Appetizer
The plot above shows Spitzer IRS low-resolution spectra of ULIRGs sorted by spectral shape. The three spectra at the top are continuum-dominated (AGN-like), the next four are PAH-dominated (starburst-like) and the rest are absorption-dominated (burried nuclei). Vertical lines indicate the positions of the 6.2, 7.7 and 11.2 micron PAH emission bands. Note the increasing depth of silicate absorption and the decreasing contribution of PAH emission towards the bottom of the plot and the strong presence of absorption bands of water ice and aliphatic hydrocarbons in the middle spectra.
Crystalline silicates in the ISM of ULIRGs
The above comparison of the silicate absorption profiles of the Galactic ISM line of sight towards Galactic Center Source GCS3 and the line of sight into the northwestern nucleus of the ULIRG IRAS 08572+3915 shows some striking differences. While the Galactic ISM silicate profile is exceedingly smooth, the ULIRG profile shows significant substructure at 11, 16, 19, 23 and 28 microns. This substructure is consistent with the presence of the crystalline silicate Forsterite in the ISM of this ULIRG.
Similar substructure was discovered towards several other ULIRGs, all of which have appreciable to strong silicate absorption features in their spectra. Note that identification of absorption features of crystalline silicates can only be attempted in spectra in which the PAH emission features are weak or absent.
Warm/hot CO gas in ULIRGs
The short wavelength cut-off of the IRS low-resolution spectrograph occurs at 5.2 microns. For ULIRGs with redshifts of z=0.3 or more this cut-off shifts down to rest frame wavelengths of 4.0 microns or less. This opens up the near-infrared for observations of distant ULIRGs with Spitzer. The surprise was great early in the mission when the spectrum of the distant ULIRG IRAS F00183-7111 showed a broad absorption band of dense warm (T=700K) CO gas. Since then, several similarly redshifted ULIRGs have shown this feature to be present in their IRS spectra. No two profiles are the same.
Warm C2H2 and HCN gas in ULIRGs
Absorption features of C2H2, HCN and CO2 gas have been discovered in high signal-to-noise IRS high-resolution spectra (R=600) towards a number of deeply obscured ULIRG nuclei.
These features have previously been seen in ISO-SWS spectra of
deeply embedded massive Young Stellar Objects (YSOs) and recently
in the Spitzer spectra of a low mass YSO, IRS46 (above;
Lahuis et al. 2006).
In the massive YSOs, the features originate in the hot inner dense
region of the shells surrounding the young stars.
The detection of absorption features of C2H2, HCN and CO2
in our ULIRGs might suggest the presence of a population of deeply
embedded YSOs in their nuclear environments. However, an
identification with embedded protostars is not supported by spectral
evidence in other parts of our mid-infrared spectra (e.g. by the
absence of commonly detected NH4+ ice at 6.85 microns and CO2 ice
at 15 microns.
Strong absorption bands of aliphatic hydrocarbons in ULIRGs
Absorption bands of water ice (6.0 microns) and aliphatic hydrocarbons (6.90 and 7.25 microns) have been detected at high signal-to-noise and in fine spectral detail towards the nuclei of deeply obscured ULIRG nuclei. The detections of the 6.90 and 7.25 micron bands are stronger than towards our own Galactic Center -- the line of sight with the strongest detection in our Galaxy. Comparison of the 6 micron `water ice' profiles in ULIRG spectra to those of Galactic embedded protostars shows the profiles to be different. This raises the question whether species other than water ice may be contributing to the observed profile in ULIRG spectra. The issue is complicated by the presence of PAH emission features at 5.7 and 6.2 microns in most ULIRG spectra. Eg. Arp220 and IRAS 15250+3609.
Near-infrared groundbased spectroscopy has revealed absorption features of 3.0 micron water ice and 3.4 micron aliphatic hydrocarbons against the hot dust continuum in the spectra of some ULIRG nuclei (Imanishi et al. 2003). The presence of the PAH emission feature at 3.3 microns complicates the study of the 3.4 micron hydrocarbon band at low spectral resolution. Note that the depth of the 3.0 micron water ice feature is better constrained for ULIRGs with higher redshifts.
Comparison of the continuum subtracted absorption profiles of the 3.4 micron aliphatic hydrocarbon features detected towards Galactic Center Source IRS7 (Pendleton et al. 1994) and the southern nucleus of IRAS 19254-7245 (Risaliti et al. 2003,2006).
Luminous radio supernovae in Arp220
VLBI observations at 18cm have revealed the two nuclei of the closest ULIRG, Arp220, to be `covered' with sub mJy radio point sources. The result is illustrated in the image above (Diamond et al. 2003), which measures 1.6 arcseconds across. The sources are interpreted as luminous radio supernovae associated with an intense nuclear starburst.
The rich far-IR absorption spectrum of Arp220
(Gonzalez-Alfonso et al. 2004).
AGNs in both nuclei of NGC6240
Observations with the X-ray telescope Chandra revealed the existence of a supermassive black holes in each of the two nuclei of the ULIRG NGC6240 (Komossa et al. 2003). In the image above the two AGNs can be recognized by their blue color, which codes for hard radiation.
References
Spoon H.W.W., Keane J.V., Cami J., Lahuis F., Tielens A.G.G.M.,
Armus L., Charmandaris V.,
Spitzer Observations of Deeply Obscured Galactic Nuclei,
2005, invited paper in: IAU Symposium 231
"Astrochemistry: Recent Successes and Current Challenges",
eds. D.C. Lis, G.A. Blake and E. Herbst,
astro-ph/0512037
Armus L., Bernard-Salas J., Spoon H.W.W., et al.,
Detection of the Buried AGN in NGC 6240 with the
Infrared Spectrograph on The Spitzer Space Telescope
2006, ApJ, in press (astro-ph/0511381)
Spoon H.W.W., Tielens A.G.G.M., Armus L., et al.,
The Detection of Crystalline Silicates in Ultra-Luminous Infrared Galaxies,
2005, ApJ, in press (astro-ph/0509859)
Lutz D., Sturm E., Genzel R., Spoon H.W.W., Stacey G.J.,
Gas near active galactic nuclei: A search for the 4.7 micron CO band,
2004, A&A 426, L5
Armus L., Charmandaris V., Spoon H.W.W., et al.,
Observations of Ultraluminous Infrared Galaxies with the Infrared
Spectrograph on the Spitzer Space Telescope: Early Results on Mrk 1014,
Mrk463, and UGC 5101,
2004, ApJS 154, 178
Spoon H.W.W., Armus L., Cami J., Tielens A.G.G.M., Chiar J.E.,
Peeters E., Keane J.V., Charmandaris V., et al.,
Fire and Ice: IRS Mid-IR Spectroscopy of IRAS F00183--7111,
2004, ApJS 154, 184