Towards a comprehensive understanding of molecular cloud life cycle
based on HI observations with the ngVLA
Masato I.N. Kobayashi, 1 Kazuki Tokuda, 2,3
1

2

Astronomical Institute, Graduate School of Science, Tohoku University, Aoba, Sendai, Miyagi 980–8578, Japan
Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1–1 Gakuen–cho, Naka–ku, Sakai,
Osaka 599–8531, Japan
3
National Astoronomical Observatory of Japan, 2–21–1 Osawa, Mitaka-shi, Tokyo 181–8588, Japan
mskobayashi@astr.tohoku.ac.jp, masato.kobayashi@nagoya-u.jp

Abstract
Molecular cloud formation and evolution is a key initial condition of galactic star formation. The understandingof star formation and subsequent galaxy evolution needs to reveal how the galactic environments and the driving
mechanisms of cloud life cycle govern the molecular cloud properties and star formation processes. Recent observational and theoretical studies suggest that the inner structures of molecular clouds (e.g., supersonic turbulence,
filaments and hubs) are the imprints of formation conditions/mechanisms of molecular clouds. In particular, themultiphase structures of molecular clouds and their precursor, HI gas, are important to create such turbulence, filaments, and hubs, where HI gas is often the predominant gas reservoir for molecular cloud formation in massive
spiral galaxies like the Milky Way. High-resolution and high-sensitivity investigations of HI gas make significant
progress to our understanding of the origin and lifetime of molecular clouds. In this memo, we briefly summarize
the importance of HI gas observations, review recent observational and theoretical progress of molecular cloud formation, and describe our expectations to the next generation Very Large Array (ngVLA) to investigate the density
and turbulence structures of HI gas across various galactic environments.
Key words: ISM: clouds — ISM: structure — ISM: kinematics and dynamics
1. Introduction
Star formation is one of the central phenomena driving thethermal, chemical, and kinematical evolution of galaxies. In
this galactic context, star formation involves various physicalprocesses on a wide dynamic range from tens of kpc down to
sub-pc scales; e.g., gas accretion and outflow between halos
and galactic disks (Tumlinson et al., 2017), condensation fromthe diffuse interstellar medium (ISM) into molecular clouds(Baba et al., 2017), the densest part of clouds collapse to formstars (Larson, 1969) etc.. Since molecular clouds are on thesize of a few pc scales and are the formation sites of stars(Kennicutt & Evans, 2012), revealing the formation and evolution of molecular clouds bridge the galaxy evolution (>∼1Gyr) and the individual star formation (<
10
Myr).
∼
The interstellar medium (ISM) has the multiphase structure;e.g., warm neutral medium (WNM), thermally unstable neutral medium (UNM) cold neutral medium (CNM), and molecular gas. The thermal instability plays a role controlling thesephase balances (Field, 1965; Wolfire et al., 1995). The phasetransitions from the WNM to UNM/CNM is believed to be
triggered by supersonic shocks by supernovae (e.g., McKee &Ostriker, 1977), superbubbles (e.g., McCray & Snow, 1979),
galactic spirals (e.g., Kim, Kim, & Ostriker, 2020), and galaxy
mergers (e.g., Heitsch et al., 2006). To understand how starformation appears along with galaxy evolution, it is importantto reveal how the development of these structures in the shockISM interacting systems depend on the galactic environments,and how they control the inner structures of molecular clouds.
Radio and infrared observations have shown the various in-ner properties/structures of molecular clouds in detail, suchas a supersonic turbulence (Heyer & Dame, 2015), filamentary structures (André et al., 2010), and hub structures connecting filaments (Kumar et al., 2020). Meanwhile, recent
numerical simulations suggest that these rich structures are
the imprints of formation conditions/mechanisms of molecular clouds. For example, the supersonic motion with respect tomolecular gas is subsonic with respect to the WNM. Therefore,
turbulent WNM in the shock downstream maintain the supersonic motion of molecular gas (Koyama & Inutsuka, 2002).
A shock-compressed molecular clouds tend to host filamentary structures (e.g., Inoue & Inutsuka, 2012; Iwasaki et al.,2019) and magnetized cloud-cloud collisions induce the formation of massive filaments through shock and magnetic fieldinteractions (Inoue et al., 2018), whose filament structure anddynamics resemble the massive star formation at the hub pointof collapsing filaments (Peretto et al., 2013). These simulations and their synthetic observations suggest that the molecular clouds are not purely molecule but essentially the mixtureof the WNM, UNM, CNM, and dense molecular gas (Valdivia
et al., 2016; Tachihara et al., 2018b). Therefore, in addition
to ordinary molecular line surveys to probe molecular gas, observations of WNM and CNM will be crucial to understandthe thermal, dynamical, and chemical evolution of molecular
clouds.
HI gas is often the predominant gas reservoir for molecular cloud formation in massive spiral galaxies like the MilkyWay. Recent observations also give some hints of the rapid
formation of molecular clouds and stars due to HI gas accretion and collision, in the outskirt of the Milky Way disk and2[Vol. ,

Fig. 1. Schematic figure of three-dimensional converging flow simulations (revised from Kobayashi et al. (2020)). Supersonic flows arecontinuously injected from the left and right boundaries. The injectflow can be purely WNM (e.g., Hennebelle et al., 2008) or the alreadymixture of WNM and CNM (e.g. Inoue & Inutsuka, 2012). The turbulent shock-compressed layer at the center is sandwiched by two shock
fronts (black curled arrows in blue solid curves). The thermal instability develops and CNM and molecular gas form within the layer (bluepoints).

Large/Small Magellanic Clouds (Izumi et al., 2014; Fukui etal., 2017). Although the filamentary structures seem to commonly have a typical width of 0.1 pc, the line-mass has an order of magnitude variety (André et al., 2019), and the probability distribution function of the column density in star forming
regions also does (Schneider et al., 2016), it is crucial to understand the connection between the structure of WNM, CNM,
molecular gas and the variety of filamentary structures.
Therefore, the comprehensive understanding of molecular
cloud life cycle across various galactic environments requires
the understanding of the background HI gas reservoir together
with molecular clouds. The volume of HI is dominated by
WNM and UNM than by CNM (e.g., Fukui et al., 2018;
Kobayashi et al., 2020). However, current observations of suchdetailed analysis is mostly limited to the emission-absorption
feature in the line of sight against radio continuum sources,
which is more sensitive to CNM (e.g., Arecibo; Heiles &Troland, 2003a,b). Therefore, it is promising that the capabilityof the next generation Very Large Array (ngVLA) advance ourunderstanding of such HI structures by covering all the phasesof HI gas.
In the following sections, we will review the results fromrecent numerical simulations (by mainly introducing our simulations (Kobayashi et al., 2020)), and express the importance/expectations to investigate the properties of HI gas inboth microscopic and macroscopic viewpoints.
2. Brief review of converging flow simulations
Many previous studies have investigated the dynamical
condensation through the thermal instability in a shockcompressed layer. This layer is formed by supersonic converging flows, which is a technical analog to easily calculate thethermal instability in the postshock rest frame (Hennebelle &Pérault, 1999; Koyama & Inutsuka, 2000; Heitsch et al., 2006;
Vázquez-Semadeni, 2006; Inoue & Inutsuka, 2008). Figure 1
shows a schematic figure of such converging flow simulations. Two supersonic flows collide with each other at the center of the computational domain and form a turbulent shockcompressed layer. The thermal instability creates denser struc-tures (CNM and molecular clumps) through the radiative cooling, which is mostly controlled by the Lyα and CII lines in the
Solar metallicity environment. The turbulence here is naturally
developed either by the interaction between shocks and density inhomogeneity (Richtmyer, 1960), by the thermal instability, or by the thermal conduction (Koyama & Inutsuka, 2004;
Iwasaki & Inutsuka, 2014). Although there are variety in the
choice of initial conditions (e.g., the density and velocity structure of the injected flow), these series of converging flow simulations suggest that molecular clouds have multiphase structure
within them; WNM, UNM, CNM, and molecular gas (Valdivia
et al., 2016; Tachihara et al., 2018b). The WNM and UNM
govern the thermal energy and the overall dynamics whereas
the CNM and molecular gas dominate the mass and turbulent
energy. Therefore, the comprehensive understanding of molecular cloud life cycle requires to probe multiphase ISM coherently from WNM to molecular gas, which is currently often
limited to molecular lines. The sensitivity and angular resolution capability of the ngVLA Band 1 provides a great opportunity to coherently probe all the HI phases and to advance ourunderstanding of molecular cloud life cycle in the Milky Way
and nearby galaxies.
To prepare such ngVLA observations, numerical simulationsof the multiphase ISM formation and evolution should reveal
how the galactic environments impact the molecular cloud formation and how they set the initial condition of star formation. Converging flow simulations are promising in this context by filling the spatial gap between galaxy evolution andindividual star formation. In particular, the latest simulationsof galaxy evolution start to achieve the spatial resolution of< 10 pc (e.g., Baba et al., 2017) and converging-flow simula∼tions on 10 pc scales can be utilized to provide ISM sub-gridmodels for large-scale simulations. However, the authors on
the previous converging-flow simulations employ different implementations of perturbation that initiates the thermal instability, and the perturbation amplitudes also differ between theauthors accordingly. For example, a fluctuation is introducedin the upstream flow density (e.g., Koyama & Inutsuka, 2002;
Inoue & Inutsuka, 2012; Carroll-Nellenback et al., 2014), velocity (e.g., Audit & Hennebelle, 2005; Vázquez-Semadeni,2006), or a sinusoidal interface where two flows initially contact (e.g., Heitsch et al., 2005). Therefore, it is a remaining
task for converging-flow simulations to reveal the spatial resolution required for a galactic subgrid-model, and to reveal how
detailed conditions of the flow impact cloud properties, undera fixed method of perturbation seeding.
3. Microscopic Viewpoint: Density structure of HI gas
As such a first step to answer the above issue, we here introduce Kobayashi et al. (2020) where we perform converging
flow simulations over 3 Myr in the computational domain of
Lx,y,z = 20,10,10 pc. We introduce an upstream density inhomogeneity ∆ρ0 as a perturbation seed, which is motivated by
the fact that density inhomogeneity exists essentially on all spatial scales in the diffuse ISM (Armstrong et al., 1995), throughwhich supersonic flows are naturally expected to expand (Inoueet al., 2012; Kim & Ostriker, 2015). While the inhomogeneity has a fixed power spectrum of the Kolmogorov spectrum

No. ]

Fig. 2. The density slice from converging-flow simulations of
Kobayashi et al. (2020) with ∆ρ0 = 31.6% at 2.2 Myr, where the color
represents log(n[cm−3 ]). The horizontal and vertical axes are in theunit of pc. The thin black lines show the shock front positions.

(Kolmogorov, 1941), we systematically vary
√ the amplitude of
the upstream density fluctuation ∆ρ0 = ⟨δρ20 ⟩/ρ0 and thespatial resolution ∆x to reveal the dependence of the physical properties of the multiphase ISM on those conditions. We
opt to employ ρ0 /(µM mp ) = 0.57 cm−3 to calculate the earlyphase of molecular cloud formation by purely WNM flows,
where µM is the mean molecular weight and mp is the proton mass. We refer the readers to see Kobayashi et al. (2020)
for further information of the calculation schemes and settings.
Figure 2 shows an example of the density slice, in which the
turbulent multiphase structure is developed on sub-pc scales
throughout the entire shock-compressed layer. We found thatthe shock-compressed layer shows a bimodal behavior depending on ∆ρ0 . When ∆ρ0 > 10 %, the interaction between shocks
and density inhomogeneity deforms the shock fronts, leading
to a strong driving of the post-shock turbulence of > 3 km
s−1 . This strong turbulence dominates the energy budget ofthe shock-compressed layer and prevents the dynamical condensation by cooling and the following CNM formation. The
CNM mass fraction remains as ∼ 45 % in 3 Myr. In contrast,
when ∆ρ0 ≤ 10 %, the shock fronts maintain an almost straightgeometry. The CNM formation efficiently proceeds and resultin the CNM mass fraction of ∼ 70 %. The velocity dispersion
is limited to the thermal-instability mediated level of ∼ 2–3 km
s−1 . The shock-compressed layer is supported by both turbulent and thermal energy equally. We show that the level turbulence when ∆ρ0 > 10 % is as strong as the one expectedfrom the interaction between shocks and density inhomogeneity (e.g., the growth velocity of the Richtmyer–Meshkov instability; Richtmyer (1960); Inoue et al. (2013)). ∆ρ0 = 10 % is,therefore, the characteristic level of inhomogeneity at which
the speed of the dynamically condensing motion driven by thethermal instability (i.e., the sound speed of the UNM) is equal
to the growth velocity of the shock-density inhomogeneity interaction. Given that the density inhomogeneity in the ISM
typically ranges over many orders of magnitude (Armstrong etal., 1995, ; i.e., ∆ρ0 > 10 %), our results essentially suggestthat most of the observed supersonic turbulence in molecular
clouds may originate in the shock-density inhomogeneity interaction, which prevents the dynamical condensation purely
by the thermal instability and keep the CNM mass fraction less3
than 50 %.
To verify this formation mechanism of molecular clouds, it is
crucial to observe 1) the probability distribution of density (orcolumn density) of the pre-shock HI gas accreting onto molecular clouds, and 2) the mass fraction and velocity dispersion of
HI gas within molecular clouds. Our simulation results from
Kobayashi et al. (2020) indicate that the requirement of the spatial resolution and velocity is, ideally, 0.02 pc and 1.0 km s−1 ,with which one cane probe the clumpy density structure (e.g.,shown in Figure 2) and the level of turbulence.
Many previous surveys have conducted HI emission mapping by single dish and interferometer telescopes (e.g.,Kalberla et al., 2005), and observational attempts has recently
started to separate the multiphase structures into the WNM,
UNM, and CNM (Kalberla & Haud, 2018). The ngVLA Band
1 capability of its high angular/spectral resolution and high sensitivity will enable us to access such detailed structure of HI
gas inside/outside molecular clouds, mostly towards low-massclouds in the Solar neighborhood (André et al., 2010). In thecase of the Aquila rift cloud at the distance of 260 pc, as an
example, the Herschel Gould Belt Survey reveal filamentary
stuctures that host prestellar cores. To reveal the HI gas strcture
surrounding these filaments, the ngVLA Band 1 is able to mosaic the main filament region (2 pc × 2 pc fields) in 40 hours,
where each single pointing costs ∼ 20 sec with the 7.7′′ resolution (∼ 0.02 pc) over 1 km s−1 line width to achieve the rms
noise of 13 K. Besides such an emission observation, the absorption of HI combined with CO observations (Band 6) would
also play an important role to reveal dark molecular clouds insuch star-forming regions (e.g., Zuo et al., 2018), becuase thefraction of CNM in the pre-shock HI gas may alter the degreeof turbulence driven in molecular clouds.
4. Macroscopic Viewpoint: Bulk properties of HI gas
5
Among molecular clouds, massive molecular clouds >∼ 10
M⊙ host most of massive stars, imposing an important initial condition of galactic star formation. The population ofsuch massive clouds is also important to observationally estimate the typical cloud lifetime (Koda et al., 2009; Meidt etal., 2015; Kobayashi et al., 2017, 2018). Massive molecular
clumps are often observed in high-redshift star-forming galaxies as well (Tadaki et al., 2018).
The formation of massive molecular clouds require drastic
mass accretion, which cannot be easily achieved by a simple
extension of converging flows. For example, it requires thatflows continue more than 10 Myr with the gas density of atleast a few cm−3 and with the speed of more than 10 km s−1(Heitsch et al., 2006), which is difficult by a single supernova
and likely need the aid of large-scale flow (e.g., galactic spirals(Kim, Kim, & Ostriker (2020)), HI gas accretion from galactic
halos and galaxy mergers). In addition, extremely massive ones
> 106 M⊙ are likely remnant clouds that luckily survive from∼feedback processes such as stellar feedback and galactic shear
motion (Herrera et al., 2020).
There are already observational and theoretical investigations suggesting the importance of such a drastic accumulationof HI gas. For example, a massive molecular cloud complex
exists at the intersection of a spiral arm and the galactic bar

4(W43; Kohno et al., 2020), which is surrounded by ambient
HI gas (Nguyen Luong et al., 2011). Tidal interaction of the
Magellanic clouds (Tsuge et al., 2020) induce massive compact
clouds who host a young massive star cluster (Maeda, Inoue,& Fukui, 2021).
The Atacama Large Millimeter/Submillimeter Array(ALMA) started to reveal the inner structure of massive
molecular clouds in relatively nearby galaxies (e.g., Galaxy
M33; Muraoka et al. (2020); Tokuda et al. (2019), Kondo etal. submitted). With complementary surveys of HI gas, theysuggest that those massive molecular clouds are formed bylarge-scale HI accumulation such as galactic spiral shocks, HI
gas accretion (tidal interaction) from Galaxy M31 (Tachihara
et al., 2018). The level of star formation activity differs
between massive clouds, where the property of accreting HI
gas may be responsible for such variety in star formation.
One way to investigate the dependence of star formation
activity on the HI gas property is to observe the detailed HI
gas structure associated with massive clouds in the Milky Way
galaxy, similar to low-mass clouds but with limited pointings.
In the case of W43, one of the most massive molecular cloudcomplexes in the Milky Way galaxy (∼ 107 M⊙ ), it is locatedat 5.5 kpc and surrounded by diffuse HI gas (Kohno et al.,2020; Nguyen Luong et al., 2011). Albeit a single pointing
with the maximum recoverable scale of 5.3 pc, the ngVLA
Band 1 towards W43 will achieve the rms noise ∼ 40 K in asingle field with 48 hours integration with the 0.36′′ resolution(∼ 0.02 pc) over 2 km s−1 line width. Such distance massive
clouds may suffer from the line-of-sight contamination, but
the kinematic distance estimation based on CO observations
in Band 6 would mitigate this contamination (Roman-Duval etal., 2009) where CO is spatially correlated more with CNM
than with WNM (Tachihara et al., 2018). Another possibility
is targeting at clouds less-massive than W43 but with the lessline-of-sight contamination expected (e.g., Maddalena’s cloud,Cygnus OB7).
An alternative way is to investigate star-forming regionsin the outer galaxy where HI gas accreting from the galactic
halo directly trigger ongoing star formation with a <1 Myrtimescale (Izumi et al., 2014). Such investigations in the outergalaxy can be also a hint to understand the cold accretion at
high-redsfhit galaxies (Dekel & Birnboim, 2006). In the caseof The Digel Cloud 1 at the distance of ∼ 16 kpc, albeit a single pointing with the maximum recoverable scale of 150 pc,
the ngVLA Band 1 towards the Digel Cloud 1 will achieve therms noise becomes about 13 K in a single field with 48 hours
integration with the 0.6′′ resolution (∼ 0.1 pc) over 2 km s−1line width.
Although the spatial resolution is limited, the ngVLA is alsoable to mosaic HI gas in nearby galaxies. For example, it covers the central region of GMC-8 in Galaxy M33 (50 pc × 50
pc fields) in 75 hours, where each single pointing costs ∼ 3hours with the 1.2′′ resolution (∼ 10 pc) over 3 km s−1 line
width to achieve the rms noise of 14 K. The ngVLA is, therefore, promising to reveal the formation mechanism of massive
clouds by such a combination of high-resolution observations
within the Milky Way and low-resolution observations of extragalactic HI.

[Vol. ,
5. Summary
The ngVLA becomes a complementary observatory of
ALMA to observe the northern sky. Its capability of highangular/spectral resolution is promising to probe the innerdensity/velocity structure of HI gas outside/inside molecular
clouds with a ∼ 0.02 pc resolution in the solar neighborhood and with a ∼ 10 pc resolution towards nearby galaxies.
Complementary surveys in HI absorption and in CO emission
will be a powerful method to obtain further information of the
HI gas phases comprehensively from the WNM, UNM, andCNM, which will be the base of the understanding of molecular cloud life cycle including the origin of massive molecular
clouds.

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