UCD's for simulations

[Laurie Shaw]

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
---------------------
Currently, there exists no UCDs for the main cosmological
parameters. In terms of simulations, it is very important to be able
define the assumed cosmology, when interpreting the results. To
describe these parameters I propose a 'cosmology' sub-branch of
the phys branch. So,

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
------------------
People frequently analyse the output of simulations, or snapshots, at
a series of different timesteps. This is often quoted in terms of the
redshift of
the snapshot. At the moment, redshift exists under the 'src'
branch. Maybe it might be better under the `phys' branch as it is a
measure of both distance and time, and can therefore be used to label
both the distance of observed objects and to label a time-stamp for
simulation snapshots:

phys.redshift;comp.sim.snapshot

Astrophysical Objects
--------------------
Another problem is the listing of astronomical objects types under the
'src' branch. This introduces confusion when trying to describe a
simulated astrophysical object. For example, the first word in
src.class.galaxy;comp.sim (simulated galaxy) implies that this is an
observed astrophysical source, the second that it is simulated.

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 (computational astronomy)
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/data
processing)
comp.resource.processors (processors used)
comp.resource.memory (total size on disk of data)
(comp.dataReduct ?
comp.algorithm (general algorithms applied to sim/obs data) )

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)
etc