Near Perfect “Einstein Ring” Discovered

by Fraser Cain on April 29, 2005

Near perfect “Einstein Ring” gravitational lens. Image credit: ESO/VLT. Click to enlarge.
This is Einstein’s Year. One-hundred years ago a little known Swiss patent clerk in the very early years of a scientific career was confronted with a series of paradoxes related to time and space, energy and matter. Gifted with a profound intuition and a powerful imagination, Albert A. Einstein rose out of obscurity to present an entirely new way of looking at natural phenomenon. Einstein showed us all that time had very little to do with clocks, energy has less to do with quantity and more to do with quality, space was not just ?a big square box to put stuff in”, matter and energy were two sides of the same cosmic coin, and gravity had a profound effect on everything – light, matter, time, and space.

Today we use all these principles ? enunciated a century ago – to probe the most distant things in the Universe. Because of Einstein’s investigation of the photoelectric effect, we now understand why light is not continuous but curiously riddled with dark and bright lines telling us when that light was emitted, what emitted it. and the kinds of things touching it in its travels. Because of Einstein’s insight into the conversion of mass and energy, we now understand how distant suns illuminate the cosmos, and how powerful magnetic fields whip particles up to stupendous speeds later to come crashing down on the Earth’s atmosphere. And because gravity is now understood to influence everything, we have learned how distant objects can capture and focus light from even more distant objects.

Although we have yet to find an absolutely perfect instance of gravitational lensing in the Universe, today we are much closer to that ideal. In a paper entitled “Discovery of a high red-shift Einstein Ring” published April 27, 2005, Remi Cabanac of Canada-France-Hawaii Telescope, in Hawaii and associates “report the discovery of a partial Einstein ring … produced by a massive (and seemingly isolated) elliptical galaxy.” Previous to this find, the most complete Einstein ring discovered was documented in 1996 by S.J. Warren of the Imperial College in London. That ring – also one of the few visible in optical light – is slightly less than a half-circle in circumference (170 degrees).

Remi Cabanac explained that he “discovered the system while observing at the European Southern Observatory Very Large Telescope in Chile with a spectro imager called FORS1.” Remi says he was fullfilling his responsibilties as a service astronomer, “observing for Helmut Jerjen (co-author of the paper) doing deep imaging of nearby dwarf galaxies in the outskirts of a well-known nearby galaxy cluster in Fornax.” Remi continued to say that his “eye got attracted by the very unusual bright arc in the northwest of the field, I knew it was something pretty amazing because lensing arcs are usually very dim, and I was observing in red band whereas arcs are usually blueish.”

To confirm his suspicions of a new discovery Remi “went to the astronomical database but nothing existed under the coordinates.” Later Remi consulted with “Chris Lidman (another co-author and lens expert) and showed him the image. He couldn’t believe it was a lens at first because it was so bright and conspicuous, Chris thought it could be an artefact on the image.” With Chris’ support, Remi “applied for spectroscopic follow-up and realized that it was both a true gravitational lense and a very significant discovery, because the background source was highly amplified and very far away.”

According to the paper, the ring inscribes a “C-shaped” circle of 270 degrees in near-complete circumference with an apparent radius of slightly more than 1 3/4 arc seconds – roughly the size of a star’s “virtual” image seen at high power through a small amateur telescope. The lens galaxy is a giant elliptical similar to M87 in the Virgo-Coma cluster. The lens lies some 7 billion light years distant in the direction of the constellation Fornax (visible from warmer temperate northern hemisphere and southern hemisphere skies). The source galaxy bears a red shift of 3.77 – suggesting a recessionary distance of roughly 11 BLYs. Source and lens galaxy have received the designation FOR J0332-3557 3h32m59s, -35d57m51s and lie proximate to the Fornax galaxy cluster – but well beyond it in terms of real space.

What makes this particular discovery so interesting astronomically is the fact that the lens galaxy is very massive, is in a period of star-birth quiescence, lies at such a great distance from the Earth, and may be isolated from other cluster galaxies in its own spatial locale. Meanwhile the source galaxy is significantly brighter (by one absolute stellar magnitude) than other Lyman break galaxies (galaxies that red-shift the Lyman Break at 912 angstroms into the visible part of the spectrum), is poor in emission line spectra, and recently had completed a cycle of rapid star birth (“starburst”). All these factors combined mean that FOR J0332 could provide a wealth of data concerning galaxy formation before the current inflationary epoch of the Universe.

According to the science team, “One of the key issues in galaxy formation within the current LCDM (Lambda Cold Dark Matter) framework of structure formation is the mass assembly histories of galactic halos.” Current thinking is that galaxies accumulate halo mass – that huge spherical bulge of low luminosity matter surrounding galactic cores – before star formation really kicks in. One way to investigate this idea is to determine how mass-to-light ratios change over time as galaxies evolve. But to do that you need to sample the masses and luminosities of as many galaxies as possible, of a variety of types, over the broadest possible range of space and time.

The discovery of FOR J0332 – and the three other partial Einstein ring objects – helps astronomers by adding examples of galaxies normally undetectable at great distances. From the paper, “Various deep surveys have uncovered different galaxy populations, but the selection criteria produced biased samples: UV-selected and narrow-band selected samples are sensitive to actively star-forming galaxies and biased against quiescent, evolved systems while sub-millimeter and near-infrared surveys select dusty starburst galaxies and very red galaxies respectively.”

What conclusions can we draw based on this discovery?

Remi underscores the significance of this find by saying “The source amplified by the lens is the galaxy with the brightest apparent luminosity ever discovered at such a distance. It will give us unique information on the physical conditions prevailing in the interstellar medium when the universe was only 12% of its present age. The shape of the source is also very important because it gives the amount of mass within the lens at a redshift of z=1. Only a handful of Einstein rings have been discovered at such high redshift. It will give an important measurement at how elliptical galaxy mass evolved through time.”

Written by Jeff Barbour

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Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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