Dear All,
I've recently been experimenting with assigning UCDs to the results of some cosmological simulations -- specifically to catalogues of dark matter halos and their associated subhalos. In general, i've found that the existing UCD tree is able to describe most of the properties of these objects that are typically analysed in the literature, albeit with a few exceptions.
However, it is not currently possible to describe the properties and parameters of the simulations themselves. This includes some input physical parameters (i.e. cosmological parameters) that define the theoretical context of the simulation, and technical parameters that define its size,scope and resolution (e.g number of particles, length of simulation box side, gravitational softening length, time/redshift of a simulation output).
Therefore, for the latter, I propose a new branch of the UCD tree to encompass computatational techniques in astronomy: 'comp'. This branch can be used to describe both astrophysical (and cosmological) simulations, and data reduction and post-processing algorithms for both simulation and observational data. It is roughly a computing analogue of the 'instr' branch. For the case of simulations I propose the following sub-branches
comp.sim (computational simulation) comp.sim.nbody (Nbody simulation) comp.sim.sph (Smoothed Particle Hydrodynamics simulation) comp.sim.boxside (Simulation box) comp.sim.gravsoft (gravitational softening) comp.sim.particles (simulation particles (for Nbody and SPH simulations)) comp.sim.snapshot (output of a simulation box at a particular instant) comp.sim.grid (simulation grid (for hydro simulations))
The number of particles in the simulation box, number of grid points, particle mass, gravitational softening length and simulation box side length would therefore be:
meta.num;comp.sim.particles meta.num;comp.sim.grid phys.mass;comp.sim.particles phys.size;comp.sim.gravsoft phys.size;comp.sim.boxside
(For the last two, introduction of a phys.size.length UCD might provide a
more accurate description.)
The mass of an object in terms of the number of particles it contains:
phys.mass;meta.num;comp.sim.particles
Other possible sub-branches could be
comp.resourse (computational resources used in simulation/data
processing)
comp.resource.processors (processors used)
comp.resource.memory (total size of a data file)
plus those that are more specific to data-reduction/post-processing of observational data. Algorithms that might apply to both simulated and observed data (e.g. smoothing of images or particle densities) would be listed directly under the comp branch:
phys.size;comp.smooth
(or, with the introduction of a phys.size.length UCD:
phys.size.length;comp.smooth)
Physical Parameters
phys.cosmology (cosmology) phys.cosmology.omega (matter/energy density of universe) phys.cosmology.hubble (hubble constant) phys.cosmology.sigma8 (Normalisation of matter power-spectrum)
and also:
phys.matter.dark (dark matter tag) phys.matter.baryon (baryonic matter tag) phys.DarkEnergy (dark energy tag)
So, Omega_Lambda, Omega_DM, Omega_baryon would be
phys.cosmology.omega;phys.DarkEnery, phys.cosmology.omega;phys.matter.dark phys.comsology.omega;phys.matter.baryonic
Now we can also describe the number of dark matter (gas particles) in an SPH simulation, or a simulated object (star/galaxy/halo) using:
meta.num;comp.sim.particles;phys.matter.dark(/baryonic)
Furthermore, the mass and radius of dark matter halos in cosmological simulations are frequently defined in terms of a virial overdensity. Hence a phys.virial UCD would be usefull in specifying what is meant by the mass and radius of a halo:
phys.mass;phys.virial (virial mass)
phys.size.radius;phys.virial (virial radius)
Time in Simulations
phys.redshift;comp.sim.snapshot
Astrophysical Objects
Additionally, objects such as halos and subhalos are not typically
observed (though i guess people do make estimates of their mass/size
through
gravitational lensing). It seems strange to have halo and subhalo
listed under the src.class sub-branch. I therefore suggest that an
object branch be introduced in which astrophysical (and theoretical)
objects can be listed (as also proposed by others on the UCD suggestion
page):
object.galaxy;comp.sim (a simulated galaxy) object.galaxy.spiral;comp.sim (a simulated spiral galaxy)
One example that I encountered of an actual quantity where this was useful is describing the mass in substructure, or the number of subhalos, in a simulated halo:
phys.mass;object.DMhalo.subhalo
meta.num;onject.DMhalo.subhalo
It would be great to hear everyones thoughts, or ideas for alternative approaches, for all these.
Cheers,
Laurie
List of Proposed UCDs
comp.sim (simulations) comp.sim.nbody (Nbody simulation) comp.sim.sph (Smoothed Particle Hydrodynamics simulation) comp.sim.boxside (Simulation box) comp.sim.gravsoft (gravitational softening) comp.sim.particles (simulation particles (for Nbody and SPH simulations)) comp.sim.snapshot (output of a simulation box at a particular instant) comp.sim.grid (simulation grid (for hydro simulations)) comp.resourse (to describe computational resources used in simulation/dataprocessing)
phys.cosmology phys.cosmology.omega (matter/energy density of universe) phys.cosmology.hubble (hubble constant) phys.cosmology.sigma8 (Normalisation of matter power-spectrum) phys.matter.dark (dark matter tag) phys.matter.baryon (baryonic matter tag) phys.DarkEnergy (dark energy tag) phys.virial phys.size.length
object (astophysical object)
object.planet (planet) object.satellite object.star (star) object.galaxy (galaxy) object.DMhalo (DM halo) Object.DMhalo.subhalo (DM subhalo)