Detection of jet launching structure near the supermassive black hole in M 87

Approximately 10% of active galactic nuclei (AGN) exhibit relativistic jets, which are powered by accretion of matter onto super massive black holes. While the measured width profiles of such jets on large scales agree with theories of magnetic collimation, predicted structure on accretion disk scales at the jet launch point has never been detected. We report radio interferometry measurements at 1.3mm wavelength of the elliptical galaxy M87 that spatially resolve the black hole accretion disk system at the jet base. The derived size of 4.9 +/0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, and implies that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole. One Sentence Summary: Black hole sized structure is detected at the base of a relativistic jet in the center of an active galaxy. Main Text: The compact central regions of some galaxies are so luminous that they outshine the combined output of all other energy sources in the galaxy. The small size and high power output of these active galactic nuclei (AGN) are most plausibly explained by the conversion of gravitational energy through accretion onto a super massive black hole. Many AGN produce powerful collimated jets of relativistic particles that can extend for hundreds and thousands of light-years, providing an important mechanism for redistributing matter and energy on large scales that affect galactic evolution (1). Jets are thought to form through magnetic acceleration processes located within the accretion flow or at the central black hole itself (2-4), but no observations to date have had the angular resolution required to detect and confirm structure on these scales for extragalactic jet sources. High-resolution radio interferometry of these sources at cm wavelengths is limited by optical depth effects that obscure the innermost accretion region. For these reasons, it remains unclear if jet formation requires a spinning black hole (5,6), or if jets are likely to be launched from disks orbiting in the opposite direction (retrograde) of the spin of the black hole (7,8). To address these questions, we have assembled a Very Long Baseline Interferometry (VLBI) array operating at a wavelength of 1.3mm, where AGN become optically thin, and angular resolutions necessary to resolve the inner accretion disks of nearby AGN are obtained. At a distance of 16−1.1 +1.3Mpc (9) and with a mass most recently measured to be (6.6 ± 0.4) × 10 M⊙ (10), the Schwarzschild radius of the M87 black hole (RSCH = 2GM/c = (6.3 ± 0.4) × 10 parsec = (2.0 ± 0.12) × 10 cm) subtends an angle of 8.1 ± 0.8 micro arcseconds, presenting us with the best known opportunity for studying the formation of relativistic jets on scales commensurate with the black hole and accretion disk. Radiating via synchrotron emission, the relativistic jet from M87 extends for hundreds of kilo parsecs and terminates in extended lobes of emission as it slows and interacts with the intergalactic medium. Closer to the galaxy's core, on hundreds of parsec scales, the radio jet is remarkably well collimated with an opening angle of less than 5 degrees (11), and is also clearly seen in the optical, ultra-violet, and x-rays (12,13) where the emission is primarily confined to knots along the central ‘spine’ of the jet. VLBI observations at wavelengths ranging from 3.5mm to 20cm, show that within 10's of milli arcseconds of the core, the jet opening angle, delineated by edge brightening in the outflow, increases to greater than 40 (14-18). This wide opening angle is a signature of the launch point for a magnetohydrodynamically (MHD) powered jet that has not yet had time to collimate (2), and identifies the VLBI core as the most likely site of the central black hole. We observed M87 over three consecutive days with a 1.3mm wavelength VLBI array consisting of four telescopes at three geographical locations: the James Clerk Maxwell Telescope (JCMT) on Mauna Kea in Hawaii, the Arizona Radio Observatory's Submillimeter Telescope (SMT) in Arizona, and two telescopes of the Combined Array for Research in Millimeter-wave Astronomy (CARMA, located ~60m apart) in California. On Mauna Kea, the JCMT partnered with the Submillimeter Array (SMA), which housed the Hydrogen maser atomic frequency standard and wideband VLBI recording systems; the SMT and CARMA were similarly equipped. These special-purpose systems allowed two frequency bands of 512 MHz to be sampled at 2-bit precision and recorded at an aggregate rate of 4 Gigabits/second. Data recorded at all sites were shipped to MIT Haystack Observatory for processing on the Mark4 VLBI correlator. Once correlated, data for each VLBI scan (typically 10 minutes) were corrected for coherence losses due to atmospheric turbulence and searched for detections using established algorithms tailored for high frequency observations (see SI). M87 was clearly detected each day on all VLBI baselines, and the interferometric data were then calibrated to flux density units (Figure 1; see SI for details). Clear detections on the long baselines to Hawaii (CARMA-JCMT and SMT-JCMT) represent the highest angular resolution observations of M87 reported in any waveband, and when combined with the CARMA-SMT baseline data they provide a robust means to measure the size of the M87 core, which is unresolved with longer wavelength VLBI. The baseline between the two CARMA antennae corresponds to angular scales of ~4 arcseconds and is sensitive to extended and much larger scale jet structure; these data were thus used to refine calibration of the antennas, but were excluded from analysis of the core component. To extract a size for the core, we fit a two-parameter circular Gaussian model to the 1.3mm VLBI data, deriving a total flux density and full width half maximum (FWHM) size for each day of observations. Sizes and flux densities fit separately for each day are consistent with each other at the 3σ level, indicating no significant variation in the 1.3mm core structure over the three days of observation. Fits on a plot of correlated flux density vs. baseline length are shown in Figure 1. When data from all three days are combined, the weighted least-squares best-fit model for the compact component results in a flux density of 0.98 ±0.04 Jy and a FWHM of 40 ±1.8micro arcseconds (3σ errors). Conversion to units of Schwarzschild radius yields a value of 4.9± 0.4 RSCH (1σ errors) where the errors are completely dominated by uncertainties in the distance to M87 and the black hole mass. We adopt the circular Gaussian size derived using data from all three days for subsequent discussion. Our VLBI observations cannot be used to fix the absolute position of this Gaussian component. However, two separate lines of evidence imply that this ultra-compact 1.3mm emission is in immediate proximity to the central super massive black hole at the jet-launch point. First, the long history of VLBI observations of M87 at many wavelengths can be used to construct a jet width profile that begins ~100 RSCH from the core and extends to core-separations of more than 10 RSCH. Figure 2 shows this profile along with a power law fit to the data that matches the functional form and characteristics exhibited by General Relativistic MHD simulations (18-20) in which the jet opening angle widens as it nears the black hole. The best-fit power law intersects the size of the 1.3mm emission region at a core distance of only 1.25 RSCH. Since the angle of the M87 jet axis to our line of sight is estimated to be within the range 15 – 25 (21), the de-projected distance of this intersection point lies in the range 3 – 5 RSCH. A second method of locating the 1.3mm emission derives from observations of the position shift of the M87 core as a function of wavelength. The core corresponds to the point in the jet where opacity due to synchrotron self-absorption approaches unity, and this τ∼1 surface should shift towards the jet launch point with decreasing observing wavelength. Multi-wavelength astrometric VLBI observations confirm that over a wide range of frequency, the absolute position of the core moves asymptotically towards the central black hole with a ν dependence (21). This relation also places the 1.3mm emission at an apparent distance of just 1.25 RSCH from the black hole. Such strong evidence for associating the M87 core with the central black hole contrasts with the case of blazar sources, in which relativistic jets are closely aligned to our line of sight and the core becomes visible hundreds of thousands of Schwarzschild radii from the central engine (22). In M87, the favourable geometry of a misaligned jet and increased transparency of the synchrotron emission at mm wavelengths (23) allows us direct access to the innermost central engine with 1.3mm VLBI. The most plausible mechanisms for powering extragalactic jets involve conversion of the black hole rotational energy through the Blandford-Znajek (BZ) process (3), whereby magnetic fields lines cross the black hole event horizon or become locked into co-rotation with the black hole ergosphere, and launch Poynting flux dominated outflows. The inner portion of the accretion disk is not only the source of the magnetic fields threading the black hole, but also launches a disk-wind via the Blandford-Payne (BP) mechanism (4), which serves to collimate the jet. This BZ/BP combination forms a spine/sheath morphology in which a high velocity and narrow central jet from the black hole is surrounded by a slower outflow originating from the inner disk (6,24). In the case of M87, our line of sight is sufficiently off axis that the dominant contribution to the 1.3mm VLBI emission is from the slower moving sheath, anchored within the accretion disk (21). Thus, the critical size scale associated with the jet footprint is the Innermost Stable Circular Orbit (ISCO) of the black hole, within which matter quickly p