The more flattened the mass distribution

The more flattened the mass distribution - In the most...

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The more flattened the mass distribution, the flatter the rotation curve - the less it acts like a point mass. Rotation curves of real disk galaxies, once it was possible to measure them to large radii, gave one of the big surprises of modern astronomy. M(r) continues to increase with r almost as far as can be measured. A first hint came when the rotation curve of the Milky Way was pieced together from optical, H I, and CO observations (from Burton 1976 Ann. Rev. 14, 275, shown from the ADS): Here, there is little evidence of a Keplerian drop at large radii. A flat rotation curve implies ρ (r) ~ 1/r 2 , though in the interior of all but the dimmest dwarf galaxies the luminous mass must dominate over the dark matter. This behavior is universal among spirals. Here's a sample in NGC 5746, from Lowell imaging and Kitt Peak optical spectroscopy:
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and further ones as found by Rubin and collaborators, shown in Fig. 3 of Rubin, Ford, and Thonnard 1978 ApJL 225, L107 (reproduced courtesy of the AAS):
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Unformatted text preview: In the most extreme cases, the rotation curve is flat to almost 100 kpc. Clearly, most of the mass is in a more extended distribution than stars or gas - the "missing mass" (even though what is missing is in our brains, not in the mass). There are trends of the mass distribution with Hubble type, as mapped by Rubin et al 1985 ApJ 289, 81: the form of the rotation curve is fairly constant, with the amplitude changing from Sa to Sc. The maximum rotation velocity has median values 299,222,175 km/s for Sa, Sb, and Sc, respectively. In all cases, the correlation with luminosity dominates for individual galaxies. It is traditional to trace some aspects of the mass distribution via the mass-to-light ratio M/L in solar units, running from a few for young stellar assemblies to several hundred for galaxy halos and clusters. Not only does this reduce the big galaxy-small galaxy bias, but it is also distance-independent for traditional dynamical mass estimates....
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This note was uploaded on 11/10/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.

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The more flattened the mass distribution - In the most...

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