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B. Moore

Publications and source records attributed to B. Moore.

At least 19 recordsLinked to original sources
arXiv2021

Conformer-specific Chemistry Imaged in Real Space and Time

Conformational isomers or conformers of molecules play a decisive role in chemistry and biology. However, experimental methods to investigate chemical reaction dynamics are typically not conformer-sensitive. Here, we report on a gas-phase megaelectronvolt ultrafast electron diffraction investigation of {\alpha}-phellandrene undergoing an electrocyclic ring-opening reaction. We directly image the evolution of a specific set of {\alpha}-phellandrene conformers into the product isomer predicted by the Woodward-Hoffmann rules in real space and time. Our experimental results are in quantitative agreement with nonadiabatic quantum molecular dynamics simulations, which provide unprecedented detail of how conformation influences time scale and quantum efficiency of photoinduced ring-opening reactions. Due to the prevalence of large numbers of conformers in organic chemistry, our findings impact our general understanding of reaction dynamics in chemistry and biology.

physics.chem-ph↗
arXiv2017

Infall near clusters of galaxies: comparing gas and dark matter velocity profiles

We consider the dynamics in and near galaxy clusters. Gas, dark matter and galaxies are presently falling into the clusters between approximately 1 and 5 virial radii. At very large distances, beyond 10 virial radii, all matter is following the Hubble flow, and inside the virial radius the matter particles have on average zero radial velocity. The cosmological parameters are imprinted on the infall profile of the gas, however, no method exists, which allows a measurement of it. We consider the results of two cosmological simulations (using the numerical codes RAMSES and Gadget) and find that the gas and dark matter radial velocities are very similar. We derive the relevant dynamical equations, in particular the generalized hydrostatic equilibrium equation, including both the expansion of the Universe and the cosmological background. This generalized gas equation is the main new contribution of this paper. We combine these generalized equations with the results of the numerical simulations to estimate the contribution to the measured cluster masses from the radial velocity: inside the virial radius it is negligible, and inside two virial radii the effect is below 40%, in agreement the earlier analyses for DM. We point out how the infall velocity in principle may be observable, by measuring the gas properties to distance of about two virial radii, however, this is practically not possible today.

astro-ph.CO↗
arXiv2014

The VMC ESO Public Survey

The VISTA near-infrared YJKs survey of the Magellanic Clouds system (VMC) has entered its core phase: about 40% of the observations across the Large and Small Magellanic Clouds (LMC, SMC), the Magellanic Bridge and Stream have already been secured and the data are processed and analysed regularly. The initial analyses, concentrated in the first two completed tiles in the LMC (6_6 including 30 Doradus and 8_8 including the South Ecliptic Pole), show the superior quality of the data. The depth of the VMC survey allows the derivation of the star formation history (SFH) with unprecedented quality compared to previous wide-area surveys while reddening maps of high angular resolution are constructed using red clump stars. The multi-epoch Ks-band data reveal tight period-luminosity relations for variable stars and they permit the measurement of accurate proper motions of the stellar populations. The VMC survey continues to acquire data that will address many issues in the field of star and galaxy evolution.

astro-ph.GA↗
arXiv2012

Cusp-core transformations induced by AGN feedback in the progenitors of cluster galaxies

In a recent study (Martizzi et al. 2012), we used cosmological simulations to show that active galactic nuclei (AGN) feedback on the gas distribution in clusters of galaxies can be important in determining the spatial distribution of stars and dark matter in the central regions of these systems. The hierarchical assembly of dark matter, baryons and black holes obscures the physical mechanism behind the restructuring process. Here we use idealized simulations to follow the response of a massive dark matter halo as we feed the central black hole with a controlled supply of cold gas. This removes most of the complexity taking place in the cosmological simulations that may have biased our previous study. We confirm our previous results: gas heated and expelled from the central regions of the halo by AGN feedback can return after cooling; repeated cycles generate gravitational potential fluctuations responsible for irreversible modifications of the dark matter mass profile. The main result is the expulsion of large amounts of baryons and dark matter from the central regions of the halo. According to the work presented here, outflow induced fluctuations represent the only mechanism able to efficiently create dark matter cores in clusters of galaxies.

astro-ph.CO↗
arXiv2010

The VMC Survey - I. Strategy and First Data

The new VISual and Infrared Telescope for Astronomy (VISTA) has started operations. Over its first five years it will be collecting data for six public surveys, one of these is the near-infrared YJKsVISTA survey of the Magellanic Clouds system (VMC). This survey comprises the LMC, the SMC, the Bridge connecting the two galaxies and two fields in the Stream. This paper provides an overview of the VMC survey strategy and presents first science results. The main goals of the VMC survey are the determination of the spatially resolved SFH and 3D structure of the Magellanic system. Therefore, the VMC survey is designed to reach stars as faint as the oldest main sequence turn-off point and to constrain the mean magnitude of pulsating variable stars such as RR Lyrae stars and Cepheids. This paper focuses on observations of VMC fields in the LMC obtained between November 2009 and March 2010. These observations correspond to a 7% completeness of the LMC fields. The VMC observations consist of multi-epoch measurements organised following a specific structure. The data were reduced using the VISTA Data Flow System pipeline whose source catalogues were produced and made available via the VISTA Science Archive. The analysis of the data shows that the sensitivity in each wave band agrees with expectations. Uncertainties and completeness of the data are also derived. The first science results, aimed at assessing the science quality of the VMC data, include an overview of the distribution of stars in colour-magnitude and colour-colour diagrams, the detection of planetary nebulae and of stellar clusters, and the Ks band light-curve of variable stars. The VMC survey represents a tremendous improvement, in spatial resolution and sensitivity, on previous panoramic observations of the Magellanic system in the near-infrared and complements nicely the deep observations at other wavelengths. (Abridged)

astro-ph.CO↗
arXiv2009

Dark Matter Direct Detection with Non-Maxwellian Velocity Structure

The velocity distribution function of dark matter particles is expected to show significant departures from a Maxwell-Boltzmann distribution. This can have profound effects on the predicted dark matter - nucleon scattering rates in direct detection experiments, especially for dark matter models in which the scattering is sensitive to the high velocity tail of the distribution, such as inelastic dark matter (iDM) or light (few GeV) dark matter (LDM), and for experiments that require high energy recoil events, such as many directionally sensitive experiments. Here we determine the velocity distribution functions from two of the highest resolution numerical simulations of Galactic dark matter structure (Via Lactea II and GHALO), and study the effects for these scenarios. For directional detection, we find that the observed departures from Maxwell-Boltzmann increase the contrast of the signal and change the typical direction of incoming DM particles. For iDM, the expected signals at direct detection experiments are changed dramatically: the annual modulation can be enhanced by more than a factor two, and the relative rates of DAMA compared to CDMS can change by an order of magnitude, while those compared to CRESST can change by a factor of two. The spectrum of the signal can also change dramatically, with many features arising due to substructure. For LDM the spectral effects are smaller, but changes do arise that improve the compatibility with existing experiments. We find that the phase of the modulation can depend upon energy, which would help discriminate against background should it be found.

astro-ph.GA↗
arXiv2009

The Orbital Evolution Induced by Baryonic Condensation in Triaxial Halos

Using spectral methods, we analyse the orbital structure of prolate/triaxial dark matter (DM) halos in N-body simulations to understand the processes that drive the evolution of shapes of DM halos and elliptical galaxies in which central masses are grown. A longstanding issue is whether the change in the shapes of DM halos is the result of chaotic scattering of box orbits, or whether they change shape adiabatically in response to the evolving galactic potential. We use orbital frequencies to classify orbits, to quantify orbital shapes, and to identify resonant orbits and chaotic orbits. The frequency-based method overcomes the limitations of Lyapunov exponents which are sensitive to numerical discreteness effects. Regardless of the distribution of the baryonic component, the shape of a DM halo changes primarily due to changes in the shapes of individual orbits within a given family. Orbits with small pericentric radii are more likely to change both their orbital type and shape than orbits with large pericentric radii. Whether the evolution is regular (and reversible) or chaotic (and irreversible), depends primarily on the radial distribution of the baryonic component. The growth of an extended baryonic component of any shape results in a regular rather than chaotic change in orbital populations. In contrast the growth of a massive and compact central component results in chaotic scattering of a significant fraction of both box and long-axis tube orbits. The growth of a disk causes a significant fraction of halo particles to become trapped by major global orbital resonances. Despite the fact that shape of a DM halo is always quite oblate following the growth of a central baryonic component, a significant fraction of its orbit population has characteristics of its triaxial or prolate progenitor (ABRIDGED).

astro-ph.CO↗
arXiv2009

Blue straggler stars in dwarf spheroidal galaxies II: Sculptor and Fornax

The existence of blue straggler stars (BSSs) in dwarf spheroidal galaxies (dSphs) is still an open question. In fact, many BSS candidates have been observed in the Local Group dSphs, but it is unclear whether they are real BSSs or young stars. Shedding light on the nature of these BSS candidates is crucial, in order to understand the star formation history of dSphs. In this paper, we consider BSS candidates in Sculptor and Fornax. In Fornax there are strong hints that the BSS population is contaminated by young stars, whereas in Sculptor there is no clear evidence of recent star formation. We derive the radial and luminosity distribution of BSS candidates from wide field imaging data extending beyond the nominal tidal radius of these galaxies. The observations are compared with the radial distribution of BSSs expected from dynamical simulations. In Sculptor the radial distribution of BSS candidates is consistent with that of red horizontal branch (RHB) stars and is in agreement with theoretical expectations for BSSs generated via mass transfer in binaries. On the contrary, in Fornax the radial distribution of BSS candidates is more concentrated than that of all the considered stellar populations. This result supports the hypothesis that most of BSS candidates in Fornax are young stars and is consistent with previous studies.

astro-ph.GA↗
arXiv2009

Are ring galaxies the ancestors of giant low surface brightness galaxies?

Giant low surface brightness galaxies (GLSBs), such as Malin 1, have unusually large and flat discs. Their formation is a puzzle for cosmological simulations in the cold dark matter scenario. We suggest that GLSBs might be the final product of the dynamical evolution of collisional ring galaxies. In fact, our simulations show that, approximately 0.5-1.5 Gyr after the collision which lead to the formation of a ring galaxy, the ring keeps expanding and fades, while the disc becomes very large (~100 kpc) and flat. At this stage, our simulated galaxies match many properties of GLSBs (surface brightness profile, morphology, HI spectrum and rotation curve).

astro-ph.CO↗
arXiv2008

Fossil remnants of reionization in the halo of the Milky Way

Our recently completed one billion particle Via Lactea II simulation of a Milky Way-sized dark matter halo resolves over 50,000 gravitationally bound clumps orbiting today within the virialized region of the main host. About 2,300 of these subhalos have one or more "progenitors" above 1e6 Msun at redshift 11, i.e. massive enough for their gas to have cooled via excitation of H2 and fragmented prior to the epoch of cosmic reionization. We count 4,500 such progenitors: if these were able to convert a fraction of their gas content into very metal-poor stars with a Salpeter initial mass function (IMF), they would be shining today with a visual magnitude M_V=6.7 per solar mass in stars. Assuming a universal baryon fraction, we show that mean star formation efficiencies as low as 0.1% in progenitors below 1e8 Msun would overproduce the abundance of the faint Galatic dwarf spheroidals observed by the Sloan Digital Sky Survey. Star formation at first light must have occurred either with an IMF lacking stars below 0.9 Msun, or was intrinsically very inefficient in small dark matter halos. If the latter, our results may be viewed as another hint of a minimum scale in galaxy formation.

astro-ph↗
arXiv2008

The origin of lopsidedness in galaxies

It has long been known that a large fraction of disc galaxies are lopsided. We simulate three different mechanisms that can induce lopsidedness: flyby interactions, gas accretion from cosmological filaments and ram pressure from the intergalactic medium. Comparing the morphologies, HI spectrum and m=1 Fourier components, we find that all of these mechanisms can induce lopsidedness in the gaseous component of disc galaxies. In particular, we estimate that flybys can contribute to ~20 per cent of lopsided galaxies. We compare our simulations with the observations of NGC 891, a lopsided, edge-on galaxy with a nearby companion (UGC 1807). We find that the main properties of NGC 891 favour a flyby event for the origin of lopsidedness in this galaxy.

astro-ph↗
arXiv2008

Are ring galaxies the ancestors of giant low surface brightness galaxies?

Giant low surface brightness galaxies (GLSBs) have flat discs extending up to ~100 kpc. Their formation is a puzzle for cosmological simulations in the cold dark matter scenario. We suggest that GLSBs might be the final product of the evolution of collisional ring galaxies. In fact, our simulations show that, approximately 0.5-1.5 Gyr after the collision which led to the formation of the ring galaxy, the ring keeps expanding and fades, while the disc becomes very large (~100 kpc) and flat. At this stage, our simulated galaxies match many properties of GLSBs (surface brightness profile, morphology, HI spectrum and rotation curve).

astro-ph↗
arXiv2008

Merger and Ring Galaxy Formation Rates at z<=2

We compare the observed merger rate of galaxies over cosmic time and the frequency of collisional ring galaxies (CRGs), with analytic models and halo merger and collision rates from a large cosmological simulation. In the Lambda cold dark matter (LCDM) model we find that the cosmic {\it merger fraction} does not evolve strongly between 0.2<z<2, implying that the observed decrease of the cosmic star formation rate since z~1 might not be tied to a disappearing population of major mergers. Halos hosting massive galaxies undergo on average ~2 mergers from z~2 up to present day, reflecting the late assembly time for the massive systems and the related downsizing problem. The cosmic {\it merger rate} declines with redshift: at the present time it is a factor of 10 lower than at z~2, in reasonable agreement with the current available data. The rate of CRG formation derived from the interactions between halo progenitors up to z=2 is found to be a good tracer of the cosmic merger rate. In the LCDM model the rate of CRGs as well as the merger rate do not scale as (1+z)^m, as suggested by previous models. Our predictions of cosmic merger and CRG rates may be applied to forthcoming surveys such as GOODS and zCOSMOS.

astro-ph↗
arXiv2008

Clumps and streams in the local dark matter distribution

In cold dark matter cosmological models, structures form and grow by merging of smaller units. Numerical simulations have shown that such merging is incomplete; the inner cores of halos survive and orbit as "subhalos" within their hosts. Here we report a simulation that resolves such substructure even in the very inner regions of the Galactic halo. We find hundreds of very concentrated dark matter clumps surviving near the solar circle, as well as numerous cold streams. The simulation reveals the fractal nature of dark matter clustering: Isolated halos and subhalos contain the same relative amount of substructure and both have cuspy inner density profiles. The inner mass and phase-space densities of subhalos match those of recently discovered faint, dark matter-dominated dwarf satellite galaxies and the overall amount of substructure can explain the anomalous flux ratios seen in strong gravitational lenses. Subhalos boost gamma-ray production from dark matter annihilation, by factors of 4-15, relative to smooth galactic models. Local cosmic ray production is also enhanced, typically by a factor 1.4, but by more than a factor of ten in one percent of locations lying sufficiently close to a large subhalo. These estimates assume that gravitational effects of baryons on dark matter substructure are small.

astro-ph↗
arXiv2008

Lopsided galaxies: the case of NGC 891

It has been known for a long time that a large fraction of disc galaxies are lopsided. We simulate three different mechanisms that can induce lopsidedness: flyby interactions, gas accretion from cosmological filaments and ram pressure from the intergalactic medium. Comparing the morphologies, HI spectrum, kinematics and m=1 Fourier components, we find that all of these mechanisms can induce lopsidedness in galaxies, although in different degrees and with observable consequences. The timescale over which lopsidedness persists suggests that flybys can contribute to ~20 per cent of lopsided galaxies. We focus our detailed comparison on the case of NGC 891, a lopsided, edge-on galaxy with a nearby companion (UGC 1807). We find that the main properties of NGC 891 (morphology, HI spectrum, rotation curve, existence of a gaseous filament pointing towards UGC 1807) favour a flyby event for the origin of lopsidedness in this galaxy.

astro-ph↗
arXiv2007

Intermediate-mass black holes and ultraluminous X-ray sources in the Cartwheel ring galaxy

Chandra and XMM-Newton observations of the Cartwheel galaxy show ~17 bright X-ray sources (>~5x10^38 erg s^-1), all within the gas-rich outer ring. We explore the hypothesis that these X-ray sources are powered by intermediate-mass black holes (IMBHs) accreting gas or undergoing mass transfer from a stellar companion. To this purpose, we run N-body/SPH simulations of the galaxy interaction which might have led to the formation of Cartwheel, tracking the dynamical evolution of two different IMBH populations: halo and disc IMBHs. Halo IMBHs cannot account for the observed X-ray sources, as only a few of them cross the outer ring. Instead, more than half of the disc IMBHs are pulled in the outer ring as a consequence of the galaxy collision. However, also in the case of disc IMBHs, accretion from surrounding gas clouds cannot account for the high luminosities of the observed sources. Finally, more than 500 disc IMBHs are required to produce <~15 X-ray sources via mass transfer from very young stellar companions. Such number of IMBHs is very large and implies extreme assumptions. Thus, the hypothesis that all the observed X-ray sources in Cartwheel are associated with IMBHs is hardly consistent with our simulations, even if it is still possible that IMBHs account for the few (<~1-5) brightest ultraluminous X-ray sources (ULXs).

astro-ph↗
arXiv2007

Are ring galaxies the ancestors of giant low surface brightness galaxies?

We simulate the collisional formation of a ring galaxy and we integrate its evolution up to 1.5 Gyr after the interaction. About 100-200 Myr after the collision, the simulated galaxy is very similar to observed ring galaxies (e.g. Cartwheel). After this stage, the ring keeps expanding and fades. Approximately 0.5-1 Gyr after the interaction, the disc becomes very large (~100 kpc) and flat. Such extended discs have been observed only in giant low surface brightness galaxies (GLSBs). We compare various properties of our simulated galaxies (surface brightness profile, morphology, HI spectrum and rotation curve) with the observations of four well-known GLSBs (UGC6614, Malin 1, Malin 2 and NGC7589). The simulations match quite well the observations, suggesting that ring galaxies could be the progenitors of GLSBs. This result is crucial for the cold dark matter (CDM) model, as it was very difficult, so far, to explain the formation of GLSBs within the CDM scenario.

astro-ph↗