University of Chicago


Spectra of narrow absorption line quasars in a 7,650 square degree survey area

Finding Targets

An object whose ANCILLARY_TARGET1 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Number of Fibers Number of Unique Primary Objects
QSO_AAL 22 Radio-quiet, with one absorption system associated with the quasar 509 466
QSO_AALS 23 Radio-quiet, with multiple absorption systems associated with the quasar 916 833
QSO_IAL 24 Radio-quiet, with one intervening absorption system 295 275
QSO_RADIO 25 Radio-loud, with multiple absorption systems, either local or intervening 251 220
QSO_RADIO_AAL 26 Radio-loud, with one absorption system associated with the quasar 151 138
QSO_RADIO_IAL 27 Radio loud, with one intervening absorption system 80 70
QSO_NO_AALS 28 Radio-quiet, with multiple intervening absorption systems and no associated absorption systems 73 66


Quasar absorption lines are plentiful in SDSS-I and II, and have been documented in a catalog of all lines and systems (QSOALS; York et al. 2005). This catalog, now updated through Data Release 7 (Hacker, Brunner, Lundgren, & York 2013), contains 60,000 uniformly-detected quasar absorption line systems in which two or more transitions from common metal absorption lines (e.g., Mg II, Fe II, C IV) are identified at the same redshift. This dataset has been used to study the statistics of quasar absorption lines (York et al. 2006), and to confirm correlations with quasars (Wild, Hewett, & Pettini 2007) and foreground galaxies projected along the line of sight (Lundgren et al. 2009).

It has been shown that smaller equivalent width BALs are more prone to variation on short timescales (e.g., Barlow 1994; Lundgren et al. 2007). A large survey of variability in narrow absorption lines (NALs) is therefore required in order to examine if this trend applies across a larger range in equivalent widths.

Complementary to the BAL quasar variability ancillary target selection program, this program seeks to compile the largest dataset of multi-epoch observations of quasar sight lines with known narrow absorption along the line of sight. Detections of variability in narrow absorption line systems hold great promise for identifying high-velocity intrinsic quasar absorption and mini-BAL emergence and for providing limits on the sizes of cold gas clouds in the extended haloes of luminous galaxies in the foreground.

Target Selection

The targets of this program include quasars with 16.5 < ifib2 < 17.9 and redshift 0.7 < z < 2.2 from the DR7 quasar catalog (Schneider et al. 2010), which would otherwise be ignored by the primary BOSS target selection. Sight lines with known BALs (Gibson, Brandt, Schneider, & Gallagher 2008) are ignored, as this parameter space is being covered by the BAL quasar variability program. Each of the quasar sight lines targeted in this program contains an NAL system detected at > 4σ (including multiple unambiguous transitions of Mg II, C IV, or both) that has been identified in York et al. (2005). However, this program is not limited only to cases in which previously identified NALs disappear in later epochs, since NALs should have the same probability of emerging along these lines of sight with and without identified NALs. In total, this program targets about 2,000 quasars with a target density of ~0.35 deg-2.

As one of the science goals of this program is to determine the extent of variable NALs in velocity space relative to the quasar, the target list includes sight lines with NALs over a wide range in velocity. The following sub-groups allow for the identification of quasars and absorbers with particular characteristics:

  • QSO_RADIO_AAL: radio-loud with one associated absorption system (AAL; v ≤ 5000 km/s in the quasar rest frame)
  • QSO_RADIO_IAL: radio-loud with one intervening absorption system (IAL; v > 5000 km/s in the quasar rest frame)
  • QSO_AAL: radio-quiet source with one AAL
  • QSO_IAL: radio-quiet source with one IAL
  • QSO_RADIO: radio-loud source with multiple AALs and/or IALs
  • QSO_AALs: radio-quiet source with multiple AALs and/or IALs
  • QSO_NOAALS: radio-quiet source with no AALs and multiple IALs


Barlow, T. A. 1994, PASP, 106, 548

Gibson, R. R., Brandt, W. N., Schneider, D. P., & Gallagher, S. C. 2008, ApJ, 675, 985

Hacker, T. L., Brunner, R. J., Lundgren, B. F., & York, D. G. 2013 (Abstract from ADS), MNRAS, 434, 163, doi:10.1093/mnras/stt1022 (Catalog contained in data supplment)

Lundgren, B. F., Wilhite, B. C., Brunner, R. J., Hall, P. B., Schneider, D. P., York, D. G., Vanden Berk, D. E., & Brinkmann, J. 2007, ApJ, 656, 73

Lundgren, B. F., et al. 2009, ApJ, 698, 819

Schneider, D. P., et al. 2010, AJ, 139, 2360

Wild, V., Hewett, P. C., & Pettini, M. 2007, MNRAS, 374, 292

York, D. G., et al. 2005, in IAU Colloq. 199: Probing Galaxies through Quasar Absorption Lines, ed. P. Williams, C.-G. Shu, & B. Ménard, 58-64

York, D. G., et al. 2006, MNRAS, 367, 945