The Physical Conditions of Low-Mass Galaxies at z=3.5-7.0 from JWST Spectroscopy: The Behaviour of Spectral Line Ratios with Burstiness

Abstract: We present a study of the physical conditions of galaxies at redshifts $z=3.5-7.0$, based on emission-line diagnostics of JWST NIRSpec spectra for 1889 star-forming sources spanning more than 4~dex in stellar mass. We find that the median H$α$/H$β$ and [OIII]/H$β$ are constant throughout these redshifts, indicating a slow evolution of the interstellar medium conditions from the Epoch of Reionization to Cosmic Noon. The stellar-mass evolution of these line ratios follow the expected trend for chemical enrichment and dust build-up, with \ohb~being maximum for galaxies with $$\rm M_{\star} \approx 10⁹ \, \rm M_\odot$$, and the medians of H$α$/H$β$ and [OII]/H$β$ monotonically increasing from $\rm{M}<em>{\rm \star} \approx 10<sup>7$ through $\approx 10^{10}\, \rm M</em>\odot$. Focusing on the 1552 low stellar-mass ($&lt;10⁹ \, \rm M_\odot$) galaxies, we particularly investigate the behaviour of the main spectral-line ratios with burstiness. We find a significantly higher degree of scatter in [OIII]/H$β$ and [OII]/H$β$ for high-burstiness than for low-burstiness galaxies. Values of log([OIII]/H$β$)~$\gtrsim 0.8$ are almost exclusively seen amongst those with high burstiness. Meanwhile, log([OIII]/H$β$)~$\lesssim 0.3$ are virtually not found amongst high-burstiness galaxies, except at the lowest stellar masses $\mathrm{log(M_\star/M_\odot)}&lt;7.5$. Our results are consistent with a scenario in which star-formation bursts produce temporary chemical enrichment that is quickly lost via stellar winds. They also suggest that metallicity measures in bursting galaxies are unstable, and that the search for the most metal-poor objects should optimally be conducted amongst low-burstiness or very low-stellar mass sources.