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