Hi Hassan (cc'ing trident help list), A number density (like H I number density) is in units of cm**-3 representing the number of particles per unit volume, whereas a column density (like H I column density) is in units of cm**-2 representing the number of particles per unit area on the sky. If you take some number density in a given volume and project it along one axis, it compresses the dimensions of that axis (calculating a line integral along that axis) to give you a column density. That's what is happening in your second example. You project the H_p0_number_density field along the z axis and it yields a 2D map of H I column densities, where each pixel is a line integral along the z direction. On the other hand, in your first example, you're generating a ProfilePlot, which isn't doing any projecting at all, so it's just returning the average H I number density value at each galactocentric radius. What you're trying to do, to make an HI column density plot as a function of radius, doesn't make sense unless you pick a viewing angle. One thing you could do is to make the column density plot like you did for the second example, using the ProjectionPlot functionality. Then each pixel is a separate line integral with a distinct impact parameter (i.e. projected radius from the galactic center). Then you could identify all the pixels that are in different radial bins and average them. That would give you a plot showing H I column density vs projected radius from that viewing angle. This is similar to what I do (from many different viewing angles) in my research analyses of the CGM. I hope this helps! Cameron On Wed, Feb 24, 2021 at 11:26 AM <h.fathie@gmail.com> wrote:

Hello,

I would like to plot the HI column density as a function of the radius. To do this I follow these steps. But instead of the column density (in cm-2) the y-axis is number density (in cm-3):

fn = '../IsolatedGalaxy/galaxy0030/galaxy0030' ds = yt.load(fn)

trident.add_ion_fields(ds, ions = ['H I'], ftype = 'gas') sphere = ds.sphere('c', (100., 'kpc')) plot = yt.ProfilePlot(sphere, 'radius', 'H_p0_number_density', weight_field = None) plot.save('my_fig.png')

The field 'H_p0_number_density' should return the column density because it does so when I use the following line of code: plot = yt.ProjectionPlot(ds, 'z', 'H_p0_number_density') plot.save('fig.png') Here the 'H_p0_number_density' is interpreted as column density but not as number density.

I would be very grateful if some one could help me with this issue. Many thanks

Best regards, Hassan _______________________________________________ yt-users mailing list -- yt-users@python.org To unsubscribe send an email to yt-users-leave@python.org https://mail.python.org/mailman3/lists/yt-users.python.org/ Member address: chummels@gmail.com

-- Cameron Hummels Computational Astrophysicist California Institute of Technology http://chummels.org