Massive dusty multiphase outflow in local merger shows no sign of slowing on kiloparsec scales

TL;DR

This study presents a detailed, multiphase characterization of the local ULIRG IRAS20100-4156 by combining ALMA CO(1-0) mapping with VLT/MUSE optical spectroscopy. By anchoring molecular gas kinematics to the stellar velocity field, the authors decompose the CO(1-0) profiles into quiescent and outflowing components, revealing a very massive $8\times10^{9}\,M_\odot$ cold molecular outflow that accounts for ~40% of the system’s molecular gas and extends to ~5 kpc with little evidence of deceleration. Ionized and neutral gas phases show faster outflows but substantially lower masses, with shock-like ionization in the outflow and dust embedded in the molecular component, including an estimated $M_{dust,out}\approx3.5\times10^{7}\,M_\odot$. The overall energetics and morphology favor a starburst-driven outflow, with radiation pressure likely contributing to acceleration, and suggest that the outflow may accelerate gas on kiloparsec scales rather than slow down, potentially influencing the host galaxy’s evolution on short timescales.

Abstract

We use ALMA CO(1-0) observations and VLT/MUSE rest-frame optical data of the ultraluminous infrared galaxy (ULIRG) IRAS20100-4156 at $z=0.1297$ to characterize its powerful outflow in multiple phases using tracers of cold molecular, ionized, and neutral atomic gas and dust as well. Our analysis uses the correspondence with the stellar velocity field to split the complex emission line profiles of the CO(1-0) line into components in gravitational and non-gravitational motion. We find a massive ($8\times10^{9}\,M_\odot$) molecular outflow containing about 40% of the total molecular gas mass in the system. The outflow shows a bi-conical morphology centered on the brightest galaxy in the merger, oriented along its minor axis and extending to $\sim5\,\mathrm{kpc}$. This outflow has a characteristic velocity of $170\,\mathrm{km/s}$, an outflow mass rate of $700\,M_\odot/\mathrm{yr}$, a depletion time of $16\,\mathrm{Myr}$, and energetics consistent with star formation as a driver. The neutral atomic and ionized gas phases traced by NaI absorption and H$α$ emission show counterparts to the blueshifted cold molecular outflow but are only 15% and 3% as massive. None of the three gas phases show any signs of slowing down over the extent at which we detected the outflow, suggesting an acceleration mechanism acting on the outflowing gas at kiloparsec scales. We also detect $3.5\times 10^7\,M_\odot$ of dust, traced by optical extinction in the MUSE data, in the blueshifted outflowing cold molecular gas. The ionization state of the non-outflowing gas is consistent with star formation, while the outflowing component shows shock-like ionization. We conclude that the multiphase outflow in IRAS20100-4156 originates in the southeast nucleus of the merger and is driven by the starburst activity there, with radiation pressure likely playing a significant role in its acceleration.

Figures (21)

• Figure 1: Integrated intensity map of IRAS20100-4156 produced from the MUSE cube. The black-and-white crosses show the positions of the two main merger components determined from the high-resolution ALMA observations of CO(3-2), while the green contours show the zeroth moment of the same CO(3-2) data. The first two contour levels correspond to three and five times the noise level of the moment map, respectively, and every following step is doubled.
• Figure 2: Velocity map of the stellar population in Voronoi bins with a target S/N of 40 in the continuum. The black contours show the stellar continuum emission integrated over the entire spectral range of the fit.
• Figure 3: Moment 0 map of CO(1-0) emission. The color map is cut at half the maximum to highlight fainter structure, saturating the SE nucleus. The black-and-white star markers show the positions of the CO(3-2) emission peaks in the SE and NW nuclei. The black-and-white dashed line shows the estimated orientation of the major axis in the SE disk. The cyan-and-blue (magenta-and-red) contours show the integrated intensity of emission from gas with velocities below -150 km/s (above 150 km/s) relative to the systemic velocity. The first two contour levels correspond to the 3$\sigma$ and 5$\sigma$ significance levels and every following step is doubled. The cyan and red framed panels show spectra of the CO(1-0) line (black) extracted from two positions along the minor axis. The vertical magenta dashed line in the extracted spectra shows the line-of-sight velocity of the stellar component at that position. The red line profile corresponds to the fitted quiescent gas components. The bottom panel shows the spectrum of the outflowing gas in orange. In both panels, dashed horizontal lines show the zero-level and dotted horizontal lines the 1$\sigma$ noise level.
• Figure 4: Velocity maps of quiescent and outflowing gas in the cold molecular, ionized and neutral gas phases. Top row: Maps of $v_{50}$ for the quiescent CO(1-0) emission (a), blueshifted outflowing (b), and redshifted outflowing (c) gas. The cyan-and-blue (magenta-and-red) contours show the integrated intensity of emission from gas with velocities below -150 km/s (above 150 km/s) relative to the systemic velocity. The first two contour levels correspond to the 3$\sigma$ and 5$\sigma$ significance levels and every following step is doubled. The dotted lines in the outflow panels show the borders according to which bins are associated with the SE and NW nuclei when calculating total outflow properties. Bottom row: Velocity maps based on MUSE data. Quiescent ionized gas traced by H$\alpha$ (d), and outflowing ionized gas traced by H$\alpha$ (e) and by [OIII]$\lambda$5007 emission (f). Panel (g) shows the best-fit velocity map of the neutral outflowing gas traced by NaID absorption. The black contours show the stellar continuum emission integrated over the spectral range. We do not show the quiescent component traced by [OIII]$\lambda$5007, as kinematics are tied between this line and H$\alpha$ in the fit and the quiescent component is detected in every bin, so that this map would be identical to that of quiescent H$\alpha$.
• Figure 5: BPT classification of outflowing gas by Voronoi bins. In the maps, purple bins are dominated by SF/composite-like, red bins by shock and LINER-like, and mint bins by AGN-like (Seyfert) ionization. Contours correspond to the stellar continuum flux. In the BPT diagrams symbol color corresponds to central velocity of the fit component and symbol size (diameter) corresponds to velocity dispersion $\sigma$. The solid line shows the theoretical maximum for ionization due to star-formation from kewleyTheoreticalModelingStarburst2001 and the dashed line shows the demarcation between Seyfert (above the line) and LINER- or shock-like ionization (below the line) suggested by kauffmannHostGalaxiesActive2003.