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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).
![ALMA press release ALMA press release](eso1530a.jpg)
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).
![cicone et al. 14 cicone et al. 14](qso_outflow.jpg)
Discovery of quasar-driven massive molecular outflows through the detection
of broad CO(1-0) wings in nearby quasars (Feruglio,
Maiolino et al. 2010, Cicone,
Maiolino et al. 2014).
![cicone et al. 2015 cicone et al. 2015](cicone15.jpg)
Discovery of a super-massive outflow driven by a quasar at z=6.4, through
the detection of extended broad wings of the [CII]158um line (Maiolino
et al. 2012, Cicone,
Maiolino et al. 2015).
![quasar quenching star formation quasar quenching star formation](cano_diaz_maiolino_2012.jpg)
Quasar driven outflow (high velocity gas traced by the [OIII]5007 line,
white contours) quenching star formation (traced by Halpha emission, color)
in a galaxy at z=2.4 (Cano-Diaz,
Maiolino et al. 2012).
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.
Left: expected evolution of the stellar mass and stellar metallicity in two
different quenching scenarios: sudden gas removal (e.g. by outflows) and
"starvation"/"strangulation". Right: observed distribution of the average
stellar metallicities of passive and star forming galaxies in the local
universe, supporting starvation/strangulation as primary quenching mechanism
in low/intermediate mass galaxies (Peng,
Maiolino, Cochrane 2015).
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.
Map of the emission line flux (~Star Formation Rate), velocity field and
metallicity in a galaxy at z=3.5; the metallicity dip in the central region
is interpreted in terms recent accretion of pristine gas (Cresci
et al. 2010).
![belfiore et al. 2015 belfiore et al. 2015](belfiore.jpg)
Radial budget of metals in a nearby galaxy: the blue line indicates the
total amount of metals produced by the observed stars at various
galactocentric radii; the red line shows the total amount of metals observed
(in stars and in the interstellar medium), clearly highlighting a metal
deficit relative to the metals produced, implying a major loss of metals at
all galactic radii (Belfiore
et al. 2015).
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.
![dust_highz dust_highz](dust_highz.jpg)
Observed extinction curve of a quasar at z=6.2, which nicely matches the
extinction curve expected from Supernova-produced dust (Maiolino
et al. 2004).
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
Observed distribution of the dust mass, in bins of stellar mass and star
formation rate, at different redshifts, and by combing all redshifts
together. (From Santini,
Maiolino et al. 2014).
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.
![comets around black hole comets around black hole](comet.jpg)
Model of the clouds orbiting and eclipsing the supermassive accreting black
hole as inferred from hard X-ray monitoring data (Maiolino
et al. 2010).
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).
![obscured supernova 2a obscured supernova 2a](sn2.jpg)
Left: near-infrared images of a galaxy at two epochs showing the discovery
of a supernova. Right: optical and near-IR spectra confirming that the
supernova is a core-collapse one (from Maiolino
et al. 2002).
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).
![whim detection whim detection](zappacosta1.gif)
Diffuse soft X-ray emission, associated with an overdensity of galaxies at
z=0.47, associated with Warm-Hot Intergalactic Medium (from Zappacosta,
Mannucci, Maiolino et al. 2002)