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HyperLEDA documentation: parameter: m21

Neutral Hydrogen HI flux: m21

HI line flux (flux corresponding to the area under the 21-cm line profile) is generally expressed in converted in magnitude m21 according to the formula adopted in RC3: where f is the area of the 21-cm line profile expressed in Jy*km/s (the constant is 16.6 if the flux is in 10**-22 W/m**2).

Non-detections (or upper limits) are not stored in the database, therefore galaxies with no HI measurements available may either be too faint or not have been observed.

The m21 magnitude is available for 13984 galaxies (May 2003 version of the database).

Flux at other wavelengths are given in the parameters: bt, it, mfir. The corrected m21 is m21c.

HyperLeda Questions: prugniel@obs.univ-lyon1.fr