A Far Ultraviolet Spectroscopic Explorer survey of interstellar molecular hydrogen in the Small and Large Magellanic Clouds

We describe a moderate-resolution Far Ultraviolet Spectroscopic Explorer (FUSE) survey of H(2) along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H(2) Lyman and Werner bands between 912 and 1120 Angstrom. Our survey is sensitive to N(H(2)) greater than or equal to 10(14) cm(-2); the highest column densities are log N(H(2)) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H(2) abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions [f(H2)] = 0.010(-0.002)(+0.005) for the SMC and [f(H2)] = 0.012(-0.003)(+0.006) for the LMC, compared with [f(H2)] = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic di+erences between 21 cm radio emission and Lyalpha in pencil beam sight lines as measures of N(H I). These results imply that the diffuse H(2) masses of the LMC and SMC are 8 x 10(6) and 2 x 10(6) M., respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H(2) formation rate coefficient, Rsimilar to3 x 10(-18) cm(3) s(-1), and incident radiation fields ranging from 10-100 times the Galactic mean value. We find that these high-radiation, low formation rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H(2) column densities in low rotational states (J = 0 and 1) to derive kinetic and/or rotational temperatures of diffuse interstellar gas, and we find that the distribution of rotational temperatures is similar to Galactic gas, with [T(01)] = 82 +/- 21 K for clouds with N(H(2)) greater than or equal to 10(16.5) cm(-2). There is only a weak correlation between detected H(2) and far-infrared fluxes as determined by IRAS, perhaps as a result of differences in the survey techniques. We find that the surface density of H(2) probed by our pencil beam sight lines is far lower than that predicted from the surface brightness of dust in IRAS maps. We discuss the implications of this work for theories of star formation in low-metallicity environments.