An Updated View of Giant Molecular Clouds, Gas Flows and Star Formation in M51 with PAWS (original) (raw)
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The Astrophysical Journal, 2014
Using data from the PdBI Arcsecond Whirlpool Survey (PAWS), we have generated the largest extragalactic giant molecular cloud (GMC) catalog to date, containing 1507 individual objects. GMCs in the inner M51 disk account for only 54% of the total 12 CO(1-0) luminosity of the survey, but on average they exhibit physical properties similar to Galactic GMCs. We do not find a strong correlation between the GMC size and velocity dispersion, and a simple virial analysis suggests that ∼30% of GMCs in M51 are unbound. We have analyzed the GMC properties within seven dynamically motivated galactic environments, finding that GMCs in the spiral arms and in the central region are brighter and have higher velocity dispersions than inter-arm clouds. Globally, the GMC mass distribution does not follow a simple power-law shape. Instead, we find that the shape of the mass distribution varies with galactic environment: the distribution is steeper in inter-arm region than in the spiral arms, and exhibits a sharp truncation at high masses for the nuclear bar region. We propose that the observed environmental variations in the GMC properties and mass distributions are a consequence of the combined action of large-scale dynamical processes and feedback from high-mass star formation. We describe some challenges of using existing GMC identification techniques for decomposing the 12 CO(1-0) emission in molecule-rich environments, such as M51's inner disk.
The Astrophysical Journal, 2013
The PdBI (Plateau de Bure Interferometer) Arcsecond Whirlpool Survey (PAWS) has mapped the molecular gas in the central ∼ 9 kpc of M 51 in its 12 CO(1-0) line emission at cloud-scale resolution of ∼40 pc using both IRAM telescopes. We utilize this dataset to quantitatively characterize the relation of molecular gas (or CO emission) to other tracers of the interstellar medium (ISM), star formation and stellar populations of varying ages. Using 2-dimensional maps, a polar cross-correlation technique and pixel-by-pixel diagrams, we find: (a) that (as expected) the distribution of the molecular gas can be linked to different components of the gravitational potential, (b) evidence for a physical link between CO line emission and radio continuum that seems not to be caused by massive stars, but rather depend on the gas density, (c) a close spatial relation between the PAH and molecular gas emission, but no predictive power of PAH emission for the molecular gas mass, (d) that the I-H color map is an excellent predictor of the distribution (and to a lesser degree the brightness) of CO emission, and (e) that the impact of massive (UVintense) young star-forming regions on the bulk of the molecular gas in central ∼9 kpc can not be significant due to a complex spatial relation between molecular gas and star-forming regions that ranges from co-spatial to spatially offset to absent. The last point, in particular, highlights the importance of galactic environment-and thus the underlying gravitational potential-for the distribution of molecular gas and star formation.
A Comparative Study of Giant Molecular Clouds in M51, M33, and the Large Magellanic Cloud
The Astrophysical Journal, 2013
We compare the properties of giant molecular clouds (GMCs) in M51 identified by the Plateau de Bure Interferometer Whirlpool Arcsecond Survey (PAWS) with GMCs identified in wide-field, high resolution surveys of CO emission in M33 and the Large Magellanic Cloud (LMC). We find that GMCs in M51 are larger, brighter and have higher velocity dispersions relative to their size than equivalent structures in M33 and the LMC. These differences imply that there are genuine variations in the average mass surface density Σ H2 of the different GMC populations. To explain this, we propose that the pressure in the interstellar medium surrounding the GMCs plays a role in regulating their density and velocity dispersion. We find no evidence for a correlation between size and linewidth in any of M51, M33 or the LMC when the CO emission is decomposed into GMCs, although moderately robust correlations are apparent when regions of contiguous CO emission (with no size limitation) are used. Our work demonstrates that observational bias remains an important obstacle to the identification and study of extragalactic GMC populations using CO emission, especially in molecule-rich galactic environments.
DYNAMICALLY DRIVEN EVOLUTION OF THE INTERSTELLAR MEDIUM IN M51
The Astrophysical Journal, 2009
Massive star formation occurs in Giant Molecular Clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (Giant Molecular Associations -GMAs) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H 2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by largescale galactic dynamics -their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the inter-arm region and into the next spiral arm passage.
The Astrophysical Journal, 2013
We use the high spatial and spectral resolution of the PAWS CO(1-0) survey of the inner 9 kpc of the iconic spiral galaxy M51 to examine the effects of gas streaming motions on the star-forming properties of individual giant molecular clouds (GMCs). We compare our view of gas flows in M51-which arise due to departures from axisymmetry in the gravitational potential (i.e., the nuclear bar and spiral arms)-with the global pattern of star formation as traced by Hα and 24 μm emission. We find that the dynamical environment of GMCs strongly affects their ability to form stars, in the sense that GMCs situated in regions with large streaming motions can be stabilized, while similarly massive GMCs in regions without streaming go on to efficiently form stars. We argue that this is the result of reduced surface pressure felt by clouds embedded in an ambient medium undergoing large streaming motions, which prevent collapse. Indeed, the variation in gas depletion time expected based on the observed streaming motions throughout the disk of M51 quantitatively agrees with the variation in the observed gas depletion time scale. The example of M51 shows that streaming motions, triggered by gravitational instabilities in the form of bars and spiral arms, can alter the star formation law; this can explain the variation in gas depletion time among galaxies with different masses and morphologies. In particular, we can explain the long gas depletion times in spiral galaxies compared with dwarf galaxies and starbursts. We suggest that adding a dynamical pressure term to the canonical free-fall time produces a single star formation law that can be applied to all star-forming regions and galaxies across cosmic time.
Gas distribution, star formation and giant molecular cloud evolution in nearby spiral galaxies
2013
In this thesis, I present a detailed study of the resolved properties of the cold gas in nearby galaxies at different size scales, starting from the whole galactic disk to the size of the Giant Molecular Clouds (GMCs). Differences in the shape and width of global CO and HI spectra of resolved disks of spiral galaxies are systematically investigated using a nearby sample for which high-resolution CO and HI maps are available. I find that CO line widths can be wider than HI widths in galaxies where the rotation curve declines in the outer parts, while they can be narrower in galaxies where the CO does not adequately sample the flat part of the rotation curve. Limited coverage of the CO emission by the telescope beam can mimic the latter effect. A physically based prescription linking the CO and HI radial profiles with the stellar disk is consistent with these findings. Then, I present an analysis performed on high spatial resolution observations of Giant Molecular Clouds in the three nearby spiral galaxies NGC 6946, NGC 628 and M101 obtained with the Combined Array for Research in Millimeter-wave Astronomy (CARMA). Using the automated CPROPS algorithm I identified 112 CO cloud complexes in the CO(1 → 0) map and 145 GMCs in the CO(2 → 1) maps. The properties of the GMCs are similar to values found in other extragalactic studies. Clouds located on-arm present in general higher star formation rates than clouds located in inter-arm regions. Also, I find differences in the distribution of star formation efficiencies in the disk of these galaxies. These differences may be related to the underlying dynamical process that drives the observed spiral arm structure in the disks. In this scenario, in galaxies with nearly symmetric arm shape (e. g., NGC 628), the spiral shocks are triggering star formation along the arms. On other hand, galaxies with flocculent or multi-arm spiral structure (e. g., NGC 6946 and M101) show regions of high star formation efficiency at specific regions of the spiral arms, as the result of gas flow convergence or regions where previous spiral arms may have collided. The work presented here has been the result of an extraordinary collaboration with my adviser Tony Wong. This project would not have been possible without his continuous support and critical help in several stages of my research. I thank Tony for his suggestions and comments in the numerous revisions on my proposals and science papers. I truly believe that his feedback improved significantly the quality of my work in many ways. Also, I thank Tony for his constant concern about my financial support in the time we were working together. Also thanks to the thesis committee members, Professor Charles Gammie, Professor You-Hua Chu, and Leslie Looney for several useful comments. Thanks to the Astronomy Department of the University of Illinois for accepting me in the Ph. D. program, and thanks to the CARMA telescope for the financial support that allowed me to complete my program. This thesis would not have been possible without the support of my collaborators. Many thanks to Adam
The PdBI Arcsecond Whirlpool Survey (PAWS): The Role of Spiral Arms in Cloud and Star Formation
The Astrophysical Journal
The process that leads to the formation of the bright star forming sites observed along prominent spiral arms remains elusive. We present results of a multi-wavelength study of a spiral arm segment in the nearby grand-design spiral galaxy M 51 that belongs to a spiral density wave and exhibits nine gas spurs. The combined observations of the (ionized, atomic, molecular, dusty) interstellar medium (ISM) with star formation tracers (HII regions, young < 10 Myr stellar clusters) suggest (1) no variation in giant molecular cloud (GMC) properties between arm and gas spurs, (2) gas spurs and extinction feathers arising from the same structure with a close spatial relation between gas spurs and ongoing/recent star formation (despite higher gas surface densities in the spiral arm), (3) no trend in star formation age either along the arm or along a spur, (4) evidence for strong star formation feedback in gas spurs, (5) tentative evidence for star formation
The Astrophysical Journal, 2013
We present the data of the Plateau de Bure Arcsecond Whirlpool Survey, a high spatial and spectral resolution 12 CO (1-0) line survey of the inner ∼10 × 6 kpc of the M51 system, and the first wide-field imaging of molecular gas in a star-forming spiral galaxy with resolution matched to the typical size of giant molecular clouds (40 pc). We describe the observation, reduction, and combination of the Plateau de Bure Interferometer (PdBI) and IRAM-30 m "short spacing" data. The final data cube attains 1. 1 resolution over the ∼270 × 170 field of view, with sensitivity to all spatial scales from the combination of PdBI and IRAM-30 m data, and a brightness sensitivity of 0.4 K (1σ) in each 5 km s −1-wide channel map. We find a CO luminosity of 9 × 10 8 K km s −1 pc 2 , corresponding to a molecular gas mass of 4 × 10 9 M for a standard CO-to-H 2 conversion factor. Unexpectedly, we find that a large fraction of this emission, (50 ± 10)%, arises mostly from spatial scales larger than 36 1.3 kpc. Through a series of tests, we demonstrate that this extended emission does not result from a processing artifact. We discuss its origin in light of the stellar component, the 12 CO/ 13 CO ratio, and the difference between the kinematics and structure of the PdBI-only and hybrid synthesis (PdBI + IRAM-30 m) images. The extended emission is consistent with a thick, diffuse disk of molecular gas with a typical scale height of ∼200 pc, substructured in unresolved filaments that fill ∼0.1% of the volume.
Cloud-scale ISM Structure and Star Formation in M51
The Astrophysical Journal
We compare the structure of molecular gas at 40pc resolution to the ability of gas to form stars across the disk of the spiral galaxy M51. We break the PAWS survey into 370pc and 1.1kpc resolution elements, and within each we estimate the molecular gas depletion time (t Dep mol), the star-formation efficiency per free-fall time ( ff), and the mass-weighted cloud-scale (40 pc) properties of the molecular gas: surface density, Σ, line width, σ, and s a º S µb 2 vir Dep mol 0.9. The sense of the correlation is that gas with stronger self-gravity (higher b) forms stars at a higher rate (low t Dep mol). The different regions of the galaxy mostly overlap in t Dep mol as a function of b, so that low b explains the surprisingly high t Dep mol found toward the inner spiral arms found by Meidt et al. (2013).
Giant Molecular Cloud Evolutions in the Nearby Spiral Galaxy M33
The Astrophysical Journal, 2012
We present a giant molecular cloud (GMC) catalog of M33, containing 71 GMCs in total, based on wide-field and high-sensitivity CO(J = 3-2) observations with a spatial resolution of 100 pc using the ASTE 10 m telescope. Employing archival optical data, we identify 75 young stellar groups (YSGs) from the excess of the surface stellar density, and estimate their ages by comparing with stellar evolution models. A spatial comparison among the GMCs, YSGs, and H ii regions enable us to classify GMCs into four categories: Type A, showing no sign of massive star formation (SF); Type B, being associated only with H ii regions; Type C, with both H ii regions and <10 Myr old YSGs; and Type D, with both H ii regions and 10-30 Myr YSGs. Out of 65 GMCs (discarding those at the edges of the observed fields), 1 (1%), 13 (20%), 29 (45%), and 22 (34%) are Types A, B, C, and D, respectively. We interpret these categories as stages in a GMC evolutionary sequence. Assuming that the timescale for each evolutionary stage is proportional to the number of GMCs, the lifetime of a GMC with a mass >10 5 M is estimated to be 20-40 Myr. In addition, we find that the dense gas fraction as traced by the CO(J = 3-2)/CO(J = 1-0) ratio is enhanced around SF regions. This confirms a scenario where dense gas is preferentially formed around previously generated stars, and will be the fuel for the next stellar generation. In this way, massive SF gradually propagates in a GMC until gas is exhausted.