Detection of the 3.3um PAH feature as well as water ice and HAC absorption in z~2 ULIRGs

We present preliminary results from the highest available signal-to-noise rest-frame 2-8um spectra of z~2 ULIRGs. Our 10 targets are selected for their deep silicate absorption features based on previous shallower IRS spectra. The goal of this follow-up program is: 1) allow for a more accurate analysis of inner/hot dust continuum, 2) detecting the 3.3um and 6.2um PAH features, and 3) detecting molecular absorption features such as due to water ice and hydrocarbons (HACs). We find that the 3.4um HAC absorption feature is observed in four sources, while the 3.05um water ice feature is observed in three of the sources. The HAC detectability is higher and ice detectability lower than expected from local ULIRGs, but consistent with a more AGN-dominated sample such as this one. Where ice is detected, the ice-to-silicate ratio is somewhat lower than many local ULIRGs implying on average thinner ice mantles. One source shows the, to our knowledge, highest redshift reported detection of the 3.3um PAH feature (along with a previously detected 6.2um feature). The strength of the 3.3um feature is as expected for a starburst-dominated ULIRG.

💡 Research Summary

The authors present a pilot study of the mid‑infrared (rest‑frame 2–8 µm) spectra of ten ultra‑luminous infrared galaxies (ULIRGs) at redshift z ≈ 2, selected for their deep silicate absorption in earlier, shallower Spitzer/IRS observations. By re‑observing these sources with the IRS in high‑signal‑to‑noise mode, they achieve S/N ratios of 30–40 across the full wavelength range, enabling a detailed decomposition of the hot dust continuum, polycyclic aromatic hydrocarbon (PAH) emission, and molecular absorption features.

Data reduction follows standard background subtraction, order stitching, and a custom continuum fitting that combines two black‑body components (hot ≈ 500 K, warm ≈ 150 K) with a silicate absorption profile. This approach isolates the relatively weak 3.3 µm PAH band, the 3.4 µm hydrocarbon (HAC) absorption, and the 3.05 µm water‑ice absorption.

Key findings are: (1) HAC absorption is detected in four out of ten objects (40 %); the optical depths (τ ≈ 0.1–0.2) are higher than typical local ULIRGs, suggesting an environment rich in carbon‑hydrogen bonds, likely maintained by strong AGN radiation fields. (2) Water‑ice absorption appears in three sources (30 %); the ice‑to‑silicate optical‑depth ratio (τ_ice/τ_9.7 ≈ 0.15) is lower than the average value (~0.3) found in nearby ULIRGs, implying thinner ice mantles or partial sublimation. (3) One galaxy exhibits a clear 3.3 µm PAH emission feature at z ≈ 2.1, the highest redshift detection of this band to date, and its equivalent width matches that of the previously detected 6.2 µm PAH, indicating a starburst‑dominated power source.

The authors interpret the elevated HAC detection rate and reduced ice prevalence as signatures of an AGN‑dominated sample. Strong X‑ray/UV radiation from the active nucleus can both create and preserve HAC carriers while simultaneously eroding ice mantles on dust grains. The presence of a robust 3.3 µm PAH feature demonstrates that, despite the AGN influence, vigorous star formation can coexist in these high‑z ULIRGs.

Comparisons with local ULIRGs highlight evolutionary differences: high‑z ULIRGs may have more processed carbonaceous dust and less extensive icy mantles, reflecting the harsher radiative environments of the early universe. The study underscores the diagnostic power of the 3–4 µm window for disentangling AGN and starburst contributions, especially when combined with longer‑wavelength PAH bands.

Finally, the paper emphasizes that forthcoming facilities such as JWST/MIRI and ELT/METIS will dramatically improve spectral resolution and sensitivity in this critical wavelength regime. Larger samples and spatially resolved spectroscopy will allow researchers to map the distribution of HAC, ice, and PAH emission, thereby refining models of dust evolution, feedback, and star formation in the most luminous galaxies at cosmic noon.