mJy Rms Research Articles

The Northern Cross fast radio burst project – I. Overview and pilot observations at 408 MHz

ABSTRACT Fast radio bursts (FRBs) remain one of the most enigmatic astrophysical sources. Observations have significantly progressed over the last few years, due to the capabilities of new radio telescopes and the refurbishment of existing ones. Here, we describe the upgrade of the Northern Cross radio telescope, operating in the 400–416 MHz frequency band, with the ultimate goal of turning the array into a dedicated instrument to survey the sky for FRBs. We present test observations of the pulsar B0329+54 to characterize the system performance and forecast detectability. Observations with the system currently in place are still limited by modest sky coverage (∼9.4 deg2) and biased by smearing of high dispersion measure events within each frequency channels. In its final, upgraded configuration, however, the telescope will be able to carry out unbiased FRB surveys over a ∼350 deg2 instantaneous field of view up to z ∼ 5, with a (nearly constant) $\sim 760 \, (\tau /{\rm ms})^{-0.5}$ mJy rms sensitivity.

How Far Can We Push Deconvolution? A SCUBA-2 Test Case

How far can we use multi-wavelength cross-identifications to deconvolve far-infrared images? In this short research note I explore a test case of CLEAN deconvolutions of simulated confused 850 micron SCUBA-2 data, and explore the possible scientific applications of combining this data with ostensibly deeper TolTEC Large Scale Structure (LSS) survey 1.1mm-2mm data. I show that the SCUBA-2 can be reconstructed to the 1.1mm LMT resolution and achieve an 850 micron deconvolved sensitivity of 0.7 mJy RMS, an improvement of at least ~1:5x over naive point source filtered images. The TolTEC/SCUBA-2 combination can constrain cold (<10K) observed-frame colour temperatures, where TolTEC alone cannot.

Polarized point sources in the LOFAR Two-meter Sky Survey: A preliminary catalog

The polarization properties of radio sources at very low frequencies (&lt;200 MHz) have not been widely measured, but the new generation of low-frequency radio telescopes, including the Low Frequency Array (LOFAR: a Square Kilometre Array Low pathfinder), now gives us the opportunity to investigate these properties. In this paper, we report on the preliminary development of a data reduction pipeline to carry out polarization processing and Faraday tomography for data from the LOFAR Two-meter Sky Survey (LOTSS) and present the results of this pipeline from the LOTSS preliminary data release region (10h45m–15h30m right ascension, 45°–57° declination, 570 square degrees). We have produced a catalog of 92 polarized radio sources at 150 MHz at 4.′3 resolution and 1 mJy rms sensitivity, which is the largest catalog of polarized sources at such low frequencies. We estimate a lower limit to the polarized source surface density at 150 MHz, with our resolution and sensitivity, of 1 source per 6.2 square degrees. We find that our Faraday depth measurements are in agreement with previous measurements and have significantly smaller errors. Most of our sources show significant depolarization compared to 1.4 GHz, but there is a small population of sources with low depolarization indicating that their polarized emission is highly localized in Faraday depth. We predict that an extension of this work to the full LOTSS data would detect at least 3400 polarized sources using the same methods, and probably considerably more with improved data processing.

THE HOST GALAXIES OF X-RAY QUASARS ARE NOT STRONG STAR FORMERS

We use ultradeep SCUBA-2 850um observations (~0.37 mJy rms) of the 2 Ms Chandra Deep Field-North (CDF-N) and 4 Ms Chandra Deep Field-South (CDF-S) X-ray fields to examine the amount of dusty star formation taking place in the host galaxies of high-redshift X-ray AGNs. Supplementing with COSMOS, we measure the submillimeter fluxes of the 4-8 keV sources at z>1, finding little flux at the highest X-ray luminosities but significant flux at intermediate luminosities. We determine gray body and MIR luminosities by fitting spectral energy distributions to each X-ray source and to each radio source in an ultradeep Karl G. Jansky Very Large Array (VLA) 1.4 GHz (11.5uJy at 5-sigma) image of the CDF-N. We confirm the FIR-radio and MIR-radio correlations to z=4 using the non-X-ray detected radio sources. Both correlations are also obeyed by the X-ray less luminous AGNs but not by the X-ray quasars. We interpret the low FIR luminosities relative to the MIR for the X-ray quasars as being due to a lack of star formation, while the MIR stays high due to the AGN contribution. We find that the FIR luminosity distributions are highly skewed and the means are dominated by a small number of high-luminosity galaxies. Thus, stacking or averaging analyses will overestimate the level of star formation taking place in the bulk of the X-ray sample. We conclude that most of the host galaxies of X-ray quasars are not strong star formers, perhaps because their star formation is suppressed by AGN feedback.

SPITZER/IRAC OBSERVATIONS OF THE VARIABILITY OF Sgr A* AND THE OBJECT G2 AT 4.5 μm

We present the first detection from the Spitzer Space Telescope of 4.5 micron variability from Sgr A*, the emitting source associated with the Milky Way's central black hole. The >23 hour continuous light curve was obtained with the IRAC instrument in 2013 December. The result characterizes the variability of Sgr A* prior to the closest approach of the G2 object, a putative infalling gas cloud that orbits close to Sgr A*. The high stellar density at the location of Sgr A* produces a background of ~250 mJy at 4.5 microns in each pixel with a large pixel-to-pixel gradient, but the light curve for the highly variable Sgr A* source was successfully measured by modeling and removing the variations due to pointing wobble. The observed flux densities range from the noise level of ~0.7 mJy rms in a 6.4-s measurement to ~10 mJy. Emission was seen above the noise level ~34% of the time. The light curve characteristics, including the flux density distribution and structure function, are consistent with those previously derived at shorter infrared wavelengths. We see no evidence in the light curve for activity attributable to the G2 interaction at the observing epoch, ~100 days before the expected G2 periapsis passage. The IRAC light curve is more than a factor of two longer than any previous infrared observation, improving constraints on the timescale of the break in the power spectral distribution of Sgr A* flux densities. The data favor the longer of the two previously published values for the timescale.

The nature of the low-frequency emission of M 51

Context. Low-frequency radio continuum observations (<300 MHz) can provide valuable information on the propagation of low-energy cosmic ray electrons (CRE). Nearby spiral galaxies have hardly been studied in this frequency range because of the technical challenges of low-frequency radio interferometry. This is now changing with the start of operations of LOFAR. Aims. We aim to study the propagation of low-energy CRE in the interarm regions and the extended disk of the nearly face-on spiral galaxy Messier 51. We also search for polarisation in M 51 and other extragalactic sources in the field. Methods. The grand-design spiral galaxy M 51 was observed with the LOFAR High Frequency Antennas (HBA) and imaged in total intensity and polarisation. This observation covered the frequencies between 115 MHz and 175 MHz with 244 subbands of 8 channels each, resulting in 1952 channels. This allowed us to use RM synthesis to search for polarisation. Results. We produced an image of total emission of M 51 at the mean frequency of 151 MHz with 20′′ resolution and 0.3 mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300 MHz so far. The integrated spectrum of total radio emission is described well by a power law, while flat spectral indices in the central region indicate thermal absorption. We observe that the disk extends out to 16 kpc and see a break in the radial profile near the optical radius of the disk. The radial scale lengths in the inner and outer disks are greater at 151 MHz, and the break is smoother at 151 MHz than those observed at 1.4 GHz. The arm-interarm contrast is lower at 151 MHz than at 1400 MHz, indicating propagation of CRE from spiral arms into interarm regions. The correlations between the images of radio emission at 151 MHz and 1400 MHz and the FIR emission at 70 μm reveal breaks on scales of 1.4 and 0.7 kpc, respectively. The total (equipartition) magnetic field strength decreases from about 28 μG in the central region to about 10 μG at 10 kpc radius. No significant polarisation was detected from M 51, owing to severe Faraday depolarisation. Six extragalactic sources are detected in polarisation in the M 51 field of 4.1◦ × 4.1◦ size. Two sources show complex structures in Faraday space. Conclusions. Our main results, the scale lengths of the inner and outer disks at 151 MHz and 1.4 GHz, arm-interarm contrast, and the break scales of the radio-FIR correlations, can be explained consistently by CRE diffusion, leading to a longer propagation length of CRE of lower energy. The distribution of CRE sources drops sharply at about 10 kpc radius, where the star formation rate also decreases sharply. We find evidence that thermal absorption is primarily caused by H ii regions. The non-detection of polarisation from M 51 at 151 MHz is consistent with the estimates of Faraday depolarisation. Future searches for polarised emission in this frequency range should concentrate on regions with low star formation rates.

NO EVIDENCE FOR EVOLUTION IN THE FAR-INFRARED–RADIO CORRELATION OUT TOz∼ 2 IN THE EXTENDED CHANDRA DEEP FIELD SOUTH

We investigate the 70 um Far-Infrared Radio Correlation (FRC) of star-forming galaxies in the Extended Chandra Deep Field South (ECDFS) out to z > 2. We use 70 um data from the Far-Infrared Deep Extragalactic Legacy Survey (FIDEL), which comprises the most sensitive (~0.8 mJy rms) and extensive far-infrared deep field observations using MIPS on the Spitzer Space Telescope, and 1.4 GHz radio data (~8 uJy/beam rms) from the VLA. In order to quantify the evolution of the FRC we use both survival analysis and stacking techniques which we find give similar results. We also calculate the FRC using total infrared luminosity and rest-frame radio luminosity, qTIR, and find that qTIR is constant (within 0.22) over the redshift range 0 - 2. We see no evidence for evolution in the FRC at 70 um which is surprising given the many factors that are expected to change this ratio at high redshifts.

AzTEC half square degree survey of the SHADES fields – I. Maps, catalogues and source counts

We present the first results from the largest deep extragalactic millimetre-wavelength survey undertaken to date. These results are derived from maps covering over 0.7 deg^2, made at 1.1mm, using the AzTEC continuum camera mounted on the James Clerk Maxwell Telescope. The maps were made in the two fields originally targeted at 0.85mm with SCUBA in the SHADES project, namely the Lockman Hole East (mapped to a depth of 0.9-1.3 mJy rms) and the Subaru XMM Deep Field (1.0-1.7 mJy rms). The wealth of existing and forthcoming deep multi-frequency data in these two fields will allow the bright mm source population revealed by these images to be explored in detail in subsequent papers. Here we present the maps themselves, a catalogue of 114 high-significance sub-millimetre galaxy detections, and a thorough statistical analysis leading to the most robust determination to date of the 1.1mm source number counts. Through careful comparison, we find that both the COSMOS and GOODS North fields, also imaged with AzTEC, contain an excess of mm sources over the new 1.1mm source-count baseline established here. In particular, our new AzTEC/SHADES results indicate that very luminous high-redshift dust enshrouded starbursts (S_{1.1} > 3 mJy) are 25-50% less common than would have been inferred from these smaller surveys, thus highlighting the potential roles of cosmic variance and clustering in such measurements. We compare number count predictions from recent models of the evolving mm/sub-mm source population to these SMG surveys, which provide important constraints for the ongoing refinement of semi-analytic and hydrodynamical models of galaxy formation, and find that all recent models over-predict the number of bright sub-millimetre galaxies found in this survey.

Large dust particles in disks around T Tauri stars

We present 7-mm continuum observations of 14 low-mass pre-main-sequence stars in the Taurus-Auriga star-forming region obtained with the Very Large Array with ~1.5 resolution and ~0.3 mJy rms sensitivity. For 10 objects, the circumstellar emission has been spatially resolved. The large outer disk radii derived suggest that the emission at this wavelength is mostly optically thin. The millimetre spectral energy distributions are characterised by spectral indices alpha = 2.3 to 3.2. After accounting for contribution from free-free emission and corrections for optical depth, we determine dust opacity indices beta in the range 0.5 to 1.6, which suggest that millimetre-sized dust aggregates are present in the circumstellar disks. Four of the sources with beta > 1 may be consistent with submicron-sized dust as found in the interstellar medium. Our findings indicate that dust grain growth to millimetre-sized particles is completed within less than 1 Myr for the majority of circumstellar disks.

Discovery of Water Maser Emission in Eight AGNs with 70 m Antennas of NASA’s Deep Space Network

We report the discovery of water maser emission in eight active galactic nuclei (AGNs) with the 70 m NASA Deep Space Network (DSN) antennas at Tidbinbilla, Australia, and Robledo, Spain. The positions of the newly discovered masers, measured with the VLA, are consistent with the optical positions of the host nuclei to within 1 σ (03 radio and 13 optical) and most likely mark the locations of the embedded central engines. The spectra of two sources, NGC 3393 and NGC 5495, display the characteristic spectral signature of emission from an edge-on accretion disk, with orbital velocities of ~600 and ~400 km s-1, respectively. In a survey with DSN facilities of 630 AGNs selected from the NASA Extragalactic Database, we have discovered a total of 15 water maser sources. The resulting incidence rate of maser emission among nearby (vsys < 7000 km s-1) Seyfert 1.8-2.0 and LINER systems is ~10% for a typical rms noise level of ~14 mJy over 1.3 km s-1 spectral channels. As a result of this work, the number of nearby AGNs (vsys < 7000 km s-1) observed with <20 mJy rms noise has increased from 130 to 449.

A Laboratory for Constraining Cosmic Evolution of the Fine‐Structure Constant: Conjugate 18 Centimeter OH Lines toward PKS 1413+135 atz= 0.24671

We report the detection of the satellite 18 cm OH lines at 1612 and 1720 MHz in the z = 0.24671 molecular absorption system toward the radio source PKS 1413+135. The two OH lines are conjugate; the 1612 MHz line is seen in absorption while the 1720 MHz line is seen in weak maser emission of equal but negative optical depth. We do not detect the main 18 cm OH lines at 1667 and 1665 MHz down to 1.1 mJy rms in 4.0 km s-1 channels. The detected and undetected 18 cm OH lines support a scenario of radiatively pumped stimulated absorption and emission with pumping dominated by the intraladder 119 μm line of OH, suggesting a column density N(OH) ≃ 1015-1016 cm-2. Combined with simultaneous H I 21 cm observations and published CO data, we apply the OH redshifts to measurements of cosmic evolution of the fine-structure constant (α ≡ e2/ℏc). We obtain highly significant (~25 σ) velocity offsets between the OH and H I lines and the OH and CO lines, but measurements of α-independent systematics demonstrate that the observed velocity differences are entirely attributable to physical velocity offsets between species rather than a change in α. The OH alone, for which conjugate line profiles guarantee that both lines originate in the same molecular gas, provides a weak constraint of Δα/α0 = (+0.51 ± 1.26) × 10-5 at z = 0.24671. Higher frequency OH line detections can provide a larger lever arm on Δα and can increase precision by an order of magnitude. The OH molecule can thus provide precise measurements of the cosmic evolution of α that include quantitative constraints on systematic errors. Application of this technique is limited only by the detectability of |τ| ~ 0.01 OH lines toward radio continuum sources and may be possible to z ~ 5.

Large grains in the disk of CQ Tau

We present 7 mm observations of the dusty disk surrounding the 10 Myr old pre-main-sequence star CQ Tauri obtained at the Very Large Array with 0.8 arcsec resolution and 0.1 mJy rms sensitivity. These observations resolve the 7 mm emission in approximately the north-south direction, confirming previous results obtained with lower resolution. We use a two-layer flared disk model to interpret the observed fluxes from 7 mm to 1.3 mm together with the resolved 7 mm structure. We find that the disk radius is constrained to the range 100 to 300 AU, depending on the steepness of the disk surface density distribution. The power law index of the dust opacity coefficient, β, is constrained to be 0.5 to 0.7. Since the models indicate that the disk is optically thin at millimeter wavelengths for radii greater than 8 AU, the contribution of an optically thick region to the emission is less than 10%. This implies that high optical depth or complex disk geometry cannot be the cause of the observed shallow millimeter spectral index. Instead, the new analysis supports the earlier suggestion that dust particles in the disk have grown to sizes as large as a few centimeters. The dust in the CQ Tauri system appears to be evolved much like that in the TW Hydra system, a well-studied pre-main-sequence star of similar age and lower mass. The survival of gas-rich disks with incomplete grain evolution at such old ages deserves further investigations.

Radio Continuum Emission at OH and H2O Maser Sites

A search for centimeter radio continuum emission associated with OH and H2O masers has been made toward 26 fields containing 45 separate maser sites using the Australia Telescope Compact Array (ATCA) at 8.2 and 9.2 GHz. The sensitivity achieved for fields not limited by dynamic range is ~0.15 mJy rms. This is the deepest radio search for embedded stellar sources at sites containing OH and H2O masers made to date. Only 17 compact continuum sources were detected within 2'' of the 45 maser sites searched. The absence of 3 cm continuum emission at the mJy level in 62% of the maser sites shows that most OH/H2O maser sites do not contain well-developed ultracompact H II regions (UCHIIs). The implications of this result on the nature of the source pumping the maser emission are discussed. While other possibilities cannot be ruled out, we argue that many maser sites without detected UCHIIs are most likely massive stars still in the pre-main-sequence contraction stage.

An HI Search for M81 Group Dwarf Galaxies

The M81 group has great advantages for dwarf galaxy studies: it has about three times the dwarf content of the Local Group but is at only about a quarter of the Virgo cluster distance. We searched for HI in 23 optically selected dwarf members and possible members of the M81 group with the Nançay decimetric radio telescope in the velocity range of −529 to 1826 km s−1 with considerably better sensitivity (2–4 mJy rms.) than previous surveys. Half the objects observed are irregular dwarfs, expected to be HI-rich and detectable at Nançay if at the distance of the M81 group (4 Mpc).

A 2.8 Millimeter Survey of the Hubble Deep Field

We present a continuum survey of the Hubble Deep Field at a wavelength of 2.8 mm made with the BIMA array. A mosaic of seven pointings was constructed to span the entire 47 × 47 area observed by the Hubble Space Telescope with 47 resolution and a nearly uniform 0.71 mJy rms sensitivity. No significant sources of emission were found (>5 σ). We use these observations to place limits on the presence of far-infrared emission from dusty star-forming galaxies at high redshifts. The most stringent limits apply for a critically closed universe (Ω -->0=1), dust temperatures typical of nearby starbursts (40 K), and long-wavelength dust emissivity power-law index β=1. For these parameters, the data rule out systems at z>5 with far-infrared luminosities in excess of 1.3×10 -->13 L☉. The lack of a large population of such luminous systems supports the view that galaxies are assembled in a hierarchical fashion instead of forming the bulk of their stars in single bursts of short duration. For an open universe and/or substantially higher dust temperatures, the limits are higher by an order of magnitude or more and allow for undetected massive starbursts.

A deep ROSAT survey - III. Deep radio observations of a selected field

We report on a deep (27 μJy rms) radio observation made at 1.472 GHz with the Australia Telescope Compact Array of the Durham/AAT QSO survey field QSF3, which has also previously been observed with ROSAT. In total, 132 sources with S 1.47 > 125 μJy have been identified in an area that also contains 58 faint X-ray identifications, S(0.5-2 keV) > 5 × 10 −15 erg s −1 cm −2 . At separations 125 μJy. Of these, a significant fraction (> 50 per cent) are likely to be QSOs

The Parkes-Tidbinbilla interferometer

view Abstract Citations (39) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Parkes-Tidbinbilla Interferometer Norris, R. P. ; Kesteven, M. J. ; Wellington, K. J. ; Batty, M. J. Abstract The Parkes-Tidbinbilla Interferometer is a radio-linked interferometer using the 64m telescope at Parkes together with one of the NASA antennas (34m or 64m) at Tidbinbilla. With a baseline of 275 km, it is currently the world's longest real-time interferometer, and is usable at frequencies of 1.7, 2.3 and 8.4 GHz to give angular resolutions of 0.13", 0.09", and 0.03", respectively, with a sensitivity of 1-3 mJy rms in 3-15 minutes at a bandwidth of up to 10 MHz. Here we describe its design and construction and give some examples of astronomy programs currently being pursued using it. Publication: The Astrophysical Journal Supplement Series Pub Date: May 1988 DOI: 10.1086/191267 Bibcode: 1988ApJS...67...85N Keywords: Astrometry; Pulsars; Radio Interferometers; Systems Engineering; Astronomical Observatories; Radio Telescopes; Stellar Motions; Astronomy; INSTRUMENTS; INTERFEROMETRY full text sources ADS |

The Parkes - Tidbinbilla Interferometer

The Parkes - Tidbinbilla Interferometer is a radio-linked interferometer using the 64-m telescope at Parkes together with one of the NASA antennas (34-m or 64-m) at Tidbinbilla. With a baseline of 275km, it is currently the world's longest real-time interferometer, and is usable at frequencies of 1.6, 2.3, and 8.4 GHz to give angular resolutions of 0.13, 0.09, and 0.03 arcsec respectively, with a sensitivity of 1–2 mJy rms in 5 minutes at a bandwidth of up to 10MHz.