Cool dense hydrogen gas around the first quasars

November 01, 2019
Quasars are amongst the brightest non-transient sources in the sky. Thanks to their high luminosity, they can be observed even at early cosmic times, where – surprisingly – these first quasars appear as already evolved systems: with black holes with masses exceeding one billion solar masses hosted by massive and heavily star forming galaxies. To explain such rapid growth, theorists believe these systems must reside in peculiarly dense environments, where huge gas reservoirs favour efficient inflow of material onto seed super-massive black holes. An international team of astronomers has recently found the first clear observational evidence that this is indeed the case. The new “panoramic” spectrograph called MUSE unveiled, for the first time, the almost ubiquitous presence of large amounts of cool gas in close proximity to the first quasars. This pristine fuel will fall on the primordial galaxies and sustain their growth in both stellar and black hole mass.

A prime objective of observational astrophysics is to peer deep into the young Universe and study how the first stars, galaxies, and black holes formed. For decades, astronomers exploited the brightness of quasars to study galaxy formation and evolution at all cosmic times, both as silhouettes against the luminous quasars, and in emission around them. Despite significant progress, we still do not understand the detailed processes whereby super-massive black holes with masses a billion times larger than the Sun assemble their mass in less than one billion years after the Big Bang, a small fraction of the current Universe age (13.7 billion years).

Hydro-dynamical cosmological simulations and analytical arguments suggest that to grow such massive systems in such a short time scale, the host galaxies of the first quasars need a continuous replenishment of fresh fuel. This gas has to be provided by cold filamentary streams from the so-called intergalactic medium down to the quasar's host galaxy and/or by mergers with other gas rich galaxies. While a merger is a violent short episode, the aforementioned filaments should be present around each quasar.

Emission from this large-scale gas is, however, typically too faint to be detected unless it is illuminated by the intense radiation from the quasar. In this case, the hydrogen in the gas reprocesses the incident radiation and shines as an extended "fuzz" of Ly-Alpha emission, now detectable with top-notch facilities. Recently, a team of astronomers from Garching, Heidelberg, and Santa Barbara took advantage of this boosted emission and embarked on a large survey aimed at uncovering the presence of this fuzz around more than 30 luminous quasars in the young Universe.

An investment of more than 50 hours with the panoramic integral-field spectrograph MUSE on the Very Large Telescope revealed that around 40% of first quasars are embedded in Ly-Alpha halos (see Figure 1) with a total extent of up to a hundred thousand light years. These halos are directly tracing the presence of cool dense hydrogen gas around the first quasars. In particular, the researchers discovered that this gas is bound within the dark matter halo of the quasar host galaxies and that it is abundant enough to maintain both the observed high-rate of gas consumption of the central supermassive black holes and their highly star forming host galaxies.

The presence of these extended nebulae is an important piece of the puzzle that astronomers are building to picture the formation of large cosmic structures more than 12 billion years ago. By providing detailed constraints on the fuel supply, these new observations can be used to test current theories and models for the growth of massive galaxies and black holes from the Big Bang to the present (see Figure 2). While additional observations are already planned to fully capture the physical status of the gas, current data already pose new challenges to theoretical models. They indicate that, rather than being smooth, "Lyman-alpha" nebulae take on the consistency of a mist comprising an enormous number of tiny droplets. Reproducing the structure of these clouds may prove to be a key challenge for the next generation of theoretical models of galaxy evolution.

3D visualization of the extended Ly-Alpha halo around the quasar P308-21 at z=6.23. The “hole” at the center represents the quasar location, which has been removed so as not to contaminate the measurement with light coming from the central black hole. The gas appears to be in a relatively quiescent motion, suggesting that it is moving within the dark matter halo where the central quasar resides.

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