The ionising continuum from active galactic nuclei (AGN) is fundamental for interpreting their broad emission lines and understanding their impact on the surrounding gas. Furthermore, it provides hints on how matter accretes onto supermassive black holes. Using HST's Wide Field Camera 3 we have constructed the first stacked ultraviolet (rest-frame wavelengths 600-2500Å) spectrum of 53 luminous quasars at z 2.4, with a state-of-the-art correction for the intervening Lyman forest and Lyman continuum absorption. The continuum slope (f ν ∝ ν αν ) of the full sample shows a break at ∼912Å with spectral index α ν = −0.61 ± 0.01 at λ > 912Å and a softening at shorter wavelengths (α ν = −1.70 ± 0.61 at λ 912Å). Our analysis proves that a proper intergalactic medium absorption correction is required to establish the intrinsic continuum emission of quasars. We interpret our average ultraviolet spectrum in the context of photoionisation, accretion disk models, and quasar contribution to the ultraviolet background. We find that observed broad line ratios are consistent with those predicted assuming an ionising slope of α ion =−2.0, similar to the observed ionising spectrum in the same wavelength range. The continuum break and softening are consistent with accretion disk plus X-ray corona models when black hole spin is taken into account. Our spectral energy distribution yields a 30% increase to previous estimates of the specific quasar emissivity, such that quasars may contribute significantly to the total specific Lyman limit emissivity estimated from the Lyα forest at z < 3.2.

We use a particle tracking analysis to study the origins of the circumgalactic medium (CGM), separating it into (1) accretion from the intergalactic medium (IGM), (2) wind from the central galaxy, and (3) gas ejected from other galaxies. Our sample consists of 21 FIRE-2 simulations, spanning the halo mass range M h ∼ 10 10 − 10 12 M , and we focus on z = 0.25 and z = 2. Owing to strong stellar feedback, only ∼ L halos retain a baryon mass 50% of their cosmic budget. Metals are more efficiently retained by halos, with a retention fraction 50%. Across all masses and redshifts analyzed 60% of the CGM mass originates as IGM accretion (some of which is associated with infalling halos). Overall, the second most important contribution is wind from the central galaxy, though gas ejected or stripped from satellites can contribute a comparable mass in ∼ L halos. Gas can persist in the CGM for billions of years, resulting in well mixed-halo gas. Sight lines through the CGM are therefore likely to intersect gas of multiple origins. For low-redshift ∼ L halos, cool gas (T < 10 4.7 K) is distributed on average preferentially along the galaxy plane, however with strong halo-to-halo variability. The metallicity of IGM accretion is systematically lower than the metallicity of winds (typically by 1 dex), although CGM and IGM metallicities depend significantly on the treatment of subgrid metal diffusion. Our results highlight the multiple physical mechanisms that contribute to the CGM and will inform observational efforts to develop a cohesive picture.

We develop a new method to constrain the physical conditions in the cool (∼ 10 4 K) circumgalactic medium (CGM) from measurements of ionic column densities, by assuming that the cool CGM spans a large range of gas densities and that small high-density clouds are hierarchically embedded in large low-density clouds. The new method combines the information available from different sightlines during the photoionization modeling, thus yielding tighter constraints on CGM properties compared to traditional methods which model each sightline individually. Applying this new technique to the COS-Halos survey of low-redshift ∼ L * galaxies, we find that we can reproduce all observed ion columns in all 44 galaxies in the sample, from the low-ions to O vi, with a single universal density structure for the cool CGM. The gas densities span the range 50 ρ/ρ b 5 × 10 5 (ρ b is the cosmic mean), while the physical size of individual clouds scales as ∼ ρ −1 , from ≈ 35 kpc of the low density O vi clouds to ≈ 6 pc of the highest density low-ion clouds. The deduced cloud sizes are too small for this density structure to be driven by self-gravity, thus its physical origin is unclear. The implied cool CGM mass within the virial radius is (1.3 ± 0.4) × 10 10 M (∼1% of the halo mass), distributed rather uniformly over the four decades in density. The mean cool gas density profile scales as R −1.0±0.3 , where R is the distance from the galaxy center. We construct a 3D model of the cool CGM based on our results, which we argue provides a benchmark for the CGM structure in hydrodynamic simulations. Our results can be tested by measuring the coherence scales of different ions.

We investigate the impact of cosmic rays (CRs) on the circumgalactic medium (CGM) in FIRE-2 simulations, for ultra-faint dwarf through Milky Way (MW)-mass haloes hosting star-forming (SF) galaxies. Our CR treatment includes injection by supernovae, anisotropic streaming and diffusion along magnetic field lines, and collisional and streaming losses, with constant parallel diffusivity $\kappa \sim 3\times 10^{29}\, \mathrm{cm^2\ s^{-1}}$ chosen to match γ-ray observations. With this, CRs become more important at larger halo masses and lower redshifts, and dominate the pressure in the CGM in MW-mass haloes at z ≲ 1–2. The gas in these ‘CR-dominated’ haloes differs significantly from runs without CRs: the gas is primarily cool (a few ${\sim}10^{4}\,$ K), and the cool phase is volume-filling and has a thermal pressure below that needed for virial or local thermal pressure balance. Ionization of the ‘low’ and ‘mid’ ions in this diffuse cool gas is dominated by photoionization, with O vi columns ${\gtrsim}10^{14.5}\, \mathrm{cm^{-2}}$ at distances ${\gtrsim}150\, \mathrm{kpc}$. CR and thermal gas pressure are locally anticorrelated, maintaining total pressure balance, and the CGM gas density profile is determined by the balance of CR pressure gradients and gravity. Neglecting CRs, the same haloes are primarily warm/hot ($T\gtrsim 10^{5}\,$K) with thermal pressure balancing gravity, collisional ionization dominates, O vi columns are lower and Ne viii higher, and the cool phase is confined to dense filaments in local thermal pressure equilibrium with the hot phase.

We analyse the emission properties of a new sample of 3579 type 1 AGN, selected from Sloan Digital Sky Survey (SDSS) Data Release 7 based on the detection of broad Hα emission. The sample extends over a broad Hα luminosity L bHα of 10 40 −10 44 erg s −1 and a broad Hα full width at half-maximum (FWHM) of 1000−25 000 km s −1 , which covers the range of black hole mass 10 6 < M BH /M < 10 9.5 and luminosity in Eddington units 10 −3 < L/L Edd < 1. We combine ROSAT, GALEX and 2MASS observations to form the spectral energy distribution (SED) from 2.2 µm to 2 keV. We find the following. (1) The distribution of the Hα FWHM values is independent of luminosity. (2) The observed mean optical-ultraviolet (optical-UV) SED is well matched by a fixed-shape SED of luminous quasars, which scales linearly with L bHα , and a host galaxy contribution. (3) The host galaxy r-band (fibre) luminosity function follows well the luminosity function of inactive non-emission-line galaxies (NEGs), consistent with a fixed fraction of ∼3 per cent of NEGs hosting an AGN, regardless of the host luminosity. (4) The hosts of lower luminosity AGN have a mean z-band luminosity and u − z colour which are identical to NEGs with the same redshift distribution. With increasing L bHα the AGN hosts become bluer and less luminous than NEGs. The implied increasing star formation rate with L bHα is consistent with the relation for SDSS type 2 AGN of similar bolometric luminosity. (5) The optical-UV SED of the more luminous AGN shows a small dispersion, consistent with dust reddening of a blue SED, as expected for thermal thin accretion disc emission. (6) There is a rather tight relation between νL ν (2 keV) and L bHα , which provides a useful probe for unobscured (true) type 2 AGN. (7) The primary parameter that drives the X-ray to UV emission ratio is luminosity, rather than M BH or L/L Edd .