93 research outputs found
The Self-Regulated Growth of Supermassive Black Holes
We present a series of simulations of the self--regulated growth of
supermassive black holes (SMBHs) in galaxies via three different fueling
mechanisms: major mergers, minor mergers, and disk instabilities. The SMBHs in
all three scenarios follow the same black hole fundamental plane (BHFP) and
correlation with bulge binding energy seen in simulations of major mergers, and
observed locally. Furthermore, provided that the total gas supply is
significantly larger than the mass of the SMBH, its limiting mass is not
influenced by the amount of gas available or the efficiency of black hole
growth. This supports the assertion that SMBHs accrete until they reach a
critical mass at which feedback is sufficient to unbind the gas locally,
terminating the inflow and stalling further growth. At the same time, while
minor and major mergers follow the same projected correlations (e.g., the
and Magorrian relations), SMBHs grown via disk instabilities do
not, owing to structural differences between the host bulges. This finding is
supported by recent observations of SMBHs in pseudobulges and bulges in barred
systems, as compared to those hosted by classical bulges. Taken together, this
provides support for the BHFP and binding energy correlations as being more
"fundamental" than other proposed correlations in that they reflect the
physical mechanism driving the co-evolution of SMBHs and spheroids.Comment: 15 pages, 16 figures, accepted for publication in Ap
Baryonic Collapse within Dark Matter Halos and the Formation of Gaseous Galactic Disks
This paper constructs an analytic framework for calculating the assembly of
galactic disks from the collapse of gas within dark matter halos, with the goal
of determining the surface density profiles. Gas parcels (baryons) fall through
the potentials of dark matter halos on nearly ballistic, zero energy orbits and
collect in a rotating disk. The dark matter halos have a nearly universal form,
as determined previously through numerical simulations. The calculation is
first carried out for a variety of pre-collapse mass distributions and rotation
profiles, including polytropic spheres in hydrostatic equilibrium with the halo
potential. The resulting disk surface density profiles have nearly power-law
forms, with well-defined edges. This idealized scenario is generalized to
include non-spherical starting states and multiple accretion events (due to gas
being added to the halo via merger events). This latter complication is
explored in detail and considers a log-normal distribution for the angular
momenta of the pre-collapse states. If this distribution is wide, then the
composite surface density approaches a universal power-law form, independent of
the shape of the constituent profiles. When the angular momentum distribution
has an intermediate width, the composite surface density attains a nearly
exponential form, roughly consistent with profiles of observed galaxies.Comment: 47 pages including 12 figures, accepted to Ap
The inhomogeneous reionization times of present-day galaxies
Today's galaxies experienced cosmic reionization at different times in different locations. For the first time, reionization (50% ionized) redshifts, z R , at the location of their progenitors are derived from new, fully coupled radiation-hydrodynamics simulation of galaxy formation and reionization at z > 6, matched to N-body simulation to z = 0. Constrained initial conditions were chosen to form the well-known structures of the local universe, including the Local Group and Virgo, in a (91 Mpc)3 volume large enough to model both global and local reionization. Reionization simulation CoDa I-AMR, by CPU-GPU code EMMA, used (2048)3 particles and (2048)3 initial cells, adaptively refined, while N-body simulation CoDa I-DM2048, by Gadget2, used (2048)3 particles, to find reionization times for all galaxies at z = 0 with masses M(z = 0) ≥ 108 M ⊙. Galaxies with reionized earlier than the universe as a whole, by up to ~500 Myr, with significant scatter. For Milky Way–like galaxies, z R ranged from 8 to 15. Galaxies with typically reionized as late or later than globally averaged 50% reionization at , in neighborhoods where reionization was completed by external radiation. The spread of reionization times within galaxies was sometimes as large as the galaxy-to-galaxy scatter. The Milky Way and M31 reionized earlier than global reionization but later than typical for their mass, neither dominated by external radiation. Their most-massive progenitors at z > 6 had z R =9.8 (MW) and 11 (M31), while their total masses had z R = 8.2 (both)
AN ULTRA-FAINT GALAXY CANDIDATE DISCOVERED in EARLY DATA from the MAGELLANIC SATELLITES SURVEY
We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness (μ = 28.5+1 -1 mag arcsec-2 within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of 45+5 -4 kpc. The physical size (r1/2 = 46+15 -11) and low luminosity (Mv = -3.2+0.4 -0.5 mag) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located 11.3+3.1 -0.9 kpc from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.Peer reviewe
Bailing Out the Milky Way: Variation in the Properties of Massive Dwarfs Among Galaxy-Sized Systems
Recent kinematical constraints on the internal densities of the Milky Way's
dwarf satellites have revealed a discrepancy with the subhalo populations of
simulated Galaxy-scale halos in the standard CDM model of hierarchical
structure formation. This has been dubbed the "too big to fail" problem, with
reference to the improbability of large and invisible companions existing in
the Galactic environment. In this paper, we argue that both the Milky Way
observations and simulated subhalos are consistent with the predictions of the
standard model for structure formation. Specifically, we show that there is
significant variation in the properties of subhalos among distinct host halos
of fixed mass and suggest that this can reasonably account for the deficit of
dense satellites in the Milky Way. We exploit well-tested analytic techniques
to predict the properties in a large sample of distinct host halos with a
variety of masses spanning the range expected of the Galactic halo. The
analytic model produces subhalo populations consistent with both Via Lactea II
and Aquarius, and our results suggest that natural variation in subhalo
properties suffices to explain the discrepancy between Milky Way satellite
kinematics and these numerical simulations. At least ~10% of Milky Way-sized
halos host subhalo populations for which there is no "too big to fail" problem,
even when the host halo mass is as large as M_host = 10^12.2 h^-1 M_sun.
Follow-up studies consisting of high-resolution simulations of a large number
of Milky Way-sized hosts are necessary to confirm our predictions. In the
absence of such efforts, the "too big to fail" problem does not appear to be a
significant challenge to the standard model of hierarchical formation.
[abridged]Comment: 12 pages, 3 figures; accepted by JCAP. Replaced with published
versio
Merging history of three bimodal clusters
We present a combined X-ray and optical analysis of three bimodal galaxy
clusters selected as merging candidates at z ~ 0.1. These targets are part of
MUSIC (MUlti--Wavelength Sample of Interacting Clusters), which is a general
project designed to study the physics of merging clusters by means of
multi-wavelength observations. Observations include spectro-imaging with
XMM-Newton EPIC camera, multi-object spectroscopy (260 new redshifts), and
wide-field imaging at the ESO 3.6m and 2.2m telescopes. We build a global
picture of these clusters using X-ray luminosity and temperature maps together
with galaxy density and velocity distributions. Idealized numerical simulations
were used to constrain the merging scenario for each system. We show that A2933
is very likely an equal-mass advanced pre-merger ~ 200 Myr before the core
collapse, while A2440 and A2384 are post-merger systems ~ 450 Myr and ~1.5 Gyr
after core collapse, respectively). In the case of A2384, we detect a
spectacular filament of galaxies and gas spreading over more than 1 h^{-1} Mpc,
which we infer to have been stripped during the previous collision. The
analysis of the MUSIC sample allows us to outline some general properties of
merging clusters: a strong luminosity segregation of galaxies in recent
post-mergers; the existence of preferential axes --corresponding to the merging
directions-- along which the BCGs and structures on various scales are aligned;
the concomitance, in most major merger cases, of secondary merging or accretion
events, with groups infalling onto the main cluster, and in some cases the
evidence of previous merging episodes in one of the main components. These
results are in good agreement with the hierarchical scenario of structure
formation, in which clusters are expected to form by successive merging events,
and matter is accreted along large--scale filaments
The cosmic web for density perturbations of various scales
We follow the evolution of galaxy systems in numerical simulation. Our goal
is to understand the role of density perturbations of various scales in the
formation and evolution of the cosmic web. We perform numerical simulations
with the full power spectrum of perturbations, and with spectrum cut at long
wavelengths. Additionally, we have one model, where we cut the intermediate
waves. We analyze the density field and study the void sizes and density field
clusters in different models. Our analysis shows that the fine structure
(groups and clusters of galaxies) is created by small-scale density
perturbations of scale \Mpc. Filaments of galaxies and clusters are
created by perturbations of intermediate scale from to \Mpc,
superclusters of galaxies by larger perturbations. We conclude that the scale
of the pattern of the cosmic web is determined by density perturbations of
scale up to \Mpc. Larger perturbations do not change the pattern of
the web, but modulate the richness of galaxy systems, and make voids emptier.
The stop of the increase of the scale of the pattern of the cosmic web with
increasing scale of density perturbations can probably be explained as the
freezing of the web at redshift .Comment: 12 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Orientation bias of optically selected galaxy clusters and its impact on stacked weak-lensing analyses
Weak-lensing measurements of the averaged shear profiles of galaxy clusters binned by some proxy for cluster mass are commonly converted to cluster mass estimates under the assumption that these cluster stacks have spherical symmetry. In this paper, we test whether this assumption holds for optically selected clusters binned by estimated optical richness. Using mock catalogues created from N-body simulations populated realistically with galaxies, we ran a suite of optical cluster finders and estimated their optical richness. We binned galaxy clusters by true cluster mass and estimated optical richness and measure the ellipticity of these stacks. We find that the processes of optical cluster selection and richness estimation are biased, leading to stacked structures that are elongated along the line of sight. We show that weak-lensing alone cannot measure the size of this orientation bias. Weak-lensing masses of stacked optically selected clusters are overestimated by up to 3–6 per cent when clusters can be uniquely associated with haloes. This effect is large enough to lead to significant biases in the cosmological parameters derived from large surveys like the Dark Energy Survey, if not calibrated via simulations or fitted simultaneously. This bias probably also contributes to the observed discrepancy between the observed and predicted Sunyaev–Zel’dovich signal of optically selected clusters
A Measurement of the Correlation of Galaxy Surveys with CMB Lensing Convergence Maps from the South Pole Telescope
We compare cosmic microwave background lensing convergence maps derived from South Pole Telescope (SPT) data with galaxy survey data from the Blanco Cosmology Survey, WISE, and a new large Spitzer/IRAC field designed to overlap with the SPT survey. Using optical and infrared catalogs covering between 17 and 68 deg^2 of sky, we detect a correlation between the SPT convergence maps and each of the galaxy density maps at >4σ, with zero correlation robustly ruled out in all cases. The amplitude and shape of the cross-power spectra are in good agreement with theoretical expectations and the measured galaxy bias is consistent with previous work. The detections reported here utilize a small fraction of the full 2500 deg^2 SPT survey data and serve as both a proof of principle of the technique and an illustration of the potential of this emerging cosmological probe
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