New Understanding of Galaxies, Quasars and Extra-galactic Shock Features using Optical Spectroscopy and Low-frequency Radio Observations

Abstract

The study of Galaxy Formation & Evolution serves as a cornerstone in our quest to unravel the mysteries of the Universe. Galaxies are not only its building blocks, but also the key to understanding its vast complexities. With the advent of new ground-based and space-based facilities, we have acquired multi-wavelength data of millions of galaxies, including active galactic nuclei (AGN), both locally and at higher redshifts, which allow us to trace the evolution of their properties through cosmic time. However, while statistical analyses of large galaxy and AGNsamples across different redshifts are invaluable, they are not the sole avenue for exploration. High-quality data of unique objects also play a pivotal role, offering invaluable insights into the underlying physical processes that shape galaxies into the entities that we observe today. In this thesis, we make use of low-frequency radio observations and optical spectroscopy to demonstrate how such studies deepen our understanding of the Universe. Firstly, we combine 144MHz observations from the LOFAR Two-metre Sky Survey (LoTSS) and spectroscopic information from the 14th data release of the Sloan Digital Sky Survey (SDSS-DR14) to compile the largest sample of uniformly-selected, spectroscopically-confirmed quasars, the most luminous representations of AGN, and use it to investigate whether radioloud (RL) and radio-quiet (RQ) quasars are physically distinct populations. Employing the classical definition of radio-loudness, 𝑅 = log(𝐿1.4GHz/𝐿𝑖), we identify 3,697 RL and 111,132 RQ sources at 0.6 < 𝑧 < 3.4. To study their properties, we develop a new rest-frame spectral stacking algorithm, designed with forthcoming massively-multiplexed spectroscopic surveys in mind, and use it to create high signal-to-noise composite spectra of each class, matched in redshift and absolute 𝑖-band magnitude. We show that RL quasars have redder continuum and enhanced [O ii] emission compared to their RQ counterparts. These results persist when additionally matching in black hole mass, suggesting that this parameter is not the defining factor in making a RL QSO. We find that these features are not gradually varying as a function of radio-loudness but are maintained even when probing deeper into the RQ population, indicating that a clear-cut division in radio-loudness is not apparent. Upon examining the star formation rates (SFRs) inferred from the [O ii] emission line, with the contribution from AGN removed using the [Ne v] line, we find that RL quasars have a significant excess of star-formation relative to RQ quasars out to 𝑧 = 1.9 at least. Given our findings, we suggest that RL sources reside in systems which preferably have a rich gas supply and rapidly spinning black holes, or represent an earlier obscured phase of QSO evolution. We then present a detailed study of Stephan’s Quintet (SQ), an interacting nearby group of galaxies which contains a large-scale shock front. This extragalactic shock feature is thought to be the result of an ongoing collision between NGC 7318b and the complex intergalactic medium of SQ, thereby allowing us to study the effects of galaxy mergers and interactions. By combining the integral field spectroscopy from the first light data of the new William Herschel Telescope Enhanced Area Velocity Explorer (WEAVE) with new 144MHz observations from LoTSS, plus archival data from the Very Large Array and the James Webb Space Telescope (JWST), we are able to see SQ in a new light. Harnessing WEAVE large integral field unit’s (LIFU) field of view (90 × 78 arcsec2), spectral resolution (𝜆/Δ𝜆 ∼ 2500) and continuous wavelength coverage across the optical band, we perform robust emission line fitting. This allows us to dynamically infer the location of the shock region with higher precision than previously possible. We find that the ionised gas in the shock is of low density (𝑛𝑒 < 140 cm−3), with a low temperature 𝑇𝑒 < 14,000K and metallicity consistent with the surrounding hot X-ray plasma. The Mach number (M ∼ 2.2) of the shock suggests that it is relatively weak and is not efficient in accelerating particles. Instead, it has adiabatically compressed the medium, leading to a boost in the radio emission by a factor of ∼ 10, during which dust has survived the collision event. This is further demonstrated by comparing the extinction distribution seen with WEAVE to the molecular gas and hot dust observed with JWST. Finally, we use the most sensitive low-frequency radio data from the LoTSS Deep Fields and spectroscopic information from the Dark Energy Spectroscopic Instrument (DESI) to classify 2,033 radio sources as star-forming galaxies (SFGs), radio-quiet AGN (RQ AGN), emission-line low-excitation radio galaxies (LINELERGs) and high excitation radio-galaxies (HERGs). This was done by combining two diagnostics: (i) the identification of a radio excess compared to star-forming processes as traced by the Balmer lines, and (ii) the use of emission line ratios to separate sources producing higher ionisation energies than those produced from stellar radiation alone. These spectroscopic classifications allow us to evaluate the performance of recent photometric classifications, created for the same sample of radio sources, by using the deepest wide-field optical, near- and mid-infrared data available. This is important because while optical spectroscopy is widely regarded as a highly reliable method to classify sources, it is not as readily accessible as photometry. Preliminary results show that while photometric classifications can successfully recover the SFG class (above 90 per cent agreement with the spectroscopic classifications), there are some discrepancies involved amongst the three AGN classes, which require further investigation.