Why stellar feedback promotes disc formation in simulated galaxies

We study how feedback influences baryon infall onto galaxies using cosmological, zoom-in simulations of haloes with present mass $M_{vir}=6.9\times10^{11} M_{\odot}$ to $1.7\times10^{12} M_{\odot}$. Starting at z=4 from identical initial conditions, implementations of weak and strong stellar feedback produce bulge- and disc-dominated galaxies, respectively. Strong feedback favours disc formation: (1) because conversion of gas into stars is suppressed at early times, as required by abundance matching arguments, resulting in flat star formation histories and higher gas fractions; (2) because 50% of the stars form in situ from recycled disc gas with angular momentum only weakly related to that of the z=0 dark halo; (3) because late-time gas accretion is typically an order of magnitude stronger and has higher specific angular momentum, with recycled gas dominating over primordial infall; (4) because 25-30% of the total accreted gas is ejected entirely before z~1, removing primarily low angular momentum material which enriches the nearby inter-galactic medium. Most recycled gas roughly conserves its angular momentum, but material ejected for long times and to large radii can gain significant angular momentum before re-accretion. These processes lower galaxy formation efficiency in addition to promoting disc formation.Comment: 23 pages, 29 figures, accepted for publication in MNRA

Galaxy kinematics during the peak epoch of cosmic star formation

Diese Arbeit befasst sich mit der Kinematik von Sterne bildenden Galaxien (SFGs) während der Hochzeit der kosmischen Sternentstehung, bei Rotverschiebungen 0.5<z<3. Basierend auf den genommenen Beobachtungen wird abgeleitet, welche Massenkomponenten die Galaxien dynamisch stabilisieren, und wie sich deren Beitrag im Laufe von 6 Milliarden Jahren verändert. Um einen Zusammenhang zwischen einerseits der beobachtbaren Masse in der Form von Sternen und Gas und andererseits der Dunklen Materie in Galaxien herzustellen, werden die Tully-Fisher-Beziehungen genutzt. Es zeigt sich, dass die dynamische Stabilität der SFGs bei z~2.3 durch Gas und Sterne dominiert wird, während bei z~0.9 Dunkle Materie relevanter wird. Bei gleichbleibender Kreisgeschwindigkeit haben SFGs bei z~2.3 und z~0.9 die gleiche stellare Masse, aber ihre Gasmasse ist bei höherer Rotverschiebung größer. Auf der Grundlage von vorhandenen Modellen der Galaxienentwicklung wird ein Toy-Modell entwickelt, das die zeitlichen Änderung in der stellaren und gasförmigen Masse typischer SFGs in Betracht zieht, um die beobachtete, nicht-monotone Entwicklung der Tully-Fisher-Beziehungen von z~2.3 bis z=0 zu erklären. Durch die graduelle Umwandlung von Gas zu Sternen verändert sich das interstellare Medium und dessen Einfluss auf die Galaxienkinematik. Die Entwicklung der intrinsischen Geschwindigkeitsdispersion des ionisierten Gases in typischen SFGs wird diskutiert sowie die Streuung und mögliche Ursachen dieser turbulenten Bewegungen. Durch Beobachtungsdaten sowie theoretische Überlegungen wird gezeigt, dass die galaktische Turbulenz bei z>2 höchstwahrscheinlich durch gravitative Instabilitäten dominiert wird, während diese zu späterer kosmischer Zeit weniger bedeutsam werden, und so der Einfluss von stellaren Feedbackprozessen an Relevanz gewinnen kann. Eine genauere Analyse der Kinematik individueller, massiver SFGs wird vorgenommen, um die Beiträge sichtbarer und Dunkler Materie zur Galaxiendynamik mit höherer räumlicher Auflösung und bis zu größeren galaktischen Radien zu untersuchen. Besonders bei z>2 finden sich sehr turbulente und extrem baryonisch dominierte Systeme mit fallenden Rotationskurven auf der Basis von ionisiertem Gas. In einer detaillierten Fallstudie, die Messungen des ionisierten sowie molekularen Gases kombiniert, wird gezeigt, dass die Kinematik dieser beiden Gasphasen ausgezeichnet übereinstimmt. Dieses Ergebnis ist eine wichtige Demonstration dessen, dass die Bewegungen des ionisiertes Gases das Gravitationspotential abbilden. Durch einen Vergleich der Beobachtungsdaten mit modernen kosmologischen Simulationen werden Unterschiede im Gasgehalt und in der Kinematik massiver z~2 SFGs identifiziert, die vermutlich auf Unzulänglichkeiten in den Simulationen aufgrund nicht aufgelöster physikalischer Prozesse im interstellaren Medium und deren Implementierung hinweisen.In this thesis we discuss the kinematics of star-forming galaxies (SFGs) during the peak epoch of cosmic star formation rate density, at redshifts 0.5<z<3. Based on our observations, we deduce information on their mass budget and dynamical support, and we follow its evolution over 6 billion years of cosmic history. We use the Tully-Fisher relations to connect the observable stellar and total baryonic mass to dark matter on galactic scales, and find that at z~2.3 the galactic dynamical support is dominated by gas and stellar mass, while at z~0.9 dark matter becomes more important. At fixed circular velocity, SFGs have the same amount of stellar mass at z~2.3 and z~0.9, but their gas masses are higher at higher redshift. Based on existing models of galaxy evolution, we develop a toy model taking into account changes in the stellar and gas content of typical SFGs, to explain the observed, non-monotonic evolution of the Tully-Fisher relations from z~2.6 to z=0. Through the gradual conversion of gas into stars, the dynamical state of the interstellar medium and its impact on the galaxy kinematics changes. We discuss the evolution of the intrinsic velocity dispersion of ionized gas in typical SFGs, its scatter, and possible mechanisms driving these turbulent motions. Based on both observational and theoretical evidence we conclude that at z>2 gas turbulence is likely dominated by gravitational instabilities, while towards lower redshift these mechanisms become less important and therefore the impact of stellar feedback may become comparable. We zoom in on the kinematics of individual, massive SFGs to investigate in more detail the dynamical contributions of luminous and dark matter with higher spatial resolution and out to larger galactic radii. Especially at z>2 we find very turbulent, strongly baryon-dominated systems with dropping outer rotation curves traced by ionized gas emission. In a detailed case study combining measurements from ionized and molecular gas, we show that the observed kinematics in both tracers are in excellent agreement. This result is an important demonstration that the ionized gas reliably traces the gravitational potential. Through comparison of our observations with modern cosmological simulations, we identify differences in gas content and kinematics of massive z~2 SFGs that likely point towards shortcomings in the simulations introduced by unresolved physics in the interstellar medium, and their implementation via sub-grid recipes

Galaxy kinematics during the peak epoch of cosmic star formation

Diese Arbeit befasst sich mit der Kinematik von Sterne bildenden Galaxien (SFGs) während der Hochzeit der kosmischen Sternentstehung, bei Rotverschiebungen 0.5<z<3. Basierend auf den genommenen Beobachtungen wird abgeleitet, welche Massenkomponenten die Galaxien dynamisch stabilisieren, und wie sich deren Beitrag im Laufe von 6 Milliarden Jahren verändert. Um einen Zusammenhang zwischen einerseits der beobachtbaren Masse in der Form von Sternen und Gas und andererseits der Dunklen Materie in Galaxien herzustellen, werden die Tully-Fisher-Beziehungen genutzt. Es zeigt sich, dass die dynamische Stabilität der SFGs bei z~2.3 durch Gas und Sterne dominiert wird, während bei z~0.9 Dunkle Materie relevanter wird. Bei gleichbleibender Kreisgeschwindigkeit haben SFGs bei z~2.3 und z~0.9 die gleiche stellare Masse, aber ihre Gasmasse ist bei höherer Rotverschiebung größer. Auf der Grundlage von vorhandenen Modellen der Galaxienentwicklung wird ein Toy-Modell entwickelt, das die zeitlichen Änderung in der stellaren und gasförmigen Masse typischer SFGs in Betracht zieht, um die beobachtete, nicht-monotone Entwicklung der Tully-Fisher-Beziehungen von z~2.3 bis z=0 zu erklären. Durch die graduelle Umwandlung von Gas zu Sternen verändert sich das interstellare Medium und dessen Einfluss auf die Galaxienkinematik. Die Entwicklung der intrinsischen Geschwindigkeitsdispersion des ionisierten Gases in typischen SFGs wird diskutiert sowie die Streuung und mögliche Ursachen dieser turbulenten Bewegungen. Durch Beobachtungsdaten sowie theoretische Überlegungen wird gezeigt, dass die galaktische Turbulenz bei z>2 höchstwahrscheinlich durch gravitative Instabilitäten dominiert wird, während diese zu späterer kosmischer Zeit weniger bedeutsam werden, und so der Einfluss von stellaren Feedbackprozessen an Relevanz gewinnen kann. Eine genauere Analyse der Kinematik individueller, massiver SFGs wird vorgenommen, um die Beiträge sichtbarer und Dunkler Materie zur Galaxiendynamik mit höherer räumlicher Auflösung und bis zu größeren galaktischen Radien zu untersuchen. Besonders bei z>2 finden sich sehr turbulente und extrem baryonisch dominierte Systeme mit fallenden Rotationskurven auf der Basis von ionisiertem Gas. In einer detaillierten Fallstudie, die Messungen des ionisierten sowie molekularen Gases kombiniert, wird gezeigt, dass die Kinematik dieser beiden Gasphasen ausgezeichnet übereinstimmt. Dieses Ergebnis ist eine wichtige Demonstration dessen, dass die Bewegungen des ionisiertes Gases das Gravitationspotential abbilden. Durch einen Vergleich der Beobachtungsdaten mit modernen kosmologischen Simulationen werden Unterschiede im Gasgehalt und in der Kinematik massiver z~2 SFGs identifiziert, die vermutlich auf Unzulänglichkeiten in den Simulationen aufgrund nicht aufgelöster physikalischer Prozesse im interstellaren Medium und deren Implementierung hinweisen.In this thesis we discuss the kinematics of star-forming galaxies (SFGs) during the peak epoch of cosmic star formation rate density, at redshifts 0.5<z<3. Based on our observations, we deduce information on their mass budget and dynamical support, and we follow its evolution over 6 billion years of cosmic history. We use the Tully-Fisher relations to connect the observable stellar and total baryonic mass to dark matter on galactic scales, and find that at z~2.3 the galactic dynamical support is dominated by gas and stellar mass, while at z~0.9 dark matter becomes more important. At fixed circular velocity, SFGs have the same amount of stellar mass at z~2.3 and z~0.9, but their gas masses are higher at higher redshift. Based on existing models of galaxy evolution, we develop a toy model taking into account changes in the stellar and gas content of typical SFGs, to explain the observed, non-monotonic evolution of the Tully-Fisher relations from z~2.6 to z=0. Through the gradual conversion of gas into stars, the dynamical state of the interstellar medium and its impact on the galaxy kinematics changes. We discuss the evolution of the intrinsic velocity dispersion of ionized gas in typical SFGs, its scatter, and possible mechanisms driving these turbulent motions. Based on both observational and theoretical evidence we conclude that at z>2 gas turbulence is likely dominated by gravitational instabilities, while towards lower redshift these mechanisms become less important and therefore the impact of stellar feedback may become comparable. We zoom in on the kinematics of individual, massive SFGs to investigate in more detail the dynamical contributions of luminous and dark matter with higher spatial resolution and out to larger galactic radii. Especially at z>2 we find very turbulent, strongly baryon-dominated systems with dropping outer rotation curves traced by ionized gas emission. In a detailed case study combining measurements from ionized and molecular gas, we show that the observed kinematics in both tracers are in excellent agreement. This result is an important demonstration that the ionized gas reliably traces the gravitational potential. Through comparison of our observations with modern cosmological simulations, we identify differences in gas content and kinematics of massive z~2 SFGs that likely point towards shortcomings in the simulations introduced by unresolved physics in the interstellar medium, and their implementation via sub-grid recipes

Metal enrichment and evolution in four z > 6.5 quasar sightlines observed with JWST/NIRSpec

We present JWST/NIRSpec R$\approx$2700 spectra of four high-redshift quasars: VDES J0020-3653 (z = 6.860), DELS J0411-0907(z = 6.825), UHS J0439+1634 (z = 6.519) and ULAS J1342+0928 (z = 7.535). The exquisite data quality, signal-to-noise ratio of 50-200, and large $0.86\!~\mu{\rm m}\le \lambda \le 5.5\!~\mu{\rm m}$ spectral coverage allows us to identify between 13 and 17 intervening and proximate metal absorption line systems in each quasar spectrum, with a total number of 61 absorption-line systems detected at $2.42<z<7.48$ including the highest redshift intervening OI $\lambda$1302 and MgII systems at $z=7.37$ and $z=7.44$. We investigate the evolution of the metal enrichment in the epoch of reionization at $z>6$ and find: i) A continued increase of the low-ionization OI, CII, and SiII incidence, ii) Decreasing high-ionization CIV and SiIV incidence with a transition from predominantly high- to low-ionization at $z\approx6.0$, and iii) a constant MgII incidence across all redshifts. The observations support a change in the ionization state of the intergalactic medium in the EoR rather than a change in metallicity. The abundance ratio of [Si/O] in five $z>6$ absorption systems show enrichment signatures produced by low-mass Pop III pair instability supernovae, and possibly Pop III hypernovae. In the Gunn-Peterson troughs we detect transmission spikes where Ly$\alpha$ photons can escape. From 22 absorption systems at $z>5.7$, only a single low-ionization system out of 13 lies within 2000 km s$^{-1}$ from a spike, while four high-ionization systems out of nine lie within $\sim$2000 km s$^{-1}$ from a spike. This confirms that galaxies responsible for the heavy elements that are transported into the circumgalactic medium lie in predominantly in high-density, neutral environments, while lower density environments are ionized without being polluted by metals at $z\approx$ 6-7. [abridged]Comment: 50 pages including 30 pages of appendices. Submitted to A&

The MOSDEF Survey: Kinematic and Structural Evolution of Star-Forming Galaxies at 1.4≤z≤3.81.4\leq z\leq 3.8

We present ionized gas kinematics for 681 galaxies at $z\sim 1.4-3.8$ from the MOSFIRE Deep Evolution Field survey, measured using models which account for random galaxy-slit misalignments together with structural parameters derived from CANDELS Hubble Space Telescope (HST) imaging. Kinematics and sizes are used to derive dynamical masses. Baryonic masses are estimated from stellar masses and inferred gas masses from dust-corrected star formation rates (SFRs) and the Kennicutt-Schmidt relation. We measure resolved rotation for 105 galaxies. For the remaining 576 galaxies we use models based on HST imaging structural parameters together with integrated velocity dispersions and baryonic masses to statistically constrain the median ratio of intrinsic ordered to disordered motion, $V/\sigma_{V,0}$. We find that $V/\sigma_{V,0}$ increases with increasing stellar mass and decreasing specific SFR (sSFR). These trends may reflect marginal disk stability, where systems with higher gas fractions have thicker disks. For galaxies with detected rotation we assess trends between their kinematics and mass, sSFR, and baryon surface density ($\Sigma_{\mathrm{bar},e}$). Intrinsic dispersion correlates most with $\Sigma_{\mathrm{bar},e}$ and velocity correlates most with mass. By comparing dynamical and baryonic masses, we find that galaxies at $z\sim 1.4-3.8$ are baryon dominated within their effective radii ($R_E$), with Mdyn/Mbaryon increasing over time. The inferred baryon fractions within $R_E$, $f_{\mathrm{bar}}$, decrease over time, even at fixed mass, size, or surface density. At fixed redshift, $f_{\mathrm{bar}}$ does not appear to vary with stellar mass but increases with decreasing $R_E$ and increasing $\Sigma_{\mathrm{bar},e}$. For galaxies at $z\geq2$, the median inferred baryon fractions generally exceed 100%. We discuss possible explanations and future avenues to resolve this tension.Comment: Accepted to ApJ. Added Figure 9, corrected sample size (main results unchanged). 28 pages, 13 figure

Physics of ULIRGs with MUSE and ALMA: The PUMA project: III. Incidence and properties of ionised gas disks in ULIRGs, associated velocity dispersion, and its dependence on starburstiness

CONTEXT: A classical scenario suggests that ultra-luminous infrared galaxies (ULIRGs) transform colliding spiral galaxies into a spheroid-dominated early-type galaxy. Recent high-resolution simulations have instead shown that, under some circumstances, rotation disks can be preserved during the merging process or rapidly regrown after coalescence. Our goal is to analyse in detail the ionised gas kinematics in a sample of ULIRGs to infer the incidence of gas rotational dynamics in late-stage interacting galaxies and merger remnants. AIMS: We analysed integral field spectrograph MUSE data of a sample of 20 nearby (z < 0.165) ULIRGs (with 29 individual nuclei) as part of the Physics of ULIRGs with MUSE and ALMA (PUMA) project. We used multi-Gaussian fitting techniques to identify gaseous disk motions and the 3D-Barolo tool to model them. METHODS: We found that 27% (8 out of 29) individual nuclei are associated with kiloparsec-scale disk-like gas motions. The rest of the sample displays a plethora of gas kinematics, dominated by winds and merger-induced flows, which makes the detection of rotation signatures difficult. On the other hand, the incidence of stellar disk-like motions is ∼2 times larger than gaseous disks, as the former are probably less affected by winds and streams. The eight galaxies with a gaseous disk present relatively high intrinsic gas velocity dispersion (σ0 ∈ [30 − 85] km s−1), rotationally supported motions (with gas rotation velocity over velocity dispersion vrot/σ0 ∼ 1 − 8), and dynamical masses in the range (2 − 7)×1010 M⊙. By combining our results with those of local and high-z disk galaxies (up to z ∼ 2) from the literature, we found a significant correlation between σ0 and the offset from the main sequence (δMS), after correcting for their evolutionary trends. RESULTS: Our results confirm the presence of kiloparsec-scale rotating disks in interacting galaxies and merger remnants in the PUMA sample, with an incidence going from 27% (gas) to ≲50% (stars). Their gas σ0 is up to a factor of ∼4 higher than in local normal main sequence galaxies, similar to high-z starbursts as presented in the literature; this suggests that interactions and mergers enhance the star formation rate while simultaneously increasing the velocity dispersion in the interstellar medium

The PUMA project. III. Incidence and properties of ionised gas disks in ULIRGs, associated velocity dispersion and its dependence on starburstiness

A classical scenario suggests that ULIRGs transform colliding spiral galaxies into a spheroid dominated early-type galaxy. Recent high-resolution simulations have instead shown that, under some circumstances, rotation disks can be preserved during the merging process or rapidly regrown after coalescence. Our goal is to analyze in detail the ionised gas kinematics in a sample of ULIRGs to infer the incidence of gas rotational dynamics in late-stage interacting galaxies and merger remnants. We analysed MUSE data of a sample of 20 nearby (z<0.165) ULIRGs, as part of the "Physics of ULIRGs with MUSE and ALMA" (PUMA) project. We found that 27% individual nuclei are associated with kpc-scale disk-like gas motions. The rest of the sample displays a plethora of gas kinematics, dominated by winds and merger-induced flows, which make the detection of rotation signatures difficult. On the other hand, the incidence of stellar disk-like motions is ~2 times larger than gaseous disks, as the former are probably less affected by winds and streams. The eight galaxies with a gaseous disk present relatively high intrinsic gas velocity dispersion (sigma = 30-85 km/s), rotationally-supported motions (with gas rotation velocity over velocity dispersion vrot/sigma > 1-8), and dynamical masses in the range (2-7)x1e10 Msun. By combining our results with those of local and high-z disk galaxies from the literature, we found a significant correlation between sigma and the offset from the main sequence (MS), after correcting for their evolutionary trends. Our results confirm the presence of kpc-scale rotating disks in interacting galaxies and merger remnants, with an incidence going from 27% (gas) to ~50% (stars). The ULIRGs gas velocity dispersion is up to a factor of ~4 higher than in local normal MS galaxies, similar to high-z starbursts as presented in the literature

The Kinematics and Dark Matter Fractions of TNG50 Galaxies at z=2 from an Observational Perspective

We contrast the gas kinematics and dark matter contents of $z=2$ star-forming galaxies (SFGs) from state-of-the-art cosmological simulations within the $\Lambda$CDM framework to observations. To this end, we create realistic mock observations of massive SFGs ($M_*>4\times10^{10} M_{\odot}$, SFR $>50~M_{\odot}$ yr$^{-1}$) from the TNG50 simulation of the IllustrisTNG suite, resembling near-infrared, adaptive-optics assisted integral-field observations from the ground. Using observational line fitting and modeling techniques, we analyse in detail the kinematics of seven TNG50 galaxies from five different projections per galaxy, and compare them to observations of twelve massive SFGs by Genzel et al. (2020). The simulated galaxies show clear signs of disc rotation but mostly exhibit more asymmetric rotation curves, partly due to large intrinsic radial and vertical velocity components. At identical inclination angle, their one-dimensional velocity profiles can vary along different lines of sight by up to $\Delta v=200$ km s$^{-1}$. From dynamical modelling we infer rotation speeds and velocity dispersions that are broadly consistent with observational results. We find low central dark matter fractions compatible with observations ($f_{\rm DM}^v(<R_e)=v_{\rm DM}^2(R_e)/v_{\rm circ}^2(R_e)\sim0.32\pm0.10$), however for disc effective radii $R_e$ that are mostly too small: at fixed $R_e$ the TNG50 dark matter fractions are too high by a factor of $\sim2$. We speculate that the differences in gas kinematics and dark matter content compared to the observations may be due to physical processes that are not resolved in sufficient detail with the numerical resolution available in current cosmological simulations.Comment: 25 pages, 16 figures, accepted for publication in MNRA

Galaxy kinematics and mass estimates at z~1 from ionised gas and stars

We compare ionised gas and stellar kinematics of 16 star-forming galaxies ($\log(M_\star/M_\odot)=9.7-11.2$, SFR=6-86 $M_\odot/yr$) at $z\sim1$ using near-infrared integral field spectroscopy (IFS) of H$\alpha$ emission from the KMOS$^{\rm 3D}$ survey and optical slit spectroscopy of stellar absorption and gas emission from the LEGA-C survey. H$\alpha$ is dynamically colder than stars, with higher disc rotation velocities (by ~45 per cent) and lower disc velocity dispersions (by a factor ~2). This is similar to trends observed in the local Universe. We find higher rotational support for H$\alpha$ relative to [OII], potentially explaining systematic offsets in kinematic scaling relations found in the literature. Regarding dynamical mass measurements, for six galaxies with cumulative mass profiles from Jeans Anisotropic Multi-Gaussian Expansion (JAM) models the H$\alpha$ dynamical mass models agree remarkably well out to ~10 kpc for all but one galaxy (average $\Delta M_{\rm dyn}(R_{e,\rm F814W})<0.1$ dex). Simpler dynamical mass estimates based on integrated stellar velocity dispersion are less accurate (standard deviation 0.24 dex). Differences in dynamical mass estimates are larger, for example, for galaxies with stronger misalignments of the H$\alpha$ kinematic major axis and the photometric position angle, highlighting the added value of IFS observations for dynamics studies. The good agreement between the JAM models and the dynamical models based on H$\alpha$ kinematics at $z\sim1$ corroborates the validity of dynamical mass measurements from H$\alpha$ IFS observations also for higher redshift rotating disc galaxies.Comment: 29 pages, 16 figures, 4 tables; accepted for publication in MNRA

From Nuclear to Circumgalactic:Zooming in on AGN-driven Outflows at z ∼ 2.2 with SINFONI

We use deep adaptive optics assisted integral field spectroscopy from SINFONI on the VLT to study the spatially resolved properties of ionized gas outflows driven by active galactic nuclei (AGN) in three galaxies at z~2.2 -- K20-ID5, COS4-11337 and J0901+1814. These systems probe AGN feedback from nuclear to circumgalactic scales, and provide unique insights into the different mechanisms by which AGN-driven outflows interact with their host galaxies. K20-ID5 and COS4-11337 are compact star forming galaxies with powerful $\sim$1500 km s$^{-1}$ AGN-driven outflows that dominate their nuclear H$\alpha$ emission. The outflows do not appear to have any impact on the instantaneous star formation activity of the host galaxies, but they carry a significant amount of kinetic energy which could heat the halo gas and potentially lead to a reduction in the rate of cold gas accretion onto the galaxies. The outflow from COS4-11337 is propagating directly towards its companion galaxy COS4-11363, at a projected separation of 5.4 kpc. COS4-11363 shows signs of shock excitation and recent truncation of star formation activity, which could plausibly have been induced by the outflow from COS4-11337. J0901+1814 is gravitationally lensed, giving us a unique view of a compact (R = 470 $\pm$ 70 pc), relatively low velocity ($\sim$650 km s$^{-1}$) AGN-driven outflow. J0901+1814 has a similar AGN luminosity to COS4-11337, suggesting that the difference in outflow properties is not related to the current AGN luminosity, and may instead reflect a difference in the evolutionary stage of the outflow and/or the coupling efficiency between the AGN ionizing radiation field and the gas in the nuclear regions.Comment: Accepted for publication in ApJ. Main text 23 pages, 15 figures and 4 tables, plus Appendix (3 pages, 3 figures, 1 table