July 01, 2016

In Search of vanishing stars and galaxies

A team of astronomers say that the next search for advanced extraterrestrial civilizations should look for stars – or even galaxies – that have vanished without a trace, as anything so unexplainable could only be due to life far more intelligent than us.

Beatriz Villarroel at Uppsala University in Sweden says this crazy idea has been gnawing at her since her first year of graduate studies. Now she and two undergraduates have finally taken the plunge. They scoured multiple surveys of the night’s sky by eye in order to see if any of nearly 300,000 light sources disappeared from one survey to the next.

So far the results are mixed. The team found one interesting artefact that looks like it might have vanished, but they can’t be sure. “It was a depressing case in the sense that we neither could reject it and neither could we say that it was a real candidate,” says Villarroel. Although the team checked for so-called false positives, throwing out hundreds of similar disappearing objects, this one withstood all tests – but only just.

Even if the disappearance is real, there could still be an astrophysical explanation. Quasars – the bright centres of galaxies powered by supermassive black holes – can shut down in less than a decade and drop drastically in brightness. Stars, too, can be highly variable.





Arxiv - Our Sky now and then − searches for lost stars and impossible effects as probes of advanced extra-terrestrial civilisations



That’s why Villarroel and her colleagues plan to search for this missing object (and any others found in the future), on the largest telescopes. If it is still not visible, then they will be able to rule out most astrophysical phenomena and say with more likelihood that it has vanished. Only then will they begin speculating about extraterrestrial causes

“I think it’s a very reasonable thing to do,” says Jay Olson at Boise State University, Idaho. “It’s a deliberate search for something very unusual, which could be hiding in existing data across time. At this stage of the game, it’s a very limited search, but it illustrates well what can be accomplished.”

Unlike Tabby’s Star, a baffling star that dips in brightness and made headlines last year when astronomers suggested that “alien megastructures” could be the culprit, these objects would have no physical culprit behind their sudden disappearance – it’s just not possible for something to suddenly vanish from the universe without a trace.

Abstract

Searches for extra-terrestrial intelligence (SETI) using large survey data often look for possible signatures of astroengineering. We propose to search for physically impossible effects caused by highly advanced technology, by carrying out a search for disappearing galaxies and Milky Way stars. We select ∼ 10 million objects from USNO-B1.0 with low proper motion (μ < 20 milli arcseconds / year) imaged on the sky in two epochs. We search for objects not found at the expected positions in the Sloan Digital Sky Survey (SDSS) by visually examining images of ∼ 290 000 USNO-B1.0 objects with no counterpart in the SDSS. We identify some spurious targets in the USNO-B1.0. We find one candidate of interest for follow-up photometry, although it is very uncertain. If the candidate eventually is found, it defines the probability of observing a disappearing-object event the last decade to less than one in one million in the given samples. Nevertheless, since the complete USNO-B1.0 dataset is 100 times larger than any of our samples, we propose an easily accessible citizen science project in search of USNO-B1.0 objects which have disappeared from the SDSS.

SOURCES - New Scientists, Arxiv

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