11institutetext: Key Laboratory for Research in Galaxies and Cosmology,
Department of Astronomy, University of Science and Technology of China, Hefei
230026, China
11email<EMAIL_ADDRESS><EMAIL_ADDRESS>22institutetext: School of
Astronomy and Space Science, University of Science and Technology of China,
Hefei 230026, China 33institutetext: Department of Astronomy and Astrophysics,
University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064,
USA 44institutetext: School of Astronomy and Space Science, Nanjing
University, Nanjing 210093, China 55institutetext: Key Laboratory of Modern
Astronomy and Astrophysics (Nanjing University), Ministry of Education,
Nanjing 210093, China

# Unveiling the formation of NGC 2915 with MUSE:
A counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A)

Yimeng Tang Unveiling the formation of NGC 2915 with MUSE:  A counter-rotating
stellar disk embedded in a disordered gaseous environment††thanks: Based on
observations made with ESO Telescope at Paranal Observatory under program ID
094.B-0745(A)Unveiling the formation of NGC 2915 with MUSE:  A counter-
rotating stellar disk embedded in a disordered gaseous environment††thanks:
Based on observations made with ESO Telescope at Paranal Observatory under
program ID 094.B-0745(A)Unveiling the formation of NGC 2915 with MUSE:  A
counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A)Unveiling the formation of NGC 2915
with MUSE:  A counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A)Unveiling the formation of NGC 2915
with MUSE:  A counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A)Unveiling the formation of NGC 2915
with MUSE:  A counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A) Bojun Tao Unveiling the formation
of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in a
disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A) Hong-Xin Zhang
Corresponding authorUnveiling the formation of NGC 2915 with MUSE:  A counter-
rotating stellar disk embedded in a disordered gaseous environment††thanks:
Based on observations made with ESO Telescope at Paranal Observatory under
program ID 094.B-0745(A)Unveiling the formation of NGC 2915 with MUSE:  A
counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A)Unveiling the formation of NGC 2915
with MUSE:  A counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A)Unveiling the formation of NGC 2915
with MUSE:  A counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A) Guangwen Chen Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A) Yulong Gao
Unveiling the formation of NGC 2915 with MUSE:  A counter-rotating stellar
disk embedded in a disordered gaseous environment††thanks: Based on
observations made with ESO Telescope at Paranal Observatory under program ID
094.B-0745(A)Unveiling the formation of NGC 2915 with MUSE:  A counter-
rotating stellar disk embedded in a disordered gaseous environment††thanks:
Based on observations made with ESO Telescope at Paranal Observatory under
program ID 094.B-0745(A)Unveiling the formation of NGC 2915 with MUSE:  A
counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A)Unveiling the formation of NGC 2915
with MUSE:  A counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A) Zesen Lin Unveiling the formation
of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in a
disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A) Yao Yao
Unveiling the formation of NGC 2915 with MUSE:  A counter-rotating stellar
disk embedded in a disordered gaseous environment††thanks: Based on
observations made with ESO Telescope at Paranal Observatory under program ID
094.B-0745(A)Unveiling the formation of NGC 2915 with MUSE:  A counter-
rotating stellar disk embedded in a disordered gaseous environment††thanks:
Based on observations made with ESO Telescope at Paranal Observatory under
program ID 094.B-0745(A)Unveiling the formation of NGC 2915 with MUSE:  A
counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A)Unveiling the formation of NGC 2915
with MUSE:  A counter-rotating stellar disk embedded in a disordered gaseous
environment††thanks: Based on observations made with ESO Telescope at Paranal
Observatory under program ID 094.B-0745(A) Yong Shi Unveiling the formation of
NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in a disordered
gaseous environment††thanks: Based on observations made with ESO Telescope at
Paranal Observatory under program ID 094.B-0745(A)Unveiling the formation of
NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in a disordered
gaseous environment††thanks: Based on observations made with ESO Telescope at
Paranal Observatory under program ID 094.B-0745(A)Unveiling the formation of
NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in a disordered
gaseous environment††thanks: Based on observations made with ESO Telescope at
Paranal Observatory under program ID 094.B-0745(A)Unveiling the formation of
NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in a disordered
gaseous environment††thanks: Based on observations made with ESO Telescope at
Paranal Observatory under program ID 094.B-0745(A) Xu Kong Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)Unveiling the
formation of NGC 2915 with MUSE:  A counter-rotating stellar disk embedded in
a disordered gaseous environment††thanks: Based on observations made with ESO
Telescope at Paranal Observatory under program ID 094.B-0745(A)

(Received XXX / Accepted XXX)

NGC 2915 is a unique nearby galaxy that is classified as an isolated blue
compact dwarf based on its optical appearance but has an extremely extended H
i gas disk with prominent Sd-type spiral arms. To unveil the starburst-
triggering mystery of NGC 2915, we performed a comprehensive analysis of deep
VLT/MUSE integral field spectroscopic observations that cover the star-forming
region in the central kiloparsec of the galaxy. We find that episodes of
bursty star formation have recurred in different locations throughout the
central region, and the most recent one peaked around 50 Myr ago. The bursty
star formation has significantly disturbed the kinematics of the ionized gas
but not the neutral atomic gas, which implies that the two gas phases are
largely spatially decoupled along the line of sight. No evidence for an active
galactic nucleus is found based on the classical line-ratio diagnostic
diagrams. The ionized gas metallicities have a positive radial gradient, which
confirms the previous study based on several individual H ii regions and may
be attributed to both the stellar feedback-driven outflows and metal-poor gas
inflow. Evidence for metal-poor gas infall or inflow includes discoveries of
high-speed collisions between gas clouds of different metallicities, localized
gas metallicity drops and unusually small metallicity differences between gas
and stars. The central stellar disk appears to be counter-rotating with
respect to the extended H i disk, implying that the recent episodes of bursty
star formation have been sustained by externally accreted gas.

###### Key Words.:

Galaxies: individual: NGC 2915 – Galaxies: dwarf – Galaxies: blue compact
dwarf – Galaxies: kinematics and dynamics – Galaxies: star clusters

##  1 Introduction

Blue compact dwarfs (BCDs) are dwarf galaxies with compact optical appearance
and H ii-region-like spectra due to highly concentrated starburst activity
(e.g., Thuan & Martin 1981; Hunter & Elmegreen 2004, 2006). They are generally
metal-poor (e.g., Kunth et al. 1988; Izotov & Thuan 1999), and rich in gas
which has a much more extended distribution than stars (e.g., Brinks & Klein
1988; Taylor et al. 1996; Putman et al. 1998; Pustilnik et al. 2001). Such
properties once led people to think that BCDs could be young systems that are
experiencing their first episodes of star formation activity. However, later
studies revealed the existence of extended ancient stellar populations in
almost all of the known BCDs (e.g., Loose & Thuan 1986; Kunth et al. 1988;
Papaderos et al. 1996; Cairós et al. 2001; Cairós & González-Pérez 2017a, b).
Several scenarios have been put forward to understand the formation mechanisms
of BCDs, including merger (e.g., Bekki 2008; Lelli et al. 2012; Zhang et al.
2020a, b), violent disk instabilities (e.g., Elmegreen et al. 2012), and
cosmic web gas inflow(e.g., López-Sánchez et al. 2012; Verbeke et al. 2014),
etc., but it is unclear which mechanism is the most relevant one.

NGC 2915 is an extreme case of a BCD in the local Universe, which is also
known as one of the closest BCDs ($M_{B}=-15.9$ mag; $D\sim 4.1$ Mpc, Meurer
et al. 2003), making it an ideal laboratory for understanding the evolution of
this kind of galaxies. The H i gas content of this galaxy is very high
($M_{\rm HI}$/$L_{B}\sim 1.7\leavevmode\nobreak\ M_{\odot}/L_{\odot,B}$;
$M_{\rm HI}\sim 4.5\times 10^{8}M_{\odot}$, Elson et al. 2010) and extremely
extended in distribution ($\sim 6\ R_{25}$, Becker et al. 1988), showing well-
defined spiral structures that are not seen in optical images (Meurer et al.
1996). In the central region, there are two H i gas clumps similar in mass and
size (Elson et al. 2010), which were mistaken for a bar structure in low-
resolution Hi maps (Meurer et al. 1996) for a long time. The H i velocity
field shows the regular rotation, and its rotation curve becomes flat at large
radii (Becker et al. 1988; Meurer et al. 1996; Elson et al. 2010). Its dark
matter content is extremely high ($M_{\rm tot}/L_{B}\sim
140M_{\odot}/L_{\odot,B}$, Elson et al. 2010) and dominant at all radii. Also,
the dark matter halo has a dense and compact core (Bureau et al. 1999),
resulting in a very deep potential well in the center. Signs of a warp in the
H i disk are found (Meurer et al. 1996), indicating possible environmental
interaction with a partner. A low-surface-brightness galaxy named SGC
0938.1-7623 (Corwin et al. 1985; Karachentseva & Karachentsev 1998), $\sim
42^{\prime}$ away from NGC 2915 (or projection distance is about 50 kpc by
assuming the distance of NGC 2915), is the only candidate neighbor, without
confirmation by either radial velocity or distance.

According to a study of multi-wavelength images of NGC 2915 by Meurer et al.
(1994), almost all the young stellar populations and H ii regions are located
near the center of the galaxy. Werk et al. (2010) studied five H ii regions
within the 1.2 $\times$ Holmberg radius and found a non-negative radial
gradient of gas-phase metallicity. The apparent under-abundant inner disk and
overabundant outer disk in metallicities indicate that there may be certain
mixing mechanisms redistributing the heavy elements produced by star
formation. In addition, Sersic & Donzelli (1992) and Meurer et al. (1994)
found that the optical surface brightness profiles of NGC 2915 are well
described by an up-bending double-exponential form, with a sharp turning at a
radius of $\sim$30 arcsec. The inner exponential disk, the so-called BCD part,
mainly consists of the blue and young stellar population. whereas the outer
exponential is red and old, similar to a dE galaxy. Meurer et al. (2003)
discovered three metal-poor globular clusters based on HST images that cover a
small portion of the outer disk, implying an abnormally high specific
frequency of globular clusters for the whole system. Although being extremely
rich in gas, star formation activity is barely detected in the extended outer
H i disk. The rotational shear may play a role in inhibiting the star
formation there (Meurer et al. 1996; Elson et al. 2012).

Figure 1: From top to bottom: H i intensity map of NGC 2915 obtained from the
Australia Telescope Compact Array by Elson et al. (2010), $V$-band image from
the Cerro Tololo Inter-American Observatory’s 1.5m telescope in the Spitzer
Nearby Galaxies Survey (SINGS, Kennicutt et al. 2003), and $V$-band image
obtained by collapsing the wavelength dimension of the MUSE data cube. In all
panels, the blue and green dashed ellipses represent the Holmberg radius and
the break radius of the up-bending surface brightness profile of NGC 2915,
respectively. The MUSE FoV is plotted as a red box, which measures $\sim$
1.2$\times$1.2 kpc at the distance of NGC 2915. The galactic center is marked
as a black cross and the same for all following plots.

The triggering mechanism of the highly concentrated star formation in NGC 2915
is still unclear. In this study, taking advantage of the high-quality integral
field spectroscopy from MUSE (Bacon et al. 2010) on the Very Large Telescope
(VLT), we present a study of the stellar population distributions and
kinematics of NGC 2915, in an effort to shed light on the formation and
evolution of this galaxy. The rest of this paper is structured as follows.
Section 2 describes the method used in our data processing and analysis. The
main results and discussion are given in Section 4. A summary follows in
Section 5.

##  2 Observations and data reduction

###  2.1 MUSE data

MUSE (Bacon et al. 2010) is an integral-field spectrograph installed on the
VLT of the European Southern Observatory at Cerro Paranal, Chile. The
observations of NGC 2915 were taken in the wide-field mode (WFM) of MUSE,
which covers a field of view (FoV) of $1^{\prime}\times 1^{\prime}$, and has a
spatial sampling of $0.2^{\prime\prime}\times 0.2^{\prime\prime}$, a spectral
resolution of $\sim 2.5$ Å (at 7000 Å), and a spectral sampling of 1.25 Å
across the wavelength range from 4750 to 9350 Å.

The MUSE observations of NGC 2915 (Figure 1) were acquired over four nights in
February 2015 (Program ID: 094.B-0745(A)) for a total exposure time of 12480
($13\times 960$) seconds on target, without separate sky exposures. The
observations cover the central 1.2$\times$1.2 kpc of NGC 2915. We downloaded
the raw science data and calibration data from the ESO Science Archive. The
data were reduced with the MUSE data reduction pipeline (v2.8.3, Weilbacher et
al. 2020) packaged in the ESO recipe execution tool (EsoRex, v3.13.3). We
followed the standard procedure for calibration and reduction except for the
sky subtraction. For observations without separate sky exposures, the default
strategy adopted by the pipeline is to select the dimmest part of the white-
light image (obtained by collapsing the data cube in the wavelength dimension)
for sky background estimation. We adopt a more conservative (or stricter)
strategy for background selection. In particular, we first select the faintest
10% pixels in continuum and the faintest 10% pixels in [O iii]$\lambda$5007
emission, and then the common pixels from the two selections ($\sim 2.5$% of
the FoV) are used for background estimation. Finally, we refine the sky
subtraction by applying the Zurich Atmosphere Purge (ZAP, Soto et al. 2016)
principal component analysis algorithm. We note that the seeing conditions
vary from exposure to exposure ($1.16^{\prime\prime}$ \-
$2.00^{\prime\prime}$), and the final combined data cube has a full width at
half maximum (FWHM) of the point spread function (PSF) of
$1.54^{\prime\prime}$, with little dependence on wavelength.

###  2.2 HST images

There exist multi-band archival HST/WFPC2 (Wide Field and Planetary Camera 2)
images of NGC 2915, including the $U$ (F336W), $B$ (F439W), $V$ (F555W), and
$I$ (F814W)-band images that were taken with exposure times of 4400, 2100, 600
and 600 seconds respectively (Program ID: 11987). The fully processed images
of NGC 2915 are retrieved from the Hubble Legacy Archive, and are used in the
current study for the following two purposes. First, to select a clean sample
of star clusters from the relatively low-spatial-resolution MUSE data, we use
the HST images to help remove contaminations from single stars (either
foreground or within NGC 2915). Second, to have a better wavelength coverage
for reliable stellar population modeling, the $U$ and $B$ band photometry are
used jointly with the MUSE spectra for full-spectrum fitting (see Section
3.1). For the second purpose, we degrade the HST images to match the spatial
resolution of MUSE data, using a Gaussian convolution kernel.

##  3 Data analysis

###  3.1 2D spectrophotometric fitting

To prepare for resolved spectral fitting, we first rebin the MUSE datacube to
have a $0.6^{\prime\prime}\times 0.6^{\prime\prime}$ spaxel size. Then, all
the spaxels that have ${\rm S/N}<3$ near 5500 Å or are contaminated by
foreground stars or background galaxies are masked out. Finally, we use the
Voronoi tessellation method (Vorbin, Cappellari & Copin 2003) to adaptively
rebin the spaxels to a minimum S/N of 50/Å (near 5500Å). The resultant spaxels
with S/N $<$ 50/Å are excluded from subsequent analysis. The same Voronoi
binning mask is applied to the HST $U$, $B$, and $V$ band images for a joint
fit of spectroscopy and photometry.

We use the Penalized Pixel Fitting (pPXF, Cappellari & Emsellem 2004;
Cappellari 2017) software to perform a simultaneous spectral fit with stellar
population models and Gaussian emission line templates to each adaptively
rebinned spaxel. For the stellar population models, we use single stellar
population (SSP) models generated from the Flexible Stellar Population
Synthesis (FSPS, Conroy et al. 2009) code. These SSP models are built with the
MILES stellar library, Padova+07 isochrones and the Kroupa initial mass
function (Kroupa 2001). We also include nebular continuum emission in our
models. However, we note that whether or not the nebular continuum is included
makes no significant difference to our results. Our SSP models cover 42
different ages ranging from 0.001 Gyr to 12.6 Gyr and 10 different
metallicities from [M/H] $=$ $-1.98$ to $-0.20$. The model spectra are
smoothed to match the spectral resolution of MUSE data.

By largely following the common practice of using pPXF in the literature
(e.g., Kacharov et al. 2018), we developed the following procedure to perform
a joint fit of the MUSE spectra and $UB$ broadband photometry, in order to
obtain the stellar and gaseous velocity field, velocity dispersion, emission-
line flux and star formation histories.

Figure 2: Distribution of the compact sources detected based on the MUSE
$V$-band image. Different types of sources are plotted with different symbols,
as indicated in the legend. The background is the $V$-band image whose diffuse
stellar light has been subtracted (see Section 3.2). Bright star clusters
($V<22.8$ mag), faint star clusters ($V>22.8$ mag), stars, and background
galaxies are shown as green filled circles, green open circles, red crosses,
and blue squares.

Step 1. We obtain the stellar radial velocity and velocity dispersion of each
spaxel by performing a pPXF fitting without regularization. Emission lines are
masked out at this point. In order to minimize effects such as template
mismatch, imperfect sky subtraction, flux calibration and wavelength-dependent
reddening, a combination of tenth-degree additive and multiplicative Legendre
polynomials is used in the fitting. In the next steps, the stellar velocity
and velocity dispersion are fixed to the values determined here.

Step 2. A pPXF fitting without regularization is performed again for each
spaxel, using multiplicative tenth-degree Legendre polynomials only. As in
step 1, emission lines are masked out at this point. We then use the Calzetti
et al. (2000) reddening law to fit the multiplicative polynomials in order to
determine the wavelength-dependent reddening. After dividing the
multiplicative Legendre polynomials by the wavelength-dependent reddening, we
obtain a residual continuum shape correction curve for each spaxel. These
correction curves are median averaged over all spaxels to form a ”master”
correction curve, which is applied to all spaxels. We note that the typical
correction factor is about 1%.

Step 3. The wavelength-dependent reddening curve determined above (and
extrapolated to shorter wavelengths with the Calzetti law) for each spaxel is
used to de-redden the corrected MUSE spectra and HST broadband photometry.

Step 4. Before performing a joint fit to the HST broadband photometry and MUSE
spectra, we need to correct for the flux calibration difference (if any)
between the two datasets. The HST $V$ band overlaps fully with the MUSE
wavelength coverage. We therefore use the $V$ band to determine the overall
flux calibration difference between HST photometry and MUSE spectra, and
correct the HST $U$ and $B$ band photometry for this difference ($\lesssim$
0.05 mag). We then rescale the full spectra+$UB$ noise array such that the
corresponding pPXF fitting (without regularization) gives a minimum
$\chi^{2}/{\rm DOF}$ = 1. Then, the Gaussian emission line templates are
included in the fitting, and the relevant emission line parameters (i.e. flux,
equivalent widths, velocity, and velocity dispersion) are measured.

Step 5. With all of the above corrections applied to the spectrum+$UB$, we
perform pPXF fitting with third-order regularization (Boecker et al. 2020) to
iteratively find the best regularization parameter for each spaxel. The
emission lines are masked out at this point. The best regularization parameter
should increase the $\chi^{2}$ by $\Delta\chi^{2}\approx\sqrt{2N}$ compared to
the unregularized best fitting, where $N$ is the number of wavelength pixels
used in the fitting. The most likely nonparametric star formation histories
are derived based on this final round of pPXF fitting.

###  3.2 Detection and stellar population fitting of star clusters

Figure 3: $V$-band magnitude distribution of the star clusters in NGC 2915.
The cyan curve indicates how completeness changes with source brightness. The
red vertical dashed line marks the magnitude limit ($V=22.8$ mag)
corresponding to the 90% completeness. Only the star clusters brighter than
this limit are used for our analysis.

Star clusters are the best approximation to SSPs, and their distribution,
although being affected by cluster dissolution processes, provides a
relatively clean view of the star formation histories of their host galaxies.
To maximize the cluster-detection efficiency in NGC 2915, we create a new data
cube by combining the two MUSE exposures observed under the best seeing
conditions (PSF FWHM = $1.16^{\prime\prime}$ and $1.18^{\prime\prime}$
respectively), and construct a $V$ band image by convolving each spectrum with
a $V$ band filter curve and then collapsing the convolved data cube along the
wavelength direction (Figure 2). In the rest of this section, we describe the
cluster detection based on this $V$ band image, extraction of the spectra of
the clusters, and fitting for stellar populations.

Detection. In order to better reveal star clusters, we use the Background2D
task in the Photutils package to create a 2D map of the diffuse ”background”
light, with the parameter box_size set to $10\times 10$ pixels, and subtract
this 2D background map from the $V$ band image. Next, the background-
subtracted $V$ band image is used for cluster detection with the DAOFIND
algorithm (Stetson 1987). A total of 127 cluster candidates at least 5$\sigma$
above the local background are found. In order to estimate the completeness
limit of our cluster detection, we adopt the artificial-star-test method, and
find a 90% completeness limit at $V\leq 22.8$ mag (Figure 3).

Spectro-photometric extraction. A PSF-weighted aperture photometry is
performed for the 127 cluster candidates in each wavelength slice of the
original data cube. The PSF-weighted photometry is equivalent to the optimal
extraction algorithm commonly used in long-slit spectroscopy. A Gaussian PSF
model with ${\rm FWHM}=1.2^{\prime\prime}$ and a cutoff radius of
$2.4^{\prime\prime}$ is adopted in the photometry. The local background for
each cluster candidate is estimated with a circular annulus of inner radius of
$2.2^{\prime\prime}$ and outer radius of $2.6^{\prime\prime}$, and the
background spectrum is a median of all spaxels in the circular annulus. The
same aperture photometry is also performed on the HST $U$ and $B$-band images.

Sample cleaning. The above-selected sample of cluster candidates is subject to
contamination by single stars and background galaxies. We adopt two criteria
to remove contaminants. First, the sources with radial velocities of greater
than 150 km/s lower or higher than the systemic velocity of NGC 2915 ($\simeq
465$ km/s) are regarded as foreground stars or background galaxies. Second, as
typical star clusters at the distance of NGC 2915 are expected to be at least
partially resolved on the HST images, sources with concentration indices ${\rm
CI}_{2-3.5}$ measured on the HST white-light images (a combination of F336W,
F439W, F555W, and F814W) smaller than 0.75 are regarded as single stars, where
${\rm CI}_{2-3.5}$ is the magnitude difference between an aperture of 2 pixels
in radius and one of 3.5 pixels in radius. The ${\rm CI}_{2-3.5}$ criterion is
based on the ${\rm CI}_{2-3.5}$ distribution of foreground stars confirmed
with radial velocities in the NGC 2915 field. By applying the two criteria to
the candidate sample, we are left with 78 confirmed star clusters, of which 49
are brighter than the 90% completeness limit of $V=22.8$ mag. The following
analysis is focused on these 49 clusters.

Fitting with SSP Models. To estimate the ages and metallicities of the star
clusters, we largely follow the fitting procedure elaborated in Section 3.1
except that the star clusters are fitted as SSPs instead of combinations of
multiple SSPs. We perform a joint fit of MUSE spectra and HST photometry with
each SSP model and calculate the $\chi^{2}$ parameter. The most likely ages
and metallicities are derived as the $\exp(-\chi^{2}/2)$-weighted median
values, and the corresponding uncertainties correspond to the central 68% of
the cumulative distributions of $\exp(-\chi^{2}/2)$ for ages and metallicities
respectively.

##  4 Results

Figure 4: Velocity field of the stars (left), H$\alpha$ emission line
(middle), and [O iii] $\lambda 5007$ emission line (right) of NGC 2915.
Velocity field of the H i 21-cm emission line (Elson et al. 2010) is
overplotted as contours. Figure 5:  Radial variations of the average line-of-
sight velocities along the photometric major (upper panel) and minor (lower
panel) axes. A 8′′-wide slit is used to extract the average velocities for
both panels. The meanings of symbols of different shapes and colors are
indicated in the upper panel.  Figure 6:  Velocity difference maps of the
stellar and gaseous velocity fields. The left panel is the ($\upsilon_{\rm
H\alpha}$$-$$\upsilon_{\rm star}$) map at the original spatial resolution of
MUSE data, and the other three panels to the right are respectively the
($\upsilon_{\rm H\alpha}$$-$$\upsilon_{\rm star}$), ($\upsilon_{\rm H{\sc
i}}$$-$$\upsilon_{\rm star}$) and ($\upsilon_{\rm H\alpha}$$-$$\upsilon_{\rm
H{\sc i}}$) maps, where the MUSE data have been smoothed to match the spatial
resolution (FWHM $\sim$ 18′′) of H i data.  Figure 7: Left column: Velocity
field and velocity dispersion map of H$\alpha$ emission line. The velocity
dispersion is determined by enforcing single Gaussian profile fitting. Right
column: Emission line profiles of [O iii]$\lambda$5007, H$\alpha$, [N
ii]$\lambda\lambda$6548,6584, and [S ii]$\lambda\lambda$6717,6731 of the four
regions with the highest velocity dispersion, as indicated in the left panels.
The dashed curves overplotted on the line profiles represent the best fit with
either single or double Gaussian components. The velocity scale (in km/s) for
each spectral line profile is indicated in the right panels. Figure 8: Maps of
H$\alpha$ intensity (left), [O iii]$\lambda 5007$ intensity (middle) and the
equivalent width of H$\alpha$ (EW(H$\alpha$), right). The H i 21-cm line
intensity distribution (Elson et al. 2010) is overplotted as contours. Figure
9: Distributions of the light-weighted ages (left) and stellar metallicities
(middle), and the metallicities of ionized gas calculated with the O3N2 method
(right). The gas metallicities are calculated only for spaxels with ${\rm
S/N}>3$ in H$\beta$, [O iii]$\lambda 5007$, H$\alpha$, and [N ii]$\lambda
6584$ emission lines.

###  4.1 Kinematics

####  4.1.1 The overall velocity gradients: a counter-rotating stellar disk

Velocity fields of stars and ionized gas (as traced by H$\alpha$ and [O
iii]$\lambda$5007 emission lines) of NGC 2915 are shown in Figure 4. The
neutral H i gas velocity field (synthesized beam $\sim$
17${}^{\prime\prime}\times 18^{\prime\prime}$) from Elson et al. (2010) is
overplotted as contours. As with the neutral gas, the ionized gas exhibits an
obvious southeast–northwest velocity gradient that is approximately along the
direction of the photometric major axis and of the H i kinematic major axis.
In contrast to the gas, the stars exhibit a much weaker velocity gradient that
appears to be in an opposite sense to the gas.

To further quantify the radial-velocity variations, Figure 5 presents the
radial variation of average velocities along 8′′-wide slits that are aligned
with either the photometric major (upper panel) or minor axes (lower panel).
We note a negative southeast–northwest stellar velocity gradient, running from
$\sim$ 472 km/s at $\sim-$20′′ to $\sim$ 460 km/s at $\sim$ 30′′ along the
major axis. By adopting a photometric inclination angle of 55∘, the major-axis
stellar velocity gradient corresponds to a maximum rotational velocity of
$\sim$ 7 km/s within 30′′. The stellar rotation is not just in the opposite
direction to but also significantly lower than the rotational velocities
($\simeq$ 19 km/s at 30′′) derived by tilted-ring fitting to the overall
velocity field of the extended H i disk (Elson et al. 2010). We checked the
stellar velocity dispersion profile (not shown here) and found a nearly
constant velocity dispersion of 39 km/s across the observed field, with the
typical measurement error of $\sim$ 6 km/s and a scatter of $\sim$ 2 km/s
along the major axis. The relatively low stellar rotational velocities may be
attributed to a more important pressure support. In the stellar velocity map,
we note that a small area near $\Delta$(RA) $\sim$ $10^{\prime\prime}$ and
$\Delta$(Dec) $\sim$ $-15^{\prime\prime}$ has a significantly higher velocity
than its surroundings. This high-velocity “clump” is associated (in
projection) with several bright star clusters close to each other.

Elson et al. (2010) found that the H i kinematics in the central region of NGC
2915 is best modeled by a highly inclined (85∘) disk together with radial
expansion velocities of the order of $\sim$ 30 km/s (comparable to the
rotational velocities). The ionized gas follows, albeit with significant local
deviations, the overall velocity gradient of H i gas, running from $\sim$ 420
km/s to $\sim$ 480 km/s along the major axis. The remarkably larger gaseous
velocity gradient than the stellar ones (even if allowing for the possible
inclination difference between the gaseous and stellar disks) confirms that
the ionized and neutral atomic gas components are highly disturbed in the
inner disk. It is noteworthy that the major-axis gaseous velocities decline
abruptly beyond a radius of $\sim$ 20′′ to the southeast. Based on the whole H
i velocity field of Elson et al. (2010), this abrupt decline of major-axis
velocities appears to be caused by a narrow gas stream projected across the
inner and outer disks (see Figure 6 in Elson et al. (2010)).

####  4.1.2 A complex gaseous kinematics: decoupling of ionized and neutral
gas

To obtain a clear view of the disturbed gas kinematics, we subtract the
stellar velocity field from the gaseous velocity fields and present the
velocity difference maps in Figure 6. In Figure 6, the left panel is the map
of line-of-sight velocity difference of H$\alpha$ ($\upsilon_{\rm H\alpha}$)
and stars ($\upsilon_{\rm star}$) at their original spatial resolution, and
the middle and right panels are respectively the maps of ($\upsilon_{\rm
H\alpha}$$-$$\upsilon_{\rm star}$) and ($\upsilon_{\rm H{\sc
I}}$$-$$\upsilon_{\rm star}$), where the MUSE maps have been smoothed to match
the spatial resolution (FWHM $\sim$ 18′′) of H i data.

The convex iso-velocity contour lines of the H$\alpha$ emission to the
northwest (middle panel of Figure 6) are in contrast to the concave iso-
velocity contour lines (as expected for a circular rotation) of the neutral H
i gas there, suggesting that the distribution of H i gas is largely decoupled
from the ionized gas along the line of sight. The velocity difference map of
($\upsilon_{\rm H\alpha}$$-$$\upsilon_{\rm H{\sc I}}$) shown in the right
panel of Figure 6 more clearly illustrates the decoupling of ionized and
neutral atomic gas. On the northwest side of NGC 2915, the major-axis of
position–velocity curves of H$\alpha$ and H i intersect at $R$ $\sim$ 15′′
(Figure 5), which roughly coincides with the peak of the most active star-
forming region (Figure 8). The major-axis variation of $\upsilon_{\rm
H\alpha}$$-$$\upsilon_{\rm H{\sc I}}$ on the northwest side is approximately
symmetric with respect to the peak of star formation at $R$ $\sim$ 15′′
(Figure 5). The spatial alignment between the H$\alpha$ kinematics and the
active star-forming regions implies stellar feedback-driven outflows.

In addition to the above difference between the ionized and neutral gas toward
the northwest, there are patches of redshifted regions to the east
($\Delta$(RA) $>$ 0; $\Delta$(Dec) $\sim$
$-5^{\prime\prime}-15^{\prime\prime}$) that are seen in the velocity maps of
the ionized gas but not the neutral H i gas. To explore the origin of these
redshifted patches, Figure 7 shows the H$\alpha$ velocity dispersion map
derived from pPXF fitting (lower left panel). Several eastern regions with
remarkably higher $\sigma_{{\rm H}\alpha}$ ($\gtrsim 70$ km/s) are indicated
as A, B, C and D. The right panels of Figure 7 show the line profiles,
together with Gaussian profile fitting, of several nebular emission lines of
the four indicated regions. For regions A and B which have the highest
velocity dispersion, we find double-component emission line profiles. The two
kinematic subcomponents have velocity differences of $\sim 160$ km/s, and none
of them (409 and 579 km/s for the two subcomponents of A, and 387 and 534 km/s
for B) are close to the expected disk velocities ($\sim$ 460 km/s), indicating
either expanding super-bubbles or colliding gas clouds (e.g., Nath &
Shchekinov 2013; Sharma et al. 2014). Nevertheless, the redshifted
subcomponents have lower $\log$([N ii]$\lambda\lambda$6584/H$\alpha$) values
than the blueshifted subcomponents ($0.25\pm 0.12$ dex lower for A, $0.31\pm
0.42$ dex for B), suggesting lower metallicities for the redshifted
subcomponents. Therefore, the double-component line profiles are best
explained by cloud–cloud collisions. None of the colliding clouds is
kinematically associated with the disk. These cloud–cloud collisions reflect
ongoing gas infall toward the disk from opposite directions.

A comparison of the velocity dispersion map and the H$\alpha$ equivalent width
map (Figure 8) suggests that the local star formation in these
high-$\sigma_{{\rm H}\alpha}$ areas is significantly lower than its
surroundings. This is probably caused by high-speed cloud–cloud collisions
(Maeda et al. 2021). For regions C and D, the line profiles are well fitted by
single Gaussian functions, without the need of second Gaussian components. A
local suppression of star formation is seen (albeit over a much smaller area
than that for regions A and B) in region C but not in region D.

###  4.2 Star formation, age and metallicities

To explore the spatial distribution of the current star formation activities,
Figure 8 presents the H$\alpha$ and [O iii]$\lambda 5007$ line intensity maps
and the H$\alpha$ equivalent width EW(H$\alpha$) maps. Regions with bright
emission lines are the sites with active current star formation. It is clear
that the current star formation is concentrated towards the northwest side of
the galaxy center, and the most intense star formation ($\Delta$(RA) $\sim$
$-10^{\prime\prime}$; $\Delta$(Dec) $\sim 8^{\prime\prime}$) appears to be
associated with the northwest peak of H i column densities.

The light-weighted ages, stellar metallicities and ionized gas (oxygen) metal
abundances are shown in Figure 9, and the azimuthally averaged radial profiles
of these parameters are presented in Figure 10. The gas metallicities are
estimated with the O3N2 method (Alloin et al. 1979), which was recently found
to give reliable metallicity estimations even in the low H$\alpha$ surface
brightness regions where significant contribution from diffuse ionized gas
(DIG) may be expected (Kumari et al. 2019; Vale Asari et al. 2019).
Specifically, we adopt the formula derived by Marino et al. (2013):

$12+{\rm log(O/H)}=8.505-0.221\times{\rm O3N2}$

where

${\rm O3N2}\equiv\log\left(\frac{[{\rm O\ III}]\lambda 5007}{{\rm
H}\beta}\times\frac{{\rm H}\alpha}{[{\rm N\ II}]\lambda 6584}\right)$

The regions with the youngest stellar ages are located in the northwest
quadrant and roughly along (albeit with an obvious offset) the photometric
major axis. This is in agreement with the distribution of H$\alpha$ and [O
iii]$\lambda 5007$ emission lines. The youngest light-weighted stellar ages
are $\gtrsim$ 30 Myr, which is in line with the post-starburst nature of NGC
2915. The azimuthally averaged stellar ages steadily increase with
galactocentric radii, in agreement with what is usually found in dwarf
galaxies. For the stellar metallicity distribution, the azimuthally averaged
profile is nearly flat at R $\lesssim$ 20′′, beyond which the average
metallicities decrease significantly with radius, with a negative gradient of
$\simeq$ $-$0.73 dex/kpc. There is no significant azimuthal variation of
stellar metallicities at a given galactocentric radius (Figure 9). The
metallicity difference map of the gas and stars is shown in Figure 11.

The most active star-forming sites have lower gas metallicities than their
immediate surroundings (right panel of Figure 9). This suggests that recent
metal-poor gas inflow has played a role in triggering the active star
formation (e.g., Kumari et al. 2017). The radial profile of average gas
metallicities appears to be flatter than the stellar metallicity profile, but
it has an overall positive gradient of $\sim$ 0.14 dex/kpc. This positive
metallicity gradient confirms the finding of $\sim$ 0.12 dex/kpc in Werk et
al. (2010) based on spectroscopy of five H ii regions in NGC 2915.

A positive gas metallicity gradient is not expected for a galaxy whose metal
production, mixing and loss are in equilibrium (Sharda et al. 2021). Werk et
al. (2010) discussed three possible scenarios to explain the positive
gradient: metal mixing across the disk, supernova-driven blowout and fallback,
and galaxy interactions. Our kinematic analysis in Section 4.1.2 suggests
significant stellar feedback-driven outflows. Metal-enriched gas ejected by
the feedback may eventually fall back to the disk, with larger angular
momentum and thus larger galactocentric distances than before (e.g., Brook et
al. 2012), contributing to a less negative or even positive metallicity
gradient. Moreover, in addition to the clear evidence for gas infall (e.g.,
Section 4.1.2), signature of gas inflow is implied by the localized gas
metallicity drops (right panel of Figure 9). Therefore, metal-poor gas infall
or inflow has also played a role in shaping the gas-phase metallicity
gradient. We note that the logarithmic differences between gas-phase and
stellar metallicities (both normalized to the solar units; Figures 10 and 11)
are $\sim$ 0.2 dex at small radii ($<$ 20′′) and $\sim$ 0.3-0.5 dex at large
radii. These differences are $>$ 0.5 dex smaller than the typical values found
in low-mass galaxies (e.g., Lian et al. 2018), implying a strong impact from
either metal outflow or gas inflow.

Figure 10: Radial profiles of light-weighted stellar age and EW(H$\alpha$)
(upper panel), light-weighted stellar metallicity and nebular gas oxygen
abundances (lower panel). Elliptical annuli are used to extract these
profiles. The lower and upper edges of the error shades correspond to the 16th
and 84th percentiles of the azimuthal distribution of each plotted parameter.
Figure 11: Metallicity difference map of the ionized gas and stars. [O/H]gas
is defined as $12+\log({\rm O/H})-8.69$, where 8.69 is the solar oxygen
abundance (Allende Prieto et al. 2001). Figure 12: Star cluster distributions
of NGC 2915. Top: Distribution of star clusters in the mass–age plane. The
plotted mass is the mass at birth. Each circle is a star cluster color-coded
by its metallicity. The purple solid curve is the 90% mass completeness limit
converted from the $V=22.8$ mag brightness limit by using age-dependent mass-
to-light ratios. The purple dashed line is plotted as a guide for the expected
relation between maximum cluster mass and age by assuming a fiducial cluster
mass function and constant cluster formation history. The dynamical
evaporation limit is derived using a galactocentric radius of 0.47 kpc (24
arcsec) and a rotation velocity of 15 km/s, as appropriate for the inner disk
of NGC 2915. Bottom: Spatial distribution of star clusters color-coded by the
stellar age. The filled circles are plotted with sizes proportional to the
fourth root of stellar mass. The EW(H$\alpha$) distribution is overplotted as
contours.

###  4.3 Star cluster distribution

The age–mass distribution and spatial distribution of star clusters above the
90% completeness detection limit are shown in Figure 12. The maximum star
cluster mass per logarithmic time interval is expected to increase with age
($M_{max}\propto{\rm Age}$ for a cluster mass function with a fiducial power-
law slope of $-2$; Krumholz et al. 2019) if the cluster formation rate is
constant with time. A comparison with the $M_{max}\propto{\rm Age}$ relation
implies that the age–mass distribution of the star clusters in NGC 2915 is
inconsistent with a constant cluster formation history. There is an obvious
enhancement of cluster formation at $\sim$ 10 $-$ 100 Myr ago. In addition,
the most massive cluster is $\sim$ 2 Gyr old, implying a lower cluster
formation rate at earlier times (more than a couple of gigayears). The long-
term dynamical dissolution effect (Baumgardt & Makino 2003) on our clusters is
not important above the 90% completeness limit, as indicated in Figure 12. We
note that, although a quantitative understanding of the observed age–mass
distribution of star clusters may be hindered by various very uncertain
disruption and fading effects (e.g., Elmegreen & Hunter 2010), the features
for NGC 2915 mentioned above cannot be easily explained by any known
disruption mechanism.

Spatial distribution of the clusters is highly asymmetrical about the major
axis (lower panel of Figure 12), with more clusters in the southwestern half
of the galaxy. The spatial concentration of clusters largely coincides with
the active star-forming area (Section 4.2). With that said, the region with
the highest H$\alpha$ equivalent width ($\Delta$(RA) $\sim$
$-10^{\prime\prime}$; $\Delta$(Dec) $\sim$ 8′′) has lower spatial
concentration of clusters than regions at smaller galactocentric distances
along the major axis, suggesting that the most active star formation very
recently propagated to the current site from smaller galactocentric distances.
Finally, we note that the two most massive clusters lie outside the active
star-forming area.

Figure 13: Spatial distribution of star formation rate averaged over different
timescales. Left three panels: Surface density distributions of star formation
rate averaged over the last $10^{7}$, $10^{8}$ and $10^{9}$ yr (i.e., the
birth mass of stars younger than the age limits divided by the age limits),
respectively. The black contours overplotted in the left three panels
correspond to 50% of the maximum $\Sigma_{\rm SFR}$. Rightmost panel: Surface
density distribution of stellar mass.

###  4.4 Star formation history

####  4.4.1 Spatially resolved star formation histories

Spatially resolved star formation histories derived from pPXF fitting are
presented in Figure 13. In order to alleviate the potential degeneracies
between the estimate of star formation rate in adjacent age bins, we analyze
the cumulatively averaged star formation histories. Surface density
distribution of star formation rate or stellar mass averaged in cumulatively
broader age bins (i.e. $\leq$ $10^{7}$ yr, $\leq$ $10^{8}$ yr, $\leq$ $10^{9}$
yr and throughout the whole lifetime) are shown in different panels of Figure
13.

We perform exponential radial profile fitting to the four maps shown in Figure
13, and find scale lengths of 3.9, 4.5, 11.1, and 18.4 arcsec respectively for
the four age intervals. This means younger stellar populations are more
spatially confined towards the central region of NGC 2915, which implies that
either the star formation activities have become more and more centrally
concentrated over time or older stars have had more time to migrate or
disperse to larger radii. Nevertheless, the spatial distribution of star
formation is not uniform along the azimuthal direction, as indicated by the
iso-$\Sigma_{\rm SFR}$ contours in Figure 13. When averaged over the past
$10^{9}$ yr, the southeastern side of NGC 2915 appears to have higher
$\Sigma_{\rm SFR}$ than the other side. Over the past $10^{8}$ yr, the
northwestern side has significantly stronger $\Sigma_{\rm SFR}$ than the
southeastern side, as opposed to the average over $10^{9}$ yr timescales. In
the last $10^{7}$ yr, star formation has continued to be more active in the
northwestern side of the galaxy, and at the same time has been significantly
enhanced along a $\sim$ 5′′-wide ($\sim$ 0.1 kpc) stripe that is parallel to
the direction of the major axis. The linear correlation of the stellar line-
of-sight velocities with the radius shown in Figure 5 suggests a solid-body
rotating stellar disk, as with the solid-body H i rotation curve within the
central $\sim$ 3 kpc (Elson et al. 2010), so the above-mentioned azimuthal
asymmetry is (in a relative sense) not subject to significant rotational
mixing.

####  4.4.2 Global star formation history

The global star formation history of NGC 2915 is derived by integrating the
spatially resolved star formation histories and shown as blue curves in Figure
14. In addition, number counts of clusters in logarithmic age intervals are
overplotted as red curves in the lower panel of Figure 14. As a compromise
between sample size, completeness and lookback time, here we only consider
clusters more massive than the 90% completeness limit at the age of $10^{9}$
yr ($\sim 1.25\times 10^{4}M_{\odot}$, Figure 12).

Theoretical studies suggest that cluster dissolution or fading may result in a
approximately uniform $\log$(Age) distribution of clusters for a constant
cluster formation history (e.g., Elmegreen & Hunter 2010), and therefore
deviations of the observation from a uniform distribution may indicate a
nonconstant cluster formation history. It is encouraging to see that the
overall trend of cluster distribution roughly matches that of the star
formation history derived from full-spectrum fitting for NGC 2915. Cluster
distributions have been used to probe the star formation histories of
interacting galaxies in the literature (e.g., Bastian et al. 2005; Zhang et
al. 2020a). Nevertheless, it is unclear to what extent (and how) the observed
cluster distributions have been affected by disruption or fading effects, so
it is not straightforward to convert the observed number counts to cluster
formation rates.

Figure 14: Integrated star formation history of the whole galaxy (blue) and
the age distribution of star clusters (red, bottom panel). The bottom panel is
an enlarged view of the last 1 Gyr. The leftmost data point of the top panel
represents the average star formation rate of the last 1 Gyr, and the other
three points correspond to the star formation rate averaged over logarithmic
age intervals of 0.4 dex. In the lower panel, the red horizontal error bars
correspond to the (logarithmic) age intervals used for cluster counting, while
the red vertical error bars account for the measurement uncertainties of ages
and counting noise.

The global star formation history peaked around $\sim$ 2 Gyr ago, which is
also when the most massive cluster in NGC 2915 formed (Figure 12). In the last
$\sim$ 1 Gyr, the global star formation rate has been declining and $\sim$ 10%
of the total stellar mass was formed. The most recent episode of active star
formation reached its maximum around $\sim$ 50 Myr ago (lower panel of Figure
14), with a duration of $\sim$ 50 Myr, and has formed $\simeq$ 3% of the total
stellar mass. The cluster age distribution also peaks at $\sim$ 50 Myr or so,
in broad agreement with the recent star formation history.

It is conceivable that episodes of bursty star formation like the most recent
one may have recurred in the past, but the coarser time resolution at older
ages precludes us from recovering them. Nevertheless, we can estimate that
such bursty star formation events have recurred less than 4 times (10%/3%) in
the past 1 Gyr. Our spatially resolved star formation histories and the
distribution of star clusters (Section 4.3) imply that recurrence of bursty
star formation episodes occurred in different spatial locations.

###  4.5 Excitation sources of the gaseous nebulae: no evidence for an active
galactic nucleus

Figure 15: The Veilleux & Osterbrock (1987) line-ratio diagnostic diagrams of
NGC 2915. Each point represents one spaxel and color-coded by the
galactocentric radius. The dashed curve in the first panel is the boundary
between empirical pure H ii region emission and “composite” emission proposed
by Kauffmann et al. (2003). The solid curves in the three panels are the
Kewley et al. (2001) maximum starburst line. The gray straight lines in the
middle and right panels are the empirical boundary between Seyferts and
LI(N)ERs proposed by Kewley et al. (2006). The model grids are the MAPPINGS
III shock (solid grids) and shock+precursor (dashed grids) models from (Allen
et al. 2008) for a SMC-like metallicity. The grid lines are color-coded
according to shock velocity $v_{s}$ and magnetic field $B$. Figure 16: Spatial
variation of the emission line flux ratio [O iii]$\lambda 5007$/H$\beta$, [S
ii]$\lambda\lambda 6717,6731$/H$\alpha$ and [O i]$\lambda 6300$/H$\alpha$.
Only spaxels with ${\rm S/N}>10$ in the involved emission lines are plotted.
The rightmost panel indicates the spatial distribution of possible excitation
sources implied by the [O iii]$\lambda 5007$/H$\beta$ vs. [O i]$\lambda
6300$/H$\alpha$ diagnostic diagram.

In order to explore the excitation mechanisms of the gaseous nebulae in NGC
2915, we turn to the three commonly used line-ratio diagnostic diagrams
(Veilleux & Osterbrock 1987), involving [O iii]/H$\beta$, [N ii]/H$\alpha$, [S
ii]/H$\alpha$, and [O i]/H$\alpha$. The [O iii]/H$\beta$ versus [N
ii]/H$\alpha$ diagram is known as the BPT diagram (Baldwin et al. 1981), which
is one of the most important diagnostics for distinguishing the excitation by
star formation and active galactic nuclei (AGN). [S ii] and [O i] lines are
produced by partially ionized zones of gaseous nebulae and are not as
sensitive to metallicities as [N ii], which makes them ideal tracers of shocks
and weak or low-metallicity AGNs.

Figure 15 shows the distribution of the Voronoi-binned spaxels of NGC 2915 on
the three diagnostic diagrams. The data points are color-coded by
galactocentric radius and only spaxels with ${\rm S/N}>10$ in all the six
involved emission lines are plotted. Almost all of the spaxels in NGC 2915 are
well below the theoretical “maximum starburst line” (Kewley et al. 2001) and
are also below the Kauffmann et al. (2003) empirical “purely star-forming
line” on the BPT diagram. On the other two line-ratio diagrams, a fraction of
spaxels fall into the regime of AGN- or LINER-like excitation, and the
fraction is higher in the [O iii]/H$\beta$ versus [O i]/H$\alpha$ diagram. All
but several unconnected spaxels with AGN- or LINER-like spaxels are
distributed at large galactocentric distances (rightmost panel of Figure 16),
which basically rules out AGN as a possible excitation source.

We also make an attempt to search for the forbidden high-ionization coronal
lines (e.g., [Fe VII]$\lambda$6087, [Fe X]$\lambda$6374, [Fe XI]$\lambda$7892)
that have ionization potentials $\gtrsim$ 100 eV (e.g., Gelbord et al. 2009;
Negus et al. 2021). Detection of these emission lines is considered to be a
signature of AGN activity (e.g., Molina et al. 2021). No such emission lines
with ${\rm S/N}>3$ are detected across the observed field, again disfavoring
the presence of an AGN.

While there is no evidence for an AGN, excitation sources other than pure star
formation, such as shocks, are still possible. In the two diagnostic diagrams
involving [S ii] and [O i] lines, the Allen et al. (2008) shock and
shock+precursor models (with one-fifth of solar metallicity) overplotted in
the middle and right panels of Figure 15 appear to explain the excitation of
spaxels above the maximum starburst line to a great extent. Figure 16 further
shows the line-ratio maps, where the highest values of the shock-sensitive [S
ii]/H$\alpha$ and [O i]/H$\alpha$ are found at the outer edges of the observed
field. Similar findings are also reported in several other dwarf galaxies
(Fensch et al. 2016; Cairós & González-Pérez 2017a; Bik et al. 2018).
Nevertheless, an inspection of the emission line velocity dispersion map
(Figure 7) reveals that the regions with the highest [S ii]/H$\alpha$ and [O
i]/H$\alpha$ tend to have relatively low velocity dispersion ($\lesssim$ 30
km/s), which raises doubt about their shock-excited nature and instead favors
diffuse ionized gas (possibly) ionized by photons leaking from H ii regions.

##  5 Summary

In this work, we used the deep integral field spectroscopic data from the
VLT/MUSE instrument to investigate the kinematics, stellar populations, and
excitation sources of the blue compact dwarf galaxy NGC 2915. The MUSE
observations cover the inner exponential disk of the galaxy. In particular, we
performed a joint fitting of the MUSE spectra and HST broadband photometry to
obtain the spatially resolved star formation histories, studied the spatial
distribution and stellar populations of bright star clusters ($V<22.8$ mag;
$M_{V}<-5.3$ mag) detected based on the MUSE data, compared the stellar and
gaseous kinematics, and explored the excitation sources of the gaseous nebulae
based on the classical line-ratio diagnostic diagrams. Our main results are
summarized below.

  * 1.

The stellar disk exhibits a relatively weak but significant rotation within
the central $\sim$ 0.5 kpc in radius. The stellar rotation axis appears to be
anti-parallel to that of the extended neutral H i disk (Section 4.1.1). The
kinematics of the ionized gas is significantly disturbed either by stellar
feedback associated with the active star formation or gas infall/inflow. In
contrast to the ionized gas, the neutral H i gas does not appear to deviate
significantly from regular rotation (e.g., Figure 6), implying that the two
gas phases are largely spatially decoupled along the line of sight.

  * 2.

The global star formation history peaked around a couple of gigayears ago
(Section 4.4), when the most massive star cluster was formed. The most recent
episode of bursty star formation happened around $\sim$ 50 Myr ago and has
lasted for $\sim$ 50-100 Myr. It is intriguing to note that the observed
cluster age distribution roughly follows the trend of star formation
histories. This recent episode of bursty star formation has formed $\sim$ 3%
of the total stellar mass. Episodes of bursty star formation similar to the
most recent one may have recurred less than 4 times in different locations
(but largely confined within the central 0.4 kpc in radius) in the past 1 Gyr.

  * 3.

The average stellar metallicities have a nearly flat radial profile in the
central 0.4 kpc in radius, beyond which the metallicities drop steeply with
radius. In contrast to the stellar metallicities, the average gas-phase
metallicities have a positive radial gradient ($\sim$ 0.14 dex/kpc), which is
unexpected for a system whose metal production, mixing, and loss are in
equilibrium. This confirms the finding of Werk et al. (2010) based on
spectroscopy of five H ii regions. We found evidence for both metal-rich gas
outflow and metal-poor gas infall or inflow that may collectively contribute
to the positive gas-phase metallicity gradient. In particular, the metal-poor
gas infall is unveiled by both signatures of cloud–cloud collisions (Section
4.1.2). An unusually small difference between the gaseous and stellar
metallicities (Section 4.2; Figure 10) also points to either significant
inflow or outflow.

  * 4.

Based on the classical line-ratio diagnostic diagrams of Veilleux & Osterbrock
(1987), no evidence for an AGN is found in NGC 2915 (Figures 15 and 16).
Regions outside the active star-forming area tend to have higher [S
ii]/H$\alpha$ and [O i]/H$\alpha$ line ratios but lower gas velocity
dispersions ($\lesssim$ 30 km/s) than the central regions, suggesting a
dominant contribution from diffuse ionized gas.

The analysis presented in this work points to a highly complex inner disk of
NGC 2915 in terms of the relative distribution of stars and gas. Indeed, Elson
et al. (2010) found that the complex H i gas distribution of the inner $\sim$
2 kpc cannot be explained by either pure circular rotation (even allowing for
radial variation of disk inclinations) or highly turbulent velocity
dispersion. There are at least two nonaxisymmetric H i features in the central
$\sim$ 2-3 kpc, one is a stream-like feature in the southeast along the major
axis that is visible in the velocity field and causes an abrupt decline in
radial velocities (Section 4.1.1), and the other is a plume-like feature in
the northwest also along the major axis that is visible in the column density
and moment-2 map (Elson et al. 2010). These features demonstrate an ongoing
tidal interaction or gaseous accretion. The discovery of a counter-rotating
central stellar disk implies an external origin of the gaseous material that
has sustained the recent episodes of bursty star formation.

###### Acknowledgements.

We thank the anonymous referee for their comments on the manuscript. We thank
E. C. Elson for sharing his H i data. We acknowledge support from the China
Manned Space Project (Nos. CMS-CSST-2021-A07, CMS-CSST-2021-B02), the NSFC
grant (Nos. 11421303, 11973038, 11973039, 12122303, 12233008), and the CAS
Pioneer Hundred Talents Program, the Strategic Priority Research Program of
Chinese Academy of Sciences (Grant No. XDB 41000000) and the Cyrus Chun Ying
Tang Foundations. Z.S.L. acknowledges the support from China Postdoctoral
Science Foundation (2021M700137).

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