# An ALMA study of the massive molecular clump N159W-North in the Large
Magellanic Cloud:
A possible gas flow penetrating one of the most massive protocluster systems
in the Local Group

Kazuki Tokuda Department of Earth and Planetary Sciences, Faculty of Sciences,
Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan  National Astronomical
Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa,
Mitaka, Tokyo 181-8588, Japan  Department of Physics, Graduate School of
Science, Osaka Metropolitan University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka
599-8531, Japan  Taisei Minami Department of Physics, Graduate School of
Science, Osaka Metropolitan University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka
599-8531, Japan  Yasuo Fukui Department of Physics, Nagoya University, Furo-
cho, Chikusa-ku, Nagoya 464-8601, Japan  Tsuyoshi Inoue Department of Physics,
Konan University, Okamoto 8-9-1, Kobe, Japan  Takeru Nishioka  Department of
Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
Kisetsu Tsuge Dr. Karl Remeis Observatory and ECAP, Universit$\ddot{a}$t
Erlangen-N$\ddot{u}$rnberg, Sternwartstrasse 7, 96049, Bamberg, Germany
Sarolta Zahorecz National Astronomical Observatory of Japan, National
Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
Hidetoshi Sano Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu
501-1193, Japan  National Astronomical Observatory of Japan, National
Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
Ayu Konishi Department of Physics, Graduate School of Science, Osaka
Metropolitan University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
C.-H. Rosie Chen Max Planck Institute for Radio Astronomy, Auf dem Huegel 69,
D-53121 Bonn, Germany  Marta Sewiło CRESST II and Exoplanets and Stellar
Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD
20771, USA  Department of Astronomy, University of Maryland, College Park, MD
20742, USA  Center for Research and Exploration in Space Science and
Technology, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
Suzanne C. Madden AIM, CEA, CNRS, Université Paris-Saclay, Université Paris
Diderot, Sorbonne Paris Cité, F-91191 Gif-sur-Yvette, France  Omnarayani Nayak
Space Telescope Science Institute, Baltimore, MD 21218, USA  Kazuya Saigo
Department of Physics and Astronomy, Graduate School of Science and
Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima, Kagoshima
890-0065, Japan  Atsushi Nishimura Nobeyama Radio Observatory, National
Astronomical Observatory of Japan (NAOJ), National Institutes of Natural
Sciences (NINS), 462-2 Nobeyama, Minamimaki, Minamisaku, Nagano 384-1305,
Japan  Kei E. I. Tanaka Center for Astrophysics and Space Astronomy,
University of Colorado Boulder, Boulder, CO 80309, USA  National Astronomical
Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa,
Mitaka, Tokyo 181-8588, Japan  Tsuyoshi Sawada National Astronomical
Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa,
Mitaka, Tokyo 181-8588, Japan  Joint ALMA Observatory, Alonso de Córdova 3107,
Vitacura, Santiago 763-0355, Chile  Remy Indebetouw Department of Astronomy,
University of Virginia, PO Box 400325, Charlottesville, VA 22904, USA
National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA
22903, USA  Kengo Tachihara Department of Physics, Nagoya University, Furo-
cho, Chikusa-ku, Nagoya 464-8601, Japan  Akiko Kawamura National Astronomical
Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa,
Mitaka, Tokyo 181-8588, Japan  Toshikazu Onishi Department of Physics,
Graduate School of Science, Osaka Metropolitan University, 1-1 Gakuen-cho,
Naka-ku, Sakai, Osaka 599-8531, Japan

###### Abstract

Massive dense clumps in the Large Magellanic Cloud can be an important
laboratory to explore the formation of populous clusters. We report multiscale
ALMA observations of the N159W-North clump, which is the most CO-intense
region in the galaxy. High-resolution CO isotope and 1.3 mm continuum
observations with an angular resolution of $\sim$0$\farcs$25($\sim$0.07 pc)
revealed more than five protostellar sources with CO outflows within the main
ridge clump. One of the thermal continuum sources, MMS-2, shows especially
massive/dense nature whose total H2 mass and peak column density are $\sim$104
$M_{\odot}$ and $\sim$1024 cm-2, respectively, and harbors massive ($\sim$100
$M_{\odot}$) starless core candidates identified as its internal
substructures. The main ridge containing this source can be categorized as one
of the most massive protocluster systems in the Local Group. The CO high-
resolution observations found several distinct filamentary clouds extending
southward from the star-forming spots. The CO (1-0) data set with a larger
field of view reveals a conical-shaped, $\sim$30 pc long complex extending
toward the northern direction. These features indicate that a large-scale gas
compression event may have produced the massive star-forming complex. Based on
the striking similarity between the N159W-North complex and the previously
reported other two high-mass star-forming clouds in the nearby regions, we
propose a $``$teardrops inflow model$"$ that explains the synchronized,
extreme star formation across $>$50 pc, including one of the most massive
protocluster clumps in the Local Group.

ISM: clouds – ISM: kinematics and dynamics – ISM: molecules – stars: formation

††software: CASA (v5.6.1; McMullin et al. 2007), Astropy (Astropy
Collaboration et al., 2018), APLpy (Robitaille & Bressert, 2012)

##  1 Introduction

High-mass stars and the associated events in their formation and evolution
dramatically change the surrounding interstellar environment and eventually
regulate galaxy evolution. Its astrophysical importance has motivated a large
number of both observational and theoretical efforts in the past few decades
(see reviews by e.g., Zinnecker & Yorke 2007; Tan et al. 2014). One of the
basic ideas to explain the formation of an individual high-mass star or
binary/multiple systems is that a massive cloud core, whose mass is $\sim$100
$M_{\odot}$, collapses into stellar objects in a similar manner to low-mass
star formation (e.g., McKee & Tan, 2003). The dynamical process produces more
massive protostellar disks and more energetic outflow than those in the low-
mass case (e.g., Matsushita et al., 2017), which is consistent with recent
observational findings achieved by millimeter/submillimeter facilities (e.g.,
Beuther et al., 2002; Hirota et al., 2017; Matsushita et al., 2019; Motogi et
al., 2019; Tanaka et al., 2020; Torii et al., 2021).

Because most stars form as clusters (e.g., Lada & Lada, 2003), understanding
the cluster formation mechanism is a key toward establishing the conclusive
picture of high-mass star formation. The 24 most massive giant molecular
clouds (GMCs) are currently responsible for most of the star formation in the
Galaxy (Lee et al., 2012), assuming the GMCs will make clusters. Observational
characterizations of such massive cradles forming young massive clusters (YMC)
whose total stellar mass exceeds $\sim$104 $M_{\odot}$ (Portegies Zwart et
al., 2010) are thus a reasonable approach to investigate their formation
origin. According to the review by Longmore et al. (2014), $``$in-situ star
formation$"$ requires sufficient gas to be packed into the final cluster
volume, and thus predicts the formation of a starless massive clump before the
onset of extreme star formation. The mass and size (radius) requirements are
$\gtrsim$104 $M_{\odot}$ and $\lesssim$1 pc, respectively. Although the
probability of discovering a completely starless phase is very small due to
the extremely short lifetime (Motte et al. 2018; Vázquez-Semadeni et al. 2019,
and references therein), there should be a sufficient number of ongoing star-
forming clouds (Ginsburg et al., 2012). In order to form such massive clouds,
mass accumulation, i.e., gas flow, from a large-scale surrounding environment
is necessary, and some promising mechanisms have been proposed, mainly
motivated by observations, such as cloud-cloud collision (Fukui et al., 2021),
and global hierarchical collapse (Motte et al., 2018). Revealing a high-
dynamic-range molecular gas spatial/velocity distribution from a giant
molecular cloud to an individual core inside the parental cloud is still a key
method to further consider what mass accumulation mechanism is efficiently
working at each physical scale.

The most nearby cluster-forming regions, such as Orion, whose distance from
the Sun is $\lesssim$1 kpc, are not necessarily appropriate to pursue the YMC
formation judging from the current gas distribution and star formation
activities therein (Portegies Zwart et al., 2010). In this regard, several
outstanding regions along with the Galactic disk, e.g., Cygnus X, Carina, W43,
W49, and W51, are the nearest neighbors as extreme star-forming environments,
because they have a sufficient molecular gas mass, $\gtrsim$106 $M_{\odot}$
(e.g., Nguyen-Luong et al., 2016). Unfortunately, almost all of these regions
are located within a $\pm$1$\arcdeg$ galactic latitude range on the inner
Galactic plane where line-of-sight contaminations from unrelated objects are
significant. Because dense clumps/cores rarely appear by chance in the same
observed direction, source confusion may not have serious impact on such a
small-scale study itself. However confusion from larger ($\gtrsim$10 pc) scale
foreground/background emission to individual massive clumps makes it difficult
to examine the formation mechanisms of progenitors leading to YMCs in these
outstanding Galactic regions. Extragalactic targets at a distance of
$\lesssim$100 kpc can be a frontier to obtain a much clearer view around YMC
progenitor clumps. Atacama Large Millimeter/submillimeter Array (ALMA)
provides an unprecedented view resolving individual proto/prestellar sources
with a spatial resolution of $\lesssim$0.1 pc (e.g., Indebetouw et al., 2020).

The Large Magellanic Cloud (LMC) is one of the ideal laboratories to
investigate molecular clouds clearly thanks to its proximity ($D\sim$50 kpc,
Schaefer 2008; de Grijs et al. 2014) and face-on view (Balbinot et al., 2015).
The star formation activity is different from that in the Galaxy; populous
clusters are actively forming (e.g., Hodge, 1961; van den Bergh, 1981). Their
stellar masses are 104–105 $M_{\odot}$, which is larger than those of Galactic
open clusters (Kumai et al., 1993; Hunter et al., 2003). The most extreme
star-forming region is 30 Dor, which is the brightest H ii region in the
galaxy, inhering a total stellar mass of $\sim$105 $M_{\odot}$ (e.g., Cignoni
et al., 2015). However, the total amount of the remaining molecular material
in this region is $\sim$105 $M_{\odot}$ (e.g., Minamidani et al., 2008, 2011;
Wong et al., 2011), which is not large compared to the above-listed GMCs in
the Galaxy. Among the $\sim$300 entities discovered by the NANTEN CO(1–0)
survey in the LMC (Fukui et al., 2008; Kawamura et al., 2009), the molecular
cloud associated with the N159 H ii region (hereafter, the N159 GMC) is the
most massive one with a total gas mass of $\sim$2 $\times$106 $M_{\odot}$ and
a less evolved region than 30 Dor (see also Johansson et al. 1994, 1998).

Using the single-dish Atacama Submillimeter Telescope Experiment (ASTE),
Minamidani et al. (2008) performed CO(3–2) observations on several GMCs along
the molecular ridge region at the south-east side of the galaxy and confirmed
that the N159W cloud shows the strongest intensity. The subsequent
comprehensive CO(1–0) study with the Mopra telescope (Wong et al., 2011) also
confirmed that it shows the strongest peak along with the N113 region. Using
the same telescope, Nishimura et al. (2016) reported marginal detections of
N2H+ emission in N159W and N113 despite of a significant deficient of
N-bearing molecules in LMC (Millar & Herbst, 1990). Measurements at
millimeter/submillimeter windows (Paron et al., 2016; Galametz et al., 2020)
detected several high-density gas tracers, HCO+, HCN, CS, and (tentative)
C18O, toward the N159W region. Ott et al. (2010) performed an NH3 survey on
such CO prominent regions, including N159W and N113, using the Australia
Telescope Compact Array (ATCA). The NH3 emission was only found in the
N159W-North region, indicating the molecule is also deficient in the LMC, and
likely traces the densest best-shielded cores. The CO peak and the position of
NH3 core is $\sim$10 pc displaced from the H$\alpha$ peak (e.g., Smith & MCELS
Team, 1999), indicating that the molecular gas dissipation has not been
started yet. Nevertheless, the Spitzer-based studies discovered several
luminous ($\gtrsim$105 $L_{\odot}$) young stellar object (YSO) candidates
inside the cloud (e.g., Seale et al., 2009, 2014; Gruendl & Chu, 2009; Chen et
al., 2010; Carlson et al., 2012; Jones et al., 2017). These observational
signatures strongly demonstrate that active star formation is ongoing in the
massive cloud and its evolutionary stage must be early.

Early ALMA observations in Cycle 1 clearly revealed molecular filaments in the
N159W and N159E regions (Fukui et al., 2015; Saigo et al., 2017). Figure 1
shows an overview of the ALMA CO observations in N159E/W. One of the
interesting findings is that the locations of the bright YSOs are the
intersections of multiple filamentary clouds, which partially resemble
galactic hub filaments (e.g., Myers, 2009; Peretto et al., 2013). The N159
studies hypothesized that filament-filament collisions promoted high-mass star
formation activity (Fukui et al., 2015; Saigo et al., 2017). The follow-up
ALMA high-resolution studies in Cycle 4 toward N159E-Papillion (Fukui et al.
2019, hereafter Paper I; Sharda et al. 2021) and N159W-South (Tokuda et al.
2019, hereafter Paper II) further resolved complex web-type systems with a
filament widths of $\sim$0.1 pc (Figure 1b,c). Papers I and II concluded that
the coalescence/collision of many individual filaments is unlikely to form
such a complex system almost simultaneously across 50 pc. Alternatively, they
suggested that a large-scale H i gas flow driven by the last tidal interaction
between the LMC and Small Magellanic Cloud (SMC) triggered the filament and
subsequent high-mass star formation at the two systems in N159E/W.

Figure 1: A multi-wavelength overview of N159E/W. (a) Three-color composite
image combining the ALMA Cycle 1 12CO ($J$ = 2–1) (red; Fukui et al. 2015;
Saigo et al. 2017; Nayak et al. 2018), the Spitzer/IRAC 8.0 $\mu$m (green;
Meixner et al. 2006), and the MCELS H$\alpha$ (blue; Smith & MCELS Team 1999)
images. Regions delimited by white dashed lines represent the ALMA Cycle 1
field coverages. The white ellipse in the lower left corner gives the ALMA
beam size of 1$\farcs$3 $\times$ 0$\farcs$8\. The magenta contours show the
12CO($J$ = 3–2) integrated intensity map with ASTE (Minamidani et al., 2008).
The lowest and subsequent contour steps are 10 K km s-1.

The present study is the third paper to show the same observing project with
Papers I and II, but we describe the high-resolution ($\sim$0$\farcs$26) ALMA
1.3 mm wavelength CO, its isotopes, and continuum images in the N159W-North
region for the first time. The total molecular gas mass of star-forming dense
clump are one order of magnitude larger than the previously reported two
regions (see the comparison in Sect. 3.3). Taking into account the fact that
high-density gas tracers such as NH3 have also been detected, this region is
even more favorable for exploring the formation of massive dense clump leading
to YMC. We also present one more additional data set, CO(1–0), at a lower
resolution ($\sim$1$\farcs$5) covering a wide field coverage. Section 2
explains the observations and data reduction. Section 3 provides a multiscale
molecular cloud view from $\sim$100 pc down to $\sim$0.06 pc and characterizes
the individual proto/prestellar cores. Section 4 discusses the formation
scenario of the massive clump in the N159W-North region as well as the high-
mass star formation throughout the N159 E/W region.

##  2 Observations and data reduction

In the ALMA Cycle 4, we carried out Band 6 (211–275 GHz) observations toward
the N159W-North region with the ALMA 12 m array as part of a multiobjects
survey in N159 (P.I.: Y. Fukui #2016.1.01173.S). Because the previous studies
(Papers I and II) described the observation and data reduction details, we
briefly summarize the data quality here. The frequency setting mainly targeted
the molecular lines of 12CO($J$ = 2–1), 13CO($J$ = 2–1), C18O($J$ = 2–1) and
1.3 mm continuum. The synthesized beam sizes of the 12CO line and continuum
images are 0$\farcs$27 $\times$ 0$\farcs$23, and 0$\farcs$26
$\times$0$\farcs$23, respectively. The rms noise (1$\sigma$) levels of the
12CO line and the continuum are $\sim$3.7 mJy beam-1 ($\sim$1.3 K) at a
velocity resolution of 0.2 km s-1 and $\sim$0.027 mJy beam-1. Table 5 in
Appendix A presents the beam propeties and sensitivities of the analyzed lines
in this paper. Because the 12CO and 13CO emission of the N159W-North region in
the Cycle 4 data have non negligible missing flux ($\sim$40–60%), we used the
combined data with the new Cycle 4 and the previously obtained Cycle 1 data
sets (Fukui et al., 2015; Nayak et al., 2018) throughout the manuscript. Note
that the previously obtained CO sets did not include the compact array (7
m+TP) data, but the 12 m array data alone sufficiently covers the total flux
judging from single-dish measurements in this region (see Fukui et al., 2015;
Nayak et al., 2018).

In addition to the Band 6 data, we retrieved an ALMA archival Band 3 (84–116
GHz) CO($J$ = 1–0) data (P.I.: T. Sawada #2019.1.00915.S) of the same source
covering a much larger spatial scale. The field coverage was 170$\arcsec$
$\times$ 180$\arcsec$ centered at ($\alpha_{\rm J2000.0}$, $\delta_{\rm
J2000.0}$) = (5h39m35$\fs$0,$-$69°45$\farcs$31$\farcs$2). Using CASA (Common
Astronomy Software Application) version 5.6.1, we individually imaged the 12
m, 7 m , and TP(Total Power) array data and then combined them together using
the feather task. The resultant beam size and sensitivity of the CO($J$ = 1–0)
data cube are 2$\farcs$2 $\times$ 1$\farcs$8 and $\sim$0.6 mJy beam-1
($\sim$1.5 K) at a velocity resolution of 0.2 km s-1. Note that we made all of
the molecular line cube data in a unified velocity frame of LSRK (Local
Standard of Rest, Kinematic), and describe the velocity in the same frame
throughout the manuscript.

##  3 Results

###  3.1 High-resolution 12CO and 13CO images of the N159W-North region

Figure 2 shows peak brightness temperature maps of 12CO and 13CO($J$ = 2–1) at
a spatial resolution of $\sim$0.06 pc. The high-spatial dynamic range
observations have revealed the molecular cloud distribution with a size scale
of $\sim$10 pc down to $\lesssim$0.1 pc (the velocity-channel maps are
presented in Figures 10 and 11 of Appendix B). Within the molecular cloud,
there are at least two spectroscopically confirmed YSOs, whose luminosity are
$\sim$105 $L_{\odot}$ identified by the infrared observations using Spitzer
(e.g., Seale et al., 2009; Gruendl & Chu, 2009; Chen et al., 2010; Seale et
al., 2014; Jones et al., 2017). In this paper, we call the two sources as
YSO-N1 and YSO-N2, which are 053937.56-694525.4 and 053937.04-694536.7,
respectively, in the Chen et al. (2010) catalog. For the two YSOs, we detected
1.3 mm continuum emission (see black contours in Figure 2), tracing free-free
emission from ionized gas and/or thermal dust components in the cold/dense
condensations (see more details in Sect. 3.2). With respect to the continuum
and YSO sources, the 12CO and 13CO cloud asymmetrically distributes in the
northwest(NW)-southeast(SE) direction. We see a highly extended emission on
the NW side, while the SE region has a sharp boundary at the cloud edge.

Figure 2:  (a) The color-scale image illustrates the 12CO($J$ = 2–1) peak
brightness temperature map. The white ellipse in the lower-left corner gives
the angular resolution, 0$\farcs$27 $\times$ 0$\farcs$23\. The white dashed
ellipse indicates where the mosaic sensitivity falls to 50%. The purple dashed
line marks the warm side of the cloud (see details in Sect. 4.1.2). The black
contours show the 1.3 mm continuum image. The lowest contour and subsequent
steps are 0.1 mJy beam-1. The red crosses denote positions of two YSOs (Chen
et al., 2010). (b) Same as (a) but for the 13CO($J$ = 2–1) peak brightness
temperature map.

The 12CO peak brightness temperature map (Figure 2a) shows that most of the
emitting area exceed 30 K and the maximum value is $\sim$80 K. Assuming the
abundance (isotope) ratio, 12CO/13CO of 50 (Johansson et al., 1994; Mizuno et
al., 2010), the typical intensity ratio between 12CO and 13CO of $\sim$2 tells
us that the optical thickness of 12CO is $\gtrsim$30, which is an optically
thick regime. The peak brightness map in 12CO thus reflects the temperature
distribution in this region under the local thermodynamical equilibrium (LTE)
assumption. The most intense region is around YSO-N2, but the area near the
cloud edge at the SE side is also uniformly high, $\sim$50 K. In addition to
the feedback from YSO-N1, there may be some heating mechanisms changing the
temperature of the molecular cloud across several parsecs. We call the
distinct feature $``$warm side$"$ hereafter as indicated in the dotted line in
Figure 2a and discuss the possible origins in Sect. 4.1.2.

13CO emission is optically thin at a molecular cloud scale generally, and thus
it is a good tracer of column density. The LTE calculation (Rohlfs & Wilson,
2004) tells us that the median and maximum H2 column densities are $\sim$3
$\times$1023 cm-2 and $\sim$1 $\times$1024 cm-2, respectively, assuming
[H2]/[13CO] = 3${\times}$106 for consistency with the previous analysis (e.g.,
Mizuno et al., 2010). Adopting the mean molecular weight per hydrogen molecule
(2.8), the total H2 gas mass is $\sim$7$\times$ 104 $M_{\odot}$ within the
13CO detected region above 5$\sigma$ noise level on the integrated intensity
image. This mass is almost comparable to a single giant molecular cloud traced
by 13CO($J$ = 2–1) (e.g., the Orion A cloud; Nishimura et al. 2015).

####  3.1.1 Filaments extending from the main ridge

One of the notable characteristics in the 12CO and 13CO images (Figure 3) is
that there are several filamentary structures, which extend to the south
direction from the vicinity of YSO-N1 and N2. We call these features as
$``$filaments$"$, hereafter. Only filament D has a discontinuous distribution
with weaker intensity than the others and is not detected in 13CO, and thus we
treat it as marginal. The projected lengths of the 12CO and 13CO filaments are
$\sim$4 pc and $\sim$3 pc, respectively. Since we could not measure the actual
length of the 12CO filaments, especially for A and B, due to the limited field
coverage (Figure 3), the current estimate is thus the lower limit. The column
density and line masses of the filaments are $\sim$(2–6)${\times}$1022 cm-2
and $\sim$(1–3)$\times$102 $M_{\odot}$ pc-1, respectively, which are listed in
Table 1.

The velocity-widths, $\Delta v$ are $\sim$1–3 km s-1 in FWHM, corresponding to
velocity-dispersion, $\sigma_{v}$ ( = $\Delta v$/2$\sqrt{2\rm ln2}$) of
0.4–1.3 km s-1. The virial line masses of the filaments, $M_{\rm
line,vir}=2\sigma_{v}^{2}/G$, are calculated to be (0.9–8) $\times$102
$M_{\odot}$ pc-1, which is somewhat larger than those values obtained from the
12CO luminosity mass mentioned above. Sect. 4.1.1 discuss the formation of
these filaments and the relation between them and the star formation in this
region.

Figure 3: Spatial distributions of the filaments in the N159W-North region.
(a1–a3) The color-scale images illustrate the velocity channel maps of the
12CO($J$ = 2–1) with velocity ranges of 232.0–234.0 km s-1 (a1), 240.0–242.6
km s-1 (a2), and 244.0–245.0 km s-1 (a3). The ellipse at the lower-left corner
in each panel gives angular resolution, 0$\farcs$27 $\times$ 0$\farcs$23\.
White dashed lines highlight the filaments by eye. Red crosses denote the
positions of YSO, same as those in Figure 2. (b1–b3) Same as panels (a1–a3),
but for the 13CO($J$ = 2–1) maps. Table 1: Physical quantities of filaments
traced in 12CO($J$ = 2–1)

filament name |  $v_{\rm cent}$ [km s-1]a |  ${\Delta}v$ [km s-1]a |  $L$ [pc]b |  $M$ [$M_{\odot}$]c |  $N_{\rm H_{2}}^{\rm ave}$ [cm-2]c |  $M_{\rm line}$ [$M_{\odot}$ pc-1]d
---|---|---|---|---|---|---
A | 232.3 | 2.3 | 3.5 | 4${\times}$102 | 2${\times}$1022 | 1${\times}$102
B | 234.5 | 2.4 | 2.9 | 4${\times}$102 | 2${\times}$1022 | 1${\times}$102
C | 242.4 | 3.1 | 1.9 | 3${\times}$102 | 2${\times}$1022 | 1${\times}$102
D | 244.0 | 1.1 | $\cdots^{\rm e}$ | 2${\times}$102 | 2${\times}$1022 | $\cdots^{\rm e}$

aafootnotetext: Central velocity ($v_{\rm cent}$) and width in FWHM
(${\Delta}v$) determined by a single Gaussian fitting to the 12CO spectra at
the cyan cross positions in panels (a,b) of Figure 3.

bbfootnotetext: Projected length between the southern edge of filaments and
the boundary of the main ridge (see also the definition of the main ridge in
Sect. 3.3).

ccfootnotetext: 12CO luminosity based total mass and averaged column density
of each filament. We assumed a constant conversion factor of $X_{\rm CO}$ = 5
$\times$1020 cm-2 (K km s-1)-1 (Hughes et al., 2010) and CO(2–1)/CO(1–0)
intensity ratio of 1.0.

ddfootnotetext: Line mass inferred from $M/L$.

eefootnotetext: We did not measure the length because filament D shows a
discontinuous spatial distribution.

####  3.1.2 Velocity structures of the molecular cloud

Figure 4 shows the velocity structure of 12CO and 13CO in the N159W-North
region. Overall, the structure is very complex, with a mixture of various
components over a velocity range of 10 km s-1. One of the outstanding features
is that the north side is redshifted, while the south side is blueshifted.
This trend is quantitatively similar to the N159E-Papillon and N159W-South
regions (Papers I and II). We extracted position-velocity (PV) diagrams across
the two YSO locations to examine the velocity structure around them in more
detail (Figure 4c,d). The velocity is distributed discontinuously with a
boundary around the offset axis of $\sim$3 pc. As one can see, there are
velocity width enhancements toward the two YSO positions. A particular feature
seen in 12CO is the outflow contribution from the YSOs (Sect. 3.2). Although
the outflow itself likely contributes only a few $\times$ 0.1 pc, there is
also a V-shaped structure around YSO-N2 on the PV diagram over $\sim$1 pc.
This feature may represent a cloud with multiple velocity components
interacting with each other (see discussion in Sect. 4.2).

Figure 4:  (a) The color-scale image illustrates the first-moment intensity-
weighted velocity map of 12CO($J$ = 2–1). The white ellipse in the lower-left
corner gives the angular resolution, 0$\farcs$27 $\times$ 0$\farcs$23\. The
portion of a white dashed ellipse indicates where the mosaic sensitivity falls
to 50%. The rectangle defined by the white thick line marks the extracted
region to make the position-velocity diagram in panels (c,d). The black
contours show the 1.3 mm continuum image. The lowest contour and subsequent
steps are 0.1 mJy beam-1. The red crosses denote positions of two YSOs (Chen
et al., 2010). (b) Same as (a) but for the 13CO($J$ = 2–1). (c) The color-
scale and orange contours show the 12CO($J$ = 2–1) position-velocity diagram
along the rectangle shown in panel (a) The contour levels are
[5,10,15,20,25,30,35,40] K. The two dashed lines represent the positions of
YSO-N1 and -N2. (d) Same as (c) but for 13CO($J$ = 2–1). The contour levels
are [5,10,15,20] K.

###  3.2 Millimeter continuum sources and molecular outflow

Figure 5 shows the 1.3 mm continuum distributions in the N159W-North region.
We focus on the stronger than 5$\sigma$ emission within the 3$\sigma$ contour,
and we find that there are five spatially well-separated entities. We labeled
them as MMS-1, -2, -3, -4, and -5 (see rectangles in Figure 5). MMS-1–5 have a
projected distance of 1–2 pc from the nearest neighbors. The largest/brightest
one is MMS-1, coinciding with YSO-N1. Indebetouw et al. (2004) detected 3 cm
continuum emission toward this source, and our ALMA observations of
H30$\alpha$ show a similar spatial extension to the 1.3 mm emission. These
results demonstrate that the free-free emission from ionized material largely
contribute to the continuum emission from MMS-1. The N159E-Papillon region
shows a similar 1.3 mm continuum structure whose size scale is
$\sim$5$\arcsec$ (Paper I; see also Figure 14b), but it is visible in the
optical H$\alpha$ band, unlike MMS-1 in N159W-North. The Papillon nebula is
more evolved than N159W-North MMS-1 and/or one of the complex velocity
components in the foreground hides the ionized gas. In fact, a part of 1.3 mm
emission in MMS-1 overlaps with the C18O distribution (see Figure 6a). In the
other 1.3 mm sources, the previous measurements, such as longer wavelength
radio observations, and hydrogen recombination lines, did not find any
indication of ionized material. The thermal dust emission is the main
contributor, which is consistent with that of the continuum emission spatially
correlates with C18O well (see Sect. 3.3).

Figure 5: 1.3 mm continuum and 12CO high-velocity emission toward N159W-North.
(a) The color-scale image illustrates the 1.3 mm continuum emission. The white
ellipse in the lower-left corner gives the beam size of 0$\farcs$26 $\times$
0$\farcs$23\. The red crosses indicate the positions of YSOs (Chen et al.,
2010) identified using the Spitzer data with an aperture radius of
3$\farcs$6\. (b–f) Enlarged views of each millimeter source. Red and blue
contours shows the redshifted and blueshifted high-velocity emission of
12CO($J$ = 2–1). The integrated velocity range are given in lower right
corners in each panel. We arbitrarily adjusted the contour level to emphasize
the weaker components in each target. Black ellipses in each lower left corner
show the beam size of 1.3 mm continuum.

Our visual inspection of MMS-2 and MMS-4 revealed six and two local peaks,
respectively (Figure 5a). We defined their boundary as 50% intensity in each
peak, including MMS-3, and -5, to estimate the physical quantities (Table 2).
Although multiple peaks can be also identified in MMS-1, we treat it as a
single continuous source in this paper, since a large free-free contribution
makes it difficult for us to estimate the physical quantities of thermal
component with the 1.3 mm measurement alone. The flux, $F_{\rm 1.3mm}^{\rm
MMS}$, is converted to molecular gas mass, $M_{\rm MMS}=D^{2}F_{\rm
1.3mm}^{\rm MMS}/R_{\rm dg}\kappa_{\rm 1.3mm}B(T_{\rm d})$, where $D$ is the
distance to the source, $R_{\rm dg}$ is the dust-to-gas mass ratio,
$\kappa_{\rm 1.3mm}$ is the absorption coefficient per unit dust mass at 1.3
mm, and $B(T_{\rm d})$ is the Planck function at a dust temperature $T_{\rm
d}$. Our assumptions are following; $R_{\rm dg}$ = 3.5 $\times$10-3, and
$\kappa_{\rm 1.3mm}$ = 1 cm2 g-1 (see, Ossenkopf & Henning 1994; Herrera et
al. 2013; Gordon et al. 2014). Assuming that the dust and gas temperatures
couple in the dense medium at a core scale (e.g., Young et al., 2004;
Ceccarelli et al., 2007), we applied the gas temperature determined from
ammonia observations (16 K, Ott et al. 2010) as $T_{\rm d}$. As summarized in
Table 2, the peak column density and mass of the sources are as high as
(0.4–1)${\times}$1024 cm-2, and (0.7–3)${\times}$102 $M_{\odot}$,
respectively.

Table 2: Physical quantities of 1.3 mm continuum sources in N159W-North

Name |  ${\alpha}$ (J2000.0)a |  ${\delta}$(J2000.0)a |  $F_{\rm 1.3mm}^{\rm max}$ [mJy beam-1] |  $F_{\rm 1.3mm}^{\rm MMS}$ [mJy]b |  $N_{\rm peak}$[cm-2]c |  $M_{\rm MMS}$ [$M_{\odot}$]d | I.R. source
---|---|---|---|---|---|---|---
MMS-1e | 05:39:37.55 | -69:45:26.11 | 1.36 | 12.2 | $\cdots^{\rm e}$ | $\cdots^{\rm e}$ | Y
MMS-2a | 05:39:37.09 | -69:45:36.56 | 0.56 | 0.8 | 1${\times}$1024 | 1${\times}$102 | Y
MMS-2b | 05:39:37.03 | -69:45:36.40 | 0.53 | 1.1 | 1${\times}$1024 | 2${\times}$102 | N
MMS-2c | 05:39:36.78 | -69:45:35.67 | 0.24 | 1.1 | 5${\times}$1023 | 2${\times}$102 | N
MMS-2d | 05:39:36.71 | -69:45:38.09 | 0.63 | 2.0 | 1${\times}$1024 | 3${\times}$102 | Y
MMS-2e | 05:39:36.68 | -69:45:37.39 | 0.31 | 1.1 | 7${\times}$1023 | 2${\times}$102 | N
MMS-2f | 05:39:36.57 | -69:45:38.24 | 0.25 | 0.5 | 5${\times}$1023 | 1${\times}$102 | N
MMS-3 | 05:39:36.13 | -69:45:24.31 | 0.20 | 0.4 | 4${\times}$1023 | 7${\times}$101 | N
MMS-4a | 05:39:36.00 | -69:45:41.42 | 0.39 | 0.7 | 8${\times}$1023 | 1${\times}$102 | N
MMS-4b | 05:39:35.94 | -69:45:41.10 | 0.34 | 0.7 | 7${\times}$1023 | 1${\times}$102 | N
MMS-5 | 05:39:35.19 | -69:45:24.31 | 0.32 | 0.4 | 7${\times}$1023 | 7${\times}$101 | N

aafootnotetext: Peak sky coordinate in 1.3 mm.

bbfootnotetext: Flux of the 1.3 mm continuum emission above $\sim$50%
intensity level from each peak.

ccfootnotetext: H2 column density at each peak.

ddfootnotetext: Total gas mass within the same region to measure $F_{\rm
1.3mm}^{\rm MMS}$.

eefootnotetext: MMS-1 shows multiple peaks, but we treated it as a single
source and did not derive the column denisty and mass, because there is a
large free-free contamination (see the text).

We investigated the presence or absence of known infrared counterparts in each
continuum source to characterize the star formation activities. The Spitzer-
based YSO identifications do not have a sufficient angular resolution to
determine which sources, MMS-2a and -2b, are brighter in the mid-infrared
wavelength. Bernard et al. (2016) performed high-resolution near-infrared
observations using the VLT (Very Large Telescope) at an angular resolution of
$\sim$0$\farcs$1 with a sensitivity of 21.85 mag in $K_{\rm s}$. We
independently verified the positional accuracy of the VLT detected sources
using the GAIA DR2 catalog (Bailer-Jones et al., 2018). MMS-2a is visible in
$K_{\rm s}$ band, while MMS-2b was not detected. MMS-2d is detected in $K_{\rm
s}$, but the position is slightly ($\sim$0$\farcs$1) shifted to the east
direction from the continuum peak. According to Bernard et al. (2016), the
source is categorized as a YSO candidate based on the color-magnitude diagram.
Figure 6b visualizes the presence/absence of the infrared sources in MMS-2. In
the other continuum sources, we could not find any known infrared
counterparts.

Our previous studies (Papers I and II) discovered molecular outflows in 12CO
at some of the 1.3 mm continuum sources in the N159E-Papillon and W-South
regions. The new ALMA measurements enable us to make a powerful guide map
additionally searching for star formation activity in infrared quiescent
sources (see also some Galactic studies on infrared dark clouds (IRDC), e.g.,
Kong et al. 2019). In the same manner, described in Paper II, we manually
searched for high-velocity wing emission more than $\sim$10 km s-1 away from
the systemic velocity determined by the 13CO spectra at each continuum source.
Figures 5b–f present the spatial distributions of 12CO high-velocity emission
(see also the spectra of Figure 12 in Appendix C). We conclude that the high-
velocity emission is most likely due to molecular outflows from embedded
protostellar object(s) inside the continuum sources. The high-resolution 12CO
data reveals the outflow population in this region for the first time, except
for the monopolar one at YSO-N1 inside MMS-1, which was already reported in
the previous lower resolution ALMA study (Fukui et al., 2015). The
characteristics of the outflows are comparable to those in
N159E-Papillon/N159W-South (Papers I and II); the typical size and maximum
velocity of the flows are $\sim$0.2 pc, and $\sim$20 km s-1, respectively.
Only one exception is the blueshifted emission in MMS-1 whose maximum velocity
is not captured by the current observing setting in the Cycle 4 data. An
alternative ALMA project using the CO(3–2) line (K., Tanaka et al. in prep.)
detected more than $\sim$60 km s-1 emission in this source.

Among the 1.3 mm continuum sources, MMS-2c, e, and f do not harbor either
infrared nor 12CO high-velocity emission, indicating that these sources are
purely starless at least in the high-mass regime (Figure 6b). The inferred gas
mass is as massive as $\sim$100 $M_{\odot}$ (see Table 2). Although some
Galactic observations using ALMA discovered similar sources in high-mass star-
forming regions (e.g., Kong et al., 2017; Molet et al., 2019; Zhang et al.,
2021), our finding is the first discovery of starless massive core candidates
in extragalactic studies at less than 0.1 pc resolution.

###  3.3 Dense molecular gas traced by C18O in N159W-North

Figure 6a illustrates the C18O integrated intensity maps in the N159W-North
region. The emitting regions of the rare isotope are more spatially compact
than those of 12CO and 13CO, and have good spatial correlation with the
millimeter sources. These features clearly demonstrate that the C18O traces a
high density inside of the molecular cloud. We defined the dust and gas rich
region as $``$main ridge$"$ where we detect both the 1.3 mm and C18O emission
and the 13CO integrated intensity map enables to enclose the dense
condensations with a single contour level of $\sim$70 K km s-1. The maximum
integrated intensity is $\sim$12 K km s-1 around MMS-2c, which is the most
massive starless source inferred from the 1.3 mm measurement (Sect. 3.2). In
the MMS-1, the C18O emission is relatively weak compared to that in MMS-2, and
the extended 1.3 mm component is not bright in C18O. This is because the dense
gas around the high-mass protostellar object, YSO-N1, is beginning to
dissipate, and the extended 1.3 mm emission is mainly arising from the ionized
gas as described in Sect. 3.2.

Figure 6: C18O and 1.3mm continuum emission in the main ridge of N159W-North.
(a) The color-scale image illustrates the C18O integrated intensity image with
an angular resolution of 0$\farcs$6\. The beam size is shown by the white
ellipse in the lower left corner. The contours shows the 1.3 mm continuum
emission. The lowest and subsequent contour steps are 0.1 mJy beam-1. (b) The
color-scale image and contours shows an enlarged view of the 1.3 mm continuum
emission toward MMS-2. The contour levels are the same as panel (a). The white
ellipse at the lower left corner shows the beam size of 0$\farcs$26 $\times$
0$\farcs$23\. The blue and green crosses represent positions of the infrared
and starless sources, respectively.

We quantitively characterize the C18O clumps in the N159E/W region (Table 3,
see also Figure 14 in Appendix E). Our assumption to estimate the virial mass,
$M_{\rm vir}$ = 210$R_{\rm clump}\Delta v_{\rm clump}^{2}$, where $R_{\rm
clump}$ is the C18O clump radius in parsec and $\Delta v_{\rm clump}$ is the
FWHM of the line profile in km s-1, are a uniform density inside the clump and
ignoring the magnetic field contribution. To estimate the luminosity-based H2
mass using the LTE assumption, we need to apply a conversion factor, i.e.,
relative abundance, [H2]/[C18O]. According to Galactic studies (e.g., Frerking
et al., 1982), a commonly used value of [H2]/[C18O] is $\sim$5.9 $\times$106.
However, the early single-dish measurements by Johansson et al. (1994) showed
that C18O emission in the LMC is quite weak, indicating that [H2]/[C18O] in
the LMC is about 20 times higher than in the Milky Way (MW), $\sim$1.2
$\times$108. We adopted the higher value as the conversion factor to derive
the LTE mass. The resulting masses are one order of magnitude larger than the
virial mass (Table 3), which is typical for such a dense, gravitationally
bound system (see also Nayak et al. 2018). The current observational evidence
suggests that the main ridge in N159W-North and its subregions have dense gas,
whose density and total masses are $\sim$105 cm-3 and $\gtrsim$104
$M_{\odot}$, respectively.

Table 3: Physical properties of C18O clumps in N159E/W

Region name |  $R_{\rm clump}$ [pc]a |  $\Delta v_{\rm clump}$ [km s-1]b |  $V_{\rm cent}$ [km s-1]b |  $M_{\rm vir}$ [$M_{\odot}$]c |  $M_{\rm LTE}$ [$M_{\odot}$]d
---|---|---|---|---|---
MMS-1 | 0.63 | 5.9 | 239.9 | 5${\times}$103 | 1${\times}$104
MMS-2 | 0.71 | 4.6 | 235.6 | 3${\times}$103 | 2${\times}$104
MMS-4 | 0.36 | 2.6 | 236.2 | 5${\times}$102 | 1${\times}$103
MMS-5 | 0.31 | 2.5 | 235.0 | 4${\times}$102 | 1${\times}$103
main ridgee | $\cdots$ | $\cdots$ | $\cdots$ | $\cdots$ | 4${\times}$104
N159E-Papillon | 0.41 | 3.1 | 232.4 | 8${\times}$102 | 7${\times}$103
N159W-South | 0.38 | 4.6 | 236.1 | 2${\times}$103 | 5${\times}$103

aafootnotetext: Radius of a circle having the same area above the C18O
integrated intensity of $\sim$4 K km s-1, which corresponds to $\sim$5$\sigma$
noise level.

bbfootnotetext: Velocity width in FWHM ($\Delta v_{\rm clump}$) and central
velocity ($V_{\rm cent}$) of the C18O spectra at the peak sky coordinate in
each clump.

ccfootnotetext: Virial mass, $M_{\rm vir}$ = 210$R_{\rm clump}\Delta v_{\rm
clump}^{2}$ (see the text).

ddfootnotetext: The LTE mass assuming the uniform excitation temperature of 20
K and [H2]/[C18O] of $\sim$1.2 $\times$108 (see the text) within the region
having $>$5$\sigma$ C18O emission.

eefootnotetext: We defined only the $M_{\rm LTE}$, because the C18O emitting
regions are not spatially connected.

###  3.4 A comprehensive CO view around/in the N159W-North region

Figure 7a illustrates the CO($J$ = 1–0) peak brightness temperature map. The
field coverage is the widest among the N159W ALMA studies using CO and its
isotope lines with a sub-pc resolution. Note that we arbitrarily extracted a
velocity range of 234.7–239.5 km s-1 to highlight the N159W-North related
structures in Figure 7a. The velocity-channel map is presented in Figure 13 of
Appendix D.

Figure 7: Multi-spatial-scale molecular gas view around the N159W-North
region. (a) The color-scale image illustrates the 12CO($J$ = 1–0) peak
brightness temperature map extracted from a velocity range of 234.7–239.5 km
s-1. The white dashed contours indicates where the mosaic sensitivity falls to
50%. The white ellipse in the lower-left corner gives the angular resolution,
2$\farcs$2 $\times$ 1$\farcs$8\. The region determined by white dashed
triangle represent the CO emitting regions defined as the conical cloud. The
yellow and cyan dashed rectangles indicate the N159W-North and -South regions
studied in this work and Paper II, respectively. Black crosses with white
circles indicate positions of YSO candidates (Chen et al., 2010) within the
Cycle 4 observed field. (b) An explanatory map of distinct features (see the
text) in the N159W-North region on the high-resolution 12CO($J$ = 2–1) peak
brightness temperature image, which is the same as the color-scale image in
Figure 2a. The black ellipse in the lower-left corner gives the angular
resolution, 0$\farcs$27 $\times$ 0$\farcs$23\. Orange contour shows a smoothed
13CO($J$ = 2–1) velocity-integrated intensity image with contour levels of 70
and 140 K km s-1.

As one can see, there are at least two elongated or filamentary features
intersecting around the main ridge region as indicated by the triangle in
Figure 7a. The total projected lengths of the western and eastern clouds are
$\sim$20 pc and $\sim$30 pc, respectively. Although their central velocities
differ from each other by a few km s-1, they are spatially connected via the
main ridge, making a single large system. We call this chain of features
$``$conical cloud$"$ hereafter. These characteristics qualitatively resemble
those of the V-shaped filament in the N159W-South region (Fukui et al. 2015;
Paper II). The southern tip of the conical cloud, which roughly lies to the
Cycle 4 field coverage, has CO peak brightness temperature of $\gtrsim$30 K.
Compared to the entire conical cloud, the temperature rises across the main
ridge region, not only the internal subregion, the warm side introduced in
Sect. 3.1.

The total CO(1–0) luminosity of the conical cloud is 3.7${\times}$103 K km s-1
pc2, corresponding to $\sim$1.6${\times}$105 $M_{\odot}$ if we assume a CO-
to-H2 conversion factor, 5 $\times$1020 cm-2 (K km s-1)-1 (Hughes et al.,
2010). The conical cloud is an order of magnitude more massive than the
N159W-South cloud, $\sim$104 $M_{\odot}$ (Paper II). Because the comprehensive
single-dish survey in the LMC shows that the N159W region is the most CO
intense spot (see also Sect. 1), the conical cloud is likely one of the
largest and brightest structures in this galaxy. Based on the spatial
distribution of YSO candidates (Chen et al., 2010), the western part of the
observed field (RA(J2000)$\lesssim$5h39m30s) seems quiescent in high- and
intermediate-mass star formation, although we see a lot filamentary structure
with a strong intensity contrast. The total molecular gas mass of this
quiescent region is $\sim$1.2${\times}$105 $M_{\odot}$. In summary, about half
of the total mass of molecular material is associated with the current high-
mass star formation in this observed field.

Figure 7b highlights the characteristic features presented in Sect. 3.1, and
3.2 to guide the positional relationship with each other on the high-
resolution CO map.

##  4 Discussion

###  4.1 Dynamical status of the characteristic components

This section discusses the physical states of the newly discovered features in
N159W-North with our ALMA observations described in Sect. 3.

####  4.1.1 Filaments traced by 12CO and 13CO

Several filaments are seen extending from the southern edge of the main ridge
(Sect. 3.1.1). They are relatively ordered in the north-south direction and
their roots appear to be spatially attached to the protostellar systems in
MMS-1 and MMS-2. We discuss here the dynamical state (stability) of the
filaments to infer a possible fate. As shown in Sect. 3.1.1, the virial line
mass of filaments is somewhat larger than those of the 12CO luminosity mass.
If we ignore the stabilizing effect by magnetic force, the filaments are not
gravitationally stable objects. In this case, the filaments can be dissipated
within a turbulent crossing time, which is an order of $\sim$105 yr = $W_{\rm
fil}$/$\sigma_{v}$, where $W_{\rm fil}$ is the typical width of filament
($\sim$0.1–0.2 pc). In fact, the current star formation is inactive in all of
the filaments except for their junctions with the millimeter sources. Among
them, filament D has the weakest and shows the discontinuous 12CO distribution
(Figure 3(a3)), suggesting that this filament is the oldest one and is
currently in the process of dissipating.

We discuss possible formation scenarios of the filaments in the N159W-North
region. The filaments spatially connect to the protostellar source, and thus
they are likely related to the star formation activity. Schneider et al.
(2010) found similar 13CO filaments toward the most massive clump in the
Galactic Cygnus X DR21 region. The morphological features and physical
properties, such as mass ($M$ $\sim$2600 $M_{\odot}$) and length ($L$ $\sim$5
pc), are very similar to N159W-North. The authors interpreted this as
filaments falling onto the central ridge due to the global collapse. However,
this interpretation is not necessarily supported by gas kinematic features,
e.g., accelerated motion toward the star-forming main ridge. A robust common
feature of the filaments in the N159W-North regions is that they are
distributed only on one side of the main ridge, suggesting the existence of an
asymmetric gas flow through the main ridges. The warm side nature can be
explained by the past shock wave event in this region (see Sect. 4.1.2), and
thus we propose that an extensive gas flow penetrates the N159W-North main
ridge and promotes the high-mass star formation with remarkable filaments. We
will discuss this possibility further in Sect. 4.2.

####  4.1.2 Warm side

We found that the peak brightness temperature of 12CO at the southeast edge of
the main ridge is remarkably high ($\gtrsim$50 K) compared to the typical
value ($\sim$30 K) of this region, indicating that something is contributing
to the heating. The probable reasons are (case 1) external radiation field and
cosmic ray heating, (case 2a) shock heating in the turbulent material within
the cloud, and/or (case 2b) shock heating due to external gas compression. Lee
et al. (2016) obtained multi-line CO transitions up to $J$ = 12–11 in the
N159W region using Herschel Space Observatory and ground-based telescopes, and
estimated the molecular gas kinematic temperature of $>$100 K and density of
$\sim$103 cm-3 on $\sim$10 pc scale. Their modeling showed that case 1 is
unlikely to produce the warm environment, and instead, suggested that low-
velocity C-type shocks (case 2a and/or 2b) with $\sim$10 km s-1 appear to be
more plausible. Our narrow field coverage and high-resolution observations
highlight the detection region of the rare isotopes of CO, and we are
selectively looking only at denser and colder regions than the Lee et al.
(2016) study. However, the origin of the cloud nature can still be understood
in the same context, because there is no significant external heating source
near the ALMA observed region too. For case 2a, Tokuda et al. (2018) actually
found warm CO (30–60 K) filamentary components in a turbulent low-mass star-
forming dense core in Taurus. However, the physical scale of the gas is quite
small as 1000 au that our present study could not resolve, and shock heating
with internal turbulence is expected to realize a more stochastic temperature
distribution, instead of asymmetric heating region over several pc as seen in
N159W-North. We conclude that mechanical heating due to an external gas
colliding flow (case 2b) is probable.

####  4.1.3 Dense clumps in the main ridge traced by C18O and 1.3 mm and star-
formation activities therein

We found that the molecular mass of the main ridge of N159W-North and the most
massive inside clump exceeds 104 $M_{\odot}$, despite their compactness with a
size of less than a few parsecs. Such a massive clump, which is not frequently
discovered in the MW disk region, agrees with the mass and size requirements
to evolve into a young massive cluster (e.g., Longmore et al., 2014). Table 4
lists several massive clumps studied in the Local Group of galaxies (MW, LMC,
and M33). Note that a large-scale survey of molecular gas has been conducted
at the SMC, but such a massive clump has not been discovered yet (Tokuda et
al., 2021). The mass estimates based on the thermal dust emission and/or
molecular line emission are subject to a factor of two or three uncertainties
due to methodological differences and systematic errors arising from, e.g.,
dust properties and molecular abundance. Therefore, we cannot immediately
conclude that our characterization of the N159W-North main ridge is the most
massive in the Local Group, but it is certainly one of the most massive. The
N113 region also has very massive clumps, and thus it could be one of the top
contenders in the LMC. However, because this source was not detected in the
ATCA NH3 observations (Ott et al., 2010), the dense gas fraction is likely
smaller than that of the N159W-North main ridge.

Table 4: Physical properties of cluster-forming massive clumps in the Local
Group

Clump Name | Hosting Galaxy | Sizea | $M_{\rm H_{2}}$ | $N_{\rm H_{2}}^{\rm peak}$ |  $n_{\rm H_{2}}^{\rm ave}$ b | Beam | Tracers | Referencesc
---|---|---|---|---|---|---|---|---
|  | [ pc] | [M⊙] | [cm-2] | [cm-3] | [pc] |  |
N159W-N main ridged | LMC | 1.1$\times$3.3 |  $\sim$7$\times$104 | $\sim$1$\times$1024 |  $\sim$1.0$\times$105 | $\sim$0.1 |  13CO/C18O/1.3 mm | This work
MMS-2 e |  $\sim$1.5 |  $\sim$2$\times$104 |  $\sim$2$\times$105 | C18O/1.3 mm
N113 Region B | LMC | 1.2$\times$2.7 |  $\sim$2$\times$104 |  $\sim$9$\times$1023 |  $\sim$9$\times$104 |  $\sim$0.2 |  13CO/C18O/1.3 mm | [1]
W43 MM1 | MW | 3.9$\times$2.0 |  $\sim$2$\times$104 |  $\sim$3$\times$1023 |  $\sim$4$\times$104 |  $\sim$0.5 | C18O/1.3 mm | [2,3,4]
W49-N |  $\sim$4 |  $\sim$5$\times$104 |  $\sim$2$\times$1024 |  $\sim$2$\times$104 |  $\sim$0.1 |  13CO/C18O | [5]
W51 G49.5$-$0.4 |  $\sim$2.2 | (1–2)$\times$104 |  $\sim$4$\times$1023 | (3–5)$\times$104 |  $\sim$0.5 | C18O | [6]
Cygnus X DR21 |  $\sim$4$\times$1 |  $\sim$3$\times$104 |  $\sim$5$\times$1023 |  $\sim$105 |  $\sim$0.2 |  13CO/N2H+ | [7,8]
NGC 604 MMS-1 | M33 | 2.9$\times$1.9 |  $\sim$5$\times$104 | $\cdots$ |  $\sim$2$\times$104 |  $\sim$2$\times$1 | C18O/1.3 mm | [9]
GMC-16 MMS | 2.2$\times$1.1 |  $\sim$2$\times$104 | $\cdots$ |  $\sim$2$\times$104 |  $\sim$2$\times$1 | C18O/1.3 mm | [10]

aafootnotetext: Projected length of major and minor axes or size in diameter.

bbfootnotetext: Averaged density assuming the uniform, spherical geometry.

ccfootnotetext: References: [1]We retrieved an ALMA archival data
(2015.1.01388, see also Sewiło et al. 2018) of CO and its isotope lines at 1.3
mm wavelength (Nishioka et al. in prep.) and applied the same mass estimation
method as in N159W-North.; [2]Motte et al. (2003); [3]Louvet et al. (2014);
[4]Kohno et al. (2021); [5]Galván-Madrid et al. (2013); [6]Fujita et al.
(2021); [7]Schneider et al. (2010); [8]Dobashi et al. (2019); [9]Muraoka et
al. (2020); [10]Tokuda et al. (2020)

ddfootnotetext: The parameters are determined by the 13CO intense region as
indicated with the orange contours in Figure 7b.

eefootnotetext: This source is part of the main ridge. The physical properties
are the same as those in Tables 2 and 3.

###  4.2 Colliding flows promoting the protocluster formation in N159W-North

We summarize the features described in Sect. 3 and the possibilities discussed
in Sect. 4.1. The cluster forming N159W-North main ridge resides at the apex
of a conical-shaped structure traced by the CO(1-0) emission (Sect. 3.4). The
presence of several southward filaments from the main ridge, together with the
warm side features, suggests that a north-south penetrating colliding flow may
have triggered the formation of a massive clump.

We compare these features with a molecular cloud collision model. Many, if not
all, of these qualitative characteristics, are explained by the results of
numerical simulation of molecular cloud collision by Inoue et al. (2018). They
simulated that a small cloud collided with a larger cloud with a density of
103 cm-3 at a relative velocity of 10 km s-1 by solving isothermal
magnetohydrodynamic (MHD) equations with self-gravity. Generally, a gas clump
colliding with the larger cloud is decelerated once it interacts or sweeps up
the same amount of the larger cloud gas. Since turbulence creates many dense
clumps in the small cloud, some dense clumps go beyond the arc-like structure.
Figure 8 summarizes such phenomenon schematically, and we call the penetrated
filamentary feature $``$funnel-type flow$"$. In the N159W-North region, the
conical cloud and warm side show the shock compression layer, and the main
ridge with a mass of $\gtrsim$104 $M_{\odot}$ is supposed to be the product of
the colliding event. If the filaments represent funnel-type flows described
above, the feature could also be a piece of indirect evidence of a colliding
flow.

Figure 8: Cloud-cloud collision as a possible formation origin of several
distinct features, such as the conical cloud, dense cores, and filaments, in
the N159W-North region. Panels (a) and (b) schematically illustrate the
initial condition and compressed product after the collision, respectively.
(c) The final time step of the MHD simulation by Inoue et al. (2018).

We caveat the discrepancy between our observations and the simulation by Inoue
et al. (2018). The N159W-North system is heavier and larger than that in the
simulation. Although it is beyond the scope of this paper to perform an
optimized calculation for N159W-North, we expect to see further massive clumps
with multiple funnel-type flows, if we perform a colliding flow simulation
with a higher mass initial condition, because the hydrodynamic phenomenon is
basically scale-free.

We presented the morphological features, temperature distribution, and total
mass of the dense clump as indirect evidence for molecular cloud interaction,
but the velocity analysis enables us to find additional relevant
characteristics (Sect. 3.1.2). Paper I shows that the molecular cloud in the
N159W-South region has V-shaped structures around protostellar objects on PV
diagrams. This feature was originally designed based on numerical simulation
reproducing molecular cloud collision phenomena (Takahira et al. 2014 and the
subsequent synthetic observations in CO emission Torii et al. 2017; Fukui et
al. 2018). In N159W-North, the V-shaped structure is more prominent around
YSO-N2, while the same feature around YSO-N2 is less distinct (see Figure 4)
possibly due to the molecular gas ionization (see Sect. 3.2).

###  4.3 High-mass star formation across the N159E/W region: an updated view
of the synchronized massive cluster formation

We discuss the massive cluster formation throughout the N159E/W regions by
combining current knowledge obtained with molecular and atomic gas
observations. Paper I and Paper II found that the massive protostars in
N159E-Papillon and N159W-South, respectively, are accompanied by filamentary
molecular clouds with their line mass of 300–1000 $M_{\odot}$ pc-1. Although
these two filamentary systems are separated by about 50 pc in the sky, their
physical and morphological features are quite similar, suggesting the
formation of filaments and massive protostars simultaneously on the order of
0.1 Myr. Papers I and II proposed that the trigger for these events could be
the H i colliding flow with a velocity of 50–100 km s-1 from the northern
direction driven by the last close encounter with the Small Magellanic Cloud
$\sim$2 Gyr ago (Fukui et al., 2017; Tsuge et al., 2019).

Since the newly revealed N159W-North system has a very similar head-tailed
filamentary molecular cloud, it is possible that we can interpret its
formation scenario in the same context. Figure 9 shows our updated colliding
flow star formation scenario, $``$teardrops inflow model$"$ in the N159
region. In this scenario, the H i gas flow containing multiple dense spots
interacts with a pre-existing GMC. Locally, this event can be regarded as a
collision between large and small clouds, resulting in the morphological and
dynamical features described in Sect. 4.2. N159E-Papillon and N159W-South are
impacted by relatively low-mass blobs, while a larger one impacts the
N159W-North region, producing a 30 pc scale compression layer. This scenario
potentially explains multiple generations of high-mass star formation of
N159E/W. N159E already started the main star formation activities 3-4 Myr ago
with its numerous O and early B optically-visible stars, except for the above-
mentioned young Papillon region, while N159W just started its main episode
with mostly massive YSOs (Chen et al., 2010). In this line of thought,
assuming that the two regions were in the same pre-existing GMC and only N159E
was actively star-forming earlier, the H i dense blob seems likely to be the
main contributor to forming many massive cores, i.e., the YMC progenitor,
especially in N159W.

Figure 9: Schematic view of the star formation process in N159E/W motivated by
the ALMA studies.

Numerical simulations by Maeda et al. (2021) demonstrated $\sim$104
$M_{\odot}$ massive clump formation driven by a large-scale H i gas flow that
mimics galactic interactions. Abe et al. (2021) showed that fast ($>$10 km
s-1) gas flow is more favorable for massive filament formation with a line
mass of $>$100 $M_{\odot}$ pc-1. From observational perspectives, recent ALMA
studies in M33 also found massive filaments with high-mass cluster formation
associated with galaxy-galaxy interactions and other galaxy-scale gas motions
related to a spiral arm (Tokuda et al., 2020; Muraoka et al., 2020). Galactic
high-resolution, large-scale molecular gas surveys toward active cluster-
forming regions listed in Table 4 exclusively show complex velocity structures
originated from multiple-gas components (Miyawaki et al., 2009, 2021; Dobashi
et al., 2019; Kohno et al., 2021; Fujita et al., 2021). The total mass of
their parent clouds exceeds 106 $M_{\odot}$, which is an order of magnitude
higher than the typical GMC mass, and thus the collision/coalescence of clouds
and/or the large-scale diffuse gas inflow from the surrounding environment
likely contribute to the formation of gravitationally bound massive clumps
that leads to YMC formation.

In the N159E-Papillon and N159W-South regions, the total filamentary gas and
dense gas masses are $\sim$104 $M_{\odot}$ and $\sim$103 $M_{\odot}$,
respectively, suggesting they may be just medium-sized clusters. On the other
hand, in N159W-North, the total gas mass of the large-scale filamentary cloud
(conical cloud) and dense gas (main ridge) exceed 105 $M_{\odot}$ and 104
$M_{\odot}$, respectively, indicating that the system grows into a more
massive cluster. Fukushima & Yajima (2021) numerically demonstrated that a 104
$M_{\odot}$ molecular cloud packed into a few pc volume is converted into a
cluster with a high star formation efficiency because the gravitational
potential is stronger than the negative feedback force of the stars formed.
This case allows forming a massive cluster with a mass of $\gtrsim$104
$M_{\odot}$ in the N159W-North region. Furthermore, since a part of the
conical cloud with a total mass of $\sim$105 $M_{\odot}$ is connected to the
main ridge, it may grow into a massive cluster comparable to R136 in 30 Dor if
the conveyor belt-like gas supply mechanism where gas accretion and star
formation occur simultaneously (Longmore et al., 2014; Krumholz & McKee, 2020)
works in this region.

Although the large-scale H i flow scenario was originally designed to explain
the R136 formation (Fukui et al., 2017), the fact that the same flow
distributes over a kpc scale motivates us to apply it describing the star
formation history in N159. In a reimportation of this context, an ALMA wide-
field imaging study found a N159-type conical-shaped molecular filament system
around R136 (T. Wong et al. submitted). These observational pieces of evidence
may demonstrate that galaxy-scale (H i) gas flows and the subsequent more than
a few $\times$ 10 pc scale filament system development are universally
responsible for the super star cluster formation that can drive galaxy
evolution itself. Since clusters larger than 105 $M_{\odot}$ are only found in
a limited number in the Local Group, further searches for similar filament
systems in more remote galaxies will help to elucidate the super star cluster
formation mechanism in the local universe.

##  5 Conclusions

Our ALMA observations revealed a comprehensive molecular gas view of the
N159W-North region in the LMC with a high-dynamic spatial range from a few
$\times$10 pc to $\lesssim$0.1 pc. The main results and conclusions are
summarized as follows:

  * •

The 12CO and 13CO high-resolution data depicted highly complex spatial and
velocity structures of the N159W-North molecular cloud. The 12CO brightness
temperature is higher than $\sim$30 K throughout the observed region and even
higher than $\sim$50 K, especially in the southern side of the cloud. One of
the outstanding features is that several filaments, whose length and line
masses are a few parsecs and $\gtrsim$100 $M_{\odot}$ pc-1, extend from high-
mass star-forming dense cores toward the southern direction.

  * •

Based on 12CO high-velocity emission, which appears to be of outflow origin,
we found five new bright star-forming cores in 1.3 mm continuum emission at
the infrared quiescent spots. We also identified $\sim$0.1 pc scale starless
cores with a mass of $\sim$100 $M_{\odot}$, which were not discovered in the
previous extragalactic studies. The dense molecular region (main ridge) traced
by thermal dust continuum and C18O harbors the above-mentioned proto- and
prestellar cores. The total mass of the cluster-forming clump exceeds 104
$M_{\odot}$ despite its compact size of a few pc, making it one of the most
massive, dense categories in the Local Group of galaxies.

  * •

The wide-field CO($J$ = 1–0) map deciphered a head-tailed conical shape
feature, whose size scale and total molecular gas mass are $\sim$30 pc and
$\sim$2 $\times$105 $M_{\odot}$, respectively. The strong impacts of a cloud-
cloud collision presumably explain the observed large-scale and small-scale
subfeatures, such as the warm side nature and filaments in the southern side
of the main ridge.

  * •

Combining our current observational understanding of two other regions in
N159, E-Papillon (Paper I) and W-South (Paper II), three systems across more
than 50 pc show active star formation simultaneously. Massive clumps,
especially N159W-North like objects, are highly rare in the Local Group,
indicating that a typical galactic environment cannot easily produce YMC
precursors. We hypothesize $``$teardrops inflow model$"$ to explain the
synchronized, extreme cluster formation, possibly driven by a quite dynamic,
substructured flow induced by a galactic-scale phenomenon.

We would like to thank the anonymous referee for useful comments that improved
the manuscript. This paper makes use of the following ALMA data: ADS/ JAO.
ALMA#2012.1.00554.S, 2016.1.01173.S, and 2019.1.00915.S. ALMA is a partnership
of ESO (representing its member states), NSF (USA) and NINS (Japan), together
with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in
cooperation with the Republic of Chile. The Joint ALMA Observatory is operated
by ESO, AUI/NRAO, and NAOJ. This work was supported by NAOJ ALMA Scientific
Research grant Nos. 2016-03B, Grants-in-Aid for Scientific Research (KAKENHI)
of Japan Society for the Promotion of Science (JSPS; grant Nos. JP18K13582,
JP18H05440, JP21H00049, and JP21K13962), and NSF award 2009624. The National
Radio Astronomy Observatoryis a facility of the National Science Foundation
operatedunder cooperative agreement by Associated Universities,Inc. The
material is based upon work supported by NASA under award number 80GSFC21M0002
(M. S.). Dr. Benoit Neichel kindly provided us the VLT photometric data
(Bernard et al., 2016) to compare spatial distributions of the ALMA millimeter
continuum and infrared sources.

##  Appendix A Data qualities

Table 5 gives the beam sizes and r.m.s. sensitivities of each molecular line
data.

Table 5: Beam Properties and Sensitivities in the molecular line observations

line name |  $B_{\rm maj}$ [arcsec] |  $B_{\rm min}$ [arcsec] |  $B_{\rm P.A.}$ [deg] | r.m.sch [K]a | r.m.si.i. [K km s-1]b
---|---|---|---|---|---
12CO($J$ = 2–1) | 0.27 | 0.23 | 6.1 | 1.3 | 1.1
13CO($J$ = 2–1) | 0.28 | 0.24 | 5.6 | 1.4 | 1.1
C18O($J$ = 2–1) | 0.28 | 0.24 | 4.7 | 1.0 | 0.9

aafootnotetext: Noise level of data cube at a velocity resolution of 0.2 km
s-1.

bbfootnotetext: Noise level of integrated intensity image over a velocity
range of 5 km s-1, which roughly corresponds to the typical line width of the
N159W-North cloud.

##  Appendix B High-resolution CO channel maps

Figures 10 and 11 show the 12CO($J$ = 2–1) and 13CO($J$ = 2–1) channel maps,
respectively, with a velocity bin of 2 km s-1.

Figure 10: Velocity channel maps of N159W-North in 12CO($J$ = 2–1). The lowest
velocity in unit of km s-1 of each panel are given in the upper right corners.
The white ellipse in the lower left corner in the lower left panel shows the
beam size, 0$\farcs$29 $\times$ 0$\farcs$23, of the 12CO data. Red dotted
lines show the field coverage. The white contours show the 1.3 mm continuum
emission, same as Figure 2.  Figure 11: Same as Figure 10, but for 13CO($J$ =
2–1).

##  Appendix C Outflow spectra and properties

Figure 12 shows 12CO spectra with high-velocity wing emission originated from
protostellar sources in MMSs. Table 6 lists the outflow properties.

Figure 12: 12CO spectra with high-velocity wing components toward millimeter
continuum sources in N159W-North. Red and blue profiles show the 12CO
redshifted and blueshifted components, respectively, averaged within the red
and blue contours in Figure 5b–f. The red and blue vertical lines indicate the
velocity ranges of high-velocity emission (see also Table 6). Gray profiles
show 13CO($J$ = 2–1) spectra at the MMS peaks. Gray dotted vertical lines
represent the central velocity determined by Gaussian fitting the 13CO spectra
with a single Gaussian profile.  Table 6: Outflow properties associated with
millimeter sources (MMSs) in N159W-North

Source | Blue lobe |  | Red lobe
---|---|---|---
Name |  $L_{\rm CO}$ [K km s-1 pc2]a |  $v_{\rm max}$ [km s-1]b |  $v_{\rm range}$ [km s-1] |  |  $L_{\rm CO}$ [K km s-1 pc2]a |  $v_{\rm max}$ [km s-1]b |  $v_{\rm range}$ [km s-1]
MMS-1 | 2.87 |  $>$24.8 |  $<$210–231 |  | $\cdots$ | $\cdots$ | $\cdots$
MMS-2a | 0.57 | 15.8 | 218–225 |  | 0.47 | 18.0 | 242–252
MMS-2b | 0.27 | 18.0 | 217–228 |  | $\cdots$ | $\cdots$ | $\cdots$
MMS-2d | 0.05 | 12.8 | 223–229 |  | 0.70 | 14.2 | 242–250
MMS-3 | 0.21 | 14.6 | 224-234 |  | 0.48 | 18.4 | 247-257
MMS-4 | 0.04 | 12.8 | 223–230 |  | 0.27 | 16.2 | 243–252
MMS-5 | 0.23 | 11.8 | 223–230 |  | 0.20 | 21.2 | 243–253

aafootnotetext: Total CO($J$ = 2–1) luminosity of outflow lobes above
$\sim$3$\sigma$ detection.

bbfootnotetext: Maximum radial velocity of outflow lobes with respect to the
systemic velocity determined with the 13CO(2–1) spectra.

##  Appendix D Large-scale CO channel maps

Figure 13 illustrates the velocity channel maps of 12CO($J$ = 1–0) across the
N159W region.

Figure 13: Velocity channel maps of N159W in 12CO($J$ = 1–0). The lowest
velocity in unit of km s-1 of each panel are given in the upper right corners.
The white ellipse in the lower left corner in the lower left panel shows the
beam size, 2$\farcs$2 $\times$ 1$\farcs$8, of the 12CO data. Red dotted lines
show the field coverage.

##  Appendix E C18O dense clumps traced in the N159E-Papillon and W-South
regions

We compare the C18O distribution in N159W-North and that in the two dense
clumps in N159E-Papillon (Figure 14a) and N159W-South (Figure 14b). Both C18O
emitting regions are elongated in the direction along the major axis of the
filamentary structures traced in 12CO and 13CO (Figure 1, see also Papers I
and II). The emitting region reaches a high-density of $\sim$106 cm-3 at
maximum in the N159W-South region (Paper I). However, the peak integrated
intensities are $\sim$1–2 K which are significantly lower than those in
N159W-North. The currently available ALMA surveys (Fukui et al. 2015; Saigo et
al. 2017; Papers I and II) detected C18O emission only in these three spots,
N159W-North/South, and E-Papillon, across the N159E/W region, indicating that
the C18O detection itself is highly rare in the LMC (see also the 30 Dor case
in Figure 7 of Indebetouw et al. 2013). Among all known C18O emitters, the
N159W-North clump is the strongest one in the LMC.

Figure 14: C18O and 1.3 mm continuum emission toward N159E-Papillon and
N159W-South regions (Papers I; II). (a) The color-scale image shows the C18O
velocity-integrated intensity map of N159E-Papillon with an angular resolution
of 0$\farcs$6 shown in the lower left corner. The white contours show the 1.3
mm continuum emission. The lowest and subsequent contour steps are 0.15 mJy
beam-1. Note that we coordinate to the intensity scale with that in Figure 6a
for the comparison purpose. (b) Same as panel (a), but for N159W-South.

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