Starbursts and High-Redshift Galaxies are Radioactive: High Abundances of ${}^{26}$Al and Other Short Lived Radionuclides

Abstract

Short lived radionuclides (SLRs) like ${}^{26}$Al are synthesized by massive stars and are a byproduct of star formation. The abundances of SLRs in the gas of a star-forming galaxy are inversely proportional to the gas consumption time. The rapid evolution of specific star formation rate (SSFR) of normal galaxies implies they had mean SLR abundances ~3--10 times higher at z = 2. During the epoch of Solar system formation, the background SLR abundances of the Galaxy were up to twice as high as at present, if SLR yields from massive stars do not depend on metallicity. If SLRs are homogenized in the gas of galaxies, the high SSFRs of normal galaxies can partly explain the elevated abundance of SLRs like ${}^{60}$Fe and ${}^{26}$Al in the early Solar system. Starburst galaxies have much higher SSFRs still, and have enormous mean abundances of ${}^{26}$Al (${}^{26}$Al/${}^{27}$Al ~ $10^{-3}$ for Solar metallicity gas). The main uncertainty is whether the SLRs are mixed with the star-forming molecular gas: they could be trapped in hot gas and decay before entering the colder phases, or be blown out by starburst winds. I consider how variability in star-formation rate affects the SLR abundances, and I discuss how SLR transport may differ in these galaxies. The enhanced ${}^{26}$Al of starbursts might maintain moderate ionization rates ($10^{-18}$ -- $10^{-17}$ s${}^{-1}$), possibly dominating ionization in dense clouds not penetrated by cosmic rays. Similar ionization rates would be maintained in protoplanetary discs of starbursts, if the SLRs are well-mixed, and the radiogenic heating of planetesimals would likewise be much higher. In this way, galaxy evolution can affect the geological history of planetary systems.

Published in MNRAS, 12 pages, 3 figures

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