arXiv subjects

Bruce G. Elmegreen

Publications and source records attributed to Bruce G. Elmegreen.

At least 181 records · Page 10Linked to original sources

arXiv2009

Clumpy Galaxies in GOODS and GEMS: Massive Analogs of Local Dwarf Irregulars

Clumpy 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. There is an evolutionary trend from clump clusters with no evident interclump emission to clump clusters with faint red disks, to spiral galaxies of the flocculent or grand design types. Along this sequence, the interclump surface density increases and the mass surface density contrast between the clumps and the interclump regions decreases, suggesting a gradual dispersal of clumps to form disks. Also along this sequence, the bulge-to-clump mass ratios and age ratios increase, suggesting a gradual formation of bulges. All of these morphological types occur in the same redshift range, indicating that the clump cluster morphology is not the result of bandshifting. Comparisons to local galaxies with the same rest wavelength and spatial resolution show that clump clusters resemble local dwarf Irregulars. This resemblance is consistent with a model in which the clumpy morphology comes from gravitational instabilities in gas with a high turbulent speed compared to the rotation speed and a high mass fraction compared to the stars.

astro-ph.CO↗

arXiv2009

Bulge formation by the coalescence of giant clumps in primordial disk galaxies

The 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 a bulge. The model requires smooth gas accretion during galaxy growth.

astro-ph.CO↗

arXiv2009

Unstable disks at high redshift: Evidence for smooth accretion in galaxy formation

Galaxies 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 have formed through a clumpy phase. The clumps may form by gravitational instabilities in gas-rich turbulent disks; they do not appear to be separate galaxies merging together. We show here that the formation of the observed clumps requires initial disks of gas and stars with almost no stabilizing bulge or stellar halo. This cannot be achieved in models where disk galaxies grow by mergers. Mergers tend to make stellar spheroids even when the gas fraction is high, and then the disk is too stable to make giant clumps. The morphology of high-redshift galaxies thus suggests that inner disks assemble mostly by smooth gas accretion, either from cosmological flows or from the outer disk during a grazing interaction.

astro-ph.CO↗

arXiv2008

Star Formation in Disks: Spiral Arms, Turbulence, and Triggering Mechanisms

Star 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 familiar beads on a string with 1-2 kpc separation. Similar instabilities occur in resonance rings and tidal tails. Sequential triggering from stellar pressure occurs in two ways. For short times and near distances, it occurs in the bright rims and dense knots that lag behind during cloud dispersal. For long times, it occurs in swept-up shells and along the periphery of cleared regions. The first case should be common but difficult to disentangle from independent star formation in the same cloud. The second case has a causality condition and a collapse condition and is often easy to recognize. Turbulent triggering produces a hierarchy of dense cloudy structure and an associated hierarchy of young star positions. There should also be a correlation between the duration of star formation and the size of the region that is analogous to the size-linewidth relation in the gas. The cosmological context is provided by observations of star formation in high redshift galaxies. Sequential and turbulent triggering is not yet observable, but gravitational instabilities are, and they show a scale up from local instabilities by a factor of ~3 in size and ~100 in mass. This is most easily explained as the result of an increase in the ISM turbulent speed by a factor of ~5. In the clumpiest galaxies at high redshift, the clumps are so large that they should interact with each other and merge in the center, where they form or contribute to the bulge.

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arXiv2008

Bulge and Clump Evolution in Hubble Ultra Deep Field Clump Clusters, Chains and Spiral Galaxies

Clump 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 and colors are determined for these bulge-like objects and for the bulges in spiral galaxies, and for all of the prominent star-formation clumps in these three galaxy types. The colors are fitted to population evolution models to determine the bulge and clump masses, ages, star-formation rate decay times, and extinctions. The results indicate that bulge-like objects in clump cluster and chain galaxies have similar ages and 2 to 5 times larger masses compared to the star-formation clumps, while the bulges in spirals have ~6 times larger ages and 20 to 30 times larger masses than the clumps. All systems appear to have an underlying red disk population. The masses of star-forming clumps are typically in a range from 10^7 to 10^8 Msun; their ages have a wide range around ~10^2 Myr. Ages and extinctions both decrease with redshift. Star formation is probably the result of gravitational instabilities in the disk gas, in which case the large clump mass in the UDF is the result of a high gas velocity dispersion, 30 km/s or more, combined with a high gas mass column density, ~100 Msun/pc^2. Because clump clusters and chains dominate disk galaxies beyond z~1, the observations suggest that these types represent an early phase in the formation of modern spiral galaxies, when the bulge and inner disk formed.

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arXiv2008

Bulge Formation by the Coalescence of Giant Clumps in Primordial Disk Galaxies

Gas-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 migrate to the center to form a bulge. Here we study the properties of the bulges formed by this mechanism. They are all thick, slowly rotating, and have a high Sersic index, like classical bulges. Their rapid formation should also give them relatively high alpha-element abundances. We consider fourteen low-resolution models and four high-resolution models, three of which have supernova feedback. All models have an active halo, stellar disk, and gaseous disk, three of the models have a pre-existing bulge and three others have a cuspy dark matter halo. All show the same basic result except the one with the highest feedback, in which the clumps are quickly destroyed and the disk thickens too much. The coalescence of massive disk clumps in the center of a galaxy is like a major merger in terms of orbital mixing. It differs by leaving a bulge with no specific dark matter component, unlike the merger of individual galaxies. Normal supernova feedback has little effect because the high turbulent speed in the gas produces tightly bound clumps. A variety of indirect observations support the model, including clumpy disks with young bulges at high redshift and bulges with relatively little dark matter.

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arXiv2008

Nuclear Black Hole Formation in Clumpy Galaxies at High Redshift

Massive 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 young clusters of these disk clumps. We find that the BHs move inward with the clumps and reach the inner few hundred parsecs in only a few orbit times. There they could merge into a supermassive BH by dynamical friction. The ratio of BH mass to stellar mass in the disk clumps is approximately preserved in the final ratio of BH to bulge mass. Because this ratio for individual clusters has been estimated to be ~10^{-3}, the observed BH-to-bulge mass ratio results. We also obtain a relation between BH mass and bulge velocity dispersion that is compatible with observations of present-day galaxies.

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arXiv2008

On the Constancy of the Characteristic Mass of Young Stars

The characteristic mass M_c in the stellar initial mass function (IMF) is about constant for most star-forming regions. Numerical simulations consistently show a proportionality between M_c and the thermal Jeans mass M_J at the time of cloud fragmentation, but no models have explained how it can be the same in diverse conditions. Here we show that M_J depends weakly on density, temperature, metallicity, and radiation field in three environments: the dense cores where stars form, larger star-forming regions ranging from GMCs to galactic disks, and the interiors of HII regions and super star clusters. In dense cores, the quantity T^{3/2}n^{-1/2} that appears in M_J scales with core density as n^{0.25} or with radiation density as U^{0.1} at the density where dust and gas come into thermal equilibrium. On larger scales, this quantity varies with ambient density as n^{-0.05} and ambient radiation field as U^{-0.033} when the Kennicutt-Schmidt law of star formation determines U(n). In super star clusters with ionization and compression of pre-stellar globules, M_J varies as the 0.13 power of the cluster column density. These weak dependencies on n, U, and column density imply that most environmental variations affect the thermal Jeans mass by at most a factor of ~2. Cosmological increases in M_J, which have been suggested by observations, may be explained if the star formation efficiency is systematically higher at high redshift for a given density and pressure, if dust grains are smaller at lower metallicity, and so hotter for a given radiation field, or if small pre-stellar cores are more severely ionized in extreme starburst conditions.

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arXiv2008

The Stellar Initial Mass Function in 2007: A Year for Discovering Variations

The 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, as are the limitations imposed by these variations. Cosmological star formation appears to have both a higher M_c and a slightly shallower slope at intermediate to high stellar mass. The center of the Milky Way may have a shallow slope too. Field regions have slightly steeper slopes than clusters, but this could be the result of enhanced drift of low mass stars out of clusters and associations. Dwarf galaxies also have steeper slopes. Results from the observation of pre-stellar clumps are reviewed too. Pre-stellar clumps appear to have about the same mass function as stars and are therefore thought to be the main precursors to stars. If this is the case, then the IMF is generally determined by gas-phase processes. Brown dwarf formation also shares many characteristics of star formation, suggesting that they form by similar mechanisms.

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arXiv2007

Variation of Galactic Bar Length with Amplitude and Density as Evidence for Bar Growth over a Hubble Time

K_s-band images of 20 barred galaxies show an increase in the peak amplitude of the normalized m=2 Fourier component with the R_25-normalized radius at this peak. This implies that longer bars have higher $m=2$ amplitudes. The long bars also correlate with an increased density in the central parts of the disks, as measured by the luminosity inside 0.25R_25 divided by the cube of this radius in kpc. Because denser galaxies evolve faster, these correlations suggest that bars grow in length and amplitude over a Hubble time with the fastest evolution occurring in the densest galaxies. All but three of the sample have early-type flat bars; there is no clear correlation between the correlated quantities and the Hubble type.

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arXiv2007

Variations in Stellar Clustering with Environment: Dispersed Star Formation and the Origin of Faint Fuzzies

The 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 characteristic density as which individual stars become well defined. Also in this case, the efficiency of star formation increases with the dispersion of the density probability distribution function (pdf). Models with log-normal pdf's illustrate these effects. The difference between star formation in bound clusters and star formation in loose groupings is attributed to a difference in cloud pressure, with higher pressures forming more tightly bound clusters. This correlation accounts for the observed increase in clustering fraction with star formation rate and with the observation of Scaled OB Associations in low pressure environments. ``Faint fuzzie'' star clusters, which are bound but have low densities, can form in regions with high Mach numbers and low background tidal forces. The proposal by Burkert, Brodie & Larsen (2005) that faint fuzzies form at large radii in galactic collisional rings, satisfies these constraints.

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arXiv2007

Evolution of the Bar Fraction in COSMOS: Quantifying the Assembly of the Hubble Sequence

We have analyzed the redshift-dependent fraction of galactic bars over 0.2<z<0.84 in 2,157 luminous face-on spiral galaxies from the COSMOS 2-square degree field. Our sample is an order of magnitude larger than that used in any previous investigation, and is based on substantially deeper imaging data than that available from earlier wide-area studies of high-redshift galaxy morphology. We find that the fraction of barred spirals declines rapidly with redshift. Whereas in the local Universe about 65% of luminous spiral galaxies contain bars (SB+SAB), at z ~0.84 this fraction drops to about 20%. Over this redshift range the fraction of strong (SB) bars drops from about 30% to under 10%. It is clear that when the Universe was half its present age, the census of galaxies on the Hubble sequence was fundamentally different from that of the present day. A major clue to understanding this phenomenon has also emerged from our analysis, which shows that the bar fraction in spiral galaxies is a strong function of stellar mass, integrated color and bulge prominence. The bar fraction in very massive, luminous spirals is about constant out to z ~ 0.84 whereas for the low mass, blue spirals it declines significantly with redshift beyond z=0.3. There is also a slight preference for bars in bulge dominated systems at high redshifts which may be an important clue towards the co-evolution of bars, bulges and black holes. Our results thus have important ramifications for the processes responsible for galactic downsizing, suggesting that massive galaxies matured early in a dynamical sense, and not just as a result of the regulation of their star formation rate.

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arXiv2007

On the Rapid Collapse and Evolution of Molecular Clouds

Stars 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 cloud (GMC) cores and GMC envelopes, GMC core formation in an initially supercritical state, and compression-induced triggering in dispersing GMC envelopes. The classical issues related to star formation timescales are addressed: high molecular fractions, low efficiencies, long consumption times for CO and HCN, rapid GMC core disruption and the lack of a stable core, long absolute but short relative timescales with accelerated star formation, and the slow motions of protostars. We consider stimuli to collapse from changes in the density dependence of the ionization fraction, the cosmic ray ionization rate, and various dust properties at densities above ~10^5 cm^{-3}. We favor the standard model of subcritical GMC envelops and suggest they would be long lived if not for disruption by rapid star formation in GMC cores. The lifecycle of GMCs is illustrated by a spiral arm section in the Hubble Heritage image of M51, showing GMC formation, star formation, GMC disruption with lingering triggered star formation, and envelope dispersal. There is no delay between spiral arm dustlanes and star formation; the classical notion results from heavy extinction in the dust lane and triggered star formation during cloud dispersal. Differences in the IMF for the different modes of star formation are considered.

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arXiv2007

Smooth and Starburst Tidal Tails in the GEMS and GOODS Fields

GEMS 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 galaxies with diffuse plumes; equal-mass grazing-collisions; and thick J-shaped tails beaded with star formation and double cores. One type is not common locally and is apparently a loose assemblage of smaller galaxies. Photometric measurements were made of the tails and clumps, and physical sizes were determined assuming photometric redshifts. Antennae tails are a factor of ~3 smaller in GEMS and GOODS systems compared to local antennae; their disks are a factor of ~2 smaller than locally. Collisions among early type galaxies generally show no fine structure in their tails, indicating that stellar debris is usually not unstable. One exception has a 5x10**9 Msun smooth red clump that could be a pure stellar condensation. Most tidal dwarfs are blue and probably form by gravitational instabilities in the gas. One tidal dwarf looks like it existed previously and was incorporated into the arm tip by tidal forces. The star-forming regions in tidal arms are 10 to 1000 times more massive than star complexes in local galaxies, although their separations are about the same. If they all form by gravitational instabilities, then the gaseous velocity dispersions in interacting galaxies have to be larger than in local galaxies by a factor of ~5 or more; the gas column densities have to be larger by the square of this factor.

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arXiv2007

Resolved Galaxies in the Hubble Ultra Deep Field: Star Formation in Disks at High Redshift

The photometric redshift distributions, spectral types, Sersic indices, and sizes of all resolved galaxies in the Hubble Space Telescope Ultra Deep Field (UDF) are studied in order to understand the environment and nature of star formation in the early Universe. Clumpy disk galaxies that are bright at short wavelengths (restframe <5000 Angstroms) dominate the UDF out to z~5.5. Their uniformity in V/V_max and co-moving volume density suggest they go even further, spanning a total time more than an order of magnitude larger than their instantaneous star formation times. They precede as well as accompany the formation epoch of distant red galaxies and extreme red objects. Those preceding could be the pre-merger objects that combined to make red spheroidal types at z~2 to 3. Clumpy disks that do not undergo mergers are likely to evolve into spirals. The morphology of clumpy disks, the size and separation of the clumps, and the prevalence of this type of structure in the early Universe suggests that most star formation occurs by self-gravitational collapse of disk gas.

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arXiv2006

Formation and Evolution of Young Massive Clusters

Clusters 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 and a only smooth cluster or smooth unbound group remains. The stellar hierarchy follows a similar structure in the interstellar gas, which is presumably scale-free because of supersonic motions in the presence of turbulence and self-gravity. The efficiency of star formation increases automatically with density in a hierarchical ISM, causing most dense stellar groups to be initially bound for local conditions. In lower pressure environments, the infant mortality rates should be higher. Also following from hierarchical structure is the cluster mass distribution function and perhaps also the cluster size distribution function, although the predicted mass-size relation is not observed. Cluster destruction is from a variety of causes. The destruction time should depend on cluster mass, but the various groups who have studied this dependence have gotten significantly different results so far.

astro-ph↗

arXiv2006

The Initial Mass Function in Clusters

The 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 additional formation processes for massive stars that are not present in low density regions, making the slope of the upper mass IMF somewhat shallower in clusters. Observations of shallow IMFs in some super star clusters and in elliptical galaxies are reviewed. We also review mass segregation and the likelihood that peculiar IMFs, as in the Arches cluster, result from segregation and stripping, rather than an intrinsically different IMF. The theory of the IMF is reviewed in some detail. Several problems introduced by the lack of a magnetic field in SPH simulations are discussed. The universality of the IMF in simulations suggests that something more fundamental than the physical details of a particular model is at work. Hierarchical fragmentation by any of a variety of processes may be the dominant cause of the power law slope. Physical differences from region to region may make a slight difference in the slope and also appear in the low-mass turnover point.

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arXiv2006

Rings and Bent Chain Galaxies in the GEMS and GOOD Fields

Twenty-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 go up to several x10^8 Msun, based on color evolution models. The clump ages are consistent with the expected lifetimes of ring structures if they are formed by collisions. Fifteen other galaxies that resemble the arcs in partial ring galaxies but have no evident disk emission were also studied. Their clumps have bluer colors at all redshifts compared to the clumps in the ring and partial ring sample, and their clump ages are younger than in rings and partial rings by a factor of ~10. In most respects, they resemble chain galaxies except for their curvature; we refer to them as ``bent chains.'' Several rings are symmetric with centered nuclei and no obvious companions. They could be outer Lindblad resonance rings, although some have no obvious bars or spirals to drive them. If these symmetric cases are resonance rings, then they could be the precursors of modern resonance rings, which are only ~30% larger on average. This similarity in radius suggests that the driving pattern speed has not slowed by more by ~30% during the last ~7 Gy. Those without bars could be examples of dissolved bars.

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