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Bruce G. Elmegreen

Publications and source records attributed to Bruce G. Elmegreen.

At least 235 records · Page 13Linked to original sources
arXiv2002

Arm Structure in Anemic Spiral Galaxies

Anemic galaxies have less prominent star formation than normal galaxies of the same Hubble type. Previous studies showed they are deficient in total atomic hydrogen but not in molecular hydrogen. Here we compare the combined surface densities of HI and H2 at mid-disk radii with the Kennicutt threshold for star formation. The anemic galaxies are below threshold, which explains their lack of prominent star formation, but they are not much different than other early type galaxies, which also tend to be below threshold. The spiral wave amplitudes of anemic and normal galaxies were also compared, using images in B and J passbands from the OSU Bright Spiral Galaxy Survey. Anemic galaxies have normal spiral wave properties too, with the same amplitudes and radial dependencies as other galaxies of the same arm class. Because of the lack of gas, spiral waves in early type galaxies and anemics do not have a continuous supply of stars with low velocity dispersions to maintain a marginally stable disk. As a result, they are either short-lived, evolving toward lenticulars and S0 types in only a few rotations at mid-disk, or they are driven by the asymmetries associated with gas removal in the cluster environment.

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arXiv2002

Environmental Dependencies for Star Formation Triggered by Expanding Shell Collapse

Criteria 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, and the resultant probabilities for triggering and unstable masses were determined as functions of 8 dimensionless parameters. When the ratio of the midplane gas density to the midplane total density is small, an expanding shell reaches the disk scale height and vents to the halo before it collapses. When the Toomre instability parameter Q, or a similar shear parameter, Q_A, are large, Coriolis forces and shear stall or reverse the collapse before the shell accumulates enough mass to be unstable. With large values of C=c_sh/(GL)^0.2, for rms velocity dispersion c_sh in the swept-up matter and shell-driving luminosity L, the pressure in the accumulated gas is too large to allow collapse during the expansion time. Considering ~5000 models covering a wide range of parameter space, the common properties of shell collapse as a mechanism for triggered star formation are: (1) the time scale is 4*sqrt(C/2 pi G rho) for ambient midplane density rho, (2) the total fragment mass is ~2x10^7 Msun, of which only a small fraction is likely to be molecular, (3) the triggering radius is ~2 times the scale height, and the triggering probability is ~0.5 for large OB associations. Star formation triggered by shell collapse should be most common in gas-rich galaxies, such as young galaxies or those with late Hubble types.

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arXiv2001

Compact Star Clusters in Nearby Dwarf Irregular Galaxies

Nearby dwarf irregular galaxies were searched for compact star clusters using data from the HST archives. Three of the galaxies were found to host both populous clusters (M_V<-9.5 at a fiducial age of 10 Myr) and super-star clusters (M_V<-10.5 at 10 Myr). Four other dwarf galaxies, two of which contain populous and super-star clusters, were drawn from the literature. We find upper limits to integrated galactic M_B and star formation rate for the formation of these clusters. However, galaxies brighter and more active than these limits rarely form them. Yet when they do form, the associated star formation activity is very high, with numerous compact clusters of similar age in the same complex and often evidence for a galaxy-wide perturbation as the trigger. This tendency to concentrate star formation in localized regions of high column density is consistent with previous suggestions that self-gravity must be strong and the pressure must be high to allow a cool phase of gas to exist in equilibrium. Statistical considerations emphasize the peculiarity of super-star clusters in dwarf galaxies, which are too small to sample the cluster mass function to that extreme. We suggest that triggered large-scale flows and ambient gravitational instabilities in the absence of shear make the clouds that form super-star clusters in small galaxies. This is unlike the case in spirals where density wave flows and scale-free compression from turbulence dominate. Further compar- isons with spirals give insight into Larsen & Richtler's relation between star formation rate per unit area and the fraction of young stars in massive dense clusters. We suggest that this relation is the result of a physical connection between maximum cluster mass, interstellar pressure, column density, and star formation rate, combined with a size-of-sample effect.

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arXiv2001

CO Observations of the Interacting Galaxy Pair NGC 5394/95

BIMA 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 an encounter simulation that reproduces some of the main features of this galaxy pair, a considerable amount of gas in NGC 5394 falls into the central region early in the collision. The observed total gas distribution in the disk of NGC 5394 is lopsided, with more HI, CO, and H-alpha emission coming from the western or southwestern side. The innermost western arm of NGC 5394 is seen in CO and H-alpha emission, but the eastern inner-disk arm, which is very bright in the optical continuum, is not detected in CO or H-alpha emission. From a comparison of the radio continuum, H-alpha, 60 micron, and CO luminosities, we estimate that the average visual extinction of the starburst is 3 - 4 mag and the conversion factor N(H2)/I(CO) in the starburst is a factor of 3 - 4 below the standard value. Comparison of NGC 5394 with two other systems previously studied suggests that in prograde grazing encounters a central starburst may not develop until near the end of the ocular phase. Very little of the CO emission from NGC 5395 found in previous single-dish observations is detected by BIMA.

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arXiv2001

History of Star Formation in the Universe

The 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 globular clusters appear to have formed in about the same way as today's clusters, with the same IMF and stellar densities, and with a larger mass reflecting the higher pressures of their environments. The bulge of the Milky Way appears to have evolved according to the dissipational collapse model, but other bulges may differ.

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arXiv2001

Theory of star formation in the central kiloparsec

Star 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 very little is known about inner galaxy triggering mechanisms, there appear to be examples that parallel the well-known mechanisms around us. The balance may shift slightly, however, because of the high densities in nuclear regions. The sequential triggering mode, in which one generation of stars triggers another, should be less prominent compared to gravitational instabilities and turbulence triggering when the ambient density is so high that the local dynamical time scale is less than the lifetime of an O-type star. Regardless of this shift in balance, the star formation rate seems to follow the same column density scaling law as in main disks, probably because it is everywhere saturated to the maximum possible value allowed by the fractal structure of the interstellar medium.

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arXiv2000

Fractal Structure in Galactic Star Fields

The 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, and a count of these aggregates versus smoothing scale gives the fractal dimension. Fractal and Poisson models confirm the procedure. The fractal dimension of star formation in all of the galaxies is ~2.3. This is the same as the fractal dimension of interstellar gas in the Milky Way and nearby galaxies, suggesting that star formation is a passive tracer of gas structure defined by self-gravity and turbulence. Dense clusters like the Pleiades form at the bottom of the hierarchy of structures, where the protostellar gas is densest. If most stars form in such clusters, then the fractal arises from the spatial distribution of their positions, giving dispersed star fields from continuous cluster disruption. Dense clusters should have an upper mass limit that increases with pressure, from ~1000 Msun in regions like the Solar neighborhood to one million Msun in starbursts.

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arXiv2000

The Origin of The Stellar Initial Mass Function

Observations 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 times of young clusters suggest a further connection to turbulence. There is also a similarity between the characteristic mass at the lower end of the power law and the minimum self-gravitating mass in a typical star-forming cloud. All of this suggests that the power law part of the IMF comes from star formation in self-gravitating cloud pieces that are formed by compressible turbulence. Timing constraints involving cloud destruction and competition for gas might limit the upper stellar mass to several hundred suns. This upper limit has to be less than the mass of a clump that has a dynamical time equal to several times the dynamical time of the characteristic mass. The smallest stars and brown dwarfs may not come directly from cloud clumps, which are not self-gravitating at this mass in the cloudy phase, but from collapsed fragments or other pieces connected with the formation of more massive stars.

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arXiv2000

A Fractal Analysis of the HI Emission from the Large Magellanic Cloud

A 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 in 1D, 2D, and in the azimuthal direction were found to be approximate power laws over 2 decades in length. Delta-variance methods also showed the same power-law structure. Detailed models of these data were made using line-of-sight integrals over fractals that are analogous to those generated by simulations of turbulence with and without phase transitions. The results suggested a way to measure directly for the first time the line-of-sight thickness of the cool component of the HI disk of a nearly face-on galaxy. The signature of this thickness was found to be present in all of the measured power spectra. The character of the HI structure in the LMC was also viewed by comparing positive and negative images of the integrated emission. The geometric structure of the high-emission regions was found to be filamentary, whereas the geometric structure of the low-emission (intercloud) regions was found to be patchy and round. This result suggests that compressive events formed the high-emission regions, and expansion events, whether from explosions or turbulence, formed the low-emission regions. The character of the structure was also investigated as a function of scale using unsharp masks. All of these results suggest that most of the ISM in the LMC is fractal, presumably the result of pervasive turbulence, self-gravity, and self-similar stirring.

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arXiv2000

Modeling a high mass turn down in the stellar initial mass function

Statistical 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 is not necessary for dense clusters because the number of stars sampled is always too small. Here we explore several mechanisms for an upper mass cutoff, including an exponential decline of the star formation probability after a turbulent crossing time. The results are in good agreement with the observed IMF over the entire stellar mass range, and they give a gradual turn down compared to the Salpeter function above ~100 Msun for normal thermal Jeans mass, M_J. The upper mass turn down should scale with M_J in different environments. A problem with the models is that they cannot give both the observed power-law IMF out to the high-mass sampling limit in dense clusters, as well as the observed lack of supermassive stars in whole galaxy disks. Either there is a sharper upper-mass cutoff in the IMF, perhaps from self-limitation, or the IMF is different for dense clusters than for the majority of star formation that occurs at lower density. Dense clusters seem to have an overabundance of massive stars relative to the average IMF in a galaxy.

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arXiv2000

The detection of spiral arm modulation in the stellar disk of an optically flocculent and an optically grand design galaxy

Two dimensional Fourier spectra of near-infrared images of galaxies provide a powerful diagnostic tool for the detection of spiral arm modulation in stellar disks. Spiral arm modulation may be understood in terms of interference patterns of outgoing and incoming density wave packets or modes. The brightness along a spiral arm will be increased where two wave crests meet and constructively interfere, but will be decreased where a wave crest and a wave trough destructively interfere. Spiral arm modulation has hitherto only been detected in grand design spirals (such as Messier 81). Spiral arm amplitude variations have the potential to become a powerful constraint for the study of galactic dynamics. We illustrate our method in two galaxies: NGC 4062 and NGC 5248. In both cases, we have detected trailing and leading m=2 waves with similar pitch angles. This suggests that the amplification mechanism is the WASER type II. In this mechanism, the bulge region reflects (rather than refracts) incoming waves with no change of pitch angle, but only a change of their sense of winding. The ratio between the amplitudes of the leading and the trailing waves is about 0.5 in both cases, wherein the higher amplitude is consistently assigned to the trailing (as opposed to leading) mode. The results are particularly significant because NGC 5248 is an optically grand design galaxy, whereas NGC 4062 is optically flocculent. NGC 4062 represents the very first detection of spiral arm modulation in the stellar disk of an optically flocculent galaxy.

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arXiv2000

Structure and Star Formation in NGC 925

We present the results from an optical study of the stellar & star formation properties of NGC 925 using the WIYN 3.5m telescope. Images in B,V,R, & H-alpha reveal a galaxy that is fraught with asymmetries. From isophote fits we discover that the bar center is not coincident with the center of the outer isophotes nor with the dynamical center (from Pisano et al. 1998). Cuts across the spiral arms reveal that the northern arms are distinctly different from the southern arm. The southern arm not only appears more coherent, but the peaks in stellar and H-alpha emission are found to be coincident with those of the HI distribution, while no such consistency is present in the northern disk. We also examine the gas surface density criterion for massive star formation in NGC 925, and find that its behavior is more consistent with that for irregular galaxies, than with late-type spirals. In particular, star formation persists beyond the radius at which the gas surface density falls below the predicted critical value for star formation for late-type spirals. Such properties are characteristic of Magellanic spirals, but are present at a less dramatic level in NGC 925, a late-type spiral.

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arXiv1999

The Specific Frequency of Globular Clusters in Galaxies

Variations 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 properties of the associated galactic clusters are discussed, as are the color distributions of globular cluster populations. Several models for the origin of these variations are summarized, including the tidal stripping and interaction/starburst models. Most of the variations in the specific frequency for elliptical galaxies result from variations in the ratio of total mass to light with a nearly constant efficiency for conversion of gas into globular cluster mass. Smaller additional variations may come from tidal stripping, particularly in cD galaxies.

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arXiv1999

Formation and loss of hierarchical structure in two-dimensional MHD simulations of wave-driven turbulence in interstellar clouds

Two 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 incoming non-linear Alfven waves. These structures and motions are similar to what are commonly observed in weakly self-gravitating interstellar clouds, suggesting that these clouds get their fractal structures from non-linear magnetic waves generated in the intercloud medium; no internal source of turbulent energy is necessary. The clumps in the simulated clouds are slightly warmer than the interclump medium as a result of magnetic dissipational and compressive heating. Thus the interclump medium has a lower pressure than the clumps, demonstrating that the clumps owe their existence to transient compressive motions, not pressure confinement. Clump lifetimes increase with size, and are about one sound crossing time. Two specific problems are addressed. We find that low ionization fractions in molecular clouds do not necessarily lead to increased cloud smoothing, but that enhanced density in a self-gravitating cloud does, because of external wave self-shielding. Bulk cloud self-gravity may therefore promote star formation in the relatively quiescent gas pools that contain a thermal Jeans mass or more.

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arXiv1999

Star Formation in a Crossing Time

Observations 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, (2) measurements of the age difference versus separation for clusters in the Large Magellanic Clouds in comparison with the crossing time versus size correlation for molecular clouds, (3) hierarchical structure of embedded young clusters, and (4) the high fraction of dense clouds that contain star formation. Such a short overall time scale for star formation implies that sources of turbulent energy or internal feedback are not required to explain or extend cloud lifetimes, and that star and protostar interactions cannot be important for the stellar initial mass function. Stars appear in a cloud as if they freeze out of the gas, preserving the turbulent-driven gas structure in their birth locations. The galaxy-wide star formation rate avoids the Zuckerman-Evans catastrophe, which has long been a concern for molecular clouds that evolve this quickly, because the multifractal structure of interstellar gas ensures that only a small fraction of the mass is able to form stars. Star formation on large scales operates slower than on small scales, but in most cases the whole process is over in only a few dynamical times.

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arXiv1999

The IMF and its Evolution

Observations 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 in the mass of the flattened part are important for understanding star formation; e.g., starburst galaxies may be an example where the characteristic mass is larger than it is locally. A steeper high-mass slope in the extreme field studied by Massey et al. may be the result of cloud destruction and the termination of star formation by ionization, with a normal IMF in each separate cluster. The lack of a density dependence in cluster IMFs suggests that star and protostar interactions play little role in star formation or the IMF. These considerations, along with indirect observations of the IMF in the early Universe, suggest that the IMF does not vary much in its basic form over position and time, but that shifts in the characteristic mass might occur in regions with extremely high or low star formation activity.

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arXiv1999

A Prediction of Brown Dwarfs in Ultracold Molecular Gas

A 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 found in nearby galaxies by Allen and collaborators, should make an abundance of Brown Dwarfs with relatively few normal stars. This result comes from the low value of the thermal Jeans mass, considering that the hierarchical cloud model always gives the Salpeter IMF slope above this lower mass limit. The ultracold CO clouds in the inner disk of M31 have T~3K and pressures that are probably 10 times higher than in the solar neighborhood. This gives a mass at the peak of the IMF equal to 0.01 Msun, well below the Brown Dwarf limit of 0.08 Msun. Using a functional approximation to the IMF, the ultracold clouds would have 50% of the star-like mass and 90% of the objects below the Brown Dwarf limit. The brightest of the Brown Dwarfs in M31 should have an apparent, extinction-corrected K-band magnitude of ~21 mag in their pre-main sequence phase.

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