Submm/fir Astronomy in Antarctica Potential for a Large Telescope Facility

Preliminary site testing datasets suggest that Dome C in Antarctica is one of the best sites on Earth for astronomical observations in the 200 to 500-μm regime, i.e. for far-infrared (FIR) and submillimetre (submm) astronomy. We present an overview of potential science cases that could be addressed with a large telescope facility at Dome C. This paper also includes a presentation of the current knowledge about the site characterics in terms of atmospheric transmission, stability, sky noise and polar constraints on telescopes. Current and future site testing campaigns are finally described. The participants of the ARENA workshop on Submillimetre Far-InfraRed Astronomy from Antarctica that was held at CEA Saclay, France in 2007, are greatly acknowledged, especially the speakers, the session chairmen and the SOC members: Ph. André (CEA Saclay), W. Ansorge (RAMS-CON), J. Braine (L3AB Bordeaux), P. Calisse (Cardiff University), P. Cernicharo (DAMIR Madrid), C. De Breuck (ESO), N. Epchtein (ARENA coordinator), E. Fossat (LUAN Nice), Y. Frénot (IPEV), L. Giacomel (EIE), P. Godon (IPEV), F. Helmich (SRON), F. Israel (Leiden), P. Lapeyre (Thales Alenia Space), R. Neri (IRAM), H. Olofsson (Onsala Space Observatory), P. Persi (INAF Rome), M. Piat (APC Paris), L. Rodriguez (CEA Saclay), M. Sarazin (ESO), R. Siebenmorgen (ESO), T. Stark (CfA Harvard), J. Storey (UNSW Sydney), J.-P. Swings (Liège University), M. Vaccari (Padova University), C. Walter (CEA Saclay), M. Wiedner (Universitaet zu Koeln), H. Zinnecker (AIP Potsdam) http://irfu.cea.fr/Sap/Antarctica 1 CEA Saclay/DSM/IRFU/Service d’Astrophysique, France. email: [email protected] 2 Osservatorio di Arcetri, INAF, Italy and University of Puerto Rico 3 IFSI, INAF Roma, Italy 4 IASF, INAF Bologna, Italy 5 LGGE, Grenoble, France 6 Universitaet zu Koeln, Germany 7 NRAO/University of Chicago, US 8 Dipartimento di Fisica, Roma 3, Italy 9 Dept of Physics, University of Exeter, Exeter, UK 10 CEA Saclay/DSM/IRFU/SIS, France c © EDP Sciences 2008 DOI: (will be inserted later) 2 2 ARENA Conference: The Astrophysical Science Case at Dome C 1 Submm/FIR astronomy context and Antarctica 1.1 A new generation of FIR/submm facilities Far-infrared/submillimetre (FIR/submm 100 to 1000 μm) astronomy is the prime technique to study the cold Universe and unveil the birth and early evolution of planets, stars and galaxies. It is a relatively new branch of astronomy at the frontier between IR and radio astronomy. FIR/submm continuum observations are particularly powerful to measure the luminosities, temperatures and masses of cold dust emitting objects because dust enshrouded star-forming regions emit the bulk of their energy between 60 and 500 μm. The submm/FIR range of the spectrum or THz regime is also rich in several lines that are the only means to study the kinematical structure of the interstellar medium (ISM) of galaxies. They allow to probe different physical regimes, i.e. regions of widely different densities and temperatures, depending on their excitation levels and critical abundances. Observations at these wavelengths with a large telescope will primarily lead to breakthroughs in the study of star formation at all scales and understand its cosmic history back to the early Universe as well as in the understanding of galaxy evolution. Asteroids, Debris disks, Planet formation, Dust origin in evolved stars, Interstellar dust and Polarisation of dust in the Universe are also potential science drivers for FIR/submm astronomy. An overview of FIR/submm astronomical science at Dome C was presented at the Saclay ARENA workshop in June 2007 and is reviewed in the present paper. What is the context today of submm astronomy ? Two major submm facilities will become available in the coming years: the Herschel Space Observatory, a FIR/submm (60-500 μm) telescope in Space and ALMA, a ground-based mmwave (350 μm-7 mm) interferometer on the Chajnantor plateau in the northern Atacama desert. Both facilities will have their specific niches. Herschel will have the ability to carry out large area imaging surveys of both the distant Universe (Franceschini 2001) and the nearby interstellar medium in our own Galaxy (André & Saraceno 2005). ALMA will make possible ultra deep searches for primordial galaxies (Blain 2001), as well as detailed kinematical investigations of individual protostars (Evans 2001). However, both Herschel and ALMA will have their own limitations. The Herschel telescope (3.5 m) will suffer from its only moderate angular resolution, implying a fairly high extragalactic confusion limit (Oliver 2001) and preventing the study of individual protostars in all but the nearest star-forming clusters of our Galaxy. ALMA will suffer from a small field of view (10) and limited observable conditions in the FIR/submm, making extensive wide-field mapping impossible given the amount of time necessary to cover large star-forming complexes and fields of primordial galaxies. Beside these two major facilities, there is a constellation of submm/FIR telescope projects in operation or in study, aboard balloons (e.g. BLAST, OLIMPO, PILOTE etc), aboard an airplane (SOFIA), aboard satellites (e.g. Akari; SPICA) and on high-altitude plateaux and mountains (e.g. APEX; CCAT, a 25-m telescope project). For instance, the APEX telescope at Chajnantor could allow 450-μm observations Minier et al.: Submm/FIR Astronomy in Antarctica . . . 3 as recently demonstrated in 2007 by the CEA ArTéMiS project. BLAST , the Balloon-borne Large Aperture Submillimeter Telescope, observes simultaneously in 3 broadband filters at 250, 350 and 500 μm with detector arrays, from an altitude of 40 km. Two science flights have been performed: a 4-day flight from northern Sweden in 2005 and an 11-day flight in Antarctica in 2006 (e.g. Chapin et al. 2008). Why evaluating potential science with a FIR/submm telescope at Dome C ? Beside Herschel and ALMA, there is thus a clear need for a large (> 10 m) single-dish telescope (1) operating at 200-450 μm and providing (2) better angular resolution than Herschel and (3) wider-field mapping capabilities than ALMA, making large-scale mapping with a relatively good angular resolution (∼ 1) possible and well matched with thermal infrared space telescope (e.g. Spitzer). New sites are therefore intensively tested because the 200-350-450-μmwindows at Chajnantor open less than 30% of wintertime at an observable level, probably less than 10% at 200 μm. The stability of the atmosphere is an equally important parameter when comparing the sites and Dome C may stand out as being far more stable than Chilean sites (Minier et al. 2007 and reference therein). Equipped with FIR/submm imagers and spectrometers, a European telescope at Dome C in Antarctica might be able to operate in all atmospheric windows between 200 μm and 1 mm, and very regularly at 450 μm in wintertime. As a demonstration Antarctica is indeed a very good site for submm/mm astronomy, US astronomers have built a submm telescope of 10 m (the SPT) at the South Pole (Ruhl et al. 2004). On a higher and more stable site, Dome C could become the European observatory in Antarctica. 1.2 Dome C in Antarctica: a potential site for FIR/submm astronomy Dome C is the location of the French-Italian Concordia station that is connected to Dumont-D’Urville on the coast by either light planes or ground motorised raids for transporting heavy material. The Station building can now host ∼ 15 people during winter and therefore allows experiments all year long . Site testing and qualification of the site for optical and near-infrared astronomy have been conducted for many years by French, Italian and Australian teams. However, little efforts have been produced so far to evaluate the quality of the atmosphere, the climate constraints and the specificity of Dome C for a potential submm/FIR telescope. Calisse et al. (2004) undertook the measurement of the atmosphere opacity at 350 μm during a summer period and found it comparable to that at South Pole. No direct assessment of the wintertime atmosphere transmission has ever been performed. A major obstacle to carry out submm observations from ground is the water vapour in the atmosphere. Astronomical observations in the FIR/submm spectral bands (e.g. 200, 350, 450 μm) can only be achieved from cold, dry and stable sites with ground-based telescopes or from space to overcome the atmosphere opacity http://www.concordiastation.org/ 4 2 ARENA Conference: The Astrophysical Science Case at Dome C Fig. 1. Precipitable Water Vapour content (PWV) distribution over Antarctica as simulated by the regional climate model MAR (june-august 2004 average). Mean PWV at Dome C is around 0.35 mm while at Dome A it is around 0.2 mm. From Gallée, 2008, Monthly Weather Review, in prep. and instability that are mainly due to water vapour absorption and fluctuations in the low atmosphere. Chile currently offers the driest accessible (all-year long) sites on Earth, where the precipitable water vapour (PWV) content is often less than 1 mm. The Chajnantor plateau (5100 m) hosts the ESO facilities for submm astronomy: the APEX 12-m telescope and the coming ALMA interferometer. However, FIR/submm observations at 200, 350, 450 μm are only possible when PWV drops below 0.5 mm, which occurs less than 30% of wintertime at the ALMA site. In addition, observing conditions in Chilean sites can progressively and frequently be degraded by climate phenomena like the Bolivian winter or El Niño that brings weather instabilities. Possibly global warming has a more severe impact on these sites than on Antarctica. Other potential Chilean sites at higher altitudes (e.g. Cerro Chajnantor, Sairecabur) presently undergo site testing. The plateau of Antarctica might possibly be a very privileged and alternative area for achieving astronomical observations in the submm/FIR range. The geographical and climate conditions are extreme, which favour low PWV in the atmosphere: the low sun cover, the isolation by the circumpolar stream of the Antarctic ocean and the high power of reflection of ice make Antarctica the coldMinier et al.: Submm/FIR Astronomy in Antarctica . . . 5 est continent on Earth. It is also important to emphasize that snow precipitation is very low on the Antarctic plateau. The low pressure fronts do not penetrate into the inner plateau and remain located on the coast lines. In fact the inner part of the continent is a true desert: on an area of 5 millions of km, snow precipitation is about 5 cm, and often less than 2 cm on the highest Domes. As a consequence, the PWV at Dome C is very low and expected lower than at Chajnantor in average (Fig. 1). However, a proper comparison between Dome C and other sites should be based on the comparison of the transmission, the linewidth of the atmospheric windows and their stability, and the level of skynoise. The transmission and the linewidth of the windows mainly depend on the PWV, temperature and pressure, while the sky noise depends on the water vapour cell fluctuations in the low atmosphere (Fig. 2). Preliminary site testing in summertime and modelling predict that the FrenchItalian Concordia base at Dome C in Antarctica is a potentially remarkable site on Earth for FIR/submm astronomy. Measurements of the humidity at different altitudes with radiosounding techniques (Fig. 3) and derivation of the PWV (Fig. 4) over a statistically valid time tend to demonstrate that Dome C is a slightly better site than the best Chajnantor site (i.e. cerros or mountains at > 5500 m) in terms of PWV percentiles. However, when taking into account the temperature and pressure effects, it is not clear that Dome C is a better site in terms of overall transmission (amplitude, linewidth and time percentiles; see inset in Fig. 2). In addition, the PWV does not fall below 0.2 mm very often, which is critical for opening the 200-μm windows. Nonetheless the atmospheric transmission and stability are probably always ideal for observations at 350 and 450 μm. For nonexclusive use of telescope time for these wavelengths, the Chajnantor plateau is to date the optimal site as submm observatories are already in place. For regular (> 100 days) use of telescope time for the 350-450 μm windows and complementary observations at 200 μm, Dome C might become, however, a much more attractive site. A complete assessment of the 200-μm opacity of the atmosphere is therefore crucial for concluding whether Dome C is better than any other known sites in the Chilean Andes and more generally on Earth. Skynoise will also be measured by CAMISTIC in 2010 (Minier et al. 2007). Note that site testing experiments at Dome A will be undertaken (PLATO project) in 2008 with preHeat a specific experiment that is dedicated to assess the site for submm astronomy (Tothill et al. this volume). Another crucial assessment is the understanding of the effects of temperature gradients in the inversion temperature layer between 0 and ∼ 30 m and icing formation during polar nights on telescope hardware and operation (Fig. 5). Nighttime cooling of ice sheet by radiation to space creates a powerful temperature gradient and generates near surface level inversion winds where air, cooled by contact with the surface, flows down the gently sloped interior of the plateau (Swain & Gallée 2006). The studies of these constraints are realised under the GIVRE experiments (Fig. 5), which are currently in operation at Dome C. 6 2 ARENA Conference: The Astrophysical Science Case at Dome C Fig. 2. Modelled transmission for Dome C and Chajnantor/ALMA site with the MOLIERE code. Transmissions were estimated using PWV=0.2 mm for Dome C (blue) and 0.5 mm for ALMA site (grey) that represent the expected first quartile PWV values. 200-μm (1500 GHz) windows open at Dome C. Note: MOLIERE (Microwave Observation LIne Estimation and REtrieval) is a versatile forward and inversion model for the millimetre and submillimetre wavelengths range, used in many aeronomy and some astronomy applications (Urban 2004). Plots produced by N. Schneider-Bontemps. Inset plot with am code shows the comparison of the 200-μm transmission for PWV=0.1 mm at Dome C, at Sairecabur (a 5525-m Chilean site) and Dome X, a fictive site with atmospheric profile as Dome C but at a pressure altitude of 5525 m. The Chilean site has the highest transmission. At equal altitudes and pressures, Chilean site vs. Dome X, Chilean site would win over Antarctica lower temperature conditions. This is because low temperatures bias H2O partition function and strengthen THz absorption lines. Comparing Dome C with Sairecabur, higher atmospheric pressure at Dome C broadens very strong lines that bound THz windows. Overall: the transmission for PWV=0.2 mm at Dome C is comparable to the transmission for PWV=0.35 mm at 5500m in Chile. However, this comparison does not take into account the percentiles of PWV at both sites and need to be confirmed by observations. The models were done with the am modeling software: http://sma-www.cfa.harvard.edu/private/memos/152-03.pdf. Plots produced by D. Marrone