Subhalo indexing and Data Spec for SubhaloStellarPhotometrics

Andrew Thibeault
  • 12 Mar '20

I have two questions about how data is stored in the Subfind Subhalos Group Catalogs. Note that I am working with this data locally via Python.

1) If I access the data for a given field (say 'SubhaloMass') of a given subhalo (in my case accessed using the example script's loadSubhalos(), and getting the value of a field's corresponding data list at an arbitrary index (subhalos['SubhaloMass'][i])), will the data for a different field (say 'SubhaloCM') of the same subhalo exist at the same index in the dataset corresponding to 'SubhaloMass' as in the dataset corresponding to 'SubhaloCM'? In other words, are the datasets for each field in the chunked HDF5 files ordered to correspond to the same subhalo across fields at a given index in the field datasets?

2) In the data specifications documentation for the Subfind Subhalos Group Catalogs, are the 'SubhaloStellarPhotometrics' bands ordered in the 8-element arrays in the same order as in the description on the documentation page, such that the 0th index element corresponds to U-band, 1st index element corresponds to B-band, etc.?

I assume the answer to both of these questions is yes, but I just want to be sure.

Thank you!

Dylan Nelson
  • 13 Mar '20

Hi Andrew,

1) Yes this is correct. Your i we call the "subhalo ID" or "subhalo index", these are interchangeable. So all the Subhalo* fields have this same order, you could think of them just as columns in a table, where all the rows are the same. Note that the Group* fields are different objects.

2) Yes this is also correct, the only way to verify this is a sanity check (e.g. that the g-r color values are reasonable).

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