Luminous and Ultraluminous Galaxies
Ultraluminous Infrared Galaxies (ULIRGs) are among the most luminous galaxies in the local universe. The nature of their power source, almost totally obscured by dust at UV, optical and even NIR wavelengths, remains a controversial issue, but it is commonly recognized that their energetic environment and their dense interstellar medium constitute the nearest examples of the typical conditions of formation of massive galaxies at early epochs.
The model is a superposition of a starburst component - diffuse emission (short dashed), emission from dense molecular gas (dotted), thermal (three dots-dashed) and non-thermal (long dashed) radio emission- and an AGN component (dot-dashed cyan line). Upward displaced is the fit of the model (black solid) to the Spitzer IRS spectrum (green solid, from SSC archive). The broad band fluxes, combined with the IRS 10μm silicate absorption, uniquely constrain the AGN contribution to 20% of the IR flux (from Vega et al 2008).
The presence of either an AGN or a starburst does not prove that one or the other is the principal power source for their infrared luminosity. The observation that ULIRGs fall on the same FIR-radio correlation as star forming galaxies was taken as evidence that these sources are powered predominantly by star formation. However, the evidence that AGN activity and starburst luminosities are correlated over a wide range of IR luminosities has made such conclusions less secure. Since ULIRGs may commonly host both an AGN and a starburst, their relative contributions need to be quantified. Our approach to the energetic environment within ULIRGs is to compare the panchromatic spectral energy distribution (SED), from the near-infrared to the radio, with our own models (GRASIL) including both starburst and AGN components (Prouton et al. 2004). Results: We have analysed 31 compact ULIRGs (see Clemens et al 2008, Vega et al. 2008). New VLA radio data have been combined with existing data in literature (2MASS, ISO, IRAC, MIPS, IRS, SCUBA at JCMT) to construct aperture matched panchromatic SEDs. The above method allows disentangling the AGN and starburst contribution from the luminosity. In our sample only 25% of objects show an AGN contribution larger than 10% (only one larger than 50%). Besides, we derive ages, SFR timescale, evolutionary status optical depth and mass of dense molecular gas. The latter compare well with the masses obtained with HCN line luminosities and allow us to make predictions of other molecular lines that are observed with the next generation of millimetric telescopes in high redshift galaxies.
People: A. Bressan, M.S. Clemens
CollaborationM. Chavez, O. Vega (INAOE, Mex), P. Panuzzo (CEA, FR), G.L. Granato, L. Silva (INAF OA Trieste)
Recent Publications: Vega et al. (2008), A&A 484,631; Clemens et al. (2008), A&A 477,95