121 research outputs found
Detectability of Free Floating Planets in Open Clusters with JWST
Recent observations have shown the presence of extra-solar planets in
Galactic open stellar clusters, as in the Praesepe (M44). These systems provide
a favorable environment for planetary formation due to the high heavy-element
content exhibited by the majority of their population. The large stellar
density, and corresponding high close-encounter event rate, may induce strong
perturbations of planetary orbits with large semimajor axes. Here we present a
set of N-body simulations implementing a novel scheme to treat the tidal
effects of external stellar perturbers on planetary orbit eccentricity and
inclination. By simulating five nearby open clusters we determine the rate of
occurrence of bodies extracted from their parent stellar system by
quasi-impulsive tidal interactions. We find that the specific free-floating
planet production rate (total number of free-floating planets per unit of time,
normalized by the total number of stars) is proportional to the stellar density
of the cluster, with a constant of proportionality equal to (23 +/- 5)10^-6
pc^3 Myr^-1. For the Pleiades (M45) we predict that about 26% of stars should
have lost their planets. This raises the exciting possibility of directly
observing these wandering planets with the James Webb Space Telescope in the
NIR band. Assuming a surface temperature of the planet of 500 K, a
free-floating planet of Jupiter size inside the Pleiades would have a specific
flux @4.4 micron of approximately 400 nJy, which would lead to a very clear
detection (S/N of order 100) in only one hour of integration.Comment: Accepted for publication in ApJ Letters on 4 November 201
Feedback Limits to Maximum Seed Masses of Black Holes
The most massive black holes observed in the Universe weigh up to , nearly independent of redshift. Reaching these
final masses likely required copious accretion and several major mergers.
Employing a dynamical approach, that rests on the role played by a new,
relevant physical scale - the transition radius - we provide a theoretical
calculation of the maximum mass achievable by a black hole seed that forms in
an isolated halo, one that scarcely merged. Incorporating effects at the
transition radius and their impact on the evolution of accretion in isolated
haloes we are able to obtain new limits for permitted growth. We find that
large black hole seeds ()
hosted in small isolated halos ()
accreting with relatively small radiative efficiencies () grow optimally in these circumstances. Moreover, we show that the
standard relation observed at cannot be
established in isolated halos at high-, but requires the occurrence of
mergers. Since the average limiting mass of black holes formed at is in the range , we expect to observe them
in local galaxies as intermediate-mass black holes, when hosted in the rare
haloes that experienced only minor or no merging events. Such ancient black
holes, formed in isolation with subsequent scant growth, could survive, almost
unchanged, until present.Comment: Accepted for publication in ApJ Letter
Early galaxy formation in warm dark matter cosmologies
We present a framework for high-redshift () galaxy formation that
traces their dark matter (DM) and baryonic assembly in four cosmologies: Cold
Dark Matter (CDM) and Warm Dark Matter (WDM) with particle masses of
1.5, 3 and 5 . We use the same astrophysical parameters regulating
star formation and feedback, chosen to match current observations of the
evolving ultra violet luminosity function (UV LF). We find that the assembly of
observable (with current and upcoming instruments) galaxies in CDM and WDM results in similar halo mass to light ratios (M/L),
stellar mass densities (SMDs) and UV LFs. However the suppression of
small-scale structure leads to a notably delayed and subsequently more rapid
stellar assembly in the WDM model. Thus galaxy assembly in WDM cosmologies is characterized by: (i) a dearth of
small-mass halos hosting faint galaxies; and (ii) a younger, more UV bright
stellar population, for a given stellar mass. The higher M/L ratio (effect ii)
partially compensates for the dearth of small-mass halos (effect i), making the
resulting UV LFs closer to CDM than expected from simple estimates of halo
abundances. We find that the redshift evolution of the SMD is a powerful probe
of the nature of DM. Integrating down to a limit of for the
James Webb Space Telescope (JWST), the SMD evolves as (SMD) in WDM, as compared to (SMD) in CDM. Thus high-redshift stellar assembly provides a powerful testbed
for WDM models, accessible with the upcoming JWST.Comment: Accepted for publication in Ap
Shining in the Dark: the Spectral Evolution of the First Black Holes
Massive Black Hole (MBH) seeds at redshift are now thought to
be key ingredients to explain the presence of the super-massive () black holes in place after the Big
Bang. Once formed, massive seeds grow and emit copious amounts of radiation by
accreting the left-over halo gas; their spectrum can then provide crucial
information on their evolution. By combining radiation-hydrodynamic and
spectral synthesis codes, we simulate the time-evolving spectrum emerging from
the host halo of a MBH seed with initial mass ,
assuming both standard Eddington-limited accretion, or slim accretion disks,
appropriate for super-Eddington flows. The emission occurs predominantly in the
observed infrared-submm () and X-ray () bands. Such signal should be easily detectable by JWST around
up to , and by ATHENA (between and
, up to ). Ultra-deep X-ray surveys like the
Chandra Deep Field South could have already detected these systems up to . Based on this, we provide an upper limit for the MBH
mass density of assuming standard Eddington-limited accretion. If accretion
occurs in the slim disk mode the limits are much weaker, in the most
constraining case.Comment: Submitted for publication in MNRA
Triggering the formation of direct collapse black holes by their congeners
Direct collapse black holes (DCBHs) are excellent candidates as seeds of
supermassive black holes (SMBHs) observed at z \gsim 6. The formation of a
DCBH requires a strong external radiation field to suppress formation
and cooling in a collapsing gas cloud. Such strong field is not easily achieved
by first stars or normal star-forming galaxies. Here we investigate a scenario
in which the previously-formed DCBH can provide the necessary radiation field
for the formation of additional ones. Using one-zone model and the simulated
DCBH Spectral Energy Distributions (SEDs) filtered through absorbing gas
initially having column density , we derive the critical field
intensity, , to suppress formation and
cooling. For the SED model with cm,
cm and cm, we obtain
, 35 and 54, all much smaller than the critical
field intensity for normal star-forming galaxies (J_{\rm LW}^{\rm crit}\simgt
1000). X-ray photons from previously-formed DCBHs build up a high- X-ray
background (XRB) that may boost the . However, we find
that in the three SED models only increases to
, 170 and 390 respectively even when \dt{\rho}_\bullet reaches the
maximum value allowed by the present-day XRB level (yrMpc), still much smaller than the galactic value.
Although considering the XRB from first galaxies may further increase , we conclude that our investigation supports a scenario in
which DCBH may be more abundant than predicted by models only including
galaxies as external radiation sources.Comment: 18 pages, 14 figures, 5 tables, ApJ in pres
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