Zooming into the parsec-scale cores of the e-MERLIN legacy sample LeMMINGs with EVN and VLBA
April 2025
Using the EVN the parsec-scale cores of some nearby galaxies have been revealed for the first time
A new synoptic survey mode for EVN+e-MERLIN observations will revolutionise statistical studies of the VLBI sky
The centers of many nearby galaxies are powered by a weakly accreting super-massive black hole (SMBH), known as a "low-luminosity active galactic nucleus" (LLAGN). Unlike their more luminous quasar cousins, LLAGN are not merely low luminosity analogues. Instead, the accretion flow may turn into an advection dominated accretion flow, leading to a different fueling of the central SMBH and thus different observational characteristics. Previously, optical emission line ratios have been used to separate sources where photoionsation in the nucleus is too large to be from star formation alone, and these sources are known as Seyferts. An intermediate case are the low-ionisation nuclear emission line regions (LINERs) which may be powered by a LLAGN and/or star formation. Finally, the HII galaxies show no clear photoionisation consistent with an accreting SMBH, however this doesn't preclude a very weakly accretion SMBH. X-ray emission can also be used to find an accreting SMBH but nuclear absorption can make it difficult to detect their presence. Perhaps the 'gold standard' to detect an LLAGN is a high brightness temperature (TB > 107K) compact radio core with, occasionally, resolved radio jets. To achieve this goal, e-MERLIN and EVN/VLBA are required to search for the jets and the high brightness temperature cores in the nuclear regions of nearby galaxies, helped by the gaia positions of the optical core.
To study the accretion process as a whole, a large statistically complete sample is required. The Legacy e-MERLIN Multi-band Imaging of Nearby Galaxies survey (LeMMINGs) provides such an opportunity: a statistically-complete sample of nearby galaxies with high quality optical spectra and multi-wavelength data. Using the preliminary 5 GHz LeMMINGs results, Xiaopeng Cheng and collaborators searched the VLBA and EVN archives for observations of these sources. Out of the sources not observed by the VLBA or EVN, 36 had flux densities of >1mJy amongst the sources with likely AGN activity, and >3mJy for the HII galaxies where an AGN is less likely. Of the 36 sources, 23 three were detected, with the majority coming from the sources with known LLAGN activity like Seyferts and LINERs, but 3/8 HII galaxies were also detected. This included NGC 2146 which showed a compact radio core with a high brightness temperature and two components either side, the characteristic morphology of a LLAGN.
The new sample has nearly doubled the number of VLBI-detected sources of the LeMMINGs sample. The source compactness was investigated and showed that most sources bar NGC 2655 were core-dominated. Using the combined archival and new datasets, various multi-wavelength correlations were shown to follow those found in the LeMMINGs survey, namely a strong correlation between the radio luminosity and black hole mass and also the fundamental plane of black hole activity. But, only a weak correlation was found between the [O III] line luminosity and the radio emission, potentially due to the different scales probed in the two wavelengths. Finally, the ratio of the radio to X-ray luminosities showed that the Seyfert galaxies were similar in origin to radio quiet quasars, whereas the radio emission in the other LLAGN classes were more consistent with being dominated by a jet.
Using the EVN the parsec-scale cores of some nearby galaxies have been revealed for the first time
SWEEPS (Synoptic Wide-field EVN–e-MERLIN commensal Public Survey) is a proposed commensal survey mode for the EVN+e-MERLIN, where single-target PI-led observations are re-correlated at the position of all known radio sources within 12 arcmin. Initially, the phase centres are selected using the LOFAR Two Metre Sky Survey (LoTSS; Shimwell et al. 2022), in the future however, additional phase centres will be provided by wide-field images using the short baselines of e-MERLIN. Using the integrated EVN+e-MERLIN observations, these wide-field images can be generated on-the-fly during post processing.
This project leverages recent technical advances, such as multiple phase centre correlation and multi-source self-calibration, to bring VLBI into the realm of large statistical studies with ~8000 sources observed per year (Herbé-George et al. in prep.). These will be essential in advancing the key topics studied with VLBI such as, the role of AGN feedback (e.g. Zhuang et al. 2021, King & Pounds 2015), detecting binary black holes to study the hierarchical galaxy formation models (e.g. Rodriguez et al. 2006) and testing different dark matter models with gravitational lenses (e.g. Spingola et al. 2018; Powell et al. 2023)
Here, Célestin Herbé-George and collaborators demonstrator a proof-of-concept of this methodology by detecting a 5.6 mJy core-jet object at 1.7 GHz shown in Figure 1. This object, which was present in the raw visibilities of the observation, would have otherwise been lost from the parent data set and is the first object to be recovered as part of the SWEEPS pilot programme. The other objects present within the primary beams of the EVN+e-MERLIN telescopes are shown in figure 2, and illustrate the potential for increasing sample sizes of VLBI-detected radio sources with commensal observing modes in the near future.
High-resolution e-MERLIN observations of the continuum emission in G9.62+0.20E suggest optically-thick emission at lower frequencies
Variable Methanol masers at 6.7 GHz and OH masers at 1.6 GHz have been observed in the hypercompact HII region G9.62+0.20E with the methanol having a period of 244 days. A number of models have been put forward to explain the cause of the variability. It is unclear which model best describes these variations, however, the pulsating star, colliding wind and accretion disc models all remain plausible explanations. In these models the variability is driven by, rapid mass accretion onto the protostar leading to a bloated pulsating star (pulsating star model), variations in seed photons from a background HII region (colliding wind model) or from an eccentric binary system causing changes in accretion rate resulting in a flare (accretion disc model). The colliding wind binary model in G9.62+0.20E requires the existence of an HII region, making the study of its radio continuum emissions particularly important. Previous astrometric observations have determined the positions of masers relative to the 43 GHz UCHII region in G9.62+0.20E. However, the continuum data at 43 GHz does not accurately represent the distribution of seed photons in the continuum at 1.6 GHz and 6.7 GHz. Hence radio continuum observations at 1.6 GHz and 6.7 GHz are needed to understand the seed photon population responsible for this model. e-MERLIN observations of the HII region G9.62+0.20E were obtained by Benedicta Woode and colleagues with the goal of exploring this seed photon population in detail.
The high resolution observations at the requisite frequencies enabled the detection of the continuum. At C-band a flux density of 2.25±0.20 mJy and an angular size of 127±22 mas were measured. At L-band, an upper limit of 0.162 mJy on the integrated flux density was calculated at the continuum’s position, but the lack of detection at 1.6 GHz was unexpected as the spectral index from archival higher-frequency observations suggest a source of order 0.6 mJy.
The e-MERLIN data provide the lowest frequency observations of this source to date, and enabled an extended SED constraining the density distribution. The non-detection at 1.6 GHz resulted in a turnover of the radio spectral index at lower frequencies. Using an inverse square density distribution of ionised gas model, the data fit the model much better than a simple power law and suggest that the low frequency turnover is caused by emission becoming progressively more optically thick below 5 GHz. This information is crucial for understanding what may be causing the periodic variability in the maser emission pumping in both the methanol masers, and the lower frequency OH masers at 1.6 GHz.
Join a plethora of other SKA Pathfinder scientists including e-MERLIN at NAM 2025 in Durham.
We are happy to announce a parallel session entitled “Radio Astronomy in the build up to the SKAO” will be held at NAM 2025 hosted at Durham University 7-11 July. The session has 2 blocks and we are accepting abstracts with the deadline of 30th March.
The field of radio astronomy will soon be revolutionised by the arrival of the Square Kilometre Array telescopes. Now that the first fringes have been recorded with SKA-Low, and the first SKA-Mid dish is in place, it is time to look to the future of radio astronomy and the scientific opportunities coming up, and to reflect on the state-of-the-art science with precursor/pathfinder telescopes.
The goal of this session is to showcase the breadth of work conducted with SKA precursor and pathfinder instruments, such as e-MERLIN, MeerKAT and LOFAR across a diverse range of science goals, and to provide an opportunity for discussion and connection over plans for future science with the SKA Observatory (SKAO). We also will likely include discussion from invited speakers to update on the SKAO and the Science Regional Centres.
The session will be organised and led by members of the SKAO UK Early Career Researcher (ECR) committee, and will build on the success of the equivalent session at NAM2024.
High-resolution e-MERLIN observations revealed three compact AGN components contaminating the candidate star’s mid-infrared light.
The Dyson Sphere is an intriguing concept in the search for extraterrestrial intelligence (SETI). As proposed by Dyson (1960), advanced extraterrestrial civilisations may construct megastructures to capture starlight for energy. The resulting waste heat would cause the host stars to exhibit unusual infrared excess. By using Gaia, 2MASS, and ALLWISE measurements, Project Hephaistos (Suazo et al., 2024) identified seven Dyson Sphere candidate stars from 5 million sources. However, follow-up studies by Tongtian Ren, Michael Garrett and Andrew Siemion have revealed that three of these candidates exhibit radio emission detected in multiple archival radio surveys, suggesting that these sources may be contaminated by hot, dust-obscured galaxies (hot DOGs).
To verify this hypothesis, Ren, Garrett and Siemion conducted e-MERLIN and EVN radio observations of J2335-0004, the brightest radio source associated with candidate G of the Suazo et al. study. High-resolution e-MERLIN imaging resolved this source into three compact components, showing characteristics typical of a radio-loud AGN. The EVN (e-VLBI) observations found that the central component has a brightness temperature exceeding 10^8 K, reinforcing the AGN hypothesis. No radio emission was detected at the position of the star.
Additional analysis of archival WISE and VLA-SE data reveals that the WISE W3 and W4 emissions—previously attributed to a Dyson Sphere—are actually dominated by this AGN. Furthermore, the W1 and W2 dropout suggests that this AGN should be a hot DOG galaxy, as they initially hypothesised. These findings underscore the importance of high-resolution radio imaging in identifying the true nature of anomalous astrophysical data.
Radio-quiet Quasars show evidence for winds at high Eddington ratios.
The centres of galaxies can have an actively accreting super-massive black hole, known as an active galactic nucleus (AGN). In the most powerful AGN, this accretion process can give rise to radio jets which can extend from kilo-parsec to mega-parsec scales. In the weakest AGN, the radio emission can be from low-power jets, or from a mixture of other processes like winds, an accretion disk corona or star formation. Disentangling these different processes can be tricky without multi-frequency sensitivity high-resolution radio observations.
In this work by Sina Chen and collaborators, they explore the association of the parsec scale radio emission with a wind from the broad line region (BLR) surrounding the AGN. Their sample of 19 radio quiet Palomar-Green quasars includes archival data using the VLBA and new EVN+e-MERLIN data providing multi-frequency (1.5 and 5 GHz) observations of six new sources. They then compared the radio emission with the C IV emission line from the BLR to find indications of a wind in these systems and correlations with their Eddington ratios.
Of the six new datasets included in this work, three have radio emission indicative of a wind, two show likely coronal emission and one further source is likely a jet. In the overall sample of 19 sources, nearly three quarters show signs of either both radio and BLR winds or neither. Most of these wind sources are consistent with high Eddington ratios, while nearly all the non-wind sources have low Eddington ratios, indicating that the AGN winds are likely driven by radiation pressure.
EVN+e-MERLIN observations investigate the cold gas outflows in the very young radio source, 4C 31.04
The role of radio jets emitted by supermassive black holes (SMBHs) in the centres of galaxies in halting the growth of their host galaxies is not yet well understood. Models of galaxy evolution consider these jets to have an impact only at the circumgalactic level, where they prevent the accretion of gas onto the host galaxy, essentially cutting off the fuel for star formation. However, in recent years, it has been found that radio jets also directly impact the gas within their host galaxies. As the jets are launched and expand through the interstellar medium (ISM), they interact with the gas clouds and drive them out; in the process, they also make the ISM highly turbulent. To assess this impact in detail, we need to ‘zoom in’ to these sites of jet-ISM interaction and study the properties of the gas. Fortunately, it is possible to conduct such very high spatial resolution studies by observing the 21 cm transition of neutral atomic hydrogen (HI) using the Very Long Baseline Interferometry (VLBI) networks.
In this work, Suma Murthy and colleagues observed a very young (<~ 5000 years old) radio source, 4C 31.04, that is expanding into a gas-rich environment in the very centre of its host galaxy. Using the Global VLBI array, including the e-MERLIN stations, they mapped the distribution and kinematics of cold gas at a spatial resolution of 2 pc and found that the cold gas envelops the expanding radio jets. As a result of the interaction the gas is also highly turbulent. They also found that the southern radio jet is driving an outflow of cold gas, just 35 pc in projection, from the SMBH. The e- MERLIN baselines helped to eliminate the possibility that the outflowing gas was at a much larger distance from the centre of the galaxy. This study reinforces the growing evidence that radio jets have a role in shaping the evolution of their host galaxies from within, starting from a very early stage in their evolution. It also raises a puzzle about how cold gas can survive in such extreme environments.
The nature and origin of radio emission in most radio-quiet quasars remain ambiguous based on all-sky, low-resolution surveys.
For galaxy evolution models and simulations to fully explain the properties of massive galaxies, Active Galactic Nuclei (AGN) are required to inject energy into their environments. This produces a ‘feedback’ process, which re-distributes baryons and regulates galaxy growth. However, the role of powerful ‘radiatively dominated’ AGN (i.e. quasars with LAGN ≳1045 erg s−1) remains controversial from both a theoretical and observational perspective. There is an ongoing debate around the most important mechanism of energy injection from these sources (i.e., jets, accretion-disk winds or direct radiation pressure on the host galaxy gas) and to what level they can have an appreciable impact on galaxy evolution. Observations show that understanding the origin of radio emission in typical quasar host galaxies is a key piece of the AGN feedback puzzle.
In this work, Ann Njeri and collaborators present the Quasar Feedback Survey by the eMERLIN. The 42 low redshift z < 0.2 type 1 and type 2 quasars of moderate radio luminosities L1.4 GHz > 1023.45 W Hz−1, selected from a parent sample of spectroscopically identified AGN in the SDSS, were observed with eMERLIN at C-band. These targets are predominantly ‘radio quiet’ quasars. The e-MERLIN data enabled measurement of radio structures on ∼ 10s-100s pc scales, which is an order of magnitude smaller than the structures measured in the existing VLA images of the same targets.
The targets showed a wide range of morphology on sub-kiloparsec scales, with compact cores, knots, extended collimated structures and more diffuse structures. Based on visual inspection only, 15/42 (35%) targets were classified as compact sources as they showed no extended radio features, 21/42 (50%) targets were classified as jet-like since they showed either one-sided jets or well collimated multiple radio components. Although there was a large variety across the sample, the team found that a significant fraction of the total radio luminosity in this sample was typically not located in the compact, sub-kiloparsec structures traced by e-MERLIN. Indeed, most targets have moderate brightness temperatures, with only 13–21 targets showing clear evidence for AGN-like values (i.e., TB 104.8 K). Previous VLA maps of the same targets showed that this resolved-away emission can be sometimes located in diffuse, loosely collimated lobes, likely associated with AGN jets or outflows.
By combining diagnostics of brightness temperature and morphology, the team identified radio AGN activity in 32/42 (76%) of the sample. This was an increase on ∼57 per cent, based only on the VLA maps, thanks to the increased constraints on morphology and brightness temperatures at the higher resolution of e-MERLIN. This work highlights the importance of using a wide range of radio images (tracing sub-kiloparsec scales through to 10s of kiloparsec scales), to obtain a full characterisation of the radio morphologies in a representative sample of quasars. Through these data, the team finds a high prevalence of radio emission associated with an AGN (∼90%) for a sample which is traditionally characterised as ‘radio quiet’. This sample shares many radio properties with other AGN populations, including Seyferts AGN, FR0s, and GPS/CSS sources; highlighting limited evidence for these being distinct populations.
The e-MERLIN Time Allocation Group met on 3rd December 2024 to consider proposals for Cycle-19. Proposals were received for L-Band (1.3-1.7 GHz), C-Band (4.5-7.5GHz), and K-Band (19-25 GHz). The amount of time approved is set by existing programme observation requirements, scheduling and engineering constraints, and by EVN commitments within the Cycle-19 period.
The e-MERLIN Cycle 19 call for proposals list of accepted proposals is below (in no particular order). The next call for proposals is anticipated in Spring 2025.
e-MERLIN observations of OH masers confirm outflowing gas in Zw049.057
Some of the most luminous galaxies in our (local) universe are covered by thick layers of gas and dust, rendering them only visible at radio and millimeter wavelengths. Recent observations are uncovering that behind the curtain of dust lie dynamical systems, where gas flows inward to fuel the supermassive black hole, while powerful outflows expel gas out of the galaxy.
Observations using the e-MERLIN telescope have now revealed a powerful tracer of these processes. In the dynamical galaxy nucleus of the luminous infrared galaxy Zw049.057, hydroxyl (OH) molecule emissions are amplified through a laser-like process, forming OH megamasers. While these OH megamasers are well known to emanate from luminous galaxies, high-resolution e-MERLIN observations were able to give the first interferometric confirmation that the OH megamaser is coming from outflowing gas.
In combination with ALMA data, Lankhaar et al. (2024) were able to make a reconstruction of the complex gas flows within Zw049.057. They discovered that the OH megamaser traces a slow, wide-angle outflow that is enveloping a faster, collimated outflow detected through HCN emission. These two distinct outflows illustrate the dynamic processes in the galaxy’s nucleus, with the slower outflow likely falling back into the galaxy, while the faster outflow may escape into the intergalactic medium. The combination of inflows and outflows in Zw049.057 provides key insights into the evolution and powering of such luminous, dust-enshrouded galaxies.
e-MERLIN joins a new EU-funded project dedicated to multi-messenger astronomy
On the 16th and 17th of September 2024 the kick-off meeting for the Astrophysics Centre for Multimessenger studies in Europe (ACME) was held in Paris. This EU-funded project is coordinated by Centre national de la recherche scientifique (CNRS) and aims to realise an ambitious coordinated European-wide optimization of the accessibility and cohesion between multiple leading astroparticle and astronomy research infrastructures. The e-MERLIN/VLBI National facility which the University of Manchester operates on behalf of STFC, and expertise from the UK’s SKA Regional Centre will be key part of this project, offering access to instruments, data and expertise, focused on the new science of multi-messenger astrophysics.
With 40 world-class collaborating institutions, from 15 countries, including the Jodrell Bank Centre for Astrophysics, ACME brings together the astroparticle and astronomy communities in a joint effort to forge a basis for strengthened long-term collaboration between these research infrastructures irrespective of location and level-up access opportunities across Europe and beyond.
The ACME project coordinator Prof. Antoine Kouchner (CNRS/Université Paris Cité), and co-coordinator Paolo D’Avanzo (INAF), represent each community to ensure balance and drive cross-domain collaboration.
“ACME is an incredibly exciting opportunity. This project will bring together a wide range of world-class researchers and astronomical research infrastructure spanning astroparticle and gravitational wave facilities along the entire electromagnetic spectrum, with a common focus to advance multi-messenger astrophysics,” says Prof Rob Beswick who co-leads ACME’s transnational access programme and the University of Manchester’s involvement.
ACME will also develop six new multi-messenger Centres of Excellence across Europe. These will be new hubs of expertise supporting all researchers in all aspects of direct and multimessenger science programmes (from proposals to data analysis and science interpretation). Dr David Williams-Baldwin, who leads JBCA’s involvement in these new Centres of Excellence says “The ACME project will bring many infrastructures and groups together across Europe in a unique collaboration to provide the astronomy and astro particle communities unprecedented access to data, workflows and expertise. ACME will revolutionise how researchers in multi-messenger fields work and collaborate in the future.”
The e-MERLIN Time Allocation Group met on 5th July 2024 to consider proposals for Cycle-18. Proposals were received for L-Band (1.3-1.7 GHz), C-Band (4.5-7.5GHz), and K-Band (19-25 GHz). The amount of time approved is set by existing programme observation requirements, scheduling and engineering constraints, and by EVN commitments within the Cycle-18 period.
The e-MERLIN Cycle 18 call for proposals list of accepted proposals is below (in no particular order). The next call for proposals is anticipated in Autumn 2024.
20 years of monitoring using MERLIN, e-MERLIN, KVN, VERA and EVN have enabled a reconstruction of the complex structures surrounding the high mass star.
The processes that form high-mass stars can be probed by high-resolution studies of Methanol masers, which are bright tracers of the 3D structure of the disk-like accretion flow. Methanol masers are excited closer to high-mass protostars and can be used to infer the location of the star and study the circumstellar kinematics. However, the distribution of observed Methanol masers is often complex when observed with VLBI which may indicate the aforementioned rotating disc-like structures or in some cases expanding and/or infalling motions. In addition some Methanol masers can be excited by outflows, so ascertaining the direct cause of the maser is often difficult and may point to multiple processes.
In this work by Momotaro Nakamura, Kazuhito Motogi and colleagues, archival MERLIN and new e-MERLIN data from 2019 were combined with VERA and JVN observations of the methanol (at 6.7 GHz) and water (at 22 GHz) masers in a high-mass star forming region named G59.783+0.065 to study the distribution of masering components. Two decades worth of data enabled the researchers to see how the maser clusters changed over a prolonged period. For example, archival MERLIN data (Darwish et al. 2020) only saw the western cluster of water masers, but the first, second and third epochs of the new data confirmed a ‘pair-like’ distribution with an eastern cluster, which disappeared in the fourth epoch along with the water masers. However, the distribution of methanol masers remained stable throughout all epochs from 2004 to 2019, despite some individual spots only showing up for short periods. This suggests that the environment for pumping the methanol masers has remained consistent over a long time period, whereas the water masers were only found in the western cluster in pairs in specific epochs, suggesting episodic mass accretion leading to an outflow/shocks producing the water masers.
In context, the variety of structures and clusters of the eastern and western methanol and water masers provides evidence for: 1) a bipolar outflow, traced mostly by the water masers; 2) the methanol masers in the eastern structure show some inward motions which may be explained by them being produced at the edge of the outflow and the infalling envelope; 3) the western methanol masers are perpendicular to the outflow which may indicate that they are in a rotating disc-like structure. These multi-epoch, multi-frequency observations studying the different types of maser have produced the schematic model in the figure above, which can only be performed with high-resolution high-sensitivity VLBI arrays.
Join a plethora of other SKA Pathfinder scientists including e-MERLIN at NAM 2024 in Hull.
We are happy to announce a parallel session entitled “Era of SKAO Pathfinders – Pushing the Boundaries of the Radio Sky” will be held at NAM 2024 hosted in Hull. The session has 2, 2-hour slots on the Monday of the conference (15th July) and we are accepting abstracts with the deadline of 3rd June.
The Square Kilometre Array Observatory (SKAO) will revolutionise our understanding of the radio sky and with the SKAO-Mid prototype dish having achieved first light, the future of radio astronomy is in sight. To prepare for such an impressive instrument, SKAO pathfinders (such as, LOFAR, e-MERLIN, MeerKAT) have already pushed, and are continuing to push, the boundaries of what is possible. It is important to show the wider astronomical community the vast science that has been conducted at low to mid radio frequencies and what has already been done to prepare for the transformative results the SKAO will bring.
This session will enable the UK astronomy community, particularly early-career researchers (ECRs), to present their cutting-edge research conducted using SKAO precursors. We will welcome a wide scope of science to this session to emphasize the widespread impact that current radio facilities are having on science topics such as galaxy evolution, AGN, transients, pulsars and cosmic magnetism as well as technical challenges such as the confrontation of large data sets in radio imaging. Discussion of how such research prepared us for the SKAO will help us understand the path toward its survey operations.
Using a novel method to decompose AGN activity from star formation, the star formation rate is found to be higher than previous studies in four Ultra-Luminous Infrared Galaxies.
The class of objects called Ultra-Luminous and Luminous Infrared Galaxies (U/LIRGs) are bright infrared emitters. They are characteristic of massive merger interactions and high rates of star formation, at different spatial scales, both in nuclear regions (<0.2-1 kpc) and on large-scale structures. Understanding how star formation takes place in these systems plays a pivotal role in studying their dynamics and evolution. Alongside this, characterising local U/LIRGs with high-angular resolution helps us to understand the star-formation and mass assembly of the Universe at higher red-shifts.
The radio emission in the nuclear regions of these systems is a mixture of AGN activity, starburst, jets and star formation. Decoupling these individual processes to study them separately has proven difficult and led to inaccurate measurements of star formation rates or AGN power. The PARADIGM project, alongside with the e-MERLIN Legacy Project LIRGI, aims to measure the radio emission on 10s pc-scales, thus disentangling the emission mechanisms.
In this recent work, Geferson Lucatelli and collaborators investigated an alternative approach that uses combined interferometric arrays (e-MERLIN + VLA) at 6 GHz to obtain a multi-scale tracer for star-formation. Using e-MERLIN, the compact AGN emission is disentangled from the nuclear extended radio emission, and was thus used to quantify the nuclear star-formation. The VLA data helped to probe the larger-scale emission, translating it to star formation without the contamination from possible AGN emission, which was removed with the e-MERLIN data.
The main result of the pilot study showed that the total star formation rate (SFR) of these sources can be significantly higher when using a multi-scale metric, since it takes into account the nuclear star formation. This effect provides a boost to the total SFR output, and in some occasions, dominates the large-scale SFR.
The next phase of this work, is to expand the same concepts for the full LIRGI sample (42 U/LIRGs), and using multi-wavelength observations, separate different emission mechanisms (thermal and non-thermal) across various frequency and spatial scales, on a component-by-component basis.
An astonishingly active repeating fast radio burst source has been pinpointed to milliarcsecond precision, thanks to an ad hoc array of European VLBI Network and e-MERLIN dishes.
The majority of fast radio bursts (FRBs) are observed as single, millisecond-duration radio pulses originating from extragalactic distances. While a small fraction (~3%) of these sources repeat sporadically and infrequently, FRB 20220912A stands out as one of the most active FRB sources known to date. Discovered by the Canadian Hydrogen Intensity Mapping Experiment FRB project (CHIME/FRB), this single source contributed a few percent of the entire all-sky rate of FRBs above a fluence threshold of 100 Jy ms during its peak activity! Consequently, it has become the focus of numerous follow-up campaigns, including localization efforts involving e-MERLIN.
In a recent paper, Danté Hewitt and colleagues from the PRECISE FRB-localisation programme, detected 150 bursts from the hyperactive repeater FRB 20220912A, allowing it to be pinpointed to a precision of a few milliarcseconds - hundreds of times more precise than previous localisations. This revealed that FRB 20220912A resides closer to (albeit still offset from) the center of its host galaxy, which was earlier identified using a localisation from the Deep Synoptic Array 110 (DSA-110). The observations also ruled out the presence of a compact persistent radio source at the FRB location, a phenomenon observed in some other active repeaters, and theorised to be hypernebulae powered by neutron stars. Instead, the persistent radio emission on larger angular scales is most likely due to star formation in the host galaxy.
PRECISE (Pinpointing REpeating ChIme/frb Sources with EVN dishes) is a project to localise repeating FRBs to the best-possible precision by making use of ad-hoc arrays of radio telescopes. Approximately half of these dishes also form part of e-MERLIN. Typically, angular resolution on the order of milliarcseconds is achieved in PRECISE FRB localisations. In the past this has facilitated not only the identification of host galaxies of FRB sources, but also enabled detailed studies of their local environments using, e.g., the Hubble Space Telescope. Such milliarcsecond localizations will prove particularly valuable in the coming era, where optical telescopes like the Extremely Large Telescope will match the milliarcsecond resolution of VLBI, allowing us to zoom-in on the local environments of FRB sources at the (sub-)parsec level in their host galaxy; perhaps even leading to the discovery of extragalactic stellar counterparts to FRB sources.
Extragalactic Survey fields provide a rich hunting ground for Active nuclei, but multiple resolution interferometers are needed to tell them apart from star formation.
Deep radio surveys of extragalactic legacy fields trace a large range of spatial and brightness temperature sensitivity scales, and therefore have differing biases to radio-emitting physical components within galaxies. This is particularly true of radio surveys performed at ≲1 arcsec angular resolutions, and so robust comparisons are necessary to better understand the biases present in each survey.
Ann Njeri and her team performed a multi-resolution analysis of 24 sources detected in a new Very Long Baseline Array (VLBA) survey of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey Great Observatories Origins Deep Survey-North field. This VLBA data were augmented by ~0.1-1 arcsec angular resolution data provided by the European VLBI Network, the Very Large Array (VLA) and the enhanced-Multi Element Remotely Linked Interferometry Network (e-MERLIN).
The high brightness temperatures of these sources (> 1 million Kelvin) confirm AGN cores as measured by VLBI. These sources would often be missed or ambiguous in lower-resolution radio data of the same sources. By combining VLA and VLBI measurements, they find evidence that most of the extended radio emission is also AGN dominated, with only ~13% of the sources showing a likelihood of being star formation-dominated. Combining e-MERLIN and VLBI reveals potentially hybrid systems that are missed in either individual surveys. This survey demonstrates the importance of wide-field multi-resolution (arcsecond-to-milliarcsecond) coverage of the faint radio source population, for a complete picture of the multiscale processes within these galaxies.
An opportunity to propose for 18 facilities simultaneously in the radio and optical has been announced through the Opticon RadioNet Pilot programme.
The Opticon RadioNet Pilot (ORP) has launched as new Multi-Facility Call for Proposals to use multiple observatories for the same scientific project. 18 radio and optical facilities are available and proposers are asked to submit for time on more than one facility as part of this call. The deadline is the 9th April 2024 at 1400UTC.
The 2nd e-MERLIN data school has been announced, taking place in Manchester from 13-15 May 2024
e-MERLIN support staff, with funding support from the Opticon RadioNet Pilot, will be running a data school at the University of Manchester from 13-15 May 2024. The purpose of this school is to show participants how to reduce e-MERLIN data using the e-MERLIN CASA Pipeline. The school is designed for those with some radio astronomy knowledge. Priority will be given to PhD students and early career researchers. The three-day school will include calibration, imaging and self-calibration tutorials, with an opportunity to bring your own data (BYOD) to get expert help with your own e-MERLIN projects. We expect to be able to have space for up to 10 participants.
The complex OH line activity in Arp 220 has been detected by combining MERLIN and EVN data.
Arp 220 is a nearby system comprised of two merging galaxies triggering significant star formation in the merger. It was the first system to be detected with hydroxyl (OH) MegaMaser emission, afterwards identified as an archetypal Ultra-Luminous infrared Galaxy (ULIRG). Since that first discovery many more ULIRGs have been discovered with OH MegaMaser emission. ULIRGs tend to be found at higher redshift, making Arp 220 a useful nearby laboratory for the ULIRG population.
In this work by Willem Baan and colleagues, observations obtained in 2003 using MERLIN and EVN have been analysed and combined to study the multiple masering components in Arp 220. The data combination was performed in order to make an intermediate resolution dataset to provide a clearer view of the source. MERLIN on its own does not provide the requisite resolution but detects the larger scale regions; EVN resolves those extended emission regions and only detects higher brightness temperature components.
At intermediate resolution with MERLIN and EVN, the spectral line emission in Arp 220 separates into compact and extended regions distributed across the two nuclei. The strong compact components represent star-formation-related maser sources and provide the first accurate estimate of the systemic velocities of these two very obscured nuclei. On the other hand, the more dominant extended emission regions belong to foreground material at a much lower velocity that is part of the surrounding debris structures, generated by the galactic merger. The far-infrared radiation field from the ongoing star-formation in the nuclear regions serves to excite the OH molecules in those foreground regions. In turn, this results in maser amplification of the radio background as well as re-amplification of the compact components in the nuclear regions.
The results of the MERLIN/EVN synthesis have provided a detailed look at the masering process in Arp 220 showing that indeed excited foreground material can result in amplification of the radio background in the galaxy. This can generate the observed powerful MegaMaser emissions not only of OH, but also those of other masering molecular species. Such a scenario has been proposed along with the first discovery of the OHMM emission in Arp 220, but it had not been verified in detail.
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