Molecular Outflows in z > 6 Unobscured QSO Hosts Driven by Star Formation

The rest-frame 119 μm dust continuum is detected at high S/N and marginally resolved in each of our sources (Figure 2). The integrated spectra shown in Figure 1 are created by stacking all spaxels with a continuum level >8σ in the corresponding continuum map. This is done for both side bands. The OH 119 μm doublet is clearly detected in absorption for two sources (J2310+1855 and P183+05), both blueshifted with respect to the systemic velocity of their respective host galaxies. OH 119 μm absorption is not found at systemic velocities in any of the sources. OH 119 μm is also detected in emission at systemic velocity in P183+05, forming a P-Cygni profile in the spectra of this source, and tentatively detected at the systemic velocity in P036+03. This is in contrast with the sample of high-z DSFGs studied by Spilker et al. (2020a), where OH 119 μm emission was not detected in any of the sources. We discuss this result in Section 5.4.

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We fit each spectrum across both side bands, with a combination of one or two double Gaussians and an underlying linear dust-continuum slope. The corresponding spectral ranges used are 344.0–360.0 GHz, 335.5–351.0 GHz, and 330.9–346.5 GHz for J2310+1855, P183+05, and P036+03, respectively. The double-Gaussian component consists of two Gaussians of equal amplitude and width, and placed at a fixed rest-frame separation of 4.37 GHz, as required of the OH Λ–doublet at the redshift under consideration. The continuum flux density and slope are left as free parameters as well as the amplitude and width of the double Gaussian. The frequency offset of the absorption features in J2310+1855 and P183+05 is left as a free parameter while the OH emission features in P183+05 and P036+03 are fixed at the systemic velocity of their hosts. The resulting spectral fits are shown in Figure 1 along with the residual spectra. The residuals of J2310+1855 display possible excess absorption at more blueshifted velocities than is captured by the single double-Gaussian fit, possibly indicating an outflow at even higher velocities. This feature lies at the edge of the side band, however, and additional observations extending to higher frequencies are needed to confirm this additional outflow component. The narrow (108 ± 22 km s⁻¹) line width of the tentative OH emission in P036+03 is comparable to that found for the weak water vapor emission of the blended 3₂₂–3₁₃ and 5₂₃–4₃₂ H₂O transitions (Bañados et al. 2015), which similarly traces warm, dense molecular gas. The best-fit parameter values and uncertainties for each source are presented in Table 3. Line maps, integrated over the FWHM, are shown in Figure 2.

Table 3. Measured and Derived Host Galaxy and Outflow Properties

		J2310+1855	P183+05	P036+03
119 μm Contintuum
R1/2,119μm	[pc]	637 ± 53	1280 ± 45	707 ± 22
S119μm	[mJy]	4.73 ± 0.0016	6.66 ± 0.0010	5.00 ± 0.0020
OH 119 μm Emission (systemic)
SOH 119μm	[Jy km s−1]	⋯	0.28 ± 0.028	0.12 ± 0.022
σEmis.	[km s−1]	⋯	123 ± 13	45.9 ± 9.5
FWHMEmis	[km s−1]	⋯	289 ± 31	108 ± 22
OH 119 μm Absorption (outflow)
SOH 119μm	[Jy km s−1]	−0.42 ± 0.04	−0.19 ± 0.03	⋯
NOH 119μm	[1015 cm−2]	8.90 ± 2.1	3.10 ± 0.059	⋯
EWOF	[km s−1]	619 ± 140	215 ± 4.1	⋯
σOF	[km s−1]	140 ± 12	123 ± 19	⋯
FWHMOF	[km s−1]	330 ± 29	290 ± 45	⋯
V50	[km s−1]	−334 ± 14	−534 ± 18	⋯
V84	[km s−1]	−473 ± 21	−656 ± 79	⋯
${V}_{{\max }}$	[km s−1]	−622 ± 85	−787 ± 222	⋯
MOF	[108 M⊙]	25–99	13–140	⋯
${\dot{M}}_{\mathrm{OF}}$	[M⊙yr−1]	1300–5300	560–5900	⋯
${\dot{p}}_{\mathrm{OF}}$	[1035 dyne]	28–110	19–200	⋯
${\dot{E}}_{\mathrm{OF}}$	[1043 ergs−1]	4.7–19	5.0–530	⋯
τOF	[Myr]	8.3–33	8.4–90	⋯
τOF+SFR	[Myr]	4.9–8.8	6.1–18	23

Note. Measured properties of the 119 μm dust continuum (size and flux), OH emission and absorption lines (with formal uncertainties from the fitting procedure), and derived properties of the molecular outflows.

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As is commonly done in the literature, we derive additional quantities of the absorption lines from our best-fit parameters in order to compare our work with previous OH studies (Veilleux et al. 2013; Spilker et al. 2020a; Herrera-Camus et al. 2020). These include the FWHM, EW (equal to the optical depth in the optically thin regime), and three definitions of the absorption line velocity: v₅₀, v₈₄, and v_max, referring to the velocities above which 50%, 84%, and 98% of the absorption lies, respectively. These values are also found in Table 3 and presented in Figure 3 as functions of the far-infrared (FIR) luminosity and EW.

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For comparison, Figure 3 also shows results from previous samples of OH absorption in star-forming and AGN hosts at both low-z (Veilleux et al. 2013; Stone et al. 2016) and high-z (Spilker et al. 2020a; Herrera-Camus et al. 2020). As we are specifically interested in the outflowing molecular gas we additionally plot the outflow-only components of the high-z DSFGs studied by Spilker et al. (2020a) (the separate components are not provided for the low-z sample). The outflow-only components in general show stronger correlations than the full absorption line values. Furthermore, we find that the strongest correlation with outflow velocity is associated with V₅₀ and not V_max, as is found in studies that use the total (systemic and blueshifted) absorption lines. The tighter relation found with V_max for the full absorption line may be due to the increasing fractional contribution of the outflowing gas to the total absorption signature toward more negative velocities of the absorption line. We provide a fit to the outflow-only components of the DSFG sample in each panel of Figure 3 to guide the reader's eye when comparing these values with the OH outflows in J2310+1855 and P183+05. These fits are illustrative only and are not intended to imply true physical trends in all panels.

A notable difference evident between the OH outflows in the DSFGs and those of J2310+1855 and P183+05 is the offset to smaller FWHMs at similar L_FIR for the two QSOs. The central outflow velocities (v₅₀) of the two QSOs appear on or near the DSFG v₅₀ versus L_FIR trend. The narrower line widths of the QSO outflow lines, however, cause the QSO outflows to drift toward slower than expected velocities for more extreme velocity definitions (v₈₄ and v_max versus L_FIR). This is also seen in the plots of EW versus V.

A second, notable observation is the large EW of the OH absorption in J2310+1855. Although less extreme in the case of P183+05, the EWs of the OH absorption lines are relatively larger for both J2310+1855 and P183+05 than seen in the DSFGs, given the narrower line widths.

Care should be taken when interpreting the position of ULAS J131911+095051 (Herrera-Camus et al. 2020) in Figure 3 as these values are taken from a single-Gaussian fit. The lower-frequency transition of the OH 119 μm doublet is likely canceled out by emission at systemic velocities, as is seen in P183+05. Some absorption from the higher-frequency transition is also likely lost on the red side of the line, meaning a fit to this absorption feature alone will underestimate the absorption strength and line width and will overestimate the blueshifted velocity offset.

For each source we estimate the size of the 119 μm continuum-emitting area. Taking only line-free channels, the visibilities of each data set are binned in radial uv bins. We perform the procedure for bins of uv widths of 10, 25, and 50 k λ taking the mean value of the real visibilities in each bin. We perform the fit on the resulting binned data using scipy's curve_fit routine, adopting the scatter in the visibilities as the uncertainty. We fit the continuum data with (1) a single Gaussian and (2) a combination of a Gaussian and a point source. A single-Gaussian fit is preferred for all the sources except P183+05. We find mean 119 μm continuum half-light radii of 0109 ± 0009=637 pc, 0228 ± 0008=1280 pc, and 0127 ± 0004=707 pc, for J2310+1855, P183+05, and P036+03, respectively. The errors include the formal uncertainties from curve_fit and the scatter between different uv-binnings. The results are robust with respect to the uv bin size. All sources are marginally resolved by the beam as seen by the distinct drop-off in continuum flux toward larger uv distances in each source (Figure 4).

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Our measured half-light radii of J2310+1855 and P183+05 are comparable to previous high-resolution measurements of the dust continuum at 260 GHz (R_1/2 = 011 × 0095; Tripodi et al. 2022) and 158 μm (0225 × 0175; Venemans et al. 2020), respectively. P036+03's measured half-light radius is large compared to previous high-resolution measurements of 0095 × 008 at 158 μm (Venemans et al. 2020). All three QSO sizes are within the range reported for the full 27 QSO hosts studied by Venemans et al. (2020) and the high-z comparison sample of DSFGs (Spilker et al. 2020b).

The same procedure is also applied to continuum-subtracted channels within the FWHM of the lower-frequency line of the OH emission doublet of P183+05, and in both emission components of P036+03 (bottom row of Figure 4). This provides an OH 119 μm emitting radius of 063 ± 013=3.54 kpc for P183+05. The OH emission is not resolved in the case of P036+03.

A limiting factor of absorption line work is that only gas lying between the observer and the background continuum may be detected. For some geometries, outflowing or inflowing gas may or may not intervene the line of sight (LOS) and background continuum given particular viewing angles. Consequently, a non-detection does not immediately imply the absence of an out/inflow. Likewise, when a detection is made, the observed covering fraction f_cov of the intervening gas (the fraction of the background continuum with an absorption signature), may not be indicative of the overall 4π f_cov, unless the geometry is known to be isotropic. Assumptions must therefore be made when converting observed column densities to total, galaxy-wide masses (see Section 4).

Measuring the LOS covering fraction requires resolved observations of the intervening gas. Our observations do not resolve the structure of the outflows detected in OH absorption for J2310+1855 and P183+05. We can, however, derive a hard lower bound on the LOS f_cov by assuming an optically thick outflow and taking the fractional absorption depth of the OH line. This gives f_cov ≥ 25% and 9.4% for J2310+1855 and P183+05, respectively, which are comparable to the 5%–20% fractional depths measured by Spilker et al. (2020a) for the high-z DSFGs sample.

The spatial resolution achieved by Spilker et al. (2020a) allowed them to detect distinct clumps in the outflowing OH absorption and measure LOS f_cov between 30% and 85% in their lensing reconstructions. They note that these values are upper limits as the individual clumps remain unresolved, accounting for (at least partially) the discrepancy between the LOS f_cov and the aforementioned fractional absorption depths. They argue that the true covering fractions are likely closer to those estimated from the peak absorption depth as OH is expected to be highly optically thick. This has similarly been assumed in low-redshift studies (e.g., González-Alfonso et al. 2017).

In any case, f_cov measured by Spilker et al. (2020a) is typically smaller than the outflow detection rate of 73% in their sample, interpreted by the authors to imply that the ouflowing gas traced by OH absorption must exit the galaxies in some "fortuitous geometry," one that does not require a special viewing angle. Spilker et al. (2020a) also find positive trends between the covering fraction, FIR luminosity, and outflow velocity, further suggesting that more luminous galaxies, capable of driving more extreme outflows, are either more likely to drive an outflow or drive more widespread outflows (i.e., a wider opening angle/more clumps).

The detection rate of OH outflows in the four currently observed QSOs at z > 6 is either 50% or 75%, depending on whether we include the tentative outflow detection of the z = 6.13 QSO ULAS J131911+095051 (Herrera-Camus et al. 2020). In the following section we use the fractional absorption depth of the OH line to derive lower limits of the outflow properties. As we cannot measure an upper limit of the covering fraction as Spilker et al. (2020a) did with their high spatial resolution data, we simply use an upper limit of f_cov of 100%. We caution the reader when interpreting these upper limits, as a 100% covering fraction is likely unphysical, given the evidence from the literature that OH traces optically thick, widespread and clumpy outflows, thus resulting in covering fractions more comparable with that of the fractional absorption depth.

In the optically thin regime, the EW of an absorption line is equal to the integrated optical depth. In this scenario, the EW can directly replace the optical depth integrated over the velocity, ∫τ_ν dv, when deriving the column density of the OH119 μm, given by,

$$\begin{eqnarray}&&{N}_{\mathrm{OH}}=\int {\tau }_{\nu }{\text{}}{dv}\displaystyle \frac{{\nu }^{3}{g}_{{\rm{l}}}8\pi }{{c}^{3}{A}_{\mathrm{ul}}{g}_{{\rm{u}}}},\end{eqnarray} \tag{ 1 }$$

where ν is the rest frequency of the line, g_u = 6 and g_l = 4 are the upper and lower statistical weights, respectively, and A_ul = 10^−0.86618 s⁻¹ is the Einstein coefficient of the transition. N_OH is then converted to a total molecular hydrogen column density assuming a conversion factor; here we assume a ${N}_{\mathrm{OH}}/{N}_{{{\rm{H}}}_{2}}=1\times {10}^{-7}$ from Nguyen et al. (2018).

To convert a column density to a full outflow mass and mass outflow rate, the geometry of the outflow must first be assumed. Our marginally resolved observations do not allow the geometry of the outflows observed for J2310+1855 and P183+05 to be determined. We therefore assume a simple spherical thin shell outflow geometry, commonly used elsewhere in the literature (e.g., Rupke et al. 2005), which provides a conservative estimate of the mass and mass outflow rate,

$$\begin{eqnarray}&&{\dot{M}}_{\mathrm{OF}}=4\pi {{\rm{R}}}_{\mathrm{OF}}^{2}\,\mu {{\rm{m}}}_{{{\rm{H}}}_{2}}\,{{\rm{f}}}_{\mathrm{cov}}\,{{\rm{N}}}_{{{\rm{H}}}_{2}}\displaystyle \frac{{{\rm{V}}}_{\mathrm{OF}}}{{{\rm{R}}}_{\mathrm{OF}}}=\displaystyle \frac{{{\rm{M}}}_{\mathrm{OF}}{{\rm{V}}}_{\mathrm{OF}}}{{{\rm{R}}}_{\mathrm{OF}}},\end{eqnarray} \tag{ 2 }$$

where μ = 1.36 accounts for the universal helium fraction, R is the outflow radius, assumed to be R_cont, V_OF is the outflow velocity V₅₀, and f_cov is the covering fraction of the outflowing gas.

As discussed in Section 3.3, we cannot measure f_cov with our spatially unresolved data. Thus, for the purpose of simply comparing our two QSOs and with the sample of DSFGs, we derive all values of ${\dot{M}}_{\mathrm{OF}}$ using f_cov = 1, providing upper limits on M_OF and ${\dot{M}}_{\mathrm{OF}}$ for the given geometry. For J2310+1855 this gives M_OF < 99 × 10⁸ M_⊙ and ${\dot{M}}_{{\rm{O}}{\rm{F}}}\lt 5300\,{M}_{\odot }\,{{\rm{y}}{\rm{r}}}^{-1}$ and for P183+05 M_OF < 140 × 10⁸ M_⊙ and ${\dot{M}}_{{\rm{O}}{\rm{F}}}\lt 5900\,{M}_{\odot }\,{{\rm{y}}{\rm{r}}}^{-1}$.

Figure 5 presents the derived values of ${\dot{M}}_{\mathrm{OF}}$ as a function of FIR luminosity and derived SFR. As previously reported by Spilker et al. (2020b), DSFGs with larger L_FIR drive outflows with higher ${\dot{M}}_{\mathrm{OF}}$. Focusing on the upper-limit relations, we find that J2310+1855 and P183+05 both sit above the DSFG relation. We note however that there is only one DSFG in the L_FIR range of the two QSO hosts, and therefore we do not view the QSO offsets to higher ${\dot{M}}_{\mathrm{OF}}$ as significant. In fact if the highest L_FIR DSFG is removed then a fit to the remaining DSFGs will extrapolate between the two QSOs. With the assumed geometry and a f_cov = 1, we derive ${\dot{M}}_{\mathrm{OF}}{\rm{s}}$ greater than the SFRs of both QSOs, giving lower limits on the gas-depletion time of τ_OF > 8.3 Myr and τ_OF > 8.4 Myr for J2310+1855 and P183+05, respectively. If star formation is included and assumed to remain constant, the combined depletion times reduce to τ_OF+SFR > 4.9 Myr and τ_OF+SFR > 6.1 Myr for J2310+1855 and P183+05, respectively.

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González-Alfonso et al. (2017) found an average f_cov for OH absorption in their low-z star-forming galaxies of 0.3, which if adopted here would shift all but one of the DSFG and both QSO ${\dot{M}}_{\mathrm{OF}}{\rm{s}}$ below the ${\dot{M}}_{\mathrm{OF}}={SFR}$ line. Whether f_cov = 0.3 is a suitable covering fraction for high-z galaxies and moreover high-z QSOs, is still yet to be shown. As mentioned in Section 3.3, however, the fractional absorption depths of the OH absorption lines provides a hard lower limit of f_cov, which in turn can be used to derive a lower limit of M_OF and ${\dot{M}}_{\mathrm{OF}}$. This gives M_OF > 25 × 10⁸ M_⊙ and ${\dot{M}}_{{\rm{O}}{\rm{F}}}\gt 1300\,{M}_{\odot }\,{{\rm{y}}{\rm{r}}}^{-1}$ for J2310+1855 and M_OF > 13 × 10⁸ M_⊙ and ${\dot{M}}_{{\rm{O}}{\rm{F}}}\gt 560\,{M}_{\odot }\,{{\rm{y}}{\rm{r}}}^{-1}$ for P183+05 This corresponds to outflow depletion timescales of τ_OF < 33 Myr and τ_OF < 90 Myr and combined timescales of τ_OF+SFR < 8.8 Myr and τ_OF+SFR < 18 Myr for J2310+1855 and P183+05, respectively.

The energy ${\dot{E}}_{\mathrm{OF}}$ and momentum ${\dot{p}}_{\mathrm{OF}}$ flux of an outflow can provide insight into the mechanisms needed to drive it. We derive both these values, for J2310+1855, P183+05, and the DSFGs with the same size and geometrical assumptions used to derive ${\dot{M}}_{\mathrm{OF}}$, giving,

$$\begin{eqnarray}&&{\dot{E}}_{{\rm{OF}}}=\displaystyle \frac{1}{2}{\dot{{\rm{M}}}}_{{\rm{OF}}}{{\rm{V}}}_{{\rm{OF}}}^{2},\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{\dot{p}}_{\mathrm{OF}}={\dot{M}}_{\mathrm{OF}}{V}_{\mathrm{OF}}.\end{eqnarray} \tag{ 4 }$$

As in the last section, we derive these values assuming a covering fraction of 1 for all objects to compare between the DSFGs and the QSOs and again with f_cov = 0.3, for the DSFGs, and with the fractional absorption depths of the two QSOs as lower limits (Figure 6). This gives limits of the energy fluxes of $4.7\times {10}^{43}\ \mathrm{erg}\,{{\rm{s}}}^{-1}\lt {\dot{{\rm{E}}}}_{\mathrm{OF}}\lt 19\times {10}^{43}\ \mathrm{erg}\,{{\rm{s}}}^{-1}$ and $5.0\times {10}^{44}\ \mathrm{erg}\,{{\rm{s}}}^{-1}\lt {\dot{{\rm{E}}}}_{\mathrm{OF}}\lt 530\times {10}^{44}\ \mathrm{erg}\,{{\rm{s}}}^{-1}$, and of the momentum fluxes of $28\times {10}^{36}\ \mathrm{dyne}\lt {\dot{{\rm{p}}}}_{\mathrm{OF}}\lt 110\times {10}^{36}\ \mathrm{dyne}$ and $19\times {10}^{36}\ \mathrm{dyne}\lt {\dot{{\rm{p}}}}_{\mathrm{OF}}\lt 200\times {10}^{36}\ \mathrm{dyne}$, for J2310+1855 and P183+05, respectively.

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Again focusing on the upper limit relations: the DSFG sample displays a clear positive trend for both fluxes with L_IR. The QSOs sit below and above these relations for J2310+1855 and P183+05, respectively. J2310+1855's deviation below the L_FIR-${\dot{E}}_{\mathrm{OF}}$ relation, however, is particularly significant. For all sources, the energy and momentum injected by star formation is sufficient to drive the observed outflow, even at the upper limit values. This is perhaps surprising for J2310+1855 and P183+05, where feedback from both star formation and the central AGN is available and may have been expected to collectively drive a stronger outflow than star formation alone. In Section 5 we discuss possible explanations for why we do not see evidence of an AGN contribution and instead see the opposite in the case of J2310+1855.

Not all material ejected via an outflow will escape the host galaxy or the dark matter halo's gravitational potential. Instead some fraction of the material will remain bound to the galaxy and may re-accrete via a galactic fountain at a later time, unless further heating or acceleration is experienced. Some fraction of the outflow, however, may be sufficiently accelerated to pollute the CGM or even IGM, removing mass, momentum, and metals from the galaxy altogether. Below we attempt to estimate a possible escape fraction of the molecular outflow in our two QSOs.

Assuming that the outflow is at a radius of R_cont and that the outflow will not be decelerated by swept-up material, or accelerated by radiation pressure, the dynamical masses (Table 1) are enough to retain effectively all of the outflowing molecular gas in both J2310+1855 and P183+05.

When treated under the same assumptions, the OH molecular outflows observed for+1855 and P183+05 are less or comparably energetic to those driven by the high-z DSFGs (Figure 6). This is despite harboring comparable SFRs and an additional luminous AGN.

Returning to Figure 3, we highlight the key difference in the basic host galaxy and blueshifted absorption properties between the QSOs and the DSFG sample. First, both QSOs display narrower than expected absorption signatures for their L_IR when compared to the DSFG sample. This may be due to a more cohesive outflow velocity structure (i.e., sheet), fewer velocity components contributing to the overall absorption signature (fewer clumps), or an overall less turbulent outflow. P183+05 consistently deviates toward narrower FWHMs from all of the DSFG relations, but does not significantly deviate in any other way with regards to both outflow and host galaxy properties. J2310+1855, on the other hand, deviates in three additional ways: (1) toward much higher outflow absorption strengths (EW), (2) toward slower outflow velocities, and (3) toward a smaller dust continuum size, giving J2310+1855 the highest FIR surface brightness of both samples. We investigate the possible effects of the dust continuum size in Section 5.3.

Our upper limits on the outflow ${\dot{M}}_{\mathrm{OF}}$ and energetics, assuming a covering fraction of 100% around J2310+1855 and P183+05, provide values within the range where the star formation in these systems is sufficient. Not only is additional energy or momentum from an AGN simply not required to drive these outflows, but the fact that J2310+1855 and P183+05's do not even appear to be boosted with respect to the DSFGs in energetics, ${\dot{M}}_{\mathrm{OF}}$, or velocity, suggests that these outflows may indeed be driven by the same mechanisms—those associated with star formation.

At low-z, it is consistently found that higher AGN fractions and luminosities drive faster and more energetic molecular outflows (Sturm et al. 2011; Spoon et al. 2013; González-Alfonso et al. 2017). Outflow properties also correlate best with AGN luminosity or AGN fraction, while only weakly correlating with the star-forming properties of the host galaxy (Calderón et al. 2016). Studies measuring 9.7 μm silicate absorption, which indicates the degree of obscuration originating in a nuclear torus and/or host galaxy disk, find strong correlations with the strength of the OH absorption (EW) (Spoon et al. 2013; Veilleux et al. 2013; Stone et al. 2016). Together, these results suggest that not only the strength of the radiation field, be it from star formation or an AGN, but the obscuration and thus ability of these energy and momentum sources to couple with the surrounding ISM, is crucial in driving a strong molecular outflow. Indeed González-Alfonso et al. (2017) find in their sample of 14 local ULIRGs, that the highest molecular outflow velocities traced by OH absorption are found in buried sources, where slower but more massive expansion of the nuclear gas is found.

But how comparable are low-z AGN hosts compared with our high-z unobscured QSOs? The entire sample studied by González-Alfonso et al. (2017) were galaxies undergoing gas-rich mergers, and indeed AGNs found in the low-z universe are ubiquitously inbedded in such host galaxies (Sanders & Mirabel 1996), where AGN feedback is thought to be stochastically injected into the chaotic surrounding ISM throughout the merging process (González-Alfonso et al. 2017). The three z > 6 QSOs studied in this paper, however, all display signatures of orderly, rotationally dominated gas reservoirs (Table 1; Neeleman et al. 2021; Tripodi et al. 2022), opposing a merger scenario. As unobscured QSOs, they have already removed the obscuring material surrounding the AGN, clearing a path through the ISM along the LOS (note: this need not be aligned with the host galaxy).

At high-z, the interaction between central AGNs and their host galaxies is a matter under investigation. Rojas-Ruiz et al. (2021) found that strong synchrotron emission extending into millimeter wavelengths for the radio-loud QSO PSO J352.4034-15.3373 contributed significantly to its FIR luminosity. This source, however is one of the brightest radio-loud sources in the early universe and the subsequent study of Khusanova et al. (2022), considering five radio-loud z > 6 QSOs, found that the SFR distribution and [C ii] luminosities of their sources were comparable with the radio-quiet population, suggesting that radio jets do not contribute significantly to these properties.

Significant AGN contributions to the FIR luminosity via dust heating have been measured in some high-z QSOs. Schneider et al. (2015) found an AGN contribution between 30% and 70% for the z = 6.4 QSO J1148+5251. Although chosen as a prototype for the general population of high-z QSOs, this source harbors an X-ray luminosity twice that of J2310+1855 and has been detected in CO(17-16) (Gallerani et al. 2014) which remains undetected for J2310+1855. Carniani et al. (2019) inspected the CO spectral line energy distributions (SLEDs) and FIR properties of these sources and concluded that the differences found were likely due to different gas heating mechanisms present, those being mechanisms associated with the central AGN for J1148+5251 and star formation for J2310+1855. Decarli et al. (2022) similarly find, using multiline diagnostics and the non-detection of high-J CO transitions, that star formation also appears to drive the molecular gas excitation in P183+05. Duras et al. (2017) found an average AGN contribution of 50% in their sample of 16 IR-bright QSOs. This sample was chosen to represent an intermediate population of QSOs emerging from the heavily obscured phase, but preceding the blue QSO phase where intervening material has already been swept away. This again differs from the population of unobscured, UV- and FIR-bright QSOs represented by our QSOs.

In a luminosity-limited sample of z > 6 unobscured QSOs (including our sources), no correlation is seen between quasar UV luminosity (M₁₄₅₀, see Table 1) and FIR luminosity (Venemans et al. 2018). This is true even in the central regions surrounding the QSOs (Venemans et al. 2020), suggesting little interaction between the QSOs and host galaxy ISM. Our assumption that the SFR dominates the L_FIR of our sources is thus likely a good assumption and it is perhaps also unsurprising that we do not find evidence of a boost in the molecular outflow properties measured and derived for our unobscured QSO hosts, compared to the DSFG sample.

In any case, we cannot rule out a contribution from an AGN to the FIR luminosity with this information alone, nor can we ignore the large uncertainties associated with estimating this value (Venemans et al. 2018, 2020; Tripodi et al. 2022). Therefore, as an extreme case, we consider an AGN contribution of 50% to the L_FIR, as found for high-z broad absorption line (BAL) QSOs (Duras et al. 2017). We expect, for the reasons stated above, that this value is an upper limit of the AGN contribution for our unobscured sources. This effect would move our sources left, by half, in Figures 5 and 6 and in the left two columns of Figure 3, if one is to consider only the input from star formation. In Figure 3 (note log scales), this makes little difference to our conclusions. In Figures 5 and 6, P183+05 indeed begins to appear more energetic than expected for its SFR, if it is assumed that all the sources have an outflow covering fraction of 100%. This moves the upper limit of the energy flux within the "high coupling efficiency or AGN required" region (indicated by the smaller faint orange symbols). We remind the reader that a covering fraction of 100% is an extreme scenario, with the true values expected to be close to the fractional absorption depth, which in the case of P183+05 is only 9.5%. Assuming an AGN contribution to the FIR of 50%, a high coupling efficiency or an AGN contribution to drive the molecular outflow seen in P183+05 is not required unless the outflow covering fraction is ≥53%. This is higher than all of the absorption depths measured by Spilker et al. (2020a) for the outflowing or systemic gas in their sample. Thus, while it is possible the molecular outflow in P183+05 lies in the parameter space requiring either a high coupling fraction or an AGN contribution, this would require either a significant AGN contribution to the FIR luminosity and/or a high outflow covering fraction (≥53%). For reasons stated above we believe both the condition of a significant L_FIR AGN contribution and/or high covering fraction to move P183+05 into this parameter space to be unlikely, but do not rule it out. For our other source, J2310+1855, a L_FIR 50% AGN contribution does not display higher than expected values and does not require a high coupling efficiency or AGN contribution even with a 100% outflow covering fraction. In summary, an AGN contribution to the driving of the molecular outflow in J2310+1855 is not required and is only possibly required for P183+05 under particular extreme scenarios.

Recent results studying the ionized gas phase, however, have found extremely fast outflows up to 17% the speed of light in z ∼ 6 QSOs using C i v absorption (Bischetti et al. 2022). Such outflows must transport enormous amounts of energy into the GCM and IGM from the central engine. Included in this sample is J2310+1855, with a detected ionized gas outflow traveling between 18,500 and 27,000 km s⁻¹. Although clearly not associated with the molecular outflowing gas traveling at just ∼300 km s⁻¹, this may suggest a scenario where a combination of star-formation-driven molecular outflows and AGN-driven ionized outflows work in conjunction to quench these massive active galaxies and transfer energy to the atomic outflows found at higher radii in the surrounding halos of diffuse turbulent gas (as found for J2310+1855 in OH⁺; Shao et al. 2022). As seen in low-z studies, molecular gas outflows dominate the mass and momentum transport of matter out of the galaxy, while the warmer ionized phases dominate the kinetic energy budget (Fluetsch et al. 2021). The molecular outflows presented here are indeed capable of transporting large amounts of gas, providing short depletion times on the order of megayears predicted for these galaxies (Simpson et al. 2012; Costa et al. 2018). As discussed in Section 4.3, however, a negligible fraction of the outflowing molecular gas will escape the gravitational bind of their host galaxy, and will be re-accreted unless additional energetic input is provided at higher radii. Thus, while the molecular phase may predominantly remove the fuel for future star formation directly from the galaxy, the ionized phase may be responsible for keeping that material at higher radii by efficiently injecting energy into the CGM and thus providing a scenario where these massive gas-rich galaxies can quench on short timescales.

During the QSOs' embedded or blowout phase, however, trapped radiation from the central engine may play a much greater role in launching molecular outflows (Costa et al. 2018). Observing QSOs in these early stages will therefore be crucial in our studies determining the full impact of QSOs on their surrounding ISMs and the role they play in ejecting gas from the galaxy.

The conclusion of the previous subsection does not, however, explain the differences in spectral properties of the OH outflows observed between our z > 6 QSO and the high-z DSFGs. The combination of narrower line widths and comparable or higher EWs of the total outflow absorption may imply an increase in the outflow covering fraction or clump positions that preferentially block brighter regions of the background dust emission. Both scenarios may be true in the case of J2310+1855 whose small 119 μm dust size and high surface brightness (Figure 3) means any clumps intervening the LOS will have the tendency to cover brighter regions, and will block a larger fraction of the overall surface area. For galaxy disks at higher inclinations along the observer's LOS, such effects of intervening clouds are further intensified.

We illustrate this effect in Figure 7, for two galaxies of equal luminosity but different sizes (different surface brightnesses), observed at four different inclinations. We consider the effect of the background continuum size and inclination on the observability of foreground clumps traveling in a galactic fountain trajectory, ejected from the center of the galaxy. We show this trajectory for five radial directions but note that clumps may be ejected from anywhere in the galactic plane and travel outwards in any direction. Clumps that intervene the observer's LOS and background dust continuum and are therefore observable via absorption are colored orange. We find that for smaller disk sizes and higher inclinations, absorption line observations become limited to clumps at low radial distances or those lying more toward the foreground. Individual clumps in these circumstances cover a greater fraction of the background dust area and tend to block regions of higher surface brightness, thus imprinting a stronger absorption signature on the global spectrum. Fewer clumps are therefore needed to absorb the same or more background emission, limiting the number of velocity components contributing to the velocity dispersion of the global absorption line.

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Without resolving the molecular outflow we cannot say for certain what the geometry or trajectory of the outflowing gas are. Figure 7 does, however, illustrate that clumps at smaller scale heights are preferentially observed. If a clump is initially ejected perpendicular to the disk (the center or farther out), and then continues along a trajectory that deviates from this initial direction, (such as the fountain scenario depicted in Figure 7), the restriction of our observations to low-altitude clumps also restricts our observations to clumps with higher perpendicular velocities. The combined outflow velocity therefore becomes more dependent on the inclination of the disk.

Thus, the smaller disk size of J2310+1855 may cause the absorption signature to be more sensitive to projection effects, contributing to the offset in outflow velocity shown in the first panel of Figure 3. Neither J2310+1855 nor P183+05, however, are found to have significantly inclined disks (25° and <22°), and so it is not expected that projection effects play a dominating role in the slower than expected outflow velocities. We note, however, that Tripodi et al. (2022) found similar results for their dynamical models of J2310+1855 for inclinations over the range of [20, 45]°, and earlier dynamical modeling by Feruglio et al. (2018) using lower-resolution data have previously found a higher inclination of 53°. Tripodi et al. (2022) discarded models with i > 30° due to the gas mass exceeding the dynamical mass, however, this cut off is dependent on the reliability and suitability of the CO conversion factor for a high-z unobscured QSO. In the case of P183+05, which does not display an unusually small continuum size and has only an upper limit on the host galaxy inclination, it is unclear what causes the smaller than expected FWHM of the absorption line and we do not rule out simple natural variance.

Small QSO host dust sizes have been measured for a number of samples at high-z (Ikarashi et al. 2015; D'Amato et al. 2020; Venemans et al. 2020; Stacey et al. 2021), although evidence for an additional shallower extended component is found in the stacking of 27 z ≥ 6 QSO hosts (Novak et al. 2020). Stacey et al. (2021) divided their z = 1.5–2.8 QSO hosts into two categories: those exhibiting clumpy dust distributions, with sizes and SFR densities typical of sub-mm-selected DSFGs, and those with no evidence of clumpy features and characterized by compact (R_eff < 1 kpc) sizes and high star formation densities. The latter category is believed to indicate a phase of dissipative contraction before becoming a compact spheroid, and describes both the QSO hosts presented in this paper and the DSFGs presented by Spilker et al. (2020a). The optical/IR sizes of QSO hosts at z ∼ 1.5 (Silverman et al. 2019), place them between main-sequence and quiescent galaxies, while Ikarashi et al. (2015) found in their sample of z ∼ 1–3 QSOs and starbursts, that composite sources harbor the smallest sub-mm dust sizes. While all three of our QSO sources lie within the range of dust continuum sizes of the DSFG comparison sample, they do lie at the extreme end of the surface brightnesses. In particular, J2310+1855 exhibits a much smaller continuum size for its IR luminosity and the highest surface brightness of the two samples, possibly indicating that this source is in a more advanced stage in the transformation into a quiescent spheroidal, consistent with the shorter depletion timescale estimated for this source. We note again that our size estimate of P036+03 was significantly larger than previous measurements (Venemans et al. 2020), which would instead make P036+03 our most compact source, and possibly also further along in its dissipative collapse. This may also provide an explanation for the absence of OH 119 μm absorption in P036+03, as outflows from a more-evolved system may have traveled farther away from the bright central region, and a more compact background emission decreases the total area in which an outflow may be observed. The combination of these effects would decrease the likelihood of an outflowing component intercepting the LOS with the background continuum.

OH 119 μm emission has been detected in many of the low-z galaxy samples (Sturm et al. 2011; Spoon et al. 2013; Veilleux et al. 2013; Calderón et al. 2016; Stone et al. 2016), and is typically associated with the presence of an AGN. AGN fraction, and not simply AGN luminosity, was found to be key in setting the OH 119 μm character (relative strength of emission to absorption) (Veilleux et al. 2013; Stone et al. 2016). In the most AGN-dominated systems where OH is seen purely in emission, Veilleux et al. (2013) notes that the line widths remain modest, possibly indicating a phase after which the galaxy has cleared away the obscuring molecular gas.

Deeper 9.7 μm silicate absorption (a measure of obscuration in both nuclear torii and host galaxy disks) has been found to correlate with fainter OH 119 μm emission and deeper OH 119 μm absorption, indicating that OH 119 μm emission is strongly affected by the geometry of the obscuring material (Stone et al. 2016). While earlier studies argued that OH 119 μm emission originates from the buried nuclear regions of galaxies (e.g., Spoon et al. 2013), where radiative pumping would predominantly excite the OH, Stone et al. (2016) later showed that in some Type 2 AGNs (where the nucleus is obscured), and where the silicate 9.7 μm line is found in absorption, some of the OH 119 μm is still found in emission. Given the apparent impact of obscuring material on OH 119 μm emission this may imply that the OH 119 μm emission also originates farther out than the nucleus, such as in a circumnuclear starburst. Here, physical conditions favor collisional excitation over radiative pumping. Similar conclusions have emerged from a studies measuring the relative strengths of OH transitions (e.g., in NGC 1068; Spinoglio et al. 2005) and in studies using both silicate absorption and multi OH transition data (Runco et al. 2020).

OH 119 μm emission was not detected in any of the Spilker et al. (2020a) high-z DSFGs, consistent with their measured subdominant AGN fractions (all upper limits). OH 119 μm emission is likely present in the z = 6.13 QSO ULAS J131911+095051 (Herrera-Camus et al. 2020), causing the blueshifted absorption observed in it is spectra to exhibit only a single absorption peak, due to superimposed emission and absorption of the low- and high-frequency doublet transitions, respectively. A similar scenario is more clearly observed for P183+05 in our sample. Tentative emission is also detected for a second of our sources, P036+03, but not in that of J2310+1855, which instead displays the strongest OH absorption. Thus, as seen at low-z, it appears the presence of an AGN in high-z sources is also required to provide an environment in which OH 119 μm emission may be observed. Also consistent with low-z observations, emission line widths appear to be modest.

All three of our QSOs are unobscured and as mentioned in our previous discussion sections, it is suspected that the outflowing OH observed in absorption is ejected via star formation feedback and thus likely not to have originated from the nucleus region. It may then be expected that the gas traced by OH 119 μm emission, is also, at least in part, located farther out from the AGN, perhaps in a circumnuclear starburst as predicted for some low-z galaxies (Spinoglio et al. 2005; Spoon et al. 2013; Runco et al. 2020). Furthermore, we find in the case of P183+05, that the OH 119 μm emission is spatially more extended than the background continuum. We cannot however determine with a single OH transition what excitation mechanism is dominant in these sources. Future multiline and spatially resolved OH observations will be needed to elucidate the nature and location of these environments.