Are cosmological gas accretion streams multiphase and turbulent?

Abstract

Simulations of cosmological filamentary accretion reveal flows ("streams") of warm gas, T∼10 4 K, which are efficient in bringing gas into galaxies. We present a phenomenological scenario where gas in such flows -if it is shocked as it enters the halo as we assume -become biphasic and, as a result, turbulent. We consider a collimated stream of warm gas that flows into a halo from an over dense filament of the cosmic web. The post-shock streaming gas expands because it has a higher pressure than the ambient halo gas, and fragments as it cools. The fragmented stream forms a two phase medium: a warm cloudy phase embedded in hot post-shock gas. We argue that the hot phase sustains the accretion shock. During fragmentation, a fraction of the initial kinetic energy of the infalling gas is converted into turbulence among and within the warm clouds. The thermodynamic evolution of the post-shock gas is largely determined by the relative timescales of several processes. These competing timescales characterize the cooling, the expansion of the post-shock gas, the amount of turbulence in the clouds, and the dynamical time of the halo. We expect the gas to become multiphase when the gas cooling and dynamical times are of the same order-of-magnitude. In this framework, we show that this occurs in the important mass range of M halo ∼10 11 to 10 13 M , where the bulk of stars have formed in galaxies. Due to expansion and turbulence, gas accreting along cosmic web filaments may eventually loose coherence and mix with the ambient halo gas. Through both the phase separation and "disruption" of the stream, the accretion efficiency onto a galaxy in a halo dynamical time is lowered. De-collimating flows make the direct interaction between galaxy feedback and accretion streams more likely, thereby further reducing the overall accretion efficiency. As we discuss, moderating the gas accretion efficiency through these mechanisms may help to alleviate a number of significant challenges in theoretical galaxy formation.

ALMA Reveals Strong Emission in a Galaxy Embedded in a Giant Lyα Blob at z = 3.1

“…ALMA3 hosts intense star formation activity, as the dust continuum detection shows, and appears to be a relatively evolved system with large stellar mass M * ≈10 11 M e (Kubo et al 2015) comparable to the derived dynamical mass (we need to recognize both estimates contain large uncertainties). Therefore, galactic outflow may interact with intergalactic gas stream (e.g., Cornuault et al 2016). Thus, shock heating might be a contributor of [C II] emission from ALMA3.…”

Are we seeing accretion flows in a 250 kpc Lyα halo at z = 3?

“…Simulations of gas accretion imply that streams will be visible via their Lyα emission if we detect down to a surface brightness of ≈10 −19 erg s Rosdahl & Blaizot 2012). It is not clear whether these simulations capture all the physics necessary to model the emission and evolution of the accreting gas (e.g., Cornuault et al 2016). Observational constraints are needed.…”

“…Shock heating in the outer halo. Another possibility for exciting the Lyα emission in the halo of MRC 0316-257 is an accretion shock from inflowing gas at the halo boundary and/or shocks from cloud-cloud collisions in a multiphase stream (Cornuault et al 2016). The morphology of the most extended emission to the west-southwest (region 11) is shell-like and certainly suggestive of compression (a "splash").…”

“…However, we view aspects of this agreement as fortuitous. Simulations generally lack the spatial and temporal resolution necessary to capture thermal and dynamical instabilities, resolve high Mach shock fronts, fragmentation of the post-shock gas, and turbulence, which naturally occur in astrophysical gas flows (Kritsuk & Norman 2002;Cornuault et al 2016). It is not clear if simulations show conspicuously large shock fronts and strong post-shock cooling at approximately the halo boundary, as we have suggested.…”

Implications for the Origin of Early-type Dwarf Galaxies: A Detailed Look at the Isolated Rotating Early-type Dwarf Galaxy LEDA 2108986 (CG 611), Ramifications for the Fundamental Plane’s Kinematic Scaling, and the Spin–Ellipticity Diagram

“…White & Frenk 1991 cold streams (e.g. Kereš et al 2005;Dekel & Birnboim 2006;Brooks et al 2009;Dekel et al 2009;Cornuault et al 2016). This cold mode of delivery may be the most efficient of the mechanisms (Genel et al 2010;van de Voort et al 2011;Lu et al 2011) andPichon et al (2011) have revealed how these cold gas streams can build a disk in a coherent planar manner (see also Danovich et al 2012Danovich et al , 2014Prieto et al 2013;and Stewart et al 2013).…”