PKS 1353-341

A 10 min I image (FWHM = 18) and a 30 m B image (FWHM = 19) were obtained with EFOSC at the 3.6m telescope on August 10 and 11, 1996 and three 15 min I images (FWHM = 13) with the NTT on May 2, 1992. These images show the object to be associated with a large luminous galaxy in a crowded field with many galaxies and stars (fig. ). One of these is seemingly situated within the galaxy.

 

Figure:  I image of PKS 1353-341 taken with EFOSC in 1996. North is up, east to the left; the field is $\sim$155x 155. The straight line indicates the position of the spectrograph slit.

To establish its nature we have obtained on Aug 11, 1996 two 5 min spectra through the EFOSC B300 and R300 grims which yielded a useful spectral range of 3740-9280 Å$\;$with a FWHM of 15 Å$\;$for the night sky lines. The 15 slit was oriented in PA = 19$^{\circ}$, so as to obtain the spectrum of the nucleus, object 1 at 48 NE and object 2 at 69 NE (fig. ).

 

1353-341 1814-637 1934-638

$$\; \; \;$$ z

$$_{\rm B-V} ^{\rm m} ^{\rm m} ^{\rm m}$$

$$^{\rm m} ^{\rm m} ^{\rm m}$$ B-I

$$\Delta ^{\rm m} ^{\rm m} ^{\rm m}$$

B 16.53 17.1 19.51

$$_{\rm B} ^{\rm m} ^{\rm m} ^{\rm m}$$

$$_{\rm B_{0}}$$ -25.6 -21.4 -22.1

$$\; \; \;$$ S₅

log P₅ 26.2 25.9 27.0 VLBI

sep. (mas) 300 42

The nucleus shows a continuum typical for a gE galaxy with probably Mg I b and Na I D in absorption and with a set of emission lines typical for a Seyfert 2 galaxy with [N II]$\lambda$6583/ H$\alpha$=2.1, [O I]$\lambda$6300/H$\alpha$= 0.4 and [OIII]$\lambda$5007/H$\beta$=5.5, measured on the spectrum after subtraction of a template gE galaxy spectrum (fig. ). For H$\alpha$/H$\beta$ we measured an uncertain value of 5.5 and for [OII]$\lambda$3727/H$\beta$$\sim$3. The FWHM of the lines near H$\beta$ is 23 Å or, upon correction for instrumental broadening of 15Å, about 700 km s^-1 in the rest frame. We obtained z = 0.2230 $\pm$ 0.0001, very close to z = 0.2227 found by White et al. (1988).

Object 1 also has a continuum compatible with a gE, with H$\beta$, Mg I b and Na I D in absorption and H$\alpha$ weakly in emission. The rather noisy spectrum yielded a redshift z = 0.2252 $\pm$ 0.0004. This corresponds to a relative velocity of 540$\pm$100 km s^-1 in the rest frame. It may well be, therefore, that object 1 is a galaxy being captured by PKS 1353-341, though it could also be an independent galaxy in the surrounding cluster. As we shall discuss later the velocity derived for PKS 1353-341 from emission lines may not necessarily represent the systemic velocity and the velocity difference might well be smaller.

Object 2 turns out to have a blue continuum with Mg I b in absorption and emission lines corresponding to a low excitation H II region ($\lambda$6583/H$\alpha$=0.6; $\lambda$6300/H$\alpha$< 0.04 and $\lambda$5007/H$\beta$= 0.6); the emission lines are unresolved at our resolution. The redshift z = 0.2157$\pm$0.0001 which corresponds to a velocity of 1800 km s^-1, in the rest frame of PKS 1353-341, probably indicating the presence of a substantial cluster of gala- xies.

 

Figure:  Low dispersion spectrum of PKS 1353-341 taken with EFOSC on the ESO 3.6m telescope.

Elliptical isophotes with b/a=0.7 and major axis in PA=40$^{\circ}$ were fitted to the I and B images of PKS 1353-341, excluding the part influenced by object 1. To obtain the surface brightness in V we have corrected for the (1 + z)⁴ factor, the k-term and for galactic absorption (table ) and we have assumed q₀=0 and V₀-I₀=1.15$^{\rm m}$ and B₀-V₀= 0.95$^{\rm m}$, the standard values for gE. The resulting surface brightness distribution is shown in fig. , where also some of Schombert's (1987) standard curves for gE are indicated. PKS 1353-341 appears to have a slightly flatter profile than a typical gE for r less than $\sim$40 kpc (r^1/4=2.5). Further from the center the distribution flattens, indicative of a cD galaxy. Between 3 and 10 from the center where the photometry is most reliable we obtain B-I=3.01$^{\rm m}$ about the value expected for a standard gE at this redshift.

 

Figure:  Surface brightness profile in V magnitude arcsec^-2 as a function of r^1/4 [r=(ab)^0.5] deduced from the profiles in I (NTT image, open square, EFOSC image, filled square) and in B (EFOSC image, crosses). The lines are average profiles of elliptical from Schombert (1986) for M$_{\rm V}$(<16 kpc)=-21.0, -22.0, -23.0, -23.5.

Upon subtracting an r^1/4 model and a PSF centered on the nucleus from the observed NTT image one clearly sees the extended nature of the object 1 which may be traced to approximately 5 (30 kpc) from its center. The integrated magnitude of object 1 within a 22 kpc radius is I$_{\rm KC}\sim$18.6$^{\rm m}$with B-I roughly 3.0$^{\rm m}$ corresponding, with the normal colour of a gE, to M$_{\rm V}=-$21.4$^{\rm m}$, probably a minimum estimate for its total luminosity. Such a galaxy certainly could perturb an object like PKS 1353-341. The more distant object 2 has B=20.40$^{\rm m}$ and I=18.42$^{\rm m}$ corresponding to a late type galaxy.

Beyond 10 from the center the light distribution flattens and a cD envelope begins to dominate. The integrated magnitudes of the whole system to 120 (780 kpc) from the center are I=13.62$^{\rm m}$, and B=16.57$^{\rm m}$ from the 10 min and B=16.49$^{\rm m}$ from the 30 min exposures. As a result of the great extent of the envelope it is possible that our sky level is still contaminated by light from the galaxy. A larger CCD would be needed to determine the integrated magnitudes and their uncertainty with confidence. Our measured magnitudes give M$_{\rm I_{\rm KC}}$=-27.5 and M$_{\rm B}$=-25.6 which would correspond to M$_{\rm V}$=-26.4 or to log(L$_{\rm V}$/L$_{\rm V,\odot}$)= 12.5. Hence PKS 1353-341 is a very luminous galaxy indeed. In the list of 26 cD galaxies by Schombert (1988) there are only two that are still more luminous.