Yuri Shchekinov

Mixing metals in the early Universe

ABSTRA C T We investigate the evolution of the metallicity of the intergalactic medium (IGM) with particular emphasis on its spatial distribution. We propose that metal enrichment occurs as a two-step process. First, supernova (SN) explosions eject metals into relatively small regions confined to the surroundings of star-forming galaxies. From a comprehensive treatment of blowout we show that SN by themselves fail by more than one order of magnitude to distribute the products of stellar nucleosynthesis over volumes large enough to pollute the whole IGM to the metallicity levels observed. Thus, an additional (but as yet unknown) physical mechanism must be invoked to mix the metals on scales comparable to the mean distance between the galaxies that are most efficient pollutants. From this simple hypothesis we derive a number of testable predictions for the evolution of the IGM metallicity. Specifically, we find that: (i) the fraction of metals ejected over the starformation history of the Universe is about 50 per cent at za 0; that is, approximately half of the metals today are found in the IGM; (ii) if the ejected metals were homogeneously mixed with the baryons in the Universe, the average IGM metallicity would be kZla V ej =Vb . 1=25Z( at za 3: However, due to spatial inhomogeneities, the mean of the distribution of metallicities in the diffusive zones has a wide (more than 2 orders of magnitude) spread around this value; (iii) if metals become more uniformly distributed at z & 1; as assumed, at za 0 the metallicity of the IGM is narrowly confined within the range Z < 0:1 ^ 0:03Z(: Finally, we point out that our results can account for the observed metal content of the intracluster medium.

Photoelectric heating for dust grains at high redshifts

Lyman-α absorption systems at z ∼ 3 with NH I≥ 3 × 1014 cm-2 have been found to be enriched with a mean metallicity of Z/ Z⊙∼ 10-2.5, and a large scatter in the metallicity. It is reasonable to assume that the process of initial enrichment of the intergalactic medium (IGM) at z ≥ 3 also produced dust grains. We explore the implications of the presence of dust grains in the IGM at high redshift, in particular, the contribution of photoelectric emission from grains by hard background photons to the net heating rate of the IGM. We show that (i) the charge on dust particles and the characteristics of photoemitted electrons differ substantially from those in the interstellar medium (ISM) in several respects: (a) grains are exposed to and charged by photons beyond the Lyman limit, and (b) because of this, the photoelectrons have typical energy of tens of eV. We also show that: (ii) silicates are more efficient heating agents than graphites; (iii) small grains contribute mostly to the net heating; (iv) at densities typical of the IGM at z ∼ 3 and for Ly α absorbers, dust heating can be comparable to or exceed photoionization heating within an order of magnitude; and (v) this increases the temperature of overdense regions, compared to the case of no dust heating, by a factor of ∼ 2. We discuss the implications of this extra heating source in Ly α absorbing systems.

In a hot bubble: why does superbubble feedback work, but isolated supernovae do not?

Using idealized one-dimensional Eulerian hydrodynamic simulations, we contrast the behaviour of isolated supernovae with the superbubbles driven by multiple, collocated supernovae. Continuous energy injection via successive supernovae exploding within the hot/dilute bubble maintains a strong termination shock. This strong shock keeps the superbubble over-pressured and drives the outer shock well after it becomes radiative. Isolated supernovae, in contrast, with no further energy injection, become radiative quite early (less than or similar to 0.1Myr, tens of pc), and stall at scales less than or similar to 100 pc. We show that isolated supernovae lose almost all of their mechanical energy by 1 Myr, but superbubbles can retain up to similar to 40 per cent of the input energy in the form of mechanical energy over the lifetime of the star cluster (a few tens of Myr). These conclusions hold even in the presence of realistic magnetic fields and thermal conduction. We also compare various methods for implementing supernova feedback in numerical simulations. For various feedback prescriptions, we derive the spatial scale below which the energy needs to be deposited in order for it to couple to the interstellar medium. We show that a steady thermal wind within the superbubble appears only for a large number (greater than or similar to 10(4)) of supernovae. For smaller clusters, we expect multiple internal shocks instead of a smooth, dense thermalized wind.

Dynamics of conductive/cooling fronts : cloud implosion and thermal solitons

We investigate the evolution of interfaces among phases of the interstellar medium with different temperatures. It is found that, for some initial conditions, the dynamical effects related to conductive fronts are very important even if radiation losses, which tend to decelerate the front propagation, are taken into account. We also explored the consequences of the inclusion of shear and bulk viscosity, and we have allowed for saturation of the kinetic effects. Numerical simulations of a cloud immersed in a hot medium have been performed; depending on the ratio of conductive to dynamical time, the density is increased by a huge factor and the cloud may become optically thick

Conditions for supernovae driven galactic winds

We point out that the commonly assumed condition for galactic outflows, that supernovae (SNe) heating is efficient in the central regions of starburst galaxies, suffers from invalid assumptions. We show that a large filling factor of hot (≥10{sup 6} K) gas is difficult to achieve through SNe heating, irrespective of the SNs initial gas temperature and density, its uniformity, or its clumpiness. We instead suggest that correlated supernovae from OB associations in molecular clouds in the central region can drive powerful outflows if the molecular surface density is >10{sup 3} M {sub ☉} pc{sup –2}.

Evolution of multiple supernova remnants

(Abridged) Heating of the interstellar medium by multiple supernovae (SNe) explosions is at the heart of producing galaxy-scale outflows. We use hydrodynamical simulations to study the efficiency of multiple SNe in heating the interstellar medium (ISM) and filling the volume with gas of high temperatures. We argue that it is important for SNe remnants to have a large filling factor {\it and} a large heating efficiency. For this, they have to be clustered in space and time, and keep exploding until the hot gas percolates through the whole region, in order to compensate for the radiative loss. In the case of a limited number of SNe, we find that although the filling factor can be large, the heating efficiency declines after reaching a large value. In the case of a continuous series of SNe, the hot gas (

DUST-DRIVEN WIND FROM DISK GALAXIES

T \ge 3 \times 10^6

Limits on dust and metallicity evolution of Lyα forest clouds from COBE

K) can percolate through the whole region after the total volume filling factor reaches a threshold of

Superbubble breakout and galactic winds from disc galaxies

\sim 0.3

Extended O VI haloes of star-forming galaxies

. The efficiency of heating the gas to X-ray temperatures can be