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Register a new userSearching for high-redshift centimeter-wave continuum, line and maser emission using the Square Kilometer Array
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Authors
Andrew W. Blain
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We discuss the detection of redshifted line and continuum emission at radio wavelengths using a Square Kilometer Array (SKA), specifically from low-excitation rotational molecular line transitions of CO and HCN (molecular lines), the recombination radiation from atomic transitions in almost-ionized hydrogen (radio recombination lines; RRLs), OH and water maser lines, as well as from synchrotron and free-free continuum radiation and HI 21-cm line radiation. The detection of radio lines with the SKA offers the prospect to determine the redshifts and thus exact luminosities for some of the most distant and optically faint star-forming galaxies and active galactic nuclei (AGN), even those galaxies that are either deeply enshrouded in interstellar dust or shining prior to the end of reionization. Moreover, it provides an opportunity to study the astrophysical conditions and resolved morphologies of the most active regions in galaxies during the most active phase of star formation at redshift z~2. A sufficiently powerful and adaptable SKA correlator will enable wide-field three-dimensional redshift surveys at chosen specific high redshifts, and will allow new probes of the evolution of large-scale structure (LSS) in the distribution of galaxies. The detection of molecular line radiation favours pushing the operating frequencies of SKA up to at least 26 GHz, and ideally to 40 GHz, while very high redshift maser emissions requires access to about 100 MHz. To search for LSS the widest possible instantaneous field of view would be advantageous.
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We present the results from recent VLA 8.5-GHz and WSRT 1.4 and 4.9-GHz monitoring campaigns of the CLASS gravitational lens B1600+434 and show how the observed variations argue strongly in favor of microlensing by MACHOs in the halo of a dark-matter dominated edge-on disk galaxy at z=0.4. The population of flat-spectrum radio sources with micro-Jy flux-densities detected with the Square-Kilometer-Array is expected to have dimensions of micro-arcsec. They will therefore vary rapidly as a result of Galactic scintillation (diffractive and refractive). However, when positioned behind distant galaxies they will also show variations due to microlensing, even more strongly than in the case of B1600+434. Relativistic or superluminal motion in these background sources typically leads to temporal variations on time scales of days to weeks. Scintillation and microlensing can be distinguished, and separated, by their different characteristic time scales and the frequency dependence of their modulations. Monitoring studies with Square-Kilometer-Array at GHz frequencies will thus probe both microscopic and macroscopic properties of dark matter and its mass-function as a function of redshift, information very hard to obtain by any other method.
HI 21~cm absorption spectroscopy provides an excellent probe of the neutral gas content of absorbing galaxies, yielding information on their kinematics, mass, physical size and ISM conditions. The high sensitivity, unrivaled frequency coverage and RFI suppression techniques of the SKA will enable it to use HI absorption to study the ISM of high column density intervening systems along thousands of lines of sight out to high redshifts. Blind SKA 21~cm surveys will yield large, unbiased absorber samples, tracing the evolution of normal galaxies and active galactic nuclei from $z gtrsim 6$ to the present epoch. It will thus be possible to directly measure the physical size and mass of typical galaxies as a function of redshift and, hence, to test hierarchical models of structure formation.
The recent detections of extrasolar giant planets has revealed a surprising diversity of planetary system architectures, with many very unlike our Solar System. Understanding the origin of this diversity requires multi-wavelength studies of the structure and evolution of the protoplanetary disks that surround young stars. Radio astronomy and the Square Kilometer Array will play a unique role in these studies by imaging thermal dust emission in a representative sample of protoplanetary disks at unprecedented sub-AU scales in the innermost regions, including the ``habitable zone that lies within a few AU of the central stars. Radio observations will probe the evolution of dust grains up to centimeter-sized ``pebbles, the critical first step in assembling giant planet cores and terrestrial planets, through the wavelength dependence of dust emissivity, which provides a diagnostic of particle size. High resolution images of dust emission will show directly mass concentrations and features in disk surface density related to planet building, in particular the radial gaps opened by tidal interactions between planets and disks, and spiral waves driven by embedded protoplanets. Moreover, because orbital timescales are short in the inner disk, synoptic studies over months and years will show proper motions and allow for the tracking of secular changes in disk structure. SKA imaging of protoplanetary disks will reach into the realm of rocky planets for the first time, and they will help clarify the effects of the formation of giant planets on their terrestrial counterparts.
The redshifted spectral line radiation emitted from both atomic fine-structure and molecular rotational transitions in the interstellar medium (ISM) of high-redshift galaxies can be detected in the centimetre, millimetre and submillimetre wavebands. Here we predict the counts of galaxies detectable in an array of molecular and atomic lines. This calculation requires a reasonable knowledge of both the surface density of these galaxies on the sky, and the physical conditions in their ISM. The surface density is constrained using the results of submillimetre-wave continuum surveys. Follow-up OVRO Millimeter Array observations of two of the galaxies detected in the dust continuum have provided direct measurements of CO rotational line emission at redshifts of 2.56 and 2.81. Based on these direct high-redshift observations and on models of the ISM that are constrained by observations of low-redshift ultraluminous infrared galaxies, we predict the surface density of line-emitting galaxies as a function of line flux density and observing frequency. We incorporate the sensitivities and mapping speeds of existing and future millimetre/submillimetre-wave telescopes and spectrographs, and so assess the prospects for blank-field surveys to detect this line emission from gas-rich high-redshift galaxies.
SKA is a new technology radio-telescope array, about two orders of magnitude more sensitive and rapid in sky surveys than present instruments. It will probe the dark age of the universe, just afer recombination, and during the epoch of reionisation (z=6-15); it will be the unique instrument to map the atomic gas in high redshift galaxies, and determine the amount and distribution of dark matter in the early universe. Not only it will detect and measure the redshifts of billions of galaxies up to z=2, but also it will discover and monitor around 20 000 pulsars in our Milky Way. The timing of pulsars will trace the stretching of space, able to detect gravitational waves. Binary pulsars will help to test gravity in strong fields, and probe general relativity. These exciting perspectives will become real beyond 2020.
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