###### Abstract

The issue of the difference between optical and UV properties of radio-quiet
and radio-loud (relativistically "jetted") active galactic nuclei (AGN) is a
long standing one, related to the fundamental question of why a minority of
powerful AGN possess strong radio emission due to relativistic ejections. This
paper examines a particular aspect: the singly-ionized iron emission in the
spectral range 4400 — 5600 Å, where the prominent HI H$\beta$ and
[Oiii]$\lambda\lambda$4959,5007 lines are also observed. We present a detailed
comparison of the relative intensity of Feii multiplets in the spectral types
of the quasar main sequence where most jetted sources are found, and
afterwards discuss radio-loud narrow-line Seyfert 1 (NLSy1) nuclei with
$\gamma$-ray detection and with prominent Feii emission. An Feii template
based on I Zw 1 provides an accurate representation of the optical Feii
emission for RQ and, with some caveats, also for RL sources. CLOUDY
photoionization simulations indicate that the observed spectral energy
distribution can account for the modest Feii emission observed in composite
radio-loud spectra. However, spectral energy differences alone cannot account
for the stronger Feii emission observed in radio-quiet sources, for similar
physical parameters. As for RL NLSy1s, they do not seem to behave like other
RL sources, likely because of their different physical properties, that could
be ultimately associated with a higher Eddington ratio.

###### keywords:

active galactic nuclei; optical spectroscopy; ionized gas; broad line region

Optical singly-ionized iron emission in radio-quiet and relativistically
jetted active galactic nuclei Paola Marziani1, Marco Berton2,3,4111ESO fellow,
Swayamtrupta Panda5,6,7222CNPq fellow, and Edi Bon8 Paola Marziani
Correspondence<EMAIL_ADDRESS>Tel.: +39-0498293415

##  1 Introduction

The wide majority of active galactic nuclei (AGN) are characterised by the
presence of broad and narrow optical and UV lines emitted by ionic species
over a wide range of ionization potential $\chi$.333For an introduction to AGN
spectra and the interpretation of their spectra in physical terms, see e.g.,
Netzer (1990); Peterson (1997); Osterbrock and Ferland (2006), and references
therein. Restricting the attention to broad lines, type-1 AGN444 Type-2 AGN do
not show broad permitted lines in natural light; they are believed to be
mostly obscured type-1 and will not be further considered here because they
lack the diagnostics offered by the broad lines measurements. spectra
invariably show the same low ionization lines ($\chi\lesssim$ 20 eV) that
include HI Balmer lines (H$\beta$, H$\alpha$), Mgii$\lambda$2800, the CaII IR
Triplet, and Feii features, in addition to higher ionization lines (with
CIV$\lambda$1549 as prototypical representative in the rest-frame UV domain).

The relative intensities of the emission lines and their profiles do not
scatter around an average, but instead change in a systematic way along the
so-called quasar main sequence (MS, Boroson and Green (1992); Sulentic et al.
(2000); Marziani et al. (2001); Shen and Ho (2014)). The MS can be represented
in a plane where the full-width at half maximum (FWHM) of H$\beta$ is
diagrammed against the parameter $R_{\rm FeII}$, defined as the intensity
ratio between Feii$\lambda$4570 and the broad component of H$\beta$ i.e.,
$R_{\rm FeII}$= F(Feii$\lambda$4570)/F(H$\beta_{\rm BC}$). Sketches or scatter
plots of the occupation of the quasar MS in this parameter plane have been
often shown in the literature (e.g., Gaskell, 1985; Boroson and Green, 1992;
Shen and Ho, 2014; Sulentic and Marziani, 2015). Here we show a simple sketch
of the quasar occupation in the optical parameter plane. Fig. 1 identifies
several spectral bins in the plane (Sulentic et al., 2002), and the domain of
two populations: Population A, with FWHM H$\beta$$\lesssim 4000$ km s-1, and a
Population B of broader sources. The rationale for the subdivision in two
Populations has been provided in several papers (Sulentic et al., 2000, 2008;
Fraix-Burnet et al., 2017). Population A objects are predominantly sources
with high Eddington ratio, with evidence of powerful winds originating from
the accretion disk (Leighly and Moore, 2004; Richards et al., 2011; Coatman et
al., 2016; Marziani et al., 2016; Sulentic et al., 2017; Vietri, 2017).
Conversely, Population B sources have lower Eddington ratio, and their line
asymmetries are predominantly toward the red (Marziani et al., 2003, 2009).
The origin of the asymmetry is a topic of current debate: partial obscuration
and infall (Wang et al., 2017), and gravitational redshift (Corbin, 1995;
Popovic et al., 1995; Zheng and Sulentic, 1990; Bon et al., 2015; Punsly et
al., 2020) are two processes that have been proposed.

The importance of the $R_{\rm FeII}$ parameter stems from the fact that the
Feii emission extends from UV to the IR and can dominate the thermal balance
of the low-ionization broad-line region (BLR) (Marinello et al., 2016). Feii
emission is self-similar in type-1 AGN but the relative intensity to H$\beta$
(parameterized by $R_{\rm FeII}$) can vary from undetectable to $R_{\rm FeII}$
$\gtrsim 2$, with values larger than $\approx$ 2 being rare (less than 1% in
optically selected samples, (Marziani et al., 2013)). The $R_{\rm FeII}$ trend
of the MS in turn provides systematic constraints on the physical conditions
of the line emitting gas (Marziani et al., 2010; Panda et al., 2018, 2019),
and is related to several multi-frequency properties as well. For instance,
Feii emission is also correlated with narrow-line properties such as weak Feii
implies strong [Oiii]$\lambda\lambda$4959,5007. Boroson and Green (1992)
formulated this and the FWHM H$\beta$– $R_{\rm FeII}$ correlation in the
context of the Eigenvector 1 in a principal component analysis, the main
forerunner of the quasar MS, although this result was known since much earlier
time (see for instance Ref. (Steiner, 1981)). Furthermore, strong Feii is
associated with a high value of the soft-X ray photon index (Boller et al.,
1996; Wang et al., 1996; Sulentic et al., 2000; Grupe et al., 2001), a tenet
of the 4D Eigenvector 1 introduced by Sulentic et al. (2000). Nonetheless, a
full understanding of the Feii emission remains a daunting task (Panda et al.,
2018, 2021, 2020, 2021). Even the main ionization mechanism is not fully
clear. Early suggestions of a contribution of collisional ionization (e.g.,
Joly et al., 2008, and references therein) have fallen into disfavor following
the extensive monitoring of several objects (Barth et al., 2013; Du et al.,
2018): the Fe iiopt emission responds to observed continuum variations with a
delay larger than the one of H$\beta$, but still fairly well defined.

The MS has been instrumental to the establishment of a second major result, in
addition to the realisation of the importance of Feii emission. RL sources are
not uniformly distributed along the main sequence (Sulentic et al., 2003;
Zamfir et al., 2008). In low-$z$ samples, they occupy the region of low
$R_{\rm FeII}$ and predominantly broad or very broad H$\beta$. The numbers in
square brackets in Fig. 1 report the fraction of sources along the sequence,
and the fraction of core-dominated (CD) and Fanaroff-Riley II (FR-II) sources
in each spectral bin555We consider here as radio loud (RL) only powerful
jetted radio sources with radio-to-optical specific flux ratio $R\gtrsim 80$..
This excludes a population of radio-detected sources that are likely
associated with star formation, and are confined at the high-$R_{\rm FeII}$
end of the main sequence (Bonzini et al., 2015; Caccianiga et al., 2015; Ganci
et al., 2019). CD and FR-II sources show systematic differences, in the sense
that CD sources show stronger Feii emission and narrower Balmer line
(Osterbrock, 1977; Miley and Miller, 1979; Zamfir et al., 2008).

Figure 1: Left: Sketch of the optical plane of the MS, FWHM H$\beta$ vs.
$R_{\rm FeII}$. Numbers in square brackets provide, from top to bottom, the
full sample fractional occupation, the fraction of RL (CD + FR-II) sources in
each spectral bin. Right: Same section of the optical plane, but with a rough
indication of the FWHM domain spanned by FR-I, FR-II, and CD along with the
location of the RL NLSy1 galaxies considered in this study (dark green area).

In this small contribution we focus the attention on the Feii optical spectrum
of type 1 AGN, and we first provide a summary description of the optical Feii
emission in the spectral region of H$\beta$ sample (Sect. 2). The aim is not
to solve the still-open problem of the Feii emission in type-1 AGN (Panda et
al., 2018, 2019), but rather to establish whether the optical Feii emission
might be different in radio-quiet (RQ) and RL AGN. We then present an overview
of the sample used for the analysis of the differences between RQ and RL
sources, keeping the distinction between CD and FR-II among jetted sources.
The comparison is carried out within the MS context, namely comparing sources
that are RQ and RL belonging to the same spectral type (§3), considering a
“solid” Feii template (i.e, a template with fixed relative line intensities),
and a template in which the relative multiplet strengths are free to vary
(§4). The results (§5) confirm a substantial equality for the Feii emission,
within the non-trivial constraints imposed by signal-to-noise ratio (S/N) and
resolution. Possible implications for the line emitting region structure in RQ
and RL are briefly discussed, also with the help of photoionization
computations (§6).

##  2 The Feii emission lines

The main Feii optical lines in the spectral region between 4400 Å and 5600 Å
are associated with 5 main multiplets. It is expedient to distinguish 3 main
features:

  * •

the blend on the blue side of H$\beta$, made up of lines from the m37 and m38,
and usually referred to as Feii$\lambda$4570, whose intensity is the Feii
measure that enters in the definition of $R_{\rm FeII}$ and applied in most
papers following Boroson and Green (1992).

  * •

m42, with two lines appearing as satellite lines of
[Oiii]$\lambda\lambda$4959,5007. The line at $\lambda$5018 might be affected
by contamination from He i$\lambda 5016$, and is strongly affected also by the
red wing of [Oiii]$\lambda$5007\. Given these difficulties the m42 lines were
included in the fits but no result concerning this multiplet is considered;

  * •

the lines of multiplets m48 and m49 that provide the bulk of the emission of
the Feii blend on the red side of H$\beta$ (referred to as Feii$\lambda$5270
or as the Feii red blend).

The transitions giving rise to these lines are schematically shown in the
highly-simplified Grotrian diagram of Fig. 2 (Phillips, 1978; Osterbrock and
Ferland, 2006; Pradhan and Nahar, 2015).

Figure 2: Highly simplified Grotrian diagram of the energy levels leading to
the strongest Feii transitions in the spectral range 4400 Å — 5600 Å.

The observed Fe iiopt emission is produced by a combination of collisional and
resonance-fluorescence processes (Verner et al., 1999, 2004; Sigut and
Pradhan, 1998, 2003; Pradhan and Nahar, 2015). Collisional excitation,
resonance fluorescence, in which a line or continuum photon is absorbed in a
line transition, both play an important role. Ly$\alpha$ fluorescence can
populate the upper levels of Fig. 2 via population of much higher energy level
at $\approx 10$ eV, producing a cascade with two branches leading to the $z$
${}^{6}D$,${}^{6}F$,${}^{6}P$,${}^{4}D$, ${}^{4}F$, and ${}^{4}P$ terms
between 4.8 and 5.6 eV responsible for the optical emission between 4500 and
5600 Å. Collisional excitation, albeit significant (Phillips, 1978), alone
cannot explain the emission of the stronger Feii emitters.

##  3 Sample

This paper considers two samples of broad-line, type-1 sources:

  * •

a sample of bright quasars from the Sloan Digital Sky Survey (SDSS) database
in the redshift range $0.4\lesssim z\lesssim 0.7$ (Marziani et al., 2013,
2013). Composite median spectra were extracted for the RQ, CD and FR-II radio
classes. A significant number of RL sources ( out of the 680 quasars of the
original sample) is found only in the spectral bins A1, B1, B1+: 8, 16, 9 CDs
and 10, 23, 11 FR-IIs in each bin, respectively. All sources (save a
borderline one) in the sample that are RL with extended emission satisfy the
power criterion at 1.5 GHz $\log P\nu\gtrsim 31.7$ erg s-1 Hz-1 that separate
FR-II sources from the lower luminosity FR-I (de Young, 2002; Kembhavi and
Narlikar, 1999).666Both FR-I and FR-II sources are characterized by extended
radio emission. FR-Is however show lower radio surface brightness toward the
outer extremities of the lobes, at variance with FR-II sources that are often
described as “edge brightened.” According to the unification schemes (Urry and
Padovani, 1995), both classes are observed at relatively high inclination.
Their optical spectra can be of both type 1 and type 2 AGN. In the last bin,
however, the Feii intensity is consistent with 0, so that only sources in bin
A1 and B1 are considered. In addition, in A1 FR-II the Fe iiopt emission is
too weak for a meaningful analysis.In conclusion, our bright quasar sample
belongs in the following composite spectra: A1 RQ, A1 CD, B1 RQ, B1 CD, B1 FR-
II;

  * •

three individual relativistically jetted NLSy1s with $\gamma$-ray detections,
that show more prominent Feii emission than the composites. The $\gamma$-ray
detection supports the presence of a relativistic jet as the origin of the
radio power for these sources (Komossa et al., 2006). They were selected
because high-quality optical spectra were available for the three of them.

The three $\gamma$-ray sources are 1H 0323+342 ($\equiv$ B2 0321+33 $\equiv$
1H 0323+342 $\equiv$ J03246+341), 3C 286 ($\equiv$J13311+305), and PKS
2004-447 ($\equiv$J20079–445). J03246 is the closest $\gamma$-ray NLSy1
($z=0.063$), hosted either by a spiral or an interacting late-type galaxy
(Zhou et al., 2006; Antón et al., 2008), and it is the only known
$\gamma$-NLSy1 showing a Fe K$\alpha$ emission line in X-rays (Kynoch et al.,
2018). This object was detected by the Fermi Gamma-ray Space Telescope soon
after its launch (Abdo et al., 2009). J13311 is a rather high redshift
($z\approx 0.850$) object, identified as a $\gamma$-ray source in all Fermi
catalogs (e.g., (Ackermann et al., 2015)). Originally it was classified as a
compact steep-spectrum (CSS) source based on its radio morphology (Peacock and
Wall, 1982), and later as a NLSy1 from optical spectroscopy (Berton et al.,
2017; Yao and Komossa, 2021). Also J20079 ($z\approx 0.240$) is classified as
both a CSS and a NLSy1 (Oshlack et al., 2001; Gallo et al., 2006; Berton et
al., 2021) and, as J03246, it belongs to the first batch of NLSy1s detected by
Fermi (Abdo et al., 2009). The spectra of these three sources were obtained
using the Asiago 1.22m Telescope for J03246 (full description of the data
reduction in Foschini et al. (2015)), the SDSS archive for J13311, and the
FORS2 instrument on the Very Large Telescope for J20079–445 (Berton et al.,
2021).

##  4 Analysis

The analysis is mainly empirical and, as mentioned, there is no pretension to
solve the problem of the optical Feii emission in AGN. Two approaches are
followed in the measurement of the relative intensity of the multiplets:

  * •

the modeling of the spectrum using a "solid" Feii template, scaled and
broadened to minimize the $\chi^{2}$ in a multi-component fit. It is basically
the one of Boroson and Green (1992) actualized with a higher resolution
spectrum and a model of the Feii emission underlying H$\beta$ (Marziani et
al., 2009). In this case, a multi-component fit was carried out including all
known emission components, as detailed in several recent works (Sulentic et
al., 2015; Negrete et al., 2018; Marziani et al., 2019). Specifically, the
redward asymmetric H$\beta$ has been modeled by the use of three components -
a narrow, a broad and a very broad component (Wolf et al., 2020, and
references therein). After verification that the host galaxy spectral emission
is not contributing significantly, the local continuum was fit with a power
law;

  * •

the use of a "liquid" template that permits to change the relative intensity
of the multiplets in the optical spectral range (Kovačević et al., 2010;
Kovačević-Dojčinović and Popović, 2015) 777http://servo.aob.rs/FeII_AGN/. In
this case, the continuum subtracted spectrum was fit to a set of variable
multiplets and to H$\beta$ and [Oiii]$\lambda$5007 profiles that were
approximated as Gaussians.

##  5 Results

###  5.1 Feii emission comparison between RQ and RL

Figure 3: From top to bottom: Non-linear $\chi^{2}$ multi-component analysis
with a solid Feii template of RQ, CD, and of the residuals in Feii emission
for composites of spectral type A1. In the bottom panel the brown line traces
the difference in the template (positive, since Feii is stronger in RQ than in
CD). The black filled lines traces the difference between the observed Feii
emission after rescaling of the CD Feii to obtain a minimum average $\sim 0$.
The dashed line traces the relative difference, i.e., (Feii RQ - Feii CD)/Feii
RQ. Wavelengths of multiplet lines are from Moore (1945).

The results of the application of the multi-component fit with the solid Feii
template are shown in Figs. 3 and 4 for the spectral types A1 and B1,
respectively. In Fig. 3, the only comparison we show is between A1 RQ and A1
CD sources, as there is no A1 FR-II composite. In Fig. 4, the left panel shows
the comparison between B1 RQ and B1 CD composites, while on the right the B1
RQ composites are compared to B1 FR-II. The immediate result is that RL (CD
and FR-II) composites show weaker Feii emission (Zamfir et al., 2008). The
second immediate result (already reported in Marziani et al., 2003) is the
similar appearance of the H$\beta$ profiles. The H$\beta$ profiles of Figs. 3
and 4 confirm that the prominent redward asymmetry is not a prerogative of RL
sources, but is a common feature in both RQ and RL sources, provided that they
belong to Population B (Marziani et al., 1996; Sulentic et al., 2008; Punsly,
2010).

The bottom panels of Figs. 3 and 4 provide constraints on the nature of the
difference between Feii emissions in the RQ and RL composite spectra. The
brown line traces the difference in the templates, which is positive since
Feii is stronger in RQ than in CD and FR-II as well. The black filled lines
traces the difference between the observed Feii emission after rescaling of
the RL Feii emission by a factor $k$ to obtain an average of 0. The difference
is computed between the observed Feii spectra, as follows:

$\delta\mathrm{Fe}\textsc{ii}_{\lambda,\mathrm{i}}=f_{\lambda,\mathrm{rq}}-(\mathcal{M}_{\lambda,\mathrm{rq}}-\mathcal{F}e_{\lambda,\mathrm{rq}})-k_{\mathrm{i}}\cdot[f_{\lambda,\mathrm{i}}-(\mathcal{M}_{\lambda,\mathrm{i}}-\mathcal{F}e_{\lambda,\mathrm{i}})]$
(1)

$\frac{\delta\mathrm{Fe}\textsc{ii}_{\lambda,\mathrm{i}}}{\mathcal{F}e_{\lambda,\mathrm{rq}}}=\frac{f_{\lambda,\mathrm{rq}}-(\mathcal{M}_{\lambda,\mathrm{rq}}-\mathcal{F}e_{\lambda,\mathrm{rq}})-k\cdot[f_{\lambda,\mathrm{i}}-(\mathcal{M}_{\lambda,\mathrm{i}}-\mathcal{F}e_{\lambda,\mathrm{i}})]}{\mathcal{F}e_{\lambda,\mathrm{rq}}}$
(2)

where the index $i$ refers to CD or FR-II, the $f_{\lambda}$ to the observed
flux as a function of wavelength, and the calligraphic symbols refer to model
spectrum ($\mathcal{M}_{\lambda}$) and model Feii ($\mathcal{F}e)$ via the
scaled and broadened template. Model parameters were obtained with a multi-
component, non-linear minimum $\chi^{2}$ technique, as implemented in IRAF
(Kriss, 1994).

The basic results of the spectral fits are reported in Table 1. The Table
columns list, in the following order, the spectrum identification, the rest-
frame equivalent width in Å for Feii$\lambda$4570, $R_{\rm FeII}$, FWHM
H$\beta$, the spectral type defined on the basis of the reported $R_{\rm
FeII}$ and FWHM H$\beta$, and the rescaling factor $k$.

Table 2 lists the average of the absolute difference between the observed iron
blends and the solid Feii template. The averages are computed over the full
ranges 4434 Å – 4684 Å (hereafter indicated as B blend or Feii$\lambda$4570),
and 5100 Å – 5600 Å (hereafter R blend) also used for the CLOUDY
photoionization simulations (Sect. 6.1). The next columns yield the average
and relative absolute differences between RL and RQ after rescaling, for the
restricted spectral region $\tilde{B}$ between 4500 and 4590 Å (roughly
corresponding to multiplets 38 and 39) and $\tilde{R}$ (between 5200 and 5330
Å, with the strongest features of m48 and m49). The reason of this restriction
is to avoid the contamination by Heii on the blue side of H$\beta$. The
Heii$\lambda$4686 line can have a significant effect and its estimation is
based only from fit.

Figure 4: In the left panel, comparison between RQ and CD composites of spectral type B1. In the right panel, comparison between B1 RQ, the same as in the left panel, and B1 FR-II. Bottom panels as in Fig. 3.  Table 1: Basic results  Spectrum |  W(Feii$\lambda$4570) | $R_{\rm FeII}$ |  FWHM H$\beta$ | ST | $k$
---|---|---|---|---|---
Composite spectra
A1 RQ | 31.6 | 0.291 | 3230 | A1 | …
A1 CD | 16.2 | 0.397 | 3290 | A1 | 1.815
B1 RQ | 44.1 | 0.414 | 6940 | B1 | …
B1 CD | 26.0 | 0.281 | 7060 | B1 | 1.657
B1 FR-II | 21.5 | 0.184 | 6790 | B1 | 1.900
Feii template | … | … | 990 | A | …
$\gamma$ RL NLSy1s
1H 0323+342 | 50.1 | 1.03 | 1280 | A3 | …
3C 286 | 33.5 | 0.792 | 3170 | A2 | …
PKS 2004-447 | 36.7 | 0.5455 | 1470 | A2 | …
Table 2: Feii results

Spectrum | $\overline{f_{\lambda}-{\mathcal{M}_{\lambda}}(B)}^{a}$ | $\overline{f_{\lambda}-{\mathcal{M}_{\lambda}}(R)}^{b}$ | $\overline{\delta f_{\lambda}(\tilde{B})}^{c}$ | $\overline{\delta f_{\lambda}/f_{\lambda}(\tilde{B})}^{c}$ | $\overline{\delta f_{\lambda}(\tilde{R})}^{c}$ | $\overline{\delta f_{\lambda}/f_{\lambda}(\tilde{R})}^{c}$ |
---|---|---|---|---|---|---|---
Composite spectra |
A1 RQ |  0.00146 $\pm$ 0.00942 |  0.00257 $\pm$ 0.01333 | … | … | … | … |
A1 CD |  0.00014 $\pm$ 0.01265 |  -0.00117 $\pm$ 0.01671 | 0.00244 | -9.986E-05 | 0.0146 | -0.00354 |
B1 RQ |  0.00149 $\pm$ 0.00665 |  0.00277 $\pm$ 0.00982 | … | … | … | … |
B1 CD |  -0.00188 $\pm$ 0.01698 |  9.44E-05$\pm$ 0.01745 | 0.01420 | 0.07467 | -0.00403 | -0.02626 |
B1 FR-II |  0.00297 $\pm$ 0.01554 |  0.0047$\pm$ 0.01574 | 0.03138 | 0.15740 | -0.00294 | -0.01834 |
$\gamma$ RL NLSy1s |
1H 0323+342 |  0.00509 $\pm$ 0.05126 |  0.00949 $\pm$ 0.04280 | -0.00598 | -0.0388 | 0.02072 | 0.1297 |
3C 286 |  -0.00262 $\pm$ 0.03654 |  0.005087 $\pm$ 0.05126 | -0.00304 | -0.0254 | 0.00516 | 0.0475 |
PKS 2004-447 |  -0.00145 $\pm$ 0.02275 |  0.005514 $\pm$ 0.02115 | -0.0008 | -0.0149 | 0.00866 | 0.1117 |

a: Averages of the normalized flux difference between the continuum subtracted
spectrum and the solid template in the range 4434–4684 Å; b: same, in the
range 5100–5600 Å; c: absolute and relative average difference following Eq. 1
and Eq. 2, for the restricted B and R ranges, respectively.

In the case of spectral type A1, the comparison between RQ and CD relative
intensities of the multiplets is shown in the panels of Fig. 3. On the blue
side, the $\delta\mathrm{Fe}\textsc{ii}_{\lambda,\mathrm{i}}$ is consistent
with 0 over the range where most of the emission due to multiplets 37 and 38
is expected. Similar considerations apply to the red side emission; the main
fluctuations according to Eq. 2 occur when Feii emission is weak.

The comparison for the composite spectra of bin B1 is shown in Fig. 4. On the
blue side of H$\beta$ in correspondence of the m37 and m38 emission there
could be a slight excess for the CD composite, accompanied by a deficit on the
red side of H$\beta$ where emission is associated with multiplets m48 and m49.
The average relative difference is a few positive and negative percents for
the B and R regions, respectively (Table 2). Accepted as real the absolute
difference is constrained within $\lesssim 3$ %, and the relative difference
is contained within $\pm$ 20 %. The $\approx 3$% average excess translates
into an error in terms of $\delta$$R_{\rm FeII}$$\approx 0.03$, and in $W$
Feii$\lambda$4570 the difference is also $\approx$ 3 Å implying that there is
no effect on the placement in the MS optical plane. To estimate the
measurement errors on $R_{\rm FeII}$ for the same S/N, equivalent width and
FWHM of the lines, we used the quality parameter defined in an unrelated
investigation, and derived from Monte Carlo simulations estimates of the
errors for H$\beta$ and $R_{\rm FeII}$. The errors on Feii are the dominant
one, and are $\delta$$R_{\rm FeII}$$\lesssim$ 0.05 for the RQ composites and
$\delta$$R_{\rm FeII}$$\approx$ 0.10 for the radio-loud ones.

Figure 5: Non-linear $\chi^{2}$ multi-component analysis with a solid Feii
template (green) of three jetted NLSy1s with $\gamma$-ray detections. The
spectra are shown after continuum subtraction. The thin filled line traces the
observed spectrum. The dashed magenta line represents the model, and the thick
black line the H$\beta_{\rm BC}$. The [Oiii]$\lambda\lambda$4959,5007 is shown
in yellow. The lower panel show the residuals of the subtraction of the model
from the observed spectrum. From left to right panel, J03246+341, J13311+305,
and J20079-445.

###  5.2 $\gamma$-ray detected RL NLSy1s

We apply the same fitting techniques to three relativistically jetted NLSy1s
with $\gamma$-ray detection. Fig. 5 shows that the solid template used for the
fitting of the composites is providing a good agreement, in a case with higher
values of $R_{\rm FeII}$ and much narrower lines that is better posed to
appreciate possible differences. The fits are successful, and the residuals
consistent with 0, with average systematic differences are $\lesssim$ 1 %, and
the rms scatter is $\lesssim 5$% of the normalized continuum value at 5100 Å.

Table 3: “Liquid” Feii results

Spectrum |  B/Ra | $\tilde{B}/\tilde{R}^{b}$ | $\frac{m37+m38z}{m48+m49}^{c}$ | $\tilde{B}_{\mathrm{k}}/\tilde{R}_{\mathrm{k}}^{d}$ | $\overline{\tilde{B}-\tilde{B}_{\mathrm{k}}}^{e}$ | $\overline{\tilde{R}-\tilde{R}_{\mathrm{k}}}^{e}$ |
---|---|---|---|---|---|---|---
Composite spectra |
A1 RQ | 0.993 | 0.994 | 0.82 | 0.974 | 0.00447 | -0.0182 |
A1 CD | 1.141 | 1.045 | 0.72 | 0.942 | 0.0117 | 0.0075 |
B1 RQ | 0.884 | 0.891 | 0.97 | 0.899 | 0.00934 | 0.00419 |
B1 CD | 0.666 | 0.698 | 0.79 | 0.699 | -0.0375 | -0.0198 |
B1 FR-II | 0.685 | 0.726 | 0.65 | 0.890 | -0.051 | -0.0169 |
Feii template | 0.792 | 0.979 | 0.65 | 0.967 | 0.00264 | 0.00813 |
$\gamma$ RL NLSy1s |
1H 0323+342 | 0.743 | 0.780 | 0.59 | 0.765 | 0.0048 | -0.00020 |
3C 286 | 0.821 | 0.905 | 0.78 | 0.867 | 0.00591 | -0.00218 |
PKS 2004-447 | 0.980 | 0.882 | 0.62 | 0.882 | 0.00093 | -0.0000466 |

a: B/R ratio measured on the observed spectrum; b: same as in the previous
column, but $\tilde{B}/\tilde{R}$ in the restricted range; c: ratio between
the sum of m37+m38 and m48+m49 intensity obtained with the fits following
Kovačević et al. (2010). Note that the ratio includes only the sums of these
four multiplets, while both the standard and the restricted range include
additional Feii emission as well as a possible residual contribution of Hei
and Heii lines. d: ratio between the Feii emission of the B and R restricted
range computed with the simulations of Kovačević et al. (2010). e: absolute
average difference for the restricted B and R ranges, respectively, using the
total  Feii emission following Kovačević et al. (2010).

###  5.3 Models with relative intensity of the multiplets free to vary

We considered first the measurements based on the continuum-subtracted spectra
in the full and restricted ranges (second and third columns of Table 3), and
we utilized the web-based fitting
tool888http://servo.aob.rs/FeII$\\_$AGN/uploads.php for implementing the
scheme of Kovačević et al. (2010) for the optical Feii emission. The
measurement of the ratio between the sum of the m37+m38, and m48+m49 blends is
reported in the fourth column of Table 3. The fifth column reports the ratio
$\tilde{B}_{\mathrm{k}}/\tilde{R}_{\mathrm{k}}$ measured on the full spectral
model of Kovačević et al. (2010). The last two columns report the average
differences between the observed and the Kovačević et al. (2010) modelling for
the restricted ranges $\tilde{B}_{\mathrm{k}}$ and $\tilde{R}_{\mathrm{k}}$,
respectively, and are meant to provide an estimate of how well the model Feii
reproduces the actual observations. The agreement is very good with all cases
having the average difference less than 0.05, and most less than 0.01. The
same procedure was also applied to the solid Feii template used in the
previous analysis (Fig. 6). The values reported in Table 3 explain why the
solid Feii template provides satisfactory results in all cases, and especially
in the RQ cases.

We stress three main results:

  * •

There is a significant difference (by a factor $\approx$2) between RQ and RL
sources in the same spectral types (meaning similar mass, Eddington ratio
$\lesssim$ 0.1, and luminosity).

  * •

The B/R ratio for the RQ A1 and B1 spectral types is $\approx 0.9-1.1$ (Fig. 7
and Table 3).

  * •

The various measurements of the blue and red blend ratios suggest a somewhat
higher values for the A1 and B1 RQ than for the RL sources ($\approx 0.9-1\
vs\ 0.7$). The effect is not strong: yet, it is apparent especially in Fig. 7.

It is advisable to consider that there could be effects not related to the
estimation of the Feii intensity. The main reason could be a difference in the
intensity of Heii$\lambda$4686, even if Heii$\lambda$4686 is included in the
fit. Other possibilities involve internal extinction (Gaskell, 2017; Baskin
and Laor, 2018) affecting the Feii emitting region, limb-darkening (Marziani
et al., 2001), incorrect continuum placement, contamination by lines other
than Heii, and blueshifted H$\beta$ emission. To overcome at least the problem
of the Heii$\lambda$4686 uncertain contribution, and of the contribution of
the H$\beta$ wings, we defined a restricted range for B and R, $\tilde{B}$ and
$\tilde{R}$. The effect is however not significantly reduced (Table 2).

If we apply the same technique to the $\gamma-$detected sources, the intensity
ratio between B and R is closer to the value of the template (Fig. 8 and Table
3) but not for 1H 0323+342. The values reported in Table 3 confirm that the
template is providing a good approximation for the RQ sources in bin A1 and
B1. This is likely true also for spectral bins with higher $R_{\rm FeII}$ (A2
and A3), as the template is based on the spectrum of I Zw 1, a NLSy1 accreting
at a high rate and especially emitting strong Feii, with $R_{\rm FeII}$
slightly larger than 1, and so of spectral type A3999I Zw 1 is however not a
very extreme object: the most extreme accretors show spectra similar to the
one of PHL 1092 (Marinello et al., 2020).. At the other end of the MS, Feii
emission is faint and systematic differences might be not appreciable. So the
issue of a possible, significant disagreement concerns only $\approx 3$% of
all type 1 AGN.

Figure 6: Non-linear $\chi^{2}$ multi-component analysis of the adopted solid
Feii template with relative multiplet intensity free to vary. The blue solid
line represents the B blends, while the red solid line indicates the R blends.

In summary, the use of a solid Feii template appears fully justified in this
work. There could be a genuine effect associated with radio-loudness, in the
sense of a less prominent blue blend with respect to the red one.

##  6 Discussion

The composites that we have considered in the present work all show modest
Feii emission and satisfy the condition that $R_{\rm FeII}$$\lesssim 0.5$. In
principle, on the context of photoionization, they do not pose a serious
challenge to the conventional view of the of the BLR as a system of emitting
clouds characterized by typical densities $n_{\mathrm{H}}\sim 10^{9.5}$ cm-3,
and column density $N_{\mathrm{c}}=10^{23}$ [cm-2] (Davidson and Netzer,
1979). Indeed, physical parameters similar to these $n_{\mathrm{H}}\sim
10^{10-10.5}$ cm-3, along with metallicity solar or slightly sub-solar, and a
flattish spectral energy distribution (SED) (Laor et al., 1997; Korista et
al., 1997), can account for the emission in the $R_{\rm FeII}$ range of
spectral bins A1 and B1, at BLR distances from the continuum sources
consistent with their luminosity (Panda et al., 2019).

Figure 7: Non-linear $\chi^{2}$ multi-component analysis with a Feii template
with relative multiplet intensity free to vary. Colors as in Fig. 6.

Figure 8: Non-linear $\chi^{2}$ multi-component analysis with a Feii template
with relative multiplet intensity free to vary, from left to right for the A1
RQ vs. A1 CD, B1 RQ vs. B1 CD, B1 RQ vs. B1 FR-II composites. The bottom
panels show the ratios between the B, m42, and R for the three cases. Colors
as in Fig. 6.

Thanks especially to the results of reverberation mapping and Marinello et al.
(2016), the emitting region radius was found to be a factor of 10 smaller than
previously thought Peterson et al. (2004); Peterson (2017). The conventional
view outlined in the previous paragraph may be only partially valid in
Population A sources, as it is unable to account for the strong Feii emission
in quasars Collin-Souffrin and Dumont (1989); Collin and Joly (2000).
Different physical conditions are required for sources with stronger Feii
emission: higher densities are needed to maintain the ionization parameter
within reasonable limits. In spectral type A3, $n_{\mathrm{H}}\sim 10^{12}$
cm-3 (strong Feii emission appears possible only if $n_{\mathrm{H}}\gtrsim
10^{10.5}$ cm-3, (Matsuoka et al., 2008; Martínez-Aldama et al., 2015; Panda
et al., 2018)). High column density and super-solar metallicity provide a
further boost to the Feii intensity and to the strength of the metal lines in
the UV (Panda et al., 2018; Śniegowska et al., 2021).

###  6.1 Photoionization computations

Figure 9: The SEDs used for photoionization computations. From top to bottom:
1) SED of RL and RQ sources from Laor et al. (1997), represented by the blue
and red solid line, respectively, and from Korista et al. (1997) for RQ shown
with black solid line. The y-axis for the upper panel is shown with arbitrary
units (Ar. Un.) for the specific flux; 2) SED of J03426. The observed data
points are shown in grey, while the SED used in the models is represented by
the black solid line, 3) as in (2) for J13311; 4) as in (2) for J20079.

To ascertain the role of the SED in the strength of Feii we performed
exploratory calculations of the RQ and RL differences, as well as for the
$\gamma$-detected NLSy1s. As a first attempt, we assumed a column density
$N_{\mathrm{c}}=10^{23}$ cm-2, solar metallicity, and 0 micro-turbulence. We
focused on the exploration of the parameter ranges of the density
$n_{\mathrm{H}}$ ($\sim 10^{9-11}$ cm-3), and of BLR radius within the range
expected from the Bentz et al. (2013) scaling law, corrected for high-
accretion rates ((Martínez-Aldama et al., 2019; Du and Wang, 2019); see also
(D’Onofrio et al., 2021)). We utilized the SEDs from Laor et al. (1997) and
Korista et al. (1997), which are most appropriate for bins A1 and B1, for RQ
and RL separately (top panel of Fig. 9). The two RQ SEDs most likely bracket
the distribution of SEDs in the two bins, as also assumed in previous work
(Panda et al., 2019). For the composites, we consider a fixed luminosity
$\lambda L_{\lambda}(5100$ Å) $\approx 10^{45}$ erg s-1, consistent with the
values derived for the original sample (Marziani et al., 2013). For RL NLSy1s,
we built a specific SED for each one of them by using the multi-frequency data
of the SED builder available at the Space Science Data Center of the Italian
Space Agency101010https://tools.ssdc.asi.it/SED/. The results are shown in the
second, third, and fourth panel of Fig. 9.

###  6.2 Composites RL and RQ

Fig. 10 shows the expected $R_{\rm FeII}$, the ratio between the red blend R
and H$\beta$, and the ratio B/R for the RQ and RL SEDs, as a function of
$r_{\rm BLR}$ and $n_{\mathrm{H}}$. As mentioned above, the range in $r_{\rm
BLR}$ has been centered on the value expected from the correlation $r_{\rm
BLR}$ \- $L$ by Bentz et al. (2013). The density range includes the most
likely values for the BLR along the MS. Typical values are lower for
Population B, and much higher for Population A. (Marziani et al., 2001, 2018).

Figure 10: Results of CLOUDY simulations. First two rows: W(Feii$\lambda$4570)
for RQ and RL (i.e., computed for the same physical parameters but with the
two different SEDs of Laor et al. (1997)) and their ratios; third and fourth
row from top: $R_{\rm FeII}$, for RQ and RL and their ratios. Last four rows:
same, for the red (R) Feii blend peaked at $\lambda$ 5312 Å.

The most important result is that the SED for RL sources is able to account
for the modest $R_{\rm FeII}$ and W(Feii$\lambda$4570) reported in Table 1.
The observed equivalent width $\sim 10-20$ Å and $R_{\rm FeII}$$\sim$ 0.15 are
explained at the $r_{\rm BLR}$ expected from the $r_{\rm BLR}$ – $L$ relation,
and by the moderate density $n_{\mathrm{H}}$$\sim 10^{11}$ cm-3. The result
seems to be especially robust against changes in $n_{\mathrm{H}}$ and $r_{\rm
BLR}$ (Fig. 10, middle column). Restricting the ranges to $\tilde{B}$ and
$\tilde{R}$ confirms this conclusion (Fig. 1; the result of the analysis for
the restricted range are shown in Appendix A).

Regarding the interpretation of the composite spectra, a second important
issue is whether the difference in RQ and RL SED can explain the differences
in their Feii strength. We stress that the comparison is being carried out
between sources that are in the same spectral types, meaning that they have
similar $M_{\rm BH}$, $L$, and Eddington ratio. The $R_{\rm FeII}$ computed
over a large area of the parameter plane $n_{\mathrm{H}}$ – $r_{\rm BLR}$
shows differences that are at most 0.05, i.e., less than $15$% (Fig. 10). This
has the important implication that the parameter $k\approx 2$, i.e., $R_{\rm
FeII}$ twice as strong in RQ than in RL AGNs, cannot be explained on the basis
of the SED only. However, the RQ SED that we adopt is not necessarily
equivalent to the one of the sources used to construct the Population A and B.
Using the SED from Korista et al. (1997) that has a stronger big blue bump
(implying for fixed $r_{\rm BLR}$ and $n_{\mathrm{H}}$ an increase in
ionization parameter by a factor 3) cannot lead to a higher $R_{\rm FeII}$
(Fig. 11 and Fig. 2).

Figure 11: Results of CLOUDY simulations, same as in Fig. 12 but for the RQ
SED of Korista et al. (1997).

The second issue investigated in the present paper is the possibility of a
systematic difference in the B/R Feii ratio. This result should be viewed with
care, and confirmatory data are needed, also considering that we are dealing
mainly with weak Feii emitters. The agreement between the observed and
photoionization-predicted B/R is good, and in most cases the B/R $\sim$ 1,
with little dependence on the $r_{\rm BLR}$ and $n_{\mathrm{H}}$ parameters.
Fig. 12 (Fig. 3 for the restricted ranges) shows that there is a possible
lowering of the B/R ratio in the B1 RL (both CD and FR-II) with respect to the
RQ. Accepted at face value, it might imply a lower density, and a higher
ionization parameter for the RL sources (Fig. 12), although Figs. 10 and 11
show that this implies fainter Feii emission than what is expected for the
typical values of $r_{\rm BLR}$.

Sulentic et al. (2016) speculated that the cocoon associated with the shock
wave due to the expansion of the jet could push outward the line emitting
regions. It is known that the Civ high-ionization line reaches lower blueshift
amplitudes in RL than in RQ sources (Sulentic et al., 2007; Richards et al.,
2011). The present paper confirms that an effect on the low-ionization lines
is minor. Considering the layout of the atomic levels in the Grotrian diagram
of Fig. 2, the lower B in radio loud sources could be due a lower electron $T$
in RL than in RQ, which could be, according to Fig. 12, in turn associated
with a lower $U$ and a larger distance.

Figure 12: Results of CLOUDY simulations: the three panels yield the ratio
$R_{\rm FeII}$ B to $R_{\rm FeII}$R (in practice B/R) as a function of density
and BLR radius, for the RQ Korista et al. (1997) (leftmost), the RQ Laor et
al. (1997) (middle) and RL SED (rightmost).

Figure 13: Results of CLOUDY simulations for the 1H 0323+342 SED: the two top
panel yields the equivalent width of Feii$\lambda$4570 and the two middle
panels $R_{\rm FeII}$ for the blue and red blends, as a function of density
and BLR radius. The bottom panel shows the ratio of the blue and red Feii
emission.

###  6.3 $\gamma$-detected NLSy1s

The photoionization calculations performed with the parameters appropriate to
model 1H 0323+342 (Fig. 13, repeated in Appendix A for the restricted range as
Fig. 4) reveal that is not possible to achieve $R_{\rm FeII}$$\sim 1$ with the
SED shown in Fig. 9. This raises an important issue about the Feii emission in
these jetted sources. A change in SED within the limit of Population B, such
as the one provided by Korista et al. (1997) is likely not enough to explain
the increase in Feii. This leaves several other possibilities to account for
this intrinsic difference between jetted and non-jetted sources. They can be
loosely grouped in "evolutionary" (chemical evolution of the host and of the
circumnuclear regions, in turn related to the host morphology), and
"intrinsic" (density, column density, ionization degree, covering fraction of
the line emitting gas). The host morphologies of RL and RQ are systematically
different, and recent work has validated the long-held paradigm that RL are
hosted in earlier morphological types than RQ (Kozie$\lambda$-Wierzbowska et
al., 2017a, b). This result might not be applicable to NLSy1s, and especially
to the ones that are moderate-to-strong Feii emitters. Basically all of the
recent studies on the host of jetted RL NLSy1s (Järvelä et al., 2018; Berton
et al., 2019; Olguín-Iglesias et al., 2020) showed that they are in late-type
hosts. Merging is instead rather common, and it may constitute the biggest
difference with respect to RQ NLSy1. In particular, Olguín-Iglesias et al.
(2020) clearly pointed out on a rather large sample that disk galaxies
dominate among jetted NLSy1s.

The visual inspection of almost all known RL NLSy1s suggests that they never
reach the extreme $R_{\rm FeII}$ value obtained by RQ sources. This impression
– even if consistent with the difference found for weak Feii emitters – should
be validated by a systematic study that goes beyond the scope of the present
paper. Enrichment in chemical composition appears a likely possibility, as the
strongest Feii emitters are also believed to have the highest metal content
(Shin et al., 2013; Sulentic et al., 2014; Nagao et al., 2006; Śniegowska et
al., 2021). Photoionization analyses indicate that significant Feii emission
is associated with super-solar metallicity, in addition to high density and
high column density (Panda et al., 2018, 2019), although a complete
observational analysis with matching $R_{\rm FeII}$ and UV line diagnostics
for metallicity and physical parameters estimates along the main sequence is
still lacking.

##  7 Summary and conclusion

This work presented an analysis of the Feii emission in the spectral bins
along the main sequence where both RQ and RL coexist (A1, B1) with measurable
Feii strength, and a comparison between the emission of RQ and RL composite
template, along with the analysis of 3 RL sources found to have a stronger
Feii emission. The main results can be summarized as follows:

  * •

the template based on the I Zw 1 spectrum works equally well for RQ and RL
objects. Deviation in $R_{\rm FeII}$ and equivalent width measurements due to
differences between the template and the observed spectra have been found to
be $\delta$$R_{\rm FeII}$$\lesssim$0.03, and $\delta W\lesssim$ a few Å. In
particular, no correction is needed for the placement of RL in the MS built
from RQ-dominated samples.

  * •

The RL SED of Laor et al. (1997) can account for the modest emission in RL
sources, for conditions that are very likely in Population B objects (Panda et
al., 2019).

  * •

Somewhat surprisingly, SED differences between RQ and RL cannot account for
the stronger Feii emission in RQ111111Note that the equivalent width is
increased, as shown in Fig. 11 and Fig. 2 but, since there is a corresponding
increase in H$\beta$, the $R_{\rm FeII}$ is left unaffected.. We suggest that
other factors related to the evolutionary pattern of the circumnuclear regions
of the active nucleus should be investigated.

  * •

Last, the SED shape does not seem so important in determining the optical Feii
prominence with respect to H$\beta$ for such sources with intermediate
Eddington ratios ($\sim$0.1), as the change from the Laor et al. (1997) to the
Korista et al. (1997) SED produces no significant change in $R_{\rm FeII}$,
even if the number of ionizing photons is increased by a factor $\approx 3$.
We remark that, for sources at higher Eddington ratios, this might be
different, as has been tested and shown in Panda et al. (2019). Those authors
found that the SED shape matters for pushing the $R_{\rm FeII}$ higher,
especially when the contribution from a soft-X-ray excess is accounted for.

  * •

The ratio between the B and the R blend is found to be $\approx$ 0.9 – 1.0 and
to be consistent with the predictions of the photoionization computation. A
slightly lower value ($\approx 0.7$) might be possible for the B1 RL
composites. However, the measurement is excruciatingly difficult, considering
the uncertain influence of the Heii$\lambda$4686 emission that is known to be
very strong in cases where Feii is negligible (e.g., Marziani and Sulentic,
1993), and of several other factors that should play a lesser role. More
importantly, the B/R value around $\approx$ 1 is found also in higher
Eddington ratio sources which are stronger Feii emitters such as the 3
$\gamma$-detected NLSy1s considered in this study, all with Eddington ratio
$\approx$ 0.5, typical of Population A quasars. Since $R_{\rm FeII}$ is
correlated with Eddington ratio (Sun and Shen, 2015; Du et al., 2016), and the
B/R is apparently independent from $R_{\rm FeII}$, a lower B/R might be a
genuine radio loudness effect or, perhaps more likely, an effect dependent on
other RQ/RL sample differences.

In summary, we find that the Feii emission is stable, and we confirm that a
solid template based on I Zw 1 provides an accurate representation of the
optical Feii emission in the spectral region of H$\beta$ within the limit of
precision allowed by moderate dispersion spectroscopy.

All authors contributed equally to this paper.

###### Acknowledgements.

SP would like to acknowledge the financial support by the Polish Funding
Agency National Science Centre, project 2017/26/A/ST9/00756 (MAESTRO 9), and
from CNPq Fellowship (164753/2020-6). EB acknowledge the support of Serbian
Ministry of Education, Science and Technological Development, through the
contract number 451-03-68/2020-14/200002. Based on observations made with ESO
Telescopes at the La Silla Paranal Observatory under program ID 096.B-0256.
This work is partially based on observations made with the Galileo 1.22 m
telescope of the Asiago Astrophysical Observatory operated by the Department
of Physics and Astronomy “G. Galilei” of the University of Padova. Funding for
the Sloan Digital Sky Survey has been provided by the Alfred P. Sloan
Foundation, and the U.S. Department of Energy Office of Science. The SDSS web
site is http://www.sdss.org. SDSS-III is managed by the Astrophysical Research
Consortium for the Participating Institutions of the SDSS-III Collaboration
including the University of Arizona, the Brazilian Participation Group,
Brookhaven National Laboratory, Carnegie Mellon University, University of
Florida, the French Participation Group, the German Participation Group,
Harvard University, the Instituto de Astrofisica de Canarias, the Michigan
State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence
Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max
Planck Institute for Extraterrestrial Physics, New Mexico State University,
University of Portsmouth, Princeton University, the Spanish Participation
Group, University of Tokyo, University of Utah, Vanderbilt University,
University of Virginia, University of Washington, and Yale University. The
authors declare no conflict of interest. The following abbreviations are used
in this manuscript:
AGN | Active Galactic Nucleus
---|---
BLR | Broad Line Region
CD | Core dominated
FR-I | Fanaroff-Riley I
FR-II | Fanaroff-Riley II
FWHM | Full Width Half-Maximum
MS | Main Sequence
NLSy1 | Narrow-Line Seyfert 1
RL | Radio loud
RQ | Radio quiet
SDSS | Sloan Digital Sky Survey
SED | Spectral energy distribution
S/N | Signal-to-noise ratio

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The Appendix shows the results of the CLOUDY simulations in the case of the
restricted ranges 4500 – 4590 Å and 5200 – 5300 Å ($\tilde{B}$ and $\tilde{R}$
through the paper).

##  Appendix A Restricted range

Figure 1: Results of CLOUDY simulations. First two rows: W(Feii$\lambda$4570)
for RQ and RL (i.e., computed for the same physical parameters but with the
two different SEDs of Laor et al. (1997)) and their ratios; third and fourth
row from top: $R_{\rm FeII}$, for RQ and RL and their ratios. Last four rows:
same, for the red (R) Feii blend peaked at $\lambda$ 5312 Å.

Figure 2: Results of CLOUDY simulations, same as in Fig. 12 but for the RQ SED
of Korista et al. (1997).

Figure 3: Results of CLOUDY simulations: the three panels yield the ratio
$R_{\rm FeII}$ B to $R_{\rm FeII}$R (in practice B/R) as a function of density
and BLR radius, for the RQ Korista et al. (1997) (leftmost), the RQ Laor et
al. (1997) (middle) and RL SED (rightmost).

Figure 4: Results of CLOUDY simulations for the 1H0323+342 SED: the two top
panel yields the equivalent width of Feii$\lambda$4570 and the two middle
panels $R_{\rm FeII}$ for the blue and red blends, as a function of density
and BLR radius. The bottom panel shows the ratio of the blue and red Feii
emission.