SDSS J153636.22+044127.0 – Another high redshift NGC 1275 clone

Boroson & Lauer (2009) report about another high redshift (z ~ 0.4) system that resembles the situation in NGC 1275. Recently, I wrote about SDSSJ092712.65+294344.0, which is very similar to this new one. Both have two emission line sets at different redshifts. SDSS J153636.22+044127.0 has redshift difference of about 3500 km/s. There is one big difference between these two systems, however, the SDSSJ092712.65+294344.0 has third emission line set at intermediate redshift, but SDSS J153636.22+044127.0 has an absorption line set at intermediate redshift.

Boroson & Lauer (2009) first point out how extremely exceptional systems these two are:

Of the 17,500 objects in the entire sample, only two objects have multiple redshift systems, SDSS J092712.65+294344.0, described above, and SDSS J153636.22+044127.0 (J1536+0441).

They then go through the basic properties of the system (magnitudes etc.), and they describe the emission line systems:

The spectrum of J1536+0441 shows two broad-line emission systems and one system of narrow absorption lines. The higher redshift “r-system” at z = 0.3889 shows broad Balmer lines (Ha, Hß, and H?) and the usual narrow lines seen in low redshift quasars. The lower redshift “b-system” at z = 0.3727, shows broad Balmer lines (Ha 2 through Hd) and broad Fe II emission, seen most strongly around 3000 Å in the rest frame. A strong narrow absorption-line “a-system” is also present, including 6 unresolved resonance lines, at z =0.38783, which, in the quasar rest frame, is 240 km s-1 less than that of the r-system and 3300 km s-1 greater than that of the b-system.

The “red system” has spectral features like a low redshift quasar typically has, but the “blue system” is nothing like a quasar. “Absorption system” is also not typical for a low redshift quasar. They suggest that one possibility to explain the absorption system is that it is an unrelated gas cloud in front of the object.

They analyse the system as a binary black hole candidate, and calculate system parameters based on that. They also suggest as alternative hypothesis, that the two emission line systems could arise as a chance projection of two quasars. They find a probability of 0.0032 for the chance projection, but they also note that the blue system spectral features argue against chance projection hypothesis. They conclude:

The most obvious interpretation of the presence of absorption at a redshift close to the r-system is that the b-system is background to the r-system. This would be inconsistent with an explanation involving ejection of one of the black holes, though it would be consistent with an infall interpretation, analogous with NGC 1275. In this case, however, the two broad-line systems must represent two active nuclei.

Gaskell (2009) presents an argument against the binary black hole hypothesis:

I argue here, however, that just as the Balmer line profiles in previous SMB candidates have been shown to be due to disc emission, so too the Balmer line profile of J1536+0441 is probably due to disc emission.

Gaskell then compares the spectra of SDSS J153636.22+044127.0 and ARP 102B which is an example of the disk emitter, and finds that they are similar.

Chornock et al. (2009) confirmed the existence of the two emission line sets, but they also suggest a presence of a third emission redshift that is blueshifted by 3800 km/s compared to the red system. They also perform a test to the binary black hole hypothesis. They argue that the system should show measurable redshift evolution if it would be a binary black hole. They compare the two spectra now available of the system, and don’t find evidence for the expected redshift evolution. They also suggest that the system is disk emitter.

Wrobel & Laor (2009) found from VLA images that there is two radio sources separated by 0.97″ at the position of SDSS J153636.22+044127.0. They propose two alternatives:

These could be two related radio sources, both energized by the candidate 0.1-pc binary system. Alternatively, VLA-A and VLA-B could be independent radio sources originating from a binary quasar system, with a projected separation of 5.1 kpc.

They find some similar features from this system than the features of known systems matching the first alternative. For the binary quasar alternative they note:

Boroson & Lauer (2009) noted that the probability for such a random projection in their sample is 0.0032. Therefore, the two quasars are most likely not due to a random projection, but are likely physically related, i.e. a binary quasar system.

They discuss further that it is likely to have one binary quasar in Boroson & Lauer (2009) sample. They also say that if this is a binary quasar, then it is most likely within two strongly interacting galaxies. However:

The velocity separation of 3500 km s-1 (Boroson & Lauer 2009) is rather large, but not implausible in a cluster of galaxies. About half the clusters studied by Carlberg et al. (1996) show such an extent of velocities. The maximum velocity differences in the quasar binaries studied by Hennawi et al. (2006) is 1870 km s-1, but that study imposed a cap of 2000 km s-1 on the binary velocity separation. At a projected relative velocity of 3500 km s-1, two galaxies cannot form a bound system, so the term binary quasar here does not refer to a physically bound binary system.

Decarli et al. (2009) also found two separate objects in the system from VLT-images. Here is a link to their image. Based on that, they argue that the system is a quasar pair.

In a new study based on HST images of the system, Lauer & Boroson (2009) mention another possibility for this system:

A more intriguing possibility is that the b-system is a black hole ejected from the nucleus of a galaxy now just hosting the r-system; however, the large implied ejection velocity and the light source for the absorption system are problematic.

However, they say:

These new observations appear to rule out the “chance superposition” or “ejected black hole” hypotheses, but leave the choice between the “double-peaked” or “binary black hole” hypotheses unresolved.

From the HST images, they find an optical counterpart for the second radio source that Wrobel & Laor (2009) found. Lauer & Boroson don’t find any signs of interaction between the two objects. From the available spectra (they took a new one) they determine that the different emission line systems are not separated spatially, so they must come from the same object. This means that the newly found companion doesn’t contribute to the different redshifts in the system. This also means that the chance projection hypothesis has less evidence; when there was another candidate object for different redshift, the possibility of chance projection seemed better, but now we are again back to the situation where we have apparently one object emitting different redshifts.

They don’t favor the ejected black hole hypothesis because velocity differences are so high. They seem to lean toward the double-peaked disk emitter hypothesis also. At least there seems to be little evidence against that hypothesis. They also mention that the binary black hole hypothesis does not gain any new evidence from their observations, but it can’t be ruled out either. As a binary black hole, the system is so close together that there are expected changes in the two emission velocities to be seen. That we should know within few years.

In a follow-up to their previous paper, Chornock et al. (2009b) report a new optical Keck spectrum. They confirm the third emission line system which they reported in their first paper. They still don’t find the redshift evolution expected in a binary black hole hypothesis, and now the situation is quite clearly such that the evolution should be measurable. They also show, like Lauer & Boroson, that the two main emission line sets are not separated spatially so the double-quasar hypothesis seems to be ruled out. They suggest that the system is double peaked disk emitter.

UPDATE (August 6, 2009):
Decarli et al. (2009b) published new imaging of the system. They verify that the system has two optical components by perfoming an decomposite analysis on the system, see their Figure 1. They checked the neighborhood of the system to see if there is a galaxy cluster that would help to explain the large velocity difference implied by the redshift difference between the emission sets, and the closeness of the two objects. They said:

We find a significant excess of the galaxy density within < 200 kpc from SDSS J1536+0441 with respect to average field. Thus the quasar is found in a moderately rich cluster of galaxies.

However, they only estimated amount of galaxies photometrically consistent to a z = 0.4 galaxy, so the estimate is very uncertain. At any case, the presence of the companion helps to explain the absorption in the spectrum. In the end, they also can’t make a difference between the binary black hole hypothesis and the double peaked emitter hypothesis.

UPDATE (September 3, 2009):
There is a new paper by Tang & Grindlay (2009) in arXiv. They have reanalysed the existing data of the system by fitting double peaked disk emitter model to the spectrum of the system. They found that even though the model doesn’t fully explain the spectrum, it still provides evidence that there is some disk emission in the system. They end up proposing that the system is both a binary black hole and a double peaked emitter. They provide several arguments to support that proposal. First, as mentioned above, the spectral fit with disk emission model suggests that there’s some disk emission involved. Second, any model with a single explanation doesn’t seem to provide adequate explanation. Third, they say:

The existence of a minor black hole is physically more natural than an extremely asymmetric accretion disk. In fact, we do expect SMBH binaries as natural consequences of galaxy mergers (Begelman et al. 1980; Civano et al. 2009; Comerford et al. 2009).

Fourth, the fact that double peaked disk emitters are rare objects can be neatly explained by a presence of a another black hole which helps produce disk emission by disturbing the accretion disk of the central black hole. They also discussed the question why other double peaked disk emitters don’t show evidence of binary black hole and possible answers they came up is that the other black hole might be so small that the central black hole is too dominant for the other to show up in observations and that long-term observations are generally needed (but not available currently) to detect binary black holes anyway. I think their explanation of the system is quite sensible.

Figure 1. The SDSS J153636.22+044127.0 field. Size of the image is 7 x 7 arcmin. There are no objects with measured redshifts within this field, except for SDSS J153636.22+044127.0 which is in the center of the image. Image is from Digitized Sky Survey (POSS2/UKSTU Red).

Objects and their data

1 SDSS J153636.22+044127.0 “blue system” QSO em. line system 0.3727 17.24 (g) 0
2 SDSS J153636.22+044127.0 “red system” QSO em. line system 0.3889 0
3 SDSS J153636.22+044127.0 “absorption system” QSO abs. line system 0.38783 0
4 SDSS J153636.22+044127.0 “even bluer system” QSO em. line system cz of red system – 3800 km/s 0

Boroson & Lauer, 2009, Nature, 458, 53, “A candidate sub-parsec supermassive binary black hole system”

Chornock et al., 2009, ATel, 1955, 1, “SDSS J1536+0441: An Extreme “Double-peaked Emitter,” Not a Binary Black Hole”

Chornock et al., 2009b, arXiv, 0906.0849, “The Quasar SDSS J1536+0441: An Unusual Double-Peaked Emitter”

Decarli et al., 2009, aTel, 2061, 1, “SDSS J1536+0441: A quasar pair, not a binary black hole”

Decarli et al., 2009b, arXiv, 0907.5414, “Probing the nature of the massive black hole binary candidate SDSS J1536+0441”

Gaskell, 2009, arXiv, 0903.4447, “J1536+0441 and the lack of evidence for close supermassive binary black holes”

Lauer & Boroson, 2009, arXiv, 0906.0020, “HST Images and KPNO Spectroscopy of the Binary Black Hole Candidate SDSS J153636.22+044127.0”

Tang & Grindlay, 2009, arXiv, 0909.0258, “The Quasar SDSS J153636.22+044127.0: A Double-Peaked Emitter in a Candidate Binary Black-Hole System”

Wrobel & Laor, 2009, ApJ, 699, 22, “Discovery of Radio Emission from the Quasar SDSS J1536+0441, a Candidate Binary Black-Hole System”

2 Responses

  1. There was a new paper on this system from Decarli et al., so I have added a brief discussion about it above, titled “Update (August 6, 2009)”.

  2. I added a brief discussion of a new paper on the system by Tang & Grindlay (2009), see above.

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