“The Accidental Galaxy” –Hubble Unveils Lonely Fossil from the Early Universe

NGC_6752

 

Astronomers using the NASA/ESA Hubble Space Telescope to study some of the oldest and faintest stars in the globular cluster NGC 6752 (above) have made an unexpected finding. They discovered a dwarf galaxy in our cosmic backyard, only 30 million light-years away while studying white dwarf stars within NGC 6752. The aim of their observations was to use these stars to measure the age of the globular cluster, but in the process they made a surprising discovery.

The Milky Way harbors about 150 globular clusters, most of them orbiting in the galactic outskirts, formed about 10 billion years ago. As a result, their stars contain fewer of the heavy elements needed to construct planets, since those elements (like iron and silicon) must be created in earlier generations of stars. Some scientists have argued that this makes globular cluster stars less likely to host planets. To date, only one planet has been found in a globular cluster.

In the outer fringes of the area observed with Hubble’s Advanced Camera for Surveys a compact collection of stars was visible. After a careful analysis of their brightnesses and temperatures, the astronomers concluded that these stars did not belong to the cluster — which is part of the Milky Way — but rather they are millions of light-years more distant.

Our newly discovered cosmic neighbor, nicknamed Bedin 1 by the astronomers, is a modestly sized, elongated galaxy. It measures only around 3000 light-years at its greatest extent — a fraction of the size of the Milky Way. Not only is it tiny, but it is also incredibly faint. These properties led astronomers to classify it as a dwarf spheroidal galaxy.

 

globular cluster NGC 6752

 

The composite image above shows the location of the accidentally discovered dwarf galaxy Bedin 1 behind NGC 6752. The lower image, depicting the complete cluster, is a ground-based observation from the Digitized Sky Survey 2. The upper right image shows the full field of view of the NASA/ESA Hubble Space Telescope. The upper left one highlights the part containing the galaxy Bedin 1. (ESA/Hubble, NASA, Bedin et al., Digitized Sky Survey 2)

Dwarf spheroidal galaxies are defined by their small size, low-luminosity, lack of dust and old stellar populations [1]. 36 galaxies of this type are already known to exist in the Local Group of Galaxies, 22 of which are satellite galaxies of the Milky Way.

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While dwarf spheroidal galaxies are not uncommon, Bedin 1 has some notable features. Not only is it one of just a few dwarf spheroidals that have a well established distance but it is also extremely isolated. It lies about 30 million light-years from the Milky Way and 2 million light-years from the nearest plausible large galaxy host, NGC 6744. This makes it possibly the most isolated small dwarf galaxy discovered to date.

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From the properties of its stars, astronomers were able to infer that the galaxy is around 13 billion years old — nearly as old as the Universe itself. Because of its isolation — which resulted in hardly any interaction with other galaxies — and its age, Bedin 1 is the astronomical equivalent of a living fossil from the early Universe.

The discovery of Bedin 1 was a truly serendipitous find. Very few Hubble images allow such faint objects to be seen, and they cover only a small area of the sky. Future telescopes with a large field of view, such as the WFIRST telescope, will have cameras covering a much larger area of the sky and may find many more of these galactic neighbors.

In a 2016 study, Rosanne DiStefano of the Harvard-Smithsonian Center for Astrophysics (CfA) observed that a globular cluster might be the first place in which intelligent life is identified in our galaxy. Globular star clusters are extraordinary in almost every way. They’re densely packed, holding a million stars in a ball only about 100 light-years across on average. They’re old, dating back almost to the birth of the Milky Way. And according to new research, they also could be extraordinarily good places to look for space-faring civilizations.

The outer part of the Milky Way is made up of a sparsely populated halo about 300,000 light years across of very old stars, at least twice as old as the Sun. The outer halo, at least, formed during the process which gave birth to our Galaxy from a collapsing cloud of gas about 10 billion years ago, although the inner part of the halo contains slightly younger stars and may have formed slightly later.

If life existed on any planets orbiting those stars, writes John Gribbin in Alone in the Universe, “it would have had the opportunity for 5 billion years of evolution even before the Earth formed. It could have been as advanced as we are now when our ancestors were still single-celled bacteria – a seemingly dramatic demonstration of the Fermi paradox. But since very few of the halo stars have even 10 per cent of the Sun’s metallicity, let alone the 40 per cent that seems to be necessary for planet-building, it is extremely unlikely that there are any Earth-like planets or life forms like us out there.”

The discovery of Bedin 1 was presented in the letter The HST Large Programme on NGC 6752. I. Serendipitous discovery of a dwarf galaxy in background, published in the journal Monthly Notices of the Royal Astronomical Society: Letters.

The Daily Galaxy via Hubble/ESO Information Center

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