A new class of “dark” globular clusters around Centaurus A

Centaurus A haloImage and text: ESO,ESA/Hubble, NASA. Digitized Sky Survey. Acknowledgement: Davide de Martin

Observations with ESO’s Very Large Telescope in Chile have discovered a new class of “dark” globular star clusters around the giant galaxy Centaurus A. These mysterious objects look similar to normal clusters, but contain much more mass and may either harbour unexpected amounts of dark matter, or contain massive black holes — neither of which was expected nor is understood. Globular star clusters are huge balls of thousands of stars that orbit most galaxies. They are among the oldest known stellar systems in the Universe and have survived through almost the entire span of galaxy growth and evolution.

Matt Taylor, a PhD student at the Pontificia Universidad Catolica de Chile, Santiago, Chile, and holder of an ESO Studentship, is lead author of the new study. He sets the scene: “Globular clusters and their constituent stars are keys to understanding the formation and evolution of galaxies. For decades, astronomers thought that the stars that made up a given globular cluster all shared the same ages and chemical compositions — but we now know that they are stranger and more complicated creatures.” The elliptical galaxy Centaurus A (also known as NGC 5128) is the closest giant galaxy to the Milky Way and is suspected to harbour as many as 2000 globular clusters. Many of these globulars are brighter and more massive than the 150 or so orbiting the Milky Way. Matt Taylor and his team have now made the most detailed studies so far of a sample of 125 globular star clusters around Centaurus A using the FLAMES instrument on ESO’s Very Large Telescope at the Paranal Observatory in northern Chile.

The full science paper is available via ESO (link under above image). While I ponder that with the benefit of hindsight it appears natural that massive galaxies like Cen A, totally different from our own Milky Way, should harbour some very odd (to our eyes) globular clusters, I can’t resist instead here to note some other, more uncertain and maybe even stranger dark-related news.  Atmospheric scientists may have discovered clouds of positrons associated with thunderstorms here on Earth. Why they have not annihilated is a mystery, and no excess gamma ray emission appears to be observed. Story and quote: phys.org

A terrifying few moments flying into the top of an active thunderstorm in a research aircraft has led to an unexpected discovery that could help explain the longstanding mystery of how lightning gets initiated inside a thunderstorm. University of New Hampshire physicist Joseph Dwyer and lightning science colleagues from the University of California at Santa Cruz and Florida Tech describe the turbulent encounter and discovery in a paper to be published in the Journal of Plasma Physics. In August 2009, Dwyer and colleagues were aboard a National Center for Atmospheric Research Gulfstream V when it inadvertently flew into the extremely violent thunderstorm—and, it turned out, through a large cloud of positrons, the antimatter opposite of electrons, that should not have been there. To encounter a cloud of positrons without other associated physical phenomena such as energetic gamma-ray emissions was completely unexpected, thoroughly perplexing and contrary to currently understood physics. “The fact that, apparently out of nowhere, the number of positrons around us suddenly increased by more than a factor of 10 and formed a cloud around the aircraft is very hard to understand. We really have no good explanation for it,” says Dwyer, a lightning expert and the UNH Peter T. Paul Chair in Space Sciences at the Institute for the Study of Earth, Oceans, and Space. It is known that thunderstorms can sometimes make flashes of energetic gamma rays, which may produce pairs of electrons and positrons when they interact with air. But the appearance of positrons should then coincide with a large increase in the number of gamma rays. “We should have seen bright gamma-ray emissions along with the positrons,” Dwyer says. “But in our observations, we first saw a positron cloud, then another positron cloud about seven kilometers away and then we saw a bright gamma-ray glow afterwards. So it’s all not making a whole lot of sense.”

Furthermore:

One possible explanation for the sudden appearance of positrons is that the aircraft itself dramatically influenced the electrical environment of the thunderstorm but that, Dwyer says, would be very surprising. It’s also possible the researchers were detecting a kind of exotic electrical discharge inside the thunderstorm that involves positrons. “This is the idea of ‘dark lightning,’ which makes a lot of positrons,” says Dwyer. “In detecting the positrons, it’s possible we were seeing sort of the fingerprint of dark lightning. It’s possible, but none of the explanations are totally satisfying.”

Light in dark red sky

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