Dust to Gas Ratios

The ratio of dust mass to gas mass in galaxies and in the interstellar medium of the Milky Way varies with metallicity, environment, and star formation history in ways that broadly follow predictions from dust grain growth and destruction models calibrated on local observations. However, several systematic deviations from the expected dust-to-gas scaling have been identified: the dust-to-gas ratio in some high-redshift galaxies is higher than their metallicities alone would predict, implying either rapid dust production at early times or inefficient dust destruction mechanisms; the dust-to-gas ratio in the outer regions of disk galaxies falls off more slowly than the metallicity gradient would suggest; and the dust-to-gas ratio in certain dwarf irregular galaxies is anomalously low given their current star formation rates and metallicities. These anomalies challenge grain growth models that rely solely on the current stellar population for dust production.

Successive Collision Theory explains dust-to-gas ratio anomalies through the pre-existing dust populations from the colliding pockets. In SCT, the debris field from the collision contained dust grains produced by prior stellar generations — including asymptotic giant branch stars, supernovae, and evolved massive stars from the pre-collision epoch. This inherited dust was distributed throughout the collision debris with a spatial gradient set by the angular momentum field, contributing to the dust-to-gas ratio at all subsequent epochs independently of the current star formation history. High-redshift galaxies with anomalously high dust-to-gas ratios formed from debris regions that were particularly dust-rich from the pre-collision populations, explaining the mismatch between their metallicity (set by post-collision stellar nucleosynthesis) and their dust content (partly inherited). The metallicity floor from pre-existing stellar generations and the pre-existing dust floor are both manifestations of the same physical mechanism — the inherited pre-collision stellar content of the debris field.

The slow radial falloff of dust-to-gas ratios in outer disk regions is explained by the extended angular momentum distribution of the collision debris. In SCT, the outer disk was assembled from debris with systematically higher specific angular momentum than the inner disk, and this outer debris also contained a proportionally larger fraction of pre-existing dust from the colliding pocket peripheries where stellar processing had been less intense. The pre-existing dust contribution is relatively larger at large galactocentric radii than in the inner disk where post-collision stellar nucleosynthesis has processed more gas into metals and produced more new dust. The resulting dust-to-gas profile declines more slowly with radius than the metallicity gradient predicts, consistent with observations, and SCT predicts the ratio of inherited-to-post-collision dust should increase monotonically with galactocentric radius for all disk galaxies.