Nuclear activity in galaxies

Executive summary

Many open questions remain concerning Active Galactic Nuclei (AGNs). The mechanism responsible for triggering the non-thermal nuclear activity (mergers/interactions or secular evolution of the host galaxies?), the conditions required to switch on the nuclear activity and how the properties of the host galaxy relate to the different levels of activity are not known. Both the origin of the gas accreted onto the black hole and how it loses its angular momentum before being accreted need to be elucidated. To investigate all these questions, a quantification of the relationship between AGN class (LINER, Seyfert 1 or 2), the morphological type of the host galaxy and the environment, needs to be determined. Another open question concerns the relationship between Seyfert and LINER activity, to know whether they are a result of scaling of the AGN energetics or they are connected to the galaxy evolution and the accreting mechanism onto the black hole. As a result from this project we expect to provide some hints on the unification of AGNs.

On the other hand, AGN feedback has become a popular topic in the last decade, since theoretical models demonstrate that it can solve compelling problems of the most popular galaxy formation scenarios. However, little observational evidence exists to support such models. It is important to carry out deep spectroscopic and imaging studies of different AGN classes at different epochs to evaluate the role AGN feedback plays and disentangle the impact on the evolution of the host galaxies.

Detailed research

One of the main challenges of Astrophysics in this Topic is to understand the physical processes responsible of Nuclear Activity in Galaxies (AGNs) and the characterization of their host galaxies (mass, luminosity, kinematics, stellar population). It has been shown that there exist indeed causal connections between the masses of the nuclear black holes and the galactic bulges. This important discovery should be intimately linked with the for and backwards question on the relation between Nuclear Activity and the Starburst circumnuclear phenomenon. It seems that the nuclear activity phenomenon might be related in some yet unknown way with the galaxy formation and evolution, eventually producing secular evolution in the Hubble sequence. On the other hand convincing proves on the potential for gravitational interaction to ignite a large star formation event and eventually nuclear activity have come out in the past years Moreover these gravitational processes can be also invoked to build up new and more massive Galaxies thought the coalescence between galaxies. Thus, the study of nuclear activity and their relation to circumnuclear Starburst is a central topic to disentangling the formation and evolution of galaxies (see below). Therefore, the understanding on the physical processes occurring at the early epoch of the Universe when the most massive black holes are formed have something to do with the understanding of the AGN-Starburst connection. Our group has developed this line of research with the goal to get hints into this problem through an observational multifrequency approach and taking advantage of the stellar evolution models developed in the group.

Scientific objectives

  1. To determine the frequency of nuclear and circumnuclear stellar clusters in galaxies with nuclear activity (LINERs, Seyfert 2 and Seyfert 1, and to determine the main properties (masses, luminosity, ages, etc). Investigate if there is an evolution among different types of nuclear activity. This will be done analyzing data from HST at ultraviolet and optical wavelengths.
  2. To investigate the role of starbursts and AGNs in the evolution of galaxies. Imaging and spectroscopic data from HST, GTC, VLT will be used in combination with our own evolutionary synthesis models. Determining the properties of the stellar populations in samples of Luminous and Ultraluminous Infrared galaxies (ULIRGs), and in QSOs. Investigating the possible evolutionary link between ULIRGs, QSOs and normal elliptical galaxies. Find out the origin of the extended emission line region, and the possible connection with merger processes using the morphology and the kinematics of the ionized gas.
  3.  To study feedback processes in ULIRGs, QSOs and Radio Galaxies to characterize them (energetics, time and spatial scales, origin) and evaluate their impact on the evolution of the system. This study will have direct implications on current models of galaxy formation, which predict that AGN feedback is an essential mechanism to regulate star formation activity and galaxy growth. Deep imaging, long slit and integral feld spectroscopic data obtained with the VLT, Calar Alto and GTC facilities will be used.
  4. To perform the first detailed study of the general spatial properties (morphology, ionization, physical properties, kinematics, metal abundances) of the gas in the recently discovered family of optically selected type 2 quasars from the SDSS. Imaging, long slit and integral field spectroscopic data from VLT, Calar Alto and GTC facilities will be sued. The origin of the gas and the role of feedback processes will be investigated for the first time in type 2 quasars.
  5. To perform a multifrequency characterization of different types of AGNs, from LINERs to Seyferts. To study in detail the mechanisms responsible for the AGN emission at different frequencies from high energies to radiofrequencies and to model the Spectral Energy Distribution (SED).
  6. To quantify the relationship between AGN activity class (LINER, Seyfert 1 or 2), the morphological type of the host galaxy and the environment, in order to understand whether the AGN activity is more related to merger/interacion processes or it is due to the secular evolution in the hosting galaxies.
  7. To characterize different AGN classes at X-ray frequencies to be able to test the Unification models for AGNs. Results based on our previous studies and recent large X-ray surveys have led to the discovery of a large population of obscured AGNs at different redshifts. Further studies are required to obtain a realistic census of AGNs and evaluate the contribution of these obscured objects to the X-ray background radiation. The MIR properties of these galaxies will also be characterized. This will provide information about the effects of dust obscuration and further tests to the unification models will be possible.
  8. To analyse the kinematics of the gas and the stars in the central circumnuclear regions of Active Galaxies. This will provide important information about the mechanisms responsible for funnelling the gas towards the center and the way the gas is fed to the central massive black hole.