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Measurement of the small-scale structure of the intergalactic medium using close quasar pairs

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Science  28 Apr 2017:
Vol. 356, Issue 6336, pp. 418-422
DOI: 10.1126/science.aaf9346

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  • Dissecting cosmic web with gravitationally lensed quasars

    Distant quasars play important roles in studying the properties of intergalactic medium. Recently, Rorai and colleagues(1) have demonstrate a novel approach of using close quasar pairs to probe the small-scale structure of IGM, down to 100 - 500 kpc. We applaud Rorai et al for their effort, and would like to point out that by using gravitationally lensed and multi-imaged quasars, we can probe even finer structure.

    First of all, unlike quasar pairs, in lensed quasars the lights of all the images come from the very same quasar, enabling us to probe as small scale as possible. The upper limit of the scale is set by the image separation on the lens plane, which is often in the scale of 1 arcsec, translating to 10 and 30 kpc for a z=0.5 and 1.0 lens, respectively. This enables us to probe structures that are an order of magnitude finer than the close quasar pair method.


    
In addition, thanks to the lensing nature, the quasar images will be amplified, with a typical factor of 4-10. This means that we can either reduce the exposure time by a factor of 2-3, or gain up to 3 times more S/N in the spectra, to better reduce the statistic errors. This is especially helpful as the Ly-alpha forest features are faint, and often require several hours of integration even with 10-meter class telescopes.


    With the advent of dedicated imaging and spectroscopic surveys, there are more than 100 quasar lenses reported to-date. With on-going and forth-coming surveys, several...

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    Competing Interests: None declared.