SCUBA-2 is a 10 000-bolometer submillimetre camera on the James Clerk Maxwell Telescope. The instrument commissioning was completed in 2011 September, and full science operations began in 2011 ...October. To harness the full potential of this powerful new astronomical tool, the instrument calibration must be accurate and well understood. To this end, the algorithms for calculating the line-of-sight opacity have been improved, and the derived atmospheric extinction relationships at both wavebands of the SCUBA-2 instrument are presented. The results from over 500 primary and secondary calibrator observations have allowed accurate determination of the flux conversion factors (FCF) for the 850 and 450 μm arrays. Descriptions of the instrument beam shape and photometry methods are presented. The calibration factors are well determined, with relative calibration accuracy better than 5 per cent at 850 μm and 10 per cent at 450 μm, reflecting the success of the derived opacity relations as well as the stability of the performance of the instrument over several months. The sample size of the calibration observations and accurate FCFs have allowed the determination of the 850 and 450 μm fluxes of several well-known submillimetre sources, and these results are compared with previous measurements from SCUBA.
The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation. Sagittarius A* (Sgr ...A*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. Here we report observations at a wavelength of 1.3 mm that set a size of microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.
Submillimetre Common-User Bolometer Array 2 (SCUBA-2) is an innovative 10 000 pixel bolometer camera operating at submillimetre wavelengths on the James Clerk Maxwell Telescope (JCMT). The camera has ...the capability to carry out wide-field surveys to unprecedented depths, addressing key questions relating to the origins of galaxies, stars and planets. With two imaging arrays working simultaneously in the atmospheric windows at 450 and 850 μm, the vast increase in pixel count means that SCUBA-2 maps the sky 100-150 times faster than the previous SCUBA instrument. In this paper, we present an overview of the instrument, discuss the physical characteristics of the superconducting detector arrays, outline the observing modes and data acquisition, and present the early performance figures on the telescope. We also showcase the capabilities of the instrument via some early examples of the science SCUBA-2 that have already been undertaken. In 2012 February, SCUBA-2 began a series of unique legacy surveys for the JCMT community. These surveys will take 2.5 yr and the results are already providing complementary data to the shorter wavelength, shallower, larger area surveys from Herschel. The SCUBA-2 surveys will also provide a wealth of information for further study with new facilities such as ALMA, and future telescopes such as CCAT and SPICA.
The Galactic center supermassive black hole Sagittarius A* (Sgr A*) is one of the most promising targets to study the dynamics of black hole accretion and outflow via direct imaging with very long ...baseline interferometry (VLBI). At 3.5 mm (86 GHz), the emission from Sgr A* is resolvable with the Global Millimeter VLBI Array (GMVA). We present the first observations of Sgr A* with the phased Atacama Large Millimeter/submillimeter Array (ALMA) joining the GMVA. Our observations achieve an angular resolution of ∼87 as, improving upon previous experiments by a factor of two. We reconstruct a first image of the unscattered source structure of Sgr A* at 3.5 mm, mitigating the effects of interstellar scattering. The unscattered source has a major-axis size of 120 34 as (12 3.4 Schwarzschild radii) and a symmetrical morphology (axial ratio of ), which is further supported by closure phases consistent with zero within 3 . We show that multiple disk-dominated models of Sgr A* match our observational constraints, while the two jet-dominated models considered are constrained to small viewing angles. Our long-baseline detections to ALMA also provide new constraints on the scattering of Sgr A*, and we show that refractive scattering effects are likely to be weak for images of Sgr A* at 1.3 mm with the Event Horizon Telescope. Our results provide the most stringent constraints to date for the intrinsic morphology and refractive scattering of Sgr A*, demonstrating the exceptional contribution of ALMA to millimeter VLBI.
The Dusty Galactic Center as Seen by SCUBA-2 Parsons, H.; Dempsey, J. T.; Thomas, H. S. ...
The Astrophysical journal. Supplement series,
02/2018, Letnik:
234, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present new JCMT SCUBA-2 observations of the Galactic Center region from and , covering 10 × 2 square degrees along the Galactic Plane to a depth of 43 mJy beam−1 at 850 m and 360 mJy beam−1 at ...450 m. We describe the mapping strategy and reduction method used. We present 12CO(3-2) observations of selected regions in the field. We derive the molecular-line conversion factors (mJy beam−1 per K km s−1) at 850 and 450 m, which are then used to obtain the amount of contamination in the continuum maps due to 12CO(3-2) emission in the 850 m band. Toward the fields where the CO contamination has been accounted for, we present an 850 m CO-corrected compact source catalog. Finally, we look for possible physical trends in the CO contamination with respect to column density, mass, and concentration. No trends were seen in the data despite the recognition of three contributors to CO contamination: opacity, shocks, and temperature, which would be expected to relate to physical conditions. These SCUBA-2 Galactic Center data and catalog are available via https://doi.org/10.11570/17.0009.
The Event Horizon Telescope (EHT) is a very long-baseline interferometer built to image supermassive black holes on event-horizon scales. In this paper, we investigate candidate sites for an expanded ...EHT array with improved imaging capabilities. We use historical meteorology and radiative transfer analysis to evaluate site performance. Most of the existing sites in the EHT array have median zenith opacity less than 0.2 at 230 GHz during the March/April observing season. Seven of the existing EHT sites have 345 GHz opacity less than 0.5 during observing months. Out of more than 40 candidate new locations analyzed, approximately half have 230 GHz opacity comparable to the existing EHT sites, and at least 17 of the candidate sites would be comparably good for 345 GHz observing. A group of new sites with favorable transmittance and geographic placement leads to greatly enhanced imaging and science on horizon scales.
We used Spitzer Space Telescope 3.6, 8.0, 70 and 160 μm data, James Clerk Maxwell Telescope HARP-B CO J= (3–2) data, National Radio Astronomy Observatory 12 m telescope CO J= (1–0) data and Very ...Large Array H i data to investigate the relations among polycyclic aromatic hydrocarbons (PAHs), cold (∼20 K) dust, molecular gas and atomic gas within NGC 2403, an SABcd galaxy at a distance of 3.13 Mpc. The dust surface density is mainly a function of the total (atomic and molecular) gas surface density and galactocentric radius. The gas-to-dust ratio monotonically increases with radius, varying from ∼100 in the nucleus to ∼400 at 5.5 kpc. The slope of the gas-to-dust ratio is close to that of the oxygen abundance, suggesting that metallicity strongly affects the gas-to-dust ratio within this galaxy. The exponential scale length of the radial profile for the CO J= (3–2) emission is statistically identical to the scale length for the stellar continuum-subtracted 8 μm (PAH 8 μm) emission. However, CO J= (3–2) and PAH 8 μm surface brightnesses appear uncorrelated when examining sub-kpc-sized regions.
We present large-area maps of the CO J = 3-2 emission obtained at the James Clerk Maxwell Telescope for four spiral galaxies in the Virgo Cluster. We combine these data with published CO J = 1-0, 24 ...mm, and Ha images to measure the CO line ratios, molecular gas masses, and instantaneous gas depletion times. For three galaxies in our sample (NGC 4254, NGC 4321, and NGC 4569), we obtain molecular gas masses of 7 X 108 - 3 X 109 M and disk-averaged instantaneous gas depletion times of 1.1-1.7 Gyr. We argue that the CO J = 3-2 line is a better tracer of the dense star-forming molecular gas than the CO J = 1-0 line, as it shows a better correlation with the star formation rate surface density both within and between galaxies. NGC 4254 appears to have a larger star formation efficiency (smaller gas depletion time), perhaps because it is on its first passage through the Virgo Cluster. NGC 4569 shows a large-scale gradient in the gas properties traced by the CO J = 3-2/J = 1-0 line ratio, which suggests that its interaction with the intracluster medium is affecting the dense star-forming portion of the interstellar medium directly. The fourth galaxy in our sample, NGC 4579, has weak CO J = 3-2 emission despite having bright 24 mm emission; however, much of the central luminosity in this galaxy may be due to the presence of a central active galactic nucleus.
We present the first complete CO J = 3-2 map of the nearby grand-design spiral galaxy M51 (NGC 5194), at a spatial resolution of ∼600 pc, obtained with the HARP-B instrument on the James Clerk ...Maxwell Telescope. The map covers the entire optical galaxy disc and out to the companion NGC 5195, with CO J = 3-2 emission detected over an area of ∼9 arcmin × 6 arcmin (∼21 × 14 kpc). We describe the CO J = 3-2 integrated intensity map and combine our results with maps of CO J = 2-1, CO J = 1-0 and other data from the literature to investigate the variation of the molecular gas, atomic gas and polycyclic aromatic hydrocarbon (PAH) properties of M51 as a function of distance along the spiral structure on sub-kiloparsec scales. We find that for the CO J = 3-2 and CO J = 2-1 transitions, there is a clear difference between the variation of arm and interarm emission with galactocentric radius, with the interarm emission relatively constant with radius and the contrast between arm and interarm emission decreasing with radius. For the CO J = 1-0 line and H i emission, the variation with radius shows a similar trend for the arm and interarm regions, and the arm-interarm contrast appears relatively constant with radius. We investigate the variation of CO line ratios (J = 3-2/2-1, J = 2-1/1-0 and J = 3-2/1-0) as a function of distance along the spiral structure. Line ratios are consistent with the range of typical values for other nearby galaxies in the literature. The highest CO J = 3-2/J = 2-1 line ratios are found in the central ∼1 kiloparsec and in the spiral arms and the lowest line ratios in the interarm regions. We find no clear evidence of a trend with radius for the spiral arms, but for the interarm regions there appears to be a trend for all CO line ratios to increase with radius. We find a strong relationship between the ratio of CO J = 3-2 intensity to stellar-continuum-subtracted 8 μm PAH surface brightness and the CO J = 3-2 intensity that appears to vary with radius.
Power spectra of deprojected images of late-type galaxies in gas or dust emission are very useful diagnostics of the dynamics and stability of their interstellar medium. Previous studies have shown ...that the power spectra can be approximated as two power laws, a shallow one on large scales (larger than 500 pc) and a steeper one on small scales, with the break between the two corresponding to the line-of-sight thickness of the galaxy disk. The break separates the 3D behavior of the interstellar medium on small scales, controlled by star formation and feedback, from the 2D behavior on large scales, driven by density waves in the disk. The break between these two regimes depends on the thickness of the plane, which is determined by the natural self-gravitating scale of the interstellar medium. We present a thorough analysis of the power spectra of the dust and gas emission at several wavelengths in the nearby galaxy M 33. In particular, we use the recently obtained images at five wavelengths by PACS and SPIRE onboard Herschel. The wide dynamical range (2–3 dex in scale) of most images allows us to clearly determine the change in slopes from −1.5 to −4, with some variations with wavelength. The break scale increases with wavelength from 100 pc at 24 and 100 μm to 350 pc at 500 μm, suggesting that the cool dust lies in a thicker disk than the warm dust, perhaps because of star formation that is more confined to the plane. The slope on small scales tends to be steeper at longer wavelength, meaning that the warmer dust is more concentrated in clumps. Numerical simulations of an isolated late-type galaxy, rich in gas and with no bulge, such as M 33, are carried out to better interpret these observed results. Varying the star formation and feedback parameters, it is possible to obtain a range of power spectra, with two power-law slopes and breaks, that nicelybracket the data. The small-scale power-law does indeed reflect the 3D behavior of the gas layer, steepening strongly while the feedback smoothes the structures by increasing the gas turbulence. M 33 appears to correspond to a fiducial model with an SFR of ~ 0.7 M⊙/yr, with 10% supernovae energy coupled to the gas kinematics.
PDF
