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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GAMMA-RAY EMISSION FROM THE SNR HB3

Image: At a distance of about 20,000 light years, G292.0+1.8 is one of only three supernova remnants in the Milky Way known to contain large amounts of oxygen. These oxygen-rich supernovas are of great interest to astronomers because they are one of the primary sources of the heavy elements (that is, everything other than hydrogen and helium) necessary to form planets and people. The X-ray image from Chandra shows a rapidly expanding, intricately structured, debris field that contains, along with oxygen (yellow and orange), other elements such as magnesium (green) and silicon and sulfur (blue) that were forged in the star before it exploded. Credit: NASA/CXC/SAO The processes of particles acceleration to very high energies from the supernova shock region and diffusion in the interstellar medium of such particles has not been well understood so far. Gamma-ray observations in the GeV regime are a powerful probe of these mechanisms

THE HITCHCHIKER'S GUIDE TO THE LOCAL SUPERCLUSTER

Image: Virgo Supercluster. Credit: Andrew Z. Colvin The Virgo Supercluster is a region with a diameter of 33 megaparsecs (~1000 times larger the Milky Way's diameter) containing at least 100 galaxy groups and clusters.

Boulevard of Broken Rings

Credit: ESO/Perrot This Picture illustrates the remarkable capabilities of SPHERE (the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument), a planet-hunting instrument mounted on ESO's Very Large Telescope (VLT) in Chile: It shows a series of broken rings of dust around a nearby star. These concentric rings are located in the inner region of the debris disc surrounding a young star named HD 141569A, which sits some 370 light-years away from us. In this image we see what is known as a transition disc, a short-lived stage between the protoplanetary phase, when planets have not yet formed, and a later time when planets have coalesced, leaving the disc populated only by any remaining - and predominantly dusty - debris. What we see here are structures formed of dust, revealed for the first time in near-infrared light by SPHERE - at a high enough resolution to capture remarkable detail! The area shown in this image has a diameter of just 200 times the Earth–Sun distan...

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