A WISE Measurement of the 2.4 μm Galaxy Luminosity Function and its Implications for the Extragalactic Background Light at 3.4 μm
The measurement of the the Extragalactic Background Light (EBL) has seen some controversy in recent works, with direct and indirect measures conflicting. Specifi- cally, upper limits based on analyzing the plausible opacity obscuring TeV spectra of blazars suggests that the density of radiation with wavelengths near 3.4 μm is onethirdtoonehalfasintenseasdirectmeasuresofthesame(forexample: Aharonian et al., 2006; Levenson et al., 2007; Matsumoto et al., 2005). The dominant contributor of the EBL at 3.4μm is expected to be ordinary starlight from relatively local, z < 1, galaxies, so an estimate of the amount of light emitted by galaxies based on the galaxy Luminosity Function (LF) should provide a useful lower limit to the EBL. While analyses of this sort have been done by others (Dom ́ınguez et al., 2011; Helgason et al., 2012), the full sky coverage of the All- WISE database has made it possible for us to improve the measurement of both the LF at 2.4 μm and the EBL using the large public spectroscopic redshift surveys. In order to do so, we had to develop a mathematical model for the measurement of a generalization of the LF, which is the density of galaxies per unit comoving volume per unit luminosity, to the Spectro-Luminosity Functional (SLF), which replaces the density per unit single luminosity, dL, with the density per luminosi-
ties at all frequencies, DLν. Our best combined analysis of the data yields present day Shechter Function LF parameters of: L⋆ = 6.4ï¿½[0.1stat, 0.3sys]ï¿½1010 L2.4μm ⊙ (M⋆ = −21.67ï¿½[0.02stat, 0.05sys] AB mag), φ⋆ = 5.8ï¿½[0.3stat, 0.3sys]ï¿½10−3 Mpc−3, and α = −1.050 ï¿½ [0.004stat, 0.03sys]; this implies a present day density of galax- ies of 0.08 Mpc−3 brighter that 106 L2.4 μm ⊙ (10−3 Mpc−3 brighter than L⋆) and a luminosity density equivalent to 3.8 ï¿½ 108 L2.4 μm ⊙ Mpc−3. The net EBL at 3.4μm that our synthesis model produces from galaxies closer than z = 5 is Iν = 9.0 ï¿½ 0.5 kJy sr−1 (νIν = 8.0 ï¿½ 0.4 nW m−2 sr−1), largely in agreement with similar LF based estimates of the EBL.