Astrophysics Journal Club


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    McMaster University

Current Schedule of Speakers
Fall 2018

Speaker Article Link
September 21
Samantha Benincasa From molecules to young stellar clusters: the star formation cycle across the disk of M 33

We study the association between giant molecular clouds (GMCs) and young stellar cluster candidates (YSCCs) to shed light on the time evolution of local star formation episodes in the nearby galaxy M 33. Methods. The CO (J = 2−1) IRAM all-disk survey was used to identify and classify 566 GMCs with masses between 2 × 104 and 2 × 106 M across the whole star-forming disk of M 33. In the same area, there are 630 YSCCs that we identified using Spitzer-24 µm data. Some YSCCs are embedded star-forming sites, while the majority have GALEX-UV and Hα counterparts with estimated cluster masses and ages. Results. The GMC classes correspond to different cloud evolutionary stages: inactive clouds are 32% of the total and classified clouds with embedded and exposed star formation are 16% and 52% of the total, respectively. Across the regular southern spiral arm, inactive clouds are preferentially located in the inner part of the arm, possibly suggesting a triggering of star formation as the cloud crosses the arm. The spatial correlation between YSCCs and GMCs is extremely strong, with a typical separation of 17 pc. This is less than half the CO (2–1) beam size and illustrates the remarkable physical link between the two populations. GMCs and YSCCs follow the HI filaments, except in the outermost regions, where the survey finds fewer GMCs than YSCCs, which is most likely due to undetected clouds with low CO luminosity. The distribution of the non-embedded YSCC ages peaks around 5 Myr, with only a few being as old as 8–10 Myr. These age estimates together with the number of GMCs in the various evolutionary stages lead us to conclude that 14 Myr is the typical lifetime of a GMC in M 33 prior to cloud dispersal. The inactive and embedded phases are short, lasting about 4 and 2 Myr, respectively. This underlines that embedded YSCCs rapidly break out from the clouds and become partially visible in Hα or UV long before cloud dispersal.
September 28
Matthew Alessi How cores grow by pebble accretion

Planet formation by pebble accretion is an alternative to classical core accretion. In this scenario, planets grow by the accretion of cm-to-m-sized pebbles instead of km-sized planetesimals. One of the main differences with classical core accretion is the increased thermal ablation rate experienced by pebbles. This provides early enrichment to the planet’s envelope, which influences its subsequent evolution and changes the process of core growth. Aims. To describe and compute core growth in the pebble accretion model. We aim to predict core masses and compositions that can form by pebble accretion and compare them to the case of planetesimals. Methods. We have written a code containing both an impact and a planet evolution model to simulate the early growth of a proto-planet self-consistently. The region where high-Z material (in our case SiO2) can exist in vapor form is determined by the temperaturedependent vapor pressure. We include enrichment effects by locally modifying the mean molecular weight of the envelope and determine when direct core growth of the planet terminates. Results. We have identified three phases of core growth in pebble accretion. In the first phase (Mcore < 0.23–0.39 M⊕ ), pebbles impact the core without significant ablation. During the second phase (Mcore < 0.5 M⊕ ), ablation becomes increasingly severe. A layer of high-Z vapor starts to form around the core that absorbs a small fraction of the ablated mass. The rest of the material either rains out to the core or mixes outwards instead, slowing core growth. In the third phase (Mcore > 0.5 M⊕ ), the high-Z inner region expands outwards, absorbing an increasing fraction of the ablated material as vapor. Rainout ends before the core mass reaches 0.6 M⊕ , terminating direct core growth. Conclusions. Our results indicate that pebble accretion can directly form rocky cores up to only 0.6 M⊕ , and is unable to form icy cores. This result contrasts classical core accretion models, which can directly form massive cores of both rocky and icy compositions. Subsequent core growth can proceed indirectly when the planet cools, provided it is able to retain its high-Z material.
October 5
Ryan Chown Mapping the millimeter-wave sky with South Pole Telescope and Planck data

Measurements of fluctuations in the cosmic microwave background (CMB) have revolutionized the field of observational cosmology. Although the CMB dominates the millimeter-wave sky, emission from dusty star-forming galaxies, AGN, dust in our Galaxy, and the thermal Sunyaev-Zel’dovich effect are all significant at millimeter wavelengths. High-sensitivity mm-wave maps from space-based, ground-based, and balloon-borne telescopes have been used to test cosmological models, to study foregrounds, and to study systematics such as CMB lensing. The need for mm-wave maps with high sensitivity over a wide range of angular scales (particularly for lensing analyses) motivated my MSc work, which focused on combining temperature maps from the South Pole Telescope (covering roughly 2500 square degrees) with maps from the Planck Satellite in a nearly optimal way based on anisotropic filtering and noise. I will talk about the components of the mm-wave sky and the combining algorithm. I will show results of the 2500 deg^2 combining analysis, a CMB lensing analysis using one of the combined maps, as well as results of a separate analysis where we combined SPT and Planck data covering the Magellanic Clouds.
October 12
October 19
Samantha Benincasa Practice for UW Astro Lunch Talk & UVic Astro Seminar

A tale of two clump masses – a different approach to studying clump formation in simulations

In the Milky Way now, star formation proceeds almost exclusively in Molecular Clouds (MCs) under 106 Msun . This is not necessarily true at higher redshifts. The CANDELS survey has shown us a fraction of galaxies host massive kpc-scale stellar clumps (Guo et. al 2015). This suggests that star formation may proceed in objects at least 100 times more massive than present day MCs. However, lensing source reconstruction, which has enhanced resolution, has shown us that star formation can still proceed in MC-scale objects at these redshifts (e.g. Johnson et al. 2017). A natural way to piece two opposing datasets together would be to turn to simulations of clump formation. However, this produces a similar dichotomy where both large (e.g. Inoue et al. 2016) and small (Tamburello et al. 2016, Behrendt et al. 2016) objects can be formed. With a wealth of conflicting observational and theoretical data, we find ourselves at a crossroad. We propose a new approach to studying clump formation in simulations. We seed clump formation events in isothermal simulations of galaxy disks. In this way, we can explore a large parameter space in both perturbation size and strength. We can find a space of likely clump masses for a given galaxy. We propose this method as a way to stitch together the wealth of data we now have.
October 26
Alison Sills Discussion of the recent LIGO results
November 2
Fraser Evans The Relationship Between Dust and Star Formation: A Spatially Resolved Perspective in the SAMI Survey

One way to understand the physical mechanisms driving galaxy evolution is to study how different galaxy properties correlate. There is now a deep body of literature describing these correspondences, however, due to observational constraints and the need for large sample sizes, these studies have primarily looked at integrated galaxy properties averaged across entire galaxies. Studying the spatial variation of properties within a galaxy itself and the correlations between galaxy properties on the sub-galaxy scale will allow for a more complete understanding of the physics driving galaxy evolution. With the advent of modern, multiplexed integral field spectroscopy (IFS) surveys such as SAMI, we are finally able to marry the wealth of information provided by IFS observations with the statistical power of large samples. In this talk I will describe the research I conducted in Leiden over the summer, using the SAMI survey to study the relationships between dust, stellar mass and star formation (and, if time permits, gas-phase metallicity) on spatially-resolved scales. Our preliminary results show that correlations seen between dust, stellar mass and star formation in integrated data do not necessarily hold on spatial scales.
Ian Roberts Connecting optical and X-ray tracers of galaxy cluster relaxation

Substantial effort in recent years has been devoted to determining the ideal proxy for quantifying the morphology of the hot intracluster medium in clusters of galaxies -- a proxy for cluster relaxation state. These proxies, based on X-ray emission, typically require expensive, high-quality X-ray observations making them difficult to apply to large surveys of groups and clusters. Here, we compare optical relaxation proxies with X-ray asymmetries and centroid shifts for a sample of SDSS clusters matched to high-quality, archival X-ray data from the Chandra and XMM-Newton X-ray observatories. The three optical relaxation measures that we consider are: the shape of the member-galaxy projected velocity distribution – as measured by the Anderson-Darling statistic, the stellar mass gap between the most-massive and second-most-massive cluster galaxy, and the offset between the position of the most-massive galaxy (MMG) and the luminosity-weighted cluster centre. The Anderson-Darling statistic and stellar mass gap show significant correlations with X-ray relaxation proxies, with the Anderson-Darling statistic being the stronger correlator. Conversely, we find no evidence for a correlation between X-ray morphologies and the MMG offset. High-mass clusters are systematically more disturbed than low-mass clusters, as measured by both X-ray and optical techniques. Finally, when considering the discrete dichotomy of Gaussian and non-Gaussian clusters (as measured by the Anderson-Darling test), we show that the probability of being a non-Gaussian cluster correlates with X-ray asymmetry. These results confirm that the shape of the member galaxy velocity distribution is a useful proxy for cluster relaxation, which can then be applied to large redshift surveys without extensive X-ray coverage.
November 9
Ashley Bemis Inefficient jet-induced star formation in Centaurus A: High resolution ALMA observations of the northern filaments

NGC 5128 is one of the best targets to study AGN-feedback in the local Universe. At 13.5 kpc from the galaxy, optical filaments with recent star formation lie along the radio-jet direction. It is a testbed region for positive feedback (jet-induced star formation). APEX revealed strong CO emission in star-forming regions but also in regions with no detected tracers of star formation. When observed, star formation appears to be inefficient compared to the Kennicutt-Schmidt relation. We used ALMA to map the 12CO(1-0) emission all along the filaments at 1.3"~ 23.8 pc resolution. The CO emission is clumpy and distributed in two main structures: (i) the Horseshoe complex, outside the HI cloud, where gas is mostly excited by shocks and no star formation is observed; (ii) the Vertical filament, at the edge of the HI shell, which is a region of moderate star formation. We identified 140 molecular clouds. A statistical study reveals that they have very similar physical properties that in the inner Milky Way. However, the range of radius available with the present observations does not enable to investigate whether the clouds follow the Larson relation or not. The large virial parameter of the clouds suggests that gravity is not dominant. Finally, the total energy injection in the filaments is of the same order as in the inner part of the Milky Way. The strong CO emission detected in the filaments is an indication that the energy injected by the jet acts positively in the formation of dense molecular gas. The relatively high virial parameter of the molecular clouds suggests that the injected kinetic energy is too strong for star formation to be efficient. This is particularly the case in the Horseshoe complex where the virial parameter is the largest and where strong CO is detected with no associated star formation. This is the first evidence of inefficient AGN positive feedback.
November 16
Colin McNally Wind driven protoplanetary discs and how planets move in them

Protoplanetary discs are believed to accrete onto their central T Tauri star because of magnetic stresses. At the same time, planets form in these discs and are forced to radially migrate when subject to unbalanced torques from the disc gas, contributing to the final architecture of the planetary system. Simulations indicate that the non-ideal MHD of the very poorly ionised disc plays an important roles in its evolution. In the presence of a vertical magnetic field, the disc remains laminar between 1–5 au, and a magnetocentrifugal disc wind forms that provides an important mechanism for removing angular momentum. Although not unstable to the magnetorotational instability, the laminar dead zone can be threaded by large laminar magnetic fields which exert a accretion driving stress at the midplane. Motivated by these models, we have re-examined low mass planet migration in such an environment. Our results show that planet migration torques in a inviscid disc threaded with a magnetic field are phenomenologically different from those previously considered in a traditional viscous accretion disc. We have predicted, and then experimentally demonstrated the existence of four regimes of low-mass 'Type-I' migration, including a rapid outward migration. In addition, we demonstrate that midplane magnetic stresses induce a bifurcation dependent on the disc surface density leading to torque and migration reversal.
November 23
Toby Brown Galaxy Evolution and the Gas Cycle

The importance of cold gas in the picture of galaxy evolution is well known, as is its role as a probe of recent environmental effects on galaxies. However, sensitivity limitations mean the extent to which environment impacts the gas-star formation cycle of galaxies remains unclear. With this talk I will show how we take full advantage of the powerful HI spectral stacking technique to overcome this obstacle and quantify the gas content for the entire gas-poor to -rich regime. This was accomplished using the largest sample of HI and multi-wavelength information available (28,000 galaxies), selected according to stellar mass (M*>10^9 Msol) and redshift (0.02 <= z <= 0.05). I will present HI scaling relations with key structural, star formation and environmental metrics, using stacking to provide strong observational evidence of significant and systematic environment driven gas suppression across the group regime, well before galaxies enter the cluster. Furthermore, I will show that gas depletion is more closely associated to halo mass than local density and cannot be reproduced by starvation of the gas supply alone, invoking systematic ram-pressure stripping of the cold gas to explain this. Finally, I will show results highlighting the role of HI in regulating the correlation between stellar mass, star formation and gas-phase metal abundance known as the “fundamental" mass-metallicity relation.
November 30
Ben Pearce The Road to Protocells: Meteoritic Abundances of Fatty Acids and Potential Reaction Pathways

The origin of fatty acids on the prebiotic Earth is important as they likely formed the encapsulating membranes of the first protocells. Carbon-rich meteorites (i.e., carbonaceous chondrites) such as Murchison and Tagish Lake are well known to contain these molecules, and their delivery to the early planet by intense early meteorite bombardments constitutes a key prebiotic source. We collect the fatty acid abundances measured in various carbonaceous chondrites from the literature and analyze them for patterns and correlations. Fatty acids in meteorites include straight-chain and branchedchain monocarboxylic and dicarboxylic acids up to 12 carbons in length|fatty acids with at least 8 carbons are required to form vesicles, and modern cell membranes employ lipids with ∼12–20 carbons. To understand the origin of meteoritic fatty acids, we search the literature for abiotic fatty acid reaction pathways and create a candidate list of reactions that potentially produced these fatty acids in meteorite parent bodies. Straight-chain monocarboxylic acids (SCMA) are the dominant fatty acids in meteorites, followed by branched-chain monocarboxylic acids (BCMA). SCMA are most abundant in CM2 and Tagish Lake (ungrouped) meteorites, ranging on average from 10^2 to 4×10^5 ppb, and 10^4 to 5×10^6 ppb, respectively. In CM, CV, and Tagish Lake meteorites, SCMA abundances decrease with increasing carbon chain length. Conversely, SCMA abundances in CR meteorites peak at 5 and 6 carbons in length, and decrease on either side of this peak. We find that Fischer-Tropsch-type synthesis was likely the dominant reaction pathway producing SCMA and BCMA in meteorite parent bodies. Finally, because CO was a likely reactant producing fatty acids and other biomolecules (e.g., nucleobases) in carbonaceous chondrite parent bodies, we suggest that these bodies may have originated from beyond the CO ice line in the protoplanetary disk, which was located between ∼20–90 AU.
Nathan Brunetti The shapes of column density PDFs. The importance of the last closed contour

The probability distribution function of column density (PDF) has become the tool of choice for cloud structure analysis and star formation studies. Its simplicity is attractive, and the PDF could offer access to cloud physical parameters otherwise difficult to measure, but there has been some confusion in the literature on the definition of its completeness limit and shape at the low column density end. In this letter we use the natural definition of the completeness limit of a column density PDF, the last closed column density contour inside a surveyed region, and apply it to a set of large-scale maps of nearby molecular clouds. We conclude that there is no observational evidence for log-normal PDFs in these objects. We find that all studied molecular clouds have PDFs well described by power laws, including the diffuse cloud Polaris. Our results call for a new physical interpretation of the shape of the column density PDFs. We find that the slope of a cloud PDF is invariant to distance but not to the spatial arrangement of cloud material, and as such it is still a useful tool for investigating cloud structure.
December 7
Melanie Demers Galaxy Zoo: The interplay of quenching mechanisms in the group environment

Does the environment of a galaxy directly influence the quenching history of a galaxy? Here we investigate the detailed morphological structures and star formation histories of a sample of SDSS group galaxies with both classifications from Galaxy Zoo 2 and NUV detections in GALEX. We use the optical and NUV colours to infer the quenching time and rate describing a simple exponentially declining SFH for each galaxy, along with a control sample of field galaxies. We find that the time since quenching and the rate of quenching do not correlate with the relative velocity of a satellite but are correlated with the group potential. This quenching occurs within an average quenching timescale of ∼2.5 Gyr from star forming to complete quiescence, during an average infall time (from ∼10 R200 to 0.01 R200) of ∼2.6 Gyr. Our results suggest that the environment does play a direct role in galaxy quenching through quenching mechanisms which are correlated with the group potential, such as harassment, interactions or starvation. Environmental quenching mechanisms which are correlated with satellite velocity, such as ram pressure stripping, are not the main cause of quenching in the group environment. We find that no single mechanism dominates over another, except in the most extreme environments or masses. Instead an interplay of mergers, mass & morphological quenching and environment driven quenching mechanisms dependent on the group potential drive galaxy evolution in groups.
January 4
Fereshteh Rajabi Dicke's Superradiance -- From millisecond fast radio bursts (FRBs) to multiyear maser bursts

We apply Dicke's superradiance, a coherent quantum mechanical radiation mechanism, to explain burst phenomena in astrophysics. We show that bursts lasting from a few milliseconds (FRBs) to a few years (e.g. OH masers) can be produced by very large groups of entangled atoms/molecules. This is in contrast with the common assumption that, in the interstellar medium, the atoms/molecules in a radiating gas act independently from each other. Superradiance, a well-known and intensely studied phenomenon in the physics community, was first discussed by R. H. Dicke in 1954. I will present our superradiance models developed to explain some maser flares and FRBs, and discuss our results for the 6.7-GHz methanol, 1612-MHz OH, and 22-GHz water spectral lines [1,2]. Our analyses suggest that the aforementioned groups of entangled atoms/molecules, developing over distances of up to a few kilometers for maser flares and 1000 AU for FRBs, can reproduce the observed light curves [1,2,3].
January 11
Joey Rucska Overcoming planet formation barriers with dust and gas simulations in SPH

The mechanisms by which grains in protoplanetary disks grow to planetesimals are poorly constrained. Small grains can collide and stick to build larger objects, however, around one metre in diameter, current models expect collisions to be primarily destructive. Metre-sized objects also radially drift rapidly into the central star. Currently, it is believed instabilities are present in protoplanetary disks due to relative motion between the dust and gas phases and the drag forces that couple their motions together. These instabilities could overcome these barriers by quickly forming larger objects. In this talk I will provide an overview of how dust and gas in protoplanetary disks can be simulated using smoothed particle hydrodynamics (SPH) methods, and the numerical difficulties present with these methods. I will discuss how the previously mentioned instabilities have been numerically investigated in other studies, and how they can be tackled with SPH.
January 18
Jeremy Webb Do Globular Clusters Have an Identity Problem? - The Dynamics of Mass Segregation and Tidal Stripping

Globular clusters, once thought to be simple dynamical systems, are proving to be increasingly complex as our observational capabilities continue to grow. Advancements in ground based telescopes in particular have made it possible to study how various cluster properties change as a function of clustercentric distance, as such studies require multiple pointings per cluster. Being able to study the radial dependence of cluster properties allows for the internal dynamical evolution of clusters to be directly probed. Of particular interest is the ability to measure the degree of mass segregation in a cluster as it provides an indication of a cluster's dynamical age. Coupled with estimates of how much mass a cluster has lost since formation, based on its global mass function, it becomes possible to begin piecing together each cluster's dynamical history. In this talk, I will first present a new method for quantifying the degree of mass segregation in a cluster based on the radial variation of its mass function and relate this measurement to a cluster's kinematic properties. I will then illustrate the application of this method to select Milky Way Globular Clusters and discuss how the degree of mass segregation within a cluster is inconsistent with its present day mass function. Potential remedies to this discrepancy will also be discussed.
January 25
Astro-ph session View
February 8
Astro-ph session View
February 15
Astro-ph session View
February 22
Paolo Bianchini The dynamical life of globular clusters.

Globular clusters (GCs) are old stellar systems and their current properties are shaped by the interplay between stellar evolution, dynamical evolution and interaction with their host galaxy. Understanding these evolutionary ingredients is the key to unveil the formation properties of GCs in the earliest phases of galaxy formation. In this talk, I will show how to trace the dynamical evolution of GCs exploiting their internal kinematics. In particular, I will introduce a novel method to measure their dynamical state and to identify clusters that underwent core collapse. This approach will be the starting point for exploiting at full power the comprehensive amount of 3D kinematic data (e.g. Gaia data, HST proper motions) necessary to unveil the complex evolution of GCs.
March 1
Astro-ph session View
March 8
Jasper Grond Implementing and Characterizing Adaptive Refinement in TREVR (Tree Based Reverse Ray Tracing)

I will present my masters thesis work, focusing on an adaptive refinement criteria for our tree based radiative transfer solver TREVR and how to characterize the computational cost of such a refinement method.
Ralph Pudritz The origin of filamentary star forming clouds in magnetised galaxies

Observations show that galaxies and their interstellar media are pervaded by strong magnetic fields with energies in the diffuse component being at least comparable to the thermal and even as large or larger than the turbulent energy. Such strong magnetic fields prevent the formation of stars because patches of the interstellar medium are magnetically subcritical. Here we present the results from global numerical simulations (using FLASH AMR MHD) of strongly magnetised and self-gravitating galactic discs, which show that the buoyancy of the magnetic field due to the Parker instability leads at first to the
March 15
Michael Radica A High Resolution Study of NGC6822 with SITELLE

I will present my work while a science intern at the CFHT last Fall. The project focused on using the SITELLE Imaging Fourier Transform Spectrometer (IFTS) to characterize the line of sight variations in dust extinction for high resolution optical surveys. I will expand on my contributions to this main project, as well as another subproject I participated in: improving and optimizing SITELLE's ORCs data reduction pipeline.
March 22
Ian Roberts TBA
March 29
Matt Alessi TBA
April 5
Joey Rucska TBA
April 12
Nathan Brunetti TBA
April 19
Melanie Demers TBA
April 26
Ashley Bemis TBA