Most distant gravitational lens helps weigh galaxies
– but also deepens a galactic mystery
A team of astronomers led by Arjen van der Wel from the Max Planck Institute for Astronomy (MPIA) has found the most distant gravitational lens yet – a galaxy that, as predicted by Albert Einstein's general theory of relativity, deflects and intensifies the light of an even more distant object. The discovery provides a rare opportunity to directly measure the mass of a distant galaxy. But it also poses a mystery: Lenses of this kind should be exceedingly rare. Given this and recent other finds, astronomers either have been phenomenally lucky – or, more likely, they have underestimated substantially the number of small, very young galaxies in the early universe.
Figure 1: Hubble Space Telescope image of J1000+0221, the most distant gravitational lens discovered to date. Light from the massive object that is acting as a lens needs 9.4 billion years to reach us (z=1.53). The foreground galaxy (the lensing mass) shows up in orange, and the background galaxy that is magnified by the lens into an Einstein Ring is seen in blue.
The diameter of the Einstein ring is 0.7 arcseconds (corresponding to a size of 19,000 light-years at the distance of the lens) – less than 1/2500th of the diameter of the full Moon, and at the very limit of what ordinary Earth-based telescopes would be able to image. The object is so small that the pixel structure of the detector chip becomes visible.
The color image was created from three separate images from two different instruments aboard the Hubble Space Telescope: two near-infrared images from the Wide Field Camera 3 (Credit: NASA, ESA, and CANDELS), and one image from the Advanced Camera for Surveys (Credit: NASA, ESA, CANDELS and COSMOS.
Light is affected by gravity, and light passing a distant galaxy will be deflected as a result. Since the first find in 1979, numerous such gravitational lenses have been discovered. In addition to providing tests of Einstein's theory of general relativity, gravitational lenses have proved to be valuable tools. Notably, one can determine the mass of the matter that is bending the light – including the mass of the still-enigmatic Dark Matter, which does not emit or absorb light and can only be detected via its gravity. Also, the lens magnifies the background light source, acting as a "natural telescope" that allows astronomers a more detailed look at distant galaxies than what is normally possible.
Gravitational lenses consist of two objects: One that is further away and supplies the light, and the other, the lensing mass or gravitational lens, which sits between us and the distant light source, and whose gravity deflects the light. When the observer, the lens, and the distant light source are precisely aligned, the observer sees an "Einstein ring": a perfect circle of light that is the projected and greatly magnified image of the distant light source.
Now, astronomers have found the most distant gravitational lens yet. MPIA's Arjen van der Wel explains: "The discovery was completely by chance. I had been reviewing observations from an earlier project with the goal of measuring masses of old, distant galaxies by looking at the motion of their stars. Among the galaxy spectra" – the rainbow-like split of a galaxy's light into myriads of different shades of color – I noticed a galaxy that was decidedly odd. It looked like an extremely young galaxy, and at an even larger distance than I was aiming for. It shouldn't even have been part of our observing program!
Van der Wel followed up the spectra, which were taken with the Large Binocular Telescope in Arizona, by looking at images taken with the Hubble Space Telescope as part of the CANDELS and COSMOS surveys. The object looked like an old galaxy, a plausible target for the original observing program, but with some irregular features which, he suspected, meant that he was looking at a gravitational lens. Combining the available images and removing the haze of the lensing galaxy's collection of stars, the result was very clear: an almost perfect Einstein ring, indicating a gravitational lens with very precise alignment of the lens and the background light source (0.01 arcseconds).
The lensing mass is so distant that the light, after having been deflected, has traveled 9.4 billion years to reach us (redshift z = 1.53; compare this with the total age of the universe of 13.8 billion years). The previous record holder was found thirty years ago, and it took less than 8 billion years for its light to reach us (z ∼ 1).
Not only is this a new record, the object also serves an important purpose: The amount of distortion caused by the lensing galaxy allows for a direct measurement of its mass. This provides an independent test for astronomers' usual methods of estimating distant galaxy masses – which rely on extrapolation from their nearby cousins. Fortunately for astronomers, their usual methods pass the test.
But the discovery also poses a puzzle. Gravitational lenses are the result of a chance alignment. In this case, the alignment is very precise. To make matters worse, the magnified object is a so-called star-bursting dwarf galaxy: a comparatively light galaxy (only about 100 million solar masses' worth of stars), but extremely young (about 10 – 40 million years old) and producing new stars at an enormous rate (cf. MPIA press release 2011-11-10). The chances for such peculiar galaxies to be gravitationally lensed are very small. Yet this is the second star-bursting dwarf galaxy found to be lensed. Either the astronomers have been phenomenally lucky. Or starbursting dwarf galaxies are much more common than previously thought, forcing astronomers to re-think their models of galaxy evolution.
Van der Wel concludes: "This has been a weird and interesting discovery. It was a completely serendipitous find, it combines two rather disparate topics I have been working on – massive, old galaxies, and young, starbursting dwarfs –, and it has the potential to start a new chapter in our description of galaxy evolution in the early universe."
The team is composed of Arjen van der Wel, Glenn van de Ven, Michael Maseda, Hans-Walter Rix (all Max Planck Institute for Astronomy [MPIA]), Gregory Rudnick (University of Kansas and MPIA), Andrea Grazian (INAF), Steven Finkelstein (University of Texas at Austin), David Koo, Sandra M. Faber (both University of California, Santa Cruz), Henry Ferguson, Anton Koekemoer, Norman Grogin (all STScI), and Dale Kocevski (University of Kentucky).
The Large Binocular Telescope (LBT) on Mount Graham (Arizona, USA), which combines two giant 8.4 meter mirrors on a single mount, is a collaboration among the Italian astronomical community (National Institute of Astrophysics – INAF), The University of Arizona, Arizona State University, Northern Arizona University, the LBT Beteiligungsgesellschaft in Germany (Max-Planck-Institut für Astronomie in Heidelberg, Zentrum für Astronomie der Universität Heidelberg, Astrophysikalisches Institut in Potsdam, Max-Planck-Institut für Extraterrestrische Physik in Munich, and Max-Planck-Institut für Radioastronomie in Bonn), The Ohio State University and Research Corporation (Ohio State University, University of Notre Dame, University of Minnesota, and University of Virginia).
LUCI, the Large Binocular Telescope Near-infrared Utility with Camera and Integral Field Unit, is a multi-purpose instrument combining a large field of view with a high resolution and the capability of simultaneous spectroscopy of about two dozen objects in the infrared through laser-cut slit-masks. The LUCI instruments have been built by a consortium of five German institutes led by the Center for Astronomy of Heidelberg University (Landessternwarte Heidelberg, LSW) together with the Max Planck Institute for Astronomy in Heidelberg (MPIA), the Max Planck Institute for Extraterrestrial Physics in Garching (MPE), the Astronomical Institute of the Ruhr-University in Bochum (AIRUB) as well as the University of Applied Sciences in Mannheim (Hochschule Mannheim).
The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is a powerful survey of the distant universe being carried out with the Hubble Space Telescope (HST). The largest project in the history of Hubble, it has been allocated observing time amounting to 900 of the space telescope's orbits around Earth. Taken together with other observations covering the same region, the CANDELS researchers can use what amounts to a combined exposure time of nearly 4 months of Hubble data. CANDELS uses two instruments aboard the HST: the near-infrared WFC3 camera and the visible-light ACS camera. Jointly, these two cameras give unprecedented coverage of galaxies from optical wavelengths to the near-infrared. This will allow CANDELS to study different stages in the formation of galaxies, from the first billion years of cosmic evolution to the present.
Which telescopes and instruments were used in the study?
The initial find was made in spectra taken with the LUCI near-infrared imager/spectroscope. The study also used images taken with the Wide Field Camera 3 and the Advanced Camera for Surveys on board the Hubble Space Telescope (HST/WFC3 and HST/ACS) as part of the CANDELS survey, and HST/ACS images from the COSMOS survey. Distortion by the telescope optics was removed ("deconvolution") for the images shown here.
What is the geometry of the newly discovered gravitational lens?
Based on distance measurements of the lens and the background source, van der Wel concluded that 11.8 billion years ago a very young galaxy emitted light roughly in our direction. Then, 2.4 billion years later, a massive galaxy acting as gravitational lens, deflected the path of the light by 0.35 arcseconds, and another 9.4 billion later the light reached our telescopes. The light deflection happened such that 40 times more light arrived than would have been the case in the absence of the lens. The 9.4 billion year light travel time, corresponding to redshift z=1.53, turns out be the largest for any known galaxy lens, breaking the previous record by almost 2 billion years.
What is new/important about the results?
This is the most distant lensing mass found so far. (As for objects whose images we see thanks to a gravitational lens, more distant examples have been found. In fact, the most distant object known is gravitationally lensed by a massive cluster of galaxies – cf. http://www.spacetelescope.org/news/heic1217/)
The previous record-holder was the triple radio source MG2016+112, discovered almost 30 years ago at a redshift z = 1 (Lawrence et al. 1984; Schneider et al. 1986), no strong lenses at higher redshifts have been found, despite the large number of z > 1 lenses discovered since then (e.g., Bolton et al. 2006; Faure et al. 2008; More et al. 2012). Here, we are not counting a handful of tentative z ∼ 1.2 candidates (More et al. 2012) for which it is doubtful whether or not they are gravitational lenses at all. All of the above, as well as the record, refers to strong lenses, where the gravitational lensing effect is so pronounced that multiple images or arcs, or Einstein rings, are produced. On a cosmological scale, weaker distortion effects, caused by the large-scale inhomogeneities of matter distribution in the universe, but not traceable to specific objects, can be detected.
Since the mass of a lensing system can be deduced from the distortion of the lensed image, the find provides a much-needed consistency check for the usual methods of determining distant galaxy masses. Those are based on colors/luminosities of galaxies, which in turn are related to stellar mass; the total mass is then estimated using relations derived from galaxies that are less distant, and whose mass can be measured by other means. This is potentially problematic because of the intervening billions of years of galaxy evolution. Thus, van der Wel 's discovery provided a way to put standard methods of galaxy mass determination to the test – showing that those methods are on the right track.
The fact that we even see the lens poses an interesting puzzle. Going by probability estimates alone, such chance alignments are very rare – much more rare for objects in the early universe than at later times. By chance, one would expect about five gravitational lenses showing galaxies of this kind in the whole area covered by the CANDELS survey – and there is only a chance of 1 in 200 to find one so precisely aligned as the one actually found! Either the astronomers are very lucky to find the lens. Or else, chance alignments with these components – in particular the distant starburst dwarf galaxy – are more probable than previously thought, due to the fact that the distant starburst dwarfs are much more numerous than estimated based on current observations. If the latter, then there could be a whole population of such galaxies just below the detection threshold of the CANDELS survey. The existence of this population could force major revisions of our current models of the early stages of galaxy evolution.