GIM2D/GRART

Parameters

    in_image = "gx1687y184"    Name of input image
    msk_image = "gx1687y184_m"  Name of segmentation image
    res_image = "gx1687y184_r"  Name of residual image
        (mode = "q")

GRART measures 4 different asymmetry indices from the residual image. It is a stand-alone version of the subroutine ra_rt in GIMFIT2D. These indices are denoted RT, RA, Az and Dz. Each index is described below. Note that bad pixels (flagged by -2.0 in segmentation image) and their 180 degree symmetrical counterparts are rejected from all asymmetry index calculations.

RT and RA indices

The RT and RA indices are defined as

and

where R_ij is the flux at (i,j) in the residual image res_image, R180_ij is the flux at (i,j) in the residual image rotated by 180 degrees, and I_ij is the flux at (i,j) in the input original image in_image. The total and asymmetric indices RT and RA (computed within a radius of 5 kpc) are approximately equal to the total and asymmetric residual flux, respectively, expressed as a fraction of the total galaxy flux (Schade et al. 1995). The absolute value in the sum of both indices makes them vulnerable to the effect of noise. RT and RA can have large values in the presence of noise even though there may be no asymmetry present. Background corrections have therefore been applied by GRART to the output RT and RA values. GRART computes RT and RA with different median-filtering box sizes (1x1 --> RT1 and RA1, 3x3 --> RT3 and RA3, 5x5 --> RT5 and RA5). The indices are also calculated through 10 different apertures with radii ranging from 1 to 10 half-light radii.

The Az index

The Az index is calculated over the pixels belonging to the object or the background. Each pixel is compared to its symmetrical counterpart about the center of the object. If the flux in that pixel is n*sigma(background) higher than the flux in its symmetrical counterpart, then the pixel is taken to be part of an asymmetrical component. Az is the sum of the fluxes of all such pixels normalized by the total object flux. GRART computes the Az index for three values of n:2, 3 and 5. These indices are denoted Az2, Az3 and Az5. They are computed within annuli with the following radii (expressed as multiples of the half-ligh radius): 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10.

The Dz index

The Dz index takes advantage of the object isophotal shape of the object as measured by SExtractor. Dz is the sum of the fluxes of the object pixels (as given by the segmentation image) with symmetrical counterparts about the object center which do not belong to the object. This sum is normalized by the total object flux. Dz is computed outside of one half-light radius, and it should be sensitive to asymmetries such as tidal arms.

GRART output

GRART outputs background parameters followed by the half-light radius of the object computed by numerical integration of the best structural parameters. This radius is therefore the intrinsic radius of the object before PSF convolution. The last 10 lines of output corresponds to RT1, RA1, RT3, RA3, RT5, RA5, Az2, Az3, Az5 and Dz respectively. RA and RT indices are computed within 10 different apertures. The Az indices are calculated within 9 annuli. IMPORTANT: These last lines of output are exactly the same lines as the lines in the GIMFIT2D output log file.

Last Update: 21 September 2000
Comments/questions to Luc.Simard(at)nrc-cnrc.gc.ca