11institutetext: Department of Space, Earth and Environment, Chalmers
University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
22institutetext: National Radio Astronomy Observatory, Charlottesville, VA
22903, USA 33institutetext: Department of Astronomy, University of Florida,
Gainesville, FL 32611, USA 44institutetext: Department of Astronomy,
University of Virginia, Charlottesville, VA 22904, USA 55institutetext: The
Institut de radioastronomie millimétrique (IRAM), Domaine Universitaire, 38406
Saint Martin d’Hères, France 66institutetext: Wisconsin IceCube Particle
Astrophysics Center, Madison, WI 53703, USA 77institutetext: Jodrell Bank
Centre for Astrophysics, Department of Physics and Astronomy, The University
of Manchester, M13 9PL, UK 88institutetext: Department of Astronomy,
University of Virginia, 530 McCormick Road, Charlottesville, VA 22903, USA
99institutetext: Observatoire de Paris, LERMA, Collège de France, CNRS, PSL
University, Sorbonne University, Paris, France 1010institutetext: Institute of
Astrophysics, Foundation for Research and Technology Hellas (FORTH),
Heraklion, GR-70013, Greece 1111institutetext: National Radio Astronomy
Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
1212institutetext: Department of Physics, University of Oxford, Keble Road,
Oxford OX1 3RH, UK 1313institutetext: Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69, 53121 Bonn, Germany 1414institutetext: Astronomy Department,
Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589,
Saudi Arabia 1515institutetext: National Astronomical Observatory of Japan,
National Institutes of Natural Sciences (NINS), 2-21-1 Osawa, Mitaka, Tokyo
181-8588, Japan 1616institutetext: European Southern Observatory, Alonso de
Córdova, 3107, Vitacura, Santiago, 763-0355, Chile 1717institutetext: Joint
ALMA Observatory, Alonso de Córdova, 3107, Vitacura, Santiago, 763-0355, Chile
1818institutetext: Department of Astronomy, The University of Tokyo, 7-3-1,
Hongo, Bunkyo, Tokyo 113-0033, Japan 1919institutetext: Nućleo de Astronomía
de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército
Libertador 441, Santiago 22, Chile 2020institutetext: Kavli Institute for
Astronomy and Astrophysics, Peking University, Beijing 100871, People’s
Republic of China 2121institutetext: Leiden Observatory, Leiden University, PO
Box 9513, 2300 RA Leiden, The Nether- lands 2222institutetext: Department of
Physics and Astronomy, University College London, Gower Street, WC1E 6BT,
London, UK

# Double, double, toil, and trouble:

The tails, bubbles, and knots of the local compact obscured nucleus galaxy NGC
4418

C. F. Wethers 11<EMAIL_ADDRESS>S. Aalto 11 G. C. Privon 223344 F.
Stanley 55 J. Gallagher 66 M. Gorski 11 S. König 11 K. Onishi 11 M. Sato 11 C.
Yang 11 R. Beswick 77 L. Barcos-Munoz F. Combes 8899 T. Diaz-Santos 1010 A. S.
Evans 111188 I. Garcia-Bernete 1212 C. Henkel 13131414 M. Imanishi 1515 S.
Martín 16161717 S. Muller 11 Y. Nishimura 1818 C. Ricci 19192020 D. Rigopoulou
22 S. Viti;
21212222

(Received ????; accepted ????)

###### Abstract

Context. Compact obscured nuclei (CONs) are an extremely obscured
(N${}_{\rm{H2}}>$1025 cm-2) class of galaxy nuclei thought to exist in 20-40
per cent of nearby (ultra-)luminous infrared galaxies While they have been
proposed to represent a key phase of the active galactic nucleus (AGN)
feedback cycle, the nature of these CONs – what powers them, their dynamics,
and their impact on the host galaxy – remains unknown.

Aims. This work analyses the galaxy-scale optical properties of the local CON
NGC 4418 ($z$=0.00727). The key aims of the study are to understand the impact
of nuclear outflows on the host galaxy and infer the power source of its CON.
Through the mapping of the galaxy spectra and kinematics, we seek to identify
new structures in NGC 4418 to ultimately reveal more about the CON’s history,
its impact on the host, and, more generally, the role CONs play in galaxy
evolution.

Methods. We present new, targeted integral field unit observations of the
galaxy with the Multi-Unit Spectroscopic Explorer (MUSE). For the first time,
we mapped the ionised and neutral gas components of the galaxy, along with
their dynamical structure, to reveal several previously unknown features of
the galaxy.

Results. We confirm the presence of a previously postulated, blueshifted
outflow along the minor axis of NGC 4418. We find this outflow to be
decelerating and, for the first time, show it to extend in both directions
from the nucleus. We report the discovery of two further outflow structures: a
redshifted southern outflow connected to a tail of ionised gas surrounding the
galaxy and a blueshifted bubble to the north. In addition to these features,
we find the [O iii] emission reveals the presence of knots across the galaxy,
which are consistent with regions of the galaxy that have been photoionised by
an AGN.

Conclusions. We identify several new features in NGC 4418, including a bubble
structure, a reddened outflow, and [O iii] knot structures throughout the
galaxy. We additionally confirm the presence of a bilateral blueshifted
outflow along the minor axis. Based on the properties of these features, we
conclude that the CON in NGC 4418 is most likely powered by AGN activity.

###### Key Words.:

galaxies: nuclei – galaxies: kinematics and dynamics – galaxies: evolution –
galaxies: active

##  1 Introduction

Compact obscured nuclei (CONs) have been shown to exist in as many as 40 per
cent of nearby (ultra-)luminous infrared galaxies ((U)LIRGs; e.g. Falstad et
al., 2021; García-Bernete et al., 2022). These compact (¡100 pc) nuclear
structures are characterised by their extreme nuclear column densities, NH2
$>$1025 cm-2 (e.g. Sakamoto et al., 2013), which make them almost invisible at
mid-IR, optical, and even X-ray wavelengths due to the heavy attenuation (e.g.
Treister et al., 2010; Lusso et al., 2013; Roche et al., 2015). Currently, the
only way to identify CONs is through the rare $\nu$2 = 1f transition of
vibrationally excited HCN (HCN-VIB), which is usually radiatively excited by
intense mid-IR emission from hot dust at high column densities (Aalto et al.,
2015). While a handful of alternative identification methods are being
developed (e.g. Donnan et al., 2023), including using the 1 millimetre surface
brightness (Falstad et al., 2021) and the silicate absorption slope (García-
Bernete et al., 2022), no CONs have yet been confirmed via these methods.

Although the exact nature of CONs remains unknown, they likely play a critical
role in the evolution of galaxies as an early, obscured starburst phase in
which remnant material is being expelled from the centre of the galaxy via,
for example, active galactic nucleus (AGN) driven outflows. Such a phase could
represent the early stages of a feedback cycle, where a fraction of the
expelled gas will eventually fall back into the galactic plane. Indeed, while
only a handful of CONs have been confirmed in the nearby Universe (z$<$0.05),
all show signatures of molecular outflows in HCN and/or CO, and several host
molecular gas inflows (Falstad et al., 2021). Even in the CONs with no obvious
inflow signatures, the CO outflows appear to be slow (V$<$400 km s-1), meaning
this gas will likely remain within the galaxy. This inflow and outflow
feedback cycle provides a key mechanism by which a super-massive black hole
(SMBH) affects its host galaxy on scales larger than its sphere of influence.
Thus, identifying these CON outflows, understanding what drives them, and
quantifying their effect on the larger-scale galaxy properties is critical in
building a co-evolutionary model of galaxies.

Two main interpretations of the CON phenomenon exist. The first is a hidden
AGN scenario, in which the nuclear region is powered by active accretion onto
the central SMBH. In this scenario, the galaxy nucleus is completely obscured
by remnant material, following a merger or interaction event (e.g. Kocevski et
al., 2015; Ricci et al., 2017; Blecha et al., 2018; Boettcher et al., 2020).
The frequency of such interaction events, and their severity, has been shown
to increase with (U)LIRG luminosity (Sanders & Mirabel, 1996), meaning that if
CONs are associated with rapid gas inflow onto a SMBH, they should reside in
more luminous LIRGs and merger-rich environments. Indeed, Falstad et al.
(2021) find the CON fraction of ULIRGs (L${}_{\rm{IR}}>$ 1012 L⊙; $\sim$40 per
cent) is almost double that of LIRGs (10${}^{11}<$L${}_{\rm{IR}}>$1012 L⊙;
$\sim$20 per cent). Furthermore, the compact nature of CONs strongly favours a
hidden AGN scenario (e.g. Evans et al., 2003; Costagliola & Aalto, 2010;
Sakamoto et al., 2010). The second interpretation is that CONs are powered by
a nuclear nascent starburst, deeply embedded in a dusty cocoon. This so-called
embedded starburst scenario was initially put forward to explain the unusual
far-IR and radio properties observed in CONs (e.g. Yun et al., 2001; Bressan
et al., 2002; Roussel et al., 2003). The scenario postulates that CONs host a
very young starburst ($\sim$4-6 Myr) with little to no non-thermal radio
emission arising from supernovae (SNe). These starbursts are rare by
definition, owing to their short lifespans, and become ’ordinary’ starbursts
containing many SNe. However, several groups have argued that dust is unlikely
to completely cover an extended starburst to such an extent as to cause the
deep absorption features observed in CONs (Roche et al., 1986; Dudley & Wynn-
Williams, 1997; Spoon et al., 2001).

Distinguishing between the embedded starburst and hidden AGN scenario for CONs
has proven challenging, in part due to the fact that the extreme nuclear
column densities of CONs can completely erase many of the traditional AGN
signatures in the galaxy spectra. Nevertheless, identifying the dominant power
source of CONs remains critical in building a cohesive model of galaxy
evolution. AGN are currently considered a short, but reoccurring, phase in the
lifetime of a galaxy. However, if CONs are found to be universally powered by
AGN, this would increase the predicted number of local AGN by a factor of
$\sim$2 (Maiolino et al., 2003) and thus revolutionise our understanding of
AGN phase(s) in the evolution of local galaxies. Alternatively, if CONs are
powered by a nascent starburst, they represent a rare and unusually intense
compact nuclear starburst, which likely plays an important role in the growth
of galactic nuclei.

NGC 4418 ($z$=0.00727) has been described as a prototypical CON (e.g.
Costagliola et al., 2013) and has been the subject of several studies into the
nature of CONs. NGC 4418 is part of an interacting system with the nearby
dwarf galaxy VV655, which lies to the south-east at a distance of $\sim$30 kpc
(2-3 arcmin) from NGC 4418 (e.g. Varenius et al., 2017; Boettcher et al.,
2020) and may be in part responsible for several of the outflowing and
inflowing features observed throughout the galaxy. A study by Sakamoto et al.
(2013) finds evidence for an outflow on scales of $\sim$1 kpc along the minor
axis of NGC 4418. More recently, Ohyama et al. (2019) confirm the presence of
this outflow and classify it as a dusty superwind, based on both shock-heated
emission lines and the enhanced stellar NaD absorption. Additionally, work by
González-Alfonso et al. (2012) finds a substantial inflow in NGC 4418 with a
mass inflow rate $\dot{M}$ $\gtrsim$ 12 M⊙yr-1. The existence of both inflows
and outflows in NGC 4418 implies that its nucleus is evolving very rapidly,
yet the structure of these features and their impact on the galaxy remain
unknown. As a result, identifying the mechanism(s) by which cool gas is
funnelled into the centre of the NGC 4418 has been the subject of several
recent studies. Within the local galaxy population, such gas infall events are
likely triggered by major gas-rich mergers, yet to date observations of NGC
4418 have shown none of the prominent tidal tails, shells, or stellar
asymmetries typically associated with such interactions. Instead, Boettcher et
al. (2020) postulate the trajectory of the neighbouring dwarf galaxy, VV655,
has likely caused tidal torques in NGC 4418, leading to material from the
initial gas disk being driven towards the galaxy centre. Indeed, models
demonstrate that a satellite galaxy with a Small Magellanic Cloud-like mass,
such as VV655, can induce torques sufficient to trigger a significant inward
migration of gas from the disk (Pettitt et al., 2016; Ramón-Fox & Aceves,
2020). Confirming the mechanism(s) by which so much dust and gas migrates to
the galaxy centre remains an important question, both for determining the
nature of CONs and galaxy evolution in general.

Despite extensive multi-wavelength studies, the nuclear power source of NGC
4418 is also unknown. On the one hand, X-ray observations with Chandra show
NGC 4418 to have a flat, hard X-ray spectrum, usually indicative of a Compton-
thick AGN (Maiolino et al., 2003). This scenario is supported by the apparent
compactness of the CON in NGC 4418, with near- and mid-IR observations
indicating an optically thick source with a diameter $<$70 pc (Evans et al.,
2003), and vibrationally excited HC3N (Costagliola & Aalto, 2010) and HCN
(Sakamoto et al., 2010) studies deriving sizes $<$10 pc in diameter. On the
other hand, Roussel et al. (2003) suggest the low radio-to-far-IR luminosity
ratio of NGC 4418 is entirely consistent with a young starburst ($<$5 Myr),
and Varenius et al. (2014) find several compact nuclear structures believed to
be super star clusters. Many of these structures, however, lie outside the
optically thick 5 $\mu$m photosphere identified from the near-IR CO absorption
from warm molecular gas (Ohyama et al., 2023), within which most of the galaxy
luminosity is expected to arise. It is therefore unlikely that the SNe and
super star clusters seen by Varenius et al. (2014) are the dominant CON power
source. With the advent of optical integral field unit (IFU) observations with
optical instruments such as the Multi-Unit Spectroscopic Explorer (MUSE), we
may be able to distinguish between these scenarios in NGC 4418 for the first
time. Despite the heavy obscuration at their nuclei, optical IFU data allow
the kinematics of the ionised and neutral gas to be spatially resolved and
shed light on the inflow and outflow structures and their impact on the host
galaxy.

Here we present the first results from new IFU observations of NGC 4418 with
MUSE. In section 2 we outline the observations, reduction pipeline, and data
products. Section 3 presents our key findings, including the 2D mapping of the
dust extinction, dynamical structure, and inflow and outflow features in NGC
4418. Possible interpretations of these results are discussed in section 4 and
are summarised in section 5. Throughout this paper, we assume a flat $\Lambda$
cold dark matter cosmology with $H_{0}$ = 70 km s-1 Mpc-1, $\Omega_{M}$ = 0.3,
and $\Omega_{\Lambda}$ = 0.7. We adopt a distance of 34 Mpc, at which 1 arcsec
= 165pc (Sakamoto et al., 2013). Unless otherwise specified, all quoted
magnitudes are given in the AB system.

##  2 Data

###  2.1 Data overview

We present new observations from MUSE for the local CON galaxy, NGC 4418. MUSE
is a panoramic IFU spectrograph on the Very Large Telescope (VLT), based at
Cerro Paranal, Chile. The IFU capabilities of MUSE allow the kinematics of the
ionised and neutral gas to be mapped throughout the galaxy for the first time.
Observations of NGC 4418 were acquired in 2020 February, during ESO period 104
(ESO0104.B-0668; P.I. F. Stanley). Observations were carried out in wide-field
mode (WFM), covering a 60 $\times$ 60 arcsec field of view (FoV) with a
spatial sampling of 0.2 $\times$ 0.2 arcsec and a theoretical instrument
spatial resolution of 0.4 arcsec (full-width at half-maximum; FWHM) at 7000 Å.
The nominal instrument setup provides a spectral coverage from 4750 to 9300 Å
at a mean resolution of 2.5 Å (FWHM) and provides a dataset of $>$90,000
individual spectra (323 $\times$ 324 spaxels). The total on-source integration
time was 2.8 h, with an average seeing of 0.8 arcsec. The raw data products
were reduced via the MUSE EsoReflex pipeline (v.2.6.2) to produce a fully
calibrated and combined science-ready data cube with a point spread function
(PSF) FWHM of 0.8$\times$1.0 arcsec at 5000 Å. Any residual sky contamination
was identified using the Zurich Atmosphere Purge (ZAP) software package (Soto
et al. 2016) and subtracted from the final data cube.

###  2.2 Continuum subtraction

Following the initial reduction of the MUSE data cube, we fitted and
subtracted the stellar contribution from the galaxy continuum in order to
isolate light from the emission features. The continuum fitting makes use of
single stellar population (SSP) synthesis models from v9.1 of the Medium
resolution INT Library of Empirical Spectra (MILES; Falcón-Barroso et al.,
2011). The complete MILES library consists of spectra for $\sim$1000 stars
spanning a large range in atmospheric parameters, obtained with the 2.5m INT
telescope. The available stellar templates span ages 0.03 - 17.78 Gyr and
metallicities 0.0001 $<Z<$ 0.04. The SSP templates provide composite spectral
energy distributions (SEDs) comprised from multiple stellar population
templates, each with a single age and metallicity. Each template spans
wavelengths 3525-7500 Å (Sánchez-Blázquez et al., 2006) with a spectral
resolution of 2.5 Å (FWHM), corresponding to the mean instrumental resolution
of MUSE. Templates are fitted pixel-by-pixel to the MUSE data cube using a
penalised pixel fitting (pPXF) method, based on a non-linear least squares
fitting routine (Cappellari, 2017). As part of this process, the spectral data
were re-binned such that each channel had a fixed velocity width, $\Delta$V =
55.9 km s-1. Throughout the fitting, all strong emission lines in the MUSE
spectra are masked to prevent them from biasing the fit of the stellar
templates and overestimating the continuum level. In this way, we ensured the
fitted stellar continuum templates are constrained only by the strength of the
stellar absorption and the galaxy continuum, which traces the local
interstellar reddening, and the age and metallicity of the stellar
populations.

Following the continuum modelling and subtraction there remains a non-
negligible H$\beta$ ($\lambda$4861$\AA$) absorption feature in several
spaxels. This residual absorption is centrally concentrated. It appears strong
towards the galaxy nucleus, but becomes negligible at larger radii. To account
for this residual absorption in the continuum-subtracted data cube, we
performed a spaxel-by-spaxel non-linear least squares fitting over the
spectral region 4750-4950 Å, covering the H$\beta$ feature. The model for the
fitting combines two Gaussian functions: a broad, negative component to model
the H$\beta$ absorption, and a narrower positive component to model the
H$\beta$ emission. During the fitting, we accounted only for spaxels in which
the residual H$\beta$ absorption is significant, namely, $>$3$\sigma$ of the
spectral noise as measured from a nearby, featureless region of the spectrum.
The absorption component of the best-fit model is then removed from the
original continuum-subtracted cube to produce an H$\beta$-corrected data cube.
Unless otherwise stated, all analysis in this work is performed on the
continuum-subtracted data cube corrected for any residual H$\beta$ absorption.

##  3 Results

###  3.1 Overview

In this work we identify five new features in NGC 4418, which we summarise
here as follows: (1) An outflow along the minor axis of the galaxy, extending
in both directions from the galaxy nucleus. Throughout this work, we refer to
this feature as the minor axis dusty superwind. (2) A bubble-like structure to
the north, which we refer to as the northern bubble. (3) Two outflow features
to the south, which we denote as S1 and S2. (4) Knots tracing regions of
ionised gas in the galaxy. These are predominantly traced by [O iii], but a
subset are also traced by H$\alpha$. (5) A tail of redshifted material
surrounding the galaxy, which appears to originate from the S1 outflow. We
name this the H$\alpha$ tail. Fig. 1 illustrates the location of these
features in NGC 4418 for reference. Throughout this work, we explore these
features in more detail, outlining their key properties and using them to
infer information about the CON in NGC 4418 and its power source.

Figure 1: Schematic illustrating the key features of NGC 4418 identified in
this work.

###  3.2 Spectral features

####  3.2.1 Emission lines

Figure 2: RGB colour images for left: H$\beta$ (red), [O iii] (green), and [O
I] (blue) and right: H$\alpha$ (red), the [N ii] doublet (green), and the [S
II] doublet (blue) with key features labelled. North is up, and east is to the
left.

In this section we focus on six key diagnostic emission lines tracing the
ionised gas in NGC 4418: H$\beta$ ($\lambda$4861), [O iii] ($\lambda$5007), [O
I]($\lambda$6300), [N ii] ($\lambda\lambda$6548,6583), H$\alpha$
($\lambda$6563), and [S II] ($\lambda\lambda$6716,6731). The effect of line
blending in the MUSE data cube is negligible, allowing for the effective
separation of these different emission lines via channel selection alone. Fig.
2 presents the Lupton red, green, blue (RGB) colour images for H$\beta$, [O
iii] and [O I], and [N ii], H$\alpha,$ and [S II] (Lupton et al., 2004). The
component line maps are created by summing the flux across the range of
channels associated with each line to create flux-integrated images. These
images are then weighted in order optimise the visual output of the resulting
colour image Fig. 2. This method was chosen to minimise the flux disparity
between structures traced by intrinsically brighter (e.g. H$\alpha$) and
fainter (e.g. [S II]) lines and thus to best illustrate the key features
identified in NGC 4418. The line emission generally shows an extension along
the direction of the minor axis (south-east to north-west), extending in both
directions from the galaxy nucleus. In addition to this minor axis structure,
we highlight several key features of NGC 4418 traced by individual emission
lines as follows;

H$\beta$ ($\lambda$4861): We note the presence of two emission features
towards the south: one to the south-west, aligning with the major axis of the
galaxy, and the other to the south-east. In the interests of clarity, we
denote these two southern features as S1 and S2, respectively. In addition to
these features, we identify a void to the north-east, surrounded by a shell-
or bubble-like emission structure. This structure is reminiscent of that
observed in the so-called teacup galaxy, J1430+1339 (Treister et al. in
prep.), which has previously been proposed as a fading AGN candidate (Gagne et
al., 2014). Hereafter, we refer to this feature as the northern bubble.

[O iii] ($\lambda$5007): We identify an emission structure towards the galaxy
centre, tracing both the southern features (S1 and S2) and the northern
bubble. In addition, the [O iii] emission reveals bright clumps, or knots,
throughout the galaxy, which have never before been observed in NGC 4418.
Several of these knots appear extended, yet the majority appear compact and
trace a seemingly random and fragmented structure throughout the galaxy. The
nature of these knots is explored in detail in section 4.2.

[O I]($\lambda$6300): The [O I] emission appears visually less extended than
that of both H$\beta$ and [O iii]. [O I] shows a more rapid radial light
depletion than either H$\beta$ or [O iii], indicating a smaller half-light
radius. While we identify prominent [O I] emission tracing the S1 feature, we
find no evidence that [O I] traces either the S2 feature or the northern
bubble.

H$\alpha$ ($\lambda$6563): We discover several knots traced by the H$\alpha$
emission. Although similar to those identified in the [O iii] emission, they
are fewer in number and generally appear more spatially extended. Towards the
centre of the galaxy, a handful of these knots trace the S1 and S2 features,
as well as the westward edge of the northern bubble. At larger radii, the
knots appear coincident with a small subset of the knots traced by [O iii].

[N ii] ($\lambda\lambda$6548,6583): The [N ii] doublet also traces a bright
knot to the south-west of the nucleus, aligning with the S1 feature. We note
the presence of an additional, isolated south-western knot further out in the
galaxy disk, along with a diffuse knot to the south-east, coinciding with the
locations of two of the knots traced by both H$\alpha$ and [O iii]. With the
exception of these two knots, we find no evidence that [N ii] is enhanced in
the knot structures traced by [O iii]. While we identify faint [N ii] emission
tracing the northern bubble, this feature is much less pronounced than in
H$\beta$, [O iii] or H$\alpha$.

[S II] ($\lambda\lambda$6716,6731): The [S II] emission doublet traces both
the S1 and S2 features, but not the northern bubble. With the exception of two
bright knots to the far north, we find no evidence that [S II] is enhanced in
any of the knots traced by [O iii].

While the features identified above may be tracing outflowing or inflowing
material, it is also possible that the emission is instead mapping dust
obscuration and/or star forming regions within the galaxy. In order to
distinguish between these scenarios and develop a physical interpretation of
the line emission observed in NGC 4418, we therefore require additional
kinematic information. In section 3.4.2 we analyse the kinematics of both the
stellar disk and the ionised gas traced by the line emission in Fig. 2. For
now however, we note only the presence of several prominent structures in the
line emission.

####  3.2.2 The NaD$\lambda\lambda$5890,5896 Å absorption doublet

The NaD$\lambda\lambda$5890,5896 $\AA$ absorption doublet traces two distinct
components of the interstellar medium (ISM): cold molecular gas and neutral
atomic gas. Although the NaD absorption observed in NGC 4418 is a sum of both
these ISM components, the stellar templates used to fit the continuum (see
section 2.2), include only the NaD absorption arising in stellar photospheres.
Here, we separate the two ISM components traced by NaD by masking the
absorption doublet in the spectra when fitting the continuum. In this way, the
stellar component of the NaD absorption included in the stellar templates is
constrained only by the continuum strength. Subtracting the best-fit continuum
model therefore removes only the absorption due to stellar photospheres and
isolates the NaD absorption from neutral atomic gas in the resulting spectra.
This remaining NaD absorption is normalised to the galaxy continuum to remove
the degeneracy between the strength of the continuum and the absorption depth
(i.e. a brighter continuum has a higher absorption potential). Fig. 3 shows
the continuum-normalised NaD absorption, tracing the neutral gas in NGC 4418.
The peak of the absorption coincides with the position of the CON in NGC 4418
and extends towards both the north and the west, forming a ‘U’ shape.

Figure 3: Integrated line map of the NaD$\lambda\lambda$ 5890,5896 Å
absorption doublet normalised to the galaxy continuum level. Colours have been
inverted such that stronger absorption corresponds to brighter regions on the
map. Black cross denotes the location of the CON.

####  3.2.3 H$\beta$ absorption

The equivalent width of the H$\beta$ absorption is highly sensitive to the
initial mass function (IMF) of the galaxy and thus is often used as an
indicator of the underlying stellar population (e.g. Copetti et al., 1986),
primarily tracing the presence of A-type stars in the galaxy. To explore the
stellar properties of NGC 4418, we therefore map the H$\beta$ absorption
across the galaxy. To this end, we perform a spaxel-by-spaxel non-linear least
squares fitting to the original data cube, prior to any continuum modelling
and subtraction. The region of the spectrum covering the H$\beta$ absorption
feature (4750 - 4950 Å) is isolated and each spaxel is fit with a two-
component Gaussian model, consisting of both a narrow emission and a broad
absorption component. The continuum strength is estimated from a nearby,
featureless region of the spectrum and is also included in the fitting. A
signal-to-noise ratio cut of three is applied prior to this fitting such that
we consider only spaxels for which the H$\beta$ absorption strength exceeds
three times the standard deviation of the spectral noise. The total H$\beta$
absorption in each spaxel is calculated by integrating over the broad
absorption component of the best-fit model. As in section 3.2.2, the resulting
H$\beta$ absorption is then normalised to the strength of the continuum and is
plotted in Fig. 4, where we reveal several notable features in the H$\beta$
absorption of NGC 4418. We identify a strong absorption feature to the south-
east of the galaxy nucleus, which appears elongated along the east-west axis.
We additionally find an absorption feature to the north-east, which we suggest
is associated with the void of the bubble structure seen on the H$\beta$
emission (Fig. 2). To the south, we also find an enhancement in the absorption
strength of H$\beta$, potentially associated with the S1 and S2 features.

Figure 4: Map of H$\beta$ absorption strength, normalised to the continuum
level in each individual spaxel. A 3$\sigma$ S/N cut has been applied and the
image has been smoothed by a 1$\sigma$ Gaussian kernel to improve visual
output. Colours have been inverted such that stronger absorption corresponds
to brighter regions on the map. White cross denotes the location of the CON.

###  3.3 Dust extinction

Having mapped the ionised (emission) and neutral (absorption) components of
the gas in NGC 4418, we now map the dust extinction in the galaxy. The total
extinction, $A_{v}$, is calculated from the Balmer decrement of the continuum-
subtracted spectra (Eq. 1),

$\frac{F(H\alpha)}{F(H\beta)}=\frac{F_{0}(H\alpha)}{F_{0}(H\beta)}10^{\frac{-0.4A_{v}(k(H\alpha)-k(H\beta))}{R_{V}}},$
(1)

where $F$ is the peak flux of the given emission line, RV is the effective
total obscuration in the V-band and $k$ is the chosen dust extinction law.
During the line fitting, the width of the emission features, $\sigma$, were
fixed to be the same, meaning the peak flux ratio is equivalent to the ratio
of the integrated fluxes of the lines. We assume a Calzetti et al. (2000)
attenuation law, which gives RV=4.05 and $k(H\alpha)-k(H\beta)$=-0.58. This
law has been empirically derived from nearby starburst galaxies and thus
provides a suitable approximation for NGC 4418. Based on the work of
Osterbrock & Ferland (2006), we assume an intrinsic flux ratio:

$\frac{F_{0}(H\alpha)}{F_{0}(H\beta)}=2.86.$ (2)

The value of $A_{v}$ for each spaxel is plotted in Fig. 5, where a signal-to-
noise ratio cut of three has been imposed to remove any spurious measurements.
The resulting extinction map (Fig. 5) shows a similar ’U’ shape to that
observed in the NaD neutral gas (Fig. 3), with an extended structure to the
north. Furthermore, we identify two prominent dust structures to the south,
aligning with both the S1 and S2 emission line features. We note the lack of
dust along the direction of the minor axis of the galaxy, implying the dust in
this region may have been cleared out by, for example, an outflow.

Figure 5: Map of $A_{v}$ values, derived from the Balmer decrement. The map
has been smoothed with a 1$\sigma$ Gaussian kernel to improve the visual
output and a signal-to-noise ratio cut of three has been applied.

###  3.4 Galaxy dynamics

Although the line emission and absorption features in NGC 4418 provide
valuable information on the morphological structure, stellar populations and
dust composition within the galaxy, many of the structures we observe cannot
be physically interpreted without dynamical information. To this end, we
exploit the IFU capabilities of MUSE to present the kinematic properties of
NGC 4418.

####  3.4.1 Stellar kinematics

The rotation of the stellar disk of NGC 4418 is mapped from the wavelength
shift of the best-fit stellar templates during the continuum modelling
(section 2.2). During this modelling process, all prominent emission lines in
the spectra are masked, meaning the fitted stellar templates are constrained
only by the spectral absorption. While the stellar velocities can therefore be
measured for each spaxel in the cube individually, we instead opt to bin the
data into regions with a fixed signal-to-noise. To achieve this, we implement
a Voronoi adaptive binning system (Cappellari & Copin, 2003), using the
5000$\AA$ flux as a reference for the binning. The continuum modelling was re-
run on the binned data cube, with a target signal-to-noise ratio of 20 in each
bin. Fig. 6 maps the resulting stellar velocities derived in each bin. We
discover an asymmetric disk rotation in NGC 4418, with the north-east side
approaching with velocities, V$\sim$-115 km s-1 and the south-west side
receding with V$\sim$80 km s-1. Such asymmetries can be caused by a major
tidal disruption event, such as a major merger. It is therefore possible that
NGC 4418 underwent such an event in the past, potentially with the nearby
galaxy VV655 (e.g. Boettcher et al., 2020), although we note that we see no
conclusive evidence for this.

Within the central kiloparsec we identify a structure that does not appear to
be rotating with the stellar disk. Although difficult to resolve due to the
spatial resolution of MUSE (0.4 arcsec), the velocities in this central region
appear counter-aligned with the major axis rotation of the galaxy. Given the
region’s proximity to the galaxy centre, the peculiar velocities can be seen
in both the major and minor axis rotation curves of NGC 4418 (Fig. 7). Along
the major axis (Fig. 7), we identify a small ‘bump’ in the velocity structure.
Likewise, the minor axis rotation curve (Fig. 7) also shows a ‘bump’, offset
by $\sim$0.25 kpc from the galaxy centre.

We suggest that the anomalous velocity structure we observe may indicate the
presence of a counter-rotating nuclear disk. This conclusion is consistent
with the results of Ohyama et al. (2019), who also find evidence for a
counter-rotating structure in the centre of NGC 4418 based on single-slit
spectroscopy. We note however that Ohyama et al. (2019) do not explicitly
distinguish between a counter-rotating gas disk and other non-rotational
motions of the central gas. Despite the lack of a large-scale bar in NGC 4418
we suggest that the peculiar velocity structure may alternatively mark the
presence of a nuclear bar. One test to help distinguish between these
scenarios is to look for any (anti-)correlation in the rotation velocities of
the stellar disk and the third Gauss-Hermite coefficient, h3. The h3 parameter
is output from the pPXF fitting described in section 2.2. An anti-correlation
between these parameters would indicate the likely existence of a counter-
rotation as opposed to a nuclear bar. Indeed, we observe an anti-correlation
between the h3 parameter and the rotation of the galaxy disk (Fig. A.1), with
the positive (red skewed) h3 coefficients to the north-east (blue side) of the
stellar disk. While we emphasise that the spatial resolution of MUSE is
insufficient to map the detailed kinematics of the central kiloparsec, the
broad anti-correlation between the h3 coefficient and the rotation of the
galaxy disk supports our conclusion of a counter-rotation.

We additionally identify a clear enhancement in the stellar velocity
dispersion, $\sigma$, towards the centre of NGC 4418 (Fig. A.1). This enhanced
dispersion forms a small cross-like structure extending both along the minor
axis of the galaxy, coincident with the location of the ionised outflow, and
along the major axis tracing the galaxy disk rotation. Due to the orientation
of this feature in, we suggest it is likely that the enhanced velocity
dispersion is associated with the outflow and galaxy disk. It is also possible
however, that we are instead observing two misaligned nuclear disks; a
reasonable scenario given the merger history of NGC 4418. While the limited
resolution of the MUSE imaging prevents us from robustly distinguishing
between these scenarios, observations with ALMA at higher spatial resolution
confirm that the nuclear gas disk in NGC 4418 is counter-rotating with respect
to the galaxy disk (Sakamoto et al., 2021). Although Sakamoto et al. (2021)
attribute this feature to be a disturbance resulting from an outflow, nuclear
rotation, or turbulence, Varenius et al. (2017) also suggest gas inflow from a
companion galaxy as a potential explanation. Regardless of the exact nature of
the central velocity structure observed in NGC 4418, it is a clear indicator
of a major ongoing or recent disturbance to the nuclear region, perhaps
associated with a recent merger or interaction event.

Figure 6: Map of the stellar velocities derived from the pPXF fitting. The
black square denotes the extent of the 7$\times$7 arcsec2 zoom-in region shown
in the insert. The position of the CON is marked with a black cross and dashed
lines mark the positions of the major and minor axes. Pixels have been binned
to achieve a signal-to-noise ratio of 20. An initial signal-to-noise ratio cut
of 2 has been applied, with pixels below this threshold omitted from the
binning. Figure 7: Galaxy rotation curves along the kinematic major (solid
line) and minor (dashed line) axes of NGC 4418. In each case, velocities are
averaged over a simulated slit with a width of $\pm$3 pixels ($\pm$0.6
arcsec). Shaded regions denote a standard deviation from the average velocity.

####  3.4.2 Ionised gas velocities

Fig. 8 shows the moment-1 velocity maps for [O iii] ($\lambda$5007), H$\alpha$
and [N ii] ($\lambda$6583), tracing the motions of the ionised gas in NGC
4418. The moment maps were created using the Scipy package scipy.stats.moment,
and have undergone a signal-to-noise ratio cut of three to remove any spurious
velocity features. All three tracers show clear non-circular motions,
independent from the rotation of the stellar disk. In each case, we identify a
blueshifted structure to the east of the galaxy nucleus with a peak velocity,
$V\sim$75 km s-1. In the moment-1 maps of H$\alpha$ and [N ii] (Fig. 8), this
eastern blueshifted structure comprises multiple distinct regions, appearing
as three separate structures in the H$\alpha$ velocity map and two in the [N
ii] map. In both cases, the northern-most structure is the most prominent.
This northern-most blueshifted structure appears spatially aligned with the
void of the northern bubble seen in the H$\beta$ line emission (section
3.2.1). The moment-1 maps of H$\alpha$ and [N ii] also reveal a blueshifted
feature to the west of the nucleus with velocities up to $V\sim$140 km s-1.
This feature is much larger than that to the east, extending $>$1 kpc from the
galaxy centre, and appears aligned with the minor axis elongation traced by
the emission lines (Fig. 2).

In addition to these blueshifted structures, we detect a slower ($V<$50 km
s-1) redshifted feature to the south. This feature is present in all three of
the velocity maps in Fig. 8, albeit faint in [O iii]. This redshifted
structure appears aligned with the S1 feature we identify to the south. In the
H$\alpha$ and [N ii] velocity maps, there is some indication that this
structure also traces the S2 feature, but this feature is comparatively faint
in terms of its velocity shift. We therefore conclude that the S2 feature is
likely tracing slower material than S1.

Figure 8: Moment-1 velocity maps of the ionised gas traced by [O iii] (left),
H$\alpha$ (centre), and [N ii]$\lambda$6583 (right). In each case, a signal-
to-noise ratio cut of three has been applied. Maps have been smoothed by a
$\sigma$ = 1 spaxel Gaussian kernel to improve visual output.

####  3.4.3 Velocity dispersion of the ionised gas

Fig. 9 shows the moment-2 velocity dispersion maps for [O iii]
($\lambda$5007), H$\alpha$ and [N ii] ($\lambda$6583), tracing the random
motions of the ionised gas along the line of sight and in turn probing the
motions of the stars in the gravitational potential of the galaxy. The
H$\alpha$ and [N ii] maps (Figs. 9) show an inverted ‘3’ shaped structure,
angled towards the north-west, over which the velocity dispersion is enhanced.
The central part of this structure appears aligned with the extension along
the minor axis we observe in the line emission (Fig. 2) and with the strongly
blueshifted structure seen in the moment-1 velocity maps (Fig. 8). If this
blueshifted structure is an outflow, we therefore expect it to contain a
significant amount of mass. The regions at either side of this minor axis
structure have a lower velocity dispersion, implying that either the mass in
these regions has been cleared out or the mass is in some way collimated along
the minor axis of NGC 4418. We also identify an enhancement in the velocity
dispersion towards the south, in apparent alignment with the redshifted S1 and
S2 features.

Figure 9: Moment-2 velocity dispersion maps of the ionised gas traced by [O
iii] (left), H$\alpha$ (centre), and [N ii]$\lambda$6583 (right). A signal-to-
noise ratio cut of three has been applied. Maps have been smoothed by a
$\sigma$ = 1 spaxel Gaussian kernel to improve visual output. North is up,
east is to the left.

###  3.5 Inflows and outflows

To explore the outflow and inflow structures in NGC 4418 in more detail, we
separate the H$\alpha$ line emission into different velocity regions. During
the continuum modelling, all spectra in the MUSE cube were re-binned such that
each channel has a fixed velocity width of $\Delta$V = 55.9 km s-1. Based on
these velocities, we separate the H$\alpha$ emission into four velocity
regions, tracing the gas at different velocities (Fig. 10). Regions 1 and 4
trace the highest velocity gas (140 $<v<$ 300 km s-1) on the blue and red
wings, respectively, while regions 2 and 3 trace the slightly slower gas
(85$<v<$140 km s-1). By isolating the emission within each of these different
velocity regions, we are able to spatially map the high- and intermediate-
velocity gas in NGC 4418 for the first time, to reveal several new structures
in the galaxy. Combined with the kinematic information presented in section
3.4, we identify four key features: 1. the minor-axis dusty superwind, 2. the
northern bubble, 3. the southern outflow and 4. the H$\alpha$ ’tail’.

Figure 10: Emission features in the H$\alpha$ line wings. Upper: Spectrum of
H$\alpha$, extracted from a 2.5 arcsec (diameter) aperture centred on the CON
position. Blue and red shaded regions are overlaid and labelled from 1-4.
Regions 1 and 4 contain the highest velocity (140$<V>$305 km s-1) blue- and
redshifted gas, respectively. Regions 2 and 3 map include the slower
(85$<V>$140 km s-1) gas. Middle: Emission of the fast blueshifted (region 1;
blue contours) and slower redshifted (region 3; orange contours) H$\alpha$
gas, overlaid onto the integrated H$\alpha$ line map (log-scaled). Lower:
Emission of the fast (region 4; orange contours) and slower (region 3; red
contours) redshifted H$\alpha$ gas. White box shows the region shown in the
middle panel.

####  3.5.1 The dusty superwind

The blueshifted high velocity gas (region 1 in Fig. 10; $V<$-140 km s-1)
traces a notable asymmetry along the minor axis of NGC 4418. The position of
this asymmetry is consistent with the location of both the strongly
blueshifted regions traced by the H$\alpha$ moment-1 velocity map (Fig. 8) and
the region of enhanced velocity dispersion (Fig. 9). We attribute this feature
to a large-scale ($>$1 kpc) outflow along the direction of the minor axis. The
existence of such a large-scale outflow in NGC 4418 was first proposed by
Sakamoto et al. (2013), who observed a ’U’-shaped region of heavy extinction
along the minor axis of the galaxy in their optical colour map. This
’U’-shaped region is very similar to the structure we observe in the NaD
absorption and the reddening map with MUSE (Figs. 3 and 5, respectively).
Sakamoto et al. (2013) proposed that this dust structure was likely a result
of an outflow cone, appearing aligned with the minor axis of the galactic disk
on the plane of the sky. Indeed, follow up observations by Ohyama et al.
(2019) also found evidence for this structure based on slit spectroscopy from
the Subaru telescope. They concluded that NGC 4418 hosts an outflow, or dusty
superwind, launched from the nucleus and extending in the north-west direction
with a velocity of $\sim$-190 km s-1. We suggest that the minor-axis structure
observed here with MUSE is likely linked to this outflow cone, or dusty
superwind, postulated by both Sakamoto et al. (2013) and Ohyama et al. (2019).
The position of the outflow structure seen in the high velocity channels of
H$\alpha$ aligns with the inner section of the ’U’ shape seen in the dust. Our
results therefore support a scenario in which much of the dust and neutral gas
has been expelled from this region of the galaxy via this superwind, or in
which this is part of an outflowing cone structure.

In addition to the previously postulated north-west component of this outflow
however, we also discover a blue component to the SE. We therefore suggest
that the outflow observed in NGC 4418 does not just extend to the north-west,
but rather on either side of the nucleus. The high-velocity blueshifted
emission tracing this outflow feature is shown in the blue contours of Fig.
10, where we identify a clear elongation along the projected minor axis of the
galaxy. The outflow appears to extend to $\sim$1 kpc in either direction from
the nucleus and is blueshifted in both directions. Furthermore, the outflow is
found to be decelerating, with the low velocity ($V<$85 km s-1) gas appearing
much more extended than the fastest gas, which is concentrated in the nuclear
region and depletes with distance from the galaxy centre.

####  3.5.2 The northern bubble

In addition to the minor axis superwind, we identify a blueshifted shell- or
bubble-like structure to the NE, traced by the highest velocity gas (region
1). This structure, which we refer to as the northern bubble appears connected
to the SE side of the minor axis superwind and is illustrated in Fig. 10. This
northern bubble is spatially aligned with the position of the bubble structure
traced by the line emission (Fig. 2), particularly H$\beta$. Furthermore,
recent Hubble Space Telescope (HST) observations of NGC 4418 (Gallagher et al.
in prep.) support the presence of such a bubble, showing dust arcs extending
northwards of the galaxy centre and tracing a similar structure. Our results
show that the fastest gas traces the edge of the bubble, with the slower gas
($V>$-85 km s-1) tracing clumpy structures in the centre, or void, of the
bubble. This suggests that the outflowing bubble may be accelerating, with the
edges of the bubble expanding much faster than the inner, void region.
Alternatively, this could be an orientation effect. Given that the outer edge
of the bubble appears connected to the minor axis outflow, it is therefore
possible they are part of the same structure, oriented in such a way so as to
appear as a bubble in the plane of the sky. Some tentative evidence for this
scenario is seen in the H$\alpha$ moment-1 map (Fig. 8), where the northern-
most edge of the bubble is shown to be slightly redshifted relative to the
rest of the bubble. It is therefore possible that this northern-most edge is
instead tracing inflowing material, that is to say, material is being ejected
towards the observer via the dusty superwind before falling back onto the
plane of the galaxy.

####  3.5.3 The southern outflow

The intermediate-velocity redshifted gas (region 3 in Fig. 10; 85 $<V<$ 140 km
s-1) traces a notable feature to the south of the galaxy nucleus in NGC 4418.
This feature appears aligned with the location of both the S1 and S2 emission
features identified in the line emission (Fig. 2) and moment-1 maps of the
ionised gas (Fig. 8). Given the velocity structure of this feature, we
conclude the presence of two redshifted outflows to the south of the galaxy
nucleus: a primary outflow, S1, extended on scales $>$1 kpc, and a secondary,
more compact outflow, S2. Due to their redshifted velocities, we infer that
both outflows are located behind the galaxy disk. However, it is also possible
that these structures are instead inflows located in front of this disk.

####  3.5.4 The H$\alpha$ tail

On larger scales, the same redshifted intermediate-velocity gas traces a
large-scale feature extending south from the nucleus and to the north-east
(Fig. 10), which we refer to as the H$\alpha$ tail. Moreover, the highest
velocity redshifted gas (region 4; $V>$ 140 km s-1) also traces much of this
tail, extending further into a ring shape surrounding the galaxy. While the
high velocity ring is much fainter than both the tail and the southern
outflow, they appear to be connected. Both the tail and the ring are likely
indicative of a past large-scale disturbance to the galaxy, such as a major
interaction event. This interpretation would be consistent with the asymmetry
in the stellar disk identified in section 3.4.1. Alternatively, the tail
feature we identify could be part of a spiral structure in the galaxy. Indeed,
imaging from the Sloan Digital Sky Survey (SDSS) reveals what appears to be a
faint spiral arm originating from the south-west (red) side of the stellar
disk and aligning with the tail structure. This could also explain the
asymmetry observed in the stellar disk and provides a more elegant explanation
for the unusual redshifted tail structure we observe.

##  4 Discussion

###  4.1 Comparing dust extinction diagnostics

In section 3.3 we map the dust structure of NGC 4418 based on the Balmer
decrement. Here, we compare the total dust extinction derived in this work,
hereafter $A_{v,\rm{balmer}}$, with that derived from the Kilo-Degree Survey
(KiDS) $g^{\prime}-r^{\prime}$ colours, $A_{v,\rm{colour}}$, presented in
Ohyama et al. (2019). We derive $A_{v,\rm{colour}}$ from the
$g^{\prime}-r^{\prime}$ colours, $E(g^{\prime}-r^{\prime})$, presented in
Ohyama et al. (2019), following the methods outlined in Tian et al. (2014)
(Eq. 3):

$E(g-r)=\left(\left(a_{g}+\frac{b_{g}}{R_{V}}\right)-\left(a_{r}+\frac{b_{r})}{R_{V}}\right)\right)\times
A_{V},$ (3)

where the wavelength-dependent coefficients, $a$ and $b$, are calculated for
the optical/near-IR (1.1$\leq x\leq$3.3 $\mu$ m-1) as follows (Eqns. 3a,b in
Cardelli et al., 1989):

$\displaystyle\begin{split}a(x)=&1+0.17699y-0.50447y^{2}-0.02427y^{3}+0.72085y^{4}\\\
&+0.01979y^{5}-0.77530y^{6}+0.32999y^{7}\end{split}$ (4a)
$\displaystyle\begin{split}b(x)=&1.41338y+2.28305y^{2}+1.07233y^{3}-5.38434y^{4}\\\
&-0.62251y^{5}+5.30260y^{6}-2.09002y^{7},\end{split}$ (4b)

where $x$ is the wavenumber of the band, $1/\lambda_{\rm{eff}}$, and
$y=x-1.82$. Fig. 11 shows the resulting $A_{v,\rm{colour}}$ map, with contours
of the $A_{v,\rm{balmer}}$ overlaid. While both $A_{v,\rm{colour}}$ and
$A_{v,\rm{balmer}}$ trace the same characteristic ’U’ structure, we note that
$A_{v,\rm{balmer}}$ traces an additional prominent dust feature to the south,
which is not present in the map of $A_{v,\rm{colour}}$. This feature is
spatially coincident with the redshifted southern features, S1 and S2,
discussed in this work. We suggest that this discrepancy in the dust
distribution within the galaxy is primarily due to the different methods of
measuring the dust. Extinction measurements based on ionised emission, such as
the $A_{v,\rm{balmer}}$ value we derive, are typically more sensitive to dust
behind the disk of the galaxy than $A_{v,\rm{colour}}$, which has been derived
from the broad band colours. In principle, the dust map derived from MUSE may
therefore be tracing additional features not traced by the KiDS colour map. As
such, we suggest that the dusty southern features likely lie behind the plane
of the galaxy. Given that we find these features to be kinematically
redshifted, we suggest that they are indeed outflowing components as
postulated in section 3.5.3.

Figure 11: Map of the total dust extinction, $A_{v,\rm{(g-r)}}$, derived from
the KiDS g’-r’ (AB) colours. Contours mapping $A_{v,\rm{balmer}}$ values
derived from the Balmer decrement of the MUSE spectra are overlaid (white).

###  4.2 The [O iii] knots

In section 3.2.1 we identify clumpy structures in the ionised [O iii]
emission, which have never been observed before in NGC 4418. To further
investigate the nature of these ionised regions, or knots, we extract the
location of each knot by convolving the integrated [O iii] line map (Fig. 12)
with a Gaussian kernel and subtracting it from the original map in a
rudimentary high-pass filter. A threshold is applied to this residual image to
remove any obvious noise signals and isolate the brightest knots. The
threshold value was selected by eye such that all spurious sources in the
image are removed and all knots appearing as separate structures under visual
inspection remain separate in the threshold image (i.e. the threshold was
sufficiently large to avoid blending multiple nearby knots into a single
structure). Contours are fitted to the resulting threshold map and the centre
of each contour is chosen to denote the location of the corresponding knot.
Through this method, we identify positions for 51 of the brightest knots in
NGC 4418 (K01-K51; Fig. 12), the details of which are given in Table 1. We
highlight that many other, fainter structures similar to the bright knots also
exist throughout the galaxy, but were too faint to be selected via this
method.

Figure 12: Locations of the [O iii] knots.

Once the positions of the [O iii] knots have been identified, we extract their
spectra, integrated within circular 0.4 arcsec (radius) apertures centred on
each of the knots. Identical apertures are placed at a distance of 0.8 arcsec
from each of the knots to provide reference spectra from nearby regions of the
galaxy. We opt to select nearby regions rather than randomly placed apertures
to eliminate any potential radial bias in the reference spectra. Each of the
reference apertures is visually inspected to ensure it does not contain [O
iii] emission from any of the knots. The mean spectrum for the knots and
reference apertures are shown in Fig. 13. Based on the residual (knots minus
nearby regions) spectrum, we find the knots have stronger [O iii] emission
than the underlying galaxy on average, with a peak [O iii] flux $\sim$60 per
cent brighter than that of the average reference spectrum. Conversely, the
peak of both the [N ii] and H$\alpha$ emission are weaker in the average knot
spectrum, appearing $\sim$20-30 per cent stronger in the nearby regions of the
galaxy. Here we discuss the potential ionising source(s) of these [O iii]
knots, presenting four possible scenarios: stellar clusters, AGN
photoionisation, extended HII regions, and planetary nebulae (PNe).

Figure 13: Average spectrum of the [O iii] knots compared to nearby regions of
the galaxy. Upper: Mean spectrum for the [O iii] knots (pink). Middle: Average
spectrum of nearby regions of the galaxy (blue). Lower: Knot minus nearby
region residual spectrum. Shaded regions denote the 32nd and 68th percentiles
of the spectra in each case.

####  4.2.1 Stellar clusters

One potential source of the [O iii] knots observed in NGC 4418 is stellar
clusters: regions of the galaxy ionised by young stars. If the knot structures
we observe are ionised in this way, we expect to detect them at shorter
optical wavelengths where the light from young stars dominates. However, U
(F336W) and V (F555W) band imaging from the HST (Gallagher et al. in prep.)
reveals no evidence for young stellar populations at the locations of the [O
iii] knots. Although two knots (K08 and K50) lie within $<$0.3 arcsec of
detected HST sources, K50 corresponds to an unresolved pair of redder stellar
sources, likely globular clusters, and K08 is detected only in the V band,
suggesting it is an extremely bright PN. For the majority of the knots (49 of
51) we find no HST counterpart, meaning stellar clusters are likely not the
dominant source of the [O iii] knots in NGC 4418. Furthermore, stellar
clusters tend to follow the rotation of the galaxy (stellar) disk, yet the
moment-1 velocity map of [O iii] (Fig. 8) reveals that the knots are
kinematically distinct from the galaxy disk. Based both on this kinematic
information from MUSE, and the lack of detection in HST imaging, we therefore
conclude that the [O iii] knots in NGC 4418 are not ionised by stellar
clusters.

####  4.2.2 AGN photoionisation

Another possible mechanism behind the knot structures seen in NGC 4418 is the
presence of so-called AGN echoes. These are regions of the galaxy that have
been photoionised by an AGN and are predominantly characterised by their AGN-
like line ratios on scales $>$10 kpc from the galaxy centre. To test whether
AGN echoes are a plausible source of the [O iii] knots observed in NGC 4418,
we measure the [O iii] ($\lambda$5007) / H$\beta$ and [N ii] ($\lambda$6583) /
H$\alpha$ line ratios of each knot and plot them on a Baldwin, Phillips, and
Terlevich (BPT) diagram (Fig. 14), designed to use line diagnostics to
distinguish star formation from low-ionisation emission-line regions (LINERs)
and Seyferts. In each case, spectra are integrated within a 0.4 arcsec
(radius) circular apertures, centred on the knot position and Gaussian
profiles are fit to the relevant emission features. We highlight that all
spectra are corrected for the dust extinction measured in section 3.3, prior
to the fitting. The fluxes for each line are then measured by integrating over
the best-fit Gaussian line profiles. In cases where one of the features does
not exceed the spectral noise, we instead measure an upper limit on the flux,
assuming the same FWHM as the other features in the spectrum. These are shown
as upper and lower limits on the line ratios.

Fig. 14 shows all of the knots in NGC 4418 to be inconsistent with heating via
star formation alone. Even based on the more conservative constraints of
Kewley et al. (2006), 37 of the 51 knots (72.5 per cent) are classified as
Seyfert- or LINER-like. The remaining 14 knots are consistent with a
transitional phase. Objects in this transition region are often interpreted in
the literature as either composite systems in which a central LINER is
contaminated by surrounding H ii regions (e.g. Ho et al., 1993; Menezes et
al., 2022), or accretion powered systems with a lower accretion rate (e.g. Ho,
2008). It has even been suggested that evolved hot stars and turbulent mixing
layers in the interstellar medium (ISM) can also impact the H$\alpha$ emission
in these transition objects (Shields et al., 2007). We further note that the
LINER classification remains somewhat controversial, with low-ionisation AGN
(e.g. Ferland & Netzer, 1983; Halpern & Steiner, 1983), hot low-mass evolved
stars (HOLMESs), or post-asymptotic giant branch stars (e.g. Binette et al.,
1994; Eracleous et al., 2010; Cid Fernandes et al., 2011) and shock heating
(e.g. Heckman, 1980; Dopita & Sutherland, 1995, 1996) all proposed to explain
the emission line spectra of LINERs. Indeed, all but one of the knots and all
but four of the nearby regions are also consistent with ionisation resulting
from shock-heating (Alatalo et al., 2016). Nevertheless, the unanimous
Seyfert- or LINER- like classification of the knots suggests the presence of
an AGN in the CON of NGC 4418.

An AGN photoionisation scenario is further supported by the velocities of the
ionised gas (Fig. 8), where we find the [O iii] knots are kinematically
independent from the rotation of the galaxy disk and are therefore likely to
be part of a 3D gas structure surrounding the central disk. We note that while
the knots appear to have been ionised by an AGN, albeit with a low ionisation
parameter as indicated by the LINER classification, it is unlikely that the
nucleus of NGC 4418 is entirely powered by this AGN. Previous high resolution
radio imaging of the nuclear region of NGC 4418 reveals several compact
features, thought to be super star clusters (Varenius et al., 2014). It is
therefore likely that some additional star formation component is contributing
to the central luminosity. This conclusion is consistent with our finding that
several of the knots exist in the transition region of the diagram.
Furthermore, we find the central spaxel, marking the position of the CON in
NGC 4418, to also lie in this transition region (red cross, Fig. 14). While
the extreme dust obscuration in the nucleus, along with the spatial resolution
of the MUSE imaging, severely limit our ability to interpret the emission line
ratios of the CON itself, existing in the transition region is again
consistent with the presence of an AGN with some starburst contribution. We
therefore conclude that the CON in NGC 4418 is likely powered by a combination
of AGN and starburst activity.

Figure 14: BPT diagram showing the dominant ionising source of the knots (blue
circles) and nearby regions (yellow squares) compared to a sample of known HII
regions in the nearby LIRG Arp 86 from Zhou et al. (2014) (green triangles),
based on the diagnostic models of Kauffmann et al. (2003) (dotted line),
Kewley et al. (2001) (dashed line), and Kewley et al. (2006) (solid line). The
region of the diagram consistent with shock heating (Alatalo et al., 2016) is
overlaid (pink). Red cross marks the position of the central spaxel denoting
the position of the CON in NGC 4418.

####  4.2.3 Extended HII regions

H ii regions, or diffuse nebulae, are regions of interstellar gas that have
been ionised by one or more O- or early B-type star(s). The spectra of H ii
regions typically contain strong hydrogen recombination lines and
collisionally excited [N ii] and [OIII], with additional [O iii] and [N ii]
emission that varies strongly with the temperature of the ionising source. Due
to their association with young stars, H ii regions, like stellar clusters,
are expected to be associated with optical U- and V- band emission. The lack
of detection in the HST U (F336W) and V (F555W) band imaging (Gallagher et al.
in prep.), therefore, makes a young stellar source of ionisation unlikely. To
further test this possibility, we compare the BPT line ratios of the knots in
NGC 4418 (Fig. 14) with a sample of 69 known H ii regions in the nearby
($z\sim$0.0162), interacting system Arp86 (Zhou et al., 2014). Much like NGC
4418, Arp 86 is a LIRG and is undergoing an interaction with a much smaller
companion galaxy. These H ii regions occupy a similar region in the BPT
diagram to the reference spectra taken from regions of the galaxy close to the
knots (Fig. 14). In general, the H ii regions lie below the knots in the BPT
diagram, with the majority of H ii regions lying below the upper limit of star
formation. Indeed, Zhou et al. (2014) conclude star formation to be the
dominant ionising source related to the H ii regions in Arp86, with a handful
of regions consistent with both ionisation by star formation and the presence
of shocks, AGB stars or AGN photoionisation. Nevertheless, Fig. 14 shows a
clear distinction between these H ii regions and the knots identified in NGC
4418, implying a different source of ionisation.

####  4.2.4 Planetary nebulae

Finally, we considered the possibility that the [O iii] knots trace PNe in NGC
4418. PNe are shells of ionised gas that have been lost by stars. The remnant
central stars of PNe are typically much older and less luminous than the
O-type stars associated with H ii regions and are thus not expected to be
detected in HST imaging. One clear identifier of PNe is the presence of a
helium recombination line, He ii ($\lambda$4686), whose intensity exceeds a
few per cent of H$\beta$. We do not detect this feature in the individual knot
spectra or in the average spectrum of the knots (Fig. 13), although we
emphasise that the lack of this feature cannot rule out the existence of PNe.
PNe can also be identified through an [O iii]/H$\beta$ ratio $\gtrsim$4, which
is only present in PNe. Indeed, we find the [O iii]/H$\beta$ ratio in the
knots generally exceeds that measured from the reference spectra. However,
while six of the 51 knots ($\sim$12 per cent) have [O iii]/H$\beta$ ratios
$\gtrsim$4, the majority of the knots lie below this threshold and thus cannot
be confirmed as PNe. Despite finding little evidence to support the PNe
scenario, we highlight that these two diagnostics (the presence of He ii and
the [O iii]/H$\beta$ line ratio) are typically biased against central stars
with low effective temperatures, Teff, such as young PNe. We therefore cannot
rule out young PNe as the source of the ionising radiation based on these
diagnostics alone. We note however that PNe shells expand with velocities of a
few tens of km s-1, meaning they become rapidly unobservable on timescales of
about a few 104 years. Given their short lifetimes, it is unlikely that we are
seeing such widespread PNe simultaneously on scales $>$10 kpc in NGC 4418.
Additionally, at the redshift of NGC 4418, PNe are expected to appear as point
sources, yet we find many of the [OIII] knots to be extended and irregular.
Finally, PNe are expected to follow the rotation of the galaxy disk, yet
velocity information of the ionised gas shows this is not the case. We
therefore suggest that PNe are unlikely to be the dominant ionising source of
the [O iii] knots in NGC 4418, although they could be responsible for a small
fraction of the knots observed. For the majority of the knots, we instead
conclude AGN photoionisation to be the most likely cause.

##  5 Conclusions

We have presented new, targeted MUSE IFU observations for the local CON NGC
4418. Our main conclusions are as follows;

(i) For the first time, we have mapped the stellar velocity field of NGC 4418.
We find the stellar disk to be approaching on the north-eastern and receding
on the south-western side with line-of-sight velocities of $V\sim$-115 km s-1
and $V\sim$80 km s-1, respectively. Within the central kiloparsec of the
galaxy, we also reveal significant non-circular motions. We suggest these to
be due to either a counter-rotating central stellar disk, perhaps caused by a
recent interaction event, or the presence of a nuclear bar. While the data
presented in this work are insufficient to conclude on the nature of this
feature, we hope that future, high-resolution optical IFU studies will allow
us to distinguish between these scenarios.

(ii) We confirm the presence of a kiloparsec-scale blueshifted outflow along
the minor axis of NGC 4418, extending in the north-west direction. The
presence of this outflow has previously been postulated in the work of
Sakamoto et al. (2013) and Ohyama et al. (2019), where it is referred to as a
dusty superwind. Indeed, we show this outflow to be associated with a region
of enhanced velocity dispersion, and to be surrounded by a ‘U’-shaped dust
structure. Our findings are consistent either with an outflow cone in which
the edges appear dustier due to the cone’s orientation or with a collimated
outflow that has cleared the bulk of the dust from its path. Based on the line
wings of H$\alpha$, we find the outflow to be decelerating, with a projected
velocity of between -300$<V<$-140 km s-1 at the outer tip. For the first time,
we discover that this outflow also extends to the south-east, appearing
blueshifted on both sides of the galaxy nucleus.

(iii) In addition to discovering the south-eastern side of the minor axis
outflow, we identify several other new features of NGC 4418. We detect a
redshifted feature to the south composed of two seemingly distinct regions, S1
and S2. We conclude that this feature is likely outflowing material located
behind the galaxy disk with an estimated velocity of 85$<V<$140 km s-1, based
on the H$\alpha$ line wings. This redshifted outflow appears to be connected
to a ‘tail’ of high velocity gas surrounding the galaxy, which we refer to as
the southern tail. In addition to these redshifted features, we also discover
a bubble-like structure to the north of the galaxy nucleus: the northern
bubble. The highest velocity gas ($V<$-140 km s-1) traces the edges of the
bubble, whilst the apparent void of the bubble consists of slower gas ($V>$-85
km s-1) with enhanced H$\beta$ absorption and dust extinction. We propose that
the observed bubble could either be an accelerating shell of gas expanding
outwards from the galaxy centre or part of the minor axis outflow structure
beginning to infall back onto the plane of the galaxy and appearing as a
bubble due to orientation effects.

(iv) For the first time, we uncover the presence of [O iii] knots throughout
NGC 4418. Their lack of detection in the HST U (F336W) and V (F555W) bands,
AGN-like line ratios, and distinct velocity profiles imply they are likely
regions of the galaxy that have been photoionised by an AGN. From this, we
postulate that the CON in NGC 4418 is powered by AGN activity, with the
photoionised knots tracing the dust structure of the inner nuclear region.

Overall, our results are consistent with NGC 4418 being in a phase in the AGN
feedback cycle where dust is being expelled from the galactic nucleus via
outflows. The large-scale structure of NGC 4418 shows evidence for regions
ionised by an AGN. Along with the fast outflows ($V>$140 km s-1) and bubble
structures, we suggest it is unlikely that the CON is powered solely by
starburst activity. Instead, we suggest NGC 4418 has either recently undergone
or is currently in an active AGN phase. Future IFU studies of the wider CON
population are needed to determine whether all CONs are consistent with this
classification. If so, the number of AGN in the local Universe may double with
respect to what is currently estimated, completely revolutionising our
understanding of how AGN form and their role in galaxy evolution as a whole.

###### Acknowledgements.

C.W. gratefully acknowledges support from a European Research Council (ERC)
Advanced Grant, 789410. The work of YN was supported by JSPS KAKENHI grant No.
JP23K13140.

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##  Appendix A Supplementary figures and tables

Figure A.1: Stellar kinematics of NGC 4418 output from the pPXF fitting. Left: Map of the stellar velocity dispersion, $\sigma$. Right: Map of the h3 Gauss-Hermite coefficient. In each case, pixels have been binned to achieve a signal-to-noise ratio of 20. An initial signal-to-noise ratio cut of 2 has been applied, with pixels below this threshold omitted from the binning. Knot ID | RA (deg) | Dec (deg) | RA (J200) | Dec (J200)
---|---|---|---|---
K01 | 186.7285 | -0.8842 | 12:26:54.8319 | -0:53:02.9919
K02 | 186.7249 | -0.8839 | 12:26:53.9797 | -0:53:02.0972
K03 | 186.7228 | -0.8827 | 12:26:53.4798 | -0:52:57.6230
K04 | 186.7271 | -0.8824 | 12:26:54.5129 | -0:52:56.7076
K05 | 186.7213 | -0.8823 | 12:26:53.1209 | -0:52:56.1114
K06 | 186.7261 | -0.8814 | 12:26:54.2610 | -0:52:52.8641
K07 | 186.7236 | -0.8814 | 12:26:53.6609 | -0:52:53.0114
K08 | 186.7292 | -0.8813 | 12:26:55.0003 | -0:52:52.5563
K09 | 186.7233 | -0.8810 | 12:26:53.5869 | -0:52:51.4411
K10 | 186.7298 | -0.8804 | 12:26:55.1441 | -0:52:49.4525
K11 | 186.7225 | -0.8802 | 12:26:53.4036 | -0:52:48.6314
K12 | 186.7270 | -0.8800 | 12:26:54.4727 | -0:52:47.8284
K13 | 186.7259 | -0.8798 | 12:26:54.2130 | -0:52:47.1355
K14 | 186.7325 | -0.8788 | 12:26:55.8074 | -0:52:43.5477
K15 | 186.7313 | -0.8799 | 12:26:55.5145 | -0:52:47.6114
K16 | 186.7280 | -0.8798 | 12:26:54.7197 | -0:52:47.1688
K17 | 186.7209 | -0.8797 | 12:26:53.0117 | -0:52:46.9480
K18 | 186.7251 | -0.8790 | 12:26:54.0210 | -0:52:44.2481
K19 | 186.7236 | -0.8791 | 12:26:53.6609 | -0:52:44.6114
K20 | 186.7325 | -0.8787 | 12:26:55.8092 | -0:52:43.2477
K21 | 186.7267 | -0.8789 | 12:26:54.4143 | -0:52:43.9114
K22 | 186.7281 | -0.8787 | 12:26:54.7531 | -0:52:43.4307
K23 | 186.7225 | -0.8782 | 12:26:53.4054 | -0:52:41.4955
K24 | 186.7268 | -0.8781 | 12:26:54.4270 | -0:52:41.2389
K25 | 186.7281 | -0.8779 | 12:26:54.7377 | -0:52:40.3342
K26 | 186.7275 | -0.8780 | 12:26:54.6077 | -0:52:40.8114
K27 | 186.7250 | -0.8779 | 12:26:54.0020 | -0:52:40.3366
K28 | 186.7274 | -0.8775 | 12:26:54.5852 | -0:52:38.9465
K29 | 186.7290 | -0.8769 | 12:26:54.9543 | -0:52:36.8135
K30 | 186.7261 | -0.8777 | 12:26:54.2529 | -0:52:39.5539
K31 | 186.7234 | -0.8775 | 12:26:53.6220 | -0:52:39.1011
K32 | 186.7270 | -0.8770 | 12:26:54.4848 | -0:52:37.1241
K33 | 186.7304 | -0.8765 | 12:26:55.2969 | -0:52:35.4750
K34 | 186.7337 | -0.8764 | 12:26:56.0970 | -0:52:35.1167
K35 | 186.7326 | -0.8763 | 12:26:55.8327 | -0:52:34.7045
K36 | 186.7316 | -0.8761 | 12:26:55.5844 | -0:52:34.0764
K37 | 186.7264 | -0.8759 | 12:26:54.3354 | -0:52:33.2953
K38 | 186.7280 | -0.8759 | 12:26:54.7265 | -0:52:33.3392
K39 | 186.7301 | -0.8757 | 12:26:55.2193 | -0:52:32.5183
K40 | 186.7335 | -0.8756 | 12:26:56.0412 | -0:52:32.3114
K41 | 186.7327 | -0.8755 | 12:26:55.8489 | -0:52:31.8991
K42 | 186.7257 | -0.8754 | 12:26:54.1610 | -0:52:31.6114
K43 | 186.7343 | -0.8751 | 12:26:56.2342 | -0:52:30.2879
K44 | 186.7273 | -0.8747 | 12:26:54.5453 | -0:52:28.7759
K45 | 186.7341 | -0.8743 | 12:26:56.1946 | -0:52:27.4114
K46 | 186.7276 | -0.8738 | 12:26:54.6319 | -0:52:25.5774
K47 | 186.7311 | -0.8738 | 12:26:55.4572 | -0:52:25.6667
K48 | 186.7335 | -0.8738 | 12:26:56.0345 | -0:52:25.7638
K49 | 186.7302 | -0.8734 | 12:26:55.2493 | -0:52:24.2681
K50 | 186.7266 | -0.8733 | 12:26:54.3769 | -0:52:23.9732
K51 | 186.7246 | -0.8731 | 12:26:53.8921 | -0:52:23.1144
K52 | 186.7328 | -0.8725 | 12:26:55.8812 | -0:52:21.0114
K53 | 186.7297 | -0.8713 | 12:26:55.1270 | -0:52:16.6783
Table 1: World coordinate system (WCS) positions of the [O iii] $\lambda 5007$
knots identified in NGC 4418.