The Detection of the Diffuse Interstellar Bands in Dusty Starburst Galaxies

We report the detection of the Diffuse Interstellar Bands (``$DIBs$'') in theoptical spectra of seven far-infrared-selected starburst galaxies. The$\lambda$6283.9 \AA and $\lambda$ 5780.5 \AA features are detected withequivalent widths of $\sim$ 0.4 to 1 \AA and 0.1 to 0.6 \AA respectively. Inthe two starbursts with the highest quality spectra (M82 and NGC2146), fourother weaker $DIBs$ at $\lambda$ 5797.0 \AA, 6010.1 \AA, 6203.1 \AA, and 6613.6\AA are detected with equivalent widths of $\sim$ 0.1 \AA. The region overwhich the $DIBs$ can be detected ranges from $\sim$ 1 kpc in the less powerfulstarbursts, to several kpc in the more powerful ones. The gas producing the$DIBs$ is more kinematically quiescent on-average than the gas producing thestrongly-blueshifted $NaI\lambda\lambda$5890,5896 absorption in the samestarbursts. We show that the $DIBs$ in these intense starbursts are remarkablysimilar to those in our Galaxy: the relative strengths of the features detectedare similar, and the equivalent widths follow the same dependence as Galactic$DIBs$ on $E(B-V)$ and $NaI$ column density. While the ISM in starbursts isheated by a photon and cosmic ray bath that is $\sim$ 10$^3$ times more intensethan in the diffuse ISM of the Milky Way, the gas densities and pressures arealso correspondingly larger in starbursts. This ``homology'' may help explainthe strikingly similar $DIB$ properties.Comment: 18 pages, 5 figures, in press in Ap