For the past years, the idea of attaining an analogous metric to the stellar Hertzsprung- Russell (H-R) diagram to classify the diverse spectral properties of quasars, has always been appealing. The search came to fruition when Boroson & Green(1992) applied principal component analysis to a sample of 87 low redshift z < 0.5 Palomar-Green quasars and subsequently identified a set of measurable parameters with meaningful correlations. They found that the largest source of dispersion, referred to as the
6.1. BACKGROUND
Figure 6.1: Quasar main sequence in Eigenvector 1 optical plane given by Hβ FWHM vs. RFe iifrom Marziani et al.(2018). The shaded region shows an indication of a quasar main sequence usingZamfir et al.(2010) quasar sample.
Eigenvector 1 (E1), is attributed to the strong anti-correlation between the strength of Fe ii and [O iii] λ5007. Additionally, E1 is also related to FWHM and asymmetry of Hβ line. These trends have been verified in other studies (e.g.,Sulentic et al. 2000;
Marziani et al. 2001; Shen & Ho 2014). It is speculated that the Eddington ratio is
mainly responsible for driving the observed E1 pattern, but other physical drivers, such as black hole mass, spin, and orientation, might also contribute (Boroson & Green
1992;Marziani et al. 2001;Boroson 2002;Shen & Ho 2014;Marziani et al. 2018). The
secondary source of dispersion or Eigenvector 2 is described by the inverse relationship between strength of He ii λ4686 and optical luminosity.
This breakthrough has provided a foundation to construct a potential H-R diagram for AGN with broad lines. In particular, there appears to be a systematic pattern in the optical plane of E1 parameters Hβ FWHM and RFe ii=EW(Fe ii λ4570/Hβ) (e.g.,
Sulentic et al. 2000; Marziani et al. 2001; Sulentic et al. 2003; Zamfir et al. 2008, 2010),
dubbed the main sequence of quasars (Marziani et al. 2001). Figure 6.1 shows the plot of E1 parameters fromMarziani et al.(2018), with possible indication of quasar main sequence usingZamfir et al.(2010) quasar sample shaded in green (see review by e.g.,Marziani et al. 2018). The main sequence of quasars resembles a wedge-shaped, compared to that of stellar H-R diagram. The difference is primarily because quasars do not radiate isotropically unlike stars, which hints to additional dimensions in the parameter space required to reflect the complexity.
The four dimensional Eigenvector 1 (4DE1) parameter space is introduced as complimentary to E1. It contains E1 parameters (Sulentic et al. 2000) plus 2 extras, the soft X-ray photon index, Γsoft(Wang et al. 1996), and C iv λ1549 broad line centroid shift
CHAPTER 6. QUASAR ORIENTATION
at half maximum, c 1 2
(Sulentic et al. 2007). Basically, two populations can be identified
within the 4DE1 formalism for low redshift quasars, population A and B. Population A has FWHM(Hβ). 4000 km s−1, strong R
Fe ii≈ 0.7, blueshifted C iv c 12 ≈ −800 km s−1, and large Γsoft > 2. In contrast, population B has FWHM(Hβ)> 4000 km s−1, weak RFe ii≈ 0.3, no blueshift C iv c 12
, and Γsoft≈ 2. Several other physical properties that are associated with each population have also been found (see Table 1 ofSulentic et al.
2011;Fraix-Burnet et al. 2017).
The pursuit for a quasar H-R diagram and main sequence have been built upon the context of E1 parameter space. Although a precise H-R diagram for quasars has yet to be established, the E1 provides a useful framework to distinguish the majority of low redshift broad line sources according to their spectral types. As will be discussed in the next section, hints of orientation indicator have also been proposed in the E1 plane.
6.1.2.1 [O iii] EW as Orientation Tracer
The [O iii] λ5007 is a prominent NEL in the optical spectrum. The line, originating from the NLR, emits isotropically (Mulchaey et al. 1994, but see alsodi Serego Alighieri et al. 1997), while the continuum is often assumed to exhibit anisotropic emission from the optically thick and geometrically thin accretion disk (Shakura & Sunyaev 1973). Hence, the flux of the optical continuum from the disk is expected to decrease with increasing angle from face-on to edge-on, while the variation in the line intensity is insignificant.
By inspecting the distribution of [O iii] EW for ∼ 6000 SDSS quasar spectra,
Risaliti et al. (2011) demonstrated that the [O iii] EW parameter plays a part in
orientation effect. A follow-up study with twice the sample size also supports the idea
(Bisogni et al. 2017). These studies argued that the power-law tail slope of -3.5 of
the EW distribution above 30 Å is a strong signature of orientation and indicates near edge-on quasar. Values below this may be due to the intrinsic variance due to factors such as the continuum properties and narrow line region structure. In relation to E1 parameter space, the Hβ width is also expected to be broad in this case due to the contribution from the virial component. As the orientation moves towards pole-on, the EW of [O iii] decreases by a factor of cosine angle, between the disk and line-of-sight, along with narrower Hβ width. Their analyses also favoured a disk-like BLR geometry, which has been reported by several authors (e.g.,Netzer 1987;Collin-Souffrin & Dumont
6.1. BACKGROUND
6.1.2.2 Fe ii Strength and Hβ FWHM as Eddington Ratio and Orientation Tracer
As aforementioned, the Eddington ratio is thought to be the primary driver of E1 (e.g.,
Boroson & Green 1992). Shen & Ho(2014) took a step further by developing a unification
scheme solely defined by the Eddington ratio and orientation. Using 20 000 quasars at low redshift 0.1 ≤ z ≤ 0.9 from the SDSS, they mapped the quasar distribution in the E1 optical parameter space, as shown in Fig.6.2a.
The FWHM of Hβ is plotted against the EW ratio of Fe ii line and broad Hβ line, denoted by RFe ii. With stronger RFe ii and weaker EW [O iii], the Eddington ratio is higher. Information on orientation is provided at any constant value of RFe ii. At pole-on, the FWHM of Hβ is narrow and becomes broader as the inclination angle increases approaching edge-on (e.g.,Wills & Browne 1986;Boroson & Green 1992;Shen & Ho
2014).
The radio morphologies and orientation relationships are also demonstrated in Fig.6.2b. Their findings are consistent with the prediction that core-dominated objects are mainly viewed pole-on, and hence a less broad FWHM of Hβ than lobe-dominated objects. Furthermore, the RL sample dwells in region with larger Hβ FWHM and smaller range of RFe ii compared to the RQ sample. This suggests that RL AGN are likely to harbour massive black holes, accreting at lower Eddington ratio, as has been determined in earlier studies (e.g.,Laor 2000;Ho 2002).
0 1 2 3 RFeII 0 5000 10000 FWHM H β [km s −1] 0.8 1.0 1.2 1.4 1.6 logEW[OIII] [Å] all SDSS quasars
(a) Quasar distribution
0 1 2 3 RFeII 0 5000 10000 FWHM H β [km s −1] core−dominant lobe−dominant radio−quiet radio−loud
(b) Radio-quiet and radio-loud quasars distri- butions. The points with error bar indicate the
median FWHM Hβ at constant RFe ii for core-
and lobe-dominated radio quasars
Figure 6.2: Eigenvector 1 for quasars from Shen & Ho (2014). 126
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