Here I summarize some of the projects that I am currently working on and in which I have a main role.
The search of the highest-redshift quasars
Luminous high-redshift quasars provide direct probes of the evolution of supermassive black holes and the intergalactic medium at early cosmic time. The discovery of more high-redshift quasars is crucial in order to have a complete sample to further study and understand the Universe when it was about one billion years old. We can give a fundamental step forward in this point using the Panoramic Survey telescope & Rapid Response System 1 (Pan-STARRS1), which gives us the unique opportunity to search for quasars up to z~7.2 in an area covering 3/4 of the sky. We started our quasar search in the redshift range 5.7< z < 6.2 and we present our initial results -- the discovery of eight additional z~6 quasars -- in Bañados et al. 2014 . The new sample shows a variety of quasars properties, in terms of both luminosities and spectral features. The fraction of weak-line emission quasars found in this work is much higher than in previous studies, implying that the z~6 quasar population could be more diverse than previously thought. We are now extending our selection criteria to discover quasars at even higher redshifts.
The galaxy environment of high-redshift quasars
High-redshift quasars are thought to reside in the most massive dark matter halos in the early Universe and should therefore be located in fields with overdensity of galaxies, which will eventually evolve into galaxy clusters seen in the local Universe. So far, despite many efforts, no unambiguous relation between galaxy overdensities and high-redshift quasars has been found, mainly due to the difficulty of finding galaxies with accurate redshifts above z~6. We use broad/narrow band imaging in 8m class telescopes to select and analyze galaxies in quasars' vicinity. Our first study of Lyman Alpha Emitters (LAEs) around the quasar ULAS J0203+0012 at z=5.72 showed that this quasar does not reside in an overdense region. The density of galaxies around ULAS J0203+0012 is comparable to the density expected in a blank field. This work is published in Bañados et al. 2013. This result is in strong contrast with some predictions based on current galaxy and black hole formation models. However, this conclusion is currently only based on one source, then more studies are critical.
The CO molecular gas in high-redshift quasars
In order to understand the physical properties under which star formation takes place in different starburst environments and find a connection with AGN activity, the most promising technique is to study directly the properties of the molecular gas as it is the material that is potentially both fueling the starburst and feeding the AGN. The differences in the CO excitation of quasars and other high-redshift galaxies are still unclear, mainly because there are not many quasars with multiple CO rotational level transitions measurements (low- and high-J), thus, no constraint on the gas excitation for most of them. We are building a sample of quasars at 2.06 < z < 5.03 with mid- and high-J CO transitions. This sample is important to understand the differences in the CO excitation of quasars and other high-redshift sources, and to get an idea for the range in CO excitation observed within the population (Bañados et al. in prep.).