Exploring the active galactic nucleus and starburst content of local ultraluminous infrared galaxies through 5–8 μm spectroscopy

• Published in: Monthly notices of the Royal Astronomical Society Vol. 399; no. 3; pp. 1373 - 1402
• Main Authors: Nardini, E., Risaliti, G., Salvati, M., Sani, E., Watabe, Y., Marconi, A., Maiolino, R.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.11.2009; Wiley-Blackwell
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; galaxies: active; galaxies: starburst; infrared: galaxies; galaxies: active; galaxies: starburst; infrared: galaxies; Spectroscopy; Red shift; Accretion; Starburst galaxies; Luminosity; Continuum; Active galaxies; Geometrical structure; Star formation; Active galaxy nuclei; Galaxy nuclei; Infrared galaxies; Models
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2009.15357.x

We present a 5–8 μm analysis of the Spitzer spectra of 71 ultraluminous infrared galaxies (ULIRGs) with redshift z < 0.15, devoted to the study of the role of active galactic nuclei (AGN) and starbursts (SBs) as the power source of the extreme infrared emission. Around ∼5 μm, an AGN is much brighter (by a factor of ≈30) than an SB of equal bolometric luminosity. This allows us to detect the presence of even faint accretion-driven cores inside ULIRGs: signatures of AGN activity are found in ∼70 per cent of our sample (50/71 sources). Through a simple analytical model, we are also able to obtain a quantitative estimate of the AGN/SB contribution to the overall energy output of each source. Although the main fraction of ULIRG luminosity is confirmed to arise from star formation events, the AGN contribution is non-negligible (∼23 per cent) and is shown to increase with luminosity. The existence of a rather heterogeneous pattern in the composition and geometrical structure of the dust among ULIRGs is newly supported by the comparison between individual absorption features and continuum extinction.