Recent radio observations show that giant molecular cloud (GMC) mass functions noticeably vary across galactic disks. High-resolution magnetohydrodynamics simulations show that multiple episodes of ...compression are required for creating a molecular cloud in the magnetized interstellar medium. In this article, we formulate the evolution equation for the GMC mass function to reproduce the observed profiles, for which multiple compressions are driven by a network of expanding shells due to H ii regions and supernova remnants. We introduce the cloud-cloud collision (CCC) terms in the evolution equation in contrast to previous work (Inutsuka et al.). The computed time evolution suggests that the GMC mass function slope is governed by the ratio of GMC formation timescale to its dispersal timescale, and that the CCC effect is limited only in the massive end of the mass function. In addition, we identify a gas resurrection channel that allows the gas dispersed by massive stars to regenerate GMC populations or to accrete onto pre-existing GMCs. Our results show that almost all of the dispersed gas contributes to the mass growth of pre-existing GMCs in arm regions whereas less than 60% contributes in inter-arm regions. Our results also predict that GMC mass functions have a single power-law exponent in the mass range <105.5 (where represents the solar mass), which is well characterized by GMC self-growth and dispersal timescales. Measurement of the GMC mass function slope provides a powerful method to constrain those GMC timescales and the gas resurrecting factor in various environments across galactic disks.
Abstract
Recent millimeter/submillimeter observations towards nearby galaxies have started to map the whole disk and to identify giant molecular clouds (GMCs) even in the regions between galactic ...spiral structures. Observed variations of GMC mass functions in different galactic environments indicates that massive GMCs preferentially reside along galactic spiral structures whereas inter-arm regions have many small GMCs. Based on the phase transition dynamics from magnetized warm neutral medium to molecular clouds, Kobayashi et al. (2017, ApJ, 836, 175) proposes a semi-analytical evolutionary description for GMC mass functions including a cloud–cloud collision (CCC) process. Their results show that CCC is less dominant in shaping the mass function of GMCs than the accretion of dense H i gas driven by the propagation of supersonic shock waves. However, their formulation does not take into account the possible enhancement of star formation by CCC. Millimeter/submillimeter observations within the Milky Way indicate the importance of CCC in the formation of star clusters and massive stars. In this article, we reformulate the time-evolution equation largely modified from Kobayashi et al. (2017, ApJ, 836, 175) so that we additionally compute star formation subsequently taking place in CCC clouds. Our results suggest that, although CCC events between smaller clouds are more frequent than the ones between massive GMCs, CCC-driven star formation is mostly driven by massive GMCs $\gtrsim 10^{5.5}\,M_{\odot }$ (where M⊙ is the solar mass). The resultant cumulative CCC-driven star formation may amount to a few 10 percent of the total star formation in the Milky Way and nearby galaxies.
Abstract
Supersonic flows in the interstellar medium (ISM) are believed to be a key driver of the molecular cloud formation and evolution. Among molecular clouds’ properties, the ratio between the ...solenoidal and compressive modes of turbulence plays important roles in determining the star formation efficiency. We use numerical simulations of supersonic converging flows of the warm neutral medium (WNM) resolving the thermal instability to calculate the early phase of molecular cloud formation, and we investigate the turbulence structure and the density probability distribution function (density PDF) of the multiphase ISM. We find that both the solenoidal and compressive modes have their power spectrum similar to the Kolmogorov spectrum. The solenoidal (compressive) modes account for ≳80% (≲20%) of the total turbulence power. When we consider both the cold neutral medium (CNM) and the thermally unstable neutral medium (UNM) up to
T
≲ 400 K, the density PDF follows the lognormal distribution, whose width
σ
s
is well explained by the known relation from the isothermal turbulence as
σ
s
=
ln
(
1
+
b
2
2
)
(where
b
is the parameter representing the turbulence mode ratio and
is the turbulent Mach number). The density PDF of the CNM component alone (
T
≤ 50 K), however, exhibits a narrower
σ
s
by a factor of ∼2. These results suggest that observational estimations of
b
based on the CNM density PDF requires the internal turbulence within each CNM clump but not the interclump relative velocity, the latter of which is instead powered by the WNM/UNM turbulence.
Abstract
The formation of molecular clouds out of H
i
gas is the first step toward star formation. Its metallicity dependence plays a key role in determining star formation throughout cosmic history. ...Previous theoretical studies with detailed chemical networks calculate thermal equilibrium states and/or thermal evolution under one-zone collapsing background. The molecular cloud formation in reality, however, involves supersonic flows, and thus resolving the cloud internal turbulence/density structure in three dimensions is still essential. We here perform magnetohydrodynamics simulations of 20 km s
−1
converging flows of warm neutral medium (WNM) with 1
μ
G mean magnetic field in the metallicity range from the solar (1.0
Z
⊙
) to 0.2
Z
⊙
environment. The cold neutral medium (CNM) clumps form faster with higher metallicity due to more efficient cooling. Meanwhile, their mass functions commonly follow
dn
/
dm
∝
m
−
1.7
at three cooling times regardless of the metallicity. Their total turbulence power also commonly shows the Kolmogorov spectrum with its 80% in the solenoidal mode, while the CNM volume alone indicates the transition toward Larson’s law. These similarities measured at the same time in units of the cooling time suggest that the molecular cloud formation directly from the WNM alone requires a longer physical time in a lower-metallicity environment in the 1.0–0.2
Z
⊙
range. To explain the rapid formation of molecular clouds and subsequent massive star formation possibly within ≲10 Myr as observed in the Large/Small Magellanic Clouds, the H
i
gas already contains CNM volume instead of pure WNM.
Abstract
We present Atacama Large Millimeter/submillimeter Array (ALMA) imaging of molecular gas across the full star-forming disk of the barred spiral galaxy M83 in CO(
J
= 1–0). We jointly ...deconvolve the data from ALMA’s 12 m, 7 m, and Total Power arrays using the MIRIAD package. The data have a mass sensitivity and resolution of 10
4
M
⊙
(3
σ
) and 40 pc—sufficient to detect and resolve a typical molecular cloud in the Milky Way with a mass and diameter of 4 × 10
5
M
⊙
and 40 pc, respectively. The full disk coverage shows that the characteristics of molecular gas change radially from the center to outer disk, with the locally measured brightness temperature, velocity dispersion, and integrated intensity (surface density) decreasing outward. The molecular gas distribution shows coherent large-scale structures in the inner part, including the central concentration, offset ridges along the bar, and prominent molecular spiral arms. However, while the arms are still present in the outer disk, they appear less spatially coherent, and even flocculent. Massive filamentary gas concentrations are abundant even in the interarm regions. Building up these structures in the interarm regions would require a very long time (≳100 Myr). Instead, they must have formed within stellar spiral arms and been released into the interarm regions. For such structures to survive through the dynamical processes, the lifetimes of these structures and their constituent molecules and molecular clouds must be long (≳100 Myr). These interarm structures host little or no star formation traced by H
α
. The new map also shows extended CO emission, which likely represents an ensemble of unresolved molecular clouds.
We systematically perform hydrodynamics simulations of 20 converging flows of the warm neutral medium (WNM) to calculate the formation of the cold neutral medium (CNM), focusing especially on the ...mean properties of the multiphase interstellar medium (ISM), such as the mean density on a 10 pc scale. Our results show that convergence in those mean properties requires a 0.02 pc spatial resolution that resolves the cooling length of the thermally unstable neutral medium (UNM) to follow the dynamical condensation from the WNM to the CNM. We also find that two distinct postshock states appear in the mean properties depending on the amplitude of the upstream WNM density fluctuation ( ). When , the interaction between shocks and density inhomogeneity leads to a strong driving of the postshock turbulence of >3 km s−1, which dominates the energy budget in the shock-compressed layer. The turbulence prevents dynamical condensation by cooling, and the CNM mass fraction remains at ∼45%. In contrast, when , the CNM formation proceeds efficiently, resulting in the CNM mass fraction of ∼70%. The velocity dispersion is limited to the thermal-instability-mediated level of ∼2-3 km s−1, and the layer is supported by both turbulent and thermal energy equally. We also propose an effective equation of state that models the multiphase ISM formed by the WNM converging flow as a one-phase ISM in the form of P ∝ γeff, where γeff varies from 0.9 (for a large pre-shock Δ 0) to 0.7 (for a small pre-shock Δ 0).
Abstract
The tip of the tidal tail, resulting from an encounter between galaxies, features gas concentrations and some star-forming regions, such as tidal dwarf galaxies (TDGs). This region provides ...a unique laboratory for examining the star formation process in a dynamical environment distinct from that of disk galaxies. Using the Nobeyama 45 m telescope, we conducted
12
CO(1−0) position-switching observations at the tips of the southern tidal tail in the Antennae galaxies. We detected CO emission not only from the two star-forming TDG candidates but also in regions with no significant star formation. Adopting a Galactic CO-to-H
2
conversion factor without helium correction, the H
2
gas surface density is ∼5–12
M
⊙
pc
−2
. In most regions, the molecular-to-atomic gas ratio is around unity (0.6–1.9), but we find a region with a high ratio with a 3
σ
lower limit of >7.2. The star formation efficiency (SFE) of molecular gas is notably low (<0.15 Gyr
−1
), indicating less active star formation than in both nearby disk galaxies (∼0.5–1.0 Gyr
−1
) and other TDGs previously observed. Including previous observations, the molecular gas SFEs vary widely among TDGs/tidal tails, from 10
−2
to 10 Gyr
−1
, demonstrating significant variations in star formation activity. Potential factors contributing to the low SFE in the Antennae tail tips include extensive tides and/or the young age of the tail.
ABSTRACT
We studied the H2 column density probability distribution function (N-PDF) based on molecular emission lines using the Nobeyama 45-m Cygnus X CO survey data. Using the DENDROGRAM and SCIMES ...algorithms, we identified 124 molecular clouds in the 13CO data. From these identified molecular clouds, an N-PDF was constructed for 11 molecular clouds with an extent of more than 0.4 deg2. From the fitting of the N-PDF, we found that the N-PDF could be well fitted with one or two lognormal distributions. These fitting results provided an alternative density structure for molecular clouds from a conventional picture. We investigated the column density, dense molecular cloud cores, and radio continuum source distributions in each cloud and found that the N-PDF shape was less correlated with the star-forming activity over a whole cloud. Furthermore, we found that the lognormal N-PDF parameters obtained from the fitting showed two impressive features. First, the lognormal distribution at the low-density part had the same mean column density (∼1021.5 cm−2) for almost all the molecular clouds. Second, the width of the lognormal distribution tended to decrease with an increasing mean density of the structures. These correlations suggest that the shape of the N-PDF reflects the relationship between the density and turbulent structure of the whole molecular cloud but is less affected by star-forming activities.
Abstract
Cylindrical molecular filaments are observed to be the main sites of Sunlike star formation, while massive stars form in dense hubs at the junction of multiple filaments. The role of ...hub–filament configurations has not been discussed yet in relation to the birth environment of the solar system and to infer the origin of isotopic ratios of short-lived radionuclides (SLR, such as
26
Al) of calcium–aluminum-rich inclusions (CAIs) observed in meteorites. In this work, we present simple analytical estimates of the impact of stellar feedback on the young solar system forming along a filament of a hub–filament system. We find that the host filament can shield the young solar system from stellar feedback, both during the formation and evolution of stars (stellar outflow, wind, and radiation) and at the end of their lives (supernovae). We show that a young solar system formed along a dense filament can be enriched with supernova ejecta (e.g.,
26
Al) during the formation timescale of CAIs. We also propose that the streamers recently observed around protostars may be channeling the SLR-rich material onto the young solar system. We conclude that considering hub–filament configurations as the birth environment of the Sun is important when deriving theoretical models explaining the observed properties of the solar system.
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