Osservatorio Astronomico di Roma

The WISSH quasars project: revealing ultra-massive black-holes and powerful winds in the most luminous AGN
2017-06-13 11:45 - 12:45 (Europe/Rome) | Aula Gratton
The systematic investigation of hyper-luminous quasars shining at the golden epoch of AGN activity offers the unique opportunity of studying the power and the effect of AGN feedback at its extreme.
The WISE/SDSS selected hyper-luminous (WISSH) quasar survey is an extensive
multi-band observing program (from millimeter wavelengths to hard X rays) designed to accurately probe the role of nuclear activity in SMBH-galaxy self-regulated growth via extended outflows.
Our on-going project aims at constraining both AGN and host galaxy ISM and star-formation properties in a large sample of ~90 broad-line quasars at the brightest end of the AGN luminosity function (L_bol > 1e14 L_sun), and at the peak of their number density (z ~ 2.5 – 3.5).
I will review the most important results of the near-IR spectroscopic follow-up of WISSH quasars (available for ~40% of the sample) performed with the LUCI at LBT and SINFONI.
We found that WISSH quasars are typically powered by highly accreting (0.3-3 Ledd), ten billion solar masses SMBHs, demonstrating that WISSH provides a simple and valuable tool to complete the census of the extreme SMBH population in the universe. We also succeeded in discovering [OIII] emission lines with a broad, skewed profile and exceptional luminosities (> 6e44 erg/s), tracing very powerful ionized outflows (up to ~4% of L_bol) in ~30% of the sample.
Remarkably, the remaining 70% of quasars lacks [OIII] emission but shows strong winds traced by 3,000-8,000 km/s blueshifts of the high-ionization (CIV) with respect to low-ionization (Hbeta) broad emission lines, revealing strong radiatively driven winds that dominate the BLR kinematics. Finally, I will discuss the possible causes behind this intriguing dichotomy which involve fundamental parameters such as bolometric luminosity, SMBH mass, Eddington ratio and the shape of the UV-X-ray spectrum.

Dwarf spheroidal satellite galaxies under the eye of Cherenkov telescopes: indirect search for dark matter by MAGIC and opportunities and prospects for CTA
2017-06-06 11:45 - 12:45
Searching for gamma-ray signals from dark matter annihilation in dwarf
spheroidal satellite galaxies (dSphs) of the Milky Way plays a critical
role within the framework of indirect dark matter searches. This study,
aimed at unveiling the nature of the most elusive component of the
Universe, is potentially able to confirm the dark matter interpretation of
the observed gamma-ray excess in the Galactic Center.
Thanks to deep observation campaigns on dSphs, MAGIC achieved stringent
constraints to dark matter annihilation cross section, testing new
analysis tools for the investigation of extended regions. In this talk, I
will present both results achieved by MAGIC and the importance of DM
tailored analysis tools and the proper selection of new dSph targets, in
order to propose scientific cases for CTA, the upcoming generation of
Cherenkov telescopes.

What process(es) produced barium in the Early Universe?
2017-05-23 11:45 - 12:45
In the last years our group has found that many chemical anomalies
observed in very metal-poor halo stars in the light elements suggest
that the first stellar generations were fast rotators
(spinstars). Recently, theoretical computations have found that
spinstars can also play a role in the chemical enrichment of neutron
capture elements providing a early contribution of s-process. By means
of a stochastic chemical evolution model, it is possible to identify
the spinstars s-process contribution as the missing component
responsible for the spread in the ratio between light (Sr) to heavy
(Ba) neutron capture elements. A specific distribution is predicted
for the isotopic ratio of Ba in halo stars and this imprint could be
the smoking gun of the role played by spinstars in the spread of
[Sr/Ba] ratio. In this context, regarding the still unknown origin of
the complementary r-process component, I present new constraints on
the rate and time scales of r-process events, based on the recent
discovery of the r-process rich stars in the ultra faint galaxy
Reticulum 2.

Orbital evolution of AMXP in the context of non-conservative mass transfer
2017-05-16 11:45 - 12:45 | Aula Gratton
In this seminar I present:
1) a detailed discussion of the orbital evolution of SAXJ1808.4-3658 and other Accreting Millisecond Pulsar in the context of the Applegate & Shaham mechanism.
2) a discussion of the orbital evolution of AMXP in the context of
non-conservative accretion models, and in particular of the so called
Radio-Ejection hypothesis (Burderi et al. 2001, ApJ; Di Salvo et al. 2008, ApJ).
In particular I will apply these models to the orbital evolution of
the accreting millisecond pulsar SAXJ1808.

A review in mixed-chemistry of low-mass evolved stars
2017-05-08 11:45 - 12:45 | Aula Gratton
During the late stages of their evolution, Sun-like stars bring the products of nuclear burning to the surface. There is a chemical dichotomy between oxygen-rich and carbon-rich evolved stars, however, in the last three decades, a few stars have been shown to display both carbon-and oxygen-rich material in their circumstellar envelopes. These phenomena is seen in both Galactic Disk and Bulge planetary nebulae. For the Galactic Disk objects the mixed chemistry phenomenon is best explained through a recent dredge-up of carbon produced by nucleosynthesis inside the star during the Asymptotic Giant Branch that changed the surface chemistry of the star. On the contrary, the mixed chemistry phenomenon occurring in the Galactic Bulge planetary nebulae is best explained through hydrocarbon chemistry in an ultraviolet (UV)-irradiated, dense torus.

The chemical composition of globular clusters throughout the Local Group
2017-05-02 11:45 - 12:45 | Aula Gratton
I will discuss results from our recent work on the detailed chemistry of GCs in Local Group galaxies. This work is based on a combination of integrated-light spectroscopy at high resolution and resolved imaging with HST in colour combinations that are sensitive to light-element abundance variations. Through the resolved imaging studies, it is now clear that the abundance anomalies that are the hallmark of multiple stellar populations are not restricted to old GCs, but are also found in LMC/SMC clusters as young as 2 Gyrs. This rules out formation mechanisms that were unique to the high redshift Universe. More generally, we find that GCs in dwarfs are, on average, more metal-poor than those in larger galaxies, although no clusters more metal-poor than [Fe/H]approx-2.5 have been found so far. Overall, a large fraction of the most metal-poor stars in dwarf galaxies tend to belong to GCs. This has important implications for scenarios that invoke heavy cluster mass loss to account for the large fractions of enriched “second-generation” stars in GCs, as well as for globular cluster disruption and its contribution to the field star populations in halos in general.

The detection of magnetic waves in the solar atmosphere, using the Sun as plasma physics laboratory.
2017-04-26 11:45 - 12:45 | Aula Gratton
The Sun is one of our best plasma physics laboratory. The recent direct identification of magnetic waves in the solar atmosphere (i.e. Alfvén and alfvénic waves) is an example of how the Sun can be exploited to advance plasma physics research. In this talk, starting from this recent discovery, I will discuss the implications that these new results can have in a number of fields, from astrophysics to fusion research. Further, I will briefly discuss what to expect from the next generation solar telescopes (DKIST and EST).

Elemental Abundances across Cosmic Time
2017-04-18 11:45 - 12:45 | Aula Gratton
I show how chemical enrichment took place in the Milky Way and in a cosmic scale, using my hydrodynamical simulations that includes star formation and feedback from stars, supernovae, and active galactic nuclei (AGN). Thanks to nuclear astro-physics collaboration, we now have good understanding of the origin of elements from carbon to zinc, and theoretical models have well reproduced the observations of these elemental abundances in the Milky Way Galaxy. Some new observations challenge these models for a few elements, which is due to the problems in stellar astrophysics. In our cosmological simulations, we also include super-massive blackholes that originate the first stars, which had only ~100-1000 Msun initially, contrary to the merger products in other simulations. Our AGN cause large-scale metal outflows, which result in the enrichment of intergalactic medium, as well as the quenching of star formation in massive galaxies. The simulated results are in good agreement with many observations of galaxies, including cosmic star formation rates, blackhole mass-galaxy mass relation, size-mass relation, and mass-metallicity relations of galaxies (which evolve with a steeper slope at higher redshifts), and metallicity radial gradients within galaxies (which can trace the merging history of the galaxies). I also show elemental abundances at high-redshift galaxies, which can be used to understand the formation and evolution of galaxies.

Unveiling the most elusive star-forming galaxies
2017-04-11 11:45 - 12:45 | Aula Gratton
Our knowledge of the cosmic star formation history is crucially dependent on completeness of the observed samples, and the best efforts must be made to ensure the best completeness levels against two classes of sources: intrinsically faint galaxies and heavily dust-obscured ones.
In the first part of the talk I will show recent results on the relation between the SFR and the stellar mass, i.e. the Main Sequence (MS) relation of star-forming galaxies, in the HST Frontier Fields. Gravitational lensing allows us to extend the analysis of the MS down to stellar masses as low as 10^7.5 Msun at z<4 and 10^8 Msun at higher redshift, i.e. a factor of ~10 below previous results. I will show that the scatter around the MS increases at such low stellar masses, suggesting a larger variety of star formation histories, in agreement with theoretical predictions.
In the second part of the talk I will present a detailed study of a representative of the elusive dust-enshrouded star-forming population that is typically not included in the measured SFH nor in the stellar mass density. Thanks to ALMA data, an apparently extremely dust-rich galaxy turned out to be a faint submillimeter galaxy (SMG) lensed by a foreground source and completely obscured at optical wavelengths. A photo-z of z~3.3 and the physical parameters are estimated thanks to an accurate reconstruction of the SED. With a SFR lower than 300 Msun/yr, this galaxy belongs to the class of faint SMGs (S870um~2.5mJy) that are important contributors to the cosmic SFH. This faint galaxy population thus likely represents an important and missing piece of our understanding of the cosmic SFH and galaxy evolution.

The dust mass in Cassiopeia A from spatially resolved Herschel photometry
2017-04-04 11:45 - 12:45 | Aula Gratton
The large reservoirs of dust observed in some high redshift galaxies have been hypothesised to originate from dust produced by supernovae from massive stars. Theoretical models predict that core-collapse supernovae (CCSN) can be efficient dust producers (0.1-1 Msun) potentially responsible for most of the dust production in the early Universe. Observational evidence for this dust production efficiency is however currently limited to only a few CCSN remnants (e.g., SN1987A, Crab Nebula) that confirm this scenario.
We revisit the dust mass produced in Cassiopeia A (Cas A), a ~330-year old O-rich Galactic supernova remnant (SNR) embedded in a dense interstellar foreground and background. We present the first spatially resolved analysis of Cas A based on Spitzer and Herschel infrared and submillimetre data at a common resolution of ~0.6 arcmin for this 5arcmin diameter remnant following a careful removal of contaminating line emission and synchrotron radiation. We fit the dust continuum from 17 to 500 micron with a four-component interstellar medium (ISM) and supernova (SN) dust model.
We find a concentration of cold dust in the unshocked ejecta of Cas A and derive a mass of 0.3-0.5 Msun of silicate grains freshly produced in the SNR. For a mixture of 50% of silicate-type grains and 50% of carbonaceous grains, we derive a total SN dust mass between 0.4 Msun and 0.6 Msun. These dust masses estimates are higher than from most previous studies of Cas A and support the scenario of supernova dominated dust production at high redshifts. Our resolved analysis shows that the cold SN dust component is mainly distributed interior to the reverse shock of Cas A, suggesting that part of the newly formed dust has already been destroyed by the reverse shock. We furthermore derive an interstellar extinction map which towards Cas A gives average values of AV=6-8 mag, up to a maximum of AV=15 mag.

The AGB-Supernovae mass transition
2017-03-27 09:00 - 2017-03-31 18:00 | Aula Gratton

Dwarf Galaxies - Fossils of Galaxy Evolution
2017-03-21 11:45 - 12:45 | Aula Gratton
Dwarf galaxies are the most common type of galaxy in the Universe and
include the most dark-matter-dominated objects known. They offer
intriguing insights into evolutionary processes at low halo masses
and low metallicities. Moreover, as survivors of a once much more
numerous population of building blocks of larger galaxies, they are
key to understanding very early star formation processes. The Local
Group and particularly the Milky Way’s dwarf galaxy entourage offer
us the unique possibility to compare in detail dwarf and Galactic
populations. This is an important step towards quantifying the
magnitude and time scales of dwarf contributions to the build-up of
the Milky Way and allows us to test predictions of cosmological
theories and hierarchical structure formation

Highly Ionized X-ray Absorbers in AGNs and their Energetic: the case of NGC 4051
2017-03-14 11:45 - 12:45 | Aula Gratton
Highly ionized absorbers are found in 50% of type 1 Seyferts and QSOs. They presence, along our line of sight, imprints a number of absorption lines from highly ionized metals in the far UV and X-ray spectral bands, like FeX-XVII, CIV-VI, OVI-OVIII, NeVII-X, etc. These absorption lines are always detected blue-shifted compared to the host galaxy reference system, which implies that the ionized material is being ejected from the nucleus and driven outwards, possibly by the AGN radiation pressure. Outflow velocities span a wide range, from few hundreds to few thousands km/s in the so called Narrow-Absorption Line (NAL) and/or Warm Absorber objects, to non negligible fractions of the speed of light in UV Broad Absorption Line (BAL) or X-ray Ultra-Fast Outflows (UFOs). These outflows are believed to be photo-ionized by the central source and their energetic depends on their mass outflow rate which, in turn, depends on the volume density n_e of the material, which is difficult to measure because degenerate with the square of the distance R of the absorber from the photo-ionizing source.
In this talk, after a brief overview of the phenomenon, I will show how time-evolving photo-ionization works and how it can be used to break the degeneracy between the absorber volume density n_e and its distance R from the central engine. I will then present the particularly fortunate case of NGC 4051 and show the puzzling results of the analysis performed here at OAR of the long 2009 XMM-Newton monitoring of this source.

From the first stars to the first quasars
2017-03-07 11:45 - 12:45 | Aula Gratton
Observations of the most luminous quasars at redshift z>6 reveal the existence of numerous supermassive black holes (SMBHs, > 10^9 Msun) already in place about twelve billion years ago. The growth of these first SMBHs is still a major challenge for theoretical models. If it starts from black hole (BH) remnants of Population III stars (light seeds with mass ~100 Msun), it requires super-Eddington accretion. An alternative route is to start from heavy seeds formed by the direct collapse of gas on to a ~10^5 Msun BH. In addition, the interstellar medium (ISM) of the galaxies hosting these quasars are observed to be chemically mature systems, with metallicities Z>Zsun and dust masses >10^8 Msun, similar to that of more evolved, local galaxies. The connection between the rapid growth of the SMBHs and the fast chemical evolution of the host galaxy is one of the most puzzling issues for theoretical models.
I will present the results of our cosmological, data constrained semi-analytic model in which we investigate the relative role of light and heavy seeds as BH progenitors of the first SMBHs, self-consistently following the evolution of the chemical properties (metals and dust abundance) in the host galaxies ISM.

AGB dust production: what we learn from the Local Group galaxies
2017-02-28 11:45 - 12:45 | Aula Gratton
Asymptotic giant branch (AGB) stars are among the most important gas and dust polluters of the Universe. The composition and the amount of dust produced in the winds of AGB stars is strongly correlated with the evolution of the central object. Detailed knowledge of the physical structure and chemical composition Is an essential ingredient to estimate the impact of these stars in the evolution history of their host environment.
A new release of AGB+dust models were developed in the last years to this scope. Their application in characterizing the AGB population and its contribution to the dust budget in the MCs, as well as in dwarf galaxies of the Local Group will be discussed in this talk.

The challenge of massive black hole binary coalescence; from galaxy formation to gravitational waves
2017-02-21 11:45 - 12:45 | Aula Gratton
I will review recent progress in understanding how a pair of massive black holes can evolve from galactic scale separations, kpc and above, to milliparsec separations, where gravitational wave emission takes over
and leads to signals that should be detected by the Laser Interferometer
Space Antenna (LISA). I will show how, contrary to common belief, the
orbital decay of massive black holes in gaseous environments can be
less efficient and more stochastic than in stellar backgrounds, where
the only two processes at play are dynamical friction and 3-body
encounters with passings stars. Novel supercomputer simulations that attempt to study orbital decay in realistic galactic hosts show that coalescence timescales
can become very short, about 10 Myr, in high redshit massive galaxies that are on their way to become quenched spheroids, but can be longer than a
Gyr in massive star forming gas-rich disks as those observed at z > 1.
Hosts of LISA black holes are expected to have a different nature. The complex interplay between
the physical properties of galactic nuclei and the orbital decay process
poses a huge challenge to computational model, but at the same time
holds the promise for LISA and other gravitational wave detection
experiments to become a powerful probe of galaxy formation and evolution.
