Opportunities for maser studies with the Square Kilometre Array
aa r X i v : . [ a s t r o - ph ] S e p Astrophysical Masers and their EnvironmentsProceedings IAU Symposium No. 242, 2007J.M. Chapman & W.A. Baan, eds. c (cid:13) Opportunities for maser studies with theSquare Kilometre Array
Anne J. Green and Willem A. Baan School of Physics, University of Sydney, NSW 2006, Australiaemail: [email protected] ASTRON, 7991PD Dwingeloo, The Netherlands
Abstract.
The Square Kilometre Array (SKA) is the radio telescope of the next generation, pro-viding an increase in sensitivity and angular resolution of two orders of magnitude over existingtelescopes. Currently, the SKA is expected to span the frequency range 0.1 −
25 GHz with capa-bilities including a wide field-of-view and measurement of polarised emission. Such a telescopehas enormous potential for testing fundamental physical laws and producing transformationaldiscoveries. Important science goals include using H O megamasers to make precise estimatesof H , which will anchor the extragalactic distance scale, and to probe the central structures ofaccretion disks around supermassive black holes in AGNs, to study OH megamasers associatedwith extreme starburst activity in distant galaxies and to study with unprecedented precisionmolecular gas and star formation in our Galaxy.
1. Overview
The Square Kilometre Array (SKA) is a paradigm-shifting radio telescope for the nextgeneration, providing an increase in sensitivity and angular resolution of two orders ofmagnitude over existing telescopes. It will be a truly global machine with an expected life-time of at least 50 years. Such a telescope has enormous potential for testing fundamentalphysical laws and producing transformational discoveries. The aim of the telescope is toanswer some of the ”big” questions in astronomy, such as what is the nature of darkenergy, are we alone in the Universe, how did galaxies and black holes form, what is theorigin and evolution of cosmic magnetism, can pulsars be used to detect gravity waves?The project is an international consortium of 50 institutions, spread over 17 coun-tries. Present governance is through the International SKA Steering Committee, with 21members, which oversees the International SKA Project Office and 6 targeted WorkingGroups. Engagement with inter-governmental agencies has begun to establish a gover-nance model and funding strategies. Currently, there are several pathfinder and demon-strator projects in progress for both science and technology developments.
2. SKA Concept
The concept of this instrument is based on the following principles: • A data network of sensors of the electromagnetic field are connected using a corre-lator to produce an interferometric array. • The sensing antennas will be highly concentrated in a central core, with 20% of thecollecting area within 1 km, 50% within 5 km and 75% within 150 km. Outlier stationswill be distributed at distances up to at least 3000 km from the core. • Antennas and stations will be connected via wide-band optic fibre links (data ratesat 100 Gbits/sec) to the central processor, which will need to process 10 – 100 Pflops/sec.1 Green & Baan • The telescope will be built in stages, with Phase 1 planned to be 10% of the totalcollecting area and able to undertake unique science.
3. Status at March 2007
Following a rigorous and objective assessment, two sites to host the SKA have beenshortlisted for further evaluation and development. The locations encompass Australiaand New Zealand, and South Africa with 7 partner countries. The key issues in the siteselection process were a very low RFI environment, a large unencumbered site and lowionospheric and tropospheric turbulence. A site decision is expected about 2010 with thecomplete SKA operational in 2020.A second milestone was the selection of a Reference Design, which was developed tofocus engineering and science efforts, to provide the basis for detailed costing models andto provide a recognisable image for the SKA. The design is likely to evolve, but at presentit comprises small dishes with smart feeds with aperture arrays for the lower frequencies.
4. Expected capabilities
The current SKA model has an estimated cost of about 1 Billion Euro for constructionwith an annual operating budget of about 70 Million Euro. This is based on the followingintended capabilities and specifications: • A sensitivity at least 50 times more than the EVLA. This will enable detection ofatomic hydrogen and other molecules right to the edge of the Universe. The specificationsare for a continuum sensitivity of 0.4 µ Jy in 1 hour and a spectral line sensitivity of 5 µ Jy/channel after 12 hours (both 5 σ detections). To achieve this requires a very largecollecting area, ∼ . • A fast survey speed, up to 10,000 times better than currently possible. This requiresa very large field of view, projected to be 1 square degree at 1.4 GHz and 18 squarearcminutes at 20 GHz . • A wide frequency range of 0.1 – 25 GHz , to handle the key science priorities. • Moderately high angular resolution to make detailed images of structures includingdisks, outflows and planetary gaps. To do this requires a large physical extent, at least3000 km , to produce beamsizes of 20 /f GHz mas. The size is limited by the Earth, if oneassumes a real-time connected ground-based array. • Good spectral resolution, with more than 4000 dual polarization channels to givevelocity resolution of at least 0.2 km/sec.
5. Science goals for maser research
The strength of the SKA will be its great sensitivity with a wide field of view. Angularresolution is constrained by the size of the Earth. There are two main threads for themaser science projects, namely, one which focuses on the early Universe, dark energy andgalaxy evolution, and one which will make discoveries in our Galaxy on star formationmechanisms and the interstellar medium.5.1.
Masers in the early Universe
Megamasers (MM) of the OH and H O molecules will be used to study the properties ofprominent populations of active galaxies at cosmological distances. Extragalactic maser-ing activity relies on amplification of radio continuum by foreground pumped moleculargas and the large pathlengths in galactic nuclei. asers and the SKA O MMs are a signpost of AGN activity that may be used to study dark energy,make precise estimates of H to anchor the extragalactic distance scale, and to probethe central structures of accretion disks around supermassive black holes (see Green-hill, these proceedings). Direct mapping of nuclear Keplerian disks such as found in thearchetypal NGC 4258 (e.g. Claussen et al. et al. et al. et al. O maser found to dateis in quasar SDSS J0804+3607 at a distance of 2.4 Gps (Barvainis & Antonucci 2005),which shows that molecular gas exists at very early epochs. A second class of H O MMprobes the interaction between radio jets and encroaching molecular clouds away fromthe AGN, such as seen in nearby NGC 1052 (Claussen et al. et al. et al. et al. et al. et al. et al. et al. et al. et al.
Galactic and extragalactic masers of many flavours
The SKA will be used to study with unprecedented precision molecular gas and starformation in our Galaxy and nearby galaxies. The increased sensitivity will make up forthe absence of massive stars in the solar neighbourhood and enable detection of largenumbers of protostellar Keplerian disks and mapping of outflows. Molecular masers arecommon in the vicinity of newly formed massive stars, with H O and CH OH being thesignposts of massive star formation and some 70% of UCHIIs in the Galaxy are asso-ciated with H O masers (Churchwell et al. et al. et al. et al. O maser emission in othergalaxies may trace weak nuclear activity (as in H O MM) or massive star-forming regionssimilar to regions found in the Galaxies. NGC 2146 shows that the kilomaser H O emis-sion from UCHII regions may be detected up to distances of 50 Mpc (Tarchi et al. et al. et al.
6. Conclusions
A summary of what the SKA is likely to produce for maser science include: • A large increase in the number H O masers in sub-parsec disks around AGNS to beused to study structure and black hole properties. For resolved nuclear disks, precisiondistances and an improved measure of H can be determined out to distances of about500 Mpc. • Probing the high-density ISM of high redshift galaxies using H O for parsec nuclearregions and jet interaction regions, and using OH for the nuclear ISM and torus structuresin luminous (circum-)nuclear starbursts will be possible without a distance limit. • Many more disks and outflows will be detected for star-forming regions and dustyproto-clusters in our Galaxy. Masers associated with SNRs and Compact HII regions canbe studied in nearby galaxies. • Proper motion and parallax studies will give a more precise picture of the structureand the peculiar motion of the spiral arms in our Galaxy.A more detailed explanation can be found in ”Science with the Square Kilometre Array” ,edited by Carilli & Rawlings in New Astronomy Reviews (2004) volume 48.
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