A Fast, Hot Wind from a Nuclear Starburst

Abstract

Galaxies with intense star formation often host multiphase, galaxy-scale winds powered by supernovae and fast stellar winds. These are strong enough to disrupt the star-forming interstellar medium, and they chemically enrich the surrounding circumgalactic medium. However, their launching mechanism remains unknown. Here we show that thermal gas pressure is sufficient to drive the multiphase wind in the prototypical starburst galaxy M82. Using a high energy-resolution ($ΔE = 4.5$ eV) XRISM Resolve spectrum, including detections of FeXXV 6.7 keV, ArXVII 3.1 keV, and SXVI 2.6 keV, we measure the temperature ($T = 2.3^{+0.5}_{-0.2} \times 10^7$ K) and mass ($M \approx 6 \pm 2 \times 10^5$ M$_\odot$) of the hot gas in the starburst and provide the first direct measurement of its line-of-sight velocity dispersion ($σ= 595^{+464}_{-128}$ km s$^{-1}$). These values are consistent with a freely-expanding wind exceeding the galactic escape velocity. The size of the FeXXV-emitting region suggests a hot gas outflow rate of $\dot{M} \approx 4$ M$_\odot$ yr$^{-1}$, carrying a total energy of $\dot{E} \approx 2 \times 10^{42}$ erg s$^{-1}$. This is sufficient to drive the molecular, atomic, and ionized outflows while transporting up to $\approx 2$ M$_\odot$ yr$^{-1}$ of hot gas to the intergalactic medium. The estimated supernova rate implies that $\approx$ 60% of the supernova energy must be thermalized in hot gas. Our results suggest that additional driving mechanisms, such as cosmic-ray pressure, are not required to launch the wind.

Figures (6)

• Figure 1: The hot starburst wind in M82. The soft X-ray wind is seen at left in a Chandra ACIS-S narrowband image at Si xiv 2.0 keV and contour plot of O viii 0.65 keV emission. Chandra reveals that the spatial extent of Si xiv emission ($R \lesssim 1$ kpc) is representative of the $E < 4$ keV emission lines seen by Resolve, including S xv 2.4 keV, S xvi 2.6 keV, and Ar xvii 3.1 keV. Softer X-ray emission is extended to $R \gtrsim 2$ kpc, as shown by the extent of the O viii emission. As shown at right, Fe xxv 6.7 keV emission is largely contained within a projected ellipsoid with $a \approx 300$ pc, $c \approx 75$ pc (white contours). This suggests that the Fe xxv emission largely traces the thermalization zone, while the $E < 4$ keV lines are produced in and around the starburst nucleus and in the biconical outflow. The nucleus falls fully within the Resolve field of view ($3' \times 3' = 3~\text{kpc} \times 3~\text{kpc}$; black outline at left, with the excluded pixel 27 shaded in gray). Point sources have been masked in both images. The blue arrow lies along the major axis of the galaxy.
• Figure 2: Resolve spectrum of the starburst nucleus of M82. At top, the Resolve spectrum (black histogram) shows a suite of resolved emission lines, including strong Fe xxv 6.7 keV emission. The best-fit model (red) consists of a hot, $kT = 2.0$ keV wind fluid (purple), a cooler, $kT = 0.7$ keV phase, a powerlaw continuum from X-ray binaries (XRBs; orange), and a Gaussian Fe K$\alpha$ 6.4 keV emission line (gray). This fluorescence line arises from neutral gas irradiated by hard X-ray photons from compact objects 2007ApJ...658..258S2014MNRAS.437L..76L. In the bottom panel, the significant velocity broadening of the $kT = 2$ keV gas ($\sigma = 595^{+464}_{-128}$ km s$^{-1}$) is evident from the visible blending of the Fe xxv triplet lines. The gray shading shows the suite of best-fit models obtained by varying $\sigma$ within the uncertainty.
• Figure 3: Geometry of the starburst nucleus of M82. The hot, Fe xxv-bearing wind fluid is found within an ellipsoidal thermalization zone (purple), where star clusters demarcated by H ii regions (gray clouds) shock-heat the ambient medium. This zone is bounded in the plane of the galaxy by a molecular torus 1987PASJ...39..685N (white ring, shown as a cross-section) seen in CO emission. This torus nozzles the hot wind fluid as it leaves the thermalization zone (purple vectors), reducing the surface area through which the hot wind can freely stream (red vectors) and producing the biconical outflow.
• Figure 4: Power budget and thermalization efficiency of the hot wind. At left, probability distributions are shown for the thermal (blue), kinetic (red), and total power (purple) carried by the hot wind, as derived from the measurement error on the spectral fit. The total power in the hot wind, $\dot{E}_{\text{hot}}$, yields the thermalization efficiency, or $\alpha = \dot{E}_{\text{hot}}/\dot{E}_{\text{SF}}$, at right. The shaded gray regions indicate where $\dot{E}_{\text{hot}}$ exceeds the $\dot{E}_{\text{SF}} \approx 3 \times 10^{42}$ erg s$^{-1}$ available from SNe; this is indicative of measurement error and systematic uncertainty on the wind geometry and SN rate.
• Figure 5: A one-dimensional, free wind model of M82. A free wind model 1985Natur.317...44C well describes the hot outflow in M82, as shown by temperature (left) and velocity (right) profiles as functions of radius. The black lines show the Resolve measurements and the dashed lines indicate the 90% confidence intervals. We assume a spherical driving region with a radius of $R_\star = 200$ pc (shown by the red line) and adopt the mass and energy loading inferred from the XRISM measurements ($\dot{M} = 4~M_\odot$ yr$^{-1}$, $\dot{E} = 2 \times 10^{42}$ erg s$^{-1}$).