- ID:
- ivo://CDS.VizieR/J/ApJS/234/24
- Title:
- VLA 33GHz obs. of star-forming regions
- Short Name:
- J/ApJS/234/24
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present 33GHz imaging for 112 pointings toward galaxy nuclei and extranuclear star-forming regions at ~2" resolution using the Karl G. Jansky Very Large Array (VLA) as part of the Star Formation in Radio Survey. A comparison with 33GHz Robert C. Byrd Green Bank Telescope single-dish observations indicates that the interferometric VLA observations recover 78%+/-4% of the total flux density over 25" regions (~kpc scales) among all fields. On these scales, the emission being resolved out is most likely diffuse non-thermal synchrotron emission. Consequently, on the ~30-300pc scales sampled by our VLA observations, the bulk of the 33GHz emission is recovered and primarily powered by free-free emission from discrete HII regions, making it an excellent tracer of massive star formation. Of the 225 discrete regions used for aperture photometry, 162 are extranuclear (i.e., having galactocentric radii rG>=250pc) and detected at >3{sigma} significance at 33GHz and in H{alpha}. Assuming a typical 33GHz thermal fraction of 90%, the ratio of optically-thin 33GHz to uncorrected H{alpha} star formation rates indicates a median extinction value on ~30-300pc scales of A_H{alpha}_~1.26+/-0.09mag, with an associated median absolute deviation of 0.87mag. We find that 10% of these sources are "highly embedded" (i.e., A_H{alpha}_>~3.3mag), suggesting that on average, HII regions remain embedded for <~1Myr. Finally, we find the median 33GHz continuum-to-H{alpha} line flux ratio to be statistically larger within rG<250pc relative to the outer disk regions by a factor of 1.82+/-0.39, while the ratio of 33GHz to 24{mu}m flux densities is lower by a factor of 0.45+/-0.08, which may suggest increased extinction in the central regions.
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Search Results
- ID:
- ivo://CDS.VizieR/J/AJ/151/74
- Title:
- VLA/VLBA obscured radio-loud active galactic nuclei
- Short Name:
- J/AJ/151/74
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- This is the second paper directed toward finding new highly redshifted atomic and molecular absorption lines at radio frequencies. To this end, we selected a sample of 80 candidates for obscured radio-loud active galactic nuclei (AGNs) and presented their basic optical/near-infrared (NIR) properties in Yan et al. 2012AJ....144..124Y. In this paper, we present both high-resolution radio continuum images for all of these sources and H I 21 cm absorption spectroscopy for a few selected sources in this sample. A-configuration 4.9 and 8.5 GHz Very Large Array continuum observations find that 52 sources are compact or have substantial compact components with size <0.5" and flux densities >0.1 Jy at 4.9 GHz. The 36 most compact sources were then observed with the Very Long Baseline Array at 1.4 GHz. One definite and 10 candidate Compact Symmetric Objects (CSOs) are newly identified, which is a detection rate of CSOs ~three times higher than the detection rate previously found in purely flux-limited samples. Based on possessing compact components with high flux densities, 60 of these sources are good candidates for absorption-line searches. Twenty-seven sources were observed for H I 21 cm absorption at their photometric or spectroscopic redshifts with only six detections (five definite and one tentative). However, five of these were from a small subset of six CSOs with pure galaxy optical/NIR spectra (i.e., any AGN emission is obscured) and for which accurate spectroscopic redshifts place the redshifted 21 cm line in a radio frequency intereference (RFI)-free spectral "window" (i.e., the percentage of H I 21 cm absorption-line detections could be as high as ~90% in this sample). It is likely that the presence of ubiquitous RFI and the absence of accurate spectroscopic redshifts preclude H I detections in similar sources (only 1 detection out of the remaining 22 sources observed, 13 of which have only photometric redshifts); that is, H I absorption may well be present but is masked by the RFI. Future searches for highly redshifted H I and molecular absorption can easily find more distant CSOs among bright, "blank field" radio sources, but will be severely hampered by an inability to determine accurate spectroscopic redshifts due to their lack of rest-frame UV continuum.
- ID:
- ivo://CDS.VizieR/J/ApJ/749/47
- Title:
- VLBA 22.2GHz H_2_O maser features in G28.87+0.07
- Short Name:
- J/ApJ/749/47
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We used the Very Long Baseline Array and the European VLBI Network to perform phase-referenced very long baseline interferometry observations of the three most powerful maser transitions associated with the high-mass star-forming region G28.87+0.07: the 22.2GHz H_2_O, 6.7GHz CH_3_OH, and 1.665GHz OH lines. We also performed Very Large Array (VLA) observations of the radio continuum emission at 1.3 and 3.6cm and Subaru observations of the continuum emission at 24.5{mu}m. Two centimeter-continuum sources are detected and one of them (named hot molecular core (HMC)) is compact and placed at the center of the observed distribution of H_2_O, CH_3_OH, and OH masers. The bipolar distribution of line-of-sight velocities and the pattern of the proper motions suggest that the water masers are driven by a (proto)stellar jet interacting with the dense circumstellar gas. The same jet could both excite the centimeter-continuum source named HMC (interpreted as free-free emission from shocked gas) and power the molecular outflow observed at larger scales--although one cannot exclude that the free-free continuum is rather originating from a hypercompact H II region. At 24.5{mu}m, we identify two objects separated along the north-south direction, whose absolute positions agree with those of the two VLA continuum sources. We establish that ~90% of the luminosity of the region (~2x10^5^L_{sun}_) is coming from the radio source HMC, which confirms the existence of an embedded massive young stellar object exciting the masers and possibly still undergoing heavy accretion from the surrounding envelope.
- ID:
- ivo://CDS.VizieR/J/A+A/638/A17
- Title:
- VLBA SiO masers toward V838 Monocerotis
- Short Name:
- J/A+A/638/A17
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present multi-epoch observations with the VLBA of SiO maser emission in the v=1, J=1-0 transition at 43GHz from the remnant of the red nova V838 Mon. We model the positions of maser spots to derive a parallax of 0.166+/-0.060mas. Combining this parallax with other distance information results in a distance of 5.6+/-0.5kpc, which agrees with an independent geometric distance of 6.1+/-0.6kpc from modeling polarimetry images of V838 Mon's light echo. Combining these results, and including a weakly constraining Gaia DR2 parallax, yields a best estimate of distance of 5.9+/-0.4kpc. The maser spots are located close to the peaks of continuum at ~225GHz and SiO J=5-4 thermal emission detected with ALMA. The proper motion of V838 Mon confirms its membership in a small open cluster in the Outer spiral arm of the Milky Way.
- ID:
- ivo://CDS.VizieR/J/A+A/520/A113
- Title:
- VLBI detection of 398 extragalactic radio sources
- Short Name:
- J/A+A/520/A113
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The European space astrometry mission Gaia will construct a dense optical QSO-based celestial reference frame. For consistency between optical and radio positions, it will be important to align the Gaia and VLBI frames with the highest possible accuracy. It has been found that only 70 (10%) of the sources from the International Celestial Reference Frame (ICRF) are suitable for establishing this link, either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the situation, we have initiated a VLBI survey dedicated to finding additional suitable radio sources for aligning the two frames. The sample consists of 447 optically- bright (magnitude <=18) extragalactic radio sources, typically 20 times weaker than the ICRF sources, which have been selected by cross-correlating an optical quasar catalog with the NRAO VLA Sky Survey (NVSS, Cat. VIII/65). This paper presents the observing strategy to detect, image, and measure accurate radio positions for these sources. It also provides results on the VLBI detectability of the sources, as derived from initial observations with the European VLBI Network in June and October 2007. Based on these observations, a high detection rate of 89% is found, which is promising for the continuation of this project. This high VLBI detection rate for sources from the NVSS catalog is probably due to the selection process, suggesting that optically-bright quasars have compact radio structures.
- ID:
- ivo://CDS.VizieR/J/ApJS/230/13
- Title:
- VLBI Ecliptic Plane Survey: VEPS-1
- Short Name:
- J/ApJS/230/13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present here the results of the first part of the VLBI Ecliptic Plane Survey (VEPS) program. The goal of the program is to find all compact sources within 7.5{deg} of the ecliptic plane that are suitable as calibrators for anticipated phase referencing observations of spacecraft, and determine their positions with accuracy at the 1.5 nrad level. We run the program in two modes: search and refine. In the search mode, a complete sample of all sources brighter than 50mJy at 5GHz listed in the Parkes-MIT-NRAO and Green Bank 6cm (GB6) catalogs, except those previously detected with VLBI, is observed. In the refining mode, the positions of all ecliptic plane sources, including those found in the search mode, are improved. By 2016 October, thirteen 24hr sessions that targeted all sources brighter than 100mJy have been observed and analyzed. Among 3320 observed target sources, 555 objects have been detected. We also conducted a number of follow-up VLBI experiments in the refining mode and improved the positions of 249 ecliptic plane sources.
317. VLBI ICRF2
- ID:
- ivo://CDS.VizieR/J/AJ/150/58
- Title:
- VLBI ICRF2
- Short Name:
- J/AJ/150/58
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the second realization of the International Celestial Reference Frame (ICRF2) at radio wavelengths using nearly 30 years of Very Long Baseline Interferometry observations. ICRF2 contains precise positions of 3414 compact radio astronomical objects and has a positional noise floor of ~40{mu}as and a directional stability of the frame axes of ~10{mu}as. A set of 295 new "defining" sources was selected on the basis of positional stability and the lack of extensive intrinsic source structure. The positional stability of these 295 defining sources and their more uniform sky distribution eliminates the two greatest weaknesses of the first realization of the International Celestial Reference Frame (ICRF1). Alignment of ICRF2 with the International Celestial Reference System was made using 138 positionally stable sources common to both ICRF2 and ICRF1. The resulting ICRF2 was adopted by the International Astronomical Union as the new fundamental celestial reference frame, replacing ICRF1 as of 2010 January 1.
- ID:
- ivo://CDS.VizieR/J/A+A/526/A102
- Title:
- VLBI imaging of 105 extragalactic radio sources
- Short Name:
- J/A+A/526/A102
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The European space astrometry mission Gaia, to be launched by 2012, will construct a dense optical QSO-based celestial reference frame which will need to be linked to the International Celestial Reference Frame (ICRF; the IAU fundamental frame), with the highest accuracy. However, it has been found that only 10% of the ICRF sources (70 sources) are suitable to establish this link. The remaining sources are not useful either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the accuracy of this alignment, we have developed a program of VLBI observations based on three steps to detect, image and measure astrometric positions of weak extragalactic radio sources, with bright optical counterparts, from a sample of 447 candidate sources. The experiments devoted to VLBI detection, carried out with the European VLBI Network (EVN) in June and October 2007, were very successful, with 398 sources detected at both S- and X-bands. From these, 105 sources were observed in March 2008 with a global VLBI array (EVN and VLBA; Very Long Baseline Array) for imaging their VLBI structures. All sources were successfully imaged in both bands and about 50% (47 sources) were found to be point-like on VLBI scales. These images are available at http://www.obs.u-bordeaux1.fr/BVID/GC030/. VLBI positions of these sources will be measured accurately in future astrometric experiments.
- ID:
- ivo://CDS.VizieR/J/ApJ/772/13
- Title:
- VLBI observations of 3C 279 at 230GHz
- Short Name:
- J/ApJ/772/13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report results from five day very long baseline interferometry (VLBI) observations of the well-known quasar 3C 279 at 1.3mm (230GHz) in 2011. The measured nonzero closure phases on triangles including stations in Arizona, California, and Hawaii indicate that the source structure is spatially resolved. We find an unusual inner jet direction at scales of ~1pc extending along the northwest-southeast direction (P.A.=127{deg}+/-3{deg}), as opposed to other (previously) reported measurements on scales of a few parsecs showing inner jet direction extending to the southwest. The 1.3mm structure corresponds closely with that observed in the central region of quasi-simultaneous super-resolution Very Long Baseline Array images at 7mm. The closure phase changed significantly on the last day when compared with the rest of observations, indicating that the inner jet structure may be variable on daily timescales. The observed new direction of the inner jet shows inconsistency with the prediction of a class of jet precession models. Our observations indicate a brightness temperature of ~8x10^10^K in the 1.3mm core, much lower than that at centimeter wavelengths. Observations with better uv coverage and sensitivity in the coming years will allow the discrimination between different structure models and will provide direct images of the inner regions of the jet with 20-30{mu}as (5-7 light months) resolution.
- ID:
- ivo://CDS.VizieR/J/A+A/558/A149
- Title:
- VLTI/MIDI AGN Large Program observations
- Short Name:
- J/A+A/558/A149
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The AGN-heated dust distribution (the "torus") is increasingly recognized not only as the absorber required in unifying models, but as a tracer for the reservoir that feeds the nuclear Super-Massive Black Hole. Yet, even its most basic structural properties (such as its extent, geometry and elongation) are unknown for all but a few archetypal objects. In order to understand how the properties of AGN tori are related to feeding and obscuration, we need to resolve the matter distribution on parsec scales. Since most AGNs are unresolved in the mid-infrared, even with the largest telescopes, we utilize the MID-infrared interferometric Instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) that is sensitive to structures as small as a few milli-arcseconds (mas). We present here an extensive amount of new interferometric observations from the MIDI AGN Large Program (2009-2011) and add data from the archive to give a complete view of the existing MIDI observations of AGNs. Additionally, we have obtained high-quality mid-infrared spectra from VLT/VISIR to provide a precise total flux reference for the interferometric data. We present correlated and total fluxes for 23 AGNs (16 of which with new data) and derive flux and size estimates at 12m using simple axisymmetric geometrical models.
