# Chemical Abundance of the LINER galaxy UGC 4805 with SDSS-IV MaNGA

A.C. Krabbe,1 C. B. Oliveira Jr.,1 I. A. Zinchenko,2,3 J. A. Hernández-
Jiménez,4 O. L. Dors Jr.,1 G. F. Hägele,5,6 M. V. Cardaci,5,6 N. R. Telles1
1 Universidade do Vale do Paraíba, Av. Shishima Hifumi, 2911, Zip Code
12244-000, São José dos Campos, SP, Brazil
2 Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679
Munich, Germany
3 Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27
Akad. Zabolotnoho St 03680 Kyiv, Ukraine
4 Departamento de Ciencias Físicas, Universidad Andrés Bello, Fernández
Concha, 700, Las Condes, Santiago, Chile.
5 Instituto de Astrofísica de La Plata (CONICET La Plata–UNLP), Argentina.
6 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La
Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina
E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

Chemical abundance determinations in Low-Ionization Nuclear Line Regions
(LINERs) are especially complex and uncertain because the nature of the
ionizing source of this kind of object is unknown. In this work, we study the
oxygen abundance in relation to the hydrogen abundance (O/H) of the gas phase
of the UGC 4805 LINER nucleus. Optical spectroscopic data from the Mapping
Nearby Galaxies (MaNGA) survey was employed to derive the O/H abundance of the
UGC 4805 nucleus based on the extrapolation of the disk abundance gradient, on
calibrations between O/H abundance and strong emission-lines for Active
Galactic Nuclei (AGNs) as well as on photoionization models built with the
Cloudy code, assuming gas accretion into a black hole (AGN) and post-
Asymptotic Giant Branch (p-AGB) stars with different effective temperatures.
We found that abundance gradient extrapolations, AGN calibrations, AGN and
p-AGB photoionization models produce similar O/H values for the UGC 4805
nucleus and similar ionization parameter values. The study demonstrated that
the methods used to estimate the O/H abundance using nuclear emission-line
ratios produce reliable results, which are in agreement with the O/H values
obtained from the independent method of galactic metallicity gradient
extrapolation. Finally, the results from the WHAN diagram combined with the
fact that the high excitation level of the gas has to be maintained at kpc
scales, we suggest that the main ionizing source of the UGC 4805 nucleus
probably has a stellar origin rather than an AGN.

###### keywords:

galaxies:abundances – ISM:abundances – galaxies:nuclei

††pubyear: 2020††pagerange: Chemical Abundance of the LINER galaxy UGC 4805
with SDSS-IV MaNGA–Chemical Abundance of the LINER galaxy UGC 4805 with SDSS-
IV MaNGA

##  1 Introduction

Determinations of the chemical abundances of Active Galactic Nuclei (AGNs) and
Star-Forming regions (SFs) are essential for understanding the chemical
evolution of galaxies and, consequently, of the Universe.

Among the heavy elements present in the gas phase of AGNs and SFs (e.g., O, N,
S), oxygen is the element with more accurate abundance determinations. This is
because prominent emission-lines from the main ionic stages of oxygen can be
easily detected in the optical spectra of these objects, making it a good
tracer of the metallicity (e.g., Kennicutt et al. 2003; Hägele et al. 2008;
Dors et al. 2015, 2020a). Therefore, hereafter we use metallicity ($Z$) and
oxygen abundance [12 + $\log$(O/H)] interchangeably. Abundance estimations
based on the direct method, also known as $T_{\rm e}$-method, are commonly
used to determine chemical abundances of gas phase of SFs (for a review see
Peimbert et al. 2017; Pérez-Montero 2017). These estimations seem to be more
reliable than those derived using empirical or theoretical relations between
the different electron temperatures (Hägele et al., 2006, 2008). In fact, the
compatibility between oxygen abundances in nebulae located in the solar
neighborhood and the ones derived from observations of the weak interstellar O
i$\lambda$1356 line towards the stars (see Pilyugin 2003 and references
therein) sustains the accuracy of the $T_{\rm e}$-method. This method is based
on determinations of nebular electron temperatures, which requires
measurements of auroral emission-lines, such as [O iii]$\lambda$ 4363 and [N
ii]$\lambda$ 5755, generally weak (about 100 times weaker than H$\beta$) or
not measurable in objects with high metallicity and/or low excitation (e.g.,
van Zee et al. 1998; Díaz et al. 2007). In the cases that auroral lines can
not be measured, indirect or strong-line methods can be used to estimate the
oxygen abundance, as proposed by Jensen et al. (1976) and Pagel et al. (1979).
This method is based on calibrations between the oxygen abundance or
metallicity and strong emission-lines, easily measured in SF spectra (for a
review see López-Sánchez & Esteban 2010b; Maiolino & Mannucci 2019; Kewley et
al. 2019).

For AGNs, chemical abundance determinations are preferably carried out in
Narrow Line Regions (NLRs) of Seyfert 2 nuclei due to the relatively low
velocity ($v\>\la\>400\>\rm km\>s^{-1}$, Contini 2017) of the shock waves
present in the gas and their low electron density ($N_{\rm e}\>\la\>2000\>\rm
cm^{-3}$, Zhang et al. 2013; Dors et al. 2014; for a review see Dors et al.
2020a). Oxygen abundance estimations for NLRs of Seyfert 2 have been obtained
by using the $T_{\rm e}$-method (Alloin et al. 1992; Izotov & Thuan 2008; Dors
et al. 2015, 2020a) and strong-line methods (e.g., Storchi-Bergmann et al.
1998; Castro et al. 2017; Carvalho et al. 2020). Studies based on strong-line
methods have indicated that Seyfert 2 nuclei in the local universe
($z\><\>0.4$) present similar metallicity (or abundances) as those in metal
rich H ii regions, i.e., no extraordinary enrichment has been observed in
AGNs, with these objects exhibiting solar or slightly over-solar
metallicities. This result agrees with predictions of chemical evolution
models for spiral and elliptical galaxies (e.g., Mollá & Díaz 2005).

An opposite situation is found for Low-Ionization Nuclear Emission-line
Regions (LINERs), whose chemical abundance studies are rare in the literature.
This class of objects appear in 1/3 of galaxies in the local universe (Netzer,
2013), and their ionization sources are still an open problem in astronomy.
Heckman (1980) suggested that these nuclei have gas shocks as their main
ionization/heating source. Later, Ferland & Netzer (1983) proposed that LINERs
could be ionized by accretion gas into a central black hole (AGN) but with
lower ionization parameters (U) than those found in Seyferts. Therefore, the
difference between LINERs and other AGN types would consist of the order of
the ionization parameter (Ho et al., 1993). However, Terlevich & Melnick
(1985) and Shields (1992) proposed a new ionization model, i.e., LINERs are
ionized by hot stars, but contrary to SFs, they are old stars (0.1-0.5 Gyr)
that came out from the main sequence (e.g., in the post-Asymptotic Giant
Branch, p-AGB). Based on this scenario, Taniguchi et al. (2000) showed that
photoionization models considering Planetary Nebula Nuclei (PNNs) with a
temperature of $10^{5}$ K as ionizing sources can reproduce the region
occupied, at least, by a subset of type 2 LINERs in optical emission-line
ratio diagnostic diagrams. Winkler (2014) found that these objects have
composite ionizing sources, i.e., more than one mechanism is responsible for
the ionization of the gas. This explanation was also proposed by Yan & Blanton
(2012), Singh et al. (2013), and Bremer et al. (2013).

The unknown nature of the ionizing sources and excitation mechanisms of LINERs
hinder determination of their metallicity using the $T_{\rm e}$-method and/or
strong-line methods (e.g., Storchi-Bergmann et al. 1998). Annibali et al.
(2010) analysed intermediate-resolution optical spectra of a sample of LINERs
and derived oxygen abundances considering these objects as AGNs (by using the
Storchi-Bergmann et al. 1998 calibrations) and as SFs (by using the Kobulnicky
et al. 1999 calibration). These authors found that when AGNs are assumed as
ionizing sources, higher oxygen values are derived than for those assuming hot
stars, which provide sub-solar abundances. On the other hand, oxygen abundance
estimations based on the extrapolation of disk abundance gradients to the
central part of the galaxies (an independent method) by Florido et al. (2012)
indicate over-solar oxygen abundances for three LINERs (NGC 2681, NGC 4314,
and NGC 4394).

Recently, semi-empirical calibrations between the oxygen abundance (or
metallicity) and strong-emission lines of Seyfert 2 were obtained by Castro et
al. (2017) and Carvalho et al. (2020). In addition, several methods to
determine the oxygen abundance gradients in spiral galaxies are available in
the literature (see Vila-Costas & Edmunds, 1992; Zaritsky et al., 1994; van
Zee et al., 1998; Pilyugin et al., 2004, 2007; Lopez-Sanchez & Esteban,
2010a). These methods, together with data from the Mapping Nearby Galaxies at
the Apache Point Observatory (MaNGA, Bundy et al., 2015), offer a powerful
opportunity to determine the chemical abundances of LINERs and to produce
insights about the ionization mechanisms of these objects.

In previous papers, we have analysed oxygen abundance in Seyfert 2 nuclei
using the $T_{\rm e}$-method, photoionization model grids, and HCM code (see
Dors et al. 2014; Castro et al. 2017; Pérez-Montero et al. 2019; Dors et al.
2019; Carvalho et al. 2020; Dors et al. 2020a, b). Although, the semi-
empirical calibrations between metallicity and strong-emission lines of
Seyfert 2 obtained by Castro et al. 2017 and Carvalho et al. 2020 along with
the AGN photoionization model grids and SF calibrations, are applied in this
paper, the object class studied here and the methodology applied are also
different. The main goal of this work is to determine the oxygen abundance in
relation to the hydrogen abundance (O/H) in the central region of the LINER
galaxy UGC 4805 (redshift $z=0.02698$), in combination with data from the
Mapping Nearby Galaxies at the Apache Point Observatory (MaNGA, Bundy et al.
(2015). We assumed a spatially flat cosmology with $H_{0}$ = 71 $\rm
km\>s^{-1}Mpc^{-1}$, $\Omega_{m}=0.270$, and $\Omega_{\rm vac}=0.730$ (Wright,
2006), which leads to a spatial scale of 0.535 kpc/arcsec at the distance of
UGC 4805. This paper is organized as follows: in Section 2 the observational
data of UGC 4805 are described; Section 3 contains the methodology used to
estimate the oxygen abundance of the nucleus and along the disk of UGC 4805;
in Section 4, the results for the nuclear oxygen abundance are given; while
discussion and conclusions of the outcome are presented in Sections 5 and 6,
respectively.

##  2 Data

Figure 1: Left panel: SDSS false colour image combining the $gri$ bands of UGC
4805 taken from the MaNGA survey (Blanton et al., 2017). The IFU field of view
is indicated in purple. Right panel: Map of the H$\alpha$ flux (in units of
$10^{-17}$ erg/cm2/spaxel).

MaNGA survey is an Integral Field Spectroscopy (IFS)
survey111sdss.org/surveys/manga/ (Blanton et al., 2017) developed to observe
about 10 000 galaxies until 2020 using Integral Field Units (IFUs). This
survey is part of the fourth version of the Sloan Digital Sky Survey (SDSS-IV,
Blanton et al. 2017) and utilises the 2.5 m Sloan Telescope in its
spectroscopic mode. The spectra have a wavelength coverage of 3 600 - 10 300
Å, with a spectral resolution of $R\sim$ 1 400 at $\lambda\sim$4 000 Å and
$R\sim$ 2 600 at $\lambda\sim$9 000 Å. The angular size of each spaxel is 0.5
arcsec, and the average Full Width Half Maximum (FWHM) of the MaNGA data is
2.5 arcsec. For details about the strategy of observations and data reduction
see Law et al. (2015) and Law et al. (2016), respectively. From the objects
whose data are available in the MaNGA survey, we selected those presenting
LINER nuclei and disk emission, preferably from objects classified as SFs.
Based on these selection criteria, we selected 81 objects. In this work, we
present a detailed analysis of the spiral galaxy UGC 4805, an object with a
classical LINER nuclear emission and with the largest number of star-forming
emission spaxels along the disk. The spectrum of each spaxel was processed
according to the steps listed below:

  * •

To obtain the nebular spectrum of each spaxel not contaminated by the stellar
population continuum, i.e., the pure nebular spectrum, we use the stellar
population synthesis code  STARLIGHT developed by Cid Fernandes et al. (2005);
Mateus et al. (2006); Asari et al. (2007). This code fits the observed
spectrum of a galaxy using a combination of Simple Stellar Populations (SSPs),
in different proportions and excluding the emission lines. We use a spectral
basis of 45 synthetic SSP spectra with three metallicities $Z$ = 0.004, 0.02
($Z_{\odot}$), and 0.05, and 15 ages from 1 Myr up to 13 Gyr, taken from the
evolutionary synthesis models of Bruzual & Charlot (2003). The reddening law
by Cardelli et al. (1989) was considered. The stellar spectra of the SSPs were
convolved with an elliptical Gaussian function to achieve the same spectral
resolution as the observational data and transformed to the rest frame.

  * •

The emission lines are fitted with Gaussian profiles. For more details about
the synthesis method and the fitting of emission lines, see Zinchenko et al.
(2016).

  * •

The residual extinction associated with the gaseous component for each spatial
bin was calculated by comparing the observed value of the H$\alpha$/H$\beta$
ratio to the theoretical value of 2.86 obtained by Hummer & Storey (1987) for
an electron temperature of 10 000 K and an electron density of 100 cm-3.

Fig. 1 presents the SDSS false colour image obtained combining the $gri$ bands
of UGC 4805 and the resulting 2D map of the H$\alpha$ flux. Observe the very
separated and clear nucleus and a bright star-forming ring in the disk at
$\sim$8 arcsec ($\sim 4.2$ kpc). In Fig. 2 (top panel), the observed (in
black) and synthetic (in red) spectra of the central region of UGC 4805 are
shown. Fig. 2 (bottom panel) also presents the pure emission spectrum, i.e.,
after the SSP subtraction, as well as emission line identifications. The
nuclear emission was obtained by integrating the flux of the central region
considering a radius of 1.5 arcsec ($\sim$1 kpc), which corresponds
approximately to the mean value of the seeing during the observations. In
Table 1 the reddening corrected emission-line intensities (in relation to
H$\beta$=100), the reddening function $f(\lambda)$, the logarithmic extinction
coefficient $c$(H$\beta$), the visual extinction AV, and the equivalent width
of H$\alpha$ [$\rm W_{H\alpha}$] of the LINER nucleus of UGC 4805 are listed.
The H$\beta$ luminosity (in units of erg/s) was also calculated and listed in
Table 1, considering a distance of 119 Mpc.

Figure 2: Upper panel: Stellar population synthesis for the nuclear region of
UGC 4805 within a circular aperture with a radius equal to 1.5 arcsec ($\sim$1
Kpc). Observed and synthetic spectra are in black and red, respectively. Lower
Panel: Pure emission spectrum of the UGC 4805 nucleus. Emission lines are
identified in the plot. The flux is in units of
$10^{-15}\rm{ergs}\,\rm{cm^{-2}}\,\rm{s^{-1}}\,\AA$.

The identification of the dominant ionization mechanism of the emitting gas
across the galaxy is essential to determine chemical abundances. To do that,
we used the $[\text{O\,{iii}}]\lambda 5007/\rm H\beta$ versus
$[\text{N\,{ii}}]\lambda 6584/\rm H\alpha$, $[\text{O\,{iii}}]\lambda 5007/\rm
H\beta$ versus $[\text{S\,{ii}}](\lambda\lambda 6716+31)/\rm H\alpha$, and
$[\text{O\,{iii}}]\lambda 5007/\rm H\beta$ versus $[\text{O\,{i}}]\lambda
6300/\rm H\alpha$ diagnostic diagrams proposed by Baldwin et al. (1981),
commonly known as BPT diagrams, to classify each spaxel of UGC 4805. The
empirical and theoretical criteria proposed by Kewley et al. (2001) and
Kauffmann et al. (2003), respectively, were considered to classify objects in
H ii-like regions, composite, and AGN-like objects. Furthermore, the
separation between Seyferts and LINERs proposed by Kewley et al. (2006) was
used. Fig. 3 shows these BPT diagrams for each spaxel of UGC 4805 and the
distribution of the regions in the galaxy according to $[\text{O\,{iii}}]/\rm
H\beta$ versus $[\text{N\,{ii}}]/\rm H\alpha$ diagram. As can be seen in these
diagrams, the central area of the galaxy is classified as LINER. Fig. 4 shows
the same $[\text{O\,{iii}}]\lambda 5007/\rm H\beta$ versus
$[\text{N\,{ii}}]\lambda 6584/\rm H\alpha$ diagram as Fig. 3 (top left panel),
but as a function of the distance to the centre of the galaxy. The colour of
each point corresponds to its distance from the galaxy centre, with the
reddest points representing the central spaxels. As can be noted in this
figure, the points closest to the centre lie in the LINER region. In Addition,
the distance to the galaxy centre and the location in the diagram are
connected, so that the points that approach the centre of the galaxy moves
away from the line that separates SF-like objects from AGN-like ones.

On the other hand, the diagram introduced by Cid Fernandes et al. (2011) uses
the equivalent width of H$\alpha$ ($\rm W_{H\alpha}$) and is known as a WHAN
diagram. This diagram can to discriminate genuine low-ionization AGNs from
galaxies that are ionized by evolved low-mass stars, i.e. the post-Asymptotic
Giant Branch (post-AGB). The WHAN diagram identifies 5 classes of galaxies,
namely:

  1. 1.

Pure star forming galaxies: $\log(\text{N\,{ii}}/\rm H\alpha)\><\>-0.4$ and
$\rm W_{H\alpha}\>>\>3$ Å.

  2. 2.

Strong AGN (i.e., Seyferts): $\log(\text{N\,{ii}}/\rm H\alpha)\>>\>-0.4$ and
$\rm W_{H\alpha}\>>\>6$ Å.

  3. 3.

Weak AGN: $\log(\text{N\,{ii}}/\rm H\alpha)\>>\>-0.4$ and $\rm W_{H\alpha}$
between 3 and 6 Å.

  4. 4.

Retired galaxies (i.e., fake AGN): $\rm W_{H\alpha}\><\>3$ Å.

  5. 5.

Passive galaxies (actually, line-less galaxies): $\rm W_{H\alpha}$ and $\rm
W_{\text{N\,{ii}}}\><\>0.5$ Å.

According to this classification, the UGC 4805 nucleus is a Retired Galaxy
and, thus, it is ionized by post-AGB stars.

Figure 3: Top left panel: $\log([\text{O\,{iii}}]\lambda 5007/\rm H\beta)$ versus $\log([\text{N\,{ii}}]$ $\lambda 6584/\rm H\alpha)$ diagnostic diagram. Black solid curve represents the theoretical upper limit for the star-forming regions proposed by Kewley et al. 2001 (Ke01), the black dashed curve is the empirical star-forming limit proposed by Kauffmann et al. (2003) (Ka03), and the blue solid line represents the separation between Seyferts and LINERs from Kewley et al. (2006) (Ke06). The region between the Ke01 and Ka03 lines is denominated the composite region (black points). Top right panel: Distribution of the UGC 4805 regions accordingly to their main excitation mechanism as showed in the $\log([\text{O\,{iii}}]\lambda 5007/\rm H\beta)$ versus $\log([\text{N\,{ii}}]$ $\lambda 6584/\rm H\alpha)$ diagram (top left panel). Bottom left panel: $\log([\text{O\,{iii}}]\lambda 5007/\rm H\beta)$ versus $\log([\text{S\,{ii}}](\lambda\lambda 6716+31)/\rm H\alpha$) diagram. Bottom right panel: $\log([\text{O\,{iii}}]\lambda 5007/\rm H\beta)$ versus $\log([\text{O\,{i}}]\lambda 6300/\rm H\alpha)$ diagram. Red points represent the AGN-like spaxels and blue points the SF-like spaxels of UGC 4805, according to $\log([\text{O\,{iii}}]\lambda 5007/\rm H\beta)$ versus $\log([\text{N\,{ii}}]$ $\lambda 6584/\rm H\alpha)$ diagram.  Figure 4: $\log([\text{O\,{iii}}]\lambda 5007/\rm H\beta)$ versus $\log([\text{N\,{ii}}]$ $\lambda 6584/\rm H\alpha)$ diagnostic diagram. The colour of each point corresponds to its distance from the galaxy centre, with the reddest points representing the central spaxels. Table 1: Reddening corrected emission-line intensities (in relation to H$\beta$=100), reddening function $f(\lambda)$, the logarithmic extinction coefficient $c$(H$\beta$), the visual extinction AV, and the H$\beta$ luminosity (erg/s) of the UGC 4805 nucleus. The estimations were obtained considering a radius of 1 kpc. |  $f(\lambda$) | Measurements
---|---|---
[O ii] $\lambda$3727 | 0.33 | 327 $\pm$ 5
[O iii] $\lambda$4959 |  $-$0.02 | 91 $\pm$ 2
H$\beta$ $\lambda$4861 | 0.00 | 100 $\pm$ 3
[O iii] $\lambda$5007 |  $-$0.04 | 242 $\pm$ 3
[N ii] $\lambda$6548 |  $-$0.35 | 126 $\pm$ 3
[O i] $\lambda$6300 |  $-$0.29 | 21 $\pm$ 5
H$\alpha$ $\lambda$6563 |  $-$0.35 | 286 $\pm$ 3
[N ii] $\lambda$6584 |  $-$0.35 | 321 $\pm$ 4
[S ii] $\lambda$6717 |  $-$0.36 | 135 $\pm$ 3
[S ii] $\lambda$6731 |  $-$0.37 | 96 $\pm$ 3
$c$(H$\beta$) | — | 0.19 $\pm$ 0.005
WHα | — | 1.65 $\pm$ 0.21 [Å]
AV | — | 0.37 [mag]
$\log$[$L$(H$\beta$)] | — | 38.86 [erg/s]

##  3 Oxygen abundance determination

To obtain the oxygen abundance of the UGC 4805 nucleus, five calibrations of
SFs were used to extrapolate the radial oxygen abundance for the central
region. This method has been used by several authors (e.g., Vila-Costas &
Edmunds 1992; van Zee et al. 1998; Pilyugin et al. 2004; Zinchenko et al.
2019) and it produces an independent estimation of the oxygen abundance of
nuclear regions. Recently, Mingozzi et al. (2020) measured gas-phase
metallicity, ionisation parameter and dust extinction for a representative
sample of 1795 local star-forming galaxies using integral field spectroscopy
from the SDSS-IV MaNGA survey, showing the extensive reliability of this
survey in this type of study. In addition, calibrations between the gas phase
O/H abundance (or metallicity) and strong emission-lines for Seyfert 2 AGNs
and photoionization model results were considered to estimate the UGC 4805
nucleus oxygen abundance. Each method is described below.

###  3.1 Star-forming regions

The goal of this work is to determine the oxygen abundance in the nuclear
region of UGC 4805. In principle, determinations of oxygen abundances based on
measurements of temperature sensitive line ratios, for example
$[\text{O\,{iii}}]\lambda\,4363$ and $[\text{N\,{ii}}]\lambda\,5755$, should
provide more accurate estimates of O/H (Kennicutt et al., 2003), because these
are free from the uncertainties of photoionization models (e.g., Viegas 2002;
Kennicutt et al. 2003), considered in the majority of strong-line methods
(e.g., Kewley & Dopita 2002). Unfortunately, electron temperature sensitive
line ratios were not measured in the UGC 4805 spectra. In these cases, only
strong-line methods would be used to determine the oxygen abundances in the H
ii regions along the UGC 4805 disk and, then, to obtain the central intersect
O/H abundance. The strong-line methods considered in this work to derive the
O/H gradient are briefly described below.

  * •

Edmunds & Pagel (1984): This theoretical calibration, obtained by using the
model calculations by Dufour et al. (1980) and Pagel et al. (1980), is based
on the $R_{23}$=([O ii]$\lambda$3727+[O
iii]$\lambda\lambda$4959+5007)/H$\beta$ index and the equations are given by

${}12+\log{\rm(O/H)_{up}}=8.76-0.69\log R_{23}$ (1)

and

${}12+\log{\rm(O/H)_{low}}=6.43+1.67\log R_{23},$ (2)

where "up" and "low" mean the equations for the upper and lower branch of the
(O/H)-$R_{23}$ calibration, respectively.

  * •

Denicoló et al. (2002): These authors proposed a calibration between the O/H
abundance and the $N2=\log$([N ii]$\lambda$6584/H$\alpha$) line ratio,
originally proposed by Storchi-Bergmann et al. (1994) as a metallicity
indicator for H ii regions. For the low metallicity regime ($\rm
12+\log(O/H)\><\>8.4$), Denicoló et al. (2002) considered O/H values
calculated through the $T_{\rm e}$-method and for the high metallicity regime
abundance estimations based on calibrations by McGaugh (1991) and Díaz &
Pérez-Montero (2000). The expression proposed by Denicoló et al. (2002) is

$\displaystyle 12+\mathrm{\log(O/H)}$ $\displaystyle=9.12+0.73\times N2.$

This calibration is valid for the range of $7.2<12+\mathrm{\log(O/H)}<9.1$.

  * •

Pettini & Pagel (2004): These authors used a sample of extragalactic H ii
regions and the
$O3N2=\log\left(\frac{[\mathrm{OIII}]\lambda\,5007/\mathrm{H}\beta}{[\mathrm{NII}]\lambda\,6583/\mathrm{H}\alpha}\right)$
parameter to derive the calibration:

$\displaystyle 12+\mathrm{\log(O/H)}$
$\displaystyle=8.73-0.32\times\textit{O3N2},$

valid for the range of $8.0<12+\mathrm{\log(O/H)}<9.0$. Pettini & Pagel (2004)
considered O/H values calculated using the $T_{\rm e}$-method for most cases
and a few estimations based on detailed photoionization models.

  * •

Dors & Copetti (2005): These authors built photoionization model sequences to
reproduce the emission-line ratio intensities of H ii regions located along
the disks of a sample of spiral galaxies to derive O/H gradients. Dors &
Copetti (2005) obtained the semi-empirical calibration

$\displaystyle 12+\mathrm{\log(O/H)}$
$\displaystyle=8.96-0.03x-0.1x^{2}-0.21x^{3}-0.26x^{4},$

with $x=\log{R_{23}}$. This calibration is valid for the upper branch of the
(O/H)-$R_{23}$ relation (i.e., $12+\log({\rm O/H})\>>\>8.2$).

  * •

Pilyugin & Grebel (2016): These authors used a sample of H ii regions with
abundances determined by the ‘counterpart’ method ($C$ method) to derive a
calibration based on oxygen and nitrogen emission lines. These empirical
calibrations use the excitation parameter $P=R_{3}/(R_{2}+R_{3})$, and $N2$,
where $R_{2}$ = [O ii]($\lambda\,$3726 + $\lambda\,$3729)/H$\beta$ and $R_{3}$
= [O iii]($\lambda$ 4959 + $\lambda$ 500 7)/H$\beta$.

Two equations were obtained, one for H ii regions with $N2\lid-0.6$ (the lower
branch), defined by

$\displaystyle 12+\log(\mathrm{O/H})$
$\displaystyle=7.932+0.944\times\log(R_{3}/R_{2})+0,695\times N2+$
$\displaystyle\quad+(0.970-0.291\times\log(R_{3}/R_{2})+$
$\displaystyle\quad-0.019\times N2)\times\log R_{2},$

and another for $N2\>\gid\>-0.6$ (the upper branch), where the following
equation is valid

$\displaystyle 12+\log(\mathrm{O/H})$
$\displaystyle=8.589+0.022\times\log(R_{3}/R_{2})+0.399\times N2+$
$\displaystyle\quad+(-0.137+0.164\times\log(R_{3}/R_{2})+$
$\displaystyle\quad+0.589\times N2)\times\log R_{2}.$

This method is similar to the $C_{NS}$ method proposed by Pilyugin et al.
(2012) and it yields O/H abundance values similar to those derived through the
$T_{\rm e}$-method.

###  3.2 Active Galactic Nuclei

  * •

Storchi-Bergmann et al. (1998): The first calibrations between the oxygen
abundance and strong narrow emission-line ratios of AGNs were the theoretical
ones proposed by Storchi-Bergmann et al. (1998). These authors used
photoionization model sequences, built with the Cloudy code (Ferland, 1996),
and proposed the calibrations

$\displaystyle\begin{matrix}(\mathrm{O/H})&=8.34+0.212x-0.012x^{2}-0.002y+0.007xy+\\\
&\quad-0.002x^{2}y+6.52\times 10^{-4}y^{2}+2.27\times 10^{-4}xy^{2}+\\\
&\quad+8.87\times 10^{-5}x^{2}y^{2},\end{matrix}$
$\displaystyle\begin{matrix}(\mathrm{O/H})&=8.643-0.275u+0.164u^{2}+0.655v-0.154uv+\\\
&\quad-0.021u^{2}v+0.288v^{2}+0.162uv^{2}+0.0353u^{2}v^{2},\end{matrix}$

where $x$ = [N ii]($\lambda\lambda$6548,6584)/H$\alpha$, $y=$ [O
iii]($\lambda\lambda$4949,5007)/H$\beta$, $u=\log$([O
ii]($\lambda\lambda$3726,3729)/[O iii]($\lambda\lambda$4959,5007), and
$v=\log$ ([N ii]($\lambda\lambda$6548,6584)/H$\alpha)$.

These calibrations are valid for the range of
$8.4\><12+\>\mathrm{\log(O/H)}\>\><9.1$. Differences between O/H estimations
derived using these calibrations are in of order of 0.1 dex (Storchi-Bergmann
et al., 1998; Annibali et al., 2010; Dors et al., 2020a, 2015). For LINERs,
Storchi-Bergmann et al. (1998) found that the calibrations above yield lower
values than those derived from the extrapolation of O/H abundance gradients,
suggesting that the assumptions of their models are not representative for
LINERs. It should be mentioned that they indicated that their sample of LINERs
was too small (only four objects) to provide a firm conclusion about the
application of their method to this kind of object. They also suggest that a
more extensive sample needs to be used to test their calibrations.

  * •

Castro et al. (2017) proposed a semi-empirical calibration between the
metallicity and the
N2O2$=\log([\mathrm{\text{N\,{ii}}}]\lambda\,6584/[\mathrm{\text{O\,{ii}}}]\lambda\,3727)$
index. The calibration derived by these authors was obtained upon a comparison
between observational and photoionization model predictions of the [O
iii]$\lambda$5007/[O ii]$\lambda$3727 versus $N2O2$ line ratios and given by

$\displaystyle(Z/{\rm Z_{\odot}})$ $\displaystyle=1.08(\pm 0.19)\times
N2O2^{2}+1.78(\pm 0.07)\times N2O2+$ $\displaystyle\quad 1.24(\pm 0.01).$

This calibration is valid for
$-1.4\>\la\>([\text{O\,{iii}}]/[\text{O\,{ii}}])\>\la\>2$ and
$-1.0\>\la\>N2O2\>\la\>1$.

  * •

Carvalho et al. (2020) used the same methodology as Castro et al. (2017) to
calibrate NLRs metallicities of Seyfert 2 nuclei with the $N2$ emission-line
ratio. This ratio is practically independent of the flux calibration and
reddening correction. These authors proposed the following calibration

$(Z/Z_{\odot})=a^{N2}+b,$ (3)

where $a=4.01\pm 0.08$ and $b=-0.07\pm 0.01$. This calibration is valid for
$-1.4\>\la\>([\text{O\,{iii}}]/[\text{O\,{ii}}])\>\la\>2$ and
$-0.7\>\la\>(N2)\>\la\>0.6$. Carvalho et al. (2020) also proposed a relation
between the ionization parameter ($U$) and the [O iii]$\lambda$5007/[O
ii]$\lambda$3727 line ratio, almost independent of other nebular parameters,
and given by

$\log U=(0.57\pm 0.01\>x^{2})+(1.38\pm 0.01\>x)-(3.14\pm 0.01),$ (4)

where $x=\log$([O iii]$\lambda$5007/[O ii]$\lambda$3727).

Although the AGN calibrations above were developed for Seyfert 2 nuclei, in
this paper, we consider them to derive the O/H abundance in the LINER nucleus
of UGC 4805, and we compared the resulting values to those derived from
extrapolation of oxygen abundance gradients for central parts of this galaxy.

###  3.3 Photoionization models

To reproduce the observed line ratios of UGC 4805 LINER nucleus with the goal
of deriving the O/H abundance and the ionization parameter ($U$), we built
photoionization model grids using version 17.00 of the CLOUDY code (Ferland et
al., 2017). These models are similar to the ones used in Carvalho et al.
(2020), and a brief description of the input parameters is presented below:

  1. 1.

SED: The models consider two distinct Spectral Energy Distributions (SEDs):
one to represent an AGN and another representing p-AGB stars. The AGN SED is a
multi-component continuum, similar to that observed in typical AGNs. As
described in the Hazy manual of the Cloudy code
222http://web.physics.ucsb.edu/~phys233/w2014/hazy1_c13.pdf, it is composed by
the sum of two components. The first one is a Big Bump component peaking at
$\approx$ 1 Ryd, parametrized by the temperature of the bump, assumed to be
$5\>\times\>10^{5}$ K, with a high-energy exponential cutoff and an infrared
exponential cutoff at 0.01 Ryd. The second component is an X-ray power law
with spectral index $\alpha_{x}=-1$ that is only added for energies greater
than 0.1 Ryd to prevent it from extending into the infrared. The X-ray power
law is not extrapolated below 1.36 eV or above 100 keV: for energies lower
than 1.36 eV it is set to zero (since the bump dominates for these energies),
and for energies above 100 keV, the continuum falls off as $\nu^{-2}$. The
$\alpha_{ox}$ spectral index defined as the slope of a power law between 2 keV
and 2500 Å is the parameter that provides the normalization of the X-ray power
law to make it compatible with the thermal component. It is given by

$\alpha_{ox}=\frac{\log[F(2\>{\rm
keV})/F(2500\>\textrm{\AA})]}{\log[\nu(2\>{\rm
keV})/\nu(2500\>\textrm{\AA})]},$ (5)

where $F$ is the flux at 2 keV, 2500 Å and $\nu$ are the corresponding
frequencies (Tananbaum et al., 1979). This AGN SED generates a continuum
similar to that used by Korista et al. (1997). In all our AGN models, a fixed
value of $\alpha_{ox}=-1.0$ is assumed. Carvalho et al. (2020) found that
models with $\alpha_{ox}\>\la\>-1.0$ trend not to reproduce optical emission
line ratios of Seyfert 2 nuclei (see also Dors et al. 2017b; Pérez-Montero et
al. 2019). Moreover, $\alpha_{ox}\sim-1.0$ has been derived in observational
studies of LINERs and low luminosity AGNs (see Ho 1999; Eracleous et al. 2010;
Maoz 2007; Younes et al. 2012).

In the case of the stellar SED, we consider p-AGB stars atmosphere models by
Rauch (2003) assuming the available values for the effective temperatures:
$T_{\rm eff}=50,100$, and 190 kK, with the logarithm of the surface gravity
$\log(\rm g)=6$. In Fig. 5, we present a comparison between the SEDs assumed
in our models. The AGN SED maintains a high ionization flux even at high
energies (low wavelengths) somewhat similar to the p-AGB one with the highest
$T_{\rm eff}$ value. Some soft emission is noted for p-AGB stars with 100 kK
and mainly with 50 kK. Both AGN and p-AGB SED models can be considered as the
main ionizing source, i.e., responsible for the ionization of the gas, and
underlying stellar population was not considered in the models. Therefore, our
models are designed to investigate what kind of object would be producing the
gas ionization in UGC 4805 based on emission line intensity ratios. These
models are not intended for analysing the equivalent width of lines, as
performed by Cid Fernandes et al. (2011), which also strongly depends on the
underling stellar population (Dottori & Bica, 1981).

  2. 2.

Metallicity: We assumed ($Z/{\rm Z_{\odot}}$) = 0.2, 0.5, 0.75, 1.0, 2.0, and
3.0 for the models. We assumed the solar oxygen abundance to be 12 + $\log$
(O/H)⊙ = 8.69 (Asplund et al., 2009; Prieto et al., 2001) and it is equivalent
to ($Z/{\rm Z_{\odot}}$)=1.0. All the abundances of heavy elements were scaled
linearly with the metallicity, except the nitrogen for which we assumed the
relation $\rm\log(N/O)=1.29\times[12+\log(O/H)]-11.84$ derived by Dors et al.
(2017b), who considered abundance estimations for type 2 AGNs and H ii
regions.

  3. 3.

Electron Density: We assumed for the models an electron density value of
$N_{\rm e}$ = 500 $\rm cm^{-3}$, constant in the nebular radius. This value is
very similar to the one estimated for UGC 4805 nucleus through the relation
between $N_{\rm e}$ and $R_{S2}=$[S ii]$\lambda 6716/\lambda 6731$ line ratio
and using the IRAF/TEMDEN task. Observational estimations of $N_{\rm e}$ based
on the Ar iv$\lambda$4711/$\lambda$4740 ratio, which map a denser gas region
than the one based on [S ii] ratio, for two Seyfert nuclei (IC 5063 and NGC
7212) by Congiu et al. (2017), indicate $N_{\rm e}$ ranging from $\sim 200$ to
$\sim 13\,000\>\,\rm cm^{-3}$. Furthermore, radial gradients with electron
densities decreasing from the centres to the edges have been found in star-
forming regions (e.g., Copetti et al. 2000) and in AGNs (e.g., Revalski et al.
2018). However, Carvalho et al. (2020) showed that models with $N_{\rm
e}\><2\,000\>\rm cm^{-3}$ produce practically the same optical emission-line
ratios. In addition, photoionization models assuming electron density
variations along the radius have an almost negligible influence on predicted
optical line ratios as demonstrated by Dors et al. (2019). For a detailed
discussion about the $N_{\rm e}$ influence on metallicity estimates in Seyfert
2 AGNs, see Dors et al. (2020b).

  4. 4.

Ionization Parameter: This parameter is defined as

$U=\frac{Q({\rm H)}}{4\,\pi\,R_{{\rm 0}}^{2}\,n(\rm H)\,\rm c},$ (6)

in which $Q(\rm H)$ [$\rm s^{-1}$] is the number of hydrogen-ionizing photons
emitted by the central object, $R_{0}$ [cm] is the distance from the
ionization source to the inner surface of the ionized gas cloud, $n(\rm H)$
[cm-3] is the total hydrogen density (ionized, neutral and molecular), and
$\rm c$ is the speed of light [cm s-1]. We assumed logarithm of $U$ in the
range of -4.0 $\leq\log U\leq$ -0.5, with a step of 0.5 dex, which is about
the same range of values assumed by Feltre et al. (2016), who used a
photoionization model grid to reproduce ultraviolet and optical emission-line
ratios of active and normal galaxies. Different ionization parameter values
simulate gas excitation differences, owing to variations in the mass of the
gas phase and several geometrical conditions covering a wide range of possible
scenarios (Pérez-Montero, 2014).

In our models, a plane-parallel geometry is adopted, and the outer radius is
assumed to be the one where the gas temperature falls to 4 000 K (default
outer radius value in the CLOUDY code), since cooler gas practically does not
produce optical emission lines. Models with different combinations of $Q(\rm
H)$, $R_{0}$, and $n(\rm H)$, resulting in similar values of $U$, are
homologous models, i.e., they predict very similar emission-line intensities.

For the ionizing sources, Cloudy is a unidimensional code that assumes a
central ionization source, which is a good approach for AGNs. However, in
giant star-forming regions (e.g., Monreal-Ibero et al. 2011), stars are
spreaded out through the gas. In this sense, in most cases, a central
ionization source usage would not constitute a genuine representation of the
situation. Ercolano et al. (2009) and Jamet & Morisset (2008) showed that the
distribution of the O-B stars in relation to the gas alters the ionisation
structure and the electron temperature. Hence, the ionization parameter
partially depends on the distance of the ionizing source to the gas. However,
in our case, we are considering an integrated spectrum of the UGC 4805
nucleus; thus, the stellar distribution may have a minimum effect on the
emergent spectrum. In the case of giant H ii regions ionized by stellar
clusters (e.g. Mayya & Prabhu 1996; Bosch et al. 2001), the hottest stars
dominate the gas ionization (Dors et al., 2017a). Therefore, the assumption of
a single star with a representative effective temperature as the main ionizing
source, as assumed in our p-AGB models, is a good approximation (see e.g.,
Zinchenko et al., 2019).

To estimate the O/H and $U$ values for the UGC 4805 nucleus, we compare some
observational emission line intensity ratios with photoionization model
results using diagnostic diagrams and perform a linear interpolation between
models.

Figure 5: Comparison between the p-AGB star and AGN SEDs assumed the ionizing
source in the photoionization models. The atmosphere models by Rauch (2003)
and three effective temperature values (as indicated) are considered. The AGN
SED is represented by a multi-component continuum with spectral index
$\alpha_{ox}=-1.0$ (see Eq. 5).

Figure 6: Left panels: oxygen abundance maps obtained through the calibrations
described in Sect. 3.1 and indicated in each plot. Right panels: radial
abundance distributions along the UGC 4805 disk. The line in each plot
represents the linear fitting (Eq. 7) to the estimations, whose coefficients
are listed in Table 2.

Figure 7: Same as Fig. 6, but for the indicated calibrations. Figure 8: Upper
left panel: $\log$([O iii]$\lambda 5007$/H$\beta$) versus $\log$([N
ii]$\lambda 6584$/H$\alpha$) diagnostic diagram. Upper right panel: $\log$([O
iii]$\lambda 5007$/H$\beta$) versus $\log$([S ii]$\lambda\lambda
6717,6731$/H$\alpha$) diagnostic diagram. Gray lines represent the separating
criteria of the BPT diagrams, from Kewley et al. (2006) (Ke06), Kauffmann et
al. (2003) (Ka03), and Kewley et al. (2001) (Ke01). Lower left panel:
$\log$([O iii]$\lambda 5007$/[O ii] $\lambda 3727$) versus $\log$([N
ii]$\lambda 6584$/H$\alpha$) diagnostic diagram. Lower right panel: $\log$([O
iii]$\lambda 5007$/[O ii] $\lambda 3727$) versus $\log$([N ii]$\lambda
6584$/[O ii]$\lambda 3727$) diagnostic diagram. Coloured solid lines connect
AGN photoionization model results (see Sect. 3.3) with the same metallicity
$(Z/Z_{\odot})$ and dotted lines models with the same ionization parameter
($U$), as indicated. The blue point represents the observational line ratios
for the UGC 4805 nucleus (see Sect. 2). Figure 9: Same as Fig. 8 but
considering p-AGB photoionization models (see Sect. 3.3) assuming $T_{\rm
eff}$ = 50 kK. Figure 10: Same as Fig. 9 but considering p-AGB
photoioniazation models (see Sect. 3.3) assuming $T_{\rm eff}$ = 100 kK.
Figure 11: Same as Fig. 9 but considering p-AGB photoioniazation models (see
Sect. 3.3) assuming $T_{\rm eff}$ = 190 kK.

##  4 Results

###  4.1 O/H calibrations

To apply some of the strong-line calibrations developed for SFs described in
Sect. 3.1 to the UGC 4805 disk H ii regions, it is necessary to select which
branch of the (O/H)-$R_{23}$ relation is adequate. We consider the Kewley &
Ellison (2008) criteria to break the degeneracy, i.e., for objects with
$\log$([N ii]$\lambda$6584/[O ii]$\lambda$3727)$\>>\>-1.2$, the upper $R_{23}$
branch must be used. The O/H abundances were estimated only for objects
classified as pure star-forming regions, i.e., those with line ratios under
the Kauffmann et al. (2003) line in the diagnostic diagram in the left panel
of Fig. 3. Figures. 6 and 7 present the abundance maps (left panels) and the
O/H values along the disk (right panels). Note that all the strong-line
calibrations applied exhibited a linear decrease of O/H along the disk in
agreement with previous results (e.g., Pilyugin et al. 2004). We derive the
central oxygen abundance $\rm 12+\log(O/H)_{0}$ extrapolating to the centre of
the galaxy the linear fit:

${\rm 12+\log(O/H)=12+\log(O/H)_{0}}+(grad\>\times R),$ (7)

where $\rm 12+\log(O/H)$ is the oxygen abundance at a given galactocentric
distance $R$ (in units of arcsec) and $grad$ is the regression slope. The
parameters of the linear regressions for the distinct calibrations used are
listed in Table 2. The star-forming ring, clearly visible in the H$\alpha$ map
(see Fig. 1), does not present any oxygen abundance discrepancy in comparison
to its neighbour regions.

The calibration proposed by Edmunds & Pagel (1984) resulted in 12 +
$\log$(O/H) values ranging from $\rm\sim 8.2$ to $\sim 8.8$ along the galactic
disk, while the abundance value extrapolated to the nucleus ($R=0$ arcsec) is
$\rm 12+\log(O/H)_{0}=8.72$. Considering the Denicoló et al. (2002)
calibration, we derive 12 + $\log$ (O/H)0 = 8.81 for the nucleus. By using the
calibration by Pettini & Pagel (2004), we derive a nuclear abundance of 12 +
$\log$ (O/H)0 = 8.79. Estimates of oxygen abundances obtained using the
calibration by Dors & Copetti (2005) yield a flatter gradient than the
gradients derived with other calibrations, i.e., O/H values vary along the
galactic disk in the narrow range of 8.85 < 12 + $\log$(O/H) < 9.0. The
estimated nuclear abundance is 12 + $\log$ (O/H)0 = 8.98. Note that a large
part of our estimated values are close to the upper metallicity limit for this
calibration, where the metallicity is practically constant, i.e., the O/H
abundance is saturated with the variation of $R_{23}$. Finally, the
application of the calibration proposed by Pilyugin & Grebel (2016) indicates
abundances in the range of 8.5 < 12 + $\log$(O/H) < 8.7, with an inferred
central abundance of 12 + $\log$ (O/H)0 = 8.76, which is close to the
abundance obtained through the Pettini & Pagel (2004) calibration. In summary,
the extrapolation for the UGC 4805 LINER nucleus based on the calibrations
considered above indicates an over solar oxygen abundance, with an averaged
value of $\rm 12+\log(O/H)_{0}=8.82$.

To estimate the O/H abundance by using the nuclear emission of UGC 4805, we
used the line intensity ratios listed in Table 1 and applied the Storchi-
Bergmann et al. (1998), Castro et al. (2017), and Carvalho et al. (2020)
calibrations. The estimated values of O/H abundance are listed in Table 2. As
suggested by Storchi-Bergmann et al. (1998), the final (O/H) abundance derived
from their methods should be the average of the values calculated from the two
equations (Sect. 3.2), which provides 12 + $\log$ (O/H)0 $=8.93\pm$ 0.04. The
Castro et al. (2017) and Carvalho et al. (2020) calibrations provide a value
of 12 + $\log$ (O/H)0 = $8.77\pm$ 0.01 and 12 + $\log$ (O/H)0 = $8.69\pm$
0.01, respectively. An average value of 12 + $\log$ (O/H)${}_{0}=8.81\pm 0.02$
was derived considering the three calibrations.

Table 2: Oxygen abundance results for the UGC 4805 nucleus. The first set of
values are the central Z/Z⊙ estimations and the coefficients of the linear
fitting (Eq. 7) to the O/H estimations along the UGC 4805 disk (see Figs. 6
and 7) considering different calibrations for H ii regions proposed by
different authors as indicated (see Sect. 3.1). The second set of values are
the metallicities, the oxygen abundances, and $\log U$ (only for one case)
obtained by using the AGN calibrations (see Sect. 3.2). The third set of
values are metallicities, O/H abundances and $\log U$ obtained from linear
interpolations of the photoionization model results shown in Fig. 8, 9, 10 and
11. The diagnostic diagrams and the model ionizing sources considered are
indicated. Central intersect method – H ii region calibrations
---
| Z/Z⊙ | 12 + $\log$(O/H)0 |  grad (dex/arcsec)
Edmunds & Pagel (1984) | 1.07 | 8.72 $\pm$ 0.003 | $-0.016\pm 0.001$
Denicoló et al. (2002) | 1.32 | 8.81 $\pm$ 0.002 | $-0.002\pm 0.0001$
Pettini & Pagel (2004) | 1.26 | 8.79 $\pm$ 0.003 | $-0.003\pm 0.0002$
Dors & Copetti (2005) | 1.95 | 8.98 $\pm$ 0.002 | $-0.004\pm 0.0002$
Pilyugin & Grebel (2016) | 1.17 | 8.76 $\pm$ 0.002 | $-0.007\pm 0.0001$
Average | 1.35 | 8.82 $\pm$ 0.003 |
AGN calibrations
| Z/Z⊙ | 12 + $\log$(O/H) | $\log U$
Storchi-Bergmann et al. (1998) | 1.74 |  $8.93$ $\pm$ 0.04 | —
Castro et al. (2017) | 1.20 | $8.77\pm 0.01$ | —
Carvalho et al. (2020) | 1.00 | $8.69\pm 0.01$ | $-3.09$
Average | 1.31 | 8.81 $\pm$ 0.02 |
Diagnostic diagrams – Photoionization models
| Z/Z⊙ | 12 + $\log$(O/H) | $\log U$
AGN models
$\log([\text{O\,{iii}}]/\rm H\beta)$ vs. $\log$([N ii]/H$\alpha$) | 0.95 | $8.67\pm 0.02$ | $-3.39$
$\log([\text{O\,{iii}}]/[\text{O\,{ii}}])$ vs. $\log$($[\text{N\,{ii}}]$/H$\alpha$) | 0.93 | $8.66\pm 0.02$ | $-3.22$
$\log([\text{O\,{iii}}]/[\text{O\,{ii}}])$ vs. $\log([\text{N\,{ii}}]/[\text{O\,{ii}}])$ | 1.29 | $8.80\pm 0.02$ | $-3.24$
Average | 1.06 | $8.71\pm 0.02$ |
p-AGB models ($T_{\rm eff}$= 100 kK)
$\log([\text{O\,{iii}}]/\rm H\beta)$ vs. $\log$([N ii]/H$\alpha$) | 0.85 | $8.62\pm 0.03$ | $-3.50$
$\log([\text{O\,{iii}}]/[\text{O\,{ii}}])$ vs. $\log$($[\text{N\,{ii}}]$/H$\alpha$) | 0.98 | $8.68\pm 0.01$ | $-3.26$
$\log([\text{O\,{iii}}]/[\text{O\,{ii}}])$ vs. $\log([\text{N\,{ii}}]/[\text{O\,{ii}}])$ | 1.32 | $8.81\pm 0.02$ | $-3.29$
Average | 1.06 | $8.71\pm 0.02$ |
p-AGB models ($T_{\rm eff}$= 190 kK)
$\log([\text{O\,{iii}}]/\rm H\beta)$ vs. $\log$([N ii]/H$\alpha$) | 0.72 | $8.55\pm 0.01$ | $-3.57$
$\log([\text{O\,{iii}}]/[\text{O\,{ii}}])$ vs. $\log$($[\text{N\,{ii}}]$/H$\alpha$) | 0.81 | $8.60\pm 0.01$ | $-3.26$
$\log([\text{O\,{iii}}]/[\text{O\,{ii}}])$ vs. $\log([\text{N\,{ii}}]/[\text{O\,{ii}}])$ | 1.48 | $8.86\pm 0.01$ | $-3.31$
Average | 1.00 | $8.69\pm 0.01$ |

###  4.2 Photoionization models

As mentioned previously (see Sect. 3.3), two different photoionization model
grids were built, one assuming an AGN as the ionizing source and another
assuming p-AGB stars with different $T_{\rm{eff}}$ values as the ionizing
source. In the upper panels of Fig. 8, the observational line ratios of the
UGC 4805 nucleus are plotted in the $\log$([O iii]$\lambda 5007$/H$\beta$)
versus $\log$([N ii]$\lambda 6584$/H$\alpha$) (left panel) and $\log$([O
iii]$\lambda 5007$/H$\beta$) versus $\log$([S ii]$\lambda\lambda
6717,6731$/H$\alpha$) (right panel) diagnostic diagrams and compared to those
predicted by AGN photoionization models. These plots also show the demarcation
lines proposed by Kauffmann et al. (2003) and Kewley et al. (2006). The
observational line intensity ratios are reproduced by the AGN models;
therefore, we can infer a metallicity and an ionization parameter for the UGC
4805 nucleus. Using linear interpolation between the models in the $\log$([O
iii]/H$\beta$) versus $\log$([N ii]/H$\alpha$) diagnostic diagram (Fig. 8
upper left panel), we derive a metallicity of (Z/Z⊙) $\sim$ 0.95 and $\log
U\sim-3.39$. For $\log$([O iii]/H$\beta$) versus $\log$([S ii]/H$\alpha$)
diagnostic diagram (Fig. 8 upper right panel), which is clearly bi-valuated
with the upper envelope at (Z/$\rm Z_{\odot})\sim$ 1, we adopt the models with
larger values to characterise our object, since the low metallicity models do
not represent AGN-like objects, as it is seen in the left panel. Then, we
derived (Z/Z⊙) $\sim$ 2.57 and $\log U\sim-3.26$, using the $\log$([O
iii]/H$\beta$) versus $\log$([S ii]/H$\alpha$) diagnostic diagram. The second
metallicity value is about three times the former one.

Dors et al. (2011), by using a grid of photoionization models, showed that
there are relations between different line ratios, such as
$[\text{O\,{iii}}]\lambda 5007$/$[\text{O\,{ii}}]\lambda 3727$ versus
$[\text{N\,{ii}}]\lambda 6584$/H$\alpha$ and [O iii]$\lambda 5007$ / [O ii]
$\lambda 3727$ versus [N ii]$\lambda 6584$/ [O ii]$\lambda 3727$, that are
more sensitive to the ionization parameter, and the metallicities obtained
through them are closer to those obtained using the $T_{\rm e}$-method. For
this reason, we use these diagnostic diagrams also employed by Castro et al.
(2017) and Carvalho et al. (2020) to perform a more reliable analysis. The
lower panels of Fig. 8 presents these observational line ratios for the UGC
4805 nucleus superimposed on those ratios predicted by our AGN photoionization
models. By using linear interpolation between the models we derive (Z/Z⊙)
$\sim$ 0.93 and $\log U\sim-3.22$ from the $\log$([O iii]/[O ii]) vs.
$\log$([N ii]/H$\alpha$) diagnostic diagram (lower left panel), and (Z/Z⊙)
$\sim$ 1.29 and $\log U\sim-3.24$ from the $\log$([O iii]/[O ii]) vs.
$\log$([N ii]/[O ii]) diagnostic diagram (lower right panel).

The values of the ionization parameter found using the four diagnostic
diagrams (Fig. 8) are very similar and in agreement with the typical value for
LINER galaxies estimated by Ferland & Netzer (1983).

Figs. 9, 10, and 11 contain the same diagnostic diagrams exhibited in Fig. 8
for the photoionization model results considering p-AGB stars as ionizing
sources. In Fig. 9, the models with $T_{\rm eff}=50$ kK do not reproduce the
UGC 4805 nucleus line ratios. In the upper panels of this figure, the
parameter space characterized by the models is occupied only by H ii-like
objects. Therefore, it is impossible to derive any value of $Z$ or $U$ from
these models. For models with $T_{\rm eff}=100$ and 190 kK and considering the
$\log$([O iii]/H$\beta$) versus $\log$([N ii]/H$\alpha$) (upper left panels of
Figs. 10 and 11), we derive (Z/Z⊙) $\sim$ 0.85 and $\log U\sim 3.50$, and
(Z/Z⊙) $\sim$ 0.72 and $\log U\sim 3.57$, respectively. Taking into account
$T_{\rm eff}=100$ kK and $\log$([O iii]/H$\beta$) versus $\log$([S
ii]/H$\alpha$) diagnostic diagram (upper right panel), we found $\log
U\sim-3.44$ and two values for the metallicity, i.e., Z/$\rm Z_{\odot}\sim$
2.87 and Z/$\rm Z_{\odot}\sim$ 0.42. This happens because, as in the case of
AGN models, this relation is bi-valuated for the metallicity. Analysing the
results of the same diagnostic diagram for the p-AGB models with $T_{\rm
eff}=190$ kK, we do not observe a bi-valuated relatio. Models with
metallicities larger than 0.75 occupy almost the same region. We obtain
(Z/Z${}_{\odot})$ $\sim$ 0.36 and $\log U=-3.35$. These results could indicate
that the high metallicity model solution found for the models with T${}_{\rm
eff}=100$ kK [$(Z/\rm Z_{\odot})\sim 2.0$] is not correct.

The lower panels of Figs. 10 and 11 display the same diagnostic diagrams as in
the lower panels of Fig. 8, but containing photoionization model results
considering p-AGB stars as ionizing source. For models with $T_{\rm eff}=100$
kK (Fig. 10), we derive from the $\log$([O iii]/[O ii]) versus $\log$([N
ii]/H$\alpha$) diagnostic diagram Z/$\rm Z_{\odot}\sim$ 0.98 and $\log
U\sim-3.26$. From the $\log$([O iii]/[O ii]) versus $\log$([N ii]/[O ii])
diagram we calcule Z/$\rm Z_{\odot}\sim$ 1.32 and $\log U\sim-3.29$. Finally,
we see that the models with $T_{\rm eff}=190$ kK (Fig. 11) provide from the
$\log$([O iii]/[O ii]) versus $\log$([N ii]/H$\alpha$) diagram a metallicity
of Z/$\rm Z_{\odot}\sim$ 0.81 and $\log U\sim-3.26$, and from the $\log$([O
iii]/[O ii]) versus $\log$([N ii]/[O ii]) diagram Z/$\rm Z_{\odot}\sim$ 1.48
and $\log U\sim-3.31$.

The models yield bi-valuated or saturated results for the emission-line
diagnostic diagrams that include the [S ii] emission-lines and show the more
discrepant results including super-solar metallicities values
[$(Z/Z_{\odot})\sim$ 2.57] for the AGN models and sub-solar metallicities for
the p-AGB models with $T_{\rm eff}=100$ and 190 kK [$(Z/Z_{\odot})\sim$ 0.42,
0.36, respectively]. Hence, we do not take into account the results derived
from the $\log$([O iii]/H$\beta$) versus $\log$([S ii]/H$\alpha$) diagnostic
diagrams. The adopted $(Z/Z_{\odot})$, 12 + log(O/H) and $\log U$ values
derived from Figs. 8, 10, and 11 are listed in Table 2.

The averaged values obtained from the extrapolation of the oxygen abundance
gradient from H ii region estimations and from AGN calibrations are
Z/Z${}_{\odot})\sim$ 1.35 and (Z/Z${}_{\odot})\sim$ 1.31, respectively. In
both cases, the estimated abundance values are over-solar and are in
agreement, taking into account their errors. On the other hand, all the
photoionization model produce very similar average values close to the solar
one: (Z/Z${}_{\odot})$ $\sim$ 1.06, 1.06, 1.00 for AGN, p-AGB with $T_{\rm
eff}=100$ kK, and p-AGB with $T_{\rm eff}=190$ kK, respectively.

##  5 Discussion

A widely accepted practice is to estimate the oxygen abundance at the central
part of a galaxy by the central intersect abundance [$\rm 12+\log(O/H)_{0}$]
obtained from the radial abundance gradient (e.g., Vila-Costas & Edmunds 1992;
Zaritsky et al. 1994; van Zee et al. 1998). This methodology has predicted
solar or slightly over-solar metallicities for the central region of spiral
galaxies, i.e., 12 + $\log$(O/H) from $\sim 8.6$ to $\sim 9.0$ (e.g., Pilyugin
et al. 2004; Dors et al. 2020a), depending on the method considered to derive
the individual disk H ii-region abundances. Comparisons of these extrapolated
oxygen abundance measurements ($\rm 12+\log(O/H)_{0}$) with the ones obtained
through the use of other methods that directly involve the nuclear emission
have achieved good agreement. Storchi-Bergmann et al. (1998) found that the
O/H abundances derived for a sample of seven Seyfert 2 galaxies through their
calibrations are in consonance with those obtained by the central intersect
abundance. This agreement was also found by Dors et al. (2015) using a larger
sample of objects than the one considered by Storchi-Bergmann et al. (1998).

The oxygen abundance profile along the UGC 4805 disk presents a negative
gradient, as expected, since it is a spiral galaxy. The negative gradient is
explained naturally by models assuming the inside-out scenario of galaxy
formation (Portinari & Chiosi, 1999; MacArthur et al., 2004; Barden et al.,
2005). According to this scenario, galaxies begin to form in the inner regions
before the outer regions. This was confirmed by studies of the stellar
populations (e.g., Boissier & Prantzos 2000; Bell & Jong 2000; Pohlen &
Trujillo 2006) and chemical abundances of spiral galaxies (e.g., Sánchez et
al. 2014). As previously shown, considering the O/H gradient extrapolation,
AGN calibrations, and AGN and p-AGB photoionization models, we derived
averaged oxygen abundance values for the UGC 4805 nucleus in the range of 1.00
$\><\>$ (Z/Z${}_{\odot})\><$ 1.35, i.e., ranging from solar to slightly over-
solar metallicities.

Figure 12: Comparison between central intersect oxygen abundances derived for
the UGC 4805 nucleus from the radial abundance gradients ($12+\log(\rm
O/H)_{0}$) with those derived through strong-line methods and AGN and p-AGB
models (colored points as indicated). The point of the AGN model is the
average from the AGN models. Black points represent the estimations performed
by Dors et al. (2015) using the observational data by Ho et al. (1997). Solid
line represents the equality between the estimations. Figure 13: Metallicity
sensitive line ratios $R_{23}$, $N2O2$ and $N2$ versus the ionization
parameter sensitive line ratio [O iii]$\lambda$5007/[O ii]$\lambda$3727\.
Black points represent 463 Seyfert 2 nuclei studied by Dors et al. (2020a) and
blue points represent 38 LINERs compiled by Ho et al. (1993), Eracleous &
Halpern (2001), Annibali et al. (2010), and Molina et al. (2018). The red
point represents the UGC 4805 nucleus.

In Fig. 12 the O/H average values estimated for the UGC 4805 nucleus using AGN
calibrations as well as AGN and p-AGB models are compared with the average
value derived through the central intersect method. The estimations for active
and star-forming nuclei from Dors et al. (2015) are also presented in Fig. 12.
This figure clearly illustrates that the averaged O/H value derived through
the central intersect method is in consonance with the ones derived through
the use of AGN calibrations and AGN and p-AGB models, as well as with the Dors
et al. (2015) estimations.

Annibali et al. (2010) compared intermediate-resolution optical spectra of a
sample of 49 nuclei classified as LINERs/composites with photoionization model
results assuming as ionization source accretion-rate AGN (represented by a
power law SED) using the Groves et al. (2004) models and the shock models
built by Allen et al. (2008). These authors also compared the observed and
predicted equivalent widths of the lines present on their spectra using models
with p-AGB SEDs computed by Binette et al. (1994) [see also Cid Fernandes et
al. 2009], finding that photoionization by p-AGB stars alone can explain only
$\approx 22$% of the observed LINER/composite sample. They also found that the
major fraction of their sample could be characterized by nuclear emission
consistent with excitation by a low-accretion rate AGNs and/or fast shocks.
Molina et al. (2018) compared observational optical and ultraviolet spectra of
three LINERs with model results assuming four different excitation mechanisms:
shocks, photoionization by an accreting black hole, and photoionization by
young or old hot stars. These authors concluded that the model which best
describes their data has a low-luminosity accretion-powered active nucleus
that photoionizes the gas within $\sim 20$ pc of the galaxy centre, as well as
shock excitation of the gas at larger distances. These authors also indicated
that LINERs could have more than one ionizing mechanism. In the case of the
UGC 4805 nucleus, the good agreement among all the different methods applied
to derive its metallicity does not allow discrimination of the nature of the
ionizing source.

Fig. 13 illustrates the $\log(R_{23})$, $N2O2$ and $N2$ metallicity indexes as
a function of the [O iii]$\lambda$5007/[O ii]$\lambda$3727 line ratio used as
an ionization parameter indicator for the UGC 4805 nucleus. This figure
compares our results to those of a sample of confirmed 463 Seyfert 2 nuclei
studied by Dors et al. (2020a) and obtained from the Sloan Digital Sky Survey
(York et al., 2000), as well as those of a sample of 38 LINERs obtained by Ho
et al. (1993), Eracleous & Halpern (2001), Annibali et al. (2010), and Molina
et al. (2018). Both populations LINERs and Seyfert 2s, are partially
overlapped in all of these diagrams although they display slightly different
trends with LINERs showing lower ionizations ($\log U\><\>-3.2$) following Eq.
4. As can be seen in Fig. 13, the UGC 4805 nucleus positions in these diagrams
are compatible with both populations, although they seems to follow the LINERs
sequence; therefore, they would share similar physical properties.

According to Fig. 13, LINERs have intermediate and low [O iii]/[O ii] line
ratio intensities, with the high values
[$\rm(\log[\text{O\,{iii}}]/[\text{O\,{ii}}])\>\ga\>0.0$] only observed in
Seyfert 2. Since the [O iii]/[O ii] has a strong dependence on $U$, the above
results indicate a tendency of LINERs to present lower $U$ values than the
ones in Seyfert 2, as suggested by Ferland & Netzer (1983). As an additional
test of this scenario, Fig. 14 presents $\log U$ versus $Z/\rm Z_{\odot}$,
calculated by using the Carvalho et al. (2020) calibrations (Eqs. 3 and 4),
for the same sample as the one in Fig. 13. We can see that the UGC 4805 and
the LINERs occupy the region with lower $U$ values and the highest values of
this parameter are only observed in Seyfert 2s.

Finally, the geometry of UGC 4805 nucleus can provide information about the
ionization source. In view of this, we compare the ionization parameter
derived from the AGN and pAGB photoionization models with the one estimated
from the observational data. The average value from the models is $<\log
U>\sim-3.30$. To calculate $U$ from observational data, first, we obtained the
$Q(\rm H)$ from the expression of Hekatelyne et al. (2018)

$\left(\frac{Q({\rm H)}}{{\rm s^{-1}}}\right)=1.03\times
10^{12}\left(\frac{L_{\rm H_{\alpha}}}{\rm s^{-1}}\right)$ (8)

and employing the luminosity value listed in Table 1. This luminosity value is
obtained from integrated flux of the UGC 4805 nucleus. We found $\log Q({\rm
H)}=50.87$. The value $N_{\rm e}=100\rm\>cm^{-3}$ is obtained from [S
ii]$\lambda$6716/$\lambda 6731$ line ratio intensity, also listed in Table 1.
Applying the $Q(\rm H)$ and $N_{\rm e}$ values above to Eq. 6, the innermost
radius value $R_{0}$ to conciliate the theoretical and observational $U$ value
is about 50 pc, in order of the radius assumed by Bennert et al. (2006). As
can be noted in Fig. 3, the LINER emission extends to until $\sim 2.5$ kpc,
i.e., a high excitation level (or $U$) is maintained from $\sim 50$ pc to kpc
scales. Since $U\approx R^{-2}$, the ionization source is probably spread
along the $R$. Thus, this result indicates that p-AGB is the preferable
ionization source rather than AGN. This assumption is supported by the result
obtained previously from the WHAN diagram (Cid Fernandes et al., 2011).

Figure 14: As Fig. 13 but for logarithm of the ionzation parameter ($\log U$)
versus the metallicity ($Z/\rm Z_{\odot}$) calculated by using the Carvalho et
al. (2020) calibrations (Eqs. 3 and 4).

##  6 Conclusion

We used optical emission-line fluxes taken from the SDSS-IV MaNGA survey to
determine the oxygen abundance (metallicity) of the LINER nucleus of the UGC
4805 galaxy. The oxygen abundance was derived through the extrapolation of the
radial abundance gradient for the central part of the disk by using strong-
line calibrations for AGNs and photoionization model grids assuming as
ionizing sources gas accretion into a black hole, representing an AGN and
p-AGB stars. We found that all the O/H abundance estimations agree with each
other. The results from these methods indicate that the UGC 4805 nucleus has
an oxygen abundance in the range of $1.0\>\la\>(Z/Z_{\odot})\>\la 1.35$, i.e.,
solar or slightly over-solar metallicity.

We calculated that the UGC 4805 nucleus and other LINERs present metallicity
and ionization parameter sensitive emission-line ratios similar to those
observed in confirmed Seyfert 2 nuclei,l although exhibiting a slightly
different trend. Even though LINERs present low ionization parameter values
($\log U\>\la\>-3.2$), Seyfert 2 nuclei also present low values of the
ionization parameter. Although both AGN and p-AGB models (with $T_{\rm eff}$=
100 and 190 kK) are able to reproduce the observational data, the results from
the WHAN diagram combined with the fact that the high excitation level of the
gas has to be maintained at kpc scales, suggest that the main ionizing source
of the UGC 4805 nucleus probably has a stellar origin rather than an AGN.

## Acknowledgements

ACK thanks to CNPq. CBO is grateful to the FAPESP for the support under grant
2019/11934-0, and to the CAPES. IAZ acknowledges support by the grant for
young scientist’s research laboratories of the National Academy of Sciences of
Ukraine. AHJ thanks to CONICYT, Programa de Astronomía, Fondo ALMA-CONICYT
2017, Código de proyecto 31170038.

##  7 Data Availability

The data underlying this article will be shared on reasonable request to the
corresponding author.

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