0846+51W1 – A Cosmic Coincidence With Surprises

0846+51W1 (SBS 0846+513, object 2 in Figure 1) is a high redshift quasar that is positioned very close to a low redshift galaxy (object 1) at the end of a chain of five galaxies. Arp et al. (1979) described the system. They first discussed the general properties of 0846+51W1. It is an interesting object because it brightened considerably during a month in 1975. They classified 0846+51W1 as a BL Lacertae object (although they later pointed out some features that suggested it to be a transitional object between quasars and BL Lacertae objects). Then they mentioned the situation with the galaxies:

Figures 1 and 2 also conspicuously show that the eruptive object 0846+51W1 is extremely close to one of a pair of interacting spiral galaxies (12″ south of the southernmost of the pair). Actually, the configuration north of 0846+51W1 is even rarer than an interacting pair. The five brightest galaxies in this general area form a chain stretching north and slightly west with 0846+51W1 being a sixth object aligned closely in this same direction.

Then they presented their research of the light curve of 0846+51W1. Their discussion on the color of 0846+51W1 brings out an interesting detail that might be relevant to the discordant redshift issue of the system:

It is clear that 0846+51W1 is much redder than normal quasars. It is known that BL Lacertae objects tend to be redder than quasars (for a review, see Stein, O’Dell, and Strittmatter 1976). But 0846+51W1 at minimum is redder than most BL Lacertae objects.

Later they returned to the discussion of the color of 0846+51W1:

To what is the redness of 0846+51W1 due? The normal explanation of the redness of BL Lacertae objects involves an underlying galaxy. But at a redshift of z = 1.86 for the object 0846+51W1, any normal galaxy would be much too faint in apparent magnitude to make any contribution to the observed light. So the nature of this high-redshift, BL Lacertae object – Quasar remains a mystery.

Next they discussed the energy curve, radio observations, and the optical spectrum of 0846+51W1. They measured a redshift of z = 1.86 for 0846+51W1, which they noted to be rather high for a BL Lacertae object. From spectral features, they suggested a hypothesis about 0846+51W1:

However, the most attractive among a poor choice of hypotheses may still be that the slit is observing a central blue stellar object embedded in a slightly resolved redder object.

They measured an emission line that didn’t fit to the redshift of z = 1.86. They questioned the reality of the line but also mentioned that if the line were real, it likely had a redshift of z = 0.747. They also noted a presence of a small nebulosity very close to 0846+51W1, and suggested that the nebulosity actually is a normal red galaxy that might be a part of a loose galaxy cluster or associated with 0846+51W1.

They calculated the probability for the chance projection of 0846+51W1 close to the small nebulosity/galaxy, and found out that it is quite possibly a chance projection with that object. They also calculated the chance projection probability for 0846+51W1’s association with the nearby interacting pair of galaxies, and find that the association is quite rare.

0846+51W1 was included in the sample of Arp (1981). Sitko et al. (1984) studied the polarimetry of 0846+51W1 and concluded it to be highly polarized, highly variable quasar.

A gravitational lens?

Nottale (1986) suggested that 0846+51W1 is a gravitational lens:

The aim of this paper is to suggest that all these characteristics may be accounted for in a simple model where most of the variability is not intrinsic to the object, but instead due to the gravitational amplification by a star lying in the intervening galaxy at z = 0.072.

They then continued to show in detail how the different features of 0846+51W1 can be explained by gravitational lens hypothesis. Barnothy & Barnothy (1986) also suggested the gravitational lens hypothesis, although they also invoked a hypothesis of a supernova going off in the object of the lens to explain some of the observed features. Additionally, they noted that 0846+51W1 is positioned so close to the galaxy that the sight line to it passes through the halo of the galaxy. Crampton et al. (1989) published a new image on the system, and also suggested gravitational amplification.

Stickel et al. (1989) presented new imaging of the system and noticed that 0846+51W1 was elongated towards the north (in which direction also the object 1 lies by the way), which they found to be due to an intervening galaxy almost exactly positioned on the line of sight to the 0846+51W1. Their images show that the luminous areas of 0846+51W1 and object 1 apparently connect, but there’s no evidence of any bridge-like feature, just two objects overlapping slightly. From the images, they suggested:

Due to the high redshift (z = 1.86), this northern elongation is not attributable to the BL Lac host galaxy, but more likely to an intervening galaxy with considerably smaller redshift lying nearly on the line of sight.

They even utilized a decomposition technique to show each object separately in the images. That way they were also able to show the small nebulosity/galaxy Arp et al. (1979) had noted. Stickel et al. (1989) noted that it also lies on the line of sight to 0846+51W1, and they called that galaxy a “southern companion galaxy”, because they assumed that it is a companion to the intervening galaxy (and hence should have about the same redshift). They also took a new spectrum of 0846+51W1 confirming the redshift of z = 1.86. They also determined the redshift of the intervening galaxy (z = 0.235), but that was done based on energy distribution of standard elliptical galaxy, not from spectral lines.

Additionally, they noted that their spectral measurements recorded an emission line from a galaxy they named “G1” (object 9 in figure 1). They gave most likely redshift for it as z = 0.249. From this they went on to suggest that as there are lot of apparently similar objects nearby, there might be a group or cluster at z ~ 0.24.

New spectral information

Zhou et al. (2005) discussed the SDSS spectrum of 0846+51W1:

However, the narrow wavelength coverage of Arp et al. and Stickel et al. led 0846+51W1 be taken as a high redshift BL Lacs (z=1.86) by these authors whereas the SDSS spectrum clearly shows that its true redshift is z = 0.5835.

They note that they see no signs of any intervening galaxies in the SDSS spectrum, but they suggest that the spectrum is a composite of three basic features:

Its optical spectrum can be well decomposed into three components, a power law component from the relativistic jet, a stellar component from the host galaxy, and a component from a typical NLS1 nucleus.

Here NLS1 stands for narrow line Seyfert 1. They suggest further that:

In 0846+51W1 and SDSS J0948+0022 we are very likely observing the innermost part of the jet pointing toward us and the fact that all of the radio sources in NLS1s are compact can be understood according to this interpretation.

Zhou et al. (2005) practically turned the situation upside down; redshift was all wrong and there’s no intervening objects to support the gravitational amplification hypothesis. However, as the redshift of 0846+51W1 got much smaller, there’s less need for amplification to explain the object. Unfortunately, Zhou et al. didn’t address this issue explicitly. So, we still have a high redshift object very close to a low redshift galaxy, and the high redshift object is elongated towards the low redshift object, and even a visible “connection” is a possibility (although there’s no signs of any connections in Digitized Sky Survey images). There’s also the line of objects 1, 2, 3, 4 mentioned by Arp et al. (1979) (and the two additional ones they mentioned which are quite far outside the field shown in Fig. 1), and there’s also additional object that falls to this line (object 6 in Fig. 1).

0846_51W1
Figure 1. The objects with measured redshifts near SDSS J084957.48+510842.3. Size of the image is 7 x 7 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Blue).

Objects and their data

NBR NAME TYPE REDSHIFT (cz) MAG SEPARATION
1 SDSS J084957.48+510842.3 galaxy 0.073485 (22030 km/s) 16.8 (g) 0
2 SBS 0846+513 QSO FSRQ 0.583715 18.8 (g) 0.235
3 SDSS J084956.70+510927.1 galaxy 0.073379 (21998 km/s) 17.4 (g) 0.757
4 SDSS J084953.16+511151.6 galaxy 0.056646 (16982) 17.3 (g) 3.227
5 SDSS J084949.11+510538.0 galaxy 0.044816 (13436 km/s) 16.4 (g) 3.341
6 SDSS J085003.35+510524.8 galaxy 0.267347 19.5 (g) 3.418
7 small nebulosity/galaxy galaxy 19.6 ~0.04 from obj. 2
8 intervening galaxy galaxy 0.235 19.2 ~0.005 from obj. 2
9 G1 galaxy 0.249 ~ 1

Objects in NED within 10 arcmin from SDSS J084957.48+510842.3

References

Arp et al., 1979, ApJ, 230, 68, “An eruptive BL Lacertae object with a high redshift, 0846 + 51 W1”

Arp, 1981, ApJ, 250, 31, “Quasars near companion galaxies”

Barnothy & Barnothy, 1986, AJ, 91, 755, “Morphological aberration in gravitational-lens images”

Crampton et al., 1989, AJ, 98, 1188, “A search for closely spaced gravitational lenses”

Nottale, 1986, A&A, 157, 383, “The eruptive BL Lac object 0846 + 51W1 – A gravitationally lensed QSO?”

Sitko et al., 1984, PASP, 96, 402, “Optical and radio polarimetry of the quasar 0846 + 513”

Stickel et al., 1989, A&A, 224, 27, “The gravitational lens hypothesis for 0846 + 51W1 supported by new observations”

Zhou et al., 2005, ChJAA, 5, 41, “The Hybrid Nature of 0846+51W1: a BL Lac Object with a Narrow Line Seyfert 1 Nucleus”

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