velocity offset for absorption line

[d.defe...@columbia.edu]

Hello,

I'm running the development version of trident on some TNG data and I'm having an issue that may be conceptual on my part but I'm not sure.

I've been manually editing the velocities of the gas in a TNG halo and generating absorption spectra with this edited halo. However, I've just tried setting all the gas velocities in the halo to 0. I expected to get a spectrum centered on 0 km/s (i.e. the line center) but I didn't: it's consistently negative (around -25 km/s) for all sightlines I've chosen so far. I don't understand how I can be getting absorption lines at nonzero velocities if all the gas cells have velocity = 0. 

I'm also wondering if this means all the other absorption spectra I've made are offset somehow. Any thoughts? 

Thank you,

Daniel DeFelippis

[Cameron Hummels]

Hi Daniel,

Thanks for reaching out.  It does indeed seem odd that the velocity is shown as being negative when you set it to 0 km/s, but I have an idea for this.  I assume you're getting these results by using the `SpectrumGenerator` class with `bin_space=velocity` keyword?  Can you check what the redshift is for the elements of your `LightRay` that you used to generate the spectrum?  When Trident generates a `LightRay` through a simulation volume, it usually starts with the redshift of the simulation volume at the *back* of the LightRay and then decrements in redshift as the LightRay gets closer to the observer.  This means that usually, you have lower values at the front of the LightRay than at the back, usually lower than the redshift of the simulation snapshot by a little bit.  I imagine this might be happening here, in that you've set the velocities to 0, representing the peculiar velocity of the gas, but if the redshift is slightly lower than you expect, then you'll get cosmological velocities that are slightly negative, which might give you the behavior you're showing.

I'm on vacation this week and return to "my office" on Tuesday.  What I'd recommend doing is :

(1) checking these redshift values of the LightRay that generated these spectra to confirm that the redshift is slightly lower than you anticipated on the near side. This may be the answer.  

(2) checking to see if you get the same behavior if you change to `bin_space=wavelength`.

(3) checking to see if you get the same behavior with a non-TNG dataset, like the sample gizmo or enzo datasets using in our examples: https://trident.readthedocs.io/en/latest/annotated_example.html

I can help with this next week if you don't make any progress before then.  

Cameron

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Cameron Hummels

Computational Astrophysicist

California Institute of Technology

http://chummels.org

[Cameron Hummels]

Hi Daniel,

I just wanted to follow up on this.  Did you check my suggestions?  Was it the cosmological redshift that was responsible for the slight blue-shifting you were seeing in the velocity of your absorption features?  Hopefully that was the issue, but I'm happy to work with you if this remains a problem.

Cameron