BROAD ABSORPTION LINE DISAPPEARANCE ON MULTI-YEAR TIMESCALES IN A LARGE QUASAR SAMPLE


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Ak N., Brandt W. N. , Hall P. B. , Schneider D. P. , Anderson S. F. , Gibson R. R. , ...More

ASTROPHYSICAL JOURNAL, vol.757, no.2, 2012 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Volume: 757 Issue: 2
  • Publication Date: 2012
  • Doi Number: 10.1088/0004-637x/757/2/114
  • Journal Name: ASTROPHYSICAL JOURNAL
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Keywords: quasars: absorption lines, DIGITAL SKY SURVEY, ACTIVE GALACTIC NUCLEI, DATA RELEASE, SDSS-III, STELLAR OBJECTS, MILKY-WAY, SI IV, C IV, OUTFLOWS, VARIABILITY
  • Erciyes University Affiliated: Yes

Abstract

We present 21 examples of C IV broad absorption line (BAL) trough disappearance in 19 quasars selected from systematic multi-epoch observations of 582 bright BAL quasars (1.9 < z < 4.5) by the Sloan Digital Sky Survey-I/II (SDSS-I/II) and SDSS-III. The observations span 1.1-3.9 yr rest-frame timescales, longer than have been sampled in many previous BAL variability studies. On these timescales, ≈2.3% of C IV BAL troughs disappear and ≈3.3% of BAL quasars show a disappearing trough. These observed frequencies suggest that many C IV BAL absorbers spend on average at most a century along our line of sight to their quasar. Ten of the 19 BAL quasars showing C IV BAL disappearance have apparently transformed from BAL to non-BAL quasars; these are the first reported examples of such transformations. The BAL troughs that disappear tend to be those with small-to-moderate equivalent widths, relatively shallow depths, and high outflow velocities. Other non-disappearing C IV BALs in those nine objects having multiple troughs tend to weaken when one of them disappears, indicating a connection between the disappearing and non-disappearing troughs, even for velocity separations as large as 10,000-15,000 km s-1. We discuss possible origins of this connection including disk-wind rotation and changes in shielding gas.

We present 21 examples of CIV broad absorption line (BAL) trough disappearance in 19 quasars selected from systematic multi-epoch observations of 582 bright BAL quasars (1.9 < z < 4.5) by the Sloan Digital Sky Survey-I/II (SDSS-I/II) and SDSS-III. The observations span 1.1-3.9 yr rest-frame timescales, longer than have been sampled in many previous BAL variability studies. On these timescales, approximate to 2.3% of C iv BAL troughs disappear and approximate to 3.3% of BAL quasars show a disappearing trough. These observed frequencies suggest that many CIV BAL absorbers spend on average at most a century along our line of sight to their quasar. Ten of the 19 BAL quasars showing C iv BAL disappearance have apparently transformed from BAL to non-BAL quasars; these are the first reported examples of such transformations. The BAL troughs that disappear tend to be those with small-to-moderate equivalent widths, relatively shallow depths, and high outflow velocities. Other non-disappearing CIV BALs in those nine objects having multiple troughs tend to weaken when one of them disappears, indicating a connection between the disappearing and non-disappearing troughs, even for velocity separations as large as 10,000-15,000 km s(-1). We discuss possible origins of this connection including disk-wind rotation and changes in shielding gas.