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Register a new userSupermassive Black Hole Demographics: Evading $M - {sigma}$
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We consider black hole - galaxy coevolution using simple analytic arguments. We focus on the fact that several supermassive black holes are known with masses significantly larger than suggested by the $M - {sigma}$ relation, sometimes also with rather small stellar masses. We show that these are likely to have descended from extremely compact `blue nugget galaxies born at high redshift, whose very high velocity dispersions allowed the black holes to reach unusually large masses. Subsequent interactions reduce the velocity dispersion, so the black holes lie above the usual $M - {sigma}$ relation and expel a large fraction of the bulge gas (as in WISE J104222.11+164115.3) that would otherwise make stars, before ending at low redshift as very massive holes in galaxies with relatively low stellar masses, such as NGC 4889 and NGC 1600. We further suggest the possible existence of two new types of galaxy: low-mass dwarfs whose central black holes lie below the $M - {sigma}$ relation at low redshift, and galaxies consisting of very massive ($gtrsim 10^{11}$M$_{odot}$) black holes with extremely small stellar masses. This second group would be very difficult to detect electromagnetically, but potentially offer targets of considerable interest for LISA.
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Precision timing of large arrays (>50) of millisecond pulsars will detect the nanohertz gravitational-wave emission from supermassive binary black holes within the next ~3-7 years. We review the scientific opportunities of these detections, the requirements for success, and the synergies with electromagnetic instruments operating in the 2020s.
Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds - a departure from the hot mode accretion model - although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z=0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities can precipitate from this hot gas, producing a rain of cold clouds that fall toward the galaxys centre, sustaining star formation amid a kiloparsec-scale molecular nebula that inhabits its core. The observations show that these cold clouds also fuel black hole accretion, revealing shadows cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole in the galaxy centre, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.
We present the first constraint on Faraday rotation measure (RM) at submillimeter wavelengths for the nucleus of M 87. By fitting the polarization position angles ($chi$) observed with the SMA at four independent frequencies around $sim$230 GHz and interpreting the change in $chi$ as a result of emph{external} Faraday rotation associated with accretion flow, we determine the rotation measure of the M 87 core to be between $-$7.5$times$10$^{5}$ and 3.4$times$10$^{5}$ rad/m$^{2}$. Assuming a density profile of the accretion flow that follows a power-law distribution and a magnetic field that is ordered, radial, and has equipartition strength, the limit on the rotation measure constrains the mass accretion rate $dot{M}$ to be below 9.2$times$10$^{-4}$ M$_{odot}$~yr$^{-1}$ at a distance of 21 Schwarzchild radii from the central black hole. This value is at least two orders of magnitude smaller than the Bondi accretion rate, suggesting significant suppression of the accretion rate in the inner region of the accretion flow. Consequently, our result disfavors the classical emph{advection dominated accretion flow} (ADAF) and prefers the emph{adiabatic inflow-outflow solution} (ADIOS) or emph{convection-dominated accretion flow} (CDAF) for the hot accretion flow in M 87.
Relativistic jets in active galactic nuclei represent one of the most powerful phenomena in the Universe. They form in the surroundings of the supermassive black holes as a by-product of accretion onto the central black hole in active galaxies. The flow in the jets propagates at velocities close to the speed of light. The distance between the first part of the jet that is visible in radio images (core) and the black hole is still a matter of debate. Only very-long-baseline interferometry observations resolve the innermost compact regions of the radio jet. Those can access the jet base, and combining data at different wavelenghts, address the physical parameters of the outflow from its emission. We have performed an accurate analysis of the frequency-dependent shift of the VLBI core location for a multi-wavelength set of images of the blazar CTA 102 including data from 6 cm down to 3 mm. The measure of the position of the central black hole, with mass $sim 10^{8.93},M_odot$, in the blazar CTA 102 reveals a distance of $sim 8times10^4$ gravitational radii to the 86 GHz core, in agreement with similar measures obtained for other blazars and distant radio galaxies, and in contrast with recent results for the case of nearby radio galaxies, which show distances between the black hole and the radio core that can be two orders of magnitude smaller.
Spin measurements of supermassive black holes (SMBHs) provide crucial constraints on the accretion processes that power active galactic nuclei (AGN), fuel outflows, and trigger black hole growth. However, spin measurements are mainly limited to a few dozen nearby sources for which high quality, high S/N spectra (e.g., from Chandra, XMM-Newton, Suzaku, NuSTAR) are available. Here we measure the average SMBH spin of $sim$1900 AGN in the Chandra COSMOS-Legacy survey using spectral stacking analysis. We find broad Fe K$alpha$ line emission in the average COSMOS spectrum (Gaussian width $sigma=0.27pm0.05$ keV), and by fitting this emission line profile with relativistic line models, we measure the average black hole spin parameter $a=0.62~substack{+0.07 -0.17}$. The sample size, availability of multiwavelength data, and spatial resolution of the COSMOS Legacy field also provide a unique environment to investigate the average SMBH spin as a function of other observables (e.g., redshift, luminosity) up to $zsim5.3$. We find that optically classified Type 1 sources have broader Fe K$alpha$ line emission than Type 2 sources. X-ray unobscured and obscured sources, as defined by their column densities, have widths that are consistent with the optically defined unobscured and obscured sources, respectively. There is some evidence for evolution of the Fe K$alpha$ width and black hole spin parameter with luminosity, but not conclusively with redshift. The results of this work provide insights into the average spins of SMBHs in AGN, shedding light on their growth mechanisms and observed co-evolution with their host galaxies.
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