The ISM is governed by supersonic turbulence on a range of scales. We use this to develop a rigorous excursion-set model for the formation and time evolution of dense gas structures (GMCs, massive clumps, and cores). Supersonic turbulence drives the density distribution to a lognormal with dispersion increasing with Mach number; we generalize this to include scales >h (the disk scale height), and use it to construct the statistical properties of the density field smoothed on a scale R. We then compare conditions for self-gravitating collapse including thermal, turbulent, and rotational support. We show this becomes a well-defined barrier crossing problem. As such, an exact 'bound object mass function' can be derived, from scales of the sonic length to above the disk Jeans mass. This agrees remarkably well with observed GMC mass functions in the MW and other galaxies; the only inputs are the mass and size of the galaxies (to normalize the model). This explains the mass function cutoff and its power-law slope (close to, but shallower than, -2). The model also predicts the linewidth-size and size-mass relations of clouds and the dependence of their residuals on surface density/pressure. We use this to predict the spatial correlation function/clustering of clouds and star clusters; these also agree well with observations. We predict the size/mass function of ISM 'bubbles' or 'holes', and show this can account for observed HI hole distributions without any local feedback. We generalize the model to construct time-dependent 'merger/fragmentation trees' which can be used to follow cloud evolution and construct semi-analytic models for the ISM. We provide explicit recipes to construct the trees. We use a simple example to show that, if clouds are not destroyed in ~1-5 crossing times, then all ISM mass would be trapped in collapsing objects even if the large-scale turbulence were maintained.
We present a detailed study of the evolution of GMCs in a galactic disc simulation. We follow individual GMCs (defined in our simulations by a total column density criterion), including their level of star formation, from their formation to dispersal. We find the evolution of GMCs is highly complex. GMCs often form from a combination of smaller clouds and ambient ISM, and similarly disperse by splitting into a number of smaller clouds and ambient ISM. However some clouds emerge as the result of the disruption of a more massive GMC, rather than from the assembly of smaller clouds. Likewise in s…
We perform calculations of isolated disc galaxies to investigate how the properties of the ISM, the nature of molecular clouds, and the global star formation rate depend on the level of stellar feedback. We adopt a simple physical model, which includes a galactic potential, a standard cooling and heating prescription of the ISM, and self gravity of the gas. Stellar feedback is implemented by injecting energy into dense, gravitationally collapsing gas, but is independent of the Schmidt-Kennicutt relation. We obtain fractions of gas, and filling factors for different phases of the ISM in reasona…
By resimulating a region of a global disc simulation at higher resolution, we resolve and study the properties of molecular clouds with a range of masses from a few 100's M$_{\odot}$ to $10^6$ M$_{\odot}$. The purpose of our paper is twofold, i) to compare the ISM and GMCs at much higher resolution compared to previous global simulations, and ii) to investigate smaller clouds and characteristics such as the internal properties of GMCs which cannot be resolved in galactic simulations. We confirm the robustness of cloud properties seen in previous galactic simulations, and that these properties …
We report on a multi parameter analysis of giant molecular clouds (GMCs) in the nearby spiral galaxy M33. A catalog of GMCs identifed in 12CO(J=3-2) was used to compile associated 12CO(J=1-0), dust, stellar mass and star formation rate. Each of the 58 GMCs are categorized by their evolutionary stage. Applying the principal component analysis on these parameters, we construct two principal components PC1 and PC2 which retain 75% of the information in the original dataset. PC1 is interpreted as expressing the total interstellar matter content, and PC2 as the total activity of star formation. You…
Clusters, Clouds, and Correlations: Relating Young Clusters to Giant Molecular Clouds in M33 and M31
University of Alberta（阿尔伯塔大学）；Northwestern University（西北大学）；University of Utah（犹他大学）；University of Washington（华盛顿大学）；Flatiron Institute（平顿研究所）；University of Michigan（密歇根大学）；Univ. Bordeaux（波尔多大学）；California Institute of Technology（加州理工学院）
Using hydrodynamical simulations of entire galactic discs similar to the Milky Way, reaching 4.6pc resolution, we study the origins of observed physical properties of giant molecular clouds (GMCs). We find that efficient stellar feedback is a necessary ingredient in order to develop a realistic interstellar medium (ISM), leading to molecular cloud masses, sizes, velocity dispersions and virial parameters in excellent agreement with Milky Way observations. GMC scaling relations observed in the Milky Way, such as the mass-size ($M$--$R$), velocity dispersion-size ($σ$--$R$), and the $σ$--$RΣ$ re…
Recently, we have shown that if the ISM is governed by super-sonic turbulent flows, the excursion-set formalism can be used to calculate the statistics of self-gravitating objects over a wide range of scales. On the largest self-gravitating scales ('first crossing'), these correspond to GMCs, and on the smallest non-fragmenting self-gravitating scales ('last crossing'), to protostellar cores. Here, we extend this formalism to rigorously calculate the auto and cross-correlation functions of cores (and by extension, young stars) as a function of spatial separation and mass, in analogy to the cos…
We analyse the results of four simulations of isolated galaxies: two with a rigid spiral potential of fixed pattern speed, but with different degrees of star-formation induced feedback, one with an axisymmetric galactic potential and one with a `live' self-gravitating stellar component. Since we use a Lagrangian method we are able to select gas that lies within giant molecular clouds (GMCs) at a particular timeframe, and to then study the properties of this gas at earlier and later times. We find that gas which forms GMCs is not typical of the interstellar medium at least 50 Myr before the clo…
The escape of ultraviolet photons from the densest regions of the interstellar medium (ISM) --- Giant Molecular Clouds (GMCs) --- is a poorly constrained parameter which is vital to understanding the ionization of the ISM and the intergalactic medium. We characterize the escape fraction, f$_{\text{esc,GMC}}$, from a suite of individual GMC simulations with masses in the range 10$^{4-6}$ M$_{\odot}$ using the adaptive-mesh refinement code FLASH. We find significantly different f$_{\text{esc,GMC}}$ depending on the GMC mass which can reach $>$90% in the evolution of 5$\times$10$^4$ and 10$^{5}$ …
We present numerical methods for including stellar feedback in galaxy-scale simulations. We include heating by SNe (I & II), gas recycling and shock-heating from O-star & AGB winds, HII photoionization, and radiation pressure from stellar photons. The energetics and time-dependence are taken directly from stellar evolution models. We implement these in simulations with pc-scale resolution, modeling galaxies from SMC-like dwarfs and MW analogues to massive z~2 starburst disks. Absent feedback, gas cools and collapses without limit. With feedback, the ISM reaches a multi-phase steady state in wh…
Tidal dwarf galaxies (TDGs) are gravitationally bound condensations of gas and stars formed during galaxy interactions. Here we present multi-configuration ALMA observations of J1023+1952, a TDG in the interacting system Arp 94, where we resolve CO(2-1) emission down to giant molecular clouds (GMCs) at 0.64" ~ 45pc resolution. We find a remarkably high fraction of extended molecular emission (~80-90%), which is filtered out by the interferometer and likely traces diffuse gas. We detect 111 GMCs that give a similar mass spectrum as those in the Milky Way and other nearby galaxies (a truncated p…
We report the second complete molecular line data release from the {\em Census of High- and Medium-mass Protostars} (CHaMP), a large-scale, unbiased, uniform mapping survey at sub-parsec resolution, of mm-wave line emission from 303 massive, dense molecular clumps in the Milky Way. This release is for all $^{12}$CO $J$=1$\rightarrow$0 emission associated with the dense gas, the first from Phase II of the survey, which includes $^{12}$CO, $^{13}$CO, and C$^{18}$O. The observed clump emission traced by both $^{12}$CO and HCO$^+$ (from Phase I) shows very similar morphology, indicating that, for …
We analyze age-velocity dispersion relations (AVRs) from kinematics of individual stars in eight Local Group galaxies ranging in mass from Carina ($M_{*} \sim 10^{6}$) to M31 ($M_{*} \sim 10^{11}$). Observationally the $σ$ vs. stellar age trends can be interpreted as dynamical heating of the stars by GMCs, bars/spiral arms, or merging subhalos; alternatively the stars could have simply been born out of a more turbulent ISM at high redshift and retain that larger velocity dispersion till present day - consistent with recent IFU studies. To ascertain the dominant mechanism and better understand …
We investigate the effect of star formation and diffuse photoelectric heating on the properties of giant molecular clouds (GMCs) formed in high resolution (~< 10 pc) global (~ 20 kpc) simulations of isolated Milky Way-type galaxy disks. The clouds are formed through gravitational fragmentation and structures with densities n_H>=100cm^-3 are identified as GMCs. Between 1000-1500 clouds are created in the simulations with masses M > 10^5 Msolar and 180-240 with masses M > 10^6 Msolar in agreement with estimates of the Milky Way's population. We find that the effect of photoelectric heating is to…
The 12CO J=4-3 to J=13-12 lines of the interstellar medium from nearby galaxies, newly observable with the Herschel SPIRE Fourier Transform Spectrometer (FTS), offer an opportunity to study warmer, more luminous molecular gas than that traced by 12CO J=1-0. Here we present a survey of 17 nearby infrared-luminous galaxy systems (21 pointings). In addition to photometric modeling of dust, we modeled full 12CO spectral line energy distributions from J=1-0 to J=13-12 with two components of warm and cool CO gas, and included LTE analysis of [CI], [CII], [NII] and H2 lines. CO is emitted from a low-…
The properties of young star clusters formed within a galaxy are thought to vary in different interstellar medium (ISM) conditions, but the details of this mapping from galactic to cluster scales are poorly understood due to the large dynamic range involved in galaxy and star cluster formation. We introduce a new method for modeling cluster formation in galaxy simulations: mapping giant molecular clouds (GMCs) formed self-consistently in a FIRE-2 MHD galaxy simulation onto a cluster population according to a GMC-scale cluster formation model calibrated to higher-resolution simulations, obtaini…
We use a recently-developed analytic model for the ISM structure from scales of GMCs through star-forming cores to explore how the pre-stellar core mass function (CMF) and, by extrapolation, stellar initial mass function (IMF) should depend on both local and galactic properties. If the ISM is supersonically turbulent, the statistical properties of the density field follow from the turbulent velocity spectrum, and the excursion set formalism can be applied to analytically calculate the mass function of collapsing cores on the smallest scales on which they are self-gravitating (non-fragmenting).…
Radiation feedback from young star clusters embedded in giant molecular clouds (GMCs) is believed to be important to the control of star formation. For the most massive and dense clouds, including those in which super star clusters (SSCs) are born, pressure from reprocessed radiation exerted on dust grains may disperse a significant portion of the cloud mass back into the interstellar medium (ISM). Using our radiaton hydrodynamics (RHD) code, Hyperion, we conduct a series of numerical simulations to test this idea. Our models follow the evolution of self-gravitating, strongly turbulent clouds …
With the aim of improving predictions on far infrared (FIR) line emission from Giant Molecular Clouds (GMC), we study the effects of photoevaporation (PE) produced by external far-ultraviolet (FUV) and ionizing (extreme-ultraviolet, EUV) radiation on GMC structure. We consider three different GMCs with mass in the range $M_{\rm GMC} = 10^{3-6}\,\rm{M_{\odot}}$. Our model includes: (i) an observationally-based inhomogeneous GMC density field, and (ii) its time evolution during the PE process. In the fiducial case ($M_{\rm GMC}\approx10^5 M_{\odot}$), the photoevaporation time ($t_{pe}$) increas…
We use ALMA to derive the mass, length, and time scales associated with the nuclear starburst in NGC 253. This region forms ~2 M_sun/yr of stars and resembles other starbursts in scaling relations, with star formation consuming the gas reservoir 10 times faster than in galaxy disks. We present observations of CO, the high effective density transitions HCN(1-0), HCO+(1-0), CS(2-1), and their isotopologues. We identify ten clouds that appear as peaks in line emission and enhancements in the HCN-to-CO ratio. These clouds are massive (~10^7 M_sun) structures with sizes (~30 pc) similar to GMCs in …
To understand the impact of low metallicities on giant molecular cloud (GMC) structure, we compare far infrared dust emission, CO emission, and dynamics in the star-forming complex N83 in the Wing of the Small Magellanic Cloud. Dust emission (measured by Spitzer as part of the S3MC and SAGE-SMC surveys) probes the total gas column independent of molecular line emission and traces shielding from photodissociating radiation. We calibrate a method to estimate the dust column using only the high-resolution Spitzer data and verify that dust traces the ISM in the HI-dominated region around N83. This…
Hennebelle & Chabrier 2008 (HC08) attempted to derive the stellar IMF as a consequence of turbulent density fluctuations, using an argument similar to Press & Schechter 1974 for Gaussian random fields. Like that example, however, this solution does not resolve the 'cloud in cloud' problem; it also does not extend to large scales that dominate the velocity/density fluctuations. In principle, these can change the results at the order-of-magnitude level. Here, we use the results from Hopkins 2011 (H11) to generalize the excursion set formalism and derive the exact solution in this regime. We argu…
Simulations of isolated giant molecular clouds (GMCs) are an important tool for studying the dynamics of star formation, but their turbulent initial conditions (ICs) are uncertain. Most simulations have either initialized a velocity field with a prescribed power spectrum on a smooth density field (failing to model the full structure of turbulence) or "stirred" turbulence with periodic boundary conditions (which may not model real GMC boundary conditions). We develop and test a new GMC simulation setup (called TURBSPHERE) that combines advantages of both approaches: we continuously stir an isol…
Ring-like structures in the ISM are commonly associated with high-mass stars. Kinematic studies of large structures in GMCs toward these ring-like structures may help us to understand how massive stars form. The origin and properties of the ring-like structure G345.45+1.50 is investigated through observations of the 13CO(3-2) line. The aim of the observations is to determine the kinematics in the region and to compare physical characteristics estimated from gas emission with those previously determined using dust continuum emission. The 13CO(3-2) line was mapped toward the whole ring using the…
