*** Mean Intensity of the Near Infrared Background ***
This version assumes Salpeter's initial mass function (IMF) and metal-free
(popIII) stars. 

December 29, 2010: E.Komatsu
Ref: Fernandez & Komatsu, ApJ, 646, 703 (2006)
     Fernandez et al., ApJ, 710, 1089 (2010)

This code calculates the mean intensity of the near infrared background (NIRB)
using the method of Fernandez&Komatsu, for a star formation rate (SFR) given by

SFR(z) = fstar * (omega_b/omega_m) * rho_m0 * fcoll(z) / tSF
       = fstar * omega_b * rho_c0 * fcoll(z) / tSF

where "fstar" is the star formation efficiency, "rho_m0" is the present-day
mass density of the universe (rho_m0 = omega_m rho_c0 and rho_c0=2.775d11 h^2 
is the present-day critical density of the universe), "fcoll(z)" is the 
collapse fraction, and "tSF" is the star formation time scale.

Following Fernandez et al. (2010), this code uses tSF=20Myr and fstar=0.01 as
the default values.

The collapse fraction, fcoll(z), is provided by 
"wmap5baosn_max_likelihood_pressschechter_redshift_fcoll.txt", which was 
computed from the Press-Schechter mass function with the WMAP 5-year best-fit
parameters (WMAP5+BAO+SN), and the minimum virial mass of 1.5e9 h^-1 Msun.
In the code, this fcoll will be divided by 1.2 to better match the simulation
results of Iliev et al., MNRAS, 384, 863 (2008). The cosmological parameters
used in this code are also those used by Iliev et al.:
H0=73 km/s/Mpc, Omega_m=0.24, Omega_L=0.76, and Omega_bh^2=0.02238.

The nebula spectra are computed for the gas temperature of 10,000K (instead
of 20,000K used in Fernandez&Komatsu (2006)). The case-B recombination
coefficient has been updated to alphaB=2.17d-10*Tg**(-0.7395) [cm^3/s]
from Eq.(9) of Fernandez&Komatsu (2006). These updates affect only the free-free
and free-bound emission.



The mean intensity is given by

I_nu = (c/fourpi) int_{zend}^{zstart} dz p[(1+z)nu,z]/H(z)/(1+z),

where the comoving volume emissivity, p(nu,z), is given by

p(nu,z) = SFR*c^2*[sum_i <epsilon_i>(nu)]

Here, <epsilon_i>(nu) is the "radiative efficiency", which gives 
a ratio of the mass-weighted average of radiative energy of a radiation
process i (where i=free-free, free-bound, stellar, Lyman-alpha, two-photon)
to the stellar rest-mass energy, in a unit frequency interval.

The radiative efficiency is calculated from

<epsilon_i>(nu) = (1/mstar) int_m1^m2 dm m*f(m)*[ L_i(nu,m)tau(m)/(m c^2) ],

where mstar [=int dm m*f(m)] is the mean stellar mass, f(m) is the initial
mass spectrum, and L_i(nu,m) is the luminosity of each radiation process 
in a unit frequency interval.

- To compile and use the program, edit Makefile and simply "make"
- It will generate an executable called "compute_nuinu"
- Running it will yield two files: 

"hnu_nu_ff_fb_star_lya_2g_nuInu.txt" contains nu*Inu
1st column: h*nu [eV]
2nd column: nu   [Hz]
3rd column: free-free   [nW/m^2/str]
4th column: free-bound  [nW/m^2/str]
5th column: stellar     [nW/m^2/str]
6th column: Lyman-alpha [nW/m^2/str]
7th column: two-photon  [nW/m^2/str]

"hnu_nu_ff_fb_star_lya_2g_nulnu.txt" contains nu*lnu
1st column: h*nu [eV]
2nd column: nu   [Hz]
3rd column: free-free   [nW/Msun]
4th column: free-bound  [nW/Msun]
5th column: stellar     [nW/Msun]
6th column: Lyman-alpha [nW/Msun]
7th column: two-photon  [nW/Msun]

lnu is the luminosity per stellar mass, given as "lnu = <epsilon>(nu)c^2/tSF".
(See Fig.1. of Fernandez et al. 2010)