Giant Molecular Clouds and Star Formation in the Tidal Molecular Arm of NGC 4039 (original) (raw)
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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
Ultraviolet Morphology and Star Formation in the Tidal Tails of NGC 4038/39
Astrophysical Journal, 2005
We present Galaxy Evolution Explorer far-ultraviolet (1530 Å) and near-ultraviolet (2310 Å) observations of the archetypal merging system NGC 4038/39, ``the Antennae.'' Both tails are relatively bright in the UV, especially in the vicinity of the tidal dwarf galaxy candidates at the end of the southern tail. The UV light generally falls within the optically delineated tails, although the UV light is considerably more structured, with a remarkably similar morphology to the tidal H I. The UV colors suggest that there has been continuing star formation within the tidal tails, even outside the previously studied tidal dwarf regions. Within the inner disk regions, there are interesting UV features that appear to be related to the extended soft X-ray loops and halo recently discovered by Chandra.
Molecular gas and star formation in the red-sequence counter-rotating disc galaxy NGC 4550
Monthly Notices of the Royal Astronomical Society, 2009
We present observations of the CO(1-0) emission in the central 750 pc (10 arcsec) of the counter-rotating disc galaxy NGC 4550, obtained at the Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer. Very little molecular gas is detected, only 1×10 7 M , and its distribution is lopsided, with twice as much molecular gas observed at positive relative velocities than at negative relative velocities. The velocity gradient in the CO(1-0) emission shows that the molecular gas rotates like the thicker of the two stellar discs, which is an unexpected alignment of rotations if the thinner disc was formed by a major gas accretion event. However, a simulation shows that the gas rotating like the thicker disc naturally results from the coplanar merger of two counter-rotating disc galaxies, demonstrating the feasibility of this scenario for the formation of NGC 4550. We investigate various star formation tracers to determine whether the molecular gas in NGC 4550 is currently forming stars. Ultraviolet (UV) imaging data and optical absorption line strengths both suggest a recent star formation episode; the best-fitting two-population model to the UV-optical colours yields a mass of young stars of 5.9×10 7 M with an age of 280 Myr. The best information on the current star formation rate is a far-infrared-based upper limit of only 0.02 M yr −1. We are thus witnessing NGC 4550 either in a dip within a bursty star formation period or during a more continuous low-level star formation episode.
Molecular Gas, Kinematics, and OB Star Formation in the Spiral Arms of the Southern Milky Way
The Astrophysical Journal, 2006
The rotation curve for the IV galactic quadrant, within the solar circle, is derived from the Columbia University -U. de Chile CO(J=1→0) survey of molecular gas. A new sampling, four times denser in longitude than in our previous analysis, is used to compute kinematical parameters that require derivatives w/r to galactocentric radius; the angular velocity Ω(R), the epicyclic frequency κ(R), and the parameters A(R) and B(R) describing, respectively, gas shear and vorticity. The face-on surface density of molecular gas is computed from the CO data in galactocentric radial bins for the subcentral vicinity, the same spectral region used to derive the rotation curve, where the two-fold ambiguity in kinematical distances is minimum. The rate of massive star formation per unit area is derived, for the same radial bins, from the luminosity of IRAS point-like sources with FIR colors of UC HII regions detected in the CS(J=2→1) line. Massive star formation occurs preferentially in three regions of high molecular gas density, coincident with lines of sight tangent to spiral arms. The molecular gas motion in these arms resembles that of a solid body, characterized by constant angular velocity and by low shear and vorticity. The formation of massive stars in the arms follows the Schmidt law, Σ MSF R ∝ [Σ gas ] n , with an index of n = 1.2 ± 0.2. Our results suggest that the large scale kinematics, through shear, regulate global star formation in the Galactic disk.
The Astrophysical Journal, 2013
We present the results of a study of simulated Giant Molecular Clouds (GMCs) formed in a Milky Way-type galactic disk with a flat rotation curve. This simulation, which does not include star formation or feedback, produces clouds with masses ranging between 10 4 M ⊙ and 10 7 M ⊙. We compare our simulated cloud population to two observational surveys; The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey and the BIMA All-Disk Survey of M33. An analysis of the global cloud properties as well as a comparison of Larson's scaling relations is carried out. We find that simulated cloud properties agree well with the observed cloud properties, with the closest agreement occurring between the clouds at comparable resolution in M33. Our clouds are highly filamentary-a property that derives both from their formation due to gravitational instability in the sheared galactic environment, as well as to cloud-cloud gravitational encounters. We also find that the rate at which potentially star forming gas accumulates within dense regions-wherein n thresh ≥ 10 4 cm −3-is 3% per 10 Myr, in clouds of roughly 10 6 M ⊙. This suggests that star formation rates in observed clouds are related to the rates at which gas can be accumulated into dense subregions within GMCs via filamentary flows. The most internally well-resolved clouds are chosen for listing in a catalogue of simulated GMCs; the first of its kind. The catalogued clouds are available as an extracted data set from the global simulation.
Molecular gas in the galaxy M 83
Astronomy & Astrophysics, 2004
We present the kinematics of the molecular gas in the barred spiral galaxy M 83 (NGC 5236). The study is based on 12 CO(J = 1-0 and 2-1) observations with the Swedish-ESO Submillimetre Telescope (SEST). Iso-velocity maps of the entire optical disk, 10 × 10 or 13 × 13 kpc, are produced. They show the pattern of an inclined, rotating disk, but also the effects of streaming motions along the spiral arms. A dynamical mass of about 6 × 10 10 M is estimated by fitting the rotation curve of an exponential disk model to these data. The gas constitutes about 13% of the disk mass. The pattern speed is determined from the residual velocity pattern. The locations of various resonances are discussed. The molecular gas velocity dispersion is determined, and a trend of decreasing dispersion with increasing galactocentric radius is found. A total gas (H 2 + H + He) mass surface density map is presented, and compared to the critical density for star formation of an isothermal gaseous disk. The critical density is exceeded in the spiral arms, but not in the interarm regions. The locations of Giant Molecular Associations (GMAs) and H regions are consistent with this scenario of dynamically induced star formation.
Giant Molecular Cloud Complexes in NGC6946
Combining observations of multiple CO lines with radiative transfer modeling is a very powerful tool to investigate the physical properties of the molecular gas in galaxies. Using new observations as well as literature data, we provide the most complete CO ladders ever generated for eight star-forming regions in the spiral arms and inter-arms of the spiral galaxy NGC 6946, with observations of the CO(1-0), CO(2-1), CO(3-2), CO(4-3), CO(6-5), 13 CO(1-0) and 13 CO(2-1) transitions. For each region, we use the large velocity gradient assumption to derive beam-averaged molecular gas physical properties, namely the gas kinetic temperature (T K ), H 2 number volume density (n(H 2 )) and CO number column density (N (CO)). Two complementary approaches are used to compare the observations with the model predictions: χ 2 minimisation and likelihood. The physical conditions derived vary greatly from one region to the next: T K = 10-250 K, n(H 2 )= 10 2.3 -10 7.0 cm −3 and N (CO)= 10 15.0 -10 19.3 cm −2 . The spectral line energy distribution (SLED) of some of these extranuclear regions indicates a star-formation activity that is more intense than that at the centre of our own Milky Way. The molecular gas in regions with a large SLED turnover transition (J max > 4) is hot but tenuous with a high CO column density, while that in regions with a low SLED turnover transition (J max 4) is cold but dense with a low CO column density. We finally discuss and find some correlations between the physical properties of the molecular gas in each region and the presence of young stellar population indicators (supernova remnants, H II regions, H I holes, etc).
The headlight cloud in NGC 628: An extreme giant molecular cloud in a typical galaxy disk
Astronomy & Astrophysics
Context. Cloud-scale surveys of molecular gas reveal the link between giant molecular cloud properties and star formation across a range of galactic environments. Cloud populations in galaxy disks are considered to be representative of the normal star formation process, while galaxy centers tend to harbor denser gas that exhibits more extreme star formation. At high resolution, however, molecular clouds with exceptional gas properties and star formation activity may also be observed in normal disk environments. In this paper we study the brightest cloud traced in CO(2–1) emission in the disk of nearby spiral galaxy NGC 628. Aims. We characterize the properties of the molecular and ionized gas that is spatially coincident with an extremely bright H II region in the context of the NGC 628 galactic environment. We investigate how feedback and large-scale processes influence the properties of the molecular gas in this region. Methods. High-resolution ALMA observations of CO(2–1) and CO(...
The Astrophysical Journal, 2010
We report molecular line and dust continuum observations, made with the SEST telescope, towards four young high-mass star forming regions associated with highly luminous (L > 6 × 10 5 L ⊙ ) IRAS sources (15290-5546, 15502-5302, 15567-5236 and 16060-5146). Molecular emission was mapped in three lines of CS (J=2→1, 3→2 and 5→4), two lines of SiO (J=2→1 and 3→2), two rotational transitions of CH 3 OH (J k =3 k → 2 k and 2 k → 1 k ), and in the C 34 S(J=3→2) line.