We estimate the star formation efficiency per gravitational free-fall time, , from observations of nearby galaxies with resolution matched to the typical size of a giant molecular cloud. This ...quantity, , is theoretically important but so far has only been measured for Milky Way clouds or inferred indirectly in a few other galaxies. Using new, high-resolution CO imaging from the Physics at High Angular Resolution in nearby Galaxies-Atacama Large Millimeter Array (PHANGS-ALMA) survey, we estimate the gravitational free-fall time at 60-120 pc resolution, and contrast this with the local molecular gas depletion time in order to estimate . Assuming a constant thickness of the molecular gas layer (H = 100 pc) across the whole sample, the median value of in our sample is 0.7%. We find a mild scale dependence, with higher measured at coarser resolution. Individual galaxies show different values of , with the median ranging from 0.3% to 2.6%. We find the highest in our lowest-mass targets, reflecting both long free-fall times and short depletion times, though we caution that both measurements are subject to biases in low-mass galaxies. We estimate the key systematic uncertainties, and show the dominant uncertainty to be the estimated line-of-sight (LOS) depth through the molecular gas layer and the choice of star formation tracers.
We present a new survey of HCN(1-0) emission, a tracer of dense molecular gas, focused on the little-explored regime of normal star-forming galaxy disks. Combining HCN, CO, and infrared (IR) ...emission, we investigate the role of dense gas in star formation, finding systematic variations in both the apparent dense gas fraction (traced by the HCN-to-CO ratio) and the apparent star formation efficiency of dense gas. The latter may be unexpected, given the recent popularity of gas density threshold models to explain star formation scaling relations. Our survey used the IRAM 30 m telescope to observe HCN(1-0), CO(1-0), and several other emission lines across 29 nearby disk galaxies whose CO(2-1) emission has previously been mapped by the HERACLES survey. We detected HCN in 48 out of 62 observed positions. We explore one such model in which variations in the Mach number drive many of the trends within galaxy disks, while density contrasts drive the differences between disk and merging galaxies.
Modern extragalactic molecular gas surveys now reach the scales of star-forming giant molecular clouds (GMCs; 20-50 pc). Systematic variations in GMC properties with galaxy environment imply that ...clouds are not universally self-gravitating objects, decoupled from their surroundings. Here we re-examine the coupling of clouds to their environment and develop a model for 3D gas motions generated by forces arising with the galaxy gravitational potential defined by the background disk of stars and dark matter. We show that these motions can resemble or even exceed the motions needed to support gas against its own self-gravity throughout typical galactic disks. The importance of the galactic potential in spiral arms and galactic centers suggests that the response to self-gravity does not always dominate the motions of gas at GMC scales, with implications for observed gas kinematics, virial equilibrium, and cloud morphology. We describe how a uniform treatment of gas motions in the plane and in the vertical direction synthesizes the two main mechanisms proposed to regulate star formation: vertical pressure equilibrium and shear/Coriolis forces as parameterized by Toomre Q 1. As the modeled motions are coherent and continually driven by the external potential, they represent support for the gas that is distinct from that conventionally attributed to turbulence, which decays rapidly and thus requires maintenance, e.g., via feedback from star formation. Thus, our model suggests that the galaxy itself can impose an important limit on star formation, as we explore in a second paper in this series.
We present kinematic orientations and high-resolution (150 pc) rotation curves for 67 main-sequence star-forming galaxies surveyed in CO (2-1) emission by PHANGS-ALMA. Our measurements are based on ...the application of a new fitting method tailored to CO velocity fields. Our approach identifies an optimal global orientation as a way to reduce the impact of nonaxisymmetric (bar and spiral) features and the uneven spatial sampling characteristic of CO emission in the inner regions of nearby galaxies. The method performs especially well when applied to the large number of independent lines of sight contained in the PHANGS CO velocity fields mapped at 1″ resolution. The high-resolution rotation curves fitted to these data are sensitive probes of mass distribution in the inner regions of these galaxies. We use the inner slope as well as the amplitude of our fitted rotation curves to demonstrate that CO is a reliable global dynamical mass tracer. From the consistency between photometric orientations from the literature and kinematic orientations determined with our method, we infer that the shapes of stellar disks in the mass range of log( ) = 9.0-10.9 probed by our sample are very close to circular and have uniform thickness.
We present maps of the dust properties in the Small and Large Magellanic Clouds (SMC, LMC) from fitting Spitzer and Herschel observations with the Draine & Li dust model. We derive the abundance of ...the small carbonaceous grain (or polycyclic aromatic hydrocarbon; PAH) component. The global PAH fraction ( , the fraction of the dust mass in the form of PAHs) is smaller in the SMC ( %) than in the LMC ( %). We measure the PAH fraction in different gas phases (H ii regions, ionized gas outside of H ii regions, molecular gas, and diffuse neutral gas). H ii regions appear as distinctive holes in the spatial distribution of the PAH fraction. In both galaxies, the PAH fraction in the diffuse neutral medium is higher than in the ionized gas, but similar to the molecular gas. Even at equal radiation field intensity, the PAH fraction is lower in the ionized gas than in the diffuse neutral gas. We investigate the PAH life-cycle as a function of metallicity between the two galaxies. The PAH fraction in the diffuse neutral medium of the LMC is similar to that of the Milky Way (∼4.6%), while it is significantly lower in the SMC. Plausible explanations for the higher PAH fraction in the diffuse neutral medium of the LMC compared to the SMC include: more effective PAH production by fragmentation of large grains at higher metallicity, and/or the growth of PAHs in molecular gas.
We study the Local Group spiral galaxy M33 to investigate how the observed scaling between the (kpc-averaged) surface density of molecular gas ( Delta *SH2) and recent star formation rate ( Delta ...*SSFR) relates to individual star-forming regions. To do this, we measure the ratio of CO emission to extinction-corrected H Delta *a emission in apertures of varying sizes centered both on peaks of CO and H Delta *a emission. We parameterize this ratio as the molecular gas (H2) depletion time ( Delta *tdep). On large (kpc) scales, our results are consistent with a molecular star formation law ( Delta *SSFR ~ Delta *S b H2) with b ~ 1.1-1.5 and a median Delta *tdep ~ 1 Gyr, with no dependence on type of region targeted. Below these scales, Delta *tdep is a strong function of the adopted angular scale and the type of region that is targeted. Small (300 pc) apertures centered on CO peaks have very long Delta *tdep (i.e., high CO-to-H Delta *a flux ratio) and small apertures targeted toward H Delta *a peaks have very short Delta *tdep. This implies that the star formation law observed on kpc scales breaks down once one reaches aperture sizes of 300 pc. For our smallest apertures (75 pc), the difference in Delta *tdep between the two types of regions is more than one order of magnitude. This scale behavior emerges from averaging over star-forming regions with a wide range of CO-to-H Delta *a ratios with the natural consequence that the breakdown in the star formation law is a function of the surface density of the regions studied. We consider the evolution of individual regions the most likely driver for region-to-region differences in Delta *tdep (and thus the CO-to-H Delta *a ratio).
We develop a model for the regulation of galactic star formation rates SigmaSFR in disk galaxies, in which interstellar medium (ISM) heating by stellar UV plays a key role. By requiring that thermal ...and (vertical) dynamical equilibrium are simultaneously satisfied within the diffuse gas, and that stars form at a rate proportional to the mass of the self-gravitating component, we obtain a prediction for SigmaSFR as a function of the total gaseous surface density Sigma and the midplane density of stars+dark matter rhosd. The physical basis of this relationship is that the thermal pressure in the diffuse ISM, which is proportional to the UV heating rate and therefore to SigmaSFR, must adjust until it matches the midplane pressure value set by the vertical gravitational field. Our model applies to regions where Sigma
The under-abundance of very massive galaxies in the Universe is frequently attributed to the effect of galactic winds. Although ionized galactic winds are readily observable, most of the expelled ...mass (that is, the total mass flowing out from the nuclear region) is likely to be in atomic and molecular phases that are cooler than the ionized phases. Expanding molecular shells observed in starburst systems such as NGC 253 (ref. 12) and M 82 (refs 13, 14) may facilitate the entrainment of molecular gas in the wind. Although shell properties are well constrained, determining the amount of outflowing gas emerging from such shells and the connection between this gas and the ionized wind requires spatial resolution better than 100 parsecs coupled with sensitivity to a wide range of spatial scales, a combination hitherto not available. Here we report observations of NGC 253, a nearby starburst galaxy (distance ∼ 3.4 megaparsecs) known to possess a wind, that trace the cool molecular wind at 50-parsec resolution. At this resolution, the extraplanar molecular gas closely tracks the Hα filaments, and it appears to be connected to expanding molecular shells located in the starburst region. These observations allow us to determine that the molecular outflow rate is greater than 3 solar masses per year and probably about 9 solar masses per year. This implies a ratio of mass-outflow rate to star-formation rate of at least 1, and probably ∼3, indicating that the starburst-driven wind limits the star-formation activity and the final stellar content.
The process that leads to the formation of the bright star-forming sites observed along prominent spiral arms remains elusive. We present results of a multi-wavelength study of a spiral arm segment ...in the nearby grand-design spiral galaxy M51 that belongs to a spiral density wave and exhibits nine gas spurs. The combined observations of the (ionized, atomic, molecular, dusty) interstellar medium with star formation tracers (H ii regions, young <10 Myr stellar clusters) suggest (1) no variation in giant molecular cloud (GMC) properties between arm and gas spurs, (2) gas spurs and extinction feathers arising from the same structure with a close spatial relation between gas spurs and ongoing/recent star formation (despite higher gas surface densities in the spiral arm), (3) no trend in star formation age either along the arm or along a spur, (4) evidence for strong star formation feedback in gas spurs, (5) tentative evidence for star formation triggered by stellar feedback for one spur, and (6) GMC associations being not special entities but the result of blending of gas arm/spur cross sections in lower resolution observations. We conclude that there is no evidence for a coherent star formation onset mechanism that can be solely associated with the presence of the spiral density wave. This suggests that other (more localized) mechanisms are important to delay star formation such that it occurs in spurs. The evidence of star formation proceeding over several million years within individual spurs implies that the mechanism that leads to star formation acts or is sustained over a longer timescale.
PDF
