In this paper we compare the molecular gas depletion times and midplane hydrostatic pressure in turbulent, star-forming disk galaxies to internal properties of these galaxies. For this analysis we ...use 17 galaxies from the DYNAMO sample of nearby (z ∼ 0.1) turbulent disks. We find a strong correlation, such that galaxies with lower molecular gas depletion time (tdep) have higher gas velocity dispersion ( ). Within the scatter of our data, our observations are consistent with the prediction that made in theories of feedback-regulated star formation. We also show a strong, single power-law correlation between midplane pressure (P) and star formation rate surface density ( SFR), which extends for 6 orders of magnitude in pressure. Disk galaxies with lower pressure are found to be roughly in agreement with theoretical predictions. However, in galaxies with high pressure we find P/ SFR values that are significantly larger than theoretical predictions. Our observations could be explained with any of the following: (1) the correlation of SFR−P is significantly sublinear; (2) the momentum injected from star formation feedback (p*/m*) is not a single, universal value; or (3) alternate sources of pressure support are important in gas-rich disk galaxies. Finally, using published survey results, we find that our results are consistent with the cosmic evolution of tdep(z) and (z). Our interpretation of these results is that the cosmic evolution of tdep may be regulated not just by the supply of gas but also by the internal regulation of star formation via feedback.
Abstract
We compare the molecular and ionized gas velocity dispersions of nine nearby turbulent disks, analogs to high-redshift galaxies, from the DYNAMO sample using new Atacama Large ...Millimeter/submillimeter Array and GMOS/Gemini observations. We combine our sample with 12 galaxies at
z
∼ 0.5–2.5 from the literature. We find that the resolved velocity dispersion is systematically lower by a factor 2.45 ± 0.38 for the molecular gas compared to the ionized gas, after correcting for thermal broadening. This offset is constant within the galaxy disks and indicates the coexistence of a thin molecular gas disk and a thick ionized one. This result has a direct impact on the Toomre
Q
and pressure derived in galaxies. We obtain pressures ∼0.22 dex lower on average when using the molecular gas velocity dispersion,
σ
0,mol
. We find that
σ
0,mol
increases with gas fraction and star formation rate. We also obtain an increase with redshift and show that the EAGLE and FIRE simulations overall overestimate
σ
0,mol
at high redshift. Our results suggest that efforts to compare the kinematics of gas using ionized gas as a proxy for the total gas may overestimate the velocity dispersion by a significant amount in galaxies at the peak of cosmic star formation. When using the molecular gas as a tracer, our sample is not consistent with predictions from star formation models with constant efficiency, even when including transport as a source of turbulence. Feedback models with variable star formation efficiency,
ϵ
ff
, and/or feedback efficiency,
p
*
/
m
*
, better predict our observations.
The CII 157.74 mu m transition is the dominant coolant of the neutral interstellar gas, and has great potential as a star formation rate (SFR) tracer. Using the Herschel KINGFISH sample of 46 nearby ...galaxies, we investigate the relation of CII surface brightness and luminosity with SFR. We conclude that CII can be used for measurements of SFR on both global and kiloparsec scales in normal star-forming galaxies in the absence of strong active galactic nuclei (AGNs). The uncertainty of the capital sigma sub(CII) - capital sigma sub(SFR) calibration is + or -0.21 dex. The main source of scatter in the correlation is associated with regions that exhibit warm IR colors, and we provide an adjustment based on IR color that reduces the scatter. We show that the color-adjusted capital sigma sub(CII) - capital sigma sub(SFR) correlation is valid over almost five orders of magnitude in capital sigma sub(SFR), holding for both normal star-forming galaxies and non-AGN luminous infrared galaxies. Using CII luminosity instead of surface brightness to estimate SFR suffers from worse systematics, frequently underpredicting SFR in luminous infrared galaxies even after IR color adjustment (although this depends on the SFR measure employed). We suspect that surface brightness relations are better behaved than the luminosity relations because the former are more closely related to the local far-UV field strength, most likely the main parameter controlling the efficiency of the conversion of far-UV radiation into gas heating. A simple model based on Starburst99 population-synthesis code to connect SFR to CII finds that heating efficiencies are l%-3% in normal galaxies.
We present SOFIA/FIFI-LS observations of the C ii 158 m cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO, and H i data to compare C ii emission to dust ...properties, star formation rate (SFR), H2, and H i at 560 pc scales via stacking by environment (spiral arms, interarm, and center), radial profiles, and individual, beam-sized measurements. We attribute 73% of the C ii luminosity to arms, and 19% and 8% to the center and interarm region, respectively. C ii/TIR, C ii/CO, and C ii/PAH radial profiles are largely constant, but rise at large radii ( 8 kpc) and drop in the center ("C ii deficit"). This increase at large radii and the observed decline with the 70 m/100 m dust color are likely driven by radiation field hardness. We find a near proportional C ii-SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. C ii also becomes increasingly luminous relative to CO at low SFR (interarm or large radii), likely indicating more efficient photodissociation of CO and emphasizing the importance of C ii as an H2 and SFR tracer in such regimes. Finally, based on the observed C ii and CO radial profiles and different models, we find CO to increase with radius, in line with the observed metallicity gradient. The low CO (galaxy average 2 M pc−2 (K km s−1)−1) and low C ii/CO ratios (∼400 on average) imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way.
We present a new survey for CO in dwarf galaxies using the ARO Kitt Peak 12 m telescope. This survey consists of observations of the central regions of 121 northern dwarfs with IRAS detections and no ...known CO emission. We detect CO in 28 of these galaxies and marginally detect another 16, increasing by about 50% the number of such galaxies known to have significant CO emission. The galaxies we detect are comparable in stellar and dynamical mass to the Large Magellanic Cloud, although somewhat brighter in CO and fainter in the far-IR. Within dwarfs, we find that the CO luminosity L sub(CO) is most strongly correlated with the K-band and the far-infrared luminosities. There are also strong correlations with the radio continuum (RC) and B-band luminosities and linear diameter. Conversely, we find that far-IR dust temperature is a poor predictor of CO emission within the dwarfs alone, although a good predictor of normalized CO content among a larger sample of galaxies. We suggest that L sub(CO) and L sub(K) correlate well because the stellar component of a galaxy dominates the midplane gravitational field and thus sets the pressure and density of the atomic gas, which control the formation of H sub(2) from H I. We compare our sample with more massive galaxies and find that dwarfs and large galaxies obey the same relationship between CO and the 1.4 GHz RC surface brightness. This relationship is well described by a Schmidt law with capital sigma sub(RC) proportional to capital sigma super(1) sub(C) super(.) sub(O) super(3). Therefore, dwarf galaxies and large spirals exhibit the same relationship between molecular gas and star formation rate (SFR). We find that this result is robust to moderate changes in the RC-to-SFR and CO-to-H sub(2) conversion factors. Our data appear to be inconsistent with large (order of magnitude) variations in the CO-to-H sub(2) conversion factor in the star-forming molecular gas.
Abstract
We present new 3 mm observations of the ionized gas toward the nuclear starburst in the nearby (
D
∼ 3.5 Mpc) galaxy NGC 253. With ALMA, we detect emission from the H40
α
and He40
α
lines in ...the central 200 pc of this galaxy on spatial scales of ∼4 pc. The recombination line emission primarily originates from a population of approximately a dozen embedded super star clusters in the early stages of formation. We find that emission from these clusters is characterized by electron temperatures ranging from 7000 to 10,000 K and measures an average singly ionized helium abundance 〈
Y
+
〉 = 0.25 ± 0.06, both of which are consistent with values measured for H
ii
regions in the center of the Milky Way. We also report the discovery of unusually broad line width recombination line emission originating from seven of the embedded clusters. We suggest that these clusters contribute to the launching of the large-scale hot wind observed to emanate from the central starburst. Finally, we use the measured recombination line fluxes to improve the characterization of overall embedded cluster properties, including the distribution of cluster masses and the fractional contribution of the clustered star formation to the total starburst, which we estimate is at least 50%.
Using free-free emission measured in the Ka band (26-40 GHz) for 10 star-forming regions in the nearby galaxy NGC 6946, including its starbursting nucleus, we compare a number of star formation rate ...(SFR) diagnostics that are typically considered to be unaffected by interstellar extinction. These diagnostics include non-thermal radio (i.e., 1.4 GHz), total infrared (IR; 8-1000 Delta *mm), and warm dust (i.e., 24 Delta *mm) emission, along with hybrid indicators that attempt to account for obscured and unobscured emission from star-forming regions including H Delta *a + 24 Delta *mm and UV + IR measurements. The assumption is made that the 33 GHz free-free emission provides the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24 Delta *mm, H Delta *a + 24 Delta *mm, and UV + IR SFR calibrations are in good agreement with the 33 GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimates the nuclear SFR by a factor of ~2 relative to the 33 GHz free-free SFR. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the 33 GHz free-free SFRs for the nucleus and underestimate the SFRs from the extranuclear star-forming regions by an average factor of ~2 and ~4-5 before and after subtracting local background emission, respectively. This result likely arises from the cosmic-ray (CR) electrons decaying within the starburst region with negligible escape, whereas the transient nature of star formation in the young extranuclear star-forming complexes allows for CR electrons to diffuse significantly further than dust-heating photons, resulting in an underestimate of the true SFR. Finally, we find that the SFRs estimated using the total 33 GHz flux density appear to agree well with those estimated using free-free emission due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33 GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.
Context. Carbon monoxide (CO) is widely used as a tracer of the molecular gas in almost all types of environments. However, several shortcomings of CO complicate usaging it as H2 tracer, such as its ...optical depth effects, the dependence of its abundance on metallicity, or its susceptibility to dissociation in highly irradiated regions. Neutral carbon emission has been proposed to overcome some of these shortcomings and hence to help revealing the limits of CO as a measure of the molecular gas. Aims. We aim to study the general characteristics of the spatially and spectrally resolved carbon line emission in a variety of extragalactic sources and evaluate its potential as complementary H2 tracer to CO. Methods. We used the Atacama Compact Array to map the CI(3P1–3P0) line emission in the nearby starburst galaxy NGC 253 at unprecedented angular resolution (~3′′). This is the first well-resolved interferometric CI map of an extragalactic source. Results. We have detected the CI line emission at high significance levels along the central disk of NGC 253 and its edges where expanding shells have previously been found in CO. Globally, the distribution of the CI line emission strongly resembles that of CO, confirming the results of previous Galactic surveys that CI traces the same molecular gas as CO. However, we also identify a significant increase of CI line emission with respect to CO in (some of) the outflow or shocked regions of NGC 253, namely the bipolar outflow emerging from the nucleus. A first-order estimate of the CI column densities indicates abundances of CI that are very similar to the abundance of CO in NGC 253. Interestingly, we find that the CI line is marginally optically thick within the disk. Conclusions. The enhancement of the CI/CO line ratios (~0.4−0.6) with respect to Galactic values (≤0.1), especially in the shocked regions of NGC 253, clearly indicates that mechanical perturbation such as shocks and the strong radiation fields in this starburst galaxy have a marked effect on the carbon excitation and/or abundance.
We present an update to the ultraviolet-to-radio database of global broadband photometry for the 79 nearby galaxies that comprise the union of the KINGFISH (Key Insights on Nearby Galaxies: A ...Far-Infrared Survey with Herschel) and SINGS (Spitzer Infrared Nearby Galaxies Survey) samples. The 34-band data set presented here includes contributions from observational work carried out with a variety of facilities including GALEX, SDSS, Pan-STARRS1, NOAO, 2MASS, Wide-Field Infrared Survey Explorer, Spitzer, Herschel, Planck, JCMT, and the VLA. Improvements of note include recalibrations of previously published SINGS BVRCIC and KINGFISH far-infrared/submillimeter photometry. Similar to previous results in the literature, an excess of submillimeter emission above model predictions is seen primarily for low-metallicity dwarf or irregular galaxies. This 33-band photometric data set for the combined KINGFISH+SINGS sample serves as an important multiwavelength reference for the variety of galaxies observed at low redshift. A thorough analysis of the observed spectral energy distributions is carried out in a companion paper.
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