Probing the interface between infall and outflows in a high-redshift massive halo

September 01, 2021

Cosmological simulations show that the growth of galaxies in the early Universe is regulated by the interplay between gas accretion onto dark-matter halos and ejection of matter by stars and active galactic nuclei (AGN). While these processes are routinely described in theoretical works, still little is known from observations on the complex exchanges of mass and energy within the halos of galaxies, where large-scale infall (i.e. accretion) meets outflows (i.e. ejection). Recently, an international team of astronomers was able to probe the halo gas of a massive galaxy system, SMM J02399-0136, using a novel approach. These observations unveiled – for the first time – the infall towards the galaxies of a large mass of diffuse, highly turbulent multiphase gas, pervaded by powerful outflows and more than 10 times larger than the star-forming galaxies.

Figure 1. False-colour image of the galaxy group SMM J02399-0136, which is known to host a dust-obscured highly star-forming galaxy (L2SW), a bright AGN or quasar (L1), and additional faint emissions (L2 and L1N). Contours indicate the 360micron continuum emission (dust) observed with the ALMA interferometer. The horizontal size of this image corresponds to about a 55,000 light years at the epoch of SMM J02399-0136. The field is gravitationally lensed which stretches the image roughly along the L2SW-L1 direction.

Most of the material in the early Universe lies outside galaxies, and is organized in a net of filaments, sheets and knots, where galaxies form and evolve in their dark-matter halos. The material gravitationally bound to each galaxy halo, also known as circumgalactic medium (CGM), encodes key information on the processes (like gas infall and outflows) that regulate galaxy evolution. While the importance of unravelling the CGM physics has been clear to astronomers for decades, only recent technical developments in instrumentation at optical and submillimeter wavelengths allow them to directly probe such gas in emission, and in absorption against bright background sources.

In this framework, galaxy systems hosted by the most massive halos in the early Universe, with masses greater than a trillion times the Sun’s mass, are the ideal targets for pilot studies of the CGM. These massive halos are observed to host prodigious bursts of star formation and episodes of strong AGN activity, which are predicted to drive powerful outflows that eject matter at high velocities in the CGM. Therefore, these outflows should be able to redistribute material from galaxies to larger scales, and mix it with the infalling material from intergalactic scales.

As the infall and outflow velocities are extremely large, their interface is highly turbulent. To study this, an international team of astronomers envisioned a new set of observations of the galaxy group SMM J02399-0136 (Fig. 1). This galaxy group is located in the early Universe, at about 2.3 billion years from the Big Bang, and should sit in a dark-matter halo as massive as ten trillion times the Sun’s mass.

Figure 2. The high velocity outflowing gas (red, blue) located in the vicinity of the quasar L1 and the starburst galaxy L2SW as traced by the hydrogen Lyα emission in the SMM J02399-0136 galaxy group. The arrows indicate the projected direction of the outflows in the vicinity of the quasar, with the red portion moving away from us and the blue portion approaching us.

The velocity ranges used for the figure are shown on the top-left corner, using the quasar L1 as reference. The white scale in the bottom-left corner indicates about 32,000 light years at the epoch of SMM J02399-0136.

As a first step, the astronomers targeted the hydrogen Lyman-alpha (Lyα) emission around the galaxy group to trace the gas reservoir at a temperature of about ten thousand degrees. The observations conducted with the “panoramic” integral field spectrograph ‘Keck Cosmic Web Imager’ on the Keck telescope unveiled extended, bright Lyα emission, with high velocity outflows located in the vicinity of the quasar L1 and the starburst galaxy L2SW (Fig. 2), and a turbulent nebula, which is about twice the size than the region shown in Figure 1. 

The next step was targeting the galaxy group with the ALMA interferometer to detect the J=1-0 transition of a light hydride (the methylidyne cation CH+), which requires very dense molecular gas to be excited. CH+ is a very fragile molecule: it needs molecular hydrogen H2 to form, but in low fractions to avoid rapid destruction in collisions with this partner. Moreover, its formation is highly endothermic, requiring a large amount of supra-thermal energy of Eform ~ 0.5 eV. This is why CH+ is a specific tracer of regions where the kinetic energy of molecular gas is dissipated, e.g. of turbulence or shocks. This fragile molecule can be observed only where it forms, because its lifetime is extremely short (about 1 year).

Remarkably, CH+ has been detected in both absorption and emission in the SMM J02399-0136 galaxy group. The absorption, occurring in low-density gas in front of the luminous dust continuum of the quasar L1 and the starburst galaxy L2SW, traces a massive turbulent reservoir of cool molecular gas whose extent is comparable to the large Lyα nebula. The CGM of this massive system is therefore multiphase. Importantly, the CH+ absorption is found at positive velocities with respect to the central galaxies (Fig. 3), similar to those of the large Lyα nebula. As positive velocity means motion away from the observer, the whole multiphase CGM is therefore infalling onto the galaxy group.

Figure 3. Some of the detected CH+ absorption and emission lines in the SMM J02399-0136 galaxy group.

Center: the location of the CH+ emission regions (white contours) is shown in comparison to the high-velocity Lyα.

Sides: the spectra of CH+ integrated over the indicated regions. While the broad CH+ emission lines are tracing myriad high-velocity shocks at the interface of large-scale infall and outflows, the redshifted absorption lines observed in the direction of the starburst galaxy L2SW trace a more diffuse massive and turbulent molecular reservoir, which is infalling onto the galaxy group.

CH+ emission is tentatively detected in structures roughly following the shape of the high-velocity Lyα emission (Fig. 3). These CH+ emission lines are extremely broad and trace myriad molecular shocks at the interface of the infalling CGM and the high-velocity outflows.

Thanks to these observations, the astronomers are also able to study the redistribution of energy throughout the CGM. The powerful outflows, driven by bursts of star-formation and AGN activity, inject a considerable amount of kinetic energy into the CGM, sufficient to sustain its turbulence. The substantial mass accretion rate that almost balances the consumption rate of the gas mass due to the ongoing high star formation provides a comparable energy source to the CGM turbulence because accreting matter loses gravitational energy along its infall onto the galaxy group.

Overall, these observations show a promising avenue for the direct study of the interface between infall and outflows in the CGM of massive systems. Future deep targeted observations will be able to unveil the physical properties and energy trails of the turbulent, multiphase large-scale gas reservoirs surrounding these active environments.

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