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Guest Post: Evalyn Gates on Cosmic Magnification (or -- Invasion of the Giant Blue Space Amoebas)

Cosmic Variance
By Sean Carroll
Jul 13, 2009 4:57 PMNov 5, 2019 8:20 AM


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Scientists like to argue, contra Walt Whitman, that understanding something increases our appreciation of its beauty, rather than detracting from it. The image below, as Evalyn Gates explains, is a perfect example. Evalyn is an astronomer at the University of Chicago, and the author of a great new book on the science of gravitational lensing, Einstein's Telescope: The Hunt for Dark Matter and Dark Energy in the Universe (Amazon, Barnes & Noble, Powell's). This post is an introduction to how gravitational lensing gives us some of the most visually arresting and scientifically informative images in all of astronomy. ------------------------------------------ I had the pleasure of meeting up with Sean and some other old friends at the World Science Festival in NYC last month, and over champagne at the opening night reception (science has its benefits) Sean graciously invited me to write a guest post on gravitational lensing. It’s a broad topic, mainly because lensing is proving to be such an incredibly useful tool for many areas of cosmology and astronomy, but I have to admit that the visual beauty of the images produced by lensing is part of the appeal for me. I’m also enamored of the visceral connection between these images and lensing phenomena that all of us encounter in daily life – and the access into a complex theory that this connection affords. The giant arcs, Einstein Rings, and multiple copies of a single distant galaxy or quasar that have now been observed in hundreds of images are concrete visualizations of otherwise abstract concepts of general relativity – they effectively trace out the warps in spacetime created by massive objects, revealing the outline of the cosmos much as the technique of “rubbing” can reveal the writing on an ancient gravestone. This image, from a recent paper by Adi Zitrin and Tom Broadhurst is both scientifically and visually irresistible:

First, the image itself is really cool. The bright white/yellow galaxies are members of a cluster known as MACS J1149.5+2223, while the blue amoeba-like objects that appear to be invading the cluster are actually five images of a single distant (z ~ 1) spiral galaxy. This galaxy has been lensed by the warp in spacetime created by the cluster. Light from the galaxy, which lies almost directly behind the center of the cluster but much farther away from us, travels along several curved paths through the cluster lens, producing multiple magnified images of the galaxy. The inset box shows a computer generated model of the unlensed source galaxy, enlarged by a factor of four so that the details, including the spiral arm structure, are visible. Without the lensing power of the cluster, we would see this galaxy as a single small blue smudge. In general, lensing will both magnify and distort (shear) images of a background source. This lens is fairly unique in that we see large but relatively intact images of the spiral galaxy, which implies that the mass distribution in the central region of the cluster must be nearly uniform. The images in the upper left (#1) and lower right (#2) are especially striking. #1 is magnified but very minimally distorted, while #2, the largest image with a magnification of over 80, seems to be curling its tentacles about one of the galaxies in the cluster. A close look also reveals the negative parity (mirror symmetry) of the remaining three images – the spiral arms appear to circle in the opposite direction – as expected from lensing. The total magnification of the distant galaxy (the sum of all five images) is about 200, the largest known to date – supporting the authors’s claim that this is “the more powerful lens yet discovered.” This is not just a pretty picture, however – the image packs a lot of scientific information. The authors extract the mass distribution in the cluster (which has implications for cosmological models), measure the mass-to-light ratio of the bright galaxy in the center of the cluster, and use the magnifying power of the lens to search for even more distant galaxies. The basic idea is to construct a model of the lens, starting with the cluster galaxies and a dark matter halo; then refine the model to reproduce the multiple images that are seen. Using this refined model it’s possible to predict the location of additional images of a given source, and to identify regions of high magnification that can then be examined for multiple images of other sources. Any additional images that are found can be used to further refine the model and so on. For example in this system, image #1 is “delensed” to obtain an image of the source galaxy; this model source image is then relensed and the resulting multiple images are compared (size, shape and location) with the observed images. The agreement between observed and modeled images is excellent. Using this lens model nine additional multiply-lensed galaxies (all fainter and at a higher redshift than the spiral galaxy) were found. In total there are 33 images of 10 background source galaxies. So what does this tell us? The model of the lens outlines the (projected 2D) mass profile of the cluster – which doesn’t seem to agree with numerical simulations for clusters, assuming a standard ΛCDM cosmology. The mass concentration in the center of the cluster is higher than predicted, a result that has also been found for other massive clusters studied with gravitational lensing. This implies that we’re either missing some physics in our simulations, or we may need to modify our cosmological model. And I suspect that we will hear from this lens again. The most distant galaxy discovered to date, at a redshift of z ~ 7.6, was found courtesy of the cluster lens A1689, and MACS J1149 offers another powerful magnifying glass through which to search for the earliest galaxies in the universe.

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