Milliarcsecond Resolution Research Articles

The VLBA CANDELS GOODS-North Survey – I. survey design, processing, data products, and source counts

ABSTRACT The past decade has seen significant advances in wide-field cm-wave very long baseline interferometry (VLBI), which is timely given the wide-area, synoptic survey-driven strategy of major facilities across the electromagnetic spectrum. While wide-field VLBI poses significant post-processing challenges that can severely curtail its potential scientific yield, many developments in the km-scale connected-element interferometer sphere are directly applicable to addressing these. Here we present the design, processing, data products, and source counts from a deep (11 μJy beam−1), quasi-uniform sensitivity, contiguous wide-field (160 arcmin2) 1.6 GHz VLBI survey of the CANDELS GOODS-North field. This is one of the best-studied extragalactic fields at milli-arcsecond resolution and, therefore, is well-suited as a comparative study for our Tera-pixel VLBI image. The derived VLBI source counts show consistency with those measured in the COSMOS field, which broadly traces the AGN population detected in arcsecond-scale radio surveys. However, there is a distinctive flattening in the S1.4GHz ∼100–500 μJy flux density range, which suggests a transition in the population of compact faint radio sources, qualitatively consistent with the excess source counts at 15 GHz that is argued to be an unmodelled population of radio cores. This survey approach will assist in deriving robust VLBI source counts and broadening the discovery space for future wide-field VLBI surveys, including VLBI with the Square Kilometre Array, which will include new large field-of-view antennas on the African continent at ≳1000 km baselines. In addition, it may be useful in the design of both monitoring and/or rapidly triggered VLBI transient programmes.

A lensed radio jet at milli-arcsecond resolution – II. Constraints on fuzzy dark matter from an extended gravitational arc

ABSTRACT Using a single gravitational lens system observed at ≲ 5 mas resolution with very long baseline interferometry, we place a lower bound on the mass of the fuzzy dark matter (FDM) particle, ruling out mχ ≤ 4.4 × 10−21 eV with a 20:1 posterior odds ratio relative to a smooth lens model. We generalize our result to non-scalar and multiple-field models, such as vector FDM, with mχ,vec > 1.4 × 10−21 eV. Due to the extended source structure and high angular resolution of the observation, our analysis is directly sensitive to the presence of granule structures in the main dark matter halo of the lens, which is the most generic prediction of FDM theories. A model based on well-understood physics of ultra-light dark matter fields in a gravitational potential well makes our result robust to a wide range of assumed dark matter fractions and velocity dispersions in the lens galaxy. Our result is competitive with other lower bounds on mχ from past analyses, which rely on intermediate modelling of structure formation and/or baryonic effects. Higher resolution observations taken at 10–100 GHz could improve our constraints by up to two orders of magnitude in the future.

Lensed radio arcs at milli-arcsecond resolution: Methods, science results, and current status

AbstractStrong gravitational lensing by galaxies provides us with a powerful laboratory for testing dark matter models. Various particle models for dark matter give rise to different small-scale distributions of mass in the lens galaxy, which can be differentiated with sensitive observations. Th. The sensitivity of a gravitational lens observation to the presence (or absence) of low-mass dark structures in the lens galaxy is determined mainly by the angular resolution of the instrument and the spatial structure of the source. Here, I discuss results from the analysis of a global VLBI observation of a gravitationally lensed radio jet. With an angular resolution better than 5 milli-arcseconds and a highly extended, spatially resolved source, we are able to place competitive constraints on the particle mass in fuzzy dark matter models using this single observation. I also discuss preliminary results from our analysis of warm dark matter models using this lens system.

First light for GRAVITY Wide

GRAVITY+ is the upgrade for GRAVITY and the Very Large Telescope Interferometer (VLTI) with wide-separation fringe tracking, new adaptive optics, and laser guide stars on all four 8 m Unit Telescopes (UTs) to enable ever-fainter, all-sky, high-contrast, milliarcsecond interferometry. Here we present the design and first results of the first phase of GRAVITY+, known as GRAVITY Wide. GRAVITY Wide combines the dual-beam capabilities of the VLTI and the GRAVITY instrument to increase the maximum separation between the science target and the reference star from 2 arcseconds with the 8 m UTs up to several 10 arcseconds, limited only by the Earth’s turbulent atmosphere. This increases the sky-coverage of GRAVITY by two orders of magnitude, opening up milliarcsecond resolution observations of faint objects and, in particular, the extragalactic sky. The first observations in 2019–2022 include the first infrared interferometry of two redshift z ~ 2 quasars, interferometric imaging of the binary system HD 105913A, and repeat observations of multiple star systems in the Orion Trapezium Cluster. We find the coherence loss between the science object and fringe-tracking reference star well described by the turbulence of the Earth’s atmosphere. We confirm that the larger apertures of the UTs result in higher visibilities for a given separation due to the broader overlap of the projected pupils on the sky and provide predictions for visibility loss as a function of separation to be used for future planning.

A lensed radio jet at milliarcsecond resolution I: Bayesian comparison of parametric lens models

ABSTRACT We investigate the mass structure of a strong gravitational lens galaxy at z = 0.350, taking advantage of the milliarcsecond (mas) angular resolution of very long baseline interferometric (VLBI) observations. In the first analysis of its kind at this resolution, we jointly infer the lens model parameters and pixellated radio source surface brightness. We consider several lens models of increasing complexity, starting from an elliptical power-law density profile. We extend this model to include angular multipole structures, a separate stellar mass component, additional nearby field galaxies, and/or a generic external potential. We compare these models using their relative Bayesian log-evidence (Bayes factor). We find strong evidence for angular structure in the lens; our best model is comprised of a power-law profile plus multipole perturbations and external potential, with a Bayes factor of +14984 relative to the elliptical power-law model. It is noteworthy that the elliptical power-law mass distribution is a remarkably good fit on its own, with additional model complexity correcting the deflection angles only at the ∼5 mas level. We also consider the effects of added complexity in the lens model on time-delay cosmography and flux-ratio analyses. We find that an overly simplistic power-law ellipsoid lens model can bias the measurement of H0 by ∼3 per cent and mimic flux ratio anomalies of ∼8 per cent. Our results demonstrate the power of high-resolution VLBI observations to provide strong constraints on the inner density profiles of lens galaxies.

Wandering Black Hole Candidates in Dwarf Galaxies at VLBI Resolution

Thirteen dwarf galaxies have recently been found to host radio-selected accreting massive black hole (MBH) candidates, some of which are “wandering” in the outskirts of their hosts. We present 9 GHz Very Long Baseline Array (VLBA) observations of these sources at milliarcsecond resolution. Our observations have beam solid angles ∼104 times smaller than the previous Very Large Array (VLA) observations at 9 GHz, with comparable point-source sensitivities. We detect milliarcsecond-scale radio sources at the positions of the four VLA sources most distant from the photocenters of their associated dwarf galaxies. These sources have brightness temperatures of >106 K, consistent with active galactic nuclei (AGNs), but the significance of their preferential location at large distances (p-value = 0.0014) favors a background AGN interpretation. The VLBA nondetections toward the other nine galaxies indicate that the VLA sources are resolved out on scales of tens of milliarcseconds, requiring extended radio emission and lower brightness temperatures consistent with either star formation or radio lobes associated with AGN activity. We explore the star formation explanation by calculating the expected radio emission for these nine VLBA nondetections, finding that about five have VLA luminosities that are inconsistent with this scenario. Of the remaining four, two are associated with spectroscopically confirmed AGNs that are consistent with being located at their galaxy photocenters. There are therefore between five and seven wandering MBH candidates out of the 13 galaxies we observed, although we cannot rule out background AGNs for five of them with the data in hand.

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions

Context. Although the role of magnetic fields in launching molecular outflows in massive young stellar objects has been convincingly demonstrated by theoretical arguments, observationally, the alignment of the magnetic field lines with the molecular outflows is still under debate. Aims. We aim to complete the measurements of the direction of the magnetic fields at milliarcsecond resolution around a sample of massive star-forming regions to determine whether the magnetic field and outflows are aligned. Methods. In 2012, we started a large very long baseline interferometry campaign with the European VLBI Network to measure the magnetic field orientation and strength toward a sample of 31 massive star-forming regions (called the flux-limited sample) by analyzing the polarized emission of 6.7 GHz CH3OH masers. In the previous papers of the series, we have presented 80% of the sample. Here, we report the linearly and circularly polarized emission of 6.7 GHz CH3OH masers toward the last five massive star-forming regions of the flux-limited sample. The sources are G30.70-0.07, G30.76-0.05, G31.28+0.06, G32.03+0.06, and G69.52-0.97. Results. We detected a total of 209 CH3OH maser cloudlets, 15% of which show linearly polarized emission (0.07–16.7%), and 2% of which show circularly polarized emission (0.2–4.2%). As reported in previous papers, in the last five sources of the flux-limited sample, we also measured well-ordered linear polarization vectors. Zeeman splitting was measured toward G30.70-0.07, G32.03+0.06, and G69.52-0.97. Conclusions. The statistical analysis of the entire flux-limited sample shows that the observations are consistent with a bimodal distribution in the difference between the 3D magnetic field direction and the outflow axis, with half the magnetic field directions being perpendicular and the other half being parallel to the outflow. In addition, we determined that typical values of the linear and circular polarization fractions for 6.7 GHz CH3OH masers are Pl = 1.0–2.5% and PV = 0.5–0.75%, respectively. From the circularly polarized spectra of the CH3OH maser features, we found that a typical Zeeman splitting is in the range between 0.5 m s−1 and 2.0 m s−1. This would correspond to 9 mG < |B||| < 40 mG if F = 3 → 4 is the most favored of the eight hyperfine transitions that might contribute to the maser emission.

Characterizing the dust content of disk substructures in TW Hydrae

Context. A key piece of information to understand the origin and role of protoplanetary disk substructures is their dust content. In particular, disk substructures associated with gas pressure bumps can work as dust traps, accumulating grains and reaching the necessary conditions to trigger the streaming instability. Aims. In order to shed some light on the origin and role that disk substructures play in planet formation, we aim to characterize the dust content of substructures in the disk of TW Hya. Methods. We present Atacama Large Millimeter Array (ALMA) observations of TW Hya at 3.1 mm with ~50 milliarcsecond resolution. These new data were combined with archival high angular resolution ALMA observations at 0.87, 1.3, and 2.1 mm. We analyze these multiwavelength data to infer a disk radial profile of the dust surface density, maximum particle size, and slope of the particle size distribution. Results. Most previously known annular substructures in the disk of TW Hya are resolved at the four wavelengths. Inside the inner 3 au cavity, the 2.1 and 3.1 mm images show a compact source of free–free emission, likely associated with an ionized jet. Our multiwavelength analysis of the dust emission shows that the maximum particle size in the disk of TW Hya is >1 mm. The inner 20 au are completely optically thick at all four bands, which results in the data tracing different disk heights at different wavelengths. Coupled with the effects of dust settling, this prevents the derivation of accurate density and grain size estimates in these regions. At r > 20 au, we find evidence of the accumulation of large dust particles at the position of the bright rings, indicating that these are working as dust traps. The total dust mass in the disk is between 250 and 330 M⊕, which represents a gas-to-dust mass ratio between 50 and 70. Our mass measurement is a factor of 4.5–5.9 higher than the mass that one would estimate using the typical assumptions of large demographic surveys. Conclusions. Our results indicate that the ring substructures in TW Hya are ideal locations to trigger the streaming instability and form new generations of planetesimals.

The 60 pc Environment of FRB 20180916B

Abstract Fast radio burst FRB 20180916B in its host galaxy SDSS J015800.28+654253.0 at 149 Mpc is by far the closest-known FRB with a robust host galaxy association. The source also exhibits a 16.35 day period in its bursting. Here we present optical and infrared imaging as well as integral field spectroscopy observations of FRB 20180916B with the WFC3 camera on the Hubble Space Telescope and the MEGARA spectrograph on the 10.4 m Gran Telescopio Canarias. The 60–90 milliarcsecond (mas) resolution of the Hubble imaging, along with the previous 2.3 mas localization of FRB 20180916B, allows us to probe its environment with a 30–60 pc resolution. We constrain any point-like star formation or H ii region at the location of FRB 20180916B to have an Hα luminosity L Hα ≲ 1037 erg s−1, and we correspondingly constrain the local star formation rate to be ≲10−4 M⊙ yr−1. The constraint on Hα suggests that possible stellar companions to FRB 20180916B should be of a cooler, less massive spectral type than O6V. FRB 20180916B is 250 pc away (in projected distance) from the brightest pixel of the nearest young stellar clump, which is ∼380 pc in size (FWHM). With the typical projected velocities of pulsars, magnetars, or neutron stars in binaries (60–750 km s−1), FRB 20180916B would need 800 kyr to 7 Myr to traverse the observed distance from its presumed birth site. This timescale is inconsistent with the active ages of magnetars (≲10 kyr). Rather, the inferred age and observed separation are compatible with the ages of high-mass X-ray binaries and gamma-ray binaries, and their separations from the nearest OB associations.

Further milliarcsecond resolution results on cool giants and binary stars from lunar occultations at Devashtal

ABSTRACT We report on 26 lunar occultation events observed in the context of a programme started at Devasthal in 2017. In addition to the customary observations with the 1.3-m telescope, we report here also the first attempts performed with the TIRCAM2 camera at the 3.6-m Devasthal Optical Telescope in the near-infrared (near-IR). The results consist in the first-time angular diameters for two late-type giants, in a measurement of the well-known asymptotic giant branch pulsating variable SW Vir, and in the measurement of eight small separation binaries one of which is detected for the first time (HR 1860). We also measured the wider binaries SAO 94431 and 55 Tau (twice). The remaining sources were found to be unresolved with upper limits as small as 1 mas. We plan to develop further the high-speed capability of the TIRCAM2 instrument, so as to include in our programme also more near-IR, highly extincted sources.

γ-ray emission in radio galaxies under the VLBI scope

Aims. This is the second paper in our series studying the evolution of parsec-scale radio emission in radio galaxies in the Southern Hemisphere. Following our study of the radio and high-energy properties of γ-ray-emitting sources, here we investigate the kinematic and spectral properties of the parsec-scale jets of radio galaxies that have not yet been detected by the Fermi Large Area Telescope (Fermi-LAT) instrument on board NASA’s Fermi Gamma-ray Space Telescope. For many sources, these results represent the first milliarcsecond resolution information in the literature. These studies were conducted within the framework of the Tracking Active Nuclei with Austral Milliarcsecond Interferometry (TANAMI) monitoring program and in the context of high-energy γ-ray observations from Fermi-LAT. Methods. We took advantage of the regular 8.4 GHz and 22.3 GHz Very Long Baseline Interferometry (VLBI) observations provided by the TANAMI monitoring program, and explored the kinematic properties of six γ-ray-faint radio galaxies. We complemented this with ∼8.5 years of Fermi-LAT data, deriving updated upper limits on the γ-ray emission from this subsample of TANAMI radio galaxies. We included publicly available VLBI kinematics of γ-ray-quiet radio galaxies monitored by the MOJAVE program and performed a consistent Fermi-LAT analysis. We combined these results with those from our previous paper to construct the largest sample of radio galaxies with combined VLBI and γ-ray measurements to date. The connection between parsec-scale jet emission and high-energy properties in the misaligned jets of radio galaxies was explored. Results. For the first time, we report evidence of superluminal motion up to βapp = 3.6 in the jet of the γ-ray-faint radio galaxy PKS 2153−69. We find a clear trend of higher apparent speed as a function of distance from the jet core, which indicates that the jet is still being accelerated on scales of tens of parsecs, or ∼105 Rs, corresponding to the end of the collimation and acceleration zone in nearby radio galaxies. We find evidence of subluminal apparent motion in the jets of PKS 1258−321 and IC 4296, and no measurable apparent motion for PKS 1549−79, PKS 1733−565, and PKS 2027−308. For all these sources, TANAMI provides the first multi-epoch kinematic analysis on parsec scales. We then compare the VLBI properties of γ-ray-detected and undetected radio galaxies, and find that the two populations show a significantly different distribution of median core flux density, and, possibly, of median core brightness temperature. In terms of correlation between VLBI and γ-ray properties, we find a significant correlation between median core flux density and γ-ray flux, but no correlation with typical Doppler boosting indicators such as median core brightness temperature and core dominance. Conclusions. Our study suggests that high-energy emission from radio galaxies is related to parsec-scale radio emission from the inner jet, but is not driven by Doppler boosting effects, in contrast to the situation in their blazar counterparts. This implies that γ-ray loudness does not necessarily reflect a higher prevalence of boosting effects.

VLBI observations of the H2O gigamaser in TXS 2226-184

Context.Outside the Milky Way the most luminous H2O masers at 22 GHz, called megamasers because of their extreme luminosity with respect to the Galactic and extragalactic H2O masers associated with star formation, are mainly detected in active galactic nuclei. In the case of the H2O maser detected in the nuclear region of the galaxy TXS 2226-184, the term gigamaser was used for the first time. However, the origin of this very luminous H2O maser emission has never been investigated in detail.Aims.We study the nature of the 22 GHz H2O gigamaser in TXS 2226-184 by measuring its absolute position for the first time at milliarcsecond resolution, by comparing the morphology and characteristics of the maser emission on very long baseline interferometry (VLBI) scales after about 20 years, and by trying to detect its polarized emission.Methods.We observed the 22 GHz H2O maser emission toward TXS 2226-184 three times: the very first time with the NRAO Very Long Baseline Array (VLBA, epoch 2017.45) and the next two times with the European VLBI Network (EVN, epochs 2017.83 and 2018.44). The first two epochs (2017.45 and 2017.83) were observed in phase-reference mode, while the last epoch (2018.44) was observed in full-polarization mode, but not in phase-reference mode to increase the on-source integration time. We also retrieved and analyzed the VLBA archival data at 22 GHz of TXS 2226-184 observed in epoch 1998.40.Results.We detected six H2O maser features in epoch 2017.45 (VLBA), one in epoch 2017.83 (EVN), and two in epoch 2018.44 (EVN). All but one are red-shifted with respect to the systemic velocity of TXS 2226-184; we detected only one blue-shifted maser feature and it is the weakest one. All the H2O maser features but the blue-shifted one are composed of two components with very different linewidths. For the first time we were able to measure the absolute position of the H2O maser features with errors below 1 milliarcsec. No linear and circular polarization was detected.Conclusions.We were able to associate the H2O maser features in TXS 2226-184 with the most luminous radio continuum clump reported in the literature. The comparison between epochs 1998.40 and 2017.45 reveals a difference in the morphology and velocity of the maser features that can be justified accounting for maser variability.

A family portrait of disk inner rims around Herbig Ae/Be stars

Context. The innermost astronomical unit (au) in protoplanetary disks is a key region for stellar and planet formation, as exoplanet searches have shown a large occurrence of close-in planets that are located within the first au around their host star. Aims. We aim to reveal the morphology of the disk inner rim using near-infrared interferometric observations with milli-arcsecond resolution provided by near-infrared multitelescope interferometry. Methods. We provide model-independent reconstructed images of 15 objects selected from the Herbig AeBe survey carried out with PIONIER at the Very Large Telescope Interferometer, using the semi-parametric approach for image reconstruction of chromatic objects. We propose a set of methods to reconstruct and analyze the images in a consistent way. Results. We find that 40% of the systems (6/15) are centrosymmetric at the angular resolution of the observations. For the rest of the objects, we find evidence for asymmetric emission due to moderate-to-strong inclination of a disk-like structure for ~30% of the objects (5/15) and noncentrosymmetric morphology due to a nonaxisymmetric and possibly variable environment (4/15, ~27%). Among the systems with a disk-like structure, 20% (3/15) show a resolved dust-free cavity. Finally, we do not detect extended emission beyond the inner rim. Conclusions. The image reconstruction process is a powerful tool to reveal complex disk inner rim morphologies, which is complementary to the fit of geometrical models. At the angular resolution reached by near-infrared interferometric observations, most of the images are compatible with a centrally peaked emission (no cavity). For the most resolved targets, image reconstruction reveals morphologies that cannot be reproduced by generic parametric models (e.g., perturbed inner rims or complex brightness distributions). Moreover, the nonaxisymmetric disks show that the spatial resolution probed by optical interferometers makes the observations of the near-infrared emission (inside a few au) sensitive to temporal evolution with a time-scale down to a few weeks. The evidence of nonaxisymmetric emission that cannot be explained by simple inclination and radiative transfer effects requires alternative explanations, such as a warping of the inner disks. Interferometric observations can therefore be used to follow the evolution of the asymmetry of those disks at an au or sub-au scale.

Peering into the heart of darkness: Radio VLBI survey of the NEP deep field

AbstractActive Galactic Nuclei (AGN), accreting supermassive black holes at the centers of galaxies, are believed to produce powerful outflows – often observed as radio jets – which significantly influence the evolution of the surrounding galaxy and inter-galactic medium. However, how these jets – which are produced in the central parsecs of the AGN – impact gas on scales thousands to millions times larger is poorly understood. Doing so requires measuring the properties on all the relevant size scales. In this talk I will present initial results from the deepest-ever radio VLBI survey of an extragalactic field, whose milli-arcsecond angular resolution allows us to probe the central parsecs around these AGN. By comparing the radio properties of the detected radio jets with the multi-wavelength properties of their host galaxies, we are better to understand what galaxies generate powerful radio jets, and how do these outflows affect their host galaxies.

Extragalactic megahertz-peaked spectrum radio sources at milliarcsecond scales

Extragalactic peaked-spectrum radio sources are thought to be the progenitors of larger, radio-loud active galactic nuclei (AGN). Synchrotron self-absorption (SSA) has often been identified as the cause of their spectral peak. The identification of new megahertz-peaked spectrum sources from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey provides an opportunity to test how radio sources with spectral peaks below 1 GHz fit within this evolutionary picture. We observed six peaked-spectrum sources selected from the GLEAM survey, three that have spectral characteristics which violate SSA and three that have spectral peaks below 230 MHz, with the Very Long Baseline Array at 1.55 and 4.96 GHz. We present milliarcsecond resolution images of each source and constrain their morphology, linear size, luminosity, and magnetic field strength. Of the sources that are resolved by our study, the sources that violate SSA appear to be compact doubles, while the sources with peak frequencies below 230 MHz have core-jet features. We find that all of our sources are smaller than expected from SSA by factors of ≳20. We also find that component magnetic field strengths calculated from SSA are likely inaccurate, differing by factors of ≳5 from equipartition estimates. The calculated equipartition magnetic field strengths more closely resemble estimates from previously studied gigahertz-peaked spectrum sources. Exploring a model of the interaction between jets and the interstellar medium, we demonstrate that free-free absorption (FFA) can accurately describe the linear sizes and peak frequencies of our sources. Our findings support the theory that there is a fraction of peaked-spectrum sources whose spectral peaks are best modeled by FFA, implying our understanding of the early stages of radio AGN is incomplete.

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions

Context. Magnetohydrodynamical simulations show that the magnetic field can drive molecular outflows during the formation of massive protostars. The best probe to observationally measure both the morphology and the strength of this magnetic field at scales of 10–100 au is maser polarization. Aims. We measure the direction of magnetic fields at milliarcsecond resolution around a sample of massive star-forming regions to determine whether there is a relation between the orientation of the magnetic field and of the outflows. In addition, by estimating the magnetic field strength via the Zeeman splitting measurements, the role of magnetic field in the dynamics of the massive star-forming region is investigated. Methods. We selected a flux-limited sample of 31 massive star-forming regions to perform a statistical analysis of the magnetic field properties with respect to the molecular outflows characteristics. We report the linearly and circularly polarized emission of 6.7 GHz CH3OH masers towards seven massive star-forming regions of the total sample with the European VLBI Network. The sources are: G23.44−0.18, G25.83−0.18, G25.71−0.04, G28.31−0.39, G28.83−0.25, G29.96−0.02, and G43.80−0.13. Results. We identified a total of 219 CH3OH maser features, 47 and 2 of which showed linearly and circularly polarized emission, respectively. We measured well-ordered linear polarization vectors around all the massive young stellar objects and Zeeman splitting towards G25.71−0.04 and G28.83−0.25. Thanks to recent theoretical results, we were able to provide lower limits to the magnetic field strength from our Zeeman splitting measurements. Conclusions. We further confirm (based on ∼80% of the total flux-limited sample) that the magnetic field on scales of 10–100 au is preferentially oriented along the outflow axes. The estimated magnetic field strength of |B||| > 61 mG and >21 mG towards G25.71−0.04 and G28.83−0.25, respectively, indicates that it dominates the dynamics of the gas in both regions.

Nowhere to Hide: Radio-faint AGN in GOODS-N field

Context. The occurrence of active galactic nuclei (AGN) is critical to our understanding of galaxy evolution and formation. Radio observations provide a crucial, dust-independent tool to study the role of AGN. However, conventional radio surveys of deep fields ordinarily have arc-second scale resolutions often insufficient to reliably separate radio emission in distant galaxies originating from star-formation and AGN-related activity. Very long baseline interferometry (VLBI) can offer a solution by identifying only the most compact radio emitting regions in galaxies at cosmological distances where the high brightness temperatures (in excess of 105 K) can only be reliably attributed to AGN activity. Aims. We present the first in a series of papers exploring the faint compact radio population using a new wide-field VLBI survey of the GOODS-N field. This will expand upon previous surveys, permitting the characterisation of the faint, compact radio source population in the GOODS-N field. The unparalleled sensitivity of the European VLBI Network (EVN) will probe a luminosity range rarely seen in deep wide-field VLBI observations, thus providing insights into the role of AGN to radio luminosities of the order 1022 WHz−1 across cosmic time. Methods. The newest VLBI techniques are used to completely cover an entire 7′̣5 radius area to milliarcsecond resolutions, while bright radio sources (S > 0.1 mJy) are targeted up to 25′ from the pointing centre. Multi-source self-calibration, and a primary beam model for the EVN array are used to correct for residual phase errors and primary beam attenuation respectively. Results. This paper presents the largest catalogue of VLBI detected sources in GOODS-N comprising of 31 compact radio sources across a redshift range of 0.11–3.44, almost three times more than previous VLBI surveys in this field. We provide a machine-readable catalogue and introduce the radio properties of the detected sources using complementary data from the e-MERLIN Galaxy Evolution survey (eMERGE).

Globular clusters vs dark matter haloes in strong lensing observations

Small distortions in the images of Einstein rings or giant arcs offer the exciting prospect of detecting dark matter haloes or subhaloes of mass below $10^9$M$_{\odot}$, most of which are too small to have made a visible galaxy. A very large number of such haloes are predicted to exist in the cold dark matter model of cosmogony; in contrast other models, such as warm dark matter, predict no haloes below a mass of this order which depends on the properties of the warm dark matter particle. Attempting to detect these small perturbers could therefore discriminate between different kinds of dark matter particles, and even rule out the cold dark matter model altogether. Globular clusters in the lens galaxy also induce distortions in the image which could, in principle, contaminate the test. Here, we investigate the population of globular clusters in six early type galaxies in the Virgo cluster. We find that the number density of globular clusters of mass $\sim10^6$M$_{\odot}$ is comparable to that of the dark matter perturbers (including subhaloes in the lens and haloes along the line-of-sight). We show that the very different degrees of mass concentration in globular clusters and dark matter haloes result in different lensing distortions. These are detectable with milli-arcsecond resolution imaging which can distinguish between globular cluster and dark matter halo signals.

VLBA+GBT observations of the COSMOS field and radio source counts at 1.4 GHz

We present very long baseline interferometry (VLBI) observations of 179 radio sources in the COSMOS field with extremely high sensitivity using the Green Bank Telescope (GBT) together with the Very Long Baseline Array (VLBA) (VLBA+GBT) at 1.4 GHz, to explore the faint radio population in the flux density regime of tens of μJy. Here, the identification of active galactic nuclei (AGN) is based on the VLBI detection of the source, meaning that it is independent of X-ray or infrared properties. The milli-arcsecond resolution provided by the VLBI technique implies that the detected sources must be compact and have large brightness temperatures, and therefore they are most likely AGN (when the host galaxy is located at z ≥ 0.1). On the other hand, this technique only allows us to positively identify when a radio-active AGN is present, in other words, we cannot affirm that there is no AGN when the source is not detected. For this reason, the number of identified AGN using VLBI should be always treated as a lower limit. We present a catalogue containing the 35 radio sources detected with the VLBA+GBT, ten of which were not previously detected using only the VLBA. We have constructed the radio source counts at 1.4 GHz using the samples of the VLBA and VLBA+GBT detected sources of the COSMOS field to determine a lower limit for the AGN contribution to the faint radio source population. We found an AGN contribution of >40−75% at flux density levels between 150 μJy and 1 mJy. This flux density range is characterised by the upturn of the Euclidean-normalised radio source counts, which implies a contribution of a new population. This result supports the idea that the sub-mJy radio population is composed of a significant fraction of radio-emitting AGN, rather than solely by star-forming galaxies, in agreement with previous studies.

Long-term millimeter VLBI monitoring of M 87 with KVN at milliarcsecond resolution: nuclear spectrum

We study the centimeter- to millimeter-wavelength synchrotron spectrum of the core of the radio galaxy M 87 at ≲0.8 mas ~ 110Rs spatial scales using four years of fully simultaneous, multi-frequency VLBI data obtained by the Korean VLBI Network (KVN). We find a core spectral index α of ≳−0.37 (S ∝ ν+α) between 22 and 129 GHz. By combining resolution-matched flux measurements from the Very Long Baseline Array (VLBA) at 15 GHz and taking the Event Horizon Telescope (EHT) 230 GHz core flux measurements in epochs 2009 and 2012 as lower limits, we find evidence of a nearly flat core spectrum across 15 and 129 GHz, which could naturally connect the 230 GHz VLBI core flux. The extremely flat spectrum is a strong indication that the jet base does not consist of a simple homogeneous plasma, but of inhomogeneous multi-energy components, with at least one component with the turn-over frequency ≳ 100 GHz. The spectral shape can be qualitatively explained if both the strongly (compact, optically thick at >100 GHz) and the relatively weakly magnetized (more extended, optically thin at <100 GHz) plasma components are colocated in the footprint of the relativistic jet.