[yt-users] Fitting an absorption spectrum

[Hilary Egan]

Hi Pengfei,

I'm glad at least some of my fixes worked! I'm not sure whats going on with
your b-value distributions. I've performed the same analysis for my
absorption spectra and I dont see anything like the peaks at ~40 km/s that
you do (see attached). At the high end, I do see a very slight excess of
absorbers at the high end, but again nothing like what you're showing.

This leads me to the conclusion that the fitting process works well for
spectra generated like mine, but behaves somewhat poorly for yours. Having
looked briefly at one example lightray of yours (offlist) I can definitely
see that yours looks qualitatively different than mine and has a lot more
lines and those lines are much narrower, although having little idea what
exactly you're looking at I can't really extrapolate beyond that.

Going forward I can relatively quickly add something that throws away lines
with these parameters as they're clearly unphysical; however, I'm somewhat
uncomfortable doing that as its probably an indication of a larger problem
for a certain class of spectra. I wish I could guarantee that I could look
at this and completely fix it for you, but I really don't have more time to
devote to this project at the moment. If you're interested in helping look
at it I'd be more than happy to point you in the right direction and we can
chat offlist/in the irc. Otherwise I'll update the pull request to just
remove those lines, and heavily comment it so any future user can
remove/play around with it if they're unsatisfied with this stop-gap
measure.

-Hilary


On Tue, Mar 4, 2014 at 11:32 PM, Pengfei Chen <madcpf at gmail.com> wrote:

> Hi Hilary,
>
> Thank you very much for your update and I'm sorry for the late reply. I've
> tested the new code with two datasets. The results are attached. The
> abnormal peaks in the statistics of N have disappeared, and the abnormal
> peaks for b are much lower now, but still visible.
>
> Thank you very much for your effort in fixing this! Please tell me if you
> need any test data from me.
>
> Pengfei
>
>
> On Fri, Feb 14, 2014 at 3:30 PM, Pengfei Chen <madcpf at gmail.com> wrote:
>
>> Hi Hilary,
>>
>> Thank you! I'm looking forward to it.
>>
>> Pengfei
>>
>>
>> On Thu, Feb 13, 2014 at 7:32 PM, Hilary Egan <hilaryye at gmail.com> wrote:
>>
>>> Hi Pengfei,
>>>
>>> I have a fix for this issue that just needs a little more testing! I'll
>>> push it shortly and reply to this message when it's been accepted.
>>>
>>> -Hilary
>>>
>>>
>>> On Thu, Feb 13, 2014 at 8:04 PM, Pengfei Chen <madcpf at gmail.com> wrote:
>>>
>>>> Hi all,
>>>>
>>>> I've used the
>>>>
>>>> yt.analysis_modules.absorption_spectrum.absorption_spectrum_fit
>>>>
>>>> script to fit an Lya absorption spectrum generated by yt.
>>>>
>>>> But there are some abnormal peaks when I did statistics of line width b
>>>> and column density N. Two figures are attached. I've tried with different
>>>> redshifts and all of them had peaks in b-distribution at init_b (which is
>>>> 40km/s here) and maxb(which is 300km/s here). For the N-distribution I got
>>>> peaks around 10^12 cm^-2 at every redshift. The parameters I used were:
>>>>
>>>> HI_parameters = {'name':'HI',
>>>>                  'f': [.4164],
>>>>                  'Gamma':[6.265E8],
>>>>                  'wavelength':[1215.67],
>>>>                  'numLines':1,
>>>>                  'maxN': 1E22, 'minN':1E11,
>>>>                  'maxb': 300, 'minb':1,
>>>>                  'maxz': 5, 'minz':0,
>>>>                  'init_b':40,
>>>>                  'init_N':1E14}
>>>>
>>>> Could someone please tell me how to solve this? All the analysis were
>>>> done with the latest development version of yt and I made no changes to the
>>>> source code.
>>>>
>>>> Please tell me if you need any datasets.
>>>>
>>>> Thank you!
>>>> Pengfei
>>>>
>>>> _______________________________________________
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>>>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>>>
>>>>
>>>
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>>
>
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