Terzan 5! — Ancient Dwarf Galaxy Remnant & Source of a Mysterious Signal


It's not the name a Japanese horror movie…Scientists have spotted stellar fossils in the center of the Milky Way, globular clusters orbiting in the central bulge which seem to have come from somewhere else.  Like all fossils these are vital evidence of the evolutionary processes which led to everything we see today.  (Though for fairness we should point out the rival suggestion of an Intelligently Designed galaxy, where over two hundred billion gigantic fusion reactors were carefully constructed as extraordinarily elaborate background for a single planet.)

Terzan 5 is a massive blob of over a million tightly packed stars, with up to 10,000 per cubic light-year.  (Out with us it's less 0.02 over the same volume.) Examining their output with the wonderfully named Very Large Telescope (which combines four eight-meter apertures into a singe instrument effectively two hundred meters across) scientists have spotted distinct stellar signatures of both old and young stars.

This composition indicates that Terzan 5 evolved its stellar populations as a dwarf galaxy, and must once have been much larger than it is now.  As old stars explode in supernovae they spread newly created material huge distances across space – so subsequently formed stars begin with more of these distinctive elements.  But the blasts are so gigantic they would throw this starborne material right out of the current globular cluster – indicating Terzan 5 and others like it were once small galaxies in their own right. 

Obscured behind galactic dust clouds the faint cluster was discovered in 1968 by Agop Terzan on photographic plates of the Haute-Provence Observatory in France. About 150 known globular clusters, concentrated spherical collection of very old stars, orbit the centre of our galaxy in form of a spherical swarm as part of the galactic halo.

Terzan 5 is located in the inner parts of our galaxy closely above the galactic plain in about 20,000 light-years distance to the earth. It has the highest density of stars of all globular clusters and contains the largest number of millisecond pulsars. The latter are rapidly rotating neutron stars which are thought to be part of close binary systems.

Terzan 5 gained particular attention in 2009 when it turned out that is has two star populations of different age (12 and 6 billion years, respectively). Due to these unique properties Terzan 5 is assumed to be the remnant of a dwarf galaxy which has been captured by our galaxy.

Researchers of the Max Planck Institute for Nuclear Physics in Heidelberg and 33 other institutions within the H.E.S.S. collaboration report the discovery of a new source (HESS J1747 – 248) of very-high-energy gamma-rays from the direction of Terzan 5. The location in the close vicinity to the cluster suggests that the source is a so far unknown part of Terzan 5. The probability of a chance coincidence with an unrelated gamma emission (derived from the abundance of known sources) is less than 1/10,000.


The Daily Galaxy via eso.org and mpg.de

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