A quick overview

The algorithm directly compares data-cubes with a disk parametric model which has 9 or 10 free parameters [which can also be fixed independently]. The algorithm uses a Markov Chain Monte Carlo (MCMC) approach with non-traditional sampling laws in order to efficiently probe the parameter space. More importantly, it uses the knowledge of the 3-dimensional spread-function to return the intrinsic galaxy properties and the intrinsic data-cube. The 3D spread-function class is flexible enough to handle any instrument.

One can use such an algorithm to constrain simultaneously the kinematics and morphological parameters of (non-merging, i.e. regular) galaxies observed in non optimal seeing conditions. The algorithm can also be used on Adaptive-Optics (AO) data or on high-quality, high-SNR data to look for non-axisymmetric structures in the residuals.

The algorithm can be (roughly) summarized as :

  1. Pick new random galaxy parameters using uniform priors [boundaries can be set]
  2. Create clean cube from parameters and convolve it with instrument’s PSF+LSF
  3. Measure closeness of resulting convolved cube to input cube
  4. Accept/Reject galaxy parameters using Metropolis-Hasting algorithm
  5. Goto 1. until max iterations are reached
  6. Fill output attributes with data (chain, cubes, etc.)
  7. Compute and return best fit galaxy parameters from chain

Warning

The output flux is in the pixel units, which may need to multiplied by CDELT3.

Warning

The algorithm should never be used blindly and we stress that one should always
  1. look at the convergence of the parameters using plot_mcmc(),
  2. investigate possible covariance in the parameters using plot_correlations() and/or fix a parameter with the option known_parameters to remove the degenerancy,
  3. adjust the MCMC algorithm with the option random_scale (lower than 1 [default] for higher acceptance rate and vice versa) to ensure an acceptance rate of 30 to 50%.

Input Cubes supported

  • Any fits cube can be used provided that the z-axis represents wavelengths or frequencies.
  • Any units are normally accepted as the algorithm works in velocity space (dlamba/lamba or dfrequency/frequency). Check the Instrument.z_step_kms value which is critial for the kinematic parameters.
  • If the header is incomplete (CRPIX3, CDELT3, CRVAL3, CUNIT3), the algorithm will try to use the default values assigned to the instrument. The user can specify these directly.
  • If the header is complete, the instrument default pixel sizes will be over-written by the the information from the cube header.

Warning

Pay attention to the LSF FWHM, which should be specified in the same units as the cube.

Instruments supported

  • ALMA

    set lsf_fwhm to 1 cdelt (default) or less

  • SINFONI (J250, H250 and K250 modes)

  • MUSE (WFM and NFW modes)

    default lsf_fwhm is 2.67 Angstrom

  • KMOS

    default xy_step is 0.2 arcsecs

  • MANGA (soon)

Note

The default Instrument values will be overridden by the cube header info (CRPIX, CDELT etc.) if present.

Incomplete Header handling overview

_images/GalPak_flow.png