Pierluigi Monaco, Giuseppe Murante, Stefano Borgani, Klaus Dolag
We study several versions of the Schmidt-Kennicutt (SK) relation obtained for
isolated spiral galaxies in TreeSPH simulations run with the GADGET3 code
including the novel MUlti-Phase Particle Integrator (MUPPI) algorithm for star
formation and stellar feedback. [...] The standard SK relation between surface
densities of cold (neutral+molecular) gas and star formation rate of simulated
galaxies shows a steepening at low gas surface densities, starting from a knee
whose position depends on disc gas fraction: for more gas-rich discs the
steepening takes place at higher surface densities. Because gas fraction and
metallicity are typically related, this environmental dependence mimics the
predictions of models where the formation of H2 is modulated by metallicity.
The cold gas surface density at which HI and molecular gas surface densities
equate can range from ~10 up to 34 Msun/pc^2. As expected, the SK relation
obtained using molecular gas shows much smaller variations among simulations.
We find that disc pressure is not well represented by the classical external
pressure of a disc in vertical hydrostatic equilibrium. Instead is well fit by
the expression P_fit = Sigma_cold sigma_cold kappa / 6, where the three
quantities on the right-hand side are cold gas surface density, vertical
velocity dispersion and epicyclic frequency. When the "dynamical" SK relation,
i.e. the relation that uses gas surface density divided by orbital time, is
considered, we find that all of our simulations stay on the same relation. We
interpret this as a manifestation of the equilibrium between energy injection
and dissipation in stationary galaxy discs, when energetic feedback is
effective and pressure is represented by the expression given above. These
findings further support the idea that a realistic model of the structure of
galaxy discs should take into account energy injection by SNe. [Abridged]
View original:
http://arxiv.org/abs/1109.0484
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