Astro Seminar
The origin of metals and dust within galaxies in the first billion years of cosmic time
Speaker: Joris Witstok (University of Cambridge)
Date: Friday 22 March 2024
Time: 15:00
Venue: N3.28/Zoom
Elements heavier than hydrogen and helium, metals, impact many facets of galaxy evolution. A significant fraction of metals condense into solid state as astrophysical dust grains which absorb, scatter, and re-emit light, reshaping the spectrum of galaxies from rest-frame UV to far-infrared (FIR) wavelengths. Interstellar dust is thus an indispensable component both in theory and observations of galaxy evolution. Dust emission is generally the primary coolant of the interstellar medium (ISM) and facilitates the gravitational collapse and fragmentation of gas clouds from which stars form, while altering the emission spectrum of galaxies from ultraviolet (UV) to far-infrared wavelengths through the reprocessing of starlight. However, the astrophysical origin of various types of dust grains remains an open question, especially in the early Universe where galaxies were previously thought to be largely dust-free. In this talk, I will present direct evidence for the presence of carbonaceous grains from the detection of the broad UV absorption feature around 2175 A in deep JWST/NIRSpec spectra of galaxies up to the first billion years of cosmic time, at a redshift of z ~ 7. This dust attenuation feature has previously only been observed spectroscopically in older, more evolved galaxies at redshifts of z < 3. The carbonaceous grains giving rise to this feature are often thought to be produced on timescales of hundreds of millions of years by asymptotic giant branch (AGB) stars, while our results suggest a more rapid evolutionary scenario for the growth of dust grains. I will further show that substantial dust production on such short timescales is required to explain the high dust yields seen in a sample of 17 galaxies in the early Universe (4 < z < 8) with well-sampled far-infrared spectral energy distributions (SEDs) compiled from the literature. The observed dust masses, whose degeneracy with dust temperature can only be mitigated with a well-constrained SED, in several sources significantly exceed the prediction of stellar dust build-up even under a highly optimistic dust yield, pointing towards additional and/or accelerated dust production channels.