444,207 research outputs found
Wandering Black Holes in Bright Disk Galaxy Halos
We perform SPH+N-body cosmological simulations of massive disk galaxies,
including a formalism for black hole seed formation and growth, and find that
satellite galaxies containing supermassive black hole seeds are often stripped
as they merge with the primary galaxy. These events naturally create a
population of "wandering" black holes that are the remnants of stripped
satellite cores; galaxies like the Milky Way may host 5 -- 15 of these objects
within their halos. The satellites that harbor black hole seeds are comparable
to Local Group dwarf galaxies such as the Small and Large Magellanic Clouds;
these galaxies are promising candidates to host nearby intermediate mass black
holes. Provided that these wandering black holes retain a gaseous accretion
disk from their host dwarf galaxy, they give a physical explanation for the
origin and observed properties of some recently discovered off-nuclear
ultraluminous X-ray sources such as HLX-1.Comment: Accepted for publication in ApJ Letter
Giant enhanced optical nonlinearity of colloidal nanocrystals with a graded-index host
The effective linear and third-order nonlinear optical properties of metallic
colloidal crystal immersed in a graded-index host fluid are investigated
theoretically. The local electric fields are extracted self-consistently based
on the layer-to-layer interactions, which are readily given by the Lekner
summation method. The resultant optical absorption and nonlinearity enhancement
show a series of sharp peaks, which merge in a broadened resonant band. The
sharp peaks become a continuous band for increasing packing density and number
of layers. We believe that the sharp peaks arise from the in-plane dipolar
interactions and the surface plasmon resonance, whereas the continuous band is
due to the presence of the gradient in the host refractive index. These results
have not been observed in homogeneous and randomly-dispersed colloids, and thus
would be of great interest in optical nanomaterial engineering.Comment: Submitted to Applied Physics Letter
Do Moderate-Luminosity Active Galactic Nuclei Suppress Star Formation?
The growth of supermassive black holes and their host galaxies are thought to
be linked, but the precise nature of this symbiotic relationship is still
poorly understood. Both observations and simulations of galaxy formation
suggest that the energy input from active galactic nuclei (AGN), as the central
supermassive black hole accretes material and grows, heats the interstellar
material and suppresses star formation. In this Letter, we show that most host
galaxies of moderate-luminosity supermassive black holes in the local universe
have intermediate optical colors that imply the host galaxies are transitioning
from star formation to quiescence, the first time this has been shown to be
true for all AGN independent of obscuration. The intermediate colors suggest
that star formation in the host galaxies ceased roughly 100 Myr ago. This
result indicates that either the AGN are very long-lived, accreting for more
than 1 Gyr beyond the end of star formation, or there is a ~100 Myr time delay
between the shutdown of star formation and detectable black hole growth. The
first explanation is unlikely given current estimates for AGN lifetimes, so
low-lumiosity AGN must shut down star formation before substantial black hole
accretion activity is detected. The scarcity of AGN host galaxies in the blue
cloud reported here challenges scenarios where significant star formation and
black hole growth are coeval. Lastly, these observations also strongly support
the `Unified Model' of AGN as the host galaxy colors are independent of
obscuration towards the central engine.Comment: Accepted for publication in ApJ Letters. 7 pages, 4 figure
Bath Hebrew Ladies Society (1944) (USO)
Bath Hebrew Ladies Society (1944) (USO) The Bath Hebrew Ladies Society was a womenâs organization, made up of members of the Beth Israel Congregation (originally, Base Isroall), an unaffiliated synagogue, serving the Jewish community in Bath, Maine and vicinity. This includes two letters from the USO in 1944. The first letter provides instructions for local organizations that are willing to host a meal for servicemen and industrial workers. The second is a letter of appreciation to the âOrder of Hebrew Ladiesâ in recognition of their participation in this program.https://digitalcommons.usm.maine.edu/jud_povich/1037/thumbnail.jp
Low and High Surface Brightness Galaxies at Void Walls
We study the relative fraction of low and high surface brightness galaxies
(LSBGs and HSBGs) at void walls in the SDSS DR7. We focus on galaxies in equal
local density environments. We assume that the host dark-matter halo mass (for
which we use SDSS group masses) is a good indicator of local density. This
analysis allows to examine the behavior of the abundance of LSBG and HSBG
galaxies at a fixed local density and distinguish the large-scale environment
defined by the void geometry. We compare galaxies in the field, and in the void
walls; the latter are defined as the volume of void shells of radius equal to
that of the void. We find a significant decrement, a factor , of the
relative fraction of blue, active star-forming LSBGs in equal mass groups at
the void walls and the field. This decrement is consistent with an increase of
the fraction of blue, active star-forming HSBGs. By contrast, red LSBGs and
HSBGs show negligible changes. We argue that these results are consistent with
a scenario where LSBGs with blue colors and strong star formation activity at
the void walls are fueled by gas from the expanding void regions. This process
could lead to LSBG to HSBG transformations.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
First detection of [CII]158um at high redshift: vigorous star formation in the early universe
We report the detection of the 2P_3/2 -> 2P_1/2 fine-structure line of C+ at
157.74 micron in SDSSJ114816.64+525150.3 (hereafter J1148+5251), the most
distant known quasar, at z=6.42, using the IRAM 30-meter telescope. This is the
first detection of the [CII] line at high redshift, and also the first
detection in a Hyperluminous Infrared Galaxy (L_FIR > 10^13 Lsun). The [CII]
line is detected at a significance level of 8 sigma and has a luminosity of 4.4
x 10^9 Lsun. The L_[CII]/L_FIR ratio is 2 x 10^-4, about an order of magnitude
smaller than observed in local normal galaxies and similar to the ratio
observed in local Ultraluminous Infrared Galaxies. The [CII] line luminosity
indicates that the host galaxy of this quasar is undergoing an intense burst of
star formation with an estimated rate of ~3000 Msun/yr. The detection of C+ in
SDSS J1148+5251 suggests a significant enrichment of metals at z ~ 6 (age of
the universe ~870 Myr), although the data are consistent with a reduced carbon
to oxygen ratio as expected from chemical evolutionary models of the early
phases of galaxy formation.Comment: 5 pages, 2 figures, accepted by A&A Letter
"Dark energy" in the Local Void
The unexpected discovery of the accelerated cosmic expansion in 1998 has
filled the Universe with the embarrassing presence of an unidentified "dark
energy", or cosmological constant, devoid of any physical meaning. While this
standard cosmology seems to work well at the global level, improved knowledge
of the kinematics and other properties of our extragalactic neighborhood
indicates the need for a better theory. We investigate whether the recently
suggested repulsive-gravity scenario can account for some of the features that
are unexplained by the standard model. Through simple dynamical considerations,
we find that the Local Void could host an amount of antimatter
() roughly equivalent to the mass of a typical
supercluster, thus restoring the matter-antimatter symmetry. The antigravity
field produced by this "dark repulsor" can explain the anomalous motion of the
Local Sheet away from the Local Void, as well as several other properties of
nearby galaxies that seem to require void evacuation and structure formation
much faster than expected from the standard model. At the global cosmological
level, gravitational repulsion from antimatter hidden in voids can provide more
than enough potential energy to drive both the cosmic expansion and its
acceleration, with no need for an initial "explosion" and dark energy.
Moreover, the discrete distribution of these dark repulsors, in contrast to the
uniformly permeating dark energy, can also explain dark flows and other
recently observed excessive inhomogeneities and anisotropies of the Universe.Comment: 6 pages, accepted as a Letter to the Editor by Astrophysics and Space
Scienc
How to make an ultra-faint dwarf spheroidal galaxy: tidal stirring of disky dwarfs with shallow dark matter density profiles
In recent years the Sloan Digital Sky Survey has unraveled a new population
of ultra-faint dwarf galaxies (UFDs) in the vicinity of the Milky Way (MW)
whose origin remains a puzzle. Using a suite of collisionless N-body
simulations, we investigate the formation of UFDs in the context of the tidal
stirring model for the formation of dwarf spheroidal galaxies in the Local
Group (LG). Our simulations are designed to reproduce the tidal interactions
between MW-sized host galaxies and rotationally supported dwarfs embedded in
10^9 M_sun dark matter (DM) halos. We explore a variety of inner density slopes
\rho \propto r^{-\alpha} for the dwarf DM halos, ranging from core-like (\alpha
= 0.2) to cuspy (\alpha = 1), and different dwarf orbital configurations. Our
experiments demonstrate that UFDs can be produced via tidal stirring of disky
dwarfs on relatively tight orbits, consistent with a redshift of accretion by
the host galaxy of z \sim 1, and with intermediate values for the halo inner
density slopes (\rho \propto r^{-0.6}). The inferred slopes are in excellent
agreement with those resulting from both the modeling of the rotation curves of
dwarf galaxies and recent cosmological simulations of dwarf galaxy formation.
Comparing the properties of observed UFDs with those of their simulated
counterparts, we find remarkable similarities in terms of basic observational
parameters. We conclude that tidal stirring of rotationally supported dwarfs
represents a viable mechanism for the formation of UFDs in the LG environment.Comment: 6 pages, 4 figures, revised version accepted for publication in ApJ
Letter
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