The broad and narrow high-ionization emission lines (HILs) in the optical and UV spectra of quasars frequently show significant blueshifts with respect to the quasar rest frame (e.g., Gaskell, 1982; Tytler and Fan, 1992; Corbin and Boroson, 1996; Marziani et al., 1996; Zamanov et al., 2002, for some early papers). The interpretation involves the Doppler shift of line radiation due to the emitting gas motion toward the observer, with the part of line emitted by receding gas suppressed by obscuration. In the following we will adhere with this interpretation (for a dissenting view see however, Gaskell and Goosmann, 2013 who posit that we see light originally emitted by gas falling toward the black hole and backscattered toward us), and consider [OIII]λ4959,5007 as representative of narrow high-ionization lines (HILs), and CIVλ1549 as a prototypical broad HIL.

The diversity of quasar spectroscopic properties as found in single epoch observations of large samples has been organized along a quasar main sequence (Sulentic et al., 2000a,b; Marziani et al., 2001; Shen and Ho, 2014). Boroson and Green (1992) identified a first eigenvector in their sample of ≈80 Palomar-Green quasars which is associated with an anticorrelation between FWHM (Hβ) and a parameter measuring the prominence of FeII emission (the intensity ratio between the FeII blend at λ4570 and Hβ). Along the main sequence defined by this anti-correlation, Sulentic et al. (2000a) suggested a change in properties in correspondence of FWHM Hβ ≈ 4,000 km/s, and distinguished two populations: Population A (FWHM ≲ 4,000 km/s) and B (where the B stands for broader than 4,000 km/s; e.g., Marziani et al., 2014; Fraix-Burnet et al., 2017, for reviews; Table 1 of Fraix-Burnet et al., 2017 summarized parameter systematic differences between the two populations). Population A and B have been associated with high and low accretion, respectively.

The CIVλ1549 large blueshifts (above 1,000 km/s) are a Population A phenomenon, likely associated with a disk wind (see Figure 7 of Sulentic and Marziani, 2015). Population A sources, at low z (≲ 1), encompass relatively low black hole mass quasars (~107-108M⊙) radiating at a relatively high Eddington ratio (≳ 0.1 − 0.2). In many ways Pop. A sources can be considered as an extension of Narrow Line Seyfert 1 (NLSy1) with moderate or strong FeII emission: the limit FWHM ≈ 4,000 km/s (valid at bolometric luminosity log L ≲ 47 [erg s⁻¹]) allows one to include sources with the same Balmer line profiles and intensity ratios as observed in NLSy1s. This is not to imply that Pop. B sources (with FWHM Hβ≳ 4,000 km/s) do not show evidence of outflows. Evidence of outflow is, for example, overwhelming in the prototypical Pop. B source NGC 5548 (Kaastra et al., 2014). The latest developments suggest that outflows are ubiquitous, also in forms that may not provide striking optical/UV spectral phenomenologies (e.g., Harrison et al., 2014; Tombesi et al., 2015). Only, outflows are more difficult to trace in Population B single-epoch spectra, as the CIV integrated line profiles are relatively symmetric. In both Pop. A and B, the CIVλ1549 line profile can be represented as a scaled Hβ profile plus an excess of blueshifted emission: the two components are assumed to be representative of a “virialized” low-ionization broad line region (producing a fairly symmetric and unshifted line) plus an outflow/wind component with different physical conditions. In Pop. B, CIVλ1549 shows only a small blueshifted excess if compared to Hβ.

Similarly, large blueshifts of [OIII]λλ4959,5007 above 250 km/s are rare in z samples (they are real statistical outliers, called “blue outliers” [BOs] by Zamanov et al., 2002) and have been preferentially found among Population A sources (e.g., Zamanov et al., 2002; Xu et al., 2012; Zhang et al., 2013; Cracco et al., 2016). Sulentic and Marziani (2015) show the distribution of [OIII]λλ4959,5007 peak shifts for the spectral types defined along the Eigenvector 1 sequence: the prevalence of [OIII]λλ4959,5007 large blueshifts increases in Pop. A and reaches a maximum in extreme sources with FeIIλ4570/Hβ ≳ 1 (Figure 5 of Sulentic and Marziani, 2015; Negrete et al., in preparation). Blueshifts of [OIII]λλ4959,5007 trace larger scale outflows than CIVλ1549, outside of the broad line region (BLR).

A recent result is that the prevalence of large blueshifts in both CIVλ1549 and [OIII]λ4959,5007 quasars is much higher at high L in intermediate-z samples (1 ≲ z ≲ 2.5, Coatman et al., 2016; Marziani et al., 2016a,b; Zakamska et al., 2016; Bischetti et al., 2017; Vietri, 2017). Blueshifts of CIVλ1549 reach several thousands km/s in Pop. A. BOs become much more frequent in the high z and L samples. The [OIII] shift and FWHM distributions at high-L are remarkably different from those of low-z, low L samples. Figure 1 shows an example of a luminous Pop. A source in the sample of Sulentic et al. (2017): the left panel shows, overlaid to the continuum-subtracted spectrum, a decomposition of the CIVλ1549 profile into an unshifted and symmetric component (thick black line) and a blue shifted component (blue line). Without involving any profile decomposition (the caveats of the technique are discussed in Negrete et al., 2014), it is easy to see that about 80% of the line flux is emitted short-wards of the rest wavelength. At the same time, the Hβ remains symmetric. Figure 1 (rightmost panel) shows an enlargement of the [OIII]λ4959,5007 profile: the FWHM ≈ 3,600 km/s is extremely broad by [OIII]λ5007 standards (at low-z, [OIII]λ5007 FWHM is ≲ 1, 000 km/s). The profile appears boxy, and fully displaced to the blue.

Figure 1 represents the diagnostic provided by single epoch observations for quasars at intermediate-to-high z (1–5), with CIVλ1549 covered by optical spectrometers and [OIII]λλ4959,5007 requiring near-IR spectroscopic observations. The latter have become possible for relatively faint quasars only in recent times, with the advent of second generation IR spectrometers mounted at the foci of large aperture telescopes; two major examples are XSHOOTER at VLT and LUCI at LBT. Near IR observations provide a reliable estimate of the quasar systemic redshift if the narrow component of Hβ or the [OII]λ3727 can be effectively measured. If these lines are detected above noise, then a good precision in the rest frame can be achieved, and the uncertainty is typically δz ≲ 3 · 10⁻⁴. An accurate knowledge of the rest frame is not an end in itself, since an important physical parameter such as the outflow kinetic power depends on the third power of the outflow velocity υ_o. The availability of such observations should increase dramatically in the next few years, providing useful data (even with lack of real spatial resolution) for a better understanding of the outflow prevalence and power as a function of luminosity and cosmic epoch. At low-z, partial spatial resolution of the [OIII]λλ4959,5007 emitting regions is currently obtained (and will be more frequently obtained in the coming years) with the use of integral-field unit spectrographs with adaptive optics (e.g., Cresci et al., 2015). Observations of CIVλ1549 are and will remain challenging for sources at z ≲ 1.2 i.e., for all the low-z quasar population.

Computing the kinetic power and the thrust from single-epoch spectra is possible under several caveats and assumptions. Here we briefly recall a simplified way to estimate the mass of ionized gas, the mass outflow rate, the thrust, and the kinetic power of the outflow for collisional excited lines in photoionized gases (Cano-Díaz et al., 2012). The formulation of these authors allows to write the outflow parameters in a form independent from the covering and filling factor, provided that all emitting gas has the same density. The simplified approach is well-suited to elucidate the role of the most relevant outflow parameters in the computation of the thrust and kinetic power. We will then consider the specific assumptions needed to apply the following relations to [OIII] and CIV measurements from single epoch spectra.

The luminosity of any collisionally-excited line¹ is given by L(line)=∫VjlineffdV, where j_line is the line emissivity per unit volume, and can be written as: j_line = hνq_lun_en_l, where n_e the electron density, and n_l the number density of ions at the lower level of the transition. The collisional excitation rate at electron temperature T is qlu=βTϒluglexp(-ϵlukT), where g_l is the statistical weight of the lower level, and ϒ_lu is the effective collision strength. The line luminosity can be connected to the mass of ionized gas since Moution∝Lline(ZZ⊙)-1n-1. Up to this point the main assumptions are: (1) constant density; (2) all emitting gas being in the ionization stage that is producing the line; (3) well defined chemical abundances.

The mass outflow rate at a distance r can be written as, if the flow is confined to a solid angle of Ω of volume 43πr3Ω4π: Ṁoion=ρ Ωr2υo=MoionVΩr2υo∝Lυor-1. This requires the knowledge of (4) a typical emitting region radius, and (5) the outflow velocity υ_o. Assuming a single r value is already a strong simplification, especially for [OIII]. If the line emitting gas is still being accelerated (as in the CIV case), and the terminal velocity is υ_term = kυ_o, then the thrust should be ∝ Ṁkυ_o and the kinetic power of the outflow ϵ˙ ~ 12Ṁionk2υo2∝Llinek2υo3r-1. A value of k=1 can be assumed for [OIII] (as explained in section 4.2). The parameters of Table 1 can be all written in the form ∝Lliner-1n-1(Z/Z⊙)-1υon, with 0 ≤ n ≤ 3. The BLR gas exhibits highly super-solar chemical composition (Nagao et al., 2006; Shin et al., 2013), so that assuming Z = 5Z_⊙ is a reasonable choice for both [OIII] and CIV outflow, if the [OIII] emission is ascribed to a nuclear outflow. Finally, to be consistent with the idea of a bipolar outflow, all the quantities in Table 1 have been multiplied by a factor 2.

It is interesting to make some considerations on the most likely values of the outflow parameters in very luminous quasars, and somehow constrain their upper limits. The considerations below are focused on very luminous quasars such as the ones studied by the WISSH project and by Sulentic et al. (2017) and Marziani et al. (2016b). We will consider here the 14 Pop. A objects of the “HE” sample of Marziani et al. (2016a,b) with 47.5 ≲ log L ≲ 48.5 erg s⁻¹. The relations of Table 1 are scaled to values typical of the HE sample. In the redshift range 1 ≲ z ≲ 2.5 where the population of most luminous quasars peaks, the angular distance is not increasing anymore with redshift (Hogg, 1999), implying a fairly constant scale around 8 Kpc/arcsec. For standard ground based observations, with a slit width of 0.5–1 arcs, all emission from CIV is collected (obviously) and most or at least a significant fraction of [OIII] should be collected as well, although not necessarily all of it: [OIII] emission may extend to the outer boundaries of optical galaxies and even beyond (see for instance, the impressive case of NGC 5252, Tadhunter and Tsvetanov, 1989).

Evidence of HIL blueshifts are ubiquitous, and at high luminosity they become impressive involving very large shifts in broad and narrow HILs. The mass outflow rates indicated by both [OIII] and CIV are extremely high, only somewhat lower than the accretion mass influx needed to sustain the observed luminosity for modest radiative efficiency (~100 M_⊙ yr⁻¹ at efficiency 0.1). Even in the most luminous quasars, it is not obvious whether powerful outflows can have the ability to disrupt the host galaxy gas. However, it is likely that [OIII] and CIV trace only a part of the mass outflow. Accounting for multiphase outflows will be one of the major challenges of present and future observational and theoretical studies.

PM wrote the review. AD contributed to some of the papers reviewed in the present contribution. Other authors contributed with suggestions and critical reading.