High mass distributions in galaxies in the spotlight

Two surveys of mass in the vicinity of galaxies have produced surprising but theoreticaly supported results. The first is to do with the distribution of massive stars within a galaxy and the second, the dark matter halo around it.

In the first survey, presented at the National Astronomy Meeting, 2010 and then in Astronomy Now’s article, a series of galaxies were observed and supernovae within them recorded. Supernovae are the death calls of the most massive stars. As massive stars are short lived, supernova rates are good ways of estimating rates of star formation in the galaxy’s recent history. As the supernova outshine anything else the galaxy will have to offer, they’re also easier to spot than say the emerging smaller stars that were thought to make up the bulk of new star birth.

Stacey Habergham of Liverpool John Moores University and colleagues examined the supernovae in 140 galaxies, amounting to 178 explosions, using the Liverpool Robotic Telescope and the Isaac Newton Telescope in the Canary Islands. They divided the galaxies into two classes – those that showed tidal disruption due to probable interactions with others and those that didn’t – and also divided the supernovae into two categories – Type 1 b/c, which are metal rich with little hydrogen, indicative of the most massive stars and Type II’s, which have hydrogen and are likely to be the end of less massive (but still, very massive) stars.

The result of marking the positions of the two supernova types onto the two galaxy types was the suggestion that in tidally disrupted galaxies, the more massive stars ended their lives more clustered to the centre than the evenly distributed less massive stars. In undisturbed galaxies, both types were more evenly distributed.

The analysis suggests that when galaxies merge, the result is starburst with an emphasis on larger stars, happening at the point of interaction. The deaths of these stars creates high metallicity gas that then gets pushed to the centre of the galaxy where it can give rise to the kinds of stars that undergo Type 1b/c supernovae.

The team now plans to perform infrared observations to better record the disturbances the galaxies are showing and also spectroscopic campaigns to find the metallicity of the remaining gas and dust around the galaxies’ bodies.

One facility that may be able to help her with that is the Large Binocular Telescope in Arizona, which has twin 8 metre mirrors operating together in the infrared. It is to be joined by the LBT near infrared spectroscopic Utility with Camera and Integral Field unit for Extragalactic Research (LUCIFER – are we really that low on acronyms?).

A second survey of relevance to galactic scale mass distributions was performed by the Subaru telescope in Hawaii. Masamune Oguri of the National Astronomical Observatory of Japan and an international team used the process known as gravitational lensing to measure the clumping of mass between the telescope’s aperture and a galaxy cluster.

Gravitational lensing is the effect of space-time bending more near to the surface of a higher mass object than a lower mass one. Light travelling past finds its direction altered, sending it away from the centre of mass, with the displacement greater the closer to the mass it travels. Eddington used this effect to provide some experimental proof of the predictions of General Relativity in 1919 and the effect has been going strong ever since, becoming a new branch of astronomy.

The survey’s results have been analysed and show that dark matter, the component of the Universe’s matter-like stuff that cannot be directly seen with ordinary telescopes due to its low level of interactivity with other matter or light in any way other than gravitationally, clumps in cigar shaped formations. This is consistent with the idea of Dark Matter being composed of cold WIMPs – Weakly Interacting Massive Particles – ie just another type of particle that ignores the electromagnetic force, doesn’t exert pressure on ordinary matter, but does have gravitational influence. The term cold in this case refers to the velocity of motion of the particles. Hot would imply near light speed particles like neutrinos, cold refers to relatively slow particles. In this case, the bulge is the response of the Dark Matter cloud to its own spinning. The exact shape is determined by the temperature of the Dark Matter and its interactions with the matter component of the region, allowing more determinations to be made and compared with other observations.


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