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Leo Blitz - One of the best experts on this subject based on the ideXlab platform.
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Giant Molecular Clouds in the early type galaxy ngc4526
arXiv: Astrophysics of Galaxies, 2015Co-Authors: Dyas Utomo, Leo Blitz, Erik Rosolowsky, Timothy A Davis, M Bureau, Michele Cappellari, Marc SarziAbstract:We present a high spatial resolution ($\approx 20$ pc) of $^{12}$CO($2-1$) observations of the lenticular galaxy NGC4526. We identify 103 resolved Giant Molecular Clouds (GMCs) and measure their properties: size $R$, velocity dispersion $\sigma_v$, and luminosity $L$. This is the first GMC catalog of an early-type galaxy. We find that the GMC population in NGC4526 is gravitationally bound, with a virial parameter $\alpha \sim 1$. The mass distribution, $dN/dM \propto M^{-2.39 \pm 0.03}$, is steeper than that for GMCs in the inner Milky Way, but comparable to that found in some late-type galaxies. We find no size-linewidth correlation for the NGC4526 Clouds, in contradiction to the expectation from Larson's relation. In general, the GMCs in NGC4526 are more luminous, denser, and have a higher velocity dispersion than equal size GMCs in the Milky Way and other galaxies in the Local Group. These may be due to higher interstellar radiation field than in the Milky Way disk and weaker external pressure than in the Galactic center. In addition, a kinematic measurement of cloud rotation shows that the rotation is driven by the galactic shear. For the vast majority of the Clouds, the rotational energy is less than the turbulent and gravitational energy, while the four innermost Clouds are unbound and will likely be torn apart by the strong shear at the galactic center. We combine our data with the archival data of other galaxies to show that the surface density $\Sigma$ of GMCs is not approximately constant as previously believed, but varies by $\sim 3$ orders of magnitude. We also show that the size and velocity dispersion of GMC population across galaxies are related to the surface density, as expected from the gravitational and pressure equilibrium, i.e. $\sigma_v R^{-1/2} \propto \Sigma^{1/2}$.
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angular momentum in Giant Molecular Clouds ii m33
The Astrophysical Journal, 2011Co-Authors: Nia Imara, Frank Bigiel, Leo BlitzAbstract:We present an analysis comparing the properties of 45 Giant Molecular Clouds (GMCs) in M33 and the atomic hydrogen (H I) with which they are associated. High-resolution Very Large Array observations are used to measure the properties of H I in the vicinity of GMCs and in regions where GMCs have not been detected. The majority of Molecular Clouds coincide with a local peak in the surface density of atomic gas, though 7% of GMCs in the sample are not associated with high surface density atomic gas. The mean H I surface density in the vicinity of GMCs is 10 M{sub sun} pc{sup -2} and tends to increase with GMC mass as {Sigma}{sub HI} {proportional_to} M{sup 0.27}{sub GMC}. Thirty-nine of the 45 H I regions surrounding GMCs have linear velocity gradients of {approx}0.05 km s{sup -1} pc{sup -1}. If the linear gradients previously observed in the GMCs result from rotation, 53% are counterrotating with respect to the local H I. And if the linear gradients in these local H I regions are also from rotation, 62% are counterrotating with respect to the galaxy. If magnetic braking reduced the angular momentum of GMCs early in their evolution, the angular velocity ofmore » GMCs would be roughly one order of magnitude lower than what is observed. Based on our observations, we consider the possibility that GMCs may not be rotating. Atomic gas not associated with GMCs has gradients closer to 0.03 km s{sup -1} pc{sup -1}, suggesting that events occur during the course of GMC evolution that may increase the shear in the atomic gas.« less
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angular momentum in Giant Molecular Clouds ii m33
arXiv: Cosmology and Nongalactic Astrophysics, 2011Co-Authors: Nia Imara, Frank Bigiel, Leo BlitzAbstract:We present an analysis comparing the properties of 45 Giant Molecular Clouds (GMCs) in M33 and the atomic hydrogen (HI) with which they are associated. High-resolution VLA observations are used to measure the properties of HI in the vicinity of GMCs and in regions where GMCs have not been detected. The majority of Molecular Clouds coincide with a local peak in the surface density of atomic gas, though 7% of GMCs in the sample are not associated with high-surface density atomic gas. The mean HI surface density in the vicinity of GMCs is 10 M_sol/pc^2 and tends to increase with GMC mass as Sigma_HI ~ M_GMC^0.27. 39 of the 45 HI regions surrounding GMCs have linear velocity gradients of ~0.05 km/s/pc. If the linear gradients previously observed in the GMCs result from rotation, then 53% are counterrotating with respect to the local HI. If the linear gradients in these local HI regions are also from rotation, 62% are counterrotating with respect to the galaxy. If magnetic braking reduced the angular momentum of GMCs early in their evolution, the angular velocity of GMCs would be roughly one order of magnitude lower than what is observed. Based on our observations, we consider the possibility that GMCs may not be rotating. Atomic gas not associated with GMCs has gradients closer to 0.03 km/s/pc, suggesting that events occur during the course of GMC evolution that may increase the shear in the atomic gas.
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the resolved properties of extragalactic Giant Molecular Clouds
arXiv: Astrophysics, 2008Co-Authors: Alberto D Bolatto, A K Leroy, Erik Rosolowsky, Fabian Walter, Leo BlitzAbstract:We use high spatial resolution observations of CO to systematically measure the resolved size-line width, luminosity-line width, luminosity-size, and the mass-luminosity relations of Giant Molecular Clouds (GMCs) in a variety of extragalactic systems. Although the data are heterogeneous we analyze them in a consistent manner to remove the biases introduced by limited sensitivity and resolution, thus obtaining reliable sizes, velocity dispersions, and luminosities. We compare the results obtained in dwarf galaxies with those from the Local Group spiral galaxies. We find that extragalactic GMC properties measured across a wide range of environments are very much compatible with those in the Galaxy. We use these results to investigate metallicity trends in the cloud average column density and virial CO-to-H2 factor. We find that these measurements do not accord with simple predictions from photoionization-regulated star formation theory, although this could be due to the fact that we do not sample small enough spatial scales or the full gravitational potential of the Molecular cloud. We also find that the virial CO-to-H2 conversion factor in CO-bright GMCs is very similar to Galactic, and that the excursions do not show a measurable metallicity trend. We contrast these results with estimates of Molecular mass based on far-infrared measurements obtained for the Small Magellanic Cloud, which systematically yield larger masses, and interpret this discrepancy as arising from large H2 envelopes that surround the CO-bright cores. We conclude that GMCs identified on the basis of their CO emission are a unique class of object that exhibit a remarkably uniform set of properties from galaxy to galaxy (abridged).
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Giant Molecular Clouds in M64
The Astrophysical Journal, 2005Co-Authors: Erik Rosolowsky, Leo BlitzAbstract:(abridged) We investigate the properties of Giant Molecular Clouds (GMCs) in the molecule-rich galaxy M64 (NGC 4826). In M64, the mean surface density of Molecular gas is 2N(H_2) ~ 10^22 over a 2 kpc region, equal to the surface densities of individual GMCs in the Milky Way. We observed the J=1->0 transitions of CO, 13CO, and HCN. The line ratio W_CO/W_13CO for 200 pc < R_gal < 800 pc is comparable to that found in the Milky Way and increases significantly outside this region, in part due to a large contribution to the CO emission from diffuse gas, which composes 25% of the Molecular mass in the galaxy. We developed a modified CLUMPFIND algorithm to decompose the 13CO emission into 25 resolved Clouds. The Clouds have a luminosity--linewidth relationship L ~ DV^{2.2+/-0.4}, substantially different from the Milky Way trend reported by Solomon et al. (1987): L ~ DV^5. Similarly, the Clouds have a linewidth--size relationship of DV ~ R_e^{1.0 +/- 0.3} compared to DV ~ R_e^0.5 in the Milky Way. Estimates of the kinetic and binding energies of the Clouds suggest that the Clouds are self-gravitating and significantly overpressured with respect to the remainder of the ISM in M64. The M64 Clouds have a mean surface density at least 2.5 times larger than observed in Local Group GMCs, and the surface density is not independent of mass as it is in the Local Group: \Sigma_H2 ~ M^{0.7+/-0.2}. The Clouds are correlated with the recombination emission from the galaxy, implying that they are star forming; the rate is comparable to that in other galaxies despite the increased densities of the Clouds. We note that the internal pressures of Clouds in several galaxies scales with the external pressure exerted on the Clouds by the ambient ISM: P_int ~ P_ext^{0.75+/-0.05}.
Erik Rosolowsky - One of the best experts on this subject based on the ideXlab platform.
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Giant Molecular Clouds in the early type galaxy ngc4526
arXiv: Astrophysics of Galaxies, 2015Co-Authors: Dyas Utomo, Leo Blitz, Erik Rosolowsky, Timothy A Davis, M Bureau, Michele Cappellari, Marc SarziAbstract:We present a high spatial resolution ($\approx 20$ pc) of $^{12}$CO($2-1$) observations of the lenticular galaxy NGC4526. We identify 103 resolved Giant Molecular Clouds (GMCs) and measure their properties: size $R$, velocity dispersion $\sigma_v$, and luminosity $L$. This is the first GMC catalog of an early-type galaxy. We find that the GMC population in NGC4526 is gravitationally bound, with a virial parameter $\alpha \sim 1$. The mass distribution, $dN/dM \propto M^{-2.39 \pm 0.03}$, is steeper than that for GMCs in the inner Milky Way, but comparable to that found in some late-type galaxies. We find no size-linewidth correlation for the NGC4526 Clouds, in contradiction to the expectation from Larson's relation. In general, the GMCs in NGC4526 are more luminous, denser, and have a higher velocity dispersion than equal size GMCs in the Milky Way and other galaxies in the Local Group. These may be due to higher interstellar radiation field than in the Milky Way disk and weaker external pressure than in the Galactic center. In addition, a kinematic measurement of cloud rotation shows that the rotation is driven by the galactic shear. For the vast majority of the Clouds, the rotational energy is less than the turbulent and gravitational energy, while the four innermost Clouds are unbound and will likely be torn apart by the strong shear at the galactic center. We combine our data with the archival data of other galaxies to show that the surface density $\Sigma$ of GMCs is not approximately constant as previously believed, but varies by $\sim 3$ orders of magnitude. We also show that the size and velocity dispersion of GMC population across galaxies are related to the surface density, as expected from the gravitational and pressure equilibrium, i.e. $\sigma_v R^{-1/2} \propto \Sigma^{1/2}$.
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the resolved properties of extragalactic Giant Molecular Clouds
arXiv: Astrophysics, 2008Co-Authors: Alberto D Bolatto, A K Leroy, Erik Rosolowsky, Fabian Walter, Leo BlitzAbstract:We use high spatial resolution observations of CO to systematically measure the resolved size-line width, luminosity-line width, luminosity-size, and the mass-luminosity relations of Giant Molecular Clouds (GMCs) in a variety of extragalactic systems. Although the data are heterogeneous we analyze them in a consistent manner to remove the biases introduced by limited sensitivity and resolution, thus obtaining reliable sizes, velocity dispersions, and luminosities. We compare the results obtained in dwarf galaxies with those from the Local Group spiral galaxies. We find that extragalactic GMC properties measured across a wide range of environments are very much compatible with those in the Galaxy. We use these results to investigate metallicity trends in the cloud average column density and virial CO-to-H2 factor. We find that these measurements do not accord with simple predictions from photoionization-regulated star formation theory, although this could be due to the fact that we do not sample small enough spatial scales or the full gravitational potential of the Molecular cloud. We also find that the virial CO-to-H2 conversion factor in CO-bright GMCs is very similar to Galactic, and that the excursions do not show a measurable metallicity trend. We contrast these results with estimates of Molecular mass based on far-infrared measurements obtained for the Small Magellanic Cloud, which systematically yield larger masses, and interpret this discrepancy as arising from large H2 envelopes that surround the CO-bright cores. We conclude that GMCs identified on the basis of their CO emission are a unique class of object that exhibit a remarkably uniform set of properties from galaxy to galaxy (abridged).
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Giant Molecular Clouds in M31: I - Molecular Cloud Properties
The Astrophysical Journal, 2007Co-Authors: Erik RosolowskyAbstract:We present Berkeley Illinois Maryland Association (BIMA) millimeter interferometer observations of Giant Molecular Clouds (GMCs) along a spiral arm in M31. The observations consist of a survey using the compact configuration of the interferometer and follow-up, higher-resolution observations on a subset of the detections in the survey. The data are processed using an analysis algorithm designed to extract GMCs and correct their derived properties for observational biases thereby facilitating comparison with Milky Way data. The algorithm identifies 67 GMCs of which 19 have sufficient signal-to-noise to accurately measure their properties. The GMCs in this portion of M31 are indistinguishable from those found in the Milky Way, having a similar size-line width relationship and distribution of virial parameters, confirming the results of previous, smaller studies. The velocity gradients and angular momenta of the GMCs are comparable to the values measured in M33 and the Milky Way; and, in all cases, are below expected values based on the local galactic shear. The studied region of M31 has a similar interstellar radiation field, metallicity, Toomre Q parameter, and midplane volume density as the inner Milky Way, so the similarity of GMC populations between the two systems is not surprising.
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Giant Molecular Clouds in m31 i Molecular cloud properties
The Astrophysical Journal, 2007Co-Authors: Erik RosolowskyAbstract:We present Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometer observations of Giant Molecular Clouds (GMCs) along a spiral arm in M31. The observations consist of a survey using the compact configuration of the interferometer and follow-up, higher resolution observations on a subset of the detections in the survey. The data are processed using an analysis algorithm designed to extract GMCs and correct their derived properties for observational biases, thereby facilitating comparison with Milky Way data. The algorithm identifies 67 GMCs, of which 19 have a sufficient signal-to-noise ratio to accurately measure their properties. The GMCs in this portion of M31 are indistinguishable from those found in the Milky Way, having a similar size-line width relationship and distribution of virial parameters, confirming the results of previous, smaller studies. The velocity gradients and angular momenta of the GMCs are comparable to the values measured in M33 and the Milky Way, and in all cases are below expected values based on the local galactic shear. The studied region of M31 has an interstellar radiation field, metallicity, Toomre Q parameter, and midplane volume density similar to those of the inner Milky Way, so the similarity of GMC populations between the two systems is not surprising.
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Giant Molecular Clouds in M64
The Astrophysical Journal, 2005Co-Authors: Erik Rosolowsky, Leo BlitzAbstract:(abridged) We investigate the properties of Giant Molecular Clouds (GMCs) in the molecule-rich galaxy M64 (NGC 4826). In M64, the mean surface density of Molecular gas is 2N(H_2) ~ 10^22 over a 2 kpc region, equal to the surface densities of individual GMCs in the Milky Way. We observed the J=1->0 transitions of CO, 13CO, and HCN. The line ratio W_CO/W_13CO for 200 pc < R_gal < 800 pc is comparable to that found in the Milky Way and increases significantly outside this region, in part due to a large contribution to the CO emission from diffuse gas, which composes 25% of the Molecular mass in the galaxy. We developed a modified CLUMPFIND algorithm to decompose the 13CO emission into 25 resolved Clouds. The Clouds have a luminosity--linewidth relationship L ~ DV^{2.2+/-0.4}, substantially different from the Milky Way trend reported by Solomon et al. (1987): L ~ DV^5. Similarly, the Clouds have a linewidth--size relationship of DV ~ R_e^{1.0 +/- 0.3} compared to DV ~ R_e^0.5 in the Milky Way. Estimates of the kinetic and binding energies of the Clouds suggest that the Clouds are self-gravitating and significantly overpressured with respect to the remainder of the ISM in M64. The M64 Clouds have a mean surface density at least 2.5 times larger than observed in Local Group GMCs, and the surface density is not independent of mass as it is in the Local Group: \Sigma_H2 ~ M^{0.7+/-0.2}. The Clouds are correlated with the recombination emission from the galaxy, implying that they are star forming; the rate is comparable to that in other galaxies despite the increased densities of the Clouds. We note that the internal pressures of Clouds in several galaxies scales with the external pressure exerted on the Clouds by the ambient ISM: P_int ~ P_ext^{0.75+/-0.05}.
Pieter C. Van Der Kruit - One of the best experts on this subject based on the ideXlab platform.
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The volume densities of Giant Molecular Clouds in M83
Astronomy & Astrophysics, 2008Co-Authors: J. S. Heiner, Ronald J. Allen, O. Ivy Wong, Pieter C. Van Der KruitAbstract:Using observed GALEX far-ultraviolet (FUV) fluxes and VLA images of the 21-cm HI column densities, along with estimates of the local dust abundances, we measure the volume densities of a sample of actively star-forming Giant Molecular Clouds (GMCs) in the nearby spiral galaxy M83 on a typical resolution scale of 170 pc. Our approach is based on an equilibrium model for the cycle of Molecular hydrogen formation on dust grains and photodissociation under the influence of the FUV radiation on the cloud surfaces of GMCs. We find a range of total volume densities on the surface of GMCs in M83, namely 0.1 - 400 cm-3 inside R25, 0.5 - 50 cm-3 outside R25 . Our data include a number of GMCs in the HI ring surrounding this galaxy. Finally, we discuss the effects of observational selection, which may bias our results.
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The Volume Densities of Giant Molecular Clouds in M81
The Astrophysical Journal, 2008Co-Authors: J. S. Heiner, Ronald J. Allen, Bjorn Emonts, Pieter C. Van Der KruitAbstract:H I features near young star clusters in M81 are identified as the photodissociated surfaces of Giant Molecular Clouds (GMCs) from which the young stars have recently formed. The H I column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 × 1021 cm −2 near R ≈ 7.5 kpc. This trend is similar to that of the radially averaged H I distribution in this galaxy, and implies a constant area covering factor of ≈0.21 for GMCs throughout M81. The incident UV fluxes G0 of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the H I column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly Molecular hydrogen. Volume densities of 1 < n < 200 cm −3 are derived, with a geometric mean of 17 cm−3. These values are similar to the densities of GMCs in the Galaxy, but somewhat lower than those found earlier for M101 with similar methods. Low values of Molecular density in the GMCs of M81 will result in low levels of collisional excitation of the CO(1-0) transition, and are consistent with the very low surface brightness of CO(1-0) emission observed in the disk of M81.
Asao Habe - One of the best experts on this subject based on the ideXlab platform.
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properties of Giant Molecular Clouds in the strongly barred galaxy ngc 1300
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: Fumiya Maeda, Kouji Ohta, Yusuke Fujimoto, Asao HabeAbstract: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.
Guo-xuan Song - One of the best experts on this subject based on the ideXlab platform.
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The effect of rigid rotation in the formation of Giant Molecular Clouds by aggregation
Chinese Astronomy and Astrophysics, 2000Co-Authors: Tong-jie Zhang, Guo-xuan SongAbstract:The formation of Giant Molecular Clouds (GMCs) by aggregation in a rigidly rotating disk galaxy is studied. The result shows that the GMCs so formed consist mainly of individual Clouds located close to one another from inelastic collision and self-gravitation under a velocity dispersion. The Clouds so formed are not very massive. If there is differential rotation, then there is a greater chance of more massive Clouds being formed.
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Formation of Giant Molecular Clouds by aggregation in a Galactic disc with rigid rotation
Journal of Astrophysics and Astronomy, 1999Co-Authors: Tong-jie Zhang, Guo-xuan SongAbstract:We have explored a model for the formation of Giant Molecular Clouds through accretion of smaller Clouds; the star disc is assumed to be rigidly rotating in this model. The main results are: (1) The aggregates consist mainly of the neighbouring Clouds whose collision was induced by the velocity dispersion. In this model aggregates take a shorter time to form compared to the model which includes differential rotation, although the aggregates tend to be less massive. (2) By the same token, the mass of these aggregates does not increase further. The inclusion of differential rotation would have allowed for the possibility of distinct aggregates colliding amongst themselves to form larger aggregates.
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Formation of Giant Molecular Clouds Associated with Spiral Structure
Publications of the Astronomical Society of Australia, 1991Co-Authors: Guo-xuan SongAbstract:Molecular hydrogen in spiral galaxies is distributed in clumps, i.e., Molecular Clouds, which have mass between 10 3 M ⊙ and 10 6 M ⊙ and a mass spectrum of n ( m ) ∝ m −1.6 . Molecular Clouds with masses greater than 10 5 M ⊙ , are called Giant Molecular Clouds (GMCs). It is generally accepted that GMCs are formed by the coalescence of Molecular Clouds through their collision. This process is studied by both numerical simulation and numerical integration. The observation with high resolution identified a great number of CO emission cores in galaxies. Based on this result, the aggregation or clustering formation of GMCs is numerically simulated. In the process of either coalescence or clustering, spiral perturbation plays an important role.