Current black hole spin measurements, in X-rays, radio and gravitational waves, are already constraining models for the growth of black holes, the dynamics of stellar core-collapse and the physics of relativistic jet production.
Black holes were theoretically established in 1915, shortly after Albert Einstein published his theory of General Relativity. Since then, observations have confirmed black holes as actual astrophysical objects. In this Insight we publish ten long- and short-format pieces (in the “Reviews & Perspectives” and “Views & Comments” tabs, respectively) discussing key aspects of black holes, from their masses, to their spins, to the ways in which they impact their surroundings and are studied. This collection also showcases some of the black hole-related content that Nature Astronomy has published since our launch (in the “Primary research” and “Further reading” tabs). Also, please view our previous collection of landmark black hole discoveries published in Nature and other Springer Nature journals.
Reviews & Perspectives
Supermassive black holes are fed through galaxy interactions and mergers, chaotic cold accretion in galaxy clusters and secular processes that may include stellar bars. Observations constraining these mechanisms at different scales are reviewed.
KAGRA is a new gravitational wave detector being built in Japan. Unlike LIGO/Virgo, it will operate at cryogenic temperatures with sapphire mirrors. KAGRA will help improve the localization of gravitational wave detections and determination of the source parameters.
Examining and comparing many of the definitions of a black hole, it is concluded that the profusion of different definitions is a virtue that makes the investigation of black holes possible and fruitful in many different kinds of problems.
Views & Comment
Black holes have the distinct honour of being the most popular and potentially the least well-understood objects in the Universe. This issue’s Insight explores how far black hole research has come since its inception, though it still has a long way to go.
Mitchell C. Begelman, Professor in the Department of Astrophysical and Planetary Sciences at the University of Colorado Boulder and a black hole expert, discusses the start of the field with Nature Astronomy.
Intermediate-mass black holes (BHs) in local dwarf galaxies are considered the relics of the early seed BHs. However, their growth might have been impacted by galaxy mergers and BH feedback so that they cannot be treated as tracers of the early seed BH population.
The masses of supermassive black holes, key to many cosmological studies, are highly uncertain beyond our local Universe. The main challenge is to establish the spatial and kinematic structure of the broad-line emitting gas in active galactic nuclei.
The detection of a gravitational-wave background at nanohertz frequencies can tell us if and how supermassive black holes merge, and inform our knowledge of galaxy merger rates and supermassive black hole masses. All we have to do is time pulsars.
The James Webb Space Telescope may detect and distinguish a young galaxy that hosts a direct-collapse black hole and nearby massive metal-free star formation at redshift 15 with as little as a 20,000-second total exposure time across four filters.
We calculate the continuous nanohertz gravitational-wave emission from individual supermassive black hole binaries and the gravitational-wave background they generate, which will be observable with pulsar timing arrays.
A candidate dual supermassive black hole system with a projected separation of 0.35 pc is found in the gas-rich interacting spiral galaxy NGC 7674, evidenced by a ∼0.7 kpc Z-shaped radio jet and two, possibly inverted-spectrum, compact radio cores.
The X-ray polarization properties of the black hole binary Cygnus X-1 in its hard state, combined with spectral and timing data, reveals that the accretion disk corona is either an extended structure or located far from the black hole.
A bright X-ray outburst from a massive star cluster 12.5 kpc from a galactic centre fits the profile of a tidal disruption event (TDE), indicating the likely presence of an intermediate-mass black hole (IMBH). TDEs could be the most effective way of identifying IMBHs.
The authors predict the ability of the Event Horizon Telescope (in its 2017 campaign) to distinguish between different theories of gravity based on images of Sagittarius A*; they suggest that it will not be possible.
Interferometric observations of 3C84 reveal a broad cylindrical jet a few hundred gravitational radii from the black hole, implying that the jet either undergoes a rapid lateral expansion on even smaller scales or is launched from the accretion disk.
During reconfinement in unmagnetized relativistic jets, a centrifugal instability develops that leads to a turbulent state. This instability likely lies behind the division of active galactic nuclei jets into the two Fanaroff–Riley classes.
Black hole masses derived from the properties of the accretion disk and virial mass estimates differ by a factor that is inversely proportional to the width of the broad emission lines. An inclined planar gas distribution may account for this effect.
The discovery of the most energetic transient event to date is reported. Its spectroscopic properties and temporal evolution imply it is powered by shock interaction between expanding material and large quantities of surrounding dense matter.
An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system
A delay between rapid optical and X-ray flux variations from an accreting black-hole binary is reported together with a brightening radio jet, indicating a characteristic elevation of the radiative jet base of 0.1 light-seconds above the black hole.
A candidate intermediate-mass black hole is reported within a molecular cloud near Sgr A*, the centre of our Galaxy. High-resolution observations with ALMA reveal extreme gas kinematics and a compact source consistent with a quiescent black hole.
Low-mass black holes that accrete stars from locally dense environments grow over the Hubble time above a minimal mass of 105 solar masses, independently of their initial mass. This explains why there are no convincing cases of intermediate-mass black holes to date.
The key ingredients for a massive cloud of gas to collapse and directly form a black hole without fragmenting and forming stars are a strong ionizing background emission and a closely timed burst of star formation in its vicinity.
A magnetohydrodynamic model for outflows around supermassive black holes can also reproduce the X-ray properties of an outflow around a stellar black hole. This indicates that magnetic forces have a universal role to play in driving these winds.
Transient object ASASSN-15lh was previously cast as the most luminous supernova ever discovered. Now, however, there is convincing evidence that its flare was a tidal disruption event: a rapidly-spinning black hole tearing apart a neighbouring star.
Arguably, no mission changed X-ray astronomy in as short a time as did Hitomi. The planned X-ray Astronomy Recovery Mission, XARM, will carry its legacy forward.
Black holes and spacetime singularities are fundamental in science. While observational proof for black holes is hard to come by, alternatives can be ruled out or confirmed to exist through precision gravitational wave observations.
The material surrounding accreting supermassive black holes connects them with their hosts. From studies in the infrared and X-rays, the structure of this material is found to be complex, clumpy and dynamic.
Quantifying the effect of active galactic nuclei (AGNs) on their hosts requires knowledge of their life cycle. This review on AGN archaeology summarizes the main recent findings regarding the AGN life cycle from optical and radio observations.
Feedback from actively accreting supermassive black holes is thought to be important in the evolution of galaxies. Theoretical and observational results are reviewed with regard to the impact of this feedback on star formation in galaxies.
Radiative efficiency in radio-loud active galactic nuclei is governed by the accretion rate onto the central black hole rather than directly by the type of accreted matter; while it correlates with real differences in host galaxies and environments, it does not provide unambiguous information about particular objects.
Galaxy-scale outflows powered by actively accreting supermassive black holes are routinely detected, and they have been associated with both the suppression and triggering of star formation. Recent observational evidence and simulations are favouring a delayed mechanism that connects outflows and star formation.
Different mechanisms can drive outflows in active galactic nuclei (AGNs), but it is often unclear which mechanism dominates, if any. To quantify the impact of AGN feedback on galaxy evolution, the driving mechanism of outflows must be better understood.
Observations and simulations show that outflows in active galactic nuclei contain gas in different phases. To understand their true impact on galaxy evolution, we advocate consistent and unbiased investigation of these multiphase winds in large active galactic nuclei samples.
Active galactic nuclei (AGNs) are empirically divided into ‘radio-loud’ and ‘radio-quiet’. These 50-year-old labels are obsolete, misleading and wrong. I argue that AGNs should be classified as ‘jetted’ and ‘non-jetted’ based on a physical difference — the presence (or lack) of strong relativistic jets.
Millimetre-wavelength interferometry and gravitational-wave detectors currently provide the most stringent tests for the existence of cosmic black holes. Complementary measurements of magnetic fields near their event horizon would be decisive.
Black holes present a profound challenge to our current foundations of physics, and an exciting era of astronomy is just opening in which gravitational-wave observation and very-long-baseline interferometry may provide important hints about the new principles of physics needed.
A phenomenon recently studied in theoretical physics may hold considerable interest for astronomers: the explosive decay of primordial black holes through quantum tunnelling. Their detection would be of major theoretical importance.
Feedback from active galactic nuclei (AGNs) remains controversial despite its wide acceptance as necessary to regulate massive galaxy growth. Consequently, we held a workshop in October 2017, at Leiden’s Lorentz Center, to distinguish between the reality and myths of feedback.
The Event Horizon Telescope, an Earth-sized interferometer, aims to capture an image of a black hole’s event horizon to test the theory of general relativity and probe accretion processes, explains project director Shep Doeleman.
Recent polarization measurements of the stellar-mass black hole in Cygnus X-1 reveal an extended corona in the inner parts of the accretion flow and open the path for a new era in high-energy astrophysics.
Supermassive binary black holes are thought to lie at the centres of merging galaxies. The blazar OJ 287 is the poster child of such systems, showing strong and periodic variability across the electromagnetic spectrum. A new study questions the physical origin of this variability.
A study suggests that the gas clouds in the vicinity of rapidly accreting supermassive black holes are distributed in a planar distribution, impacting the estimation of the mass of the black hole based on the motion of these clouds.
Orbiting supermassive black holes in the centres of nearby galaxies contribute to a gravitational-wave background over the whole sky. Networks of millisecond pulsars are sensitive to this signal. Creating maps of this background using information from known galaxies can help us to project when (and how) we may observe it.
The first detection of electromagnetic emission from a gravitational wave source bridges the gap between one of the most energetic phenomena in the Universe and their dark, difficult to detect progenitors.
The biggest black holes in the Universe were in place soon after the Big Bang. Explaining how they formed so rapidly is a daunting challenge, but the latest simulations give clues to how this may have occurred.
The destruction of stars by supermassive black holes appears to be rarer than predicted. A candidate stellar disruption in a kind of galaxy that is usually obscured may explain why.
Black holes grow by accreting mass, but the process is messy and redistributes gas and energy into their environments. New evidence shows that magnetic processes mediate both the accretion and ejection of matter.