60 research outputs found
Non-linear violent disc instability with high Toomre's Q in high-redshift clumpy disc galaxies
We utilize zoom-in cosmological simulations to study the nature of violent
disc instability (VDI) in clumpy galaxies at high redshift, --. Our
simulated galaxies are not in the ideal state assumed in Toomre instability, of
linear fluctuations in an isolated, uniform, rotating disk. There, instability
is characterised by a parameter below unity, and lower when the disk is
thick. Instead, the high-redshift discs are highly perturbed. Over long periods
they consist of non-linear perturbations, compact massive clumps and extended
structures, with new clumps forming in inter-clump regions. This is while the
galaxy is subject to frequent external perturbances. We compute the local,
two-component parameter for gas and stars, smoothed on a
scale to capture clumps of . The regions are
confined to collapsed clumps due to the high surface density there, while the
inter-clump regions show significantly higher than unity. Tracing the
clumps back to their relatively smooth Lagrangian patches, we find that
prior to clump formation typically ranges from unity to a few. This is unlike
the expectations from standard Toomre instability. We discuss possible
mechanisms for high- clump formation, e.g. rapid turbulence decay leading to
small clumps that grow by mergers, non-axisymmetric instability, or clump
formation induced by non-linear perturbations in the disk. Alternatively, the
high- non-linear VDI may be stimulated by the external perturbations such as
mergers and counter-rotating streams. The high may represent excessive
compressive modes of turbulence, possibly induced by tidal interactions.Comment: Accepted for publication in MNRAS. 20 pages, 21 figure
Instability of Supersonic Cold Streams Feeding Galaxies II. Nonlinear Evolution of Surface and Body Modes of Kelvin-Helmholtz Instability
As part of our long-term campaign to understand how cold streams feed massive
galaxies at high redshift, we study the Kelvin-Helmholtz instability (KHI) of a
supersonic, cold, dense gas stream as it penetrates through a hot, dilute
circumgalactic medium (CGM). A linear analysis (Paper I) showed that, for
realistic conditions, KHI may produce nonlinear perturbations to the stream
during infall. Therefore, we proceed here to study the nonlinear stage of KHI,
still limited to a two-dimensional slab with no radiative cooling or gravity.
Using analytic models and numerical simulations, we examine stream breakup,
deceleration and heating via surface modes and body modes. The relevant
parameters are the density contrast between stream and CGM (), the Mach
number of the stream velocity with respect to the CGM () and the
stream radius relative to the halo virial radius (). We
find that sufficiently thin streams disintegrate prior to reaching the central
galaxy. The condition for breakup ranges from for
to for
. However, due to the large stream
inertia, KHI has only a small effect on the stream inflow rate and a small
contribution to heating and subsequent Lyman- cooling emission.Comment: The main astrophysical results are Figure 22 and Figure 23. Final 7
pages are appendices. Accepted to MNRA
Cosmological simulations of the circumgalactic medium with 1 kpc resolution: enhanced HI column densities
The circumgalactic medium (CGM), i.e. the gaseous haloes around galaxies, is
both the reservoir of gas that fuels galaxy growth and the repository of gas
expelled by galactic winds. Most cosmological, hydrodynamical simulations focus
their computational effort on the galaxies themselves and treat the CGM more
coarsely, which means small-scale structure cannot be resolved. We get around
this issue by running zoom-in simulations of a Milky Way-mass galaxy with
standard mass refinement and additional uniform spatial refinement within the
virial radius. This results in a detailed view of its gaseous halo at
unprecedented (1 kpc) uniform resolution with only a moderate increase in
computational time. The improved spatial resolution does not impact the central
galaxy or the average density of the CGM. However, it drastically changes the
radial profile of the neutral hydrogen column density, which is enhanced at
galactocentric radii larger than 40 kpc. The covering fraction of Lyman-Limit
Systems within 150 kpc is almost doubled. We therefore conclude that some of
the observational properties of the CGM are strongly resolution dependent.
Increasing the resolution in the CGM, without increasing the resolution of the
galaxies, is a promising and computationally efficient method to push the
boundaries of state-of-the-art simulations.Comment: Accepted for publication in MNRAS Letters. Revised version: minor
change
Efficient Formation of Massive Galaxies at Cosmic Dawn by Feedback-Free Starbursts
JWST observations indicate a surprising excess of luminous galaxies at and above, consistent with efficient conversion of the accreted gas into
stars, unlike the suppression of star formation by feedback at later times. We
show that the high densities and low metallicities at this epoch {\it
guarantee} a high star-formation efficiency (SFE) in the most massive
dark-matter haloes. Feedback-free starbursts (FFBs) occur when the free-fall
time is shorter than Myr, below the time for low-metallicity massive
stars to develop winds and supernovae. This corresponds to a characteristic
density of cm. A comparable threshold density permits
a starburst by allowing cooling to star-forming temperatures in a free-fall
time. The galaxies within haloes at are
expected to have FFB densities. The halo masses allow efficient gas supply by
cold streams in a halo crossing time Myr. The FFBs gradually turn all
the accreted gas into stars in clusters of within
galaxies that are rotating discs or shells. The starbursting clouds are
insensitive to radiative feedback and are shielded against feedback from
earlier stars. We predict high SFE above thresholds in redshift and halo mass,
where the density is cm. The haloes of are predicted to host galaxies of
with SFR yr and sub-kpc sizes. The metallicity is
with little gas, dust, outflows and hot circumgalactic gas,
allowing a top-heavy IMF but not requiring it. The compact galaxies with
thousands of young FFB clusters may have implications on reionization,
black-hole growth and globular clusters.Comment: 20 pages, 7 figure
Star Formation and Clumps in Cosmological Galaxy Simulations with Radiation Pressure Feedback
Cosmological simulations of galaxies have typically produced too many stars
at early times. We study the global and morphological effects of radiation
pressure (RP) in eight pairs of high-resolution cosmological galaxy formation
simulations. We find that the additional feedback suppresses star formation
globally by a factor of ~2. Despite this reduction, the simulations still
overproduce stars by a factor of ~2 with respect to the predictions provided by
abundance matching methods for halos more massive than 5E11 Msun/h (Behroozi,
Wechsler & Conroy 2013).
We also study the morphological impact of radiation pressure on our
simulations. In simulations with RP the average number of low mass clumps falls
dramatically. Only clumps with stellar masses Mclump/Mdisk <= 5% are impacted
by the inclusion of RP, and RP and no-RP clump counts above this range are
comparable. The inclusion of RP depresses the contrast ratios of clumps by
factors of a few for clump masses less than 5% of the disk masses. For more
massive clumps, the differences between and RP and no-RP simulations diminish.
We note however, that the simulations analyzed have disk stellar masses below
about 2E10 Msun/h.
By creating mock Hubble Space Telescope observations we find that the number
of clumps is slightly reduced in simulations with RP. However, since massive
clumps survive the inclusion of RP and are found in our mock observations, we
do not find a disagreement between simulations of our clumpy galaxies and
observations of clumpy galaxies. We demonstrate that clumps found in any single
gas, stellar, or mock observation image are not necessarily clumps found in
another map, and that there are few clumps common to multiple maps.Comment: 13 pages, 6 figures, submitted to MNRA
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