N-body simulations in modified Newtonian dynamics

We describe some results obtained with N-MODY, a code for N-body simulations of collisionless stellar systems in modified Newtonian dynamics (MOND). We found that a few fundamental dynamical processes are profoundly different in MOND and in Newtonian gravity with dark matter. In particular, violent relaxation, phase mixing and galaxy merging take significantly longer in MOND than in Newtonian gravity, while dynamical friction is more effective in a MOND system than in an equivalent Newtonian system with dark matter.Comment: 4 pages, no figures. To appear in EAS Publication Series (Proceedings of Symposium 7 of the JENAM 2008, Vienna

Phase mixing in MOND

Dissipationless collapses in Modified Newtonian Dynamics (MOND) have been studied by using our MOND particle-mesh N-body code, finding that the projected density profiles of the final virialized systems are well described by Sersic profiles with index m<4 (down to m~2 for a deep-MOND collapse). The simulations provided also strong evidence that phase mixing is much less effective in MOND than in Newtonian gravity. Here we describe "ad hoc" numerical simulations with the force angular components frozen to zero, thus producing radial collapses. Our previous findings are confirmed, indicating that possible differences in radial orbit instability under Newtonian and MOND gravity are not relevant in the present context.Comment: 10 pages, 3 figures. To appear in the Proceedings of the International Workshop "Collective Phenomena in Macroscopic Systems", G. Bertin, R. Pozzoli, M. Rome, and K.R. Sreenivasan, eds., World Scientific, Singapor

Galactic fountains and gas accretion

Star-forming disc galaxies such as the Milky Way need to accrete \gsim 1 $M_{\odot}$ of gas each year to sustain their star formation. This gas accretion is likely to come from the cooling of the hot corona, however it is still not clear how this process can take place. We present simulations supporting the idea that this cooling and the subsequent accretion are caused by the passage of cold galactic-fountain clouds through the hot corona. The Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas causes coronal gas to condense in the cloud's wake. For likely parameters of the Galactic corona and of typical fountain clouds we obtain a global accretion rate of the order of that required to feed the star formation.Comment: 2 pages, 1 figure, to appear in "Hunting for the Dark: The Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista & C.C. Popescu, AIP Conf. Se

Fountain-driven gas accretion by the Milky Way

Accretion of fresh gas at a rate of ~ 1 M_{sun} yr^{-1} is necessary in star-forming disc galaxies, such as the Milky Way, in order to sustain their star-formation rates. In this work we present the results of a new hydrodynamic simulation supporting the scenario in which the gas required for star formation is drawn from the hot corona that surrounds the star-forming disc. In particular, the cooling of this hot gas and its accretion on to the disc are caused by the passage of cold galactic fountain clouds through the corona.Comment: 2 pages, 1 figure. To appear in the proceedings of the conference "Assembling the Puzzle of the Milky Way", Le Grand-Bornand 17-22 April 2011, European Physical Journal, editors C. Reyl\'e, A. Robin and M. Schulthei

Dissipationless collapses in MOND

Dissipationless collapses in Modified Newtonian Dynamics (MOND) are studied by using a new particle-mesh N-body code based on our numerical MOND potential solver. We found that low surface-density end-products have shallower inner density profile, flatter radial velocity-dispersion profile, and more radially anisotropic orbital distribution than high surface-density end-products. The projected density profiles of the final virialized systems are well described by Sersic profiles with index m~4, down to m~2 for a deep-MOND collapse. Consistently with observations of elliptical galaxies, the MOND end-products, if interpreted in the context of Newtonian gravity, would appear to have little or no dark matter within the effective radius. However, we found impossible (under the assumption of constant mass-to-light ratio) to simultaneously place the resulting systems on the observed Kormendy, Faber-Jackson and Fundamental Plane relations of elliptical galaxies. Finally, the simulations provide strong evidence that phase mixing is less effective in MOND than in Newtonian gravity

Dissipationless collapse, weak homology and central cores of elliptical galaxies

By means of high-resolution N-body simulations we revisited the dissipationless collapse scenario for galaxy formation. We considered both single-component collapses and collapses of a cold stellar distribution in a live dark matter halo. Single-component collapses lead to stellar systems whose projected profiles are fitted very well by the Sersic R^(1/m) law with 3.6 < m < 8. The stellar end-products of collapses in a dark matter halo are still well described by the R^(1/m) law, but with 1.9 < m < 12, where the lowest m values are obtained when the halo is dominant. In all the explored cases the profiles at small radii deviate from their global best-fit R^(1/m) model, being significantly flatter. The break-radius values are comparable with those measured in `core' elliptical galaxies, and are directly related to the coldness of the initial conditions. The dissipationless collapse of initially cold stellar distributions in pre-existing dark matter haloes may thus have a role in determining the observed weak homology of elliptical galaxies.Comment: Accepted for publication in MNRAS (11 pages, 10 figures

DNA quantification to assess Zymoseptoria tritici on a susceptible cultivar of durum wheat to establish the best timing for fungicide application in an italian environment

Zymoseptoria tritici, a globally distributed pathogen, is responsible of Septoria tritici blotch (STB), one of the most damaging wheat diseases. In Italy the incidence of STB has increased during the past few years. The presence of Z. tritici on flag leaves of susceptible durum wheat plants, cultivar San Carlo, after a single artificial inoculation with two inoculum concentrations at different vegetative stages has been evaluated in the plain of Bologna (North of Italy), in a two year field study (2012–2013). The pathogen presence was also assessed in natural infection conditions after a fungicide application in the second year (2013). The results obtained, by visual examination (Incidence, Disease Severity) and DNA quantification by Real time PCR, demonstrated that BBCH 39 (flag leaf stage) is the most susceptible vegetative stage, independently of inoculum concentration and climatic conditions. A good correlation between Disease Severity and DNA quantity was observed in either sampling methods, entire flag leaves and flag leaf discs. Thereafter the most suitable period to obtain the best crop protection with only one fungicide treatment is the flag leaf stage

Study of fungal colonization of wheat kernels in Syria with a focus on Fusarium species

Wheat is one of the main crops in Mediterranean countries, and its cultivation has an important role in the Syrian economy. In Syria, Fusarium head blight (FHB) has not been reported so far. Mycological analysis of 48 samples of wheat kernels collected from cultivation areas with different climatic conditions were performed in 2009 and 2010. Fungal isolates were identified at the genus level morphologically; Fusarium species were characterized morphologically and by species-specific PCR. The most frequent fungal genera found were Alternaria spp. and Cladosporium spp., with frequencies of 24.7% and 8.1%, respectively, while the frequency of Fusarium spp. was 1.5% of kernels. Most frequent Fusarium species were F. tricinctum (30% of all Fusarium isolates), F. culmorum (18%), F. equiseti (14%) and F. graminearum (13%). The mycotoxin production potential of selected Fusarium isolates was assessed by HPLC-MS analysis of rice cultures; chemotyping by PCR was carried out for comparison. All six F. graminearum strains tested produced small amounts (&lt;3 mg/kg) of nivalenol (NIV). All ten F. culmorum strains tested produced large amounts of trichothecenes (&gt;100 mg/kg); four strains produced NIV and six strains produced deoxynivalenol (DON) and 3-acetyl-deoxynivalenol (3Ac-DON). PCR chemotyping lead to an oversimplified picture, because all 3Ac-DON chemotype strains produced more DON than 3Ac-DON; furthermore, the strongest NIV producers produced significant amounts of DON. All tested strains of F. culmorum, F. graminearum, F. pseudograminearum (two strains) and most F. equiseti strains (five of six strains) produced zearalenone. Grains of durum wheat were more frequently colonized by Fusarium spp. than grains of soft wheat. Incidence of Fusarium spp. in irrigated fields was higher than in rainfed fields. The incidence of Fusarium strains producing mycotoxins raises concerns about the risk of Fusarium head blight to Syria and its consequences for public healt

Separable triaxial potential-density pairs in MOND

We study mass models that correspond to MOND (triaxial) potentials for which the Hamilton-Jacobi equation separates in ellipsoidal coordinates. The problem is first discussed in the simpler case of deep-MOND systems, and then generalized to the full MOND regime. We prove that the Kuzmin property for Newtonian gravity still holds, i.e., that the density distribution of separable potentials is fully determined once the density profile along the minor axis is assigned. At variance with the Newtonian case, the fact that a positive density along the minor axis leads to a positive density everywhere remains unproven. We also prove that (i) all regular separable models in MOND have a vanishing density at the origin, so that they would correspond to centrally dark-matter dominated systems in Newtonian gravity; (ii) triaxial separable potentials regular at large radii and associated with finite total mass leads to density distributions that at large radii are not spherical and decline as ln(r)/r^5; (iii) when the triaxial potentials admit a genuine Frobenius expansion with exponent 0<epsilon<1, the density distributions become spherical at large radii, with the profile ln(r)/r^(3+2epsilon). After presenting a suite of positive density distributions associated with MOND separable potentials, we also consider the important family of (non-separable) triaxial potentials V_1 introduced by de Zeeuw and Pfenniger, and we show that, as already known for Newtonian gravity, they obey the Kuzmin property also in MOND. The ordinary differential equation relating their potential and density along the z-axis is an Abel equation of the second kind that, in the oblate case, can be explicitly reduced to canonical form.Comment: 17 pages, 4 figures (low resolution), accepted by MNRA