Feeding Massive High-z Galaxies with Cosmic Web Streams

Gas accretion onto galaxies is perhaps the most fundamental process driving their evolution, supplying fuel for star-formation, setting the angular momentum and size of disk galaxies, and driving turbulence and disk instabilities. Over the last two decades, a coherent picture has emerged whereby gas is accreted onto dark matter halos from the intergalactic medium (IGM) primarily in a smooth flow along filaments and sheets comprising the cosmic web of large-scale structure, rather than through mergers. During cosmic-noon, at redshifts z~(2-6) near the peak of cosmic structure-formation, intergalactic filaments manifest as narrow streams of cold gas (T~10^4 K) that feed galaxies directly from the cosmic web, penetrating their dark matter haloes and free-falling to the central disk, even in massive halos filled with hot gas (T>~10^6 K) with cooling times of order the Hubble time. However, the thermal, morphological, and kinematic properties of gas that eventually reaches the galaxy – setting disk size, spin, and turbulence – depend sensitively on how streams interact with the circumgalactic medium (CGM) of gas within DM haloes.  We therefore cannot understand galaxy evolution without a detailed understanding of accretion, and we cannot understand accretion without a detailed understanding of the multiphase C/IGM and its interaction with cold streams.

I will present the latest results from a systematic study of this interaction, using a combination of analytic models, idealized high-resolution numerical simulations, and cosmological simulations. We study the effects of hydrodynamics, radiative cooling, self-gravity, the halo potential, and magnetic fields, separately and in tandem, in order to gain insight into stream evolution in different limits. We find that while hydrodynamic instabilities can disrupt streams in the CGM, these are stabilized by cooling, gravity, and MHD. Radiative cooling in the turbulent mixing layer between the stream and the CGM is observable in Lyman alpha, and can explain several observed Lyman alpha blobs. Self-gravity in the streams can lead to star-formation in the CGM at redshifts z > 4. MHD leads to magnetically dominated interface regions which appear out of thermal pressure equilibrium and impact the phase structure of the high-z CGM. Finally, I will discuss how the streams that eventually reach the central galaxy can efficiently drive turbulence in galactic disks, leading to the violent disk instability observed at high-z.