SDSSJ092712.65+294344.0 – A high redshift copy of NGC 1275

Komossa et al. (2008) found two sets of emission lines from the spectrum of SDSSJ092712.65+294344.0. They described the object generally:

SDSS J0927+2943 at redshift z = 0.713 is a luminous quasar, observed in the course of the SDSS (Adelman-Mc-Carthy et al. 2007), and was found by us in a systematic search for active galactic nuclei (AGNs) with high [O iii] velocity shifts.

Then they described the spectrum:

Two systems of strong emission lines can be identified in the spectrum, which we refer to as the “red” (r) and “blue” (b) systems. … All lines of the blue system are blueshifted by about 2650 [km/s] relative to the [red system].

(I will use their names (“red system” and “blue system”) for the two emission line systems.)

They also noted:

The difference in velocity of the two sets of emission lines exceeds the peculiar velocities observed in galaxy clusters, and is too large for the two galaxies to form a bound merger. … Consequently, we would then have a very unlikely projection effect, including not just one, but two intrinsically extremely unusual AGNs…

They then suggested a hypothesis that in the past there has been a merger, where central SMBH would have recoiled ejecting the blue system from the core at the velocity of 2650 km/s.

Not a recoil but a massive black hole binary?

Dotti et al. (2008) considered the system as binary black hole system. They first mentioned some shortcomings of the recoil hypothesis of Komossa et al. (2009). Dotti et al. note that in the blue system the redshifts of both narrow- and broadband emission lines are the same, and that it is not possible that emitted gas would emit narrow emission lines. However, they also say that the narrow emission lines in the blue system are not such that are typically found in active galaxy nuclei. They also make some arguments relating to the probabilities in the system if it would be a recoil system. They then describe their hypothesis:

In our model SDSSJ0927 hosts a MBHB surrounded by a circum-binary thin disc feeding the secondary MBH. In the tidal interaction of the binary with the gas, a gap open, i.e. a low density region surrounding the binary.

They explain the blueshifted emission lines (the ones with lower redshift) followingly, first the broadband lines:

…while the b–BELs are produced in the broad line region bound to M2, and can be blue–shifted or red–shifted depending on the orbital phase of the secondary.

“Secondary” here refers to one of the hypothesized black holes. The explanation for the blueshifted narrowband lines:

The b–NELs come from the region of the gap near M2, where forbidden lines can be emitted because of the low density of the gas. SinceM2 and the gas orbiting in the gap at comparable annuli are subject to the same gravitational potential of M1, so that they move with approximately the same Keplerian velocity. Accordingly, the same blue–shift for the b–BELs and b–NELs can be explained if M2 emits an anisotropic ionizing radiation normal to the plane of the discs so that the ionizing photons interact only with the gas in the gap near to M2. In this case, the assumption of an accretion disc around M1 can be relaxed, and no red–shifted NELs are produced even if M1 is able to accrete on a short time–scale all its gas reservoir

But I have to admit that the explanation goes over my head, but I think the basic idea here is that the blueshifts can arise without the black holes having to have huge velocities themselves. They then go on to provide technical details of the system, when considered as a binary black hole system. They conclude:

SDSSJ0927 is clearly exceptional. Either it is the first example of a MBH ejected from its nucleus due to the gravitational recoil, or it harbours one of the smallest separation MBHBs ever detected.

Bogdanovic et al. (2009) suggested the same thing as Dotti et al. in a very similar paper. These two works were probably done in parallel to each other.

Another hypothesis

Heckman et al. (2009) provided some criticism for the binary black hole model:

In this model, the broad lines are made in more or less conventional fashion, even though the second, more massive black hole, is close enough that its gravity must influence the broad line gas—after all, its gravity is strong enough to force an orbital speed for the quasar black hole ≥ 2650 km s−1. The associated narrow lines are attributed to gas flowing between the circumbinary disk and the lower-mass black hole, and therefore traverse a region where, by construction, the characteristic orbital speed is & 2000 km s−1. It is hard to understand how, under these circumstances, the narrow lines could be centered on the velocity of the broad lines and have FWHMs as small as 450 km s−1.

They suggested another possibility for this system:

…that this system is a high-redshift analog to the familiar nearby Seyfert galaxy NGC 1275.

NGC 1275 is similar system that has two emission line systems separated by 3000 km/s, and it is currently thought to be an interacting galaxy pair within a galaxy cluster. The location within a cluster is important, because it makes the suggestion of high peculiar velocities involved more plausible. Heckman et al. (2009) pointed out that the NGC 1275 system and this new SDSSJ0927 system are almost exactly alike. So, in this system we would have a quasar at the redshift of the blue system, and in front of it there would be an infalling galaxy at the redshift of the red system. The higher redshift of the red system would be then caused simply by the velocity of the infall. They went on to make some arguments from the properties of the spectrum of the system, that they are fitting to this view, and they concluded:

Thus, in all respects that we can measure from the existing data, the properties of the redshifted narrow emission-line system are fully compatible with our hypothesis that SDSSJ0927 is a higher redshift version of the NGC 1275 system: a galaxy falling into the deep potential well of a rich cluster of galaxies where it interacts with the host galaxy of an AGN.

But what about the implied galaxy cluster, is it there? They made an initial analysis based on SDSS images of the field, and the photometric redshifts (which are very inaccurate) of the objects there. They found some galaxies that might suggest a presence of a massive cluster, but it is very uncertain. Another outstanding issue is to pinpoint the objects where the two sets of emission lines are produced. They approached this problem by looking at the closeup SDSS images, and found potential candidate for the red system in addition to blue system being the quasar itself. There is an apparent companion galaxy NE from the quasar that might overlap with quasar image. They suggested that further spectroscopy might be able to solve the question.

Shields et al. (2009) also suggested the same thing as Heckman et al. (2009). More from Shields et al. below.

Decarli et al. (2009) performed a photometric study in order to look for the galaxy cluster. They note about the situation:

Such a high velocity difference between the two galaxies is inconsistent with a simple on–going merger, and requires the deep potential well of a rich galaxy cluster.

They then proceeded to do a follow-up study for the Heckman et al. (2009) initial galaxy cluster search. Decarli et al. (2009) took new images of the system and base their study on those. They studied each object within a few square-arcmin field to see which ones are such that they could be a part of z = 0.7 galaxy cluster. They found 11 candidates. They compared the situation by virtually moving couple of known nearby galaxy clusters to the higher redshift (note that there are some corrections involved with this, see their paper for details), and they found that there should be at least 50 candidate objects expected to be detected within their search field in these imaginary cases. So, the simple coonclusion is that there just is not enough candidate objects within this field for massive enough galaxy cluster to be there.

Third emission line system

Shields et al. (2009) obtained a new spectrum of the system, which confirmed the basic properties known so far. In addition to that, they found a new emission line system at intermediate redshift compared to the two previous ones. They note:

The i-system offers an alternative candidate for the host galaxy of the QSO, or it may represent a close companion. In either case, the presence of a galaxy at a velocity close to the broad-line velocity lends credence to the possibility that the r-system may not represent the host galaxy and that the broad line system may be an ordinary QSO. The i-system could also represent gas ejected from the QSO.

They emphasized that finding absorption lines from the quasar host galaxy would be important. They then went on to make similar arguments of the system as Heckman et al. (2009) did, about the possibility of galaxy cluster from SDSS images and photometric redshifts. See also their discussion about other previously suggested explanations for the system (which were discussed above).

UPDATED (September 2, 2009): A new study by Vivek et al.

Vivek et al. (2009) published a new spectra and compared it to a four years earlier taken SDSS spectra. They didn’t find any detectable acceleration between the two spectra, which seems to make the binary black hole hypothesis quite unlikely. They found out that the red system is an extended region of size ~8 kpc, and that would seem to rule out the binary black hole hypothesis. They were also able to determine that the red system is in slightly different position than the blue system, and they concluded that the red system is separated from the quasar by 1 kpc. They couldn’t rule out the recoiling black hole hypothesis or the high redshift analog of NGC 1275 hypothesis. They also confirmed the existence of the third emission line set.

sdss0927
Figure 1. The objects with measured redshifts near SDSSJ092712.65+294344.0. Size of the image is 7 x 7 arcmin. Image is from Digitized Sky Survey (POSS2/UKSTU Red).

Some notes

There’s not much more to say of this system. Nearby objects within the field, shown in Fig. 1, show a line alignment containing objects 1, 4, and 6. Coincidentally, redshift of object 4 is quite accurately twice the redshift of object 6 (z6 / z4 = 1.96), redshift of object 1 is then 2.5 times the redshift of 4. Curious numerical coincidence, which doesn’t mean anything without decent statistical analysis of the situation. Another thing worth mentioning is that the object 7 seems to be very large to be such a high redshift object (see SDSS images), so large that I’m wondering if it’s a mistake in position or somewhere else. [UPDATE (November 23, 2009): My astronomer friend took a peek at the object 7 issue I mentioned and he showed me that the size of the object is not that special, there’s plenty of objects with similar or bigger size at redshift z ~ 0.1 (thank you, John, for the help).]

UPDATED: Here is the link to my website’s entry on NGC 1275

Objects and their data

NBR NAME TYPE REDSHIFT MAG SEPARATION
1 SDSSJ092712.65+294344.0 “blue system” QSO em. line system 0.69798 18.4 (g) 0
2 SDSSJ092712.65+294344.0 “red system” QSO em. line system 0.71279 0
3 SDSSJ092712.65+294344.0 “intermediate system” QSO em. line system 0.7028 0
4 SDSS J092704.52+294401.6 galaxy 0.273648 20.4 (g) 1.788
5 SDSS J092706.01+294503.9 Sc 0.025972 16.5 (g) 1.962
6 SDSS J092656.35+294415.2 galaxy 0.139845 18.5 (g) 3.576
7 SDSS J092728.41+294641.1 Sb 0.103412 17.2 (g) 4.519

NED descriptions for the objects: objects 1-3, object 4, object 5, object 6, object 7.

SDSS object descriptions: objects 1-3, object 4, object 5, object 6, object 7.

SDSS image of the system

References

Bogdanovic et al., 2009, ApJ, 697, 288, “SDSS J092712.65+294344.0: Recoiling Black Hole or a Subparsec Binary Candidate?”

Decarli et al., 2009, arXiv, 0904.2999, “A photometric study of the field around the candidate recoiling/binary black hole SDSS J092712.65+294344.0”

Dotti et al., 2008, arXiv, 0809.3446, “SDSSJ092712.65+294344.0: a candidate massive black hole binary”

Heckman et al., 2009, ApJ, 695, 363, “SDSSJ092712.65+294344.0: NGC 1275 at z = 0.7?”

Komossa et al., 2008, ApJ, 678, 81, “A Recoiling Supermassive Black Hole in the Quasar SDSS J092712.65+294344.0?”

Shields et al., 2009, ApJ, 696, 1367, “Comment on the Black Hole Recoil Candidate Quasar SDSS J092712.65+294344.0”

Vivek et al., 2009, arXiv, 0909.0018, “SDSS J092712.64+294344.0: recoiling black hole or merging galaxies?”

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2 Responses

  1. I added a brief description of the new Vivek et al. paper that was just inserted to arXiv, see above.

  2. My astronomer friend showed me that the object 7 issue is not an issue, I added a note on this, see above.

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