Saturday, July 17, 2010

Quasars in the very early universe

Quasars are powered by the gravitational (potential) energy of their central supermassive black holes. However, their distinctive features – their extremely high luminosity in particular – are very dependent on characteristics of matter close to the black hole.

Most supermassive black holes (SMBH), including those at the centers of the Milky Way and our close neighbor M31 (Andromeda), are responsible for fairly small amounts of radiation in any part of the electromagnetic spectrum. This is generally because the radiation of a quasar is produced mainly by the infall of matter during a relatively brief period of the object's life – a few percent of the total, i. e. a few hundred million years. Once the nearby matter is used up, the lights go out.

(For earlier articles on quasars, see here.)

In a quasar, where matter in significant quantities is still being accreted, most of the radiation originates in a central "accretion disk" close to the black hole. The radiation is thermal ("black body") produced by very hot gas consisting mostly of hydrogen and helium. This radiation covers the spectrum from infrared to X-rays. Since quasars are so bright, they can be seen individually at high redshifts – z≥6, which is not true of ordinary galaxies. That corresponds to times within a billion years of the big bang. At z≈6 photon wavelengths are stretched by a factor of 7, so what we actually see is not the rest-frame spectrum, but a considerably red-shifted version of it.

If rs is the Schwarzschild radius, then the accretion disk extends from a radius of about 3 times rs outward to a few hundred times rs. To give a sense of the scale, a largish SMBH has a mass of a billion solar masses. So for such an object rs is about the radius of the orbit of Uranus, by a simple calculation given here.

Quasars and active galaxies (i. e. just smaller versions of the same thing) have been intensively studied for several decades. In that time, a fairly clear picture has emerged of how matter is distributed further out from the accretion disk. The most prominent feature of this region is a thick (compared to the accretion disk) torus-shaped ring of cooler gas and dust. The "dust" consists of very small particles composed of various elements heavier than helium. Electromagnetic radiation from the torus is mostly in the infrared part of the spectrum (rest frame), and is produced by re-emission (at lower energies) of higher energy photons from the accretion disk.

The shape of this region is not strictly a torus, since it's considerably flattened, especially at higher distances from the center, but it's referred to as a torus for simplicity. The outer limits of quasar tori are hard to determine, but probably extend hundreds of light years from the center. However, the inner parts are thick enough that unless we are seeing the quasar almost face-on (i. e., along the symmetry axis of the accretion disk and torus), we cannot clearly see the accretion disk itself, because it's obscured by the dust in the torus.

One interesting thing about quasars is that as far as we can tell (until quite recently), their characteristics are very similar no matter how distant they are. Although the present-day universe is quite different in many respects from what it was a billion years after the big bang, quasars seem hardly different at all.

Research published just this year is starting to change this story:

Dust-free quasars in the early Universe
The most distant quasars known, at redshifts z ≈ 6, generally have properties indistinguishable from those of lower-redshift quasars in the rest-frame ultraviolet/optical and X-ray bands. This puzzling result suggests that these distant quasars are evolved objects even though the Universe was only seven per cent of its current age at these redshifts. Recently one z ≈ 6 quasar was shown not to have any detectable emission from hot dust, but it was unclear whether that indicated different hot-dust properties at high redshift or if it is simply an outlier. Here we report the discovery of a second quasar without hot-dust emission in a sample of 21 z ≈ 6 quasars. Such apparently hot-dust-free quasars have no counterparts at low redshift. Moreover, we demonstrate that the hot-dust abundance in the 21 quasars builds up in tandem with the growth of the central black hole, whereas at low redshift it is almost independent of the black hole mass. Thus z ≈ 6 quasars are indeed at an early evolutionary stage, with rapid mass accretion and dust formation. The two hot-dust-free quasars are likely to be first-generation quasars born in dust-free environments and are too young to have formed a detectable amount of hot dust around them.

Some things in these results are actually more interesting than that a few very early quasars are different from all other quasars. In the very early universe at z>6, less intergalactic dust is to be expected. This is because the dust – which is composed of elements heavier than helium – is (by conventional accounts) produced mostly in stars. It is expelled from stars only either gradually, as the star evolves, or suddenly in the rare case of supernova explosions.

We still don't know very precisely when the very first stars formed (see here), but that probably happened only a few hundred million years after the big bang. Since the very first stars must have consisted almost entirely of hydrogen and helium, star-formation models indicate they should have been much more massive than typical later stars, and they should have expoded as supernovae after only a few tens of millions of years. As heavier elements gradually accumulated in the universe, stars of a more modern sort, initially containing small amounts of heavier elements, began to form. These later stars were small enough so that most never ended as supernovae, but they continued to manufacture and expel heavier elements – and hence dust.

Observations of very early quasars therefore tell us a little about the pace of this process. We now know, for example, of at least two quasars that formed so early that they do not have any dust around them that we can observe. The evidence for this is that emissions of these two quasars at rest-frame wavelengths from ultraviolet to very near infrared appear normal. Dust, however, should also produce rest-frame emissions in farther infrared – and that's not seen in these two examples, although it is in all other of the sampled z≈6 quasars.

One other feature of z≈6 quasars is particularly interesting. The mass of a quasar can be estimated from total luminosity and certain spectral features. In all z≈6 quasars that do have evidence of dust, the amount of dust is roughly proportional to the SMBH mass. However, in low-redshift quasars, dust abundance is almost uncorrelated with mass. The implication, then, is that in their youngest stages, but not later, quasars accumulate dust at about the same rate as SMBH mass. And indeed, the two quasars without apparent dust also have the smallest SMBH mass of any in the sample, about 2 to 3×108 M.

This raises another interesting, unanswered question. Just where does this dust come from? It's generally thought that in the present universe most of the existing dust originated from ordinary stars. However, at z≈6 most stars would be less than 500 million years old, and it's not at all clear this would have allowed enough time for the production of sufficient dust. Supernovae are another possibility, but even in the early universe we don't know whether they would have been common enough.

Further, the existence of elements heavier than helium is necessary but not sufficient to produce dust. Most models assume also a proper combination of relatively low temperature (<2000 K) and high density is also required for dust to form. But a theoretical study (astro-ph/0202002) suggests that dust could actually form in the vicinity of a quasar itself. Some combination of all these possibilities may be the answer, but a lot more research will probably be needed to clear this up.

Every time we learn something new, it seems, we also find new questions.
Jiang, L., Fan, X., Brandt, W., Carilli, C., Egami, E., Hines, D., Kurk, J., Richards, G., Shen, Y., Strauss, M., Vestergaard, M., & Walter, F. (2010). Dust-free quasars in the early Universe Nature, 464 (7287), 380-383 DOI: 10.1038/nature08877

Further reading:

NASA's Spitzer Unearths Primitive Black Holes (3/17/10)

Quasar Dust in the Early Universe (3/26/10)

Primordial Black Holes Formed Just After Big Bang (3/17/10)

First generation of quasars (3/17/10)

Related articles:

Where the action is in black hole jets (5/12/10)

Active galaxies and supermassive black hole jets (4/25/10)

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