444 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
Star Clusters with Primordial Binaries: III. Dynamical Interaction between Binaries and an Intermediate Mass Black Hole
We present the first study of the dynamical evolution of an isolated star
cluster that combines a significant population of primordial binaries with the
presence of a central black hole. We use equal-mass direct N-body simulations,
with N ranging from 4096 to 16384 and a primordial binary ratio of 0-10%; the
black hole mass is about one percent of the total mass of the cluster. The
evolution of the binary population is strongly influenced by the presence of
the black hole, which gives the cluster a large core with a central density
cusp. Starting from a variety of initial conditions (Plummer and King models),
we first encounter a phase, that last approximately 10 half-mass relaxation
times, in which binaries are disrupted faster compared to analogous simulations
without a black hole. Subsequently, however, binary disruption slows down
significantly, due to the large core size. The dynamical interplay between the
primordial binaries and the black hole thus introduces new features with
respect to the scenarios investigated so far, where the influence of the black
hole and of the binaries have been considered separately. A large core to half
mass radius ratio appears to be a promising indirect evidence for the presence
of a intermediate-mass black hole in old globular clusters.Comment: 11 pages, 11 figures, accepted for publication in MNRA
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
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
Dust absorption along the line of sight for high-redshift objects
We estimate the optical depth distribution of dust present in absorption
systems along the line of sight of high redshift galaxies and the resulting
reddening. We characterize the probability distribution of the transmission to
a given redshift and the shape of the effective mean extinction law by means of
analytical estimates and Monte Carlo simulations. We present our results in a
format useful for applications to present samples of high redshift galaxies and
discuss the implications for observations with the James Webb Space Telescope.
Our most realistic model takes into account the metallicity evolution of Damped
Lyman Alpha absorbers and predicts that the effects of dust absorption are
modest: at redshift z \gtrsim 5 the transmission is above 0.8 at an emitted
wavelength \lambda_e = 0.14 \mu m with probability 90%. Therefore dust
obscuration along the line of sight will affect only marginally observations at
very high redshift.Comment: 27 pages, 9 figures, ApJ accepte
First Frontier Field Constraints on the Cosmic Star-Formation Rate Density at z~10 - The Impact of Lensing Shear on Completeness of High-Redshift Galaxy Samples
We search the complete Hubble Frontier Field dataset of Abell 2744 and its
parallel field for z~10 sources to further refine the evolution of the cosmic
star-formation rate density (SFRD) at z>8. We independently confirm two images
of the recently discovered triply-imaged z~9.8 source by Zitrin et al. (2014)
and set an upper limit for similar z~10 galaxies with red colors of
J_125-H_160>1.2 in the parallel field of Abell 2744. We utilize extensive
simulations to derive the effective selection volume of Lyman-break galaxies at
z~10, both in the lensed cluster field and in the adjacent parallel field.
Particular care is taken to include position-dependent lensing shear to
accurately account for the expected sizes and morphologies of highly-magnified
sources. We show that both source blending and shear reduce the completeness at
a given observed magnitude in the cluster, particularly near the critical
curves. These effects have a significant, but largely overlooked, impact on the
detectability of high-redshift sources behind clusters, and substantially
reduce the expected number of highly-magnified sources. The detections and
limits from both pointings result in a SFRD which is higher by 0.4+-0.4 dex
than previous estimates at z~10 from blank fields. Nevertheless, the
combination of these new results with all other estimates remain consistent
with a rapidly declining SFRD in the 170 Myr from z~8 to z~10 as predicted by
cosmological simulations and dark-matter halo evolution in LambdaCDM. Once
biases introduced by magnification-dependent completeness are accounted for,
the full six cluster and parallel Frontier Field program will be an extremely
powerful new dataset to probe the evolution of the galaxy population at z>8
before the advent of the JWST.Comment: 10 pages, 7 figures, changed to match accepted version to appear in
Ap
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