Dark matter merging induced turbulence as an efficient engine for gas cooling

Abstract

We have performed a cosmological numerical simulation of primordial baryonic gas collapsing on to a 3 × 107 M⊙ dark matter (DM) halo. We show that the large scale baryonic accretion process and the merger of few ∼ 106 M⊙ DM haloes, triggered by the gravitational potential of the biggest halo, are enough to create supersonic () shocks and develop a turbulent environment. In this scenario, the post‐shocked regions are able to produce both H2 and deuterated H2 molecules very efficiently, reaching maximum abundances of and nHD∼ few × 10−6 nH, enough to cool the gas below 100 K in some regions. The kinetic energy spectrum of the turbulent primordial gas is close to a Burgers spectrum, , which could favour the formation of low‐mass primordial stars. The solenoidal‐to‐total kinetic energy ratio is 0.65 ≲Rk≲ 0.7 for a wide range of wavenumbers; this value is close to the Rk≈ 2/3 natural equipartition energy value of a random turbulent flow. In this way, turbulence and molecular cooling seem to work together in order to produce potential star formation regions of cold dense gas in primordial environments. We conclude that both the mergers and the collapse process on to the main DM halo provide enough energy to develop supersonic turbulence which favours the molecular coolant formation: this mechanism, which could be universal and the main route towards the formation of the first galaxies, is able to create potential star‐forming regions at high redshift.

Primordial star formation under the influence of far ultraviolet radiation: 1540 cosmological haloes and the stellar mass distribution

“…fHD/fH 2 increases sharply in the density range 10 6 cm −3 < nH,cen < 10 7 cm −3 and eventually exceeds 10 −3 . We classify our 1540 samples of primordial clouds as one of three possible cases, depending on fHD/fH 2 during the collapse: H2-cooling cases (1186 cases), HD-cooling cases (151), and the intermediate cases (203), where the numbers 6 Another possible route for HD-cooling primordial starformation is suggested by some studies (Uehara & Inutsuka 2000;Shchekinov & Vasiliev 2006;Prieto et al 2012;Bovino et al 2014;Prieto et al 2014). In this scenario, the merging of multiple dark matter haloes induces the formation of shockwaves in which HD formation is enhanced.…”

Star formation in the first galaxies – III. Formation, evolution, and characteristics of the first metal-enriched stellar cluster

“…The first galaxies are anticipated to form in dark matter halos with virial temperatures Tvir ∼ 10 4 K corresponding to virial masses Mvir ∼ 10 8 [(1 + z)/10] −3/2 M (Oh & Haiman 2002;Bromm & Yoshida 2011). In these 'atomic cooling halos', energetic supernovae, infall of baryonic matter from the cosmic web, and perhaps even thermal instability, all contribute to the excitation of turbulence in the star-forming gas (ISM; Wise & Abel 2007;Greif et al 2008;Prieto et al 2012;Safranek-Shrader et al 2012). Previous stellar generations could have polluted these halos to average metallicities Z > Zcrit (e.g., Tornatore et al 2007;Greif et al 2010;Maio et al 2010;Ritter et al 2012), which would have enabled the formation of Pop II stars (but metal pollution to Z > Zcrit cannot be taken for granted, see Ritter et al 2014).…”

Dark-matter halo mergers as a fertile environment for low-mass Population III star formation

“…assuming a temperature of 60 K at n ∼ 10 6 cm −3 ). A first attempt to investigate the merger-induced HD cooling was made by Prieto et al (2012) who performed hydro-cosmological simulations of 3×10 7 M⊙ halo resulting from the merging of two minihalos of a few 10 6 M⊙. A nonequilibrium treatment of the chemistry was included as well as cooling from H2 and HD.…”

“…In addition, we note that also an increase of the gas temperature will boost the formation of H − , and thus H2 and HD. Another possible route for primordial star formation (Pop III.2) which also involves the HD cooling has been proposed by Shchekinov & Vasiliev (2006), and Prieto et al (2012Prieto et al ( , 2014. The idea suggested by Shchekinov & Vasiliev (2006) is based on the merging of dark-matter halos within the context of the hierarchical scenario of structure formation (Barkana & Loeb 2001;Ciardi & Ferrara 2005).…”

“…Their mass scale can therefore just serve as a rough estimate. In a recent study, Prieto et al (2012Prieto et al ( , 2014 performed cosmological N-body simulations and computed the statistic of the halos fulfilling the Shchekinov & Vasiliev (2006) criterion. Their findings suggest that the fraction of halos going through this phase is significant.…”