Consistency checks

Before obtaining parameters for the radio galaxy sample, a series of test runs were carried out on galaxies previously studied, and on simulated galaxies in order to test the validity and robustness of the results provided by the program. It is important that the test runs are carried out on a single galaxy or a set of galaxies. The results can then be used to decide the approach for the galaxies to be studied. The tests were also meant to decide on the range of radius over which the fitting could be reliably done. With the above in mind, the following kinds of fits/tests were tried:

• Fit just the effective radius and central bulge intensity.
• Fit the effective radius, bulge ellipticity and central bulge intensity.
• Fit both the bulge and disk scales and the respective ellipticities. Also fit the central bulge intensity.
• Fit including all points of the profile upto the point where the error as indicated by ellipse falls to 0.1 magnitude in the surface brightness.
• Fit leaving out a different number of innermost points i.e., start at , , , , and from the center.
• Fit profiles obtained with different filters for the same galaxy.

The results for NGC 661 are tabulated in Table  for the V, R and I filters. Here n1 and n2 refer to the innermost and outermost points from the profile that were used for the fit. The first column indicates the corresponding numbers in arcsec along with the filter and fit type, b indicating a bulge only fit and d a bulge+disk fit. The graphical results are shown in Figure . The extracted is shown as a function of the innermost profile point that was used. The last point was always chosen to be the one where exceeded 0.1. For NGC 661, we also carried out fits with a point source. However, we did not find evidence for a point source. Any such source which may be present is weak.

 

 

code D/B n1 n2

Ib 5.73 0.02 - - 1 41 9922.6

Ib15 8.21 0.03 - - 11 41 5049.6

Ib20 9.36 0.04 - - 14 41 3229.9

Ib25 12.21 0.08 - - 16 41 719.1

Ib30 13.25 0.12 - - 18 41 59.6

Ib40 12.54 0.15 - - 21 41 12.1

Id 14.64 0.10 1.19 0.02 0.30 0.01 1 41 4.6

Id15 14.14 0.11 1.26 0.03 0.27 0.01 11 41 1.5

Id20 14.00 0.12 1.29 0.04 0.26 0.01 14 41 0.9

Id25 13.85 0.13 1.32 0.05 0.24 0.01 16 41 0.4

Id30 13.81 0.20 1.39 0.20 0.12 0.34 18 41 0.4

Id40 13.79 0.19 1.36 0.11 0.18 0.29 21 41 0.4

Rb 6.02 0.02 - - 1 42 14956.6

Rb15 8.01 0.03 - - 11 42 8086.6

Rb20 9.98 0.04 - - 14 42 3557.9

Rb25 12.36 0.07 - - 16 42 960.7

Rb30 13.49 0.10 - - 18 42 103.3

Rb40 12.91 0.12 - - 21 42 16.6

Rd 13.73 0.03 100.00 67.72 0.01 0.01 1 42 36.0

Rd15 14.19 0.09 1.33 0.04 0.21 0.07 11 42 2.5

Rd20 13.98 0.10 1.33 0.04 0.18 0.12 14 42 1.6

Rd25 14.02 0.21 1.40 0.08 0.07 0.02 16 42 1.4

Rd30 14.33 0.26 1.48 0.08 0.06 0.00 18 42 1.3

Rd40 14.02 0.09 1.27 0.08 0.14 0.04 21 42 0.9

Vb 5.95 0.02 - - 1 40 11876.7

Vb15 7.85 0.03 - - 11 40 6898.7

Vb20 11.12 0.06 - - 14 40 1299.3

Vb25 13.17 0.10 - - 16 40 321.0

Vb30 13.24 0.11 - - 18 40 47.1

Vb40 12.69 0.14 - - 21 40 11.1

Vd 13.90 0.09 1.34 0.04 0.12 0.04 1 40 1.5

Vd15 13.86 0.10 1.32 0.04 0.19 0.01 11 40 1.1

Vd20 13.75 0.15 1.35 0.09 0.10 0.13 14 40 0.6

Vd25 13.72 0.13 1.36 0.07 0.18 0.01 16 40 0.7

Vd30 12.70 0.03 1.78 1.05 0.02 0.00 18 40 0.7

Vd40 13.78 0.17 1.28 0.15 0.11 0.20 21 40 0.8

Note: The ``--'' in columns and D/B indicates these parameters were not relevant in the run because the disk parameters were not used during the fits. n1 is the innermost point and n2 is the outermost point used during the fit.

 

The results obtained reveal a number of interesting points.

1. If only the bulge parameters are free, the seeing affected central points, which also have the highest signal to noise, tend to affect the results. The value of rises gradually from to about when successively larger number of points are left out. The FWHM for the PSF for the images in the three filters is and the is seen to stabilize when the points in the inner are omitted. The stabilized is smaller than that obtained by other observers ( e.g. in the band obtained by Djorgovski and Davis, 1987). The value of will be called the actual value during the course of this discussion.
2. When the disk as well as the bulge parameters are free, the obtained is close to the actual value ( ). This is because the inner points are absorbed in a seemingly false disk (seeing disk?) which has a small increasing scale length ( ) and a decreasing disk-to-bulge ratio (0.23-0.06). For a typical disk, e.g. that of a spiral or an S0 ( Burstein, 1979) one expects to be in excess of and a D/B of well over 0.2. being smaller than the FWHM of the PSF indicates that though the inner points are being absorbed in a disk, the disk luminosity is small and is more likely to be a deviation from de Vaucouleurs' law rather than a dynamical disk.
3. The typical range for only-bulge (very large to about 1.3) and for bulge-disk (exceeds 2 only once) indicates that a statistically better fit is obtained when the disk is left free. The reason why the is very large for the bulge-only case when the inner points are included, is that the errors on the innermost points are the smallest since the signal there is highest. As a result, the parameters are constrained to fit the innermost points better. This decides the parameter values without concern to the outer points. If there is even a small departure from, say, the de Vaucouleurs model near the center, the parameters turn out to be wrong and result in a large .

Thus, the following conclusions can be drawn from the above results: (1) Fitting for a disk as well as a bulge does not lead to the detection of a large false disk if one is not present and (2) inner points within about FWHM of the PSF should be omitted for reliable results.

Hence, for fitting the radio galaxies and the control sample galaxies, we chose to fit for bulge as well as disk parameters. We also chose to uniformly mask out the center and not fit for a central point source.