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Register a new userStochastic angular momentum slews and flips and their effect on discs in galaxy formation models
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Nelson David Padilla
Publication date
2013
fields
Physics
and research's language is
English
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The angular momentum of galactic discs in semi-analytic models of galaxy formation is usually updated in time as material is accreted to the disc by adopting a constant dimensionless spin parameter and little attention is paid to the effects of accretion with misaligned angular momenta. These effects are the subject of this paper, where we adopt a Monte-Carlo simulation for the changes in the direction of the angular momentum of a galaxy disc as it accretes matter based on accurate measurements from dark-matter haloes in the Millennium II simulation. In our semi-analytic model implementation, the flips seen the dark matter haloes are assumed to be the same for the cold baryons; however, we also assume that in the latter the flip also entails a difficulty for the disc to increase its angular momentum which causes the disc to become smaller relative to a no-flip case. This makes star formation to occur faster, specially in low mass galaxies at all redshifts allowing galaxies to reach higher stellar masses faster. We adopt a new condition for the triggering of starbursts during mergers. As these produce the largest flips it is natural to adopt the disc instability criterion to evaluate the triggering of bursts in mergers instead of one based on mass ratios as in the original model. The new implementation reduces the average lifetimes of discs by a factor of 2, while still allowing old ages for the present-day discs of large spiral galaxies. It also provides a faster decline of star formation in massive galaxies and a better fit to the bright end of the luminosity function at z = 0.
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This paper provides a summary of our recent work on the scaling relations between the specific angular momentum j_* and mass M_* of the stellar parts of normal galaxies of different bulge fraction beta_*. We find that the observations are consistent with a simple model based on a linear superposition of disks and bulges that follow separate scaling relations of the form j_*d ~ M_*d^alpha and j_*b ~ M_*b^alpha with alpha = 0.67 +/- 0.07 but offset from each other by a factor of 8 +/- 2 over the mass range 8.9 <= log (M_*/M_Sun) <= 11.8. This model correctly predicts that galaxies follow a curved 2D surface in the 3D space of log j_*, log M_*, and beta_*.
Star forming molecular clouds are observed to be both highly magnetized and turbulent. Consequently the formation of protostellar disks is largely dependent on the complex interaction between gravity, magnetic fields, and turbulence. Studies of non-turbulent protostellar disk formation with realistic magnetic fields have shown that these fields are efficient in removing angular momentum from the forming disks, preventing their formation. However, once turbulence is included, disks can form in even highly magnetized clouds, although the precise mechanism remains uncertain. Here we present several high resolution simulations of turbulent, realistically magnetized, high-mass molecular clouds with both aligned and random turbulence to study the role that turbulence, misalignment, and magnetic fields have on the formation of protostellar disks. We find that when the turbulence is artificially aligned so that the angular momentum is parallel to the initial uniform field, no rotationally supported disks are formed, regardless of the initial turbulent energy. We conclude that turbulence and the associated misalignment between the angular momentum and the magnetic field are crucial in the formation of protostellar disks in the presence of realistic magnetic fields.
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