Osservatorio Astronomico di Roma

Quasars and their host galaxies in the first Gyr of the Universe
2016-10-12 11:45 - 12:45 (Europe/Rome) | Aula Gratton
Quasars are the brightest (non-transient) objects observed at the highest
redshifts, z>7. Such high redshift quasars are important for several
reasons. Firstly, detailed analysis of quasar spectra provide unique
information about the baryonic and physical condition of the Universe
during the epoch of reionisation. Furthermore, the density of high
redshift quasars puts powerful constraints on the mechanisms that are
required to seed and grow >10^9 Msun supermassive black holes less than a
Gyr after the Big Bang. Lastly, as these massive black holes are thought
to be located in the progenitors of the massive early type galaxies we see
in the local Universe, studying the host galaxies of the most distant
quasars enables us to probe the formation of massive galaxies in the early
Universe.
Because high redshift quasars are rare, multi-band surveys covering large
areas on the sky are required to discover such objects. In this talk I
will describe our successful programme aimed at discovering quasars at the
highest redshifts in various optical and near-infrared surveys, such as
Pan-STARRS1, UKIDSS and VIKING. I will highlight the results of our
search, which includes the discovery of more than 100 new quasars in the
first Gyr of the Universe. I will describe our on-going multi-wavelength
follow-up observations to characterise the distant quasars, their power
source and their environment, in particular our ALMA programmes to study
the galaxies hosting these luminous objects. I will present the results of
these observations, and discuss the implications for massive galaxy and
black hole formation at high redshift and our understanding of the
relation between the black hole mass and the bulge mass of galaxies.

Gamma-ray busrts as probes of high redshift galaxies and their environments
2016-10-11 11:45 - 12:45 | Aula Gratton
The launch of the Swift mission has revolutionised the study
of the Gamma-Ray Bursts, enabling the optical follow-up of the early afterglow.
Thanks to the fast broadcast of the GRB coordinates
and to the rapid response mode systems built up in 8-meter class
telescopes, these afterglows have be observed from hours down to a few
minutes after the GRB explosion. The high signal-to noise achievable
allows us to acquire medium-to-high resolution spectra, which carry a lot of
information both on the GRB environment and the Inter-Stellar Medium
of high redshift galaxies. I will give a short review of what we have
learned through the medium-to-high resolution spectroscopy of GRB afterglows,
mainly with UVES and X-shooter at the VLT.

Cosmic rays, gamma-rays, and neutrinos from AGN winds
2016-10-04 11:45 - 12:00 | Aula Gratton
Galaxy scale outflows driven by active galactic nuclei (AGN) are likely to play a key role in the evolution of galaxies. In fact, they can perturb and possibly expel most of the gas out of the galaxy, offering a plausible physical origin for the close connection between the properties of black holes and their host galaxies. AGN-driven outflows can also induce shocks in the interstellar medium, which, in turn, can accelerate relativistic particles.
I will present a study of the non-thermal emissions produced by relativistic particles accelerated by the AGN-driven shocks in NGC 1068. NGC 1068 is a nearby prototypical Seyfert 2 galaxy which shows both starburst and nuclear activities in its central region. NGC 1068 is a strong gamma-ray emitter. It is the brightest of the few non-blazar galaxies detected by the Fermi gamma-ray space telescope.
I will show the gamma-ray and radio spectra predicted by the AGN outflow model, and I will discuss how these results compare with those corresponding to other commonly assumed sources of gamma-ray and radio emissions, like Supernova remnants or AGN jets.
I will compare the neutrino flux expected in the AGN outflow model with constraints from current experiments, and I will provide predictions for the detections by the upcoming KM3NeT neutrino telescope.

Indirect Dark Matter searches with the MAGIC telescopes and prospects with the ASTRI mini-array and the Cherenkov Telescope Array
2016-09-27 11:45 - 12:45 | Aula Gratton

Gas accretion and star formation in disk galaxies
2016-09-13 11:45 - 12:45 | Aula Gratton
Numerical simulations predict that metal-poor gas accretion from the cosmic web fuels the
formation of disk galaxies. My presentation will summarize observational evidence
for this gas accretion process driving star formation. In particular, I will discuss
a number of properties characterizing large samples of star-forming galaxies that
can be explained by metal-poor gas accretion (e.g., the fundamental metallicity relation,
or the relationship between galaxy morphology and metallicity). Some properties of the
(neutral and ionized) gas around galaxies will be examined too. I will also review the
properties of the extremely metal poor galaxies. They show chemical and dynamical
inhomogeneities that can be naturally explained as due to star-formation feeding from
infall of external metal-poor gas.

The Milky Way bulge: chemical content, kinematics and 3D structure
2016-06-30 11:45 - 12:45 | Aula Gratton
During the last decade large observational efforts have been focused on understanding the formation and evolution of the Milky Way bulge by studying the spectroscopic and photometric properties of its stellar population over large and unexplored regions.
The emerging picture is substantially different from what previously thought and by far more complicated.
I will review the results obtained by the recent surveys focusing in particular on the observational evidences concerning the Bulge chemical content, kinematics and 3D structure.

Stellar evolution modeling of low-mass stars with magnetic fields: advances and challenges
2016-06-14 11:45 - 12:45 | Aula Gratton
Though magnetic fields are commonly believed to play an important role
on the structure and evolution of stars, there have been few attempts so
far to include their effects on stellar evolution models. Specifically
in the case of low-mass stars, those effects could account for some
disparities between standard stellar models predictions and
observations, such as their radii and effective temperatures. In this
talk I will briefly review the modeling techniques for considering the
magnetic field effects on 1-D evolution codes, and also present some
recent results of those techniques as implemented in the ATON stellar
evolution code. Some theoretical and observational challenges to those
results will also be highlighted.

The two-families scenario of compact stars: signatures and implications on GRBs
2016-06-09 11:45 - 12:45 | Aula Gratton
In the last years the possibility of discriminating between the various theoretically possible compositions of a compact “neutron” star has become more and more convincing and it is based on the measure of a few fundamental quantities, such as the radii (NICER, LOFT), the moment of inertia (SKA) and/or the gravitational wave signal before and after the merging of two compact stars (VIRGO, LIGO).
I will outline the so-called two-families scenario in which both stars having very small radii (of the order of about 10.5 km) and stars having very large masses (of the order or larger than about 2 Ms) can exist. In that scenario very dense stars, having small radii, are made of neutron, protons, hyperons and other hadronic resonances, while the most massive stars are strange-quark stars, i.e. objects entirely made of deconfined quark matter. I will discuss the implications of the two-families scenario on short and long Gamma Ray Bursts within the proto-magnetar model of GRBs, showing that quark deconfinement can be responsible both of the rapid termination of the prompt emission in short GRBs and of late time activity in long GRBs.

Dust-driven stellar winds in stellar evolution models
2016-05-31 11:45 - 12:45 | Aula Gratton
Mass loss from stars during their evolution in the HR-diagram is an essential ingredient in stellar evolution models. Mass loss on the AGB is of particular interest when dust production is included in the model, since it is well established that the winds of AGB stars are largely dust driven. For practical reasons, the wind models adopted in stellar evolution codes are relatively simplistic: the actual mass-loss rate is prescribed and net effect of thermal gas pressure is usually not taken into account. In this talk I will discuss how wind these models can be improved without turning them into inhibiting computational challenges. I will also discuss (or speculate about) the effects such an improved wind model can have on the resultant dust yields for AGB stars.

The square kilometre array
2016-05-24 11:45 - 12:45 | Aula Gratton
Building on the major scientific achievements of the current generation of centimetre and metre-wavelength telescopes, phase 1 of the Square Kilometre Array (SKA1) will be the next global radio astronomy observatory. It is currently being designed by engineers and scientists from around the world, with the aim of beginning construction in 2018 and early science operations in 2020. SKA1 will conduct key tests of general relativity through surveys and timing of pulsars in our Galaxy. It will help answer fundamental questions related to the evolution of atomic Hydrogen in the Universe, from the present day back to the Cosmic Dawn. I will give an overview of the wide breadth of science objectives that could be addressed by SKA1, along with a description of the planned telescopes and the sites that will host them

Serata Estiva
2016-05-20 20:00 - 23:00 | OAR Monte Porzio Catone, Via Frascati, 33, Monte Porzio Catone , RM, 00040, Italia
Birra e astronomia

Evolution of close binary systems
2016-05-17 11:45 - 12:45 | Aula Gratton
A general outlook of the evolution of close binary stars and of population synthesis as a method of their study will be presented. I will pay special attention to the evolution of low- and intermediate-mass stars (M < (8-10) solar) and to formation of different still hypothetical progenitors of SNe Ia. Next, I will consider evolution of massive binaries with an accent on scenarii of evolution leading to the recently discovered gravitational waves burst GW150914

A closer look at the reionization sources: clustering, line emission and feedback
2016-05-10 11:45 - 12:45 | Aula Gratton
The epoch of reionization marks a major phase transition of the Universe,
during which the intergalactic medium became transparent to UV photons.
Over the last few years, searches have intensified to identify the
population of high redshift galaxies that might be responsible for this
process and to constrain their properties. In this talk I will discuss
recent efforts to address the open problems in our understanding of the
sources of reionization. I will present the results from a deep HST
imaging program providing the first observational match to the theoretical
prediction that reionization fronts are generated within significant
overdensities of faint galaxies. I will show our ongoing analysis of the
HST Frontier Fields survey and the resulting limits on the cut-off
imprinted by photo-ionization feedback on the UV luminosity function at
high redshift. Finally, I will discuss a multi-band study of z~7 galaxies
providing evidence for strong line emission in the optical rest-frame
region and the first constraints on the escape fraction of ionizing
photons from these early galaxies.

The dawn of star formation: a local perspective
2016-05-03 11:45 - 12:45 | Aula Gratton
At the end of the Dark Ages gravity began to make matter
collapse around over-densities, resulting in the formation
of the first stars, the Pop III. These stars are very important in the
cosmological context, the most massive ones contributed ionizing
photons that played a role in the reionization of the Universe
and synthetised the first metals. We know that the formation mode
of these first stars was different than the one operating today.
The lack of metals implies it is difficult to cool a contracting cloud,
which would argue for a preferential formation of massive or very massive
stars. However from the theoretical point of view it is not yet possible
to determine the initial mass function of the first stars. Simulations
show that, even in the absence of metals, a collapsing cloud may fragment
into smaller clouds giving rise to stars that span a range of masses, even
sub-solar. Once the first metals begin to be shed in the clouds cooling
becomes possible, either through atomic lines or through dust. It is likely
that there is a transition from Pop III to “normal” Pop II star formation
at some critical metallicity, however the precise value of this critical
metallicity cannot be presently determined theoretically.
In this seminar I will describe the observational efforts that our group
is conducting in the local Universe, in order to derive as many constraints
as possible on the nature of the Pop III stars. These include searches
of the most metal-poor stars, and their chemical characterization.
Both the metallicity distribution function and the chemical pattern
of the most metal-poor stars contain precious information of the first stars.

Machine learning search for gamma-ray burst afterglows in optical images
2016-04-26 11:45 - 12:45 | Aula Gratton
Thanks to the rapid advances in robotic telescopes and wide field surveys, the time domain astronomy reveals a huge number of transient detections every night. Therefore, automatic classification of sources in the optical images is essential. A special attention is brought to the identification of gamma-ray burst afterglows using the combined observational data from the GROND telescope and the SDSS survey. Their successful identification can lead to further follow-up observations by dedicated telescopes and for statistical studies of these transients.

Escape of Lyman photons from galaxies
2016-04-12 11:45 - 12:45 | Aula Gratton
One of the key questions in observational cosmology is the identification of the sources responsible for cosmic
reionization. The general consensus is that a population of faint low-mass galaxies must be responsible for the
bulk of the ionizing photons. However, until recently, attempts at identifying individual galaxies showing
Lyman continuum (LyC) leakage have only found very few such galaxies, both at high and low redshifts.
We recently proposed a new indirect diagnostic for LyC leakage based on the shape of the Lyman-alpha line,
that I will first review (Verhamme+15).
A breakthrough was recently achieved by our team (Izotov et al. 2016a,b), detecting LyC emission in
low-redshift (z~0.3) luminous compact star-forming galaxies (LCGs) with high LyC escape fractions
using HST/COS observations. I will present these new results, and the Lyman-alpha emission from these galaxies,
confirming our theoretical predictions.
Finally, I will propose another complementary Lyman-alpha signature for LyC leakage, discussing theoretical predictions
on the Lyman-alpha spatial extend of LyC emitters: LyC leakers should have no/faint Lyman-alpha halos.
