The age of stars reveal the story of the collision between galaxies

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MCG+08-11-002 is an odd-looking galaxy with a spectacular dark band of absorbing dust in front of the galaxy's centre, making it resemble a "Black Eye". Scientists believe that it is the remnant of an earlier collision of two separate galaxies. This peculiar galaxy is at the centre of a rich field of foreground stars, close to the plane of our own Milky Way galaxy. MCG+08-11-002 is about 250 million light-years away in the constellation of Auriga, the Charioteer. This image is part of a large collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24th April 2008.Interacting galaxy pairs are spectacular events where normal galaxies approach and collide with each other under the influence of gravity, where their gaseous content loses angular momentum and funnels towards the centers of these merging systems in the process. This gas provides the material that eventually triggers the growth of supermassive black holes as well as bursts of star formation. As such, these galaxy mergers are the ideal laboratories for studying the buildup of stellar mass and the relationship between star formation and black hole activity in galaxies. 

Accurate classification during the merger stage is required to understand how star formation and black hole activity evolve over the course of the collision, and one advantageous approach involves studying how stars formed as time progressed in these galaxies by determining the ages of star clusters found throughout the galaxy system.  Through knowing the age of the stars in the merging system, we can then trace the interaction history of the progenitor galaxies. This requires high resolution infrared observations that would allow us to resolve individual star clusters through the dust obscuration in these merging galaxies. 

The team from the Australian National University, the University of California (Riverside + Santa Cruz) and the University of Hawaii has acquired imaging and spectroscopic data of one such galaxy merger, MCG+08-11-002, using the NIRC2 and OSIRIS instruments on the Keck Telescopes. With the combination of high resolution near-infrared images and integral-field spectroscopic data, researchers were able to identify 25 star clusters around MGC+08-11-002 and deduce most of their ages through comparisons of the stellar absorption line widths to those predicted by computer models of star formation. We found that these star clusters peak at ages younger than 20 million years, and that there may be one or two bursts of star formation in the history of interaction. This analysis offers valuable insights into the timelines of galaxy interactions and may therefore play an important role in the future development of precise classification of systems due to their merger state.

Postdoctoral scholar Vivian U from UC Riverside shares authorship of the research paper with colleagues  of the Australian National University, the University of California (Riverside + Santa Cruz) and the University of Hawaii.

The paper can be found here:

Figures: [upper right] Galaxy MCG+08-11-002 in the visual band by NASA/ESA/HST; [bottom] Keck Telescopes by Laurie Hatch.keck