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Astronomers have unexpectedly spotted a low-luminance "ghost-surface" dwarf galaxy on the outer edges of our own Milky Way.
What would Antlia 2 look like if you could see it, for example from Chile.Credit: G. Torrealba (Sinica Academy, Taiwan), V. Belokurov (Cambridge, UK and CCA, New York, USA) according to the image of ESO / S. Brunier
After scanning the latest batch of data from the European Space Agency (ESA) Gaia spacecraft, an international team of astronomers has discovered the most diffuse and weakest surface galaxy ever detected. Nicknamed now Antlia 2, for the constellation in which it is located, it is officially a new satellite galaxy of the Milky Way.
"We do not know exactly how this galaxy has become so ghostly," said Gabriel Torrealba, team leader and astrophysicist at the Institute of Astronomy and Astrophysics Academia Sinica of Taiwan (ASIAA).
About a third of the diameter of the Milky Way, Torrealba notes that Antlia 2 is about the same size as the nearby Great Cloud Magellan (LMC), but shines about 10,000 times weaker.
Such low surface gloss galaxies have very low star formation rates and produce very little, if any, supernovae as previously reported.
Part of the reason why Antilia 2 had not been spotted to date was simply that it was in an inherently difficult part of the galactic plane to be observed. It is a region full of dust and an overabundance of bright stars near the galactic center.
But the team was able to use a hundred pulsating stars, called "RR Lyrae", poor in metals, to probe the interior and identify Antlia 2 in this galactic avoidance zone.
"The avoidance area is basically the part of the sky darkened by the Milky Way disk seen from Earth," Torrealba said. "The Milky Way's drive has a lot of gas and stars, which makes it extremely cluttered and complex."
Because of this complexity, says Torrealba, any study becomes very difficult. Gaia is able to search the avoidance area, he says, as it provides high quality star motions behind the central disk of our galaxy, the Milky Way. That is, he is able to follow the stars as they move through the celestial sphere.
Antlia 2 is a giant dwarf galaxy but of low mass.Credit: J. Sanders (Cambridge, UK) from the image of the Gaia Data Processing and Analysis Consortium (DPAC); A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal.
Located behind the galactic disk, the team reports in the newspaper Monthly Notices from the Royal Astronomical Society Antlia 2 was discovered from data from the latest version of Gaia's Data Release 2 (DR-2), as well as from ground tracking observations with the Anglo-Australian Telescope in Australia. And although it's clearly a satellite, it's never less than 130,000 light-years away from the Milky Way.
Torrealba says that Antlia 2 is probably one of the oldest dwarf galaxies in the universe, but he and his colleagues are still questioning how it has become so diffuse.
"One of the possibilities is that Antlia 2 was much more massive in the past and, falling into the Milky Way, it lost its mass to become more diffuse," said Torrealba.
A problem with this idea Torrealba says is that rather than grow, galaxies tend to shrink at the same time as they lose stars.
"There seems to be no limit (up to now) on the ability of a galaxy to get a shiny, shiny surface," said Stacy McGaugh, astronomer at La Case Western Reserve University of Cleveland, who did not participate in the study.
One of the biggest questions is how many similar dwarf galaxies are hiding around our own galaxy. The authors note that the Milky Way may still contain between one and three undetected dwarf satellite galaxies, each with 100,000 stellar masses or more.
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Astronomers have unexpectedly spotted a low-luminance "ghost-surface" dwarf galaxy on the outer edges of our own Milky Way.
What would Antlia 2 look like if you could see it, for example from Chile.Credit: G. Torrealba (Sinica Academy, Taiwan), V. Belokurov (Cambridge, UK and CCA, New York, USA) according to the image of ESO / S. Brunier
After scanning the latest batch of data from the European Space Agency (ESA) Gaia spacecraft, an international team of astronomers has discovered the most diffuse and weakest surface galaxy ever detected. Nicknamed now Antlia 2, for the constellation in which it is located, it is officially a new satellite galaxy of the Milky Way.
"We do not know exactly how this galaxy has become so ghostly," said Gabriel Torrealba, team leader and astrophysicist at the Institute of Astronomy and Astrophysics Academia Sinica of Taiwan (ASIAA).
About a third of the diameter of the Milky Way, Torrealba notes that Antlia 2 is about the same size as the nearby Great Cloud Magellan (LMC), but shines about 10,000 times weaker.
Such low surface gloss galaxies have very low star formation rates and produce very little, if any, supernovae as previously reported.
Part of the reason why Antilia 2 had not been spotted to date was simply that it was in an inherently difficult part of the galactic plane to be observed. It is a region full of dust and an overabundance of bright stars near the galactic center.
But the team was able to use a hundred pulsating stars, called "RR Lyrae", poor in metals, to probe the interior and identify Antlia 2 in this galactic avoidance zone.
"The avoidance area is basically the part of the sky darkened by the Milky Way disk seen from Earth," Torrealba said. "The Milky Way's drive has a lot of gas and stars, which makes it extremely cluttered and complex."
Because of this complexity, says Torrealba, any study becomes very difficult. Gaia is able to search the avoidance area, he says, as it provides high quality star motions behind the central disk of our galaxy, the Milky Way. That is, he is able to follow the stars as they move through the celestial sphere.
Antlia 2 is a giant dwarf galaxy but of low mass.Credit: J. Sanders (Cambridge, UK) from the image of the Gaia Data Processing and Analysis Consortium (DPAC); A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal.
Located behind the galactic disk, the team reports in the newspaper Monthly Notices from the Royal Astronomical Society Antlia 2 was discovered from data from the latest version of Gaia's Data Release 2 (DR-2), as well as from ground tracking observations with the Anglo-Australian Telescope in Australia. And although it's clearly a satellite, it's never less than 130,000 light-years away from the Milky Way.
Torrealba says that Antlia 2 is probably one of the oldest dwarf galaxies in the universe, but he and his colleagues are still questioning how it has become so diffuse.
"One of the possibilities is that Antlia 2 was much more massive in the past and, falling into the Milky Way, it lost its mass to become more diffuse," said Torrealba.
A problem with this idea Torrealba says is that rather than grow, galaxies tend to shrink at the same time as they lose stars.
"There seems to be no limit (up to now) on the ability of a galaxy to get a shiny, shiny surface," said Stacy McGaugh, astronomer at La Case Western Reserve University of Cleveland, who did not participate in the study.
One of the biggest questions is how many similar dwarf galaxies are hiding around our own galaxy. The authors note that the Milky Way may still contain between one and three undetected dwarf satellite galaxies, each with 100,000 stellar masses or more.
