Investigating active galactic nuclei in low-mass systems: feedback and seeds of supermassive black holes

Abstract

Massive black hole (BH) populations in dwarf galaxies and their outflows provide key insights into supermassive BH seed formation and galaxy evolution. This dissertation examines active galactic nuclei (AGNs) in low-mass systems through two projects. First, I identify 388 new AGN candidates in low-mass galaxies (M x < 10 10M circle dot and z < 0.3) from 23,460 Galaxy and Mass Assembly (GAMA) survey spectra and using four optical diagnostics: two narrow emission-line ratio diagnostic diagrams and two high-ionization coronal lines. Of these, 47 exhibit broad H alpha emission, indicating virial BH masses of M BH ~ 10 5 - 10 7.7M circle dot. This multi-diagnostic approach reveals a diverse AGN population, from blue star-forming dwarfs to "miniquasars", extending prior Sloan Digital Sky Survey searches to higher redshifts and new sky regions. Second, from 39,612 GAMA galaxies, I detect 398 with outflow signatures, of which 45 are among low-mass objects. I searched for second velocity components in the [O III] lambda lambda4959, 5007 doublet emission-line profiles and found that the outflow velocities are generally faster in AGNs than those in star-forming galaxies across massive and low-mass systems. This work emphasizes the key role of AGN- driven outflows in low-mass galaxies, requiring their integration into galaxy evolution models. These results advance our understanding of BH seeding and feedback in low-mass regimes.