Chemodynamic evidence of pristine gas accretion in the void galaxy VGS 12

Context. Accretion of metal-poor gas is expected to be an important channel of gas replenishment in galaxy evolution studies. However, observational evidence of this process is still relatively scarce. Aims. The unusual polar disk galaxy VGS 12 was found in the Void Galaxy Survey. It appears to be isolated and resides in the cosmological wall between two large voids. The suggested formation scenario for this peculiar system is accretion of metal-poor gas from the void interior. To confirm or refute the accretion scenario, information on the chemical properties of VGS 12 is crucial and provides novel insights into gas accretion mechanisms when combined with kinematic constraints on the polar material. Methods. We present for the first time the data on the gas-phase chemical abundance of VGS 12 obtained with the Russian 6m telescope BTA. We complement our analysis with HI data obtained with VLA and the data on the kinematics of the ionized gas. Results. VGS 12 appears to be a strong outlier from the “metallicity–luminosity” relation, with gas oxygen abundance ∼0.7 dex lower than expected for its luminosity. The nitrogen abundance, on the other hand, is higher than what is typically observed in galaxies with similar metallicity, but is consistent with the metallicity expected given its luminosity. Such behavior is what is expected in the case of metal-poor gas accretion. The H I reveals clear morphological and kinematical asymmetry between the northern and southern parts of the disk, which are likely related to its unsettled state due to the recent accretion event. The kinematics of the ionized gas seen in H α reveal prolate rotation and follow closely the rotation of the H I disk, so we suggest this is accreted H I gas ionized by the stars in the central region of the galaxy. Conclusions. Together, our findings provide strong, multiwavelength evidence of ongoing cold gas accretion in a galaxy caught in the act of growing from the cosmic web. This is one of the very few individual galaxies for which a convincing case can be made for such a process, and demonstrates the potential for cold accretion to contribute to galaxy growth even in the low-redshift Universe.