593 research outputs found
External Mass Accumulation onto Core Potentials: Implications for Star Clusters, Galaxies and Galaxy Clusters
Accretion studies have been focused on the flow around bodies with point mass
gravitational potentials, but few general results are available for non-point
mass distributions. Here, we study the accretion flow onto non-divergent, core
potentials moving through a background medium. We use Plummer and Hernquist
potentials as examples to study gas accretion onto star clusters, dwarf and
large galaxy halos and galaxy clusters in a variety of astrophysical
environments. The general conditions required for a core potential to
collectively accrete large quantities of gas from the external medium are
derived using both simulations and analytic results. The consequences of large
mass accumulation in galaxy nuclei, dwarf galaxies and star clusters are
twofold. First, if the gas cools effectively star formation can be triggered,
generating new stellar members in the system. Second, if the collective
potential of the system is able to alter the ambient gas properties before the
gas is accreted onto the individual core members, the augmented mass supply
rates could significantly alter the state of the various accreting stellar
populations and result in an enhanced central black hole accretion luminosity.Comment: 24 pages, 15 figures, accepted to Ap
A New Technique for Finding Needles in Haystacks: A Geometric Approach to Distinguishing Between a New Source and Random Fluctuations
We propose a new test statistic based on a score process for determining the
statistical significance of a putative signal that may be a small perturbation
to a noisy experimental background. We derive the reference distribution for
this score test statistic; it has an elegant geometrical interpretation as well
as broad applicability. We illustrate the technique in the context of a model
problem from high-energy particle physics. Monte Carlo experimental results
confirm that the score test results in a significantly improved rate of signal
detection.Comment: 5 pages, 4 figure
Riparian vegetation restoration: Does social perception reflect ecological value?
Special Issue PaperSocial‐ecological contexts are key to the success of ecological restoration projects.
The ecological quality of restoration efforts, however, may not be fully evident to
stakeholders, particularly if the desired aesthetic experience is not delivered. Aesthetically
pleasing landscapes are more highly appreciated and tend to be better
protected than less appealing landscapes, regardless of their ecological value. Positive
public perception of restoration actions may therefore facilitate stakeholder involvement
and catalyse recognition of ecological improvement. Here we aim to contrast
aesthetical perception and ecological condition in headwater river reaches restored
through passive ecological restoration in study areas in Portugal (Alentejo) and France
(Normandy). We recorded structural and functional indicators of riparian vegetation
to monitor the ecological condition of study sites along a passive restoration trajectory.
Aesthetical perception indicators were assessed through stakeholder inquiries
developed under a semantic differential approach. We analysed perception responses
to changes in the riparian ecosystems resulting from passive ecological restoration
across different geographical contexts and social groups. The analysed social groups
comprised stakeholders (environmental managers and landowners) and university students
(landscape architecture and geography students). Results indicate that (a) visual
preferences often do not reflect changes in ecological condition, (b) perception of the
restoration process is strongly context dependent, and (c) experience and cultural
background affect perception of ecological condition across the different social
groups analysed. Clear identification of relevant stakeholder groups (those interested
in or directly affected by restoration), effective communication, and stakeholder
engagement are therefore essential for assuring the success of river restoration
projectsinfo:eu-repo/semantics/publishedVersio
Predicting language learners' grades in the L1, L2, L3 and L4: the effect of some psychological and sociocognitive variables
This study of 89 Flemish high-school students' grades for L1 (Dutch), L2 (French), L3 (English) and L4 (German) investigates the effects of three higher-level personality dimensions (psychoticism, extraversion, neuroticism), one lower-level personality dimension (foreign language anxiety) and sociobiographical variables (gender, social class) on the participants' language grades. Analyses of variance revealed no significant effects of the higher-level personality dimensions on grades. Participants with high levels of foreign language anxiety obtained significantly lower grades in the L2 and L3. Gender and social class had no effect. Strong positive correlations between grades in the different languages could point to an underlying sociocognitive dimension. The implications of these findings are discussed
Gas Accretion by Star Clusters and the Formation of Ultraluminous X-ray Sources from Cusps of Compact Remnants
Here we show that the overabundance of ultra-luminous, compact X-ray sources
(ULXs) associated with moderately young clusters in interacting galaxies such
as the Antennae and Cartwheel can be given an alternative explanation that does
not involve the presence of intermediate mass black holes (IMBHs). We argue
that gas density within these systems is enhanced by the collective potential
of the cluster prior to being accreted onto the individual cluster members and,
as a result, the aggregate X-ray luminosity arising from the neutron star
cluster members can exceed . Various observational
tests to distinguish between IMBHs and accreting neutron star cusps are
discussed.Comment: 4 pages, 3 figures, accepted to ApJ
First results from the IllustrisTNG simulations: a tale of two elements - chemical evolution of magnesium and europium
The distribution of elements in galaxies provides a wealth of information about their production sites and their subsequent mixing into the interstellar medium. Here we investigate the elemental distributions of stars in the IllustrisTNG simulations. We analyse the abundance ratios of magnesium and europium in Milky Way-like galaxies from the TNG100 simulation (stellar masses log (M⋆/M⊙) ∼ 9.7–11.2). Comparison of observed magnesium and europium for individual stars in the Milky Way with the stellar abundances in our more than 850 Milky Way-like galaxies provides stringent constraints on our chemical evolutionary methods. Here, we use the magnesium-to-iron ratio as a proxy for the effects of our SNII (core-collapse supernovae) and SNIa (Type Ia supernovae) metal return prescription and as a comparison to a variety of galactic observations. The europium-to-iron ratio tracks the rare ejecta from neutron star–neutron star mergers, the assumed primary site of europium production in our models, and is a sensitive probe of the effects of metal diffusion within the gas in our simulations. We find that europium abundances in Milky Way-like galaxies show no correlation with assembly history, present-day galactic properties, and average galactic stellar population age. We reproduce the europium-to-iron spread at low metallicities observed in the Milky Way, and find it is sensitive to gas properties during redshifts z ≈ 2–4. We show that while the overall normalization of [Eu/Fe] is susceptible to resolution and post-processing assumptions, the relatively large spread of [Eu/Fe] at low [Fe/H] when compared to that at high [Fe/H] is quite robust
Simulating galaxy formation with black hole driven thermal and kinetic feedback
The inefficiency of star formation in massive elliptical galaxies is widely believed to be caused by the interactions of an active galactic nucleus (AGN) with the surrounding gas. Achieving a sufficiently rapid reddening of moderately massive galaxies without expelling too many baryons has however proven difficult for hydrodynamical simulations of galaxy formation, prompting us to explore a new model for the accretion and feedback effects of supermassive black holes. For high-accretion rates relative to the Eddington limit, we assume that a fraction of the accreted rest mass energy heats the surrounding gas thermally, similar to the \u2018quasar mode\u2019 in previous work. For low-accretion rates, we invoke a new, pure kinetic feedback model that imparts momentum to the surrounding gas in a stochastic manner. These two modes of feedback are motivated both by theoretical conjectures for the existence of different types of accretion flows as well as recent observational evidence for the importance of kinetic AGN winds in quenching galaxies. We find that a large fraction of the injected kinetic energy in this mode thermalizes via shocks in the surrounding gas, thereby providing a distributed heating channel. In cosmological simulations, the resulting model produces red, non-star-forming massive elliptical galaxies, and achieves realistic gas fractions, black hole growth histories and thermodynamic profiles in large haloes
- …