First detection of PAHs and warm molecular hydrogen in tidal dwarf galaxies

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 mu m, which match models of groups of similar to 100 carbon atoms with an equal mixture of neutral and ionized PAHs. The TDGs have a dominant warm (similar to 140K) dust component in marked contrast to the cooler (40 - 60K) dust found in starburst galaxies. For the first time we detect the low-J rotational lines from molecular hydrogen. Adopting LTE, there is similar to 10(5) M-circle dot of similar to 400 K gas, which is < 0.1% of the cold gas mass inferred from (CO)-C-12 (1-0) measurements. The combination of one-third solar metallicity with a recent (< 5 million year) episode of star formation is reflected in the S and Ne ratios. The excitation is higher than typical values for starburst galaxies and similar to that found in BCDs. Using the Infrared Array Camera, we identify an additional 13 PAH-rich candidate TDGs. These sources occupy a distinct region of IRAC color space with [3.6] - [4.5] < 0.4 and [4.5] - [8.0] > 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) x 10(8) M-circle dot, 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.