Special Talk

• Special Talks

  Speaker: Kalina Nedkova (STScI)
  Title: PASSAGE: Measuring the Mass-Metallicity Relation for Low-Mass Galaxies at z<3.4​​​​​​​
  Abstract: Galaxy evolution is governed by the baryon cycle, which describes the complex interplay between gas accretion, star formation, chemical enrichment, and gas outflows. These processes impact galaxies’ gas-phase metallicities, making the stellar mass–metallicity relation (MZR) a key diagnostic of galaxy formation. Early JWST results suggest a flattening in the MZR slope at the low-mass end at z > 3, hinting at different feedback mechanisms in low- versus high-mass galaxies. However, these findings are limited by small low-mass galaxy sample sizes, and rely on strong-line metallicity diagnostics that require empirical calibrations. Thus, it remains unclear whether the observed flattening is physical.
  We address this with deep JWST/NIRISS grism spectroscopy from the PASSAGE survey, a pure-parallel program that completed an unbiased spectroscopic census of galaxies without photometric pre-selection. Covering 63 independent fields, PASSAGE mitigates cosmic variance and provides large, uncorrelated galaxy samples. These data enable robust MZR measurements for hundreds of individual galaxies, expanding low-mass samples by over an order of magnitude, down to M* ~ 10^6.5 Msun at z ~ 3.4—a regime near in time to where calibrations for strong-line diagnostics have been validated. In this talk, I will present preliminary PASSAGE MZR results and discuss whether the slope flattening at high redshift is due to sample limitations, nuances in strong-line calibrations, or varying feedback mechanisms with galaxy mass.

  Speaker: Calum Hawcroft (STScI)
  Title: Towards a Solution to the Massive Star Weak-Wind Problem with JWST-MIRI​​​​​​​
  Abstract: One of the biggest unsolved mysteries in the study of massive stars is the sudden weakening of spectroscopic wind emission signatures below a luminosity of log(L/Lsol) = 5.2. This phenomenon was first identified 20 years ago, and has been labeled the 'weak-wind' problem as hydrodynamical simulations of O-type star atmospheres (which match observations above log(L/Lsol) = 5.2) predict mass-loss rates two orders of magnitude higher than those required to reproduce the observed optical and UV spectra. A breakthrough moment is now happening with the detection of highly ionised fine structure emission lines (of [Ne VI], [Ne V] and [O IV]) formed in the stellar wind of late O-type stars with JWST/MIRI. These lines are able to provide strong, independent constraints on the stellar mass-loss rate and terminal wind speed of 'weak-wind’ stars. Here we present the latest results from our work to constrain the true mass-loss rates and terminal wind speeds in the domain of the 'weak-wind' problem using MIRI spectroscopic observations of late O-type stars. With these findings we gain insights into the strength and structure of the winds of low-luminosity O-type stars with broad implications in our understanding of stellar evolution, feedback, CCSN progenitors and UCHII regions.