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"Enormous Ghost Galaxy" – Hidden in the Milky Way's "Zone of Avoidance" (Weekend Feature Film)



Posted on May 18, 2019

  Galactic Center of the Milky Way

A giant 'ghost' galaxy, considered one of the oldest in the universe, was discovered on the edge of the Milky Way in November 201

8 by a team of astronomers who discovered the massive object as they searched new data from the Gaia satellite of the European Space Agency. The object named Antlia 2 could not be detected due to its extremely low density and perfect hiding in the avoidance zone, behind the Milky Way's shroud – a region of dust and an abundance of bright stars near the galactic center.

"This is a ghost of a galaxy," said Gabriel Torrealba, astrophysicist at Taiwan's Academia Sinica Institute for Astronomy and Astrophysics (ASIAA) and lead author of the newspaper. "Objects as diffuse as Ant 2 have not been seen yet. Our discovery was only possible thanks to the quality of the Gaia data. "Gaia is able to dig into the zone of avoidance, as it provides high-quality proper motion of stars behind the central disk of our galaxy galaxy. That is, it can track stars as they move across the celestial sphere.

Visually, the zone of avoidance is like "trying to look through a velvet cloth – black as black can be," says Thomas Dame, director of the Radio Telescope Data Center at the Harvard-Smithsonian Center for Astrophysics and senior radio astronomer at Smithsonian Astrophysical Observatory. "In terms of the pursuit and understanding of the spiral structure, essentially half of the Milky Way is terra incognito."

"The Most Important in Astrophysics" – the & # 39; Holy Grail & # 39; Astronomy is to provide a clear perspective for our Milky Way relationship to the physical universe. The map of our Milky Way is part of it, a map that is still incomplete. Our solar system floats between two spiral arms at its outer edges, about 27,000 light-years from its center. Moreover, like the old sailors, no spacecraft has yet driven beyond the opaque center-plate to return and take a picture.

"Monsters & Dragons?" – Mapping the Terra Incognito of the Invisible Milky Way

"The evasion zone is basically the part of the sky that is hidden from Earth through the disc of the Milky Way," Torrealba said. "The disk of the Milky Way contains many gases and stars, making them extremely overcrowded and complex." However, the team was able to use about one hundred old and low-metal pulsating so-called "RR Lyrae" stars to examine Antlia 2 inside and ultimately identify it.

Visually, intrusion into the zone of avoidance is like "trying to look through a velvet cloth – black, as black can be," says Thomas Dame, director of the Radio Telescope Data Center at the Harvard-Smithsonian Center for Astrophysics and senior radio astronomer at the Smithsonian Astrophysical Observatory. "In terms of the pursuit and understanding of the spiral structure, essentially half of the Milky Way is terra incognito."

"The Most Important in Astrophysics" – the & # 39; Holy Grail & # 39; Astronomy is to provide a clear perspective for our Milky Way relationship to the physical universe. The map of our Milky Way is part of it, a map that is still incomplete. Our solar system floats between two spiral arms at its outer edges, about 27,000 light-years from its center. Moreover, like the old sailors, no spaceship has yet driven beyond the opaque center-plate to return and take a picture.

The swarm of weak dwarf galaxies circling the Milky Way – "Much more hidden yet to be discovered"

"Ant 2 is a curiosity compared to the roughly 60 Milky Way satellites," said co-author Matthew Walker, also of Carnegie Mellon University. "We wonder if this galaxy is just the tip of an iceberg and the Milky Way is surrounded by a large population of almost invisible dwarfs that are similar to it."

 Antlia2 Dwarf Galactic [19659005]

Torrealba says that Antlia 2 is probably one of the oldest dwarf galaxies in the universe, but he and his colleagues are still not sure how it has become so diffuse. "One possibility is that Antlia 2 was much more massive in the past, and when it fell into the Milky Way, it lost its mass to become more diffuse," Torrealba said. One problem with this idea, according to Torrealba, is that galaxies do not grow, but shrink and lose stars at the same time.

Extreme disk galaxy discovered – "Seven times the width of the Milky Way"

Astronomer Sergey Koposov of Carnegie Mellon University says that the size of the object is a mystery that matches Torrealba. "Normally, galaxies shrink when they lose mass and do not grow."

"Another possible explanation for the extraordinary appearance of Antlia 2," Koposov wrote in an email to dailygalaxy.com, "is that there is something wrong with the currently-favored theory of cold dark matter that predicts that Dark matter should be densely packed in centers of galaxies, but if the distribution of dark matter is more flaky, it can facilitate the formation of galaxies like Antlia 2. "

Antlia 2 is known as a dwarf galaxy. Dwarfs were the first galaxies to form, and so most of their stars are old, low in mass, and metal poor, but compared to the other known dwarf satellites in our galaxy, Ant 2 is immense: it's the size of the Large Magellanic Cloud (LMC). and one-third the size of the Milky Way itself.

What makes Ant 2 even more unusual is how little light it emits. Compared to the LMC, a w On the other satellites of the Milky Way, Ant 2 is 10,000 times weaker. In other words, it's either too big for its luminosity or too dark for its size.

ESA's Gaia mission has created the richest star catalog ever, including high-precision measurements of nearly 1.7 billion stars previously revealed as invisible details of our home galaxy. In early 2018, Gaia's second release of data provided scientists worldwide with new details about stars in the Milky Way.

The researchers behind the current study – from Taiwan, the UK, the US, Australia, and Germany – looked for the new Gaia data for Milky Way satellites with RR Lyrae stars. These stars are old and poor in metal, typical of those in a dwarf galaxy. RR Lyrae change their brightness within half a day and can be located thanks to these well-defined impulses.

"RR Lyrae was found in every known dwarf satellite when we found a group of them above the satellite Galactic disk, we were not totally surprised," said co-author Vasily Belokurov of the Cambridge Institute of Astronomy. "However, looking closer at the location in the sky, it turned out that we had found something new because none of the databases searched previously found any previously identified objects."

The team turned to colleagues at the Anglo-Australian Telescope (AAT) in Australia, but when they checked the coordinates for Ant 2, they found that they had a limited time window to get follow-up data. They were able to measure the spectra of more than 100 red giant stars just before Earth's movement around the Sun Ant 2 was no longer observable for months.

The spectra allowed the team to confirm that the ghostly object they were discovering was real: all the stars moved together. Ant 2 never gets too close to the Milky Way and always stays at least 40 kiloparsecs (about 130,000 light-years away). The researchers also succeeded in finding the mass of the galaxy that was much smaller than expected for an object of this size.

When it was impossible to inflate the dwarf by removing matter, Ant 2 had to have been born enormously. The team still has to figure out the exact process that has extended Ant 2. While objects of this size and luminosity have not been predicted by current models of galaxy formation, it has recently been speculated that some dwarfs might be inflated by vigorous star formation. Star winds and supernova explosions would push away the unused gas, weaken the gravitational forces that bind the galaxy, and also cause the dark matter to drift outward.

"Even if star formation could alter the distribution of dark matter in Ant 2 as it was put together, it must have acted with unprecedented efficiency," said fellow author Jason Sanders, also from Cambridge.

Alternatively, the low density of Ant 2 could mean that a modification of the properties of dark matter is required. The currently favored theory predicts that dark matter is in the centers of galaxies. Given the fluffiness of the new dwarf, a particle of dark matter may be required that is less susceptible to clusters.

The gap between Ant 2 and the rest of the galactic dwarfs is so large that this could well be a clue to it. The models of dwarf galaxy formation lacked important physics. Solving the ant-2 puzzle can help researchers understand the formation of the first structures in the early Universe.

The Daily Galaxy on Imperial College London


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