Role of AGN feedback in galaxy evolution

Over the past two decades, detailed studies in the nearby Universe have shown that accreting supermassive black holes (SMBHs; or active galactic nuclei; AGN) can have a significant effect on their host galaxies, suppressing star formation and regulating their growth (known as AGN feedback). One of the most striking examples of AGN feedback in action comes from detailed studies of local AGN which exhibit powerful bi-polar jet outflows that can deposit significant energy into the galaxy halo, heating the surrounding gas and thereby regulating their own growth and suppressing star-formation activity. Therefore, studying the properties of AGN and the galaxies in which they reside is crucial in understanding and further developing our current models of galaxy formation and evolution. AGN can be split into two distinct categories, based on the accretion efficiency of the SMBH: radiative-mode AGN, and jet-mode AGN. Radiative-mode AGN are associated with efficient accretion, typically from cold gas, resulting in the formation of a geometrically thin, optically thick accretion disk that is typically surrounded by a dusty obscuring structure; these AGN are known to drive powerful outflows. Jet-mode AGN are associated with inefficient accretion, typically from hot gas, and display powerful bi-polar synchrotron radio jets that emit the bulk of their energetic output into the surrounding medium in the form of mechanical energy; these AGN are identified as such based on radio observations, showing no signs of AGN activity (e.g. accretion disk or torus) at other wavelengths. Based on the nature of the excitation lines, the jet-mode and radio-loud radiative-mode populations are also known as low-excitation radio galaxies (LERGs) and high-excitation radio galaxies (HERGs), respectively. However, our understanding of these AGN and their feedback effect is built primarily from detailed local Universe observations. Determining the physical mechanisms underpinning triggering and fuelling of AGN and how this affects AGN feedback activity across cosmic time is crucial but lacking. In this thesis, I address this shortcoming using deep observations carried out by the LOw Frequency ARray (LOFAR) telescope: the LOFAR Deep Fields; this forms the deepest radio continuum survey to date at low frequencies. I generated key science-enhanced datasets using this survey and studied the cosmic evolution of AGN feedback from low-luminosity radio-AGN within the past 10 Gyrs and how this feedback affects the growth and evolution of galaxies. In the first science chapter of the thesis, I detail the pipeline I developed to generate new, more robust multi-wavelength catalogues in the LOFAR Deep Fields. The existing catalogues in the literature either did not include the deepest available datasets in each survey field, or were created using different methodologies for detecting sources and measuring their fluxes; all this meant the catalogues were not sufficiently robust for the scientific aims of the thesis. To overcome these issues, I generated new catalogues in two of the Deep Fields by combining information from the ultraviolet to the mid-infrared wavelengths to detect sources and extract their properties in a clean and homogeneous manner. These are some of the best-studied regions of the sky and therefore these catalogues are also expected to provide a legacy value beyond the aims of the LOFAR surveys. Then, in the next chapter, using the multi-wavelength catalogues generated, I identify the host-galaxy counterparts of the radio-detected sources in the LOFAR Deep Fields. Host-galaxy identification and characterisation is crucial, in particular for radio surveys, in determining the photometric redshifts and physical properties (e.g. stellar masses, luminosities, star-formation rates) of the radio-source host galaxies, greatly expanding the scientific scope of the survey. I identified the host-galaxy counterparts of the LOFAR sources using a combination of the statistical Likelihood Ratio method and a visual classification scheme, using a workflow to decide the most appropriate method of identification for each source. This process results in a value-added catalogue of over 80 000 radio sources with multi-wavelength counterparts identified for > 97% of them. In this chapter, I then also investigate the properties of host galaxies of the faint radio population in the LOFAR Deep Fields. In the fourth chapter I focus on studying the evolution of the radio-AGN population and their properties in the LOFAR Deep Fields and how feedback from these AGN evolves across cosmic time. Of particular interest, and the focus of this chapter are the LERGs, which dominate at low radio luminosities and are thought to play a key role in the formation of massive galaxies in the local Universe; however, the evolution of this population beyond z ∼ 1 is not well known. In this chapter, I present the first robust measurement of the LERG luminosity functions out to z ∼ 2.5 and characterise the evolution of their host galaxy properties. This population shows relatively mild evolution across the redshifts examined; this is explained by the different evolution of the LERGs hosted in star-forming galaxies and those hosted in quiescent galaxies. The evolution of the quiescent LERGs show a strong decline in their space densities with increasing redshift, in accordance with the available host galaxies, while there is also an increase in the characteristic luminosity. I also find that unlike in the local Universe, the bulk of the LERGs are hosted by star-forming galaxies at higher redshifts and that the AGN in these galaxies appear to be fuelled by a different mechanism, likely associated with the cold gas, as compared to the LERGs in quiescent galaxies. In the final chapter I present the conclusions and look towards further exploration of this dataset. In particular, I discuss the characterisation of 3% of the radio-sources that were found to be completely invisible at optical and near-infrared wavelengths; an investigation of the far-infrared and radio properties of this subset found that the vast majority of these sources are likely high-redshift star-forming galaxies hosting a radio-AGN. To understand the nature of these extreme sources at early epochs, I present preliminary analysis from recent sub-millimetre follow-up via the sub-millimetre array (SMA) and the James Clerk Maxwell Telescope (JCMT). In addition, I also discuss the spectroscopic follow-up of the radio-AGN population found in the previous chapter with the upcoming multi-object WEAVE spectrograph on the William Herschel Telescope to study the prevalence of AGN activity as a function of different galaxy properties out to high redshifts. Finally, I also describe plans to compare the observational results found in the previous chapters with predictions from the latest cosmological simulations

The lofar two-metre sky survey: Deep fields data release 1. V. Survey description, source classifications, and host galaxy properties

Source classifications, stellar masses, and star-formation rates are presented for ≈80 000 radio sources from the first data release of the Low Frequency Array Two-metre Sky Survey (LoTSS) Deep Fields, which represents the widest deep radio surv e y ev er undertaken. Using deep multi-wavelength data spanning from the ultraviolet to the far-infrared, spectral energy distribution (SED) fitting is carried out for all of the LoTSS Deep host galaxies using four different SED codes, two of which include modelling of the contributions from an active galactic nucleus (AGN). Comparing the results of the four codes, galaxies that host a radiative AGN are identified, and an optimized consensus estimate of the stellar mass and star-formation rate for each galaxy is derived. Those galaxies with an excess of radio emission o v er that e xpected from star formation are then identified, and the LoTSS Deep sources are divided into four classes: star-forming galaxies, radio-quiet AGN, and radio-loud high-excitation and low-excitation AGN. Ninety-five per cent of the sources can be reliably classified, of which more than two-thirds are star-forming galaxies, ranging from normal galaxies in the nearby Universe to highly-starbursting systems at z > 4. Star-forming galaxies become the dominant population below 150-MHz flux densities of ≈1 mJy, accounting for 90 per cent of sources at S 150MHz ∼100 μJy. Radio-quiet AGN comprise ≈10 per cent of the o v erall population. Results are compared against the predictions of the SKADS and T-RECS radio sky simulations, and improvements to the simulations are suggested

Faint satellite population of the NGC-3175 Group – a Local Group analogue

In this paper, we identify and study the properties of low-mass dwarf satellites of a nearby Local Group analogue – the NGC-3175 galaxy group with the goal of investigating the nature of the lowest mass galaxies and the ‘Missing Satellites’ problem. Deep imaging of nearby groups such as NGC-3175 is one of the only ways to probe these low-mass galaxies which are important for problems in cosmology, dark matter and galaxy formation. We discover 553 candidate dwarf galaxies in the group, the vast majority of which have never been studied before. We obtained R and B band imaging, with the European Southern Observatory 2.2 m, around the central ∼500 kpc region of NGC-3175, allowing us to detect galaxies down to ∼23 mag (MB ∼ −7.7 mag) in the B band. In the absence of spectroscopic information, dwarf members and likely background galaxies are separated using colour, morphology, and surface brightness criteria. We compare the observed size, surface brightness, and mass scaling relations to literature data. The luminosity function with a faint end slope of α = −1.31, is steeper than that observed in the Local Group. In comparison with simulations, we find that our observations are between a pure Λ cold dark matter model and one involving baryonic effects, removing the apparent problem of finding too few satellites as seen around the Milky Way

Cosmic evolution of low-excitation radio galaxies in the LOFAR two-meter sky survey deep fields

Abstract Feedback from low-excitation radio galaxies (LERGs) plays a key role in the lifecycle of massive galaxies in the local Universe; their evolution, and the impact of these active galactic nuclei on early galaxy evolution, however, remain poorly understood. We use a sample of 10 481 LERGs from the first data release of the LOFAR Two-meter Sky Survey Deep Fields, covering ∼ 25 deg2, to present the first measurement of the evolution of the radio luminosity function (LF) of LERGs out to z ∼ 2.5; this shows relatively mild evolution. We split the LERGs into those hosted by quiescent and star-forming galaxies, finding a new dominant population of LERGs hosted by star-forming galaxies at high redshifts. The incidence of LERGs in quiescent galaxies shows a steep dependence on stellar-mass out to z ∼ 1.5, consistent with local Universe measurements of accretion occurring from cooling of hot gas haloes. The quiescent-LERGs dominate the LFs at z < 1, showing a strong decline in space density with redshift, tracing that of the available host galaxies, while there is an increase in the characteristic luminosity. The star-forming LERG LF increases with redshift, such that this population dominates the space densities at most radio-luminosities by z ∼ 1. The incidence of LERGs in star-forming galaxies shows a much weaker stellar-mass dependence, and increases with redshift, suggesting a different fuelling mechanism compared to their quiescent counterparts, potentially associated with the cold gas supply present in the star-forming galaxies

The wide-field, multiplexed, spectroscopic facility WEAVE : survey design, overview, and simulated implementation

Funding for the WEAVE facility has been provided by UKRI STFC, the University of Oxford, NOVA, NWO, Instituto de Astrofísica de Canarias (IAC), the Isaac Newton Group partners (STFC, NWO, and Spain, led by the IAC), INAF, CNRS-INSU, the Observatoire de Paris, Région Île-de-France, CONCYT through INAOE, Konkoly Observatory (CSFK), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Lund University, the Leibniz Institute for Astrophysics Potsdam (AIP), the Swedish Research Council, the European Commission, and the University of Pennsylvania.WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, will see first light in late 2022. WEAVE comprises a new 2-degree field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable 'mini' integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366-959 nm at R ∼ 5000, or two shorter ranges at R ∼ 20,000. After summarising the design and implementation of WEAVE and its data systems, we present the organisation, science drivers and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy's origins by completing Gaia's phase-space information, providing metallicities to its limiting magnitude for ∼ 3 million stars and detailed abundances for ∼ 1.5 million brighter field and open-cluster stars; (ii) survey ∼ 0.4 million Galactic-plane OBA stars, young stellar objects and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey  ∼ 400 neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionised gas in z 1 million spectra of LOFAR-selected radio sources; (viii) trace structures using intergalactic/circumgalactic gas at z > 2. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator.PostprintPeer reviewe

The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design, overview, and simulated implementation

WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, will see first light in late 2022. WEAVE comprises a new 2-degree field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable 'mini' integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366$-$959\,nm at $R\sim5000$, or two shorter ranges at $R\sim20\,000$. After summarising the design and implementation of WEAVE and its data systems, we present the organisation, science drivers and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy's origins by completing Gaia's phase-space information, providing metallicities to its limiting magnitude for $\sim$3 million stars and detailed abundances for $\sim1.5$ million brighter field and open-cluster stars; (ii) survey $\sim0.4$ million Galactic-plane OBA stars, young stellar objects and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey $\sim400$ neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionised gas in $z<0.5$ cluster galaxies; (vi) survey stellar populations and kinematics in $\sim25\,000$ field galaxies at $0.3\lesssim z \lesssim 0.7$; (vii) study the cosmic evolution of accretion and star formation using $>1$ million spectra of LOFAR-selected radio sources; (viii) trace structures using intergalactic/circumgalactic gas at $z>2$. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator.Comment: 41 pages, 27 figures, accepted for publication by MNRA

The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design, overview, and simulated implementation

WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, will see first light in late 2022. WEAVE comprises a new 2-degree field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable 'mini' integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366−959\,nm at R∼5000, or two shorter ranges at R∼20000. After summarising the design and implementation of WEAVE and its data systems, we present the organisation, science drivers and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy's origins by completing Gaia's phase-space information, providing metallicities to its limiting magnitude for ∼3 million stars and detailed abundances for ∼1.5 million brighter field and open-cluster stars; (ii) survey ∼0.4 million Galactic-plane OBA stars, young stellar objects and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey ∼400 neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionised gas in z1 million spectra of LOFAR-selected radio sources; (viii) trace structures using intergalactic/circumgalactic gas at z>2. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator

A machine learning classifier for LOFAR radio galaxy cross-matching techniques

New-generation radio telescopes like LOFAR are conducting extensive sky surveys, detecting millions of sources. To maximise the scientific value of these surveys, radio source components must be properly associated into physical sources before being cross-matched with their optical/infrared counterparts. In this paper, we use machine learning to identify those radio sources for which either source association is required or statistical cross-matching to optical/infrared catalogues is unreliable. We train a binary classifier using manual annotations from the LOFAR Two-metre Sky Survey (LoTSS). We find that, compared to a classification model based on just the radio source parameters, the addition of features of the nearest-neighbour radio sources, the potential optical host galaxy, and the radio source composition in terms of Gaussian components, all improve model performance. Our best model, a gradient boosting classifier, achieves an accuracy of 95 per cent on a balanced dataset and 96 per cent on the whole (unbalanced) sample after optimising the classification threshold. Unsurprisingly, the classifier performs best on small, unresolved radio sources, reaching almost 99 per cent accuracy for sources smaller than 15 arcsec, but still achieves 70 per cent accuracy on resolved sources. It flags 68 per cent more sources than required as needing visual inspection, but this is still fewer than the manually-developed decision tree used in LoTSS, while also having a lower rate of wrongly accepted sources for statistical analysis. The results have an immediate practical application for cross-matching the next LoTSS data releases and can be generalised to other radio surveys.Comment: Accepted for publication in MNRAS. 24 pages, 13 Figures. Submission includes a data table which is also available at https://drive.google.com/file/d/1mwKWJW6oCqh-joNR1uV3PPPYDdYz0n_J/view?usp=sharin