NH (Nov96) ftools.heasarc NH (Nov96) NAME nh - Return hydrogen column density for a given Ra and Dec USAGE nh equinox ra dec disio DESCRIPTION This program returns, for a specified right ascension and declination, a value for the galactic hydrogen column density, Nh. This value is derived from the HI map by Kalberla et al. 2005, Astronomy & Astrophysics, 440, 775, known as the Leiden/Argentine/Bonn (LAB) survey. Alternatively, the Nh value can be derived from 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 Radioastronomía 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. 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. 'nh' starts by reading a submap M degrees by M degrees, where M is an input parameter, 'size' (default is 3 x 3), from which only the subset of values within N degrees are used for the average Nh value. The calculated simple and weighted average are written in the paramater file. There are two parameters for the LAB map (parameter names avnh and avwnh) and two for the DL (parameter names alnh and alwnh). These parameters are populated depending on whether the nh values are calculated for both maps or a single map. 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. PARAMETERS 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. The default is 1 degree by 1 degree. 'disio' should be always less than or equal to 'size' parameter. (size) [real] Size in degrees of the submap. Defaults to 3 degrees by 3 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) then the LAB map will be used, and the 'avnh' and 'avwnh' parameters will be populated with the calculated values. If usemap=1 then the DL map will be used, and the 'alnh' and 'alwnh' parameters will be populated with the calculated values. If usemap=2 then the Nh values will be calculated with both maps, and the outputs printed separately. In this case, all of the 'avnh', 'avwnh', 'alnh', and 'alwnh' output parameters will be set (see the individual parameter descriptions). (map) [string] Name and location of the LAB HI map. This parameter should not be changed. This is the default map used by this program and is a FITS file produced by K. Kuntz with a pixel size of 0.675 deg (see DESCRIPTION section). (altmap) [string] Name and location of the DL HI map. This parameter should not be changed. This map is a FITS file produced by Steve Snowden with a pixel size of 0.675 deg (see DESCRIPTION section). (avnh) [real] Output average Nh from the LAB map, calculated as described above. This will be set to zero if either usemap=1 or there is an error. Otherwise, it will be set to the calculated Nh value. (avwnh) [real] Output weighted average Nh from the LAB map, calculated as described above. This will be set to zero if either usemap=1 or there is an error. Otherwise, it will be set to the calculated weighted Nh value. (alnh) [real] Output average Nh from the DL map. This will be set to zero if usemap=0. Otherwise, it will be set to the calculated Nh value. (alwnh) [real] Output weighted average Nh from the DL map. This will be set to zero if usemap=0. Otherwise, it will be set to the calculated Nh value. (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 (lchat = 0) [integer] Logfile chattiness level. EXAMPLES 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 both the LAB and DL maps > nh equinox=2000 ra=326.17166 dec=38.32166 usemap=2 BUGS No known bugs.