Astronomy and Astrophysics – Astrophysics – Cosmology and Extragalactic Astrophysics
Scientific paper
2012-04-11
Astronomy and Astrophysics
Astrophysics
Cosmology and Extragalactic Astrophysics
Submitted to ApJ, 6 pages, 1 figure, emulateapj format
Scientific paper
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 is inversely proportional to its gas consumption time. The rapid evolution of specific star formation rate (SSFR) of normal galaxies implies they had mean SLR abundances ~10 times higher at z = 2. During the epoch of Solar System formation, the mean SLR abundances of the Galaxy were twice as high as at present, if SLR yields from massive stars do not depend on metallicity. If SLRs are well-mixed with 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 would 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 molecular gas: they may decay before propagating from their origin sites, or be blown out by starburst winds. I show the enhanced ^{26}Al of starbursts can 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 disks of starbursts, and the radiogenic heating of planetesimals would likewise be much higher. In this way, galaxy evolution can affect the geological history of planetary systems.
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