On the Formation of Star-forming Galaxies Having Anomalously Low-metallicity Regions: The Role of Galaxy Interaction and Circumgalactic Medium/Intergalactic Medium Accretion

Abstract Gas accretion from both the circumgalactic medium (CGM)/intergalactic medium (IGM) and interacting companion galaxy can dilute the gas phase metallicity of a galaxy. However, their relative contribution to the chemical evolution of galaxies remains to be quantified. To this end, in this work we study a sample of 510 star-forming galaxies (SFGs) having anomalously low-metallicity (ALM) regions selected from the MaNGA data available in the Data Release 17 from the Sloan Digital Sky Survey. ALM regions are defined as those having gas-phase metallicities that are at least ∼2 σ lower than the empirical relation between stellar mass surface density (Σ * ) and gas phase metallicity, i.e., the Σ * – Z relation. We find that ALM galaxies have higher star formation rates and H i gas fractions than normal SFGs at fixed M * . Also, ∼25% of the ALM galaxies exhibit tidal features, while the tidal fraction is only ∼12% for normal SFGs, indicating that galaxy interaction is an important factor responsible for the ALM phenomenon. To explore the origin of nontidal ALM galaxies, we compare their morphologies and environments with those of mass-matched normal SFGs. We find that nontidal ALM galaxies tend to have more disk-dominated morphologies and reside in less-dense environment. These findings suggest that cold gas accretion from the CGM/IGM is the primary cause for the ALM phenomenon, while galaxy interaction plays a minor but nonnegligible role.