NGC 0720 – close QSOs and pair alignments

Arp (2003) discussed the X-ray sources (not shown in Figure 1) near NGC 0720. There was one of the most luminous X-ray cluster known, the RXJ 0152.7-1357, and roughly on the other side of NGC 0720 there was an extended X-ray source. Arp noted that RXJ 0152.7-1357 was elongated towards NGC 0720. He also noted two elongated groups of galaxies near NGC 0720 and aligned across it, alignment line being quite well along the minor axis of NGC 0720. He mentioned that there were a possibly associated galaxy (at z = 0.17) to the other galaxy group. I assume that this galaxy is APMUKS(BJ) B015008.79-140221.9, object 5 in the object table below. He then mentioned that there is a quasar at z = 1.35 further out and in the same direction than RXJ 0152.7-1357. Arp also noted that there is a BSO quite well aligned with RXJ 0152.7-1357 but there were no redshift available for that object.

Arp et al. (2004) studied some ultra-luminous X-ray sources (ULX) spectroscopically. ULX’s are objects that have high X-ray luminosity and are located so that they can be considered to belong to galaxies. Two ULX’s (objects 2 and 3 in Figure 1) were known to be near NGC 0720. Somewhat surprisingly, Arp et al. (2004) found out that the two objects were quasars.

Then came another surprise (well, if it was a surprise). Burbidge et al. (2006) performed a spectroscopic study on the BSO mentioned earlier that was aligned across NGC 0720 with RXJ 0152.7-1357. First, they suggested that the BSO and the extended X-ray source have positions close enough so that they can be considered to be the same object. Then they gave the results of the spectroscopy: they found out that the BSO was a quasar and that it’s redshift was almost the same as the redshift of RXJ 0152.7-1357. They suggested:

The agreement in redshift between the mean value for the double cluster RX J0152.7-1357 and our newly discovered QSO 2XRP/BSO suggests that the QSO belongs to the cluster, and that unlike the many QSOs we have studied in the fields around AGN galaxies, its redshift is of cosmological origin.

They then presented some calculations of the configuration of the cluster, the QSO, and NGC 0720. The probability for the configuration is 2.5 x 10-6, and the alignment deviates from a straight line by 2.2 degrees. The elongation of RXJ 0152.7-1357 points about 20 degress off NGC 0720. They also note that NGC 0720’s X-ray structure is similar to RXJ 0152.7-1357 and points toward RXJ 0152.7-1357. Despite these, they conclude:

However, since the cluster galaxies have quite normal spectra, there is no reason to doubt that this is a normal distant cluster of galaxies. The fact that the QSO has the same redshift suggests that contrary to many other situations, this is a cluster and QSO that delineates a large structure (~10 Mpc) at a cosmological redshift of 0.83, and that NGC 720, despite the low probability, is a chance foreground galaxy.

However, they have added a note to the end saying that Arp (one of the et al. of the paper) doesn’t agree with that conclusion.

Figure 1. The objects with measured redshifts near NGC 0720. Size of the image is 7.5 x 7.5 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue), and has been doubled in size.

Objects and their data

1 NGC 0720 E5 0.005821 11.16 0
2 IXO 02 QSO 0.959000 19.2 2.689
3 IXO 01 QSO 2.216000 20.6 3.382
4 RXJ 0152.7-1357 X-ray cluster 0.83 ~14.2
5 APMUKS(BJ) B015008.79-140221.9 galaxy 0.170000 19.30 7.047
6 QSO 1.35 <30
7 GALEX 2674551073866252974 QSO (the BSO) 0.831200 19.0 (R) 14.466

NED page for object 1.
NED page for object 2.
NED page for object 3.
NED page for object 5.
NED page for object 7.


Arp, 2003, book, ISBN: 0968368999, “Catalogue of discordant redshift associations”

Arp et al., 2004, A&A, 418, 877, “New optical spectra and general discussion on the nature of ULXs”

Burbidge et al., 2006, PASP, 118, 124, “A QSO Discovered at the Redshift of the Extended X-Ray Cluster RX J0152.7-1357”


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