Quasars Basic Components

Quasars are thought to be mainly composed by:

• A central, supermassive black hole (SMBHs; 107_.M

BH/M.1010)

• A surrounding accretion disk, i.e. material with non-null angular momentum, being ac- creted by the black hole and emitting a large amount of energy in the rest–frame UV/optical range.

• A broad line region, BLR, composed by high-velocity gas “clouds” in the proximities of the black hole.

• A X-ray corona, a hot (T& 107 K) region above and below the accretion disk, strongly emitting in the X–ray regime.

1.4. Quasars: Discovery and Basic Elements 11

FIGURE1.5: Discovery spectrum, acquired with the Palomar 5m Telescope, of the first quasar 3C273, at z=0.16. The figure is adapted from Hazard et al.,2018.

• An obscuring, dusty torus, found at several pc from the black hole, absorbing part of the energy from the accretion disk, and re-emitting it in the infrared (λ∼3 µm) range. • A narrow line region, NLR, gas regions observed at larger distances (∼0.1 kpc), moving

at velocities of∼300–500 km s−1_{, and producing narrow emission lines in the UV/optical}

spectrum.

• Two powerful jets, which convey materials moving at relativistic speed; they are thought to be present in the 10%–20% of the objects.

All these elements are visually summarized in Figure1.6.

In this thesis, we will mainly focus on the characterization of the central black holes and of the emission from the accretion disk and the BLR in high–redshift quasars, via observations of their rest–frame UV spectra. We provide here few additional details on the latter two compo- nents. This discussion is mainly adapted from Ghisellini,2013and Vanden Berk et al., 2001. We also list current methods of measuring MBHin the next section (§ 1.4.3).

The accretion disk is composed of matter infalling into the central black hole, which, while loosing angular momentum, is emitting radiation as:

L_disk =e ˙MBHc2 (1.20)

where e is the radiative efficiency, usually around 10%, and ˙MBHis the mass accretion rate. As-

suming that this structure can be divided in annuli emitting black body radiation, than the total accretion disk emission can be modeled as a sum of black bodies with different temperatures, with higher temperatures closer to the black hole:

T(R) =  3RSLdisk 16πσMBR3 1/4 1− 3RS R 1/21/4 ∝ R−3/4 _{if R>> R} S (1.21)

12 Chapter 1. Introduction

FIGURE1.6: Schematic representation of an active galactic nucleus (AGN) basic components, which are listed in Section1.4.2(adapted from Urry and Padovani

1995).

where σMB is the Maxwell–Boltzmann constant, and RS = 2MBHG/c2 is the Schwarzchild ra-

dius. No radiation is emitted from orbits with R ≤ 3RS. Assuming that each annuli emits

luminosity as:

dL=4πRdRσMBT4 (1.22)

and considering only the peak frequency corresponding to the black body temperature (hν ∝ kT), one can derive:

L_disk ∝ ν1/3 (1.23)

This is valid up to the limit set by the maximum temperature, close to the internal radius (Rin)

of the accretion disk. In this case, we will observe only an exponential drop (Ldisk ∝ exphν/kT).

On the other hand, only the Rayleigh-Jeans contribution will be observed at the outer radius (Rout; Ldisk ∝ ν2). We show all this components in Figure1.7, right.

In the present work, we will model the emission from the quasars accretion disks with a power law relation (see Section2.5.3).

Another important quantity is the Eddington luminosity, i.e. the theoretically maximum lu- minosity permitted for the quasar. This quantity is derived assuming that: 1) the radiation pressure and the gravitational attraction are in equilibrium (F_rad = Fg); 2) the radiation pres-

sure acts on electrons through the Thomson scattering, while the gravitational force acts on the protons; 3) the black hole accretion is spherically symmetric (i.e. Bondi accretion; Bondi1952).

1.4. Quasars: Discovery and Basic Elements 13

FIGURE1.7: Left: Theoretical models of the spectrum of a quasar accretion disk, following a power law form, in case of different values of maximum radii. Right: Main broad (an narrow) emission lines in the UV/optical wavelength window, observed in the composite spectrum obtained using a sample of SDSS quasars.

The figures are adapted from Ghisellini,2013and Vanden Berk et al.,2001.

From these assumptions, we can write: L_EddσT

4πR2_c =

GMBHmp

R2 (1.24)

from which we derive

LEdd = 4πGmp σT ·MBH =1.3×1038 MBH M (1.25)

with σT the Thomson scattering section and mp the proton mass. Even considering the strict

aforementioned assumptions, the Eddington limit seems to be generally respected among the observed AGNs, i.e. no black hole has been observed so far whose luminosity largely and securely surpasses the Eddington one (e.g. Trakhtenbrot, Volonteri, and Natarajan2017; Bian and Zhao2003). Also, one can define the Eddington ratio, i.e. the ratio between the bolometric (L_bol) and the Eddington luminosity (L_bol/L_Edd). The Eddington ratio is commonly considered a proxy of the efficiency of matter accretion onto the central SMBH.

We will largely make use of LEddand its relation with the quasar bolometric luminosity in the

present thesis.

In Figure1.7, left, we show an example of emission from broad lines, overimposed to the continuous radiation from the accretion disk. In Table1.1, we report the rest–frame wave- lengths of the main emission lines, that we will utilize in the present work. The BLRs are composed by gaseous regions with a temperature of∼104K, a density of∼ 109−1011cm−3, and a covering factor of∼0.1 (e.g. Ghisellini2013). The full widths at half maximum (FWHM) of the lines are typically 1000–10,000 km s−1, while the sizes of the BLRs are RBLR ∼ 0.1pc

14 Chapter 1. Introduction

TABLE 1.1: Main quasar broad emission lines observed in the rest–frame UV/optical spectrum. The laboratory wavelength of the rest frame emission and the relative strenght of the line with respect to that of Lyα are taken from Vanden

Berk et al.,2001.

ID λrest Relat. Flux [Å] [100×F/F(Lyα)] Lyβ 1025.72 9.615±0.484 Lyα 1215.67 100.000±0.753 NV 1240.14 2.461±0.189 SiV 1396.76 8.916±0.097 CIV 1549.06 25.291±0.106 MgII 2798.75 14.725±0.030 Hβ 4862.68 8.649±0.030 Hα 6564.61 30.832±0.098