We present the first kinematical detection of embedded protoplanets within a protoplanetary disk. Using archival Atacama Large Millimetre Array (ALMA) observations of HD 163296, we demonstrate a new technique to measure the rotation curves of CO isotopologue emission to sub-percent precision relative to the Keplerian rotation. These rotation curves betray substantial deviations caused by local perturbations in the radial pressure gradient, likely driven by gaps carved in the gas surface density by Jupiter-mass planets. Comparison with hydrodynamic simulations shows excellent agreement with the gas rotation profile when the disk surface density is perturbed by two Jupiter-mass planets at 83 and 137 au. As the rotation of the gas is dependent upon the pressure of the total gas component, this method provides a unique probe of the gas surface density profile without incurring significant uncertainties due to gas-to-dust ratios or local chemical abundances that plague other methods. Future analyses combining both methods promise to provide the most accurate and robust measures of embedded planetary mass. Furthermore, this method provides a unique opportunity to explore wide-separation planets beyond the mm continuum edge and to trace the gas pressure profile essential in modeling grain evolution in disks.
The motions of gas and stars in the nuclei of nearby galaxies have demonstrated that massive black holes are common and that their masses correlate with the stellar velocity dispersion σ★ of the bulge. This correlation suggests that massive black holes and galaxies influence each other’s growth. Dynamical measurements are less reliable when the sphere of influence is unresolved; thus, it remains unknown whether this correlation exists in galaxies much smaller than the Milky Way. Light echoes from photoionized clouds around accreting black holes, in combination with the velocity of these clouds, yield a direct mass measurement that circumvents this difficulty. Here we report an exceptionally low reverberation delay of 83 ± 14 min between variability in the accretion disk and Hα emission from the nucleus of the dwarf galaxy NGC 4395. Combined with the Hα velocity dispersion σline = 426 ± 1 km /s, this lag determines a mass of about 10,000 M⊙ for the black hole (MBH). This mass is among the smallest central black hole masses reported, near the low end of expected masses for heavy ‘seeds’, and the best direct mass measurement for a galaxy of this size. Despite the lack of a bulge, NGC 4395 is consistent with the MBH–σ★ relation, indicating that the relation need not originate from hierarchical galaxy assembly nor from black hole feedback.
Galaxies are surrounded by large reservoirs of gas, mostly hydrogen, that are fed by inflows from the intergalactic medium and by outflows from galactic winds. Absorption-line measurements along the lines of sight to bright and rare background quasars indicate that this circumgalactic medium extends far beyond the starlight seen in galaxies, but very little is known about its spatial distribution. The Lyman-α transition of atomic hydrogen at a wavelength of 121.6 nanometres is an important tracer of warm (about 104 kelvin) gas in and around galaxies, especially at cosmological redshifts greater than about 1.6 at which the spectral line becomes observable from the ground. Tracing cosmic hydrogen through its Lyman-α emission has been a long-standing goal of observational astrophysics, but the extremely low surface brightness of the spatially extended emission is a formidable obstacle. A new window into circumgalactic environments was recently opened by the discovery of ubiquitous extended Lyman-α emission from hydrogen around high-redshift galaxies. Such measurements were previously limited to especially favourable systems or to the use of massive statistical averaging because of the faintness of this emission. Here we report observations of low-surface-brightness Lyman-α emission surrounding faint galaxies at redshifts between 3 and 6. We find that the projected sky coverage approaches 100 per cent. The corresponding rate of incidence (the mean number of Lyman-α emitters penetrated by any arbitrary line of sight) is well above unity and similar to the incidence rate of high-column-density absorbers frequently detected in the spectra of distant quasars. This similarity suggests that most circumgalactic atomic hydrogen at these redshifts has now been detected in emission.
The Hubble constant (H0) measures the current expansion rate of the Universe, and plays a fundamental role in cosmology. Tremendous effort has been dedicated over the past decades to measure H0. Gravitational wave (GW) sources accompanied by electromagnetic (EM) counterparts offer an independent standard siren measurement of H0, as demonstrated following the discovery of the neutron star merger, GW170817. This measurement does not assume a cosmological model and is independent of a cosmic distance ladder. The first joint analysis of the GW signal from GW170817 and its EM localization led to a measurement of H0=74+16−8kms−1Mpc−1 (median and symmetric 68% credible interval). In this analysis, the degeneracy in the GW signal between the source distance and the observing angle dominated the H0 measurement uncertainty. Recently, tight constraints on the observing angle using high angular resolution imaging of the radio counterpart of GW170817 have been obtained. Here, we report an improved measurement H0=70.3+5.3−5.0 kms−1Mpc−1 by using these new radio observations, combined with the previous GW and EM data. We estimate that 15 more GW170817-like events, having radio images and light curve data, as compared with 50–100 GW events without such data will potentially resolve the tension between the Planck and Cepheid–supernova measurements.
Permanently shadowed regions near the poles of Mercury and the Moon may cold-trap water ice for geologic time periods. In past studies, thick ice deposits have been detected on Mercury, but not on the Moon, despite their similar thermal environments. Here we report evidence for thick ice deposits inside permanently shadowed simple craters on both Mercury and the Moon. We measure the depth/diameter ratio of approximately 2,000 simple craters near the north pole of Mercury using Mercury Laser Altimeter data. We find that these craters become distinctly shallower at higher latitudes, where ice is known to have accumulated on their floors. This shallowing corresponds to a maximum infill of around 50 m, consistent with previous estimates. A parallel investigation of approximately 12,000 lunar craters using Lunar Reconnaissance Orbiter data reveals a similar morphological trend near the south pole of the Moon, which we conclude is also due to the presence of thick ice deposits. We find that previously detected surface ice deposits in the south polar region of the Moon are spatially correlated with shallow craters, indicating that the surface ice may be exhumed or linked to the subsurface via diffusion. The family of lunar craters that we identify are promising targets for future missions, and may also help resolve the apparent discrepancy between the abundance of frozen volatiles on Mercury and the Moon.
Bars are elongated structures that extend from the centre of galaxies, and about one-third of disk galaxies are known to possess bars. These bars are thought to form either through a physical process inherent in galaxies, or through an external process such as galaxy–galaxy interactions. However, there are other plausible mechanisms of bar formation that still need to be observationally tested. Here we present the observational evidence that bars can form via cluster–cluster interaction. We examined 105 galaxy clusters at redshift 0.015 < z < 0.060 that are selected from the Sloan Digital Sky Survey data, and identified 16 interacting clusters. We find that the barred disk-dominated galaxy fraction is about 1.5 times higher in interacting clusters than in clusters with no clear signs of ongoing interaction (42% versus 27%). Our result indicates that bars can form through a large-scale violent phenomenon, and cluster–cluster interaction should be considered an important mechanism of bar formation.
Several millimeter and submillimeter facilities with linear polarization observing capabilities have started operating during the last years. These facilities, as well as other previous millimeter telescopes and interferometers, require bright and stable linear polarization calibrators to calibrate new instruments and to monitor their instrumental polarization. The current limited number of adequate calibrators implies difficulties in the acquisition of these calibration observations.
Looking for additional linear polarization calibrators in the millimeter spectral range, we started monitoring 3C 286 in mid-2006. This source is a standard and highly stable polarization calibrator for radio observations.
Here we present the 3 mm and 1 mm monitoring observations obtained between September 2006 and January 2012 with the XPOL polarimeter on the IRAM 30 m Millimeter Telescope.
Our observations show that 3C 286 is a bright source of constant total flux with 3 mm flux density S3 mm = (0.91 ± 0.02) Jy. The 3 mm linear polarization degree (p3 mm = [13.5 ± 0.3] %) and polarization angle (χ3 mm = [37.3 ± 0.8] °, expressed in the equatorial coordinate system) were also constant during the time span of our observations. Although with poorer time sampling and signal-to-noise ratio, our 1 mm observations of 3C 286 are also reproduced by a constant source of 1 mm flux density (S1 mm = [0.30 ± 0.03] Jy), polarization fraction (p1 mm = [14.4 ± 1.8] %), and polarization angle (χ1 mm = [33.1 ± 5.7] °).
This, together with the previously known compact structure of 3C 286 – extended by ~3.5′′ in the sky – allow us to propose 3C 286 as a new calibrator for both single-dish and interferometric polarization observations at 3 mm, and possibly at shorter wavelengths.
C-type asteroids are among the most pristine objects in the Solar System, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith-covered surface with low thermal conductivity and particle sizes in the centimetre range. This includes observations of C-type asteroid (162173) Ryugu. However, on arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover of sand- to pebble-sized particles was found to be absent (R.J. et al., manuscript in preparation). Rather, the surface is largely covered by cobbles and boulders, seemingly incompatible with the remote-sensing infrared observations. Here we report on in situ thermal infrared observations of a boulder on the C-type asteroid Ryugu. We found that the boulder’s thermal inertia was much lower than anticipated based on laboratory measurements of meteorites, and that a surface covered by such low-conductivity boulders would be consistent with remote-sensing observations. Our results furthermore indicate high boulder porosities as well as a low tensile strength in the few hundred kilopascal range. The predicted low tensile strength confirms the suspected observational bias in our meteorite collections, as such asteroidal material would be too frail to survive atmospheric entry.