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Interstellar gas helps spiral arms survive longer, finds study by IISc

A two member astrophysics team (Mr. Soumavo Ghosh and Prof. Chanda J. Jog) from the Indian Institute of Science, have carried out investigations for understanding the role of gas in the persistence issue of the spiral arms in galaxies. They have shown that the interstellar gas present in spiral galaxies reduces the time for propagation of a wavepacket and thus helps maintain the spiral structure intact for longer time periods.

The grand-design two-armed spiral structure, as seen in many galaxies, including our own Milky Way, makes a striking visual impression against the sky background.

The origin and persistence of this pattern has been studied for over five decades or so, including via the well-known density wave theory, but the details are not yet fully understood. Density wave theory tells that the spiral arms are not material in nature, instead are made up of areas of enhanced density and the spiral arms sweep across a galactic disk and is maintained in a self-consistent way.

The current consensus is that the grand two-armed spirals are density waves in a galactic disk, governed mainly by gravity.

In a classic paper, Alar Toomre (1969) of M.I.T., USA showed that due to 'Group Transport', the life-span of these patterns is reduced to be about a billion years. This was contrary to the common idea that these features last for the lifetime of a galaxy.

It is also well-known that the interstellar gas is present in any spiral galaxy. Earlier studies by Prof. Jog and collaborators have shown that the gaseous component has a profound effect on various dynamical issues related to a galactic disk. Motivated by these theoretical and observational facts,

Ghosh & Jog investigated the role of gas in the context of persistence of the grand-design spiral structure.

They included gas with stars on an equal footing in the formulation of density wave theory, and studied its effect on group transport. Their work shows that the inclusion of gas allows the spiral pattern to survive longer, for several billion years.

The study also showed that the inclusion of gas has other far-reaching consequences. The interstellar gas is found to be essential to get a stable density wave for the observed pattern speed of spiral arms of the Milky Way Galaxy, which otherwise is not possible for a pure stellar disk. “Right now we are investigating this particular aspect for several other galaxies in a more quantitative way”, hints Ghosh on describing the future of this research.

About the authors:

Chanda J Jog is a Professor in the Department of Physics, Indian Institute of Science, Bangalore. Soumavo Ghosh is a PhD student working with Prof. Chanda J Jog and Dr. Tarun Deep Saini.

Contact:

Soumavo Ghosh: sg.deep89@gmail.com

About the paper:

The research publication appeared in the Monthly Notices of the Royal Astronomical Society. It can also be accessed at http://arxiv.org/abs/1506.01038 

Tag: 
physics