.. _physics-hiiRegionEscapeFraction:

HII Regions Escape Fractions
============================

Class providing models of the escape fraction :math:`f_\mathrm{esc}` of hydrogen-ionizing (Lyman-continuum) photons from individual H**ii** regions---the fraction of photons produced by massive stars that escape the H**ii** region into the surrounding :term:`ISM` or :term:`IGM` rather than being absorbed by gas within the region. The escape fraction is returned as a function of H**ii** region age, reflecting the time evolution as stellar winds and supernovae clear channels through the surrounding gas.

**Default implementation:** ``hiiRegionEscapeFractionFixed``

Methods
-------

``escapeFraction`` → ``double precision``
   Returns the escape fraction for hydrogen-ionizing photons.

   * ``double precision, intent(in ) :: ageHIIRegion``

.. _physics-hiiRegionEscapeFractionFixed:

``hiiRegionEscapeFractionFixed``
--------------------------------

Computes the escape fraction of hydrogen ionizing photons from HII regions. A fixed escape fraction of :math:`f_\mathrm{esc}`\ ``[escapeFraction]`` is assumed for HII regions with ages less than :math:`\tau_\mathrm{limit}=`\ ``{ageLimit}``.

**(Default implementation)**

**Parameters**

* ``[mass]`` (default ``100.0d0``) — The fixed mass (in :math:`\mathrm{M}_\odot`) assigned to all newly-formed seed black holes in this implementation, representing the initial black hole mass when a halo first forms a central black hole.
* ``[spin]`` (default ``0.0d0``) — The dimensionless spin parameter (between :math:`-1` and :math:`+1`) assigned to all newly-formed seed black holes, where :math:`0` corresponds to a non-rotating Schwarzschild black hole and :math:`\pm 1` to a maximally rotating Kerr black hole.
* ``[fraction]`` (default ``0.01d0``) — The fixed fraction :math:`f_\mathrm{outflow}` of the stellar energy input rate (normalized to a canonical :math:`1\,\mathrm{M}_\odot` population) that drives gas outflows, setting the mass loading factor for stellar feedback in the galaxy.
* ``[escapeFraction]`` (default ``0.006d0``) — Escape fraction of ionizing photons from young HII regions.
* ``[ageLimit]`` (default ``0.03d0``) — The age beyond which all ionizing photons are assumed to escape from HII regions.
* ``[timescale]`` (default ``1.0d0``) — The timescale for star formation in the fixed timescale model.
* ``[rateStarFormation]`` (default ``1.0d9``) — The rate of star formation in units of :math:`\mathrm{M}_\odot \hbox{Gyr}^{-1}`.
* ``[proposalSize]`` — The fixed value of the proposal scaling parameter :math:`\gamma` used to scale the vector difference between two randomly selected chain states when forming differential evolution proposals.
* ``[exponentValue]`` — The fixed value of the temperature-scaling exponent :math:`\alpha` by which the proposal size :math:`\gamma` is scaled as :math:`\gamma \propto T^{\alpha}` in tempered differential evolution runs.
* ``[massResolution]`` (default ``5.0d9``) — The mass resolution to use when building merger trees.
* ``[rootVariance]`` — The root variance of the random error distribution.
* ``[velocityRadial]`` (default ``-0.90d0``) — The radial velocity (in units of the host virial velocity) to used for the fixed virial orbits distribution. Default value matches approximate peak in the distribution of :cite:t:`benson_orbital_2005`.
* ``[velocityTangential]`` (default ``0.75d0``) — The tangential velocity (in units of the host virial velocity) to used for the fixed virial orbits distribution. Default value matches approximate peak in the distribution of :cite:t:`benson_orbital_2005`.
* ``[rateCoefficient]`` — The rate coefficient (in units of cm\ :math:`^3` s\ :math:`^{-1}`) for radiative recombination.
* ``[gamma]`` (default ``0.67d0``) — The multiplicative factor, :math:`\gamma`, used to compute the cooling coefficient.
* ``[fractionLossAngularMomentum]`` (default ``0.3d0``) — Specifies the fraction of angular momentum that is lost from cooling/infalling gas.
* ``[concentration]`` — The fixed NFW concentration parameter :math:`c = r_\mathrm{virial}/r_\mathrm{scale}` assigned to all halos regardless of mass or redshift, representing the ratio of the virial radius to the scale radius of the dark matter density profile.
* ``[metallicity]`` — The metallicity (relative to Solar) of the :term:`IGM`.
* ``[factor]`` (default ``sqrt(0.5d0)``) — The ratio of galaxy radius to :math:`\lambda r_\mathrm{vir}` in the "fixed" galactic structure radius solver algorithm. This will be applied to any component for which no component-specific value is provided.
* ``[factorDisk]`` (default ``sqrt(0.5d0)``) — The ratio of galaxy radius to :math:`\lambda r_\mathrm{vir}` in the "fixed" galactic structure radius solver algorithm for disks. This will override the generic value supplied by ``[factor]`` for disks.
* ``[factorSpheroid]`` (default ``sqrt(0.5d0)``) — The ratio of galaxy radius to :math:`\lambda r_\mathrm{vir}` in the "fixed" galactic structure radius solver algorithm for spheroids. This will override the generic value supplied by ``[factor]`` for spheroids.
* ``[radiusFixed]`` (default ``var_str('virial')``) — The radius to use in the "fixed" galactic structure radius solver algorithm. Allowed options are "virial" and "turnaround".
* ``[overdensity]`` — The fixed linear overdensity :math:`\delta` of the large-scale environment assigned uniformly to all halos; a positive value places halos in an overdense region, while negative values simulate voids.
* ``[radiusEnvironment]`` (default ``0.0d0``) — The radius of the sphere used to determine the variance in the environmental density.
* ``[massEnvironment]`` (default ``1.0d15``) — The mass within the sphere sphere used to determine the variance in the environmental density.
* ``[densityContrastValue]`` (default ``200.0d0``) — The virial density contrast to use in the fixed value model.
* ``[densityType]`` (default ``var_str('critical')``) — The reference density to use in the fixed value virial density contrast model. Either of ``critical`` and ``mean`` are allowed.
* ``[turnAroundOverVirialRadius]`` (default ``2.0d0``) — The ratio of the turnaround to virial radii in the fixed value model.
* ``[criticalOverdensity]`` (default ``(3.0d0/20.0d0)*(12.0d0*Pi)**(2.0d0/3.0d0)``) — The value to use for the critical overdensity for collapse of dark matter halos when using a fixed value.