Propiedades multifrecuencia de galaxias hiperluminosas en el infrarrojo

Resumen

One of the most challenging results brought by the Infrared Astronomical Satellite (IRAS) is the discovery of a new population of extragalactic objects, the Ultraluminous Infrared Galaxies (ULIRG). These sources showed a luminosity comparable to quasars (LIR > 1E12 solar luminosities ), but their bolometric output was completely dominated by the infrared (IR) emission. The origin of their extreme IR luminosities has been in discussion for decades, but in the last few years a consensual picture has emerged, brought by multi-wavelength observations. Now is broadly accepted that ULIRG are dusty galaxies where fierce star formation processes have been triggered by mergers or interactions between rich-gas galaxies. Only half of them show Active Galactic Nuclei (AGN), which usually are minor contributors to the total IR emission, but the fraction of ULIRG harbouring an AGN and its relative contribution to the bolometric output increases with luminosity. Hyperluminous Infrared Galaxies (HLIRG) are defined as those LIRG with LIR > 1e13 solar luminosities . However the ULIRG paradigm described above is not so well-grounded for HLIRG. Most of these objects seem to be composite sources, i.e. AGN and SB phenomena are both needed to fully explain their IR emission and only about a third of them has been found in interacting systems. Further investigations are needed to fully understand the nature of HLIRG and its connection with ULIRG. Moreover, as HLIRG could represent the most vigorous stage of galaxy formation they are unique laboratories to investigate extremely high stellar formation, and its connection to super-massive black hole (SMBH) growth. HLIRG could be key objects to understand the co-evolution between SMBH and galaxies. AGN and star formation processes occur in environments enshrouded by large amounts of gas and dust. X-ray and IR observations offer the needed penetrating power to study these phenomena. While X-rays provide a largely uncontaminated view of the primary AGN emission, measures of infrared emission yield detailed information about the levels of dust and on-going star-formation surrounding AGN. Infrared and X-ray observations are therefore essential to understand the AGN and SB phenomena on-going in HLIRG. The main objective of this thesis is to draw a comprehensive and consistent picture on HLIRG derived from multiwavelength observations. Thanks to the unprecedented capabilities of state-of-the-art observatories we are able to characterize the properties of HLIRG in several wavelength ranges. We selected a moderate size sample of HLIRG observed by XMM-Newton and/or Spitzer, which allows performing our study in two complementary energy bands, X-ray and MIR, and we have also studied their broadband SED from radio to X-rays. We have applied different techniques to detect the signatures of AGN and SB emission at several wavelengths and estimate the contribution of these processes to the bolometric output. Our multi-wavelength approach provides more conclusive evidence on which is the dominant mechanism with respect to any other previous study restricted to a particular energy range. We found that all the studied sources harbour an AGN. MIR and X-ray observations have allowed its detection even in heavily enshrouded environments. Most of these sources posses a strong SB, with star forming rates up to 1000 solar masses per year. Moreover, we found that those sources with no SB contribution are in fact luminous quasars and hence they are not "bona fide" HLIRG, since their bolometric emission is not dominated by the IR output. We can confirm that both AGN and SB phenomena are indispensable to understand the extreme luminosity of HLIRG. However, the "bona fide" HLIRG do not seem to be an homogeneous population. On one hand there are sources with large amounts of gas and dust enshrouding the nucleus as it is suggested by the strong absorption shown in X-rays and MIR and by the shape of their SED. Their large dust covering factors are also consistent with a nucleus almost completely enshrouded by dust. The gas and dust fuel powerful AGN activity and strong star formation, that could be triggered by galaxy interactions and/or mergers, as suggested by the study of the morphology and environment of some of these HLIRG. The analysis of their SED also hints toward a strong feedback between both phenomena. These are common properties of ULIRG and hence we can consider these HLIRG as the objects occupying the high luminosity tail of the ULIRG population distribution. On the other hand there are HLIRG with minor MIR/optical/X-ray obscuration, suggesting lower quantities of gas and dust than in the "ULIRG-like" population. However the strong SB observed in these sources need large amounts of gas to fuel the star formation. The SB emission in these HLIRG can be modelled with young SB models. They seem to be isolated galaxies, with no signs of interactions or ongoing mergers. Further studies are needed to explain the nature of these HLIRG, but they could be young active galaxies undergoing their first major episode of star formation. Therefore, the sources studied in this thesis likely belong to three different populations: 1. Very luminous QSO with minor star formation activity. 2. Young, isolated active galaxies undergoing their first episode of major star formation with little connection with a recent major merger. 3. Galaxies which have recently experienced a merger/disturbance that brought lots of gas and dust into the inner regions. This event triggered both the star formation and the AGN activity in a heavily obscured environment. These objects are good candidates to be the high luminosity tail of the ULIRG population distribution.