- ID:
- ivo://CDS.VizieR/J/MNRAS/450/1477
- Title:
- VLA 352MHz image of the Bootes field. I.
- Short Name:
- J/MNRAS/450/1477
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a 324.5MHz image of the NOAO Bootes field that was made using Very Large Array (VLA) P-band observations. The image has a resolution of 5.6"x5.1", a radius of 2.05 degrees and a central noise of ~0.2mJy/beam. Both the resolution and noise of the image are an order of magnitude better than what was previously available at this frequency and will serve as a valuable addition to the already extensive multiwavelength data that are available for this field. The final source catalogue contains 1370 sources and has a median 325 to 1400MHz spectral index of -0.72. Using a radio colour-colour diagram of the unresolved sources in our catalogue, we identify 33 megahertz peaked-spectrum (MPS) sources. Based on the turnover frequency linear size relation for the gigahertz peaked-spectrum (GPS) and compact steep-spectrum (CSS) sources, we expect that the MPS sources that are compact on scales of tens of milliarcseconds should be young radio loud active galactic nuclei at high (z>2) redshifts. Of the 33 MPS sources, we were able to determine redshifts for 24, with an average redshift of 1.3. Given that five of the sources are at z>2, that the four faint sources for which we could not find redshifts are likely at even higher redshifts and that we could only select sources that are compact on a scale of ~5", there is encouraging evidence that the MPS method can be used to search for high-redshift sources.
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Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/461/197
- Title:
- VLA NH3 observations of 5 IRAS sources
- Short Name:
- J/A+A/461/197
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The high angular resolution images with the Very Large Array (VLA) allow us to explore the general properties of protostellar cores in massive star forming regions at large distances. We observed the NH3 (J, K)=(1, 1) and (2, 2) lines toward five massive protostar candidates (IRAS 18196-1331, IRAS 18352-0148, IRAS 18361-0627, IRAS 18414-0339 and IRAS 19474+2637) with the VLA D configuration.
- ID:
- ivo://CDS.VizieR/J/ApJ/707/283
- Title:
- VLA observation of molecular clumps
- Short Name:
- J/ApJ/707/283
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have conducted a search for ionized gas at 3.6cm, using the Very Large Array, toward 31 Galactic intermediate- and high-mass clumps detected in previous millimeter continuum observations. In the 10 observed fields, 35 HII regions are identified, of which 20 are newly discovered. Many of the HII regions are multiply peaked indicating the presence of a cluster of massive stars. We find that the ionized gas tends to be associated toward the millimeter clumps; of the 31 millimeter clumps observed, nine of these appear to be physically related to ionized gas, and a further six have ionized gas emission within 1'. For clumps with associated ionized gas, the combined mass of the ionizing massive stars is compared to the clump masses to provide an estimate of the instantaneous star formation efficiency. These values range from a few percent to 25%, and have an average of 7%+/-8%. We also find a correlation between the clump mass and the mass of the ionizing massive stars within it, which is consistent with a power law. This result is comparable to the prediction of star formation by competitive accretion that a power-law relationship exists between the mass of the most massive star in a cluster and the total mass of the remaining stars.
- ID:
- ivo://CDS.VizieR/J/ApJS/244/35
- Title:
- VLA observations of the Central Molecular Zone
- Short Name:
- J/ApJS/244/35
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present new observations of the C-band continuum emission and masers to assess high-mass (>8M_{sun}_) star formation at early evolutionary phases in the inner 200pc of the Central Molecular Zone (CMZ) of the Galaxy. The continuum observation is complete to free-free emission from stars above 10-11M_{sun}_ in 91% of the covered area. We identify 104 compact sources in the continuum emission, among which five are confirmed ultracompact HII regions, 12 are candidates of ultracompact HII regions, and the remaining 87 sources are mostly massive stars in clusters, field stars, evolved stars, pulsars, extragalactic sources, or of unknown nature that is to be investigated. We detect class II CH_3_OH masers at 23 positions, among which six are new detections. We confirm six known H_2_CO masers in two high-mass star-forming regions and detect two new H_2_CO masers toward the Sgr C cloud, making it the ninth region in the Galaxy that contains masers of this type. In spite of these detections, we find that current high-mass star formation in the inner CMZ is only taking place in seven isolated clouds. The results suggest that star formation at early evolutionary phases in the CMZ is about 10 times less efficient than expected from the dense gas star formation relation, which is in line with previous studies that focus on more evolved phases of star formation. This means that if there will be any impending, next burst of star formation in the CMZ, it has not yet begun.
- ID:
- ivo://CDS.VizieR/J/ApJ/699/186
- Title:
- VLA observations of the Galactic center
- Short Name:
- J/ApJ/699/186
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present new results based on high-resolution observations of Sgr A West at the Galactic center with the Very Large Array (VLA) at 1.3cm. By combining recent observations with those made at earlier epochs with the VLA at wavelengths of 1.3 and 3.6cm, we measured proper motions for 71 compact HII components in the central 80" (3pc, assuming D=8pc). Using VLA archival data for the H92{alpha} radio recombination line, we also investigated radial velocities in the LSR velocity range from +200 to -415km/s. Combining proper motion and radial velocity measurements, we have determined the three-dimensional velocity distribution in Sgr A West.
- ID:
- ivo://CDS.VizieR/J/A+A/336/339
- Title:
- VLA observations of ultracompact HII regions
- Short Name:
- J/A+A/336/339
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have used the Very Large Array (VLA) to search for radio continuum emission towards a sample of 67 IRAS sources selected from a previous study. All observed sources are associated with high density molecular gas, exhibit an infrared spectral energy distribution characteristic of very cold young stellar objects and many of them are associated with H_2_O masers. The observed sample is divided into two groups of sources: High, with IRAS spectral energy distributions resembling those of ultracompact HII regions, and Low, for which previously collected evidence suggests that they may contain a higher fraction of protostellar objects than the High group; such objects might not have started hydrogen burning yet. Radio continuum emission was detected towards 37 sources (55%), although only in 22 cases an association with the IRAS source is established. Of the latter, 9 (24%) objects belong to the Low type and 13 (43%) to the High type. Thus, we find that 76% of Low and 57% of High sources are not associated with a radio counterpart. Because the majority of the sources have luminosities above ~10^4^L_{sun}_, corresponding to central stars of spectral type between B2 and O7, the lack of radio emission is interpreted as being due to the action of accreting matter that chokes off the expansion of the ionised gas. We show that this require s only moderate mass accretion rates, below ~10^-4^M_{sun}_/yr. Alternatively, dust absorption can also effectively absorb UV photons and the gas column density implied by our observations indicates values in excess of 10^22^cm^-2^. The physical properties of IRAS sources with associated radio counterpa rt derived from the present observations do not distinguish between High and Low sources. These sources are likely to be ZAMS stars with variable amounts of dust within the ionised region which acts as UV field absorber. The large majority of detected sources (75%) have spherical or unresolved morphology, while 15% are irregular or multiply peaked and only 10% have a core-halo structure. These results agree with the known properties of ultracompact HII regions, even though the average luminosity of the present sample is an order of magnitude lower than that in previous studies.
- ID:
- ivo://CDS.VizieR/J/ApJ/884/140
- Title:
- VLA obs. of the H2O maser source G25.65+1.05
- Short Name:
- J/ApJ/884/140
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The source G25.65+1.05 (RAFGL7009S, IRAS 18316-0602) is the least studied of the three regions of massive star formation known to show exceptionally powerful H_2_O maser bursts. We report spectral line observations of the H_2_O maser at 22GHz, the methanol maser transitions at 6.7, 12.2, and 44GHz, and the continuum in these same frequency bands with The Karl G. Jansky Very Large Array at the post-burst epoch of 2017. For the first time, maps of 22GHz H_2_O and 44GHz CH_3_OH maser spots are obtained and the absolute position of the 22GHz H_2_O bursting feature is determined with milliarcsecond precision. We detected four continuum components, three of which are closely spaced in a linear orientation, suggesting a physical link between them.
- ID:
- ivo://CDS.VizieR/J/ApJS/133/77
- Title:
- VLA radio continuum survey of Seyfert galaxies
- Short Name:
- J/ApJS/133/77
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have used the Very Large Array (VLA) to conduct a survey for radio continuum emission in the sample of 52 Seyfert nuclei selected from the optical spectroscopic galaxy catalog of Ho, Filippenko, & Sargent (1995, Cat. <J/ApJS/98/477>. This Seyfert sample is the most complete and least biased available, and, as such, it will be useful for a variety of statistical analyses. Here we present the observations, measurements, and an atlas of radio maps. The observations were made at 6 cm in the B array and at 20 cm in the A array, yielding matched angular resolutions of ~1". We detected 44 objects (85%) at 6 cm and 37 objects (71%) at 20 cm above a 3{sigma} threshold of 0.12mJy/beam. The sources have a wide range of radio powers (P~10^19^-10^25^W/Hz), spectral indices ({alpha}^20^_6_~+0.5 to 1), and linear sizes (L~few tens pc - 15kpc). The morphology of the radio emission is predominantly that of a compact core, either unresolved or slightly resolved, occasionally accompanied by elongated, jetlike features. Linearly polarized emission was detected at 6 cm in 12 sources, nine of which were also detected at 20 cm.
- ID:
- ivo://CDS.VizieR/J/ApJ/728/97
- Title:
- VLA rotation measures in the Galactic plane
- Short Name:
- J/ApJ/728/97
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have determined 194 Faraday rotation measures (RMs) of polarized extragalactic radio sources using new, multi-channel polarization observations at frequencies around 1.4GHz from the Very Large Array in the Galactic plane at 17{deg}<=l<=63{deg} and 205{deg}<=l<=253{deg}. This catalog fills in gaps in the RM coverage of the Galactic plane between the Canadian Galactic Plane Survey (CGPS; Taylor et al. 2003, Cat. VI/128; Brown et al. 2003, Cat. J/ApJS/145/213) and Southern Galactic Plane Survey (SGPS; Haverkorn et al. 2006ApJS..167..230H; Brown et al. 2007, Cat. J/ApJ/663/258).
- ID:
- ivo://CDS.VizieR/J/ApJ/740/65
- Title:
- VLA search for 5GHz radio transients
- Short Name:
- J/ApJ/740/65
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of a 5GHz survey with the Very Large Array (VLA) and the expanded VLA, designed to search for short-lived (<~1day) transients and to characterize the variability of radio sources at milli-Jansky levels. A total sky area of 2.66deg^2^, spread over 141 fields at low Galactic latitudes (b~6-8{deg}), was observed 16 times with a cadence that was chosen to sample timescales of days, months, and years. Most of the data were reduced, analyzed, and searched for transients in near real-time. Interesting candidates were followed up using visible light telescopes (typical delays of 1-2hr) and the X-ray Telescope on board the Swift satellite. The final processing of the data revealed a single possible transient with a peak flux density of f_{nu}_~2.4mJy. Furthermore, we use the data to measure the source variability on timescales of days to years, and we present the variability structure function of 5GHz sources.
