Astro Colloquium 2020
Weekly virtual colloquium | Departamento de Astronomia UdeC
Nuclear star clusters (NSCs) are a common characteristic of dwarf galaxies, particularly higher-mass dwarfs. There are two main scenarios discussed in the literature to explain their formation, in-situ formation and migration, and clues to the formation of an NSC lie in its SFH. However, NSC spectra are faint and their light is often contaminated by light of the host galaxy, making it difficult to truly isolate the stellar populations of the NSC itself. As part of the Next Generation Fornax Survey (NGFS), I am using BUDDI (Bulge-Disc Decomposition of IFU data) to overcome this issue. BUDDI uses information from the entire datacube to create a wavelength-dependent model of each each component within a galaxy, from which one can extract a spectrum representing purely the light from that component. In this talk, I'll introduce BUDDI and present our results from its application to a sample of Fornax dwarfs observed with MUSE. I will conclude with a discussion on the formation of NSCs and their role in the evolution of dwarf galaxies.
The astrophysical origin of merging black hole binaries is still a mystery. Two main pathways are usually advocated: isolated binaries merging in the field, and dynamically interacting binaries formed in star clusters. The coarse localization of gravitational wave events cannot indicate the environment in which the binary formed, but discerning among these two scenarios can be possible via the progenitor binary parameters that are observable via gravitational wave interferometry, namely masses, eccentricity, and spin. However, present models of the aforementioned formation pathways do not indicate an appreciable difference in the mass distributions, and eccentricity is unlikely to be inferred with current ground-based detectors. On the other hand, the magnitude and orientation of black hole spins is a promising indicator of the formation history of merging binaries. Merging black hole binaries from isolated binaries will likely have spins aligned with the orbit due to tidal spin-up, while binaries ejected from star clusters will have some degrees of misalignment. We quantify the expected spin parameters distributions of black hole binaries ejected from globular and open clusters by combining population synthesis and gravitational few-body simulations. Using such information, we may be able to link black hole spin to the binary formation pathway, thus leading to a more detailed picture of their environment and origin.
Dust is found frequently in early-type galaxies. The traditional view that dust is produced by young stars, can only be rescued if dust in old stellar populations would have an "external origin" (whatever that means). I shall show a gallery of dust manifestations in various galaxies. A few galaxies like NGC 3311, NGC 1316 and NGC 5102 will be discussed in more detail with regard to their dust and gas content. The lesson to be learned is that apparently a significant part of the dust is produced and distributed by dusty winds from galactic nuclei.
Identifying the most likely sources for high-energy neutrino emission has been one of the main topics in high-energy astrophysics ever since the first observation of high-energy neutrinos by the IceCube Neutrino Observatory. Active galactic nuclei with relativistic jets, blazars, have been considered to be one of the main candidates due to their ability to accelerate particles to high energies. In my talk, I will describe a study where we examined the connection between radio emission and IceCube neutrino events using data from the Owens Valley Radio Observatory and Metsähovi Radio Observatory blazar monitoring programs. We identified sources in our radio monitoring sample, which are positionally consistent with IceCube high-energy neutrino events. We estimated their mean flux density and variability amplitudes around the neutrino arrival time, and compared these with values from random samples to establish the significance of our results. Based on our results we conclude that although it is clear that not all neutrino events are associated with strong radio flaring blazars, when we see large amplitude radio flares in a blazar at the same time as a neutrino event, it is unlikely to happen by random coincidence.
Remote sensing and in situ observations in the solar wind show that the particle velocity distributions usually present characteristics of systems out of thermal equilibrium. An open question is related to why minor ions such as alpha particles (He+2) flow away from the Sun faster than protons, and why minor ions are also preferentially heated to temperatures near mass proportionality. In this talk, we show that kinetic instabilities are able to enhance spontaneously thermal magnetic fluctuations, and that these fluctuations may be related to the processes leading to plasma heating and acceleration. We also show that ion-cyclotron resonances may explain the distribution of alpha heating, and the limitations of the alpha-to-proton drift at low plasma beta, as observed in the solar wind at 1 AU.
The gravitational three-body problem is one of the longest-standing open problems in physics, dating all the way back to Newton. The problem statement is quite simple: given 3 masses with their initial position and velocities, solve for the subsequent motion. Despite this apparent simplicity, an analytical solution for the three-body problem still evades us. The main reason for this is that the three-body system exhibits chaos. Even though the underlying force evolving the system is known, which is a simple 1/r^2 gravitational force, it is difficult to predict the system’s future. However, with the development of computational methods that enable us to run numerical simulations, we have been able to study the general three-body problem in a statistical fashion. As a result, the focus over past few decades has been on providing a statistical description of the general three-body problem instead of an analytical one. In this talk, I will give an overview of the chaotic three-body problem from the statistical perspective and discuss some essential properties like phase space maps, lifetime distributions and ejection probabilities. I will also highlight some ongoing efforts in developing a statistical theory for the three-body problem and some preliminary comparisons between theory and simulations.
Not nearly as famous as Cepheids and RR Lyrae stars, Delta Scuti stars, pulsators at the intersection of the instability strip and the main sequence, will be the most numerous pulsating star to be discovered by Rubin/LSST in our Galaxy. In this talk I will discuss the role of Delta Scuti in the phenomenon of extended main-sequence turn-offs in intermediate-age Magellanic Clouds star clusters, and the unexpected behaviour of its period-luminosity relation. I will finish by introducing a new survey, the MAgellanic Delta Scuti survey (MADS), which will refresh our knowledge of these variables across the Magellanic Clouds using Blanco/DECa
Most stars and planets form in clusters/associations with hundreds of low-mass stars forming alongside each high-mass star. Stellar feedback permeates these regions and plays a central role in shaping the demographics and habitability of exoplanets. Ionising radiation from high-mass stars truncates and destroys protoplanetary disks around nearby low-mass stars, reducing the timescale for planet formation. At the same time, short-lived radioactive elements synthesised in the death of these same high-mass stars may regulate the water budget of terrestrial (Earth-like) planets. As the exoplanet community steps ever closer to detecting Earth-analogs, constraints are urgently needed to constrain the role of feedback in the environments where the majority of stars/planets form. I will talk about on-going surveys using ALMA, MUSE/VLT, and M2FS/Magellan to measure gas and stellar kinematics in order to test the role of the environment in shaping the outcome of star and planet formation.
Although substantial progress has been made over the last decades on our understanding of star formation it is mostly in terms of formation of individual isolated stars. However, we know that many stars are formed in star clusters or in clustered environments. Here, the proximity of other stars and interactions with them can alter the formation of each star and thus their final properties (e.g. their final mass). However, little is known about the formation of particularly massive star clusters and their content. This is due to a combination of limited suitable targets to observe and limited sensitivity and spatial resolution of observations. As a consequence there are limited observational constraints on cluster formation models. However, in recent years suitable candidates of massive star clusters in their formation have been found. I will present our findings for several of those covering an order of magnitude in mass. I will further discuss the end product of the star formation, the Initial Mass Function (IMF) in massive resolved star clusters. Through deep high spatial resolution imaging we have probed the low-mass content of the clusters. From this we can directly compare the derived IMF with that determined for the field and low-mass clusters. Further, through dynamic mass estimates we can determine if the clusters will remain bound or dissolve and be a component of the future field star population.
Stars are the fundamental building blocks of galaxies and stellar clusters. They are often part of small stellar systems, such as binaries and triples in which the stars can interact with each other. These interactions give rise to some of the most energetic events in the universe, e.g. supernovae Type Ia explosions and gravitational wave sources. The advent and development of large-scale time domain surveys are revealing the existence of a large and diverse zoo of transients, but their origin or progenitor evolution is often unknown. Here, I will present our latest results regarding the evolution of white dwarf binaries and their mergers, with implications for Galactic archaeology. Secondly, I will focus on the evolution of triple star systems, their evolution, interactions and resulting transients. Even though the principles of binary evolution theory have been accepted for a long time, the evolution of triples is an uncharted territory. There is a need to understand the evolution of triples, as they are common and often invoked to explain compact and exotic binaries.
Magnetic fields are ubiquitous on all astrophysical scales and objects. They also permeate entire galaxies, where the energy density of magnetic fields is comparable or even larger than the thermal energy. Hence magnetic fields have an wide impact on the dynamics of the interstellar medium (ISM). For instance, they could prevent the gravitational collapse of molecular cloud cores (similar to the thermal pressure) and therefore would not allow the formation of stars. In this talk, I'll discuss how stars are formed in galaxies in the presence of strong magnetic fields.
Gravitational-wave detections are starting to reveal the properties of the population of merging binary blackholes (BBH). Stellar theory predicts a gap in the black hole mass function between 45 and 130 Msun, referred to as the pair-instability supernovae (PISN) mass gap. This prediction of a mass gap is remarkably robust against model variations to the extent that it could be considered among the most robust predictions from stellar theory available today. The first ten binary BBH detections already indicate a dearth of BBH mergers with component masses greater than ~45\Msun. In mere weeks, O3 will increase our sample size to approximately 50. The planned 3rd generation gravitational wave detectors are estimated to detect about 10^4 BBH mergers per year and will reveal exactly how rare these PISN mass gap events are. Since the prediction of the gap is so robust, it’s lower limit could be used to constrain stellar physics. Understanding if and how BBHs with a mass in the gap can be formed is essential to understand the final stages of massive stellar evolution and the different formation channels of binary black holes. In this talk, I will explore possibilities for creating black holes with masses in the gap, and I will discuss several formation channels, ranging from dynamical interactions to isolated binary evolution including super-Eddington mass accretion onto black holes and compare their rates and predictions.
FIR emission in galaxies is produced in the ISM, by dust (continuum) and metal lines as [CII] and [OIII]. FIR emission lines are considered good ISM tracers, and [CII], one of the brightest lines at all redshifts, is often used as a SFR tracer. However, while in normal star-forming galaxies the SFR-[CII] correlation is tight, recent results have shown that different ISM conditions as a low metallicity or a strong radiation field can result in significant deviations, especially at high redshift. In this talk, I will discuss how state-of-the-art numerical simulations can help addressing the reliability of [CII] (and other FIR lines) as SFR tracers, by means of self-consistent chemo-dynamical cosmological simulations of both low and high redshift galaxies.
The principle of similitude (Rayleigh 1915) or dimensional homogeneity, states that only commensurable quantities (ones having the same dimension) may be compared, therefore, meaningful laws of nature must be homogeneous equations in their various units of measurement, a result which was formalized in the Π theorem (Vaschy 1892; Buckingham 1914 and others). However, in many areas such as Biology, Economics or even partially in Astronomy (a sub-branch of Physics), the most fundamental empirical relations do not satisfy this basic mathematical requirement. In this talk, we show (using the Π theorem) that it is indeed possible to construct homogeneous equations to describe as diverse phenomena as the star formation rate in galaxies (Kenicutt-Schmidt Law; Astronomy) and the metabolic rates of animals (Kleiber Law; Biology), in agreement with data in the literature (Escala 2015; Utreras, Becerra and Escala 2016; Escala 2019). We illustrate the power of using these mathematically well defined relations, for example, by reducing the scatter of the galactic star formation relation in numerical simulations by 43% (Utreras, Becerra and Escala 2016), or by showing how from a corrected version of the Kleiber's Law is possible to derive the empirical relation for the total energy consumed in animal's lifespan (Atasanov 2007, Escala 2020) or animal's ontogenetic growth curves (West et al. 2002).
All ingredients to make stars like our Sun and planets like our Earth are present in the dense (~100,000 H2 molecules per cc) and cold (~ 10 K) interstellar clouds. In these "stellar-system precursors" an active chemistry is already at work, as demonstrated by the presence of a rich variety of organic molecules in the gas phase and icy mantles encapsulating the sub-micrometer dust grains, the building blocks of planets. Here, I’ll present a journey from the earliest phases of star formation to protoplanetary disks, with links to our Solar System, highlighting the crucial role of astrochemistry as powerful diagnostic tool of the various steps present in the journey.
The earliest phases of star formation are observationally difficult to detect and physically characterize. Protostars form deeply embedded within dense envelopes of gas and dust, making the process opaque at most wavelengths. Thus, this regime was previously restricted to theoretical investigations and simulations lacking observational information. Taking advantage of powerful interferometers, the VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey in Orion revealed the highest spatial resolution images of ~300 protostars to date. In this talk I will focus on the youngest protostars known to date in the Orion Molecular Clouds that were originally identified by Herschel and classified as PACS Bright Red Sources (PBRS). I will discuss the properties of their disks, outflows, envelopes, and multiplicity. Four of the 19 PBRS have dense, irregular structures and are optically thick at 0.87-mm within the central ~100 AU, and have slow outflows (when detected). These four PBRS provide direct observations of the innermost opaque regions of collapse at the onset of protostar formation. Their densities imply freefall times of ~ 100 yr, which would make these observations extremely unlikely. Thus these sources must have either extreme dust properties, or be evolving on longer time-scales with additional support. Their Kelvin─Helmholtz times are of order several thousand years. The relative fraction of these sources imply lifetimes of ~ 6000 yr, in closer agreement with the Kelvin─Helmholtz time. In this case, rotational and/or magnetic support could be slowing the collapse. Finally, I'll discuss ongoing and future observational work to quantify rotation and B-fields in these extremely young protostars.
Population synthesis models of actively accreting super-massive black holes (or active galactic nuclei -- AGN) predict a large fraction that must grow behind dense, obscuring screens of gas and dust. Deep X-ray surveys are thought to have provided the most complete and unbiased samples of AGN, but there is strong observational evidence that a portion of the population of obscured AGN is being missed. In this talk, I will highlight results from my recent work on where we use a sample of AGN derived from the deepest X-ray survey to date, the Chandra 7Ms GOODS-South Survey, to investigate the nature of low flux X-ray sources. We maximize the diverse wavelength coverage of the GOODS-South field, and cross-match our objects with wavelengths from the Radio to the IR. We find the predicted column densities are on average an order of magnitude higher than the calculated column densities via X-ray detections for X-ray faint sources. We interpret our results as evidence of obscured AGN disguising as low-luminosity AGN via their X-ray luminosities. The discovery of these objects has deep implications for future X-ray surveys and X-ray AGN selection criteria. I will also touch on our current work of using this unique sample to probe long standing problems with popular SMBH-Galaxy Co-Evolution paradigms.
Magnetic fields are ubiquitous in the interstellar medium, and they certainly play an important role during the star formation process. However, their interplay with forces such as gravity and turbulence is a topic still under great debate, also due to the intrinsic difficulties of magnetic field observations. In this talk, I will present the polarimetric observations of the protostellar core IRAS15398 performed with the SOFIA/HAWC+ camera at 214 microns. IRAS15398 is a young class 0 object embedded in Lupus I, a cloud known to be a highly magnetised environment. In our new data, the B field appears ordered and aligned with the large-scale field of the cloud and with the outflow direction. The field lines, however, present a significant bend due to the gravitational pull. We estimate a magnetic field strength of B= 78 µG, which is expected to be accurate within a factor of two. The measured mass-to-flux parameter is λ = 0.95, indicating that the core is in a transcritical regime.
The evolution of galaxies is driven by complex physical mechanisms that may have internal or environmental origins. As time passes galaxies become less star-forming and tend to acquire elliptical or lenticular morphologies. We have embarked in a simultaneous study of morphology and star formation in cluster and field galaxies with the aim of investigating the relationships between star-formation quenching and morphological transformations as a function of environment. In this talk I shall present the results of a study conducted on galaxies in the CLASH and CANDELS surveys at redshifts 0.2 z 0.9. By dividing galaxies into star-forming and quiescent we find that quiescent ellipticals are more abundant in clusters than in the field. Regardless of the environment, we observe an increase in the fraction of quiescent disc galaxies at low redshifts, supporting the notion that star-formation quenching precedes morphological changes, at least at low stellar masses. Star-forming field galaxies are mostly late type discs, while in clusters they present a diverse morphological composition with a non-negligible fraction of star-forming ellipticals that is detected in low-redshift clusters. I will discuss the implications of our results in the general context of galaxy evolution.
Large scale simulations are a key pillar of modern research in many areas and require ever increasing computational resources. While the fundamental architecture of supercomputers saw little change for a long time, different novel architectures emerged in recent years on the way towards the exascale era. These include many-core processors such as the Intel Xeon Phi, FPGA accelerator cards, or GPUs for general purpose computing that can have thousands of cores per card. Until recently, each new architecture can require a separate, non-trivial rewrite of a simulation code. To circumvent this, a current goal in computational science is the creation of parallel programming paradigms for writing performance portable code: code that can run efficiently at high performance on many different supercomputer architectures. Kokkos is one example of a performance portable on-node parallel programming paradigm realized as a C++ template library. We combined Athena++, an existing radiation general relativity magnetohydrodynamics CPU code, with Kokkos into K-Athena to allow simulations to run efficiently on both CPUs and GPUs using a single codebase. I will introduce performance portability approaches, and present profiling and scaling results for multiple architecture including Intel Skylake CPUs, Intel Xeon Phis, and NVidia Volta V100 GPUs. K-Athena achieves >10^8 cell-updates/s on a single V100 for second-order double precision MHD, and a speedup of 30 up to 24,576 GPUs on Summit (compared to 172,032 CPU cores) reaching 1.94x10^12 total cell-updates/s at 76% parallel efficiency. Using a roofline analysis I will demonstrate that the overall performance is currently limited by DRAM bandwidth on both CPUs and GPUs. Based on the roofline analysis K-Athena achieves a performance portability metric of 83.1% across 5 CPU generation, 4 GPU generations, and Intel Xeon Phis. Finally, I will present the strategies we used for implementation and the challenges we encountered while attempting to achieve maximum performance on different platforms. This will support other research groups to straightforwardly adopt this approach to prepare their own methods and codes for the exascale era.
Because astronomical observations are ultimately limited in providing a complete picture of the exoplanetary census, a comprehensive understanding of planetary systems’ formation and evolution can deliver valuable insights into key physical and chemical properties that cannot be probed by remote sensing alone. In this talk, I will discuss how the inhomogeneous enrichment of forming planetary systems with short-lived radionuclides, namely Al-26 and Fe-60, in typical star-forming environments controls the interior evolution and volatile loss of planetesimals that accrete to form terrestrial planets. Their internal geophysical evolution sub-divides rocky exoplanetary systems into distinct populations: enriched systems with Solar-like or higher levels tend to form water-depleted planets, while not- or barely-enriched systems dominantly form ocean worlds, with water levels comparable to the icy moons in the outer Solar System. This suggests a direct link between the star-forming birth environment of planetary systems and the compositional make-up and long-term evolution of rocky planets that form in them: the system-to-system deviations in the abundance of short-lived radionuclides across young star-forming regions qualitatively distinguish planetary systems’ formation and evolution, and control the distribution and prevalence of terrestrial planets with Earth-like bulk compositions.
Magnetic fields are observed on virtually all astrophysical scales of the modern Universe, from planets and stars to galaxies and galaxy clusters. Observations of blazars suggest that even the intergalactic medium is permeated by magnetic fields. Such large-scale fields were most likely generated shortly after the Big Bang and therefore are a unique window into the physics of the very early Universe. In my seminar, I will review theoretical models of magnetogenesis and confront these with observational constraints. I will address the possible origin of magnetic fields in the very early Universe, during inflation and the cosmological phase transitions, as well as their pre-recombination evolution in decaying magnetohydrodynamical (MHD) turbulence. Finally, I will present results from high-resolution numerical simulations that show an efficient amplification of magnetic energy due to the so-called chiral anomaly, a standard model effect that necessarily leads to an extension of the MHD equations at high energies.
In this talk, I will give a brief overview of my research on how stellar dynamics and stellar evolution conspire to create the most interesting stars and planets. I will then discuss why you should, and how you can, visualize your N-body (and other) simulation data.
Mass profiles as the sum of luminous and dark matter of elliptical galaxies are very different, ranging from no dark matter at all until dark matter domination already at small radial distances. This seems to depend on the environment, the two extremes being isolated elliptical galaxies and galaxies at the centers of galaxy clusters. One would expect these structural differences to show up somehow in their luminosity profiles, but this cannot be seen in my sample galaxies. I shall argue that the modified Hubble-Reynolds law can be understood as a MONDian isothermal sphere and show this explicitly at the examples of NGC 1399 and M87. This is in sharp contrast to what is commonly believed to be known about the masses of these galaxies.
The recent discovery of gravitational waves has opened new horizons. Current and upcoming missions promise to shed light on black holes (BHs) of every size and neutron star (NS) physics. The astrophysical origin of these mergers is among the most puzzling open questions of our time. Two primary channels have been proposed to explain the observed population of merging BHs and NSs: field binary evolution and dynamical formation in a cluster environment. The LIGO-Virgo events have made possible to estimate rates, masses, eccentricities, and projected spins of merging compact objects for the fist time. However, neither field binary evolution nor dynamical formation can explain all of the above quantities. Observations show that about one fourth of massive stars is in triple systems, comprised of an inner binary orbited by a third companion. Despite being rarer than binaries, a large fraction of triples can merge as a result of the Kozai-Lidov mechanism, imposed on the inner binary by the field of the third companion. Within current uncertainties, triples can potentially account for most, if not all, of the observed events and future LIGO-Virgo sources. The triple scenario is definitively the third pathway to compact object mergers.