THE Fe II EMISSION IN ACTIVE GALACTIC NUCLEI: EXCITATION MECHANISMS AND LOCATION OF THE EMITTING REGION

Abstract

We present a study of Fe ii emission in the near-infrared region (NIR) for 25 active galactic nuclei (AGNs) to obtain information about the excitation mechanisms that power it and the location where it is formed. We employ a NIR Fe ii template derived in the literature and found that it successfully reproduces the observed Fe ii spectrum. The Fe ii bump at 9200Å detected in all objects studied confirms that Lyα fluorescence is always present in AGNs. The correlation found between the flux of the 9200Å bump, the 1 µm lines and the optical Fe ii imply that Lyα fluorescence plays an important role in the Fe ii production. We determined that at least 18% of the optical Fe ii is due to this process while collisional excitation dominates the production of the observed Fe ii. The line profiles of Fe ii λ10502, O i λ11287, Ca ii λ8664 and Paβ were compared to gather information about the most likely location where they are emitted. We found that Fe ii, O i and Ca ii have similar widths and are, on average, 30% narrower than Paβ. Assuming that the clouds emitting the lines are virialized, we show that the Fe ii is emitted in a region twice as far from the central source than Paβ. The distance though strongly varies: from 8.5 light-days for NGC 4051 to 198.2 light-days for Mrk 509. Our results reinforce the importance of the Fe ii in the NIR to constrain critical parameters that drive its physics and the underlying AGN kinematics as well as more accurate models aimed at reproducing this complex emission.

Panchromatic properties of the extreme Fe ii emitter PHL 1092

“…From the virial theorem, we have that the distance D ∝ 1/FWHM 2 , which translates in a Fe ii emission region 2.5 times more distant than that where the bulk of Hydrogen lines are produced. Marinello et al (2016) obtained an averaged FWHM for Fe ii that is 3/4 smaller than that of Paβ . In particular, for IZw1, the strongest Fe ii emitter of their sample, the results are quite similar to PHL 1092, with the FWHM(Fe ii)= 1/2 FWHM(Paβ ).…”

“…The residual emission left after subtraction of the template is composed of the residual contribution of Fe ii and the Pa9 line. In order to obtain a more accurate measurement of the Fe ii emission in the bump we followed the technique presented by Marinello et al (2016), which consists of subtracting the Pa9 contribution to the bump after scaling down Paγ by the theoretical Paγ/Pa9 ratio (Case B). After this subtraction, the residual should consist of a pure Fe ii emission, which can be fit with a proper function (Lorentzian in this case).…”

“…In order to study the Fe ii excitation mechanisms we follow the approach of Marinello et al (2016), which consists of using line flux ratios between Fe ii lines and the closest H i lines in the same spectrum. The observable quantities are: (i) R 4570 = F(Fe ii 4570)/F(Hβ ); (ii) the flux ratio of the Fe ii bump centered at 9200Å and the broad component of Paγ, R 9200 = F(Fe ii 9200)/F(Paγ); and (iii) the flux ratio of the Fe ii 1µm lines (λ 9998, λ 10502, λ 10890, λ 11127) and The optical Fe ii emission arises from photons emitted via levels z 4 (D,F)→b 4 F e .…”

“…The importance of this plot is on the fact that the levels z 4 (D,F), from which the optical Fe ii emission is produced, are populated after the 1µm lines are emitted via b 4 G→z 4 (D,F). The energy necessary to excite b 4 G is ∼ 6eV and can be reached by two main excitation mechanisms: collisional excitation and Lyα fluorescence (Marinello et al 2016). Figure 7(a) confirms the trend between R 9200 and R 1µm , with some scatter for strong Fe ii emitters, suggesting Lyα fluorescence as an important mechanism for the 1µm lines.…”

“…For instance, the most conspicuous Fe ii lines are nearly isolated features, allowing an accurate analysis of their intensities and line profiles. Moreover, emission lines, such O i and Ca ii are isolated or moderately blended with adjacent lines (Martínez-Aldama et al 2015;Marinello et al 2016). While in the optical Hβ is severely affected by the underlying Fe ii, the Paschen lines in the NIR are completely isolated (Rodríguez-Ardila et al 2002).…”