#1
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Illustris usage
I'm trying to tease some data out of Illustris, and am
having trouble parsing the data access specifications[1]. For instance, about one-fifth of the way down, it says (in part): The general unit system is kpc/h for lengths, 10^10M_Sol/h for masses, [...] The two meanings for "h" for which I'm aware are "Planck's constant" and "hours". Obviously, neither one of those makes any sense in this context. Where can I find out what "h" means in this context? Also, I'm going to need to follow the histories of various subhalos. In the section on Merger Trees, it says: The number inside each circle from the figure is the unique ID (within the whole simulation) of the corresponding subhalo [...] Is this "SubhaloID"? If I find a way to extract data about SubhaloID == 42 (for example) from each snapshot, will I be following a single subhalo through time? Is there a similar field for halos? I'm trying to follow the changes in a galaxy after its capture, and I think that I'll need to be able to find out things like the mass of the halo that captures it. This page is quite comprehensive, but that makes it overwhelming for somebody who's never used Illustris before. Is there a novice's introduction or some documentation that explains the documentation? I'm sure that once I get a handle on their conventions, I'll be able to bootstrap pretty quickly. It's getting up the first part of the learning curve that has me stumped. Thanks for any help that you're willing to provide. [1] http://www.illustris-project.org/data/docs/specifications/ -- Michael F. Stemper Deuteronomy 10:18-19 |
#2
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Illustris usage
In article ,
"Michael F. Stemper" writes: The general unit system is kpc/h for lengths, 10^10M_Sol/h for masses, [...] The documentation doesn't define h (at least that I see), but it is almost certainly the current value of the Hubble constant in units of 100 km s^-1 Mpc-1. That means h = 0.7 (or very nearly so). Don't overlook that quantities are given in _co-moving_ coordinates. To get physical coordinates, you have to multiply by the appropriate power of the scale factor at the epoch in question. Also, I'm going to need to follow the histories of various subhalos. In the section on Merger Trees, it says: The number inside each circle from the figure is the unique ID (within the whole simulation) of the corresponding subhalo [...] Is this "SubhaloID"? That's how it looks to me. If I find a way to extract data about SubhaloID == 42 (for example) from each snapshot, will I be following a single subhalo through time? It looks to me as though any given SubhaloID will exist in only one snapshot. You have to follow the Progenitor or Descendent IDs to look at evolution through time. If you don't want to follow each step, you could select by FirstProgenitorID or RootDescendantID. There's also TreeID. Is there a similar field for halos? I'm trying to follow the changes in a galaxy after its capture, and I think that I'll need to be able to find out things like the mass of the halo that captures it. I think you need to use SubfindID as an index into the Group Catalog. ("Group" = Halo as far as I can tell.) This page is quite comprehensive, but that makes it overwhelming for somebody who's never used Illustris before. Is there a novice's introduction or some documentation that explains the documentation? There are numerous published papers and a general introduction at http://www.illustris-project.org/about/#astronomers Isn't there anyone locally you could ask for help? I expect some of the people who worked on the simulation would be willing to answer questions by email. [1] http://www.illustris-project.org/data/docs/specifications/ -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#3
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Illustris usage
On 2018-03-19 13:44, Steve Willner wrote:
In article , "Michael F. Stemper" writes: The general unit system is kpc/h for lengths, 10^10M_Sol/h for masses, [...] The documentation doesn't define h (at least that I see), but it is almost certainly the current value of the Hubble constant in units of 100 km s^-1 Mpc-1. That means h = 0.7 (or very nearly so). Don't overlook that quantities are given in _co-moving_ coordinates. To get physical coordinates, you have to multiply by the appropriate power of the scale factor at the epoch in question. Okay, thanks for the warning. I would have blindly gone ahead with the same value of h at all epochs. Also, I'm going to need to follow the histories of various subhalos. In the section on Merger Trees, it says: The number inside each circle from the figure is the unique ID (within the whole simulation) of the corresponding subhalo [...] Is this "SubhaloID"? That's how it looks to me. If I find a way to extract data about SubhaloID == 42 (for example) from each snapshot, will I be following a single subhalo through time? It looks to me as though any given SubhaloID will exist in only one snapshot. You have to follow the Progenitor or Descendent IDs to look at evolution through time. If you don't want to follow each step, you could select by FirstProgenitorID or RootDescendantID. There's also TreeID. These sound promising. I'll read up on them. This page is quite comprehensive, but that makes it overwhelming for somebody who's never used Illustris before. Is there a novice's introduction or some documentation that explains the documentation? There are numerous published papers and a general introduction at http://www.illustris-project.org/about/#astronomers I'll take a look at that as well. Isn't there anyone locally you could ask for help? Well, I shot an email to my TAs. When I didn't hear back from them, I spread my net further. I did hear back from one of them this morning. She confirmed your interpretation of "h", but didn't point out the impact of scale factor. Thanks for the pointers -- Michael F. Stemper This sentence no verb. |
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