Figure 1: The local effective surface brighness vs. radius for NGC 4477. Boxes correspond for the R- and stars for the B-band.
Nordic-Baltic research course in applied astronomical photometry, Moletai, 1999
1VIRAC, e-mail:kberzins@acad.latnet.lv,
2Tuorla Observatory, e-mail: eianko@astro.utu.fi.
The data was obtained in 1998 using the 1.8m telescope in McDonald Observatory, USA, with the integrated exposure time of 308.9 and 183.58 sec in the B- and R- bands, respectively.
First we identified the objects on our images. After that we selected (masked) them in order to account their effect to the galaxy photometry. Especially, one should take care about nearby galaxies and close bright stars. For example although faint galaxies seem to have much less effects to a main galaxy than bright stars they actually do not, because galaxies are extended objects and stars are point sources.
After the masking procedure, we were modelling intensities of galaxies by fitting them with ellipses. This task was done by using ellipse-package inside IRAF. At the beginning of this procedure, one defines a center of galaxy and its size, then it was separated from the background. This was iteratively repeated.
To calculate the effective radius re and mean surface brightness < µe >, the relation between the colour and r¼ was defined. Our selected results are represented in Fig. 1-4 and in the Table 1. The effective mean surface brightness is defined by (cf. Caon et al., 1993)
< µe >=A+cn-1.39 (1)
where cn=8.3268 is some empirical constant, and the effective surface brightness radius as
re=(cn/B)n (2)
where n = 4 and the following equation was used for the surface profile fit:
µ = A +B*r1/n (3)
where A and B are linear fitting coefficients, and r is projected radius from a center of galaxy.
The total magnitude can be calculated as:
mT =< µe >+2.5log(2PI*re2) (4)
The colour is defined as µB-µR, i.e. the difference between B- and R-band surface brightness respecitvely.
We have obtained the following values for the colour gradients (cf. Fig. 3): For NGC4459 it is -0.07±0.01 and for NGC4477 it is -0.08±0.005
| A | B | log( re) | < µe > | mT | log( re)1 | mT1 | |
| NGC4459 B-band | 14:17±0.04 | 3.38±0.01 | 1.59 | 21.11 | 11.18 | 1.55 | 11.32 |
| NGC4459 R-band | 12.57±0.02 | 3.36±0.01 | 1.58 | 19.51 | 9.63 | -- | -- |
| NGC4477 B-band | 14.06±0.05 | 3.47±0.03 | 1.52 | 20.99 | 11.37 | 1.58 | 11.38 |
| NGC4477 R-band | 12.27±0.02 | 3.60±0.01 | 1.45 | 19.21 | 9.93 | -- | -- |
Table 1: Photometrical parameters for NGC4459 and NGC4477 in Johnson B- and R-bands.
We had obtained quite nice galaxy surface models. The residuals, however, are not perfect because galaxies actually do not follow perfectly r¼ law. The residuals can be quantified in terms of Fourier transformation coefficients, which were calculated by the IRAF ellipse package. We see that NGC 4459 can be fitted very well by elliptical functions. However, NGC 4477 has large deviations indicating that its surface brightness can not easily fit by elliptical functions at least at larger radii (cf. Fig. 4). Some other possible sources of uncertainties are introduced during the masking of other objects because this step can be done in many different ways. To obtain the best results, from the fitting procedure the very center of galaxies (which are effected by seeing) and their edges (that contain larger signal to background noise) were not used. Comparing our results to publicly available ones we conclude that they are in a good agreement (cf. Table 1).
Figure 1: The local effective surface brighness vs. radius for NGC 4477. Boxes correspond for the
R- and stars for the B-band.
Figure 2: The local effective surface brightness vs. radius for NGC 4459. Boxes correspond for the R- and stars for the B- band.
Figure 3: The color µB-µR vs. radius for NGC4459 (crosses) and NGC4477 (boxes)
Figure 4: The Fourier c4 coefficients (corresponding to the cos term) vs. radius show deviations from the elliptical mode for NGC 4459 (crosses) and NGC 4477 (boxes).