科研之友
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摘要
We present a physical model for the evolution of the ultraviolet (UV) luminosity function of high-redshift galaxies, taking into account in a self-consistent way their chemical evolution and the associated evolution of dust extinction. Dust extinction is found to increase fast with halo mass. A strong correlation between dust attenuation and halo/stellar mass for UV selected high-z galaxies is thus predicted. The model yields good fits of the UV and Lyman-alpha (Ly alpha) line luminosity functions at all redshifts at which they have been measured. The weak observed evolution of both luminosity functions between z = 2 and z = 6 is explained as the combined effect of the negative evolution of the halo mass function; of the increase with redshift of the star formation efficiency due to the faster gas cooling; and of dust extinction, differential with halo mass. The slope of the faint end of the UV luminosity function is found to steepen with increasing redshift, implying that low luminosity galaxies increasingly dominate the contribution to the UV background at higher and higher redshifts. The observed range of the UV luminosities at high z implies a minimum halo mass capable of hosting active star formation M-crit less than or similar to 10(9.8) M-circle dot , which is consistent with the constraints from hydrodynamical simulations. From fits of Ly alpha line luminosity functions, plus data on the luminosity dependence of extinction, and from the measured ratios of non-ionizing UV to Lyman-continuum flux density for samples of z similar or equal to 3 Lyman break galaxies and Ly alpha emitters, we derive a simple relationship between the escape fraction of ionizing photons and the star formation rate. It implies that the escape fraction is larger for low-mass galaxies, which are almost dust-free and have lower gas column densities. Galaxies already represented in the UV luminosity M-UV less than or similar to -18) can keep the universe fully ionized up to z similar or equal to 6. This is consistent with (uncertain) data pointing to a rapid drop of the ionization degree above z similar or equal to 6, such as indications of a decrease of the comoving emission rate of ionizing photons at z similar or equal to 6, a decrease of sizes of quasar near zones, and a possible decline of the Lya transmission through the intergalactic medium at z > 6. On the other hand, the electron scattering optical depth, tau(es), inferred from cosmic microwave background (CMB) experiments favor an ionization degree close to unity up to z similar or equal to 9-10. Consistency with CMB data can be achieved if M-crit similar or equal to 10(8.5)M(circle dot) , implying that the UV luminosity functions extend to MUV similar or equal to -13, although the corresponding tau(es) is still on the low side of CMB-based estimates.
- 出版日期2014-4-10
- 单位厦门大学
