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Dynamical effect of gas on spiral pattern speed in galaxies

Ghosh, Soumavo and Jog, Chanda J (2016) Dynamical effect of gas on spiral pattern speed in galaxies. In: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 459 (4). pp. 4057-4065.

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Official URL: http://dx.doi.org/10.1093/mnras/stw914

Abstract

In the density wave theory of spiral structure, the grand-design two-armed spiral pattern is taken to rotate rigidly in a galactic disc with a constant, definite pattern speed. The observational measurement of the pattern speed of the spiral arms, though difficult, has been achieved in a few galaxies such as NGC 6946, NGC 2997, and M 51 which we consider here. We examine whether the theoretical dispersion relation permits a real solution for wavenumber corresponding to a stable wave, for the observed rotation curve and the pattern speed values. We find that the disc when modelled as a stars-alone case, as is usually done in literature, does not generally give a stable density wave solution for the observed pattern speed. Instead the inclusion of the low velocity dispersion component, namely, gas, is essential to obtain a stable density wave. Further, we obtain a theoretical range of allowed pattern speeds that correspond to a stable density wave at a certain radius, and check that for the three galaxies considered, the observed pattern speeds fall in the respective prescribed range. The inclusion of even a small amount (15 per cent) of gas by mass fraction in the galactic disc is shown to have a significant dynamical effect on the dispersion relation and hence on the pattern speed that is likely to be seen in a real, gas-rich spiral galaxy.

Item Type: Journal Article
Additional Information: Copy right for this article belongs to the OXFORD UNIV PRESS, GREAT CLARENDON ST, OXFORD OX2 6DP, ENGLAND
Department/Centre: Division of Physical & Mathematical Sciences > Physics
Depositing User: Id for Latest eprints
Date Deposited: 22 Oct 2016 06:49
Last Modified: 22 Oct 2016 06:49
URI: http://eprints.iisc.ac.in/id/eprint/54659

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