Tsukuba Uchu Forum

159th Uchu Forum

The Cosmic Himalayas; An Extreme Quasar Concentration at Cosmic Noon Bridging AGNs, Galaxies, and IGM

Yongming Liang

ICRR, The University of Tokyo


Abstract
Roughly 10 billion years ago, during the Universe’s most active phase of star formation and black hole accretion, commonly known as Cosmic Noon, an extreme structure was discovered: a dense concentration of 11 luminous quasars within a comoving volume of $\sim 40$ cMpc$^3$ at redshift $z \approx 2.2$. This extreme environment, which we call the Cosmic Himalayas, was identified in the BOSSJ0210 field as part of the MAMMOTH-Subaru survey. It shows a quasar overdensity more than 30 times the cosmic average, corresponding to a $\sim 17\sigma$ outlier, assuming a Gaussian distribution of quasar number density. This quasar density is the highest known to date. In the first part of this talk, I will provide an introduction to the cosmic web, how galaxies and gas are traced at high redshift, and what different observational tracers (e.g., Ly$\alpha$ emitters, submillimeter galaxies, and quasars) reveal about galaxy evolution. I will then present the Cosmic Himalayas as a case study. Multi-wavelength data reveal a striking configuration: quasars align perpendicular to a $\sim 100$-cMpc-scale filament traced by Ly$\alpha$ emitters, yet avoid the densest galaxy nodes. Meanwhile, submillimeter galaxies detected with SCUBA-2 ($S_{850\mu\mathrm{m}} > 3$ mJy) occupy dusty starburst regions offset from both LAEs and quasars. Using SDSS/eBOSS background quasars, we construct IGM tomography maps revealing strong spatial ionization variations consistent with quasar-driven feedback. Taken together, these findings suggest an evolutionary sequence within a shared environment: from dusty starbursts to quasar phase and eventually to massive quenched galaxies. These findings highlight the importance of rare, quasar-rich environments as natural laboratories for probing the interplay between black hole growth, galaxy evolution, and the large-scale structure of the Universe. Ongoing and future work will aim to clarify the physical mechanisms that trigger quasar activity within the cosmic web, quantify the extent of AGN feedback on the surrounding intergalactic medium, and assess whether such extreme overdensities represent the early stages of massive cluster formation.


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