11institutetext: Dipartimento di Fisica e Astronomia, Università di Padova,
vicolo dell’Osservatorio 3, I-35122 Padova, Italy
11email<EMAIL_ADDRESS>22institutetext: Institute of Astrophysics,
Foundation for Research and Technology–Hellas (FORTH), Heraklion, GR-70013,
Greece 33institutetext: INAF-Osservatorio Astronomico di Padova, vicolo
dell’Osservatorio 5, I-35122, Padova, Italy 44institutetext: University of
Bologna, Department of Physics and Astronomy ”Augusto Righi” (DIFA), Via
Gobetti 93/2, I-40129, Bologna, Italy 55institutetext: INAF-Osservatorio di
Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, I-40129, Bologna, Italy

# Ancient stellar populations in the outskirts of nearby grand-design spirals:
Investigation of their star formation histories

Cristina Maria Lofaro 1122 Giulia Rodighiero 1133 Andrea Enia 44 5 5 Ariel
Werle 33 Laura Bisigello 1133 Paolo Cassata 1133 Viviana Casasola 55 Alvio
Renzini 33 Letizia Scaloni 4455 Alessandro Bianchetti 1133

(Received date / Accepted date )

###### Abstract

Context. The main sequence (MS) of star-forming galaxies (SFGs) is the tight
relation between the galaxy stellar mass (M$\star$) and its star formation
rate (SFR) and was observed up to z $\sim$ 6\. The MS relation can be used as
a reference for understanding the differences among galaxies, which are
characterised by different rates of stellar production (starbursts, SFGs, and
passive galaxies), and those inside a galaxy that is made up of different
components (bulge, disk, and halo). To investigate peculiar features found in
our sample galaxies in more depth, we focus here on their star formation
history (SFH).

Aims. The SFHs are a fundamental tool for revealing the galaxy path from the
earlier stages of formation to the present time. The various phases of galaxy
evolution are imprinted on the source spectrum globally and locally. Thus, we
are able to interpret the dynamical origin of the spirals quantitatively and
distinguish between in situ or ex situ formation processes.

Methods. We performed a spectral energy distribution fitting procedure that
accounted for the energetic balance between UV (observed) and far-IR
(optically obscured) radiation on a sample of eight nearby face-on grand-
design spiral galaxies from the DustPedia sample. This approach allowed us to
study the spatially resolved MS of the sample and to recover the past SFH by
accounting for attenuation due to the presence of dust. By exploiting the
BAGPIPES code, we constrained the SFHs for each galaxy with a delayed
exponentially declining model to derive their mass-weighted age (tMW).

Results. The spiral galaxies in our sample have similar radial tMW trends
overall. A central old region (tMW up to $\sim 7$Gyr, consistent with the
presence of a bulge for various systems) is followed by younger regions in
which the disks are still forming stars (t$\textsubscript{MW}\sim 4$Gyr). At
larger distances from the centre of the galaxies, tMW increases mildly in
general. Strikingly, in two galaxies (NGC4321 and NGC5194), we found a steep
increase in tMW that reached levels similar to those of the bulge. These old
stellar populations in the very galaxy outskirts, which are also detectable as
”quenched rings” below the spatially resolved MS, is unexpected. We discuss
their potential origin by considering the different gas phases (HI and H2) of
the source with the most prominent quenched ring, NGC4321, and argue for two
main possibilities: 1) some environmental effect (e.g. starvation) could
affect the outer edge of the galaxies or 2) the circumgalactic medium of
sources outside of high-density clusters might have stopped to supply pristine
gas to the galaxy (e.g. if its specific angular moment is too high for being
accreted).

###### Key Words.:

galaxies: spirals – galaxies: Star Formation History – galaxies: mass-weighted
age

##  1 Introduction

Galaxies appear to build their stellar mass in a steady cold-gas accretion
mode, as indicated by the tight relation between the galaxy stellar mass (M∗)
and its star formation rate (SFR): the main sequence (MS) of star-forming
galaxies (SFGs), observed up to $z\sim 6$ with a fairly constant scatter of
$\sim$0.3 dex (see, e.g. Noeske et al., 2007; Daddi et al., 2007; Rodighiero
et al., 2014; Speagle et al., 2014; Whitaker et al., 2014; Wuyts et al., 2011;
Schreiber et al., 2015; Renzini & Peng, 2015; Popesso et al., 2019). Galaxies
seem to fluctuate around the MS relation as a consequence of stochastic
variations in their gas accretion rate and ensuing SFR, with occasional more
dramatic events such as disk instabilities (Tacchella et al., 2016). One
outstanding issue remains: to understand how the star formation activity is
eventually quenched and leads to the bimodality between star-forming and
passive galaxies (e.g. Rodighiero et al., 2011; Peng et al., 2015; Saintonge
et al., 2016). The tight relation between stellar mass surface density
($\Sigma_{*}$) and SFR surface density ($\Sigma\textsubscript{SFR}$ ) found in
HII regions of nearby galaxies further suggests that the global MS relation
originates in local processes that set the conversion of gas into stars
(Rosales-Ortega et al., 2012; Sánchez et al., 2012). Following this, several
works have exploited the advent of large integral field spectroscopic surveys
to analyse the spatially resolved MS relation in low-redshift galaxies using
the $H\alpha$ flux as SFR tracer (e.g. Cano-Díaz et al., 2019; Hsieh et al.,
2017; Abdurro’uf & Akiyama, 2019; Medling et al., 2018).

Enia et al. (2020) studied the main biases affecting most previous works in
which the SFRs were derived from emission lines (i.e. $H\alpha$) or UV-to-
optical tracers (Sánchez et al., 2012, e.g.), and which therefore likely
underestimated the SFR of more dust-extincted regions. To achieve this, they
made use of a more complete photometric coverage that extended from the far-UV
to the far-IR (i.e. from GALEX to Herschel) to provide a reliable measure of
the SFR in galaxies by directly accounting for both the observed and the dust-
obscured components. This can be performed by a spectral energy distribution
(SED) fitting procedure that accounts for the energetic balance between dust-
absorbed and re-emitted radiation, hence offering a more complete account of
the SFRs. The sample presented in Enia et al. (2020) is restricted to grand-
design spirals with a low inclination, large spatial extension, and regular
spiral arms structures. This analysis was extended to ten thousands of
physical cells on typical scales of $\sim$0.5 kpc and over very different
internal galactic regions (bulges, spiral arms, inter-arms regions, and
outskirts). This set of galaxies offers a local reference to explore the
evolutionary processes regulating star formation in rotationally supported
systems all the way to their high-redshift counterpart (Law et al., 2009;
Förster Schreiber et al., 2009; Glazebrook, 2013; Wuyts et al., 2013; Simons
et al., 2017; Förster Schreiber et al., 2018; Übler et al., 2019). However,
individual galaxies show specific variations around the
$\Sigma\textsubscript{SFR}$-$\Sigma_{*}$ relation, indicating that a
comprehensive analysis of a larger sample is required. Before we attempt this
extension, we take one step farther here than Enia et al. (2020) by estimating
the star formation histories (SFH) (see Walcher et al., 2011, for a review) of
individual galactic cells on the same 0.5 kpc scale.

We therefore undertake a comprehensive analysis of the spatially resolved MS
of a sample of nearby grand-design spiral galaxies in which, while following a
global scaling relation on average (Enia et al., 2020; Ellison et al., 2021;
Casasola et al., 2022), some galaxies can reveal peculiar features that are
not observed in others. In particular, in some physical regions within our
galaxies, the observed SFR significantly exceeds that predicted by the main
relation at fixed stellar mass (i.e. starbursting regions). On the other hand,
we observe some areas that are characterised by a suppression of the SFR at a
given stellar mass, which we consider as quenched regions (lying up to more
than 1 dex below the expectation of the star-forming main sequence). We focus
on these latter regions with the goal of using spatially resolved SFHs to
understand the physical origin of the quenching mechanisms in star-forming
galaxies. We limited our analysis to eight sources as a pilot program to test
the method, which we plan to extend in the future to a much wider galaxy
sample.

The paper is structured as follows. In Section 2 we describe the dataset. In
Section 3 we present our SED fitting method (e.g. libraries and image
processing). In Section 4 we present our results for the spatially resolved
MS. We assume a $\Lambda$CDM cosmology Planck Collaboration et al. (2016) and
Chabrier (2003) initial mass function (IMF) throughout.

##  2 DUSTPEDIA sample

###  2.1 DustPedia archive

DustPedia is a research project to characterise the dust in the local
Universe. Posed as the legacy of the Hershel Space Observatory, it contains
the imagery and photometry study of 875 nearby galaxies, spanning over five
orders of magnitude in wavelength, from the ultraviolet to the microwave, and
was made publicly available to the scientific community through the DustPedia
database (Clark et al., 2018) 111The DustPedia website is available at
http://dustpedia.astro.noa.gr. Subsequently, the considerable data improvement
of the past years and exploration of a wide spectral range (from the far-
infrared to the sub-millimetre) enabled a relevant development in the cosmic
dust investigation through the Herschel, Planck, Spitzer, the James Clerk
Maxwell Telescope (JCMT), and the Atacama Large Millimetre/sub-millimetre
Array (ALMA) observatories. These observatories were well suited for the study
of nearby galaxies through their rapid mapping abilities, which enabled them
to observe a sizeable portion of the galaxies in the local Universe with mixed
sensitivity and resolution, and with a broad wavelength coverage.
The Dustpedia sample consists of galaxies observed within a distance of 41 Mpc
assuming $H\textsubscript{0}$=73.24 km/s/Mpc and $D\textsubscript{25}$ ¿
1$\textsuperscript{$\prime$}.$ $D\textsubscript{25}$ is the major axis
isophote at which the optical surface brightness falls beneath
$25mag/arcsec^{2}$.

###  2.2 Multiwavelength data

The ancillary data consist of observations from the following facilities (for
further details, see Tab 1):
\- the GALaxy Evolution eXplorer, GALEX (Morrissey et al., 2007). The
ultraviolet part of the electromagnetic spectrum is sampled by GALEX. Near-UV
(NUV) and far-UV (FUV) data sample the light from newborn massive stars that
traces the unobscured star formation activity of galaxies.
\- the Sloan Digital Sky Survey (SDSS; York et al., 2000; Eisenstein et al.,
2011). The SDSS provides ultraviolet, optical, and near-infrared imaging of
the 35 $\%$ of the sky and samples the young stellar content.
\- the 2 Micron All-Sky Survey, 2MASS (Skrutskie et al., 2006),
\- the Wide-field Infrared Survey Explorer, WISE (Wright et al., 2010), and
\- the Spitzer Space Telescope (Werner et al., 2004). The NIR and MIR
observations from the 2MASS, WISE, and Spitzer surveys trace the old stellar
component, the stellar mass distribution, and the carbonaceous-to-silicate
materials in the dust.
\- Herschel Space Observatory (Pilbratt et al., 2010) and Planck (Planck
Collaboration et al., 2016). Herschel and Planck cover the spectral range from
the far-infrared up to the sub-millimeter, which allows us to probe the
reprocessed emission from dust and to thus constrain the dust-obscured star
formation processes.

###  2.3 HI and CO observations

To further support the interpretation of our results, we took advantage of
atomic and molecular gas observations, in particular, for galaxy NGC4321 (see
Section 2.4). HI data are available from the public VIVA survey (VLA IMAGING
OF VIRGO SPIRALS IN ATOMIC GAS, Chung et al., 2009). These observations were
carried out with the Very Large Array (VLA) and are characterised by an
angular resolution of $\sim 15"$. To compute the HI surface brightness
$\Sigma_{HI}$, we followed the approach detailed in Morselli et al. (2020). We
convolved the 21cm natural-weighted intensity maps, given in Jy beam-1m s-1,
to the resolution of the worst of the 23 photometric bands used in the SED
fitting (the one of SPIRE350, 24 arcsec, see Enia et al., 2020) using a
Gaussian kernel. Consistently with Morselli et al. (2020), we computed the
sensitivity limit as the rms of the $\Sigma_{HI}$ maps over an aperture
corresponding to a diameter of 500pc, which we measured as
log$\Sigma_{H_{I},lim}=$0.3 M☉ pc-2.

The molecular gas surface density, $\Sigma_{H_{2}}$, was computed using the
12CO(2-1) intensity maps from the HERACLES survey (The HERA CO-Line
Extragalactic Survey, Leroy et al., 2008). These observations were made with
the IRAM 30m telescope and have an angular resolution of 11 arcsec. As for
$\Sigma_{HI}$, we convolved the images using a Gaussian kernel to the
resolution of SPIRE350. We refer to Morselli et al. (2020) for a detailed
description of the assumptions that we applied to convert CO fluxes into
$\Sigma_{H_{2}}$ measurements. We computed the sensitivity limit as the rms of
the $\Sigma_{H_{2}}$ maps over an aperture corresponding to a diameter of
500pc, which we measured as log$\Sigma_{H_{2},lim}$= 0.4 M☉ pc-2.

Facility | Effective wavelength | Pixel width
---|---|---
|  | (′′)
GALEX | 153 nm | 3.2
GALEX | 227 nm | 3.2
SDSS | 353 nm | 0.45
SDSS | 475 nm | 0.45
SDSS | 622 nm | 0.45
SDSS | 763 nm | 0.45
SDSS | 905 nm | 0.45
2MASS | 1.24 $\mu$m | 1
2MASS | 1.66 $\mu$m | 1
2MASS | 2.16 $\mu$m | 1
WISE | 3.4 $\mu$m | 1.375
WISE | 4.6 $\mu$m | 1.375
WISE | 12 $\mu$m | 1.375
WISE | 22 $\mu$m | 1.375
Spitzer | 3.6 $\mu$m | 0.75
Spitzer | 4.5 $\mu$m | 0.75
Spitzer | 5.8 $\mu$m | 0.6
Spitzer | 8.0 $\mu$m | 0.6
PACS | 70 $\mu$m | 2
PACS | 100 $\mu$m | 3
PACS | 160 $\mu$m | 4
SPIRE | 250 $\mu$m | 6
Table 1: Name, effective wavelength, and pixel width of the DustPedia
facilities.

###  2.4 Galaxy sample

From the many galaxies in the DustPedia archive, we chose a sample of eight
grand-design spiral galaxies that were previously selected in Enia et al.
(2020): NGC0628, NGC3184, NGC3938, NGC4254, NGC4321, NGC4535, NGC5194, and
NGC5457. The sample includes different morphologies and environments: isolated
galaxies (NGC3184, NGC0628, and NGC3938), cluster galaxies (NGC4321, NGC4254,
NGC4535, and NGC5457), and galaxies involved in merger processes (NGC4254 and
NGC5194). Further details of each galaxy are shown in Table 2.

Galaxy name | RA | DEC | D | i | r25 | logM$\star$ | SFR | RC3 Type | Cell size
---|---|---|---|---|---|---|---|---|---
| [deg] | [deg] | [Mpc] | [$\circ$] | [kpc] | [M$\odot$] | [M$\odot$ / yr] |  | 8′′[kpc]
NGC0628 | 24.1740 | 15.7833 | 10.14 | 19.8 | 14.74 | 10.41$\pm$0.15 | 1.90$\pm$0.41 | Sc | 0.39
NGC3184 | 154.5708 | 41.4244 | 11.64 | 14.4 | 12.55 | 10.14$\pm$0.10 | 0.98$\pm$0.10 | SABc | 0.45
NGC3938 | 178.2057 | 44.12088 | 19.41 | 14.1 | 10.04 | 10.16$\pm$0.20 | 2.19$\pm$0.19 | Sc | 0.75
NGC4254(M99) | 184.7065 | 14.4164 | 12.88 | 20.1 | 9.40 | 10.02$\pm$0.18 | 2.44$\pm$0.23 | Sc | 0.50
NGC4321 | 185.7282 | 15.8219 | 15.92 | 23.4 | 14.30 | 10.74$\pm$0.15 | 3.27$\pm$0.37 | SABb | 0.62
NGC4535 | 188.5845 | 8.1978 | 14.93 | 23.8 | 17.62 | 10.19$\pm$0.19 | 1.30$\pm$0.08 | Sc | 0.58
NGC5194 (M51) | 202.4695 | 47.1952 | 8.59 | 32.6 | 17.23 | 10.70$\pm$0.20 | 4.08$\pm$0.26 | Sbc | 0.33
NGC5457(M101) | 210.8025 | 54.3491 | 7.11 | 16.1 | 24.81 | 10.38$\pm$0.13 | 2.48$\pm$0.15 | SABc | 0.28

Table 2: DustPedia galaxy sub-sample. The galaxy name, coordinates in J2000
system reference, distances D (in Mpc), inclinations, r25 sizes, and
morphological classifications shared in the DustPedia archive come from the
HyperLEDA database (Makarov et al., 2014). The value of M$\star$ was obtained
by fitting the DustPedia photometry with the SED-fitting code MAGPHYS, and the
SFRs values were computed in Enia et al. (2020).

In addition to the morphological classification and the environment, the
selection of the eight galaxies was also based on inclination, distance, and
photometry. In order to limit corrections for dust and/or disk inclination,
the galaxy inclination i was limited to less than 40°, so that they can be
said to be nearly face-on galaxies. Furthermore, a cutoff in distance was
applied (around 1000 km/s, corresponding to approximately 22 Mpc) because for
galaxies beyond this distance, the galaxy is scarcely resolved in the sub-
millimeter. Finally, to robustly perform an SED fitting study, each galaxy of
the sample was observed in at least 20 bands, based upon which, it is possible
to estimate their physical parameters.

####  2.4.1 Image processing, data reduction, and flux evaluation

For each galaxy of the sample, the DustPedia archive contains multi-wavelength
photometric observations (flux and error maps). For each map, the background
estimation and subtraction and the PSF degradation were performed. The
background estimation and subtraction procedure follows the indications given
in Clark et al. (2018) (see Enia et al. (2020) for further details);
therefore, the background-subtracted maps were degraded to the SPIRE350 PSF
($8\textprime\textprime$). To do this, the maps were convolved using the
kernels provided by Aniano et al. (2011) (see Enia et al. (2020) for further
details). We analysed the galaxies through an SED-fitting procedure by
considering $8\textprime\textprime$ x $8\textprime\textprime$ cells.
Finally, in order to measure the flux in each band inside the apertures, the
photutils v0.6 Python package Bradley et al. (2023) was used. On the other
hand, the error can be estimated, when available, from the Dustpedia database.
When this is not possible, the signal-to-noise ratio (S/N) of the DustPedia
photometry in that particular band can help to compute the error in each cell.
As an assumption, the S/N threshold was equal to 3: pixels lower than this
value were ignored. Moreover, for a pixel for which the flux measurement could
not be performed in more than ten bands, the pixel was rejected (see Enia et
al. (2020) for further details).

##  3 Results and discussion

###  3.1 Spatially resolved star formation histories

The motivation of our work is understanding the origin of the significant
deviations from the average MS that were observed in the sample presented in
Enia et al. (2020). For the purpose of our studies, a further investigation of
the SFH of the galaxies examined is therefore highly recommended. This was
allowed through the BAGPIPES SED-fitting code (Carnall et al., 2019). An
example of an SED plot for three representative apertures (at a distance of
$0$, $0.5R_{25}$, and $R_{25}$) of the NGC4321 galaxy is provided in C. We
note that in the original paper (Enia et al., 2020), the physical parameters
of the sources were derived with MAGPHYS (da Cunha et al., 2008). However,
this code does not allow us to easily recover the adopted SFH for each object.
We therefore relied for this work on the more flexible BAGPIPES fitting
procedure. However, we found that the two codes provide almost consistent
results in terms of the derived stellar masses and SFR (see B for quantitative
details).

In the following, we refer to stellar masses derived from BAGPIPES, while SFRs
(as in Enia et al. (2020) and Morselli et al. (2020)) are obtained as SFR =
SFRUV \+ SFRIR. We note that
SFR${}_{UV}/(M\textsubscript{$\odot$}/yr)=0.88\times
10\textsuperscript{-28}\,\textit{L}_{\nu}/\textit{L}_{\odot}$ with L$\nu$, in
erg/s/Hz, as the luminosity per unit frequency evaluated at 150 $n$m (from
Bell & Kennicutt, 2001) and SFR${}_{IR}=2.64\times
10\textsuperscript{-44}\,\textit{L}_{IR}$ with LIR, in erg/s, as the
luminosity evaluated from the SED fit between 8$\mu$m and 1000$\mu$m
(Kennicutt, 1998).

####  3.1.1 Delayed exponentially declining model

We adopted a conservative approach by exploring the sample through a delayed
exponentially declining SFH model,

$SFR(\textit{t})\propto\left\\{\begin{array}[]{rl}(\textit{t}-\textit{T}\textsubscript{0})\>exp\>(-\frac{\textit{t}-\textit{T}\textsubscript{0}}{\tau})&\textit{t}>\textit{T}\textsubscript{0}\\\
0&\textit{t}<\textit{T}\textsubscript{0}\par\end{array}\right.$ (1)

, where t is the time since the Big Bang, $T_{0}$ is the time when star
formation starts, and $\tau$ is the timescale of the exponentially declining
star formation.

We adopted the following specifications for the model parameters: $T_{0}$
varying from 0.1 to 9 Gyr (since the Big Bang, to avoid stellar ages younger
than $\sim$5Gyr, that would be inconsistent with the oldest stellar
populations observed in local spiral galaxies, e.g. Peterken et al., 2020, and
references therein), $\tau$ ranging from 1 to 10 Gyr (again since the Big
Bang), the total mass formed over the whole history (in
$log_{10}(M_{\star}/M_{\odot}$)) spanning from 1 to 15, and metallicities
varying between 0 and 2.5 $Z_{\odot}$ (for further details, see Carnall et
al., 2019). The shape of the attenuation curve is provided by Calzetti et al.
(2000), with a V-band attenuation (AV) from 0 to 2 mag. The redshift was
finally provided by the DustPedia database.

We divided each galaxy in apertures with diameters of 8” (see Enia et al.,
2020, for more details) , and we performed the SED-fitting procedure in each
of them. As a first step, we retrieved the stellar mass surface density and
the SFR surface density, and we calculated the distance from the main sequence
for each aperture.

Fig.1 shows (upper panels) as an example the $\Sigma\textsubscript{$\star$}$
and $\Sigma\textsubscript{SFR}$ of NGC4321, that is, the stellar mass density
and SFR density. The two were obtained by dividing the M$\star$ and SFR
quantities with the area given by the square of the side aperture (8′′-side
aperture). As previously mentioned, the stellar mass surface density was
computed as an output of the SED fitting, while the SFRs used in the SFR
surface density were still computed using the $SFR=SFR_{UV}+SFR_{IR}$.

The stellar mass and SFR distributions clearly both have a marked pick in the
centre and along the spiral arms. This shows that both the M$\star$ and the
SFR trace the spiral pattern of the galaxies. In the same figure, we also show
again for NGC4321 the distance from the MS of each cell (bottom left panel)
and the spatially resolved MS, highlighting the distance of each cell from the
galaxy centre (bottom right panel). It is worth pointing out that the galaxy
has a very remarkable quenched ring: The portions of NGC4321 at the lowest
stellar mass densities (i.e. at large galactocentric distances) clearly lie
below the MS, which is an indication of suppressed star formation.

The spatially resolved MS plot (Fig.1, bottom right panel), NGC4321 presents a
skewed distribution in the stellar mass-SFR relation: At
log($\Sigma_{*}$)$>7.0$ M⊙kpc-2, the data follow the average MS scaling
relation, whereas at lower stellar mass densities, the data sit well below the
expected values considering the average of all points across the sample. At
fixed stellar mass, the observed SFR is suppressed, indicating quenched
galactic regions.

The original sample of eight face-on spirals (see C for the galaxy-by-galaxy
results) contains other galaxies with a potential indication of a lower
specific SFR toward the outskirts (e.g. NGC4535 and NGC5194). This corresponds
to $\sim 40\%$ of the total (including NGC4321). Even within the limited
statistic available to us, it is clear that this peculiar feature is not rare.
It is thus crucial to characterise the physical properties of the stellar
populations in these under-dense and passive external regions of the galaxies.
We therefore explored the information that can be drawn from a systematic
study of the SFHs, taking advantage of the performances of the BAGPIPES
fitting procedure.

Figure 1: Summary plot for NGC4321. The panels are organised as follows:
stellar mass surface density (upper left), SFR surface density (upper right),
distance from the MS (bottom left), and spatially resolved MS (bottom right).
The solid black line shows the spatially resolved main sequence by Enia et al.
(2020), and each point is colour-coded by the cell distance from the galaxy
centre. In the first panel, three apertures (A, B, andC) are highlighted. The
corresponding SED plot for each is reported in Appendix A.

###  3.2 Mass-weighted ages for the full sample

In this section, we discuss the implications of the SED fitting results from
the BAGPIPES code. By selection, the galaxies in the sample (Enia et al.,
2020) represent the final product of secular processes that convert the gas
into stars. Spiral galaxies are built across the Hubble time: They form stars
at a pacific rate (excluding major merger events), driven by the density wave
turbulence crossing the interstellar medium within the gaseous disks. The
regulation of this smooth star formation mode is at the origin of the observed
main sequence (MS) of star-forming galaxies and holds at different spatial
scales (from $\sim$0.5 kpc to the whole galaxy).
Here, we specifically focused on the apertures of each galaxy underlying the
MS in their peripheral regions (see Appendix B for the plots showing the
distance from the MS for each galaxy). We report the results of the mass-
weighted age (tMW) as an output from BAGPIPES. tMW gives an indication of the
epoch at which the stellar masses of galaxies were assembled, and it was
computed by weighting the mass of stars at the time of formation,

$t\textsubscript{MW}=\frac{\int_{0}^{t\textsubscript{obs}}tSFR(t)\textit{dt}}{\int_{0}^{t\textsubscript{obs}}SFR(t)\textit{dt}},$
(2)

where SFR(t) is the SFH, and $t\textsubscript{obs}=t(z\textsubscript{obs})$.
Consequently, for a visual inspection and to derive more quantitative
conclusions, we derived the radial profiles of the mass-weighted age for each
galaxy (see C). In particular, we show in Fig. 2 the radial profiles obtained
by averaging all pixels at the same distance from the corresponding galaxy
centre for each galaxy separately. The galactocentric distances are normalised
to the R25 radius for a more meaningful comparison of sources with different
physical sizes and at different distances (this approach to radial profiles of
different properties of DustPedia galaxies was also used in Casasola et al.
(2017). The statistical errors on $t_{MW}$ were computed as the standard
deviation on the mean value in each distance bin. To also account for the
effect of the model uncertainties on $t_{MW}$, we used the information
included in the posterior distributions obtained by the BAGPIPES run. For each
cell, we derived the difference between the 84th and 16th percentile of the
$t_{MW}$ distribution, and assumed half of it as the error on each $t_{MW}$
($err_{t_{MW}}$=($t_{MW}$(84th)-$t_{MW}$(16th))/2). We assumed the median of
the distributions of $err_{t_{MW}}$ for all the cells in each radial bin as a
systematic error that was added in quadrature to the statistical error on the
average $t_{MW}$ in each radial bin.

Figure 2: Average radial distribution of the mass weighted age of the eight
galaxies of our sample. The distance from the center is normalised to their
R25 radius. As regards the uncertainties computation, see Sec. 3.2

We point out that the mass-weighted age ($t_{MW}$) was not analysed in
absolute terms because it is well known that an underestimation of mass-
weighted ages can be observed in SED-fitting analyses that adopt both
parametric and non-parametric SFHs (Carnall et al., 2019, among others). We
rather aim at detecting relative variations of $t_{MW}$ within each galaxy and
at deriving average trends as a function of the distance from the centre.

The radial distribution of $t_{MW}$ of our sample (see Fig. 2) shows that most
of the sources have older $t_{MW}$ towards the central region (R$<$0.1R25), as
expected given the presence (in some cases) of a stellar bulge. In the
observed wide age spread ($\Delta(t)\sim 3Gyr$), $t_{MW}$ can reach up to
7Gyr. At increasing distances from the centre, the disk component dominates
the central bulge. Consistently, at R$>$ 0.2R25, a general flattening of
$t_{MW}$ is observed, indicating that younger stellar populations have
populated these galaxy regions (with typical $t_{MW}\sim 4$Gyr). At larger
distances, R$>$ 0.4R25, $t_{MW}$ can follow different patterns: 1) it remains
almost flat, 2) it smoothly increases (up to $\sim$ 5Gyr), and 3) it steeply
rises to higher age values, as old as the central bulges (up to $\sim$7Gyr),
which is indicative of an old stellar populations in the peripheral regions.

Therefore, we can derive a broad and general result: The stellar populations
in the outermost regions of the disk that are probed by our photometric
measurements are older (up to 2 - 3 Gyr) on average than those of the same
disk component at intermediate galactocentric distances (where they are not
contaminated by the bulge component). We note that the rise is steeper for at
least two sources, NGC5194 and NGC4321. The former has an interacting
companion (NGC5195), and the latter shows a more extended and dead disk (from
0.8 up to R=R25). We separated the galaxies in our sample into two groups:
those with a moderate rising trend in the outer region (NGC3184, NGC0628,
NGC5457, NGC4254, NGC4535, and NGC3938), and those with a steeper rising trend
(NGC4321 and NGC5194, and even though less extreme, NGC4535).

Casasola et al. (2017) have measured the scale length (h) 222a radial SFR
surface density profile fit was performed with a simple exponential curve,
$S=S_{0}exp(-h/h_{0})$, where $h_{0}$ is the scale length at the radius $h=0$
(Casasola et al., 2017). in GALEX FUV and NUV (tracing young star emission)
and at 3.6$\mu$m emission (mostly related to old stellar populations) for a
sample of 18 nearby spiral galaxies. Three of them belong to the sample
considered in this paper: NGC0628, NGC5457, and NGC5194. From Casasola et al.
(2017), we can quantify the average ratio of the scale lengths over $R_{25}$
for the whole sample within the disk and in the different observed UV and
near-IR spectral ranges:

$<h_{FUV}/R_{25}>=0.40$
$<h_{NUV}/R_{25}>=0.34$
$<h_{3.6\mu m}/R_{25}>=0.24$,

corresponding to

$<h_{FUV}/h_{3.6\mu m}>=1.67$
$<h_{NUV}/h_{3.6\mu m}>=1.42$.

These scale lengths and ratios of the scale lengths indicate that young stars
are on average located in regions of the disk in which the bulge component
dominates, as expected. NGC0628 and NGC5457 have ratios of the scale lengths
that are consistent with the mean ratios of spiral galaxies:

$h_{FUV}/h_{3.6\mu m}(NGC0628)=1.87(\;\gtrsim 1.67)$
$h_{NUV}/h_{3.6\mu m}(NGC0628)=1.59(\;\gtrsim 1.42)$

and

$h_{FUV}/h_{3.6\mu m}(NGC5457)=1.75(\;\gtrsim 1.67)$
$h_{NUV}/h_{3.6\mu m}(NGC5457)=1.58(\;\gtrsim 1.42)$.

The $h_{FUV}/h_{3.6\mu m}$ and $h_{NUV}/h_{3.6\mu m}$ values are clearly
higher than the reference values (1.67 and 1.42) for the galaxies NGC0628 and
NGC5457. This is consistent with our results: We do not find a particular
increasing trend for the mass-weighted age within the optical radius for
NGC0628 and NGC5457. This scenario is different from that for NGC5194, which
shows a different behaviour with respect to the average trend of spiral
galaxies within the disk (and the same might hold for NGC4321, which,
unfortunately, is missing in the 18 galaxies sample). In this particular case,

$h_{FUV}/h_{3.6\mu m}(NGC5194)=1.03(<1.67)$
$h_{NUV}/h_{3.6\mu m}(NGC5194)=0.91(<1.42)$.

The lower than the average values of this ratio for NGC5194 indicate that for
this source, the stellar emission from the older populations (whose emission
is traced by the 3.6$\mu m$ light) has a flatter and more extended radial
profile than the companion spirals. This is in line with our finding for
NGC5194 (Fig. 2).

We note that older stars

in the outskirts of the disk

appear to be counter-intuitive with respect to the predictions of the inside-
out quenching mechanism that was widely discussed in the literature to explain
the formation of bulges in the local Universe (e.g. Morselli et al., 2020;
Tacchella et al., 2015). High-$z$ observations found that in the most massive
galaxies, star formation is quenched from the inside out, on timescales
shorter than one billion years in the inner region and up to a few billion
years in the outer disks. These galaxies sustain high star formation activity
at large radii while hosting fully grown and already quenched bulges in their
cores (Tacchella et al., 2015). If this is observed at cosmic noon ($z\sim
2$), extrapolations at $z=0$ could naively predict that the progressive
quenching of the SFR toward the edges of the disks will lead to an almost
passive spheroid as of today (Tacchella et al., 2015).
In the next sections, we explore NGC4321 in detal to investiage these
features. We have a plethora of ancillary data for this galaxy (gas
information as well). After this, we explore a possible explanation for the
observed quenched rings and the rising age trend, which are visible at large
radii in NGC5457 as well.

###  3.3 Red and dead stellar ring surrounding NGC4321

In this section, we further discuss the specific feature observed in NGC4321
(see Fig. 1), which is identified as a region of suppressed SFR in the galaxy
outskirts, when compared to the expected spatially resolved main sequence.
Before attempting any interpretation, we investigated the significance of the
quenched ring to exclude that this is an artefact due to a low S/N in the
optical/IR bands. We recall that only the pixels with a S/N$>$3 in at least
ten photometric bands were considered for the SED fits. However, we note that
this condition selected cells with a much higher S/N even in the outskirts of
galaxies (e.g. see examples in Appendix A, where S/N$>20$ for all photometric
data).

Fig. 4 (left panel) shows the infrared emission probed by the IRAC 3.6 $\mu$m
image. This emission is a direct tracer of the stellar mass. NGC4321 shows
faint and diffuse starlight emission toward the outskirts, highlighting the
existence of old stars in these regions, without evidence of ongoing SF. Fig.
3 shows the mass-weighted ages for NGC4321 cells, with an evident peripheral
region around the galaxy with mass-weighted ages of several billion years that
correspond to the quenched ring already identified for its suppressed specific
SFR.

In Fig. 4 (left panel) we overplot as small open white squares the positions
of the cells that correspond to the passive ring below the MS. The round
distribution of these points at large galactocentric distances further
highlights the diffuse stellar emission. Interestingly, the red and dead zone
begins where the spiral arms end. In Fig. 4 (central and right panels) we
further plot the HI and H2 surface densities. The $H_{2}$ gas phase is traced
by the CO emission from the HERACLES survey hrtr (Leroy et al., 2008), while
the HI comes from the public VIVA database (Chung et al., 2009). The observed
CO emission was converted into an H2 gas mass following the prescription
described in Morselli et al. (2020): We estimated $\Sigma_{H_{2}}$ using Eq. 4
of Leroy et al. (2008), considering a metallicity-independent conversion
factor XCO (XCO = N(H2)/ICO, where N(H2) is the H2 column density, and ICO is
the line intensity) equal to 2$\times$1020 cm-2 (K km s-1)-1 (the typical
value for disc galaxies, see e.g. Bolatto et al., 2013), and a CO line ratio
ICO(2-1)/ICO(1-)=0.8 (e.g. Leroy et al., 2008; Schruba et al., 2011; Casasola
et al., 2015). We divided by a factor 1.36, which was included in Eq. 4 of
Leroy et al. (2009), to remove the helium contribution.

The maps are reported in the same astrometric system. Again, the open squares
mark the position of the galaxy cells building the quenched ring. For both HI
and CO, the S/N is relatively low in the outskirts of the galaxy, and
therefore, we stacked the data in all the white squares pertaining to the
quenched ring. We then obtained log$(\Sigma_{\rm HI})=(0.43\pm
0.11)M_{\odot}pc^{-2}$ and log$(\Sigma_{\rm H_{2}})=(0.03\pm
0.65)M_{\odot}pc^{-2}$, which means that there is a significant detection of
neutral hydrogen, whereas molecular hydrogen is only marginally detected, but
with a large uncertainty. A low H2 surface density is also to be expected
because HI is well below the $\sim 10\,M_{\odot}$ pc-2 threshold for the
production of molecular hydrogen (Bigiel et al., 2008). Thus, in the quenched
ring, H2 appears to be lacking, while the atomic HI is still present, as is
often observed in local spiral galaxies (Casasola et al., 2017).

Figure 3: Spatial distribution of the mass weighted age for NGC4321.

Quantitatively, we report in Fig. 5 the spatially resolved Kennicutt-Schmidt
relation for NGC4321, that is, we compare the surface density of SFR
($\Sigma_{SFR}$) versus $\Sigma_{HI}$ (left panel), $\Sigma_{H_{2}}$ (central
panel) and the total (HI+H2) gas surface density (right panel). As already
mentioned, the H2 signal is almost undetected in these very external regions,
with most of the marginal signal falling below the sensitivity limit of the
HERACLES survey (marked by the vertical dashed line in the central panels).

Figure 4: Spatially resolveed properties of the face-on galaxy NGC4321Left
panel: IRAC/Spitzer view of NGC4321. The map at 3.6$\mu$m highlights the
diffuse stellar emission around the disk and shows the structure of the spiral
arms. Central panel: HI emission from the public VIVA database. The maps are
reported in the same astrometric system. The open squares mark the position of
the galaxy cells building the quenched ring, and they have a size of
8”$\times$8”. Right panel: The H2 gas phase for NGC4321 is traced by the CO
emission from the Heracles survey. Each image is presented at the native
angular resolution (Spitzer = 1.6”, HI = 15”, CO = 11”).

A low $\Sigma_{HI}$ like this, associated with a paucity of $H_{2}$, in
addition to a distribution of an old stellar population, indicates that
”starvation” has started at the outer edge of the galaxy as a result of
insufficient gas supply, whereas star formation is sill going on in the inner
regions.

Figure 5: Spatially resolved Kennicutt-Schimdt relation for NGC4321. The
surface density of SFR ($\Sigma_{SFR}$) is reported as a function of
$\Sigma_{HI}$ (left panel), $\Sigma_{H2}$ (central panel), and the total
(HI+H2) gas surface density (right panel). The three upper panels are colour-
coded as a function of the distance to the MS. The lower panels indicate the
galacto-centric distance of each physical galaxy cell. Diamonds correspond to
regions associated with the quenched ring.

This is also supported by Fig. 6, where the ratio of the surface densities of
H2 /HI ($\Sigma_{H_{2}/HI}$) is shown as a function of the total gas density.
The quenched ring regions (represented as diamonds) follow the general trend
observed for local spiral galaxies (see Fig. 8 in Morselli et al., 2020),
being well below the MS.

Figure 6: Spatially resolved ratio of the surface densities of H2/HI,
$\Sigma_{H2/HI}$, shown as a function of the total gas density. The colour
code marks the distance to the MS. Diamonds correspond to regions associated
with the quenched ring.

The direct evidence is that the quenched ring contains insufficient gas to
form enough H2 to sustain star formation. This may have two reasons: i) either
gas has been removed from the ring, or ii) after consumption to form stars, it
has not been replaced by fresh gas from the circumgalactic medium (CGM).

NGC4321 is located in the Virgo cluster, and it therefore moves through its
intracluster medium (ICM), and ram pressure stripping from the diffuse plasma
is well known to deplete the gas from galaxies within a cluster, quenching the
ongoing SFR (e.g. Poggianti et al., 2017, 2019; Boselli et al., 2021). For
example, in the case of the cluster galaxy IC3476, Boselli et al. (2021)
showed that the IGM-ISM interaction causes the overall rise in the star
formation activity within the galaxy (as testified by its multiple giant HII
regions) and the sudden drop in star formation at the edge of the disk (some
$\sim 50$ Myr ago), which is completely quenched, having been stripped of its
gas content. Therefore, it is natural to entertain the possibility that while
moving through the Virgo cluster, NGC4321 received the same treatment.
However, other sources from the Enia et al. (2020) sample (in particular,
NGC5194; see Fig. 2 show trends very similar to that of NGC4321, including the
connection between quenched stellar populations and low gas surface density.
However, these objects are not associated with any cluster.

The second alternative may apply to them, namely that the CGM has ceased to
supply fresh gas to the galaxy. According to cosmological models, to sustain
their SFR across cosmic time, galaxies are fed by cold gas streams (Dekel et
al., 2009). The gas appears to be accreted primarily in a co-planar, co-
rotating fashion (e.g. Bouché, 2017), hence with increasing angular momentum.
On this basis, Peng & Renzini (2020) have argued that a time may come in the
life of a galaxy when the residual CGM is left with too high specific angular
momentum to be accreted effectively, thus leading to starvation and quenching,
starting from the outer edge of the disk.

##  4 Summary and conclusions

We used the BAGPIPES code to perform a spatially resolved SED-fitting in a
sample of eight local grand-design spiral galaxies, drawn from the DustPedia
database. Accounting for photometric information from the UV up to the far-
infrared, we produced maps of stellar mass density, star formation rate
density, and distance from the MS, using as reference the relation derived by
Enia et al. (2020). Focusing on the analysis of the SFHs in each galaxy
region, which were conservatively assumed in a parametric form (delayed-
exponential models), we studied the distribution of the mass-weighted ages
($t_{MW}$ as indicator of the main epoch of the stellar build up) at different
galacto-centric distances, and compared the average trends of the whole galaxy
sample.

The average distribution of the mass-weighted age of our sample shows two
different behaviours from the centre toward the peripheral regions: $t_{MW}$
indicates older populations in the central region ($t_{MW}$ up to $\sim$7Gyr
at $R_{25}<0.3$), consistent with the presence of a bulge for various systems;
and at larger distances, the disks are dominated by younger regions that are
still forming stars ($t_{MW}\sim$4Gyr).

If an average mild rise of $t_{MW}$ is seen at $R_{25}>0.3$, mostly consistent
with a flat relation, NGC4321 and NGC5194 instead present a dramatic steep
increase of $t_{MW}$ that reaches levels as high as to those of the bulges.
The galaxy loci that are characterised by old stellar populations like this
belong to the external regions of the disk and lie immediately beyond the
influence of the spiral arms. Their specific star formation rates fall well
below the corresponding spatially resolved MS. We thus call them quenched
rings.

We discussed the origin of these peculiar feature by exploiting further
physical information about the different gas phases (HI and H2) available in
particular for NGC4321, the galaxy with the most evident quenched ring. One
possibility is that the outer ring has been depleted in gas by ram pressure
stripping, and indeed, NGC4321 is a member of the Virgo cluster. However,
NGC5194 is not associated with any cluster, and ram pressure stripping could
not be invoked for its quenched ring. Alternatively, accretion of cold gas may
have ceased or diminished in this case because the angular momentum of the
remaining circumgalactic medium is too high to be incorporated into the
galaxy.

We plan to expand this type of study to a statistically more significant
sample from the DustPedia database.

###### Acknowledgements.

We thank the referee for valuable suggestions that improved the paper. G.R.,
A.E. and L.B. acknowledge the support from grant PRIN MIUR 2017 -
20173ML3WW_001. GR also acknowledges INAF under the Large Grant 2022 funding
scheme (project ”MeerKAT and LOFAR Team up: a Unique Radio Window on
Galaxy/AGN co-Evolution). VC acknowledges funding from the INAF Mini Grant
2022 program “Face-to-Face with the Local Universe: ISM’s Empowerment
(LOCAL)”.

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##  Appendix A BAGPIPES SED plots

In this section, three SED plots are reported. Each plot corresponds to an
aperture located at a different distance from the centre of the galaxy
NGC4321.

Figure 7: BAGPIPES SED-fitting plot for an aperture located in the centre of
NGC4321 (aperture A in 1). The data are shown in blue, and the posterior
distribution, given by the difference of the $16^{th}$ and the $84^{th}$
percentiles, is shown in orange. The error bars are reported for the observed
data. When they not visible, they are smaller than the corresponding symbols.
Figure 8: Same as 7 for an aperture located at 0.5$R_{25}$ of NGC4321
(aperture B in 1). Figure 9: Same as 7 for an aperture located at $R_{25}$ of
NGC4321 (aperture C in 1).

##  Appendix B Comparison with Enia et al. (2020)

###  B.1 Setting the frame: Photometry and MAGPHYS results

Here, we briefly set the frame for the BAGPIPES SED-fitting procedure by
reporting the approach adopted and widely discussed in Enia et al. (2020). The
strategy consists of three steps:
\- The background-subtracted maps were degraded to the SPIRE350 PSF (
8$\prime\prime$ ,the worst one in the available photometric bands).
\- In a grid of 8$\prime\prime$ x 8$\prime\prime$ cells, we measure the flux
at each wavelength in each aperture; (therefore, there are variations in the
physical scale length among galaxies, as shown in 1);
\- By applying the SED-fitting technique to the given photometry, we can
estimate the physical properties of each cell. To perform SED fitting, Enia et
al. (2020) used the publicly available code MAGPHYS (da Cunha et al. (2008)).
Here, we briefly report the main results.
The values of M$\star$ were obtained by fitting the DustPedia photometry with
MAGPHYS, while SFRs were obtained as $SFR=SFR_{UV}+SFR_{IR}$ where
$SFR_{UV}/(M\textsubscript{$\odot$}/yr)=0.88\times
10\textsuperscript{-28}\,\textit{L}_{\nu}/\textit{L}_{\odot}$ with L$\nu$, in
erg/s/Hz, as the luminosity per unit frequency evaluated at 150 $n$m (from
Bell & Kennicutt 2001) and $SFR_{IR}=2.64\times
10\textsuperscript{-44}\,\textit{L}_{IR}$ with LIR, in erg/s, as the
luminosity evaluated from the SED fit between 8$\mu$m and 1000$\mu$m
(Kennicutt 1998). For each source, it was possible to plot the maps of stellar
mass, star formation rate, the distribution of the cells in the
$\log\Sigma\textsubscript{$\star$}$-$\log\Sigma\textsubscript{SFR}$ plane
(i.e. the spatially resolved MS)and $\Delta\textsubscript{MS}$ evaluated as
the perpendicular distance from the MS relation of a point in the
$\log\Sigma\textsubscript{$\star$}$-$\log\Sigma\textsubscript{SFR}$ plane. By
adopting the EMCEE (Foreman-Mackey et al. 2013) fitting tool, Enia et al.
(2020) performed the fit to compute the MS relation. The fit, a log-linear
relation, described as
$\log\Sigma\textsubscript{SFR}=m\,\log\Sigma\textsubscript{$\star$}+q$ gave m
= 0.82 $\pm$ 0.12 and q = -8.69 $\pm$ 0.97. The M$\star$ and SFR MAGPHYS
outputs were compared with the values obtained from this work.

###  B.2 MAGPHYS and BAGPIPES initial mass function

Before we describe the details, we briefly discuss in this section potential
offsets in terms of the initial fass Function (IMF) adopted in the MAGPHYS
code (a log-normal IMF, Chabrier 2003) and in the BAGPIPES code (a broken
power law IMF, Kroupa (2001)), which may cause residual inconsistency in
stellar mass and star-formation rate results with respect to the previous work
(Enia et al. 2020). For a straightforward comparison of the two codes, we then
converted all outputs into the Chabrier IMF Chabrier (2003). As an example, in
11, we show the direct correlation between the stellar mass and SFR from Enia
et al. (2020) and those obtained in this work: the stellar masses are almost
consistent, with an average offset of $\sim 0.05$ dex and a scatter of 0.29
dex. In the SFR outputs, the discrepancy between the results from the two SED-
fitting codes is only slightly larger: The SFRs from MAGPHYS are $\sim$0.07
dex higher on average than those from BAGPIPES, with a scatter of $\sim$0.30
dex. We can then safely assume that the two codes provide consistent results,
in particular, for the stellar mass. The agreement for the SFR from the two
SED-fitting procedures is also within the typical uncertainties from different
SFR tracers. We further note that the SFR, averaged over the past 100Myr,
derived from an SED-fitting adopting a parametric SFH, can be biased
(overestimated or underestimated) because it is sensitive to instantaneous
bursts (see Haskell et al. 2023). For these reasons, we used the empirical SFR
obtained by combining the UV and far-IR luminosities (see Section 3.1).

Figure 10: Stellar mass comparison from MAGPHYS and BAGPIPES. Figure 11: Star
formation rate comparison from MAGPHYS and BAGPIPES.

##  Appendix C Galaxy-by-galaxy outputs

In this section, we present the outputs for each galaxy as shown for NGC4321
in Sec. 3.1.

Figure 12: Same as Fig.1, for NGC3184

Figure 13: Same as Fig.1 for NGC0628

Figure 14: Same as Fig.1 for NGC4535

Figure 15: Same as Fig.1 for NGC4254

Figure 16: Same as Fig.1 for NGC3938

Figure 17: Same as Fig.1 for NGC5457

Figure 18: Same as Fig.1 for NGC5194

Figure 19: Mass-weighted age for NGC3184 and NGC0628

Figure 20: Mass-weighted age for NGC5194 and NGC4254

Figure 21: Mass-weighted age for NGC3938 and NGC5457

Figure 22: Mass-weighted age for NGC4535