arXiv subjects

A. P. Thomson

Publications and source records attributed to A. P. Thomson.

At least 19 recordsLinked to original sources

arXiv2024

COALAS III: The ATCA CO(1-0) look at the growth and death of H$α$ emitters in the Spiderweb protocluster at z=2.16

We obtain CO(1-0) molecular gas measurements with ATCA on a sample of 43 spectroscopically confirmed H$α$ emitters in the Spiderweb protocluster at $z=2.16$ and investigate the relation between their star formation and cold gas reservoirs as a function of environment. We achieve a CO(1-0) detection rate of $\sim23\pm12\%$ with 10 dual CO(1-0) and H$α$ detections at $10<\log M_{*}/M_\odot<11.5$. In addition, we obtain upper limits for the remaining sources. In terms of total gas fractions ($F_{gas}$), our sample is divided into two different regimes with a steep transition at $\log M_{*}/M_\odot\approx10.5$. Galaxies below that threshold have gas fractions that in some cases are close to unity, indicating that their gas reservoir has been replenished by inflows from the cosmic web. However, objects at $\log M_{*}/M_\odot>10.5$ display significantly lower gas fractions and are dominated by AGN (12 out of 20). Stacking results yield $F_{gas}\approx0.55$ for massive emitters excluding AGN, and $F_{gas}\approx0.35$ when examining only AGN candidates. Furthermore, depletion times show that most H$α$ emitters may become passive by $1<z<1.6$, concurrently with the surge and dominance of the red sequence in the most massive clusters. Our analyses suggest that galaxies in the outskirts of the protocluster have larger molecular-to-stellar mass ratios and lower star formation efficiencies than in the core. However, star formation across the protocluster remains consistent with the main sequence, indicating that evolution is primarily driven by the depletion of the gas reservoir towards the inner regions. We discuss the relative importance of in-/outflow processes in regulating star formation during the early phases of cluster assembly and conclude that a combination of feedback and overconsumption may be responsible for the rapid cold gas depletion these objects endure.

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arXiv2024

The VLBA CANDELS GOODS-North Survey. II -- Wide-field source catalogue comparison between the VLBA, EVN, e-MERLIN and VLA

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 less than 1 arcsec angular resolutions, and so robust comparisons are necessary to better understand the biases present in each survey. We present a multi-resolution and multi-wavelength analysis of the sources detected in a new Very Long Baseline Array (VLBA) survey of the CANDELS GOODS-North field. For the 24 VLBA-selected sources described in Paper I, we augment the VLBA data with EVN data, ~0.1-1 arcsecond angular resolution data provided by VLA and e-MERLIN. This sample includes new AGN detected in this field, thanks to a new source extraction technique that adopts priors from ancillary multi-wavelength data. The high brightness temperatures of these sources (Tb > 10^6 K) confirm AGN cores, that would often be missed or ambiguous in lower-resolution radio data of the same sources. Furthermore, only 15 sources are identified as 'radiative' AGN based on available X-ray and infrared constraints. By combining VLA and VLBA measurements, we find evidence that the majority of the extended radio emission is also AGN dominated, with only 3 sources with evidence for extended potentially star-formation dominated radio emission. We demonstrate the importance of wide-field multi-resolution (arcsecond-milliarcsecond) coverage of the faint radio source population, for a complete picture of the multi-scale processes within these galaxies.

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arXiv2023

Revisiting a flux recovery systematic error arising from common deconvolution methods used in aperture-synthesis imaging

The point-spread function (PSF) is a fundamental property of any astronomical instrument. In interferometers, differing array configurations combined with their $uv$ coverage, and various weighting schemes can produce an irregular but deterministic PSF. As a result, the PSF is often deconvolved using CLEAN-style algorithms to improve image fidelity. In this paper, we revisit a significant effect that causes the flux densities measured with any interferometer to be systematically offset from the true values. Using a suite of carefully controlled simulations, we show that the systematic offset originates from a mismatch in the units of the image produced by these CLEAN-style algorithms. We illustrate that this systematic error can be significant, ranging from a few to tens of per cent. Accounting for this effect is important for current and future interferometric arrays, such as MeerKAT, LOFAR and the SKA, whose core-dominated configuration naturally causes an irregular PSF. We show that this offset is independent of other systematics, and can worsen due to some factors such as the goodness of the fit to the PSF, the deconvolution depth, and the signal-to-noise of the source. Finally, we present several methods that can reduce this effect to just a few per cent.

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arXiv2022

SPARCS-North Wide-field VLBI Survey: Exploring the resolved MicroJy extra-galactic radio source population with EVN+e-MERLIN

The SKA PAthfinder Radio Continuum Surveys (SPARCS) are providing deep-field imaging of the faint (sub-mJy) extra-galactic radio source populations through a series of reference surveys. One of the key science goals for SPARCS is to characterize the relative contribution of radio emission associated with AGN from star-formation (SF) in these faint radio source populations, using a combination of high sensitivity and high angular resolution imaging over a range of spatial scales (arcsec to mas). To isolate AGN contribution from SF, we hypothesise that there exists a brightness temperature cut-off point separating pure AGN from SF. We present a multi-resolution (10-100 mas) view of the transition between compact AGN and diffuse SF through a deep wide-field EVN+e-MERLIN, multiple phase centre survey of the centre of the Northern SPARCS (SLOAN) reference field at 1.6 GHz. This is the first (and only) VLBI (+e-MERLIN) milliarcsecond angular resolution observation of this field, and of the wider SPARCS reference field programme. Using these high spatial resolution (9 pc - 0.3 kpc at z ~ 1.25) data, 11 milliarcsecond-scale sources are detected from a targeted sample of 52 known radio sources from previous observations with the e-MERLIN, giving a VLBI detection fraction of ~ 21%. At spatial scales of ~ 9 pc, these sources show little to no jet structure whilst at ~ 0.3 kpc one-sided and two-sided radio jets begin to emerge on the same sources, indicating a possible transition from pure AGN emissions to AGN and star-formation systems.

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arXiv2022

Deep sub-arcsecond widefield imaging of the Lockman Hole field at 144 MHz

High quality low-frequency radio surveys have the promise of advancing our understanding of many important topics in astrophysics, including the life cycle of active galactic nuclei (AGN), particle acceleration processes in jets, the history of star formation, and exoplanet magnetospheres. Currently leading low-frequency surveys reach an angular resolution of a few arcseconds. However, this resolution is not yet sufficient to study the more compact and distant sources in detail. Sub-arcsecond resolution is therefore the next milestone in advancing these fields. The biggest challenge at low radio frequencies is the ionosphere. If not adequately corrected for, ionospheric seeing blurs the images to arcsecond or even arcminute scales. Additionally, the required image size to map the degree-scale field of view of low-frequency radio telescopes at this resolution is far greater than what typical soft- and hardware is currently capable of handling. Here we present for the first time (to the best of our knowledge) widefield sub-arcsecond imaging at low radio frequencies. We derive ionospheric corrections in a few dozen individual directions and apply those during imaging efficiently using a recently developed imaging algorithm (arXiv:1407.1943, arXiv:1909.07226). We demonstrate our method by applying it to an eight hour observation of the International LOw Frequency ARray (LOFAR) Telescope (ILT) (arXiv:1305.3550). Doing so we have made a sensitive $7.4\ \mathrm{deg}^2$ $144\ \mathrm{MHz}$ map at a resolution of $0.3''$ reaching $25\ μ\mathrm{Jy\ beam}^{-1}$ near the phase centre. The estimated $250,000$ core hours used to produce this image, fit comfortably in the budget of available computing facilities. This result will enable future mapping of the entire northern low-frequency sky at sub-arcsecond resolution.

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arXiv2022

Quasar Feedback Survey: Multi-phase outflows, turbulence and evidence for feedback caused by low power radio jets inclined into the galaxy disk

We present a study of a luminous, z=0.15, type-2 quasar (log [L([OIII])/(erg/s)]=42.8) from the Quasar Feedback Survey. It is classified as 'radio-quiet' (log [L(1.4 GHz)/(W/Hz)]=23.8); however, radio imaging reveals ~1 kpc low-power jets (log [Pjet/(erg/s)]=44) inclined into the plane of the galaxy disk. We combine MUSE and ALMA observations to map stellar kinematics and ionised and molecular gas properties. The jets are seen to drive galaxy-wide bi-conical turbulent outflows, reaching W80 = 1000-1300 km/s, in the ionised phase (traced via optical emission-lines), which also have increased electron densities compared to the quiescent gas. The turbulent gas is driven perpendicular to the jet axis and is escaping along the galaxy minor axis, reaching 7.5 kpc on both sides. Traced via CO(3-2) emission, the turbulent material in molecular gas phase is one-third as spatially extended and has 3 times lower velocity-dispersion as compared to ionised gas. The jets are seen to be strongly interacting with the interstellar medium (ISM) through enhanced ionised emission and disturbed/depleted molecular gas at the jet termini. We see further evidence for jet-induced feedback through significantly higher stellar velocity-dispersion aligned, and co-spatial with, the jet axis (<5 deg). We discuss possible negative and positive feedback scenarios arising due to the interaction of the low-power jets with the ISM in the context of recent jet-ISM interaction simulations, which qualitatively agree with our observations. We discuss how jet-induced feedback could be an important feedback mechanism even in bolometrically luminous 'radio-quiet' quasars.

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arXiv2021

Fundamental differences in the radio properties of red and blue quasars: kiloparsec-scale structures revealed by e-MERLIN

Red quasi-stellar objects (QSOs) are a subset of the quasar population with colours consistent with reddening due to intervening dust. Recent work has demonstrated that red QSOs show special radio properties that fundamentally distinguish them from normal blue QSOs, specifically a higher incidence of low-power radio emission (1.4 GHz luminosities L$_{\rm 1.4} \approx 10^{25}$ - $10^{27}$ W Hz$^{-1}$) that is physically compact when imaged by arcsecond-resolution radio surveys such as FIRST. In this work, we present e-MERLIN imaging of a set of intermediate-redshift ($1.0 2$ kpc). We report a statistically significant difference in the incidence of extended kpc-scale emission in red QSOs. From an analysis of the radio size distributions of the sample, we find that the excess radio emission in red QSOs can be attributed to structures that are confined to galaxy scales ($< 10$ kpc), while we confirm previous results that red and normal QSOs have similar incidences of radio jets and lobes on circumgalactic or larger scales ($> 10$ kpc). Our results indicate that the primary mechanism that generates the enhanced radio emission in red QSOs is not directly connected with the nuclear engine or accretion disc, but is likely to arise from extended components such as AGN-driven jets or winds.

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arXiv2021

The radio emission from Active Galactic Nuclei

For nearly seven decades astronomers have been studying active galaxies, that is to say galaxies with actively accreting central supermassive black holes, AGN. A small fraction of these are characterized by luminous, powerful radio emission: this class is known as radio-loud. A substantial fraction, the so-called radio-quiet AGN population, displays intermediate or weak radio emission. However, an appreciable fraction of strong X-rays emitting AGN are characterized by the absence of radio emission, down to an upper limit of about $10^{-7}$ times the luminosity of the most powerful radio-loud AGN. We wish to address the nature of these - seemingly radio-silent - X-ray-luminous AGN and their host galaxies: is there any radio emission, and if so, where does it originate? Focusing on the GOODS-N field, we examine the nature of these objects employing stacking techniques on ultra-deep radio data obtained with the JVLA. We combine these radio data with Spitzer far-infrared data. We establish the absence, or totally insignificant contribution of jet-driven radio-emission in roughly half of the otherwise normal population of X-ray luminous AGN, which appear to reside in normal star-forming galaxies. We conclude that AGN- or jet-driven radio emission is simply a mechanism that may be at work or may be dormant in galaxies with actively accreting black holes. The latter can be classified as radio-silent AGN.

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arXiv2021

Nowhere to Hide: Radio-faint AGN in the GOODS-N field. II. Multi-wavelength AGN selection techniques and host galaxy properties

Obtaining a census of active galactic nuclei (AGN) activity across cosmic time is critical to our understanding of galaxy evolution and formation. Many AGN classification techniques are compromised by dust obscuration. However, very long baseline interferometry (VLBI) can be used to identify compact emission that can only be attributed to AGN activity. This is the second in a series of papers dealing with the compact radio population in the GOODS-N field. We review 14 different AGN classification techniques in the context of a VLBI-detected sample, and use these to investigate the nature of the AGN as well as their host galaxies. We find that no single identification technique can identify all VLBI objects as AGN. Infrared colour-colour selection is most notably incomplete. However, the usage of multiple classification schemes can identify all VLBI-selected AGN, independently verifying similar approaches used in other deep field surveys. In the era of large area surveys with instruments such as the SKA and ngVLA, multi-wavelength coverage, which relies heavily upon observations from space, is often unavailable. Therefore, VLBI remains an integral component in detecting AGN of the jetted efficient and inefficient accretion types. A substantial fraction (46%) of the VLBI AGN have no X-ray counterpart, which is most likely due to lack of sensitivity in the X-ray band. A high fraction of the VLBI AGN reside in low or intermediate redshift dust-poor early-type galaxies. These most likely exhibit inefficient accretion. Finally, a significant fraction of the VLBI AGN reside in symbiotic dusty starburst - AGN systems. We present an extensive compilation of the multi-wavelength properties of all the VLBI-selected AGN in GOODS-N in the Appendix.

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arXiv2021

COALAS: I. ATCA CO(1-0) survey and luminosity function in the Spiderweb protocluster at z=2.16

We report a detailed CO(1-0) survey of a galaxy protocluster field at $z=2.16$, based on 475 hours of observations with the Australia Telescope Compact Array. We constructed a large mosaic of 13 individual pointings, covering an area of 21 arcmin$^2$ and $\pm6500$ km/s range in velocity. We obtain a robust sample of 46 CO(1-0) detections spanning $z=2.09-2.22$, constituting the largest sample of molecular gas measurements in protoclusters to date. The CO emitters show an overdensity at $z=2.12-2.21$, suggesting a galaxy super-protocluster or a protocluster connected to large-scale filaments with ~120 cMpc size. We find that 90% CO emitters have distances $>0'.5-4'$ to the center galaxy, indicating that small area surveys would miss the majority of gas reservoirs in similar structures. Half of the CO emitters have velocities larger than escape velocities, which appears gravitationally unbound to the cluster core. These unbound sources are barely found within the $R_{200}$ radius around the center, which is consistent with a picture in which the cluster core is collapsed while outer regions are still in formation. Compared to other protoclusters, this structure contains relatively more CO emitters with relatively narrow line width and high luminosity, indicating galaxy mergers. We use these CO emitters to place the first constraint on the CO luminosity function and molecular gas density in an overdense environment. The amplitude of the CO luminosity function is 1.6$\pm$0.5 orders of magnitudes higher than observed for field galaxy samples at $z\sim2$, and one order of magnitude higher than predictions for galaxy protoclusters from semi-analytical SHARK models. We derive a high molecular gas density of $0.6-1.3\times10^{9}$ $M_\odot$ cMpc$^{-3}$ for this structure, consistent with predictions for cold gas density of massive structures from hydro-dynamical DIANOGA simulations.

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arXiv2021

The Quasar Feedback Survey: Discovering hidden Radio-AGN and their connection to the host galaxy ionised gas

We present the first results from the Quasar Feedback Survey, a sample of 42 z<0.2, [O III] luminous AGN (L[O III]>10^42.1 ergs/s) with moderate radio luminosities (i.e. L(1.4GHz)>10^23.4 W/Hz; median L(1.4GHz)=5.9x10^23 W/Hz). Using high spatial resolution (~0.3-1 arcsec), 1.5-6 GHz radio images from the Very Large Array, we find that 67 percent of the sample have spatially extended radio features, on ~1-60 kpc scales. The radio sizes and morphologies suggest that these may be lower radio luminosity versions of compact, radio-loud AGN. By combining the radio-to-infrared excess parameter, spectral index, radio morphology and brightness temperature, we find radio emission in at least 57 percent of the sample that is associated with AGN-related processes (e.g. jets, quasar-driven winds or coronal emission). This is despite only 9.5-21 percent being classified as radio-loud using traditional criteria. The origin of the radio emission in the remainder of the sample is unclear. We find that both the established anti-correlation between radio size and the width of the [O III] line, and the known trend for the most [O III] luminous AGN to be associated with spatially-extended radio emission, also hold for our sample of moderate radio luminosity quasars. These observations add to the growing evidence of a connection between the radio emission and ionised gas in quasar host galaxies. This work lays the foundation for deeper investigations into the drivers and impact of feedback in this unique sample.

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arXiv2020

An ALMA survey of the SCUBA-2 CLS UDS field: Physical properties of 707 Sub-millimetre Galaxies

We analyse the physical properties of a large, homogeneously selected sample of ALMA-located sub-mm galaxies (SMGs). This survey, AS2UDS, identified 707 SMGs across the ~1 sq.deg. field, including ~17 per cent, which are undetected at $K$>~25.7 mag. We interpret their ultraviolet-to-radio data using MAGPHYS and determine a median redshift of z=2.61+-0.08 (1$σ$ range of z=1.8-3.4) with just ~6 per cent at z>4. Our survey provides a sample of massive dusty galaxies at z>~1, with median dust and stellar masses of $M_d$=(6.8+-0.3)x10$^{8}$M$_\odot$ (thus, gas masses of ~10$^{11}$M$_\odot$) and $M_\ast=$(1.26+-0.05)x10$^{11}$M$_\odot$. We find no evolution in dust temperature at a constant far-infrared luminosity across z~1.5-4. The gas mass function of our sample increases to z~2-3 and then declines at z>3. The space density and masses of SMGs suggest that almost all galaxies with $M_\ast$>~3x10$^{11}$M$_\odot$ have passed through an SMG-like phase. The redshift distribution is well fit by a model combining evolution of the gas fraction in halos with the growth of halo mass past a threshold of $M_h$~6x10$^{12}$M$_\odot$, thus SMGs may represent the highly efficient collapse of gas-rich massive halos. We show that SMGs are broadly consistent with simple homologous systems in the far-infrared, consistent with a centrally illuminated starburst. Our study provides strong support for an evolutionary link between the active, gas-rich SMG population at z>1 and the formation of massive, bulge-dominated galaxies across the history of the Universe.

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arXiv2020

An ALMA survey of the S2CLS UDS field: Optically invisible submillimetre galaxies

We analyse a robust sample of 30 near-infrared-faint (K>25.3, 5 sigma) submillimetre galaxies selected across a 0.96 deg^2 field, to investigate their properties and the cause of their lack of detectable optical/near-infrared emission. Our analysis exploits precise identifications based on ALMA 870um continuum imaging, combined with the very deep near-infrared imaging from the UKIDSS-UDS survey. We estimate that K>25.3 submillimetre galaxies represent 15+/-2 per cent of the total population brighter than S870=3.6mJy, with an expected surface density of ~450/deg^2 above S870>1mJy. As such they pose a source of contamination in surveys for both high-redshift "quiescent" galaxies and very-high-redshift Lyman-break galaxies. We show that these K-faint submillimetre galaxies are simply the tail of the broader submillimetre population, with comparable dust and stellar masses to K<25.3 mag submillimetre galaxies, but lying at significantly higher redshifts (z=3.44+/-0.06 versus z=2.36+/-0.11) and having higher dust attenuation (Av=5.2+/-0.3 versus Av=2.9+/-0.1). We investigate the origin of the strong dust attenuation and find indications that these K-faint galaxies have smaller dust continuum sizes than the K<25.3 galaxies, as measured by ALMA, which suggests their high attenuation is related to their compact sizes. We find a correlation of dust attenuation with star-formation rate surface density (Sigma_SFR), with the K-faint submillimetre galaxies representing the higher-Sigma_SFR and highest-Av galaxies. The concentrated, intense star-formation activity in these systems is likely to be associated with the formation of spheroids in compact galaxies at high redshifts, but as a result of their high obscuration these are completely missed in UV, optical and even near-infrared surveys.

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arXiv2020

An ALMA Survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS Field: The Far-infrared/Radio correlation for High-redshift Dusty Star-forming Galaxies

We study the radio properties of 706 sub-millimeter galaxies (SMGs) selected at 870$μ$m with the Atacama Large Millimeter Array from the SCUBA-2 Cosmology Legacy Survey map of the Ultra Deep Survey field. We detect 273 SMGs at $>4σ$ in deep Karl G. Jansky Very Large Array 1.4 GHz observations, of which a subset of 45 SMGs are additionally detected in 610 MHz Giant Metre-Wave Radio Telescope imaging. We quantify the far-infrared/radio correlation through parameter $q_\text{IR}$, defined as the logarithmic ratio of the far-infrared and radio luminosity, and include the radio-undetected SMGs through a stacking analysis. We determine a median $q_\text{IR} = 2.20\pm0.03$ for the full sample, independent of redshift, which places these $z\sim2.5$ dusty star-forming galaxies $0.44\pm0.04$ dex below the local correlation for both normal star-forming galaxies and local ultra-luminous infrared galaxies (ULIRGs). Both the lack of redshift-evolution and the offset from the local correlation are likely the result of the different physical conditions in high-redshift starburst galaxies, compared to local star-forming sources. We explain the offset through a combination of strong magnetic fields ($B\gtrsim0.2$mG), high interstellar medium (ISM) densities and additional radio emission generated by secondary cosmic rays. While local ULIRGs are likely to have similar magnetic field strengths, we find that their compactness, in combination with a higher ISM density compared to SMGs, naturally explains why local and high-redshift dusty star-forming galaxies follow a different far-infrared/radio correlation. Overall, our findings paint SMGs as a homogeneous population of galaxies, as illustrated by their tight and non-evolving far-infrared/radio correlation.

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arXiv2020

High molecular gas content and star formation rates in local galaxies that host quasars, outflows and jets

We use a sample of powerful z~0.1 type 2 quasars ('obscured'; log[L(AGN)/erg/s]>~45), which host kiloparsec-scale ionized outflows and jets, to identify possible signatures of AGN feedback on the total molecular gas reservoirs of their host galaxies. Specifically, we present Atacama Pathfinder EXperiment (APEX) observations of the CO(2-1) transition for nine sources and the CO(6-5) for a subset of three. We find that the majority of our sample reside in starburst galaxies (average specific star formation rates of 1.7/Gyr), with the seven CO-detected quasars also having large molecular gas reservoirs (average Mgas = 1.3x10^10Msun), even though we had no pre-selection on the star formation or molecular gas properties. Despite the presence of quasars and outflows, we find that the molecular gas fractions (Mgas/Mstar = 0.1-1.2) and depletion times (Mgas/SFR = 0.16-0.95Gyr) are consistent with those expected for the overall galaxy population with matched stellar masses and specific star formation rates. Furthermore, for at least two of the three targets with the required measurements, the CO(6-5)/CO(2-1) emission-line ratios are consistent with star formation dominating the CO excitation over this range of transitions. The targets in our study represent a gas-rich phase of galaxy evolution with simultaneously high levels of star formation and nuclear activity; furthermore, the jets and outflows do not have an immediate appreciable impact on the global molecular gas reservoirs.

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arXiv2020

The e-MERLIN Galaxy Evolution Survey (e-MERGE): Overview and Survey Description

We present an overview and description of the eMERLIN Galaxy Evolution survey (eMERGE) Data Release 1 (DR1), a large program of high-resolution 1.5 GHz radio observations of the GOODS-N field comprising $\sim140$ hours of observations with eMERLIN and $\sim40$ hours with the Very Large Array (VLA). We combine the long baselines of eMERLIN (providing high angular resolution) with the relatively closely-packed antennas of the VLA (providing excellent surface brightness sensitivity) to produce a deep 1.5 GHz radio survey with the sensitivity ($\sim 1.5μ$Jy beam$^{-1}$), angular resolution ($0.2"$--$0.7"$) and field-of-view ($\sim15' \times 15'$) to detect and spatially resolve star-forming galaxies and AGN at $z\gtrsim 1$. The goal of eMERGE is to provide new constraints on the deep, sub-arcsecond radio sky which will be surveyed by SKA1-mid. In this initial publication, we discuss our data analysis techniques, including steps taken to model in-beam source variability over a $\sim20$ year baseline and the development of new point spread function/primary beam models to seamlessly merge eMERLIN and VLA data in the $uv$ plane. We present early science results, including measurements of the luminosities and/or linear sizes of $\sim500$ galaxes selected at 1.5 GHz. In combination with deep Hubble Space Telescope observations, we measure a mean radio-to-optical size ratio of $r_{\rm eMERGE}/r_{\rm HST}\sim1.02\pm0.03$, suggesting that in most high-redshift galaxies, the $\sim$GHz continuum emission traces the stellar light seen in optical imaging. This is the first in a series of papers which will explore the $\sim$kpc-scale radio properties of star-forming galaxies and AGN in the GOODS-N field observed by eMERGE DR1.

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arXiv2020

Cosmic evolution of molecular gas mass density from an empirical relation between $\rm L_{1.4GHz}$ and $\rm L^{\prime}_{CO}$

Historically, GHz radio emission has been used extensively to characterize the star-formation activity in galaxies. In this work, we look for empirical relations amongst the radio luminosity, the infrared luminosity, and the CO-based molecular gas mass. We assemble a sample of 278 nearby galaxies with measurements of radio continuum and total infrared emission, and the $^{12}$CO (J = 1-0) emission line. We find a correlation between the radio continuum and the CO emission line (with a scatter of 0.36 dex), in a large sample of different kind of galaxies. Making use of this correlation, we explore the evolution of the molecular gas mass function and the cosmological molecular gas mass density in six redshift bins up to $z = 1.5$. These results agree with previous semi-analytic predictions and direct measurements: the cosmic molecular gas density increases up to $z=1.5$. In addition, we find a single plane across five orders of magnitude for the explored luminosities, with a scatter of 0.27 dex. These correlations are sufficiently robust to be used for samples where no CO measurements exist.

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arXiv2020

Searching for obscured AGN in z $\sim$ 2 submillimetre galaxies

Submillimetre-selected galaxies (SMGs) at high redshift ($z$ $\sim$ 2) are potential host galaxies of active galactic nuclei (AGN). If the local Universe is a good guide, $\sim$ 50$\%$ of the obscured AGN amongst the SMG population could be missed even in the deepest X-ray surveys. Radio observations are insensitive to obscuration; therefore, very long baseline interferometry (VLBI) can be used as a tool to identify AGN in obscured systems. A well-established upper limit to the brightness temperature of 10$^5$ K exists in star-forming systems, thus VLBI observations can distinguish AGN from star-forming systems via brightness temperature measurements. We present 1.6 GHz European VLBI Network (EVN) observations of four SMGs (with measured redshifts) to search for evidence of compact radio components associated with AGN cores. For two of the sources, e-MERLIN images are also presented. Out of the four SMGs observed, we detect one source, J123555.14, that has an integrated EVN flux density of 201 $\pm$ 15.2 $μ$Jy, corresponding to a brightness temperature of 5.2 $\pm$ 0.7 $\times$ 10$^5$ K. We therefore identify that the radio emission from J123555.14 is associated with an AGN. We do not detect compact radio emission from a possible AGN in the remaining sources (J123600.10, J131225.73, and J163650.43). In the case of J131225.73, this is particularly surprising, and the data suggest that this may be an extended, jet-dominated AGN that is resolved by VLBI. Since the morphology of the faint radio source population is still largely unknown at these scales, it is possible that with a $\sim$ 10 mas resolution, VLBI misses (or resolves) many radio AGN extended on kiloparsec scales.

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