Combined views of the galactic collision from the Hubble Space Telescope and the Keck-II telescope, along with the ALMA images shown in red.
Even the most powerful telescopes are limited in their ability to study very distant objects, but sometimes a fortuitous arrangement of galaxies, known as a gravitational lens, can help astronomers see farther. An international team has used gravitational lensing, along with an arsenal of telescopes at different wavelengths, to reveal an unprecedented view of a galactic collision that occurred when the Universe was just half its current age (Astron. Astrophys., doi:10.1051/0004-6361/201424410).
Gravitational lenses are caused when light from a background source, like a distant galaxy, is deflected by the strong gravity around a closer, massive cosmic structure, like another galaxy or galaxy cluster. The foreground galaxy acts as a lens, bending and magnifying the light from the distant source and allowing scientists to study the background galaxy in much greater detail than would otherwise be possible. Until recently, gravitational lenses have been very difficult to detect and found mostly by chance—the precise alignment of galaxies needed for the lens to work is quite rare. In the past few years, however, data from wide-area far-infrared and submillimeter wavelength surveys, like the Herschel Astrophysical Terahertz Large Area Survey (HATLAS), have allowed scientists to develop criteria for more easily identifying these systems.
The researchers focused on a submillimeter-bright galaxy, HATLAS J142935.3-002836, found in the HATLAS field. They imaged it with numerous telescopes, including the Atacama Large Milimeter/Submilimeter Array (ALMA), the Keck Observatory and the Karl G. Jansky Very Large Array (VLA), among others, in combination with the gravitational lens. Images from the Hubble Space Telescope and Keck revealed that the foreground, lensing object is an edge-on disk galaxy containing a large dust cloud that obscured some of the deflected light, but ALMA and the VLA were able to overcome this obstacle by observing it at longer wavelengths. ALMA in particular provided a rich dataset of information at mm wavelengths.
Combined high-resolution, multi-wavelength data from the various telescopes and observatories confirm the lensing system and show that the background object is an ongoing collision between two galaxies, forming new stars at a rate of more than 400 times the mass of the sun each year. According to co-author Shane Bussman, this far-reaching, coordinated effort has “dramatically improved our understanding of galaxy mergers when the Universe was half its present age.”