Under Pressure: UV Emission Line Ratios as Barometers of AGN Feedback Mechanisms

Abstract

Feedback from active galactic nuclei (AGN) is widely acknowledged to regulate the growth of massive galaxies, though its driving mechanisms are debated. Prevailing theories suggest that AGN-driven outflows are driven either by radiation pressure acting directly on the dusty interstellar medium (ISM) or by hot winds entraining cooler ISM gas, but the relative contribution of each mechanism remains uncertain. By combining optical emission line measurements with highly ionized UV emission lines, it is possible to constrain whether the pressure source applied to ionized clouds is primarily radiation or primarily hydrodynamic, and thus constrain the dominant driver. This study presents the first multi-object analysis of far-ultraviolet (FUV) spectra from galactic-scale AGN-driven outflows in obscured quasars, based on Cosmic Origins Spectrograph observations of five low-redshift targets. By comparing narrow-line region UV emission line ratios to theoretical models that vary the importance of the two pressure sources, we find three out of five targets fall within the radiation pressure-dominated regime. A fourth target exhibits intermediate emission-line ratios that suggest radiation pressure and pressure from a hot wind are both dynamically important. Finally, the lowest-luminosity object in our sample may have a dynamically important hot wind component, but non-detections prevent a clear conclusion in this case. These results suggest radiation pressure dominates circum-nuclear narrow-line region cloud dynamics, but pressure from a hot wind also plays a role in some cases. This is consistent with AGN feedback scenarios mediated by radiation pressure or a short-lived hot wind phase that dissipates after initially accelerating outflows.

Figures (5)

• Figure 1: Gaussian fits to SDSS optical emission lines for our five target AGN. "Narrow" components are in gold, "broad" components in blue, and "additional" components in green. Emission line regions span similar ranges between objects of $\approx$2000 km/s (H$\beta$ region), $\approx$5000 km/s ([O3] region), and $\approx$2500 km/s (H$\alpha$+[N2] region) on either side of the central emission line (or the midpoint between both, in the case of [O3]). For more on these fits, see Section \ref{['subsection: optical fluxes']}.
• Figure 2: COS UV nuclear spectra for our five AGN targets, with flux in black and its uncertainty in gray. Dashed light blue lines mark the expected location of the labeled spectral line given the target's redshift. For observed doublets (O6 $\lambda\lambda$1031, 1037 $\textrm{\AA}$, N5 $\lambda\lambda$1238, 1242 $\textrm{\AA}$, and C4 $\lambda\lambda$1548, 1550 $\textrm{\AA}$) the blue line is located at the midpoint of the expected location of each emission line. The two reported non-detections, O6 and C4 for J1255$-$0339, are denoted on their respective labels with open circles. To aid the visual inspection of emission lines, we have masked out the geocoronal Ly$\alpha$ and O I lines, leaving gaps at approximately 1215 and 1304 Å, where applicable
• Figure 3: Left panel: BPT diagram confirming identification of objects as AGN, with target obscured quasars as colored points and SDSS galaxies as small grey points. The solid green and dotted blue lines are the extreme starburst and pure star formation classification lines from Kewley:2006wa. The Seyfert-LINER line from Kauffmann_2003 is a dashed yellow-orange line. The range predicted by radiation pressure-dominated models ($P_{\rm rad}\gg P_{\rm hot}$) discussed in Section \ref{['sec:models']} for the NLR is shown as a hatched teal region. Targets are shown as multi-colored points, while error estimates are smaller than the datapoints. Right panel: [O3] luminosities measured from SDSS spectra versus redshift for our five targets plotted against those of the Type 2 Quasars catalog presented in Reyes_2008. The 68% confidence interval errors, plotted in similar colors atop the data, are smaller than the data points.
• Figure 4: Top: Observed UV and optical diagnostic line ratios and errors for our five obscured quasars, shown as colored points (Left: O6/[O3]$_{5008}$ vs. O6/N5, Middle: O6/[O3]$_{5008}$ vs. O6/C4, Right: O6/N5 vs. O6/C4). Upper-limit ratios from the J1255$-$0339 non-detections of O6 and C4 are shown as light blue points with arrows. Hydrostatic models of AGN photoionized gas varying $P_{\rm rad}/P_{\rm hot}$ from Stern_2016 with $Z/{\rm Z}_\odot=2{\rm Z}_\odot$, $\alpha_{\rm ion}=-1.6$ and $r\approx1\,{\rm kpc}$ are shown in solid black, with ticks marking steps of $P_{\rm rad}/P_{\rm hot}=0.1\,{\rm dex}$. The teal, shaded regions plot the predicted ranges for $P_{\rm rad} \gg P_{\rm hot}$ models with $1 < Z/{\rm Z}_\odot < 4$, $-1.8 < \alpha_{\textrm{ion}} < -1.4$, and $100\,{\rm pc} < r < 10\,{\rm kpc}$. A rough estimate of the hot wind-dominated regime ($P_{\rm hot} > P_{\rm rad}$) is shown in light yellow. Constant density models are shown in red and at high density are equivalent to hydrostatic models with $P_{\rm hot}\gg P_{\rm rad}$ The constant density model values within the same range of dust-to-gas ratios as the hydrostatic models are plotted as light red bands for log(O6/[O3]) only, as it is impacted most by the extinction effects discussed in Section \ref{['subsec:dust']}. Three out of five objects are close to the radiation pressure-dominated limit in all three panels, with two exceptions (J1222$-$0007 and J1255$-$0339). Bottom: Predicted line ratios versus the relative importance of radiation pressure and hot gas pressure for the hydrostatic photoionization models from Stern_2016, shown with the UV diagnostic line ratios and errors for the sample of five obscured quasars. Each object is plotted with a different color to aid visual inspection. Their errors are shown as bars on the left side of their respective plots at arbitrary x-axis positions chosen to aid visibility. The legend in the upper left panel applies to all three plots. The J1255$-$0339 upper-limit O6 and C4 flux estimates are shown with a dashed light blue line and arrows in the left and bottom right diagnostic plots, but omitted from the top right plot since both O6 and C4 are non-detections for this object. In all three plots, a light grey vertical line indicates the $\log(P_{\rm rad}/P_{\rm hot})=0$ models.
• Figure 5: Measured Balmer decrement (H$\alpha$/H$\beta$) ratios versus model $\log (P_{\rm rad}/P_{\rm hot})$(left) and Ly$\alpha$/H$\alpha$ (right), with fluxes corrected for stellar absorption. For reference, both plots include the expected Case B nebula recombination values, as a solid black line in the left panel and a black point in the right panel. To indicate how the line ratio evolves if Case B recombination emission is subject to an external screen of dust characterized by $A_{\rm V}$, the left panel includes a black arrow and the right panel shows a colored line with $A_{\rm V}$ indicated by the color bar. Also included on both plots are dashed black lines representing the $P_{\rm rad}/P_{\rm hot}$ hydrostatic photoionized AGN models which include dust internal to the NLR clouds, with a shaded grey region representing ratios given by the range of dust-to-gas ratios in the $P_{\rm{rad}}\gg P_{\rm{hot}}$ models described in the caption of Figure \ref{['fig:line_ratios']}. Also shown as a dotted gold line is the fiducial model with an additional external dust screen of $A_{\rm V}$=1.5, which best matches the ratios of J1255$-$0339.