סמינר באסטרונומיה ובאסטרופיזיקה: The Shattering (or not) of Cold Streams after the Penetration of the Virial Shock

Zoom: https://tau-ac-il.zoom.us/j/87185167411?pwd=FpTUyIgXaOturzOFSetA6pPVYQilen.1

Abstract:

Theory and observations indicate that the circumgalactic medium (CGM) and the cosmic web at high redshift are multiphase, with small cold gas clouds embedded in a hot, diffuse medium. We investigate the possibility that cold streams undergo “shattering” triggered by a sudden pressure boost from the surrounding medium after crossing the virial shock. Shattering was originally identified in large thermally unstable clouds that fragment into numerous small cloudlets after cooling to a stable temperature. Its key requirement is a sufficiently large pressure contrast between the cold gas and the background, such that a reflected shock fragments the cloud through the Richtmyer–Meshkov instability.

We extend previous work by exploring the roles of cloud geometry, metallicity, an ionizing ultraviolet background, and velocity shear. In the absence of shear, we find a critical density contrast of approximately 100 for cold streams in a CGM-like environment. Below this threshold, the initially fragmented cloudlets coagulate back into a coherent stream in a few sound crossing timescales, whereas above it, sustained shattering is possible. When shear is included, however, strong Kelvin–Helmholtz instabilities fundamentally alter the outcome by disrupting fragmented cloudlets below the survival radius, with only the large ones surviving. As a result, larger cold streams are expected to produce more cloudlets in the CGM due to their broader cloud size distribution.

We apply these results to cold streams feeding massive galaxies (Mv > 10^12 Msun) at z > 2 from the cosmic web via the analytic toy model. We find that streams entering halos at higher redshift are more prone to shattering upon penetrating the hot CGM through the virial shock. This mechanism naturally explains the large clumping factors and high covering fractions of Lyman-alpha gas observed around massive high-redshift galaxies.

מארגן הסמינר: ד"ר יהונתן שטרן