Scientific Interests
A dusty Universe
The Universe reveals a totally different beauty at mm-wavelengths compared with optical observations. The emission of dust in distant galaxies, powered by intense star formation events, reveals the build-up of galaxies in remote systems. A plethora of emission lines trace the presence of molecular, atomic and ionized gas. Thanks to recent enormous upgrades in the capabilities of mm interferometers we are now facing unprecedented opportunities to understand how galaxies formed and grew.
The fuel of star formation
The rate of star formation in a certain region of the universe evolved across cosmic time. It increased slowly from the very early epochs until redshift 1-3, when it reached a peak (the "epoch of galaxy assembly", when half of present-day stars were formed), and subsequently dropped by an order of magnitude. What drives such an evolution? The answer may lie in the evolution of the molecular gas reservoir in galaxies. The molecular phase of the interstellar medium is considered the fuel of star formation. Posing observational constraints on how much molecular gas galaxies have across the cosmic time can shed new light on what drives the growth of galaxies. Such an endeavor is now possible for the first time, thanks to the technological progress of available instruments.
Comoving cosmic mass density of molecular gas in galaxies, ρ(H2), as a function of redshift, based on molecular deep fields (boxes). Predictions from semi-analytical models and empirical studies are also shown. The global molecular content of galaxies at the peak of galaxy formation appears 3–10 times higher than in galaxies in the local universe, a similar trend as observed in the cosmic star formation history. Figure adapted from Decarli et al. (2016).
The forbidden universe
In local galaxies, studies of the physical properties (temperature, density, metal abundances, ionization conditions) of the interstellar medium (ISM) are traditionally based on the luminosity of various emission lines ([OI], [OII], [OIII], [NII], etc) associated with forbidden transitions of electrons in atoms and ions in the diffuse medium. This is not feasible at high-redshift, since these lines are shifted into the MIR range, where sensitive spectroscopy is out of reach. An alternative, yet relatively unexplored way to probe gas conditions in distant galaxies is offered by fine-structure forbidden emission lines. These lines arise from the same atomic species as mentioned before, but they have rest-frame wavelengths of 50-500 micron, i.e., they are redshifted into the (sub-)mm transparent windows of the atmosphere at high-z. Over the last few years, several lines have been observed in high-redshift galaxies: [CII], [NII], [OI], [OIII], [CI]. By comparing the properties of all these lines we can constrain the gas density, temperature, ionization conditions and metallicity; we can also use these lines to infer ionized and neutral gas masses; finally, we can compute dynamical masses of the host galaxies. The observation of these lines opens up a new insight on star formation in the young universe.
Top left: Continuum dust emission at 1 mm in MM18423+5938, a strongly-lensed sub-mm galaxy at z=3.930. The Einstein ring is clearly resolved. Top right: Continuum-subtracted [NII] emission in MM18423+5938. Bottom left: Maps of the red and blue wings of the [NII] emission. A velocity gradient is apparent. Bottom right: Map of three different ingredients of the star-forming medium: Dust, ionized gas (contours) and molecular gas (color scale). Differences in the morphologies reveal the complexity in the star-forming medium in this galaxy. Taken from Decarli, et al. (2012).
Quasars
Quasars are massive black holes (millions to billions of Solar masses) which experience dramatic accretion of gas from their host galaxies. This process is normally associated to intense energy releases, especially in terms of electromagnetic radiation, which make the quasar shine. The luminosity of a quasar can be several order of magnitudes larger than the one of the entire host galaxy. This allows us to study the properties of accreting black holes in distant galaxies. Moreover, quasars represent excellent probes of the Universe throughout all of its history.
Massive black holes and their formation
The central engine of quasars, the accreting massive black hole, is an extreme concentration of mass (billion times the mass of the Sun) in a region of universe that could fit within the solar system. Models to explain their formation can be tested by searching for the earliest massive black holes. By pushing the redshift frontier in quasar searches, we push constraints on the early growth of massive black holes, their formation mechanisms, and the first stages of their evolution.
Constraints on the build-up of early massive black holes set by high redshift quasars (points), assuming Eddington-limited growth (lines). By pushing the redshift frontier, larger initial masses, higher formation redshift, and/or phase of super-Eddington accretion need to be taken into account. Figure from Banados et al. (2018).
Black holes and host galaxies
One of my main research topic is the study of the interplay between massive black holes and their host galaxies. The singularity and its host galaxy share a joint evolution, with the gaseous reservoirs feeding both black hole accretion and star formation in the host galaxy, and feedback processes regulating both processes. By characterizing the stellar population of the host galaxies, as well as the physical properties of the interstellar medium, we obtain a unique insight on the co-evolution between black holes and host galaxies, the physical processes that ruled this evolution.
Maps of the unobscured star formation, mapped with the Hubble Space Telescope sampling the rest-frame UV light; of the gas, mapped via the [CII] 158 micron emission line with ALMA; and of the obscured star formation, mapped in the dust 1.2 mm continuum with ALMA, in a quasar host galaxy at z~6.2. The comparison between these maps highlight the complex interplay between different components in the host galaxy. Figure adapted from Decarli et al. (in prep).