111 research outputs found
On the formation of dwarf galaxies and stellar halos
Using analytic arguments and a suite of very high resolution (10^3 Msun per
particle) cosmological hydro-dynamical simulations, we argue that high
redshift, z ~ 10, M ~ 10^8 Msun halos, form the smallest `baryonic building
block' (BBB) for galaxy formation. These halos are just massive enough to
efficiently form stars through atomic line cooling and to hold onto their gas
in the presence of supernovae winds and reionisation. These combined effects,
in particular that of the supernovae feedback, create a sharp transition: over
the mass range 3-10x10^7 Msun, the BBBs drop two orders ofmagnitude in stellar
mass. Below ~2x10^7 Msun, galaxies will be dark with almost no stars and no
gas. Above this scale is the smallest unit of galaxy formation: the BBB.
A small fraction (~100) of these gas rich BBBs fall in to a galaxy the size
of the Milky Way. Ten percent of these survive to become the observed LG dwarf
galaxies at the present epoch. Those in-falling halos on benign orbits which
keep them far away from the Milky Way or Andromeda manage to retain their gas
and slowly form stars - these become the smallest dwarf irregular galax ies;
those on more severe orbits lose their gas faster than they can form stars and
become the dwarf spheroidals. The remaining 90% of the BBBs will be accreted.
We show that this gives a metallicity and total stellar mass consistent with
the Milky Way old stellar halo (abridged).Comment: 15 pages, 7 figures, one figure added to match accepted version. Some
typos fixed. MNRAS in pres
On the formation of dwarf galaxies and stellar haloes
Using analytic arguments and a suite of very high resolution (∼103 M⊙ per particle) cosmological hydrodynamical simulations, we argue that high-redshift, z∼ 10, M∼ 108 M⊙ haloes, form the smallest ‘baryonic building block' (BBB) for galaxy formation. These haloes are just massive enough to efficiently form stars through atomic line cooling and to hold on to their gas in the presence of supernova (SN) winds and reionization. These combined effects, in particular that of the SN feedback, create a sharp transition: over the mass range 3-10 × 107 M⊙, the BBBs drop two orders of magnitude in stellar mass. Below ∼2 × 107 M⊙, galaxies will be dark with almost no stars and no gas. Above this scale is the smallest unit of galaxy formation: the BBB. We show that the BBBs have stellar distributions which are spheroidal, of low rotational velocity, old and metal poor: they resemble the dwarf spheroidal galaxies (dSphs) of the Local Group (LG). Unlike the LG dSphs, however, they contain significant gas fractions. We connect these high-redshift BBBs to the smallest dwarf galaxies observed at z= 0 using linear theory. A small fraction (∼100) of these gas-rich BBBs at high redshift fall in to a galaxy the size of the Milky Way (MW). We suggest that 10 per cent of these survive to become the observed LG dwarf galaxies at the present epoch. This is consistent with recent numerical estimates. Those infalling haloes on benign orbits which keep them far away from the MW or Andromeda manage to retain their gas and slowly form stars - these become the smallest dwarf irregular galaxies; those on more severe orbits lose their gas faster than they can form stars and become the dwarf spheroidals. The remaining 90 per cent of the BBBs will be accreted. We show that this gives a metallicity and total stellar mass consistent with the MW old stellar hal
Quantifying the Rarity of the Local Super-Volume
We investigate the extent to which the number of clusters of mass exceeding 1015M⊙h−1 within the local super-volume (<135Mpch−1) is compatible with the standard ΛCDM cosmological model. Depending on the mass estimator used, we find that the observed number N of such massive structures can vary between 0 and 5. Adopting N = 5 yields ΛCDM likelihoods as low as 2.4 × 10−3 (with σ8 = 0.81) or 3.8 × 10−5 (with σ8 = 0.74). However, at the other extreme (N = 0), the likelihood is of order unity. Thus, while potentially very powerful, this method is currently limited by systematic uncertainties in cluster mass estimates. This motivates efforts to reduce these systematics with additional observations and improved modelling
Quantifying the Rarity of the Local Super-Volume
We investigate the extent to which the number of clusters of mass exceeding 1015M⊙h−1 within the local super-volume (<135Mpch−1) is compatible with the standard ΛCDM cosmological model. Depending on the mass estimator used, we find that the observed number N of such massive structures can vary between 0 and 5. Adopting N = 5 yields ΛCDM likelihoods as low as 2.4 × 10−3 (with σ8 = 0.81) or 3.8 × 10−5 (with σ8 = 0.74). However, at the other extreme (N = 0), the likelihood is of order unity. Thus, while potentially very powerful, this method is currently limited by systematic uncertainties in cluster mass estimates. This motivates efforts to reduce these systematics with additional observations and improved modelling
Dynamical friction in constant density cores: a failure of the Chandrasekhar formula
Using analytic calculations and N-body simulations we show that in constant density (harmonic) cores, sinking satellites undergo an initial phase of very rapid (super-Chandrasekhar) dynamical friction, after which they experience no dynamical friction at all. For density profiles with a central power law profile, ρ∝r−α, the infalling satellite heats the background and causes α to decrease. For α < 0.5 initially, the satellite generates a small central constant density core and stalls as in the α= 0 case. We discuss some astrophysical applications of our results to decaying satellite orbits, galactic bars and mergers of supermassive black hole binaries. In a companion paper we show that a central constant density core can provide a natural solution to the timing problem for Fornax's globular cluster
EDGE: The origin of scatter in ultra-faint dwarf stellar masses and surface brightnesses
We demonstrate how the least luminous galaxies in the Universe, ultra-faint
dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic
reionization. We select a low-mass () dark matter halo from a cosmological volume, and perform
zoom hydrodynamical simulations with multiple alternative histories using
"genetically modified" initial conditions. Earlier forming ultra-faints have
higher stellar mass today, due to a longer period of star formation before
their quenching by reionization. Our histories all converge to the same final
dynamical mass, demonstrating the existence of extended scatter ( 1 dex)
in stellar masses at fixed halo mass due to the diversity of possible
histories. One of our variants builds less than 2 % of its final dynamical mass
before reionization, rapidly quenching in-situ star formation. The bulk of its
final stellar mass is later grown by dry mergers, depositing stars in the
galaxy's outskirts and hence expanding its effective radius. This mechanism
constitutes a new formation scenario for highly diffuse (, ), metal-poor (), ultra-faint ()
dwarf galaxies within the reach of next-generation low surface brightness
surveys.Comment: Minor edits to match the published ApJL version. Results unchange
EDGE -- Dark matter or astrophysics? Clear prospects to break dark matter heating degeneracies with HI rotation in faint dwarf galaxies
Low-mass dwarf galaxies are expected to showcase pristine `cuspy' inner dark
matter density profiles compared to their stellar sizes, as they form too few
stars to significantly drive dark matter heating through supernovae-driven
outflows. Here, we study such simulated faint systems () drawn from high-resolution (3 pc)
cosmological simulations from the `Engineering Dwarf Galaxies at the Edge of
galaxy formation' (EDGE) project. We confirm that these objects have steep and
rising inner dark matter density profiles at , little affected by galaxy
formation effects. But five dwarf galaxies from the suite showcase a detectable
HI reservoir (),
analogous to the observed population of faint, HI-bearing dwarf galaxies. These
reservoirs exhibit episodes of ordered rotation, opening windows for rotation
curve analysis. Within actively star-forming dwarfs, stellar feedback easily
disrupts the tenuous HI discs (), making rotation short-lived () and
more challenging to interpret for dark matter inferences. Contrastingly, we
highlight a long-lived () and easy-to-interpret HI
rotation curve extending to in a quiescent
dwarf, that has not formed new stars since . This stable gas disc is
supported by an oblate dark matter halo shape that drives high angular momentum
gas flows. Our results strongly motivate further searches for HI rotation
curves in the observed population of HI-bearing low-mass dwarfs, that provide a
key regime to disentangle the respective roles of dark matter microphysics and
galaxy formation effects in driving dark matter heating.Comment: Main text 10 pages, submitted to MNRAS. Comments welcome
EDGE: The shape of dark matter haloes in the faintest galaxies
Collisionless Dark Matter Only (DMO) structure formation simulations predict
that Dark Matter (DM) haloes are prolate in their centres and triaxial towards
their outskirts. The addition of gas condensation transforms the central DM
shape to be rounder and more oblate. It is not clear, however, whether such
shape transformations occur in `ultra-faint' dwarfs, which have extremely low
baryon fractions. We present the first study of the shape and velocity
anisotropy of ultra-faint dwarf galaxies that have gas mass fractions of
. These dwarfs are drawn from the
Engineering Dwarfs at Galaxy formation's Edge (EDGE) project, using high
resolution simulations that allow us to resolve DM halo shapes within the half
light radius (pc). We show that gas-poor ultra-faints (M; ) retain
their pristine prolate DM halo shape even when gas, star formation and feedback
are included. This could provide a new and robust test of DM models. By
contrast, gas-rich ultra-faints (M;
) become rounder and more oblate within half
light radii. Finally, we find that most of our simulated dwarfs have
significant radial velocity anisotropy that rises to at
. The one exception is a dwarf that forms a rotating
gas/stellar disc because of a planar, major merger. Such strong anisotropy
should be taken into account when building mass models of gas-poor
ultra-faints.Comment: 16 pages and 11 figures (excluding appendices), accepted by MNRA
Linearization of homogeneous, nearly-isotropic cosmological models
Homogeneous, nearly-isotropic Bianchi cosmological models are considered.
Their time evolution is expressed as a complete set of non-interacting linear
modes on top of a Friedmann-Robertson-Walker background model. This connects
the extensive literature on Bianchi models with the more commonly-adopted
perturbation approach to general relativistic cosmological evolution.
Expressions for the relevant metric perturbations in familiar coordinate
systems can be extracted straightforwardly. Amongst other possibilities, this
allows for future analysis of anisotropic matter sources in a more general
geometry than usually attempted.
We discuss the geometric mechanisms by which maximal symmetry is broken in
the context of these models, shedding light on the origin of different Bianchi
types. When all relevant length-scales are super-horizon, the simplest Bianchi
I models emerge (in which anisotropic quantities appear parallel transported).
Finally we highlight the existence of arbitrarily long near-isotropic epochs
in models of general Bianchi type (including those without an exact isotropic
limit).Comment: 31 pages, 2 figures. Submitted to CQ
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