[yt-users] HaloProfiler ActualOverdensity

Wed Dec 2 10:50:03 PST 2009

Shankar,

As I stated in my previous email, you will find both definitions used in the
literature.  I'm not going to spend time in a citation battle.  The good
news is that both definitions differ only by a factor of Omega_matter, so
you can change the value of virial_overdensity accordingly.

Britton

On Wed, Dec 2, 2009 at 11:27 AM, Agarwal, Shankar <sagarwal at ku.edu> wrote:

> Hi Britton,
>
> I just wanted a clarification on the definition of ActualOverdensity. In
> your mail, you said...
>
>  "ActualOverdensity has the same physical meaning as the regular
> Overdensity
>  field.  It is (Baryon Density + Dark Matter Density) / (Mean density of
> the
>  universe).  If you search the literature, you will find alternate
>  definitions that use critical density instead of mean density. The one
> used
>  here is a little more common."
>
>
> Are you sure you did not mean (Baryon Density + Dark Matter Density) /
> (critical density) ?
>
>
> Because, in halo_profiler.py, there is a virial filter...
>
> hp.add_halo_filter(HP.VirialFilter,must_be_virialized=True,
>                   overdensity_field='ActualOverdensity',
>                   virial_overdensity=200,
>                   virial_filters=[['TotalMassMsun','>=','1e14']],
>                   virial_quantities=['TotalMassMsun','RadiusMpc'])
>
>
> Isn't 200 w.r.t rho_ciritcal (not rho_mean_matter)? Look
> http://arxiv.org/abs/astro-ph/0011495
>
>
> shankar
>
>
>
>
> -----Original Message-----
> From: yt-users-bounces at lists.spacepope.org on behalf of Britton Smith
> Sent: Wed 11/25/2009 1:30 PM
> To: Discussion of the yt analysis package
> Subject: Re: [yt-users] Hop vs HaloProfiler
>
> Hi Shankar,
>
> I will answer the HaloProfiler related questions and leave the Hop
> questions
> for Stephen.  However, could you post the failure output for what you tried
> to do with Hop.  That will probably be helpful for answering your question.
>
> Is Halo_0000_profile.dat related to the first Halo listed in
> HopAnalysis.out
> > file ?
> >
>
> Yes, the HaloProfiler will always use the same indices for halos as all of
> the halo finders in yt.  So, the file Halo_N_profile.dat will always refer
> to halo N on the halo list.
>
> What is # ?
> >
>
> This is just a comment character so that plotting programs don't try to
> read
> in what's on that line.  The tab between it and the first field name is
> there to accommodate the routines that read those files back in to the
> HaloProfiler.
>
>
> > What is the meaning of ActualOverdensity ? And units ?
> >
>
> ActualOverdensity has the same physical meaning as the regular Overdensity
> field.  It is (Baryon Density + Dark Matter Density) / (Mean density of the
> universe).  If you search the literature, you will find alternate
> definitions that use critical density instead of mean density.  The one
> used
> here is a little more common (I think, but I'm not sure).  As per these
> definitions, overdensity is a unitless quantity.
>
> In yt, there is an Overdensity field that is calculated on a cell-by-cell
> basis.  For calculation of virial quantities for halos, the overdensity you
> calculate should be explicitly the total mass (baryon + dm) / total volume
> /
> mean density, where total refers to all cells enclosed within the sphere of
> the radial profile, not just the shell from r_(i-1) to r_i.  Technically,
> you could get this by doing a profile of the Overdensity field, weighted by
> CellVolume, with accumulation set to True.  However, if for some reason,
> the
> user wanted to do profiles of the overdensity field in a different way, say
> weighted by CellMass, or just counting the material shell-by-shell
> (accumulation=False), this number would not be correct for the calculation
> of virial quantities.  For that reason, the HaloProfiler automatically
> generates this ActualOverdensity field which is assured to be calculated in
> the correct way.  That way the user doesn't accidentally override a proper
> calculation of the overdensity used for the virial quantities.
>
> Other than the ActualOverdensity field (which is automatically generated by
> the HaloProfiler, and thus does not exist outside that context) you can
> always get the units of any field with:
> lagos.fieldInfo[some_field].units
> For more information, see here:
>
> http://yt.enzotools.org/doc/faq.html#how-do-i-know-what-the-units-returned-are
>
> What is the meaning of CellVolume? And units ?
>
> What is Density referring to ? And units ?
> >
> CellVolume and Density are the volume of a cell and the baryon density.  In
> this context, they are the values of those fields in the radial profile.
>  If
> you did it as per the recipe, the CellVolume is the total accumulated
> volume
> for all cells within the sphere radius.  Density SHOULD be the
> mass-weighted, mean baryon density for cells within spherical shells.
> However, after looking at the recipe on the website, I see that, in error,
> I
> set the weight_field of the Density profile to None, instead of
> CellMassMsun.  Therefore, the Density profile may be meaningless as is.  I
> will change the recipe on the website ASAP.
>
>
> > What is mywieght ?
> >
>
> The myweight field is a temporary field for keeping track of the weight
> field for a weight radial profile.  It should be ignored and we might just
> want to not write it out in the future.
>
>
> > I also got the projection/ directory. But I got errors...
> >
>
> It looks like the HaloProfiler tried to do a projection of Metallicity, but
> your simulation data did not have the MetalDensity field.  You need to set
> CosmologyUseMetallicityField (or something like that) to 1 in your enzo
> parameter file in order to get this field.  You can also just remove the
> line in the HaloProfiler recipe that adds the metallicity field to the list
> of projections.
>
> Cheers,
>
> Britton
>
>
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