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We present the results of an approximately 6 100 deg2 104–196 MHz radio sky survey performed with the Murchison Widefield Array during instrument commissioning between 2012 September and 2012 December: the MWACS. The data were taken as meridian drift scans with two different 32-antenna sub-arrays that were available during the commissioning period. The survey covers approximately 20.5 h < RA < 8.5 h, − 58° < Dec < −14°over three frequency bands centred on 119, 150 and 180 MHz, with image resolutions of 6–3 arcmin. The catalogue has 3 arcmin angular resolution and a typical noise level of 40 mJy beam− 1, with reduced sensitivity near the field boundaries and bright sources. We describe the data reduction strategy, based upon mosaicked snapshots, flux density calibration, and source-finding method. We present a catalogue of flux density and spectral index measurements for 14 110 sources, extracted from the mosaic, 1 247 of which are sub-components of complexes of sources.
Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.
Radio Source I in the Orion BN/KL region provides the closest example of high mass star formation. It powers a rich ensemble of SiO and H2O masers, and is one of only three star-forming regions known to display SiO maser emission. Previous monitoring of different SiO masers with the VLBA and VLA has enabled the resolution of a compact disk and a protostellar wind at radii <100 AU from Source I, which collimates into a bipolar outflow at radii of 100-1000 AU (see contribution by Greenhill et al., this volume). Source I may provide the best case of disk-mediated accretion and outflow recollimation in massive star formation. Here, we report preliminary results of sub-arcsecond resolution 325 GHz H2O maser observations made with the SMA. We find that 325 GHz H2O masers trace a more collimated portion of the Source I outflow than masers at 22 GHz, but occur at similar radii suggesting similar excitation conditions. A velocity gradient perpendicular to the outflow axis, indicating rotation, supports magneto-centrifugal driving of the flow.
Although most nuclear 22GHz (λ=1.35 cm) H2O masers are in Seyfert 2 and LINER galaxies, only a small fraction of such galaxies host water masers. We systematically study the optical properties of the galaxies with and without nuclear H2O maser emission to better understand the relationship between H2O maser emission and properties of the central supermassive black hole and improve the detection rates in future surveys. To this end, we cross-matched the galaxies from H2O maser surveys, both detections and non-detections, with the Sloan Digital Sky Survey (SDSS) low-redshift galaxy sample. We find that maser detection rates are higher at higher optical luminosity (MB), larger velocity dispersion (σ), and higher ion [O III] λ5007 luminosity, with [O III] λ5007 being the dominant factor, and that the isotropic maser luminosity is correlated with these variables. These correlations are natural if maser emission depends on the host SMBH mass and AGN activity. We also find that the detection rate is higher for galaxies with higher extinction. These results indicate that, by pre-selecting galaxies with high extinction-corrected [O III] λ5007 flux, future maser surveys can increase detections efficiencies by a factor of ~3 to ~5.
A wealth of new information about the structure of the maser disk in NGC 4258 has been obtained from a series of 18 VLBA observations spanning three years, as well as from 32 additional epochs of spectral monitoring data from 1994 to the present, acquired with the VLA, Effelsberg, and GBT. The warp of the disk has been defined precisely. The thickness of the maser disk has been measured to be 12 micro-arcseconds (FWHM), which is slightly smaller than previously quoted upper limits. Under the assumption that the masers trace the true vertical distribution of material in the disk, from the condition of hydrostatic equilibrium the sound speed is 1.5 km s−1, corresponding to a thermal temperature of 600K. The accelerations of the high velocity maser components have been accurately measured for many features on both the blue and red side of the spectrum. The azimuthal offsets of these masers from the midline (the line through the disk in the plane of the sky) and derived projected offsets from the midline based on the warp model correspond well with the measured offsets. This result suggests that the masers are well described as discrete clumps of masing gas, which accurately trace the Keplerian motion of the disk. However, we have continued to search for evidence of apparent motions caused by “phase effects.” This work provides the foundation for refining the estimate of the distance to NGC 4258 through measurements of feature acceleration and proper motion. The refined estimate of this distance is expected to be announced in the near future.
We have carried out and recently reported VLBA multi-epoch water maser observations toward Cepheus A with an angular and velocity resolution of 0.5 mas and 0.2 km s−1, respectively. Some of the masers detected previously with the VLA (observations made with angular and velocity resolutions of 80 mas and 1.3 km s−1, respectively) are resolved now into linear/arcuate coherent “microstructures”. These structures, while smaller than the VLA beam, are 6-200 times the size of the VLBA synthesized beam. The morphology and the observed proper motions found in these structures imply three different centers of star formation activity in a region of ⋍ 0.3 radius (⋍ 200 AU). The most remarkable result from these observations is the discovery of an arc of water masers which is perfectly fitted by a circle to one part in a thousand. This arc is expanding and suggests a spherical “puff” of ejected material ejected 33 years ago from a protostar located 600 mas south of the radio jet HW2. This spherical ejection represents a very strong challenge for theoretical studies of star formation.
Study of extragalactic H2O masers has progressed significantly in the 25 years since their discovery. Existing in star forming regions and in the accretion disks supermassive black holes, they are familiar and unfamiliar at the same time. A review of how our understanding has grown, up to the present day, is followed by comments on future prospects.
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