First Detection of PAHs and Warm Molecular Hydrogen in Tidal Dwarf Galaxies

Abstract

We observed two faint tidal dwarf galaxies (TDGs), NGC 5291 N and NGC 5291 S, with the Infrared Spectrograph on the Spitzer Space Telescope. We detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2, 7.7, 8.6, 11.3, 12.6, and 16.5 μm, which match models of groups of ~100 carbon atoms with an equal mixture of neutral and ionized PAHs. The TDGs have a dominant warm (~140 K) dust component in marked contrast to the cooler (40-60 K) dust found in starburst galaxies. For the first time we detect the low-J rotational lines from molecular hydrogen. Adopting LTE, there is ~105 M☉ of ~400 K gas, which is 3.2. Their disturbed morphologies suggest past merger events between companions; for example, NGC 5291 S has a projected 11 kpc tail. NGC 5291 N and S have stellar masses of (1.5 and 3.0) × 108 M☉, which is comparable to BCDs, although still roughly 10% of the LMC's stellar mass. The candidate TDGs are an order of magnitude less massive. This system appears to be a remarkable TDG nursery.