Stability of Cold Flows

In the developing picture of galaxy formation, the main mode of accretion in massive star-forming galaxies (SFGs) at z>~2 is through cold streams of dense gas flowing along cosmic web filaments. The high density in these streams allows them to penetrate the hot CGM and reach the central galaxy. Recent MUSE and KCWI observations reveal large quantities of dense, >~10^4K gas in the CGM with spatial and kinematic properties reminiscent of cold streams. However, the evolution of these streams in the CGM is still widely disputed, and it is unclear whether or not they remain coherent all the way to the galaxy, what their thermal and morphological properties are, and how these affect the emission and absorption signatures of the CGM as well as galaxy growth. As current cosmological simulations do not properly resolve the streams, different computational methods disagree on their fate. 

As an alternative to cosmological simulations, I have been spearheading a collaboration studying the evolution of cold streams using detailed analytic models and high resolution,
idealized simulations, run with the AMR code
RAMSES, where new physical effects are added and studied one at a time. 

For the pure hydro case, I performed a detailed linear stability analysis, finding that Kelvin-Helmholtz Instability (KHI) in streams would become highly non-linear in less than a halo crossing time (Mandelker et al 2016). Studying non-linear KHI (Padnos et al 2018, Mandelker et al 2019a), I found qualitatively different behaviour in two and three dimensions, and showed that sufficiently narrow streams will be completely shredded in the CGM prior to reaching the central galaxy.

Accounting for the self-gravity of the stream, in a study led by a student I supervised, Han Aung, we identified a critical stream mass-per-unit-length below which KHI proceeds similarly to the no-gravity case, though buoyancy forces extend stream lifetimes by a factor of ~3 (Aung et al 2019). More massive streams fragment into dense, pressure confined clumps before being shredded by KHI.

Map of gas density in a cosmological zoom in simulation run using the RAMSES code. The streams feeding the galaxy are visible. The box encompasses a sphere of twice the virial radius. See Danovich et al 2012 for additional details. While some cosmological simulations suggest that the streams remain cold and coherent until they hit the galaxy, it is not at all clear that this should indeed be so.

With radiative cooling and heating, I showed that since the cooling time in the turbulent mixing layer between the stream and the halo is almost always shorter than the non-radiative stream disruption time, KHI does not disrupt the stream (Mandelker et al 2019c). Rather, halo gas cools and condenses onto the stream, becoming entrained in the flow. This causes the stream to decelerate and lose kinetic energy and the halo to lose thermal energy, which is mostly radiated in Lyman alpha.

Accounting for the gravitational potential of the host halo, I developed a toy model to make predictions for the luminosity emitted by a stream in the CGM (Mandelker et al 2019d). For >~10^{12} solar mass  halos at z~2, a single stream can emit up to ~10^{43} erg/s over virial scales, comparable to observed Lyman alpha blobs, suggesting that these may be powered by cold streams.

Much work remains to be done, exploring the effects of additional physics, such as self-shielding from the UV background, magnetic fields, and thermal conduction!

Idealized studies of the evolution of cold streams in the CGM of massive high-z galaxies. Shown are density maps at late times from Ramses simulations of cold, dense cylindrical streams flowing supersonically through a hot, diffuse medium, representing astrophysical streams in halos. The top row, adapted from Aung et al. 2019, studies the effect of self-gravity, while the bottom row, adapted from Mandelker et al. 2019c, studies the effect of cooling. Both effects can prevent stream disruption by KHI. 

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