### Creating a plot of sfr surface density measured within Re vs. log of HI or mass of HI surface density

Arjun Karki
• 8 Mar '23

Hi, I want to make a simulation plot of star formation rate surface density (measured within twice the effective radius) versus the Log of N_HI or with the mass of HI surface density. I want to show how the simulation result (using either TNG 100 or 50) will look compared to the observation results in my paper.

I went to go TNG website and looked at the Plot Galaxy/Halo Catalogs to see if I could find those parameters to choose and plot them. However, I did not see an option to choose Log of N_HI or mass of HI surface density in x or y-axis quantity. Can you please help how to import those parameters in JupyterLab so that I can create the plot? This is the first time I have been using JupyterLab access. I went through the tutorial.ipynb file, but I did not get much help from it.

Thank you,

Dylan Nelson
• 8 Mar '23

(1) The quantity `SubhaloSFRinRad` is the SFR within twice the stellar half mass radius, and `SubhaloHalfmassRadType[4]` is that radius.

If you compute `Sigma_SFR = SubhaloSFRinRad / (pi * SubhaloHalfmassRadType[4]**2)` you could get a quick, but very rough, estimate of the star formation rate surface density.

Note this is half mass radius, not effective radius of stellar light. Both values are also computed in, and represent, 3D, not 2D in projection, so the above equation is a poor use. Ideally, you would recompute these quantities in 2D projection to make the estimate.

Note: these are instantaneous, intrinsic (no obscuration) SFRs.

(2) For neutral hydrogen column density (N_HI) you would either need to (a) draw a sightline through the simulation volume at a desired distance away from the galaxy, or (b) project all the neutral hydrogen mass onto a map, normalizing by the species mass and the pixel area, and taking the value from a pixel at a desired distance away from the galaxy.

There is no easy way to achieve (a) - one option would be to use a software package like Trident.

The "Visualize Galaxies and Halos" tool can achieve (b), under various assumptions, if you select "HI" for the field.

Note that in both cases, the simulation stores total atomic hydrogen, and to compute a realistic neutral hydrogen fraction requires a molecular hydrogen model, which means additional modeling uncertainty.

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