ZEST, the Zurich Estimator of Structural Types,

is a novel approach to quantitatively describe the morphology of faint high-redshift galaxies. To classify galaxy types and galactic structure, ZEST uses:

• Five non-parametric diagnostics of galaxy structure i.e., asymmetry A, concentration C, Gini coefficient G, the 2nd-order moment of the brightest 20% of galaxy pixels M20, and the elongation of the galaxies;
• A parametric description of the two-dimensional galaxy surface brightness through single--Sersic fits.

To fully exploit the wealth of information while reducing the redundancy present in the used diagnostics, ZEST performs a Principal Component Analysis which brings down to three the dimensionality of the parameter space that fully describes, without significant loss of information, the galaxy structure. To each unit cube of the PC1-PC2-PC3 space, the ZEST classification assigns a morphological type:

• Type 1: early types;
• Type 2: disk galaxies;
• Type 3: irregular galaxies.
  

• Type 2.0: Bulge-dominated disks;
• Type 2.1 and Type 2.2: Intermediate-bulge disks (decreasing B/D from 2.1 to 2.2);
• Type 2.3: Bulge-less disks.
  

• An irregularity and a clumpiness parameter, both ranging from 0 for regular/smooth galaxies to 2 for very irregular/clumpy objects, and
• An elongation parameter, from 0 for round galaxies to 3 for very elongated structures.
  

The ZEST classification breaks the degeneracy between different galaxy populations that is present in classification schemes that use only some of the diagnostics which are simultaneously taken into account in the PC1-PC2-PC3 space (See e.g. the figure below, where two PC1-PC2 slices corresponding to two different values of PC3 are shown; each PC1-PC2-PC3 cell is occupied by galaxies with the specific structural characteristics illustrated with the stamp of a typical galaxy in that cell).

ZEST is described in a paper in the ApJ COSMOS Special Issue (submitted).

See also the April 2006 research Highlights.