Origin of the Fe II Emission Range in Quasars

1
Origin of the Fe II Emission Range in Quasars

• Randi R. Worhatch, Greg Shields, Sarah Salviander
  (University of Texas at Austin)

Abstract Active galactic nuclei (AGN) have
significant Fe II emission from the broad line
region (BLR), and the strength of the Fe II
emission differs greatly from object to object.
Do differing Fe abundances among AGNs play a
significant role in the observed variations?
Using a sample of 527 AGN spectra from the Sloan
Digital Sky Survey in the redshift range of
0.2 lt z lt 0.35, we measure the Fe/Ne abundance of
the narrow line region (NLR) using the
Fe VII/Ne V line intensity ratio. We find no
significant difference in the abundance of Fe in
the NLR as a function of Fe II/H?. Assuming the
Fe abundance is the same in the NLR and the BLR,
we conclude that changes in excitation conditions
cause differences in Fe II emission strength.
Changes in Fe abundance among objects play an
insignificant role.
Figure 3. Upper panel Fe II/H? change among
bins. Lower panel Fe VII/Ne V change among
bins. From left to right, bins are very weak,
weak, medium, strong, and very strong.

• Results
• As is apparent from Figure 3, the modest range
  in Fe abundance is insignificant next to the
  large range in Fe II emission strength. In the
  main bins, Fe VII/Ne V varies by only a
  factor of 1.3 while Fe II/H? varies by a factor
  of 4.5. When comparing the extreme bins,
  Fe VII/Ne V shows a factor of 1.6 change,
  while Fe II/H? varies by a factor of over 22.
• We allowed for an error in our measurements of
  10-20 to account for uncertainties in continuum
  estimates and 30 error in the collision
  strengths1. However, these errors will not
  affect the changes we measure across bins. We
  also investigated intrinsic reddening (assessed
  by continuum slope and Balmer decrement) and
  found that it does not account for the factor of
  1.6 change in Fe VII/Ne V.
• The modest change in Fe VII/Ne V rules out
  changes in Fe abundance as a primary cause for
  the observed range of Fe II emission strength.
  Thus this range must result from differences in
  excitation conditions among quasars.

• Introduction
• Quasars often have notable Fe emission, and much
  of this emission is in Fe II. When investigating
  the range of Fe II emission among quasars, there
  is easily a factor of ten change between the
  weakest and strongest Fe II emission. What could
  cause such a range? While Fe abundance
  differences are not thought to completely explain
  such diversity of strengths of Fe II emission, it
  is not clear whether they are a significant
  factor. Alternatively, the strength of Fe II
  emission could be driven by physical conditions
  within the BLR, exciting more or less of the
  available Fe. We set out to determine the
  importance of Fe abundance to the observed range
  in strength of Fe II emission in quasars.

• Measurements
• We investigated the influence of differing Fe
  abundances in quasars on their optical Fe II
  emission strength. We measured Fe II emission
  strength using the ratio of Fe II (4600 Å
  feature) to H? equivalent widths (EWs). However,
  it is difficult to measure abundances within the
  BLR directly, because it is a region of high
  density, temperature, and velocities. On the
  assumption that the abundance in the NLR and BLR
  is the same for a given object, we used the Fe
  VII/Ne V intensity ratio of NLR lines to
  measure the abundance of Fe and search for
  possible Fe enhancement.
• Taking a sample from SDSS of quasars at
  redshifts 0.2 lt z lt 0.35 to ensure coverage of
  both Ne V at 3425 Å and Fe VII at 6087 Å.
  This small range in redshift isolates the
  influence of changes in abundance over cosmic
  timescales versus individual object abundance
  differences. We binned the 527 objects by Fe II
  strength (EW Fe II/EW H?) and made 3 main
  composite spectra of weak, medium, and strong Fe
  II emission, with subsets of very weak and very
  strong. The composites enabled detection of the
  weak Fe VII line.
• From these composites, we measured the abundance
  of Fe using collision strengths from Berrington
  et al. (2000)1 and Osterbrock Ferland
  (2006)2 and the relation

Figure1. Composite spectra of various Fe II
strengths. Fe II strength measured is EW of the
4600 Å blend relative to EW of H?.
References 1 Berrington, K. A. Nakasaki, S.
Norrington, P. H. 2000. AAS, 142, 313 2
Osterbrock, D. Ferland, G. 2006. The
Physics of Gaseous Nebulae and Active Galactic
Nuclei, University Science Books
The factor of 1.09 includes effects of collision
strengths, for which we assumed T15000 K.
Figure 2. Zoomed in view of all three composite
spectra. Left panel Ne V at 3425 Å. Right
panel Fe VII at 6087 Å.