Spiral galaxies with unusual azimuthal asymmetry in oxygen abundance

¹ Institute of Theoretical Physics and Astronomy, Vilnius University, Sauletekio av. 3, 10257 Vilnius, Lithuania
² Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St, 03680 Kiev, Ukraine

^⋆ Corresponding author.

Received: 1 March 2025
Accepted: 3 June 2025

Abstract

We considered five galaxies in the Mapping Nearby Galaxies at the Apache Point Observatory (MaNGA) survey that show distinct azimuthal asymmetry in the abundance, in the sense that in the inner part (more than half of the optical radius, R₂₅) of each galaxy there is a sector-like (up to semi-circle) region where the oxygen abundances (O/H)_h are higher than the abundances (O/H)_l in other sector. M-11761-12705 is a massive galaxy with a stellar mass of log(M_⋆/M_⊙) = 11.6; the masses of four other galaxies (M-8450-12701, M-8546-12704, M-8561-12701, M-9500-09102) are moderate: 10.1 ≲ log(M_⋆/M_⊙) ≲ 10.4. Abundances within both high- and low-metallicity regions show flat radial gradients (the abundances are at nearly constant levels). The histogram for the spaxel abundances demonstrates two distinct peaks, the difference between the (O/H)_h and the (O/H)_l abundances are of 0.06–0.08 dex. The high-metallicity regions are located in the O/H – N/O diagram closer to the lower envelope of the band than the low-metallicity regions. The abundance properties in the massive galaxy M-11761-12705 can be explained by the low-metallicity gas infall onto the galaxy and subsequent episode of high star formation rate (starburst) in the diluted interstellar medium occurring between 20 Myr ≲t ≲ 50 Myr ago. For moderate-mass galaxies, the higher oxygen abundance in the high-metallicity region and its shift towards the lower envelope in the N/O – O/H diagram compared to the low-metallicity region can be explained in one of two ways: either the starburst in the high-metallicity region occurred several dozens of Myr ago, or the star formation in the galaxy is accompanied by galactic winds, and the region evolved with the lower efficiency of the enriched galactic winds shows higher metallicity. Two galaxies of our sample (M-8546-12704 and M-11761-12705) are members of galaxy pairs. However, the asymmetry parameter, A, quantifying the asymmetry of a light distribution across the galaxy, is above the canonical threshold (A = 0.35) between non-interacting and interacting galaxies in the massive galaxy M-11761-12705 only. The values of the A parameter in four moderate-mass galaxies (including a member of the galaxy pair) are below the threshold value.