nh - Return hydrogen column density for a given Ra and Dec


        nh equinox ra dec disio


'nh' returns, for a specified right ascension and declination, a value for the hydrogen column density. This value is derived from the 2D HI4PI map, a full-sky HI survey by the HI4PI collaboration 2016, Astronomy & Astrophysics, 594, A116. The HI4PI survey is based on data from the recently completed first coverage of the Effelsberg-Bonn HI Survey (EBHIS) and from the third revision of the Galactic All-Sky Survey (GASS). The pixel size of the HI4PI map is 0.0833 deg x 0.0833deg. NOTE: The map has negative values (about 15 pixels) set for the galactic center.

Previous 'nh' versions support the following maps: a) HI map by Kalberla et al. 2005, Astronomy & Astrophysics, 440, 775, known as the Leiden/Argentine/Bonn (LAB) survey and b) the HI map by Dickey & Lockman (DL), 1990, Ann. Rev. Ast. Astr. 28, 215. The LAB map was obtained by merging two surveys covering HI radial velocities from -400km/s to +400 km/s - the Leiden/Dwingeloo Survey (Hartmann & Burton 1997), and the Instituto Argentino de Radioastronomia Survey (Arnal et al. 2000 and Bajaja et al. 2005). The resulting velocity integrated combination had a resolution of approximately 0.5 degrees, and was resampled onto 0.675 by 0.675 degree bins in L and B. The DL map was obtained by merging several surveys (see Dickey & Lockman 1990) and averaged into 1 by 1 degree bins in L and B. These maps were provided by K.Kuntz and S.Snowden respectively and still released with the software for back compatibility.

The Nh values are in units of hydrogen atoms cm**-2. The software calculates an average Nh using values within N degree from the request position (N is an input parameter, 'disio'). Two Nh average values are output for the requested map (see the 'usemap' parameter): a simple average and an average weighted by the inverse of the distance from the request position. The output "Dist" values are in degrees.

'nh' starts by reading a submap M degrees by M degrees, where M is an input parameter, 'size' (default is 1 x 1 deg), from which only the subset of values within N degrees (N is set by the parameter 'disio') are used for the average Nh value. The calculated averages (simple and weighted) are written in the paramater file in the parameters avnh and avwnh. If no pixels are found within the N degrees of the value in the parameter 'disio', the closer value is reported.

By default 'nh ' calculates only the value using the HI4PI survey (h1_map_HI4PI.fits) and the parameters 'size' and 'disio' are optimized for this map. 'nh' provides within the same run also the Nh value for one the alternative map if the parameter usemap is set to 2 and the calculated values are stored in the parameters alnh and alwnh.

NOTE: The maximun Nh value from the DL map is 2.58e22 at RA (2000)=15h 59m 29.383s Dec (2000)=-53d 04m 40.04s corresponding to (l, b) = (329.0, 0.0). In Dickey & Lockman this is instead printed as (l, b) = (339.0, 0.0), which is assumed to be a typo.


equinox [equinox]
Equinox for the input equatorial coordinates.

ra [string]
Right ascension value. It can be input as hh mm ss.s or degrees.

NOTE: if input as degrees it is necessary to write the value as decimal (eg. "270.") otherwise for values bigger than 24 it is interpreted incorrectly.

dec [string]
Declination value. It can be input as dd mm ss.s or degrees. See NOTE in parameter ra.

(disio) [real]
Search radius in degrees. Only Nh values within 'disio' degrees of (ra, dec) are used in the Nh average calculation. This paramater should be always less than or equal to 'size' parameter. The default is 0.1 degree by 0.1 degree.

(size) [real]
Size in degrees of the submap. Defaults to 1 degrees by 1 degrees. By increasing the chattiness level (>10) all the Nh values from the submap are printed on the screen.

(usemap) [integer]
Parameter to specify which map to use. If usemap=0 (default) the values 'avnh' and 'avwnh' correspond to the map specified in the paramater 'map'. If the usemap=1 the values 'avnh' and 'avwnh' correspond to the map specified in the paramater 'altmap'. If the usemap=2 the Nh values are calculated from both maps and all of the 'avnh', 'avwnh', 'alnh', and 'alwnh' output parameters are set (see the individual parameter descriptions).

(map) [string]
Name and location of the defaut map. The default map is the HI4PI map ('h1_nh_HI4PI.fits').

(altmap) [string]
Name and location of the alternative maps. The names of the other available maps are 'h1_nh_LAB.fits' and 'h1_nh_DL.fits'. NOTE: the pixel size of these maps are much larger of the pixel size of HI4PI and with the default setting of the parameter size and disio in most cases the nh values from these map are from the closest pixels rather than an average.

(avnh) [real]
Output average Nh from the default map, calculated as described above.

(avwnh) [real]
Output weighted average Nh from the default map, calculated as described above.

(alnh) [real]
Output average Nh from the alternative map, if usemap is set to 1 or 2. This value is set to zero if usemap=0.

(alwnh) [real]
Output weighted average Nh from the alternative map, if usemap is set to 1 or 2. This value is set to zero if usemap=0.

(tchat = 10) [integer]
Terminal chattiness level. If tchat < lchat then a log file named ./nh.log will be produced containing the output (at the chattiness level of lchat). If ./nh.log exists, then the log file will be appended.

(lchat = 0) [integer]
Logfile chattiness level.


1. Calculate the nh at ra="21 44 41.20" and dec="38 19 18"

> nh equinox=2000 ra="21 44 41.20" dec="38 19 18"

2. As above but with the coordinates given as decimal degrees

> nh equinox=2000 ra=326.17166 dec=38.32166

3. As above but use the default map, HI4PI, and an alternative map set to the lAB map

> nh equinox=2000 ra=326.17166 dec=38.32166 usemap=2 altmap="$LHEA_DATA/h1_nh_LAB.fits"


No known bugs. Last update May 2019


Nov96 ftools.heasarc