High-resolution Radio Observations Research Articles

The RMS survey

The Red MSX Source (RMS) survey is a multi-wavelength program of follow-up observations designed to distinguish between genuine massive young stellar objects (MYSOs) and other embedded or dusty objects, such as ultra compact (UC) HII regions, evolved stars and planetary nebulae (PNe). We have identified nearly 2000 massive YSOs candidates by comparing the colours of MSX and 2MASS point sources to those of known MYSOs. Unfortunately, there are several other types of embedded or dust enshrouded objects that have similar colours as MYSOs and contaminate our sample. Two sources of contamination are from UCHII regions and PNe, both of which can be identified from the radio emission emitted by their ionised nebulae. In order to identify UCHII regions and PNe that contaminate our sample we have conducted high resolution radio continuum observations at 3.6 and 6 cm of all southern MYSOs candidates ($235< l < 350$) using the Australia Telescope Compact Array (ATCA). These observations have a spatial resolution of $\sim$1--2\arcsec and typical image rms noise values of $\sim$0.3 mJy -- sensitive enough to detect a B0.5 star at the far side of the Galaxy. Of the 826 RMS sources observed we found 199 to be associated with radio emission, $\sim$25% of the sample. More significant than those sources associated with radio emission are the 627 RMS sources for which no radio emission was detected as these sources are still potential MYSOs. The Galactic distribution, morphologies and spectral indices of the radio sources associated with the RMS sources are consistent with these sources being UCHII regions. In addition to the 826 RMS sources observed we present observations of a further 190 fields. These observations were made towards MSX sources that passed cuts in earlier versions of the survey, but were later excluded.

3C 254: MERLIN observations of a highly asymmetric quasar

Multifrequency, high-resolution radio observations of the quasar 3C 254 using the Multi-Element Radio-linked Interferometer Network (MERLIN) are presented. The quasar has a highly asymmetric radio structure, with the eastern component of the double-lobed structure being much closer to the nucleus and significantly less polarized than the western one. However, the two lobes are more symmetric in their total flux densities. The observations show the detailed structure of the hotspots which are very different on opposite sides of the radio core, reveal no radio jet and suggest that the oppositely directed jets may be intrinsically asymmetric.

The RMS survey: radio observations of candidate massive YSOs in the southern hemisphere

AbstractThe Red MSX Source (RMS) survey (Hoare et al. 2005) is a multi-wavelength programme of follow-up observations designed to distinguish between genuine massive young stellar objects (MYSOs) and other embedded or dusty objects, such as ultra compact (UC) HII regions, evolved stars and planetary nebulae (PNe). We have identified nearly 2000 MYSOs candidates by comparing the colours of MSX and 2MASS point sources to those of known MYSOs. There are several other types of embedded or dust enshrouded objects that have similar colours as MYSOs and contaminate our sample. Two sources of contamination are from UCHII regions and PNe, both of which can be identified from the radio emission emitted by their ionised nebulae. In order to identify UCHII regions and PNe that contaminate our sample we have conducted high resolution radio continuum observations at 3.6 and 6 cm of all southern MYSOs candidates (235° &lt; l &lt; 350°) using the Australia Telescope Compact Array (ATCA).

Radio and infrared recombination studies of the southern massive star forming region G333.6-0.2

We present high spatial resolution radio and near-infrared hydrogen recombination line observations of the southern massive star-forming region G333.6 0.2. The 3.4-cm continuum peak is found slightly offset from the infrared source. The H90α spectra show for the first time a double peak profile at some positions. The complex velocity s tructure may be accounted for by champagne outflows, which may also explain the offset betw een the radio and infrared sources. The 2.17-µm Brγ image and H90α map are combined to construct an extinction map which shows a trend probably set by the blister nature of the H II region. The total number of Lyman continuum photons in the central 50-arcsec is estimated to be equivalent to that emitted by up to 19 O7V stars.

Multiwavelength studies of WR 21a and its surroundings

We present results of high-resolution radio continuum observations towards the binary star WR 21a (Wack 2134) obtained with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.64 GHz. We detected the system at 4.8 GHz (6 cm) with a flux density of 0.25+-0.06 mJy and set an upper limit of 0.3 mJy at 8.64 GHz (3 cm). The derived spectral index of alpha < 0.3 [S(nu)=k.nu**(alpha)] suggests the presence of non-thermal emission, probably originating in a colliding-wind region. A second, unrelated radio source was detected ~10" north of WR 21a at (RA, Dec)(J2000)=(10h 25m 56.49, -57deg 48' 34.4", with flux densities of 0.36 and 0.55 mJy at 4.8 and 8.64 GHz, respectively, resulting in alpha = 0.72. HI observations in the area are dominated by absorption against the prominent HII region RCW 49. Analysis of a complete set of archived X-ray observations of WR 21a confirms its strong variability but throws into doubt previous suggestions by Reig (1999) of a period of years for the system. Finally, we comment on the association with the nearby EGRET source 3EG J1027-5817.

A supernova factory in Mrk 273?

We report on 1.6 and 5.0 GHz observations of the ultraluminous infrared galaxy (ULIRG) Mrk 273, using the European VLBI Network (EVN) and the Multi-Element Radio-Linked Interferometer Network (MERLIN). We also make use of published 1.4 GHz VLBA observations of Mrk 273 by Carilli & Taylor (2000). Our 5 GHz images have a maximum resolution of 5-10 mas, which corresponds to linear resolutions of 3.5-7 pc at the distance of Mrk 273, and are the most sensitive high-resolution radio observations yet made of this ULIRG. Component N1, often pinpointed as a possible AGN, displays a steep spectral index ($\alpha = 1.2 \pm 0.1; S_\nu \propto \nu^{-\alpha}$); hence it is very difficult to reconcile with N1 being an AGN, and rather suggests that the compact nonthermal radio emission is produced by an extremely high luminous individual radio supernova (RSN), or a combination of unresolved emission from nested supernova remnants (SNR), luminous RSNe, or both. Component N2 is partly resolved out into several compact radio sources --none of which clearly dominates-- and a region of extended emission about 30 pc in size. The integrated spectral index of this region is flat ($\alpha = 0.15 \pm 0.1$), which can be interpreted as due to a superposition of several unresolved components, e.g., RSNe or SNRs, whose radio emission peaks at different frequencies and is partially free-free absorbed. The overall extended radio emission from component N is typical of nonthermal, optically thin radio emission ($\alpha = 0.8 \pm 0.1$), and its 1.4 GHz luminosity ($L_{1.4 \rm GHz} = (2.2 \pm 0.1)\times 10^{23} $ WHz$^{-1}$) is consistent with being produced by relativistic electrons diffused away from supernova remnants in an outburst.

Current day mass loss rate for Luminous Blue Variable IRAS 18576+0341

In this letter we report on the first set of multi-frequency and high angular resolution radio observations of IRAS 18576+0341. The radio observations have revealed an extended, asymmetric and quite structured radio nebula and al- lowed us to locate the central core of this highly obscured new galactic Luminous Blue Variable. From the analysis of radio properties of IRAS 18576+0341 estimates of important physical parameters of the central star have been determined. In partic- ular, an effective temperature of Teff ∼ (2.6 ± 0.2) × 10 4 K, corresponding to a B0-B0.5 supergiant, has been derived. The most notable result is the determination of the current day mass-loss ( u M = 3.7 × 10 −5 Myr −1 ), which is of particular importance since mass-loss rate determination from radio observations appears to be a more reliable method compared to the others based on different diagnostics.

Star formation, massive stars, and super star clusters in nearby galaxies

The Square Kilometer Array (SKA) will enable studies of star formation in nearby galaxies with a level of detail never before possible outside of the Milky Way. Because the earliest stages of stellar evolution are often inaccessible at optical and near-infrared wavelengths, high spatial resolution radio observations are necessary to explore extragalactic star formation. The SKA will have the sensitivity to detect individual ultracompact HII regions out to the distance of nearly 50 Mpc, allowing us to study their spatial distributions, morphologies, and populations statistics in a wide range of environments. Radio observations of Wolf-Rayet stars outside of the Milky Way will also be possible for the first time, greatly expanding the range of conditions in which their mass loss rates can be determined from free-free emission. On a vastly larger scale, natal of super star clusters will be accessible to the SKA out to redshifts of nearly z ∼ 0.1. The unprecedented sensitivity of radio observations with the SKA will also place tight constraints on the star formation rates as low as 1 M ⊙ yr −1 in galaxies out to a redshift of z ∼ 1 by directly measuring the thermal radio flux density without assumptions about a galaxy’s magnetic field strength, cosmic ray production rate, or extinction.

Radio sources at low Galactic latitudes

ABSTRACT High-resolution radio observations of a sample of 65 radio sources at low Galactic lat-itudes are presented. The sources were all observed at 5 GHz with the VLA A-array.MERLIN observations at 5 GHz of the ultracompact Hii region G34.26+0.15 and oneof the extragalactic sources, B1857−000, are also presented, as are GMRT observa-tions of Hi in the direction of three sources, B1801−203, B1802−196 and B1938+229.These observations were made with the objectives of (i) finding compact componentssuitable for studying the effects of interstellar scattering at lower frequencies, (ii) iden-tifying high surface-brightness lobes of background radio sources to probe the Galacticmagnetic field on different scales via polarization observations, and (iii) searching foryoung supernova remnants. The nature of the sources found to have shell or shell-like structure and exhibiting both thermal and non-thermal spectra are discussed.Of the remaining sources, B1749−281 is coincident within the positional errors of aknown pulsar, not detected earlier at 5 GHz. The rest are likely to be backgroundextragalactic objects.Key words: Galaxy: general - supernova remnants - Hiiregions - planetary nebulae:general - radio continuum: galaxies

Further clues to the nature of composite LINER/H II galaxies

We have analyzed new, archival and published high resolution radio and X-ray observations of a sample of composite LINER/H  galaxies known to exhibit AGN-like properties. Five of the 16 AGN candidates have milliarcsecond-scale detections and are found to display a compact, flat spectrum, high brightness temperature radio core, four of which also exhibit extended radio emission. Five of the eight AGN candidates with available high resolution X-ray observations were found to possess a hard X-ray nuclear source, two of which have no milliarcsecond radio detection. The combined high resolution radio and X-ray data yield a 50% detection rate of low luminosity AGN among the AGN candidates, which translates into a 12% detection rate for the entire composite LINER/H  sample. In the sources where the AGN has been unambiguously detected, the ionizing power of the AGN is not sufficient to generate the observed emission lines, unless the hard X-rays are heavily obscured. We attempt to apply a canonical advection-dominated accretion flow (ADAF) and jet model to the sample sources in order to explain the observed radio and X-ray emission. While ADAFs may be responsible for the observed emission in submillijansky radio cores like NGC 7331, they do not appear consistent with the radio emission observed in the milliarcsecond-scale radio detected cores; the latter sources are more likely to have an energetically important contribution from a radio-emitting jet.

A relativistic helical jet in theγ-ray AGN 1156+295

We present the results of a number of high resolution radio observations of the AGN 1156+295. These include multi-epoch and multi-frequency VLBI, VSOP, MERLIN and VLA observations made over a period of 50 months. The 5 GHz MERLIN images trace a straight jet extending to similar to2 at PAsimilar to-18degrees. Extended low brightness emission was detected in the MERLIN observation at 1.6 GHz and the VLA observation at 8.5 GHz with a bend of similar to90degrees at the end of the 2 arcsec jet. A region of similar diffuse emission is also seen about 2 arcsec south of the radio core. The VLBI images of the blazar reveal a core-jet structure with an oscillating jet on a milli-arcsecond (mas) scale which aligns with the arcsecond jet at a distance of several tens of milli-arcseconds from the core. This probably indicates that the orientation of the jet structure is close to the line of sight, with the northern jet being relativistically beamed toward us. In this scenario the diffuse emission to the north and south is not beamed and appears symmetrical. For the northern jet at the mas scale, proper motions of 13.7+/-3.5, 10.6+/-2.8, and 11.8+/-2.8 c are measured in three distinct components of the jet (q(0)=0.5, H-0=65 km s(-1) Mpc(-1) are used through out this paper). Highly polarised emission is detected on VLBI scales in the region in which the jet bends sharply to the north-west. The spectral index distribution of the source shows that the strongest compact component has a flat spectrum, and the extended jet has a steep spectrum. A helical trajectory along the surface of a cone was proposed based on the conservation laws for kinetic energy and momentum to explain the observed phenomena, which is in a good agreement with the observed results on scales of 1 mas to 1 arcsec.

Emission mechanisms in composite galaxies

High resolution radio and X-ray observations of a subsample of composite LINER/H II galaxies with arcsec-scale radio cores were obtained. These observations yield a 12% detection rate of low luminosity AGN among the entire composite sample in the Palomar survey. Four of these low luminosity AGN also possess sub-parsec jet-like radio extensions. While canonical advection-dominated accretion flows may be repsonsible for the radio and X-ray emission in submillijansky cores like NGC 7331, they do not appear consistent with the radio emission in these milliarcsecond-scale radio cores; the latter sources are more likely to have an energetically important contribution from a radio-emitting jet.

AGNs in LLAGNs: High resolution radio observations of the Palomar sample

We present completed results of a high resolution radio imaging survey of the Palomar Spectroscopic Sample of all (∼470) nearby bright northern galaxies. Almost half the Palomar sample's galaxies have nuclei with emission-lines characteristic of AGN but with L Hα ≤ 10 40 erg s −1 . These are referred to as low-luminosity AGNs or LLAGNs. The power source of such LLAGNs has been long debated. High resolution radio surveys of the sample – with the VLA at 15 GHz (150 mas resolution), and the VLBA at 5 GHz (2 mas resolution) – have now revealed a high incidence of pc-scale radio cores with implied brightness temperatures [gsim ]10 8 K, and sub-parsec scale jets. The results support the presence of accreting black holes in ≥50% of all LLAGNs; there is no evidence against all LLAGNs being mini-AGNs. The detected parsec-scale radio nuclei are preferentially found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad Hα emission). These nuclei follow the usual correlations between radio and emission-line gas properties found in more powerful AGNs. The radio luminosity function (RLF) of Palomar Sample LLAGNs extends three orders of magnitude below, and is continuous with, that of ‘classical’ AGNs. We find marginal evidence for a low-power turnover in the RLF; nevertheless LLAGNs are responsible for a significant fraction of accretion in the local universe. Low accretion rates (≤10 −2 −10 −3 of the Eddington rate) are implied in both advection- and jet-type models. Within the context of jet models, the accretion energy output is dominated by the energy in the observed jets rather than the radiated bolometric luminosity. These jets would be able to dump sufficient energy into the innermost parsecs to significantly slow the accretion inflow. Detailed results can be found in Nagar et al. (2002, 2004) and references therein. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

High Resolution Radio Observations of AGN

HALCA, the first dedicated satellite for space-VLBI, was launched in February 1997, and VLBI Space Observatory Programme (VSOP) observations have been successfully undertaken at 1.6 and 5 GHz. This paper outlines the highlights of high resolution radio observations of AGN by VSOP. The planned near-future VSOP-2 mission and further future missions are introduced.

New High-Resolution Radio Observations of the Supernova Remnant CTB 80

We report new high resolution and high sensitivity radio observations of the extended supernova remnant (SNR) CTB 80 (G69.0+2.7) at 240 MHz, 324 MHz, 618 MHz, and 1380 MHz. The imaging of CTB 80 at 240 MHz and 618 MHz was performed using the Giant Metrewave Radio Telescope (GMRT) in India. The observations at 324 MHz and 1380 MHz were obtained using the Very Large Array (VLA, NRAO) in its C and D configurations. The new radio images reveal faint extensions for the asymmetric arms of CTB 80. The arms are irregular with filaments and clumps of size 1' (or 0.6 pc at a distance of 2 kpc). The radio image at 1380 MHz is compared with IR and optical emission. The correspondence IR/radio is excellent along the N arm of CTB 80. Ionized gas observed in the [SII] line perfectly matches the W and N edges of CTB 80. The central nebula associated with the pulsar PSR B1951+32 was investigated with an angular resolution of 10" x 6". The new radio image obtained at 618 MHz shows with superb detail structures in the 8' x 4' E-W ``plateau'' nebula that hosts the pulsar on its western extreme. A twisted filament, about 6' in extent (~3.5 pc), trails behind the pulsar in an approximate W-E direction. In the bright ``core'' nebula (size \~45"), located to the W of the plateau, the images show a distortion in the morphology towards the W; this feature corresponds to the direction in which the pulsar escapes from the SNR with a velocity of ~240 km/s. Based on the new observations, the energetics of the SNR and of the PWN are investigated.

Molecular LineObservations of the Tornado Nebula and its Eye

We present millimetre and NIR molecular-line observations of the Tornado nebula and its Eye. The observations were motivated by the presence of OH(1720 MHz) maser emission towards the nebula, believed to be an indicator of interaction between a supernova remnant and a molecular cloud. We found that the distribution of molecular gas around the Tornado complements its radio morphology, implying that the nebula’s appearance has been influenced by the structure of the surrou nding molecular gas. Our NIR H2 observations revealed the presence of shocked molecular gas at the location where the nebula is expanding into the surrounding molecular cloud. It has been suggested that the Eye of the Tornado is related to the nebula on the basis of their apparent proximity. Our NIR and millimetre-line observations show that the two objects are not spatially related. Br line emission, in conjuction with IR data at longer wavelengths and high-resolution radio continuum observations, suggests that the Eye is a massive protostellar sou rce deeply embedded within a dense molecular core.

Discovery of a New Pulsar Wind Nebula in the Large Magellanic Cloud

We present new high-resolution radio and X-ray observations of the supernova remnant (SNR) B0453-685 in the Large Magellanic Cloud, carried out with the Australia Telescope Compact Array and the Chandra X-Ray Observatory, respectively. Embedded in the SNR shell is a compact central nebula producing both flat-spectrum polarized radio emission and nonthermal X-rays; we identify this source as a pulsar wind nebula (PWN) powered by an unseen central neutron star. We present a new approach by which the properties of a SNR and PWN can be used to infer upper limits on the initial spin period and surface magnetic field of the unseen pulsar, and we conclude that this star was an initial rapid rotator with current properties similar to those of the Vela pulsar. As is the case for other similarly aged sources, there is currently an interaction taking place between the PWN and the SNR's reverse shock.

Detection of the “Active” Nucleus of M32

M32 hosts a 2.5 × 106 M☉ central black hole, but signs of nuclear activity in this galaxy have long eluded detection. We report the first conclusive detection of the nucleus of M32 in X-rays, based on high-resolution, sensitive observations made with Chandra. The 2-10 keV luminosity is merely 9.4 × 1035 ergs s-1, ~3 × 10-9 times the Eddington luminosity of the black hole. Weak diffuse emission, consistent with thermal gas at a temperature of 0.37 keV, is seen in an annular region ~100 pc from the center. We also present a deep, moderately high-resolution (09) radio continuum observation obtained with the Very Large Array, which places a tight upper bound on the nuclear radio power at 8.4 GHz. We combine these new measurements with upper limits at other wavelengths to discuss implications for the nature of accretion onto the central black hole.

A median redshift of 2.4 for galaxies bright at submillimetre wavelengths.

A significant fraction of the energy emitted in the early Universe came from very luminous galaxies that are largely hidden at optical wavelengths (because of interstellar dust grains); this energy now forms part of the cosmic background radiation at wavelengths near 1 mm (ref. 1). Some submillimetre (submm) galaxies have been resolved from the background radiation, but they have been difficult to study because of instrumental limitations. This has impeded the determination of their redshifts (z), which is a crucial element in understanding their nature and evolution. Here we report spectroscopic redshifts for ten submm galaxies that were identified using high-resolution radio observations. The median redshift for our sample is 2.4, with a quartile range of 1.9-2.8. This population therefore coexists with the peak activity of quasars, suggesting a close relationship between the growth of massive black holes and luminous dusty galaxies. The space density of submm galaxies at redshifts over 2 is about 1,000 times greater than that of similarly luminous galaxies in the present-day Universe, so they represent an important component of star formation at high redshifts.

The Quintuple Quasar: Radio and Optical Observations

We present results from high-resolution radio and optical observations of PMN J0134-0931, a gravitational lens with a unique radio morphology and an extremely red optical counterpart. Our data support the theory of Keeton & Winn (2003): five of the six observed radio components are multiple images of a single quasar, produced by a pair of lens galaxies. Multi-frequency VLBA maps show that the sixth and faintest component has a different radio spectrum than the others, confirming that it represents a second component of the background source rather than a sixth image. The lens models predict that there should be additional faint images of this second source component, and we find evidence for one of the predicted images. The previously-observed large angular sizes of two of the five bright components are not intrinsic (which would have excluded the possibility that they are lensed images), but are instead due to scatter broadening. Both the extended radio emission observed at low frequencies, and the intrinsic image shapes observed at high frequencies, can be explained by the lens models. The pair of lens galaxies is marginally detected in HST images. The differential extinction of the quasar images suggests that the extreme red color of the quasar is at least partly due to dust in the lens galaxies.