arXiv subjects

Bruce G. Elmegreen

Publications and source records attributed to Bruce G. Elmegreen.

239 records · Page 14Linked to original sources

arXiv1998

Bogus dust screens from well mixed exponential disks in galaxies

The V-K colours along the minor axes of spiral galaxies typically change from red to blue with increasing distance, giving the impression that the near side is systematically screened by dust. Such a preferred orientation for dust screens is unlikely. Here we show that common extinction from the embedded dust layer in an exponential disk has the same effect, making the near side systematically redder as the inclination increases. The galaxy NGC 2841 is modelled as an example, where the V-K profile is profoundly asymmetric and actually step-like across the centre. We predict that the minor axis emission profile of the same dust in the far-infrared, at wavelength 200 microns, will be much more symmetric than the optical profiles, implying nearly equal column densities of dust on both sides of the minor axis.

astro-ph↗

arXiv1998

Hierarchy of Interstellar and Stellar Structures and the Case of the Orion Star-Forming Region

Hierarchical structure from stellar clusters, to subgroups, to associations and star complexes is discussed in the context of the Orion stellar grouping and its origin. The analogous structure in gas clouds is also reviewed, with an emphasis on general fractal properties and mass distribution functions. The distinction between the cloud, cluster, and stellar mass functions is discussed in terms of different sampling statistics for hierarchically structured clumps in clouds with an interclump medium.

astro-ph↗

arXiv1997

Small-Scale Mixing of the Elements

The progressive mixing and contamination of interstellar gas by supernovae and other processes following the passage of a spiral density wave is reviewed, with an emphasis on the Solar neighborhood. Regions of star formation should begin their lives with an inhomogeneous mixture of abundances as a result of their chaotic and large-scale formation processes. These inhomogeneities should continue to increase during several generations of star formation until the gas enters the interarm region. Then cloud dispersal by large interarm tidal forces, high rates of shear, and internal star formation should all lead to cloud-to-cloud mixing, while increased ionization, heating, and evaporation should lead to mixing on the atomic level. Gradients in the dispersion of elemental abundances are expected for galaxy disks.

astro-ph↗

arXiv1997

The Relationship Between Gas, Stars, and Star Formation in Irregular Galaxies: A Test of Simple Models

We explore various instability models for the onset of star formation in irregular galaxies. Critical gas densities are calculated for gravitational instabilities with a thin, pure-gas disk, and with a thick disk composed of gas and a star-like fluid. We also calculated the stability properties of three dimensional systems including dark matter, considered the thermal state of the gas, and used a modified threshold column density written in terms of the local rate of shear instead of the epicyclic frequency. The model predictions were compared to the azimuthally-averaged present day star formation activity traced by the Halpha surface brightness, and to the 1 Gyr-integrated star formation activity represented by the stellar surface brightness. The ratio of the observed gas density to the critical gas density is lower by a factor of 2 in most of the Im galaxies than it is in spiral galaxies. Star formation ends before this ratio drops significantly in the outer regions, and it remains high in the inner regions where the ratio is often low. These results suggest that the critical gas density does not trace star formation with the same detail in irregular galaxies as it appears to trace it in giant spiral galaxies. The only azimuthally-averaged quantity that correlates with the current star formation activity in irregulars is the stellar surface density. Theoretical implications are discussed.

astro-ph↗

arXiv1997

HI Observations of the Spiral Arm Pattern Speed in the Late-Type Barred Galaxy NGC 925

HI observations with the VLA of the late-type barred galaxy NGC 925 show clear streaming motions along four spiral arms close to the minor axis. A transition from inward streaming to outward streaming is found, presumably marking the corotation resonance. The realm of inward streaming extends to very large radii, corresponding to 1.3R25 and 1.7 bar radii deprojected. This implies that most of the visible disk is inside corotation, although the HI disk extends beyond, at least to 2R25. As a result, either the bar and the spirals are rotating separately or the bar rotates very slowly compared to early-type bars. There is apparently no inner Lindblad resonance because of the slowly rising inner rotation curve, and there may be no outer Lindblad resonance if the disk ends first.

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