Related papers: On the Search for Quasar Light Echoes
Where do quasars reside? Are quasars located in environments similar to those of typical L* galaxies, and, if not, how do they differ? An answer to this question will help shed light on the triggering process of quasar activity. We use the…
With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar's environment in absorption. We use a sample of 650 projected quasar pairs to study the HI Lya absorption transverse…
We present the results of microlens ray-tracing simulations showing the effect of absorbing material between a source quasar and a lensing galaxy in a gravitational lens system. We find that, in addition to brightness fluctuations due to…
Nearly 300 quasars have now been found at $z >$ 6, including nine at $z >$ 7. They are thought to form from the collapse of supermassive primordial stars to 10$^4$ - 10$^5$ M$_{\odot}$ black holes at $z \sim$ 20 - 25, which then rapidly…
Narrowband observations have detected no Ly-alpha emission within a 70 pMpc^3 volume centered on the z = 2.168 quasar PKS 0424-131. This is in contrast to surveys of Ly-alpha emitters in the field at similar redshifts and flux limits, which…
For 15 bright (V<17.5), high redshift (z>3) quasars, we have obtained IR spectra and photometry, and optical spectrophotometry and photometry, which we use to construct their rest-frame 1285-5100 AA spectral energy distributions (SEDs).…
The Lyman-alpha forest at z >~ 5.5 shows strong scatter in the mean transmission even when smoothed over very large spatial scales, >~ 50 Mpc/h. This has been interpreted as a signature of strongly fluctuating radiation fields, or patchy…
We have performed a search over 3440 deg$^2$ of Epoch 1 (2017-2019) of the Very Large Array Sky Survey to identify unobscured quasars in the optical ($0.2 < z < 3.2$) and obscured active galactic nuclei (AGN) in the infrared that have…
The extragalactic magnetic field could be detected by searching for signatures of the electromagnetic cascade initiated by high-energy photons on the intergalactic radiation and deflected by the field. This process produces a time delay and…
Significant clustering around the rarest luminous quasars is a feature predicted by dark matter theory combined with number density matching arguments. However, this expectation is not reflected by observations of quasars residing in a…
The radio-loud/radio-quiet (RL/RQ) dichotomy in quasars is still an open question. Although it is thought that accretion onto supermassive black holes in the centre the host galaxies of quasars is responsible for some radio continuum…
We present the first results from our survey of intervening and proximate Lyman limit systems (LLSs) at $z$$\sim$2.0-2.5 using the Wide Field Camera 3 on-board the Hubble Space Telescope. The quasars in our sample are projected pairs with…
By monitoring $10^6$ quasars one could search for lensing by stars and Massive Compact Halo Objects (Machos) out to redshifts $z\sim 4$. If Machos have a present cosmological density $\Omega_{L,0}=1\%$, then the expected event rate is…
We present the results from a MUSE survey of twelve $z\simeq3.15$ quasars, which were selected to be much fainter (20<i<23) than in previous studies of Giant Ly$\alpha$ Nebulae around the brightest quasars (16.6<i<18.7). We detect HI…
Over the past few years, we have been collecting data with the Nordic Optical Telescope (NOT) on the galaxy environments around active galactic nuclei (AGN). Here we present some results from a sample of 21 radio-loud and 20 radio-quiet…
The presence of excess scatter in the Ly-$\alpha$ forest at $z\sim 5.5$, together with the existence of sporadic extended opaque Gunn-Peterson troughs, has started to provide robust evidence for a late end of hydrogen reionisation. However,…
The neutral hydrogen and the ionized helium absorption in the spectra of quasars are unique probes of structure in the early universe. We present Far-Ultraviolet Spectroscopic Explorer observations of the line of sight to the quasar…
Cool ($T\sim10^4$~K) gas is commonly observed around $z>2$ quasars as traced by extended Ly$\alpha$ emission. These large-scale nebulae are usually studied using circularly averaged surface brightness profiles, which suppress information on…
We investigate the X-ray properties of the Parkes sample of flat-spectrum radio sources using data from the ROSAT All-Sky Survey and archival pointed PSPC observations. In total, 163 of the 323 sources are detected. For the remaining 160…
The intergalactic medium (IGM) around quasars is shaped by their dense environments and by their excess ionizing radiation, forming a "quasar proximity zone" whose size and anisotropy depend on the quasar's halo mass, luminosity, age, and…