We are developing the Precision Array for Probing the Epoch of Re-ionization (PAPER) to detect 21 cm emission from the early universe, when the first stars and galaxies were forming. We describe the ...overall experiment strategy and architecture and summarize two PAPER deployments: a four-antenna array in the low radio frequency interference (RFI) environment of Western Australia and an eight-antenna array at a prototyping site at the NRAO facilities near Green Bank, WV. From these activities we report on system performance, including primary beam model verification, dependence of system gain on ambient temperature, measurements of receiver and overall system temperatures, and characterization of the RFI environment at each deployment site. We present an all-sky map synthesized between 139 MHz and 174 MHz using data from both arrays that reaches down to 80 mJy (4.9 K, for a beam size of 2.15e--5 sr at 156 MHz), with a 10 mJy (620 mK) thermal noise level that indicates what would be achievable with better foreground subtraction. We calculate angular power spectra (C l) in a cold patch and determine them to be dominated by point sources, but with contributions from galactic synchrotron emission at lower radio frequencies and angular wavemodes. Although the sample variance of foregrounds dominates errors in these power spectra, we measure a thermal noise level of 310 mK at l = 100 for a 1.46 MHz band centered at 164.5 MHz. This sensitivity level is approximately 3 orders of magnitude in temperature above the level of the fluctuations in 21 cm emission associated with re-ionization.
Aims. Our aim is to characterize the polarized continuum emission properties including intensity, polarization position angle, and polarization percentage of Sgr A* at ~100 (3.0 mm), ~230 (1.3 mm), ...~345 (0.87 mm), ~500 (0.6 mm), and ~700 GHz (0.43 mm). Methods. We report continuum emission properties of Sgr A* at the above frequency bands, based on the Atacama Large Millimeter Array (ALMA) observations. We measured flux densities of Sgr A* from ALMA single pointing and mosaic observations. We performed sinusoidal fittings to the observed (XX-YY)/I intensity ratios, to derive the polarization position angles and polarization percentages. Results. We successfully detect polarized continuum emission from all observed frequency bands. We observed lower Stokes I intensity at ~700 GHz than that at ~500 GHz, which suggests that emission at ≳500 GHz is from the optically thin part of a synchrotron emission spectrum. Both the Stokes I intensity and the polarization position angle at our highest observing frequency of ~700 GHz, may vary with time. However, as yet we do not detect variation in the polarization percentage at >500 GHz. The polarization percentage at ~700 GHz is likely lower than that at ~500 GHz. By comparing the ~500 GHz and ~700 GHz observations with the observations at lower frequency bands, we suggest that the intrinsic polarization position angle of Sgr A* varies with time. This paper also reports the measurable polarization properties from the observed calibration quasars. Conclusions. Future simultaneous multi-frequency polarization observations are required to clarify the time and frequency variation of the polarization position angle and polarization percentage.
Magnetic Field Structure of Orion Source I Hirota, Tomoya; Plambeck, Richard L.; Wright, Melvyn C. H. ...
The Astrophysical journal,
06/2020, Letnik:
896, Številka:
2
Journal Article
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We observed polarization of the SiO rotational transitions from Orion Source I (SrcI) to probe the magnetic field in bipolar outflows from this high-mass protostar. Both 43 GHz J = 1-0 and 86 GHz J = ...2-1 lines were mapped with ∼20 au resolution, using the Very Large Array (VLA) and Atacama Large Millimeter/Submillimeter Array (ALMA), respectively. The 28SiO transitions in the ground vibrational state are a mixture of thermal and maser emission. Comparison of the polarization position angles in the J = 1-0 and J = 2-1 transitions allows us to set an upper limit on possible Faraday rotation of 104 rad m−2, which would twist the J = 2-1 position angles typically by less than 10°. The smooth, systematic polarization structure in the outflow lobes suggests a well-ordered magnetic field on scales of a few hundred au. The uniformity of the polarization suggests a field strength of ∼30 mG. It is strong enough to shape the bipolar outflow and possibly lead to sub-Keplerian rotation of gas at the base of the outflow. The strikingly high fractional linear polarizations of 80%-90% in the 28SiO v = 0 masers require anisotropic pumping. We measured circular polarizations of 60% toward the strongest maser feature in the v = 0 J = 1-0 peak. Anisotropic resonant scattering is likely to be responsible for this circular polarization. We also present maps of the 29SiO v = 0 J = 2-1 maser and several other SiO transitions at higher vibrational levels and isotopologues.
Massive star formation occurs in giant molecular clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the ...highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (giant molecular associations (GMAs)) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics-their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the interarm region and into the next spiral arm passage.
We present results of lambda 1.3 mm dust-polarization observations toward 16 nearby, low-mass protostars, mapped with ~2".5 resolution at CARMA. The results show that magnetic fields in protostellar ...cores on scales of ~1000 AU are not tightly aligned with outflows from the protostars. Rather, the data are consistent with scenarios where outflows and magnetic fields are preferentially misaligned (perpendicular), or where they are randomly aligned. If one assumes that outflows emerge along the rotation axes of circumstellar disks, and that the outflows have not disrupted the fields in the surrounding material, then our results imply that the disks are not aligned with the fields in the cores from which they formed.
Aims. Our aim is to characterize the polarized continuum emission properties including intensity, polarization position angle, and polarization percentage of Sgr A* at ~492 GHz. This frequency, well ...into the submillimeter bump where the emission is supposed to become optically thin, allows us to see down to the event horizon. Hence the reported observations contain potentially vital information on black hole properties. We have compared our measurements with previous, lower frequency observations, which provides information in the time domain. Methods. We report continuum emission properties of Sgr A* at ~492 GHz, based on Atacama Large Millimeter Array (ALMA) observations. We measured flux densities of Sgr A* from the central fields of our ALMA mosaic observations. We used calibration observations of the likely unpolarized continuum emission of Titan and the observations of Ci line emission, to gauge the degree of spurious polarization. Results. The flux density of 3.6 ± 0.72 Jy which we measured during our run is consistent with extrapolations from previous, lower frequency observations. We found that the continuum emission of Sgr A* at ~492 GHz shows large amplitude differences between the XX and the YY correlations. The observed intensity ratio between the XX and YY correlations as a function of parallactic angle can be explained by a constant polarization position angle of ~158°± 3°. The fitted polarization percentage of Sgr A* during our observational period is 14% ± 1.2%. The calibrator quasar J1744-3116 we observed on the same night can be fitted to Stokes I = 252 mJy, with 7.9% ± 0.9% polarization at position angle PA = 14°± 4.2°. Conclusions. The observed polarization percentage and polarization position angle in the present work appear consistent with those expected from longer wavelength observations in the period of 1999−2005. In particular, the polarization position angle at 492 GHz expected from the previously fitted 167°± 7° intrinsic polarization position angle and (−5.6 ± 0.7) × 105 rotation measure is 155 \hbox{$^{+9}_{-8}$} + 9 -8 °, which is consistent with our new measurement of polarization position angle within 1σ. The polarization percentage and the polarization position angle may be varying over the period of our ALMA 12 m Array observations, which demands further investigation with future polarization observations.
We report the detection of variable linear polarization from Sgr A* at a wavelength of 3.5 mm, the longest wavelength yet at which a detection has been made. The mean polarization is 2.1% c 0.1% at a ...position angle of 16 c 2 with rms scatters of 0.4% and 9 over the five epochs. We also detect polarization variability on a timescale of days. Combined with previous detections over the range 150-400 GHz (750-2000 km), the average polarization position angles are all found to be consistent with a rotation measure of-(4.4 c 0.3) x 10 super(5) rad m super(-2). This implies that the Faraday rotation occurs external to the polarized source at all wavelengths. This implies an accretion rate 6(0.2-4) x 10 super(-8) M sub( )yr super(-1) for the accretion density profiles expected of ADAF, jet, and CDAF models and assuming that the region at which electrons in the accretion flow become relativistic is within 10R sub(S). The inferred accretion rate is inconsistent with ADAF/Bondi accretion. The stability of the mean polarization position angle between disparate polarization observations over the frequency range limits fluctuations in the accretion rate to less than 5%. The flat frequency dependence of the interday polarization position angle variations also makes them difficult to attribute to rotation-measure fluctuations and suggests that both the magnitude and position angle variations are intrinsic to the emission.
We report the CO(J = 1-0) observations of the Whirlpool Galaxy M51 using both the Combined Array for Research in Millimeter Astronomy (CARMA) and the Nobeyama 45 m telescope (NRO45). We describe a ...procedure for the combination of interferometer and single-dish data. In particular, we discuss (1) the joint imaging and deconvolution of heterogeneous data, (2) the weighting scheme based on the root-mean-square (rms) noise in the maps, (3) the sensitivity and uv coverage requirements, and (4) the flux recovery of a combined map. We generate visibilities from the single-dish map and calculate the noise of each visibility based on the rms noise. Our weighting scheme, though it is applied to discrete visibilities in this paper, should be applicable to grids in uv space, and this scheme may advance in future software development. For a realistic amount of observing time, the sensitivities of the NRO45 and CARMA visibility data sets are best matched by using the single-dish baselines only up to 4-6 k Delta *l (about 1/4-1/3 of the dish diameter). The synthesized beam size is determined to conserve the flux between the synthesized beam and convolution beam. The superior uv coverage provided by the combination of CARMA long baseline data with 15 antennas and NRO45 short spacing data results in the high image fidelity, which is evidenced by the excellent overlap between even the faint CO emission and dust lanes in an optical Hubble Space Telescope image and polycyclicaromatichydrocarbon emission in a Spitzer 8 Delta *mm image. The total molecular gas masses of NGC 5194 and 5195 (d = 8.2 Mpc) are 4.9 X 109 M and 7.8 X 107 M , respectively, assuming the CO-to-H2 conversion factor of X CO = 1.8 X 1020 cm-2(K km s--1)--1. The presented images are an indication of the millimeter-wave images that will become standard in the next decade with CARMA and NRO45, and the Atacama Large Millimeter/Submillimeter Array.
We present high-resolution CARMA 230 GHz continuum imaging of nine deeply embedded protostars in the Serpens Molecular Cloud, including six of the nine known Class 0 protostars in Serpens. This work ...is part of a program to characterize disk and envelope properties for a complete sample of Class 0 protostars in nearby low-mass star-forming regions. Here, we present CARMA maps and visibility amplitudes as a function of uv-distance for the Serpens sample. Observations are made in the B, C, D, and E antenna configurations, with B configuration observations utilizing the CARMA Paired Antenna Calibration System. Combining data from multiple configurations provides excellent uv-coverage (4-500 k Delta *l), allowing us to trace spatial scales from 102 to 104 AU. We find evidence for compact disk components in all of the observed Class 0 protostars, suggesting that disks form at very early times (t < 0.2 Myr) in Serpens. We make a first estimate of disk masses using the flux at 50 k Delta *l, where the contribution from the envelope should be negligible, assuming an unresolved disk. The resulting disk masses range from 0.04 M to 1.7 M , with a mean of approximately 0.2 M . Our high-resolution maps are also sensitive to binary or multiple sources with separations 250 AU, but significant evidence of multiplicity on scales <2000 AU is seen in only one source.
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