New Worlds, New Horizons
Radio astronomy in the coming decade is poised for revolutionary advances and exciting discoveries, many of which will be enabled by the transformational capabilities provided by the NRAO facilities, including the GBT. While it is difficult to predict exactly where such dramatic improvements will lead, NRAO builds this vision around the primary science themes identified by the American science community in NWNH.
The GBT's strength is its flexibility, and the high impact of its telescope discoveries. The most important science may come from projects not yet conceived. The GBT will also be key to much of the science outlined in the NWNH.
The following sections outline the many key science areas identified in NWNH in which the GBT will play a fundamental role in enabling progress. These areas are arranged according to the four paramount science themes highlighted by NWNH: Discovery, Origins, Understanding the Cosmic Order and Frontiers of Knowledge. The sections demonstrate the broad range of science that will be afforded users by the NRAO facilities.
Of the five science frontier discovery areas highlighted in NWNH, GBT observations play a crucial role in four: gravitational wave astronomy, time domain astronomy, astrometry and the epoch of reionization.
- Gravitational Wave Astronomy
- Opening the time domain
- Ultra-high precision astrometry
- High Energy Astrophysics
One of the most important pending discoveries in Astronomy and Astrophysics is the direct detection of gravitational waves. An important and competitive method to detect the stochastic background of nano-Hertz gravitational waves, generated by the ensemble of merging super-massive black hole (SMBH) binaries throughout the universe and by individual SMBH binaries, is via high precision timing of millisecond pulsars. Such detection requires the measurements at two observing frequencies, over 5 - 10 years, of the pulses from at least 20 millisecond pulsars spread across the sky at ~100 nanosecond timing accuracy. The pulsars act as the far ends of the arms of a Galactic-scale gravitational wave detector. A direct detection of these gravitational waves is likely during the next 10 years using current facilities, and possibly sooner with improved instrumentation planned for the GBT.
As the world’s premier telescope for pulsar observations, outside the range of Arecibo, the GBT is the central instrument for the North American effort, known as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). NRAO played a key role in establishing NANOGrav in order to devote long-term commitment of GBT observing time for this important scientific goal, developed the ultimate timing backend (GUPPI), and will develop a wide-band timing system essential to the eventual success of NANOGrav. NANOGrav was ranked by the NWNH survey as a compelling project for funding through the proposed Mid-Scale Innovations Program. NRAO is committed to continue applying its scientific and technical expertise to facilitate the implementation of the future goals of NANOGrav, e.g. to go beyond detection to using gravitational waves to probe the physics of the gravitational wave sources.
NANOGrav’s objectives and technology will complement the laser interferometer searches such as Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) and Laser Interferometer Space Antenna (LISA) to detect higher frequency gravitational waves.
The dynamic sky was specifically called out in NWNH as a rich area ripe for discovery. Thanks to new technical developments there are a new generation of synoptic optical and radio imaging telescopes working or under development (e.g., Pan-Starrs, Palomar Transient Factory, Large Synoptic Survey Telescope (LSST), Australian Square Kilometre Array Pathfinder (ASKAP), Westerbork Synthesis Radio Telescope (WSRT/Apertif, MeerKAT). While NRAO facilities are capable of carrying out large-scale time domain surveys, their superb sensitivity, broad frequency response, and ability to make spatially resolved "movies" of evolving objects, make them optimally suited as follow-up instruments.
The VLBA (with the GBT's sensitivity added to the array) is a unique instrument for precision astrometry. The VLBA astrometric accuracy of <10 microarcsec, for example, is better than what the European Space Agency (ESA) Gaia mission will achieve for most of its catalog stars (the catalog will be released no earlier than 2015). Precision astrometry at the VLBA+GBT will continue to improve as new receivers are installed and its sensitivity by expanding its recording bandwidth, and continue to extend the capabilities of its software correlator. For reference, a parallax of 3 micro-arcsecond corresponds to a distance of 1/3 Mpc, and a proper motion of 3 microsecond/year corresponds to 0.15 km/s at a distance of 10 Kpc, or 150 km/s at a distance of 1 Mpc. This is why the NWNH report (p 2-7) pointed out that ”Direct geometric measurements of distances to the Galactic Center, to major regions of star formation in the Milky Way, to nearby galaxies, and, most importantly to galaxies at cosmological distances are possible using precision radio astrometry.”
In the next five years, scientists will directly map for the first time the spiral structure and dynamics of the Milky Way to unprecedented precision, via VLBA+GBT measurements of the distance and proper motion of the 6.7 GHz methanol masers in massive star-forming regions throughout the Galaxy.
Closer to the solar neighborhood, a key project to measure the parallax and proper motion of all Premain- sequence (PMS) stars with detectable radio emission in the Gould’s Belt will provide accurate distances and velocities that will place the study of PMS stars on a much firmer theoretical footing. The VLBA+GBT will also continue searches for exoplanets around M-dwarf stars and PMS stars.
Beyond the Galaxy, the VLBA+GBT will also directly measure the relative motions of the Local Group galaxies, determining the past and future configurations of the Milky Way and its neighbors, and whether the Milky Way will experience a future merger. Angular diameter distance determinations to galaxies in the Hubble Flow, without resort to the extragalactic distance ladder, have been demonstrated with the VLBA and GBT observations of mega-masers, and they will continue to improve the accuracy of the expansion rate of the Local Universe – the Hubble Constant to place high precision constraints to cosmological parameters and the equation of state parameter of Dark Energy.
Astrometry precision is directly proportion to the S/N ratio. Therefore, the use of the GBT and the phased EVLA together with the VLBA, which greatly increases the total collecting area, will be critical to achieving the best possible astrometric measurements.
The VLBA has also long been a mainstay in the establishment and maintenance of the International Celestial Reference Frame (ICRF). The ICRF, anchored by dual-frequency VLBI observations at 2.3 and 8.4 GHz, provides the angular framework for all position measurements in the Universe, including practical applications such as spacecraft navigation and the understanding of the true orientation of the GPS satellite constellation. As part of a potential partnership with NASA for spacecraft navigation, the VLBA will enable the ICRF to begin a transition to the higher 33 GHz frequency. This will significantly reduce the reference-frame errors caused by propagation through the Earth’s ionosphere and the intrinsic structure of the quasar radio sources.
The NASA Fermi Gamma-ray Space Telescope was launched in mid-2008, and its first gamma-ray source catalog was released in early 2009. This catalog will likely include several thousand active galactic nuclei (AGNs) by FY 2011 – 2012. Fermi has been unexpectedly prolific in aiding the discovery of millisecond pulsars through the radio pulsar searches towards unidentified Fermi sources far off the Galactic plane. Point source detections made by Fermi will continue to be searched for pulsations using the GBT, which has already discovered more than 20 millisecond pulsars in these sources, several of which will be crucial for projects such as NANOGrav. Many of these milli-second pulsars are bright sources that can serve as timing sources in NANOGrav, and they will be targets of VLBA astrometry to determine their distances.
- First Sources of Light and the End of the Cosmic Dark Ages
NRAO is a leader in the study of cosmic reionization, and the preceding Dark Ages, through the HI 21cm line of neutral hydrogen. Reionization corresponds to the earliest epoch of galaxy formation, when the first stars and accreting black holes ionized the neutral IGM, and represents the last frontier in the study of cosmic structure formation. The HI 21cm line is widely recognized as the most direct and powerful method with which to probe this epoch, and the study of the HI 21cm signal from reionization was called out as one of the science areas with ‘extraordinary discovery potential’ by the NWNH survey.
NRAO is working closely with community groups to develop the hardware and observational techniques for the first generation pathfinder experiments. While the Green Bank site served as a site for initial hardware testing of Precision Array to Probe the Epoch of Reionization (PAPER), senior members of the NRAO staff continue to be key scientific and technical participants to the PAPER project. The same NRAO staff also helped prepare the highly rated Hydrogen Epoch of Reionization Array (HERA) A2010 white paper, and are playing crucial roles in the deployment of the 128-element PAPER in South Africa. For a realization of the subsequent full HERA program, the NRAO will play a proactive and leading role in facilitating this next generation instrument through university and international partnerships.
In parallel, the GBT, will detect and map the atomic and molecular gas, and dust, in the first generation of galaxies within the epoch of reionization. A particularly exciting prospect is the use of the atomic fine structure lines to determine redshifts for the first galaxies (z = 8 to 10). Getting redshifts for candidate galaxies during cosmic reionization is extremely difficult in the near-IR, but should be straightforward with ALMA using the atomic fine structure lines.
- Origin of Galaxies, Supermassive Black Holes, and Large Scale Structure
- Origin of Stars and Planets
Deciphering the origin of the first stars, galaxies and black-holes, planetary systems and life itself all has to do with understanding the mechanisms of their formation from gas and dust. Radio astronomy techniques are essential to answering the questions involved because the spectral lines from ionized, atomic and molecular gas, and the dust emission in the regions of formation at high redshift fall within a range from meter-wave, such as the 21 cm HI emission line from the Dark Ages and Epoch of Reionization, to submillimeter-wave, from redshifted far-IR line and continuum emission from the first galaxies.
Centimeter through submillimeter wavelength observations play crucial roles in the studies of the molecular lines that probe the fuel for star formation in galaxies, the atomic fine structure lines that are the principal coolants for the interstellar medium gas in distant galaxies, the thermal dust continuum emission that is a key star formation rate estimator, and the radio synchrotron emission that measures star formation and signals the presence of relativistic jets. The “inverse-K” correction due to the shape of the far-IR radiation distribution and lines means that it is possible to detect the gas and dust in the first galaxies beyond z ~ 10 with the GBT.
The origin of the Universe itself can be probed via the detailed studies of the cosmic microwave background and via the detection of gravitational waves from the inflationary era by pulsar timing.
The GBT opens a new window on the cold universe, imaging the dust and cold gas that are the building blocks for stars, planetary systems, galaxies, and life itself. The availability of bolometer arrays on the GBT has opened a discovery window into the properties of gas in galaxy clusters through high resolution measurement of the SZE. The GBT provides an unequaled combination of surface brightness sensitivity and high angular resolution for SZE measurements. The GBT SZE measurements, combined with X-ray data, will determine the cluster gas pressure and indicate the presence of shocks hidden from the imaging X-ray telescopes. The data will reveal evidence of past merger activity or ongoing subcluster mergers. The GBT is the most sensitive instrument in existence for such high-resolution SZE measurements and will provide extensive information on the merging history and processes of galaxy clusters, not easily accessible by other means.
There are many mysteries about star formation. It occurs on the scale of a solar system, but can be triggered by events at the scale of a galaxy, through density waves, tidal encounters, AGN activity, feedback from earlier star formation, and cloud collisions. Advances in our understanding of star formation require observations on all angular scales. The wide-area, high sensitive mapping capability of the GBT is an essential complement to the detailed high-resolution data provided by ALMA, the EVLA, and other interferometers. The GBT is ideally suited for measuring physical conditions in infrared-dark clouds, the likely progenitors of stellar clusters. Although thousands populate our Galaxy, their physical conditions and evolution are poorly understood. A 7-pixel GBT camera operating at 18 GHz to 26 GHz will be used to image and measure the temperature, density, turbulence, and kinematics of a large IDC sample, providing new quantitative insights into star formation and the initial conditions (interactions, shocks, self-gravity) that control dark cloud evolution.
The GBT will also extend molecular observations to nearby galaxies, providing critical information on the large-scale evolution of galactic systems. By measuring the properties of molecular clouds in galaxies, both nearby and distant, the GBT can also link large-scale processes with star formation in a necessary complement to local studies.
The dynamic interplay between stars and the interstellar medium, galaxies and quasars and the intergalactic medium, influences how stars and galaxies evolve and underlies the complex cosmic order.
The NWNH posed the following frontier science questions under this scientific theme:
- How do baryons cycle in and out of galaxies?
- What are the flows of matter and energy in the circum-galactic medium?
- What controls the mass-energy-chemical cycles within galaxies?
- How do black hole work and influence their surroundings?
- How do rotation and magnetic fields affect stars?
- How do massive stars end their lives?
- What are the progenitors of Type Ia SN and how do they explode?
- How diverse are planetary systems and can we identify the telltale signs of life on an exo-planet?
Again, many of the answers require radio observations enabled by NRAO facilities, including the GBT.
- Evolution of Galaxies and Black Holes
- Solar System, Stars and Exoplanets
The GBT provides imaging of the neutral gas around nearby galaxies, and the Milky Way, with unprecedented resolution and surface brightness sensitivity. The GBT mapping of the Magellanic Stream has already revealed crucial information about the recent evolution of the Galactic Neighborhood, and the study of high velocity clouds in our Galaxy and nearby galaxies, will revolutionize our understanding of the lifecycle of gas in and around galaxies, and how it relates to, and possibly drives, star formation. These telescopes will also explore the critical frequency regime between 30 GHz and 50 GHz, which is dominated by free-free emission from star forming galaxies.
The GBT will also image nearby galaxies in CO, HCN, and numerous other molecular species to unlock the processes of star formation.
The chemical composition of star-forming gas in galaxies similar to the Milky Way will be traceable via molecular and atomic spectroscopic observations to z ~ 3 in less than 24 hours of observation. Scientists will quantify the kinematics of obscured galactic nuclei on a large range of spatial scales and assess the influence of chemical and isotopic gradients in galactic disks on the formation of spiral structure.
The numerous and diverse instruments on the GBT give insight into galaxies at all redshifts. By measuring the properties of hot gas in galaxy clusters via the SZE, to detect cold dust at high redshift, the GBT probes the evolution of gas and dust in the Universe on large scales across cosmic time. Measurements of nuclear black hole masses, with the GBT as a single instrument and as part of the High Sensitivity Array (HSA), contribute to the understanding of the role of mergers in galaxy evolution. Surveying the raw material of star formation in CO, HCN, and HI reveals the flow of baryons in and around galaxies and the processes by which galaxies evolve. The GBT will survey thousands of galaxies for their HI content, rotation, and mass. It can detect HI emission an order of magnitude fainter than any other telescope, revealing patterns of interaction and accretion in nearby systems.
How did life arise on Earth? This question is as old as humanity, and the answer will require research across many fields, from biology and chemistry, to physics and astronomy. The GBT has had a leading role in this research, detecting many new organic molecules in space through its ability to measure weak, spatially extended spectral lines over a wide range of frequencies. It will become an increasingly important facility as its capabilities are extended into the lower part of the 3mm band, outside the current ALMA coverage and in a region of the spectrum where no large telescopes operates.
Interstellar molecular clouds are host to chemical reactions that occur under conditions of temperature and density not accessible in terrestrial laboratories. Studies of chemistry in clouds give fundamental information on the nature of the chemical bond in gases and on surfaces, over time scales not achievable on Earth. The GBT will measure interstellar chemical processes and their variation throughout the Milky Way, determine the characteristics of pre-biotic chemistry in star-forming regions, and study the components necessary for the formation of life. Such observations may shed light on the provocative question of the connection, if any, between organic chemistry in space and life on Earth. With wide-field cameras, the GBT will allow rapid imaging of cometary molecules, revealing the content of the building materials of the Solar System.
The chemistry of life on Earth most likely originated in the protosolar nebula, and analogous chemical processes may be observable in interstellar clouds around the Galaxy. More than 160 molecules have already been identified in interstellar and circumstellar sources. Many of these are complex species of ten or more atoms, and they include biologically significant molecules such as formic acid (HCOOH), acetic acid (CH3COOH), ethylene glycol (HOCH2CH2OH), and the simplest member of the sugar family, glycolaldehyde (CH2OHCHO). The complexity of interstellar chemistry and the existence of plausible delivery mechanisms such as comets and meteorites suggest that some part of pre-biotic chemistry on the early Earth and similar planets occur in interstellar and proto-solar gas clouds. NRAO has the perfect suite of telescopes to pursue this fundamental area of research, the first step of astrobiology. The GBT has sensitivity both to molecular cores and extended emission fields, and the frequency agility to cover the rich molecular bands from centimeter through 3 millimeter wavelength. It is the most capable instrument for initial identification and characterization of sources and species in these wavebands. Ices containing complex molecules survive the throes of planetary formation in cold distant reaches of proto-planetary systems, and are delivered to formed planets via orbital disruption and planetary impact. Large scale variations in chemistry may lend different characters in proto-planetary systems in diverse parts of the parent molecular cloud. Such differences may lead, in turn, to different chemistry on the surface layers of inner planets. This fosters planetary diversity as well as variation in general proto-planetary disk properties and evolution, in the environment and in the dynamics of the new planetary systems.