Star Formation Histories in the nearby Universe: the HST Legacy

The aim of this thesis is to study star-forming galaxies in the local Universe, and in particular the distribution of their stellar populations and their star formation histories, to understand how the clustering of star formation evolves both in space and time. The data I analyzed have been acquired with the Hubble Space Telescope, whose spatial resolution and sensitivity allow to measure individual stars with the highest possible accuracy even in crowded and relatively distant galaxies. Thanks to its exquisite performances, HST is still the most powerful facility for this kind of studies. The method I used to derive the star formation histories of the examined galaxies is based on the color-magnitude diagram, one of the best information desks on the evolution of a galaxy or stellar system. More specifically, I applied to the observational color-magnitude diagrams the synthetic CMD technique, a reliable tool to explore the detailed star formation history of nearby galaxies and refine stellar evolution models by comparing them with the data. This method was implemented in the code SFERA, which I contributed to improve and that will be extensively described in this thesis. Within this framework, I discussed the results obtained for three galaxies of different morphological type which have been targeted by the HST Treasury program LEGUS, a large international collaboration aimed to investigate and connect the different scales of star formation, from young stellar clusters to local Universe galaxies. The galaxies I analyze are DDO 68, a dwarf irregular, NGC 4449, a Magellanic irregular, and NGC 7793, a flocculent spiral, studied both in the UV and optical bands, in order to recover their star formation histories from very recent to older epochs and to understand whether and how the star formation process may depend on the morphological, dynamical and environmental properties of the galaxies

The Impact of Patchy Reionization on Ultra-faint Dwarf Galaxies

We investigate how patchy reionization affects the star formation history (SFH) and stellar metallicity of ultra-faint dwarf galaxies (UFDs). Patchy reionization refers to varying ultraviolet (UV) background strengths depending on a galaxy's environment. Recent observations highlight the significance of this effect on UFDs, as UFDs can have different SFHs depending on their relative position with respect to their host halo during the period of reionization. However, most cosmological hydrodynamic simulations do not consider environmental factors such as patchy reionization, and the effect of reionization is typically applied homogeneously. Using a novel approach to implement patchy reionization, we show how SFHs of simulated UFDs can change. Our cosmological hydrodynamic zoom-in simulations focus on UFD analogs with M_vir~10^9solar mass, M_star < 10^5 solar mass at $z=0$. We find that patchy reionization can weaken the effect of reionization by two orders of magnitude up to $z=3$, enabling late star formation in half of the simulated UFDs, with quenching times $\sim$460 Myr later than those with homogeneous reionization. We also show that halo merger and mass assembly can affect the SFHs of simulated UFDs, in addition to patchy reionization. The average stellar iron-to-hydrogen ratio, [Fe/H], of the simulated UFDs with patchy reionization increases by 0.22-0.42 dex. Finally, our findings suggest that patchy reionization could be responsible for the extended SFHs of Magellanic UFDs compared to non-Magellanic UFDs.Comment: 27 pages, 16 figures, Accepted for publication in Astrophysical Journa

Star Formation Histories of the LEGUS dwarf galaxies. II. Spatially resolved star formation history of the Magellanic irregular NGC 4449

We present a detailed study of the Magellanic irregular galaxy NGC 4449 based on both archival and new photometric data from the Legacy Extragalactic UV Survey, obtained with the Hubble Space Telescope Advanced Camera for Surveys and Wide Field Camera 3. Thanks to its proximity ($D=3.82\pm 0.27$ Mpc) we reach stars 3 magnitudes fainter than the tip of the red giant branch in the F814W filter. The recovered star formation history spans the whole Hubble time, but due to the age-metallicity degeneracy of the red giant branch stars, it is robust only over the lookback time reached by our photometry, i.e. $\sim 3$ Gyr. The most recent peak of star formation is around 10 Myr ago. The average surface density star formation rate over the whole galaxy lifetime is $0.01$ M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$. From our study it emerges that NGC 4449 has experienced a fairly continuous star formation regime in the last 1 Gyr with peaks and dips whose star formation rates differ only by a factor of a few. The very complex and disturbed morphology of NGC 4449 makes it an interesting galaxy for studies of the relationship between interactions and starbursts, and our detailed and spatially resolved analysis of its star formation history does indeed provide some hints on the connection between these two phenomena in this peculiar dwarf galaxy.Comment: 16 pages, 25 figures. Accepted for publication in the Astrophysical Journa

Enhancing guideline-based decision support with distributed computation through local mobile application

We introduce the need for a distributed guideline-based decision support (DSS) process, describe its characteristics, and explain how we implemented this process within the European Union’s MobiGuide project. In particular, we have developed a mechanism of sequential, piecemeal projection, i.e., 'downloading' small portions of the guideline from the central DSS server, to the local DSS in the patient's mobile device, which then applies that portion, using the mobile device's local resources. The mobile device sends a callback to the central DSS when it encounters a triggering pattern predefined in the projected module, which leads to an appropriate predefined action by the central DSS, including sending a new projected module, or directly controlling the rest of the workflow. We suggest that such a distributed architecture that explicitly defines a dialog between a central DSS server and a local DSS module, better balances the computational load and exploits the relative advantages of the central server and of the local mobile device

An Open System for Social Computation

Part of the power of social computation comes from using the collective intelligence of humans to tame the aggregate uncertainty of (otherwise) low veracity data obtained from human and automated sources. We have witnessed a surge in development of social computing systems but, ironically, there have been few attempts to generalise across this activity so that creation of the underlying mechanisms themselves can be made more social. We describe a method for achieving this by standardising patterns of social computation via lightweight formal specifications (we call these social artifacts) that can be connected to existing internet architectures via a single model of computation. Upon this framework we build a mechanism for extracting provenance meta-data across social computations

An Observational Retrospective Cohort Trial on 4,828 IVF Cycles Evaluating Different Low Prognosis Patients Following the POSEIDON Criteria

Objective: To study the actual controlled ovarian stimulation (COS) management in women with suboptimal response, comparing clinical outcomes to the gonadotropins consume, considering potential role of luteinizing hormone (LH) addition to follicle-stimulating hormone (FSH).Design: Monocentric, observational, retrospective, real-world, clinical trial on fresh intra-cytoplasmic sperm injection (ICSI) cycles retrieving from 1 to 9 oocytes, performed at Humanitas Fertility Center from January 1st, 2012 to December 31st, 2015.Methods: COS protocols provided gonadotropin releasing-hormone (GnRH) agonist long, flare-up, short and antagonist. Both recombinant and urinary FSH were used for COS and LH was added according to the clinical practice. ICSI outcomes considered were: gonadotropins dosages; total, mature, injected and frozen oocytes; cumulative, transferred and frozen embryos; implantation rate; pregnancy, delivery and miscarriage rates. Outcomes were compared according to the gonadotropin regimen used during COS.Results: Our cohort showed 20.8% of low responders, defined as 1-3 oocytes retrieved and 79.2% of "suboptimal" responders, defined as 4-9 oocytes retrieved. According to recent POSEIDON stratification, cycles were divided in group 1 (6.9%), 2 (19.8%), 3 (11.7%), and 4 (61.5%). The cohort was divided in 3 groups, according to the gonadotropin's regimen. Women treated with FSH plus LH showed worst prognostic factors, in terms of age, basal FSH, AMH, and AFC. This difference was evident in suboptimal responders, whereas only AMH and AFC were different among treatment groups in low responders. Although a different result, in terms of oocytes and embryos detected, major ICSI outcomes (i.e., pregnancy and delivery rates) were similar among groups of COS treatment. Outcomes were significantly different among Poseidon groups. Implantation, pregnancy and delivery rates were significantly higher in Poseidon group 1 and progressively declined in other POSEIDON groups, reaching the worst percentage in group 4.Conclusions: In clinical practice, women with worst prognosis factors are generally treated with a combination of LH and FSH. Despite low prognosis women showed a reduced number of oocytes retrieved, the final ICSI outcome, in terms of pregnancy, is similarly among treatment group. This result suggests that the LH addition to FSH during COS could improve the quality of oocytes retrieved, balancing those differences that are evident at baseline

NKp46-expressing human gut-resident intraepithelial V\u3b41 T cell subpopulation exhibits high anti-tumor activity against colorectal cancer

\u3b3\u3b4 T cells account for a large fraction of human intestinal intraepithelial lymphocytes (IELs) endowed with potent anti-tumor activities. However, little is known about their origin, phenotype and clinical relevance in colorectal cancer (CRC). To determine \u3b3\u3b4 IEL gut-specificity, homing and functions, \u3b3\u3b4 T cells were purified from human healthy blood, lymph nodes, liver, skin, intestine either disease-free or affected by CRC or generated from thymic precursors. The constitutive expression of NKp46 specifically identifies a new subset of cytotoxic V\u3b41 T cells representing the largest fraction of gut-resident IELs. The ontogeny and gut-tropism of NKp46pos/V\u3b41 IELs depends both on distinctive features of V\u3b41 thymic precursors and gut-environmental factors. Either the constitutive presence of NKp46 on tissue-resident V\u3b41 intestinal IELs or its induced-expression on IL-2/IL-15 activated V\u3b41 thymocytes are associated with anti-tumor functions. Higher frequencies of NKp46pos/V\u3b41 IELs in tumor-free specimens from CRC patients correlate with a lower risk of developing metastatic III/IV disease stages. Additionally, our in vitro settings reproducing CRC tumor-microenvironment inhibited the expansion of NKp46pos/V\u3b41 cells from activated thymic precursors. These results parallel the very low frequencies of NKp46pos/V\u3b41 IELs able to infiltrate CRC, thus providing new insights to either follow-up cancer progression or develop novel adoptive cellular therapies