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Register a new userProperties and rotation of molecular clouds in M 33
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The sample of 566 molecular clouds identified in the CO(2--1) IRAM survey covering the disk of M~33 is explored in detail.The clouds were found using CPROPS and were subsequently catalogued in terms of their star-forming properties as non-star-forming (A), with embedded star formation (B), or with exposed star formation C.We find that the size-linewidth relation among the M~33 clouds is quite weak but, when comparing with clouds in other nearby galaxies, the linewidth scales with average metallicity.The linewidth and particularly the line brightness decrease with galactocentric distance.The large number of clouds makes it possible to calculate well-sampled cloud mass spectra and mass spectra of subsamples.As noted earlier, but considerably better defined here, the mass spectrum steepens (i.e. higher fraction of small clouds) with galactocentric distance.A new finding is that the mass spectrum of A clouds is much steeper than that of the star-forming clouds.Further dividing the sample, this difference is strong at both large and small galactocentric distances and the A vs C difference is a stronger effect than the inner/outer disk difference in mass spectra.Velocity gradients are identified in the clouds using standard techniques.The gradients are weak and are dominated by prograde rotation; the effect is stronger for the high signal-to-noise clouds.A discussion of the uncertainties is presented.The angular momenta are low but compatible with at least some simulations.The cloud and galactic gradients are similar; the cloud rotation periods are much longer than cloud lifetimes and comparable to the galactic rotation period.The rotational kinetic energy is 1-2% of the gravitational potential energy and the cloud edge velocity is well below the escape velocity, such that cloud-scale rotation probably has little influence on the evolution of molecular clouds.
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The grand-design spiral galaxy M~51 was observed at 40pc resolution in CO(1--0) by the PAWS project. A large number of molecular clouds were identified and we search for velocity gradients in two high signal-to-noise subsamples, containing 682 and 376 clouds. The velocity gradients are found to be systematically prograde oriented, as was previously found for the rather flocculent spiral M~33. This strongly supports the idea that the velocity gradients reflect cloud rotation, rather than more random dynamical forces, such as turbulence. Not only are the gradients prograde, but their $frac{partial v}{partial x}$ and $frac{partial v}{partial y}$ coefficients follow galactic shear in sign, although with a lower amplitude. No link is found between the orientation of the gradient and the orientation of the cloud. The values of the cloud angular momenta appear to be an extension of the values noted for galactic clouds despite the orders of magnitude difference in cloud mass. Roughly 30% of the clouds show retrograde velocity gradients. For a strictly rising rotation curve, as in M~51, gravitational contraction would be expected to yield strictly prograde rotators within an axisymmetric potential. In M~51, the fraction of retrograde rotators is found to be higher in the spiral arms than in the disk as a whole. Along the leading edge of the spiral arms, a majority of the clouds are retrograde rotators. While this work should be continued on other nearby galaxies, the M~33 and M~51 studies have shown that clouds rotate and that they rotate mostly prograde, although the amplitudes are not such that rotational energy is a significant support mechanism against gravitation. In this work, we show that retrograde rotation is linked to the presence of a spiral gravitational potential.
Does star formation proceed in the same way in large spirals such as the Milky Way and in smaller chemically younger galaxies? Earlier work suggests a more rapid transformation of H$_2$ into stars in these objects but (1) a doubt remains about the validity of the H$_2$ mass estimates and (2) there is currently no explanation for why star formation should be more efficient. M~33, a local group spiral with a mass $sim 10$% and a metallicity half that of the Galaxy, represents a first step towards the metal poor Dwarf Galaxies. We have searched for molecular clouds in the outer disk of M~33 and present here a set of detections of both $^{12}$CO and $^{13}$CO, including the only detections (for both lines) beyond the R$_{25}$ radius in a subsolar metallicity galaxy. The spatial resolution enables mass estimates for the clouds and thus a measure of the $N({rm H}_2) / I_{rm CO}$ ratio, which in turn enables a more reliable calculation of the H$_2$ mass. Our estimate for the outer disk of M~33 is $N({rm H}_2) / I_{rm CO(1-0)} sim 5 times 10^{20} ,{rm cm^{-2}/(K{rm km s^{-1}})}$ with an estimated uncertainty of a factor $le 2$. While the $^{12/13}$CO line ratios do not provide a reliable measure of $N({rm H}_2) / I_{rm CO}$, the values we find are slightly greater than Galactic and corroborate a somewhat higher $N({rm H}_2) / I_{rm CO}$ value. Comparing the CO observations with other tracers of the interstellar medium, no reliable means of predicting where CO would be detected was identified. In particular, CO detections were often not directly on local HI or FIR or H$alpha$ peaks, although generally in regions with FIR emission and high HI column density. The results presented here provide support for the quicker transformation of H$_2$ into stars in M~33 than in large local universe spirals.
The relative abundance of the dust grain types in the interstellar medium (ISM) is directly linked to physical quantities that trace the evolution of galaxies. We study the dust properties of the whole disc of M33 at spatial scales of ~170 pc. This analysis allows us to infer how the relative dust grain abundance changes with the conditions of the ISM, study the existence of a submillimetre excess and look for trends of the gas-to-dust mass ratio (GDR) with other physical properties of the galaxy. For each pixel in the disc of M33 we fit the infrared SED using a physically motivated dust model that assumes an emissivity index beta close to 2. We derive the relative amount of the different dust grains in the model, the total dust mass, and the strength of the interstellar radiation field (ISRF) heating the dust at each spatial location. The relative abundance of very small grains tends to increase, and for big grains to decrease, at high values of Halpha luminosity. This shows that the dust grains are modified inside the star-forming regions, in agreement with a theoretical framework of dust evolution under different physical conditions. The radial dependence of the GDR is consistent with the shallow metallicity gradient observed in this galaxy. The strength of the ISRF derived in our model correlates with the star formation rate in the galaxy in a pixel by pixel basis. Although this is expected it is the first time that a correlation between both quantities is reported. We produce a map of submillimetre excess in the 500 microns SPIRE band for the disc of M33. The excess can be as high as 50% and increases at large galactocentric distances. We further study the relation of the excess with other physical properties of the galaxy and find that the excess is prominent in zones of diffuse ISM outside the main star-forming regions, where the molecular gas and dust surface density are low.
Star formation activity depends on galactic-scale environments. To understand the variations in star formation activity, comparing the properties of giant molecular clouds (GMCs) among environments with different star formation efficiency (SFE) is necessary. We thus focus on a strongly barred galaxy to investigate the impact of the galactic environment on the GMCs properties, because the SFE is clearly lower in bar regions than in arm regions. In this paper, we present the $^{12}$CO($1-0$) observations toward the western bar, arm and bar-end regions of the strongly barred galaxy NGC1300 with ALMA 12-m array at a high angular resolution of $sim$40 pc. We detected GMCs associated with the dark lanes not only in the arm and bar-end regions but also in the bar region, where massive star formation is not seen. Using the CPROPS algorithm, we identified and characterized 233 GMCs across the observed regions. Based on the Kolmogorov-Smirnov test, we find that there is virtually no significant variations in GMC properties (e.g., radius, velocity dispersion, molecular gas mass, and virial parameter) among the bar, arm and bar-end region. These results suggest that systematic differences in the physical properties of the GMCs are not the cause for SFE differences with environments, and that there should be other mechanisms which control the SFE of the GMCs such as fast cloud-cloud collisions in NGC1300.
Context. Using observations to deduce dust properties, grain size distribution, and physical conditions in molecular clouds is a highly degenerate problem. Aims. The coreshine phenomenon, a scattering process at 3.6 and 4.5 $mu$m that dominates absorption, has revealed its ability to explore the densest parts of clouds. We want to use this effect to constrain the dust parameters. The goal is to investigate to what extent grain growth (at constant dust mass) inside molecular clouds is able to explain the coreshine observations. We aim to find dust models that can explain a sample of Spitzer coreshine data. We also look at the consistency with near-infrared data we obtained for a few clouds. Methods. We selected four regions with a very high occurrence of coreshine cases: Taurus-Perseus, Cepheus, Chameleon and L183/L134. We built a grid of dust models and investigated the key parameters to reproduce the general trend of surface bright- nesses and intensity ratios of both coreshine and near-infrared observations with the help of a 3D Monte-Carlo radiative transfer code. The grid parameters allow to investigate the effect of coagulation upon spherical grains up to 5 $mu$m in size derived from the DustEm diffuse interstellar medium grains. Fluffiness (porosity or fractal degree), ices, and a handful of classical grain size distributions were also tested. We used the near- and mostly mid-infrared intensity ratios as strong discriminants between dust models. Results. The determination of the background field intensity at each wavelength is a key issue. In particular, an especially strong background field explains why we do not see coreshine in the Galactic plane at 3.6 and 4.5 $mu$m. For starless cores, where detected, the observed 4.5 $mu$m / 3.6 $mu$m coreshine intensity ratio is always lower than $sim$0.5 which is also what we find in the models for the Taurus-Perseus and L183 directions. Embedded sources can lead to higher fluxes (up to four times greater than the strongest starless core fluxes) and higher coreshine ratios (from 0.5 to 1.1 in our selected sample). Normal interstellar radiation field conditions are sufficient to find suitable grain models at all wavelengths for starless cores. The standard interstellar grains are not able to reproduce observations and, due to the multi-wavelength approach, only a few grain types meet the criteria set by the data. Porosity does not affect the flux ratios while the fractal dimension helps to explain coreshine ratios but does not seem able to reproduce near-infrared observations without a mix of other grain types. Conclusions. Combined near- and mid-infrared wavelengths confirm the potential to reveal the nature and size distribution of dust grains. Careful assessment of the environmental parameters (interstellar and background fields, embedded or nearby reddened sources) is required to validate this new diagnostic.
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