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I have a vizier catalog with H-alpha fluxes as logF(Haplha) in units as mW/m^2. See sample data here: http://vizier.cds.unistra.fr/viz-bin/VizieR-3?-source=J/MNRAS/431/2/fluxes How do I convert mW/m^2 to W/m^2/nm or erg/s/cm^2/A? The first entry is logF(Halpha) = -12.03 mW/m^2

Adrian
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    You'll need some spectrographic information about the flux, since these units are not consistent with each other. – James K Aug 13 '23 at 15:34
  • Do you know what kind of spectra info? The H-alpha is around 656 nm – Adrian Aug 13 '23 at 15:37
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    No, but on the face of it this question is like asking to convert force to pressure (it can't be done, unless you know the shape or area) The "per nm" part of the unit is is because a wideband flux with the same flux per nm would carry greater power than a narrowband flux. The total flux is integrated over the frequency range... but this is now rapidly going above my paygrade. – James K Aug 13 '23 at 15:43

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You can't with any precision.

mW/m$^2$ is a measure of flux, presumably integrated over a H$\alpha$ emission line.

W/m$^2$/nm would be the units of the spectrum flux density - i.e. the quantity that would be integrated over wavelength to give you a total flux. This is not a single value, but a spectrum.

If you want to know the average spectrum flux density at the wavelength of H$\alpha$ is, about the best you could do is divide the flux in the H$\alpha$ line by what you think its full width half maximum is in nm. This might be set by the properties of the source, but will at a minimum be set by the spectral resolution of the instrument that measured the spectrum.

Thus in your example, suppose the line had a FWHM of 10 nm (very broad!), then we would subtract 3 from the log (divide by 1000) to put mW into W and then subtract 1 more (divide by 10) to spread that flux across 10 nm, and the rough, average spectrum flux density over the H$\alpha$ line would be $10^{-16.03}$ W/m$^2$/nm.

ProfRob
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