Discovery of a dark matter halo around a distant quasar

Summary

A team of Danish astronomers[1] has detected the trace of a dark matter halo around a distant quasar. A dark matter halo is a spherical accumulation of dark matter thought to encompass an entire galaxy. The halo was discovered by studying gas which was attracted by the enormous gravity of the halo. The gas - primarily hydrogen - is pulled to the centre of the halo, where a bright quasar ionizes the gas and makes it shine in the Lyman-alpha line of hydrogen. The resulting Lyman-alpha light is seen as an asymmetric "fuzz", extending over a distance covering the size of our own Milky Way. The dark matter halo has an estimated mass of several million-million solar masses.

Which came first?

Galaxies are made af gas and stars, in various proportions. They harbour in their central part a giant black hole, which acts as a vacuum cleaner, gobbling up the gas and dust of its host. In most active galaxies or quasars, the black hole is so massive - up to a thousand million solar masses - that it swallows gas from it surroundings at a high rate, making it a powerful source of radiation, outshining the entire host galaxy. But this is only the visible part of the iceberg. Years of observations have already provided ample evidence that galaxies and quasars are indeed surrounded by gigantic halos of dark matter, which are tens of times more massive than the visible part of the galaxy. The true nature of dark matter is yet entirely unclear.

What is also entirely unclear is how precisely the galaxy forms: did the stars and gas come first in a galaxy or was it the black hole? New observations shed new light on this problem of great cosmological interest.

Ice-cream cone

Numerical models predict that dark matter halos and quasars go together. Luminous quasars (i.e. massive black holes) are believed to be hosted by the most massive dark matter halos (more than a thousand times more massive than the central black hole).

The black hole at the centre of the quasar is surrounded by a thick ring of dust and gas. Therefore all the light from the quasar is emitted in a cone, pretty much similar in shape to the ones used to hold ice-cream (see the insert in Fig. 1). The light from the quasar is so energetic that it ionizes everything in its way, so the cone is suitably called the "ionization cone".

If intergalactic gas is being pulled towards the massive dark matter halo hosting quasars, then it should be observable as Lyman-alpha emission around quasars. This Lyman-alpha emission is thought to arise when infalling hydrogen gas enters the ionization cone of the quasar.

Very deep image in Lyman-alpha

The Danish astronomers have obtained very deep images of the quasar Q1205-30 in the constellation Hydra (the Water Serpent). In order to see the very faint glow, the image was exposed for a total of 18 hours. The quasar is so distant that the light has been travelling for 85% of the lifetime of the Universe. Thus we now observe the quasar as it appeared 11,500 million years ago, hence about 2,000 million years after the Big Bang.

The images were obtained through a special optical filter that only allows light in a narrow spectral waveband to pass. The astronomers chose this wavelength to coincide with that of the Lyman-alpha emission line from the quasar. That makes it possible to study the Lyman-alpha light from the quasar and the surrounding objects.

The astronomers were then agreeably surprised: surrounding the quasar, an asymmetric extended emission was clearly visible, extending from the quasar out into the intergalactic medium (Fig. 2). This faint envelope reaches as far out as 100,000 light years, i.e about the size of our own Milky Way!

To further investigate the nature of this extended Lyman-alpha emission, the astronomers obtained a unique spectrum of this region (Fig. 3).

The spectrum allowed the astronomers to map the motion of the gas. And the result was spectacular! The emitting hydrogen gas was indeed found to be falling towards the quasar.

This conclusion was strengthened by comparing the observations with the results of a model calculation of hydrogen gas falling into a dark matter halo with a quasar at the centre (see right-hand side of Fig. 2). The gas falling into the quasar ionization cone shines, so we are essentially seeing a dark matter halo "illuminated" by the hydrogen gas. (It isn't the dark matter that we see, that is impossible, naturally. We see the hydrogen gas, which is influenced by the presence of the dark matter. In that way it is possible to measure properties of the dark matter through the hydrogen gas).

Despite the fully formed appearance of its bright central quasar black hole, the galaxy imaged by the astronomers is likely to be in its infancy.

The observed infall velocity (Fig. 4) was used by the astronomers to estimate the mass of the dark matter halo to several million-million solar masses! This fits very well into the picture that quasars live in the most massive dark matter halos in the universe.

Despite this new discovery many important questions remain unanswered: How many quasars are surrounded by this kind of extended Lyman-alpha emission from infalling hydrogen gas? Are their dark matter halos as massive as the one around the quasar studied here? How are these quasars different from other quasars where outflows of gas are observed? The team is currently working on a larger sample of quasars, which may help answering some of these questions.

More information

The paper describing these results appears in the August 26 issue of the research journal Nature ("The Lyman-alpha glow of gas falling into the dark matter halo of a z = 3 galaxy" by M. Weidinger, P. Møller & J.P.U. Fynbo).

Figure 1: A sketch of the quasar Q1205-30 and its surroundings. The quasar (see insert) sits in the centre of a massive dark matter halo, which gravitationally attracts the surrounding intergalactic gas. The gas falls (red arrows) towards the dark matter halo, and as it enters the ionization cone of the quasar it starts glowing (yellow cone). [2]

Figure 2: Lyman-alpha imaging. (a) Lyman-alpha image of the extended emission around the quasar Q1205-30. An unrelated foreground galaxy is marked 'g1'. The vertical bar is one arcsecond (1/3600 degree) high, which at the distance of the quasar corresponds to approximately 26 thousand light years. The two yellow lines show the position of the slit through which the spectrum in Fig. 3 was obtained. (b) Model calculation of neutral hydrogen photoionized in the quasar ionization cone.

Figure 3: Two-dimensional spectrum. Here a negative colour-scheme is used, so darker colours mean more intensity, and lighter colours mean less intensity. The wavelength increases horizontally, and the position on the sky changes along the vertical axis, corresponding to the slit marked in Fig. 2 by two yellow lines. (a) The two-dimensional quasar spectrum. The extended Lyman-alpha emission is faintly visible at 4920 Å. (b) The quasar spectrum has been subtracted between 4850 Å and 5000 Å revealing the faint, extended Lyman-alpha emission.

Figure 4: Apparent infall velocity. The apparent distance from the quasar (measured along the yellow slit marked in Fig. 2) varies along the horizontal axis (measured in seconds of arc below, and in kiloparsecs[3] above). Positive distances correspond in Fig. 2 to positions above the quasar and vice versa. The apparent velocity is measured in kilometers per second. Five different models with masses between 2 and 7 million-million solar masses are overplotted.

Notes

[1] The team consists of Michael Weidinger (Aarhus, Denmark), Palle Møller (Munich, Germany), and Johan Peter Uldall Fynbo (Copenhagen, Denmark; Aarhus, Denmark).

[2] The wire-frame drawing in the insert in Fig. 1 was kindly provided by Robert A. E. Fosbury (ESO).

[3] One kiloparsec (kpc) is roughly the same as 3300 light years.

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