# Digging deeper into NGC 6868 I: stellar population

João P. V. Benedetti,1 Rogério Riffel,1 Tiago Ricci,2 Marina Trevisan,1
Rogemar A. Riffel,3 Miriani Pastoriza,1 Luis G. Dahmer-Hahn,4 Daniel Ruschel-
Dutra,5 Alberto Rodríguez-Ardila,6,7 Jose A. Hernandez-Jimenez8 and João
Steiner9
1Departamento de Astronomia, Universidade Federal do Rio Grande do Sul. Av.
Bento Gonçalves 9500, 91501-970 Porto Alegre, RS, Brazil
2Universidade Federal da Fronteira Sul, 97900-000 Campus Cerro Largo, RS,
Brazil
3Departamento de Física, Universidade Federal de Santa Maria, Centro de
Ciências Naturais e Exatas, 97105-900 Santa Maria, RS, Brazil
4Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan
road, Shanghai 200030, China
5Departamento de Física - CFM - Universidade Federal de Santa Catarina, 476,
88040-900 Florianópolis, SC, Brazil
6Laboratório Nacional de Astrofísica/MCT - Rua dos Estados Unidos 154, Bairro
das Nacões. CEP 37504-364 Itajubá, MG, Brazil
7INPE - Instituto Nacional de Pesquisas Espaciais, Av. dos Astronautas, CEP
12227-010, São José dos Campos - SP, Brazil
8Universidade do Vale do Paraíba, Av. Shishima Hifumi, 2911, Zip Code
12244-000, São José dos Campos, SP, Brazil
9Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de
São Paulo, 05508-900 São Paulo, Brazil E-mail<EMAIL_ADDRESS>(JPVB)E-mail<EMAIL_ADDRESS>(RR)In Memorian.

(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We use Gemini integral field unit observations to map the stellar population
properties in the inner region ($\sim 680\times 470$ pc2) of the galaxy NGC
6868. In order to understand the physical and chemical properties of the
stellar content of this galaxy, we performed stellar population synthesis
using the starlight code with the MILES simple stellar population models. We
measured the absorption line indices Fe4383, Mg2, Mgb, Fe5270, Fe5335 for the
whole FoV, and used them to derive Fe3 and [MgFe]’. These indices were used to
derive [/Fe]. This galaxy is dominated by old metal-rich populations (12.6
Gyr; 1.0 and 1.6 Z⊙) with a negative metallicity gradient. We also found a
recent ($\sim 63$ Myr) metal-rich (1.6 Z⊙) residual star formation in the
centre of the galaxy. A dust lane with a peak extinction in the V band of 0.65
mag is seen. No signs of ordered stellar motion are found and the stellar
kinematics is dispersion dominated. All indices show a spatial profile varying
significantly along the FoV. Mg2 shows a shallow gradient, compatible with the
occurrence of mergers in the past. Mgb and Fe3 profiles suggest different
enrichment processes for these elements. We observe three distinct regions:
for R<100 pc and R>220 pc, Mg2, Mgb anti correlate with respect to Fe3 and
[MgFe]’, and for 100 pc<R<220 pc, they correlate, hinting at different
enrichment histories. The [/Fe] profile is really complex and has a central
value of $\sim 0.2$ dex. We interpret this as the result of a past merger with
another galaxy with a different [/Fe] history, thus explaining the [/Fe] maps.

###### keywords:

galaxies: individual (NGC 6868), galaxies: nuclei, galaxies: elliptical and
lenticular, cD, galaxies: stellar content

††pubyear: 2022††pagerange: Digging deeper into NGC 6868 I: stellar
population–Digging deeper into NGC 6868 I: stellar population

##  1 Introduction

Galaxies can widely be defined as either passive, that are not actively
forming stars and host a red and old stellar population, and star-forming
galaxies, that are blue, hosting large fractions of young stellar populations.
This bi-modality was found in many studies over the years (e.g. Kauffmann et
al., 2003; Baldry et al., 2004; Noeske et al., 2007; Wetzel et al., 2012; van
der Wel et al., 2014), even at high redshifts ($z>2.5$ Brammer et al., 2009;
Muzzin et al., 2013, for example). However, it is not yet clear which
mechanisms are regulating star formation and transforming the blue galaxies
into red-and-dead ones. A major challenge in modern astrophysics is to
determine the physical mechanism acting in quenching star formation in
galaxies.

Nowadays, it is established that active galactic nuclei (AGN) feedback plays
an important role in regulating the star formation (SF) of its host galaxy (Di
Matteo et al., 2005; Hopkins & Elvis, 2010; Harrison, 2017; Storchi-Bergmann &
Schnorr-Müller, 2019; Riffel et al., 2021; Ellison et al., 2021). The gas
inflowing, responsible for SF, also feeds the supermassive black hole (SMBH),
triggering the AGN episode that can either heat (or expel) the gas, thus
shutting down the SF (Fabian, 2012; King & Pounds, 2015; Zubovas & Bourne,
2017; Trussler et al., 2020). In cosmological simulations without including
AGN and supernova (SN) feedback, the observed luminosity function of galaxies
cannot be reproduced: the larger and smaller galaxies end up with higher
masses than observed in the present-day universe (Springel et al., 2005).
Also, the ages of the stars from the most massive galaxies are underestimated
when compared with observations (Croton et al., 2006). These results show that
some form of gas striping or heating must be taking place in these objects.
However, distinguishing the nature of such processes is still challenging,
once the simulations cannot reach the physical scales involved and use ad hoc
prescriptions to account for these mechanisms (Schaye et al., 2015). In order
to really disentangle the effects of SN feedback and AGN we need to look at
the vicinity of SMBH and trace the star formation history (SFH) of that
population in order to understand the effect of the AGN in the stellar
population (Riffel et al., 2021).

Past studies have tried to establish this link, but the results have been
controversial. Despite SF being common in AGNs (Riffel et al., 2009; Ruschel-
Dutra et al., 2017; Mallmann et al., 2018; Riffel et al., 2021; Burtscher et
al., 2021; Dahmer-Hahn et al., 2021; Riffel et al., 2022), the time scale for
starting the star formation ($\sim 100$ Myr, Hickox et al., 2014; Burtscher et
al., 2021) is far greater than for the AGN triggering ($\sim 0.1-1$ Myr, Novak
et al., 2011; Schawinski et al., 2015), preventing us from connecting the two.
Some studies show a correlation between the fraction of young populations and
the AGN luminosity, where the most luminous sources present the highest
fraction (Riffel et al., 2009; Ruschel-Dutra et al., 2017; Zubovas & Bourne,
2017; Mallmann et al., 2018). However, the hard X-ray (14-195 keV) luminosity
from the galaxies does not seem to correlate with the fraction of young
populations (Burtscher et al., 2021). Instead, mass loss from intermediate-age
stars seems to be important in AGN feeding (Riffel et al., 2022).

Most of the previous studies have been done on relatively bright objects.
However, the most common form of AGN in the local Universe is low luminosity
AGN (LLAGN) in massive galaxies (Ho, 2008). Most of them are classified as
low-ionization nuclear emission-line region (LINER) objects (Heckman, 1980).
However, despite their significance, the physical nature of these objects is
still poorly understood. Since their discovery, many mechanisms have been
proposed to explain the LINER signature beyond the LLAGN paradigm (Ferland &
Netzer, 1983; Halpern & Steiner, 1983), once many other physical processes can
mimic the same spectral signatures without the LLAGN (these objects are known
as LIERs Cid Fernandes et al., 2011; Belfiore et al., 2016) such as shocks
(Heckman, 1980), hot low-mass evolved stars (HOLMES) (Binette et al., 1994;
Yan & Blanton, 2012; Papaderos et al., 2013; Singh et al., 2013; Belfiore et
al., 2016; Oliveira et al., 2022) and starbursts with ages between 3 and 5
Myr, dominated by Wolf-Rayet stars (Barth & Shields, 2000). With current
observational technology, one can disentangle the different ionization
mechanisms by performing detailed spatially resolved studies analysing both
the stellar population and the ionized gas components.

Even in the LLAGN hypothesis, the effects of such objects in their host galaxy
are still uncertain as most studies focusing on the AGN impact over galaxies
are performed with high-luminosity AGN (e.g. Seyferts and quasars, e.g.
Nayakshin & Zubovas, 2012). With the rising importance of LINERs, new studies
have been analysing such impacts, although a complete picture is yet to be
drawn and further research is needed.

Integral Field Unit (IFU) spectroscopy has expanded our view towards early-
type galaxies, their formation, and evolution. This technique allows one to
perform spatially resolved studies of stellar populations and better constrain
the kinematical structure of these objects, with, for example, the emergence
of kinematically distinct cores (KDCs), counter-rotating stellar discs (see
Cappellari, 2016, for a review). Despite previous studies being able to
reproduce the stellar population parameters of ETGs with a rapid in-situ
conversion of gas into stars (e.g. Chiosi & Carraro, 2002) including a fully
consistent chemical evolution (e.g. Vazdekis et al., 1997), the emergence of
these structures has been seen as evidence for the importance of merger
processes in ETG formation. Dry minor mergers have already been established as
a subsequent growth pathway for ETGs (the two phases of galaxy formation Oser
et al., 2010; Navarro-González et al., 2013), however, they rarely affect the
central regions of galaxies. Therefore, studying these structures in galactic
cores may help us further elucidate the formation and evolution of ETGs (e.g.
Krajnović et al., 2015).

From the above, detailed studies thoroughly analysing the nuclear regions of
galaxies probing the stellar population and gas content are fundamental to
elucidate the impact of the AGN with respect to the galaxy evolution. In this
sense, an "artisanal" approach is better at analysing the details that would
otherwise be missed in large surveys. With this in mind, here we present a
detailed GMOS IFU study of the galaxy NGC 6868, a nearby ($27.70$ Mpc, Tully
et al., 2013) elliptical galaxy (E2, de Vaucouleurs et al., 1991). Some basic
parameters extracted from NED can be seen in table 1 and three images from NGC
6868 in different scales are shown in Fig. 1. NGC 6868 is the brightest member
of the Telescopium group. Rickes et al. (2008) have shown that NGC 6868
exhibits LINER emission in its centre, which has been attributed to a
combination of photoionization by an LLAGN and shocks. They also investigate
this galaxy’s metallicity distribution and ionized gas by means of long-slit
spectroscopy and stellar population synthesis. According to the authors, Lick
indices present a negative gradient indicating an overabundance of Fe, Mg, Na
and TiO in the central parts with respect to the external regions. Mg2
correlates with Fe5270 and Fe5335, suggesting that these elements probably
underwent the same enrichment process in NGC 6868. The lack of correlation
between computed galaxy mass and the Mg2 gradient suggests that this
elliptical galaxy was formed by merger events. The stellar population
synthesis shows the presence of at least two populations with ages of 13 and 5
Gyr old. The fact that this galaxy apparently has multiple ionization
scenarios and also shows signs of complex star formation history makes NGC
6868 an excellent candidate to further investigate the mechanisms behind LINER
emission and the processes involved in the evolution of early-type galaxies.

Table 1: Table showing some basic parameters of the galaxy NGC 6868. Parameter | NGC 6868
---|---
RA (J2000) | 20h09m54s.07
Dec. (J200) | -48∘22´46.4´´
Morphologya | E2
R (mag)b | 7.91
M${}_{\text{R}}$ (mag)b | -24.7
Diameter (kpc)c | 73.0
L${}_{\text{X}}$ (erg s-1)d | $8.54\cdot 10^{40}$
Nuclear Activitye | LINER
Radio classificationf | Flat-Spectrum Radio Source
A${}_{\text{V}}$ (mag)g | 0.152
Radial Velocity (km s-1)h | 2854
Distance (Mpc)i | 27.70
Redshift (z)h | 0.00952
Velocity dispersion (km$\cdot$s-1) j | 250
Data available in NED111The NASA/IPAC Extragalactic Database (NED) is operated
by the Jet Propulsion Laboratory, California Institute of Technology, under
contract with the National Aeronautics and Space Administration
ade Vaucouleurs et al. (1991) |  bCarrasco et al. (2006)
cLauberts & Valentijn (1989) |  dBabyk et al. (2018)
eRickes et al. (2008) |  fHealey et al. (2007)
gSchlafly & Finkbeiner (2011) |  hRamella et al. (1996)
iTully et al. (2013) |  jWegner et al. (2003)
Figure 1: Images from NGC 6868 in three different scales. (a) Composite DSS
image showing NGC 6868 and close neighbours. It is the brightest group member
from the Telescopium group (AS0851). (b) Residuals from the photometric
modelling of the acquisition image in the r band. A clear dust lane, also
reported by Veron-Cetty & Veron (1988), is seen in the centre of NGC 6868. At
$\sim$11"W and 5"N a stellar-like object is found and, according to Hansen et
al. (1991), may be a cannibalised galaxy from which spiral features are
emerging. (c) Continuum image from NGC 6868 extracted from our GMOS data cube
at 5700 Å exhibiting an irregular light profile with a distortion in the SE
direction. The (0,0) is defined as the peak in the continuum image corrected
by extinction, as can be seen in Fig. 13.

NGC 6868 was already observed in different wavelengths. Machacek et al. (2010)
using X-ray data found strong evidence of a past encounter between NGC 6868
and NGC 6861, displaying tidal tails and shells. Moreover, they found X-ray
cavities, indicative of past AGN activity. Hansen et al. (1991) studied NGC
6868 using CCD images and an International Ultraviolet Explorer (IUE) low-
resolution spectrum, and they found a dust lane in the centre of the galaxy
with an extended dust component with spiral features. A series of papers have
reported the presence of ionized gas, finding a disturbed morphology and
complex kinematics for the galaxy with a possible counter-rotating disc (Buson
et al., 1993; Zeilinger et al., 1996; Macchetto et al., 1996). Caon et al.
(2000) have analysed long-slit observations with distinct position angles
(PAs). They reported a rotating disc of ionized gas, in agreement with the
stellar velocity field. However, in a different PA, a counter-rotating gas
disc with respect to the stars is found, displaying an inner component that
shows a counter-rotation. Also, a KDC was seen where the kinematical break
radius is at 3”. Bregman et al. (1998) using IRAS data confirmed the presence
of cold dust. NGC 6868 has been observed in the radio by Slee et al. (1994) at
2.3, 5, and 8.4 GHz and Mauch et al. (2003) at 843 MHz and Healey et al.
(2007) reported a low-power flat spectrum radio source in its centre
($\alpha\sim 0.07$). The brightness, temperature and spectral slope are
inconsistent with HII regions, so an AGN was the most likely source of the
radio emission. Rose et al. (2019) using ALMA observations detected molecular
gas in the centre of NGC 6868 drifting in non-circular motions. Also, they
reported an HI absorption.

In this paper, the first of a series aimed at studying in detail this object,
we will focus on the stellar content of NGC 6868. It is organized as follows:
in § 2, we describe the observations and the reduction procedures; in § 3, we
present the employed methodology; in § 4, the results are presented and a
comparison with data from other studies. Discussion of the results is made in
§ 5 and the conclusion and summary are made in § 6. Throughout this paper, we
assume that solar metallicity corresponds to $\text{Z}_{\odot}=0.019$ (Girardi
et al., 2000).

##  2 Observation and Data Reduction

NGC 6868 was observed on 2013 May 04 with the Gemini South Telescope using the
Gemini Multi-Object Spectrograph (GMOS) in the IFU mode (Allington-Smith et
al., 2002; Hook et al., 2004). This object is part of the DIVING3D survey,
which made observations of the central regions of all 170 galaxies in the
Southern hemisphere with $B<12.0$ and $|b|>15^{\circ}$ (see Steiner et al.
2022 for more details). The one slit set-up was used for the observations,
resulting in an FoV of 5.0 × 3.5 arcsec2. The B600-G5323 grating was used with
a central wavelength of 5620 Å and a spectral range from 4260 Å to 6795 Å. The
spectral resolution is 1.8 Å, estimated with the O i $\lambda$5577Å sky line.
Flat-field exposures, bias and CuAr lamp spectra were acquired for calibration
and correction purposes. The seeing of observation was estimated using stars
that are present in the acquisition image of the galaxy, taken with the GMOS
imager in the r-band (SDSS system). Moreover, the DA white dwarf EG 274 (Hamuy
et al., 1992) was observed in order to perform the spectrophotometric
calibration. These and some other basic information regarding the observation
are given in table 2.

Standard IRAF procedures (Tody, 1986, 1993) were followed to reduce the data
using the tasks contained in the Gemini IRAF package. Bias, flat-fields,
wavelength calibration, dispersion correction, and flux calibration procedures
were applied to the science data. To remove cosmic rays, we used the lacos
software (van Dokkum, 2001). The data cube was constructed with a spatial
sampling of 0.05 arcsec.

After the standard reduction procedures, other data treatments were applied by
means of improving the visualization of the data as described in Menezes et
al. (2019): removal of the high spatial noise using a Butterworth filter, the
correction of the differential atmospheric refraction (DAR), instrumental
fingerprint removal through PCA Tomography and Richardson-Lucy deconvolution.

The removal of high-frequency noise from the spatial dimension was performed
by convolving each image of the data cube with a Butterworth filter (Gonzalez
& Woods, 2008; Ricci et al., 2014). The filter order used was $n=2$ and the
cut-off frequency was $\text{F}_{\text{c}}=0.14\ \text{F}_{\text{Ny}}$ where
$\text{F}_{\text{Ny}}$ is the Nyquist frequency, corresponding to 0.5
spaxel-1. This cut-off frequency was chosen to remove only spatial frequencies
higher than the PSF of the data cube, assuring that no valid scientific
information was lost in this process.

The correction of the differential atmospheric refraction (DAR) consists of
spatially shifting the wavelength planes of the data cube so that the spectrum
of a given point in the galaxy occupies always the same position at all
wavelengths. The correction of the differential refraction effect on the data
cube of NGC 6868 was performed using the equations from Bönsch & Potulski
(1998) and Filippenko (1982), which assume a plane parallel atmosphere and
calculate the displacement of the centroid of the galaxy for each wavelength
as a function of the zenith distance, the refraction index and other
atmospheric parameters.

The PCA Tomography technique (Steiner et al., 2009, and references therein)
applies Principal Component Analysis (PCA) to data cubes. This procedure
searches for spectro-spatial correlation across a given data cube. The results
are the eigenvectors (or eigenspectra), which show the correlations between
the wavelengths caused by some physical phenomenon or an instrumental
fingerprint, and the tomograms, which correspond to the projection of the data
cube onto each eigenvector. This is an orthogonal transform meaning it can be
reversed. The eigenvectors are ordered by how much of the variance in the data
cube they are able to explain, meaning the first eigenvector explains most of
the variance and so on. Using this technique, the instrumental fingerprints
may appear as one of the eigenvectors that would otherwise be entangled with
the data. This instrumental signature may be isolated by building a data cube
containing only this issue. In the end, we subtract this fingerprint from the
science data cube.

After the removal of the instrumental fingerprints, the reddening caused by
the dust within the Milky Way was corrected using the CCM law (Cardelli et
al., 1989) and $A_{V}=0.152$ mag (Schlafly & Finkbeiner, 2011). The telluric
lines were also removed and the spectra were brought to the rest frame using
the redshift shown in Table 1.

Lastly, the Richardson-Lucy deconvolution (Richardson, 1972; Lucy, 1974) is an
iterative process that aims at reconstructing the image of the galaxy before
its convolution with the PSF when it passes through the atmosphere and the
optical apparatus of the telescope. After 10 iterations, the final estimated
PSF was 0.71 arcsec, estimated from a spatial profile obtained along the red
wing of the broad H emission. An image from the continuum of NGC 6868,
extracted from the final data cube, is shown in Fig. 1.

Table 2: Table displaying some basic observational parameters Parameter | NGC 6868
---|---
Observation date | 2013 May 04
Gemini Programme | GS-2013A-Q-52
Seeing (arcsec) | 0.77
Airmass | 1.056
T${}_{\text{exp}}$ (s) | 1800
Number of exposures | 1

##  3 Methodology

###  3.1 Stellar population synthesis

In order to derive the SFH (star formation history), we used the starlight
code (Cid Fernandes et al., 2004; Cid Fernandes et al., 2005; Cid Fernandes et
al., 2013; Cid Fernandes, 2018) which fits the continuum spectra by combining
in different proportions the contribution from different simple stellar
populations (SSPs), taking into account reddening and kinematical parameters.
In other words, it tries to match the observed spectrum ($O_{\lambda}$) with a
modelled one ($M_{\lambda}$), described by

$M_{\lambda}=M_{\lambda_{0}}\left[\sum_{j=1}^{N_{*}}x_{j}b_{j,\lambda}r_{\lambda}\right]\otimes
G(v_{*},\sigma_{*}),$ (1)

where $M_{\lambda_{0}}$ is the flux in a predetermined normalization
wavelength, $N_{*}$ is the number of elements in the SSP base, $x_{j}$ is the
j-th component of the population vector ($\vv{x}$) that stores the light
contribution from each SSP (with respect to the normalization wavelength222We
normalized our spectra in the 5700 Å region, due to the lack of significant
stellar absorption bands and having good S/N in the whole FOV., $\lambda_{0}$
). $b_{j,\lambda}$ is the spectrum of the j-th component, $r_{\lambda}$ is the
reddening factor, defined by
$r_{\lambda}=10^{-0.4(A_{\lambda}-A_{\lambda_{0}})}$) and
$A_{\lambda}=A_{\text{V}}q_{\lambda}$, where $q_{\lambda}$ is the extinction
law evaluated at $\lambda$. Lastly, there is a convolution with a Gaussian
distribution to take into account the line-of-sight velocity distribution
(LOSVD) of the stellar component in the spectra, where $v_{*}$ is the line-of-
sight stellar velocity and $\sigma_{*}$ is the stellar velocity dispersion. To
determine the best fit, the code tries to minimize a $\chi^{2}$ defined by

$\chi^{2}=\sum_{\lambda}[(O_{\lambda}-M_{\lambda})\omega_{\lambda}]^{2}$ (2)

where $\omega_{\lambda}$ is the weight. Using this parameter, we are able to
mask ($\omega_{\lambda}=0$) spurious features or contributions from other non-
stellar components (e.g. emission lines from the ionized gas) or give more
weight to important regions of our spectra (e.g. characteristic adsorptions
that allow better kinematical predictions, if that is the intended objective).
Along with the emission lines present in our spectra, the Mg i absorption (an
element) was masked during the synthesis in order to minimize possible
degeneracies that can be introduced by the -enhancement effects in the
determination of the galaxy metallicity (the [/Fe] is derived in § 3.2).

One of the fundamental ingredients in this method is the SSPs used in the fit.
We constructed our base with the models developed by Vazdekis et al. (2016,
hereafter E-MILES), using the evolutionary tracks of Girardi et al. (2000) and
the Kroupa (2001) initial mass function. These models were chosen because
their wavelength range overlaps with our data and have a better spectral
resolution (2.51 Å FWHM) when compared to other stellar population models
(e.g. Bruzual & Charlot, 2003; Maraston & Strömbäck, 2011; Conroy et al.,
2009)333The high resolution is fundamental to precisely modelling the stellar
absorptions, allowing a detailed study of the gas kinematics when discounting
the stellar component. This allows a self-consistent analysis that will be
pursued in a future paper (Benedetti et al., in preparation).. Moreover, MILES
is a modern empirical stellar library spanning a wide range of stellar
parameters, therefore, allowing us to better explore different stellar
population properties in our object. The final SSPs span 15 ages (0.0631, 0.1,
0.16, 0.28, 0.50, 0.89, 1.26, 1.41, 2.51, 3.98, 6.31, 7.94, 10.0, 11.2, 12.6
Gyr) and six metallicities (0.005, 0.02, 0.2, 0.4, 1.0, 1.6 Z⊙).

The E-MILES models, however, lack really young and hot stars, having only SSPs
with ages greater than $63$ Myr. In order to assess the possibility of such a
young population in NGC 6868, we performed stellar synthesis with the Bruzual
& Charlot (2003) models, which include stars as young as 0.1 Myr in the whole
FoV. We found no contribution from components with less than $63$ Myr.

Since this object has no ongoing star formation, we decided to use the
reddening law from Cardelli et al. (1989) to model the dust attenuation (AV)
intrinsic to our object. To account for a possible featureless continuum (FC)
emission of an AGN, we followed Riffel et al. (2009) and a power-law spectrum
with $f\sim\nu^{-1.5}$ was added to the base. In other test runs, we have
included FC with different exponents, ranging from -1.75 to -1.0, however, no
significant contribution from any of these components was found.

Finally, to better understand the age and metallicity spatial distribution, we
calculated the light-weighted mean stellar age ($\langle t\rangle_{L}$), as

$\langle t\rangle_{L}=\sum_{j}x_{j}\log(t_{j}),$ (3)

and the light-weighted mean stellar metallicity($\langle Z\rangle_{L}$, Cid
Fernandes et al., 2005), as

$\langle Z\rangle_{L}=\sum_{j}x_{j}Z_{j}.$ (4)

$\log(t_{j})$ is used in the computation of $\langle t\rangle_{L}$ because the
ages of the stellar population span many other orders of magnitude from
$10^{6}$ to $10^{10}$.

To optimize the data management, we used the megacube tool (Mallmann et al.,
2018; Riffel et al., 2021) which takes as input the data cube and prepares the
data for the synthesis procedure, executes the synthesis and also performs the
preliminary analysis (e.g. calculate the equations 3 and 4) as well as
mounting the maps with the important parameters.

###  3.2 Indices measurements and Alpha-enhancement

To better constrain the assembly history of NGC 6868, especially the
-enhancement, we have measured indices for the absorption lines. We measured
the indices for Fe4383, Mg2, Mgb, Fe5270, Fe5335 using the definitions
presented in Riffel et al. (2019) which are based on Worthey et al. (1994) and
were subsequently used to derive
Fe3444$\text{Fe}3=(\text{Fe}4383+\text{Fe}5270+\text{Fe}5335)/3$ (Kuntschner,
2000) and
[MgFe]’555$[\text{MgFe}]^{\prime}=\sqrt{\text{Mg}_{\text{b}}\left(0.72\times\text{Fe}5270+0.28\times\text{Fe}5335\right)}$
(Thomas et al., 2003). All spaxels were corrected to the rest frame using the
line-of-sight Doppler shift velocity derived by starlight. At first, we decide
not to correct the effects due to the velocity dispersion.

The in-house pacce code (Riffel & Borges Vale, 2011) was used to perform the
equivalent widths (EW) measurements of these indices. The code uses predefined
continuum bands around a given line and fits a pseudo-continuum line. Once the
line is fitted, the EW is calculated through

$W_{\lambda}=\left(1-\frac{A_{2}}{C}\right)\left(\lambda_{u}-\lambda_{i}\right)$
(5)

being $W_{\lambda}$ the measured EW of the line, $A_{2}$ and $C$ are the
integrated areas below the absorption and below the pseudo-continuum,
respectively, and $\lambda_{u}$ and $\lambda_{i}$ are the predetermined final
and initial wavelength of the absorption feature. The trapezium integration
method is used and the calculation of Wλ is iterated over the whole cube.

A problem we faced during these measurements was the contamination from the
weak [N i] $\lambda~{}5199$ Å emission line which is located between the Mg i
absorption and the pseudo continuum definition of Mgb. We were able to work
this around by modelling the emission line profile and subtracting this
contribution from the spaxels. In order to properly model this component, we
discounted the previously derived stellar spectra and fitted the remaining
continuum with a high-order polynomial. Afterwards, we adjusted this line
using the ifscube package (Ruschel-Dutra & Oliveira, 2020). We modelled the [N
i] $\lambda~{}5199$ Å emission line using two Gaussian components and coupled
its kinematics with the [N ii] $\lambda\lambda 6548,6583~{}$ Å. Additional
details on emission line fitting will be presented in Benedetti et al. (in
preparation). Once the lines were properly fitted we were able to discount
this emission and remeasure the Mg i indices.

Aimed at constraining the assembly history of NGC 6868, we derived the [/Fe]
of the stellar population. Following the approach described in La Barbera et
al. (2013) using the indices Mg${}_{\text{b}}$, Fe3 and the luminosity-
weighted age derived from starlight to obtain ZMg${}_{\text{b}}$ and ZFe3. To
measure the Mg${}_{\text{b}}$ and Fe3 indices, we first broadened our spectra
convolving with a Gaussian to match the spectral resolution of MILES models
(2.51 Å FWHM, Vazdekis et al., 2015) and the measured indices were corrected
by the velocity dispersion following the prescriptions by de la Rosa et al.
(2007). We then interpolated the Vazdekis et al. (2015) models grids by fixing
the stellar population age to obtain ZMg${}_{\text{b}}$ and ZFe3, as
illustrated in Fig. 2. As mentioned in La Barbera et al. (2013), one may need
to extrapolate the grids in -enhanced populations. This is represented as the
dotted lines in the same figure. Afterwards, we calculated the proxy
[ZMgb/ZFe3]= ZMgb-ZFe3 which finally can be used to get (Vazdekis et al.,
2015):

$[\alpha/\text{Fe}]=0.02+0.56[Z\textsubscript{Mg${}_{b}$}/Z\textsubscript{Fe3}].$
(6)

Figure 2: Diagrams showing the grids used to derive the ZMgb (top) and the
ZFe3 (bottom) using the light-weighted mean age derived from starlight and the
measured EW from each absorption.Each spaxel is represented by one dot.

##  4 Results

###  4.1 Stellar population synthesis

We present an example of the fits for an individual, nuclear spaxel in Fig. 3.
A good matching between the observed (black) and modelled (red) spectra can be
seen. The quality of the fits over the full FoV is ensured by the signal-to-
noise (S/N) ratio map and can be certified in the $\chi^{2}$ and Adev666Adev
is the Allan deviation and serves as a quality indicator of the fit. It
corresponds to the percentage mean $|O_{\lambda}-M_{\lambda}|/O_{\lambda}$
deviation over all fitted pixels. maps (Fig. 4).

Figure 3: Top: Example spectra extracted from the spaxel with the highest
continuum flux. The observed spectra (black) and the synthesised spectrum
(red) are shown as well as the residuals (blue) and the masked pixels (pink).
Bottom: Histograms showing the stellar population composition found by
starlight. Continuous lines show light-weighted parameters and dashed lines,
mass-weighted parameters. From left to right, the sum of contributions of
components with the same age but different metallicities; contribution from
each SSP in the base; age and metallicities contribution summed in predefined
age bins, being xi SSPs between 0.1 Myr until 100 Myr, x, 100 Myr until 2 Gyr
and xo, 2 Gyr until 13.7 Gyr. Further right are other parameters derived by
the synthesis: The reddening in the V band, light- and mass-weighted mean age,
light- and mass-weighted mean metallicity, $\chi^{2}$, Adev and the S/N
measured between 5670-5730 Å. The code clearly indicates the presence of old
populations followed by a small fraction of young populations. Figure 4: Maps
displaying the statistical parameters derived from the stellar population
synthesis. From left to right: the signal-to-noise ratio (measured between
5670-5730 Å), Adev and $\chi^{2}$. Together they attest to the robustness of
the modelled population.

The resulting stellar population derived over the full FoV by the fitting
procedure resulted in a contribution of mainly 3 components: two old metal-
rich (12.6 Gyr; 1.0 and 1.6 Z⊙) and a smaller contribution of a young also
metal-rich (63.1 Myr; 1.6 Z⊙). The spatial distribution of each component is
shown in Fig. 5. It is clear that the central region is dominated by an old
stellar population ($\sim 12$ Gyr), as is to be expected for massive early-
type galaxies. However, this contribution is divided into two different
components with different metallicities, having 1.0 and 1.6 Z⊙ each. They also
exhibit a distinct spatial distribution, with more metal-rich stars dominating
the central region of our FoV. The same distribution is seen for the mass
fractions derived for each component. The 1.0 Z⊙ SSP has a mean relative mass
fraction of $\sim 60$ % ranging from $\sim 41$ % to $\sim 84$ %. On the other
hand, the 1.6 Z⊙ SSP has a mean relative mass fraction of $\sim 40$ % ranging
from $\sim 14$ % to $\sim 58$ %.

Figure 5: Maps displaying the light contribution from the three most
significant components for the spectral synthesis in our FoV. The synthesis
finds two old metal-rich (12.6 Gyr; 1.0 and 1.6 Z⊙) components and a smaller
contribution of a young also metal-rich (63.1 Myr; 1.6 Z⊙) component. For
display purposes, the scale of the younger population is different from that
of the other two components.

Rickes et al. (2008) have also conducted stellar population synthesis studies
in NGC 6868 in a larger scale ( $R<17$ arcsec) and also found an ubiquitous
old population. However, they also report an intermediate-age stellar
population of 5 Gyr which peaks in the centre of the galaxy, contradicting our
findings. However, their data ranges from $5100-6800$ Å therefore lacking the
bluer end of the spectrum in order to constrain the presence of intermediate
and young stars. Also, their SSP base consists of very few elements, that most
likely do not portray all the different SFH a galaxy can have. Therefore, the
discrepancies seen are most likely the result of better data employed here
(larger wavelength coverage and S/N) and the improvement of the synthesis
method.

Despite the different FCs we tried in our base, we did not find any
contribution from an AGN. As this is a galaxy classified as a LINER, the
supermassive black hole is likely accreting at a really low rate, making its
contribution to the continuum undetectable.

In order to represent the galaxy’s age and metallicity in a single map, we
have derived the $\langle t\rangle_{L}$ and $\langle Z\rangle_{L}$ maps, which
are shown in Fig. 6. In these maps, one can observe that the galaxy’s mean age
is slightly smaller and the metallicity higher in the nucleus. Finally in Fig.
7 we show the reddening map (AV). It reaches a peak of $\sim$0.65 mag in the
centre of the image and its morphology resembles a dust lane embedded in the
centre of the galaxy. This is in agreement with literature results (e.g.
Veron-Cetty & Veron, 1988; Hansen et al., 1991; Buson et al., 1993).

Figure 6: Mean age (left) and mean metallicity (right) maps derived from
starlight output. Both are luminosity-weighted quantities. The mean age seems
to be affected by the younger component found in the synthesis and the mean
metallicity map clearly shows the gradient metallicity in this galaxy.  Figure
7: Map of the reddening in the V band extracted using starlight. The
morphology resembles a dust lane in the line of sight of the observer. This
amount of dust has disturbed the continuum image, making it seem like a double
core.

starlight also outputs the line of sight velocity and velocity dispersion.
These maps are shown in Fig. 8. It is evident that NGC 6868 does not display a
rotation profile or any ordered motion. Instead, it appears that the stars are
in random motions, as can be seen in the $\sigma_{*}$ distribution, showing a
clear peak $\sim 290$ km s-1 in the centre of the galaxy. In order to verify
our results, we have also performed the fits using pPXF (Cappellari &
Emsellem, 2004; Cappellari, 2017) with the same SSP base and got the exact
same results. Using the spectral resolution from our data and 5000 Å as our
reference wavelength, we come to the conclusion that we are not able to
distinguish any velocities with less than $~{}100$ km$\cdot$s-1. Therefore, we
would need data with a better spectral resolution to properly characterize the
kinematics of the central region of NGC 6868.

Figure 8: Kinematical maps regarding the LOSVD of the stellar component. The
velocity map (left) does not seem to follow any particular geometry or
distribution and the velocity dispersion (right) has a more defined profile
with a clear peak at the centre of the distribution.

In past studies (Caon et al., 2000), the stellar kinematics was measured using
long-slit data and they found (1) a shallow rotation profile with a peak
velocity of $\sim 45$ km s-1 at a radius of 42 arcsec ($\sim 5.8$ kpc); (2) a
KDC which exhibits a counter rotation with respect to the outer regions and;
(3) a drop in velocity dispersion at the centre of the galaxy, only seen in
one of their PAs. However, in the inner part (<150 pc) of NGC6868 we are not
detecting any sign of rotation nor drop in $\sigma_{*}$ reported. Moreover,
comparing the derived velocity dispersion profiles from other PAs in their
data, there is also no evidence of this behaviour. This is also the case for
the KDC, where the velocities within it reach at most 10 km s-1. The most
probable explanation for these mixed findings is the high variation between
different PAs as can be seen in Fig. 8. Depending on the angle observed, a
rotation (or counter-rotation) can or cannot be observed, due to the high-
velocity dispersion in that region, making precise determinations of the
velocity challenging. Moreover, an important caveat is that a KDC can only be
detected when comparing the core and the outer parts of a galaxy. The FoV of
our data does not allow such a comparison, therefore, in the present paper, we
are unable to clearly say whether this object hosts a KDC or not.

###  4.2 Absorption line indices

The maps for the absorption line indices calculated in the Lick/IDS resolution
are shown in Fig. 9. All the values shown were corrected by the intrinsic
velocity dispersion found with starlight, except for the Mg2 index because it
is almost insensitive to Doppler broadening. In our case, this correction
would be smaller than the intrinsic error of the method (< 0.003, e.g.
Kuntschner, 2000), so we chose not to apply any correction. This is not the
case for the other indices. The correction factors for each spaxel follow the
spatial distribution from the velocity dispersion (Fig.8). The derived
correction factors for the Mgb index are in the range 1.09-1.14; Fe4383,
1.12-1.17; Fe5270, 1.23-1.29; Fe5335, 1.40-1.54. The maximum error found for
each index was Mg2: 0.0044 mag, Mgb: 0.15 Å, Fe4383: 0.36 Å, Fe5270: 0.14 Å,
Fe5335: 0.18 Å, Fe3: 0.14 Å e [MgFe]’: 0.17 Å. These errors leave all the
following reported gradients unaffected.

Figure 9: Results for the index measurement. From left to right: Mg2, Mgb, Fe3
and [MgFe]’. The indices show complex profiles in contrast to the results from
the stellar population synthesis. In the Mg2 map we over-plotted lines
indicating the extractions done in our cube in order to compare with the
literature results (Fig. 10). The red "x" in the panels is the location of the
peak Mg2.

In order to assess the confidence of our results, we made extractions at PAs
from the literature (Carollo et al., 1993; Rickes et al., 2008) matching the
spatial extent of our data. This comparison can be seen in Fig. 10 and the
error bars displayed for the values derived in this work are the standard
deviation measured within each bin which is greater than the systematic
errors. Mg2 presents an offset of at least -0.5 mag between Rickes et al.
(2008) and our extractions, with both showing a negative gradient. The likely
explanation for this offset is the different ways to measure the continuum
applied in each work: they chose custom continuum bands and we followed Riffel
et al. (2019). Carollo et al. (1993), on the other hand, matches all our data
within the error bars for both of their PAs. Fe5270 and Fe5335 also present an
offset between Rickes et al. (2008) and our measurements, again preserving
gradients probably related to the continuum definitions, as previously said.
Carollo et al. (1993) measured Fe5270 (transparent points in Fig. 10) and
verified it was not in the Lick system. Therefore they derived a correction to
be applied for their data in the form $\delta\text{Fe}5270=0.87(\pm
0.07)\cdot\text{Fe}5270-2.67(\pm 0.02)$, resulting in $\delta\text{Fe}5270\sim
0.5$ Å. The corrected points are the full circles in Fig. 10 which, once
again, match all our data within the error bars. Carollo et al. (1993) did not
measure Fe5335.

Figure 10: Comparison of our Mg2, Fe5270 and Fe5335 results to past studies
using long-slit spectroscopy for different PAs (Rickes et al., 2008; Carollo
et al., 1993). Each PA is represented by a colour, literature results are
shown as circles and our measurements are shown as squares. The smaller
scatter points correspond to the spaxels within the artificial slit observed.
As can be seen, even inside the artificial slits there is significant
variation.

In contrast to the parameters derived in the stellar population synthesis
(Fig. 6), the spatial variation of the measured indices is far more intricate.
Therefore to gauge the mean behaviour of each index, in Fig. 11 we plot the
median of each index in circular apertures of $\delta r=0.05$ arcsec (7 pc)
using the peak in Mg2 as the origin. Mg2 has the clearest spatial behaviour
when compared to all the other indices, monotonically decreasing across the
majority of the FoV. Two peculiar regions deviate from this trend, however:
one at NW (+120 pc,+175 pc) in the border of the FoV, and another one SW at
(+170 pc, -140 pc). Unfortunately, we do not have any literature results in
these areas, nevertheless, given that our method resulted in an excellent
agreement with past studies, we are convinced of this detection. The Mgb
profile closely resembles Mg2 also showing the distinct regions previously
cited. Fe3 is particularly interesting because it does not follow the
metallicity map as previously shown (Fig. 6), already a hint of -enhancement
processes at play. The most curious result is that in the same SW region
previously mentioned, there is a depletion of this index. The [MgFe]’ map
closely resembles the $\langle Z_{L}\rangle$ map indicating that the centre of
this galaxy really is metal-rich and exhibits a negative gradient.

Figure 11: Correlations between Mg2, Mgb, Fe3 and [MgFe]’ indices. These are
the median values within $\delta r=0.05$ arcsec apertures that have been
colour-coded for the distance from the centre. For $R<100$ pc and $R>220$ pc
the Mg2 and Mgb anti-correlate , with Fe3 and [MgFe]’. The intermediate region
displays a correlation. The transparent points indicate the mean of each index
in each radial bin. This hints at different possible enrichment processes in
each region.

One curious behaviour that can easily be seen in Fig. 11 is that for $R<100$
pc the Mg2 and Mgb anti-correlate with Fe3 and [MgFe]’ reaching their peaks at
$R\sim 100$ pc away from the centre. Until $\sim 220$ pc we actually observe a
correlation between the two indices which turns again into an anti-correlation
beyond $R\sim 220$ pc, however shallower when compared to the inner region.

Rickes et al. (2008) found that Mg2 and Fe5270 or Fe5335 establish a strong
correlation which led them to conclude that the elements traced by both of
these indices suffered the same enrichment processes. This can be the case for
bigger scales, but in our FoV, we observe a distinct behaviour.

###  4.3 Alpha-enhancement

The grids used to derive the [/Fe] are seen in Fig. 2 as well as our
measurements. As we only find old stellar populations, the most significant
variation is given by the indices. Using these values, the [/Fe] map is shown
in Fig. 12. It is clear that the whole FoV presents -enhanced stellar
populations with values between $\sim+0.07$ and +0.24 dex. Also, we find a
really structured profile with diverse morphology. Using once again the peak
in Mg2 as our reference, we plot the median profile of the [/Fe] using
circular apertures that can also be seen in Fig. 12. What becomes apparent is
that the centre of NGC 6868 shows a peak in -enhancement followed by a region
of shallower [/Fe] which again is followed by another region with values as
big as the ones in the centre, producing the "U" shape found in the median
profile.

Figure 12: Top panel: Derived [/Fe] for the whole FoV where it becomes clear
that -enhancement processes are ubiquitous. This parameter shows a really
complex morphology, with a clear peak towards the centre, but also alpha-
enhanced populations at larger radii. Bottom panel: Radial profile of [/Fe]
with the median (red) and mean (red, transparent) of the distribution over-
plotted. It is clear that the profile exhibits a peak in the centre, followed
by a region with smaller [/Fe] followed again by an -enhanced region. The red
"x" in the top panel is the peak in Mg2 following Fig. 11 and is used as the
reference to trace the profile in the bottom panel

The only other paper where there is a measurement of the [/Fe] from NGC 6868
is Rickes et al. (2008). According to them, the central parts of NGC 6868
present lower [/Fe], between -0.3 and 0.0 dex and an above-solar metallicity
([Z/Z⊙] $\sim+0.3$ dex. Moreover, the external parts present higher [/Fe]
values ($\sim+0.3$ dex) and lower metallicities ([Z/Z⊙] $\sim-0.33$ dex). In
order to obtain these values, they use the Mg2, Fe5270 and Fe5335. However, as
we already showed in § 4.2, these indices, despite agreeing with the gradients
found in other studies, the values present a normalization problem probably
due to the continuum bands used to compute the indices. If this shift in
measurements were accounted for, the values of [/Fe] would increase, reaching
the -enhanced region in their diagram, thus matching our observations. This
would not affect the negative gradient in metallicity nor the positive
gradient in [/Fe] which are in agreement with observations of other early-type
galaxies (e.g. Kuntschner et al., 2010).

##  5 Discussion

###  5.1 Stellar population synthesis

NGC 6868 is an early-type galaxy and so what is expected is that it had a fast
phase of intense SF that suddenly stopped, forming the bulk of its stellar
mass with a subsequent growth attributed to dry minor mergers. Therefore it is
expected that the isophotes are not severely disturbed by the accreted
galaxies. However, as we will present in the forthcoming paper (Benedetti et
al. in preparation), the photometric centre of NGC6868 has an offset with
respect to the outer parts indicating a recent encounter with the NE dwarf
companion galaxy. NGC 6868 at first inspection appears to deviate from this
hypothesis, exhibiting a significant distortion in its continuum image (Fig.
1). When corrected by the stellar reddening found in the synthesis procedure,
the actual morphology is revealed as an undisturbed spheroidal, as shown in
Fig. 13.

Figure 13: Continuum image extracted from our GMOS data of NGC 6868. The
actual photometric centre is uncovered after the correction by extinction.

According to our analysis, the SFH of this galaxy is characterized by a short
burst in the early Universe with no major episodes of star formation since
then (Fig. 6). This means that NGC 6868 probably did not experience any
encounters with star-forming galaxies as they would leave signatures in the
SFH from this galaxy.

The interpretation of the young metal-rich component (63.1 Myr; 1.6 Z⊙) is
tricky. For instance, it can be due to residual star formation which can be
found in some elliptical galaxies. How common they are is still a matter of
debate. Salvador-Rusiñol et al. (2020) have detected young stars in massive
red galaxies probably related to recycled material within the galaxy with mass
fraction up to 0.5 per cent (see also de Lorenzo-Cáceres et al., 2020;
Salvador-Rusiñol et al., 2021; Salvador-Rusiñol et al., 2022). On the other
hand, Simonian & Martini (2017) interpreted early-type galaxies which
typically are UV-weak as lacking younger components with HOLMES stars being
responsible for this residual UV emission. Also, Bica et al. (1996) using IUE
spectra classified NGC 6868 as a UV-weak source and found no contribution from
young stellar populations. In addition, Cid Fernandes & González Delgado
(2010) interpreted this young component as an artefact of the fitting process,
and due to the lack of an old blue population, probably related to the
horizontal branch, that current stellar population models do not account for.

In order to better understand the behaviour of this young component, we have
used the M/L from the E-MILES models, and summing over the contributions from
all SSPs with 2 Gyr or less, we get the map available in Fig. 14 with a median
mass contribution in our FoV of $\sim 0.2$ percent. There is a gradient for
this component with higher values towards the nucleus of the galaxy (the same
behaviour as reported by Salvador-Rusiñol et al., 2020). In addition, this is
a metal-rich (1.6 $Z_{\odot}$) stellar population, thus most likely being
formed from recycled material of former stellar generations. Taking this all
together, we interpret that the 63 Myr contribution we found is due to a
recent generation of stars, most likely formed from recycled gas from stellar
evolution. However, it is worth mentioning that the present data do not allow
us to completely rule out other mechanisms (e.g. HOLMES stars) that may also
be enough to account for this fraction we found when fitting the data.

Figure 14: Map of the summed mass contribution found by starlight from
components younger than 2 Gyr. A clear increase towards the centre is seen,
reaching 0.28 % at the centre.

It is worth mentioning that, as said in § 3.1, we have tested for the presence
of younger components in this object by using different SSP models that
account for these younger stars in order to perform the stellar population
synthesis. We found no evidence for the presence of younger components than 63
Myr.

The mean metallicity map ($Z_{L}$, Fig. 6) is consistent with the scenario
where massive early-type galaxies are able to retain the material expelled by
SN, thus fixating the elements produced during stellar evolution into new
stars, also creating a gradient where central regions tend to be more metal-
rich when compared to outer regions. This is the case for the majority of
early-type galaxies (Kuntschner et al., 2010). They also show that for fast-
rotators characterized by only old populations (>9 Gyr), a stellar disc is
embedded in the stellar population of the galaxy characterized by higher
metallicity and lower [/Fe], including the central regions.

The dust found in our data (Fig. 7) is probably related to the cannibalism of
a small gas-rich companion as proposed by Hansen et al. (1991). The molecular
(Rose et al., 2019), atomic (Rose et al., 2019) and ionized gas (e.g. Buson et
al., 1993) detected are probably related with this event. For instance, as can
be seen in figure 2 of Hansen et al. (1991), there are dust filaments with
spiral features that can be followed in a ring-like structure around the
galaxy centre with a connection at NE which is aligned with the dust lane
detected in the present study (Fig. 7). The ring is also connected to a
stellar-like object in NW that they proposed is a cannibalised galaxy. In
fact, we found some recent, metal-rich, star-formation in this galaxy, as
pointed out in § 4.1 this residual star formation is most likely formed from
material ejected from the previous generation of stars. In fact, the
triggering/enhancing of AGN activity/luminosity has been related to an extra
amount of gas that is added to the regular flow and is most likely originated
from stellar evolution processes (Riffel et al., 2022). This gas (from the
filaments + stellar evolution mass-loss) is most likely the reason behind the
triggering of the AGN which is producing the LINER emission that is detected
in the centre of the galaxy. This however is highly speculative and a thorough
analysis of the ionized gas component will be carried out in an upcoming paper
(Benedetti et al. in preparation).

In the synthesis, despite including an FC component and testing with different
exponents, the code does not find any significant contribution of an AGN to
the continuum. This however does not mean there is not an AGN in the centre of
NGC 6868 as the emission can be obscured by dust, or the SMBH might not be
accreting matter at a sufficiently high rate. As was already pointed out, this
galaxy presents AGN evidence from past studies in radio (Slee et al., 1994;
Mauch et al., 2003). We already know this is a LINER source, therefore the
SMBH cannot be accreting at high rates. However, it is already known that when
comparing AGN detection in the radio and optical the ratio of detection is not
a 1:1 relation (Comerford et al., 2020).

The kinematic revealed by starlight matches what is expected for a cD galaxy,
having $\sigma_{*}$ above 250 km$\cdot$s-1. We do not find any clear sign of
rotation or strong ordered motion. Actually, the v∗ map displays values that
are below our uncertainty (see § 4.1, Fig. 8). It is clear that our whole FoV
presents values for the velocity in the line-of-sight which we are not able to
confirm, however, for completeness we decided to show it in Fig. 8. The
velocity dispersion, on the other hand, does not face this problem as all
values derived are $>250$ km$\cdot$s-1. Despite past studies reporting a KDC
in this object, we cannot confirm this result due to our small FoV. What
remains clear is the fact that the central region of NGC 6868 is really
dispersion dominated and no kinematically distinct structure is found.

###  5.2 Absorption line indices and alpha-enhancement

Analysing the profiles derived for the indices (Fig. 9), it is clear that they
are far more structures when compared to the ones derived in the synthesis
(Fig. 6). Moreover, the variation in each index is heavily dependent on the PA
one decides to look at. This hints at a more chaotic assembly history when
compared to what is derived in the synthesis that shows only contributions
from old metal-rich populations. The [MgFe]’ resembles the $\langle
Z\rangle_{L}$ profile probably because we were able to partially isolate the
sensitivity to the -enhancement processes from the synthesis.

This hypothesis is further endorsed by Fig. 11, where at least three distinct
chemically enrichment regimes are clear, with two anti-correlations
(R$\lesssim 100$ pc and R$\gtrsim 220$ pc) and a correlation (R$\gtrsim 100$
pc and R$\lesssim 220$ pc), clearer in the Mgb-[MgFe]’ plot. What this tells
us, again is that only a simple monolithic collapse cannot explain all our
findings, otherwise we would expect the elements sensitive to the indices
measured would have followed similar enrichment processes, thus producing
matching gradients. This appears to be the case for larger scales (Rickes et
al., 2008), however, this is not true for our FoV. A deviation from past
studies that [MgFe]’ reveals is a slight dip in the metallicity of the galaxy
towards the very centre (R<100 pc, Fig. 11) that is also apparent in Fe3. As
this index is insensitive to [/Fe] what we observe is that the central region
is depleted in metals with respect to the outer (100 pc < R <220 pc) region.
This behaviour is unexpected as shown by past studies (e.g. Kuntschner et al.,
2010).

The fact that we are able to detect such gradients using our observations is
another reason why detailed studies on stellar populations on ETGs like the
one presented here are necessary to further understand these objects. As shown
here, despite these gradients naturally appearing in our data, in long-slit
studies, as presented in Rickes et al. (2008) they are not able to detect any
sign of this effect as their analysis is restricted to one PA.

One way of distinguishing the formation scenario, as noted by Carollo et al.
(1993) is measuring the Mg2 gradient (d Mg2/d log r). Using only r>1 arcsec we
find a gradient of d Mg2/d log r$\approx$-0.024, which is a shallow gradient,
incompatible with the monolithic collapse scenario according to the authors.

The [/Fe] map (Fig. 12) also presents a complex profile. However, looking at
the median curve we see a much clear behaviour. The central (R<100 pc) and
outer (R>260 pc) regions appear to be significantly more -enhanced than the
intermediate region. These regions agree with our findings from the indices.
What becomes clear is that the signatures in the stellar populations of the
centre of NGC 6868 cannot be described only by a burst of star formation in
the early universe with a passive evolution since then.

In order to test if the dilution by young stellar component was affecting our
[/Fe] estimates, we followed the same procedure as described in § 3.2 and
estimated the [/Fe] measuring the indices only in the synthetic spectrum. We
found that the effects of the young population are only able to change at a
maximum of 0.04 dex the [/Fe] values. Therefore, we conclude that the young
component is not able to explain the spatial variation found in Fig. 12.

###  5.3 Possible formation scenario for NGC 6868

From our findings, NGC 6868 presents compelling evidence that its assembly
history was not just comprised of a single burst of star formation without any
significant evolution ever since. The [/Fe] map is especially suited to
understanding this galaxy. A close inspection reveals that the regions where
it is smaller are also the more metallic regions. Kuntschner et al. (2010)
found stellar populations with these exact same features (high metallicity and
low [/Fe]) in discs of fast-rotator early-type galaxies. Some of these discs
show signs of recent star formation, however, they also find these
characteristics in galaxies depleted of gas and dominated only by old stellar
populations (<9 Gyr).

Therefore, our hypothesis is that in the past NGC 6868 could have suffered an
episode of merger with another galaxy with lower [/Fe], such as the ones
previously described. This would explain why we see such structured maps in
[/Fe], the slight increase in metallicity outside from the centre and the
absence of a detectable subsequent star formation episode or a lack of
gradient in the mean age across our FoV. The fact that we do not observe any
clear kinematical signature is probably because we are looking at a really
small region and cannot compare with outer regions, despite a KDC being
previously reported (Caon et al., 2000). However, we do find an imprinted
chemical signature. We emphasize that, in this hypothesis, the accreted galaxy
could not have a mass comparable with NGC 6868, because, despite different
maps showing structured profiles, Mg2 shows a (shallow) negative gradient.
Therefore, this enrichment process must have disturbed the stars from NGC 6868
only to a certain extent. Simulations could be used to test our predictions,
however, this analysis is beyond the scope of this paper.

Finally, this might not be the only disturbance NGC 6868 has suffered. We
notice some regions that stand out, mainly in the SW in the [/Fe] map which
displays a $\sim+0.25$ dex and, as can be seen in the Fe3 map (Fig. 9) it is
depleted in [Fe/H]. This can be a recently captured small galaxy, however,
this is unlikely due to its scale and the region it is encountered. We would
need more data to properly characterize this region

##  6 Concluding remarks

In this paper, we analysed GMOS-IFU data of the inner region of the ETG NGC
6868 mapping for the first time the physical and chemical properties of the
stellar content of this source. This, together with an absorption-line indices
analysis has allowed us to constrain the assembly history of this object. Our
results can be summarized as follows:

  * •

This galaxy is dominated by an old metal-rich (12.6 Gyr; 1.0 and 1.6 Z⊙)
stellar population and presents a negative gradient in metallicity. This is
further endorsed by the [MgFe]’.

  * •

We found a recent ($\sim$ 63 Myr) metal-rich ($1.6Z_{\odot}$) stellar
population in the center of the galaxy. We suggest that this component is most
likely due to stars being formed from recycled material of former stellar
generations.

  * •

The apparent distortion in the continuum image is due to a dust lane embedded
in the centre of the galaxy and reaches a peak in AV$\sim 0.65$ mag. This
structure is coincident with the one found in other studies.

  * •

No evidence of an FC continuum is found, probably meaning the AGN in the
centre of NGC 6868 is accreting at really low rates.

  * •

The kinematics in the centre of NGC 6868 is characterized by high dispersion
velocities and no apparent circular motion of the stars is seen.

  * •

The indices Mg2, Mgb, Fe3 and [MgFe]’ all present structured profiles, with
Mg2 presenting the steepest negative gradient. However, it is too shallow to
support a formation history due to a monolithic collapse.

  * •

Three distinct regions can be found when cross-correlating the indices: anti-
correlations for R$\lesssim 100$ pc and R$\gtrsim 220$ pc and a correlation
for 100 pc$\lesssim$R$\lesssim 220$ pc. This reveals different enrichment
histories in these regions.

  * •

The [/Fe] map also does not present a clear gradient. However, the median
appears to also show three distinct regions: the central (R<100 pc) and outer
(R>260 pc) regions appear to be significantly more -enhanced than the
intermediate region.

These findings suggest that NGC 6868 was not formed on a single collapse and
has passively evolved since then. Rather we propose that it has suffered a
past merge with another galaxy. This can explain the findings in the
-enhancement and the different regions in the indices correlations together
with the stellar population synthesis ones, such as the metallicity gradient
and ubiquitous old ages. We do not find evidence of a distinct kinematic
component either because this merger supposedly happened too long ago or we
would need a larger FoV to asses if this region really is a KDC as other
studies have previously reported.

## Acknowledgements

We thank the anonymous referee for the very useful comments and suggestions
that helped to improve the manuscript. We also thank Alexandre Vazdekis for
the insightful discussions. This work was supported by Brazilian funding
agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)
and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and by
the Programa de Pós-Graduação em Física (PPGFis) at UFRGS. JPVB acknowledges
financial support from CNPq and CAPES (Proj. 0001). RR acknowledges support
from the Fundación Jesús Serra and the Instituto de Astrofísica de Canarias
under the Visiting Researcher Programme 2023-2025 agreed between both
institutions. RR, also acknowledges support from the ACIISI, Consejería de
Economía, Conocimiento y Empleo del Gobierno de Canarias and the European
Regional Development Fund (ERDF) under grant with reference ProID2021010079,
and the support through the RAVET project by the grant PID2019-107427GB-C32
from the Spanish Ministry of Science, Innovation and Universities MCIU. This
work has also been supported through the IAC project TRACES, which is
partially supported through the state budget and the regional budget of the
Consejería de Economía, Industria, Comercio y Conocimiento of the Canary
Islands Autonomous Community. RR also thanks to Conselho Nacional de
Desenvolvimento Científico e Tecnológico ( CNPq, Proj. 311223/2020-6,
304927/2017-1 and 400352/2016-8), Fundação de amparo à Pesquisa do Rio Grande
do Sul (FAPERGS, Proj. 16/2551-0000251-7 and 19/1750-2), Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Proj. 0001). TVR thanks
CNPq for support under grant 306790/2019-0. MT thanks the support of CNPq
(process 312541/2021-0) and the programme L’Oréal UNESCO ABC Para Mulheres na
Ciência. RAR acknowledges the support from Conselho Nacional de
Desenvolvimento Científico e Tecnológico and Fundação de Amparo à Pesquisa do
Estado do Rio Grande do Sul. ARA acknowledges Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq) for partial support to this
work through grant 312036/2019-1. JAHJ acknowledges support from FAPESP,
process number 2021/08920-8.

Based on observations obtained at the international Gemini Observatory and
processed using the Gemini iraf package, a programme of NSF’s NOIRLab, which
is managed by the Association of Universities for Research in Astronomy (AURA)
under a cooperative agreement with the National Science Foundation on behalf
of the Gemini Observatory partnership: the National Science Foundation (United
States), National Research Council (Canada), Agencia Nacional de Investigación
y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación
(Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações
(Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea).

## Data Availability

The data are publicly available on gemini archive under the project
GS-2013A-Q-52.

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