Bright Quasars Research Articles

Fundamental physics with ESPRESSO: Precise limit on variations in the fine-structure constant towards the bright quasar HE 0515−4414

The strong intervening absorption system at redshift 1.15 towards the very bright quasar HE 0515−4414 is the most studied absorber for measuring possible cosmological variations in the fine-structure constant, α. We observed HE 0515−4414 for 16.1 h with the Very Large Telescope and present here the first constraint on relative variations in α with parts-per-million (ppm) precision from the new ESPRESSO spectrograph: Δα/α = 1.3 ± 1.3stat ± 0.4sys ppm. The statistical uncertainty (1σ) is similar to the ensemble precision of previous large samples of absorbers and derives from the high signal-to-noise ratio achieved (≈105 per 0.4 km s−1 pixel). ESPRESSO’s design, and the calibration of our observations with its laser frequency comb, effectively removed wavelength calibration errors from our measurement. The high resolving power of our ESPRESSO spectrum (R = 145 000) enabled the identification of very narrow components within the absorption profile, allowing a more robust analysis of Δα/α. The evidence for the narrow components is corroborated by their correspondence with previously detected molecular hydrogen and neutral carbon. The main remaining systematic errors arise from ambiguities in the absorption profile modelling, effects from redispersing the individual quasar exposures, and convergence of the parameter estimation algorithm. All analyses of the spectrum, including systematic error estimates, were initially blinded to avoid human biases. We make our reduced ESPRESSO spectrum of HE 0515−4414 publicly available for further analysis. Combining our ESPRESSO result with 28 measurements, from other spectrographs, in which wavelength calibration errors have been mitigated yields a weighted mean Δα/α = −0.5 ± 0.5stat ± 0.4sys ppm at redshifts 0.6−2.4.

Emission-line Variability during a Nonthermal Outburst in the Gamma-Ray Bright Quasar 1156+295

Abstract We present multi-epoch optical spectra of the γ-ray bright blazar 1156+295 (4C +29.45, Ton 599) obtained with the 4.3 m Lowell Discovery Telescope. During a multiwavelength outburst in late 2017, when the γ-ray flux increased to 2.5 × 10−6 phot cm−2 s−1 and the quasar was first detected at energies ≥100 GeV, the flux of the Mg ii λ2798 emission line changed, as did that of the Fe emission complex at shorter wavelengths. These emission-line fluxes increased along with the highly polarized optical continuum flux, which is presumably synchrotron radiation from the relativistic jet, with a relative time delay of ≲2 weeks. This implies that the line-emitting clouds lie near the jet, which points almost directly toward the line of sight. The emission-line radiation from such clouds, which are located outside the canonical accretion-disk related broad-line region, may be a primary source of seed photons that are up-scattered to γ-ray energies by relativistic electrons in the jet.

Co-evolution of massive black holes and their host galaxies at high redshift: discrepancies from six cosmological simulations and the key role of JWST

ABSTRACT The James Webb Space Telescope will have the power to characterize high-redshift quasars at z ≥ 6 with an unprecedented depth and spatial resolution. While the brightest quasars at such redshift (i.e. with bolometric luminosity $L_{\rm bol}\geqslant 10^{46}\, \rm erg/s$) provide us with key information on the most extreme objects in the Universe, measuring the black hole (BH) mass and Eddington ratios of fainter quasars with $L_{\rm bol}= 10^{45}-10^{46}\, \rm erg\,s^{ -1}$ opens a path to understand the build-up of more normal BHs at z ≥ 6. In this paper, we show that the Illustris, TNG100, TNG300, Horizon-AGN, EAGLE, and SIMBA large-scale cosmological simulations do not agree on whether BHs at z ≥ 4 are overmassive or undermassive at fixed galaxy stellar mass with respect to the MBH − M⋆ scaling relation at z = 0 (BH mass offsets). Our conclusions are unchanged when using the local scaling relation produced by each simulation or empirical relations. We find that the BH mass offsets of the simulated faint quasar population at z ≥ 4, unlike those of bright quasars, represent the BH mass offsets of the entire BH population, for all the simulations. Thus, a population of faint quasars with $L_{\rm bol}= 10^{45}-10^{46}\, \rm erg\,s^{ -1}$ observed by JWST can provide key constraints on the assembly of BHs at high redshift. Moreover, this will help constraining the high-redshift regime of cosmological simulations, including BH seeding, early growth, and co-evolution with the host galaxies. Our results also motivate the need for simulations of larger cosmological volumes down to z ∼ 6, with the same diversity of subgrid physics, in order to gain statistics on the most extreme objects at high redshift.

The Origin and Evolution of Giant Radio Galaxies

Giant radio galaxies are arguably the least understood of jetted active galactic nuclei (AGN). We propose that radio galaxies are the product of large mergers that do not involve radio galaxies or radio quasars, such as in merging spiral galaxies, while giant radio galaxies emerge from a merger involving a parent that in the not-too-distant past harbored a radio galaxy. Predictions following from this are an upper limit to the number fraction of giant radio galaxies to radio galaxies, lower average redshift for giant radio galaxies, a higher incidence of high excitation for giant radio galaxies compared with radio galaxies, and lower average prograde black hole spin values for giant radio galaxies compared to radio galaxies and to bright radio quiet quasars.

The nature of sub-millimetre galaxies II: an ALMA comparison of SMG dust heating mechanisms

ABSTRACT We compare the contribution of active galactic nuclei (AGNs) and star formation towards dust heating in sub-mm galaxies (SMGs). We have used ALMA at 0.1-arcsec resolution to image a complete flux-limited sample of seven sub-mm sources previously shown to have spectral energy distributions that were as well-fitted by obscured AGN as star-forming galaxy templates. Indeed, two sub-mm sources were known to be quasars from their absorbed X-ray emission. We find the sub-mm sizes of all SMGs to be small (≈1−2 kpc) and generally ∼3 times smaller than any host detected in the near-infrared (NIR). In all cases, the five SMGs are comparable in sub-mm size to the two known quasars and four z ≈ 6 quasars, also observed with ALMA. We detect no evidence of diffuse spiral arms in this complete sample. We then convert the far-infrared (FIR) luminosities to star formation rate (SFR) surface densities and find that the SMGs occupy the same range as the known quasars in our sample. We conclude that in terms of sub-mm size, extent relative to host and SFR density as well as luminosity and mid-IR (MIR) colour, there is little distinction between the SMGs and sub-mm bright quasars. Finally, we present preliminary evidence that SMGs with higher MIR luminosities and sub-mm loud quasars tend to have dust components that range to hotter temperatures than their less luminous SMG counterparts. In light of these results, we continue to suggest that luminous SMGs may host dust-absorbed quasars that may simultaneously dominate the FIR and hard X-ray backgrounds.

Near-infrared spectroscopy of extreme BAL QSOs from the QUBRICS bright quasar survey

ABSTRACT We report on the spectral confirmation of 18 quasi-stellar object (QSO) candidates from the QUasars as BRIght beacons for Cosmology in the Southern hemisphere (QUBRICS) survey, previously observed in the optical band, for which we acquired new spectroscopic data in the near-infrared band with the Folded-port InfraRed Echellette (FIRE) spectrograph at the Magellan Baade telescope. In most cases, further observations were prompted by the peculiar nature of the targets, whose optical spectra displayed unexpected absorption features. All candidates have been confirmed as bona fide QSOs, with average emission redshift z ≃ 2.1. The analysis of the emission and absorption features in the spectra, performed with astrocook and QSFit, reveals that the large majority of these objects are broad absorption line (BAL) QSOs, with almost half of them displaying strong Fe ii absorption (typical of the so-called FeLoBAL QSOs). The detection of such a large fraction of rare objects (which are estimated to account for less than 1 per cent of the general QSO population) is interpreted as an unexpected (yet favourable) consequence of the particular candidate selection procedure adopted within the QUBRICS survey. The measured properties of FeLoBAL QSOs observed so far provide no evidence that they are a manifestation of a particular stage in active galactic nucleus (AGN) evolution. In this paper, we present an explorative analysis of the individual QSOs, to serve as a basis for a further, more detailed investigation.

Spatially resolved Lyman-α emission around radio bright quasars

ABSTRACT We use Southern African Large Telescope (SALT) to perform long-slit spectroscopic observations of 23 newly discovered radio-loud quasars (RLQs) at 2.7 < z < 3.3. The sample consists of powerful AGN brighter than 200 mJy at 1.4 GHz and is selected on the basis of mid-infrared colours i.e. unbiased to the presence of dust. We report seven confirmed and five tentative detections of diffuse Ly α emission in the sample. We present the properties of diffuse Ly α emission and discuss in detail its relationship to different quasar properties. We find strong dependence of Ly α halo detection rate on the extent of radio source, spectral luminosity of RLQ at 420 MHz (L420MHz), presence of associated C iv absorption and nuclear He ii emission line equivalent width. As seen in previous surveys, the full width at half-maximum of diffuse Ly α emission in the case of confirmed detections are much higher (i.e. >1000 km s−1 in all, except one). Using the samples of high-z radio-loud quasars and galaxies from literature, we confirm the correlation between the Ly α halo luminosity and its size with $L_{\rm 420\, \rm MHz}$. The same quantities are found to be correlating weakly with the projected linear size of the radio emission. Our sample is the second largest sample of RLQs being studied for the presence of diffuse Ly α emission and fills in a redshift gap between previous such studies. Integral field spectroscopy is required to fully understand the relationship between the large-scale radio emission and the overall distribution, kinematics, and overdensity of Ly α emission in the field of these RLQs.

Detection of a Multiphase Intragroup Medium: Results from the COS-IGrM Survey

Abstract We present the results of the Cosmic Origins Spectrograph-Intragroup Medium (COS-IGrM) Survey that used the COS on the Hubble Space Telescope to observe a sample of 18 UV bright quasars, each probing the IGrM of a galaxy group. We detect Lyα, C ii, N v, Si ii, Si iii, and O vi in multiple sightlines. The highest ionization species detected in our data is O vi, which was detected in eight out of 18 quasar sightlines. The wide range of ionization states observed provide evidence that the IGrM is patchy and multiphase. We find that the O vi detections generally align with radiatively cooling gas between 105.8 and 106 K. The lack of O vi detections in 10 of the 18 groups illustrates that O vi may not be the ideal tracer of the volume filling component of the IGrM. Instead, it either exists at trace levels in a hot IGrM or is generated in the boundary between the hotter IGrM and cooler gas.

High-redshift quasars at z ≥ 3 – I. Radio spectra

ABSTRACT We present the radio properties of optically selected quasars with z ≥ 3. The complete sample consists of 102 quasars with a flux density level S1.4 ≥ 100 mJy in a declination range −35° ≤ Dec. ≤ +49°. The observations were obtained in 2017–2020 using the radio telescope RATAN-600. We measured flux densities at six frequencies 1.2, 2.3, 4.7, 8.2, 11.2, and 22 GHz quasi-simultaneously with uncertainties of 9–31 per cent. The detection rate is 100, 89, and 46 per cent at 4.7, 11.2, and 22 GHz, respectively. We have analysed the averaged radio spectra of the quasars based on the RATAN and literature data. We classify 46 per cent of radio spectra as peaked-spectrum, 24 per cent as flat, and none as ultra-steep spectra (α ≤ −1.1). The multifrequency data reveal that a peaked spectral shape (PS) is a common feature for bright high-redshift quasars. This indicates the dominance of bright compact core emission and the insignificant contribution of extended optically thin kpc-scale components in observed radio spectra. Using these new radio data, the radio loudness log R was estimated for 71 objects with a median value of 3.5, showing that the majority of the quasars are highly radio-loud with log R > 2.5. We have not found any significant correlation between z and α. Several new megahertz- peaked spectrum (MPS) and gigahertz- peaked spectrum (GPS) candidates are suggested. Further studies of their variability and additional low-frequency observations are needed to classify them precisely.

MUSEQuBES: characterizing the circumgalactic medium of redshift ≈3.3 Ly α emitters

ABSTRACT We present the first characterization of the circumgalactic medium of Ly α emitters (LAEs), using a sample of 96 z ≈ 3.3 LAEs detected with the VLT/MUSE in fields centred on eight bright background quasars. The LAEs have low Ly α luminosities ($\sim 10^{42}\, \text{erg}\, \text{s}^{-1}$) and star formation rates (SFRs) $\sim 1~\text{M}_\odot \, \text{yr}^{-1}$, which for main-sequence galaxies correspond to stellar masses of only $\sim 10^{8.6}\, \text{M}_\odot$. The median transverse distance between the LAEs and the quasar sightlines is 165 proper kpc (pkpc). We stacked the high-resolution quasar spectra and measured significant excess H i and C iv absorption near the LAEs out to 500 $\text{km}\, \text{s}^{-1}$ and at least ≈250 pkpc (corresponding to ≈7 virial radii). At $\lesssim 30~\text{km}\, \text{s}^{-1}$ from the galaxies, the median H i and C iv optical depths are enhanced by an order of magnitude. The absorption is significantly stronger around the ≈1/3 of our LAEs that are part of ‘groups’, which we attribute to the large-scale structures in which they are embedded. We do not detect any strong dependence of either the H i or C iv absorption on transverse distance (over the range ≈50–250 pkpc), redshift, or the properties of the Ly α emission line (luminosity, full width at half-maximum, or equivalent width). However, for H i, but not C iv, the absorption at $\lesssim 100\, \text{km}\, \text{s}^{-1}$ from the LAE does increase with the SFR. This suggests that LAEs surrounded by more H i tend to have higher SFRs.

Observing the host galaxies of high-redshift quasars with JWST: predictions from the BlueTides simulation

ABSTRACT The bright emission from high-redshift quasars completely conceals their host galaxies in the rest-frame ultraviolet/optical, with detection of the hosts in these wavelengths eluding even the Hubble Space Telescope (HST) using detailed point spread function (PSF) modelling techniques. In this study, we produce mock images of a sample of z = 7 quasars extracted from the BlueTides simulation, and apply Markov chain Monte Carlo-based PSF modelling to determine the detectability of their host galaxies with the James Webb Space Telescope (JWST). While no statistically significant detections are made with HST, we predict that at the same wavelengths and exposure times JWST NIRCam imaging will detect $\sim 50{{\ \rm per\ cent}}$ of quasar host galaxies. We investigate various observational strategies, and find that NIRCam wide-band imaging in the long-wavelength filters results in the highest fraction of successful quasar host detections, detecting $\gtrsim 80{{\ \rm per\ cent}}$ of the hosts of bright quasars in exposure times of 5 ks. Exposure times of ≳ 5 ks are required to detect the majority of host galaxies in the NIRCam wide-band filters, however, even 10 ks exposures with MIRI result in $\lesssim 30{{\ \rm per\ cent}}$ successful host detections. We find no significant trends between galaxy properties and their detectability. The PSF modelling can accurately recover the host magnitudes, radii, and spatial distribution of the larger scale emission, when accounting for the central core being contaminated by residual quasar flux. Care should be made when interpreting the host properties measured using PSF modelling.

Sharpening quasar absorption lines with ESPRESSO. Temperature of warm gas at z ~ 2, constraints on the Mg isotopic ratio, and structure of cold gas at z ~ 0.5

Aims. We aim to study several key physical properties of quasar absorption-line systems that are subtly encoded in their absorption profiles and have not yet been thoroughly investigated or constrained. Methods. We analysed a high-resolution (R = 140 000) spectrum of the bright quasar HE 0001−2340 (zem = 2.26) obtained with ESPRESSO, which was recently installed at the Very Large Telescope. We analysed three systems at z = 0.45, z = 1.65, and z = 2.19 using multiple-component Voigt-profile fitting. We also compared our spectrum with those obtained with VLT/UVES, covering a total period of 17 years. Results. We disentangle turbulent and thermal broadening in many components spread over about 400 km s−1 in the z ≈ 2.19 sub-damped Lyman-α system. We derive an average temperature of 16 000 ± 1300 K, which is about twice the canonical value of the warm neutral medium in the Galactic interstellar medium (ISM). A comparison with other high-z, low-metallicity absorbers reveals an anti-correlation between gas temperature and total H I column density. Although requiring confirmation, this could be the first observational evidence of a thermal decrease with galactocentric distance; in other words, we may be witnessing a thermal transition between the circumgalactic medium and the cooler ISM. We revisit the Mg isotopic ratios at z = 0.45 and z = 1.65 and constrain them to be ξ = (26Mg + 25Mg)/24Mg < 0.6 and < 1.4 in these two systems, respectively. These values are consistent with the standard solar ratio; that is, we do not confirm strong enhancement of heavy isotopes previously inferred from UVES data. Finally, we confirm the partial coverage of the quasar emission-line region by a Fe I-bearing cloud in the z = 0.45 system and present evidence for velocity substructure of the gas that has Doppler parameters of the order of only ∼0.3 km s−1. This agrees well with the low kinetic temperature of T ∼ 100 K inferred from modelling of the gas physical conditions. Conslusions. This work demonstrates the unique insight provided by high-fidelity, high-resolution optical spectrographs on large telescopes when used to investigate the thermal state of the gas in and around galaxies as well as its spatial and velocity structure on small scales, and to constrain the associated stellar nucleosynthetic history.

The Luminosity Function of Bright QSOs at z ∼ 4 and Implications for the Cosmic Ionizing Background

Abstract Based on results by recent surveys, the number of bright quasars at redshifts z > 3 is being constantly revised upward. The current consensus is that at bright magnitudes (M 1450 ≤ −27) the number densities of such sources could have been underestimated by a factor of 30%–40%. In the framework of the QUBRICS survey, we identified 58 bright QSOs at 3.6 ≤ z ≤ 4.2, with magnitudes i psf ≤ 18, in an area of 12400 deg2. The uniqueness of our survey is underlined by the fact that it allows us, for the first time, to extend the sampled absolute magnitude range up to M 1450 = −29.5. We derived a bright-end slope of β = −4.025 and a space density at 〈M 1450〉 = −28.75 of 2.61 × 10−10 Mpc−3 comoving, after taking into account the estimated incompleteness of our observations. Taking into account the results of fainter surveys, active galactic nuclei (AGNs) brighter than M 1450 = −23 could produce at least half of the ionizing emissivity at z ∼ 4. Considering a mean escape fraction of 0.7 for the QSO and AGN population, combined with a mean free path of 41.3 proper Mpc at z = 3.9, we derive a photoionization rate of , produced by AGNs at M 1450 < −18, that is, ∼100% of the measured ionizing background at z ∼ 4.

Forming massive seed black holes in high-redshift quasar host progenitors

ABSTRACT The presence of massive black holes (BHs) with masses of the order of $10^9\, {\rm M_\odot }$, powering bright quasars when the Universe was less than 1 Gyr old, poses strong constraints on their formation mechanism. Several scenarios have been proposed to date to explain massive BH formation, from the low-mass seed BH remnants of the first generation of stars to the massive seed BHs resulting from the rapid collapse of massive gas clouds. However, the plausibility of some of these scenarios to occur within the progenitors of high-z quasars has not yet been thoroughly explored. In this work, we investigate, by combining dark-matter only N-body simulations with a semi-analytic framework, whether the conditions for the formation of massive seed BHs from synchronized atomic-cooling halo pairs and/or dynamically heated (DH) mini-haloes are fulfilled in the overdense regions where the progenitors of a typical high-redshift quasar host form and evolve. Our analysis shows that the peculiar conditions in such regions, i.e. strong halo clustering and high star formation rates, are crucial to produce a non-negligible number of massive seed BH host candidates: we find ≈1400 DH metal-free mini-haloes, including one of these which evolves to a synchronized pair and ends up in the massive quasar-host halo by z = 6. This demonstrates that the progenitors of high-redshift quasar host haloes can harbour early massive seed BHs. Our results further suggest that multiple massive seed BHs may form in or near the quasar host’s progenitors, potentially merging at lower redshifts and yielding gravitational wave events.

MUSE analysis of gas around galaxies (MAGG) – III. The gas and galaxy environment of z = 3–4.5 quasars

ABSTRACT We present a study of the environment of 27 z = 3–4.5 bright quasars from the MUSE Analysis of Gas around Galaxies (MAGG) survey. With medium-depth Multi Unit Spectroscopic Explorer (MUSE) observations (4 h on target per field), we characterize the effects of quasars on their surroundings by studying simultaneously the properties of extended gas nebulae and Ly α emitters (LAEs) in the quasar host haloes. We detect extended (up to ≈100 kpc) Ly α emission around all MAGG quasars, finding a very weak redshift evolution between z = 3 and z = 6. By stacking the MUSE datacubes, we confidently detect extended emission of C iv and only marginally detect extended He ii up to ≈40 kpc, implying that the gas is metal enriched. Moreover, our observations show a significant overdensity of LAEs within 300 $\rm km~s^{-1}$ from the quasar systemic redshifts estimated from the nebular emission. The luminosity functions and equivalent width distributions of these LAEs show similar shapes with respect to LAEs away from quasars suggesting that the Ly α emission of the majority of these sources is not significantly boosted by the quasar radiation or other processes related to the quasar environment. Within this framework, the observed LAE overdensities and our kinematic measurements imply that bright quasars at z = 3–4.5 are hosted by haloes in the mass range $\approx 10^{12.0}\small{--}10^{12.5}~\rm M_\odot$.

Infrared emission of z ∼ 6 galaxies: AGN imprints

ABSTRACT We investigate the infrared (IR) emission of high-redshift (z ∼ 6), highly star-forming (${{\rm SFR}\gt 100\,{\rm M}_{\odot }\, {\rm yr}^{-1}}$) galaxies, with/without active galactic nuclei (AGN), using a suite of cosmological simulations featuring dust radiative transfer. Synthetic spectral energy distributions (SEDs) are used to quantify the relative contribution of stars/AGN to dust heating. In dusty (Md ≳ 3 × 107 M⊙) galaxies, ≳50–90 per cent of the ultraviolet (UV) radiation is obscured by dust inhomogeneities on scales ≳100 pc. In runs with AGN, a clumpy, warm (≈250 K) dust component coexists with a colder (≈60 K) and more diffuse one, heated by stars. Warm dust provides up to ${50 {{\ \rm per\ cent}}}$ of the total infrared (IR) luminosity, but only ${\lesssim}0.1 {{\ \rm per\ cent}}$ of the total mass content. The AGN boosts the MIR flux by 10–100 times with respect to star-forming galaxies, without significantly affecting the far-IR. Our simulations successfully reproduce the observed SED of bright (MUV ∼ −26) z ∼ 6 quasars, and show that these objects are part of complex, dust-rich merging systems, containing multiple sources (accreting black holes and/or star-forming galaxies) in agreement with recent HST and ALMA observations. Our results show that the proposed ORIGINS missions will be able to investigate the mid-IR (MIR) properties of dusty star-forming galaxies and to obtain good-quality spectra of bright quasars at z ∼ 6. Finally, the MIR-to-FIR flux ratio of faint (MUV ∼ −24) AGN is >10 times higher than for normal star-forming galaxies. This implies that combined JWST/ORIGINS/ALMA observations will be crucial to identify faint and/or dust-obscured AGN in the distant Universe.

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Galaxy-scale ionised winds driven by ultra-fast outflows in two nearby quasars

We used MUSE adaptive optics data in narrow field mode to study the properties of the ionised gas in MR 2251−178 and PG 1126−041, two nearby (z ≃ 0.06) bright quasars (QSOs) hosting sub-pc scale ultra-fast outflows (UFOs) detected in the X-ray band. We decomposed the optical emission from diffuse gas into a low- and a high-velocity components. The former is characterised by a clean, regular velocity field and a low (∼80 km s−1) velocity dispersion. It traces regularly rotating gas in PG 1126−041, while in MR 2251−178 it is possibly associated with tidal debris from a recent merger or flyby. The other component is found to be extended up to a few kpc from the nuclei, and shows a high (∼800 km s−1) velocity dispersion and a blue-shifted mean velocity, as is expected from outflows driven by active galactic nuclei (AGN). We estimate mass outflow rates up to a few M⊙ yr−1 and kinetic efficiencies LKIN/LBOL between 1−4 × 10−4, in line with those of galaxies hosting AGN of similar luminosities. The momentum rates of these ionised outflows are comparable to those measured for the UFOs at sub-pc scales, which is consistent with a momentum-driven wind propagation. Pure energy-driven winds are excluded unless about 100× additional momentum is locked in massive molecular winds. In comparing the outflow properties of our sources with those of a small sample of well-studied QSOs hosting UFOs from the literature, we find that winds seem to systematically lie either in a momentum-driven or an energy-driven regime, indicating that these two theoretical models bracket the physics of AGN-driven winds very well.

Evolution of stars near bright quasars

Evolution of stars near bright quasars

Revealing the impact of quasar luminosity on giant Ly α nebulae

ABSTRACT We present the results from a MUSE survey of twelve z ≃ 3.15 quasars, which were selected to be much fainter (20 < iSDSS < 23) than in previous studies of giant Ly α nebulae around the brightest quasars (16.6 < iAB < 18.7). We detect H i Ly α nebulae around 100 per cent of our target quasars, with emission extending to scales of at least 60 physical kpc, and up to 190 pkpc. We explore correlations between properties of the nebulae and their host quasars, with the goal of connecting variations in the properties of the illuminating QSO to the response in nebular emission. We show that the surface brightness profiles of the nebulae are similar to those of nebulae around bright quasars, but with a lower normalization. Our targeted quasars are on average 3.7 mag (≃30 times) fainter in UV continuum than our bright reference sample, and yet the nebulae around them are only 4.3 times fainter in mean Ly α surface brightness, measured between 20 and 50 pkpc. We find significant correlations between the surface brightness of the nebula and the luminosity of the quasar in both UV continuum and Ly α. The latter can be interpreted as evidence for a substantial contribution from unresolved inner parts of the nebulae to the narrow components seen in the Ly α lines of some of our faint quasars, possibly from the inner circumgalactic medium or from the host galaxy’s interstellar medium.