Science > Highlights > 2018 Science Highlights

2018 Science Highlights

by Davis Murphy last modified Feb 15, 2019

Solving the Mystery of Gravitational-wave EM Sources

by Mark Adams last modified Jan 28, 2019 by Davis Murphy

outflow.jpgGW170817 was the first gravitational-wave detection of a binary neutron-star merger. This event was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of 40 Megaparsecs. It has been proposed that the observed γ-ray, X-ray and radio emission is due to an ultra-relativistic jet being launched during the merger successfully breaking out of the surrounding material, directed away from our line-of-sight. The presence of such a jet is predicted from models that posit neutron-star mergers as the drivers of short hard γ-ray bursts. In this contribution, Mooley et al. report that the radio light curve of GW170817 has no direct signature of the afterglow of an off-axis jet. Although the existence of a jet directed away from the line of sight cannot be completely ruled out, the observed γ-ray emission could not have originated from such a jet. Instead, the radio data require the existence of a mildly relativistic wide-angle outflow moving towards Earth. This outflow could be the high-velocity tail of the neutron-rich material that was ejected dynamically during the merger, or a cocoon of material that breaks out when a jet launched during the merger transfers its energy to the dynamical ejecta. Because the cocoon model explains the radio light curve of GW170817, as well as the γ-ray and X-ray emission, and possibly also the ultraviolet and optical emission, it is the model that is most consistent with the observational data. Cocoons may be a ubiquitous phenomenon produced in neutron-star mergers, giving rise to a hitherto unidentified population of radio, ultraviolet, X-ray and γ-ray transients in the local Universe.

Caption: [a] The flux densities Sν correspond to the detections (markers with 1σ error bars) and upper limits (downward-pointing arrows) of GW170817 from 0.6 – 15 GHz (black for ≤1 GHz; yellow for ≥10 GHz) between days 16 and 107 after the merger. The marker shapes denote measurements from different telescopes. [b] Same as a, but with flux densities corrected for the spectral index α = −0.61and with early-time, non-constraining upper limits removed. The light curve fit with temporal index δ = 0.78 is the red line and the uncertainty in δ (±0.05) as the red shaded region. [c] Residual plot after correcting for the spectral and temporal variations. 

Publication: K.P. Mooley (Oxford, NRAO, California Institute of Technology) et al., A Mildly Relativistic Wide-angle Outflow in the Neutron-star Merger Event GW170817, 8 February 2018, Nature, 554, 207.

Radio Emission in Ultracool Dwarfs

by Mark Adams last modified Jan 28, 2019 by Davis Murphy

dwarfs.jpgCool dwarf stars are the most common planetary hosts. They are also very active in radio flaring, and hence provide the best means to study star–planet magnetospheric interactions, and possibly their influence on the development of life.

To investigate the radio emission of ultracool objects, Guirado et al. carried out a targeted search in the recently discovered system VHS J125601.92–125723.9 (hereafter simply VHS 1256–1257). this system is composed by an equal-mass M7.5 binary and a L7 low-mass substellar object located at only 15.8 pc. The research tea, observed VHS 1256–1257 system with the Karl G. Jansky Very Large Array in phase-reference mode at X-band and L-band, and with the European Very Long Baseline Interferometry Network at L band in several epochs during 2015 and 2016.

Radio emission was discovered at X band that is spatially coincident with the equal-mass M7.5 binary with a flux density of 60 μJy. The measured spectral index was α = −1.1 ± 0.3 between 8 and 12 GHz, suggesting that non-thermal, optically thin, synchrotron, or gyrosynchrotron radiation is responsible for the observed radio emission. Interestingly, no signal is seen at L band to a 3σ upper limit of 20 μJy. This might be explained by strong variability of the binary or self-absorption at this frequency. By adopting the latter scenario and gyrosynchrotron radiation, the authors constrain the turnover frequency to be in the interval 5–8.5 GHz, from which they infer the presence of kiloGauss-intense magnetic fields in the M7.5 binary. These data impose a 3σ upper bound to the radio flux density of the L7 object of 9 μJy at 10 GHz.

Caption: [Left] VLA image of the VHS 1256–1257 field at X-band. The detected source is readily assigned to the M7.5 binary. The (undetected) L7-object b location is at the solid white box. The 3σ threshold detection is 9 μJy. At 15.8 pc, the separation between components AB and b corresponds to 128.4 AU. [Right] VLA image of the VHS 1256–1257 field at L-band. A solid box, with size that of the X-band image, is centered at the position of the X-band detection. None of the VHS 1256–1257 components is detected. The 3σ threshold detection is 20 μJy. The two bright knots seen in the map at the NW correspond to known extragalactic radio sources.

Publication: J.C. Guirado (Universitat de València) et al., Radio Emission in Ultracool Dwarfs: The Nearby Substellar Triple System, 2018, Astronomy & Astrophysics, 610, A23.

A Massive Galaxy in the Early Universe

by Mark Adams last modified Jan 28, 2019 by Davis Murphy

massive_galaxy.jpgAccording to our current understanding of cosmic structure formation, the precursors of the most massive structures in the Universe began to form shortly after the Big Bang, in regions corresponding to the largest fluctuations in the cosmic density field. Observing these structures during their period of active growth and assembly—the first few hundred million years of the Universe—is challenging because it requires surveys that are sensitive enough to detect the distant galaxies that act as signposts for these structures and wide enough to capture the rarest objects. As a result, very few such objects have been detected to date. 

In this contribution, Marrone et al. report observations of a far-infrared-luminous object at redshift 6.900– just 780 million years after the Big Bang–that was discovered in a wide-field survey. This object, SPT0311−58, was originally identified in the 2,500-deg2 South Pole Telescope survey as a luminous source with a steeply increasing spectrum, indicative of thermal dust emission. Observations with ALMA provide the redshift of the source. 

High-resolution imaging shows it to be a pair of extremely massive star-forming galaxies. The larger is forming stars at a rate of 2,900 Mper year, contains 270 billion Mof gas and 2.5 billion Mof dust, and is more massive than any other known object at a redshift of more than 6. Its rapid star formation is probably triggered by its companion galaxy at a projected separation of 8 kiloparsecs. This merging companion hosts 35 billion Mof stars and has a star-formation rate of 540 Mper year, but has an order of magnitude less gas and dust than its neighbor and physical conditions akin to those observed in lower-metallicity galaxies in the nearby Universe. These objects suggest the presence of a dark-matter halo with a mass of more than 100 billion M, making it among the rarest dark-matter haloes that should exist in the Universe at this epoch.

Caption: Image & spectra of the [CII] and [OIII] emission from the z=6.9 lensed starburst galaxy SPT0311-58.

Publication: D.P. Marrone (Steward Observatory, University of Arizona) et al., Galaxy Growth in a Massive Halo in the First Billion Years of Cosmic History, 4 January 2018, Nature, 553, 51.

The Onset of Star Formation in the Universe

by Mark Adams last modified Jan 28, 2019 by Davis Murphy

onset_formation.jpgAstronomers are striving to see the earliest galaxies in our Universe and study how they influenced the intergalactic medium a few hundred million years after the Big Bang. The abundance of star-forming galaxies declines from redshifts of ~ 6-10. A key question is the extent of star formation at earlier times, when the first galaxies emerged. In a recent paper in Nature (reference below), Takuya Hashimoto et al. report and analyze Atacama Large Millimeter/submillimeter Array (ALMA) spectroscopic data of MACS1149-JD1, a gravitationally-lensed galaxy observed when the Universe was less than 4% of its current age. The authors securely detect their target, the 88 μm emission line of doubly ionized oxygen, at a redshift of 9.1096 ± 0.0006 to a significance of more than 7σ. The spatial location of this emission is coincident with the rest-frame ultraviolet continuum emission detected by the Hubble Space Telescope and Lyα emission tentatively detected by the Very Large Telescope at the same redshift, indicating that the [O III] line arises from a star-forming galaxy. This precisely determined redshift indicates that the red rest-frame optical color arises from a dominant stellar component that formed ~ 250 million years after the Big Bang, corresponding to a redshift of ~ 15. These results indicate that it may be possible to characterize such early episodes of star formation in similar galaxies with future telescopes.

Image: A Hubble Space Telescope image of MACS1149-JD1with the ALMA [O III] contours overlaid.

Publication: The onset of star formation 250 million years after the Big Bang, Takuya Hashimoto (Osaka Sangyo University) et al., Nature 557, 392-395 (2018).