83 research outputs found
Towards simulating star formation in turbulent high-z galaxies with mechanical supernova feedback
Feedback from supernovae is essential to understanding the self-regulation of
star formation in galaxies. However, the efficacy of the process in a
cosmological context remains unclear due to excessive radiative losses during
the shock propagation. To better understand the impact of SN explosions on the
evolution of galaxies, we perform a suite of high-resolution (12 pc), zoom-in
cosmological simulations of a Milky Way-like galaxy at z=3 with adaptive mesh
refinement. We find that SN explosions can efficiently regulate star formation,
leading to the stellar mass and metallicity consistent with the observed
mass-metallicity relation and stellar mass-halo mass relation at z~3. This is
achieved by making three important changes to the classical feedback scheme: i)
the different phases of SN blast waves are modelled directly by injecting
radial momentum expected at each stage, ii) the realistic time delay of SNe,
commencing at as early as 3 Myr, is required to disperse very dense gas before
a runaway collapse sets in at the galaxy centre via mergers of gas clumps, and
iii) a non-uniform density distribution of the ISM is taken into account below
the computational grid scale for the cell in which SN explodes. The last
condition is motivated by the fact that our simulations still do not resolve
the detailed structure of a turbulent ISM in which the fast outflows can
propagate along low-density channels. The simulated galaxy with the SN feedback
model shows strong outflows, which carry approximately ten times larger mass
than star formation rate, as well as smoothly rising circular velocity. Other
feedback models that do not meet the three conditions form too many stars,
producing a peaked rotation curve. Our results suggest that understanding the
structure of the turbulent ISM may be crucial to assess the role of SN and
other feedback processes in galaxy formation theory.Comment: 22 pages, 18 figures, Accepted for publication in MNRA
Snap, Crackle, Pop: sub-grid supernova feedback in AMR simulations of disk galaxies
We compare 5 sub-grid models for supernova (SN) feedback in adaptive mesh
refinement (AMR) simulations of isolated dwarf and L-star disk galaxies with
20-40 pc resolution. The models are thermal dump, stochastic thermal,
'mechanical' (injecting energy or momentum depending on the resolution),
kinetic, and delayed cooling feedback. We focus on the ability of each model to
suppress star formation and generate outflows. Our highest-resolution runs
marginally resolve the adiabatic phase of the feedback events, which correspond
to 40 SN explosions, and the first three models yield nearly identical results,
possibly indicating that kinetic and delayed cooling feedback converge to wrong
results. At lower resolution all models differ, with thermal dump feedback
becoming inefficient. Thermal dump, stochastic, and mechanical feedback
generate multiphase outflows with mass loading factors , which is
much lower than observed. For the case of stochastic feedback we compare to
published SPH simulations, and find much lower outflow rates. Kinetic feedback
yields fast, hot outflows with , but only if the wind is in effect
hydrodynamically decoupled from the disk by using a large bubble radius.
Delayed cooling generates cold, dense and slow winds with , but large
amounts of gas occupy regions of temperature-density space with short cooling
times. We conclude that either our resolution is too low to warrant physically
motivated models for SN feedback, that feedback mechanisms other than SNe are
important, or that other aspects of galaxy evolution, such as star formation,
require better treatment.Comment: 22 pages, 15 figures. Accepted for publication in MNRAS with minor
revision
Are cold flows detectable with metal absorption lines?
[Abridged] Cold gas flowing within the "cosmic web" is believed to be an
important source of fuel for star formation at high redshift. However, the
presence of such filamentary gas has never been observationally confirmed. In
this work, we investigate in detail whether such cold gas is detectable using
low-ionisation metal absorption lines, such as CII \lambda1334 as this
technique has a proven observational record for detecting gaseous structures.
Using a large statistical sample of galaxies from the Mare Nostrum N-body+AMR
cosmological simulation, we find that the typical covering fraction of the
dense, cold gas in 10^12 Msun haloes at z~2.5 is lower than expected (~5%). In
addition, the absorption signal by the interstellar medium of the galaxy itself
turns out to be so deep and so broad in velocity space that it completely
drowns that of the filamentary gas. A detectable signal might be obtained from
a cold filament exactly aligned with the line of sight, but this configuration
is so unlikely that it would require surveying an overwhelmingly large number
of candidate galaxies to tease it out. Finally, the predicted metallicity of
the cold gas in filaments is extremely low (\leq 0.001 Zsun). Should this
result persist when higher resolution runs are performed, it would
significantly increase the difficulty of detecting filamentary gas inflows
using metal lines. However, even if we assume that filaments are enriched to
Zsun, the absorption signal that we compute is still weak. We are therefore led
to conclude that it is extremely difficult to observationally prove or disprove
the presence of cold filaments as the favorite accretion mode of galaxies using
low-ionisation metal absorption lines. The Ly-alpha emission route looks more
promising but due to the resonant nature of the line, radiative transfer
simulations are required to fully characterize the observed signal.Comment: MNRAS Letters in pres
FORMATION OF GLOBULAR CLUSTERS IN ATOMIC-COOLING HALOS VIA RAPID GAS CONDENSATION AND FRAGMENTATION DURING THE EPOCH OF REIONIZATION
We investigate the formation of metal-poor globular clusters (GCs) at the center of two dark matter halos with ~ 4 10 at > 10 using cosmological radiation-hydrodynamics simulations. We find that very compact (1 pc) and massive (~ 6 10 ) clusters form rapidly when pristine gas collapses isothermally with the aid of efficient Ly emission during the transition from molecular-cooling halos to atomic-cooling halos. Because the local free-fall time of dense star-forming gas is very short (1 Myr), a large fraction of the collapsed gas is turned into stars before stellar feedback processes blow out the gas and shut down star formation. Although the early stage of star formation is limited to a small region of the central star-forming disk, we find that the disk quickly fragments due to metal enrichment from supernovae. Sub-clusters formed in the fragmented clouds eventually merge with the main cluster at the center. The simulated clusters closely resemble the local GCs in mass and size but show a metallicity spread that is much wider than found in the local GCs. We discuss a role of pre-enrichment by Pop III and II stars as a potential solution to the latter issue. Although not without shortcomings, it is encouraging that a naive blind (not tuned) cosmological simulation presents a possible channel for the formation of at least some massive GCs.The research is supported in part by NSF grant AST-1108700 and NASA grant NNX12AF91G and in part by the ERC Advanced Grant 320596 “The Emergence of Structure during the epoch of Reionization.” JR was funded by the European Research Council under the European Unions Seventh Framework Programme (FP7/2007- 2013)/ERC Grant agreement 278594-GasAroundGalaxies, and the Marie Curie Training Network CosmoComp (PITN-GA- 2009-238356). SKY acknowledges support from the Korean National Research Foundation (Doyak 2014003730).This is the final version of the article. It first appeared from the Institute of Physics via http://dx.doi.org/10.3847/0004-637X/823/1/5
The energy and dynamics of trapped radiative feedback with stellar winds
In this paper, we explore the significant, non-linear impact that stellar winds have on H ii regions. We perform a parameter study using three-dimensional radiative magnetohydrodynamic simulations of wind and ultraviolet radiation feedback from a 35 M⊙ star formed self-consistently in a turbulent, self-gravitating cloud, similar to the Orion Nebula (M42) and its main ionizing source θ1 Ori C. Stellar winds suppress early radiative feedback by trapping ionizing radiation in the shell around the wind bubble. Rapid breakouts of warm photoionized gas (‘champagne flows’) still occur if the star forms close to the edge of the cloud. The impact of wind bubbles can be enhanced if we detect and remove numerical overcooling caused by shocks crossing grid cells. However, the majority of the energy in the wind bubble is still lost to turbulent mixing between the wind bubble and the gas around it. These results begin to converge if the spatial resolution at the wind bubble interface is increased by refining the grid on pressure gradients. Wind bubbles form a thin chimney close to the star, which then expands outwards as an extended plume once the wind bubble breaks out of the dense core the star formed in, allowing them to expand faster than a spherical wind bubble. We also find wind bubbles mixing completely with the photoionized gas when the H ii region breaks out of the cloud as a champagne flow, a process we term ‘hot champagne’
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