### Calculate the neutral hydrogen HI masses for each halo

yanlin wu
• 1 Oct '21

Hi,
I want to get the HI masses of each halo. But I couldn't find the neutral hydrogen abundance for group catalog data ( I only find that for the snapshot). For now, I can only find the hydrogen mass ( mass_gas = halos['GroupMassType']
gasmass = [item[0] for item in mass_gas]
hratio = halos['GroupGasMetalFractions']
hratio1 = [item[0] for item in hratio]
hmass=np.multiply(hratio1,gasmass) That is how I get the hydrogen mass for each halo. I am not sure if that is right. Even if it's right, I still cannot figure out the neutral hydrogen abundance among the total hydrogen mass.

Dylan Nelson
• 3 Oct '21

You are right that there is no information about hydrogen in the group catalogs. These will have to be calculated from the gas in the snapshots.

You can use `PartType0/NeutralHydrogenAbundance` to drive the neutral hydrogen mass per cell.

Note that this is not equal to the atomic (HI) nor molecular (H2) mass necessarily. Please see Stevens+19, Stevens+20, Diemer+18, Diemer+19 for some important details and information.

yanlin wu
• 1 Nov '21

Hi, thank you for your advice. However, I want to plot HI mass vs Halo mass and the HI mass calculated from snapshots have different dimensions with the halo mass extracted from group catalog. Therefore, I cannot plot them as x and y. How can I get the data with same dimensions?

Dylan Nelson
• 1 Nov '21

You have information "per gas cell", but need to convert it to "per halo". This means to decide which gas cells should be included in the calculation for each halo, and then to sum them. For example, you can compute a "total HI mass" summing all gas in the halo, or other common choices: all cells within twice the stellar half mass radius, or all cells within 30 pkpc. The choice depends on what question you ask / what data you compare to / etc.

yanlin wu
• 2 Nov '21