Identifying the Most Accurate Indicator of a Galaxy’s Metallicity

A group of astronomers led by Laura Sánchez-Menguiano of the University of Granada has discovered that a galaxy’s total stellar mass is not the best predictor of its abundance of heavier elements, or its metallicity. This contradicts earlier studies and instead indicates that a galaxy’s gravitational potential is a more effective predictor. The study, published in the journal Astronomy & Astrophysics, is significant as it challenges existing understanding of galaxy formations and evolutions.

Considering that nearly 98% of the universe’s ordinary mass is hydrogen or helium, astronomers refer to the remaining elements as “metals”. Their abundance, or metallicity, is an indicator of stellar activity post the Big Bang. The team used data from over 3,000 nearby star-forming galaxies from the Mapping Nearby Galaxies survey conducted at Apache Point Observatory in New Mexico, USA.

The researchers used a computer algorithm known as the “random forest regressor algorithm” to establish scaling relations between various galactic parameters and identify the one that best predicts a galaxy’s gas-phase metallicity—the metallicity of the gases in the galaxy’s interstellar medium. The gas-phase metallicity was determined using the ratio of oxygen abundance (which traces galaxy evolution) to hydrogen mass, measured at a distance of one effective radius of the galaxy.

The study found that the most accurate predictor of gas-phase metallicity was the baryonic gravitational potential of a galaxy, represented by the ratio of stellar mass to the effective radius. The team argues that this ratio is raised to the power of 0.6, less than one, to account for the inclusion of dark matter in the galaxy. The results were consistent for galactic masses ranging from 300 million to 300 billion times the mass of the sun.

Despite this finding, the study acknowledges that the baryonic gravitational potential alone cannot predict the gas-phase metallicity, and other secondary parameters may play a substantial role in determining it. Further research is being planned to investigate these relationships.

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