Molecular Hydrogen Excitation in ULIRGs
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Molecular Hydrogen Excitation in ULIRGs.

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Molecular Hydrogen Excitation in ULIRGs

In ultraluminous galaxies (ULIRGs) the H2 1-0 S(1) line is often stronger than Brγ. But the dominant excitation mechanism of the H2 is simply not known even though it potentially forms one of the best diagnostics, with the ability to distinguish between dynamical excitation in shock waves, and radiatively driven processes such as fluorescence or UV heating in PDRs or Xray irradiation. Using ISAAC on the VLT, we have studied in detail the dominant excitation mechanism of the near-infrared H2 lines in these objects.

There is surprisingly little variation in the ratios between the objects, suggesting that in all the nuclei the excitation mechanism is similar. The ν=1-0 emission appears thermalised at about 1000 K, but the 2-1 and 3-2 transitions show evidence of being radiatively excited by far-ultraviolet (FUV) photons, indicating that the H2 emission in ULIRGs arises in dense photon-dominated regions (PDRs). The line ratios in the nuclei are consistent with cloud densities between 104 and 105 cm-3, exposed to FUV radiation fields at least 103 times more intense than the ambient FUV field in the local interstellar medium. Starburst models for the ULIRGs based on the H2 properties as well as the intensities of the He I and Brγ recombination lines provide a consistent picture of young 1-5 Myr star clusters surrounded by relatively dense PDRs that are irradiated by intense FUV fluxes.

Excitation diagram showing the relative populations of the upper levels of the observed H2 transitions. The data (red) shown are the mean ratios for all the ULIRGs in the sample, with the error bars indicating the standard deviation between the objects. The model (blue), which provides an excellent match to the data, is that described above.

K-band spectra of 2 galaxies in the ULIRG sample, with the positions of the emission lines marked. Overplotted is the stellar continuum, derived in each case from a set of template spectra. This has been fitted and subtracted in order to facilitate measurement of the fluxes of weak lines.

published as: Davies, Sternberg, Lehnert, Tacconi-Garman, 2004; ApJ, 597, 907

R. I. Davies, M. Lehnert