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The cosmic ultraviolet background (UVB) heats the intergalactic medium (IGM),as a result the gas in dark matter halos below a certain mass is too hot tocool within a Hubble time. The UVB effectively suppresses the formation ofdwarf galaxies. Using high resolution cosmological hydrodynamical simulationswe show that photo heating leads to small baryon fractions in halos below ~6x10^9 h^{-1}M_sun, independent of the cosmic environment. The simulations arecarried out assuming a homogeneous UVB with flux densities as given by Haardt &Madau (1996). A halo may stop to condense gas significantly after the universeis reionised, namely when its mass falls below the characteristic mass scaleset by the photo heating. Assuming a spherical halo model we derive thischaracteristic mass analytically and identify the main mechanisms that preventthe gas from cooling in small halos. The theoretically derived characteristicmass is smaller than the one obtained from observations. Increasing the energyper ionising photon by a factor between four and eight would be sufficient toreconcile both. This is equivalent to an average temperature of the IGM of ~10^4 K. In this sense the faint end of the luminosity function may serve as acalorimeter for the IGM.

Too small to form a galaxy: How the UV background determines the baryon fraction