About the NHtot tool

The “NHtot” tool is an enhancement to the NH ftool to calculate the Galactic hydrogen absorbing column in a given direction. Whereas that tool calculates only the column density of atomic hydrogen (HI) this new tool also calculates the density of molecular hydrogen (H₂) and hence the total column density of hydrogen, in atoms cm^-2.

The HI calculation is identical to that in the NH tool, using the LAB 21-cm maps of Kalberla et al., (2005) to determine N_HI (note that the NH tool allows also for the older map of Dickey & Lockman [1990], where nhtot does not). The H₂ measurement is determined from the dust map of Schlegel et al. (1998). For full details, see Willingale et al. (2013).

To use the tool

You can either enter a single position into the web form, or upload a file containing a series of positions. The co-ordinates can be entered as decimal or sexagesimal, the latter allowing colon, comma, whitespace or h/m/s/h/'/" as delimeters. If you upload a file, there must be one position per line, each line must be of the form: ra dec equinox. The equinox is optional and must be either J2000 or B1950. If it is not specified it is assumed to be J2000.

When submitted the page returns a table giving the list of positions you entered (converted to decimal format), preceded by a ‘J’ or ‘B’ to reflect the epoch. Then the values of the extinction (from the dust map), HI, H₂ and the total column are given. Note that all are in units of atoms cm^-2. There are two values for each field; "mean" is the mean value for all entries in the dust/HI maps within 1° of the input position, "weighted" is the mean of these entries, weighted by the inverse of their distance from the input position.