Astrophysics Journal Club


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Current Schedule of Speakers
Fall 2017

Speaker Article Link
September 15
JC intro astro-ph session, featuring pizza (sign up in the main office for $5 per person)
September 22
Terrence Trico
Magnetic Fields in Star Formation: Magnetohydrodynamics with SPH

Magnetic fields are an integral component of protostar formation. They are responsible for launching majestic jets and outflows, and tempering the rate and efficiency of star formation. However, simulating star formation is challenging for numerical methods, owing to the multi-physics nature of the problem and the wide range of dynamical length and time scales. Smoothed particle hydrodynamics (SPH) is particularly well-suited to tackle these requirements. In this talk, I will discuss the significant advancements that been made to include magnetohydrodynamics (MHD) into SPH, and, using the state of the art in MHD SPH methods, simulation results of star forming molecular clouds and jets & outflows during isolated protostar formation.
September 29
Chelsea Sharon Molecular Gas Excitation and Galaxy Evolution at z~2-3

Since I was asked to introduce myself to the department, I will provide a brief broad overview of the particular questions in galaxy evolution that I study. I will then focus on how feedback from active galactic nuclei (AGN) quench star formation ah high redshift. Direct evidence for AGN affecting the molecular ISM (the gas phase that fuels star formation) has largely been limited to detections of molecular outflows in low-redshift systems and extreme excitation regions which represent a tiny fraction of galaxies' total gas reservoir. At z~2-3, the peak epoch of star formation and AGN activity, molecular excitation studies of submillimeter-bright starburst galaxies and AGN host galaxies have offered indirect evidence AGNs' impact on their star-forming cold gas. I will present our recently-published VLA survey, which nearly doubles the number of detections of the ground-state CO transition in z~2-3 starbursts and AGN, that allows for better comparisons between the cold gas properties of these populations. I will conclude by describing a followup study that I plan to work on as part of the Herschel Fellowship which will (hopefully) more effectively identify AGN feedback in high-z galaxies.
October 6
Angus Mok Environmental Variations in the Atomic and Molecular Gas Radial Profiles of Nearby Spiral Galaxies

We present an analysis of the radial profiles of a sample of 43 HI-flux selected spiral galaxies from the Nearby Galaxies Legacy Survey (NGLS) with resolved James Clerk Maxwell Telescope (JCMT) CO(3-2) and/or Very Large Array (VLA) HI maps. Comparing the Virgo and non-Virgo populations, we confirm that the HI disks are truncated in the Virgo sample, even for these relatively HI-rich galaxies. On the other hand, the molecular gas distribution is enhanced for Virgo galaxies near their centres, resulting in higher H2-to-HI ratios and steeper radial profiles. This is likely due to the effects of moderate ram pressure stripping in the cluster environment, which would preferentially remove low density gas in the outskirts while enhancing molecular gas near the centre. Combined with H-alpha star formation rate maps, we find that the star formation efficiency (SFR/H2) is relatively constant with radius for both samples, but with Virgo galaxies having a lower star formation efficiency than non-Virgo galaxies. These results suggest that the environment of spiral galaxies can play a role in the formation of molecular gas and the star formation process.
October 13
October 20
Paolo Bianchini An intimate view of the internal kinematics of globular clusters

Globular clusters (GCs) were long believed to be simple, non-rotating, isotropic and spherical stellar systems with all of their stars formed approximately 13 Gyr ago. However, growing evidence is now showing a larger degree of complexity in their structure, stellar populations and internal dynamics, deeply challenging our understanding of their origin in the early epochs of galaxy formation.
I will show how the detailed study of their current internal kinematics can provide the ideal ''fossil record'' to unveil their formation and their subsequent dynamical evolution. In particular, exploiting the synergy between state-of-the-art kinematic observations and dynamical modeling, I will describe the kinematic effects connected to the presence of significant internal rotation, presence/absence of intermediate-mass black holes, and energy equipartition, highlighting the implications for GCs origin. This will be the starting point for exploiting at full power the comprehensive amount of data that will be delivered by the Gaia mission and in the approaching era of Extremely Large Telescopes.
October 27
Sarah McKenzie-Picot A terrestrial planet candidate in a temperate orbit around Proxima Centauri

At a distance of 1.295 parsecs, the red dwarf Proxima Centauri (α Centauri C, GL 551, HIP 70890 or simply Proxima) is the Sun's closest stellar neighbour and one of the best-studied low-mass stars. It has an effective temperature of only around 3,050 kelvin, a luminosity of 0.15 per cent of that of the Sun, a measured radius of 14 per cent of the radius of the Sun and a mass of about 12 per cent of the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is about 83 days and its quiescent activity levels and X-ray luminosity are comparable to those of the Sun. Here we report observations that reveal the presence of a small planet with a minimum mass of about 1.3 Earth masses orbiting Proxima with a period of approximately 11.2 days at a semi-major-axis distance of around 0.05 astronomical units. Its equilibrium temperature is within the range where water could be liquid on its surface.
Gandhali Joshi Conference Summary Talk

Mapping the Pathways of Galaxy Transformation Across Time and Space (August 2016)
November 3
astro-ph session View
November 10
Pizza There is a sign-up sheet in the main office. Please bring $5 to Sam by Thursday morning. View
Daniel Rahner How to Kill a Molecular Cloud: Simulations of Stellar Feedback with FLASH/FERVENT

Star clusters influence their birth environment in various ways but a full assessment of all feedback mechanisms in simulations is still missing. In this study we investigate the effect of ionizing and non-ionizing radiation, stellar winds, supernova feedback and radiation pressure originating from a star cluster on its parent molecular cloud. Employing the adaptive mesh refinement code FLASH 4 with the new radiative transfer module FERVENT we study an idealized set-up of a filament with a 1e5 Msol molecular cloud and a 1e4 Msol star cluster which is either located at the center of the cloud or offset towards the edge. Our results highlight that the location of the cluster has profound effects on the eventual fate of the cloud.
November 17
Nathan Brunetti The Detection Of A Hot Molecular Core In The Large Magellanic Cloud With ALMA

We report the first detection of a hot molecular core outside our Galaxy based on radio observations with ALMA toward a high-mass young stellar object (YSO) in a nearby low metallicity galaxy, the Large Magellanic Cloud (LMC). Molecular emission lines of CO, C17O, HCO+, H13CO+, H2CO, NO, SiO, H2CS, 33SO, 32SO2, 34SO2, and 33SO2 are detected from a compact region (∼0.1 pc) associated with a high-mass YSO, ST11. The temperature of molecular gas is estimated to be higher than 100 K based on rotation diagram analysis of SO2 and 34SO2 lines. The compact source size, warm gas temperature, high density, and rich molecular lines around a high-mass protostar suggest that ST11 is associated with a hot molecular core. We find that the molecular abundances of the LMC hot core are significantly different from those of Galactic hot cores. The abundances of CH3OH, H2CO, and HNCO are remarkably lower compared to Galactic hot cores by at least 1-3 orders of magnitude. We suggest that these abundances are characterized by the deficiency of molecules whose formation requires the hydrogenation of CO on grain surfaces. In contrast, NO shows a high abundance in ST11 despite the notably low abundance of nitrogen in the LMC. A multitude of SO2 and its isotopologue line detections in ST11 imply that SO2 can be a key molecular tracer of hot core chemistry in metal-poor environments. Furthermore, we find molecular outflows around the hot core, which is the second detection of an extragalactic protostellar out flow. In this paper, we discuss the physical and chemical characteristics of a hot molecular core in the low metallicity environment.
November 24
Ian Roberts Conference Summary

Highlights from "The Changing Face of Galaxies"
Ashley Bemis The Molecular Clouds Fueling A 1/5 Solar Metallicity Starburst

Using the Atacama Large Millimeter/submillimeter Array, we have made the first high spatial and spectral resolution observations of the molecular gas and dust in the prototypical blue compact dwarf galaxy II Zw 40. The 12CO(2-1) and 12CO(3-2) emission is clumpy and distributed throughout the central star-forming region. Only one of eight molecular clouds has associated star formation. The continuum spectral energy distribution is dominated by free-free and synchrotron; at 870 μm, only 50% of the emission is from dust. We derive a CO-to-H2 conversion factor using several methods, including a new method that uses simple photodissocation models and resolved CO line intensity measurements to derive a relationship that uniquely predicts α co for a given metallicity. We find that the CO-to-H2 conversion factor is 4-35 times that of the Milky Way (18.1-150.5 M⊙ (K km s-1 pc2)-1. The star formation efficiency of the molecular gas is at least 10 times higher than that found in normal spiral galaxies, which is likely due to the burst-dominated star formation history of II Zw 40 rather than an intrinsically higher efficiency. The molecular clouds within II Zw 40 resemble those in other strongly interacting systems like the Antennae: overall they have high size-linewidth coefficients and molecular gas surface densities. These properties appear to be due to the high molecular gas surface densities produced in this merging system rather than to increased external pressure. Overall, these results paint a picture of II Zw 40 as a complex, rapidly evolving system whose molecular gas properties are dominated by the large-scale gas shocks from its ongoing merger.
December 1
Alex Cridland Research Summary Talk

I know that we have all wondered: "What does an exoplanetary atmosphere smell like?" - No? - Maybe its just me... For journal club this week I will outline the research that I have been working on over the last 3 (or so) years as part of my Ph.D thesis. Broadly speaking, I will discuss the theoretical framework that Ralph, Matt, and I (with the help of a few other collaborators) have built for connecting the physical and chemical evolution of the gas and dust in protoplanetary disks to the accretion of material that eventually builds into a planet. Specifically, my work aims to predict the bulk chemical composition of exoplanetary atmospheres that results from our formation models. Interestingly, we think that observations of the emission / transmission spectra of exoplanetary atmospheres might tell us something about the formation history of the planet. I will discuss how the existence (or abundance) of particular chemical species in the atmosphere are sensitive to the radial position in the disk where the planet accreted its gas, as well as when the planet accreted its gas. These conclusions are obviously model dependent, which is why we have begun the process of developing an advanced disk model which incorporates complicated dust physics and the ionization of the gas. I will discuss some of the details of this disk model, and importantly how the treatment of the physics impacts our predictions of the abundance of the resulting bulk chemical composition of atmospheres. Finally I will shamelessly advertise my two papers that have been published / accepted as well as a third paper that Ralph and I are in process of preparing for publication.
December 8
Gwendolyn Eadie Inferring the Total Mass of the Milky Way Galaxy with Globular Cluster Kinematics

Although there is consensus that the Galaxy's mass is dominated by its dark matter halo, the exact value of the total mass of the Galaxy is poorly constrained. The underlying reason is that there are serious challenges associated with using the motions of tracer particles like globular clusters or halo stars to deduce the mass profile; the 3D velocity measurements are often incomplete and uncertain, and there are only a few GCs at large distances from the Galactic center. In this talk, I will explain a hierarchical Bayesian method we have developed to help deal with these challenges, and will show our latest results for the mass of the Milky Way.
Fraser Evans Quenching of Star FOrmation in Sloan Digital Sky Survey Groups: Centrals, Satellites, and Galactic Conformity

We re-examine the fraction of low-redshift Sloan Digital Sky Survey satellites and centrals in which star formation has been quenched, using the environment quenching efficiency formalism that separates out the dependence of stellar mass. We show that the centrals of the groups containing the satellites are responding to the environment in the same way as their satellites (at least for stellar masses above 1010.3M), and that the well-known differences between satellites and the general set of centrals arise because the latter are overwhelmingly dominated by isolated galaxies. The widespread concept of “satellite quenching” as the cause of environmental effects in the galaxy population can therefore be generalized to “group quenching.” We then explore the dependence of the quenching efficiency of satellites on overdensity, group-centric distance, halo mass, the stellar mass of the satellite, and the stellar mass and specific star formation rate (sSFR) of its central, trying to isolate the effect of these often interdependent variables. We emphasize the importance of the central sSFR in the quenching efficiency of the associated satellites, and develop the meaning of this “galactic conformity” effect in a probabilistic description of the quenching of galaxies. We show that conformity is strong, and that it varies strongly across parameter space. Several arguments then suggest that environmental quenching and mass quenching may be different manifestations of the same underlying process. The marked difference in the apparent mass dependencies of environment quenching and mass quenching which produces distinctive signatures in the mass functions of centrals and satellites will arise naturally, since, for satellites at least, the distributions of the environmental variables that we investigate in this work are essentially independent of the stellar mass of the satellite.
January 12
Astro-ph Session TBA View
January 19
Ray Carlberg Assembling globular clusters into a galactic halo

The observed tidal tails from halo globular clusters in the Milky Way are a factor of three or more shorter than expected from a star cluster orbiting for 10 Gyr. The discrepancy is likely the result of the assumption that the clusters have been orbiting in a stationary galactic halo for a Hubble time. As a step towards more realistic stream histories, a toy model that combines an idealized merger model with a simplified model of the internal collisional relaxation of individual star clusters is developed. The star streams from the evolving clusters in the time variable potentials have properties that vary with distance from their parent cluster. On the average, the density along the stream declines with distance and the velocity dispersion increases with distance. As a consequence, the older parts of the streams are relatively less visible, meaning that the streams visible in current data will normally be a fraction of the entire length of the stream. Nevertheless, the high surface density segment of the stellar streams created from the evaporation of the more massive globular clusters should be detectable, provided that they are not obscured and within about 30~kpc. An implication is that the sum of numbers of globular clusters and stellar streams should be equal to the total number of accreted globular clusters. The Pan-STARRS1 halo volume is used to compare the numbers of halo streams and globular clusters.
January 26
Ralph Pudritz The Origin of Stars and Their Planetary Systems

JWST presents us with a unique opportunity to probe one of the deepest questions in astrophysics - namely - how do stars and their associated planetary systems form? Is the formation mechanism for stars universal or does it depend upon their mass or environment? How is star and planet formation linked to the formation and properties of protostellar disks? One of the central insights gleaned over the last decade is that star formation occurs in cores within filaments probably by gravitational instability. They also form preferentially in clusters that are born in particularly rich filamentary environments. How is cluster formation driven and how it is connected to these filamentary flows? Do feedback processes ultimately limit cluster masses? On smaller scales, to what extent do disk properties and masses depend upon cluster formation and evolution? This review will span several decades of physical scales, ranging from processes operating in clusters/filaments down to the formation of protostars and protostellar disks. and the first stages of planet formation within them.
February 2
Quang Nguyen-Luong
Star formation laws of mini-starburst complexes in the Milky Way and link to extragalactic star formation

Using the nearly-all sky CO CfA survey, we conduct a throughout search of massive molecular complexes that host most of star formation activities in the Milky Way. We construct the scaling laws and the star formation laws for a large selection of sources, from cores to galaxies, and found that these laws are not universal but exhibit variations at specific scales. We propose to use the Schmidt-Kennicutt diagram to distinguish the starburst from the normal star-forming structures by applying a gas density and star formation threshold. We define mini-starburst complexes have enhanced SFR, probably caused by dynamic events such as radiation pressure, colliding flows, or spiral arm gravitational instability. These mini-starburst MCCs have the potential of serving as a local scale for comparing with extra-galactic starburst components, especially in the era of ALMA.
February 9
Samantha Benincasa The PdBI Arcsecond Whirlpool Survey (PAWS). The Role of Spiral Arms in Cloud and Star Formation

The process that leads to the formation of the bright star forming sites observed along prominent spiral arms remains elusive. We present results of a multi-wavelength study of a spiral arm segment in the nearby grand-design spiral galaxy M51 that belongs to a spiral density wave and exhibits nine gas spurs. The combined observations of the(ionized, atomic, molecular, dusty) interstellar medium (ISM) with star formation tracers (HII regions, young <10Myr stellar clusters) suggest (1) no variation in giant molecular cloud (GMC) properties between arm and gas spurs, (2) gas spurs and extinction feathers arising from the same structure with a close spatial relation between gas spurs and ongoing/recent star formation (despite higher gas surface densities in the spiral arm), (3) no trend in star formation age either along the arm or along a spur, (4) evidence for strong star formation feedback in gas spurs: (5) tentative evidence for star formation triggered by stellar feedback for one spur, and (6) GMC associations (GMAs) being no special entities but the result of blending of gas arm/spur cross-sections in lower resolution observations. We conclude that there is no evidence for a coherent star formation onset mechanism that can be solely associated to the presence of the spiral density wave. This suggests that other (more localized) mechanisms are important to delay star formation such that it occurs in spurs. The evidence of star formation proceeding over several million years within individual spurs implies that the mechanism that leads to star formation acts or is sustained over a longer time-scale.
Matthew Alessi Initial mass function of planetesimals formed by the streaming instability

The streaming instability is a mechanism to concentrate solid particles into overdense filaments that undergo gravitational collapse and form planetesimals. However, it remains unclear how the initial mass function of these planetesimals depends on the box dimensions of numerical simulation. To resolve this, we perform simulations of planetesimal formation with the largest box dimensions to date, allowing planetesimals to form simultaneously in multiple filaments that can only emerge within such large simulation boxes. In our simulations, planetesimals with sizes between 8- km and several hundred kilometers form. We find that a power law with a rather shallow exponential cutoff at the high-mass end represents the cumulative birth mass function better than an integrated power law. The steepness of the exponential cutoff is largely independent of box dimensions and resolution, while the exponent of the power law is not constrained at the resolutions we employ. Moreoever, we find that the characteristic mass scale of the exponential cutoff correlates with the mass budget in each filament. Together with previous studies of high-resolution simulations with small box domains, our results therefore imply that the cumulative birth mass function of planetesimals is consistent with an exponentially tapered power law with a power-law exponent of approximately -1.6 and a steepness of the exponential cutoff in the range of 0.3-0.4.
February 16
Bill Harris A 17-billion-solar-mass black hole in a group galaxy with a diffuse core

Quasars are associated with and powered by the accretion of material onto massive black holes; the detection of highly luminous quasars with redshifts greater than z=6 suggests that black holes of up to ten billion solar masses already existed 13 billion years ago. Two possible present-day 'dormant' descendants of this population of 'active' black holes have been found in the galaxies NGC 3842 and NGC 4889 at the centres of the Leo and Coma galaxy clusters, which together form the central region of the Great Wall-the largest local structure of galaxies. The most luminous quasars, however, are not confined to such high-density regions of the early Universe; yet dormant black holes of this high mass have not yet been found outside of modern-day rich clusters. Here we report observations of the stellar velocity distribution in the galaxy NGC 1600-a relatively isolated elliptical galaxy near the centre of a galaxy group at a distance of 64 megaparsecs from Earth. We use orbit superposition models to determine that the black hole at the centre of NGC 1600 has a mass of 17 billion solar masses. The spatial distribution of stars near the centre of NGC 1600 is rather diffuse. We find that the region of depleted stellar density in the cores of massive elliptical galaxies extends over the same radius as the gravitational sphere of influence of the central black holes, and interpret this as the dynamical imprint of the black holes.
February 23
March 2
Elizabeth Tasker (JAXA) Aliens are not everywhere: writing for the media and busting bad science

Science communication is often undervalued as an unimportant extra duty for professional scientists. However, in truth, our funding is dependent on being able to explain our work to a general audience. This applies to both grant proposals where the assessment committee may consist of a wide range of expertise, and to the availability of such grants in the first place, which hinges on what the government wishes to support -- a factor heavily dependent on the voting public's interest. A strong media presence for a research group also boosts reputation to encourage strong students and international faculty to apply for positions. Taking the time to explain your work to a non-specialised audience is therefore key to personal career advancement and the success of a field. (The "publish or perish" phrase can also accompany "Communicate or Collapse" -- just in case you were sleeping too well at night). This talk will run-over my own experiences with writing for the media, including my continual fight with accurate exoplanet reporting (where I'm largely fighting NASA -- they may be big, I may be along, but I'm really really REALLY annoyed and I'm gonna win.)
March 9
Anna Sippel
The impact of black holes on star cluster evolution

In the recent past, globular clusters have been suggested as an ideal environment to form stellar mass black hole binaries, which can tighten via interactions with other stars and evolve to become candidates for merging. By using a direct N-body model of a globular cluster I will illustrate this process and show that it doesn't go unnoticed for the cluster and its evolution.
March 16
Ben Pearce Origin of the RNA World: The Fate of Nucleobases in Warm Little Ponds
To be given at the Astrobiology Science Conference 2017, April 24-28th

Prior to the origin of life, the building blocks of RNA had to form and assemble in favourable environments on the early Earth. Periodic wet-and-dry warm little ponds (WLPs) may have been ideal environments to which meteorites and interplanetary dust particles (IDPs) delivered organics such as nucleobases: the characteristic molecules in RNA. We investigate the delivery of nucleobases to WLPs, and the survival and evolution of these nucleobases in such environments. Our results suggest that a negligible abundance of nucleobases were delivered to the early Earth by IDPs which survived for subsequent synthesis into nucleotides and RNA. Meteorites however delivered a dominant abundance of nucleobases, which could have accumulated to ppb-ppm level concentrations in WLPs for up to a few years. Since nucleobases don't survive for long in WLPs, nucleotide synthesis was probably quick. Furthermore, since nucleotides aren't large enough to avoid being lost to seepage, nucleotide polymerization into RNA was also probably fast, occurring in just one to a few wet-dry cycles. In order for these warm little ponds to be the birthplace of the RNA world, temperatures would have to be clement ~5-35°C to allow nucleotides to survive for evolutionary timescales. Finally, it is unlikely that the late heavy bombardment played a role in the emergence of the RNA world.
March 23
Laura Parker An intermediate-mass black hole in the centre of the globular cluster 47 Tucanae

Intermediate-mass black holes should help us to understand the evolutionary connection between stellar-mass and super-massive black holes. However, the existence of intermediate-mass black holes is still uncertain, and their formation process is therefore unknown. It has long been suspected that black holes with masses 100 to 10,000 times that of the Sun should form and reside in dense stellar systems. Therefore, dedicated observational campaigns have targeted globular clusters for many decades, searching for signatures of these elusive objects. All candidate signatures appear radio-dim and do not have the X-ray to radio flux ratios required for accreting black holes. Based on the lack of an electromagnetic counterpart, upper limits of 2,060 and 470 solar masses have been placed on the mass of a putative black hole in 47 Tucanae (NGC 104) from radio and X-ray observations, respectively. Here we show there is evidence for a central black hole in 47 Tucanae with a mass of solar masses when the dynamical state of the globular cluster is probed with pulsars. The existence of an intermediate-mass black hole in the centre of one of the densest clusters with no detectable electromagnetic counterpart suggests that the black hole is not accreting at a sufficient rate to make it electromagnetically bright and therefore, contrary to expectations, is gas-starved. This intermediate-mass black hole might be a member of an electromagnetically invisible population of black holes that grow into supermassive black holes in galaxies.
March 27
Dan Taranu
Dissecting Spiral Galaxies With Integral Field Kinematics

Measuring fundamental physical properties (including size, mass, and angular momentum) of thousands of nearby (z<0.1) spiral galaxies is rapidly becoming feasible thanks to integral field spectroscopic surveys like SAMI and MaNGa. However, most spirals consist of several distinct components - disk(s), bulge(s) and a dark matter halo - so unveiling their structure requires careful analysis. I will present a new method and code (MagRite) for decomposition of disk galaxies using integral field kinematics. MagRite generates self-consistent equilibrium galaxy models with a bulge, disk and halo, allowing for Bayesian fitting of galaxy photometry and kinematics. I will show examples using data from the SAMI galaxy survey and deep imaging from KiDS, and outline how follow-up simulations can predict star formation rates and help tune stellar feedback models. Lastly, I will show preliminary comparisons to predictions from simulations, including the cutting-edge Romulus cosmological volume (Tremmel+17).
March 30
Ana Gomez de Castro
(Compultense University of Madrid)
The interstellar medium and star formation; instights from ultraviolet wavelengths

Ultraviolet wavelengths cannot be used to penetrate in the deepest parts of molecular clouds but they are extremely sensitive to low gas columns and to the presence of large aromatic molecules in the ISM. In this talk, I will show some examples of the work being done and the potential of efficient ultraviolet instrumentation to understand the star formation process. I will also review the current status of facilities and the projects to come, including proposals like EUVO or LUVOIR.
April 6
James Wadsley Nonuniversal Star Formation in Turbulent ISM

We present a study of a star formation prescription in which star formation efficiency (SFE) depends on local gas density and turbulent velocity dispersion, as suggested by direct simulations of SF in turbulent giant molecular clouds (GMCs). We test the model using a simulation of an isolated Milky-Way-sized galaxy with a self-consistent treatment of turbulence on unresolved scales. We show that this prescription predicts a wide variation of local SFE per free-fall time, ~0.1%-10%, and gas depletion time ~0.1-10 Gyr. In addition, it predicts an deffective density threshold for star formation due to suppression of the SFE in warm diffuse gas stabilized by thermal pressure. We show that the model predicts star formation rates (SFRs) in agreement with observations from scales of individual star-forming regions to the kiloparsec scales. This agreement is nontrivial, as the model was not tuned in any way and the predicted SFRs on all scales are determined by the distribution of the GMC-scale densities and turbulent velocities in the cold gas within the galaxy, which is shaped by galactic dynamics. This broad agreement of the star formation prescription calibrated int he GMC-scale simulations with observations both gives credence to such simulations and promises to put star formation modeling in galaxy formation simulations on a much firmer theoretical footing.
April 13
Corey Howard Dynamical Ejections of Stars due to an Accelerating Gas Filament

Observations of the Orion A integral shaped filament (ISF) have shown indications of an oscillatory motion of the gas filament. This evidence is based on both the wavelike morphology of the filament as well as the kinematics of the gas and stars, where the characteristic velocities of the stars require a dynamical heating mechanism. As proposed by Stutz & Gould (2016), such a heating mechanism (the ”Slingshot”) may be the result of an oscillating gas filament in a gas-dominated (as opposed to stellarmass dominated) system. Here we test this hypothesis with the first stellar-dynamical simulations in which the stars are subjected to the influence of an oscillating cylindrical potential. The accelerating, cylindrical background potential is populated with a narrow distribution of stars. By coupling the potential to N-body dynamics, we are able to measure the influence of the potential on the stellar distribution. The simulations provide evidence that the slingshot mechanism can successfully reproduce several stringent observational constraints. These include the stellar spread (both in projected position and in velocity) around the filament, the symmetry in these distributions, and a bulk motion of the stars with respect to the filament. Using simple considerations we show that star-star interactions are incapable of reproducing these spreads on their own when properly accounting for the gas potential. Thus, properly accounting for the gas potential is essential for understanding the dynamical evolution of star forming filamentary systems in the era of GAIA.
Gwen Eadie Knowledge Transfer from Calculus to Physics

This paper reviews the Scholarship of Teaching and Learning literature on the subject of undergraduate knowledge transfer from calculus to physics. I discuss studies of calculus transfer to physics under common theoretical frameworks found in the literature, and summarize why transfer between these subjects is difficult for undergraduate students. A recurring theme in the literature is that notational differences between calculus and physics impede transfer. Why notation matters can be explained by theories from cognitive science.
This paper also presents data from the U15 Canadian Universities on calculus and physics course requirements for first-year physics majors. From this data, I argue why the structure of these course requirements may be hindering knowledge transfer from calculus to physics. While some studies advocate that calculus be taken before any physics courses, other studies suggest that calculus and physics courses be taken concurrently. I summarize arguments from both sides, and provide my own opinion on the topic. I suggest that integration of first-year calculus and physics concepts could be improved by instructors in both disciplines, and that there should be more focus, both in instruction and research, on the transfer of calculus to first-year physics in Canada.
April 20
Jasper Grond The Hydrangea simulations: galaxy formation in and around massive clusters

We introduce the Hydrangea simulations, a suite of 24 cosmological hydrodynamic zoom-in simulations of massive galaxy clusters (M_200c = 10^14-10^15 M_Sun) with baryon particle masses of ~10^6 M_Sun. Designed to study the impact of the cluster environment on galaxy formation, they are a key part of the `Cluster-EAGLE' project (Barnes et al. 2017). They use a galaxy formation model developed for the EAGLE project, which has been shown to yield both realistic field galaxies and hot gas fractions of galaxy groups consistent with observations. The total stellar mass content of the simulated clusters agrees with observations, but central cluster galaxies are too massive, by up to 0.6 dex. Passive satellite fractions are higher than in the field, and at stellar masses Mstar > 10^10 M_Sun this environmental effect is quantitatively consistent with observations. The predicted satellite stellar mass function matches data from local cluster surveys. Normalized to total mass, there are fewer low-mass (Mstar < 10^10 M_Sun) galaxies within the virial radius of clusters than in the field, primarily due to star formation quenching. Conversely, the simulations predict an overabundance of massive galaxies in clusters compared to the field that persists to their far outskirts (> 5r_200c). This is caused by a significantly increased stellar mass fraction of (sub-)haloes in the cluster environment, by up to ~0.3 dex even well beyond r_200c. Haloes near clusters are also more concentrated than equally massive field haloes, but these two effects are largely uncorrelated.
Pizza and JC feedback/planning View
April 27
Joey Rucska Self-induced dust traps: overcoming planet formation barriers

Planet formation is thought to occur in discs around young stars by the aggregation of small dust grains into much larger objects. The growth from grains to pebbles and from planetesimals to planets is now fairly well understood. The intermediate stage has however been found to be hindered by the radial-drift and fragmentation barriers. We identify a powerful mechanism in which dust overcomes both barriers. Its key ingredients are i) backreaction from the dust onto the gas, ii) grain growth and fragmentation, and iii) large-scale gradients. The pile-up of growing and fragmenting grains modifies the gas structure on large scales and triggers the formation of pressure maxima, in which particles are trapped. We show that these self-induced dust traps are robust: they develop for a wide range of disc structures, fragmentation thresholds and initial dust-to-gas ratios. They are favored locations for pebbles to grow into planetesimals, thus opening new paths towards the formation of planets.
Melanie Demers TBD