Clumpy, dense gas in the outflow of NGC 1266

TL;DR

This study uses archival ALMA Cycle 0 data to probe dense molecular gas in the outflow of the nearby quenching galaxy NGC 1266 by comparing HCN(1-0) and $^{13}$CO(2-1) with prior CO measurements. The results reveal a broad, HCN-traced outflow component with $\sigma\approx145$ km s$^{-1}$, while $^{13}$CO(2-1) shows no broad wing, implying an optically thin CO outflow and a multiphase, clumpy outflow where dense gas is entrained in clumps within a diffuse CO-environment. A revised mass and outflow-rate analysis yields $M_{out}\approx2.4\times10^8$ M$_\odot$ and $\dot{M}_{out}\approx85$ M$_\odot$ yr$^{-1}$, though $\alpha_{HCN}$ uncertainties temper the interpretation; the derived escape rate is $\sim1.7$ M$_\odot$ yr$^{-1}$ and a gas-depletion timescale of about $4.5\times10^8$ yr suggests that low-level AGN feedback could gradually expel the molecular reservoir after an initial quenching event. Overall, the work supports a two-phase outflow where dense, HCN-emitting clumps coexist with diffuse CO-traced gas, highlighting how AGN feedback can regulate star formation both on short timescales (dense-gas suppression) and long timescales (gradual gas removal).

Abstract

Outflows are one of the most spectacular mechanisms through which active galactic nuclei (AGN) impact their host galaxy, though the role of AGN-driven outflows in global star formation regulation across the galaxy population is unclear. NGC 1266 is an excellent case study for investigating the outflows and star formation quenching because it is a nearby (D\sim30 Mpc) AGN host galaxy with an outflow driving shocks through the interstellar medium (ISM) and has recently quenched its star formation outside the nucleus. While previous works have studied the molecular outflow from its CO emission, to fully characterize the impact the outflow has on the ISM observations probing the dense, cold gas are necessary. Our ALMA cycle 0 observations do not detect a molecular outflow in 13CO(2-1) and yield a lower limit 12CO/13CO \geq 250, suggesting a highly optically thin CO outflow with low 13CO abundance. In contrast, we detect substantial HCN(1-0) emission in the outflow, with an HCN(1-0)/12CO(1-0) ratio of 0.09, consistent with global measurements of many star-forming galaxies and Luminous InfraRed Galaxies (LIRGs). We conclude that the CO emission traces a diffuse component of the molecular gas with a low optical depth, whereas the HCN(1-0) traces dense clumps of gas entrained in the outflow. We measure an upper limit molecular outflow rate of < 85 Msun/yr. Assuming the ongoing nuclear star formation and outflow continue at the same rates, NGC 1266 will deplete its gas reservoirs in 450 Myr or longer, indicating that relatively low-level AGN feedback is capable of gradually expelling the molecular gas reservoir after a rapid quenching event.

Figures (7)

• Figure 1: Moment 0, 1, and 2 maps of the ALMA Cycle 0 observations of NGC 1266. Only spaxels with S/N $\geq$ 5 in the moment 0 map are shown. The top row shows the moment maps for $^{13}$CO(2-1), and the bottom row shows the maps for HCN(1-0). The axes are in units of arcseconds offset from the center of the galaxy. The red ellipse in the lower left corner of the moment 0 maps show the ALMA beam size. All panels are centered on the coordinates (3:16:00.75, -2:25:38.70)
• Figure 2: Molecular gas extent in multiple gas tracers. The colormap shows the CO(2-1) emission, the teal contours show the $^{13}$CO(2-1) emission, and the green contours show the unresolved HCN(1-0) emission. The contour levels correspond to 15, 30, 60, and 90% of the maximum flux. The green, red, and teal ellipses show the beam size for the HCN(1-0), CO(2-1), and $^{13}$CO(2-1) observations respectively.
• Figure 3: ALMA HCN(1-0) and $^{13}$CO(2-1) spectra of NGC 1266. Left: HCN(1-0) spectrum in gray bars. The red solid line is the unconstrained two Gaussian fit of the spectrum (fit A). The two red dotted lines show the narrow and wide Gaussian components. Below, the colored bars show the residuals of the plotted fits. Right: all symbols are the same as the left, but with $^{13}$CO(2-1). The thick purple dashed line is the two Gaussian fit of the spectrum with the broad Gaussian component having a fixed width of $\sigma$ = 145 km/s (fit B). The two thinner purple dashed lines show the narrow and broad Gaussian components of this fit. The zero velocity is set to a redshift of 0.0072.
• Figure 4: Fitted two component Gaussian line profiles of various molecular lines in NGC 1266. Three $^{12}$CO transitions from alatalo11a are plotted: $^{12}$CO(1-0) in a dotted blue line, $^{12}$CO(2-1) in a dashed orange line, and $^{12}$CO(3-2) in a dot-dashed green line. From this work, we plot the profile of HCN(1-0) in the thick solid black line, and $^{13}$CO(2-1) in a thick dashed purple line. All profiles have been normalized for comparison. The gray bars show the observed HCN(1-0) spectrum. The $^{12}$CO (from alatalo11a) and the HCN(1-0) line profiles are similar, while the $^{13}$CO line profile is more narrow (this work).
• Figure 5: Histogram of the HCN(1-0)/$^{12}$CO(1-0) flux ratio for a variety of galaxy samples. Bars enclosed by solid lines show measured ratios, and bars enclosed by dashed or dotted lines with hatches show upper and lower limits respectively. The solid vertical line corresponds to the measured NGC 1266 value with single dish observations crocker12, while the dotted and dashed vertical lines show the decomposed systemic and outflow ratios from this work respectively. The top plot includes star-forming galaxies and LIRGs from gao04 in blue and teal bars. Pink bars show LIRGs from the GOALS survey privon15herrero-illana19. In the lower plot, red bars show early type galaxies from the ATLAS3D survey young11, and purple bars show post-starburst galaxies from french23a. The 'dense gas fraction' in all components of NGC 1266 is more similar to LIRGs than other early-type galaxies.