First Results from the TNG50 Simulation: Galactic outflows driven by supernovae and black hole feedback

TL;DR

The paper presents first results from the high-resolution TNG50 cosmological simulation, demonstrating how supernova and black hole feedback drive galactic outflows across a representative galaxy population. By resolving ISM scales down to ~100 pc within a 50 Mpc volume, it reveals that emergent outflow properties at 10 kpc are non-monotonic in stellar mass, with BH feedback causing a rapid rise in mass loading for M* > 10^10.5 M⊙ and generating high-velocity, multiphase winds that can reach >3000 km/s. Outflows are largely collimated along galactic minor axes despite isotropic energy input, and their velocities correlate with star formation activity in a mass-dependent way, including an inversion at high mass where quenched systems exhibit the fastest BH-driven winds. The work highlights that simple sub-grid wind prescriptions at injection scales can produce rich, complex behavior on galactic and halo scales, advocating forward-modeling for robust comparisons with observations. Overall, TNG50 opens a path to connecting small-scale feedback physics with large-scale baryon cycling and galaxy evolution in a statistically robust cosmological context.

Abstract

We present the new TNG50 cosmological, magnetohydrodynamical simulation -- the third and final volume of the IllustrisTNG project. This simulation occupies a unique combination of large volume and high resolution, with a 50 Mpc box sampled by 2160^3 gas cells (baryon mass of 8x10^4 Msun). The median spatial resolution of star-forming ISM gas is ~100-140 parsecs. This resolution approaches or exceeds that of modern 'zoom' simulations of individual massive galaxies, while the volume contains ~20,000 resolved galaxies with M*>10^7 Msun. Herein we show first results from TNG50, focusing on galactic outflows driven by supernovae as well as supermassive black hole feedback. We find that the outflow mass loading is a non-monotonic function of galaxy stellar mass, turning over and rising rapidly above 10^10.5 Msun due to the action of the central black hole. Outflow velocity increases with stellar mass, and at fixed mass is faster at higher redshift. The TNG model can produce high velocity, multi-phase outflows which include cool, dense components. These outflows reach speeds in excess of 3000 km/s out to 20 kpc with an ejective, BH-driven origin. Critically, we show how the relative simplicity of model inputs (and scalings) at the injection scale produces complex behavior at galactic and halo scales. For example, despite isotropic wind launching, outflows exhibit natural collimation and an emergent bipolarity. Furthermore, galaxies above the star-forming main sequence drive faster outflows, although this correlation inverts at high mass with the onset of quenching, whereby low luminosity, slowly accreting, massive black holes drive the strongest outflows.

Figures (17)

• Figure 1: The TNG50 simulation occupies a unique region of parameter space for typical cosmological hydrodynamical simulations. TNG50 includes 2$\times$2160$^3$ resolution elements, giving a baryon mass resolution of $8.5 \times 10^4$ M$_{\odot}$ with adaptive gas softening down to $74$ comoving parsecs. This approaches or exceeds that of modern 'zoom' simulations of individual galaxies, while maintaining the statistical power and unbiased sampling of the full $\sim$ 50 cMpc cosmological volume. Here we show TNG50 (dark blue) in comparison to other cosmological volumes (circles) and zoom simulation suites (diamonds) at $z \sim 0$, based on the total number of resolved galaxies (i.e. at least 100 star/gas particles) with $M_\star \geq 10^9$ M$_{\odot}$. For TNG50 we also indicate $N_{\rm gal}$ above $10^7$ M$_{\odot}$ (rightmost blue circle), which are still resolved at this level -- as in many zooms, but in contrast to other large-volume simulations. The computational difficulty of pushing towards the upper right represents the frontier for next-generation galaxy formation simulations.
• Figure 2: Visualization of black hole feedback driving a large-scale galactic outflow in TNG50. We show the time evolution with five snapshots spanning 370 Myr starting from $z=1.8$ (rows, from top to bottom), tracking a single massive galaxy with $M_\star \simeq 10^{11.4}$ M$_{\odot}$ which is currently in the process of quenching. The left column shows the gas velocity field, while the right shows gas temperature, on the scale of the virial radius (white circles, final row). Each panel is 550 kpc x 275 kpc, with a thin projection depth of 10 kpc. The dense ISM of the galaxy itself is oriented vertically, edge-on, visible as the blue disk in temperature. The central black hole with a mass of $10^{8.7}$ M$_{\odot}$ is in the low-accretion state and its kinetic feedback drives a large-scale collimated outflow.
• Figure 3: The same time evolution series of a $z \sim 2$ black hole driven outflow as in Figure \ref{['fig_timeevo1']}, where we show gas column density (left) and gas metallicity (right). The highly directional, jet-like flow drives shells of gas which pile up along an expanding front, producing under-dense cavities in their wake. These cavities are hot, over-pressurized, and expand coherently to the scale of $r_{\rm vir}$; they are able to launch high metallicity ISM gas entirely out of the halo.
• Figure 4: The versatility and power of TNG50 in combining high-resolution (top) with the unbiased statistics of a representative, cosmological galaxy population (bottom). Here we show gas density projections of galaxy disks (left), and mock stellar light images of the same galaxies in JWST {F200W, F115W, F070W} rest-frame bands (right). (Top) Two examples of large, well-resolved disks in face-on and edge-on projections. (Bottom) The most massive 754 central galaxies at $z=2$, in descending stellar mass (from left to right, top to bottom), encompassing $10^{10.9} < M_{\rm halo}/$ M$_{\odot}$$< 10^{13.4}$, and $10^{8.0} < M_{\star}/$ M$_{\odot}$$< 10^{11.7}$. All are shown face-on, and each stamp is 40 (30, 20) physical kpc across for $\log(M_{\rm halo})/$ M$_{\odot}$$\le$ 13.5 (12.5, 12.1). At this mass scale and redshift, most galaxies host a rotationally supported, gas-rich, rapidly star-forming disk -- with a black hole at the center -- and will drive extended, gaseous outflows.
• Figure 5: Mass loading factor as a function of stellar mass at $z=2$. (Top) For three different values of the minimum speed $v_{\rm rad}$ to be classified as an outflow (blue, orange, and green) the mass loading factor $\eta_{\rm M}$($M_\star$) at a distance of 10 kpc from each galaxy. Lines show median relations, shaded areas 16-84 percentile intervals, and small squares individual systems. Requiring only $v_{\rm rad}$$> 0$ km/s, $\eta_{\rm M}$ decreases sharply with stellar mass from values as high as 20-50 at $10^8$ M$_{\odot}$ to 1-5 at $10^{10.5}$ M$_{\odot}$. The black line denotes the TNG model input of $\eta_{\rm M}$ at the injection scale, derived as the mean across all star-forming gas cells in each galaxy pillepich18a, which is monotonically decreasing with mass. This is not true of the emergent 10 kpc-scale outflows, where the trend of $\eta_{\rm M}$ reverses its decline and begins to rise rapidly for $M_\star \gtrsim 10^{10.5}$ M$_{\odot}$ as highly efficient BH feedback begins to drive strong outflows at low-SFR. (Bottom left) The sensitivity of $\eta_{\rm M}$ to both distance from the galaxy (three linestyles) and the minimum $v_{\rm rad}$ cut to be classified as outflowing (three colors). (Bottom right) Redshift evolution of the mass loading factor $\eta_{\rm M}$ in six different bins of stellar mass (different colors). Although the mass outflow rates decrease towards $z=0$, star formation rates decrease faster, leading to roughly flat $\eta_{\rm M}(z)$ curves for $M_\star \lesssim 10^{10}$ M$_{\odot}$, while the onset of quenching produces rapidly rising $\eta_{\rm M}$ values for massive galaxies towards late times.