In 2014, the scientific community from around the world were invited to contribute to the making of a science book which purpose is to highlight the ground-breaking science that will be conducted under those science themes. To a large extent, the science presented in this book provides the building blocks shaping up the technical priorities of the SKA.
Astronomers at the Jodrell Bank Centre for Astrophysics played an important role in the writing of the science book, 11 chapters of which were primary authored by JBCA members. You can find below a short description highlighting the JBCA contribution and links to the original papers.
- The exceptional sensitivity of the SKA will allow observations of the Cosmic Dawn and Epoch of Reionization in unprecedented detail, both spectrally and spatially. This wealth of information is buried under emission due to our own Galaxy and extra-galactic objects, referred to as "foregrounds" because they lie between the observer and the signal. Foregrounds are at least a thousand times brighten than the cosmological signal, therefore they must be removed accurately and precisely in order to reveal it.
- An important research topic that is addressed by the Manchester SKA group is the removal of foregrounds in the context of high redshift and high sensitivity 21-cm measurements. Novel, dedicated foreground-removal methods are being developed by exploiting known features of the foreground emission components and the 21-cm signal. State-of-the-art simulations of the SKA observations are used to test the proposed approaches. We compare the recovered cosmological signal using several different statistics and explore one of the most exciting possibilities with the SKA: imaging of the ionized bubbles.
An overview of pulsar science with the SKA (Kramer and Stappers)
- On a time scale of years to decades, gravitational wave (GW) astronomy will become a reality. Low frequency (nanoHz) GWs are detectable through long-term timing observations of the most stable pulsars. Radio observatories worldwide are currently carrying out observing programmes to detect GWs, with data sets being shared through the International Pulsar Timing Array project. One of the most likely sources of low frequency GWs are supermassive black hole binaries (SMBHBs), detectable as a background due to a large number of binaries, or as continuous or burst emission from individual sources. No GW signal has yet been detected, but stringent constraints are already being placed on galaxy evolution models. The SKA will bring this research to fruition.
- The SKA's wide field-of-view, high sensitivity, multi-beaming and sub-arraying capabilities, coupled with advanced pulsar search backends, will result in the discovery of a large population of pulsars. These will enable tests of General Relativity with pulsar binary systems, investigating black hole theorems with pulsar-black hole binaries, and direct detection of gravitational waves in a pulsar timing array. Targeted searches will enable detection of exotic systems, such as the ~1000 pulsars we infer to be closely orbiting Sgr A*, the supermassive black hole in the Galactic Centre.
Three-dimensional Tomography of the Galactic and Extragalactic Magnetoionic Medium with the SKA (Han et al.)
- The magneto-ionic structures of the interstellar medium of the Milky Way and the intergalactic medium are still poorly understood, especially at distances larger than a few kiloparsecs from the Sun. SKA will increase the known pulsar population by an order of magnitude and discover pulsars in the most distant regions of our Galaxy. Their observations will enable detailed tomography of the large-scale magneto-ionic structure of both the Galactic disk and the Galactic halo. In addition, extragalactic pulsars or fast radio bursts to be discovered by SKA can be used to probe the electron density distribution and magnetic fields in the intergalactic medium beyond the Milky Way.
- The SKA will use pulsars to enable precise measurements of strong gravity effects in pulsar systems, which yield tests of gravitational theories that cannot be carried out anywhere else. Dozens of relativistic pulsar systems will be discovered including, possibly, pulsar - black hole binaries, which can be used to test the "cosmic censorship conjecture" and the "no-hair theorem".
- The SKA will discover tens of thousands of pulsars and provide unrivalled sensitivity that will enable us to explore the properties of their magnetospheric radio emission and study eclipsing systems.
- Since their discovery in the late 1960's the population of known neutron stars (NSs) has grown to ~2500. The surveys that will be performed with SKA will produce a further tenfold increase in the number of Galactic NSs known. The much larger statistical samples will allow us to study the link between the various sub-classes of neutron stars (magnetars, intermittent pulsars, rotating radio transients, etc.). Also, it should provide new insights on their extreme properties such as the maximum masses they can have and the mechanisms leading to their formation in supernovae.
- Neutron stars lose the bulk of their rotational energy in the form of a pulsar wind: an ultra-relativistic outflow of predominantly electrons and positrons. This pulsar wind significantly impacts the environment and possible binary companion of the neutron star, and studying the resultant pulsar wind nebulae is critical for understanding the formation of neutron stars and millisecond pulsars, the physics of the neutron star magnetosphere, the acceleration of leptons up to PeV energies, and how these particles impact the interstellar medium.
Probing the neutron star interior and the Equation of State of cold dense matter with the SKA (Watts et al.)
- With an average density higher than the nuclear density, neutron stars (NS) provide a unique test-ground for nuclear physics, quantum chromodynamics (QCD), and nuclear superfluidity. The most stringent observational constraints on their internal structures come from measurements of NS bulk properties: each model generates a unique mass-radius relation which predicts a characteristic radius for a large range of masses, a maximum mass above which NS collapse to black holes, and a maximum spin frequency. Among other things, the dramatic increase in the number of sources provided by the SKA will provide several new NS mass measurements and allow to probe the highest spin frequency end that they can reach.
- The SKA is an integral part of the next-generation observatories that will survey the Universe across the electromagnetic spectrum. Owing to their extreme nature and clock-like properties, pulsars discovered and monitored by SKA will enable a broad range of scientific endeavour which will benefit from coordinated efforts among SKA and other next-generation astronomical facilities.
- Timing a pulsar in orbit around a companion, provides a unique way of probing the relativistic dynamics and spacetime of such a system. The strictest tests of gravity, in strong field conditions, are expected to come from a pulsar orbiting a black hole. In this sense, a pulsar in a close orbit (Porb<1 yr) around the supermassive black hole at the centre of the Milky Way would be the ideal tool. Given the size of the orbit and the relativistic effects associated with it, even a slowly spinning pulsar would allow the black hole spacetime to be explored in great detail.
- It has become apparent that active galactic nuclei (AGN) may have a significant impact on the growth and evolution of their host galaxies and vice versa but a detailed understanding of the interplay between these processes remains elusive. Deep radio surveys provide a powerful, obscuration-independent tool for measuring both star formation and AGN activity in high-redshift galaxies. The sensitivity and resolution of the SKA will allow us to gain a detailed picture of the apparently simultaneous development of stellar populations and black holes in the redshift range where both star-formation and AGN activity peak (1<z<4).
- Radio wavelengths offer the unique possibility of tracing the total star-formation rate in galaxies, both obscured and unobscured. As such, they may provide the most robust measurement of the star-formation history of the Universe. We highlight the constraints that the SKA can place on the evolution of the star-formation history of the Universe, the survey area required to overcome sample variance, the spatial resolution requirements, along with the multi-wavelength ancillary data that will play a major role in maximising the scientific promise of the SKA.
The Astrophysics of Star Formation Across Cosmic Time at >10 GHz with the Square Kilometre Array (Murphy et al.)
- For studying the detailed astrophysics of star formation at high-redshift, surveys at frequencies >10 GHz have the distinct advantage over traditional ~1.4 GHz surveys as they are able to yield higher angular resolution imaging while probing higher rest frame frequencies of galaxies with increasing redshift, where emission of star-forming galaxies becomes dominated by thermal (free-free) radiation. In doing so, surveys carried out at >10 GHz provide a robust, dust-unbiased measurement of the massive star formation rate by being highly sensitive to the number of ionizing photons that are produced.
- Studying galaxy clusters through their Sunyaev-Zel'dovich (SZ) imprint on the Cosmic Microwave Background has many important advantages. The total SZ signal is an accurate and precise tracer of the total pressure in the intra-cluster medium and of cluster mass, the key observable for using clusters as cosmological probes. Band 5 observations with SKA-MID towards cluster surveys from the next generation of X-ray telescopes such as e-ROSITA and from Euclid will provide the robust mass estimates required to exploit these samples. This will be especially important for high redshift systems, arising from the SZ's unique independence to redshift.
- In addition, galaxy clusters are very interesting astrophysical systems in their own right, and the SKA's excellent surface brightness sensitivity down to small angular scales will allow us to explore the detailed gas physics of the intra-cluster medium.
- The study of the Universe on ultra-large scales is one of the major science cases for the SKA. The SKA will be able to probe a vast volume of the cosmos, thus representing a unique instrument, amongst next-generation cosmological experiments, for scrutinising the Universe's properties on the largest cosmic scales. Probing cosmic structures on extremely large scales will have many advantages. For instance, the growth of perturbations is well understood for those modes, since it falls fully within the linear regime. Also, such scales are unaffected by the poorly understood feedback of baryonic physics. On ultra-large cosmic scales, two key effects become significant: primordial non-Gaussianity and relativistic corrections to cosmological observables. Moreover, if late-time acceleration is driven not by dark energy but by modifications to general relativity, then such modifications should become apparent near and above the horizon scale. As a result, the SKA is forecast to deliver transformational constraints on non-Gaussianity and to probe gravity on super-horizon scales for the first time.
Unravelling the origin of large-scale magnetic fields in galaxy clusters and beyond through Faraday Rotation Measures with the SKA (Bonafede et al.)
Statistical methods for the analysis of rotation measure grids in large scale structures in the SKA era (Vacca et al.)
- The Zeeman effect splits spectral lines in proportion to the magnetic field strength in the emitting medium. Radio waves penetrate dusty star-forming regions and masers (bright, narrow lines) are used to measure the magnetic field strength and line-of-sight field direction (and plane-of-sky orientation, using linear polarization), which influences processes from the scale of molecular cloud fragmentation to protostellar discs. The SKA will not only allow much larger scale, more sensitive surveys but detect Zeeman splitting from less dense gas, including thermal emission even from HI, from large-scale structures. This will confirm the extent to which the Galactic magnetic field traces spiral arms and where this breaks down during star formation. Other phenomena investigated will include the origins of supernova/planetary nebula asymmetries and the electron density along the line of sight (via Faraday rotation). The SKA will even be able simultaneously to resolve hydroxyl megamasers from starburst galaxies spatially and spectrally, directly measuring circumnuclear magnetic fields at redshifts up to 1.
- Anomalous Microwave Emission (AME) is thought to be due to electric dipole radiation from small spinning dust grains, although thermal fluctuations of magnetic dust grains may also contribute. Studies of this mysterious component would shed light on the emission mechanism, which then opens up a new window onto the interstellar medium (ISM). AME is emitted mostly in the frequency range ~10-100 GHz, and thus the SKA has the potential of measuring the low frequency side of the AME spectrum, particularly in band 5. Science targets include dense molecular clouds in the Milky Way, as well as extragalactic sources. There is also the possibility of detecting rotational line emission from Poly-cyclic Aromatic Hydrocarbons (PAHs), which could be the main carriers of AME. Detecting PAH lines of a given spacing would allow for a definitive identification of specific PAH species.
SKA studies of nearby galaxies: star-formation & accretion processes in all environments (Beswick et al.)
- Observations of the properties of dense molecular clouds are critical in understanding the process of star-formation. One of the most important, but least understood, is the role of the magnetic fields. High-resolution, high-sensitivity radio observations with the SKA will allow for the first time, measurements of the in-situ synchrotron radiation from these molecular clouds. If the cosmic-ray (CR) particles penetrate clouds as expected, then we can measure the B-field strength directly using radio data. So far, this signature has never been detected from the collapsing clouds themselves and would be a unique probe of the magnetic field. Dense cores are typically ~0.05 pc in size, corresponding to ~arcsec at ~kpc distances, and flux density estimates are ~mJy at 1 GHz. The SKA should be able to readily detect directly, for the first time, along lines-of-sight that are not contaminated by thermal emission or complex foreground/background synchrotron emission. Polarised synchrotron may also be detectable providing additional information about the regular/turbulent fields.
- The bandwidth, sensitivity and sheer survey speed of the SKA offers unique potential for deep spectroscopic surveys of the Milky Way. Within the frequency bands available to the SKA lie many transitions that trace the ionised, radical and molecular components of the interstellar medium and which will revolutionise our understanding of many physical processes. The Milky Way will be probed in great detail by spectroscopic SKA surveys, including "out of the box" early science with radio recombination lines, Phase 1 surveys of the molecular ISM using anomalous formaldehyde absorption, and full SKA surveys of ammonia inversion lines.
The SKA and the Unknown Unknowns (Wilkinson)
- As new scientists and engineers join the SKA project and as the pressures come on to maintain costs within a chosen envelope it is worth restating and updating the rationale for the "Exploration of the Unknown" (EoU). Maintaining an EoU philosophy will prove a vital ingredient for realizing the SKA's discovery potential. Since people make the discoveries enabled by technology a further axis in capability parameter space, the "human bandwidth" is emphasised. Using the morphological approach pioneered by Zwicky, a currently unexploited region of observational parameter space can be identified viz: time variable spectral patterns on all spectral and angular scales - one interesting example would be "spectral transients". We should be prepared to build up to 10% less collecting area for a given overall budget in order to enhance the ways in which SKA1 can be flexibly utilized.
Incoherent transient radio emission from stellar-mass compact objects in the SKA era (Corbel et al.)
- The SKA will spend most of its time looking at distant stars and galaxies, but occasionally it may look a bit closer to home. By looking at the moon, or the atmosphere immediately above the telescope, it could detect radio pulses from particle cascades, which are initiated by interacting cosmic rays. Cosmic rays are fast-moving charged particles in space, some of which have extremely high energies. Detecting cosmic rays with the SKA, by studying the radio pulses they produce, would allow us to learn more about them: what they are, and where they are from.
- By searching for radio pulses from the moon, the SKA could effectively use part of the moon's surface as an enormous cosmic ray detector. This makes it suitable for detecting the extremely rare particles at the top end of the cosmic ray spectrum, with energies exceeding 1020 eV. At these energies, the arrival directions of cosmic rays are similar to the directions of their sources, making it possible to identify where they are being produced. Using the SKA in this way would give us a new, powerful way to study the origin of these ultra-high-energy cosmic rays.
- Detecting radio pulses from particle cascades in the atmosphere would allow the SKA to detect cosmic rays in the energy range from 1017 to 1019 eV. This range is believed to contain the transition between cosmic rays of galactic and extragalactic origin. By studying the properties of the radio pulses, it is possible to determine the energy of the cosmic ray, and the type of particle that it is - for example, a proton or an iron nucleus - which allows the testing of models for how the cosmic rays are produced. The low-frequency component of the SKA, with its dense array of antennas, will be uniquely well-suited to this application. It also has the potential to study the physics of the hadronic interactions that occur in an atmospheric particle cascade, at energies higher than those that can be achieved in artificial particle accelerators.