1,721,027 research outputs found
Recommended from our members
Molecular Gas and Star Formation in Nearby Galaxies
Full text linkIn the local Universe, stars form within molecular clouds. Therefore, the properties of molecular clouds may determine the star formation rate. Conversely, star formation also gives feedback to the clouds where the stars reside. In this dissertation, I present the interplay between the molecular gas and star formation, through three parts below.First, I identify and characterize the properties of molecular clouds in NGC4526, resulting in the first catalog of molecular clouds in an early-type galaxy. As a population, the molecular clouds in NGC4526 are gravitationally bound and have a steeper mass distribution than that in the Milky Way. These molecular clouds are also more luminous, denser, and have a higher velocity dispersion than their counterparts in the Milky Way. These different properties may be due to a more intense interstellar radiation field than in the Galactic disk and a weaker external pressure than in the Galactic center.Second, I combine the mm-wave interferometric data from CARMA and the optical Integral Field Unit data from CALIFA to study the molecular depletion time on kilo-parsec scales of nearby galaxies. In particular, the molecular depletion time between the galactic centers and disks is compared. I find that some galactic centers have shorter depletion time than that in the disks, which means that those centers form stars more efficiently per unit molecular gas mass. This places the galactic centers as an intermediate regime between galactic disks and starburst galaxies. The central drop of depletion time is also correlated with a central increase in the stellar mass surface density, suggesting that a shorter depletion time is associated with the molecular gas compression by the stellar gravitational potential.Third, the feedback from star formation to maintain turbulence in the interstellar matter of M33 is investigated. I show that supernovae have enough energy to maintain atomic gas turbulence inside 4 kpc radius and within molecular clouds, assuming a constant value of turbulent dissipation time of 9.8 Myrs. In the outer parts, the energy from the differential rotation of galaxy is large enough to maintain atomic gas turbulence through the magneto-rotational instability (MRI). I conclude that the sum of supernovae and MRI energy maintains turbulence at all radii where atomic hydrogen is detected in M33
Recommended from our members
The Formation and Evolution of Giant Molecular Clouds
Full text linkTo adequately address topics such as stellar and galactic evolution, it is necessary to address the question of giant molecular cloud (GMC) formation and evolution, topics that continue to be actively debated in astrophysics. In this thesis, I present new studies on the kinematic properties of individual molecular clouds in the Galaxy and M33 and on their global properties in low-metallicity environments. My primary aim in analyzing the kinematic features of GMCs is to determine the extent to which they are explained by current formation theories. Clues pointing to the origins of GMCs are revealed by comparing the large-scale linear velocity gradients, which they are frequently observed to possess, with the gradients in the high-density atomic hydrogen (HI) from which they are expected to form. Using high-resolution CO13 observations of five Milky Way GMCs, I create intensity-weighted velocity maps from which I measure the maximum gradient magnitudes and directions of the clouds. I use data from the Leiden/Argentine/Bonn Galactic HI survey to identify and measure the properties of regions of atomic gas associated with the GMCs. If the molecular cloud gradients – ranging from 0.04 to 0.20 km s-1 pc-1 – are due to rotation, their angular momentum is always less than that in the surrounding HI. Though this result is consistent with the the hypothesis that GMCs form from large-scale instabilities, one must necessarily introduce some mechanism capable of reducing the angular momentum in order to explain the discrepancies in the molecular and atomic gas. The second key result is that – with the exception of the Orion A molecular cloud – there are large differences in the gradient directions of the molecular and atomic gas.A continuation of this study is given for a much larger sample of GMCs in M33. The results are consistent with those in the Milky Way; in particular, the gradient directions of the GMCs are uncorrelated with the HI gradient directions. Additional findings include the observation that the local surface density of atomic gas slowly increases with GMC mass as &SigmaHI ∼ MGMC 0.27 ± 0.06. Also, the properties of high-density atomic hydrogen in which GMCs have not been observed generally has smaller gradients ( ∼ 0.03 km s-1 pc-1) than does the HI associated with GMCs (∼ 0.05 km s-1 pc-1). This suggests that high shear in atomic gas is either a prerequisite for or consequence of GMC formation.Studying the properties of GMCs in different environments is another avenue for enhancing our understanding of their evolution. An extinction map of the low-metallicity Large Magellanic Cloud (LMC) is presented, using near-infrared photometry from the Two Micron All Sky Survey. A mean visual extinction of 0.38 mag is found, and an extended distribution of molecular gas is observed across the face of the galaxy that was previously undetected by CO observations. The CO-to-H2 conversion factor in the LMC, 9.3 ± 0.4 × 1020 cm-2 (K km s-2)-1, is nearly 5 times greater than the average value in the Milky Way. My work demonstrates that CO is not a good tracer of H2, and caution must be applied in using the Galactic X-factor in low-metallicity environments
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Recommended from our members
Galaxies through Cosmic Time: The Role of Molecular and Atomic Gas
Full text linkIn the past decade, molecular gas observations have begun probing the high redshift universe in a systematic way using increasingly powerful millimeter instruments. This work has significantly advanced our understanding of the history of gas consumption by star formation in galaxies, revealing the high redshift universe to be similar in many ways to what we know locally. Specifically, molecular gas studies suggest that at both high and low redshift, the molecular gas reservoir in galaxies is insufficient to support on-going star formation. This is the molecular gas depletion problem, and motivates the research presented in this dissertation.I first investigate the molecular gas depletion problem on cosmic scales. Using the observed cosmic densities of the star formation rate, atomic gas and molecular gas, combined with measurements of the molecular gas depletion time in local galaxies, I derive the history of gas consumption by star formation from z = 0 to z ~ 4. I show that models in which the molecular gas is not replenished, or is only replenished by atomic gas, are not consistent with observational constraints. I find that star formation on cosmic timescales must be fueled by intergalactic ionized gas at an average rate that roughly traces the star formation rate density of the universe. Further, I predict the volume averaged density of molecular gas to increase by a factor of 1.5 - 10 to z ~ 1.5 over the currently measured value, which implies that galaxies at high redshift must, on average, be more molecular gas-rich than they are at the present epoch, consistent with observations. Next I focus on the observational constraints on the molecular gas content of galaxies from z ~ 1 - 2 to today. Recent observations suggest z ~ 1 - 2 galaxies harbor molecular gas reservoirs an order of magnitude larger than their local counterparts, implying significant evolution of the molecular gas content of galaxies over the past 8 billion years. However, this period of time has been relatively un-observed in molecular gas. To fill in this observational gap, I carry out the Evolution of molecular Gas in Normal Galaxies (EGNoG) survey, a study of molecular gas in 31 star-forming galaxies from z = 0.05 to z = 0.5. With observations of the CO(1-0) and CO(3-2) rotational lines using the Combined Array for Research in Millimeter-wave Astronomy (CARMA), the EGNoG survey accomplishes two goals: tracing the evolution of the molecular gas content of galaxies at intermediate redshifts and constraining the excitation of the molecular gas in these galaxies. With 24 detections out of 31 observed galaxies, I calculate an average molecular gas fraction of 7 - 20% at z ~ 0.05 - 0.5, which is in line with observations at high and low redshift and agrees well with the evolution predicted by a simple empirical prescription for gas consumption by star formation in galaxies from z ~ 1 - 2 to today. The EGNoG observations of four galaxies at z = 0.3 (the gas excitation subsample) yield robust detections of both lines in three galaxies (and an upper limit on the fourth). I find an average line ratio, r31 = L'(CO(3-2)) / L'(CO(1-0)), of 0.46 ± 0.07 (with systematic errors less than 40%), which implies sub-thermal excitation of the CO(3-2) line. As the EGNoG galaxies are representative of the main sequence of star-forming galaxies, I extend this result to include main sequence galaxies at high redshift.To support the observations carried out at CARMA as part of the EGNoG survey, I give two appendices. The first details the data reduction and flux measurement for the EGNoG survey, including a description of the use of polarized calibrators to calibrate data from single, linearly polarized feeds. In the second appendix, I describe the absolute flux calibration of CARMA data and the automated monitoring system I helped put in place in order to maintain a historical record of the flux of common calibrators. Finally, I return to the gas depletion problem in the local universe. I carry out a pilot study of atomic (HI) gas in groups of galaxies in order to investigate the role of tidal interactions in transporting atomic gas from the outskirts of galaxy disks to the central regions so that it may replenish the molecular gas and fuel ongoing star formation. I image three groups of galaxies in the 21 cm line of HI with the Allen Telescope Array (ATA), detecting many galaxies not previously observed in HI as well as four previously undetected clouds of HI between galaxies that account for up to 3% of the HI reservoir of the groups. To investigate the potential role of this gas in the ongoing star formation in the group, I compare the mass of the detected HI gas in and between galaxies in the group to the estimated star formation rates of the group members
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The structure of the neutral interstellar medium: a theory of interstellar turbulence
No full textWe present a model for the development of density structure in the neutral interstellar medium. In this model, bulk kinetic energy is injected mainly at small scales, by jets and expanding nebulae generated by young and/or massive stars. Subsequently, this bulk kinetic energy propagates to, and is dissipated on, larger scales.
This is in contrast to standard (incompressible) Kolmogorov turbulence, where kinetic energy is injected at larger scales, then propagates to, and is dissipated on, smaller scales. The sense in which the Second Law of Thermodynamics drives the propagation of turbulent energy is reversed in the interstellar medium because virialized self-gravitating gas clumps and ensembles of clumps (clouds) have negative effective specific heat.
The model is able to explain Larson’s relations (between the mass, size, and velocity-dispersion of molecular clouds and clumps; Larson 1981), the maximum masses of giant molecular cloud complexes, the velocity disper sions observed (at low resolution) in face-on spirals like the Milky Way, and the apparent scaling of the interstellar magnetic field with density
Recommended from our members
High-Resolution Velocity Fields of Low-Mass Disk Galaxies
Full text linkThis study aims to examine the relative distributions of dark and baryonic matter as a function of star formation history in a representative sample of low mass disk galaxies. In this study, we present the high-resolution 12CO(J=1 → 0) interferometry for a sample of 26 nearby dwarf galaxies, which were obtained from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). Among these 26 galaxies, 14 have good CO detec- tions, including 6 galaxies previously detected in single-dish CO measurements and 8 newly detected ones. We find a linear correlation between the CO flux and the mid- and far-IR flux from the WISE and IRAS catalogs. Compared to the far-IR flux, the mid-IR flux may be a better indication of whether a galaxy contains sufficient CO for detection at the level of instrument sensitivity of CARMA. This correlation might prove to be useful in future studies to help choosing other CO targets for observation. The median molecular mass (including helium and H2) of our galaxies is 2.8 × 108M⊙, which is consistent with past ob- servations for dwarf galaxies. The molecular content is weakly correlated with the dynamical mass, r-band luminosity and size of the galaxies. The median ratios of molecular mass vs. dynamical mass and molecular mass vs. r-band luminosity are Mmol/Mdyn ≈ 0.035 and Mmol/Lr ≈ 0.078M⊙/Lr,⊙, respectively, which are also consistent with past observations for dwarf galaxies. In addition, we present the rotation curves of these 14 galaxies. To examine the dark matter distribution in their central regions, the reduced CO data were fitted with simple kinematic models using two different algorithms. For all 14 galaxies, despite inhomo- geneous distribution of molecular gas in some of the sources, robust kinematic results were obtained for all sources. Most galaxies show approximately zero or small noncircular mo- tions, particularly the ones with smooth spatial distributions of CO emission. Furthermore, consistent rotation curves are obtained using both algorithms. In general, the CO rotation curves are consistent with Hα rotation curves of the same galaxies. Using the CO rotation curves, along with information on the stellar distribution from optical and infrared imaging, we model the velocity data including contributions from stars and the dark matter halo, which we parameterized using a generalized Navarro, Frenk & White profile. The results show that the inner power-law slope α of the density profile varies over a large range from below 0.38 (cored) to 1.76 (cuspy), with a mean value of 0.58 ± 0.45 (mean ± scatter) or 0.64 ± 0.49 if we assume the stellar distribution derived from r-band data or IRAC 4.5- micron data, respectively. The density profile slope is generally robust as the baryonic M/L is varied from minimum- to maximum-disk estimates. Our galaxies show low stellar mass- to-light ratios: M∗/L = 0.10±0.02 for IRAC channel-2 data and 0.49±0.33 for r-band data. Considering our results in combination with recent studies from the literature, we find weak correlations between the dark matter profile inner slope, dynamical mass and distance of the galaxies, where more distant and more massive galaxies have steeper slopes. In addition, we find no statistically significant correlation between the slope and the stellar mass, which may suggest that baryonic feedback models alone cannot fully explain the flattening behavior of the inner profiles of dwarf galaxies
The Nature of the Galactic Center Arc
Full text linkEver since the Galactic Center Arc was resolved into its component filaments a decade ago, it has been clear that its linear structure arises from the influence of a strong magnetic field. However, the origin and nature of the contributory phenomena have remained elusive. Since what is seen is synchrotron emission from relativistic particles, of prime interest is a knowledge of the acceleration mechanism involved. Interferometric imaging of the molecular gas in the vicinity of the Arc has now provided a tantalizing clue to the Arc’s origin: molecular clumps coinciding with the endpoints of a number of the Arc’s filaments point to these clumps as the source of the relativistic particles. This suggests that as dense molecular clumps course through the ambient magnetic field at the Galactic Center, magnetic energy is liberated in their leading layers via field reconnection, precipitating rapid acceleration of free charges to high energy
- …
