JSTOR

1. INTRODUCTION

NGC 5548 ( , , km s⁻¹) is a classical Seyfert 1 galaxy contained in the original list of Seyfert (1943). It has been extensively studied by individual multiwavelength spectroscopy and photometry, and it has been the subject of detailed monitoring programs (e.g., Clavel et al. 1991; Korista et al. 1995). The X‐ray emission of NGC 5548 has been studied since it was first detected by the Ariel 5 satellite (Elvis et al. 1978). Structural peculiarities of the galaxy include an extended very low surface brightness blue tail, detected by Tyson et al. (1998) and considered by those authors to be the aftermath of an encounter or merger between two galaxies.

Radecke (1997), in his study of excess numbers of ROSAT X‐ray sources in fields centered on galaxies with active nuclei, listed 20 ROSAT sources less than 1° distant from NGC 5548. Arp (1997) tabulated 18 optical objects at these positions, noting that two were cataloged QSOs with and 0.674. Thus, NGC 5548 is an example of the statistical case made by Radecke, as shown in his Figures 1b and 1c, where the excess numbers of ROSAT point sources around Seyfert galaxies are demonstrated with a significance level of 7.4 σ.

A high surface density of QSOs has been found around the Seyfert 2 galaxy NGC 1068 (Burbidge 1999). The striking QSO distribution around that galaxy and the large areal density of QSOs that was found suggested that despite the differences in redshift, the QSOs may be physically associated with that galaxy (Burbidge 1999). A remarkable distribution of QSOs has been found around a third classical Seyfert galaxy, NGC 3516 (Chu et al. 1998). Thus, we decided to look in more detail at the QSOs near NGC 5548.

2. QSOs AROUND NGC 5548

Three of the blue stellar objects (BSOs) at ROSAT X‐ray positions tabulated by Arp (1997) around NGC 5548 were observed at the Lick Observatory with the Shane telescope and Kast spectrograph on 1997 May 12 and 13. They were all found to be medium‐redshift QSOs, with , 0.727, and 0.852, respectively. The emission lines measured and the derived redshifts are given in Table 1. Figure 1 shows two of the spectra.

TABLE 1 Emission Lines, Redshifts, and ROSAT counts ks⁻¹ of Three X‐Ray QSOs near NGC 5548

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Fig. 1.— Spectra of two QSOs near NGC 5548, obtained with the Lick Observatory Shane 3 m telescope and the Kast double spectrograph in low‐resolution setup.

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The observation that three of the X‐ray–emitting BSOs in Arp’s list are indeed QSOs motivated a literature search for QSOs in the field centered on NGC 5548. In Table 2 we list 20 QSOs around NGC 5548. They come from a variety of sources and are listed by Véron‐Cetty & Véron (2000). One of our newly observed objects (EXO 1415.2+2607 in Table 1) had already been identified as a low‐redshift QSO and was listed in the Véron catalog, but without spectroscopic details. The distribution of the 20 QSOs found in 16 deg² centered on NGC 5548 is shown in Figure 2 and is quite striking, as follows:

TABLE 2 QSOs near NGC 5548

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Fig. 2.— Plot of all QSOs within the area centered on NGC 5548.

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3. DISCUSSION

Is the concentration of QSOs around NGC 5548 real? Is it an accident due to random fluctuations in surface density of QSOs, or possibly because the empty areas in Figure 2 have not been surveyed in as much detail as has been the area close to NGC 5548? It is the distribution of the QSOs that is remarkable, their apparent concentration close to the galaxy and the areas northeast and southwest apparently devoid of QSOs.

Surveys of an area within the area , which contains 13 of the QSOs shown in Figure 2, were made by Crampton, Schade, & Cowley (1985) and Anderson & Margon (1987), using a “grens” (transmission grating–prism–lens combination) on the Canada‐France‐Hawaii Telescope. The FIRST survey (White et al. 2000) covers a wide area ( , ) containing the region around NGC 5548. The surveys of Crampton et al. (1985) and Anderson & Margon (1987), together with the serendipitous discoveries of individual QSOs, have led to the list given in Table 2, which has been taken from the compilation of Véron‐Cetty & Véron (2000) and which includes our new data.

As far as the X‐ray surveys go, the area centered on NGC 5548 and mapped by Arp with the ROSAT Position Sensitive Proportional Counter (PSPC) covers about , or 3.8 deg². The distribution of the large number of detected X‐ray sources is shown in Figure 4 of Arp (1997). The eight strongest X‐ray point sources within are shown in his Figure 20, with the ROSAT counts ks⁻¹ marked for each. Coordinates of the BSOs that are candidate identifications of the ROSAT compact sources are listed, as mentioned, in Table 3 of Arp (1997). We searched Tables 1 and 3 in the Véron catalog in the range , for any additional known QSOs, but Table 2 contains all those cataloged.

Could the concentration around NGC 5548 be due to random fluctuations? On the assumption that the redshifts are cosmological we expect to see surface density fluctuations simply due to the random nature of the distribution. Thus, some clumping of QSOs would be expected and is seen in some surveys (e.g., Boyle et al. 1990). However, such clumpings have only been considered to be real when the QSO redshifts are roughly the same, but here the QSOs have very different redshifts. Since there is practically no correlation of apparent magnitude with redshift for QSOs, this would require, on the cosmological hypothesis, that there are three‐dimensional correlations of QSOs in space with one axis of the volume much greater than the other two. The results of White et al. (2000) are shown in their Figure 1. These QSOs are bright ( on the POSS I E plates) and are low‐flux radio sources. Thus, they are a less general sample than we are discussing here. However, Figure 1 of White et al. (2000) covers the region including NGC 5548, so it is worth noting that it shows essentially a random distribution over their whole area without obvious gaps or clumping.

The possibility that QSOs may be concentrated near an active galaxy follows from the results obtained by Radecke (1997) as well as those of Burbidge (1995, 1997), Chu et al. (1998), and Arp et al. (2001). It has also been shown over many years that there are many comparatively bright nearby spiral galaxies (some of which are active) that have high‐redshift QSOs lying very close (less than 3 ) to their nuclei (Burbidge 1996).

We conclude that, while the numbers are small and the statistical argument cannot be made overwhelmingly, NGC 5548 appears to be another active galaxy with a higher than average density of QSOs immediately around it. This may be evidence that some low‐redshift active galaxies and high‐redshift QSOs are physically associated. We note, however, that the FIRST survey indicates no significant clustering around NGC 5548, and other surveys and catalogs quoted for comparison are subject to incompleteness effects.