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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Protoplanetary disks in the hostile environment of Carina

Image: Star-forming region in the Carina Nebula. Credit: NASA, ESA, N. Smith (University of California, Berkeley), and The Hubble Heritage Team (STScI/AURA) In a recent paper (Mesa-Delgado et al. 2016) [1] the authors report the first direct imaging of protoplanetary disks in the star-forming region of Carina, the most distant, massive cluster in which disks have been imaged.

A Sapphire Super-Earth

Twenty-one light years away, in the constellation Cassiopeia, a planet by the name of HD219134 b orbits its star with a year that is just three days long. With a mass almost five times that of Earth, it is what is known as a super-Earth. Unlike our planet, however, these super-Earths were formed at high temperatures close to their host star and contain high quantities of calcium, aluminum and their oxides – including sapphire and ruby. HD219134 b is one of three candidates likely to belong to a new, exotic class of exoplanets. These objects are completely different from the majority of Earth-like planets. They have 10 to 20 percent lower densities than Earth. Researchers looked at different scenarios to explain the observed densities. For example, a thick atmosphere could lead to a lower overall density. But two of the exoplanets studied, 55 Cancri e and WASP-47 e, orbit their star so closely that their surface temperature is almost 3,000 degrees and they would have lost this ...

A BINARY ORIGIN FOR A CENTRAL COMPACT OBJECT (CCO)?

Figure: False-Colour X-ray and infrared emission image from the core of the infrared shell. The RGB colours correspond to Chandra X-ray 0.2-10 keV (blue), IRAC infrared 8 μm (green), and HPACS 70 μm (red) data. The intensity scale is logarithmic for all channels. Overlaid are equal brightness levels from the MIPS 24 μm band. Note that around the CCO the infrared emission is suppressed in the 70 μm band and enhanced in the 24 μm band suggesting higher dust temperature. Credit: Doroshenko et al 2016 Central compact objects (CCOs) are thought to be young isolated neutron stars that were born during the preceding core-collapse supernova explosion.

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