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Disky galaxies

 

In Chapter gif we have explained the procedure we follow to decompose the surface brightness profile of a galaxy into de Vaucouleurs bulge (B) and an exponential disk (D) component. The ratio of the luminosities of the two components, D/B, is the factor that indicates the presence of a disk. In Figure gif we show the typical D/B values for galaxies of different morphological types (Kent, 1985). As per Hubble's original classification scheme (Hubble, 1926), elliptical galaxies do not have any structure. Whenever a disk like component was suspected the galaxy was labeled S0. Recent studies have shown, however, that elliptical galaxies can also be disky. It is likely that such galaxies may be the lower end, in diskiness, of S0 galaxies.

  figure2422

  figure2427

  figure2435

In Figure gif we show the distribution of the D/B ratio for the radio and control samples. For the R filter we also depict the distribution as a histogram for both the samples in Figure gif. It is seen that in general the D/B values in both the filters are similar. Several galaxies in the radio as well as control samples, have a D/B;SPMlt;0.1. These are likely to be insignificant disks possibly arising from deviation form the tex2html_wrap_inline3761 law or a weak point source (when tex2html_wrap_inline3765 is also small). We notice that there are a few galaxies with a D/B;SPMgt;0.3 in both the filters. In this section we consider these galaxies. The value of D/B;SPMgt;0.3 was chosen so that even if the scatter around the D/B values of different galaxy types plotted in Figure gif is taken into consideration, the galaxies with these values will not be elliptical. In the radio sample, there are five such galaxies, while in the control sample there are four galaxies with this lower limit for the ratio.

There is also a class of objects for which D/B;SPMgt;0.3 but such that tex2html_wrap_inline5551 FWHM of the PSF. Such cases are likely to be the result of a genuine departure from de Vaucouleurs' law near the center or indicative of the presence of a weak AGN. We do not consider these galaxies to be disky in the normal sense of the word and have not treated them as being significant in the following discussion.

Radio galaxies

We present here results on 5 radio galaxies with prominent disks. We examine the host galaxy properties and the radio structure. 4 of the 5 galaxies have FR II morphology and even the fifth one is of type FR II?. The D/B ratio is suggestive of spiral galaxies. However, we do not see spiral arms except in one case. Relevant parameters for these galaxies are listed in Table gif.

1222-252

This galaxy is at a redshift of 0.075 and is by far the most interesting one. It has tex2html_wrap_inline5563 and tex2html_wrap_inline5565 . The radio source is of type FR II. The morphology in the two filters shows a bar like structure along with a linear structure at right angle to the 'bar', arising from one end of the bar. There are also other flocculent structures present. D/B values are consistent with the galaxy being an Sb. The disk scale length is in this case a few times the FWHM of the PSF, but only a fraction of the bulge scale length. We are therefore again detecting a small disk relative to the bulge.

1053-282

This galaxy is at a redshift of 0.061, has tex2html_wrap_inline5569 and tex2html_wrap_inline5571 . The ellipticity of this galaxy changes from 0.1 near the center to 0.4 in the outer region. There is a spiral galaxy 15'' away with which the radio galaxy is likely to be interacting. The D/B values in both filters are ;SPMgt;4. From this, and the disk and bulge scale lengths, the galaxy can be classified as Sc. This is the most likely candidate from our radio sample to be a normal spiral as defined by bulge and disk parameters. No spiral structure is seen.

1006-214

This galaxy is at a redshift of 0.250. The radio source is of type FR II with tex2html_wrap_inline5579 and tex2html_wrap_inline5581 . The galaxy is flattened near the center ( tex2html_wrap_inline5583 near the center compared to tex2html_wrap_inline5585 on the outskirts). It has a bluer center and an armlike extension is seen along the radio axis. The K' image does not show the armlike structure and hence it is likely to be the outcome of recent star forming activity. This galaxy has large D/B (;SPMgt;5) in both filters and on that basis alone would be classified as Sc. However the disk scale length is comparable to the FWHM of the PSF. It is also much smaller than tex2html_wrap_inline3763 in the R filter. In the B filter, tex2html_wrap_inline3763 and tex2html_wrap_inline3765 are comparable in size. However, the smaller bulge scale length in this filter, relative to tex2html_wrap_inline3763 in the R filter, is suspected to be a result of excess blue emission in the central region. Taking all this into account, we conclude that the profile fit has detected a small scale disk-like structure. The linear scale length of this structure in R is tex2html_wrap_inline5607 , and it is much smaller than the bulge scale length of tex2html_wrap_inline5609 . The ratio of the disk to bulge scale lengths is therefore much smaller than is observed for standard disk galaxies.

0952-224

This galaxy is at a redshift of 0.230 and is in a crowded region. It has a tex2html_wrap_inline3881 radio flux tex2html_wrap_inline5613 and has a radio luminosity of tex2html_wrap_inline5615 . The radio source is of type FR II. This galaxy has a tex2html_wrap_inline5617 , which is large enough for it to be classified as an Sb spiral. However tex2html_wrap_inline5619 is considerably smaller. This difference in parameter values is also seen in tex2html_wrap_inline3765 , as well as in bulge parameters. Due to the difference in values from one filter to another, it is not possible to confidently classify the galaxy. The differences are caused by the presence of dust as well as blue emission, which are seen in the color maps and profile fits. We show the galaxy in Figure gif.

1215-215

This galaxy is at a redshift of 0.075. It has tex2html_wrap_inline5623 and tex2html_wrap_inline5625 . D/B values in tex2html_wrap_inline5629 are consistent with SA classification. tex2html_wrap_inline3765 in tex2html_wrap_inline3807 is comparable to the FWHM of the PSF.

   sidewaystable2407

Control Sample

We describe here objects from the control sample that have D/B;SPMgt;0.3 in both filters. The control galaxies have been chosen to be of early type and we expect a few S0s but no spiral galaxies. Relevant parameters for the 4 disky galaxies are given in Table gif.

1251/190

In this case the D/B values are consistent with the galaxy being a lenticular, but the error bars are large, and the D/B value, at least in the B filter, is consistent with zero. The disk scale lengths are smaller than the FWHM of the psf.

1323/1477

This galaxy has large D/B and appreciable bulge and disk scale lengths in both filters. However, the fit is poor in both filters.

1323/1673

In this case the fit is poor in R, tolerable in B. In the latter, the D/B value is consistent with zero.

1329/234

The disk in this case has a scale length comparable to the FWHM of the psf, tex2html_wrap_inline5783 . The D/B in B is consistent with a lenticular and D/B in R consistent with a SA.

  figure2604

figure2607


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Next: Morphological Signatures of disks Up: Radio galaxies with disks Previous: Radio galaxies with disks