A Universal Neutral Gas Profile for Nearby Disk Galaxies

Based on sensitive CO measurements from HERACLES and HI data from THINGS, we show that the azimuthally averaged radial distribution of the neutral gas surface density (Sigma_HI + Sigma_H2) in 33 nearby spiral galaxies exhibits a well-constrained universal exponential distribution beyond 0.2*r25 (inside of which the scatter is large) with less than a factor of two scatter out to two optical radii r25. Scaling the radius to r25 and the total gas surface density to the surface density at the transition radius, i.e., where Sigma_HI and Sigma_H2 are equal, as well as removing galaxies that are interacting with their environment, yields a tightly constrained exponential fit with average scale length 0.61+-0.06 r25. In this case, the scatter reduces to less than 40% across the optical disks (and remains below a factor of two at larger radii). We show that the tight exponential distribution of neutral gas implies that the total neutral gas mass of nearby disk galaxies depends primarily on the size of the stellar disk (influenced to some degree by the great variability of Sigma_H2 inside 0.2*r25). The derived prescription predicts the total gas mass in our sub-sample of 17 non-interacting disk galaxies to within a factor of two. Given the short timescale over which star formation depletes the H2 content of these galaxies and the large range of r25 in our sample, there appears to be some mechanism leading to these largely self-similar radial gas distributions in nearby disk galaxies.Comment: 7 pages, 4 figures, accepted for publication in the Astrophysical Journa

Modeling the physical properties in the ISM of the low-metallicity galaxy NGC4214

We present a model for the interstellar medium of NGC4214 with the objective to probe the physical conditions in the two main star-forming regions and their connection with the star formation activity of the galaxy. We used the spectral synthesis code Cloudy to model an HII region and the associated photodissociation region (PDR) to reproduce the emission of mid- and far-infrared fine-structure cooling lines from the Spitzer and Herschel space telescopes for these two regions. Input parameters of the model, such as elemental abundances and star formation history, are guided by earlier studies of the galaxy, and we investigated the effect of the mode in which star formation takes place (bursty or continuous) on the line emission. Furthermore, we tested the effect of adding pressure support with magnetic fields and turbulence on the line predictions. We find that this model can satisfactorily predict (within a factor of ~2) all observed lines that originate from the ionized medium ([SIV] 10.5um, [NeIII] 15.6um, [SIII] 18.7um, [SIII] 33.5um, and [OIII] 88um), with the exception of [NeII] 12.8um and [NII] 122um, which may arise from a lower ionization medium. In the PDR, the [OI] 63um, [OI] 145um, and [CII] 157um lines are matched within a factor of ~5 and work better when weak pressure support is added to the thermal pressure or when the PDR clouds are placed farther away from the HII regions and have covering factors lower than unity. Our models of the HII region agree with different evolutionary stages found in previous studies, with a more evolved, diffuse central region, and a younger, more compact southern region. However, the local PDR conditions are averaged out on the 175 pc scales that we probe and do not reflect differences observed in the star formation properties of the two regions.Comment: accepted for publication in A&

Tightly Correlated HI and FUV Emission in the Outskirts of M83

We compare sensitive HI data from The HI Nearby Galaxy Survey (THINGS) and deep far UV (FUV) data from GALEX in the outer disk of M83. The FUV and HI maps show a stunning spatial correlation out to almost 4 optical radii (r25), roughly the extent of our maps. This underscores that HI traces the gas reservoir for outer disk star formation and it implies that massive (at least low level) star formation proceeds almost everywhere HI is observed. Whereas the average FUV intensity decreases steadily with increasing radius before leveling off at ~1.7 r25, the decline in HI surface density is more subtle. Low HI columns (<2 M_solar/pc^2) contribute most of the mass in the outer disk, which is not the case within r25. The time for star formation to consume the available HI, inferred from the ratio of HI to FUV intensity, rises with increasing radius before leveling off at ~100 Gyr, i.e., many Hubble times, near ~1.7 r25. Assuming the relatively short H2 depletion times observed in the inner parts of galaxies hold in outer disks, the conversion of HI into bound, molecular clouds seems to limit star formation in outer galaxy disks. The long consumption times suggest that most of the extended HI observed in M83 will not be consumed by in situ star formation. However, even these low star formation rates are enough to expect moderate chemical enrichment in a closed outer disk.Comment: Accepted for Publication in ApJ

Extremely Inefficient Star Formation in the Outer Disks of Nearby Galaxies

(Abridged) We combine data from The HI Nearby Galaxy Survey and the GALEX Nearby Galaxy Survey to study the relationship between atomic hydrogen (HI) and far-ultraviolet (FUV) emission outside the optical radius (r25) in 17 spiral and 5 dwarf galaxies. In this regime, HI is likely to represent most of the ISM and FUV emission to trace recent star formation with little bias due to extinction, so that the two quantities closely trace the underlying relationship between gas and star formation rate (SFR). The azimuthally averaged HI and FUV intensities both decline with increasing radius in this regime, with the scale length of the FUV profile typically half that of the HI profile. Despite the mismatch in profiles, there is a significant spatial correlation (at 15" resolution) between local FUV and HI intensities; near r25 this correlation is quite strong, in fact stronger than anywhere inside r25, and shows a decline towards larger radii. The star formation efficiency (SFE) - defined as the ratio of FUV/HI and thus the inverse of the gas depletion time - decreases with galactocentric radius across the outer disks, though much shallower than across the optical disks. On average, we find the gas depletion times to be well above a Hubble time (~10^11 yr). We observe a clear relationship between FUV/HI and HI column in the outer disks, with the SFE increasing with increasing HI column. Despite observing systematic variations in FUV/HI, we find no clear evidence for step-function type star formation thresholds. When compared with results from inside r25, we find outer disk star formation to be distinct in several ways: it is extremely inefficient (depletion times of many Hubble times) with column densities and SFRs lower than found anywhere inside the optical disks. It appears that the HI column is one of, perhaps even the key environmental factor in setting the SFR in outer galaxy disks.Comment: Accepted for Publication in The Astronomical Journa

Star Formation Rates in Molecular Clouds and the Nature of the Extragalactic Scaling Relations

In this paper we investigate scaling relations between star formation rates and molecular gas masses for both local Galactic clouds and a sample of external galaxies. We specifically consider relations between the star formation rates and measurements of dense, as well as total, molecular gas masses. We argue that there is a fundamental empirical scaling relation that directly connects the local star formation process with that operating globally within galaxies. Specifically, the total star formation rate in a molecular cloud or galaxy is linearly proportional to the mass of dense gas within the cloud or galaxy. This simple relation, first documented in previous studies, holds over a span of mass covering nearly nine orders of magnitude and indicates that the rate of star formation is directly controlled by the amount of dense molecular gas that can be assembled within a star formation complex. We further show that the star formation rates and total molecular masses, characterizing both local clouds and galaxies, are correlated over similarly large scales of mass and can be described by a family of linear star formation scaling laws, parameterized by $f_{DG}$, the fraction of dense gas contained within the clouds or galaxies. That is, the underlying star formation scaling law is always linear for clouds and galaxies with the same dense gas fraction. These considerations provide a single unified framework for understanding the relation between the standard (non-linear) extragalactic Schmidt-Kennicutt scaling law, that is typically derived from CO observations of the gas, and the linear star formation scaling law derived from HCN observations of the dense gas.Comment: 14 pages + 2 figures. Accepted for publication in ApJ 16 December 201

Physical Properties of Molecular Clouds at 2 parsec Resolution in the Low-Metallicity Dwarf Galaxy NGC 6822 and the Milky Way

We present the ALMA survey of CO(2-1) emission from the 1/5 solar metallicity, Local Group dwarf galaxy NGC 6822. We achieve high (0.9 arcsec ~ 2 pc) spatial resolution while covering large area: four 250 pc x 250 pc regions that encompass ~2/3 of NGC 6822's star formation. In these regions, we resolve ~150 compact CO clumps that have small radii (~2-3 pc), narrow line width (~1 km/s), and low filling factor across the galaxy. This is consistent with other recent studies of low metallicity galaxies, but here shown with a 15 times larger sample. At parsec scales, CO emission correlates with 8 micron emission better than with 24 micron emission and anti-correlates with Halpha, so that PAH emission may be an effective tracer of molecular gas at low metallicity. The properties of the CO clumps resemble those of similar-size structures in Galactic clouds except of slightly lower surface brightness and CO-to-H2 ratio ~1-2 times the Galactic value. The clumps exist inside larger atomic-molecular complexes with masses typical for giant molecular cloud. Using dust to trace H2 for the entire complex, we find CO-to-H2 to be ~20-25 times the Galactic value, but with strong dependence on spatial scale and variations between complexes that may track their evolutionary state. The H2-to-HI ratio is low globally and only mildly above unity within the complexes. The SFR-to-H2 ratio is ~3-5 times higher in the complexes than in massive disk galaxies, but after accounting for the bias from targeting star-forming regions, we conclude that the global molecular gas depletion time may be as long as in massive disk galaxies.Comment: Accepted for publication in The Astrophysical Journal; 22 pages, 10 figures, 7 table

Which feedback mechanisms dominate in the high-pressure environment of the Central Molecular Zone?

Supernovae (SNe) dominate the energy and momentum budget of stellar feedback, but the efficiency with which they couple to the interstellar medium (ISM) depends strongly on how effectively early, pre-SN feedback clears dense gas from star-forming regions. There are observational constraints on the magnitudes and timescales of early stellar feedback in low ISM pressure environments, yet no such constraints exist for more cosmologically typical high ISM pressure environments. In this paper, we determine the mechanisms dominating the expansion of HII regions as a function of size-scale and evolutionary time within the high-pressure (P/k_\rm{B}~$10^{7-8}$K cm$^{-3}$) environment in the inner 100pc of the Milky Way. We calculate the thermal pressure from the warm ionised (P_\rm{HII}; 10$^{4}$K) gas, direct radiation pressure (P_\rm{dir}), and dust processed radiation pressure (P_\rm{IR}). We find that (1) P_\rm{dir} dominates the expansion on small scales and at early times (0.01-0.1pc; $0.1$pc; $>1$Myr); (3) during the first ~1Myr of growth, but not thereafter, either $P_{\rm IR}$ or stellar wind pressure likely make a comparable contribution. Despite the high confining pressure of the environment, natal star-forming gas is efficiently cleared to radii of several pc within ~2Myr, i.e. before the first SNe explode. This `pre-processing' means that subsequent SNe will explode into low density gas, so their energy and momentum will efficiently couple to the ISM. We find the HII regions expand to a radius of 3pc, at which point they have internal pressures equal with the surrounding external pressure. A comparison with HII regions in lower pressure environments shows that the maximum size of all HII regions is set by pressure equilibrium with the ambient ISM

The evolution of HCO+^{+} in molecular clouds using a novel chemical post-processing algorithm

Modeling the internal chemistry of molecular clouds is critical to accurately simulating their evolution. To reduce computational expense, 3D simulations generally restrict their chemical modeling to species with strong heating and cooling effects. We address this by post-processing tracer particles in the SILCC-Zoom molecular cloud simulations. Using a chemical network of 39 species and 299 reactions (including freeze-out of CO and H$_2$O), and a novel iterative algorithm to reconstruct a filled density grid from sparse tracer particle data, we produce time-dependent density distributions for various species. We focus upon the evolution of HCO$^+$, which is a critical formation reactant of CO but is not typically modeled on-the-fly. We analyse the evolution of the tracer particles to assess the regime in which HCO$^+$ production preferentially takes place. We find that the HCO$^+$ content of the cold molecular gas forms in situ around n_\textrm{HCO^+}\simeq10^3-$10^4$ cm$^{-3}$, over a time-scale of approximately 1 Myr, rather than being distributed to this density regime via turbulent mixing from deeper in the cloud. We further show that the dominant HCO$^+$ formation pathway is dependent on the visual extinction, with the reaction H$_3^+$ + CO contributing 90% of the total HCO$^+$ production flux above $A_\textrm{V,3D}=3$. Using our novel grid reconstruction algorithm, we produce the very first maps of the HCO$^+$ column density, $N$(HCO$^+$), and show that it reaches values as high as $10^{15}$ cm$^{-2}$. We find that 50% of the HCO$^+$ mass is located in an $A_\textrm{V}$-range of $\sim$10-30, and in a density range of $10^{3.5}$-$10^{4.5}$ cm$^{-3}$. Finally, we compare our $N$(HCO$^+$) maps to recent observations of W49A and find good agreement.Comment: 23 pages including appendix, 20 figures, submitted to MNRAS, comments are welcom

Physical Properties of Molecular Clouds at 2 pc Resolution in the Low-metallicity Dwarf Galaxy NGC 6822 and the Milky Way

We present the Atacama Large Millimeter/submillimeter Array survey of CO(2-1) emission from the 1/5 solar metallicity, Local Group dwarf galaxy NGC 6822. We achieve high (0buildrel{primeprime}over{.} 9≈ 2 pc) spatial resolution while covering a large area: four 250 pc × 250 pc regions that encompass ˜ 2/3 of NGC 6822's star formation. In these regions, we resolve ˜ 150 compact CO clumps that have small radii (˜2-3 pc), narrow line width (˜ 1 km s-1), and low filling factor across the galaxy. This is consistent with other recent studies of low-metallicity galaxies, but here shown with a 15× larger sample. At parsec scales, CO emission correlates with 8 μ {{m}} emission better than with 24 μ {{m}} emission and anticorrelates with Hα, so that polycyclic aromatic hydrocarbon emission may be an effective tracer of molecular gas at low metallicity. The properties of the CO clumps resemble those of similar-size structures in Galactic clouds except of slightly lower surface brightness and with CO-to-H2 ratio ˜1-2× the Galactic value. The clumps exist inside larger atomic-molecular complexes with masses typical for giant molecular clouds. Using dust to trace H2 for the entire complex, we find the CO-to-H2 ratio to be ˜ 20{--}25× the Galactic value, but with strong dependence on spatial scale and variations between complexes that may track their evolutionary state. The H2-to-H I ratio is low globally and only mildly above unity within the complexes. The ratio of star formation rate to H2 is ˜ 3{--}5× higher in the complexes than in massive disk galaxies, but after accounting for the bias from targeting star-forming regions, we conclude that the global molecular gas depletion time may be as long as in massive disk galaxies