The complete catalogue of light curves in equal-mass binary microlensing

The light curves observed in microlensing events due to binary lenses span an extremely wide variety of forms, characterised by U-shaped caustic crossings and/or additional smoother peaks. However, all peaks of the binary-lens light curve can be traced back to features of caustics of the lens system. Moreover, all peaks can be categorised as one of only four types (cusp-grazing, cusp-crossing, fold-crossing or fold-grazing). This enables us to present the first complete map of the parameter space of the equal-mass case by identifying regions in which light curves feature the same number and nature of peaks. We find that the total number of morphologies that can be obtained is 73 out of 232 different regions. The partition of the parameter space so-obtained provides a new key to optimise modelling of observed events through a clever choice of initial conditions for fitting algorithms.Comment: 22 pages, 18 figures, accepted for publication in MNRA

An insight into gravitational microlensing and the search for exoplanetary systems

2013 - 2014Nature always offers astonishing shows to the humankind. Gravitational lensing is one of the most intriguing effects which can be observed, and the astrophysical applications of such a phenomenon span from particle physics to cosmology, to the search for new worlds. Although, since its formulation in the early 1900, gravitational lensing was underestimated and poorly investigated, in the last thirty years the observation of different phenomena related to gravitational lenses have revealed the real potentiality of the study of the effects of gravity on the light. Our comprehension of the Universe has been largely improved since we have studied the cosmological implications of lensing by clusters of galaxies, lensing of quasars, or effects of microlensing by non-luminous objects. The structure of the Universe, constraints on its formation theories, the mass distribution, and even proofs of the nature of dark matter can be studied by investigating magnification patterns and the effects due to gravitational lenses. Additionally, gravitational microlensing provides a unique tool to search for planetary systems in a region of the physical parameter space not probed by other methods. By observing microlensing effects to stars towards the Galactic bulge, we are able to look for small rocky planets orbiting stars in the Galactic disk or, more in general, along the line of sight. What we observe is the source lightcurve, i.e. the measured flux as a function of time. The presence of a foreground mass, or a system of masses, between the observer and a background source, causes an amplification of the source flux, so giving to the lightcurve specific features and shapes which can be attributed to different kinds of lens (single, binary, planetary, etc...). The study of this specific effect is likely to attain more promising results with the launch of space missions like EUCLID or WFIRST, since those telescopes will be able to detect Earth-like and smaller planets in their habitable zone, i.e. at a distance from their host star allowing the planet to have an equilibrium temperature at which liquid water can exist. This thesis work propose an insight into the gravitational microlensing, by starting with a theoretical work on the classification of binary microlensing lightcurves, and then by presenting the results of the modelling of one event from the 2013 observing season. A brief summary of some hot-Jupiter discovered with the transit method, and observed by the author, is finally presented at the end of the work. [edited by author]XIII n.s

The Luminosity Function of Low Mass X-Ray Binaries in the Globular Cluster System of NGC 1399

We present a study of the faint-end of the X-ray Luminosity Function of Low Mass X-ray binaries in the Globular Cluster system of the cD galaxy NGC 1399 by performing a stacking experiment on 618 X-ray undetected GCs, in order to verify the presence of faint LMXBs and to constrain the faint-end slope of the GC-LMXBs XLF below the individual detection threshold of $8\times10^{37}$ erg s$^{-1}$ in the $0.5-8$ keV band. We obtain a significant X-ray detection for the whole GC sample, as well as for the red and blue GC subpopulations, corresponding to an average luminosity per GC $_{GC}$ of $(3.6\pm1.0)\times10^{36}\ erg\ s^{-1}$, $(6.9\pm2.1)\times10^{36}\ erg\ s^{-1}$and$(1.7\pm0.9)\times10^{36}\ erg\ s^{-1}$, respectively for all, red and blue GCs. If LMXBs in red and blue GCs have the same average intrinsic luminosity, we derive a red/blu ratio$\simeq 3$of GCs hosting LMXBs ($2.5\pm1.0$or$4.1\pm2.5$depending on the surveyed region); alternatively, assuming the fractions observed for brighter sources, we measure an average X-ray luminosity of$L_{X}=(4.3\pm1.3)\times10^{37}\ erg\ s^{-1}$and$L_{X}=(3.4\pm1.7)\times10^{37}\ erg\ s^{-1}$per red and blue GC-LMXBs respectively. In the assumption that the XLF follows a power-law distribution, we find that a low-luminosity break is required at$L_{X}\leq 8\times10^{37}$erg s$^{-1}$both in the whole, as well as in the color-selected (red and blue) subsamples. Given the bright-end slopes measured above the X-ray completeness limit, this result is significant at$>3\sigma$level. Our best estimates for the faint end slope are$\beta_{L}=-1.39/-1.38/-1.36$for all/red/blue GC-LMXBs. We also find evidence that the luminosity function becomes steeper at luminosities$L_X\gtrsim 3\times 10^{39}$erg s$^{-1}$, as observed in old ellipticals.Comment: In press on A&

The VST Early-type GAlaxy Survey: Exploring the Outskirts and Intra-cluster Regions of Galaxies in the Low-surface-brightness Regime

The VST Early-type GAlaxy Survey (VEGAS) is a deep, multi-band (u, g, r, i) imaging survey, carried out with the 2.6-metre VLT Survey Telescope (VST) at ESO's Paranal Observatory in Chile. VEGAS combines the wide (1-square-degree) OmegaCAM imager and long integration times, together with a specially designed observing strategy. It has proven to be a gold mine for studies of features at very low surface brightness, down to levels of mu_g~27-30 magnitudes arcsec^(-2), over 5-8 magnitudes fainter than the dark sky at Paranal. In this article we highlight the main science results obtained with VEGAS observations of galaxies across different environments, from dense clusters of galaxies to unexplored poor groups and in the field.Comment: Published in The Messenger, vol. 183, p. 25-2

Intrinsic Morphology of Ultra-diffuse Galaxies

With the published data of apparent axis ratios for 1109 ultra-diffuse galaxies (UDGs) located in 17 low-redshift (z~ 0.020 - 0.063) galaxy clusters and 84 UDGs in 2 intermediate-redshift (z~ 0.308 - 0.348) clusters, we take advantage of a Markov Chain Monte Carlo approach and assume a ubiquitous triaxial model to investigate the intrinsic morphologies of UDGs. In contrast to the conclusion of Burkert (2017), i.e., the underlying shapes of UDGs are purely prolate ($C=B<A$), we find that the data favor the oblate-triaxial models ($C<B\lesssim A$) over the nearly prolate ones. We also find that the intrinsic morphologies of UDGs are relevant to their stellar masses/luminosities, environments, and redshifts. First, for the low-redshift UDGs in the same environment, the more-luminous ones are always thicker than the less-luminous counterparts, possibly due to the more voilent internal supernovae feedback or external tidal interactions for the progenitors of the more-luminous UDGs. The UDG thickness dependence on luminosity is distinct from that of the typical quiescent dwarf ellipticals (dEs) and dwarf spheroidals (dSphs) in the local clusters and groups, but resembles that of massive galaxies; in this sense, UDGs may not be simply treated as an extension of the dE/dSph class with similar evolutionary histories. Second, for the low-redshift UDGs within the same luminosity range, the ones with smaller cluster-centric distances are more puffed-up, probably attributed to tidal interactions. Finally, the intermediate-redshift cluster UDGs are more flattened, which plausibly suggests a `disky' origin for high-redshift, initial UDGs.Comment: Accepted for publication in ApJ; new versio

The first sample of spectroscopically confirmed ultra-compact massive galaxies in the Kilo Degree Survey

We present results from an ongoing investigation using the Kilo Degree Survey (KiDS) on the VLT Survey Telescope (VST) to provide a census of ultra-compact massive galaxies (UCMGs), defined as galaxies with stellar masses M* > 8 × 1010M⊙ and effective radii Re < 1.5 kpc. UCMGs, which are expected to have undergone very few merger events, provide a unique view on the accretion history of the most massive galaxies in the Universe. Over an effective sky area of nearly 330 deg2, we select UCMG candidates from KiDS multicolour images, which provide high quality structural parameters, photometric redshifts, and stellar masses. Our sample of ~1000 photometrically selected UCMGs at z < 0.5 represents the largest sample of UCMG candidates assembled to date over the largest sky area. In this paper, we present the first effort to obtain their redshifts using different facilities, starting with first results for 28 candidates with redshifts z < 0.5, obtained at NTT and TNG telescopes.We confirmed, as bona fide UCMGs, 19 out of the 28 candidates with new redshifts. A further 46 UCMG candidates are confirmed with literature spectroscopic redshifts (35 at z < 0.5), bringing the final cumulative sample of spectroscopically confirmed lower-z UCMGs to 54 galaxies, which is the largest sample at redshifts below 0.5. We use these spectroscopic redshifts to quantify systematic errors in our photometric selection, and use these to correct our UCMG number counts. We finally compare the results to independent data sets and simulations