When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42±3 μas, which is circular and encompasses a central depression in brightness with a flux ratio 10:1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M=(6.5±0.7)×10 9 M e . Our radiowave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.

We present the HERA CO-Line Extragalactic Survey (HERACLES), an atlas of CO emission from 18 nearby galaxies that are also part of The H I Nearby Galaxy Survey (THINGS) and the Spitzer Infrared Nearby Galaxies Survey (SINGS). We used the HERA multi-pixel receiver on the IRAM 30-m telescope to map the CO J = 2 → 1 line over the full optical disk (defined by the isophotal radius r 25 ) of each target, at 13 ′′ angular resolution and 2.6 km s −1 velocity resolution. Here we describe the observations and reduction of the data and show channel maps, azimuthally averaged profiles, integrated intensity maps, and peak intensity maps. The implied H 2 masses range from 7 × 10 6 to 6 × 10 9 M ⊙ , with four low metallicity dwarf irregular galaxies yielding only upper limits. In the cases where CO is detected, the integrated H 2 -to-H I ratios range from 0.02 -1.13 and H 2 -to-stellar mass ratios from 0.01 to 0.25. Exponential scale lengths of the CO emission for our targets are in the range 0.8 -3.2 kpc, or 0.2 ± 0.05 r 25 . The intensity-weighted mean velocity of CO matches that of H I very well, with a 1σ scatter of only 6 km s −1 . The CO J = 2 → 1/J = 1 → 0 line ratio varies over a range similar to that found in the Milky Way and other nearby galaxies, ∼ 0.6-1.0, with higher values found in the centers of galaxies. The typical line ratio, ∼ 0.8, could be produced by optically thick gas with an excitation temperature of ∼ 10 K.

We compare molecular gas traced by 12 CO(2-1) maps from the HERACLES survey, with tracers of the recent star formation rate (SFR) across 30 nearby disk galaxies. We demonstrate a first-order linear correspondence between Σ mol and Σ SFR but also find important second-order systematic variations in the apparent molecular gas depletion time, τ mol dep = Σ mol /Σ SFR . At the 1 kpc common resolution of HERACLES, CO emission correlates closely with many tracers of the recent SFR. Weighting each line of sight equally, using a fixed alpha CO equivalent to the Milky Way value, our data yield a molecular gas depletion time, τ mol dep = Σ mol /Σ SFR ≈ 2.2 Gyr with 0.3 dex 1σ scatter, in very good agreement with recent literature data. We apply a forward-modeling approach to constrain the power-law index, N , that relates the SFR surface density and the molecular gas surface density, Σ SFR ∝ Σ N mol . We find N = 1 ± 0.15 for our full data set with some scatter from galaxy to galaxy. This also agrees with recent work, but we caution that a power law treatment oversimplifies the topic given that we observe correlations between τ mol dep and other local and global quantities. The strongest of these are a decreased τ mol dep in low-mass, low-metallicity galaxies and a correlation of the kpc-scale τ mol dep with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H 2 conversion factor (α CO ) that depends on dust shielding, and thus D/G, in the theoretically expected way. This is not a unique interpretation, but external evidence of conversion factor variations makes this the most conservative explanation of the strongest observed τ mol dep trends. After applying a D/G-dependent α CO , some weak correlations between τ mol dep and local conditions persist. In particular, we observe lower τ mol dep and enhanced CO excitation associated with nuclear gas concentrations in a subset of our targets. These appear to reflect real enhancements in the rate of star formation per unit gas and although the distribution of τ dep does not appear bimodal in galaxy centers, τ dep does appear multivalued at fixed Σ mol , supporting the the idea of "disk" and "starburst" modes driven by other environmental parameters.

We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior brightness depressions) using two independent sampling algorithms that consider distinct representations of the visibility data. We show that the crescent family of models is statistically preferred over other comparably complex geometric models that we explore. We calibrate the geometric model parameters using general relativistic magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We further fit images generated from GRMHD models directly to the data. We compare the derived emission region and black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of 42±3 μas and constrain its fractional width to be <0.5. Associating the crescent feature with the emission surrounding the black hole shadow, we infer an angular gravitational radius of GM/Dc 2 =3.8±0.4 μas. Folding in a distance measurement of -+ 16.8 Mpc 0.7 0.8 gives a black hole mass of =  ´ | | M M 6.5 0.2 0.7 10 stat sys 9. This measurement from lensed emission near the event horizon is consistent with the presence of a central Kerr black hole, as predicted by the general theory of relativity.

We combine new sensitive, wide-field CO data from the HERACLES survey with ultraviolet and infrared data from GALEX and Spitzer to compare the surface densities of H 2 , Σ H2 , and the recent star formation rate, Σ SFR , over many thousands of positions in 30 nearby disk galaxies. We more than quadruple the size of the galaxy sample compared to previous work and include targets with a wide range of galaxy properties. Even though the disk galaxies in this study span a wide range of properties, we find a strong, and approximately linear correlation between Σ SFR and Σ H2 at our common resolution of 1 kpc. This implies a roughly constant median H 2 consumption time, τ H2 Dep = Σ H2 /Σ SFR , of ∼ 2.35 Gyr (including heavy elements) across our sample. At 1 kpc resolution, there is only a weak correlation between Σ H2 and τ H2 Dep over the range Σ H2 ≈ 5-100 M ⊙ pc −2 , which is probed by our data. We compile a broad set of literature measurements that have been obtained using a variety of star formation tracers, sampling schemes and physical scales and show that overall, these data yield almost exactly the same results, although with more scatter. We interpret these results as strong, albeit indirect evidence that star formation proceeds in a uniform way in giant molecular clouds in the disks of spiral galaxies.