On measuring the Galactic dark matter halo with hypervelocity stars

Hypervelocity stars (HVSs) travel from the Galactic Centre across the dark matter halo of the Milky Way, where they are observed with velocities in excess of the Galactic escape speed. Because of their quasi-radial trajectories, they represent a unique probe of the still poorly constrained dark matter component of the Galactic potential. In this paper, we present a new method to produce such constraints. Our likelihood is based on the local HVS density obtained by back-propagating the observed phase space position and quantifies the ejection probability along the orbit. To showcase our method, we apply it to simulated Gaia samples of $\sim200$ stars in three realistic Galactic potentials with dark matter components parametrized by spheroidal NFW profiles. We find that individual HVSs exhibit a degeneracy in the scale mass-scale radius plane ($M_s-r_s$) and are able to measure only the combination $\alpha = M_s/r_s^2$. Likewise, a degeneracy is also present between $\alpha$ and the spheroidal axis-ratio $q$. In the absence of observational errors, we show the whole sample can nail down both parameters with {\it sub-per cent} precision (about $1\%$ and $0.1\%$ for $\alpha$ and $q$ respectively) with no systematic bias. This remarkable power to constrain deviations from a symmetric halo is a consequence of the Galactocentric origin of HVSs. To compare our results with other probes, we break the degeneracy in the scale parameters and impose a mass-concentration relation. The result is a competitive precision on the virial mass $M_{200}$ of about $10\%$.Comment: See Fig. 8 for a summar

Weak lensing constraints on splashback around massive clusters

The splashback radius $r_\text{sp}$ separates the physical regimes of collapsed and infalling material around massive dark matter haloes. In cosmological simulations, this location is associated with a steepening of the spherically averaged density profile $\rho(r)$. In this work, we measure the splashback feature in the stacked weak gravitational lensing signal of $27$ massive clusters from the Cluster Canadian Comparison Project with careful control of residual systematics effects. We find that the shear introduced by the presence of additional structure along the line of sight significantly affects the noise at large clustercentric distances. Although we do not detect a significant steepening, the use of a simple parametric model enables us to measure both $r_\text{sp}=3.5^{+1.1}_{-0.7}$ comoving Mpc and the value of the logarithmic slope $\gamma = \log \rho / \log r$ at this point, $\gamma(r_\text{sp}) = -4.3^{+1.0}_{-1.5}$.Comment: 9 pages, 5 figures. Accepted for publication in MNRA

Il crimine di parricidio nel XIX secolo. Dal modello normativo francese alla realt\ue0 italiana dello Stato pontificio.

Nella realt\ue0 francese e italiana dello Stato pontificio (rappresentativo anche delle altre esperienze italiane preunitarie) il dato quantitativo del crimine di parricidio risulta rilevante e suggestivo perch\ue9 nel crimine di parricidio si incrociano non solo i linguaggi scientifici della dottrina e dei legislatori ma anche alcuni fattori (antropologici e fisico-sociali) criminogeni. La permanenza di una pena \u2013 capitale e infamante fin dal medioevo e speciale sin dal diritto romano \u2013 \ue8 indicativa di alcuni caratteri fondativi su cui si \ue8 costruita la societ\ue0 europea prima del secolo della democrazia. Fin dalla realt\ue0 storico-giuridica romana la legislazione civile ruota intorno all\u2019istituzionalizzazione dell\u2019ordine patriarcale-familiare e lo fa con l\u2019invenzione della patria potestas. Un potere assoluto che \ue8 matrice e modello di tutti gli altri poteri e che si poteva spingere fino allo ius vitae et necis. Il potere del padre, riconosciuto come bene meritevole di una difesa giuridica, era all\u2019origine del parricidio, figura di reato meritevole di una speciale pena, il culleo. Nel medioevo l\u2019omicidio del parente stretto \ue8 poi qualificato come atroce perch\ue9 contrario ai comandi divini e alle leggi di natura, sino ad arrivare dentro l\u2019Ottocento dei codici come fattispecie separata degna sempre della pena estrema, esemplare e teatrale. Nel XIX secolo paradigmatico \ue8 l\u2019esempio del codice penale francese che all\u2019art. 299 qualifica reo di parricidio l\u2019omicida volontario degli ascendenti legittimi (naturali o adottivi) escludendo dall\u2019incriminazione la discendenza. In corrispondenza, la pena capitale infamante e inasprita dalla mutilazione dell\u2019art. 13, magnifica, a livello penalistico, la centralit\ue0 assoluta dell\u2019autorit\ue0 paterna, nuovo cardine della famiglia post-rivoluzionaria, anche in forza della puissance paternelle e maritale reintrodotti nel Code civil 1804. La comunit\ue0 politica risponde in un\u2019apparente armonia di valutazione tra opinione pubblica e scienza penale alla gravissima violazione dell\u2019ordine patriarcale, pensato come ancestrale e fondante. Una centralit\ue0 imperitura, quella del pater familias, anche nel contesto italiano preunitario dove serve da tramite per la tutela penale di una cerchia parentale diversamente definita da quella d\u2019oltralpe. Negli stati italiani essa \ue8 comprensiva dei legami legittimi (di cui non sempre fa parte il vincolo d\u2019adozione) e naturali ma \ue8 allargata anche ai discendenti. La sua definizione \ue8 discussa dalla dottrina che fonda l\u2019incriminazione per parricidio sulla qualit\ue0 delle persone distinguendo fermamente il parricidio dall\u2019omicidio per questo. Nonostante il modello francese in cui la difesa della massima autorit\ue0 familiare, cui si deve rispetto e soggezione, richiama gerarchie politiche e sociali, nella nostra esperienza la causa prima (la ratio, il formante) del crimine \ue8 in effetti il vincolo di sangue, come elemento primigenio e naturale su cui poggia l\u2019aggregato familiare. Anche per il forte ancoraggio all\u2019antico regime, quello che simbolicamente e metaforicamente ne deriva viene dopo, tanto che l\u2019uxoricidio compreso nella stessa categoria giuridica \ue8 per\uf2 valutato come un\u2019offesa minore alla societ\ue0, quindi come parricidio improprio. Le ragioni dell\u2019ordine nella Francia napoleonica e il rispetto del sangue nella vicenda italiana preunitaria sono gli elementi di giustificazione della condivisa durezza repressiva della legislazione penale, la quale per\uf2 non ha riscontro, almeno nel caso dello Stato pontificio, con la giustizia praticata. In effetti, lo iato tra la misura delle incriminazioni e quella delle condanne effettive rende lo spazio di un temperamento indispensabile al mantenimento dell\u2019ordine pubblico e alla non enfatizzazione di un fenomeno gi\ue0 assai significativo

Lensing efficiency for gravitational wave mergers

We gain insight into the effects of gravitational lensing on the estimated distribution of merging binaries observed through gravitational waves. We quantify the efficiency of magnification for gravitational wave events in the geometric optics limit, and we compare it to the electromagnetic case by making minimal assumptions about the distribution of intrinsic properties for the source population. We show that lensing effects leave a recognizable signature on the observed rates, and that they can be prominent only in the presence of an extremely steep mass function (or redshift evolution) and mainly at low inferred redshifts. We conclude that gravitational magnification does not represent a significant systematic for gravitational wave merger studies in the LIGO-Virgo era.Comment: 6 pages, 4 figures. Accepted for publication in MNRA

The splashback radius in symmetron gravity

The splashback radius $r_\mathrm{sp}$ has been identified in cosmological $N$-body simulations as an important scale associated with gravitational collapse and the phase-space distribution of recently accreted material. We employ a semi-analytical approach to study the spherical collapse of dark matter haloes in symmetron gravity and provide insights into how the phenomenology of splashback is affected. The symmetron is a scalar-tensor theory of gravity which exhibits a screening mechanism whereby higher-density regions are screened from the effects of a fifth force. In this model, we find that, as over-densities grow over cosmic time, the inner region becomes heavily screened. In particular, we identify a sector of the parameter space for which material currently sitting at $r_\mathrm{sp}$ has followed, during the collapse, the formation of this screened region. As a result, we find that for this part of the parameter space the splashback radius is maximally affected by the symmetron force and we predict changes in $r_\mathrm{sp}$ up to around $10\%$ compared to its General Relativity value. Because this margin is within the precision of present splashback experiments, we expect this feature to soon provide constraints for symmetron gravity on previously unexplored scales.Comment: 9 pages, 3 figures. Comments are welcom

Predicting the hypervelocity star population in Gaia

Hypervelocity stars (HVSs) are amongst the fastest objects in our Milky Way. These stars are predicted to come from the Galactic center (GC) and travel along unbound orbits across the Galaxy. In the coming years, the ESA satellite Gaia will provide the most complete and accurate catalogue of the Milky Way, with full astrometric parameters for more than $1$ billion stars. In this paper, we present the expected sample size and properties (mass, magnitude, spatial, velocity distributions) of HVSs in the Gaia stellar catalogue. We build three Gaia mock catalogues of HVSs anchored to current observations, exploring different ejection mechanisms and GC stellar population properties. In all cases, we predict hundreds to thousands of HVSs with precise proper motion measurements within a few tens of kpc from us. For stars with a relative error in total proper motion below $10 \%$, the mass range extends to ~$10 M_{\odot}$ but peaks at ~$1$ $M_\odot$. The majority of Gaia HVSs will therefore probe a different mass and distance range compared to the current non-Gaia sample. In addition, a subset of a few hundreds to a few thousands of HVSs with $M$ ~ $3$ $M_\odot$ will be bright enough to have a precise measurement of the three-dimensional velocity from Gaia alone. Finally, we show that Gaia will provide more precise proper motion measurements for the current sample of HVS candidates. This will help identifying their birthplace narrowing down their ejection location, and confirming or rejecting their nature as HVSs. Overall, our forecasts are extremely encouraging in terms of quantity and quality of HVS data that can be exploited to constrain both the Milky Way potential and the GC properties.Comment: 17 pages, 18 figures, accepted for publication in MNRA

Exploring the edge

At the largest scales, two ingredients dictate the distribution of matter in the Universe. The first is dark matter, acting as an invisible scaffolding held together by gravitational forces. The second is dark energy, an enigmatic component responsible for the accelerated expansion of the Universe. Under these two forces, matter in the Universe organizes itself in the so-called cosmic web. The nodes of this network are large dark matter haloes, and this thesis explores how their boundaries provide information about the nature of dark energy and cosmology. Chapters 3 and 4 present robust theoretical predictions for this titular edge and discuss its simple physical interpretation. Chapters 2 and 5 corroborate these results by presenting measurements of this feature in weak-lensing data. The last scientific chapter of this thesis is a collection of studies in gravitational-wave physics. This chapter explores how these spacetime ripples observed from across the cosmos can be used to detect alternative theories of gravity. Large scale structure and cosmolog

The mass-size relation of galaxy clusters

The outskirts of accreting dark matter haloes exhibit a sudden drop in density delimiting their multi-stream region. Due to the dynamics of accretion, the location of this physically motivated edge strongly correlates with the halo growth rate. Using hydrodynamical zoom-in simulations of high-mass clusters, we explore this definition in realistic simulations and find an explicit connection between this feature in the dark matter and galaxy profiles. We also show that the depth of the splashback feature correlates well with the direction of filaments and, surprisingly, the orientation of the brightest cluster galaxy. Our findings suggest that galaxy profiles and weak-lensing masses can define an observationally viable mass-size scaling relation for galaxy clusters, which can be used to extract cosmological information.Comment: 9 pages, 9 figure