Probing Spectroscopic Variability of Galaxies & Narrow-Line Active Galactic Nuclei in the Sloan Digital Sky Survey

Authors:
  • CYChing-Wa Yip
    Ching-Wa Yip
  • ACAndrew Connolly
    Andrew Connolly
  • DBDaniel Vanden Berk
    Daniel Vanden Berk
  • RSRyan Scranton
    Ryan Scranton
  • SKSimon Krughoff
    Simon Krughoff
  • ASAlex Szalay
    Alex Szalay
  • LDLaszlo Dobos
    Laszlo Dobos
  • CTChristy Tremonti
    Christy Tremonti
  • MTManuchehr Taghizadeh-Popp
    Manuchehr Taghizadeh-Popp
  • TBTamas Budavari
    Tamas Budavari
  • ICIstvan Csabai
    Istvan Csabai
  • RWRosemary Wyse
    Rosemary Wyse
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Abstract
Under the unified model for active galactic nuclei (AGNs), narrow-line (Type 2) AGNs are, in fact, broad-line (Type 1) AGNs but each with a heavily obscured accretion disk. We would therefore expect the optical continuum emission from Type 2 AGN to be composed mainly of stellar light and non-variable on the time-scales of months to years. In this work we probe the spectroscopic variability of galaxies and narrow-line AGNs using the multi-epoch data in the Sloan Digital Sky Survey (SDSS) Data Release 6. The sample contains 18,435 sources for which there exist pairs of spectroscopic observations (with a maximum separation in time of ~700 days) covering a wavelength range of 3900-8900 Angstrom. To obtain a reliable repeatability measurement between each spectral pair, we consider a number of techniques for spectrophotometric calibration resulting in an improved spectrophotometric calibration of a factor of two. From these data we find that on average the spectroscopic variability of the continuum for narrow-line AGNs is 0.07 +/- 0.26 in the g band and, for the emission-line ratios log10([NII]/Ha) and log10([OIII]/Hb), the variability is 0.02 +/- 0.03 dex and 0.06 +/- 0.08 dex, respectively. We provide the corresponding upper limits for other spectral classes, including those from the BPT diagram, eClass galaxy classification, stars and quasars. If the change in the continuum flux of the narrow-line AGNs is caused by an AGN component, in the g band the AGN-to-host galaxy continuum flux ratio is typically 10% with an upper limit of 30%.