Galaxies Collide

Collisions of galaxies are ubiquitous, reports research astronomer R. Michael Rich, whose new survey provides details on outskirts of galaxies

Collisions of galaxies are ubiquitous, reports research astronomer R. Michael Rich, whose new survey provides details on outskirts of galaxies

Michael Rich, a UCLA research astronomer and adjunct professor of physics and astronomy, University of California, Los Angeles, led an international team of astronomers that has completed the largest survey ever of the faint outskirts of nearby galaxies. The researchers have also successfully tested a low-cost system for exploring these local stellar systems. Rich’s Haloes and Environments of Nearby Galaxies (HERON) collaboration has published research in Monthly Notices of the Royal Astronomical Society. The team reports that the diameters of the galactic outskirts — the haloes — appear to correlate with the brightness and type of galaxy.

The outer regions of galaxies contain ancient stars ejected in collisions with other galaxies, as well as stars that were among the first to form in the galaxy’s history, Rich said. Understanding these regions helps trace the invisible dark matter structures enmeshed with the visible stars and gas that make up the most obvious component of a galaxy.

– the elliptical galaxy NGC474 at 91 million light-years distance has a complex of shells and streamers roughly 200,000 light-years across, caused by a collision with a smaller galaxy that has likely occurred with the last 1 billion years.

The observations suggest that collisions of galaxies are ubiquitous, and can eject stars hundreds of thousands of light years from their galaxy of origin.

The team used a relatively small 28-inch telescope based near Frazier Park, California. They observed 119 galaxies in the study, a larger dataset than any previous survey of this type. They obtained images using a CCD chip similar to that found in digital cameras, with each pixel on the chip covering a larger area of sky than a similar system in larger telescopes. The result was that in a one-hour exposure, the telescope reveals the faint shells and plumes ejected in galaxy collisions as clearly as a comparable length exposure on the 3.6-meter Canada France Hawaii Telescope. The total cost of the telescope system is economical too: approximately $150,000, which is less than 10% of the cost of similar competing projects, Rich said.

“We were surprised at how powerful a telescope of such modest size could be”, Rich said. “There

– A dwarf elliptical galaxy satellite of the spiral galaxy NGC 7331, a galaxy similar to the Milky Way’s companion, the great Andromeda galaxy M31.

will be many interesting fish to catch in this particular bay of the cosmic ocean, offering a potential lifetime of studies with very large telescopes.”

Noah Brosch, an astronomer at Tel Aviv University and a member of the team, is leading two additional studies of galaxy discs and galaxy groups, using an identical 28-inch telescope that is part of the HERON collaboration, at the Wise Observatory of Tel Aviv University, near Mitzpe Ramon, Israel. Brosch says: “The low surface brightness Universe hides many surprises amongst galaxies, small groups of galaxies, and giant clusters; we expect our two telescopes to make many discoveries.”

Code developed by Aleksandr Mosenkov of the Russian Academy of Sciences allowed the team to precisely measure the extent of the galaxies. The team found that diameter of a galaxy’s halo is correlated with a galaxy’s brightness; those galaxies brighter than the Milky Way show the largest haloes, some exceeding 600,000 light-years across — significantly larger than the Milky Way and its satellites. The largest haloes are found in the round, red galaxies known as giant elliptical galaxies that weigh as much as 1 million suns, but giant haloes can also be found in galaxies like the Milky Way that have a disc.

The nearby edge-on spiral galaxy NGC 3628 is 35 million light-years distant; the tail of material has an extent far exceeding the full diameter of the Milky Way. This illustration and the two following it have four panels that progress clockwise in illustrating fainter galaxy features.

These very faint, distant stars may trace the extent of dark matter associated with these galaxies, a possibility that the team plans to explore in the future. The team had sought a connection between evidence of a galaxy collision and the amount of star formation observed in a galaxy, but none is seen: old, red and dead galaxies are as likely to have visible collisions as a disc galaxy with a youthful burst of star formation, Rich said. Instead, collisions are mostly found to have occurred in any galaxy more luminous than the Milky Way.

The trove of images is published in the paper, and shows previously unknown faint extensions and faint companion galaxies. Among the surprises is a dwarf

The disk galaxy NGC 4762 is a member of the Virgo cluster of galaxies, roughly 60 million light-years distant. Visible in amateur telescopes as a very thin disk, successively deeper exposures reveal a distortion or “warp” in the disk.

galaxy of diameter 50,000 light years, orbiting the spiral galaxy known as NGC 7331 in the Virgo cluster of galaxies. NGC 3628 appears at first to look like the Milky Way on edge, but the survey images show its flat disc to be a small part of a system looking like a dog bone connected to a long streamer.

The team will place all of its science images into the NASA/IPAC Infrared Science Community

Archive by late October, including new observations of additional galaxies not discussed in the paper. The researchers expect that data for more than 150 galaxies will become available to anyone in the world.