We present simulations of galaxy formation, based on the GADGET-3 code, in which a sub-resolution model for star formation and stellar feedback is interfaced with a new model for AGN feedback. Our sub-resolution model describes a multiphase ISM, acco
unting for hot and cold gas within the same resolution element: we exploit this feature to investigate the impact of coupling AGN feedback energy to the different phases of the ISM over cosmic time. Our fiducial model considers that AGN feedback energy coupling is driven by the covering factors of the hot and cold phases. We perform a suite of cosmological hydrodynamical simulations of disc galaxies ($M_{rm halo, , DM} simeq 2 cdot 10^{12}$ M$_{odot}$, at $z=0$), to investigate: $(i)$ the effect of different ways of coupling AGN feedback energy to the multiphase ISM; $(ii)$ the impact of different prescriptions for gas accretion (i.e. only cold gas, both cold and hot gas, with the additional possibility of limiting gas accretion from cold gas with high angular momentum); $(iii)$ how different models of gas accretion and coupling of AGN feedback energy affect the coevolution of supermassive BHs and their host galaxy. We find that at least a share of the AGN feedback energy has to couple with the diffuse gas, in order to avoid an excessive growth of the BH mass. When the BH only accretes cold gas, it experiences a growth that is faster than in the case in which both cold and hot gas are accreted. If the accretion of cold gas with high angular momentum is reduced, the BH mass growth is delayed, the BH mass at $z=0$ is reduced by up to an order of magnitude, and the BH is prevented from accreting below $z lesssim 2$, when the galaxy disc forms.
High performance computing numerical simulations are today one of the more effective instruments to implement and study new theoretical models, and they are mandatory during the preparatory phase and operational phase of any scientific experiment. Ne
w challenges in Cosmology and Astrophysics will require a large number of new extremely computationally intensive simulations to investigate physical processes at different scales. Moreover, the size and complexity of the new generation of observational facilities also implies a new generation of high performance data reduction and analysis tools pushing toward the use of Exascale computing capabilities. Exascale supercomputers cannot be produced today. We discuss the major technological challenges in the design, development and use of such computing capabilities and we will report on the progresses that has been made in the last years in Europe, in particular in the framework of the ExaNeSt European funded project. We also discuss the impact of this new computing resources on the numerical codes in Astronomy and Astrophysics.
We implement a state-of-the-art treatment of the processes affecting the production and Interstellar Medium (ISM) evolution of carbonaceous and silicate dust grains within SPH simulations. We trace the dust grain size distribution by means of a two-s
ize approximation. We test our method on zoom-in simulations of four massive ($M_{200} geq 3 times 10^{14} M_{odot}$) galaxy clusters. We predict that during the early stages of assembly of the cluster at $z gtrsim 3$, where the star formation activity is at its maximum in our simulations, the proto-cluster regions are rich of dusty gas. Compared to the case in which only dust production in stellar ejecta is active, if we include processes occurring in the cold ISM,the dust content is enhanced by a factor $2-3$. However, the dust properties in this stage turn out to be significantly different than those observationally derived for the {it average} Milky Way dust, and commonly adopted in calculations of dust reprocessing. We show that these differences may have a strong impact on the predicted spectral energy distributions. At low redshift in star forming regions our model reproduces reasonably well the trend of dust abundances over metallicity as observed in local galaxies. However we under-produce by a factor of 2 to 3 the total dust content of clusters estimated observationally at low redshift, $z lesssim 0.5$ using IRAS, Planck and Herschel satellites data. This discrepancy can be solved by decreasing the efficiency of sputtering which erodes dust grains in the hot Intracluster Medium (ICM).
Kepler-452b is currently the best example of an Earth-size planet in the habitable zone of a sun-like star, a type of planet whose number of detections is expected to increase in the future. Searching for biosignatures in the supposedly thin atmosphe
res of these planets is a challenging goal that requires a careful selection of the targets. Under the assumption of a rocky-dominated nature for Kepler-452b, we considered it as a test case to calculate a temperature-dependent habitability index, $h_{050}$, designed to maximize the potential presence of biosignature-producing activity (Silva et al. 2016). The surface temperature has been computed for a broad range of climate factors using a climate model designed for terrestrial-type exoplanets (Vladilo et al. 2015). After fixing the planetary data according to the experimental results (Jenkins et al. 2015), we changed the surface gravity, CO$_2$ abundance, surface pressure, orbital eccentricity, rotation period, axis obliquity and ocean fraction within the range of validity of our model. For most choices of parameters we find habitable solutions with $h_{050}>0.2$ only for CO$_2$ partial pressure $p_mathrm{CO_2} lesssim 0.04$,bar. At this limiting value of CO$_2$ abundance the planet is still habitable if the total pressure is $p lesssim 2$,bar. In all cases the habitability drops for eccentricity $e gtrsim 0.3$. Changes of rotation period and obliquity affect the habitability through their impact on the equator-pole temperature difference rather than on the mean global temperature. We calculated the variation of $h_{050}$ resulting from the luminosity evolution of the host star for a wide range of input parameters. Only a small combination of parameters yield habitability-weighted lifetimes $gtrsim 2$,Gyr, sufficiently long to develop atmospheric biosignatures still detectable at the present time.
We present results of cosmological simulations of disk galaxies carried out with the GADGET-3 TreePM+SPH code, where star formation and stellar feedback are described using our MUlti Phase Particle Integrator (MUPPI) model. This description is based
on simple multi-phase model of the interstellar medium at unresolved scales, where mass and energy flows among the components are explicitly followed by solving a system of ordinary differential equations. Thermal energy from SNe is injected into the local hot phase, so as to avoid that it is promptly radiated away. A kinetic feedback prescription generates the massive outflows needed to avoid the over-production of stars. We use two sets of zoomed-in initial conditions of isolated cosmological halos with masses (2-3) * 10^{12} Msun, both available at several resolution levels. In all cases we obtain spiral galaxies with small bulge-over-total stellar mass ratios (B/T approx 0.2), extended stellar and gas disks, flat rotation curves and realistic values of stellar masses. Gas profiles are relatively flat, molecular gas is found to dominate at the centre of galaxies, with star formation rates following the observed Schmidt-Kennicutt relation. Stars kinematically belonging to the bulge form early, while disk stars show a clear inside-out formation pattern and mostly form after redshift z=2. However, the baryon conversion efficiencies in our simulations differ from the relation given by Moster et al. (2010) at a 3 sigma level, thus indicating that our stellar disks are still too massive for the Dark Matter halo in which they reside. Results are found to be remarkably stable against resolution. This further demonstrates the feasibility of carrying out simulations producing a realistic population of galaxies within representative cosmological volumes, at a relatively modest resolution.
We measure and quantify properties of galactic outflows and diffuse gas at $z geq 1$ in cosmological hydrodynamical simulations. Our novel sub-resolution model, MUPPI, implements supernova feedback using fully local gas properties, where the wind vel
ocity and mass loading are not given as input. We find the following trends at $z = 2$ by analysing central galaxies having a stellar mass higher than $10^{9} M_{odot}$. The outflow velocity and mass outflow rate ($dot{M}_{rm out}$) exhibit positive correlations with galaxy mass and with the star formation rate (SFR). However, most of the relations present a large scatter. The outflow mass loading factor ($eta$) is between $0.2 - 10$. The comparison Effective model generates a constant outflow velocity, and a negative correlation of $eta$ with halo mass. The number fraction of galaxies where outflow is detected decreases at lower redshifts, but remains more than $80 %$ over $z = 1 - 5$. High SF activity at $z sim 2 - 4$ drives strong outflows, causing the positive and steep correlations of velocity and $dot{M}_{rm out}$ with SFR. The outflow velocity correlation with SFR becomes flatter at $z = 1$, and $eta$ displays a negative correlation with halo mass in massive galaxies. Our study demonstrates that both the MUPPI and Effective models produce significant outflows at $sim 1 / 10$ of the virial radius; at the same time shows that the properties of outflows generated can be different from the input speed and mass loading in the Effective model. Our MUPPI model, using local properties of gas in the sub-resolution recipe, is able to develop galactic outflows whose properties correlate with global galaxy properties, and consistent with observations.
As a contribution to the study of the habitability of extrasolar planets, we implemented a 1-D Energy Balance Model (EBM), the simplest seasonal model of planetary climate, with new prescriptions for most physical quantities. Here we apply our EBM to
investigate the surface habitability of planets with an Earth-like atmospheric composition but different levels of surface pressure. The habitability, defined as the mean fraction of the planets surface on which liquid water could exist, is estimated from the pressure-dependent liquid water temperature range, taking into account seasonal and latitudinal variations of surface temperature. By running several thousands of EBM simulations we generated a map of the habitable zone (HZ) in the plane of the orbital semi-major axis, a, and surface pressure, p, for planets in circular orbits around a Sun-like star. As pressure increases, the HZ becomes broader, with an increase of 0.25 AU in its radial extent from p=1/3 bar to p=3 bar. At low pressure, the habitability is low and varies with a; at high pressure, the habitability is high and relatively constant inside the HZ. We interpret these results in terms of the pressure dependence of the greenhouse effect, the effciency of horizontal heat transport, and the extent of the liquid water temperature range. Within the limits discussed in the paper, the results can be extended to planets in eccentric orbits around non-solar type stars. The main characteristics of the pressure-dependent HZ are modestly affected by variations of planetary properties, particularly at high pressure.
We present results from high--resolution cosmological hydrodynamical simulations of a Milky--Way-sized halo, aimed at studying the effect of feedback on the nature of gas accretion. Simulations include a model of inter-stellar medium and star formati
on, in which SN explosions provide effective thermal feedback. We distinguish between gas accretion onto the halo, which occurs when gas particles cross the halo virial radius, and gas accretion onto the central galaxy, which takes place when gas particles cross the inner one-tenth of the virial radius. Gas particles can be accreted through three different channels, depending on the maximum temperature value, $T_{rm max}$, reached during the particles past evolution: a cold channel for $T_{rm max}<2.5 times 10^5$ K, a hot one for $T>10^6$K, and a warm one for intermediate values of $T_{rm max}$. We find that the warm channel is at least as important as the cold one for gas accretion onto the central galaxy. This result is at variance with previous findings that the cold mode dominates gas accretion at high redshift. We ascribe this difference to the different supernova feedback scheme implemented in our simulations. While results presented so far in the literature are based on uneffective SN thermal feedback schemes and/or the presence of a kinetic feedback, our simulations include only effective thermal feedback. We argue that observational detections of a warm accretion mode in the high--redshift circum-galactic medium would provide useful constraints on the nature of the feedback that regulates star formation in galaxies.
We study sever
We present results based on an implementation of the Godunov Smoothed Particle Hydrodynamics (GSPH), originally developed by Inutsuka (2002), in the GADGET-3 hydrodynamic code. We first review the derivation of the GSPH discretization of the equation
s of moment and energy conservation, starting from the convolution of these equations with the interpolating kernel. The two most important aspects of the numerical implementation of these equations are (a) the appearance of fluid velocity and pressure obtained from the solution of the Riemann problem between each pair of particles, and (b the absence of an artificial viscosity term. We carry out three different controlled hydrodynamical three-dimensional tests, namely the Sod shock tube, the development of Kelvin-Helmholtz instabilities in a shear flow test, and the blob test describing the evolution of a cold cloud moving against a hot wind. The results of our tests confirm and extend in a number of aspects those recently obtained by Cha (2010): (i) GSPH provides a much improved description of contact discontinuities, with respect to SPH, thus avoiding the appearance of spurious pressure forces; (ii) GSPH is able to follow the development of gas-dynamical instabilities, such as the Kevin--Helmholtz and the Rayleigh-Taylor ones; (iii) as a result, GSPH describes the development of curl structures in the shear-flow test and the dissolution of the cold cloud in the blob test. We also discuss in detail the effect on the performances of GSPH of changing different aspects of its implementation. The results of our tests demonstrate that GSPH is in fact a highly promising hydrodynamic scheme, also to be coupled to an N-body solver, for astrophysical and cosmological applications. [abridged]
