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Nature's Best Magnifying Glass Views Early Spiral Galaxy

Sp1149.jpg

Credit: Karen Teramura, University of Hawai'i Institute for Astronomy

Astronomers in Hawaii have plucked unprecedented details from the life of an early galaxy using an unusually lucid gravitational lens coupled with the powerful 10-meter Keck II Telescope on Mauna Kea.
Gravitational lenses are Nature's largest telescopes, created by colossally massive clusters of thousands of galaxies that bend and magnify the light of more distant objects behind them in a way similar to a glass lens. But gravitational lenses are far from perfect. Though they make very distant galaxies from the early universe visible to telescopes, they also put the images through a cosmic blender. As a result, the smeared and distorted images don't offer much in the way of direct information about what the earliest galaxies looked like.
But that is not the case for an elegant little spiral galaxy called Sp1149, located 9.3 billion light-years away. The galaxy's image has come through a gravitational lens magnified 22 times and fairly intact, as seen in a Hubble Space Telescope image. The image was first observed in detail by the University of Hawaii's Tiantian Yuan and was initially taken by Harald Ebeling, also of Hawaii, and published by Graham P. Smith and colleagues in 2009. The giant cluster of galaxies that created the lens is located in the vast expanse of space between Sp1149 and Earth, and appears beside Sp1149 in the Hubble image.

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Title: Metallicity Gradient of a Lensed Face-on Spiral Galaxy at Redshift 1.49
Authors: T.-T. Yuan (1), L. J. Kewley (1), A. M. Swinbank (2), J. Richard (2,3), R. C. Livermore (2) ((1) IfA, Hawaii (2) Durham, UK (3) Dark Cosmology Centre, Niels Bohr Institute)
(Version v2)

We present the first metallicity gradient measurement for a grand-design face-on spiral galaxy at z~1.5. This galaxy has been magnified by a factor of 22 x by a massive, X-ray luminous galaxy cluster MACS\,J1149.5+2223 at z=0.544. Using the Laser Guide Star Adaptive Optics aided integral field spectrograph OSIRIS on KECK II, we target the Halpha emission and achieve a spatial resolution of 0.1", corresponding to a source plane resolution of 170 pc. The galaxy has well-developed spiral arms and the nebular emission line dynamics clearly indicate a rotationally supported disk with V_{rot}/\sigma~4. The best-fit disk velocity field model yields a maximum rotation of V_{rot} sin{i}=150 ±15 km s^{-1}, and a dynamical mass of M_{dyn}=1.3 ±0.2 x 10^{10}°^2(i) solar masses (within 2.5\,kpc), where the inclination angle i=45 ±10°. Based on the [NII] and Halpha ratios, we measured the radial chemical abundance gradient from the inner hundreds of parsecs out to ~5 kpc. The slope of the gradient is -0.16 ±0.02 dex kpc^{-1}, significantly steeper than the gradient of late-type or early-type galaxies in the local universe. If representative of disk galaxies at z~1.5, our results support an "inside-out" disk formation scenario in which early infall/collapse in the galaxy center builds a chemically enriched nucleus, followed by slow enrichment of the disk over the next 9 Gyr.

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