ROTATION CURVES OF GALAXIES AS A TEST OF MOND

Galaxies are rotating with such speed that the gravity generated by their observable matter could not possibly hold them together. According to Newtonian gravity, the rotational velocity falls with distance from the center of a galaxy, while the observed data usually show an asymptotically flat rotation curve out to the furthest observationally accessible data points.

One solution to solve this problem is assuming a dark matter halo distributed around each galaxy.

An alternative approach is to replace dark matter by a modification of the Newtonian dynamics known as Milgromian dynamics (MOND). In the MOND the gravitational dynamics of a system is influenced by the external gravitational field in which it is embedded. This so called External Field Effect (EFE) is one of the important implications of MOND and provides a special context to test Milgromian dynamics.

In a recent paper (Haghi et al. 2016) the authors study the rotation curves of 18 spiral galaxies and they show that the EFE can successfully remedy the overestimation of rotation velocities in 80% of the sample galaxies in Milgromian dynamics fits by decreasing the velocity in the outer part of the rotation curves.

Haghi et al. 2016 (Accepted for publication in MNRAS) - Declining rotation curves of galaxies as a test of gravitational theory - (arXiv)

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Image: The Kuiper Belt. Credit: NASA . The Kuiper belt is a circumstellar disc in the Solar System beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt (the circumstellar disc located roughly between the orbits of the planets Mars and Jupiter), but it is far larger-20 times as wide and 20 to 200 times as massive.

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Image: A giant gamma-ray structure was discovered in 2010 by processing Fermi all-sky data at energies from 1 to 10 billion electron volts, shown here. The dumbbell-shaped feature (center) emerges from the galactic center and extends 50 degrees north and south from the plane of the Milky Way, spanning the sky from the constellation Virgo to the constellation Grus. Credits: NASA/DOE/Fermi LAT/D. Finkbeiner et al. At a time when our earliest human ancestors mastered walking upright the heart of our Milky Way galaxy underwent a titanic eruption, driving gases and other material outward at 2 million miles per hour.

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