Spatial Evolution of Metallicity Gradients Induced by Galaxy Interactions

Hsi-An Pan^1*, Lihwai Lin², Bau-Ching Hsieh², Jorge K. Barrera-Ballesteros³

¹Department of Physics, Tamkang University, New Taipei City, Taiwan
²Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
³Instituto de Astronomía, Universidad Nacional Autónoma de México, México, Mexico

* Presenter:Hsi-An Pan, email:hapan@gms.tku.edu.tw

Central starbursts are a common phenomenon in interacting galaxies and mergers. The standard theory of interaction-triggered starbursts suggests that the global interaction-induced gas inflow is responsible for raising the average gas density and, in turn, increasing the star formation rate within the central kpc. While the direct observation of such global gas inflow can be challenging, it has the potential to rearrange the initial gas-phase metallicity gradient of the galaxy.

In this work, we focus on the evolution of metallicity gradients in galaxies at different merger stages. The samples under study are drawn from the integral field spectroscopy (IFS) survey SDSS-IV MaNGA. We quantify the impact of interactions on metallicity gradients by examining spatially resolved metallicity with respect to a control sample specifically selected for each individual galaxy in pair/merger.

Our analysis reveals that galaxy interactions exert no significant influence on metallicity during the initial phase of two galaxies approaching each other. Subsequently, following the first pericenter passage, which coincides with the onset of a high gas inflow rate and a starburst event, the metallicity profile decreases steeply from normal to suppressed metallicity for decreasing galactocentric radius. As galaxies progress further into the apocenter phase, the central metallicity experiences an enhancement, presumably due to enrichment from the starburst. Consequently, the metallicity profile is reversed compared to its state at the first pericenter passage. Although the metallicity gradient during the apocenter phase bears similarity to the initial metallicity gradient of a galaxy, the absolute metallicity values remain lower than those of the controls out to the maximum radius we have explored (~6-7 kpc). Finally, upon reaching the final coalescence phase, the metallicity profile is reversed again, and metallicity decreases to its lowest levels across all radii. This decline is presumably attributed to the substantially increased gas inflow rate, which ultimately triggers the final, and typically the most intense, starburst event.

Our findings demonstrate that galaxy interactions exert a significant, global influence on the metallicity of galaxies at all radial distances. As far as we are aware, this is the first coherent picture of the spatial evolution of metallicity gradients during galaxy interactions conducted by observations.

Keywords: galaxy interactions, gas-phase metallicity, interstellar medium, galaxy survey