Inverted metallicity gradients in two Virgo cluster star-forming dwarf galaxies: evidence of recent merging?

TL;DR

This study reveals inverted metallicity gradients in two Virgo dwarf galaxies, suggesting recent merging events with gas-rich companions as a likely cause, contrasting typical metallicity profiles in such environments.

Contribution

It provides the first evidence of positive metallicity gradients in Virgo cluster dwarfs, linking them to recent mergers and gas inflow, challenging previous assumptions about metallicity distribution in dense environments.

Findings

Positive metallicity gradients observed in VCC135 and VCC324.

Gradients likely caused by recent merging with gas-rich companions.

Inflow of metal-poor gas inferred from morphological and kinematic analysis.

Abstract

We present integral field spectroscopy observations of two star-forming dwarf galaxies in the Virgo cluster (VCC135 and VCC324) obtained with PMAS/PPak at the Calar Alto 3.5 meter telescope. We derive metallicity maps using the N2 empirical calibrator. The galaxies show positive gas metallicity gradients, contrarily to what is usually found in other dwarfs or in spiral galaxies. We measure gradient slopes of 0.20 $\pm$ 0.06 and 0.15 $\pm$ 0.03 dex/$R_e$ for VCC135 and VCC324, respectively. Such a trend has been only observed in few, very isolated galaxies, or at higher redshifts ($z >$ 1). It is thought to be associated with accretion of metal-poor gas from the intergalactic medium, a mechanism that would be less likely to occur in a high-density environment like Virgo. We combine emission line observations with deep optical images to investigate the origin of the peculiar metallicity gradient. The presence of weak underlying substructures in both galaxies and the analysis of morphological diagnostics and of ionised gas kinematics suggest that the inflow of metal-poor gas to the central regions of the dwarfs may be related to a recent merging event with a gas-rich companion.