HI column density
FITS Header Information

Primary Header

SIMPLE  =                    T / file does conform to FITS standard             
BITPIX  =                    8 / number of bits per data pixel                  
NAXIS   =                    0 / number of data axes                            
EXTEND  =                    T / FITS dataset may contain extensions            
COMMENT   FITS (Flexible Image Transport System) format is defined in 'Astronomy
COMMENT   and Astrophysics', volume 376, page 359; bibcode: 2001A&A...376..359H 
DATE    = '2003-04-23T00:00:00' /file creation date (YYYY-MM-DDThh:mm:ss UT)    
OBJECT  = 'ALL-SKY '           / Portion of sky given                           
COMMENT   This is a composite all-sky map of neutral hydrogen column density    
COMMENT   (N_HI), formed from the Leiden/Dwingeloo HI survey data               
COMMENT   (Hartmann & Burton 1997) and the composite N_HI map of Dickey and     
COMMENT   Lockman (1990).  The two datasets are not matched in sensitivity or   
COMMENT   resolution: note that discontinuities exist in the constructed        
COMMENT   composite map.  A pixel mask is provided to indicate which dataset    
COMMENT   was used for each location on the sky.                                
COMMENT   Hartmann & Burton provide a velocity integrated                       
COMMENT   (-450 km/s < V_lsr < +400 km/s) HI brightness temperature map in      
COMMENT   Galactic coordinates, sampled every 0.5 degrees.  This entire data set
COMMENT   was converted to N_HI by multiplying by their factor of               
COMMENT   1.8224e18 K km s-1 cm-2 and then interpolated to pixel centers        
COMMENT   appropriate for HEALPix Nside=512.  Since the Leiden/Dwingeloo survey 
COMMENT   does not have sky coverage for declinations < -30 deg., the lower     
COMMENT   resolution Dickey & Lockman map was also interpolated to HEALPix and  
COMMENT   used to fill in the coverage gap.  The Dickey & Lockman N_HI map is   
COMMENT   itself a composite of several surveys which had been merged and       
COMMENT   averaged onto 1 deg. bins in Galactic coordinates.  Their map         
COMMENT   includes emission between -250 km/s < V_lsr < 250 km/s (excluding the 
COMMENT   LMC and SMC).                                                         
COMMENT   The Leiden/Dwingeloo Survey data were obtained from the CDS.          
COMMENT   The Dickey & Lockman map was obtained from NCSA ADIL.                 
COMMENT   This file is distributed and maintained by LAMBDA.                    
REFERENC= 'Legacy Archive for Microwave Background Data Analysis (LAMBDA)      '
REFERENC= '                http://lambda.gsfc.nasa.gov/                        '
REFERENC= 'Atlas of Galactic Neutral Hydrogen,                                 '
REFERENC= 'Hartmann & Burton 1997 Cambridge University Press.                  '
REFERENC= 'HI in the Galaxy, Dickey & Lockman 1990 Ann. Rev. A&A, 28, 215.     '
REFERENC= 'NCSA Astronomy Digital Image Library: http://adil.ncsa.uiuc.edu/    '
REFERENC= 'CDS: http://cdsweb.u-strasbg.fr/                                    '
REFERENC= 'HEALPix Home Page: http://www.eso.org/science/healpix/              '
FREQ    = '21 cm   '           / Frequency band                                 
RESOLUTN=                    9 / Resolution index                               
SKYCOORD= 'Galactic'           / Coordinate system                              
PIXTYPE = 'HEALPIX '           / Pixel algorithm                                
ORDERING= 'NESTED  '           / Ordering scheme                                
NSIDE   =                  512 / Resolution parameter                           
NPIX    =              3145728 / # of pixels                                    
FIRSTPIX=                    0 / First pixel (0 based)                          
LASTPIX =              3145727 / Last pixel (0 based)                           
END                                                                             

Extension Header #1

XTENSION= 'BINTABLE'           /binary table extension                          
BITPIX  =                    8 /8-bit bytes                                     
NAXIS   =                    2 /2-dimensional binary table                      
NAXIS1  =                    8 /width of table in bytes                         
NAXIS2  =              3145728 /number of rows in table                         
PCOUNT  =                    0 /size of special data area                       
GCOUNT  =                    1 /one data group (required keyword)               
TFIELDS =                    2 /number of fields in each row                    
TTYPE1  = 'TEMPERATURE'        /label for field   1                             
TFORM1  = '1E      '           /data format of field: 4-byte REAL               
TUNIT1  = 'atoms cm-2'         /physical unit of field                          
TTYPE2  = 'N_OBS   '           /label for field   2                             
TFORM2  = '1E      '           /data format of field: 4-byte REAL               
TUNIT2  = 'counts  '           /physical unit of field                          
EXTNAME = 'Archive Map Table'  /name of this binary table extension             
DATE    = '2003-04-23T00:00:00' /Table creation date                            
PIXTYPE = 'HEALPIX '           /Pixel algorithm                                 
ORDERING= 'NESTED  '           /Ordering scheme                                 
NSIDE   =                  512 /Resolution parameter                            
FIRSTPIX=                    0 /First pixel (0 based)                           
LASTPIX =              3145727 /Last pixel (0 based)                            
COMMENT   The TEMPERATURE field contains the N_HI column density map in units   
COMMENT    atoms cm-2.                                                          
COMMENT   N_obs is set to 1 for those pixels for which Leiden/Dwingeloo survey  
COMMENT    data were available.  The southern declinations filled in with the   
COMMENT    Dickey & Lockman data have N_obs set to  2.                          
END                                                                             

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