Osservatorio Astronomico di Roma

Unveiling the most elusive star-forming galaxies
2017-04-11 11:45 - 12:45 (Europe/Rome) | 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.

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.

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).

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.

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.

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.

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.

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.

The WISSH quasars project: revealing ultra-massive black-holes and powerful winds in the most luminous AGN
2017-06-13 11:45 - 12:45 | 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.

Probing the Dark Halo of the Milky Way
2017-07-04 11:45 - 12:45 | Aula Gratton
The Local Group, the regime in which detailed star-by-star studies can be done, is becoming a major testbed for the cold dark matter-based model of the Universe. Critical to these tests is a better estimate of the total Milky Way halo mass. The most reliable means by which to constrain the properties of the Milky Way dark halo is through assessing the 6-D phase space distributions of tracers of its gravitational potential. This requires accurate proper motions (tangential velocities) in addition to generally known radial velocities for field stars, streams, and satellite galaxies widely distributed throughout the halo. In turn, these measurements allow us to investigate the past histories of these tracers, and thus the accretion history of the Milky Way. I will discuss some novel approaches we have been developing to obtain proper motions for a variety of tracers in the Milky Way halo, including streams, globular clusters, and satellite galaxies, to definitively constrain the Milky Way’s dark halo mass, shape and distribution. These techniques involve space-based imagers (HST), wide-field ground-based imagers, as well as imagers equipped with adaptive optics systems. I will also discuss the future prospects for this work, and the trade-off between Gaia, space missions (JWST, WFIRST, EUCLID), and ELTs.

talk by Orlagh Creevey
2017-07-11 11:45 - 12:45 | Aula Gratton

The evolution of protoplanetary disks and their host stars
2017-09-13 11:45 - 12:45 | Aula Gratton
The origins of our own planetary system was set when the sun was still young and surrounded by a circumstellar disk. Although the evolutionary sequence of star formation is now well charted, the physical phenomena that drive these changes are still poorly understood. These differences determine whether and what types of planets will form, around stars of different masses and in different environments. In this talk, I will discuss how recent and upcoming observations of protoplanetary disks in nearby star forming regions are revealing the disk evolution, including inferences of ongoing planet formation. I will also discuss expected improvements in the age dating of young stars, which will form the foundation for piecing together the sequences of disk evolution.

Appetite comes with eating: EXTraS PASTA & SOPA @ OAR
2017-09-26 11:45 - 12:45 | Aula Gratton
Data mining of public archives is a powerful tool in modern astronomy and it is expected to be more and more important in the near future since the size and number of experiments is rapidly increasing . Focus is put on the millions of objects falling by chance in the field of view of astronomical observations and left mostly ignored. In the EXTraS project we tapped into the poorly studied sources included in the 3XMM catalog, characterizing their variability with unprecedented detail. The products of the EXTraS project has been published in on-line catalogues open to the whole scientific community. At OAR, in particular, we searched for periodic variability of near 1.5 M timeseries. The main results of my work will be briefly shown.
Additionally, with the newly developed pipelines PASTA (Pulsation Accelerated Search for Timing Analysis) and SOPA (Searchfor Orbital Periods with Acceleration) we aim to dig even deeper into this huge amount of data by means of CINECA supercomputer facilities. I will tell you about these projects, some of their results and the prospectives of Data Mining and Big Data automatic analysis at the OAR.

First black holes: observability and growth
2017-10-03 11:45 - 12:45 | Aula Gratton
The first black hole seeds ​likely ​formed when the Universe was younger than 500 Myr​ old.
​They play​ an important role ​in​ the growth of early (z=7) supermassive black holes. While ​much ​progress ha​s​ been made in understanding their formation and growth, their observational signatures remain largely unexplored. As a result, ​we are yet to ​detect ​these​ sources. ​We​ present a novel photometric method to identify black hole seed candidates in deep multi-wavelength surveys. The method relies on infrared and X-ray observations and selects​ the only two objects with a robust X-ray detection found in the CANDELS/GOODS-S survey with a photometric redshift z>6. To date, these selected objects represent the most promising black hole seed candidates, possibly formed via the direct collapse black hole scenario. While this result is based on the best photometric observations of high-z sources available to date, additional ​gains are​ expected from deeper spectroscopic and X-ray data. For this reason, ​we ​explore the role ​that ​JWST ​will play in the ​detect​on of ​the first black holes​ in ​the ​U​niverse.
Moreover, we present a novel probabilistic model for black hole growth: our results have important implications for the conditions that facilitate efficient black hole growth, particularly at early cosmic epochs. By bringing in a precise definition of “growth probability”, our finding clearly demonstrates quantitatively the generally-accepted concept that the super-massive black holes that we observed at z=7 are the “luckiest” members of the ensemble of high-z black holes. Our formalism has significant and broad ​consequences​ for upcoming observations of high-redshift quasars.

L'utilità della Scienza inutile
2017-10-10 11:30 - 12:30 | Aula Gratton
Se i risultati scientifici finanziati con i soldi dei contribuenti sono “beni pubblici”, è utile o addirittura etico spendere miliardi per cercare una particella elementare? O per andare a visitare un satellite di Giove o cercare l’acqua su Marte? O per osservare una galassia ai confini dell’Universo? In altre parole, è utile finanziare la ricerca di base? E se sì, con che risorse? E per fare quale tipo di ricerca?
La competizione è diventata una delle maggiori forze trainanti per la ricerca. Ma siamo sicuri che il modello “competition-driven science” sia davvero quello migliore?
Di più, sta cambiando il concetto stesso di conoscenza? Siamo sicuri che sia ancora valido e applicabile oggi il “metodo scientifico” introdotto da Galileo più di 400 anni fa?
Le risposte a tutte queste domande non sono scontate, anzi, possono contenere molte sorprese non tutte piacevoli o rassicuranti, risposte tra le quali cercheremo di orientarci usando un punto di vista non sempre convenzionale.
La presentazione, in Italiano, è aperta a tutto il personale, anche ai non specialisti del settore.
