Cold quasars are a rare population of luminous, unobscured quasars associatedwith host galaxies that have a high star formation rate. We aimed to study thehost galaxies of sixty four of these cold quasars in order to probe how thesupermassive black holes and host galaxies were coevolving. We compiled datafrom the XXL survey and crossmatched with the VHS, WISE, and HerMES surveys toobtain multiwavelength photometry spanning the Xray to the infrared andincluding optical spectroscopy. From the data, we calculated the supermassiveblack hole masses using broad emission from the magnesium II and hydrogen betalines. We compared this with the stellar mass of the entire galaxy and findthat the black holes are significantly more massive than would be predicted bylocal relations, indicating that the majority of black hole growth precedes thebulk of the the stellar mass formation. In addition to this, we created aspectral energy distribution for each galaxy to calculate the star formationrate. We compared the star formation rate with the black hole accretion rateand find that the stellar mass is rapidly increasing at a relative rate fasterthan the black hole growth, supporting the picture where the black hole growsfirst.

The study of gas-phase metallicity and its spatial distribution at highredshift is crucial to understand the processes that shaped the growth andevolution of galaxies in the early Universe. Here we study the spatiallyresolved metallicity in three systems at $z\sim6-8$, namely A2744-YD4,BDF-3299, and COSMOS24108, with JWST NIRSpec IFU low-resolution ($R$ $\sim$100) spectroscopic observations. These are among the highest-$z$ sources inwhich metallicity gradients have been probed so far. Each of these systemshosts several spatial components in the process of merging within a fewkiloparsecs, identified from the rest-frame UV and optical stellar continuumand ionised gas emission line maps. The sources have heterogeneous properties,with stellar masses log($M_*/M_\odot) \sim 7.6-9.3$, star formation rates(SFRs) $\sim1-15$ $M_\odot$ yr$^{-1}$, and gas-phase metallicities 12+log(O/H)$\sim 7.7-8.3$, which exhibit a large scatter within each system. Theirproperties are generally consistent with those of the highest-redshift samplesto date ($z\sim3-10$), though the sources in A2744-YD4 and COSMOS24108 are atthe high end of the mass-metallicity relation (MZR) defined by the $z\sim3-10$sources. Moreover, the targets in this work follow the predicted slope of theMZR at $z\sim 6-8$ from most cosmological simulations. The gas-phasemetallicity gradients are consistent with being flat in the main sources ofeach system. Flat metallicity gradients are thought to arise from gas mixingprocesses on galaxy scales, such as mergers or galactic outflows and SN windsdriven by intense stellar feedback, which wash out any gradient formed in thegalaxy. The existence of flat gradients at $z\sim6-8$ sets also importantconstraints on cosmological simulations, whose predictions on the cosmicevolution of metallicity gradients often differ significantly, especially athigh redshift.