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"The Accidental Galaxy" – Hubble reveals the solitary fossil of the early universe

Posted on February 1, 2019


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

The Milky Way is home to about 150 globular clusters, most of them orbiting the galaxy periphery, formed about 10 billion years ago. As a result, their stars contain fewer heavy elements needed to build planets, since these elements (such as iron and silicon) must be created in previous generations of stars. Some scientists argue that this makes it less likely that globular clusters will 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 glow and temperatures, astronomers concluded that these stars did not belong to the cluster – which is part of the Milky Way – but are millions of light years away.

Our newly discovered cosmic neighbor, nicknamed Bedin 1 by astronomers, is an elongated galaxy of modest size. It measures only about 3000 light years to the greatest extent – a fraction of the size of the Milky Way. Not only is it small, but it is also incredibly weak. These properties led astronomers to classify it as a spheroidal dwarf galaxy.

globular agglomerate NGC 6752

The composite image above shows the location of the accidentally discovered Bedin 1 dwarf galaxy behind NGC 6752. The bottom image, representing the complete cluster, is a terrestrial observation of the Digitized Sky Survey 2. The upper right image shows the full field of view of the Telescope NASA / ESA Hubble Space Telescope. The upper left highlights the part that contains the Bedin 1 galaxy (ESA / Hubble, NASA, Bedin et al., Digitized Sky Survey 2)

Spheroidal dwarf galaxies are defined by their small size, low luminosity, lack of dust and old star populations. [1]. It is known that there are 36 galaxies of this type in the Local Group of Galaxies, 22 of which are satellite galaxies of the Milky Way.

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

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From the properties of their stars, astronomers were able to infer that the galaxy is 13 billion years old – almost as old as the Universe itself. Because of its isolation – which resulted in almost no interaction with other galaxies – and its age, Bedin 1 is the astronomical equivalent of a living fossil of the primordial Universe.

The discovery of Bedin 1 was a truly casual find. Very few Hubble images allow these objects to be seen and 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 encounter many of these galactic neighbors.

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

The outer part of the Milky Way is composed of a sparsely populated halo, with about 300,000 light-years of very old stars, at least twice as old as the Sun. The outer halo at least formed during the process it gave originated our galaxy from a collapsing gas cloud about 10 billion years ago, although the inner part of the halo contains stars slightly younger and may have formed a little later.

If life existed on any planets orbiting these stars, John Gribbin writes in Alone in the Universe"It would have the opportunity of 5 billion years of evolution even before the formation of Earth. It could have been as advanced as we are now, when our ancestors were still unicellular bacteria – an apparently dramatic demonstration of Fermi's paradox. But since very few of the halo stars have up to 10% of the metallicity of the Sun, not to mention the 40% that seems to be needed for the construction of the planet, it is extremely unlikely that there are Earth-like planets or life forms like us out there.

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

The Daily Galaxy via Hubble / ESO Information Center

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