Astronomy Colloquium

Observations of resolved stars in nearby galaxies anchor our knowledge of the distant Universe (e.g., stellar evolution, the distance scale, initial mass function) and encode the formation history of the local Universe. In this talk, I will highlight some of the broad science enabled by resolved star studies, with a particular emphasis on new insights into the formation histories of the lowest-mass, lowest-metallicity galaxies in the Universe. I will present results from my 1000+ orbit Hubble Space Telescope Treasury survey of M31 satellite galaxies, which has revealed a previously unknown channel for low-mass galaxy growth. I will describe my team's work on expanding resolved star "galaxy archaeology" science from dozens of galaxies in the Local Group (D~1 Mpc) to thousands of galaxies at much larger distances (D~50 Mpc) using facilities such as the James Webb Space Telescope (JWST), Euclid, and Roman. Finally, I will preview "J-Virgo", my team's new JWST Treasury program that is undertaking the first systematic study of galaxy evolution in a massive galaxy cluster, Virgo, using resolved stellar populations.

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I will first review the progress in surveys of the most distant quasars, including the latest record-breaking quasars discovered by Euclid at z>7.5. They are powered by billion solar mass black holes, possible only by a combination of massive early black hole seeds with highly efficient and sustained accretion. I will present results of surveys of early quasars and their environments using JWST. While rapid early black hole growth is accompanied by intense star formation and feedback in their host galaxies, the diverse quasar environment unveiled by these observations suggests a complex interplay between black hole accretion, galaxy assembly, the physics of reionization and the emergence of early large scale structure. JWST observations have revealed a new population of active galactic nuclei (AGN), the "Little Red Dots" (LRDs), with high abundance and multiwavelength properties different from those of typical AGN and quasars. I will discuss the observations of LRDs and constraints on their physical nature in relation to early black hole growth. A subset of LRDs, both those discovered at high-redshift with JWST, and detected in the local universe, strongly suggest the presence of optically-thick gas envelops surrounding the central energy source, consistent with scenarios of super-Eddington accretion, and could be the missing link between black hole seeds and luminous early quasars.