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تسجيل مستخدم جديدA Constant Molecular Gas Depletion Time in Nearby Disk Galaxies
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We combine new sensitive, wide-field CO data from the HERACLES survey with ultraviolet and infrared data from GALEX and Spitzer to compare the surface densities of H2, Sigma_H2, and recent star formation rate, Sigma_SFR, over many thousands of positions in 30 nearby disk galaxies. We more than quadruple the size of the galaxy sample compared to previous work and include targets with a wide range of galaxy properties. Even though the disk galaxies in this study span a wide range of properties, we find a strong and approximately linear correlation between Sigma_SFR and Sigma_H2 at our common resolution of 1kpc. This implies a roughly constant median H2 consumption time, tau_H2 = Sigma_H2 / Sigma_SFR, of ~2.35Gyr (including heavy elements) across our sample. At 1kpc resolution, there is only a weak correlation between Sigma_H2 and tau_H2 over the range Sigma_H2~5-100M_sun/pc^2, which is probed by our data. We compile a broad set of literature measurements that have been obtained using a variety of star formation tracers, sampling schemes and physical scales and show that overall, these data yield almost exactly the same results, although with more scatter. We interpret these results as strong, albeit indirect evidence that star formation proceeds in a uniform way in giant molecular clouds in the disks of spiral galaxies.
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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 univ
ersal 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.
We use the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities. Using atomic hydrogen (HI) data, mostly from THINGS, we predict the local mean CO velocity from the mean HI velocity. By renormalizing t
he CO velocity axis so that zero corresponds to the local mean HI velocity we are able to stack spectra coherently over large regions as function of radius. This enables us to measure CO intensities with high significance as low as Ico = 0.3 K km/s (H2_SD = 1 Msun/pc2), an improvement of about one order of magnitude over previous studies. We detect CO out to radii Rgal = R25 and find the CO radial profile to follow a uniform exponential decline with scale length of 0.2 R25. Comparing our sensitive CO profiles to matched profiles of HI, Halpha, FUV, and IR emission at 24um and 70um, we observe a tight, roughly linear relation between CO and IR intensity that does not show any notable break between regions that are dominated by molecular (H2) gas (H2_SD > HI_SD) and those dominated by atomic gas (H2_SD < HI_SD). We use combinations of FUV+24um and Halpha+24um to estimate the recent star formation rate (SFR) surface density, SFR_SD, and find approximately linear relations between SFR_SD and H2_SD. We interpret this as evidence for stars forming in molecular gas with little dependence on the local total gas surface density. While galaxies display small internal variations in the SFR-to-H2 ratio, we do observe systematic galaxy-to-galaxy variations. These galaxy-to-galaxy variations dominate the scatter in relations between CO and SFR tracers measured at large scales. The variations have the sense that less massive galaxies exhibit larger ratios of SFR-to-CO than massive galaxies. Unlike the SFR-to-CO ratio, the balance between HI and H2 depends strongly on the total gas surface density and radius. It must also depend on additional parameters.
Studies of nearby galaxies including the Milky Way have provided fundamental information on the evolution of structure in the Universe, the existence and nature of dark matter, the origin and evolution of galaxies, and the global features of star for
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High resolution, multi-wavelength maps of a sizeable set of nearby galaxies have made it possible to study how the surface densities of HI, H2 and star formation rate (Sigma_HI, Sigma_H2, Sigma_SFR) relate on scales of a few hundred parsecs. At these
scales, individual galaxy disks are comfortably resolved, making it possible to assess gas-SFR relations with respect to environment within galaxies. Sigma_H2, traced by CO intensity, shows a strong correlation with Sigma_SFR and the ratio between these two quantities, the molecular gas depletion time, appears to be constant at about 2Gyr in large spiral galaxies. Within the star-forming disks of galaxies, Sigma_SFR shows almost no correlation with Sigma_HI. In the outer parts of galaxies, however, Sigma_SFR does scale with Sigma_HI, though with large scatter. Combining data from these different environments yields a distribution with multiple regimes in Sigma_gas - Sigma_SFR space. If the underlying assumptions to convert observables to physical quantities are matched, even combined datasets based on different SFR tracers, methodologies and spatial scales occupy a well define locus in Sigma_gas - Sigma_SFR space.
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ral nearby galaxies that host AGNs, such as the nearest radio galaxy Centaurus A (NGC 5128), the Seyfert 2 galaxy M51 (NGC 5194), the Seyfert 2 galaxy NGC 1068, the Seyfert 1 galaxy NGC 1097, and the Seyfert 2 / starburst composite galaxy NGC 4945, in CO lines to see whether the molecular gas distribution, kinematics, and physical conditions at 10 - 100 pc scale follows the AGN unified model or not. In 100 pc scale, most of the circumnuclear molecular gas shows smooth velocity gradient, suggesting a regular rotating feature, and also shows abnormal line ratios, suggesting the existence of active sources to make the circumnuclear molecular gas dense and/or warm conditions or abnormal chemical compositions. In 10 pc scale, on the other hand, the molecular gas kinematics shows various characteristics, some shows very disturbed kinematics such as a jet-entrained feature in the galaxies that have jets, but some still shows regular rotation feature in a galaxy that does not have obvious jets. These results indicate that the kinematics and physical/chemical conditions of the circumnuclear molecular gas at the scale less than 100 pc is highly affected by the AGN activities, and at this scale, there is no clear evidence of any unified feature seen in the circumnuclear molecular gas.
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