Characterizing the Circumgalactic Medium in Emission

In this thesis I focus on understanding the physical properties of extended Lyman-alpha emitting gas at high redshift, from the circumgalactic medium (CGM) and intergalactic medium (IGM). First, I investigate the physics of giant, luminous Lyman-alpha nebulae known as Lyman alpha blobs (LABs). The mechanism powering the emission of LABs is poorly understood, although they are now often associated with active galactic nuclei (AGN). Our poor understanding results primarily from the lack of information beyond the Lyman-alpha line. Targeting 13 LABs, I thus conduct a deep search for the HeII1640 and CIV1549 lines, which probe the volume density, the metallicity, and the ionization level within the nebulae. Although I did not detect any emission down to unprecedented surface brightness levels, I show that LABs could be still consistent with photoionization from an obscured AGN. Second, I led a narrow-band imaging survey (FLASHLIGHT) targeting the Lyman-alpha line around 25 quasars at redshift of about 2. FLASHLIGHT is the deepest line imaging study ever undertaken around quasars, and aims to uncover the emission from their CGM. During this campaign, I took part in the discovery of the largest (about 500 kpc) Lyman-alpha nebula known at high redshift: UM287. Its bright large scale emission is in tension with our current understanding of the physical state of gas in massive dark matter halos. As for the LABs, I obtained even deeper HeII and CIV spectroscopy of UM287, and again failed to detect emission. Using photoionization modeling I show that the extended Lyman-alpha emission is likely arising from remarkably dense and compact clouds, which are clearly unresolved in current cosmological simulations. Lastly, by stacking the FLASHLIGHT data, I obtain the first measurement of the average Lyman-alpha emission from the typical quasar CGM. Combined with absorption line measurements on the total CGM gas mass, this provides the first measurement of the gas density in the quasar CGM. Given this low surface brightness, the next generation of telescopes is probably needed to routinely detect CGM emission around individual quasars. Nevertheless, this thesis paves the way for the understanding of the CGM in emission, which is timely given the upcoming instruments

Clustering of Lyman-alpha Emitters Around Quasars at z∼4z\sim4

The strong observed clustering of $z>3.5$ quasars indicates they are hosted by massive ($M_{\rm{halo}}\gtrsim10^{12}\,h^{-1}\,\rm{M_{\odot}}$) dark matter halos. Assuming quasars and galaxies trace the same large-scale structures, this should also manifest as strong clustering of galaxies around quasars. Previous work on high-redshift quasar environments, mostly focused at $z>5$, have failed to find convincing evidence for these overdensities. Here we conduct a survey for Lyman alpha emitters (LAEs) in the environs of 17 quasars at $z\sim4$ probing scales of $R\lesssim7\,h^{-1}\,{\rm{Mpc}}$. We measure an average LAE overdensity around quasars of 1.4 for our full sample, which we quantify by fitting the quasar-LAE cross-correlation function. We find consistency with a power-law shape with correlation length of $r^{QG}_{0}=2.78^{+1.16}_{-1.05}\,h^{-1}\,{\rm{cMpc}}$ for a fixed slope of $\gamma=1.8$. We also measure the LAE auto-correlation length and find $r^{GG}_{0}=9.12^{+1.32}_{-1.31}\,h^{-1}$\,cMpc ($\gamma=1.8$), which is $3.3$ times higher than the value measured in blank fields. Taken together our results clearly indicate that LAEs are significantly clustered around $z\sim4$ quasars. We compare the observed clustering with the expectation from a deterministic bias model, whereby LAEs and quasars probe the same underlying dark matter overdensities, and find that our measurements fall short of the predicted overdensities by a factor of 2.1. We discuss possible explanations for this discrepancy including large-scale quenching or the presence of excess dust in galaxies near quasars. Finally, the large cosmic variance from field-to-field observed in our sample (10/17 fields are actually underdense) cautions one from over-interpreting studies of $z\sim6$ quasar environments based on a single or handful of quasar fields.Comment: 19 pages, 12 figures, submitted to the Ap

AGN radiation imprints on the circumgalactic medium of massive galaxies

Active Galactic Nuclei (AGN) in cosmological simulations generate explosive feedback that regulates star formation in massive galaxies, modifying the gas phase structure out to large distances. Here, we explore the direct effects that AGN radiation has on gas heating and cooling within one high-resolution $z=3$ dark matter halo as massive as a quasar host ($M_{\rm h}=$10$^{\rm 12.5}$M$_{\rm\odot}$), run without AGN feedback. We assume AGN radiation to impact the circumgalactic medium (CGM) anisotropically, within a bi-cone of angle $\alpha$. We find that even a relatively weak AGN (black hole mass $M_{\rm\bullet}=10^{\rm 8}$M$_{\rm\odot}$ with an Eddington ratio $\lambda=0.1$) can significantly lower the fraction of halo gas that is catastrophically cooling compared to the case of gas photoionized only by the ultraviolet background (UVB). Varying $M_{\rm\bullet}$, $\lambda$ and $\alpha$, we study their effects on observables. A 10$^{\rm 9}$M$_{\rm\odot}$ AGN with $\lambda=0.1$ and $\alpha\approxeq60^{^{\rm o}}$ reproduces the average surface brightness (SB) profiles of Ly$\alpha$, HeII and CIV, and results in a covering fraction of optically thick absorbers within observational estimates. The simulated SB$_{\rm CIV}$ profile is steeper than observed, indicating that not enough metals are pushed beyond the very inner CGM. For this combination of parameters, the CGM mass catastrophically cooling is reduced by half with respect to the UVB-only case, with roughly same mass out of hydrostatic equilibrium heating up and cooling down, hinting to the importance of self-regulation around AGNs. This study showcases how CGM observations can constrain not only the properties of the CGM itself, but also those of the AGN engine.Comment: accepted for publication in MNRA

Extended and broad Ly α emission around a BAL quasar at z ∼ 5

In this work we report deep MUSE observations of a broad absorption line (BAL) quasar at z ∼ 5, revealing a Ly α nebula with a maximum projected linear size of ∼60 kpc around the quasar (down to our 2σ SB limit per layer of ∼9×10−19ergs−1cm−2arcsec−2 for a 1 arcsec2 aperture). After correcting for the cosmological surface brightness dimming, we find that our nebula, at z ∼ 5, has an intrinsically less extended Ly α emission than nebulae at lower redshift. However, such a discrepancy is greatly reduced when referring to comoving distances, which take into account the cosmological growth of dark matter (DM) haloes, suggesting a positive correlation between the size of Ly α nebulae and the sizes of DM haloes/structures around quasars. Differently from the typical nebulae around radio-quiet non-BAL quasars, in the inner regions (∼10 kpc) of the circumgalactic medium of our source, the velocity dispersion of the Ly α emission is very high (FWHM > 1000 km s−1), suggesting that in our case we may be probing outflowing material associated with the quasar.The research leading to these results has received funding from the European Research Council (ERC) under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement no. 306476. RM acknowledges support from the ERC Advanced Grant 695671 ‘QUENCH’. RM and S. Carniani acknowledge support from the Science and Technology Facilities Council (STFC). S. Cantalupo gratefully acknowledges support from Swiss National Science Foundation grant PP00P2_163824