GASP. VIII. Capturing the Birth of a Tidal Dwarf Galaxy in a Merging System at z ̃ 0.05

Abstract

Within the GAs Stripping Phenomena in galaxies with MUSE (GASP) sample, we identified an ongoing 1:1 merger between 2 galaxies and the consequent formation of a tidal dwarf galaxy (TDG). The system is observed at z = 0.05043 and is part of a poor group. Exploiting the exquisite quality of the Multi Unit Spectroscopic Explorer (MUSE)/Very Large Telescope data, we present the spatially resolved kinematics and physical properties of gas and stars of this object and describe its evolutionary history. An old (luminosity weighted age ̃2 × 10<SUP>9</SUP> yr), gas-poor, early-type-like galaxy is merging with a younger (luminosity weighted age ̃2.5 × 10<SUP>8</SUP> yr), gas-rich, late-type galaxy. The system has a quite strong metallicity gradient, which is indicative of an early-stage phase. Comparing the spatial extension of the star formation at different epochs, we date the beginning of the merger between 2 × 10<SUP>7</SUP> yr < t < 5.7 × 10<SUP>8</SUP> yr ago. The gas kinematic pattern reflects that of the late-type object and is distorted in correspondence to the location of the impact. The stellar kinematic instead is more chaotic, as expected for mergers. The gas redistribution in the system induces high levels of star formation between the two components, where we indeed detect the birth of the TDG. This stellar structure has a mass of ̃6 × 10<SUP>9</SUP> M <SUB>☉</SUB>, a radius of ̃2 kpc, and even though it has already accreted large quantities of gas and stars, it is still located within the disk of the progenitor, is characterized by a high velocity dispersion, indicating that it is still forming, is dusty, and has high levels of star formation (star formation rate ̃ 0.3 M <SUB>☉</SUB> yr<SUP>-1</SUP>). This TDG is originated in an early-stage merger, while these structures usually form in more evolved systems.