We construct merger trees of dark matter haloes and quantify their merger rates and mass growth rates using the joint data set from the Millennium and Millennium-II simulations. The finer resolution ...of the Millennium-II simulation has allowed us to extend our earlier analysis of halo merger statistics to an unprecedentedly wide range of descendant halo mass (1010≲M0≲ 1015 M⊙), progenitor mass ratio (10−5≲ξ≤ 1) and redshift (0 ≤z≲ 15). We update our earlier fitting form for the mean merger rate per halo as a function of M0, ξ and z. The overall behaviour of this quantity is unchanged: the rate per unit redshift is nearly independent of z out to z∼ 15; the dependence on halo mass is weak (∝M0.130); and it is nearly a power law in the progenitor mass ratio (∝ξ−2). We also present a simple and accurate fitting formula for the mean mass growth rate of haloes as a function of mass and redshift. This mean rate is 46 M⊙ yr−1 for 1012 M⊙ haloes at z= 0, and it increases with mass as ∝M1.1 and with redshift as (1 +z)2.5 (for z≳ 1). When the fit for the mean mass growth rate is integrated over a halo's history, we find excellent match to the mean mass assembly histories of the simulated haloes. By combining merger rates and mass assembly histories, we present results for the number of mergers over a halo's history and the statistics of the redshift of the last major merger.
Abstract
We present a measurement of the Hubble constant
H
0
from surface brightness fluctuation (SBF) distances for 63 bright, mainly early-type galaxies out to 100 Mpc observed with the WFC3/IR on ...the Hubble Space Telescope (HST). The sample is drawn from several independent HST imaging programs using the F110W bandpass, with the majority of the galaxies being selected from the MASSIVE survey. The distances reach the Hubble flow with a median statistical uncertainty per measurement of 4%. We construct the Hubble diagram with these IR SBF distances and constrain
H
0
using four different treatments of the galaxy velocities. For the SBF zero-point calibration, we use both the existing tie to Cepheid variables, updated for consistency with the latest determination of the distance to the Large Magellanic Cloud from detached eclipsing binaries, and a new tie to the tip of the red giant branch (TRGB) calibrated from the maser distance to NGC 4258. These two SBF calibrations are consistent with each other and with theoretical predictions from stellar population models. From a weighted average of the Cepheid and TRGB calibrations, we derive
H
0
= 73.3 ± 0.7 ± 2.4 km s
−1
Mpc
−1
, where the error bars reflect the statistical and systematic uncertainties. This result accords well with recent measurements of
H
0
from Type Ia supernovae, time delays in multiply lensed quasars, and water masers. The systematic uncertainty could be reduced to below 2% by calibrating the SBF method with precision TRGB distances for a statistical sample of massive early-type galaxies out to the Virgo cluster measured with the James Webb Space Telescope.
Abstract
Most stellar-dynamical determinations of the masses of nearby supermassive black holes (SMBHs) have been obtained with the orbit superposition technique under the assumption of axisymmetry. ...However, few galaxies—in particular massive early-type galaxies—obey exact axisymmetry. Here we present a revised orbit superposition code and a new approach for dynamically determining the intrinsic shapes and mass parameters of triaxial galaxies based on spatially resolved stellar kinematic data. The triaxial TriOS code described here corrects an error in the original van den Bosch et al. code that gives rise to incorrect projections for most orbits in triaxial models and can significantly impact parameter search results. The revised code also contains significant improvements in orbit sampling, mass constraints, and run time. Furthermore, we introduce two new parameter-searching strategies—a new set of triaxial shape parameters and a novel grid-free sampling technique—that together lead to a remarkable gain in efficiency in locating the best-fit model. We apply the updated code and search method to NGC 1453, a fast-rotating massive elliptical galaxy. A full 6D parameter search finds
p
=
b
/
a
=
0.933
−
0.015
+
0.014
and
q
=
c
/
a
= 0.779 ± 0.012 for the intrinsic axis ratios and
T
= 0.33 ± 0.06 for the triaxiality parameter. Despite the deviations from axisymmetry, the best-fit SMBH mass, stellar mass-to-light ratio, and dark matter enclosed mass for NGC 1453 are consistent with the axisymmetric results. More comparisons between axisymmetric and triaxial modeling are needed before drawing general conclusions.
We use the extensive catalogue of dark matter haloes from the Millennium Simulation to investigate the statistics of the mass accretion histories (MAHs) and accretion rates of ∼500 000 haloes from ...redshift z= 0 to 6. We find only about 25 per cent of the haloes to have MAHs that are well described by a 1-parameter exponential form. For the rest of the haloes, between 20 per cent (Milky Way mass) and 50 per cent (cluster mass) experience late-time growth that is steeper than an exponential, whereas the remaining haloes show plateaued late-time growth that is shallower than an exponential. The haloes with slower late-time growth tend to reside in denser environments, suggesting that either tidal stripping or the ‘hotter’ dynamics are suppressing the accretion rate of dark matter on to these haloes. These deviations from exponential growth are well fit by introducing a second parameter: M(z) ∝ (1 +z)βe−γz. The full distribution of β and γ as a function of halo mass is provided. From the analytic form of M(z), we obtain a simple formula for the mean accretion rate of dark matter, , as a function of redshift and mass. At z= 0, this rate is 42 M⊙ yr−1 for 1012 M⊙ haloes, which corresponds to a mean baryon accretion rate of . This mean rate increases approximately as (1 +z)1.5 at low z and (1 +z)2.5 at high z, reaching and 140 M⊙ yr−1 at z= 1, 2 and 3. The specific rate depends on halo mass weakly: . Results for the broad distributions about the mean rates are also discussed.
Abstract
We present estimates for the number of shadow-resolved supermassive black hole (SMBH) systems that can be detected using radio interferometers, as a function of angular resolution, flux ...density sensitivity, and observing frequency. Accounting for the distribution of SMBHs across mass, redshift, and accretion rate, we use a new semianalytic spectral energy distribution model to derive the number of SMBHs with detectable and optically thin horizon-scale emission. We demonstrate that (sub)millimeter interferometric observations with ∼0.1
μ
as resolution and ∼1
μ
Jy sensitivity could access >10
6
SMBH shadows. We then further decompose the shadow source counts into the number of black holes for which we could expect to observe the first- and second-order lensed photon rings. Accessing the bulk population of first-order photon rings requires ≲2
μ
as resolution and ≲0.5 mJy sensitivity, whereas doing the same for second-order photon rings requires ≲0.1
μ
as resolution and ≲5
μ
Jy sensitivity. Our model predicts that with modest improvements to sensitivity, as many as ∼5 additional horizon-resolved sources should become accessible to the current Event Horizon Telescope (EHT), whereas a next-generation EHT observing at 345 GHz should have access to ∼3 times as many sources. More generally, our results can help guide enhancements of current arrays and specifications for future interferometric experiments that aim to spatially resolve a large population of SMBH shadows or higher-order photon rings.
Abstract Evidence for the majority of the supermassive black holes in the local Universe has been obtained dynamically from stellar motions with the Schwarzschild orbit superposition method. However, ...there have been only a handful of studies using simulated data to examine the ability of this method to reliably recover known input black hole masses M BH and other galaxy parameters. Here, we conduct a comprehensive assessment of the reliability of the triaxial Schwarzschild method at simultaneously determining M BH , stellar mass-to-light ratio M */ L , dark matter mass, and three intrinsic triaxial shape parameters of simulated galaxies. For each of 25 rounds of mock observations using simulated stellar kinematics and the TriOS code, we derive best-fitting parameters and confidence intervals after a full search in the 6D parameter space with our likelihood-based model inference scheme. The two key mass parameters, M BH and M */ L , are recovered within the 68% confidence interval, and other parameters are recovered between the 68% and 95% confidence intervals. The spatially varying velocity anisotropy of the stellar orbits is also well recovered. We explore whether the goodness-of-fit measure used for galaxy model selection in our pipeline is biased by variable complexity across the 6D parameter space. In our tests, adding a penalty term to the likelihood measure either makes little difference, or worsens the recovery in some cases.
We use the Illustris simulation to study the relative contributions of in situ star formation and stellar accretion to the build-up of galaxies over an unprecedentedly wide range of masses (M
* = ...109-1012 M⊙), galaxy types, environments, and assembly histories. We find that the ‘two-phase’ picture of galaxy formation predicted by some models is a good approximation only for the most massive galaxies in our simulation – namely, the stellar mass growth of galaxies below a few times 1011 M⊙ is dominated by in situ star formation at all redshifts. The fraction of the total stellar mass of galaxies at z = 0 contributed by accreted stars shows a strong dependence on galaxy stellar mass, ranging from about 10 per cent for Milky Way-sized galaxies to over 80 per cent for M
* ≈ 1012 M⊙ objects, yet with a large galaxy-to-galaxy variation. At a fixed stellar mass, elliptical galaxies and those formed at the centres of younger haloes exhibit larger fractions of ex situ stars than disc-like galaxies and those formed in older haloes. On average, ∼50 per cent of the ex situ stellar mass comes from major mergers (stellar mass ratio μ > 1/4), ∼20 per cent from minor mergers (1/10 < μ < 1/4), ∼20 per cent from very minor mergers (μ < 1/10), and ∼10 per cent from stars that were stripped from surviving galaxies (e.g. flybys or ongoing mergers). These components are spatially segregated, with in situ stars dominating the innermost regions of galaxies, and ex situ stars being deposited at larger galactocentric distances in order of decreasing merger mass ratio.
Abstract
The three-dimensional intrinsic shape of a galaxy and the mass of the central supermassive black hole provide key insight into the galaxy’s growth history over cosmic time. Standard ...assumptions of a spherical or axisymmetric shape can be simplistic and can bias the black hole mass inferred from the motions of stars within a galaxy. Here, we present spatially resolved stellar kinematics of M87 over a two-dimensional 250″ × 300″ contiguous field covering a radial range of 50 pc–12 kpc from integral-field spectroscopic observations at the Keck II Telescope. From about 5 kpc and outward, we detect a prominent 25 km s
−1
rotational pattern, in which the kinematic axis (connecting the maximal receding and approaching velocities) is 40° misaligned with the photometric major axis of M87. The rotational amplitude and misalignment angle both decrease in the inner 5 kpc. Such misaligned and twisted velocity fields are a hallmark of triaxiality, indicating that M87 is not an axisymmetrically shaped galaxy. Triaxial Schwarzschild orbit modeling with more than 4000 observational constraints enabled us to determine simultaneously the shape and mass parameters. The models incorporate a radially declining profile for the stellar mass-to-light ratio suggested by stellar population studies. We find that M87 is strongly triaxial, with ratios of
p
= 0.845 for the middle-to-long principal axes and
q
= 0.722 for the short-to-long principal axes, and determine the black hole mass to be
(
5.37
−
0.25
+
0.37
±
0.22
)
×
10
9
M
⊙
, where the second error indicates the systematic uncertainty associated with the distance to M87.
The baryonic assembly of dark matter haloes Faucher-Giguère, Claude-André; Kereš, Dušan; Ma, Chung-Pei
Monthly Notices of the Royal Astronomical Society,
November 2011, Letnik:
417, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We use a suite of cosmological hydrodynamic simulations to systematically quantify the accretion rates of baryons into dark matter haloes and the resulting baryon mass fractions, as a function of ...halo mass, redshift and baryon type (including cold and hot gas). We find that the net baryonic accretion rates through the virial radius are sensitive to galactic outflows and explore a range of outflow parameters to illustrate the effects. We show that the cold gas accretion rate is in general not a simple universal factor of the dark matter accretion rate, and that galactic winds can cause star formation rates to deviate significantly from the external gas accretion rates, both via gas ejection and re-accretion. Furthermore, galactic winds can inject enough energy and momentum in the surrounding medium to slow down accretion altogether, especially in low-mass haloes and at low redshift, but the impact of outflows is suppressed with increasing halo mass. By resolving the accretion rates versus radius from the halo centres, we show how cold streams penetrate the hot atmospheres of massive haloes at z≥ 2, but gradually disappear at lower redshift. The total baryon mass fraction is also strongly suppressed by outflows in low-mass haloes, but is nearly universal in the absence of feedback in haloes above the UV background suppression scale, corresponding to circular velocities v
c∼ 50 km s−1. The transition halo mass, at which the gas mass in haloes is equal for the cold and hot components, is roughly constant at ∼1011.5 M⊙ and does not depend sensitively on the wind prescription. We provide simple fitting formulae for the cold gas accretion rate and the corresponding efficiency with which dark matter channels cold gas into haloes in the no-wind case. Finally, we show that cold accretion is broadly consistent with driving the bulk of the highly star-forming galaxies observed at z∼ 2, but that the more intense star formers likely sample the high end of the accretion rate distribution, and may be additionally fuelled by a combination of gas recycling, gas re-accretion, hot mode cooling and mergers.
Dark matter haloes in ΛCDM simulations grow by mergers with other haloes as well as accretion of ‘diffuse’ non-halo material. We quantify the mass growth rates via these two processes, and , and ...their respective dependence on the local halo environment using the ∼500 000 haloes of mass ∼1012 to 1015 M⊙ in the Millennium simulation. Adopting a local mass density parameter as a measure of halo environment, we find the two rates to show strong but opposite environmental dependence, with mergers playing an increasingly important role for halo growths in overdense regions while diffuse accretion dominating the growth in the voids. For galaxy-scale haloes, these two opposite correlations largely cancel out, but a weak environmental dependence remains that results in a slightly lower mean total growth rate, and hence an earlier mean formation redshift, for haloes in denser environments. The mean formation redshift of cluster-mass haloes, on the other hand, shows no correlation with halo environment. The origin of the positive correlation of with local density can be traced to the surrounding mass reservoir outside the virial radii of the haloes, where more than 80 per cent of the mass is in the form of resolved haloes for haloes residing in densest regions, while this fraction drops to ∼20 per cent in the voids. The negative correlation of with local density, however, is not explained by the available diffuse mass in the reservoir outside of haloes, which is in fact larger in denser regions. The non-halo component may therefore be partially composed of truly diffuse dark matter particles that are dynamically hotter due to tidal stripping and are accreted at a suppressed rate in denser regions. We also discuss the implications of these results for how to modify the analytic extended Press–Schechter model of halo growths, which in its original form does not predict environmental dependence.
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