arXiv subjects

Bruce G. Elmegreen

Publications and source records attributed to Bruce G. Elmegreen.

At least 217 records · Page 12Linked to original sources

arXiv2004

The IMF in Starbursts

The 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 results in perspective. Observed IMF variations seem to suggest that the IMF varies a little with environment in the sense that denser and more massive clusters produce more massive stars, and perhaps more brown dwarfs too, compared to intermediate mass stars.

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arXiv2004

Cluster Density and the IMF

Observed 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 of star formation, like Taurus, may have relatively steep IMFs too. Very dense and massive clusters, like super star clusters, could have flatter IMFs, or inner-truncated IMFs. We propose that these variations are the result of three distinct processes during star formation that affect the mass function in different ways depending on mass range. At solar to intermediate stellar masses, gas processes involving thermal pressure and supersonic turbulence determine the basic scale for stellar mass, starting with the observed pre-stellar condensations, and they define the mass function from several tenths to several solar masses. Brown dwarfs require extraordinarily high pressures for fragmentation from the gas, and presumably form inside the pre-stellar condensations during mutual collisions, secondary fragmentations, or in disks. High mass stars form in excess of the numbers expected from pure turbulent fragmentation as pre-stellar condensations coalesce and accrete with an enhanced gravitational cross section. Variations in the interaction rate, interaction strength, and accretion rate among the primary fragments formed by turbulence lead to variations in the relative proportions of brown dwarfs, solar to intermediate mass stars, and high mass stars.

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arXiv2004

Star Formation Properties of a Large Sample of Irregular Galaxies

We 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), current star formation activity (0 to 1.3 Msun/yr/kpc2), and relative gas content (0.02 to 5 Msun/LB). The Halpha images were used to measure integrated star formation rates and extents of star formation and to compare azimuthally-averaged radial profiles of current star formation to older starlight. The integrated star formation rates of Im galaxies normalized to the physical size of the galaxy span a range of a factor of 10^4 with 10% Im galaxies and one Sm system having no measurable star formation at the present time. The BCDs fall at the high end of the range of rates. We find no correlation between star formation activity and proximity to other catalogued galaxies. The cases with strong gradients in the L(Halpha)/LV ratios and with high central star formation rate densities, which include most of the BCDs, require a significant fraction of their gas to have migrated to the center in the last Gyr. We discuss possible torques that could have caused this without leaving an obvious signature, including dark matter bars and past interactions or mergers with small galaxies or HI clouds. There is now substantial evidence for these processes among many surveys of BCDs. The star formation process itself does not appear to differ much among BCD, Im and Sm types. There is evidence in the distribution function for Halpha surface brightness, which is always approximately log-normal, that the turbulent Mach numbers are about the same in these systems.

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arXiv2004

Variability in the stellar initial mass function at low and high mass: 3-component IMF models

Three 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 mass for most star formation is identified with the thermal Jeans mass in the molecular cloud core, and this presumably leads to the middle mass range by the usual collapse and accretion processes. Pre-stellar condensations (PSCs) observed in mm-wave continuum studies presumably form at this mass. Significantly smaller self-gravitating masses require much larger pressures and may arise following dynamical processes inside these PSCs, including disk formation, tight-cluster ejection, and photoevaporation as studied elsewhere, but also gravitational collapse of shocked gas in colliding PSCs. Significantly larger stellar masses form in relatively low abundance by normal cloud processes, possibly leading to steep IMFs in low-pressure field regions, but this mass range can be significantly extended in high pressure cloud cores by gravitationally-focussed gas accretion onto PSCs and by the coalescence of PSCs. These models suggest that the observed variations in brown dwarf, solar-to-intermediate mass, and high mass populations are the result of dynamical effects that depend on environmental density and velocity dispersion. They accommodate observations ranging from shallow IMFs in cluster cores to Salpeter IMFs in average clusters and whole galaxies to steep and even steeper IMFs in field and remote field regions. They also suggest how the top-heavy IMFs in some starburst clusters may originate and they explain bottom-heavy IMFs in low surface brightness galaxies.

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arXiv2004

Turbulence and galactic structure

Interstellar 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. Star formation is initiated on large scales by gravitational instabilities which control the overall rate through the long dynamical time corresponding to the average ISM density. Stars form at much higher densities than average, however, and at much faster rates locally, so the slow average rate arises because the fraction of the gas mass that forms stars at any one time is low, ~10^{-4}. This low fraction is determined by turbulence compression, and is apparently independent of specific cloud formation processes which all operate at lower densities. Turbulence compression also accounts for the formation of most stars in clusters, along with the cluster mass spectrum, and it gives a hierarchical distribution to the positions of these clusters and to star-forming regions in general. Turbulent motions appear to be very fast in irregular galaxies at high redshift, possibly having speeds equal to several tenths of the rotation speed in view of the morphology of chain galaxies and their face-on counterparts. The origin of this turbulence is not evident, but some of it could come from accretion onto the disk. Such high turbulence could help drive an early epoch of gas inflow through viscous torques in galaxies where spiral arms and bars are weak. Such evolution may lead to bulge or bar formation, or to bar re-formation if a previous bar dissolved. We show evidence that the bar fraction is about constant with redshift out to z~1, and model the formation and destruction rates of bars required to achieve this constancy.

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arXiv2004

A Constant Bar Fraction out to Redshift z~1 in the Advanced Camera for Surveys Field of the Tadpole Galaxy

Bar-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 primarily as isophotal twists in a contour plot, and 11 cases of peculiar bars in clump-cluster galaxies, which are face-on versions of chain galaxies. The latter bars are probably young, as the galaxies contain only weak interclump emission. Four of the clearly barred galaxies at z~0.8-1.2 have grand design spirals. The bar fraction was determined as a function of galaxy inclination and compared with the analogous distribution in the local Universe. The bar fraction was also determined as a function of galaxy angular size. These distributions suggest that inclination and resolution effects obscure nearly half of the bars in our sample. The bar fraction was also determined as a function of redshift. We found a nearly constant bar fraction of 0.23+-0.03 from z~0 to z=1.1. When corrected for inclination and size effects, this fraction is comparable to the bar fraction in the local Universe, ~0.4, as tabulated for all bar and Hubble types in the Third Reference Catalogue of Galaxies. The average major axis of a barred galaxy in our sample is ~10 kpc after correcting for redshift with a LambdaCDM cosmology. Galaxy bars were present in normal abundance at least ~8 Gy ago (z~1); bar dissolution cannot be common during a Hubble time unless the bar formation rate is comparable to the dissolution rate.

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arXiv2004

Formation of Young Star Clusters

Turbulence, 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 temperature. Self-gravity also partitions the gas into clouds, forming giant regular complexes in spiral arms and in resonance rings and contributing to the scale-free motions generated by turbulence. Dense clusters form in the most strongly self-gravitating cores of these clouds, often triggered by compression from local stars. Pre-star formation processes inside clusters are not well observed, but the high formation rates and high densities of pre-stellar objects, and their power law mass functions suggest that turbulence, self-gravity, and energy dissipation are involved there too.

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arXiv2004

Triggering the Formation of Massive Clusters

There 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, or in galactic tidal shocks where transient and peculiar high pressures make massive clouds at high densities. The result of this process is usually only one or two YMC without the usual morphologies of local star formation, i.e., without hierarchical structure and a continuous power law distribution of cluster masses up to the largest mass. The other operates in the more usual way: continuously for long periods of time in large parts of the interstellar medium where the ambient pressure is already high as a result of the deep potential well from background stars and other gas. This second process makes clusters in a hierarchical fashion with size-of-sample effects, and tends to occur in nuclear rings, merger remnants, and even the ambient ISM of normal galaxies if the star formation rate is high enough to sample out to the YMC range.

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arXiv2004

Gravity, Turbulence, and Star Formation

The 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 Kolmogorov turbulence. The reasons for this are unknown but it could be simply that star and cloud formation are the result of a mixture of processes and each gives a power spectrum similar to Kolmogorov turbulence, within the observable errors. The important point is that star and cloud formation are not random but are correlated over large distances by forces that span several orders of magnitude in scale. These forces are probably the usual combination of self-gravity, turbulence, and compression from stellar winds and supernovae, but they have to work in concert to create the structures we see in galaxies. In addition, the identification of flocculant spirals with swing amplified instabilities opens the possibility that a high fraction of turbulence in the ISM is the result of self-gravity.

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arXiv2004

Interstellar Turbulence I: Observations and Processes

This 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, global inviscid quadratic invariants, scaling arguments, energy transfer and cascade, velocity probability distributions, and turbulent pressure. Theories of magnetohydrodynamic turbulence, including collisionless MHD turbulence, are discussed, emphasizing various proposals for energy spectra. A large variety of numerical simulations of interstellar turbulence are reviewed. Models have reproduced many of the basic features of the observed scaling relations, predicted fast decay rates for supersonic MHD turbulence, and derived probability distribution functions for density. Thermal instabilities and thermal phases have a new interpretation in a turbulent medium. Large-scale models with various combinations of self-gravity, magnetic fields, supernovae, and star formation are beginning to resemble the observed interstellar medium in morphology and statistical properties. The role of self-gravity in turbulent gas is clarified, leading to new paradigms for the formation of star clusters, the stellar mass function, the origin of stellar rotation and binary stars, and the effects of magnetic fields.

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arXiv2004

Interstellar Turbulence II: Implications and Effects

Interstellar 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 fluctuations are summarized, and the theory of turbulent transport of passive tracers is reviewed. Modeling methods, turbulent concentration of dust grains, and the turbulent washout of radial abundance gradients are discussed. Interstellar chemistry is affected by turbulent transport of various species between environments with different physical properties and by turbulent heating in shocks, vortical dissipation regions, and local regions of enhanced ambipolar diffusion. Cosmic rays are scattered and accelerated in turbulent magnetic waves and shocks, and they generate turbulence on the scale of their gyroradii. Radio wave scintillation is an important diagnostic for small scale turbulence in the ionized medium, giving information about the power spectrum and amplitude of fluctuations. The theory of diffraction and refraction is reviewed, as are the main observations and scintillation regions.

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arXiv2004

Discovery of Face-On Counterparts of Chain Galaxies in the Tadpole ACS Field

The 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 in colors and apparent magnitudes of the brightest clumps in the clump-clusters and chain galaxies, and from the similarity between the distribution of axial ratios for the combined sample and the distribution for normal disk galaxies. These distributions are consistent with chains and clump-clusters being disks viewed at random angles with intrinsic axial ratios of 0.1-0.2 or less. The disks appear to be gas-rich galaxies at z=1 to 2 that are forming an early generation of stars in a small number of very large star complexes.

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arXiv2004

Chain Galaxies in the Tadpole ACS Field

Colors 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 with published color-evolution models. The linear galaxies have no red nuclear bulges like the normal disk galaxies in our field, but the star formation clumps in each have about the same colors and magnitudes. Light profiles along the linear galaxies tend to be flat, unlike the exponential profiles of normal galaxies. Although the most extreme of the linear objects look like beaded filaments, they are all probably edge-on disks that will evolve to late Hubble type galaxies. The lack of an exponential profile is either the result of a dust scale height that is comparable to the stellar scale height, or an intrinsically irregular structure. Examples of galaxies that could be face-on versions of linear galaxies are shown. They have an irregular clumpy structure with no central bulge and with clump colors and magnitudes that are comparable to those in the linears. Radiative transfer solutions to the magnitudes and surface brightnesses of inclined dusty galaxies suggest that edge-on disks should become more prominent near the detection limit for surface brightness. The surface brightness distribution of the edge-on galaxies in this field confirm this selection effect. The star formation regions are much more massive than in modern galaxies, averaging up to 10^9 Msun for kpc scales.

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arXiv2003

A Turbulent Origin for Flocculent Spiral Structure in Galaxies

The 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 as in the power spectra for azimuthal scans of HI emission in the Large Magellanic Clouds and for flocculent dust clouds in galactic nuclei. The steep part also corresponds to the slope of -3 for two-dimensional power spectra that have been observed in atomic and molecular gas surveys of the Milky Way and the Large and Small Magellanic Clouds. The same power law structure for star formation arises in both flocculent and grand design galaxies, which implies that the star formation process is the same in each. Fractal Brownian motion models that include discrete stars and an underlying continuum of starlight match the observations if all of the emission is organized into a global fractal pattern with an intrinsic 1D power spectrum having a slope between 1.3 and 1.8. We suggest that the power spectrum of optical light in galaxies is the result of turbulence, and that large-scale turbulent motions are generated by sheared gravitational instabilities which make flocculent spiral arms first and then cascade to form clouds and clusters on smaller scales.

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arXiv2003

A Turbulent Origin for Flocculent Spiral Structure in Galaxies: II. Observations and Models of M33

Fourier transform power spectra of azimuthal scans of the optical structure of M33 are evaluated for B, V, and R passbands and fit to fractal models of continuum emission with superposed star formation. Power spectra are also determined for Halpha. The best models have intrinsic power spectra with 1D slopes of around -0.7pm0.7, significantly shallower than the Kolmogorov spectrum (slope =-1.7) but steeper than pure noise (slope=0). A fit to the power spectrum of the flocculent galaxy NGC 5055 gives a steeper slope of around -1.5pm0.2, which could be from turbulence. Both cases model the optical light as a superposition of continuous and point-like stellar sources that follow an underlying fractal pattern. Foreground bright stars are clipped in the images, but they are so prominent in M33 that even their residual affects the power spectrum, making it shallower than what is intrinsic to the galaxy. A model consisting of random foreground stars added to the best model of NGC 5055 fits the observed power spectrum of M33 as well as the shallower intrinsic power spectrum that was made without foreground stars. Thus the optical structure in M33 could result from turbulence too.

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arXiv2002

Gas Phase Processes Affecting Galactic Evolution

Gas 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 that only an ISM with a pure-warm phase can stop star formation. The instabilities generate turbulence, and this turbulence influences the structure and timing of star formation through its effect on the gas distribution and density. The final trigger for star formation is usually direct compression by another star or cluster. The star formation rate is apparently independent of the detailed mechanisms for star formation, and determined primarily by the total mass of gas in a dense form. If the density distribution function is a log-normal, as suggested by turbulence simulations, then this dense gas mass can be calculated and the star formation rate determined from first principles. The results suggest that only 10**(-4) of the ISM mass actively participates in the star formation process and that this fraction does so because its density is larger than 10**5 cm-3, at which point several key processes affecting dynamical equilibrium begin to break down.

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arXiv2002

Constraints on Star Formation from the Close Packing of Protostars in Clusters

The 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 in the past if the MCS were 2.5 times larger than they are now. Such a high previous ratio was not likely, suggesting that the MCS grew from smaller sizes at a constant high density, possible behind turbulent shocks. Proximity constraints are even more important in massive clusters, where massive stars with the same or greater space density are more strongly interacting than the Ophiuchus MCS. As a result, the density contrast for MCS must be larger in massive clusters than it is in Ophiuchus, or significant coalescence will occur in the protostellar phase, possible forming a massive black hole. A proportionality to the second power of the Mach number allows the MCS cores to collapse independently. These results suggest that stars in dense clusters generally form on a dynamical time by the continuous collection and rapid collapse of turbulence-shocked gas. Implications of proximity constraints on the initial stellar mass function are also discussed.

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arXiv2002

Star Formation from Galaxies to Globules

The 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 turbulence compression and expanding shell collapse. The self-similar nature of the gas and associated young stars suggests that turbulence is more directly involved, but the small scale morphology of gas around most embedded clusters does not look like a random turbulent flow. Most clusters look triggered by other nearby stars. Such a prominent local influence makes it difficult to understand the universality of the Kennicutt and Schmidt laws on galactic scales. A unified view of multi-scale star formation avoids most of these problems. Ambient self-gravity produces spiral arms and drives much of the turbulence that leads to self-similar structures, while localized energy input from existing clusters and field supernovae triggers new clusters in pre-existing clouds. The hierarchical structure in the gas made by turbulence ensures that the triggering time scales with size, giving the Schmidt law over a wide range of scales and the size-duration correlation for young star fields. The efficiency of star formation is determined by the fraction of the gas above a critical density of around 10^5 m(H2)/cc. Star formation is saturated to its largest possible value given the fractal nature of the interstellar medium.

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