Galaxies and Cosmology Seminar

Jules Levanti, The University of Texas at Austin

Distinguishing Ionization Sources in RESOLVE & ECO Dwarf AGN: Wolf-Rayets and ULXs Minimally Contribute to a Primarily [O I]/Hα Selected Sample

Black hole (BH) seeding mechanisms at high-z are difficult to probe due to the intrinsic faintness of distant sources. Studying intermediate-mass black holes (IMBHs) to determine the active galactic nuclei (AGN) occupation fraction in dwarf galaxies at z ∼ 0 provides a constraint on high-z BH formation channels. However, ultra-luminous X-ray sources (ULXs) exist in low-metallicity environments, like dwarf galaxies, and produce ionizing spectra that can mimic optical emission lines used to identify AGN, complicating IMBH detections. We apply MULTIGRIS, a Bayesian framework that evaluates emission line fits across model grids, to a sample of primarily [O I]/Hα-selected star-forming AGN (SF-AGN) in dwarf galaxies from the volume-limited RESOLVE and ECO surveys. About 61% of our sample is best fit by the model grid that includes stellar, AGN, and ULX ionizing components (stars+AGN+ULX), suggesting that including AGN and ULXs as free parameters may be essential for accurate inferences in dwarf galaxy parameters. For the stars+AGN+ULX model grid, ≳ 99% of optical emission line flux predictions fall within 3σ of the observed values. We find that model grids with stars and ULXs combined, or pure shock excitation, are unlikely to reproduce the interstellar medium signatures in our sample; however, more complex combinations of ionization sources cannot be ruled out, such as shocks in addition to a triggering source. The inferred gas-phase metallicity decreases by 0.1 dex when including an additional excitation source beyond stars, such as a ULX or AGN. Our results enable us to refine the current SF-AGN occupation fraction from 6.5% to a range of 3.9%-6.5%, highlighting the importance of simultaneously analyzing multiple emission lines to identify active IMBHs rather than strictly applying demarcations from strong-line diagrams.

Junehyoung Jeon, The University of Texas at Austin

Formation and Evolution of Supermassive Black Holes in the Early Universe

The James Webb Space Telescope (JWST) has identified abundant supermassive black holes (SMBHs) already during the first few hundred million years of cosmic history, including a new population of objects known as the Little Red Dots (LRDs) proposed to be SMBH systems. To understand how they came to be, we use the cosmological code GIZMO to study SMBH seed formation considering two scenarios: low-mass stellar remnant and high-mass direct collapse black hole (DCBH) seeds. Forming the massive SMBHs observed at early times requires massive seeds accreting more efficiently than the fiducial Bondi–Hoyle model. Furthermore, using the semi-analytical code A-SLOTH, we show both seeding channels can reproduce the SMBH population and mass function observations at z∼5–6 but diverge at z∼10. Specifically, JWST observations have the potential to constrain the fraction of super-Eddington SMBHs as well as the existence and prevalence of DCBHs, particularly through blank-field ultra-deep or gravitational lensing surveys. Such observations will provide key insights into SMBH formation and evolution during the emergence of the first galaxies. Notably, we find that the DCBH population can reproduce the observed LRD population statistics, with individual extreme DCBH systems reproducing spectral properties of extreme LRDs.