244 research outputs found
High angular resolution integral-field spectroscopy of the Galaxy's nuclear cluster: a missing stellar cusp?
We report on the structure of the nuclear star cluster in the innermost 0.16
pc of the Galaxy as measured by the number density profile of late-type giants.
Using laser guide star adaptive optics in conjunction with the integral field
spectrograph, OSIRIS, at the Keck II telescope, we are able to differentiate
between the older, late-type ( 1 Gyr) stars, which are presumed to be
dynamically relaxed, and the unrelaxed young ( 6 Myr) population. This
distinction is crucial for testing models of stellar cusp formation in the
vicinity of a black hole, as the models assume that the cusp stars are in
dynamical equilibrium in the black hole potential. Based on the late-type stars
alone, the surface stellar number density profile, , is flat, with . Monte Carlo simulations of
the possible de-projected volume density profile, n(r) ,
show that is less than 1.0 at the 99.73 % confidence level. These
results are consistent with the nuclear star cluster having no cusp, with a
core profile that is significantly flatter than predicted by most cusp
formation theories, and even allows for the presence of a central hole in the
stellar distribution. Of the possible dynamical interactions that can lead to
the depletion of the red giants observable in this survey -- stellar
collisions, mass segregation from stellar remnants, or a recent merger event --
mass segregation is the only one that can be ruled out as the dominant
depletion mechanism. The lack of a stellar cusp around a supermassive black
hole would have important implications for black hole growth models and
inferences on the presence of a black hole based upon stellar distributions.Comment: 35 pages, 5 tables, 12 figures, accepted by Ap
Spatially-resolved spectrophotometric analysis and modelling of the Superantennae
We have performed spatially-resolved spectroscopy of the double-nucleated
Ultra-Luminous Infrared Galaxy IRAS 19254-7245, ``the Superantennae'', along
the line connecting the two nuclei. These data are analysed with a spectral
synthesis code, to derive the star formation and extinction properties of the
galaxy. The star formation history (SFH) of the two nuclei is similarly
characterized by two different main episodes: a recent burst, responsible of
the observed emission lines, and an older one, occurred roughly 1 Gyr ago. We
tentatively associate this bimodal SFH with a double encounter in the dynamical
history of the merger. We have complemented our study with a detailed analysis
of the broad band spectral energy distribution of the Superantennae, built from
published photometry, providing the separate optical-to-mm SEDs of the two
nuclei. Our analysis shows that: a) the southern nucleus is responsible for
about 80% of the total infrared luminosity of the system, b) the L-band
luminosity in the southern nucleus is dominated by the emission from an
obscured AGN, providing about 40 to 50% of the bolometric flux between 8 and
1000 microns; c) the northern nucleus does not show evidence for AGN emission
and appears to be in a post-starburst phase. As for the relative strengths of
the AGN and starburst components, we find that, while they are comparable at
FIR and sub-mm wavelengths, in the radio the Sy2 emission dominates by an order
of magnitude the starburst.Comment: 18 pages. Accepted for publication on A&
The Milky Way Nuclear Star Cluster
In the center of the Milky Way, as well as in many other galaxies, a compact
star cluster around a very massive black hole is observed. One of the possible
explanations for the formation of such Nuclear Star Clusters is based on the
'merging' of globular clusters in the inner galactic potential well. By mean of
sophisticated N-body simulations, we checked the validity of this hypothesis
and found that it may actually has been the one leading to the formation of the
Milky Way Nuclear Star Cluster.Comment: 4 pages, 2 figures, proceedings of "Stellar Clusters and Associations
- A RIA workshop on GAIA", 23-27 May 2011, Granada, Spai
On-manifold projected gradient descent
This study provides a computable, direct, and mathematically rigorous approximation to the differential geometry of class manifolds for high-dimensional data, along with non-linear projections from input space onto these class manifolds. The tools are applied to the setting of neural network image classifiers, where we generate novel, on-manifold data samples and implement a projected gradient descent algorithm for on-manifold adversarial training. The susceptibility of neural networks (NNs) to adversarial attack highlights the brittle nature of NN decision boundaries in input space. Introducing adversarial examples during training has been shown to reduce the susceptibility of NNs to adversarial attack; however, it has also been shown to reduce the accuracy of the classifier if the examples are not valid examples for that class. Realistic “on-manifold” examples have been previously generated from class manifolds in the latent space of an autoencoder. Our study explores these phenomena in a geometric and computational setting that is much closer to the raw, high-dimensional input space than what can be provided by VAE or other black box dimensionality reductions. We employ conformally invariant diffusion maps (CIDM) to approximate class manifolds in diffusion coordinates and develop the Nyström projection to project novel points onto class manifolds in this setting. On top of the manifold approximation, we leverage the spectral exterior calculus (SEC) to determine geometric quantities such as tangent vectors of the manifold. We use these tools to obtain adversarial examples that reside on a class manifold, yet fool a classifier. These misclassifications then become explainable in terms of human-understandable manipulations within the data, by expressing the on-manifold adversary in the semantic basis on the manifold
The Nuclear Stellar Disk in Andromeda: A Fossil from the Era of Black Hole Growth
The physics of angular momentum transport from galactic scales (~10-100 pc)
to much smaller radii is one of the oustanding problems in our understanding of
the formation and evolution of super-massive black holes (BHs). Seemingly
unrelated observations have discovered that there is a lopsided stellar disk of
unknown origin orbiting the BH in M31, and possibly many other systems. We show
that these nominally independent puzzles are in fact closely related.
Multi-scale simulations of gas inflow from galactic to BH scales show that when
sufficient gas is driven towards a BH, gravitational instabilities form a
lopsided, eccentric disk that propagates inwards from larger radii. The
lopsided stellar disk exerts a strong torque on the remaining gas, driving
inflows that fuel the growth of the BH and produce quasar-level luminosities.
The same disk can produce significant obscuration along many sightlines and
thus may be the putative 'torus' invoked to explain obscured active galactic
nuclei and the cosmic X-ray background. The stellar relic of this disk is long
lived and retains the eccentric pattern. Simulations that yield quasar-level
accretion rates produce relic stellar disks with kinematics, eccentric
patterns, precession rates, and surface density profiles in reasonable
agreement with observations of M31. The observed properties of nuclear stellar
disks can thus be used to constrain the formation history of super-massive BHs.Comment: 6 pages, 4 figures, accepted to MNRAS Letters (matches published
version
High resolution crystal structures of piscine transthyretin reveal different binding modes for triiodothyronine and thyroxine
Transthyretin (TTR) is an extracellular transport protein
involved in the distribution of thyroid hormones
and vitamin A. So far, TTR has only been found in vertebrates,
of which piscine TTR displays the lowest sequence
identity with human TTR (47%). Human and
piscine TTR bind both thyroid hormones 3,5,3 -triiodo-
L-thyronine (T3) and 3,5,3 ,5 -tetraiodo-L-thyronine (thyroxine,
T4). Human TTR has higher affinity for T4 than
T3, whereas the reverse holds for piscine TTR. X-ray
structures of Sparus aurata (sea bream) TTR have been
determined as the apo-protein at 1.75 Å resolution and
bound to ligands T3 and T4, both at 1.9 Å resolution. The
apo structure is similar to human TTR with structural
changes only at -strand D. This strand forms an extended
loop conformation similar to the one in chicken
TTR. The piscine TTR T4 complex shows the T4-binding
site to be similar but not identical to human TTR,
whereas the TTR T3 complex shows the I3 halogen situated
at the site normally occupied by the hydroxyl
group of T4. The significantly wider entrance of the hormone-
binding channel in sea bream TTR, in combination
with its narrower cavity, provides a structural explanation
for the different binding affinities of human
and piscine TTR to T3 and T4.We thank Anders Olofsson, Uwe H. Sauer,
Andreas Ho¨rnberg, and Terese Bergfors for valuable discussions and
critical reading of the manuscript
Active Galactic Nucleus Pairs from the Sloan Digital Sky Survey. II. Evidence for Tidally Enhanced Star Formation and Black Hole Accretion
Active galactic nuclei (AGNs) are occasionally seen in pairs, suggesting that
tidal encounters are responsible for the accretion of material by both central
supermassive black holes (BHs). In Paper I of this series, we selected a sample
of AGN pairs with projected separations r_p < 100 kpc and velocity offsets <
600 km/s from the SDSS DR7 and quantified their frequency. In this paper, we
address the BH accretion and recent star formation properties in their host
galaxies. AGN pairs experience stronger BH accretion, as measured by their [O
III]5007 luminosities (corrected for contribution from star formation) and
Eddington ratios, than do control samples of single AGNs matched in redshift
and host stellar mass. Their host galaxies have stronger post-starburst
activity and younger mean stellar ages, as indicated by stronger H-delta
absorption and smaller 4000 A break in their spectra. The BH accretion and
recent star formation in the host galaxies both increase with decreasing
projected separation in AGN pairs, for r_p ~< 10-30 kpc. The intensity of BH
accretion, the post-starburst strength, and the mean stellar ages are
correlated between the two AGNs in a pair. The luminosities and Eddington
ratios of AGN pairs are correlated with recent star formation in their host
galaxies, with a scaling relation consistent with that observed in single AGNs.
Our results suggest that galaxy tidal interactions enhance both BH accretion
and star formation in AGN pairs, even though the majority of low redshift AGNs
is not coincident with on-going interactions.Comment: 16 pages, 12 figures; to appear in Ap
Elliptical Galaxies and Bulges of Disk Galaxies: Summary of Progress and Outstanding Issues
This is the summary chapter of a review book on galaxy bulges. Bulge
properties and formation histories are more varied than those of ellipticals. I
emphasize two advances: 1 - "Classical bulges" are observationally
indistinguishable from ellipticals, and like them, are thought to form by major
galaxy mergers. "Disky pseudobulges" are diskier and more actively star-forming
(except in S0s) than are ellipticals. Theys are products of the slow
("secular") evolution of galaxy disks: bars and other nonaxisymmetries move
disk gas toward the center, where it starbursts and builds relatively flat,
rapidly rotating components. This secular evolution is a new area of galaxy
evolution work that complements hierarchical clustering. 2 - Disks of
high-redshift galaxies are unstable to the formation of mass clumps that sink
to the center and merge - an alternative channel for the formation of classical
bulges. I review successes and unsolved problems in the formation of
bulges+ellipticals and their coevolution (or not) with supermassive black
holes. I present an observer's perspective on simulations of dark matter galaxy
formation including baryons. I review how our picture of the quenching of star
formation is becoming general and secure at redshifts z < 1. The biggest
challenge is to produce realistic bulges+ellipticals and disks that overlap
over a factor of 10**3 in mass but that differ from each other as observed over
that whole range. Second, how does hierarchical clustering make so many giant,
bulgeless galaxies in field but not cluster environments? I argue that we rely
too much on AGN and star-formation feedback to solve these challenges.Comment: 46 pages, 10 postscript figures, accepted for publication in Galactic
Bulges, ed. E. Laurikainen, R. F. Peletier, & D. A. Gadotti (New York:
Springer), in press (2015
Evidence for a mass-dependent AGN Eddington ratio distribution via the flat relationship between SFR and AGN luminosity
The lack of a strong correlation between AGN X-ray luminosity (L X ; a proxy for AGN power) and the star formation rate (SFR) of their host galaxies has recently been attributed to stochastic AGNvariability. Studies using population synthesis models have incorporated this by assuming a broad, universal (i.e. does not depend on the host galaxy properties) probability distribution for AGN specific X-ray luminosities (i.e. the ratio of L X to host stellar mass; a common proxy for Eddington ratio). However, recent studies have demonstrated that this universal Eddington ratio distribution fails to reproduce the observed X-ray luminosity functions beyond z ~ 1.2. Furthermore, empirical studies have recently shown that the Eddington ratio distribution may instead depend upon host galaxy properties, such as SFR and/or stellarmass. To investigate this further, we develop a population synthesis model in which the Eddington ratio distribution is different for star-forming and quiescent host galaxies. We showthat, although this model is able to reproduce the observed X-ray luminosity functions out to z ~ 2, it fails to simultaneously reproduce the observed flat relationship between SFR and X-ray luminosity. We can solve this, however, by incorporating a mass dependency in the AGN Eddington ratio distribution for starforming host galaxies. Overall, our models indicate that a relative suppression of low Eddington ratios (λ Edd < ~0.1) in lower mass galaxies (M * < ~10 10-11 M⊙) is required to reproduce both the observed X-ray luminosity functions and the observed flat SFR/X-ray relationship
Effects of co-habitation between Anopheles gambiae s.s. and Culex quinquefasciatus aquatic stages on life history traits
<p>Abstract</p> <p>Background</p> <p>The effective measures for the control of malaria and filariasis vectors can be achieved by targeting immature stages of anopheline and culicine mosquitoes in productive habitat. To design this strategy, the mechanisms (like biotic interactions with conspecifc and heterospecific larvae) regulating mosquito aquatic stages survivorship, development time and the size of emerging adults should be understood. This study explored the effect of co-habitation between <it>An. gambiae </it>s.s. and <it>Cx. quinquefasciatus </it>on different life history traits of both species under different densities and constant food supply in the habitats of the same size under semi-natural conditions.</p> <p>Methods</p> <p>Experiments were set up with three combinations; <it>Cx. quinquefasciatus </it>alone (single species treatment), <it>An. gambiae </it>s.s. alone (single species treatment); and <it>An. gambiae </it>s.s. with <it>Cx. quiquefasciatus </it>(co-habitation treatment) in different densities in semi field situation.</p> <p>Results</p> <p>The effect of co-habitation of <it>An. gambiae </it>s.s. and <it>Cx. quinquefasciatus </it>was found to principally affect three parameters. The wing-lengths (a proxy measure of body size) of <it>An. gambiae </it>s.s. in co-habitation treatments were significantly shorter in both females and males than in <it>An. gambiae </it>s.s single species treatments. In <it>Cx. quinquefasciatus</it>, no significant differences in wing-length were observed between the single species and co-habitation treatments. Daily survival rates were not significantly different between co-habitation and single species treatments for both <it>An. gambiae </it>s.s. and <it>Cx. quinquefasciatus</it>. Developmental time was found to be significantly different with single species treatments developing better than co-habitation treatments. Sex ratio was found to be significantly different from the proportion of 0.5 among single and co-habitation treatments species at different densities. Single species treatments had more males than females emerging while in co-habitation treatments more females emerged than males. In this study, there was no significant competitive survival advantage in co-habitation.</p> <p>Conclusion</p> <p>These results suggest that co-habitation of <it>An. gambiae </it>s.s. and <it>Cx. quinquefasciatus </it>in semi-natural conditions affect mostly <it>An. gambiae </it>s.s. body size. Hence, more has to be understood on the effects of co-habitation of <it>An. gambiae </it>s.s. and <it>Cx. quinquefasciatus </it>in a natural ecology and its possible consequences in malaria and filariasis epidemiology.</p
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