The powerful space telescope JWST is peering far into space, allowing astronomers to observe distant galaxies whose light can shine upon the evolutionary history of the universe. A recent study uses JWST spectroscopy to characterize some of the first galaxies and their role in heating up the universe.
Reionization and Leaky Galaxies
A few million years after the Big Bang, the atoms of the infant universe cooled enough for stars to flicker on and galaxies to coalesce. Many of these early galaxies housed enormous young stars and massive black holes that produced such powerful radiation as to completely ionize the interstellar medium of their hosts, leaving the rest of the high-energy ultraviolet photons nowhere to go but out. Ionizing radiation leaked from these galaxies into the intergalactic medium, stripping the electrons from the previously neutral hydrogen atoms filling up space and setting alight the epoch of reionization.
While we know that massive stars and accreting black holes pump the most energetic photons into the universe, prior to JWST, observations have not been able to narrow down which of these sources primarily fueled cosmic reionization. Each ionizing source impacts the evolution of its host galaxy and the early universe in its own way, so a better understanding of the conditions in reionization-era galaxies is imperative to answer important cosmological questions. Now with JWST’s ability to observe with higher signal-to-noise and spectral resolution than ever before, the characteristics of these galaxies are well on their way to being uncovered.
Clues in Emission-Line Ratios
While reionization-era galaxies are unfortunately much too distant to directly observe their ionizing sources, we can characterize the conditions of their interstellar medium based on the relative strengths of the emission lines observed in their spectra. Especially of interest are the emission-line ratios associated with ultraviolet photons, as those photons have enough energy to ionize the universe. With this in mind, Weida Hu (Texas A&M University) and collaborators leveraged publicly available JWST data from the Cosmic Evolution Early Release Science Survey (CEERS) and the JWST Deep Extragalactic Survey (JADES) to explore the interstellar medium conditions in galaxies at redshifts between 5.6 and 9. This redshift range corresponds to roughly 500 million to 900 million years after the Big Bang — about the time in which the reionization of the universe occurred.
2D and 1D composite spectrum obtained from stacking 63 high-redshift galaxies. Emission lines of interest are labeled in blue. The lower flux UV lines are zoomed in to show their peaks. Click to enlarge. [Hu et al. 2024]
Ionization Origination
What do the measured emission-line ratios tell us about the ionizing source in reionization-era galaxies? Through comparing the ratios with those of other galaxies across a range of redshifts, Hu’s team finds that, on average, the galaxies in this study are ionized by highly energetic sources — mostly star formation with possible weak black hole activity — but the exact ionization driver is still unclear. However, based on ultraviolet diagnostics, the composite spectrum exhibits similar line ratios to local extreme dwarf galaxies, suggesting that observations of the ionization sources of nearby galaxies may provide insightful information about the conditions of galaxies from the distant past.
Plot of emission-line ratios. The composite spectrum’s position is marked with the large red circle, and it falls near local extreme star forming galaxies, green peas and blueberries. Other data points are plotted representing other galaxies across a range of redshifts. The black dashed line shows the separation between star forming galaxies (below the line) and active central black holes (above the line). Click to enlarge. [Hu et al. 2024]
Citation
“Characterizing the Average Interstellar Medium Conditions of Galaxies at z~5.6-9 with Ultraviolet and Optical Nebular Lines,” Weida Hu et al. 2024 ApJ 971 21. doi:10.3847/1538-4357/ad5015