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Register a new userRadio Astrometry towards the Nearby Universe with the SKA
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Authors
Hiroshi Imai
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This chapter summarizes radio astrometry in relation to current very long baseline interferometry (VLBI) projects and describes its perspectives with the SKA. The scientific goals of the astrometry with the SKA have been discussed in the international and Japanese communities of researchers, whose major issues are shown here. We have demonstrated some of the issues, such as censuses of possible targets and the technical feasibility of astrometry in the SKA frequency bands. The preliminary results of our case studies on SKA astrometry are also presented. In addition, possible synergy and commensality of the SKA astrometric projects with those in the optical and infrared astrometric missions, especially JASMINE (Japan Astrometry Satellite Mission for INfrared Exploration) are discussed.
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We discuss the unique opportunities for maser astrometry with the inclusion of the Square Kilometre Array (SKA) in Very Long Baseline Interferometry (VLBI) networks. The first phase of the SKA will enable observations of hydroxyl and methanol masers, positioning the latter to an accuracy of 5 microarcseconds, and the second phase may allow water maser observations. These observations will provide trigonometric distances with errors as small as 1%. The unrivalled sensitivity of the SKA will enable large-scale surveys and, through joint operations, will turn any VLBI network into a fast astrometry device. Both evolved stars and high mass star formation regions will be accessible throughout the (Southern) Milky Way, completing our understanding of the content, dynamics and history of our Galaxy. Maser velocities and proper motions will be measurable in the Local Group of galaxies and beyond, providing new insights into their kinematics and evolution.
One of the most challenging and exciting subjects in modern astrophysics is that of galaxy formation at the epoch of reionisation. The SKA, with its revolutionary capabilities in terms of frequency range, resolution and sensitivity, will allow to explore the first Gyr of structure formation in the Universe, in particular, with the detection and study of the earliest manifestations of the AGN phenomenon. The tens of QSOs that are currently known out to the highest redshifts (z~7), many of them exhibiting powerful radio emission, imply that super-massive black holes can be grown on a very short timescale and support the existence of very high redshift (z > 7) radio loud sources - sources that have so far escaped detection. Not only would such detections be paramount to the understanding of the earliest stages of galaxy evolution, they are necessary for the direct study of neutral hydrogen in the Epoch of Reionisation, through observations of the HI 21cm forest against such background sources. In order to understand how SKA and SKA1 observations can be optimised to reveal these earliest AGN, we have examined the effect of a hot CMB on the emission of powerful and young radio galaxies. By looking at the SKA1 capabilities, in particular in terms of wavelength coverage and resolution, we determine how the effects of CMB-muting of a radio loud source can be observationally minimised and how to identify the best highest-redshift radio candidates. Considering different predictions for the space density of radio loud AGN at such redshifts, we identify the survey characteristics necessary to optimize the detection and identification of the very first generation of radio loud AGN in the Universe.
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. In this chapter 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 required combination of depth and resolution means that a survey to trace the star formation in the Universe should be carried out with a facility that has a resolution of at least ~0.5arcsec, with high sensitivity at < 1 GHz. We also suggest a strategy that will enable new parameter space to be explored as the SKA expands over the coming decade.
We present detailed science cases that a large fraction of the Indian AGN community is interested in pursuing with the upcoming Square Kilometre Array (SKA). These interests range from understanding low luminosity active galactic nuclei in the nearby Universe to powerful radio galaxies at high redshifts. Important unresolved science questions in AGN physics are discussed. Ongoing low-frequency surveys with the SKA pathfinder telescope GMRT, are highlighted.
We explore the possibility of detecting hydrogen radio recombination lines from 0 < z < 10 quasars. We compute the expected Hnalpha flux densities as a function of absolute magnitude and redshift by considering (i) the range of observed AGN spectral indices from UV to X-ray bands, (ii) secondary ionizations from X-ray photons, and (iii) stimulated emission due to nonthermal radiation. All these effects are important to determine the line fluxes. We find that the combination of slopes: alpha_X,hard = -1.11, alpha_X,soft = -0.7, alpha_EUV = -1.3, alpha_UV = -1.7, maximizes the expected flux, f_Hnalpha = 10 microJy for z = 7 quasars with M_AB = -27 in the n = 50 lines; allowed SED variations produce variations by a factor of 3 around this value. Secondaries boost the line intensity by a factor of 2 to 4, while stimulated emission in high-z quasars with M_AB = -26 provides an extra boost to RRL flux observed at nu = 1 GHz if recombinations arise in HII regions with T_e = 10^3-5 K, n_e = 10^3-5 cm^-3. We compute the sensitivity required for a 5sigma detection of Hnalpha lines using the SKA, finding that the SKA-MID could detect sources with M_AB < -27 (M_AB < -26) at z < 8 (z < 3) in less than 100 hrs of observing time. These observations could open new paths to searches for obscured SMBH progenitors, complementing X-ray, optical/IR and sub-mm surveys.
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