arXiv subjects

B. G. Elmegreen

Publications and source records attributed to B. G. Elmegreen.

72 records · Page 4Linked to original sources

arXiv2001

The gravitational torque of bars in optically unbarred and barred galaxies

The relative bar torques for 45 galaxies observed at $K$- band with the 4.2m William Herschel Telescope are determined by transforming the light distributions into potentials and deriving the maximum ratios of the tangential forces relative to the radial forces. The results are combined with the bar torques for 30 other galaxies determined from our previous $K$-band survey (Buta & Block 2001). Relative bar torques determine the degree of spiral arm forcing, gas accretion, and bar evolution. They differ from other measures of bar strength, such as the relative amplitude of the bar determined photometrically, because they include the bulge and other disk light that contributes to the radial component of the total force. If the bulge is strong and the radial forcing large, then even a prominent bar can have a relatively weak influence on the azimuthal motions in the disk. Here we find that the relative bar torque correlates only weakly with the optical bar type listed in the Revised Shapley-Ames and de Vaucouleurs systems. In fact, some classically barred galaxies have weaker relative bar torques than classically unbarred galaxies. The optical class is a poor measure of azimuthal disk forcing for two reasons: some infrared bars are not seen optically, and some bars with strong bulges have their azimuthal forces so strongly diluted by the average radial force that they exert only small torques on their disks. The Hubble classification scheme poorly recognizes the gravitational influence of bars. Applications of our bar torque method to the high-redshift universe are briefly discussed.

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arXiv2001

Structure and Mass of a Young Globular Cluster in NGC 6946

Using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we have imaged a luminous young star cluster in the nearby spiral galaxy NGC 6946. The cluster has an absolute visual magnitude M(V)=-13.2, comparable to the brightest young `super-star clusters' in the Antennae merger galaxy. UBV colors indicate an age of about 15 Myr. The cluster has a compact core (core radius = 1.3 pc), surrounded by an extended envelope. We estimate that the effective radius (Reff) = 13 pc, but this number is uncertain because the outer parts of the cluster profile gradually merge with the general field. Combined with population synthesis models, the luminosity and age of the cluster imply a mass of 8.2x10^5 Msun for a Salpeter IMF extending down to 0.1 Msun, or 5.5x10^5 Msun if the IMF is log-normal below 0.4 Msun. Depending on model assumptions, the central density of the cluster is between 5300 Msun pc^-3 and 17000 Msun pc^-3, comparable to other high-density star forming regions. We also estimate a dynamical mass for the cluster, using high-dispersion spectra from the HIRES spectrograph on the Keck I telescope. The velocity dispersion is 10.0 +/- 2.7 km/s, implying a total cluster mass within 65 pc of (1.7 +/- 0.9) x 10^6 Msun. Comparing the dynamical mass with the mass estimates based on the photometry and population synthesis models, the mass-to-light ratio is at least as high as for a Salpeter IMF extending down to 0.1 Msun, although a turn-over in the IMF at 0.4 Msun is still possible within the errors. The cluster will presumably remain bound, evolving into a globular cluster-like object.

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arXiv2001

Neutral Hydrogen and Star Formation in the Irregular Galaxy NGC 2366

We present UBVJHKHalpha and HI data of the irregular galaxy NGC 2366. It is a normal boxy-shaped disk seen at high inclination angle. We do not see any unambiguous observational signature of a bar. There is an asymmetrical extension of stars along one end of the major axis of the galaxy, and this is where the furthest star-forming regions are found, at 1.3R_Holmberg. The HI is normal in many respects but shows some anomalies: 1) The integrated HI shows two ridges running parallel to the major axis that deproject to a large ring. 2) The velocity field exhibits several large-scale anomalies superposed on a rotating disk. 3) The inclination and position angles derived from the kinematics differ from those dervied from the optical and HI mor- phology. 4) There are regions in the HI of unusually high velocity dispersion that correlate with deficits of HI emission in a manner suggestive of long-range, turbulent pressure equilibrium. Star-forming regions are found where the gas densities locally exceed 6 Msolar/pc^2. NGC 2366, like other irregulars, has low gas densities relative to the critical gas densities of gravitational instability models. Because of the lack of shear in the optical galaxy, there is little competition to the slow gravitational contraction that follows energy dissipation. However, the peak gas densities in the star-forming regions are equal to the local tidal densities for gravitational self-binding of a rotating cloud. Evidently the large scale gas concentrations are marginally bound against background galactic tidal forces. This condition for self-binding may be more fundamental than the instability condition because it is local, three-dimensional, and does not involve spiral arm generation as an intermediate step toward star formation.

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arXiv2000

HST Observations of the Interacting Galaxies NGC 2207 and IC 2163

Hubble Space Telescope images of the galaxies NGC 2207 and IC 2163 show star formation and dust structures in a system that has experienced a recent grazing encounter. Tidal forces from NGC 2207 compressed and elongated the disk of IC 2163, forming an oval ridge of star formation. Gas flowing away from this ridge has thin parallel dust filaments transverse to the direction of motion. Numerical models suggest that the filaments come from flocculent spiral arms that were present before the interaction. A dust lane at the outer edge of the tidal tail is a shock front where the flow abruptly changes direction. A spiral arm of NGC 2207 that is backlit by IC 2163 is seen to contain several parallel, knotty filaments that are probably shock fronts in a density wave. Blue clusters of star formation inside these dust lanes show density wave triggering by local gravitational collapse. Spiral arms inside the oval of IC 2163 could be the result of ILR-related orbits in the tidal potential that formed the oval. Their presence suggests that tidal forces alone may initiate a temporary nuclear gas flow and eventual starburst without first forming a stellar bar. Several emission structures resembling jets 100-1000 pc long appear. There is a dense dark cloud with a conical shape 400 pc long and a bright compact cluster at the tip, and with a conical emission nebula of the same length that points away from the cluster in the other direction. This region coincides with a non-thermal radio continuum source that is 1000 times the luminosity of Cas A at 20 cm.

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arXiv2000

Dust Streamers in the Virgo Galaxy M86 from Ram Pressure Stripping of its Companion VCC 882

The giant elliptical galaxy M86 in Virgo has a ~28 kpc long dust trail inside its optical halo that points toward the nucleated dwarf elliptical galaxy, VCC 882. The trail seems to be stripped material from the dwarf. Extinction measurements suggest that the ratio of the total gas mass in the trail to the blue luminosity of the dwarf is about unity, which is comparable to such ratios in dwarf irregular galaxies. The ram pressure experienced by the dwarf galaxy in the hot gaseous halo of M86 was comparable to the internal gravitational binding energy density of the presumed former gas disk in VCC 882. Published numerical models of this case are consistent with the overall trail-like morphology observed here. Three concentrations in the trail may be evidence for the predicted periodicity of the mass loss. The evaporation time of the trail is comparable to the trail age obtained from the relative speed of the galaxies and the trail length. Thus the trail could be continuously formed from stripped replenished gas if the VCC 882 orbit is bound. However, the high gas mass and the low expected replenishment rate suggest that this is only the first stripping event. Implications for the origin of nucleated dwarf ellipticals are briefly discussed.

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arXiv2000

Observational Constraints on the Theory of the IMF

Observational constraints on the theory of the IMF are reviewed. These observations include the new result that star formation is very rapid, usually going from start to finish in only 1 or 2 dynamical times on a wide range of scales. This result, combined with the observation that the IMF is independent of cluster density over a factor of several hundred, implies that protostar coagulation during orbital motions in a cloud are not important for the IMF: there is not enough time. The observation that the IMF in individual clusters is about the same as the IMF in whole galaxies implies that stars of all masses form randomly in self-gravitating clouds of all masses. There cannot be a sequence of stellar masses in a cluster, based on stirring or heating for example, where the largest star that forms keeps increasing until the cloud is destroyed. The uniformity of the IMF over time and space argues for a process that is independent of specific properties of molecular clouds. This uniformity, along with the common observation of young stars in hierarchical clusters that resemble the structures of interstellar gas, suggests that the IMF, at least in the power law range, is a by-product of turbulence. Detailed physical processes may affect the turnover at low mass, but even this may have a universal character arising from a combination of turbulence-induced variations in the local cloud properties, and fragmentation in unstable cloud pieces.

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arXiv2000

A Pressure Anomaly for HII Regions in Irregular Galaxies

The pressures of giant HII regions in 6 dwarf Irregular galaxies are a factor of ~10 larger than the average pressures of the corresponding galaxy disks, obtained from the stellar and gaseous column densities. Either the visible HII regions in these dwarfs are all so young that they are still expanding, or there is an unexpected source of disk self-gravity that increases the background pressure. We consider the possibility that the additional self-gravity comes from disk dark matter, but suggest this is unlikely because the vertical scale heights inferred for Irregular galaxies are consistent with the luminous matter alone. Some of the HII region overpressure is probably the result of local peaks in the gravitational field that come from large gas concentrations, many of which are observed directly. These peaks also explain the anomalously low average column density thresholds for star formation that were found earlier for Irregular galaxies, and they permit the existence of a cool HI phase as the first step toward dense molecular cores. Many of the HII regions could also be so strongly over-pressured that they will expand for a long time. In this case, the observed population would be only 7% of the total, and the aging HII regions, now too faint to see, should occupy nearly the entire dwarf galaxy volume. Such prolonged HII region expansion would explain the origin of the giant HI shells that are seen in these galaxies, and account for the lack of bright central clusters inside these shells.

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arXiv2000

A Young Globular Cluster in the Galaxy NGC 6946

A globular cluster ~15 My old that contains 5x10^5 Msun of stars inside an 11 pc radius has been found in the nearby spiral galaxy NGC 6946, surrounded by clouds of dust and smaller young clusters inside a giant circular bubble 300 pc in radius. At the edge of the bubble is an arc of regularly-spaced clusters that could have been triggered during the bubble's formation. The region is at the end of a spiral arm, suggesting an origin by the asymmetric collapse of spiral arm gas. The globular is one of the nearest examples of a cluster that is similar to the massive old globulars in the Milky Way. We consider the energetics of the bubble and possible formation mechanisms for the globular cluster, including the coalescence of smaller clusters.

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arXiv1999

Curvature and Acoustic Instabilities in Rotating Fluid Disks

The stability of a rotating fluid disk to the formation of spiral arms is studied in the tightwinding approximation in the linear regime. The dispersion relation for spirals that was derived by Bertin et al. is shown to contain a new, acoustic instability beyond the Lindblad resonances that depends only on pressure and rotation. In this regime, pressure and gravity exchange roles as drivers and inhibitors of spiral wave structures. Other instabilities that are enhanced by pressure are also found in the general dispersion relation by including higher order terms in the small parameter 1/kr for wavenumber k and radius r. These instabilities are present even for large values of Toomre's parameter Q. Unstable growth rates are determined in four cases: a self-gravitating disk with a flat rotation curve, a self-gravitating disk with solid body rotation, a non-self-gravitating disk with solid body rotation, and a non-self-gravitating disk with Keplerian rotation. The most important application appears to be as a source of spiral structure, possibly leading to accretion in non-self-gravitating disks, such as some galactic nuclear disks, disks around black holes, and proto-planetary disks. All of these examples have short orbital times so the unstable growth time can be small.

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arXiv1999

Observations and Theory of Star Cluster Formation

Young stars form on a wide range of scales, producing aggregates and clusters with various degrees of gravitational self-binding. The loose aggregates have a hierarchical structure in both space and time that resembles interstellar turbulence, suggesting that these stars form in only a few turbulent crossing times with positions that map out the previous gas distribution. Dense clusters, on the other hand, are often well mixed, as if self-gravitational motion has erased the initial fine structure. Nevertheless, some of the youngest dense clusters also show sub-clumping, so it may be that all stellar clustering is related to turbulence. Some of the densest clusters may also be triggered. The evidence for mass segregation of the stars inside clusters is reviewed, along with various explanations for this effect. Other aspects of the theory of cluster formation are reviewed as well, including many specific proposals for cluster formation mechanisms. The conditions for the formation of bound clusters are discussed. Critical star formation efficiencies can be as low as 10% if the gas removal process is slow and the stars are born at sub-virial speeds. Environmental conditions, particularly pressure, may affect the fraction and masses of clusters that end up bound. Globular clusters may form like normal open clusters but in conditions that prevailed during the formation of the halo and bulge, or in interacting and starburst galaxies today. Various theories for the formation of globular clusters are summarized.

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arXiv1999

Galactic bulge formation as a maximum intensity starburst

Properties of normal galactic star formation, including the density dependence, threshold density, turbulent scaling relations, and clustering properties, are applied to the formation of galactic bulges. One important difference is that the bulge potential well is too deep to have allowed self-regulation or blow-out by the pressures from young stars, unlike galactic disks or dwarf galaxies. As a result, bulge formation should have been at the maximum rate, which is such that most of the gas would get converted into stars in only a few dynamical time scales, or ~10^8 years. The gas accretion phase can be longer than this, but once the critical density is reached, which depends primarily on the total virial density from dark matter, the formation of stars in the bulge should have been extremely rapid. Such three-dimensional star formation should also have formed many clusters, like normal disk star formation today. Some of these clusters may have survived as old globulars, but most got dispersed, although they might still be observable as concentrated streams in phase space.

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arXiv1998

Observation and Theory of The Initial Stellar Mass Function

Observations of normal galactic star-forming regions suggest there is widespread near-uniformity in the initial stellar mass function (IMF) in spite of diverse physical conditions. Fluctuations may come largely from statistical effects and observational selection. There are also tantalizing, but uncertain reports that the IMF shifts systematically in peculiar regions, giving a low mass bias in quiescent gas, and a high mass bias in active starbursts. Theoretical proposals for the origin of the IMF are reviewed. The theories generally focus on a combination of four physical effects: wind-limited accretion of stellar mass, coalescence of protostellar gas clumps, mass limitations at the thermal Jeans mass, and power-law cloud structure. Hybrid theories combining the best of each may be preferred.

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arXiv1998

The initial stellar mass function from random sampling in hierarchical clouds II: statistical fluctuations and a mass dependence for starbirth positions and times

Observed variations in the slope of the initial stellar mass function are shown to be consistent with a model in which the protostellar gas is randomly sampled from hierarchical clouds at a rate proportional to the square root of the local density. RMS variations in the IMF slope around the Salpeter value are +/- 0.4 when only 100 stars are observed, and +/- 0.1 when 1000 stars are observed. The hierarchical-sampling model also reproduces the tendency for massive stars to form closer to the center of a cloud, at a time somewhat later than the formation time of the lower mass stars. The assumed density dependence for the star formation rate is shown to be appropriate for turbulence compression, magnetic diffusion, gravitational collapse, and clump or wavepacket coalescence. The low mass flattening in the IMF comes from the inability of gas to form stars below the thermal Jeans mass at typical temperatures and pressures. Consideration of heating and cooling processes indicate why the thermal Jeans mass should be nearly constant in normal environments, and why it might increase in some starburst regions. The steep IMF in the extreme field is not explained by the model, but other origins are suggested.

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arXiv1998

Spontaneous and Induced Star Formation in the LMC

Spontaneous and triggered star formation in the LMC is discussed with data on star clusters ages and positions. The supershell LMC4 and the stellar arcs in the same region are suggested to be triggered by GRBs, the progenitors of which might have escaped the old elliptical cluster NGC1978, close to which are a number of X-ray binaries and the SGR/SNR N49.

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arXiv1998

Dust Spirals and Acoustic Noise in the Nucleus of the Galaxy NGC 2207

Observations with the Hubble Space Telescope reveal an irregular network of dust spiral arms in the nuclear region of the interacting disk galaxy NGC 2207. The spirals extend from ~50 pc to ~300 pc in galactocentric radius, with a projected width of ~20 pc. Radiative transfer calculations determine the gas properties of the spirals and the inner disk, and imply a factor of ~4 local gas compression in the spirals. The gas is not strongly self-gravitating, nor is there a nuclear bar, so the spirals could not have formed by the usual mechanisms applied to main galaxy disks. Instead, they may result from acoustic instabilities that amplify at small galactic radii. Such instabilities may promote gas accretion into the nucleus.

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arXiv1998

Hierarchical Star Formation from the Time-Space Distribution of Star Clusters in the Large Magellanic Cloud

The average age difference between pairs of star clusters in the Large Magellanic Clouds increases with their separation as the ~0.35 power. This suggests that star formation is hierarchical in space and time. Small regions form stars quickly and large regions, which often contain the small regions, form stars over a longer period. A similar result found previously for Cepheid variables is statistically less certain than the cluster result.

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arXiv1998

Giant Shells and Stellar Arcs as Relics of Gamma Ray Burst Explosions

Gamma Ray Burst (GRB) explosions are powerful and frequent enough to make kiloparsec-size shells and holes in the interstellar media of spiral galaxies. The observations of such remnants are summarized. Several observed shells contain no obvious central star clusters and could be GRB remnants, but sufficiently old clusters that could have formed them by supernovae and winds might be hard to detect.

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arXiv1997

Observations and Theory of Dynamical Triggers for Star Formation

Star formation triggering mechanisms are reviewed, including the direct compression of clouds and globules, the compression and collapse of molecular clouds at the edges of HII regions and supernovae, the expansion and collapse of giant rings and shells in galaxy disks, and the collision and collapse between two clouds. Collapse criteria are given. A comprehensive tabulation of regions where these four types of triggering have been found suggests that dynamical processes sustain and amplify a high fraction of all star formation that begins spontaneously in normal galaxy disks.

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