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

Abstract

We observed two faint tidal dwarf galaxies (TDGs), NGC 5291N and NGC 5291 S with the Infrared Spectrograph on the Spitzer Space Telescope. We detect strong polycyclic aromatic hydrocarbon (PAH) emission, 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 ~10^5 solar masses of ~400 K gas, which is <0.1 % of the cold gas mass. The combination of one-third solar metallicity with a recent, <5 million year, epsiode 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 Infared 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. NGC 5291 N and S have stellar masses of (1.5 and 3.0) 10^8 solar masses, which is comparable to BCDs. This system appears to be a remarkable TDG nursery.