T. Jarrett, IPAC
(971013)
Five contiguous scans toward the general direction of [glong=160 and glat=-40] were obtained on the night of 970923. Over 170 galaxies were detected (and confirmed using the dss) comprising the 5 scans. We apply bi-symmetric parameterization to identify candidate irregular and other "disturbed" galaxies among the set. The primary diagnostics are (1) flux ratio and (2) cross-correlation reduced chi; see Bi-Symmetric Autocorrelation & such for more information.
Before we dive into the bisymetric analysis, let's first take a look at the star-galaxy distriminents for this field.
Reduced Cross-Correlation Chi
One straight forward piece of information to glean from bi-symetric cross-correlation is a chi-square test. The reduced chi-square can be written as:
where p and p* are the points 180 deg apart that are being compared, N is the number of points being compared, and sigma is the pixel noise level (the factor of two is because the difference has a SQRT(2) larger error).
Flux Ratios
The flux ratio is computed by summing the flux for each bisymmetric piece and taking the ratio between the respective parts. The computation also includes a "sliding scale" to account for the uncertainty in the integrated flux measurements. We allow some range in the integrated flux measurements based on the predicted uncertainty in the measurement, as follows:
sliding flux measurement:
family (flux ratio) == INT ( p*) +- del_P* / INT (p) +- del_P
where del_P is the expected uncertainty in the integrated flux
measurement.
Assymetric Galaxies
The bisymmetry diagnostics provide a handy means to identify asymmetric galaxies, including intrinsically irregular galaxies (mergers, double nukes, ringed, etc), as well as galaxies contaminated by stars. The following images show the galaxies with flux ratios that significantly depart from unity.
Of the 170 or so galaxies, something like 25 have at least one band with a flux ratio less than 0.8. However, inspection of these galaxies reveal that most have only one band with an unusual flux ratio and it tends to be at K band. These galaxies do indeed look asymmetric at K if carefully examined, but at J and H (and the DSS) the galaxy looks rather ordinary (re: symmetric). We are probably seeing the affects of sky noise on the 2-D profiles (particularly apparent at K, and to a lesser extent at H). An example of one-band only asymmetry is given here:
1. J_frat = 0.975, H_frat = 0.884, K_frat = 0.628
2. J_frat = 0.998, H_frat = 0.787, K_frat = 0.996
3. J_frat = 0.826, H_frat = 0.850, K_frat = 0.502
Notice also that the cross-correlation chi ("cchi") is also much greater then 2.0 for the band with a low flux ratio (i.e., they are correlated).
A more robust determinant of asymmetry is to apply a "two-band" rule. We demand that at least two bands have a flux ratio less than 0.80. The following images show all of the galaxies that satisfy this criterion. One note: we demand that the integrated flux of the galaxy (per band) be at least 5:1 SNR before we apply a flux ratio test to the band.
1. Both J and K show asymmetry (0.79 and 0.71 ratios).
The galaxy looks normal on the dss.
2. All three bands have poor ratios (0.37, 0.33 & 0.62). The galaxy
appears to be contamined by a star next to the nucleus.
3. All three bands show asymmetry (0.26, 0.18 & 0.26). This
looks like an interacting pair of galaxies (also note the spiral
nearby -- looks like a cluster).
1. All three bands look lopsided (0.56, 0.29 & 0.75). The dss
also appears unusual.
2. Two bands (H and K) have low ratios (0 and 0.65). This galaxy
appears to be part of a complex system (there are at three galaxies
in the vicinity).
3. J and H have low flux rats (0.74 and 0.71). The galaxy appears
lopsided in all three bands (K is faint, however) as well as the dss.
The galaxy appears to be contaminated by a star near the nucleus. Flux rats: 0.24, 0.31 & 0.38.
