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Gerard Williger (University of Louisville)

Date: Tue. February 18th, 2025, 11:30 am-12:30 pm
Location: Rock 221 Foldy Room
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The Big Ring, Giant Arc and Large Structures

Abstract:  Large and ultra-large structures provide constraints to cosmology theories.  They are distinct from clusters, which are gravitationally bound, and can include superclusters and large quasar groups.  Large structures can be considered either still experiencing their first gravitational crossing, or as relics of primordial density fluctuations, and the larger/lower average density they are, the more challenging they are to identify.  Ultra-large structures over ~350 comoving Mpc can make tension with the cosmological theory, and several diverse ones have been reported in the literature.

One way to identify large structures is via MgII absorption lines found in spectroscopic quasar surveys, such as from the Sloan Digital Sky Survey SDSS.  MgII absorbers provide a low ionization gas
content-selected sample of galaxy haloes, unbiased by stellar mass or luminosity.  Univ. Central Lancashire PhD student Alexia Lopez has gone through recent SDSS data releases and serendipitously identified two structures at redshift z~0.8, the Giant Arc and Big Ring, which are on the scale of ~10 degrees (Gpc scales).  She, her advisor Roger Clowes and I have constructed several statistical tests to quantify the significance of the structures against the rest of the SDSS database, and compare the MgII distribution with those of quasars and galaxy clusters.  We find both structures significant at roughly the 3+-1 sigma level, depending on which tests are used.  In this talk, I will review the background of large structures, describe the tests to characterize the Giant Arc and Big Ring, and briefly discuss their
implications.

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