Research and Highlights

This page illustrates some of my main research activities, along with some of the associated highlights.

Galaxy formation in the early Universe

Tracing the formation of the first galaxies in the early Universe is one of the hottest topics in Astrophysics, but also one of the most challenging areas, since emission from such distant (and generally low mass) galaxies is very weak at most wavelengths. I have been pioneering the use of far-IR fine structure lines to identify and characterize primordial galaxies. Far-IR fine structure lines are generally the strongest emission lines in the spectrum of any galaxy,  hence the most promising tool to detect and trace galaxies at high redshift. Among these lines the [CII]158um is often the strongest.

The first detection of [CII]158um at high redshift, specifically at z=6.4, was obtained by Maiolino et al. (2005). This discovery has opened the era of using far-IR lines for searching and characterizing distant galaxies, nowadays one of the main science cases of major facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA). Since the first [CII] detection at high redshift, several groups world wide have used far-IR fine structure lines to identify distant galaxies and to trace their evolutionary processes, with steadily growing detection rates and delivering exquisite maps of the interstellar medium and circum-galactic medium in these primordial systems.

Additional highlights obtained by exploiting this technique by members of my team or collaborating groups include: the first fully resolved interferometric maps, revealing extended, optically elusive star forming systems (Gallerani, Neri, Maiolino et al. 2012); the detection of other far-IR lines at high redshift, used to constrain the physical properties and metal content of distant galaxies (Nagao, Maiolino, et al. 2012, De Carli et al. 2012); the discovery of exceptionally strong [CII] emitters (Maiolino et al. 2009, De Breuck et al. 2011); the detection of massive outflows and very extended star formation in primordial systems (Maiolino et al. 2012, Cicone, Maiolino et al. 2015); the detection of [CII] at z=7.1, i.e. well within the re-ionization epoch, tracing clumps of gas accreting onto primeval galaxies (Maiolino et al. 2015);  dynamical maps of massive systems at high redshift (De Breuck et al. 2014, Walter et al. 2009, Carniani et al. 2014); identification of extremely obscured primordial galaxies (Gallerani, Neri, Maiolino et al. 2012, Walter et al. 2012).

Left: First [CII] detection at high redshift (z=6.4), obtained with the IRAM 30m millimeter telescope (Maiolino et al. 2005). Right: cold gas traced by [CII] emission (red) accreting onto a primeval galaxy (green) at z=7.1 (Maiolino et al. 2015) (image credit: ALMA Press Release)

Massive galactic outflows

Galactic outflows are invoked by most models to remove gas from galaxies, hence regulating formation of stars. In particular, fast and very massive outflows driven by supermassive accreting black holes (quasars, or more generally Active Galactic Nuclei, AGNs) have have been invoked by most models of galaxy evolution to prevent massive galaxies to overgrow. Observational evidence for quasar-driven massive outflows, in the process of "cleaning" massive galaxies of their gas content, hence quenching star formation, was achieved only recently through the detection of molecular high velocity gas in nearby quasar hosts, extending on large galactic scales, traced thanks to interferometric observations of the CO transitions at millimeter wavelengths (Feruglio, Maiolino, et al. 2010). In parallel, at the same time, molecular outflows were identified through the detection of blueshifted absorption of molecular transition in the far-IR (Sturm et al. 2011). Extension of the same technique to a larger sample of local galaxies have revealed that massive outflows are commonly found in galaxies with vigorous star formation, but the presence a quasar/AGN can boost the outflow rate by a large factor (Cicone, Maiolino et al. 2014), supporting the expectation of models. The same technique is now routinely being used by several groups to identify outflows of cold gas in several galaxies. 

By exploiting far-IR fine structure lines we could identify for the first time massive quasar-driven outflows at high redshift, illustrating that this feedback mechanism was already in place in the early univese, soon after the formation of the first galaxies (Feruglio, Maiolino, et al. 2010; Cicone, Maiolino et al. 2014; Cano-Diaz, Maiolino, et al. 2012; Maiolino et al. 2012; Cicone et al. 2015).

By also mapping the distribution of star formation in the host galaxies of distant quasars, we could find the first direct evidence of suppression of star formation by quasar-driven outflows (Cano-Diaz, Maiolino et al. 2012). Similar results have subsequently been nicely confirmed by the analysis of other quasar host galaxies at high redshift (Carniani, Marconi, Maiolino et al. 2015, Cresci et al. 2015).

Galaxy starvation/strangulation

An alternative mechanism that can transform star forming galaxies into passive/quiescent galaxies is the so-called galaxy "starvation" (often refereed to as galaxy "suffocation" or galaxy "strangulation"). According to this scenario star formation in galaxies is quenched because the inflow of gas from the Intergalactic Medium is halted, as a consequence star formation in these galaxies can continue only for a limited amount of time by using the gas available in the galaxy. In this scenario, the metallicity (i.e. the content of elements heavier than helium, produced by star formation) of these "strangled" galaxies should increases rapidly, due to the lack of dilution from the inflow of external gas. The analysis of stellar metallicities of 26,000 galaxies in the local universe has revealed that, for low/intermediate mass galaxies, passive systems are on average much more metal rich than their star forming progenitors, which is in agreement with the strangulation scenario. This result supports the idea that "strangulation" is the main quenching mechanism for low/intermediate mass galaxies (log[Mstar/Msun)<11), i.e. the bulk of the galaxy population (Peng, Maiolino, Cochrane 2015).

In the low mass galaxies (log[Mstar/Msun]<10) there is some evidence that the effect depends on the environment (i.e. the overall density of galaxies) in which galaxies live. This can be explained in a scenario where small galaxies accrete into a massive hot halo which prevents further accretion if cold gas onto the galaxy. However in more massive galaxies no significant environmental effects are seen, hence in these systems other mechanisms must be responsible for the "strangulation" of galaxies.

Metallicity evolution throughout the cosmic epochs

The galaxy gas-phase metallicity (i.e. the content of metals heavier than helium in the interstellar medium) is another powerful tracer of the processes involved in galaxy evolution, indeed the gas metallicity is directly connected to the history of star formation (which is responsible for the production of metals), with the presence of outflows (which generally eject metal-enriched gas) and accretion of (~pristine) gas from the intergalactic medium, which generally dilutes the galaxy metallicity. By exploiting an extensive Large Programme ("AMAZE") at the European Southern Observatory , targeting distant star forming galaxies, it was found for the first time that the metallicity of galaxies evolves steeply at z>3 (Maiolino et al. 2008, Troncoso, Maiolino et al. 2014). The first metallicity maps of such primeval galaxies have revealed that the origin of such evolution is associated with massive gas inflows and outflows in the early universe (Cresci, Mannucci, Maiolino et al. 2010, Troncoso, Maiolino et al. 2014). The detailed, spatially resolved metal budget of nearby galaxies reveal that such gas flows, occurred throughout the galaxy lifetime, leave their clear imprint on the metallicity distribution of local galaxies (Belfiore, Maiolino & Bothwell 2015).

We have further shown that the metallicity of galaxies follow well defined scaling relations with other galaxy properties, such as stellar mass, star formation rate, atomic gas content and molecular gas content, often with small scatter, and that such relations seem persist at high redshift (Mannucci, Cresci, Maiolino et al. 2010; Bothwell, Maiolino, et al. 2013; Bothwell, Maiolino et al. 2016; Maiolino et al. 2008; Troncoso, Maiolino et al. 2008). Such tight scaling relations have been modeled by several theoretical groups and generally support a scenario in which the evolution of the bulk of the galaxy population evolves following smooth evolutionary processes, in which star formation, inflows and outflows are generally close to equilibrium (e.g. Peng & Maiolino 2014a).

We also found that the metallicity of galaxies depends strongly on the environment in which they live, with galaxies living in overdense environments being systematically characterized by higher metallicity (Peng & Maiolino 2014b), which has been interpreted as evidence that galaxies in dense environments accrete gas that has been pre-enriched (and expelled into the intergalactic medium) by other galaxies. Indication for a similar effect has been found in distant galaxies at z~1.5 (Williams, Maiolino et al. 2013), suggesting that the intergalactic medium is already highly enriched at these early epochs.

I have also investigated the metallicity of quasar and AGN host galaxies which show a remarkable and puzzling lack of evolution, out to z~6 (Maiolino et al. 2003, Nagao, Maiolino, Marconi 2006, Juarez, Maiolino et al. 2009, Nagao, Marconi, Maiolino 2006). This effect can be probably explained in terms of selection effect of quasar surveys at high redshift.

The origin of dust in the early Universe

The origin of the first solid particles (dust grains) in the early Universe is an extremely hot topic. Indeed, while in the local Universe most of the dust is produced in the atmospheres of evolved stars, which require about one billion years to evolve, the origin of dust in the early universe (z>6) when the age of the universe was comparable or shorter than this time, has been puzzling. The investigation of the dust extinction curves at of high redshift quasars and Gamma Ray Bursts has revealed that most of the dust in the early universe has been produced promptly in the ejecta of core-collapse Supernovae (Maiolino et al. 2004; Gallerani, Maiolino et al. 2010; Stratta, Maiolino, et al. 2007; Stratta, Gallerani, Maiolino 2011). This discovery has had implications on models of early galaxy evolution, early star formation (dust in primeval galaxies is an important coolant enabling the fragmentation of gas clouds and therefore the formation of the first low mass stars), and has had implications on the interpretation of the observational properties of distant galaxies.

The evolution of dust and gas content in galaxies

The content of dust in galaxies provides an alternative method to investigate the evolution of metals in galaxies (typically about half of the metals are condensed into dust grains) but can also used as a potentially powerful tool to investigate the evolution of the gas content in large samples of galaxies (by exploiting the fact that the dust-to-gas ratio follows well known scaling relations). We have used far-infrared and submillimeter observations with the Herschel satellite to measure the content of dust in thousands of galaxies in the redshift range 0<z<2.5 (Santini, Maiolino et al. 2014).  It was found that both the dust content and the inferred gas content in galaxies follow simple, constant scaling relations with the stellar mass and with the star formation rate, at any epoch, out to z~2.5. However, galaxies populate these scaling relation in a different way at different epochs. The result of this effect is a net strong evolution of the gas content in galaxies at high redshift, in a differential way for galaxies with different masses. 

The same technique has been applied to AGN host galaxies, enabling us to investigate the gas content in a large sample, as well as an extensive control sample, vastly expanding both in size and redshift distribution relative to our previous study based on CO millimeter observations (Maiolino et al. 1997). The new data reveal that AGN host galaxies are systematically more gas rich than the galaxies not hosting AGNs (Vito, Maiolino et al. 2014).  Such correlation can be interpreted simply in terms of gas rich galaxies having a higher probability for a gas cloud to fall within the sphere of influence of the supermassive black hole. The

Circum-nuclear medium, obscuration and Unified Model of Active Galactic Nuclei.

I have extensively investigated the properties of circumnuclear medium in accreting supermassive black hoes (Active Galactic Nuclei, AGNs) especially in the context of the Unified Model, according to which the observational properties of some of the main classes of AGNs can be explained in terms of orientation of our line of sight relative to an obscuring (~axysimmetric, possibly toroidal) structure.

By analyzing the properties of a well defined sample of local AGN, we proposed for the first time the need for two absorbing structures, one on small scales (~1 pc), unrelated relative to the host galaxy, responsible for the heavy obscuration of the nucleus, and a second one on large scales (~100 pc), aligned with the galaxy disc, and responsible for milder absorption and obscuration (Maiolino & Rieke 1995).

By exploiting some of the first sensitive hard-X surveys, we could establish for the first time the distribution of absorbing column densities around AGNs, revealing for the first time a large population AGNs absorbed by extremely large column densities ("Compton thick AGNs", Maiolino et al. 1998, Risaliti, Maiolino & Salvati 1999, Bassani, Dadina, Maiolino et al. 1999).

By comparing the obscuration in the optical with the  X-ray absorption, I found that the dust absorption in the optical is much lower than expected from the column of gas inferred from the X-rays. I inferred that either the absorbing gas must be dust-poor or that the dust properties must be different than in the diffuse intestellar medium (Maiolino et al. 2001a, Maiolino et al. 2001b). Subsequent studies found evidence for both scenarios. In particular, X-ray monitoring of AGNs revealing variable absorption has revealed that a significant fraction of the absorbing medium must reside within the "dust sublimation radius", close to the accretion disc, where dust cannot survive. By analyzing in detail the variation of the X-ray absorption I could study in detail the morphology and the physics of the clouds orbiting and eclipsing the black hole accretion disc. The data have revealed that such clouds have a cometary shape and that, by loosing large amount of matter in the tail, they have a lifetime of only a few months and dissolve completely afterwards (Maiolino et al. 2010).

On larger scales I could measure the distribution of dust (outside the sublimation radius) and in particular its covering factor relative to the accreting black hole, by measuring the emission of hot dust (observed in the mid-infrared) relative to the primary radiation emitted by the accretion disc; I inferred that the dust covering factor decreases with AGN luminosity (Maiolino et al. 2007). By using hard X-ray observations I also found evidence for a population of AGNs that do not show any signature of nuclear activity at optical wavelengths, probably because completely embedded and  covered by dust; such objects were dubbed "Elusive AGNs" (Maiolino et al. 2003), and subsequent surveys have found several additional examples of this population, also at high redshift.

Obscured Supernovae

Core collapse supernovae are explosions resulting from death and collapse of massive young stars. These are typically associated with regions of ongoing star formation. The latter are generally embedded in large amount of dust, which is likely to obscured a significant fraction of core-collapse supernovae, preventing their detection in typical optical surveys. I have led a campaign of monitoring of star forming galaxies at near-infrared wavelengths, where the  dust obscuration is much lower. This campaign has led to the discovery of some of the first near-infrared selected supernovae (Maiolino et al. 2002). These result  to be much more absorbed by dust than optically selected supernovae. These observations have enabled the (upward) revision of the supernova rate in star forming galaxies (Mannucci, Maiolino et al. 2005).

Warm-Hot Intergalactic Medium

Several popular cosmological models predict that a large fraction of the baryonic mass in the local Universe is located in filamentary and sheet-like structures associated with galaxy overdensities. This gas is expected to be gravitationally heated to temperatures of about one million degrees, therefore emitting in the soft X-rays. By analysing wide field soft-X ray images we have obtained some of the very first evidences of diffuse X-ray emission associated with galaxy overdensities (Zappacosta, Maiolino et al. 2005a, Zappacosta, Mannucci, Maiolino et al. 2002, Zappacosta, Maiolino et al. 2005b). The most likely interpretation of such diffuse soft X-ray emission is that it is tracing Warm-Hot Intergalactic Medium associated with the overdensity of galaxies. 

Additional evidence for Warm-Hot Intergalactic Medium was obtained through X-ray spectroscopy of quasars in the background of large scale structures traced by galaxy overdensities, which have revealed absorption features associated with highly ionized species (O VII) at the same redshift of the large scale structure (Fang et al. 2010, Zappacosta, Nicastro, Maiolino et al. 2010).