Quasars Shedding Dusty Cocoons: Extreme Outflows and Weak Lines
Six quasars similar to the LoBAL quasar GQ 1309+2904 were studied, with extremely weak emission lines except for Hα. Redshifts (z≈2.07–3.28) measured from Hα. All objects have high-velocity outflows (up to ~0.16c), five are LoBAL. Black hole masses (around 10^8.7–10^9.4 M⊙) and Eddington ratios (0.14–0.34) indicate very massive and active centers. The continua are reddish: for three, the reddening curve is close to that of the Small Magellanic Cloud, while three show additional ultraviolet suppression. The polarization of two of them and GQ 1309+2904 is low, suggesting observation along a direction close to the rotation axis. These objects are interpreted as weak-line quasars in the stage of shedding their dusty cocoon. Dust in the wind is fragmented into small particles, leading to a steeper ultraviolet absorption. An evolutionary scheme is presented.
Context
Quasars are among the brightest objects in the Universe, powered by accretion onto supermassive black holes. Their evolution is closely linked to the growth of galaxies, but the 'adolescent' stages of quasars remain poorly understood. Particularly intriguing are quasars with broad absorption lines (BAL), indicating powerful gas outflows that can be studied via spectroscopy. By examining objects that existed 2–3 billion years after the Big Bang, within a cosmology that includes dark matter, scientists are moving closer to the answer. Understanding the nature of these outflows and the accompanying weakening of emission lines could shed light on feedback mechanisms that regulate star formation and black hole growth, as predicted in the works of Karl Schwarzschild.
Methods
For seven quasars with redshifts 2.07–3.28, optical and near-infrared spectra were obtained with the NOT and GTC telescopes. Systemic redshifts were derived from the Hα emission line (discovered by Johann Balmer), using a double-Gaussian fit. Outflow velocities were computed from the shift of absorption lines relative to systemic redshift, allowing direct measurement of velocities reaching a significant fraction of the speed of light. For three objects, polarimetry was performed in BVR filters. Dust extinction was assessed by fitting a reddened quasar template spectrum to the observed data, applying extinction laws similar to SMC and steeper in the ultraviolet. Photometric variability and radio emission were also analyzed.
Results
All quasars show deep and broad blueshifted absorption lines, with outflow velocities reaching from −5000 to −47,000 km/s (up to 0.16 speed of light). Ultraviolet emission lines (C IV, He II) are extremely weak or absent, while Hα has normal intensity. Black hole masses range from 10^8.7 to 10^9.4 M☉, with Eddington ratios of 0.14–0.34. For three out of four objects with anomalously steep UV extinction, we found low polarization (<4%), suggesting small inclination angles to the disk. This indicates we are looking through a disk wind containing fine dust grains shattered by shock waves.
Implications
Our results place these objects in the class of quasars with weak emission lines (WLQ), viewed through a disk wind. This fills a gap in the unified model, which previously excluded BAL quasars from WLQ samples. The discovered link between high-velocity outflows and suppression of high-ionization lines supports a 'soft' ionizing continuum scenario, needed for radiative acceleration of the wind. The anomalous extinction points to the shattering of large dust grains in shock waves, possibly a common feature of young quasars observed via spectroscopy.
Future development
In the future, observations with the James Webb Space Telescope will allow detailed study of dusty cocoons and outflows in the infrared, especially at high redshifts. Combining with X-ray data will help constrain the shape of the ionizing continuum. High-resolution spectropolarimetry will enable mapping of wind kinematics. Numerical simulations of jet or wind interaction with the dusty envelope will clarify the dust shattering process and the transition from an obscured phase to an unobscured quasar.
Impact
The results will impact understanding of quasar evolution and active nucleus–galaxy feedback mechanisms, as well as interpretation of spectra of distant objects obtained by JWST.
Next steps
Plans include enlarging the sample of such objects and conducting multiwavelength observations, including X-ray and submillimeter, to assess the total energy of outflows and dust mass. Long-term monitoring is also needed to track variability and evolution of absorption lines via high-resolution spectroscopy.
Key open problems
The study links the problem of formation and growth of supermassive black holes with dust feedback mechanisms. It remains unclear exactly how accretion disks launch such powerful winds and why the ionizing continuum appears 'soft.' This touches on fundamental questions of accretion physics and particle acceleration in relativistic flows.
🎯 An outflow speed of 0.16c means the material moves faster than 48,000 km/s—in an hour it would cover the distance from Earth to the Sun more than three times! For one quasar, GQ 1309+2904, the emission line was so shifted that initially its systemic redshift was difficult to determine; only spatially extended Lyman-alpha emission from the host galaxy helped reveal the truth.
🎬 The idea of 'shedding a cocoon' echoes imagery from science fiction, such as Stanisław Lem's novel Solaris, where a superintelligent ocean tries to free itself from constraints, or the film Interstellar, showing powerful accretion flows around a black hole. Here, however, the cosmic scale and physics are real.
Key numbers
- Outflow velocity: up to 0.16c (≈ 48,000 km/s)
- Black hole mass: 10^8.7 – 10^9.4 M☉
- Eddington ratio: 0.14 – 0.34
- Redshift: z ≈ 2.07 – 3.28
- Polarization: < 4%