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The First Detection of Forbidden Emission Lines at the Outskirts of the AGN Broad Line Region?

Kelly F. Heckler, Rogemar A. Riffel, Daniel Marsango, Tiago V. Ricci, Angela C. Krabbe, Oli L. Dors

Abstract

Double-peaked (DP) broad emission line profiles in active galactic nuclei (AGNs) are often interpreted as signatures of rotating disk-like structures in the broad-line region (BLR) and are commonly observed in low-luminosity AGNs using recombination lines. We use optical spectroscopy to investigate the origin of double-peaked broad emission line profiles observed not only in hydrogen recombination lines but also in forbidden transitions in the LINER galaxy IC 1459. We detected DP emission in all strong optical lines except for the [S II] doublet, which has the lowest critical density among all the lines. We successfully fitted the DP broad profiles using a disk-like BLR model, assuming a circular accretion disk with an inclination of approximately 35 degrees and internal turbulence of about 500 km/s, confined within a maximum radius of 9.6 (+4.8, -1.1) light-years. We estimate a full width at half maximum of the DP profiles of about 3300 km/s. Our results provide new insights into the structure of the BLR, indicating that forbidden emission lines can be produced in lower-density regions near the outskirts of the BLR.

The First Detection of Forbidden Emission Lines at the Outskirts of the AGN Broad Line Region?

Abstract

Double-peaked (DP) broad emission line profiles in active galactic nuclei (AGNs) are often interpreted as signatures of rotating disk-like structures in the broad-line region (BLR) and are commonly observed in low-luminosity AGNs using recombination lines. We use optical spectroscopy to investigate the origin of double-peaked broad emission line profiles observed not only in hydrogen recombination lines but also in forbidden transitions in the LINER galaxy IC 1459. We detected DP emission in all strong optical lines except for the [S II] doublet, which has the lowest critical density among all the lines. We successfully fitted the DP broad profiles using a disk-like BLR model, assuming a circular accretion disk with an inclination of approximately 35 degrees and internal turbulence of about 500 km/s, confined within a maximum radius of 9.6 (+4.8, -1.1) light-years. We estimate a full width at half maximum of the DP profiles of about 3300 km/s. Our results provide new insights into the structure of the BLR, indicating that forbidden emission lines can be produced in lower-density regions near the outskirts of the BLR.
Paper Structure (7 sections, 2 equations, 3 figures, 2 tables)

This paper contains 7 sections, 2 equations, 3 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Data and spectral fitting
  3. Observations and Data Processing
  4. Subtraction of the stellar population contribution
  5. Results and discussion
  6. Conclusions
  7. Residual double-peaked broad profile

Figures (3)

  • Figure 1: Stellar population model obtained using the ppxf fit of the integrated nuclear spectrum. The black line shows the observed spectrum and the gray line represents the stellar population model. A zoomed-in view of a selected spectral region is included to illustrate the fit quality. The bottom panel shows the residuals of the fit, which are overall small expect for the locations of the emission lines.
  • Figure 2: Nuclear spectrum of IC 1459 after subtraction of the stellar population contribution (black circles). The insets show the emission line profiles, along with the best-fit model (black line). The disk-like BLR component is shown in gray line, while the narrow components of the emission lines are shown in gray dotted line.
  • Figure 3: Residual emission after subtraction of stellar continuum and narrow line components for the [O i] $\lambda \lambda 6300, 6364$ doublet region is represented by the black circles. The red lines represent the BLR disc model.