Research

The Dust-Selected Molecular Clouds in the Northeast Region of the Small Magellanic Cloud

We present a high-sensitivity (1σ<1.6 mJy beam−1) continuum observation in a 343 arcmin2 area of the northeast region in the Small Magellanic Cloud at a wavelength of 1.1 mm, conducted using the AzTEC instrument on the ASTE telescope. In the observed region, we identified 20 objects by contouring 10σ emission. Through spectral energy distribution (SED) analysis using 1.1 mm, Herschel, and Spitzer data, we estimated the gas masses of 5×103−7×104 M⊙, assuming a gas-to-dust ratio of 1000. Dust temperature and the index of emissivity were also estimated as 18−33 K and 0.9−1.9, respectively, which are consistent with previous low resolution studies. The relation between dust temperature and the index of emissivity shows a weak negative linear correlation. We also investigated five CO-detected dust-selected clouds in detail. The total gas masses were comparable to those estimated from the Mopra CO data, indicating that the assumed gas-to-dust ratio of 1000 and the Xco factor of 1×1021 cm−2 (K km s−1)−1, with uncertainties of a factor of 2, are reliable for the estimation of the gas masses of molecular or dust-selected clouds. Dust column density showed good spatial correlation with CO emission, except for an object that associates with bright young stellar objects. The 8 μm filamentary and clumpy structures also showed similar spatial distribution with the CO emission and dust column density, supporting the fact that polycyclic aromatic hydrocarbon emissions arise from the surfaces of dense gas and dust clouds.

Development of Multi-temperature Calibrator for the TES Bolometer Camera: System Design and Deployment at ASTE

We developed a simple add-on, cryogen-free, and low-power consumption calibrator for a new transition-edge sensor (TES) bolometer camera mounted on the ASTE 10-m telescope. To measure the responsivity of the TES bolometers and accurately correct for the nonlinearity and atmospheric extinction, we designed a motor-driven rotating filter wheel system installed in front of the cryostat window. This calibrator is required to cover the loading power under various atmospheric conditions, which corresponds to precipitable water vapor (PWV) of 0.5–4 mm. For this range of PWV, 25–100 K blackbodies are necessary for the observing bands of 1.1 and 0.85 mm. To simulate the temperature range, bolometers in the cryostat are also optically coupled to the low-temperature stage (<4 K) inside the cryostat by spherical mirrors. In addition, we used moderately absorptive polystyrene plates that are placed between a spherical mirror and the cryostat window. Various combinations of filters result in eight different temperatures by the filter wheel system and simulate the atmospheric emission under various weather conditions at the ASTE site. The loading powers introduced by each filter were self-calibrated by measuring the load curves of the TES bolometers when a filter was placed in front of the cryostat window. Each science observation was preceded by the calibration process, which measures the response of the TES bolometers to the atmosphere and filters of various opacities. Then, the responsivities of TES bolometers were derived to convert their output signal to the loading power and correct for the nonlinearity inherent in its response. Furthermore, the loading power falling on the TES bolometers from atmospheric emission measured at various PWV was in good correlation with the PWV measured with the radiometer, which enables the atmospheric extinction correction by fast and sensitive bolometers compared to the available radiometers with the modest sampling speeds.

AzTEC/ASTE 1.1 mm continuum survey of the Small Magellanic Cloud

The first 1.1 mm continuum survey toward the Small Magellanic Cloud (SMC) was performed using the AzTEC instrument installed on the ASTE 10 m telescope. This survey covered 4.5 deg2 of the SMC with 1σ noise levels of 5–12 mJy beam−1, and 44 extended objects were identified. The 1.1 mm extended emission has good spatial correlation with Herschel 160 μm, indicating that the origin of the 1.1 mm extended emission is thermal emission from a cold dust component. We estimated physical properties using the 1.1 mm and filtered Herschel data (100, 160, 250, 350, and 500 μm). The 1.1 mm objects show dust temperatures of 17–45 K and gas masses of 4 × 103–3 × 105 M⊙, assuming single-temperature thermal emission from the cold dust with an emissivity index, β, of 1.2 and a gas-to-dust ratio of 1000. These physical properties are very similar to those of giant molecular clouds (GMCs) in our galaxy and the Large Magellanic Cloud. The 1.1 mm objects also displayed good spatial correlation with the Spitzer 24 μm and CO emission, suggesting that the 1.1 mm objects trace the dense gas regions as sites of massive star formation. The dust temperature of the 1.1 mm objects also demonstrated good correlation with the 24 μm flux connected to massive star formation. This supports the hypothesis that the heating source of the cold dust is mainly local star-formation activity in the 1.1 mm objects. The classification of the 1.1 mm objects based on the existence of star-formation activity reveals the differences in the dust temperature, gas mass, and radius, which reflects the evolution sequence of GMCs.

Development of the Multi-color TES bolometer camera for the ASTE telescope

Wideband and high spatial resolution continuum observations in the millimeter and submillimeter wavelengths are of great importance to understand star formation history of galaxies from distant to nearby (early to recent universe), and thermal and non-thermal aspects of clusters of galaxies through Sunyaev-Zel’dovich effect. To promote such studies based on the millimeter and submillimeter continuum data, new TES (Transition Edge Sensor) bolometer camera for the ASTE telescope has been developed. We aim to observe three bands, 1100µm, 850µm and 450µm. Camera optics is designed by geometrical optics to fit into the Cassegrain system of the ASTE telescope and the spatial limitation of its receiver cabin. For the efficient observation, our optics realizes 7.5’ field of view and 2-band simultaneous observation. The two focal planes can accommodate two of three TES bolometer wafers which have 169, 271, and 881 pixels at 1100, 850, and 450µm, respectively. The camera optics is accomplished diffraction limited optics, which is confirmed from spot diagrams and Strehl ratios. It is evaluated via physical optics calculation, and all pixels on each focal plane have aperture efficiency of ∼ 30 percent in absence of reflection and absorption loss by filters and the Ruze loss. The dimensions of the optical elements are also determined based on the results of the physical optics calculations.

Detection of an Ultra-bright Submillimeter Galaxy behind the Small Magellanic Clouds

We report the discovery of a new ultra-bright submillimeter galaxy (SMG) behind the Small Magellanic Cloud (SMC). This SMG is detected as a 43.3 ± 8.4 mJy point source (MM J01071 − 7302, hereafter MMJ0107) in the 1.1 mm continuum survey of the SMC by AzTEC on the ASTE telescope. MMJ0107 is also detected in radio (843 MHz), Herschel/SPIRE, Spitzer MIPS 24 μm, all IRAC bands, Wide-field Infrared Survey Explorer, and near-infrared (J, H, K). We find an optical (U, B, V) source, which might be the lensing object, at a distance of 1.4" from near-infrared and IRAC sources. Photometric redshift estimates for the SMG using representative spectral energy distribution templates show the redshifts of 1.4–3.9. We estimate total far-infrared luminosity of (0.3–2.2) × 1014 μ-1 L⊙. and a star formation rate of 5600–39,000 μ-1 M⊙ yr-1, where μ is the gravitational magnification factor. This apparent extreme star formation activity is likely explained by a highly magnified gravitational lens system.