Why the efficiency of galaxy formation, in particular at high masses, is so incredibly low? Why galaxies during their cosmological evolution stop forming stars? Despite large observational and theoretical efforts, these questions remain largely unanswered.
Observations indicate that above a quenching mass of ~1E11MSun the star-forming galaxy (SFG) mass function drops exponentially, as the fraction of gas mass to stellar mass, and the gas metallicity. Evidences point to some form of “mass” quenching driven by the star-formation rate, but the physical process that is responsible for the quenching has not been identified yet. Energy injection from SN, does not work because the bounding energies of such massive galaxies largely exceeds the energy that SN can inject in the systems. Accreting black holes (BH) are much more efficient machines, and the energy they can deposit in their host galaxy inter-stellar medium (ISM) may be enough to quench star-formation. Indeed, huge BHs (M=1E9-1E10MSun) have been found in most local “red & dead” bulges. Most of the BH and galaxy stellar mass of local ellipticals must had been assembled at early times during highly active phases, suggesting that high-z luminous quasars may well be the cradles of local giant galaxies. However, convincing AGN feedback observations are sparse, and we do not even know the real fraction of active galaxies showing galaxy-scale winds, prompting skepticism on the relevance of AGN feedback in the context of galaxy transformations. As stated in the last Collection of Nature Astronomy dedicated to AGN outflows and feedback: “Contradicting results mean that the jury is still out”. BH feedback remains a “stone-guest” in the landscape of galaxy evolution models.
The main goal of our project is to finally solve this deadlock, by assessing or disproving BH feedback as a physical driver for the building of massive galaxies.
A breakthrough in our understanding of the physical mechanism(s) responsible for the low efficiency of SF in massive galaxies can only be obtained by overcoming the limitations of previous studies. In this project we will exploit cutting edge instrumentation that came on line in the past few years and which provided us a large amount of observational data (detailed in the tables in the next section). By analyzing this large dataset with state of the art physical models, we will be able to validate or disprove AGN feedback as the quenching mechanism.
We aim at exploiting the unprecedented spectro-imaging capabilities of MUSE, SINFONI at the ESO/VLT and ALMA to target ionized and molecular gas in AGN host galaxies. These ESO facilities are today the flagship instrumentation for astrophysics, placing the European community in a worldwide leading position. One of the goals of this proposal is to allow our team to compete on equal levels with our European colleagues on the exploitation of these key facilities.
To assess the importance of AGN feedback on massive galaxy formation and evolution, BLACKOUT aims at achieving the objectives listed below.
“Supposing is good, but finding out is better” (Mark Twain)
OB1 – Build the first unbiased census of BH winds. We aim at obtaining BH wind, AGN and galaxy physical properties in unbiased samples of galaxies at different redshifts. To this purpose, we use volume limited samples, and AGN samples extracted randomly from flux limited samples. We will then use these samples to study the correlations between wind properties, AGN properties (luminosity, luminosity/Eddigton luminosity, obscuration), and galaxy properties (stellar and gas masses, SFR, morphology), minimizing selection biases.
OB2 – Study the multiphase gas in and around accreting BH host galaxies. We will: a) search for any direct modification of the ISM by BH winds and jets with a resolution good enough to resolve at least galaxy disks and BH winds; b) probe in accreting BH host galaxies the gas fraction and the depletion timescales, which are indirect evidences of modification of the ISM by the winds [24,26]; c) probe the CGM of luminous quasars through the search for companions, the search and characterization of Ly -nebulae, which should include both enriched gas outflowing from galaxies and inflowing pristine gas, and the search for evidence of radiative feedback from the accreting BH; d) use samples of local galaxies with accreting BH as laboratories to study the ionized, neutral and molecular ISM physics, to guide models and interpretation of observations with coarser resolution.
OB3 – Obtain the first local BH mass function and build unbiased BH-bulge scaling relationships. Any AGN/galaxy co-evolution model must reproduce the local observables, such as SF and BH accretion rates, galaxy and AGN masses and bolometric luminosities. However a clear picture of the (even local) AGN accreting phases is still missing, as, until few years ago, it has been almost impossible to measure the BH masses of obscured AGN, which represent almost 2/3 of the active galaxies. As a consequence, we still miss a reliable measure of the BH mass function and its evolution. Moreover, there are indeed evidences that the measures of the BH-bulge scaling relations are biased in favor of the most massive BHs [12]. We will measure BH masses and build BH mass function and unbiased BH-bulge scaling relations.
OB4 – Build observationally motivated, physical BH winds models. Based on the results from OB1, OB2, and OB3, and on results achieved through another synergic programs based on the analysis of X-ray data to probe nuclear winds. We will develop both nuclear wind and galaxy scale wind physical models, including cooling and realistic galaxy mass (ISM, DM, stars) distributions in which the wind expands. We will then plug BH wind models into semi-analytical and numerical models for galaxy formation and evolution, and analyze the output.
