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News Article | October 26, 2016
Site: spaceref.com

Our Milky Way has a densely populated centre -- a feature common to many galaxies, but unique in that it is close enough to study in depth. A team led by Dante Minniti (Universidad Andrés Bello, Santiago, Chile) and Rodrigo Contreras Ramos (Instituto Milenio de Astrofísica, Santiago, Chile) used observations from the VISTA infrared survey telescope, as part of the Variables in the Via Lactea (VVV) ESO public survey, to carefully search the central part of the Milky Way. By observing infrared light, which is less affected by cosmic dust than visible light, and exploiting the excellent conditions at ESO's Paranal Observatory, the team was able to get a clearer view of this region than ever before. They found a dozen ancient RR Lyrae stars at the heart of the Milky Way that were previously unknown. Our Milky Way has a densely populated centre -- a feature common to many galaxies, but unique in that it is close enough to study in depth. This discovery of RR Lyrae stars provides compelling evidence that helps astronomers decide between two main competing theories for how these bulges form. RR Lyrae stars are typically found in dense globular clusters. They are variable stars, and the brightness of each RR Lyrae star fluctuates regularly. By observing the length of each cycle of brightening and dimming in an RR Lyrae, and also measuring the star's brightness, astronomers can calculate its distance [1]. Unfortunately, these excellent distance-indicator stars are frequently outshone by younger, brighter stars and in some regions they are hidden by dust. Therefore, locating RR Lyrae stars right in the extremely crowded heart of the Milky Way was not possible until the public VVV survey was carried out using infrared light. Even so, the team described the task of locating the RR Lyrae stars in amongst the crowded throng of brighter stars as "daunting". Their hard work was rewarded, however, with the identification of a dozen RR Lyrae stars. Their discovery indicate that remnants of ancient globular clusters are scattered within the centre of the Milky Way's bulge. Rodrigo Contreras Ramos elaborates: "This discovery of RR Lyrae Stars in the centre of the Milky Way has important implications for the formation of galactic nuclei. The evidence supports the scenario in which the bulge was originally made out of a few globular clusters that merged." The theory that galactic bulges form through the merging of globular clusters is contested by the competing hypothesis that these bulges are actually due to the rapid accretion of gas. The unearthing of these RR Lyrae stars -- almost always found in globular clusters -- isvery strong evidence that the Milky Way bulge did in fact form through merging. By extension, all other similar galactic bulges may have formed the same way. Not only are these stars powerful evidence for an important theory of galactic evolution, they are also likely to be over 10 billion years old -- the dim, but dogged survivors of perhaps the oldest and most massive star cluster within the Milky Way. [1] RR Lyrae stars, like some other regular variables such as Cepheids, show a simple relationship between how quickly they change in brightness and how luminous they are. Longer periods mean brighter stars. This period-luminosity relationship can be used to deduce the distance of a star from its period of variation and its apparent brightness. This research was presented in a paper to appear in The Astrophysical Journal Letters. The team is composed of D. Minniti (Instituto Milenio de Astrofísica, Santiago, Chile; Departamento de Física, Universidad Andrés Bello, Santiago, Chile; Vatican Observatory, Vatican City State, Italy; Centro de Astrofisica y Tecnologias Afines - CATA), R. Contreras Ramos (Instituto Milenio de Astrofísica, Santiago, Chile; Pontificia Universidad Católica de Chile, Instituto de Astrofísica, Santiago, Chile), M. Zoccali (Instituto Milenio de Astrofísica, Santiago, Chile; Pontificia Universidad Católica de Chile, Instituto de Astrofísica, Santiago, Chile), M. Rejkuba (European Southern Observatory, Garching bei München, Germany; Excellence Cluster Universe, Garching, Germany), O.A. Gonzalez (UK Astronomy Technology Centre, Royal Observatory, Edinburgh, UK), E. Valenti (European Southern Observatory, Garching bei München, Germany), F. Gran (Instituto Milenio de Astrofísica, Santiago, Chile; Pontificia Universidad Católica de Chile, Instituto de Astrofísica, Santiago, Chile) ESO is the foremost intergovernmental astronomy organisation in Europe and the world's most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world's most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become "the world's biggest eye on the sky". * Variables in the Via Lactea (VVV) - https://vvvsurvey.org/ ESO public survey Please follow SpaceRef on Twitter and Like us on Facebook.


Dale J.E.,Excellence Cluster Universe | Bonnell I.A.,University of St. Andrews
Monthly Notices of the Royal Astronomical Society | Year: 2012

We report on smoothed particle hydrodynamics simulations of the impact on a turbulent ∼2 × 10 3 M ⊙ star-forming molecular cloud of irradiation by an external source of ionizing photons. We find that the ionizing radiation has a significant effect on the gas morphology, but a less important role in triggering stars. The rate and morphology of star formation are largely governed by the structure in the gas generated by the turbulent velocity field, and feedback has no discernible effect on the stellar initial mass function. Although many young stars are to be found in dense gas located near an ionization front, most of these objects also form when feedback is absent. Ionization has a stronger effect in diffuse regions of the cloud by sweeping up low-density gas that would not otherwise form stars into gravitationally unstable clumps. However, even in these regions, dynamical interactions between the stars rapidly erase the correlations between their positions and velocities and that of the ionization front. © 2012 The Author Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Dale J.E.,Excellence Cluster Universe | Ercolano B.,Excellence Cluster Universe | Bonnell I.A.,University of St. Andrews
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present a smoothed particle hydrodynamics parameter study of the dynamical effect of photoionization from O-type stars on star-forming clouds of a range of masses and sizes during the time window before supernovae explode. Our model clouds all have the same degree of turbulent support initially, the ratio of turbulent kinetic energy to gravitational potential energy being set to E kin/|E pot|= 0.7. We allow the clouds to form stars and study the dynamical effects of the ionizing radiation from the massive stars or clusters born within them. We find that dense filamentary structures and accretion flows limit the quantities of gas that can be ionized, particularly in the higher density clusters. More importantly, the higher escape velocities in our more massive (10 6M ⊙) clouds prevent the Hii regions from sweeping up and expelling significant quantities of gas, so that the most massive clouds are largely dynamically unaffected by ionizing feedback. However, feedback has a profound effect on the lower density 10 4 and 10 5M ⊙ clouds in our study, creating vast evacuated bubbles and expelling tens of per cent of the neutral gas in the 3-Myr time-scale before the first supernovae are expected to detonate, resulting in clouds highly porous to both photons and supernova ejecta. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Baldi M.,Excellence Cluster Universe
Annalen der Physik | Year: 2012

A wide range of astrophysical and cosmological observations support the evidence that the energy density of the Universe is presently largely dominated by particles and fields that do not belong to the standard model of particle physics. Such cosmic dark sector appears to be made of two distinct entities capable to account for the growth of large-scale structures and for the observed acceleration of the expansion rate of the Universe, respectively dubbed dark matter and dark energy. Nevertheless, the fundamental nature of these two dark components has so far remained mysterious. In the currently accepted scenario dark matter is associated to a single new massive and weakly interacting particle beyond the standard model, while dark energy is assumed to be a simple cosmological constant. However, present cosmological constraints and the absence of a direct detection and identification of any dark matter particle candidate leave room to the possibility that the dark sector of the Universe be actually more complex than it is normally assumed. In particular,more than one new fundamental particle could be responsible for the observed dark matter density in the Universe, and possible new interactions between dark energy and dark matter might characterize the dark sector. In the present work, the possibility that two dark matter particles may exist in nature is investigated. These different species are assumed to have identical physical properties except for the sign of their coupling constant to dark energy. Extending previous works on similar scenarios, the evolution of the background cosmology as well as the growth of linear density perturbations for a wide range of parameters of such multiple dark matter model is studied. Interestingly, the results show how the simple assumption that dark matter particles carry a "charge" with respect to their interaction with the dark energy field allows for new long-range scalar forces of gravitational strength in the dark sector without conflicting with present observations both at the background and linear levels. The presented scenario does not introduce new parameters with respect to the case of a single dark matter species for which such strong dark interactions have been already ruled out. Therefore, the present investigation suggests that only a detailed study of nonlinear structure formation processesmight possibly provide effective constraints on new scalar interactions of gravitational strength in the dark sector. © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Baldi M.,Excellence Cluster Universe | Baldi M.,Ludwig Maximilians University of Munich
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present the largest set of N-body and hydrodynamical simulations to date for cosmological models featuring a direct interaction between the dark energy (DE) scalar field, responsible for the observed cosmic acceleration, and the cold dark matter (CDM) fluid. With respect to previous works, our simulations considerably extend the statistical significance of the simulated volume and cover a wider range of different realizations of the interacting DE scenario, including the recently proposed bouncing coupled DE model. Furthermore, all the simulations are normalized in order to be consistent with the present bounds on the amplitude of density perturbations at last scattering, thereby providing the first realistic determination of the effects of a DE coupling for cosmological growth histories fully compatible with the latest cosmic microwave background data. As a first basic analysis, we have studied the impact of the coupling on the non-linear matter power spectrum and on the bias between the CDM and baryon distributions, as a function of redshift and scale. For the former, we have addressed the issue of the degeneracy between the effects of the coupling and other standard cosmological parameters, e.g. σ 8, showing how the redshift evolution of the linear amplitude or the scale dependence of the non-linear power spectrum might provide a way to break the degeneracy. For the latter, instead, we have computed the redshift and scale dependence of the bias in all our different models showing how a growing coupling or a bouncing coupled DE scenario provides much stronger effects with respect to constant coupling models. Furthermore, we discuss the main features imprinted by the DE interactions on the halo and subhalo mass functions. We refer to this vast numerical initiative as the COupled Dark Energy Cosmological Simulations (codecs) project, and release all the codecs outputs for public use through a dedicated web data base, providing information on how to access and interpret the data. © 2012 The Author Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Baldi M.,Excellence Cluster Universe | Baldi M.,Ludwig Maximilians University of Munich
Monthly Notices of the Royal Astronomical Society | Year: 2012

The abundance of the most massive objects in the Universe at different epochs is a very sensitive probe of the cosmic background evolution and of the growth history of density perturbations, and could provide a powerful tool to distinguish between a cosmological constant and a dynamical dark energy field. In particular, the recent detection of very massive clusters of galaxies at high redshifts has attracted significant interest as a possible indication of a failure of the standard Λ cold dark matter model. Several attempts have been made in order to explain such detections in the context of non-Gaussian scenarios or interacting dark energy models, showing that both these alternative cosmologies predict an enhanced number density of massive clusters at high redshifts, possibly alleviating the tension. However, all the models proposed so far also overpredict the abundance of massive clusters at the present epoch, and are therefore in contrast with observational bounds on the low-redshift halo mass function. In this paper we present for the first time a new class of interacting dark energy models that simultaneously account for an enhanced number density of massive clusters at high redshifts and for both the standard cluster abundance at the present time and the standard power spectrum normalization at cosmic microwave background (CMB). The key feature of this new class of models is the 'bounce' of the dark energy scalar field on the cosmological constant barrier at relatively recent epochs. We present the background and linear perturbations evolution of the model, showing that the standard amplitude of density perturbations is recovered both at CMB and at the present time, and we demonstrate by means of large N-body simulations that our scenario predicts an enhanced number of massive clusters at high redshifts without affecting the present halo abundance. Such behaviour could not arise in non-Gaussian models, and is therefore a characteristic feature of the bouncing coupled dark energy scenario. © 2011 The Author Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Baldi M.,Excellence Cluster Universe | Baldi M.,Ludwig Maximilians University of Munich
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present a detailed numerical study of the impact that cosmological models featuring a direct interaction between the dark energy component that drives the accelerated expansion of the Universe and cold dark matter can have on the linear and non-linear stages of structure formation. By means of a series of collisionless N-body simulations, we study the influence that each of the different effects characterizing these cosmological models - which include among others a fifth force, a time variation of particle masses and a velocity-dependent acceleration - separately have on the growth of density perturbations and on a series of observable quantities related to linear and non-linear cosmic structures, as the matter power spectrum, the gravitational bias between baryons and cold dark matter, the halo mass function and the halo density profiles. We perform our analysis applying and comparing different numerical approaches previously adopted in the literature, and we address the partial discrepancies recently claimed in a similar study by Li & Barrow with respect to the first outcomes of Baldi et al., which are found to be related to the specific numerical approach adopted in the former work. Our results fully confirm the conclusions of Baldi et al. and show that when linear and non-linear effects of the interaction between dark energy and cold dark matter are properly disentangled, the velocity-dependent acceleration is the leading effect acting at non-linear scales and in particular is the most important mechanism in lowering the concentration of cold dark matter haloes. © 2011 The Author Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Baldi M.,Excellence Cluster Universe | Baldi M.,Ludwig Maximilians University of Munich
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present a complete numerical study of cosmological models with a time-dependent coupling between the dark energy component driving the present accelerated expansion of the Universe and the cold dark matter (CDM) fluid. Depending on the functional form of the coupling strength, these models show a range of possible intermediate behaviours between the standard ΛCDM background evolution and the widely studied case of interacting dark energy models with a constant coupling. These different background evolutions play a crucial role in the growth of cosmic structures and determine strikingly different effects of the coupling on the internal dynamics of non-linear objects. By means of a suitable modification of the cosmological N-body code gadget-2, we have performed a series of high-resolution N-body simulations of structure formation in the context of interacting dark energy models with variable couplings. Depending on the type of background evolution, the halo density profiles are found to be either less or more concentrated with respect to ΛCDM, contrarily to what happens for constant coupling models where concentrations can only decrease. However, for some specific choice of the interaction function, the reduction in halo concentrations can be larger than in constant coupling scenarios. We also find that different types of coupling evolution determine specific features in the growth of large-scale structures, like peculiar distortions of the matter power spectrum shape or different time-evolutions of the halo mass function. Furthermore, also for time-dependent couplings, baryons and CDM develop a bias already on large scales, which is progressively enhanced for smaller and smaller scales, and the effect can be significantly larger compared to constant coupling scenarios. The same happens to the baryon fraction of haloes, which can be more significantly reduced below its universal value in variable coupling models with respect to constant coupling cosmologies. In general, we find that time-dependent interactions between dark energy and CDM can in some cases determine stronger effects on structure formation as compared to the constant coupling case, with a significantly weaker impact on the background evolution of the universe, and might therefore provide a more viable possibility to alleviate the tensions between observations and the ΛCDM model on small scales than the constant coupling scenario. © 2010 The Author Monthly Notices of the Royal Astronomical Society © 2010 RAS.


Dale J.E.,Excellence Cluster Universe | Ercolano B.,Excellence Cluster Universe | Bonnell I.A.,University of St. Andrews
Monthly Notices of the Royal Astronomical Society | Year: 2013

We extend our previous smoothed particle hydrodynamics parameter study of the effects of photoionization from O-stars on star-forming clouds to include initially unbound clouds. We generate a set of model clouds in the mass range 104-106 M⊙ with initial virial ratios Ekin/Epot = 2.3, allow them to form stars and study the impact of the photoionizing radiation produced by the massive stars. We find that, on the 3 Myr time-scale before supernovae are expected to begin detonating, the fraction of mass expelled by ionizing feedback is a very strong function of the cloud escape velocities. High-mass clouds are largely unaffected dynamically, while low-mass clouds have large fractions of their gas reserves expelled on this time-scale. However, the fractions of stellar mass unbound are modest and significant portions of the unbound stars are so only because the clouds themselves are initially partially unbound. We find that ionization is much more able to create well-cleared bubbles in the unbound clouds, owing to their intrinsic expansion, but that the presence of such bubbles does not necessarily indicate that a given cloud has been strongly influenced by feedback.We also find, in common with the bound clouds from our earlier work, that many of the systems simulated here are highly porous to photons and supernova ejecta, and that most of them will likely survive their first supernova explosions. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Baldi M.,Excellence Cluster Universe | Baldi M.,Ludwig Maximilians University of Munich
Monthly Notices of the Royal Astronomical Society | Year: 2013

Long-range scalar forces with gravitational strength can be ruled out by accurate tests of scalar-tensor theories in the Solar system. However, such tests are based on the motion of celestial bodies made of standard baryonic particles, which leaves room for possible scalar interactions in the dark sector of the universe if a new scalar degree of freedom couples only to dark matter particles. In particular, an interaction between cold dark matter (CDM) and a classical scalar field playing the role of the cosmic dark energy (DE) might provide such long-range dark interactions without conflicting with Solar system bounds. Although presently available observations allow us to constrain such interactions to a few per cent of the gravitational strength, some recent studies have shown that if CDM is composed by two different particle species having opposite couplings to the DE field, such tight constraints can be considerably relaxed, allowing for long-range scalar forces of order gravity without significantly affecting observations both at the background and at the linear perturbations level. In the present work, we extend the investigation of such multiple dark matter scenarios to the non-linear regime of structure formation, by presenting the first N-body simulations ever performed for these cosmologies. Our results highlight some characteristic footprints of long-range scalar forces that arise only in the non-linear regime for specific models that would be otherwise practically indistinguishable from the standard ΔCDM scenario both in the background and in the growth of linear density perturbations. Among these effects, the formation of 'mirror' cosmic structures in the two CDM species, the suppression of the non-linear matter power spectrum at k≳0.1 h Mpc-1 and the fragmentation of collapsed haloes, represent peculiar features that might provide a direct way to constrain this class of cosmological models. © 2012 The Author.

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