The detection of the diffuse interstellar bands in dusty starburst galaxies

We report the detection of the diffuse interstellar bands (DIBs) in the optical spectra of seven far-IR-selected starburst galaxies. The lambda lambda 6283.9 and 5780.5 features are detected with equivalent widths of similar to 0.4-1 Angstrom and 0.1-0.6 Angstrom, respectively. In the two starbursts with the highest quality spectra (M82 and NGC 2146), four other weaker DIBs at 5797.0, 6010.1, 6203.1, and 6613.6 Angstrom are detected with equivalent widths of similar to 0.1 Angstrom. The region over which the DIBs can be detected ranges from similar to 1 kpc in the less powerful starbursts to several kpc in the more powerful ones. The gas producing the DIBs is more kinematically quiescent on average than the gas producing the strongly blueshifted Na I lambda lambda 5890, 5896 absorption in the same starbursts. We show that the DIBs in these intense starbursts are remarkably similar to those in our Galaxy; the relative strengths of the features detected are similar, and the equivalent widths follow the same dependence as Galactic DIBs on E(B-V) and Na I column density. While the ISM in starbursts is heated by a photon and cosmic-ray bath that is similar to 10(3) times more intense than in the diffuse ISM of the Milky Way, the gas densities and pressures are also correspondingly larger in starbursts. This "homology" may help explain the strikingly similar DIB properties.