Dwarfs found in colliding galaxies' wake

 作者:漆洒翳     |      日期:2019-02-28 08:19:02
By Maggie McKee (Image: NASA/JPL-Caltech/Cornell Univ) A new method to detect small, faint galaxies that spring up in the wake of violent galactic collisions has been devised by astronomers using the Spitzer Space Telescope. The method could shed light on how most of the galaxies near our own formed. The vast majority of local galaxies are “dwarfs” – our galaxy, the Milky Way, has 1000 times more mass in stars. But it is not clear how these dwarfs form. Some may have condensed directly from primordial gas soon after the big bang. But astronomers are not sure these lightweight galaxies would have been able to survive unscathed the relatively common galactic smash-ups that occurred in the early universe. Now, a team of astronomers from Cornell University in Ithaca, New York, US, is trying to answer that question by studying a population of dwarf galaxies that were definitely born when two larger galaxies slammed into each other. “It’s important to compare the properties of the ones we know [came from mergers] with those that may have lasted since the big bang,” says team member Jim Higdon. The objects studied by the team are called tidal dwarf galaxies (TDGs), because they are created when two galaxies interact gravitationally. They form in long, tail-like structures but are so small and faint that only about a dozen had previously been observed. But now the Cornell team has used Spitzer’s exquisite sensitivity at infrared wavelengths to identify 15 TDGs around a pair of merging galaxies, together called NGC 5291, that lie 200 million light years from Earth. The galaxies are strung along two arcs of stars and gas stretching about 240,000 light years behind each of the two larger galaxies. Previous ultraviolet measurements had suggested these arcs contained “knots” of star formation. Spitzer confirmed this, revealing these modest TDGs are actually powerhouses of star formation – boasting about 200 million sun-like stars that are just a few million years old. The researchers also discovered the merger-induced dwarf galaxies have a telltale infrared spectrum. They are now searching for this signature in large infrared sky surveys to understand what fraction of other dwarf galaxies formed this way. “We want to understand how the universe was built up,