A study of 37 nearby dwarf galaxies found they retain an average 10.7% of the oxygen their stars produced, leaving most of the estimated output unaccounted for in measured reservoirs.
Retention estimates ranged from 4% to 25%. The sample included gas-rich, star-forming galaxies less than 6 megaparsecs away, with stellar masses between 10^6.5 and 10^9.5 solar masses. Nearly all the oxygen the researchers accounted for was in gas.
The team compared estimated oxygen production with the oxygen found in the galaxies’ stars, gas and dust. Dwarf galaxies have weaker gravitational wells than larger galaxies, so researchers expect them to lose more heavy elements through outflows. Hubble’s optical images helped resolve individual stars and reconstruct the galaxies’ star-formation histories; the researchers also used measurements from other observatories. NASA said the paper was published in The Astrophysical Journal on Nov. 2.
The inventory does not establish where most of the missing oxygen is. Comparisons with earlier studies of low-mass galaxies suggest that highly ionized oxygen in surrounding gas may account for 1% to 7% of the oxygen produced by stars in galaxies like those in the sample. An adjustment for ionization could raise that estimate to 5% to 35%, but the authors cautioned that the figure is uncertain. They could not determine whether most of the remaining oxygen is in surrounding gas or in the space between galaxies.
Three cosmological simulations estimated higher metal retention than the study measured, while reproducing the observed relationship between galaxy mass and metal abundance. The authors said differing assumptions make direct comparisons difficult. Their models suggest winds and reduced gas accretion could help explain the low retention estimates, but do not show where the oxygen went.
Kristen Wingfield McQuinn, the GLOW principal investigator, said the oxygen inventory can help researchers understand how galaxies evolve.
