Publications by David Williams


Unveiling the 100 pc scale nuclear radio structure of NGC 6217 with e-MERLIN and the VLA

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 486 (2019) 4962-4979

DRA Williams, IM McHardy, RD Baldi, RJ Beswick, M Pahari, MK Argo, A Beri, P Boorman, E Brinks, BT Dullo, DM Fenech, J Ineson, P Kharb, JH Knapen, TWB Muxlow, J Westcott


The 2018 outburst of BHXB H1743-322 as seen with MeerKAT

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (0)

DRA Williams, SE Motta, R Fender, J Bright, I Heywood, E Tremou, P Woudt, DAH Buckley, S Corbel, M Coriat, T Joseph, L Rhodes, GR Sivakoff, AJVD Horst

In recent years, the black hole candidate X-ray binary system H1743-322 has undergone outbursts and it has been observed with X-ray and radio telescopes. We present 1.3 GHz MeerKAT radio data from the ThunderKAT Large Survey Project on radio transients for the 2018 outburst of H1743-322. We obtain seven detections from a weekly monitoring programme and use publicly available Swift X-ray Telescope and MAXI data to investigate the radio/X-ray correlation of H1743-322 for this outburst. We compare the 2018 outburst with those reported in the literature for this system and find that the X-ray outburst reported is similar to previously reported `hard-only' outbursts. As in previous outbursts, H1743-322 follows the `radio-quiet' correlation in the radio/X-ray plane for black hole X-ray binaries, and the radio spectral index throughout the outburst is consistent with the `radio-quiet' population.


Jets, arcs, and shocks: NGC5195 at radio wavelengths

Monthly Notices of the Royal Astronomical Society Oxford University Press 476 (2018) 2876-2889

H Rampadarath, R Soria, R Urquhart, M Brightman, MK Argo, RJ Beswick, TWB Muxlow, EM Schlegel, CK Lacey, RD Baldi, IM McHardy, D Williams, G Dumas

We studied the nearby, interacting galaxy NGC5195 (M 51b) in the radio, optical and X-ray bands.We mapped the extended, low-surface-brightness features of its radio-continuum emission; determined the energy content of its complex structure of shock-ionized gas; constrained the current activity level of its supermassive nuclear black hole. In particular, we combined data from the European Very Long Baseline Interferometry Network (∼1-pc scale), from our new e-MERLIN observations (∼10-pc scale), and from the Very Large Array (∼100–1000-pc scale), to obtain a global picture of energy injection in this galaxy. We put an upper limit to the luminosity of the (undetected) flat-spectrum radio core. We find steep-spectrum, extended emission within 10 pc of the nuclear position, consistent with optically thin synchrotron emission from nuclear star formation or from an outflow powered by an active galactic nucleus (AGN). A linear spur of radio emission juts out of the nuclear source towards the kpc-scale arcs (detected in radio, Hα and X-ray bands). From the size, shock velocity, and Balmer line luminosity of the kpc-scale bubble, we estimate that it was inflated by a long-term-average mechanical power ∼3–6 × 10^41 erg s^−1 over the last 3–6 Myr. This is an order of magnitude more power than can be provided by the current level of star formation, and by the current accretion power of the supermassive black hole.We argue that a jet-inflated bubble scenario associated with previous episodes of AGN activity is the most likely explanation for the kpc-scale structures.


The nuclear activity and central structure of the elliptical galaxy NGC 5322

Monthly Notices of the Royal Astronomical Society Oxford University Press 475 (2018) 4670-4682

BT Dullo, DRA Williams, JH Knapen, RJ Beswick, RD Baldi, G Bendo, T Muxlow, J Westcott, M Argo, IM McHardy

We have analysed a new high-resolution e-MERLIN 1.5 GHz radio continuum map together with HST and SDSS imaging of NGC 5322, an elliptical galaxy hosting radio jets, aiming to understand the galaxy’s central structure and its connection to the nuclear activity. We decomposed the composite HST + SDSS surface brightness profile of the galaxy into an inner stellar disc, a spheroid, and an outer stellar halo. Past works showed that this embedded disc counter-rotates rapidly with respect to the spheroid. The HST images reveal an edge-on nuclear dust disc across the centre, aligned along the major-axis of the galaxy and nearly perpendicular to the radio jets. After careful masking of this dust disc, we find a central stellar mass deficit Mdef in the spheroid, scoured by SMBH binaries with final mass MBH such that Mdef/MBH ∼ 1.3–3.4. We propose a three-phase formation scenario for NGC 5322, where a few (2–7) ‘dry’ major mergers involving SMBHs built the spheroid with a depleted core. The cannibalism of a gas-rich satellite subsequently creates the faint counter-rotating disc and funnels gaseous material directly on to the AGN, powering the radio core with a brightness temperature of TB, core ∼ 4.5 × 10^7 K and the low-power radio jets (Pjets ∼ 7.04 × 10^20 W Hz^−1), which extend ∼1.6 kpc. The outer halo can later grow via minor mergers and the accretion of tidal debris. The low-luminosity AGN/jet-driven feedback may have quenched the late-time nuclear star formation promptly, which could otherwise have replenished the depleted core.


LeMMINGs - I. The eMERLIN legacy survey of nearby galaxies. 1.5-GHz parsec-scale radio structures and cores

Monthly Notices of the Royal Astronomical Society Oxford Academic 476 (2018) 3478-3522

P Kharb, TJ Maccarone, I Martí-Vidal, CG Mundell, DJ Saikia, F Shankar, RD Baldi, DRA Williams, IM McHardy, RJ Beswick, MK Argo, BT Dullo, JH Knapen, E Brinks, TWB Muxlow, S Aalto, A Alberdi, GJ Bendo, S Corbel, R Evans, DM Fenech, DA Green, H-R Klöckner, E Körding

We present the first data release of high-resolution (≤0.2 arcsec) 1.5-GHz radio images of 103 nearby galaxies from the Palomar sample, observed with the eMERLIN array, as part of the LeMMINGs survey. This sample includes galaxies which are active (low-ionization nuclear emission-line regions [LINER] and Seyfert) and quiescent (H II galaxies and absorption line galaxies, ALGs), which are reclassified based upon revised emission-line diagrams.We detect radio emission ≳0.2 mJy for 47/103 galaxies (22/34 for LINERS, 4/4 for Seyferts, 16/51 for HII galaxies, and 5/14 for ALGs) with radio sizes typically of ≲100 pc. We identify the radio core position within the radio structures for 41 sources. Half of the sample shows jetted morphologies. The remaining half shows single radio cores or complex morphologies. LINERs show radio structures more core-brightened than Seyferts. Radio luminosities of the sample range from 10^32 to 10^40 erg s^−1: LINERs and HII galaxies show the highest and lowest radio powers, respectively, while ALGs and Seyferts have intermediate luminosities. We find that radio core luminosities correlate with black hole (BH) mass down to ∼10^7 M⊙, but a break emerges at lower masses. Using [OIII] line luminosity as a proxy for the accretion luminosity, active nuclei and jetted HII galaxies follow an optical Fundamental Plane of BH activity, suggesting a common disc–jet relationship. In conclusion, LINER nuclei are the scaled-down version of FR I radio galaxies; Seyferts show less collimated jets; HII galaxies may host weak active BHs and/or nuclear star-forming cores; and recurrent BH activity may account for ALG properties.


15-GHz radio emission from nearby low-luminosity active galactic nuclei

Astronomy and Astrophysics EDP Sciences 616 (2018) A152-

P Saikia, E Körding, DL Coppejans, H Falcke, D Williams, RD Baldi, I McHardy, R Beswick

We present a sub-arcsec resolution radio imaging survey of a sample of 76 low-luminosity active galactic nuclei (LLAGN) that were previously not detected with the Very Large Array at 15 GHz. Compact, parsec-scale radio emission has been detected above a flux density of 40 μ Jy in 60% (45 of 76) of the LLAGN sample.We detect 20 out of 31 (64%) low-ionization nuclear emission-line region (LINER) nuclei, ten out of 14 (71%) low-luminosity Seyfert galaxies, and 15 out of 31 (48%) transition objects.We use this sample to explore correlations between different emission lines and the radio luminosity.We also populate the X-ray and the optical fundamental plane of black hole activity and further refine its parameters.We obtain a fundamental plane relation of log LR = 0.48 (±0.04) log LX + 0.79 (±0.03) log M and an optical fundamental plane relation of log LR = 0.63 (±0.05) log L[O III] + 0.67 (±0.03) log M after including all the LLAGN detected at high resolution at 15 GHz, and the best-studied hard-state X-ray binaries (luminosities are given in erg s^-1 while the masses are in units of solar mass). Finally, we find conclusive evidence that the nuclear 15 GHz radio luminosity function (RLF) of all the detected Palomar Sample LLAGN has a turnover at the low-luminosity end, and is best-fitted with a broken power law. The break in the power law occurs at a critical mass accretion rate of 1.2 X 10^-3 M⊙ yr^-1, which translates to an Eddington ratio of m˙ Edd ~ 5.1 X 10^-5, assuming a black hole mass of 109 M⊙. The local group stands closer to the extrapolation of the higher-luminosity sources, and the classical Seyferts agree with the nuclear RLF of the LLAGN in the local universe.


Radio jets in NGC 4151: Where eMERLIN meets HST

Monthly Notices of the Royal Astronomical Society Oxford University Press 472 (2017) 3842-3853

DRA Williams, IM McHardy, RD Baldi, RJ Beswick, MK Argo, BT Dullo, E Brinks, JH Knapen, DM Fenech, TWB Muxlow, CG Mundell, H Rampadarath, F Panessa, J Westcott

We present high-sensitivity eMERLIN radio images of the Seyfert galaxy NGC4151 at 1.51 GHz. We compare the new eMERLIN images to those from archival MERLIN observations in 1993 to determine the change in jet morphology in the 22 yr between observations. We report an increase by almost a factor of 2 in the peak flux density of the central core component, C4, thought to host the black hole, but a probable decrease in some other components, possibly due to adiabatic expansion. The core flux increase indicates an active galactic nucleus (AGN) that is currently active and feeding the jet. We detect no significant motion in 22 yr between C4 and the component C3, which is unresolved in the eMERLIN image. We present a spectral index image made within the 512MHz band of the 1.51 GHz observations. The spectrum of the core, C4, is flatter than that of other components further out in the jet. We use HST emission-line images (H α, [OIII] and [O II]) to study the connection between the jet and the emission-line region. Based on the changing emission-line ratios away from the core and comparison with the eMERLIN radio jet, we conclude that photoionization from the central AGN is responsible for the observed emission-line properties further than 4 arcsec (360 pc) from the core, C4. Within this region, a body of evidence (radio-line co-spatiality, low [O III]/H α and estimated fast shocks) suggests additional ionization from the jet.


A high-resolution radio continuum study of the dwarf irregular galaxy IC 10

Monthly Notices of the Royal Astronomical Society Oxford University Press 467 (2017) 2113–2126-

J Westcott, E Brinks, RJ Beswick, V Heesen, MK Argo, DJB Smith, DRA Williams, RD Baldi, IM McHardy, DM Fenech

We present high-resolution e-Multi-Element Radio Linked Interferometer Network radio continuum maps of the dwarf irregular galaxy IC 10 at 1.5 and 5 GHz. We detect 11 compact sources at 1.5 GHz, five of which have complementary detections at 5 GHz. We classify three extended sources as compact HII regions within IC 10, five sources as contaminating background galaxies and identify three sources that require additional observations to classify. We do not expect that any of these three sources are supernova remnants as they will likely be resolved out at the assumed distance of IC 10 (0.7 Mpc). We correct integrated flux densities of IC 10 from the literature for contamination by unrelated background sources and obtain updated flux density measurements of 354±11 mJy at 1.5 GHz and 199 ± 9 mJy at 4.85 GHz. The background contamination does not contribute significantly to the overall radio emission from IC 10, so previous analysis concerning its integrated radio properties remains valid.