We examine the role of the large-scale galactic-dynamical environment in setting the properties of giant molecular clouds in Milky Way-like galaxies. We perform three high-resolution simulations of ...Milky Way-like discs with the moving-mesh hydrodynamics code Arepo, yielding a statistical sample of \(\sim 80,000\) giant molecular clouds and \(\sim 55,000\) HI clouds. We account for the self-gravity of the gas, momentum and thermal energy injection from supernovae and HII regions, mass injection from stellar winds, and the non-equilibrium chemistry of hydrogen, carbon and oxygen. By varying the external gravitational potential, we probe galactic-dynamical environments spanning an order of magnitude in the orbital angular velocity, gravitational stability, mid-plane pressure and the gradient of the galactic rotation curve. The simulated molecular clouds are highly overdense (\(\sim 100\times\)) and over-pressured (\(\sim 25\times\)) relative to the ambient interstellar medium. Their gravo-turbulent and star-forming properties are decoupled from the dynamics of the galactic mid-plane, so that the kpc-scale star formation rate surface density is related only to the number of molecular clouds per unit area of the galactic mid-plane. Despite this, the clouds display clear, statistically-significant correlations of their rotational properties with the rates of galactic shearing and gravitational free-fall. We find that galactic rotation and gravitational instability can influence their elongation, angular momenta, and tangential velocity dispersions. The lower pressures and densities of the HI clouds allow for a greater range of significant dynamical correlations, mirroring the rotational properties of the molecular clouds, while also displaying a coupling of their gravitational and turbulent properties to the galactic-dynamical environment.
It remains a major challenge to derive a theory of cloud-scale (\(\lesssim100\) pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic ...environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially-resolved (\(\sim100\) pc) CO-to-H\(\alpha\) flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically 10-30 Myr, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities \(\Sigma_{\rm H_2}\geqslant8\)M\(_{\odot}\)pc\(^{-2}\), the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at \(\Sigma_{\rm H_2}\leqslant8\)M\(_{\odot}\)pc\(^{-2}\) GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H\(\alpha\) (75-90% of the cloud lifetime), GMCs disperse within just 1-5 Myr once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4-10% These results show that galactic star formation is governed by cloud-scale, environmentally-dependent, dynamical processes driving rapid evolutionary cycling. GMCs and HII regions are the fundamental units undergoing these lifecycles, with mean separations of 100-300 pc in star-forming discs. Future work should characterise the multi-scale physics and mass flows driving these lifecycles.
We present a family of self-consistent axisymmetric rotating globular cluster models which are fitted to spectroscopic data for NGC 362, NGC 1851, NGC 2808, NGC 4372, NGC 5927 and NGC 6752 to provide ...constraints on their physical and kinematic properties, including their rotation signals. They are constructed by flattening Modified Plummer profiles, which have the same asymptotic behaviour as classical Plummer models, but can provide better fits to young clusters due to a slower turnover in the density profile. The models are in dynamical equilibrium as they depend solely on the action variables. We employ a fully Bayesian scheme to investigate the uncertainty in our model parameters (including mass-to-light ratios and inclination angles) and evaluate the Bayesian evidence ratio for rotating to non-rotating models. We find convincing levels of rotation only in NGC 2808. In the other clusters, there is just a hint of rotation (in particular, NGC 4372 and NGC 5927), as the data quality does not allow us to draw strong conclusions. Where rotation is present, we find that it is confined to the central regions, within radii of \(R \leq 2 r_h\). As part of this work, we have developed a novel q-Gaussian basis expansion of the line-of-sight velocity distributions, from which general models can be constructed via interpolation on the basis coefficients.
We present a new simulation suite for the star-forming interstellar medium
(ISM) in galactic disks using the TIGRESS-NCR framework. Distinctive aspects of
our simulation suite are: (1) sophisticated ...and comprehensive numerical
treatments of essential physical processes including magnetohydrodynamics,
self-gravity, and galactic differential rotation, as well as photochemistry,
cooling, and heating coupled with ray-tracing UV radiation transfer and
resolved supernova feedback and (2) wide parameter coverage including
metallicity over $Z'\equiv Z/Z_\odot\sim0.1-3$, gas surface density
$\Sigma_{\rm gas}\sim5-150 M_{\odot}{\rm pc^{-2}}$, and stellar surface density
$\Sigma_{\rm star}\sim 1-50 M_{\odot}{\rm pc^{-2}}$. The range of emergent star
formation rate surface density is $\Sigma_{\rm SFR}\sim 10^{-4}-0.5
M_{\odot}{\rm kpc^{-2}yr^{-1}}$ and ISM total midplane pressure is $P_{\rm
tot}/k_B=10^3-10^6{\rm cm^{-3}K}$, with $P_{\rm tot}$ equal to the ISM weight
$W$. For given $\Sigma_{\rm gas}$ and $\Sigma_{\rm star}$, we find $\Sigma_{\rm
SFR} \propto Z'^{0.3}$. We provide an interpretation based on the
pressure-regulated feedback-modulated (PRFM) star formation theory. We
characterize feedback modulation in terms of the yield $\Upsilon$, defined as
the ratio of each stress to $\Sigma_{\rm SFR}$. The thermal feedback yield
varies sensitively with both weight and metallicity as $\Upsilon_{\rm
th}\propto W^{-0.46}Z'^{-0.53}$, while the combined turbulent and magnetic
feedback yield shows weaker dependence $\Upsilon_{\rm turb+mag}\propto
W^{-0.22}Z'^{-0.18}$. The reduction in $\Sigma_{\rm SFR}$ at low metallicity is
due mainly to enhanced thermal feedback yield, resulting from reduced
attenuation of UV radiation. With the metallicity-dependent calibrations we
provide, PRFM theory can be used for a new subgrid star formation prescription
in cosmological simulations where the ISM is unresolved.
Molecular hydrogen (H$_2$) formation and dissociation are key processes that
drive the gas lifecycle in galaxies. Using the SImulating the LifeCycle of
Molecular Clouds (SILCC) zoom-in simulation ...suite, we explore the utility of
future observations of H$_2$ dissociation and formation for tracking the
lifecycle of molecular clouds. The simulations used in this work include
non-equilibrium H$_2$ formation, stellar radiation, sink particles, and
turbulence. We find that, at early times in the cloud evolution, H$_2$
formation rapidly outpaces dissociation and molecular clouds build their mass
from the atomic reservoir in their environment. Rapid H$_2$ formation is also
associated with a higher early star formation rate. For the clouds studied
here, H$_2$ is strongly out of chemical equilibrium during the early stages of
cloud formation but settles into a bursty chemical steady-state about 2 Myrs
after the first stars form. At the latest stage of cloud evolution,
dissociation outweighs formation and the clouds enter a dispersal phase. We
discuss how theories for the molecular cloud lifecycle and the star formation
efficiency may be distinguished with observational measurements of H$_2$
fluorescence with a space-based high-resolution FUV spectrometer, such as the
proposed Hyperion and Eos NASA Explorer missions. Such missions would enable
measurements of the H$_2$ dissociation and formation rates, which we
demonstrate can be connected to different phases in a molecular cloud's
star-forming life, including cloud building, rapidly star-forming, H$_2$
chemical equilibrium, and cloud destruction.
Early-type galaxies (ETGs) are known to harbour dense spheroids of stars but scarce star formation (SF). Approximately a quarter of these galaxies have rich molecular gas reservoirs yet do not form ...stars efficiently. We study here the ETG NGC~524, with strong shear suspected to result in a smooth molecular gas disc and low star-formation efficiency (SFE). We present new spatially-resolved observations of the \textsuperscript{12}CO(2-1)-emitting cold molecular gas from the Atacama Large Millimeter/sub-millimeter Array (ALMA) and of the warm ionised-gas emission lines from SITELLE at the Canada-France-Hawaii Telescope. Although constrained by the resolution of the ALMA observations (\(\approx37\)~pc), we identify only \(52\) GMCs with radii ranging from \(30\) to \(140\)~pc, a low mean molecular gas mass surface density \(\langle\Sigma_{\rm gas}\rangle\approx125\)~M\(_\odot\)~pc\(^{-2}\) and a high mean virial parameter \(\langle\alpha_{\rm obs,vir}\rangle\approx5.3\). We measure spatially-resolved molecular gas depletion times (\(\tau_{\rm dep}\equiv1/{\rm SFE}\)) with a spatial resolution of \(\approx100\)~pc within a galactocentric distance of \(1.5\)~kpc. The global depletion time is \(\approx2.0\)~Gyr but \(\tau_{\rm dep}\) increases toward the galaxy centre, with a maximum \(\tau_{\rm dep,max}\approx5.2\)~Gyr. However, no pure \ion{H}{II} region is identified in NGC~524 using ionised-gas emission-line ratio diagnostics, so the \(\tau_{\rm dep}\) inferred are in fact lower limits. Measuring the GMC properties and dynamical states, we conclude that shear is the dominant mechanism shaping the molecular gas properties and regulating SF in NGC~524. This is supported by analogous analyses of the GMCs in a simulated ETG similar to NGC~524.
Past studies have long emphasised the key role played by galactic stellar bars in the context of disc secular evolution, via the redistribution of gas and stars, the triggering of star formation, and ...the formation of prominent structures such as rings and central mass concentrations. However, the exact physical processes acting on those structures, as well as the timescales associated with the building and consumption of central gas reservoirs are still not well understood. We are building a suite of hydro-dynamical RAMSES simulations of isolated, low-redshift galaxies that mimic the properties of the PHANGS sample. The initial conditions of the models reproduce the observed stellar mass, disc scale length, or gas fraction, and this paper presents a first subset of these models. Most of our simulated galaxies develop a prominent bar structure, which itself triggers central gas fuelling and the building of an over-density with a typical scale of 100-1000 pc. We confirm that if the host galaxy features an ellipsoidal component, the formation of the bar and gas fuelling are delayed. We show that most of our simulations follow a common time evolution, when accounting for mass scaling and the bar formation time. In our simulations, the stellar mass of \(10^{10}\)~M\(_{\odot}\) seems to mark a change in the phases describing the time evolution of the bar and its impact on the interstellar medium. In massive discs (M\(_{\star} \geq 10^{10}\)~M\(_{\odot}\)), we observe the formation of a central gas reservoir with star formation mostly occurring within a restricted starburst region, leading to a gas depletion phase. Lower-mass systems (M\(_{\star} < 10^{10}\)~M\(_{\odot}\)) do not exhibit such a depletion phase, and show a more homogeneous spread of star-forming regions along the bar structure, and do not appear to host inner bar-driven discs or rings.
The molecular-to-atomic gas ratio is crucial to the evolution of the
interstellar medium in galaxies. We investigate the balance between the atomic
($\Sigma_{\rm HI}$) and molecular gas ($\Sigma_{\rm ...H2}$) surface densities in
eight nearby star-forming galaxies using new high-quality observations from
MeerKAT and ALMA (for HI and CO, respectively). We define the molecular gas
ratio as $R_{\rm mol} = \Sigma_{\rm H2} / \Sigma_{\rm HI}$ and measure how it
depends on local conditions in the galaxy disks using multi-wavelength
observations. We find that, depending on the galaxy, HI is detected at
$>3\sigma$ out to 20-120 kpc in galactocentric radius ($r_{\rm gal}$). The
typical radius at which $\Sigma_{\rm HI}$ reaches 1~$\rm M_\odot~pc^{-2}$ is
$r_{\rm HI}\approx22$~kpc, which corresponds to 1-3 times the optical radius
($r_{25}$). $R_{\rm mol}$ correlates best with the dynamical equilibrium
pressure, P$_{\rm DE}$, among potential drivers studied, with a median
correlation coefficient of $<\rho>=0.89$. Correlations between $R_{\rm mol}$
and star formation rate, total gas and stellar surface density, metallicity,
and $\Sigma_{\rm SFR}$/P$_{\rm DE}$ are present but somewhat weaker. Our
results also show a direct correlation between P$_{\rm DE}$ and $\Sigma_{\rm
SFR}$, supporting self-regulation models. Quantitatively, we measure similar
scalings as previous works and attribute the modest differences that we find to
the effect of varying resolution and sensitivity. At $r_{\rm gal}
{\gtrsim}0.4~r_{25}$, atomic gas dominates over molecular gas, and at the
balance of these two gas phases, we find that the baryon mass is dominated by
stars, with $\Sigma_{*} > 5~\Sigma_{\rm gas}$. Our study constitutes an
important step in the statistical investigation of how local galaxy properties
impact the conversion from atomic to molecular gas in nearby galaxies.
Connecting the gas in HII regions to the underlying source of the ionizing
radiation can help us constrain the physical processes of stellar feedback and
how HII regions evolve over time. With ...PHANGS$\unicode{x2013}$MUSE we detect
nearly 24,000 HII regions across 19 galaxies and measure the physical
properties of the ionized gas (e.g. metallicity, ionization parameter,
density). We use catalogues of multi-scale stellar associations from
PHANGS$\unicode{x2013}$HST to obtain constraints on the age of the ionizing
sources. We construct a matched catalogue of 4,177 HII regions that are clearly
linked to a single ionizing association. A weak anti-correlation is observed
between the association ages and the H$\alpha$ equivalent width EW(H$\alpha$),
the H$\alpha$/FUV flux ratio and the ionization parameter, log q. As all three
are expected to decrease as the stellar population ages, this could indicate
that we observe an evolutionary sequence. This interpretation is further
supported by correlations between all three properties. Interpreting these as
evolutionary tracers, we find younger nebulae to be more attenuated by dust and
closer to giant molecular clouds, in line with recent models of
feedback-regulated star formation. We also observe strong correlations with the
local metallicity variations and all three proposed age tracers, suggestive of
star formation preferentially occurring in locations of locally enhanced
metallicity. Overall, EW(H$\alpha$) and log q show the most consistent trends
and appear to be most reliable tracers for the age of an HII region.
We combine JWST observations with ALMA CO and VLT-MUSE H\(\alpha\) data to examine off-spiral arm star formation in the face-on, grand-design spiral galaxy NGC 628. We focus on the northern spiral ...arm, around a galactocentric radius of 3-4 kpc, and study two spurs. These form an interesting contrast, as one is CO-rich and one CO-poor, and they have a maximum azimuthal offset in MIRI 21\(\mu\)m and MUSE H\(\alpha\) of around 40\(^\circ\) (CO-rich) and 55\(^\circ\) (CO-poor) from the spiral arm. The star formation rate is higher in the regions of the spurs near to spiral arms, but the star formation efficiency appears relatively constant. Given the spiral pattern speed and rotation curve of this galaxy and assuming material exiting the arms undergoes purely circular motion, these offsets would be reached in 100-150 Myr, significantly longer than the 21\(\mu\)m and H\(\alpha\) star formation timescales (both <10 Myr). The invariance of the star formation efficiency in the spurs versus the spiral arms indicates massive star formation is not only triggered in spiral arms, and cannot simply occur in the arms and then drift away from the wave pattern. These early JWST results show that in-situ star formation likely occurs in the spurs, and that the observed young stars are not simply the `leftovers' of stellar birth in the spiral arms. The excellent physical resolution and sensitivity that JWST can attain in nearby galaxies will well resolve individual star-forming regions and help us to better understand the earliest phases of star formation.
