The mass-velocity and intensity-velocity relations in jet-driven molecular outflows

We use numerical simulations to examine the mass-velocity and intensity-velocity relations in the CO J=2-1 and H$_2$ S(1)1-0 lines for jet-driven molecular outflows. Contrary to previous expectations, we find that the mass-velocity relation for the swept-up gas is a single power-law, with a shallow slope $\simeq -1.5$ and no break to a steeper slope at high velocities. An analytic bowshock model with no post-shock mixing is shown to reproduce this behaviour very well. We show that molecular dissociation and the temperature dependence of the line emissivity are both critical in defining the shape of the line profiles at velocities above $\sim$ 20 km s$^{-1}$. In particular, the simulated CO J=2-1 intensity-velocity relation does show a break in slope, even though the underlying mass distribution does not. These predicted CO profiles are found to compare remarkably well with observations of molecular outflows, both in terms of the slopes at low and high velocities and in terms of the range of break velocities at which the change in slope occurs. Shallower slopes are predicted at high velocity in higher excitation lines, such as H$_2$ S(1)1-0. This work indicates that, in jet-driven outflows, the CO J=2-1 intensity profile reflects the slope of the underlying mass-velocity distribution only at velocities $\le$ 20 km/s, and that higher temperature tracers are required to probe the mass distribution at higher speed.Comment: 6 pages, 8 figures. Accepted for publication in Astronomy and Astrophysic

An explicit scheme for multifluid magnetohydrodynamics

When modeling astrophysical fluid flows, it is often appropriate to discard the canonical magnetohydrodynamic approximation thereby freeing the magnetic field to diffuse with respect to the bulk velocity field. As a consequence, however, the induction equation can become problematic to solve via standard explicit techniques. In particular, the Hall diffusion term admits fast-moving whistler waves which can impose a vanishing timestep limit. Within an explicit differencing framework, a multifluid scheme for weakly ionised plasmas is presented which relies upon a new approach to integrating the induction equation efficiently. The first component of this approach is a relatively unknown method of accelerating the integration of parabolic systems by enforcing stability over large compound timesteps rather than over each of the constituent substeps. This method, Super Time Stepping, proves to be very effective in applying a part of the Hall term up to a known critical value. The excess of the Hall term above this critical value is then included via a new scheme for pure Hall diffusion.Comment: 8 pages; 4 figures; accepted by MNRAS; minor corrections to equations; addition of appendi

Multifluid magnetohydrodynamic turbulent decay

It is generally believed that turbulence has a significant impact on the dynamics and evolution of molecular clouds and the star formation which occurs within them. Non-ideal magnetohydrodynamic effects are known to influence the nature of this turbulence. We present the results of a suite of 512-cubed resolution simulations of the decay of initially super-Alfvenic and supersonic fully multifluid MHD turbulence. We find that ambipolar diffusion increases the rate of decay of the turbulence while the Hall effect has virtually no impact. The decay of the kinetic energy can be fitted as a power-law in time and the exponent is found to be -1.34 for fully multifluid MHD turbulence. The power spectra of density, velocity and magnetic field are all steepened significantly by the inclusion of non-ideal terms. The dominant reason for this steepening is ambipolar diffusion with the Hall effect again playing a minimal role except at short length scales where it creates extra structure in the magnetic field. Interestingly we find that, at least at these resolutions, the majority of the physics of multifluid turbulence can be captured by simply introducing fixed (in time and space) resistive terms into the induction equation without the need for a full multifluid MHD treatment. The velocity dispersion is also examined and, in common with previously published results, it is found not to be power-law in nature.Comment: 16 pages, 15 figures, Accepted for publication in Ap

High-resolution [C II] imaging of HDF850.1 reveals a merging galaxy at z=5.185

New high-resolution maps with the IRAM Interferometer of the redshifted [C II] 158 micron line and the 0.98mm dust continuum of HDF850.1 at z = 5.185 show the source to have a blueshifted northern component and a redshifted southern component, with a projected separation of 0.3 arcsec, or 2 kpc. We interpret these components as primordial galaxies that are merging to form a larger galaxy. We think it is the resulting merger-driven starburst that makes HDF850.1 an ultraluminous infrared galaxy, with an L(IR) of 1E13 Lsun. The observed line and continuum brightness temperatures and the constant line-to-continuum ratio across the source imply (1) high [C II] line optical depth, (2) a [C II] excitation temperature of the same order as the dust temperature, and (3) dust continuum emission that is nearly optically thick at 158 microns. These conclusions for HDF850.1 probably also apply to other high-redshift submillimeter galaxies and quasar hosts in which the [C II] 158 micron line has been detected, as indicated by their roughly constant [C II]-to-158 micron continuum ratios, in sharp contrast to the large dispersion in their [C II]-to-FIR luminosity ratios. In brightness temperature units, the [C II] line luminosity is about the same as the predicted CO(1-0) luminosity, implying that the [C II] line can also be used to estimate the molecular gas mass, with the same assumptions as for CO.Comment: Accepted by Astronomy and Astrophysic