Journal Club

 

 

Journal Club Seminari 2022


   
 
18-02-2022   (AULA C - Via della Vasca Navale n. 84)
Chair:
ore 14:00 Federico Manzoni
(DOTTORATO IN FISICA)
Asymptotic symmetries in Maxwell and Einstein theories

In 1962 Bondi, van de Burg, Metzner and Sachs discovered that asymptotically flat spacetimes possess an asymptotic symmetries group much larger than the Poincarè group. Since then, the interest in the study of asymptotic symmetries has been growing, leading today to a possible extension of the AdS/CFT correspondence to asymptotically
flat spacetimes. We discuss asymptotic symmetries in Maxwell and linearized Einstein theories, calculating the asymptotic charges in the first case and the asymptotic killing vectors in the second one.

 
ore 15:00 Alessandro Brin
(DOTTORATO IN FISICA)
Bright basal reflections beneath SPLD: new laboratory tests on terrestrial analogues

Bright basal reflections detected at the base of the south polar layered deposits (SPLD) on Mars by MARSIS instrument (a low-frequency radar on the Mars Express spacecraft) have been interpreted as evidence of wet sediments or briny ponded water. Owing to the low temperature values expected at the base of the SPLD however, the presence of briny water has been questioned with alternative materials, such as clay, hydrated salts, and saline ices proposed as the source of the bright reflections. In this discussion I will briefly discuss all these different scenarios, illustrating new laboratory tests on terrestrial analogues.
 
 
07-04-2022   (AULA C - piano terra- V. della Vasca Navale n. 84)
Chair:
ore 14:15 Vittoria Elvezia Gianolli
(DOTTORATO IN FISICA)
X-ray absorption properties in a type 1 quasar sample from the SDSS survey

Generally, optical and X-ray studies support the Unified Model, which was proposed in order to explain the variety of classes and sub-classes in which Active Galactic Nuclei (AGN) have been dived. According to it, AGNs present similar inner regions and are powered by the same central engine. While the phenomenological differences arise from geometrical effects associated with an optically-thick obscuring torus that surrounds the nuclear engine, and the relative position of the observer.
Despite the fact that the majority of AGNs classified as type 1 are unabsorbed and Type 2 AGNs are usually absorbed, the number of cases in which observations show disagreement between X-ray and optical classifications has increased.
In this context, we study a sample of optically classified as Type 1 quasars (QSO) that are obscured in the X-ray band. The main aims are to estimate the rate of QSO with obscured X-ray spectra and to investigate the physical origin of the discordant optical/X-ray properties.
 
ore 15:00 Silvia Tosi
(DOTTORATO IN FISICA)
Characterization of evolved stars

During the AGB lifetime the stars eject into the surrounding environment gas, which on the chemical side is significantly altered with respect to the initial chemistry, and dust, formed in the cool and expanded circumstellar envelopes. Determining the mineralogy and the amount of dust is fundamental to establish their role as pollutants of the interstellar medium, and therefore the way in which they participate in the life cycle of the host galaxy. This is made difficult by the poor knowledge of some physical mechanisms however a valuable approach to infer information on the evolution of AGB stars is offered by the study of post-AGB stars. Indeed, main aspects of the evolution of these stars are well known, and the spectra of the latter sources can be more easily analyzed to derive the chemical composition and to characterize the dust in their surroundings. For this purpose, we have analyzed 10 post-AGB candidates from the LMC and the SMC. 


 
 
10-06-2022   (AULA A (primo piano - Via Della vasca Navale n. 84))
Chair:
ore 14:00 Giulia Degni
(DOTTORATO IN FISICA)
Cosmic Voids and their surroundings: a new cosmological probe

Abstract Journal Club  Giulia Degni
June 2022
Since the discovery of the expansion of the Universe, one of the main problems in physics relies on the investigation of the nature of dark energy and the exploration of alternative theories of gravity. According to the standard model in Cosmology, the CDM, almost 95% of our Universe, the dark components, is unknown, it is fundamental to develop new techniques to study and analyze the Universe. A powerful tool is given us from the observation of the Large Scale Structure of the Universe. Modern surveys are able to provide a huge amount of data and it is possible to develop detailed maps of the observable Universe tracing the distribution of galaxies. Inside these maps it is possible to identify
the different kind of structure which the Universe is made up: galaxy clusters (highest density regions), filaments (high density regions connecting galaxyclusters) and cosmic voids (lowest density regions). Recently, cosmic voids have proven to be powerful cosmological probes to study the nature of the accelerated expansion of the Universe by putting constraints on cosmological parameters.
The importance of cosmic voids, large empty underdense regions in space, relies mainly on their shape: on average, voids are intrinsically spherically symmetric and, once rescaled by their size, possess a universal density profile. From an observational perspective, voids are identified from spectroscopic galaxy redshift surveys, i.e., from the spatial distribution of galaxies whose observedredshifts
are used to infer distances. As a result, the observed shape of the voids is
not spherical but distorted by two effects. The first one derives from assuming an incorrect cosmological model. The second one originates from ignoring the Doppler due to the proper motion of the galaxy that, on top of the recession velocity, contributes to the observed redshift. Therefore, is it possible to use cosmic voids to extract cosmological information and constrain the nature ofthe dark components.
 
ore 14:45 Ivano Saccheo
(DOTTORATO IN FISICA)
Mean SED of hyper-luminous QSOs

-Mean SED of hyper-luminous QSOs-
The study of AGN SED provides both a direct description of the physical processes responsible
for the emission in different bands and a comprehensive picture of sources energy budgets. In this talk, I will present the characterization of the mean SED of a sample composed of the most luminous type 1 QSOs (the WISSH sample, Lbol>1047 erg/s at z~2-4.5, focusing on the differences
we find with respect to the typical SED of less luminous sources. I will also present the mean SED derived from different sub-samples of QSOs showing peculiar spectral features (BAL vs non-
BAL QSO and CIV weak vs non CIV weak QSOs) and the bolometric corrections obtained for 5100 Å and 3 μm luminosities.
By extending this analysis to the more heterogeneous SDSS QSO catalog, I will discuss whether our results can be generalized to the entire population of hyper-luminous QSOs. Finally, I will
discuss the location of WISSH QSOs in the recently proposed star-forming vs AGN dominated galaxy diagnostic diagram (Symeonidis & Page 2021) and its possible extension up to z = 4.5.

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