The Metagalactic UV Background

One of the main motivations for ARES was to be able to easily generate models for the metagalactic background. In this example, we’ll focus on the ultraviolet background , which is noteworthy given the ‘’sawtooth’’ modulation (e.g., Haiman et al. (1997)) caused by intergalactic hydrogen atoms.

In order to model this background, we need to decide on a few main ingredients:

• The spectrum of sources, which can be one of several pre-defined options (like a power-law, pl or blackbody, bb), or a Python function supplied by the user.

• How the background will evolve with redshift, which could be based on the rate of collapse onto dark matter haloes as a function of time, a parameterized form for the star-formation rate history, or more detailed models of star-formation (see Models for Star Formation in Galaxies).

• What (if any) approximations we’ll make in order to speed-up the calculation, aside from the assumption of a spatially uniform radiation background, which we make implicitly in ARES throughout.

First things first:

import ares
import numpy as np
import matplotlib.pyplot as pl
from ares.physics.Constants import erg_per_ev, c, ev_per_hz

Now, let’s set some parameters that define the properties of the source population:

alpha = 0. # flat SED
beta = 0.  # flat SFRD

pars = \
{
 'pop_sfr_model': 'sfrd-func',
 'pop_sfrd': lambda z: 0.1 * (1. + z)**beta,

 'pop_sed': 'pl',
 'pop_alpha': alpha,
 'pop_Emin': 1,
 'pop_Emax': 1e2,
 'pop_EminNorm': 13.6,
 'pop_EmaxNorm': 1e2,
 'pop_rad_yield': 1e57,
 'pop_rad_yield_units': 'photons/msun',

 # Solution method
 'pop_solve_rte': True,
 'lya_nmax': 8,
 'tau_redshift_bins': 400,

 'initial_redshift': 40,
 'final_redshift': 10,
}

To summarize these inputs, we’ve got:

• A constant SFRD of \(0.1 \ M_{\odot} \ \mathrm{yr}^{-1} \ \mathrm{cMpc}^{-3}\), given by the pop_sfrd parameter.

• A flat spectrum (power-law with index \(\alpha=0\)), given by pop_sed and pop_alpha.

• A yield of \(10^{57} \ \mathrm{photons} \ M_{\odot}^{-1}\) of star-formation in the \(13.6 \leq h\nu / \mathrm{eV} \leq 100\) band, set by pop_EminNorm, pop_EmaxNorm, pop_yield, and pop_yield_units.

See Population Parameters for a complete listing of parameters relevant to ares.populations.GalaxyPopulation objects.

Next, let’s initialize an ares.simulations.MetaGalacticBackground object (which will automatically create an ares.populations.GalaxyPopulation instance):

mgb = ares.simulations.MetaGalacticBackground(**pars)

To run the thing:

mgb.run()

The results of the calculation, as in any ares.simulations class, are stored in an attribute called history. Here, we’ll use a convenience routine to extract the redshifts, photon energies, and corresponding fluxes (a 2-D array):

z, E, flux = mgb.get_history(flatten=True)

Internally, fluxes are computed in units of \(\mathrm{s}^{-1} \ \mathrm{cm}^{-2} \ \mathrm{Hz}^{-1} \ \mathrm{sr}^{-1}\), but often it can be useful to look at the background flux in terms of its energy, i.e., in units of \(\mathrm{erg} \ \mathrm{s}^{-1} \ \mathrm{cm}^{-2} \ \mathrm{Hz}^{-1} \ \mathrm{sr}^{-1}\):

pl.semilogy(E, flux[0] * E * erg_per_ev, color='k', ls=':')

You should see the characteristic sawtooth modulation of an intrinsically flat spectrum.

Compare to the analytic solution, given by Equation A1 in Mirocha (2014) (the cosmologically-limited solution to the radiative transfer equation), which does not take into account the sawtooth modulation:

\[J_{\nu}(z) = \frac{c}{4\pi} \frac{\epsilon_{\nu}(z)}{H(z)} \frac{(1 + z)^{9/2-(\alpha + \beta)}}{\alpha+\beta-3/2} \times \left[(1 + z_i)^{\alpha+\beta-3/2} - (1 + z)^{\alpha+\beta-3/2}\right]\]

with \(\alpha = \beta = 0\) (i.e., constant SFRD, flat spectrum), \(z=10\), and \(z_i=40\),

# Grab the GalaxyPopulation instance
pop = mgb.pops[0]

# Compute cosmologically-limited solution
zi, zf = 40., 10.
e_nu = np.array(map(lambda E: pop.Emissivity(zf, E), E))
e_nu *= (1. + zf)**(4.5 - (alpha + beta)) / 4. / np.pi \
    / pop.cosm.HubbleParameter(zf) / (alpha + beta - 1.5)
e_nu *= ((1. + zi)**(alpha + beta - 1.5) - (1. + zf)**(alpha + beta - 1.5))
e_nu *= c * ev_per_hz

# Plot it
pl.semilogy(E, e_nu, color='k', ls='-')

Add some axis labels if you’d like:

pl.xlabel(ares.util.labels['E'])
pl.ylabel(ares.util.labels['flux_E'])

pl.savefig('ares_crte_uv.png')

Sawtooth modulation of the UV background compared to optically-thin solution (solid).

Note

In reality, the ionizing background before reionization should be heavily damped. This example is unphysical in some sense because while it treats the opacity of HI and HeI Lyman lines (which produce the sawtooth modulation) it ignores the continuum opacity at energies above 13.6 eV. This will be treated more carefully by setting pop_approx_tau='neutral' in the The Metagalactic X-ray Background example.