402 research outputs found
Through a Smoother Lens: An expected absence of LCDM substructure detections from hydrodynamic and dark matter only simulations
A fundamental prediction of the cold dark matter cosmology is the existence
of a large number of dark subhalos around galaxies, most of which should be
entirely devoid of stars. Confirming the existence of dark substructures stands
among the most important empirical challenges in modern cosmology: if they are
found and quantified with the mass spectrum expected, then this would close the
door on a vast array of competing theories. But in order for observational
programs of this kind to reach fruition, we need robust predictions. Here we
explore substructure predictions for lensing using galaxy lens-like hosts at
z=0.2 from the Illustris simulations both in full hydrodynamics and dark matter
only. We quantify substructures more massive than ~ 10^9 M_sun, comparable to
current lensing detections derived from HST, Keck, and ALMA. The addition of
full hydrodynamics reduces the overall subhalo mass function by about a factor
of two. Even for the dark matter only runs, most (~ 85%) lines of sight through
projected cylinders of size close to an Einstein radius contain no
substructures larger than 10^9 M_sun. The fraction of empty sight lines rises
to ~ 95% in full physics simulations. This suggests we will likely need
hundreds of strong lensing systems suitable for substructure studies, as well
as predictions that include the effects of baryon physics on substructure, to
properly constrain cosmological models. Fortunately, the field is poised to
fulfill these requirements.Comment: 11 pages, 9 figure
Direct Detection of Dark Matter Debris Flows
Tidal stripping of dark matter from subhalos falling into the Milky Way
produces narrow, cold tidal streams as well as more spatially extended "debris
flows" in the form of shells, sheets, and plumes. Here we focus on the debris
flow in the Via Lactea II simulation, and show that this incompletely
phase-mixed material exhibits distinctive high velocity behavior. Unlike tidal
streams, which may not necessarily intersect the Earth's location, debris flow
is spatially uniform at 8 kpc and thus guaranteed to be present in the dark
matter flux incident on direct detection experiments. At Earth-frame speeds
greater than 450 km/s, debris flow comprises more than half of the dark matter
at the Sun's location, and up to 80% at even higher speeds. Therefore, debris
flow is most important for experiments that are particularly sensitive to the
high speed tail of the dark matter distribution, such as searches for light or
inelastic dark matter or experiments with directional sensitivity. We show that
debris flow yields a distinctive recoil energy spectrum and a broadening of the
distribution of incidence direction.Comment: 22 pages, 7 figures; accepted for publication in PR
Galactic accretion and the outer structure of galaxies in the CDM model
We have combined the semi-analytic galaxy formation model of Guo et al.
(2011) with the particle-tagging technique of Cooper et al. (2010) to predict
galaxy surface brightness profiles in a representative sample of ~1900 massive
dark matter haloes (10^12--10^14 M_sol) from the Millennium II Lambda-CDM
N-body simulation. Here we present our method and basic results focusing on the
outer regions of galaxies, consisting of stars accreted in mergers. These
simulations cover scales from the stellar haloes of Milky Way-like galaxies to
the 'cD envelopes' of groups and clusters, and resolve low surface brightness
substructure such as tidal streams. We find that the surface density of
accreted stellar mass around the central galaxies of dark matter haloes is well
described by a Sersic profile, the radial scale and amplitude of which vary
systematically with halo mass (M_200). The total stellar mass surface density
profile breaks at the radius where accreted stars start to dominate over stars
formed in the galaxy itself. This break disappears with increasing M_200
because accreted stars contribute more of the total mass of galaxies, and is
less distinct when the same galaxies are averaged in bins of stellar mass,
because of scatter in the relation between M_star and M_200. To test our model
we have derived average stellar mass surface density profiles for massive
galaxies at z~0.08 by stacking SDSS images. Our model agrees well with these
stacked profiles and with other data from the literature, and makes predictions
that can be more rigorously tested by future surveys that extend the analysis
of the outer structure of galaxies to fainter isophotes. We conclude that it is
likely that the outer structure of the spheroidal components of galaxies is
largely determined by collisionless merging during their hierarchical assemblyComment: Accepted by MNRAS. Shortened following referee's report, conclusions
unchanged. 21 pages, 15 figure
The Absolute Age of M92
The \textit{absolute age} of a simple stellar population is of fundamental
interest for a wide range of applications but is difficult to measure in
practice, as it requires an understanding of the uncertainties in a variety of
stellar evolution processes as well as the uncertainty in the distance,
reddening and composition. As a result, most studies focus only on the
\textit{relative age} by assuming that stellar evolution calculations are
accurate and using age determinations techniques that are relatively
independent of distance and reddening. Here, we construct sets of
theoretical isochrones through Monte Carlo simulation using the Dartmouth
Stellar Evolution Program to measure the absolute age of the globular cluster
M92. For each model, we vary a range of input physics used in the stellar
evolution models, including opacities, nuclear reaction rates, diffusion
coefficients, atmospheric boundary conditions, helium abundance, and treatment
of convection. We also explore variations in the distance and reddening as well
as its overall metallicity and enhancement. We generate simulated Hess
diagrams around the main-sequence turn-off region from each set of isochrones
and use a Voronoi binning method to fit the diagrams to HST ACS data. We find
the age of M92 to be Gyr. The error in the absolute
age is dominated by the uncertainty in the distance to M92 ( of the
error budget); of the remaining parameters, only the total metallicity,
element abundance, and treatment of helium diffusion contribute
significantly to the total error.Comment: 15 Pages, 14 Figures, 2 Tables; Accepted for Publication A
The origin of ultra diffuse galaxies: stellar feedback and quenching
We test if the cosmological zoom-in simulations of isolated galaxies from the
FIRE project reproduce the properties of ultra diffuse galaxies. We show that
stellar feedback-generated outflows that dynamically heat galactic stars,
together with a passively aging stellar population after imposed quenching
(from e.g. infall into a galaxy cluster), naturally reproduce the observed
population of red UDGs, without the need for high spin halos or dynamical
influence from their host cluster. We reproduce the range of surface
brightness, radius and absolute magnitude of the observed z=0 red UDGs by
quenching simulated galaxies at a range of different times. They represent a
mostly uniform population of dark matter-dominated galaxies with M_star ~1e8
Msun, low metallicity and a broad range of ages. The most massive simulated
UDGs require earliest quenching and are therefore the oldest. Our simulations
provide a good match to the central enclosed masses and the velocity
dispersions of the observed UDGs (20-50 km/s). The enclosed masses of the
simulated UDGs remain largely fixed across a broad range of quenching times
because the central regions of their dark matter halos complete their growth
early. A typical UDG forms in a dwarf halo mass range of Mh~4e10-1e11 Msun. The
most massive red UDG in our sample requires quenching at z~3 when its halo
reached Mh ~ 1e11 Msun. If it, instead, continues growing in the field, by z=0
its halo mass reaches > 5e11 Msun, comparable to the halo of an L* galaxy. If
our simulated dwarfs are not quenched, they evolve into bluer low-surface
brightness galaxies with mass-to-light ratios similar to observed field dwarfs.
While our simulation sample covers a limited range of formation histories and
halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy
type around Ms~1e8 Msun, both in the field and in clusters.Comment: 20 pages, 13 figures; match the MNRAS accepted versio
Evolution of giant molecular clouds across cosmic time
Giant molecular clouds (GMCs) are well studied in the local Universe, however, exactly how their properties vary during galaxy evolution is poorly understood due to challenging resolution requirements, both observational and computational. We present the first time-dependent analysis of GMCs in a Milky Way-like galaxy and an Large Magellanic Cloud (LMC)-like dwarf galaxy of the FIRE-2 (Feedback In Realistic Environments) simulation suite, which have sufficient resolution to predict the bulk properties of GMCs in cosmological galaxy formation self-consistently. We show explicitly that the majority of star formation outside the galactic centre occurs within self-gravitating gas structures that have properties consistent with observed bound GMCs. We find that the typical cloud bulk properties such as mass and surface density do not vary more than a factor of 2 in any systematic way after the first Gyr of cosmic evolution within a given galaxy from its progenitor. While the median properties are constant, the tails of the distributions can briefly undergo drastic changes, which can produce very massive and dense self-gravitating gas clouds. Once the galaxy forms, we identify only two systematic trends in bulk properties over cosmic time: a steady increase in metallicity produced by previous stellar populations and a weak decrease in bulk cloud temperatures. With the exception of metallicity, we find no significant differences in cloud properties between the Milky Way-like and dwarf galaxies. These results have important implications for cosmological star and star cluster formation and put especially strong constraints on theories relating the stellar initial mass function to cloud properties
The Mass Profile and Accretion History of Cold Dark Matter Halos
We use the Millennium Simulation series to study the relation between the
accretion history (MAH) and mass profile of cold dark matter halos. We find
that the mean density within the scale radius, r_{-2} (where the halo density
profile has isothermal slope), is directly proportional to the critical density
of the Universe at the time when the main progenitor's virial mass equals the
mass enclosed within r_{-2}. Scaled to these characteristic values of mass and
density, the mean MAH, expressed in terms of the critical density of the
Universe, M(\rho_{crit}(z)), resembles that of the enclosed density profile,
M(), at z=0. Both follow closely the NFW profile, suggesting that the
similarity of halo mass profiles originates from the mass-independence of halo
MAHs. Support for this interpretation is provided by outlier halos whose
accretion histories deviate from the NFW shape; their mass profiles show
correlated deviations from NFW and are better approximated by Einasto profiles.
Fitting both M() and M(\rho_{crit}) with either NFW or Einasto profiles
yield concentration and shape parameters that are correlated, confirming and
extending earlier work linking the concentration of a halo with its accretion
history. These correlations also confirm that halo structure is insensitive to
initial conditions: only halos whose accretion histories differ greatly from
the NFW shape show noticeable deviations from NFW in their mass profiles. As a
result, the NFW profile provides acceptable fits to hot dark matter halos,
which do not form hierarchically, and for fluctuation power spectra other than
CDM. Our findings, however, predict a subtle but systematic dependence of mass
profile shape on accretion history which, if confirmed, would provide strong
support for the link between accretion history and halo structure we propose
here.Comment: 12 pages, 8 figures, MNRAS 432 1103L (2013
Dark and luminous satellites of LMC-mass galaxies in the FIRE simulations
Within lambda cold dark matter (CDM), dwarf galaxies like the Large Magellanic Cloud (LMC) are expected to host numerous dark matter subhaloes, several of which should host faint dwarf companions. Recent Gaia proper motions confirm new members of the LMC system in addition to the previously known SMC, including two classical dwarf galaxies (M∗ > 105 M; Carina and Fornax) as well as several ultrafaint dwarfs (Car2, Car3, Hor1, and Hyd1). We use the Feedback In Realistic Environments (FIRE) simulations to study the dark and luminous (down to ultrafaint masses, M∗ ∼6×103 M) substructure population of isolated LMC-mass hosts (M200m = 1–3×1011 M) and place the Gaia + DES results in a cosmological context. By comparing number counts of subhaloes in simulations with and without baryons, we find that, within 0.2 r200m, LMC-mass hosts deplete ∼30 per cent of their substructure, significantly lower than the ∼70 per cent of substructure depleted by Milky Way (MW) mass hosts. For our highest resolution runs (mbary = 880 M), ∼ 5–10 subhaloes form galaxies with M∗ ≥ 104 M , in agreement with the seven observationally inferred pre-infall LMC companions. However, we find steeper simulated luminosity functions than observed, hinting at observation incompleteness at the faint end. The predicted DM content for classical satellites in FIRE agrees with observed estimates for Carina and Fornax, supporting the case for an LMC association. We predict that tidal stripping within the LMC potential lowers the inner dark matter density of ultrafaint companions of the LMC. Thus, in addition to their orbital consistency, the low densities of dwarfs Car2, Hyd1, and Hyd2 reinforce their likelihood of Magellanic association
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