Galactic helium and stellar population colors

The sensitivity of the colors and luminosities in different spectral regions of a synthetic stellar population to its helium-Y abundance is analyzed with an evolutionary model. Possible evolutions of the luminosities at 2.4 microns and 1700 A are given. The evolution of (B-V) is presented as a function of the ratio of the present over past star-formation rate, calculated for two helium abundances Y = 0.20 and Y = 0.30 and the metallicity Z = 0.001. It is noted that a Delta(Y) = 0.1 variation induces a color variation Delta(B-V) = 0.25 for the bluest galaxies. The observational colors of blue galaxies indicate a Y = 0.25-0.30 value in agreement with the spectroscopic value. A Delta(Y)/Delta(Z) estimation could be hopefully determined by a similar analysis with the Z = 0.0001 composition.