The primary mission of the ASTRON Astronomy Group is to conduct world-leading scientific research. The Astronomy Group also plays a vital role supporting the Radio Observatory in the operation of our national facilities LOFAR and the Westerbork Synthesis Radio Telescope (WSRT). As expert users of our local telescopes, researchers in the Astronomy Group push the capabilities of these instruments to achieve exciting new science, while simultaneously working with members of the Radio Observatory to bring those new capabilities to the world-wide astronomy community. We also collaborate closely with members of the ASTRON Research & Development department to develop and exploit new technologies to improve our current facilities, such as the APERTIF upgrade to the WSRT, as well as build the next generation of radio instruments such as the SKA. Members of the ASTRON Astronomy Group were active across all these areas in 2016.
Recruitment process It was also a year of change within the Astronomy Group in 2016 with some staff departures and a number of new members joining the group. Dr. Antonia Rowlinson joined the Astronomy Group in 2016 as a joint tenure-track staff appointment between ASTRON and the University of Amsterdam to work on radio transients. Dr. Jess Broderick also joined the group as SKA Project Scientist to help support ASTRON’s contributions to the Mid Frequency and Low-Frequency Aperture Array SKA design constortia. Following the departures of Dr. George Heald to lead the low-frequency group at CSIRO in Perth, Australia at the end of 2015 and Dr. Adam Deller to join the group at Swinburne University in the summer of 2016, we began a new recruitment process to replace these positions. This process culminated in early 2017 with the addition of three new junior staff. Dr. Betsey Adams was awarded one of the inaugural NWO WISE fellowships and joined the group as a tenure-track staff appointment focussing on HI observations of nearby low-mass galaxies. Dr. Tim Shimwell from Leiden University joined the group as a staff astronomer studying galaxy clusters at low-frequency using the LOFAR all-sky surveys. Finally, Dr. André Offringa joined the group specializing in low-frequency studies of the Epoch of Reionization (EoR) and the development of cutting edge calibration and imaging techniques.
Over 200 refereed scientific publications Amidst these changes, the Astronomy Group continued to have a very productive year scientifically. Along with astronomers from the Radio Observatory, AG astronomers published over 200 refereed scientific publications in 2016 more than 30 of which were first author publications. Some select scientific highlights from the year are described below.
Acquiring grants Along with pure scientific output, ASTRON astronomers also had a successful year in terms of acquiring grants to fund their research and other development activities. Joeri van Leeuwen was awarded an NWO NLeSC center award for the AA-ALERT project to develop a system to interpret incoming data streams from the newly upgraded WSRT in real-time and trigger alerts for follow-up observations with a global network of telescopes. Gemma Janssen was awarded an NWO TOP grant to characterize the effects of space weather on our prospects for detecting gravitational waves. Michael Wise also led the successful H2020 AENEAS proposal to design a large-scale Science Data Centre in Europe for extracting SKA science.
Finally, two of our ASTRON AG staff were recognized for their achievements by the NL astronomy community in 2016. Jason Hessels won the Pastoor Schmeits Prize for astronomy. The prize is awarded every three years to a Dutch astronomer who has made a scientific contribution of exceptional importance before the age of 40. Jason was involved in a number of important discoveries in this area of research including the fastest-rotating pulsar, a pulsar in a system consisting of three stars (a double-double star), and most recently a repeating fast radio burst, which showed eleven eruptions since 2012. At the same time, Joeri van Leeuwen won the Willem de Graaff award for 2016. This prize is meant for someone who is professionally engaged in astronomy and/or space research and also contributes to the popularization of these scientific fields. The jury praised Joeri’s contributions to the popularization of astronomy on various subjects, using many forms of outreach activities including lectures and video, and for a diverse audience, from young to old.
ASTRON Astronomy Group staff were recognized for their many achievements in 2016. Jason Hessels (left) won the Pastoor Schmeits Prize for astronomy given every three years to a Dutch astronomer who has made a scientific contribution of exceptional importance before the age of 40. Joeri van Leeuwen (right) won the Willem de Graaff award for 2016 for outstanding contributes to the popularization of astronomy and space research.
A repeating fast radio burster – A team of astronomers, including Jason Hessels from ASTRON, have for the first time have detected repeating short-duration bursts of radio waves from an enigmatic source which is likely located well beyond the edge of our Milky Way galaxy. The findings indicate that these “fast radio bursts” come from an extremely powerful object which occasionally produces multiple bursts in under a minute. This discovery was made using data from the Arecibo telescope in Puerto Rico. Prior to this discovery, reported in Nature, all previously detected fast radio bursts (FRBs) have appeared to be one-off events. Consequently, most theories about the origin of these mysterious pulses have involved cataclysmic incidents that destroy their source, a star exploding in a supernova, for example, or a neutron star collapsing into a black hole. The new finding, however, shows that at least some FRBs have other origins.
A glitch in a millisecond pulsar – Recent work co-led by ASTRON astronomer Gemma Janssen has revealed a small glitch in the spin evolution of PSR J0613-0200. (Paper: McKee, Janssen et al., 2016, MNRAS 461, 2809). Including data from Jodrell Bank that precluded our EPTA data release, revealed a microglitch occuring in March 1998. A glitch is a sudden spin-up of the pulsar and is believed to be caused by transfer of angular momentum from the superfluid interior of the neutron star to its solid crust. This is the second glitch ever found in a millisecond pulsar, and moreover is the smallest glitch known to date showing a change in the rotational frequency of the pulsar of only 8E-10Hz. This result is in particular interesting as this pulsar is also used as an element in a pulsar timing array, with which we ultimately hope to detect gravitational waves. Having a glitch in one of our most stable pulsar was worrying at first, but we were able to prove that including the glitch in the timing model did not affect the long-term quality of this pulsar. The EPTA new data release was created by the EPTA timing working group, including Gemma Janssen (ASTRON), and also features EPTA members Cees Bassa (ASTRON), Jason Hessels (ASTRON/UvA), Sotiris Sanidas and Roy Smits.
Timing residuals for PSR J0613-0200, showing that the model does not correctly predict the arrival times before March 1998 due to an unmodelled glitch.
Localization and Broadband Follow-up of the Gravitational-wave Transient GW150914 – A groundbreaking new chapter has begun in gravitational wave (GW) astronomy, with the detection of GWs from two binary black hole mergers by Advanced LIGO (ALIGO). Two papers outlining the extensive electromagnetic (EM) follow-up campaign of the first detection – GW150914 – are now available in the literature: Abbott et al. 2016a, ApJ, 826, L13 and Abbott et al. 2016b, ApJS, 225, 8. ASTRON co-authors Jess Broderick (ASTRON) and Antonia Rowlinson (ASTRON/UvA) led the LOFAR follow-up effort; the observations are summarized in the following circulars: Broderick et al. 2015, GCN, 18364; Broderick et al. 2015, GCN, 18424; Rowlinson et al. 2015, GCN, 18690. LOFAR was the only northern hemisphere wide-field radio telescope contributing to this search, and will remain a prime facility for EM follow-up, given that ALIGO localizations of GW candidates are relatively poor.
While the radio telescopes did not detect anything for this event, with this infrastructure in place and now successfully tested, these SKA pathfinders will also contribute to future LIGO detections. Both the MWA and LOFAR have cooperative agreements with LIGO and other telescopes to follow up time-critical astronomical events such as gravitational wave events and gamma-ray bursts. When an alert from other instruments like LIGO is received, these facilities can immediately initiate follow-up observations of the source often within seconds.
Electromagnetic followup of GW150914. The 50% and 90% confidence levels of the initially distributed GW localization maps are shown; LOFAR observed the equatorial hotspot from the ‘cWB’ localization (thinner contours). Image courtesy of Antonia Rowlinson (ASTRON/UvA) and Jess Broderick (ASTRON).
Neutral hydrogen and magnetic fields in M83 observed with the SKA Pathfinder KAT-7 – An international team including ASTRON astronomer Erwin de Blok (ASTRON) and former ASTRON astronomer George Heald used the South-African SKA-precursor telescope KAT-7 to study neutral hydrogen and polarised radio continuum emission in the nearby spiral galaxy M83. These observations provide a sensitive probe of the outer-disc structure and kinematics, revealing a vast and massive neutral gas distribution that appears to be tightly coupled to the interaction of the galaxy with the environment. They present a new rotation curve extending out to a radius of 50 kpc. They also study the periphery of the H I distribution and reveal a sharp edge to the gaseous disc that is consistent with photoionization or ram pressure from the intergalactic medium. The radio continuum emission is not nearly as extended as the H I and is restricted to the main optical disc. Despite the relatively low angular resolution, broad conclusions can be drawn about the large-scale magnetic field topology: the magnetic field of M83 is similar in form to other nearby star-forming galaxies, and suggest that the disc-halo interface may host a large-scale regular magnetic field. (Paper: Heald, de Blok, et al., 2016, MNRAS 462, 1238).
HI column density levels of the M83 KAT-7 observations overlaid on a three-color infrared WISE image. The image measures about 1 x 1 degrees.
ALFALFA Discovery of the most metal-poor gas-rich galaxy known: AGC 198691 – As part of the SHIELD program, metallicities are being obtained for the low HI mass galaxies. Recently, an international team including Betsey Adams, identified the most metal-poor star-forming galaxy known in the local Universe through this effort. Metal-poor galaxies are one of the best testbeds for understanding conditions early in the Universe. The gas out of which metal-poor galaxies are forming stars has a similar metal content as the gas forming stars in the early Universe. In addition, the amount of helium found in extremely metal-poor galaxies can be compared to predictions for the primordial abundance from Big Bang nucleosynthesis. Leoncino was originally found by the ALFALFA HI survey and identified as a low mass dwarf galaxy. Subsequent observations of its only HII region revealed it to have an extremely low oxygen abundance. Follow-up observations with WSRT localized the ALFALFA HI detection to this galaxy. The WSRT data also hint at signs of interaction in this system, although further observations are needed to confirm this. Interestingly, two of the five lowest-abundance star-forming galaxies known have been discovered by the ALFALFA HI survey. This highlights an important parameter space for the upcoming HI surveys with Apertif. (Hirschauer+ 2016)
Left: HST image of AGC 198691 with neutral hydrogen (HI) contours from Westerbork overlaid. Right: zoom-in on the optical galaxy. The WSRT observations localize the ALFALFA HI detection to this galaxy.
Black Hole Fed by Cold Intergalactic Deluge – An international team of astronomers, including ASTRON astronomers Raymond Oonk and Michael Wise, have witnessed a cosmic weather event in the form cold molecular gas clouds falling in towards the central supermassive black hole in the centrally dominant galaxy of the bright Abell 2597 galaxy cluster. This galaxy cluster is located at a distance of about 1 billion light-years from us. These new ALMA (Atacama Large Millimeter/submillimeter Array) observations show for the first time unambiguous observational evidence for a chaotic, cold ‘rain’ feeding a supermassive black hole with a mass about 300 million times that of our Sun. The infalling clouds were revealed by narrow absorption features from carbon monoxide (CO) against the central line of sight towards the black hole. The observed molecular cloud velocities, in combination with detailed very long baseline radio interferometric observations of cold atomic gas, place these clouds very the near (less than 300 lightyears) the black hole and on rapidly infalling orbits. Each of these clouds has a size of a few tens of light-years across and contains about a million solar masses of cold molecular gas. The discovered clouds are likely part of a much larger, centralized, clumpy distribution of cold clouds that provide the necessary fuel for the black hole regulating the thermal balance in the core of this galaxy cluster. The results have been published in Nature.
The background image (blue) is from the NASA/ESA Hubble Space Telescope. The foreground (red) is ALMA data showing the distribution of carbon monoxide gas in and around the galaxy. The pull-out box shows the ALMA data of the “shadow” (black) produced by absorption of the millimetre-wavelength light emitted by electrons whizzing around powerful magnetic fields generated by the galaxy’s supermassive black hole. The shadow indicates that cold clouds of molecular gas are raining in on the black hole. Credit: B. Saxton (NRAO/AUI/NSF)/G. Tremblay et al./NASA/ESA Hubble/ALMA (ESO/NAOJ/NRA).
The LOFAR Lockman Hole project – Some of the high-priority pointings of LOFAR have been on famous fields. One of these fields is the so called “Lockman Hole”, first identified by Lockman et al. (1986) who noted the region had a very low column density of Galactic HI, making it ideal for deep observations of extragalactic sources, particularly in the IR due to the low IR background. Since then, optical and infrared data have been taken using ground-based telescopes as well as the Spitzer and Herschel satellites. Most radio telescopes have also observed this region. The latest addition to this impressive collection is the image made with LOFAR. The new LOFAR observations extend the multi-frequency radio information available for the Lockman Hole down to 60 MHz, allowing us to explore a new spectral window for the faint radio source population. This image shows a zoomed-in region approximately 2-degrees across at 150 MHz, a fraction of the full image which covers 34.7 square degrees. The image has a resolution of 18.6×14.7 arcsec and reaches an rms of 160 uJy/beam at the centre of the field. Work is in progress to further improve the quality and the resolution using the facet calibration. As expected for a low-frequency selected sample, the vast majority of sources exhibit steep spectra, with a median spectral index of -0.78 between 150 MHz and 1.4 GHz. For a bright subset we can trace the spectral properties down to lower frequencies using 60-MHz LOFAR data, finding tentative evidence for sources to have flatter spectra between 60 and 150 MHz. We also identify a sample of 100 Ultra-steep spectrum (USS) sources and 13 peaked-spectrum sources. We estimate that up to 21 percent of these are candidate high-z radio galaxies, but further observations are required to confirm the physical nature of these objects. These results are presented in a paper accepted for publication in MNRAS including a number of ASTRON (and former-ASTRON) astronomers: Elizabeth Mahony, Raffaella Morganti, Ilse van Bemmel, Marisa Brienza, Jeremy Harwood and George Heald.
Zoom-in of part of the LOFAR 150MHz Lockman Hole field.
The launching region of a protostellar outflowed resolved – Young stars are associated with prominent outflows of molecular gas. The ejection of gas is believed to remove angular momentum from the protostellar system, permitting young stars to grow by the accretion of material from the protostellar disk. Various models have been proposed to explain the outflows, differing mainly in the region where acceleration of material takes place: close to the protostar itself (‘X-wind’, or stellar wind), in a larger region throughout the protostellar disk (disk wind), or at the interface between the two. Until the publication of this observational result, outflow launching regions had only been probed by indirect extrapolation. In Bjerkeli, Van der Wiel, et al. 2016, Nature 540, 406, we report resolved images of carbon monoxide (CO) towards the outflow associated with the TMC1A protostellar system. These data show that gas is ejected from a region extending up to a radial distance of 25 astronomical units from the central protostar, and that angular momentum is removed from an extended region of the disk. This work has provided the first direct evidence of a system in which outflowing gas is launched by an extended disk wind from a Keplerian disk.
The protostar TMC1A as observed with ALMA, using interferometric baselines up to 16km. Its circumstellar disk is shown in green (dust continuum), and blue- and redshifted CO emission in blue and red, respectively.
Preparation for APERTIF Science During 2016 multiple members of the AG (B. Adams, B. Adebahr, de Blok, K.M. Hess, R. Morganti, T. Oosterloo) were involved with the commissioning of the imaging mode of Apertif. During 2016, the first images with Apertif were made. This culminated at the end of the year with the first wide-field image made with beam-forming. This image was centered on Leo T, a low mass, HI-rich dwarf galaxy. These observations were only a single subband and limited number of antennas, but highlight the potential of Apertif to discover new low-mass, gas-rich dwarf galaxies.
Layout of the Apertif Surveys.The red dots indicate pointings of the Medium-Deep Survey, while the small black dots indicate those of the Shallow Survey. The black squares give the locations of the special LOFAR fields that will also be imaged by Apertif. The red, dashed line indicates δ = 30 ̊. The blue rectangle shows the location of the Herschel-Atlas field, while the green box indicated the rough outline of the HetDex survey area. The grey, curly contour indicates the locations where the Galactic foreground extinction is 1 magnitude in the V band.
The first wide-field Apertif image. The old WSRT field of view and the full moon are shown for scale. The blue is the neutral hydrogen (HI) from Leo T, shown on the continuum image over the full field of view.
Contributions to SKA Science and Design Members of the ASTRON Astronomy Group are contributing to the many aspects of the SKA preparation process both scientifically and technically. Almost all astronomers in the group have been involved in refining the SKA science case and contributed to over 25 chapters in the recent two-volume collection “Advancing Astrophysics with the Square Kilometre Array” as either first author or co-author. In addition, several members of the group are serving or have served as chair for some of the current SKA Science Working Groups (such Erwin de Blok for the HI SWG, Jason Hessels, Joeri van Leeuwen, and Gemma Janssen for the Pulsars SWG, and John McKean for the Continuum SWG).
In terms of the SKA technical design, ASTRON is contributing to a number of the consortium working on various aspects of the design. For example, ASTRON is leading the Mid-Frequency Aperture Arrays (MFAA) consortium and developing the system design of the antennas and analog electronics as well as the science requirements. Jess Broderick from the Astronomy Group has been contributing heavily to this work as well as to the Low Frequency Aperture Arrays (LFAA) effort.
Finally, in 2016, the European Commission approved the H2020 AENEAS project. AENEAS is a 3M euro project intended to develop a design and operational model for a distributed, network of large-scale regional data centres for the SKA and includes partners from 13 countries and 28 institutions around the world. AENEAS stands for the ”Advanced European Network of E-infrastructures for Astronomy with the SKA’‘ and is designed to address the challenges the community will face in extracting science from the incredible amounts of data the SKA will produce. ASTRON is the leading institute for the project and Michael Wise from the Astronomy Group is the EC coordinator.
The new SKA science book, Advancing Astrophysics with the Square Kilometre Array, published in two volumes in 2015.
AENEAS is a 3M euro EC H2020 project intended to develop a design and operational model for a distributed, network of large-scale regional data centres for the SKA.