.. _stellar-population-continuum:

Stellar Population Continuum
=============================

This section describes how to simulate stellar continuum spectra using the ``spec1d.StellarContinuum`` module in GEHONG.

Stellar Population Templates
----------------------------

The simulation is based on a grid of single stellar population (SSP) templates, currently adopted from the EMILES library.
These templates are managed by the ``StellarContinuumTemplate`` class, which loads and prepares the template cube.

**Required Input**:

- ``config``: A ``Config`` instance that defines the wavelength grid.
- ``pathname``: Path to EMILES template files (optional; default is provided).

**Usage Example**:

.. code-block:: python

    from gehong import spec1d
    stellar_tem = spec1d.StellarContinuumTemplate(config)

Stellar Continuum Spectrum Generation
-------------------------------------

The ``StellarContinuum`` class synthesizes the stellar continuum spectrum using either single-population or composite star formation models.

Two working modes are supported:

Magnitude-calibrated mode (empirical flux scaling)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Triggered when ``mag`` is provided. The input ``sfh`` and ``ceh`` can be scalars (single SSP) or arrays (for composite populations).

**Parameters**:

- ``mag``: SDSS r-band magnitude.
- ``sfh``: Star formation history. Scalar (age in Gyr) or array [[age, SFR]].
- ``ceh``: Metallicity history. Scalar ([Fe/H] in dex) or array [[age, [Fe/H]]].
- ``vel``: Line-of-sight velocity (km/s).
- ``vdisp``: Velocity dispersion (km/s).
- ``ebv``: Dust extinction E(B-V).

**Example (single SSP)**:

.. code-block:: python

    stellar = spec1d.StellarContinuum(config, stellar_tem, 
                                      mag=17.5, sfh=5.0, ceh=-0.3,
                                      vel=8000, vdisp=120, ebv=0.1)


.. image:: /_static/image/example_spec1d_ssp1.png
   :align: center


**Example (composite SFH with magnitude scaling)**:

.. code-block:: python

    import numpy as np

    # Define a typical galaxy SFH and CEH
    age_grid = np.linspace(0.05, 13.7, 50)  # Gyr
    sfh = np.exp(-age_grid / 3.0)           # τ = 3 Gyr exponential decay
    ceh = -1.0 + (age_grid / np.max(age_grid)) * 1.0  # [Fe/H] from -1.0 to 0.0

    # Combine into [age, value] arrays
    sfh_array = np.column_stack((age_grid, sfh))
    ceh_array = np.column_stack((age_grid, ceh))

    stellar = spec1d.StellarContinuum(config, stellar_tem,
                                    mag=17.0,
                                    sfh=sfh_array,
                                    ceh=ceh_array,
                                    vel=6000, vdisp=100, ebv=0.15)


In this example, the final spectrum shape is determined by the SFH and CEH arrays, 
while the total flux is normalized to match an r-band magnitude of 17.0.


.. image:: /_static/image/example_spec1d_ssp2.png
   :align: center


Physically-calibrated mode (flux from SFH)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Triggered when ``mag`` is set to ``None``. In this mode, ``sfh`` and ``ceh`` must be 2D arrays.

**Parameters**:

- ``sfh``: Array of [[age (Gyr), SFR (M☉/yr)]].
- ``ceh``: Array of [[age (Gyr), [Fe/H] (dex)]].
- ``z``: Redshift.
- ``vpec``: Peculiar velocity (km/s).
- ``vdisp``: Velocity dispersion (km/s).
- ``ebv``: Dust extinction E(B-V).

**Example**:

.. code-block:: python

    stellar = spec1d.StellarContinuum(config, stellar_tem,
                                      sfh=sfh_array,
                                      ceh=ceh_array,
                                      z=0.015, vpec=300, vdisp=100, ebv=0.2,
                                      mag=None)


.. image:: /_static/image/example_spec1d_ssp3.png
   :align: center


Output Attributes
------------------

- ``stellar.wave``: 1D wavelength array in Ångströms.
- ``stellar.flux``: Corresponding flux array in units of :math:`10^{-17}\ \mathrm{erg\ s^{-1}\ cm^{-2}\ Å^{-1}}`.

.. note::

   In physical mode, the output flux is computed from SFH normalization and cosmological distance.  
   In magnitude-calibrated mode, flux is normalized to the input magnitude.

``StellarContinuum`` supports both empirical and physical modeling of galaxy spectra, with consistent units and interface across modes.