--- title: Loitering Outflow FeLoBAL Quasars url: https://www.emergentmind.com/topics/loitering-outflow-felobal-quasars type: topic --- # Loitering Outflow FeLoBAL Quasars Searching arXiv for recent and foundational papers on FeLoBAL quasars and “loitering” outflows. Search 1: "FeLoBAL loitering outflow SimBAL 2022". Loitering outflow FeLoBAL quasars are FeLoBAL systems in which dense, low-ionization BAL gas is inferred either to remain very close to the nucleus with unusually low velocity or to persist at kiloparsec scales after leaving the launching region. FeLoBALs themselves are BAL quasars with low-ionization absorption and prominent Fe II and/or Fe III absorption, and they are rare: roughly \(\sim 1\%\) of the BAL population and \(\sim 0.3\%\) of quasars in flux-limited samples [1312.5994], [1507.07247]. In one well-defined usage, Choi et al. identify a “loitering outflow” class with \(\log R<1\) pc, \(|v_{\rm off,FeII*}|<2000\ {\rm km\,s^{-1}}\), and \(v_{\rm width,FeII*}<2000\ {\rm km\,s^{-1}}\) [2203.11964]. In another usage, SDSS J1321−0041 and SDSS J0932+0840 are described as long-lived FeLoBAL outflows persisting at \(\sim\)kpc radii, with \(R=2.5^{+1.0}_{-1.4}\) kpc and \(R=0.7^{+0.9}_{-0.4}\) kpc, respectively [2310.06216], [2412.06929]. This suggests that the term is used for two related but distinct regimes: torus-scale stalled or slowly moving absorbers, and large-radius persistent FeLoBAL phases. ## 1. Terminology and observational scope FeLoBAL quasars are distinguished by broad absorption in low-ionization ions such as Mg II and Al III together with Fe II and often Fe III, in addition to the high-ionization BAL species typical of BAL quasars [1312.5994]. The “loitering outflow” label is therefore a refinement within an already extreme BAL subclass, not a replacement for the FeLoBAL definition. The recent literature uses the term in two main ways. Choi et al. reserve it for compact nuclear outflows characterized by small radii, low line-of-sight velocities, and high column density [2203.11964]. By contrast, the analyses of SDSS J1321−0041 and SDSS J0932+0840 use “loitering” for dense, low-ionization gas that has reached host-galaxy scales yet still persists with measurable column density and modest velocity [2310.06216], [2412.06929]. | Usage in the literature | Characteristic properties | Representative analyses | |---|---|---| | Nuclear loiterers | \(\log R<1\) pc; \(|v_{\rm off,FeII*}|<2000\ {\rm km\,s^{-1}}\); high \(U\), \(n\), and \(N_H\) | SimBAL sample [2203.11964] | | Large-radius persistent FeLoBALs | \(R\sim 0.7\)–2.5 kpc; low or intermediate velocity; dense low-ionization gas at host-galaxy scales | SDSS J0932+0840 [2412.06929], SDSS J1321−0041 [2310.06216] | This terminological duality is central. A loitering FeLoBAL is not defined by one radius alone; it is defined by a dynamical state in which unusually dense, low-ionization gas remains observable for long enough, and at modest enough velocity relative to other BAL winds, that its structure and environmental coupling can be measured. ## 2. Spectroscopic signatures and diagnostic methods Loitering FeLoBALs are identified through complex low-ionization spectra. Commonly detected ions include Mg II, Al II, Al III, Si II, Fe II, Fe III, and, in the best cases, multiple excited levels of Fe II and Si II [2310.06216]. In the SimBAL loitering objects, strong He I* \(\lambda3188\), \(\lambda3889\), Balmer absorption, and overlapping Fe II troughs are also prominent [2203.11964]. Balmer absorption is especially informative because it requires very high total column, high density, and a sustained population in \(n=2\) [2509.07611]. The standard quantitative workflow combines apparent-optical-depth column measurements with photoionization and level-population modeling. For normalized spectra, \[ I(\lambda)=I_0(\lambda)e^{-\tau(\lambda)}, \] and integrated ionic columns are obtained from \[ N_{\rm ion}=\int N(v)\,dv, \] after conversion from optical depth in velocity space [2310.06216]. Cloudy grids then constrain \((N_H,U_H)\), while excited-state ratios constrain density through collisional-equilibrium calculations with CHIANTI or related atomic data [2412.06929]. Partial covering is pervasive. In the Balmer-absorption survey of fourteen FeLoBALQs, eight objects require partial covering to reconcile the observed Balmer optical-depth ratios, and several spectra are best fit when the continuum is fully covered while the emission-line region is only partially covered [2509.07611]. FBQS J1151+3822 provides a concrete example: line-of-sight covering fractions are \(\sim 0.25\) for strong Fe II, \(\sim 0.5\) for He I*, and \(\sim 0.6\) for Mg II [1401.0605]. Such values show that loitering FeLoBALs are not homogeneous screens but structured, multiphase absorbers. Variability is the other major diagnostic. In a twelve-object FeLoBAL monitoring program, robust BAL variability at \(>8\sigma\) was detected in 3/12 sources over rest-frame timescales of \(0.6\) to \(7.6\) yr, in Fe II, Fe II*, Mg II, Mg I, and Ni II* troughs [1507.07247]. These changes can be interpreted either as ionization changes or as gas traversing the line of sight, and both interpretations are used in the literature depending on the object and the degree of saturation. ## 3. Nuclear loiterers at parsec scales The clearest formal definition of loitering FeLoBALs comes from the SimBAL analysis of 50 low-redshift FeLoBAL quasars. Eleven BAL components in eleven objects satisfy \(\log R<1\), \(|v_{\rm off,FeII*}|<2000\ {\rm km\,s^{-1}}\), and \(v_{\rm width,FeII*}<2000\ {\rm km\,s^{-1}}\) [2203.11964]. These absorbers occupy the high-ionization, high-density end of the FeLoBAL distribution, with typical \(\log U \sim -1.8\) to \(+1.1\), \(\log n \sim 6.0\)–7.8 \({\rm cm^{-3}}\), and covering-fraction-corrected \(\log N_H \sim 22.5\)–23.5. Most lie at \(R\sim 0.2\)–3 pc, inside the dusty torus region inferred for these luminosities, and they often require modified partial-covering models in which the BLR is unabsorbed while only part of the continuum is covered [2203.11964]. Variability studies independently place some FeLoBAL gas at similar scales. McGraw et al. report a representative upper limit of \(\sim 20\) pc from a typical crossing timescale \(\Delta t\sim 3.08\) yr and transverse speeds \(\gtrsim 500\ {\rm km\,s^{-1}}\) [1312.5994]. A later multi-epoch study of the same general class constrains three variable FeLoBALs to \(r\lesssim 69\), 7, and 60 pc under a transverse-motion interpretation [1507.07247]. Detailed single-object analyses sharpen the physical picture. FBQS J1151+3822 has \(\log U=-1.5\), \(5.5 \lesssim \log n_H \lesssim 8\), \(21.68 \le \log N_H \le 21.9\), and \(R \approx 7.2\) to 127 pc; its newly emerged narrow lines at \(v\approx -3400\ {\rm km\,s^{-1}}\) are interpreted as dense cores condensing inside the main outflow [1401.0605]. FBQS J1408+3054 underwent dramatic FeLoBAL weakening, and the crossing interpretation implies a transverse velocity between 2600 and \(22{,}000\ {\rm km\,s^{-1}}\) and a location between approximately 5800 and 46,000 Schwarzschild radii, or 1.7 to 14 pc [1010.3728]. Mrk 231 adds a geometrically complex nuclear case: its FeLoBAL region is placed within \(\lesssim 1\)–10 pc, with strong optical FeLoBAL troughs, no unambiguous far-UV BALs, and a faint Ly\(\alpha\) line centered at \(\sim -3500\ {\rm km\,s^{-1}}\), requiring a patchy, partially covering BAL region close to the engine [1212.2401]. These objects do not imply a static absorber. Rather, they show that a high-column, low-ionization outflow can remain at torus or inner-nuclear scales for years to centuries while evolving through covering-factor changes, ionization changes, and internal substructure. ## 4. Kpc-scale persistent FeLoBALs FeLoBALs can also loiter at host-galaxy radii. SDSS J1321−0041 is a high-resolution UVES case with strong Fe II, Si II, and C II BALs. Its best-fit parameters are \(\log N_H = 21.73^{+0.39}_{-0.26}\), \(\log U_H=-1.74^{+0.69}_{-0.27}\), \(\log n_e = 3.45^{+0.26}_{-0.20}\), and \(R = 2.5^{+1.0}_{-1.4}\) kpc, with \(\dot M = 1600^{+2600}_{-1000}\ M_\odot\,{\rm yr^{-1}}\) and \(\dot E_k = 8.4^{+13.7}_{-5.4}\times10^{45}\ {\rm erg\,s^{-1}}\) [2310.06216]. The authors explicitly describe it as dense gas that is no longer near the launching region but persists at large radii. SDSS J0932+0840 presents a slower kpc-scale case. Its narrow S2 component at \(v\simeq -720\ {\rm km\,s^{-1}}\) has \(\log N_H = 21.47^{+0.17}_{-0.16}\), \(\log U_H=-2.4^{+0.4}_{-0.5}\), \(\log n_e = 3.42^{+0.65}_{-0.46}\), \(\log n_H = 4.80^{+0.60}_{-0.52}\), and \(R = 0.7^{+0.9}_{-0.4}\) kpc [2412.06929]. Its dynamical time is \[ t_{\rm dyn}=\frac{R}{v}\sim 10^6\ {\rm yr}, \] and the absorber is interpreted as a kpc-scale, relatively slow FeLoBAL component consistent with long-lived gas persisting in the inner host galaxy [2412.06929]. Theoretical work makes this large-radius regime physically plausible. Faucher-Giguère, Quataert, and Murray argue that the extreme \(\Delta R/R \sim 10^{-5}\), kpc-scale distances, and short cloud-destruction times implied by some FeLoBAL analyses are more naturally explained if the observed FeLoBAL absorbers are formed in situ in radiative shocks when a quasar blast wave impacts moderately dense interstellar clumps [1108.0413]. Bautista and Dunn then show that year-scale variability in the ionizing continuum can drive supersonic ionization fronts, cooling fronts, and fragmentation in kpc-scale FeLoBAL clouds, and that the measured velocity differences among components in QSO 2359−1241 and SDSS J0318−0600 can be reproduced if strong magnetic fields of \(\sim 10\) mG are present within the clouds [1006.0919]. This suggests that large-radius loitering does not imply a monolithic shell. It can instead denote dense, cool clumps embedded in a larger shocked flow, with the observed FeLoBAL phase tracing only one part of a multi-phase outflow. ## 5. Energetics and feedback relevance The feedback significance of loitering FeLoBALs is highly heterogeneous. Many studies estimate energetics with a partial-shell relation such as \[ \dot M \simeq 4\pi \Omega R N_H \mu m_p v,\qquad \dot E_k = \tfrac12 \dot M v^2, \] with \(\Omega\) often set to 0.2 and \(\mu=1.4\) [2310.06216], [2412.06929]. Whether a loitering FeLoBAL is feedback-relevant then depends on \(R\), \(N_H\), and especially \(v\). At the weak end, the SimBAL loitering class is generally not feedback-relevant. Its mass outflow rates are frequently \(\dot M \lesssim 1\ M_\odot\,{\rm yr^{-1}}\), kinetic luminosities are typically \(\log L_{KE}\lesssim 42\)–43, and \(L_{KE}/L_{\rm bol}\ll 0.005\) for virtually all loitering outflows [2203.11964]. SDSS J0932+0840 is similar: \(\dot M = 43^{+65}_{-26}\ M_\odot\,{\rm yr^{-1}}\), \(\dot E_k = 0.7^{+1.1}_{-0.4}\times10^{43}\ {\rm erg\,s^{-1}}\), and \(\dot E_k/L_{\rm bol}\sim 10^{-4}\), explicitly below the level required for significant AGN feedback [2412.06929]. At the strong end, SDSS J1321−0041 reaches \[ \frac{\dot E_k}{L_{\rm Bol}}=4.8^{+8.0}_{-3.1}\%, \] above or near canonical \(0.5\%\)–5% thresholds, and is concluded to contribute to AGN feedback [2310.06216]. FBQS J1151+3822 spans \(L_{\rm kin}/L_{\rm bol}\approx 0.16\%\) to \(4.5\%\), showing that parsec- to tens-of-parsecs FeLoBALs can also enter the feedback-relevant regime [1401.0605]. Integrated IR studies support a broader feedback connection. In a sample of 31 FeLoBAL QSOs, all objects have \(L_{\rm IR,tot}>10^{12}L_\odot\), the AGN contributes 75% of the total IR luminosity on average, and the Mg II absorption-strength parameter \(BI_0\) anticorrelates with the starburst fraction \(f_{\rm SB}\): all systems with \(BI_0 \gtrsim 3500\ {\rm km\,s^{-1}}\) have \(f_{\rm SB}<25\%\), leading to the conclusion that radiatively driven outflows curtail obscured star formation to less than \(\sim25\%\) of the total IR luminosity [1112.1096]. X-ray studies add an inner, highly absorbed phase. Suzaku observations of three FeLoBALs imply intrinsic columns \(N_H \gtrsim 7\times10^{23}\), \(\gtrsim 2\times10^{24}\), and \(6\times10^{23}