Molecular and ionized gas in tidal dwarf galaxies: the spatially resolved star formation relation

Abstract

Tidal dwarf galaxies (TDGs) are low-mass objects that form within tidal and/or collisional debris ejected from more massive interacting galaxies. We use CO(1–0) observations from Atacama Large Millimeter/submillimeter Array and integral-field spectroscopy from Multi-Unit Spectroscopic Explorer to study molecular and ionized gas in three TDGs: two around the collisional galaxy NGC 5291 and one in the late-stage merger NGC 7252. The CO and H α emission is more compact than the H I emission and displaced from the H I dynamical centre, so these gas phases cannot be used to study the internal dynamics of TDGs. We use CO, H I, and H α data to measure the surface densities of molecular gas (Σmol), atomic gas (Σatom), and star formation rate (ΣSFR), respectively. We confirm that TDGs follow the same spatially integrated ΣSFR–Σgas relation of regular galaxies, where Σgas = Σmol + Σatom, even though they are H I dominated. We find a more complex behaviour in terms of the spatially resolved ΣSFR–Σmol relation on subkpc scales. The majority (⁠∼60 per cent ⁠) of star-forming regions in TDGs lie on the same ΣSFR–Σmol relation of normal spiral galaxies but show a higher dispersion around the mean. The remaining fraction of star-forming regions (⁠∼40 per cent ⁠) lie in the starburst region and are associated with the formation of massive super star clusters, as shown by Hubble Space Telescope images. We conclude that the local star formation activity in TDGs proceeds in a hybrid fashion, with some regions comparable to normal spiral galaxies and others to extreme starbursts.