Observations of Ultraluminous Infrared Galaxies with the Infrared Spectrograph on the Spitzer Space Telescope II: The IRAS Bright Galaxy SampleJournal of Astrophysics (2006)
We present spectra taken with the Infrared Spectrograph8 on Spitzer covering the 5−38µm region of the ten Ultraluminous Infrared Galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample. Among the BGS ULIRGs, we find a factor of 50 spread in the rest-frame 5.5 − 60µm spectral slope. The 9.7µm silicate optical depths range from at least τ9.7 ≤ 0.4 to τ9.7 ≥ 4.2, implying line of sight extinctions of AV ∼ 8 mag to nearly AV ≥ 78 mag. There is evidence for water ice and hydrocarbon absorption and C2H2 and HCN absorption features in four and possibly six of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [NeV] emission in three of the ten BGS ULIRGs, at flux levels of 5−18×10−14erg cm−2 sec−1 and [NeV] 14.3/[NeII] 12.8 line flux ratios of 0.12 − 0.85. The remaining BGS ULIRGs have limits on their [NeV]/[NeII] line flux ratios which range from ≤ 0.15 to ≤ 0.01. Among the BGS ULIRGs, the AGN fractions implied by either the [NeV]/[NeII] or [OIV]/[NeII] line flux ratios (or their upper limits) are significantly lower than implied by the MIR slope or strength of the 6.2µm PAH EQW feature. Fitting the SEDs, we see evidence for hot (T > 300K) dust in five of the BGS ULIRGs, with the fraction of hot dust to total dust luminosity ranging from ∼ 1 − 23%, before correcting for extinction. When integrated over the IRAC-8, IRS blue peakup, and MIPS-24 filter bandpasses, the IRS spectra imply very blue colors for some ULIRGs at z ∼ 1.3, as the strong silicate absorption enters the MIPS-24 filter. This is most extreme for sources with significant amounts of warm dust and deep silicate absorption. The large range in diagnostic parameters (line flux ratios, spectral slopes, PAH strengths) among the nearest, brightest ULIRGs suggests that matching deep survey results to a very small number of templates may lead to biased results about the fraction of luminous, dusty starbursts and AGN at high-z.
- Infrared Spectrograph,
- Ultraluminous Infrared Galaxies,
- IRAS Bright Galaxy
Publication DateOctober 6, 2006
Citation InformationSarah J.U. Higdon, L. Armus, V. Charmandaris, J. Bernard-Salas, et al.. "Observations of Ultraluminous Infrared Galaxies with the Infrared Spectrograph on the Spitzer Space Telescope II: The IRAS Bright Galaxy Sample" Journal of Astrophysics Vol. 656 Iss. 1 (2006) p. 1 - 53
Available at: http://works.bepress.com/sarah_higdon/8/