Astron. Astrophys. 327, 479-482 (1997)

2. The density enhancement around 3C 345

Fig. 1 shows the quasars found in the grens search of about 8 sq. deg. by Crampton et al. One conspicuous concentration is at R.A. = 16h 41m 18s Dec. = +39deg 41 11 (1950). The concentration is in the form of a rough line running from NE to SW exactly through the position of 3C 345. Most of these quasars can be identified in the enlarged plot of Fig. 2

Fig. 1. Quasar candidates discovered in an 8 sq. deg. area by Crampton et al. (1988). The concentration around 3C 345 can be seen at about 16h39m and +40 deg (arrow). This region is shown enlarged in Fig. 2.

Fig. 2. Quasars of redshift in a homogeneously searched area around 3C 345 and an equal area to the west. Redshifts are written to the upper right of each quasar. 3C 345 is identified HP (for high polarization) and the Seyfert galaxy NGC6212 is marked S1.

The first question that poses itself is whether the quasars in the concentration around 3C 345 are different from the quasars in the rest of the field. The answer is yes. The quasars in the rest of the field are fainter and generally of much higher redshift. This is illustrated in Fig. 2 where only the quasars of are plotted and the symbol size varies as their apparent magnitude. In this case we can see by inspection that an equal area west of 3C 345, analyzed in exactly the same way, shows almost no similar quasars.

This result can be quantified by calculating the quasar density within concentric circles centered on 3C 345. Fig. 3 plots the density per sq. deg. at different radii from 3C 345. The background density which the 3C 345 group is obviously approaching at large radii is very interesting. Taking the background density as that in the western half of Fig. 3 one obtains 2.5 background objects per sq. deg. How does this correspond to background densities in the rest of the sky? As summarized by Arp (1981), down to 20th apparent magnitude various determinations yield about 6 quasars per sq. deg. Conservatively assuming half are in the redshift range we would expect a background of 3, agreeing almost exactly with the background derived from Fig. 2 and plotted in Fig. 3. Alternatively we can use the density of quasars from the CHFT survey as determined by Arp (1990). That also comes out to be a background level of about 3 per sq. deg.

Fig. 3. Density of quasars in Fig. 2 in concentric circles around 3C 345. Background value discussed in text.

What this means is that we have a very good determination of the expected background density in Fig. 3 Therefore we can confidently compute that the over density close to 3C 345 reaches a factor of 15, falling away at greater distances to an indicated group diameter of the order of a degree. If we estimate the standard deviation of the background density as a maximum of 3, we have a minimum of 5 sigma density enhancement at this particular position in the sky.

We note that a similar density enhancement of quasars (a factor of 20) was found around the active, jet Seyfert NGC1097 (Arp, Wolstencroft and He 1984). Morever the three brightest quasars, one of which was extremely variable, were situated just between the two brightest jets of NGC1097 and had redshifts of z= .34, .53 and 1.00.

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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