Observations of ultraluminous infrared galaxies with the Infrared Spectrograph on the Spitzer Space Telescope.: II.: The IRAS Bright Galaxy Sample

We present spectra taken with the Infrared Spectrograph on Spitzer covering the 5-38 mu m region of the 10 ultraluminous infrared galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample (BGS). There is a factor of 50 spread in the rest-frame 5.5-60 mu m spectral slopes, and the 9.7 mu m silicate optical depths range from at least tau(9.7) <= 0.4 (A(V) similar to 8) to tau(9.7) >= 4.2 (A(V) >= 78). There is evidence for water ice and hydrocarbon absorption and C2H2 and HCN absorption features in 4 and possibly 6 of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [Ne v] emission in 3 of the 10 BGS ULIRGs, at flux levels of 5-18 x 10(-14) ergs cm(-2) s(-1) and [Ne v] 14.3/[Ne II] 12.8 line flux ratios of 0.12-0.85. The remaining BGS ULIRGs have limits on their [Ne v]/[Ne II]line flux ratios, which range from <= 0.15 to <= 0.01. Among the BGS ULIRGs, the AGN fractions implied by either the [Ne v]/[Ne II] or [O IV]/[Ne II] line flux ratios (or their upper limits) are significantly lower than implied by the MIR slope or strength of the 6.2 mu m PAH EQW feature. There is evidence for hot (T > 300 K) dust in five of the BGS ULIRGs, with the fraction of hot dust to total dust luminosity ranging from similar to 1% to 23%, before correcting for extinction. When integrated over the IRAC-8, IRS blue peak-up, and MIPS-24 filter bandpasses, the IRS spectra imply very blue colors for some ULIRGs at z similar to 1.3. The large range in diagnostic parameters among the nearest ULIRGs suggests that matching survey results to a small number of templates may lead to biased results about the fraction of luminous dusty starbursts and AGNs at high z.