578 research outputs found
Star Clusters with Primordial Binaries: II. Dynamical Evolution of Models in a Tidal Field
[abridged] We extend our analysis of the dynamical evolution of simple star
cluster models, in order to provide comparison standards that will aid in
interpreting the results of more complex realistic simulations. We augment our
previous primordial-binary simulations by introducing a tidal field, and
starting with King models of different central concentrations. We present the
results of N-body calculations of the evolution of equal-mass models, starting
with primordial binary fractions of 0 - 100 %, and N values from 512 to 16384.
We also attempt to extrapolate some of our results to the larger number of
particles that are necessary to model globular clusters. We characterize the
steady-state `deuterium main sequence' phase in which primordial binaries are
depleted in the core in the process of `gravitationally burning'. In this phase
we find that the ratio of the core to half-mass radius, r_c/r_h, is similar to
that measured for isolated systems. In addition to the generation of energy due
to hardening and depletion of the primordial binary population, the overall
evolution of the star clusters is driven by a competing process: the tidal
disruption of the system. We find that the depletion of primordial binaries
before tidal dissolution of the system is possible only if the initial number
is below 0.05 N, in the case of a King model with W_0=7 and N=4096 (which is
one of our longest living models). We compare our findings, obtained by means
of direct N-body simulations but scaled, where possible, to larger N, with
similar studies carried out by means of Monte Carlo methods.Comment: 15 pages, 18 figures, matches MNRAS accepted version, some sections
reorganized but no major change
Star Clusters with Primordial Binaries: I. Dynamical Evolution of Isolated Models
In order to interpret the results of complex realistic star cluster
simulations, which rely on many simplifying approximations and assumptions, it
is essential to study the behavior of even more idealized models, which can
highlight the essential physical effects and are amenable to more exact
methods. With this aim, we present the results of N-body calculations of the
evolution of equal-mass models, starting with primordial binary fractions of 0
- 100 %, with values of N ranging from 256 to 16384. This allows us to
extrapolate the main features of the evolution to systems comparable in
particle number with globular clusters. In this range, we find that the
steady-state `deuterium main sequence' is characterized by a ratio of the core
radius to half-mass radius that follows qualitatively the analytical estimate
by Vesperini & Chernoff (1994), although the N dependence is steeper than
expected. Interestingly, for an initial binary fraction f greater than 10%, the
binary heating in the core during the post collapse phase almost saturates
(becoming nearly independent of f), and so little variation in the structural
properties is observed. Thus, although we observe a significantly lower binary
abundance in the core with respect to the Fokker-Planck simulations by Gao et
al. (1991), this is of little dynamical consequence. At variance with the study
of Gao et al. (1991), we see no sign of gravothermal oscillations before 150
halfmass relaxation times. At later times, however, oscillations become
prominent. We demonstrate the gravothermal nature of these oscillations.Comment: 14 pages, 22 figures, MNRAS accepte
Distribution of the very first PopIII stars and their relation to bright z~6 quasars
We discuss the link between dark matter halos hosting the first PopIII stars
and the rare, massive, halos that are generally considered to host bright
quasars at high redshift z~6. The main question that we intend to answer is
whether the super-massive black holes powering these QSOs grew out from the
seeds planted by the first intermediate massive black holes created in the
universe. This question involves a dynamical range of 10^13 in mass and we
address it by combining N-body simulations of structure formation to identify
the most massive halos at z~6 with a Monte Carlo method based on linear theory
to obtain the location and formation times of the first light halos within the
whole simulation box. We show that the descendants of the first ~10^6 Msun
virialized halos do not, on average, end up in the most massive halos at z~6,
but rather live in a large variety of environments. The oldest PopIII
progenitors of the most massive halos at z~6, form instead from density peaks
that are on average one and a half standard deviations more common than the
first PopIII star formed in the volume occupied by one bright high-z QSO. The
intermediate mass black hole seeds planted by the very first PopIII stars at
z>40 can easily grow to masses m_BH>10^9.5 Msun by z=6 assuming Eddington
accretion with radiative efficiency \epsilon~0.1. Quenching of the black hole
accretion is therefore crucial to avoid an overabundance of supermassive black
holes at lower redshift. This can be obtained if the mass accretion is limited
to a fraction \eta~6*10^{-3} of the total baryon mass of the halo hosting the
black hole. The resulting high end slope of the black hole mass function at z=6
is \alpha ~ -3.7, a value within the 1\sigma error bar for the bright end slope
of the observed quasar luminosity function at z=6.Comment: 30 pages, 9 figures, ApJ accepte
Intermediate Mass Black Hole Induced Quenching of Mass Segregation in Star Clusters
In many theoretical scenarios it is expected that intermediate-mass black
holes (IMBHs, with masses M ~ 100-10000 solar masses) reside at the centers of
some globular clusters. However, observational evidence for their existence is
limited. Several previous numerical investigations have focused on the impact
of an IMBH on the cluster dynamics or brightness profile. Here we instead
present results from a large set of direct N-body simulations including single
and binary stars. These show that there is a potentially more detectable IMBH
signature, namely on the variation of the average stellar mass between the
center and the half-light radius. We find that the existence of an IMBH
quenches mass segregation and causes the average mass to exhibit only modest
radial variation in collisionally relaxed star clusters. This differs from when
there is no IMBH. To measure this observationally requires high resolution
imaging at the level of that already available from the Hubble Space Telescope
(HST) for the cores of a large sample of galactic globular clusters. With a
modest additional investment of HST time to acquire fields around the
half-light radius, it will be possible to identify the best candidate clusters
to harbor an IMBH. This test can be applied only to globulars with a half-light
relaxation time less than or equal to 1 Gyr, which is required to guarantee
efficient energy equipartition due to two-body relaxation.Comment: 15 pages, 3 figures, ApJ, in pres
Overdensities of Y-dropout Galaxies from the Brightest-of-Reionizing Galaxies Survey: A Candidate Protocluster at Redshift z~8
Theoretical and numerical modeling of dark-matter halo assembly predicts that
the most luminous galaxies at high redshift are surrounded by overdensities of
fainter companions. We test this prediction with HST observations acquired by
our Brightest of Reionizing Galaxies (BoRG) survey, which identified four very
bright z~8 candidates as Y-dropout sources in four of the 23 non-contiguous
WFC3 fields observed. We extend here the search for Y-dropouts to fainter
luminosities (M_* galaxies with M_AB\sim-20), with detections at >5sigma
confidence (compared to >8sigma confidence adopted earlier) identifying 17 new
candidates. We demonstrate that there is a correlation between number counts of
faint and bright Y-dropouts at >99.84% confidence. Field BoRG58, which contains
the best bright z\sim8 candidate (M_AB=-21.3), has the most significant
overdensity of faint Y-dropouts. Four new sources are located within 70arcsec
(corresponding to 3.1 comoving Mpc at z=8) from the previously known brighter
z\sim8 candidate. The overdensity of Y-dropouts in this field has a physical
origin to high confidence (p>99.975%), independent of completeness and
contamination rate of the Y-dropout selection. We modeled the overdensity by
means of cosmological simulations and estimate that the principal dark matter
halo has mass M_h\sim(4-7)x10^11Msun (\sim5sigma density peak) and is
surrounded by several M_h\sim10^11Msun halos which could host the fainter
dropouts. In this scenario, we predict that all halos will eventually merge
into a M_h>2x10^14Msun galaxy cluster by z=0. Follow-up observations with
ground and space based telescopes are required to secure the z\sim8 nature of
the overdensity, discover new members, and measure their precise redshift.Comment: Minor revision: ApJ accepted [17 pages (emulateapj style), 7 figures,
2 tables
HST followup observations of two bright z ~ 8 candidate galaxies from the BoRG pure-parallel survey
We present followup imaging of two bright (L > L*) galaxy candidates at z > 8
from the Brightest of Reionizing Galaxies (BoRG) survey with the F098M filter
on HST/WFC3. The F098M filter provides an additional constraint on the flux
blueward of the spectral break, and the observations are designed to
discriminate between low- and high-z photometric redshift solutions for these
galaxies. Our results confirm one galaxy, BoRG 0116+1425 747, as a highly
probable z ~ 8 source, but reveal that BoRG 0116+1425 630 - previously the
brightest known z > 8 candidate (mAB = 24.5) - is likely to be a z ~ 2
interloper. As this source was substantially brighter than any other z > 8
candidate, removing it from the sample has a significant impact on the derived
UV luminosity function in this epoch. We show that while previous BoRG results
favored a shallow power-law decline in the bright end of the luminosity
function prior to reionization, there is now no evidence for departure from a
Schechter function form and therefore no evidence for a difference in galaxy
formation processes before and after reionization.Comment: Accepted by ApJL, 7 pages, 4 figure
Evolution of supermassive stars as a pathway to black hole formation
Supermassive stars, with masses greater than a million solar masses, are
possible progenitors of supermassive black holes in galactic nuclei. Because of
their short nuclear burning timescales, such objects can be formed only when
matter is able to accumulate at a rate exceeding ~ 1 solar mass/yr. Here we
revisit the structure and evolution of rotationally-stabilized supermassive
stars, taking into account their continuous accumulation of mass and their
thermal relaxation. We show that the outer layers of supermassive stars are not
thermally relaxed during much of the star's main sequence lifetime. As a
result, they do not resemble n=3 polytropes, as assumed in previous literature,
but rather consist of convective (polytropic) cores surrounded by convectively
stable envelopes that contain most of the mass. We compute the structures of
these envelopes, in which the specific entropy is proportional to the enclosed
mass M(R) to the 2/3-power. By matching the envelope solutions to convective
cores, we calculate the core mass as a function of time. We estimate the
initial black hole masses formed as a result of core-collapse, and their
subsequent growth via accretion from the bloated envelopes ("quasistars") that
result. The seed black holes formed in this way could have typical masses in
the range ~ 10^4-10^5 solar masses, considerably larger than the remnants
thought to be left by the demise of Population III stars. Supermassive black
holes therefore could have been seeded during an epoch of rapid infall
considerably later than the era of Pop III star formation.Comment: 10 pages, 5 figures, to appear in Monthly Notices of the Royal
Astronomical Societ
Predictions for Triple Stars with and without a Pulsar in Star Clusters
Though about 80 pulsar binaries have been detected in globular clusters so
far, no pulsar has been found in a triple system in which all three objects are
of comparable mass. Here we present predictions for the abundance of such
triple systems, and for the most likely characteristics of these systems. Our
predictions are based on an extensive set of more than 500 direct simulations
of star clusters with primordial binaries, and a number of additional runs
containing primordial triples. Our simulations employ a number N_{tot} of equal
mass stars from N_{tot}=512 to N_{tot}=19661 and a primordial binary fraction
from 0-50%. In addition, we validate our results against simulations with
N=19661 that include a mass spectrum with a turn-off mass at 0.8 M_{sun},
appropriate to describe the old stellar populations of galactic globular
clusters. Based on our simulations, we expect that typical triple abundances in
the core of a dense cluster are two orders of magnitude lower than the binary
abundances, which in itself already suggests that we don't have to wait too
long for the first comparable-mass triple with a pulsar to be detected.Comment: 11 pages, minor changes to match MNRAS accepted versio
Expanded Search for z~10 Galaxies from HUDF09, ERS, and CANDELS Data: Evidence for Accelerated Evolution at z>8?
We search for z~10 galaxies over ~160 arcmin^2 of WFC3/IR data in the Chandra
Deep Field South, using the public HUDF09, ERS, and CANDELS surveys, that reach
to 5sigma depths ranging from 26.9 to 29.4 in H_160 AB mag. z>~9.5 galaxy
candidates are identified via J_125-H_160>1.2 colors and non-detections in any
band blueward of J_125. Spitzer IRAC photometry is key for separating the
genuine high-z candidates from intermediate redshift (z~2-4) galaxies with
evolved or heavily dust obscured stellar populations. After removing 16 sources
of intermediate brightness (H_160~24-26 mag) with strong IRAC detections, we
only find one plausible z~10 galaxy candidate in the whole data set, previously
reported in Bouwens et al. (2011). The newer data cover a 3x larger area and
provide much stronger constraints on the evolution of the UV luminosity
function (LF). If the evolution of the z~4-8 LFs is extrapolated to z~10, six
z~10 galaxies are expected in our data. The detection of only one source
suggests that the UV LF evolves at an accelerated rate before z~8. The
luminosity density is found to increase by more than an order of magnitude in
only 170 Myr from z~10 to z~8. This increase is >=4x larger than expected from
the lower redshift extrapolation of the UV LF. We are thus likely witnessing
the first rapid build-up of galaxies in the heart of cosmic reionization.
Future deep HST WFC3/IR data, reaching to well beyond 29 mag, can enable a more
robust quantification of the accelerated evolution around z~10.Comment: 13 pages, 11 figures, ApJ resubmitted after referee repor
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