Results for "Bruce Elmegreen"

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Two Stellar Mass Functions Combined into One by the Random Sampling Model of the IMFDec 08 1999The turnover in the stellar initial mass function (IMF) at low mass suggests the presence of two independent mass functions that combine in different ways above and below a characteristic mass given by the thermal Jeans mass in the cloud. In the random ... More
The Probability Distribution Function of Gas Surface Density in M33Jun 30 2018The probability distribution functions (PDFs) for atomic, molecular, and total gas surface densities of M33 are determined at a resolution of about 50~pc over regions that share coherent morphological properties to unveil fingerprints of self-gravity ... More
NOEMA Observations of a Molecular Cloud in the low-metallicity Galaxy Kiso 5639May 21 2018A giant star-forming region in a metal-poor dwarf galaxy has been observed in optical lines with the 10-m Gran Telescopio Canarias and in the emission line of CO(1-0) with the NOEMA mm-wave interferometer. The metallicity was determined to be 12+log(O/H)=7.83+-0.09, ... More
Formation of stars and clusters over cosmological timeOct 04 2014The concept that stars form in the modern era began some 60 years ago with the key observation of expanding OB associations. Now we see that these associations are an intermediate scale in a cascade of hierarchical structures that begins on the ambient ... More
The Early Years: Lyman Spitzer, Jr. and the Physics of Star FormationDec 05 2009The discovery of the interstellar medium and the early work of Lyman Spitzer, Jr. are reviewed here in the context of the remarkable observation in the early 1950's that star formation continues in the present age. Prior to this observation, stars were ... More
Formation and Evolution of Young Massive ClustersOct 23 2006Clusters are the dense inner regions of a wide-spread hierarchy of young stellar structures. They often reveal a continuation of this hierarchy inside of them, to smaller scales, when they are young, but orbital mixing eventually erases these subparts ... More
Turbulence and galactic structureJul 28 2004Interstellar turbulence is driven over a wide range of scales by processes including spiral arm instabilities and supernovae, and it affects the rate and morphology of star formation, energy dissipation, and angular momentum transfer in galaxy disks. ... More
On the Star Formation Law for Spiral and Irregular GalaxiesNov 18 2015A dynamical model for star formation on a galactic scale is proposed in which the interstellar medium is constantly condensing to star-forming clouds on the dynamical time of the average midplane density, and the clouds are constantly being disrupted ... More
Star Formation Patterns and HierarchiesJan 17 2011Star formation occurs in hierarchical patterns in both space and time. Galaxies form large regions on the scale of the interstellar Jeans length and these large regions apparently fragment into giant molecular clouds and cloud cores in a sequence of decreasing ... More
Star Formation in Spiral ArmsJan 17 2011The origin and types of spiral arms are reviewed with an emphasis on the connections between these arms and star formation. Flocculent spiral arms are most likely the result of transient instabilities in the gas that promote dense cloud formation, star ... More
A Prediction of Brown Dwarfs in Ultracold Molecular GasJun 12 1999A recent model for the stellar initial mass function (IMF), in which the stellar masses are randomly sampled down to the thermal Jeans mass from hierarchically structured pre-stellar clouds, predicts that regions of ultra-cold CO gas, such as those recently ... More
On the Initial Conditions for Star Formation and the IMFFeb 25 2011Density probability distribution functions (PDFs) for turbulent self-gravitating clouds should be convolutions of the local log-normal PDF, which depends on the local average density rho-ave and Mach number M, and the probability distribution functions ... More
Star Formation during Galaxy FormationJan 17 2011Young galaxies are clumpy, gas-rich, and highly turbulent. Star formation appears to occur by gravitational instabilities in galactic disks. The high dispersion makes the clumps massive and the disks thick. The star formation rate should be comparable ... More
Star Formation on Galactic Scales: Empirical LawsJan 17 2011Empirical star formation laws from the last 20 years are reviewed with a comparison to simulations. The current form in main galaxy disks has a linear relationship between the star formation rate per unit area and the molecular cloud mass per unit area ... More
Bulge formation by the coalescence of giant clumps in primordial disk galaxiesMar 11 2009The observations and evolution of clumpy, high-redshift galaxies are reviewed. Models suggest that the clumps form by gravitational instabilities in a gas-rich disk, interact with each other gravitationally, and then merge in the center where they form ... More
On the Similarity between Cluster and Galactic Stellar Initial Mass FunctionsMay 19 2006The stellar initial mass functions (IMFs) for the Galactic bulge, the Milky Way, other galaxies, clusters of galaxies, and the integrated stars in the Universe are composites from countless individual IMFs in star clusters and associations where stars ... More
Gas Phase Processes Affecting Galactic EvolutionDec 06 2002Gas processes affecting star formation are reviewed with an emphasis on gravitational and magnetic instabilities as a source of turbulence. Gravitational instabilities are pervasive in a multi-phase medium, even for sub-threshold column densities, suggesting ... More
What triggers star formation in galaxies?Jan 17 2012Processes that promote the formation of dense cold clouds in the interstellar media of galaxies are reviewed. Those that involve background stellar mass include two-fluid instabilities, spiral density wave shocking, and bar accretion. Young stellar pressures ... More
Star Formation in Disks: Spiral Arms, Turbulence, and Triggering MechanismsOct 30 2008Star formation is enhanced in spiral arms because of a combination of orbit crowding, cloud collisions, and gravitational instabilities. The characteristic mass for the instability is 10^7 Msun in gas and 10^5 Msun in stars, and the morphology is the ... More
The IMF in StarburstsNov 08 2004The history of the IMF in starburst regions is reviewed. The IMFs are no longer believed to be top-heavy, although some superstar clusters, whether in starburst regions or not, could be. General observations of the IMF are discussed to put the starburst ... More
Star Formation from Galaxies to GlobulesJul 04 2002The empirical laws of star formation suggest that galactic-scale gravity is involved, but they do not identify the actual triggering mechanisms for clusters in the final stages. Many other triggering processes satisfy the empirical laws too, including ... More
Star Formation in a Crossing TimeNov 10 1999Observations suggest that star formation occurs in only one or two crossing times for a range of scales spanning a factor of 1000. These observations include (1) measurements of embedded cluster ages in comparison with the cloud core dynamical times, ... More
Formation and loss of hierarchical structure in two-dimensional MHD simulations of wave-driven turbulence in interstellar cloudsNov 10 1999Two dimensional compressible magneto-hydrodynamical (MHD) simulations run for 20 crossing times on a 800x640 grid with two stable thermal states show persistent hierarchical density structures and Kolmogorov turbulent motions in the interaction zone between ... More
Triggered Star FormationJan 17 2011Triggered star formation in bright rims and shells is reviewed. Shells are commonly observed in the Milky Way and other galaxies, but most diffuse shells seen in HI or the infrared do not have obvious triggered star formation. Dense molecular shells and ... More
Bars, Spiral Structure, and Secular Evolution in Disk GalaxiesOct 09 2005Simulations and observations of galactic bars suggest they do not commonly evolve into bulges, although it is possible that the earliest bars formed bulges long ago, when galaxies were smaller, denser, and had more gas. The most highly evolved of today's ... More
Cluster Density and the IMFNov 08 2004Observed variations in the IMF are reviewed with an emphasis on environmental density. The remote field IMF studied in the LMC by several authors is clearly steeper than most cluster IMFs, which have slopes close to the Salpeter value. Local field regions ... More
Variability in the stellar initial mass function at low and high mass: 3-component IMF modelsAug 12 2004Three component models of the IMF are made to consider possible origins for the observed relative variations in the numbers of brown dwarfs, solar-to-intermediate mass stars, and high mass stars. Three distinct physical processes are noted. The characteristic ... More
Formation of Young Star ClustersMay 28 2004Turbulence, self-gravity, and cooling convert most of the interstellar medium into cloudy structures that form stars. Turbulence compresses the gas into clouds directly and it moves pre-existing clouds around passively when there are multiple phases of ... More
Modeling a high mass turn down in the stellar initial mass functionMay 23 2000Statistical sampling from the stellar initial mass function (IMF) for all star-forming regions in the Galaxy would lead to the prediction of ~1000 Msun stars unless there is a rapid turn-down in the IMF beyond several hundred solar masses. Such a turndown ... More
Variations in Stellar Clustering with Environment: Dispersed Star Formation and the Origin of Faint FuzziesOct 31 2007The observed increase in star formation efficiency with average cloud density, from several percent in whole giant molecular clouds to ~30 or more in cluster-forming cores, can be understood as the result of hierarchical cloud structure if there is a ... More
History of Star Formation in the UniverseOct 05 2001The cosmic history of star formation is briefly reviewed, starting with the Milky Way and then discussing observations relevant to the closed box and hierarchical build-up models. Observations of local star formation are reviewed for comparison. The halo ... More
Long-Lived Spiral Structure for Galaxies with Intermediate Size BulgesJul 07 2016Spiral structure in disk galaxies is modeled with nine collisionless N-body simulations including live disks, halos, and bulges with a range of masses. Two of these simulations make long-lasting and strong two-arm spiral wave modes that last for $\sim5$ ... More
Hierarchical Formation of Galactic ClustersMay 19 2006Young stellar groupings and clusters have hierarchical patterns ranging from flocculent spiral arms and star complexes on the largest scale to OB associations, OB subgroups, small loose groups, clusters and cluster subclumps on the smallest scales. There ... More
Star-Forming Complexes in GalaxiesNov 10 2004Star complexes are the largest globular regions of star formation in galaxies. If there is a spiral density wave, nuclear ring, tidal arm, or other well-defined stellar structure, then gravitational instabilities in the gaseous component produce giant ... More
Triggering the Formation of Massive ClustersMay 27 2004There are at least 2 distinct mechanisms for the formation of young massive clusters (YMC), all of which require galactic-scale processes. One operates in harrassed fragile galaxies, in the dense cores of low mass galaxies, at the ends of spiral arms, ... More
Gas accretion from halos to disks: observations, curiosities, and problemsNov 18 2015Accretion of gas from the cosmic web to galaxy halos and ultimately their disks is a prediction of modern cosmological models but is rarely observed directly or at the full rate expected from star formation. Here we illustrate possible large-scale cosmic ... More
GMC Origins and Turbulent Motions in Spiral and Dwarf GalaxiesSep 12 2013CO clouds can be non-self-gravitating in high pressure environments, while most should be strongly self-gravitating at low metallicities and ambient pressures. In the LMC, which is HI-rich, GMC formation and destruction should generally include molecule ... More
On the Rapid Collapse and Evolution of Molecular CloudsJul 16 2007Stars generally form faster than the ambipolar diffusion time, suggesting that several processes short circuit the delay and promote a rapid collapse. These processes are considered here, including turbulence compression in the outer parts of giant molecular ... More
The Initial Mass Function in ClustersOct 23 2006The stellar initial mass function (IMF) in star clusters is reviewed. Uncertainties in the observations are emphasized. We suggest there is a distinct possibility that cluster IMFs vary systematically with density or pressure. Dense clusters could have ... More
The Initial Mass Function of StarsMay 19 2006Observations and theory of the IMF are briefly reviewed. Slight variations have been observed, although they are difficult to prove unambiguously. Most detailed theoretical models reproduce the IMF, but because they use different assumptions and conditions, ... More
Gravity, Turbulence, and Star FormationMay 27 2004The azimuthal power spectra of optical emission from star formation and dust in spiral galaxies resembles the azimuthal power spectra of HI emission from the LMC. These and other power spectra of whole galaxies all resemble that of velocity in incompressible ... More
Theory of star formation in the central kiloparsecOct 05 2001Star formation by gravitational instabilities, sequential triggering, and turbulence triggering are briefly reviewed in order to compare the various mechanisms that are observed in main galaxy disks with those in the inner kiloparsec regions. Although ... More
The Origin of The Stellar Initial Mass FunctionOct 27 2000Observations and theories of the stellar initial mass function are reviewed. The universality and large total mass range of the power law portion of the IMF suggest a connection between stellar mass and cloud structure. The fractal patterns and formation ... More
Triggered Star Formation in the LMC4/Constellation III Region of the Large Magellanic CloudMay 07 1998The origin of a regular, 600 pc-long arc of young stars and clusters in the Constellation III region of the Large Magellanic Cloud is considered. The circular form of this arc suggests that the prestellar gas was uniformly swept up by a central source ... More
The IMF and its EvolutionJun 12 1999Observations of the stellar initial mass function are reviewed. The flattening at low mass is evidence for a characteristic mass in star formation, which could be the minimum stellar mass for the onset of deuterium burning or the thermal Jeans mass. Variations ... More
The Globular Cluster Mass Function as a Remnant of Violent BirthMar 03 2010Mar 10 2010The log-normal shape of the mass function for metal-poor halo globular clusters is proposed to result from an initial M^{-2} power law modified rapidly by evaporation, collisions with clouds, and mutual cluster interactions in the dense environment of ... More
Theoretical considerations for star formation at low and high redshiftNov 18 2015Star formation in strongly self-gravitating cloud cores should be similar at all redshifts, forming single or multiple stars with a range of masses determined by local magneto-hydrodynamics and gravity. The formation processes for these cores, however, ... More
Gravitational Instabilities in Two-Component Galaxy Disks with Gas DissipationJun 08 2011Growth rates for gravitational instabilities in a thick disk of gas and stars are determined for a turbulent gas that dissipates on the local crossing time. The scale heights are derived from vertical equilibrium. The accuracy of the usual thickness correction, ... More
The Nature and Nurture of Star ClustersOct 24 2009Star clusters have hierarchical patterns in space and time, suggesting formation processes in the densest regions of a turbulent interstellar medium. Clusters also have hierarchical substructure when they are young, which makes them all look like the ... More
The Stellar Initial Mass Function in 2007: A Year for Discovering VariationsMar 21 2008The characteristic mass M_c and slope Gamma of the IMF are reviewed for clusters, field regions, galaxies, and regions formed during cosmological times. Local star formation has a somewhat uniform M_c and Gamma. Statistical variations in Gamma are summarized, ... More
The Specific Frequency of Globular Clusters in GalaxiesNov 10 1999Variations in the number of globular clusters per unit galaxy luminosity, which is the specific frequency, and the mass of globular clusters per unit total available mass, are reviewed. Correlations with galaxy luminosity and morphology, and with the ... More
Small-Scale Mixing of the ElementsDec 31 1997The progressive mixing and contamination of interstellar gas by supernovae and other processes following the passage of a spiral density wave is reviewed, with an emphasis on the Solar neighborhood. Regions of star formation should begin their lives with ... More
Observations of Thick Disks in the Hubble Space Telescope Ultra Deep FieldJul 24 2006The vertical profiles of chain and spiral galaxies in the Hubble Space Telescope Ultra Deep Field (UDF) are fit to sech^2(z/z_0) functions convolved with stellar profiles in order to measure the disk scale heights z_0 in four passbands. The bulge regions ... More
Fractal Structure in Galactic Star FieldsDec 08 2000The fractal structure of star formation on large scales in disk galaxies is studied using the size distribution function of stellar aggregates in kpc-scale star fields. Achival HST images of 10 galaxies are Gaussian smoothed to define the aggregates, ... More
Rings and Bent Chain Galaxies in the GEMS and GOOD FieldsJul 26 2006Twenty-four galaxies with rings or partial rings were studied in the GEMS and GOODS fields out to z~1.4. Most resemble local collisional ring galaxies in morphology, size, and clumpy star formation. Clump ages range from 10^8 to 10^9 yr and clump masses ... More
Stellar Populations in Ten Clump-Cluster Galaxies of the Ultra Deep FieldApr 01 2005Color-color diagrams for the clump and interclump emission in 10 clump-cluster galaxies of the Ultra Deep Field are made from B,V,i, and z images and compared with models to determine redshifts, star formation histories, and galaxy masses. The clump colors ... More
The Onset of Spiral Structure in the UniverseDec 08 2013The onset of spiral structure in galaxies appears to occur between redshifts 1.4 and 1.8 when disks have developed a cool stellar component, rotation dominates over turbulent motions in the gas, and massive clumps become less frequent. During the transition ... More
Tadpole Galaxies in the Hubble Ultra Deep FieldSep 08 2010Tadpole galaxies have a head-tail shape with a large clump of star formation at the head and a diffuse tail or streak of stars off to one side. We measured the head and tail masses, ages, surface brightnesses, and sizes for 66 tadpoles in the Hubble Ultra ... More
Exponential Galaxy Disks from Stellar ScatteringAug 23 2013Stellar scattering off of orbiting or transient clumps is shown to lead to the formation of exponential profiles in both surface density and velocity dispersion in a two-dimensional non-self gravitating stellar disk with a fixed halo potential. The exponential ... More
Accretion-Driven Turbulence and the Transition to Global Instability in Young Galaxy DisksDec 05 2009Feb 15 2010A simple model of gas accretion in young galaxy disks suggests that fast turbulent motions can be driven by accretion energy for a time t_acc~2(epsilon^{0.5} GM^2/xi V^3)^{0.5} where epsilon is the fraction of the accretion energy going into disk turbulence, ... More
Unstable disks at high redshift: Evidence for smooth accretion in galaxy formationFeb 17 2009Feb 18 2009Galaxies above redshift 1 can be very clumpy, with irregular morphologies dominated by star complexes as large as 2 kpc and as massive as a few 10^8 or 10^9 Mo. Their co-moving densities and rapid evolution suggest that most present-day spirals could ... More
Interstellar Turbulence II: Implications and EffectsApr 22 2004Interstellar turbulence has implications for the dispersal and mixing of the elements, cloud chemistry, cosmic ray scattering, and radio wave propagation through the ionized medium. This review discusses the observations and theory of these effects. Metallicity ... More
The Effect of Star Formation History on the Inferred Initial Stellar Mass FunctionSep 12 2005Peaks and lulls in the star formation rate (SFR) over the history of the Galaxy produce plateaux and declines in the present day mass function (PDMF) where the main-sequence lifetime overlaps the age and duration of the SFR variation. These PDMF features ... More
Exponential Disks from Stellar Scattering: III. Stochastic ModelsJul 26 2016Stellar scattering off irregularities in a galaxy disk has been shown to make an exponential radial profile, but no fundamental reason for this has been suggested. Here we show that exponentials are mathematically expected from random scattering in a ... More
Mass Functions in Fractal Clouds: The Role of Cloud Structure in the Stellar Initial Mass FunctionAug 06 2010The possibility that the stellar initial mass function (IMF) arises mostly from cloud structure is investigated with fractal Brownian motion (fBm) clouds that have power-law power spectra. An fBm cloud with a realistic projected power spectrum slope of ... More
Bogus dust screens from well mixed exponential disks in galaxiesOct 09 1998The V-K colours along the minor axes of spiral galaxies typically change from red to blue with increasing distance, giving the impression that the near side is systematically screened by dust. Such a preferred orientation for dust screens is unlikely. ... More
Near-exponential surface densities as hydrostatic, nonequilibrium profiles in galaxy discsSep 28 2016Apparent exponential surface density profiles are nearly universal in galaxy discs across Hubble types, over a wide mass range, and a diversity of gravitational potential forms. Several processes have been found to produce exponential profiles, including ... More
Constraints on Star Formation from the Close Packing of Protostars in ClustersOct 10 2002The mm-wave continuum sources (MCS) in Ophiuchus have mutual collision rates less than their collapse rates by a factor of 10 to 100, suggesting most will form stars without further interactions. However, this ratio of rates would have exceeded unity ... More
Interstellar Turbulence I: Observations and ProcessesApr 22 2004This two-part review summarizes interstellar turbulence and its implications. The first part begins with diagnostics and energy sources. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, ... More
Nuclear Black Hole Formation in Clumpy Galaxies at High RedshiftMay 15 2008Massive stellar clumps in high redshift galaxies interact and migrate to the center to form a bulge and exponential disk in <1 Gyr. Here we consider the fate of intermediate mass black holes (BHs) that might form by massive-star coalescence in the dense ... More
Bulge Formation by the Coalescence of Giant Clumps in Primordial Disk GalaxiesAug 05 2008Gas-rich disks in the early universe are highly turbulent and have giant star-forming clumps. Models suggest the clumps form by gravitational instabilities, and if they resist disruption by star formation, then they interact, lose angular momentum, and ... More
Discovery of Face-On Counterparts of Chain Galaxies in the Tadpole ACS FieldFeb 20 2004The background field of the Tadpole galaxy image taken with the Hubble Space Telescope ACS contains 87 clump-clusters that are probably low-inclination versions of the 69 chain galaxies we found in a previous paper. This conclusion follows from the similarity ... More
A Turbulent Origin for Flocculent Spiral Structure in GalaxiesMay 04 2003The flocculent structure of star formation in 7 galaxies has a Fourier transform power spectrum for azimuthal intensity scans with a power law slope that increases systematically from -1 at large scales to -1.7 at small scales. This is the same pattern ... More
Central Blue Clumps in Elliptical Galaxies of the Hubble Ultra Deep FieldApr 01 2005Elliptical galaxies larger than 10 pixels in the Hubble Ultra Deep Field (UDF) were surveyed for internal structure; 30 out of 100 in a sample of 884 morphologically classified galaxies exhibit large blue clumps near their centers. Unsharp-masked images ... More
Chain Galaxies in the Tadpole ACS FieldJan 19 2004Colors and magnitudes were determined for 69 chain galaxies, 58 other linear structures, 32 normal edge-on galaxies, and all of their large star formation clumps in the HST ACS field of the Tadpole galaxy. Redshifts of 0.5 to 2 are inferred from comparisons ... More
A Constant Bar Fraction out to Redshift z~1 in the Advanced Camera for Surveys Field of the Tadpole GalaxyJul 28 2004Bar-like structures were investigated in a sample of 186 disk galaxies larger than 0.5 arcsec that are in the I-band image of the Tadpole galaxy taken with the HST ACS. We found 22 clear cases of barred galaxies, 21 galaxies with small bars that appear ... More
Smooth and Starburst Tidal Tails in the GEMS and GOODS FieldsApr 06 2007GEMS and GOODS fields were examined to z~1.4 for galaxy interactions and mergers. The basic morphologies are familiar: antennae with long tidal tails, tidal dwarfs, and merged cores; M51-type galaxies with disk spirals and tidal arm companions; early-type ... More
Environmental Dependencies for Star Formation Triggered by Expanding Shell CollapseApr 09 2002Criteria for gravitational collapse of expanding shells in rotating, shearing galaxy disks were determined using three-dimensional numerical simulations in the thin shell approximation. The simulations were run over a grid of 7 independent variables, ... More
Hierarchy of Interstellar and Stellar Structures and the Case of the Orion Star-Forming RegionJan 09 1998Hierarchical structure from stellar clusters, to subgroups, to associations and star complexes is discussed in the context of the Orion stellar grouping and its origin. The analogous structure in gas clouds is also reviewed, with an emphasis on general ... More
A Density Dependence for Protostellar Luminosity in Class I Sources: Collaborative AccretionJan 16 2014Class I protostars in three high-mass star-forming regions are found to have correlations among the local projected density of other Class I protostars, the summed flux from these other protostars, and the protostellar luminosity in the WISE 22 micron ... More
Star Formation Properties of a Large Sample of Irregular GalaxiesAug 12 2004We present the results of Halpha imaging of 94 irregular galaxies, including types Im, BCD, and Sm. The sample spans a large range in galactic parameters including integrated absolute magnitude (MV=-9 to -19), average surface brightness (20 to 27 mag/arcsec2), ... More
Massive Clusters in the Inner Regions of NGC 1365: Cluster Formation and Gas Dynamics in Galactic BarsJul 15 2009Jul 21 2009Cluster formation and gas dynamics in the central regions of barred galaxies are not well understood. This paper reviews the environment of three 10^7 Msun clusters near the inner Lindblad resonance of the barred spiral NGC 1365. The morphology, mass, ... More
In-spiraling Clumps in Blue Compact Dwarf GalaxiesJan 17 2012Giant star-formation clumps in dwarf irregular galaxies can have masses exceeding a few percent of the galaxy mass enclosed inside their orbital radii. They can produce sufficient torques on dark matter halo particles, halo stars, and the surrounding ... More
On the Disruption of Star Clusters in a Hierarchical Interstellar MediumFeb 15 2010The distribution of the number of clusters as a function of mass M and age T suggests that clusters get eroded or dispersed in a regular way over time, such that the cluster number decreases inversely as an approximate power law with T within each fixed ... More
Radial Profiles of Star Formation in the Far Outer Regions of Galaxy DisksSep 07 2005Star formation in galaxies is triggered by a combination of processes, including gravitational instabilities, spiral wave shocks, stellar compression, and turbulence compression. Some of these persist in the far outer regions where the column density ... More
Formation of Metal-Poor Globular Clusters in Lyman alpha Emitting Galaxies in the Early UniverseJul 21 2012Aug 09 2012The size, mass, luminosity, and space density of Lyman-alpha emitting (LAE) galaxies observed at intermediate to high redshift agree with expectations for the properties of galaxies that formed metal-poor halo globular clusters (GCs). The low metallicity ... More
The thick disks of spiral galaxies as relics from gas-rich, turbulent, clumpy disks at high redshiftOct 19 2009The formation of thick stellar disks in spiral galaxies is studied. Simulations of gas-rich young galaxies show formation of internal clumps by gravitational instabilities, clump coalescence into a bulge, and disk thickening by strong stellar scattering. ... More
Broad-Band Imaging of a Large Sample of Irregular GalaxiesSep 07 2005We present the results of UBV imaging of a large sample of irregular galaxies: 94 Im systems, 24 Blue Compact Dwarfs (BCDs), and 18 Sm galaxies. We also include JHK imaging of 41 of these galaxies. The sample spans a large range in galactic parameters. ... More
A Star Formation Law for Dwarf Irregular GalaxiesMar 15 2015The radial profiles of gas, stars, and far ultraviolet radiation in 20 dwarf Irregular galaxies are converted to stability parameters and scale heights for a test of the importance of two-dimensional (2D) instabilities in promoting star formation. A detailed ... More
Bulge and Clump Evolution in Hubble Ultra Deep Field Clump Clusters, Chains and Spiral GalaxiesOct 30 2008Clump clusters and chain galaxies in the Hubble Ultra Deep Field are examined for bulges in the NICMOS images. Approximately 50% of the clump clusters and 30% of the chains have relatively red and massive clumps that could be young bulges. Magnitudes ... More
Galaxy Morphologies in the Hubble Ultra Deep Field: Dominance of Linear Structures at the Detection LimitAug 09 2005Galaxies in the Hubble Ultra Deep Field (UDF) larger than 10 pixels (0.3 arcsec) have been classified according to morphology and their photometric properties are presented. There are 269 spirals, 100 ellipticals, 114 chains, 126 double-clump, 97 tadpole, ... More
Hierarchical Star Formation in the Spiral Galaxy NGC 628May 20 2006The distributions of size and luminosity for star-forming regions in the nearby spiral galaxy NGC 628 are studied over a wide range of scales using progressively blurred versions of an image from the Hubble Space Telescope Advanced Camera for Surveys. ... More
CO Observations of the Interacting Galaxy Pair NGC 5394/95Nov 13 2001BIMA CO 1-0 observations are presented of the spiral galaxies NGC 5394 and NGC 5395 that have undergone a recent, grazing encounter. In NGC 5394, 80% of the CO emission detected by BIMA is concentrated in the central 800 pc (FWHM) starburst region.In ... More
Clumpy Galaxies in GOODS and GEMS: Massive Analogs of Local Dwarf IrregularsJun 15 2009Clumpy galaxies in the GEMS and GOODS fields are examined for clues to their evolution into modern spirals. The magnitudes of the clumps and the surface brightnesses of the interclump regions are measured and fitted to models of stellar age and mass. ... More
Power Spectra in V-band and Halpha of Nine Irregular GalaxiesMar 14 2005Fourier transform power spectra of major axis cuts in V and Halpha images were made for a sample of 9 irregular galaxies. These power spectra reveal structure over a wide range of scales. For 6 of the galaxies the power spectrum slopes at intermediate ... More
GALEX Ultraviolet Imaging of Dwarf Galaxies and Star FormationNov 23 2009We present ultraviolet integrated and azimuthally-averaged surface photometric properties of a sample of 44 dIm, BCD, and Sm galaxies measured from archival NUV and FUV images obtained with GALEX. We compare the UV to Halpha and V-band properties and ... More
Surface Brightness Profiles of Dwarf Galaxies: II. Color Trends and Mass ProfilesJun 02 2016In this second paper of a series, we explore the B-V, U-B, and FUV-NUV radial color trends from a multi-wavelength sample of 141 dwarf disk galaxies. Like spirals, dwarf galaxies have three types of radial surface brightness profiles: (I) single exponential ... More
A Fractal Analysis of the HI Emission from the Large Magellanic CloudOct 27 2000A composite map of HI in the LMC using the ATCA interferometer and the Parkes multibeam telescope was analyzed in several ways in an attempt to characterize the structure of the neutral gas and to find an origin for it. Fourier transform power spectra ... More
Young star clusters in the outer disks of LITTLE THINGS dwarf irregular galaxiesMar 01 2016We examine FUV images of the LITTLE THINGS sample of nearby dwarf irregular (dIrr) and Blue Compact Dwarf (BCD) galaxies to identify distinct young regions in their far outer disks. We use these data, obtained with the Galaxy Evolution Explorer satellite, ... More
The Relationship Between Gas, Stars, and Star Formation in Irregular Galaxies: A Test of Simple ModelsDec 31 1997We explore various instability models for the onset of star formation in irregular galaxies. Critical gas densities are calculated for gravitational instabilities with a thin, pure-gas disk, and with a thick disk composed of gas and a star-like fluid. ... More