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Register a new userImaging Protoplanetary Disks with a Square Kilometer Array
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D. J. Wilner
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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.
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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.
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.
Very Long Baseline Interferometry (VLBI) at radio wavelengths can provide astrometry accurate to 10 micro-arcseconds or better (i.e. better than the target GAIA accuracy) without being limited by dust obscuration. This means that unlike GAIA, VLBI can be applied to star-forming regions independently of their internal and line-of-sight extinction. Low-mass young stellar objects (particularly T Tauri stars) are often non-thermal compact radio emitters, ideal for astrometric VLBI radio continuum experiments. Existing observations for nearby regions (e.g. Taurus, Ophiuchus, or Orion) demonstrate that VLBI astrometry of such active T Tauri stars enables the reconstruction of both the regions 3D structure (through parallax measurements) and their internal kinematics (through proper motions, combined with radial velocities). The extraordinary sensitivity of the SKA telescope will enable similar tomographic mappings to be extended to regions located several kpc from Earth, in particular to nearby spiral arm segments. This will have important implications for Galactic science, galactic dynamics and spiral structure theories.
Using the Gemini Multi Object Spectrograph (GMOS) we search for optical counterparts of two massive (~10^9 solar masses) neutral hydrogen clouds near the spiral galaxy IC 5270, located in the outskirts of the IC 1459 group. These two HI clouds were recently discovered using the Australian Square Kilometer Array Pathfinder (ASKAP). Two low surface brightness optical counterparts to one of these HI clouds are identified in the new Gemini data that reaches down to magnitudes of ~27.5 mag in the g-band. The observed HI mass to light ratio derived with these new data, M_(HI)/L_g =242, is among the highest reported to date. We are also able to rule out that the two HI clouds are dwarf companions of IC 5270. Tidal interactions and ram pressure stripping are plausible explanations for the physical origin of these two clouds.
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.
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