Wednesday 24 Nov 2021 @ 12:00 p.m., Zoom
Prof Will Percival, University of Waterloo; Email: will.percival[at]uwaterloo.ca
Abstract
Observational astrophysics consists of making inferences about the Universe by comparing data and models. The credible intervals placed on model parameters are often as important as the maximum a posteriori probability values, as the intervals indicate concordance or discordance between models and with measurements from other data. Intermediate statistics (e.g. the power spectrum) are usually measured and inferences made by fitting models to these rather than the raw data, assuming that the likelihood for these statistics has multivariate Gaussian form. The covariance matrix used to calculate the likelihood is often estimated from simulations, such that it is itself a random variable. This is a standard problem in Bayesian statistics, which requires a prior to be placed on the true model parameters and covariance matrix, influencing the joint posterior distribution. I will review this problem and suggest a new approach linking frequentist and Bayesian approaches that have previously appeared in the astronomical literature.
Wednesday 17 Nov 2021 @ 12:00 p.m., Zoom
Brenna Mockler, University of California, Santa Cruz; Email: bmockler[at]ucsc.edu
Abstract
Tidal disruption events (TDEs) provide an exciting opportunity to study supermassive black holes in quiescent galaxies and the stellar populations and dynamics in galactic nuclei. They regularly produce super-Eddington mass fallback rates, and their light curves encode information about the accretion processes and the masses of the star and black hole. I developed a widely used tool for measuring black hole masses with TDE light curves, however, degeneracies between the accretion efficiency and the mass of the disrupted star make it difficult to measure these parameters from the light curve alone. I will show how we can combine information from the TDE light curve with constraints from the spectra and host galaxy to learn about the properties of stars in galactic nuclei.
Wednesday 10 Nov 2021 @ 12:00 p.m., Zoom
Dr Sree Oh, ANU; Email: sree.oh[at]anu.edu.au
Abstract
We investigate the stellar kinematics of the bulge and disk components and ionised gas using SAMI 3D spectroscopy data. The spatially resolved rotation velocity (V) and velocity dispersion (σ) of bulge and disk components have been simultaneously estimated using the penalised pixel fitting (PPXF) method with photometrically defined weights for the two components. The two components show mass-rotation and mass-dispersion relations with similar slopes, but different intercepts suggesting that the galaxy stellar mass scales with both V and σ for both bulge and disk components of all galaxy types. Our findings suggest that the relative contributions of the two components explain, at least to first order, the complex kinematic behaviour of galaxies. Looking further ahead, we compare gas and stellar kinematics in various galaxy types (e.g. star-forming galaxies, AGNs), finding that the difference between stellar and gas velocity dispersions correlates with the contribution of AGN to the
ionised gas emission. We also discuss gas and stellar velocity dispersions as proxies for the dynamical masses.
Wednesday 03 Nov 2021 @ 12:00 p.m., Zoom
Associate Professor Stas Shabala, University of Tasmania; Email: stanislav.shabala[at]utas.edu.au
Abstract
Relativistic jets associated with supermassive black holes are remarkable beasts. They are the most luminous objects in the radio sky; they accelerate particles to the highest energies known; and – by transferring some fraction of their enormous kinetic energy to the surrounding gas (so-called “AGN feedback”) – they regulate the evolution of the most massive galaxies over at least the last seven billion years.\nSensitive, high-resolution radio continuum observations with Square Kilometre Array (SKA) pathfinders are uncovering details of the complex interaction between kpc-scale AGN jets and their environments. I will present theoretical work on modelling the dynamics and synchrotron emission of these interactions, using a combination of numerical and analytical techniques. In this approach, the jet dynamics is described by 3D relativistic hydrodynamic simulations, while synchrotron emissivity is calculated in post-processing using an analytical model which fully accounts for the
relevant loss processes. I will present some examples of insights into mechanisms responsible for jet production and AGN feedback, gained through combining such models with multi-wavelength observations.
Wednesday 27 Oct 2021 @ 12:00 p.m., Zoom
Dr Amit Seta, ANU; Email: amit.seta[at]anu.edu.au
Abstract
Magnetic fields are a dynamically important component of the interstellar medium of the Milky Way. Pulsars can act as an excellent probe of the Milky Way magnetic field. The average strength of the Galactic magnetic field component parallel to the line of sight can be estimated as 1.232 RM / DM, where RM and DM are the observed rotation and dispersion measure of the pulsar. However, this assumes that the thermal electron density and magnetic field of the interstellar medium are uncorrelated. Using numerical simulations and observations, we test the validity of this assumption. Based on magnetohydrodynamical simulations of driven turbulence, we show that the correlation between the thermal electron density and the small-scale magnetic field increases with increasing Mach number of the turbulence. We find that the assumption of uncorrelated thermal electron density and magnetic fields is valid only for subsonic and transsonic flows, but for supersonic turbulence, the field strength can
be severely overestimated by using 1.232 RM / DM. We then correlate existing pulsar observations from the Australia Telescope National Facility with regions of enhanced thermal electron density and magnetic fields probed by CO data of molecular clouds, magnetic fields from the Zeeman splitting of the 21 cm line, neutral hydrogen column density, and Halpha observations. Using these observational data, we show that the thermal electron density and magnetic fields are largely uncorrelated over kpc scales. Thus, we conclude that the relation 1.232 RM / DM provides a good estimate of the magnetic field on Galactic scales, but might break down on sub-kpc scales.
Wednesday 20 Oct 2021 @ 12:00 p.m., Zoom
Kaley Brauer, MIT; Email: kbrauer[at]mit.edu
Abstract
The motions and chemical composition of the stars currently present in the extended outskirts (the stellar halo) of a galaxy preserve a record of the galaxy’s formation history. While most of the stars in the center and disk of a galaxy formed in situ, many of the stars in the stellar halo originated in the many small galaxies that the central host galaxy accreted over billions of years. Currently, though, we lack ways to identify which halo stars originated in which dwarf galaxies or even reliably identify which stars were accreted. By utilizing the Caterpillar simulation suite, a suite of 32 Milky Way-mass galaxies forming, we find that stars with strong enrichment of r-process elements may have preferentially formed in the smallest dwarf galaxies that merged into the Milky Way. We also quantify how well astronomers can kinematically identify stars that accreted together from these tiny dwarf galaxies. Looking forward, we will expand on this work with several more detailed simula
tions of dwarf galaxy formation and of r-process material mixing into interstellar gas.
Wednesday 13 Oct 2021 @ 12:00 p.m., Zoom
Ryan Turner, Swinburne University; Email: rjturner[at]swin.edu.au
Abstract
A key method for understanding the physics of dark energy and gravity in the Universe is to measure the growth of cosmic structure with time. A powerful technique for tracing the growth of structure is to use the peculiar velocities of galaxies, and their correlations. Peculiar velocities describe the velocity of a galaxy relative to the background expansion of the Universe, imposed by the gravitational influence of the local environment. In this talk I will present new tests using cosmological simulations to show how combining information from the local peculiar velocity field and galaxy density field provide the tightest constraints on the growth rate. I will demonstrate how velocity and galaxy survey information can be optimally combined in a correlation function approach, discuss how these tests can be applied to existing datasets, and show how future peculiar velocity surveys will provide competitive tests of gravitational growth.
Wednesday 29 Sep 2021 @ 04:00 p.m., Zoom
Dr Charlotte Angus, University of Copenhagen; Email: angus[at]nbi.ku.dk
Abstract
The advent of wide-field time-domain optical surveys has changed the landscape of transient astronomy. Not only have they dramatically increased the rate of transient discoveries, they have allowed us to unearth new, previously unheard of, transient classes. The wide variety of different survey strategies available play a pivotal role in how we can probe the physics behind these new and exotic transients. In this talk I will demonstrate how two different surveys have done this. The Dark Energy Survey (DES), using deep multi-band imaging, has significantly broadened the definition, and understanding of Superluminous Supernovae (SLSNe), unveiling the nature of of pre-peak bumps, while the high cadence, Young Supernova Experiment (YSE), designed to probe the earliest transient epochs, has uncovered a rapidly evolving tidal disruption event uniquely hosted by an intermediate mass black hole. These two complementary approaches allow distinct insights into the nature of some of the most ex
treme physical processes in the Universe, and help us to prepare for future surveys, such as LSST.
Wednesday 22 Sep 2021 @ 04:00 p.m., Zoom
Sophie Lund Schrøder, University of Copenhagen; Email: sophie.schroder[at]nbi.ku.dk
Abstract
I will present my work on advancing binary evolution models, which are used to describe and calculate rates of merging black holes and neutrons stars observed by the LIGO/Virgo collaboration. Current models include simple approximations for the binary orbit changes during all types of mass transfer and angular momentum loss. I instead work on characterising these interactions using hydrodynamic simulations, and understand how the loss of mass and angular momentum will effect the final population of merging compact objects.
Wednesday 08 Sep 2021 @ 10:00 a.m., Zoom
Grace Olivier, Ohio State University; Email: olivier.15[at]buckeyemail.osu.edu
Abstract
Massive stars contribute incredible amounts of energy to their surroundings across a variety of environments in the universe. Before their exciting deaths, massive stars produce feedback through a number of mechanisms that are frequently used in subgrid physics models in galaxy simulations to create realistic galaxies. Observations are a key to anchoring these simulations in reality, but there has been limited work on this front, especially for the youngest, embedded HII regions. I explore the effects of direct radiation pressure, dust-processed radiation pressure, photoionization heating and shock-heating from stellar winds in a sample of young HII regions (sources with radii < 0.5 pc) and determine which is the most important for very young stars. This analysis suggests radiative feedback on dust drives the earliest stages of HII region expansion. Additionally, I study the effects of radiative feedback in an exciting population of extremely low-metallicity star-forming galaxies th
at have been discovered in the local universe which are analogues to Reionization Era galaxies. During the Epoch of Reionization radiative feedback played a significant role in shaping the universe, and such analogues give us a detailed laboratory to explore the effects from feedback. I present deep FUV and optical spectra of two of these extreme emission line galaxies that have strong very-high-ionization optical and FUV emission lines (e.g., CIV, HeII, [FeV], [ArIV]). I demonstrate that canonical photoionization models, using typical stellar population models, catastrophically fail to reproduce the high-ionization emission lines. I constrain the stellar population properties using the FUV spectral features and explore the deficiencies of current stellar models. By simultaneously fitting the stellar and nebular emission within these extremely high-ionization emission line galaxies, I provide new observational benchmarks of radiative feedback for the next generation of stellar mode
ls at very low metallicity. By studying the effects of stellar feedback from single stars in the Milky Way to entire stellar populations in analogues to the first galaxies we can begin to build a coherent picture of stellar feedback as it impacts the vast scales of the universe.
Wednesday 25 Aug 2021 @ 10:00 a.m., Zoom
Kshitij Aggarwal, West Virginia University; Email: ka0064[at]mix.wvu.edu
Abstract
Fast Radio Bursts (FRBs) are millisecond-duration, transient sources of extragalactic origin. In this talk, I will describe an end-to-end search and analysis software suite to study FRBs. All of these tools are python-based, user-friendly, robust, and have been tested rigorously. I will start with Your (Your Unified Reader) that unifies the data ingestion across multiple data formats and can also be used to do RFI mitigation, single-pulse search, visualization, etc. To classify the candidates generated from the search, we have developed Fetch, a state-of-the-art deep learning-based classifier, that has already been used to discover many FRBs. To analyze the FRBs, we developed Burstfit, which provides a framework to model any spectrogram consisting of an FRB using multiple robust methods. I will then talk about localizing FRBs using Realfast system at the Very Large Array, and some recent results from this system.
Wednesday 18 Aug 2021 @ 12:00 p.m., Zoom
Dr Hyunbae Park, Kavli Institute for the Physics and Mathematics of the Universe; Email: hcosmosb[at]g.ecc.u-tokyo.ac.jp
Abstract
We expect neutral hydrogen gas in the intergalactic space to have scattered a significant portion of Lyα emission from star-forming galaxies during the reionization era. Thus, the Lyα line strength of high-z galaxies is considered a promising probe of reionization history. In order to quantitatively understand which properties of the intergalactic medium (IGM) determine the transmission of Lya photons, we calculate the transmissivity from the simulated IGM of the Cosmic Dawn II (CoDa II) simulation and analyze the results. We find that the gravitational infall motion around the galaxy, size of the HII region, and self-shielded neutral gas systems are crucial factors. In this talk, I will elaborate on how those factors affect the observability of high redshift Lyα emitters.
Wednesday 11 Aug 2021 @ 12:00 p.m., Zoom
Dr Janakee Raste, Tata Institue of Fundamental Research; Email: -%%janakee[at]theory.tifr.res.in
Abstract
Most ongoing experiments targetting the fluctuating 21-cm cosmological signal aim to look for the signal at redshifts well above 6. This strategy is motivated by the traditional assumption that reionization ends at z >~ 6. However, recent constraints from Lyman-α and CMB data prefer a significantly delayed reionization scenario in which reionization is 50% complete at redshifts as low as z ~ 7. In these models, reionization ends at z ~ 5, with large 100 Mpc “islands” of cold, neutral hydrogen persisting in the IGM well below z = 6. We have studied the effect of these neutral hydrogen islands on the 21-cm power spectrum by analysing outputs of a state-of-the-art radiative transfer simulation of the IGM calibrated to the CMB and Lyman-α forest data. We calculate the power spectra of the 21-cm signal from this simulations and compare them with a more traditional reionization model in which reionization is completed by z = 6. Contrary to previous models, we find that thanks to the late
end of reionization the 21-cm power continues at be high (~ 1 mK^2) at k ~ 0.1 h/cMpc at z = 5-6. At z = 5.5, for example, the power spectrum can be two or more orders of magnitude higher than the traditional models. This enhanced 21-cm power spectrum signal should be easily detectable by HERA and SKA1-LOW for reasonable integration times, assuming optimistic foreground subtraction. We argue that the redshift range z = 5-6 is very attractive for 21-cm experiments due to easier thermal noise characteristics and synergies with abundant multi-wavelength observations.
Wednesday 04 Aug 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
Dr Rebecca Davies, Swinburne University; Email: rdavies[at]swinburne.edu.au
Abstract
Abstract: Outflows are likely to play an important role in shaping the growth and evolution of galaxies, especially at the peak epoch of star formation (z~1-3) where outflows are ubiquitous. Detailed measurements are now required to better constrain feedback models and to establish how outflows influence the properties of interstellar and circumgalactic gas. In this talk, I will highlight recent spatially resolved studies of outflows at z~2, focusing on the launch mechanisms of star-formation-driven outflows and the variety of ways in which outflows couple AGN accretion energy to gas on nuclear, galactic and circumgalactic scales. At the end, I will discuss ongoing efforts to make robust measurements of the abundances of metal ions in the circumgalactic medium at z>5.5, with the goal of understanding the mechanisms governing the production and transport of metals in the early Universe.
Wednesday 28 Jul 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
Dr Manodeep Sinha, Swinburne University; Email: msinha[at]swinburne.edu.au
Abstract
Software is now critical for modern research, and frequently, scientific breakthroughs are enabled by high-quality and innovative research software. Consequently, there is an increased demand for “Research Software Engineers (RSEs)” – people who are well-trained domain scientists with highly specialised software skills. However, the existing policies for rewards and recognition within academia are insufficient to hire, promote and retain these skilled RSEs. Broad community consultations are underway to finalise recommendations for a more sustainable research software ecosystem. These recommendations include both a top-down approach through funding, policy and research assessment and a bottom-up approach through actions of individual research groups and institutions. Both of these approaches are necessary to drive change in culture around research software. I will discuss the current developments surrounding research software, both nationally and in the international context. I wi
ll share various practices for software, data and communities that you can adopt to promote a more sustainable research software ecosystem.
Wednesday 21 Jul 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
Dr Anshu Gupta, Curtin Institute of Radio Astronomy; Email: anshu.gupta[at]curtin.edu.au
Abstract
I combine optical and near-infrared spectroscopy of galaxies with state-of-the-art cosmological simulations to study the galaxy formation and evolution. In this talk, I will present recent results where we find that massive galaxies grow via accretion of stars (ex situ growth) from other galaxies at z<3, and links between the size of the stellar disk and their star-formation histories.
In this second half, I will introduce recent results from the MOSEL survey which is an ongoing spectroscopic survey of emission-line galaxies at z=3-4. MOSEL sample include about 20 metal-poor (Z<0.1 Zsun), low stellar mass (10^9 Msun) galaxies with high star formation rate (5-10 times the typical star-forming galaxy) and [OIII] equivalent widths (>600 A), making them analogous to EoR galaxies. We have obtained deep KMOS/MOSFIRE spectroscopic and nearly 40-band photometric data for these targets. I will present a preliminary analysis where we find early evidence of outflows in galaxies with [OIII
] EWs > 1000A. I will argue that the EoR analogs is a unique sample to constrain the production efficiency of the ionizing photons and their escape fraction in z>6 galaxies. This sample will be an excellent comparison for the future observations of z=6-9 galaxies with JWST.
Wednesday 07 Jul 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
Dr Rory Smith, Monash University; Email: Rory.Smith[at]monash.edu
Abstract
In January 2020, the LIGO and Virgo observatories twice detected gravitational waves from the inspiral and merger of neutron stars with black holes. In this talk, I will describe the detection and characterization of this observation of a new source of gravitational waves, which also represents the first observation of neutron star-black hole binaries.
Wednesday 30 Jun 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
Adam Batten, Swinburne University; Email: abatten[at]swin.edu.au
Abstract
The Intergalactic Medium (IGM) is a difficult observe in the optical and UV due to the high temperatures (T ~ 10^6K) and low densities (n ~ 10^-6 cm^-3) leading to a lack of favourable transition lines. The dispersion measure (DM) of fast radio bursts (FRBs) provides a unique new way to probe the ionized baryons in the IGM. Cosmological models with different parameters lead to different DM-redshift (DM−z) relations. Additionally, the over/under-dense regions in the IGM and intervening galaxies’ circumgalactic medium lead to scattering around the mean DM−z relations. I will present the recent work I have done using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations to study the DM-z relation, and the scatter around it. I find that almost all of the FRBs found at low-redshifts have significantly larger observed DMs than predicted from simulations. I will also show that we would need of the order 9000 localised FRBs to constrain AGN feedback efficiency
. Finally I will talk about a future project measuring the metallicity of the IGM in EAGLE and the sizes of ‘metal bubbles’ around galaxies at the end of the epoch of reionisation.
Wednesday 23 Jun 2021 @ 16:00 p.m., Zoom
Laura Sommovigo, Scuola Normale Superiore; Email: laura.sommovigo[at]sns.it
Abstract
ALMA observations have revealed the presence of dust in the first generations of galaxies in the Universe. However, the dust temperature Td remains mostly unconstrained due to the few available FIR continuum data. This introduces large uncertainties in the derived dust masses, infrared luminosities, and obscured fraction of star formation of high-z (z>5) galaxies. We have developed a new analytical method that allows us to constrain Td using a single continuum data point at 158 microns by combining it with the overlying CII emission. With our method, one can analyse uniquely CII and continuum detections, and the targets of ALMA large programs such as ALPINE and REBELS. In particular, REBELS is providing us with unprecedented numerous continuum detections at z>7, increasing of a factor ~6 the available data so far. Our analysis of these sources will shed light on the evolution of the dust temperature and dependent quantities with redshift, glimpsing the Epoch of Reionization (EoR) for
the first time. Preliminary results indicate that the dust temperature increases at a higher redshift, with a larger scatter w.r.t. to that observed in the local universe. We physically motivate the increasing Td-redshift trend as a consequence of the decreasing total gas depletion time induced by a more vigorous cosmological accretion at early times. A higher Td has testable implications: (a) it reduces the tension between local and high-z IRX-β relation, (b) it alleviates the problem of the uncomfortably large dust masses deduced from observations of some EoR galaxies, (c) it results in a larger obscured fraction of the SFR. This is consistent with the flattening of the cosmic Star Formation Rate Density (SFRD) at z>4 suggested by recent observations at FIR and radio wavelengths of dusty UV-obscured systems.
Wednesday 16 Jun 2021 @ 12:00 p.m., Zoom
A/Prof. Randall Wayth, Curtin University; Email: R.Wayth[at]curtin.edu.au
Abstract
Fast Radio Bursts (FRB) are one of the most intriguing transient phenomena discovered in the recent years, and recently observed down to 100-MHz frequencies. I will present the first southern hemisphere all-sky real-time imaging and radio-transient monitoring system, implemented on two prototype stations of the low frequency (50 – 350 MHz) component of the Square Kilometre Array (SKA-Low), the Engineering Development Array 2 (EDA2) and Aperture Array Verification System 2 (AAVS2). For the last two years these prototypes have been regularly collecting data to verify their performance against the SKA-Low specifications and simulations, including making all-sky images every two seconds used for transient searches. The transient identification algorithm used 2-second difference images to find candidates and required their detection in the images from both stations. In approximately 360 hours of data using a single coarse channel (0.926 MHz bandwidth), we identified a few episodes of extr
emely bright pulses from the pulsar PSR B0950+08 and several transients from an unknown object, which is currently under investigation. We also determined preliminary upper limits on surface density of radio transients at a 2-second timescale. We plan to increase the bandwidth by at least 40 times (to about 40 MHz) and time resolution to 10 ms or better in order to improve the sensitivity by two orders of magnitude and start detecting hundreds of FRBs per year. This upgrade will transform the stations into low-frequency FRB survey machines looking for FRBs and signals from extraterrestrial intelligence in high-time resolution all-sky images, which will pave the way to similar searches with hundreds of SKA-Low stations.
Wednesday 09 Jun 2021 @ 12:00 p.m., Zoom
Dr James Murray and Romy Pearse, Astronomy Australia Ltd; Email: rpearse[at]swin.edu.au
Abstract
AAL will be presenting a general update to staff at The University of Melbourne on our activities of the past year (or so), as well as current projects and funding arrangements – we will also happily take questions and encourage real-time feedback via our new online platform (see more on this below). We have had some staff and Board changes since we last updated the team at The University of Melbourne, so we will touch on these as well. The update will be general in nature, but will also show The University of Melbourne’s involvement with AAL via committee and member representatives. The presentation will also serve to introduce AAL to new staff at The University of Melbourne or provide a refresher for those within the department who haven’t been in touch with us for a while.
New online format for AAL Member Updates during lockdown – information for participants
Held within a standard zoom meeting framework, AAL is currently conducting its member updates du
ring lockdown via a new interactive platform called WooClap. WooClap allows meeting attendees to participate with presenters in real-time throughout the session – they only need to have a personal device with internet capability on hand to do so (a mobile phone, tablet/ipad or laptop). At the start of the zoom meeting, we will provide everyone in the session with a URL to gain access to WooClap. Attendees need simply to enter this URL into their personal device (or they can take an image of the QR code provided on the main screen with their phones) to participate via a series of questions posed by the presenters throughout the session. As attendees respond to these prompts, real-time results from their answers will appear on the main presentation screen which the presenters can then react to.
Feedback from participant responses is captured for review at a later stage, which helps us to gain a better understanding of the individual needs of our members. All feedback i
s anonymous, however, so attendees are free (and encouraged) to be completely honest with their responses during the session. Participation via WooClap is completely voluntary, but we have found this to be a really great method of gaining meaningful feedback from our members (as well as keeping everyone engaged during the session) so we hope that most of the meeting attendees will be keen to participate during the member update.
Wednesday 02 Jun 2021 @ 12:00 p.m., Zoom
Dr Xingjiang Zhu, Beijing Normal University; Email: zhuxj[at]bnu.edu.cn
Abstract
Gravitational wave astronomy is revolutionizing our understanding of the Universe. Since the historic discovery of GW150914 five years ago, LIGO and Virgo detectors have discovered 50 compact binary merger events. These merger events, lasting from fractions of seconds to minutes, include stellar-mass binary black holes, binary neutron stars and likely neutron star-black hole binaries. Meanwhile, pulsar timing arrays have been used to search for gravitational waves with periods of years to decades. After several decades of international efforts, it is believed that we are approaching the sensitivity to detect waves from supermassive binary black holes. In this talk, I will summarize recent progresses and future prospects of gravitational wave astronomy on both short and long timescales, with a focus on the study of binary neutron stars and recent results from the Parkes Pulsar Timing Array.
Wednesday 26 May 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
Dr Rob Bassett, Swinburne University; Email: rbassett[at]swin.edu.au
Abstract
It has been nearly a century since the great debate was put to rest by Hubble, who definitively proved that many nebulae were truly “island universes” akin to our own Milky Way. Since then, millions of galaxies have been discovered, yet one relatively simple question remains open: what are the true three dimensional shapes of galaxies? The intangible nature of observational astronomy means that any one galaxy provides a single, two dimensional perspective, and it is this aspect of our field that makes this question so difficult to answer. In this talk, I will give an overview of the history of galaxy 3D shape research from statistical photometric studies up to more recent advances incorporating galaxy kinematics with integral field spectroscopy. I will then present critical tests we have performed of these recent methods that incorporate mock IFS from cosmological hydrodynamics simulations (from which the true 3D shapes are known). These tests have revealed striking biases in the rec
overed shapes of galaxies that have spurred us in a new direction: machine learning and neural networks. I will finish with our preliminary results in galaxy shape prediction with these modern techniques, which may provide an important step forward in this fundamental area of research.
Wednesday 19 May 2021 @ 11:00 a.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
A Prof Duane Hamacher, University of Melbourne; Email: duane.hamacher[at]unimelb.edu.au
Abstract
Australia’s First People developed complex knowledge systems that are committed to memory and passed to successive generations through oral tradition. The length of time oral traditions can be passed down while maintaining vitality is a topic of ongoing debate. Scientific techniques have been utilised to date natural events described in oral tradition, such as volcanic eruptions, tsunamis, and meteorite impacts to provide a terminus ante quem for the origin or development of these oral traditions. In this talk, we analyse Tasmanian Aboriginal (palawa) oral traditions recorded in the early nineteenth century that describe the flooding of the Bassian Land Bridge connecting Tasmania to mainland Australia, as well as the presence of a culturally significant “Great South Star”. Using astro-chronological and geo-chronological techniques, we show that these traditions have a terminus ante quem of approximately 12,000 years.
Wednesday 12 May 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
James Esdaile, Swinburne University; Email: jesdaile[at]swin.edu.au
Abstract
In the early Universe one might expect only star-forming galaxies but there is now substantial evidence that some massive galaxies have quenched star-formation within the first billion years of galaxy evolution. These massive quiescent galaxies (MQG) have proven difficult to reproduce in sufficient numbers in cosmological hydrodynamical simulations. While the latest generation of simulations have begun to achieve more consistent number densities through detailed prescriptions of AGN feedback, the observed quenched galaxies still appear to quench at earlier epochs. This begs the question: how early in the Universe can we still find quiescent galaxies? Additionally, high redshift MQGs host old stellar populations that can provide insights into the star-formation conditions during the epoch of reionisation. The intense star-bursts that likely formed MQGs are expected to have a different distribution of stellar masses, the initial-mass function (IMF), compared to local elliptical galaxie
s. I present work done to identify these rare MQGs at high redshift based on the FENIKS survey and determine some of their physical properties using a combination of deep HST imaging and MOSFIRE spectra.
Wednesday 28 Apr 2021 @ 12 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
Dr Nikole M. Nielsen, Swinburne University; Email: nikolenielsen[at]swin.edu.au
Abstract
The star formation history of the universe reveals that galaxies most actively build their stellar mass at “cosmic noon” (z=1-3). The gas accreting onto galaxies to drive their construction and the resulting metal-enriched material ejected from these galaxies due to feedback must pass through the circumgalactic medium (CGM). The CGM is a massive reservoir of diffuse, multiphase gas out to ~200 kpc and is the interface between the intergalactic medium and the galaxy. While the CGM is well-studied at z<1, little attention has been paid to the reservoir when star formation is most active, due to the difficulty in identifying host galaxies at cosmic noon. The installation of the Keck Cosmic Web Imager (KCWI), a sensitive integral field spectrograph, on Keck II has opened a new window to quickly identify galaxies via their Lyman alpha emission at cosmic noon as well as to directly image the CGM in emission. I will introduce two new surveys with KCWI aiming to study the CGM in (1) absorpti
on around galaxies at z=2-3 and (2) emission around local starbursting galaxies (cosmic noon analogues). These surveys are still in progress, but first results reveal strong outflows at cosmic noon and tantalising sub-kiloparsec structure in the CGM.
Wednesday 05 May 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
Dr Steven Murray, Arizona State University; Email: steven.g.murray[at]asu.edu
Abstract
21cm cosmology is growing in momentum. New low-frequency radio telescopes able to probe the neutral hydrogen in the high-redshift Universe (z~6-30) come in two flavours: single-antennas that probe the average thermal history of the Universe (such as EDGES, SARAS and REACH), and interferometers that measure the spatial fluctuations of 21cm emission (such as HERA, MWA and SKA).
Both come with an extraordinary challenge: bright foregrounds amplify small spectral “features” in the instrument, obscuring the background signal unless calibrated to one part in 10^5. Accounting for these effects is susceptible to inadvertant removal of part of the signal, which has led to several retractions of published upper-limits over the past decade. Add to this the extremely surprising results from EDGES in 2018, and we must ask: how much can we trust the results of 21cm experiments, and how can we build confidence amongst the community?
In this talk, I will discuss my role in answering
these questions with two current 21cm experiments — EDGES and HERA.
EDGES is working to verify its result from 2018 using new and improved data and improved analysis techniques. I will describe our new effort to forward-model systematics, starting with receiver calibration and simple models for the antenna reflection coefficients, showing the effects of propagating their full correlated uncertainties on the cosmological estimates.
Concerning HERA, I will report on the work of the Validation team (Aguirre et al., 2021), in support of our recent first upper limit (Kern et al., 2021). We produced a sophisticated end-to-end simulation of the full observation, including thermal noise, realistic foregrounds and many instrumental systematics. This simulation was processed with the exact analysis pipeline used for the data. I will discuss our philosophy and findings, with special regards for future improvements.
Wednesday 21 Apr 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
Dr Jielai Zhang, Swinburne University; Email: jielaizhang[at]swin.edu.au
Abstract
Fast radio bursts (FRBs) are millisecond bursts in the radio, so bright that we observe them in distant galaxies at a rate of over 1000/day over the whole sky with large radio facilities. Despite their frequency and high energy output, what causes these bursts have eluded astronomers since their discovery over a decade ago. FRBs may result from magnetar bursts, binary neutron star mergers (producing kilonovae), neutron stars collapsing to black holes (blitzars) or several other theories. Information on emission coherent with the radio burst in other wavelengths can help reveal the physical processes that create these bursts. I present results from an observational program that can do this called the Deeper Faster Wider (DWF) program. A key part of the DWF program is coordinating ~10 multi-wavelength observatories to perform deep, wide-field, fast cadence observations on the same field simultaneously to collect possible imaging on fast-evolving transients before they fade. Important
ly, DWF can collect data before, during and after fast transients. Typically, a DWF observing run goes for a week. In particular, I present results from DWF operation run 8 (DWF-O8). During DWF-O8, two FRBs were detected with the Murriyang Radio Telescope (formerly known as the Parkes Radio Telescope). Simultaneously observing with Murriayng was the Neil Gehrels Swift Observatory , Hard X-ray Modulation Telescope, AstroSat, the Korea Microlensing Telescope Network, the Huntsman Telescope, Murriyang and the Molonglo Observatory Synthesis Telescope, amongst others.
Wednesday 24 Mar 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom)
Dr Nicha Leethochawalit, University of Melbourne; Email: nicha.leethochawalit@unimelb.edu.au
Abstract
Having accurate completeness functions (and/or determining the survey effective volume) is crucial to the determination of the rest-frame ultraviolet luminosity functions (UVLFs) all the way back to the epoch of reionization. Most studies use injection-recovery simulations to determine completeness functions. Although conceptually the same, these simulations have subtle but important differences in their definition of the completeness function across existing studies. In turn, this requires implementation of different methods to obtain the final determination of the UVLFs. Here, we discuss the advantages and limitations of existing methods using a reference set of mock observations, and then compare the methods when applied to the same set of Hubble Legacy Field (HLF) images. We find that some methods may suffer limitations in a presence of substantial photometric scatter and/or steep luminosity functions.
Wednesday 17 Mar 2021 @ 12:00 p.m., David Caro building, Level 2, Hercus Theatre (+Zoom) Prof. David Jamieson, University of Melbourne Email: d.jamieson@unimelb.edu.au
Abstract
Galileo’s prolific letters to friends, colleagues, patrons and other associates are valuable treatises in their own right. A letter he wrote on 21 December 1613 to a former student in Prague sets out his forthright views on the relationship between religious doctrine and science. Galileo’s letter found its way into the hands of the Inquisition in Rome and this ultimately led to Galileo’s conviction of “vehement suspicion of heresy”. Galileo must have realised his letter would cause trouble because he claimed the letter passed to the authorities in Rome had been altered by malevolent forces out to damage him and strengthen their case for heresy. He released a new version of the letter with much less inflammatory language which he claimed was the original. In a sensational discovery in the library of the Royal Society in London UK, Galileo’s claim of alteration is shown to be false. Incorrectly filed for 250 years, the original of the inflammatory letter was accidentally rediscov
ered in 2018, with edits in Galileo’s own handwriting to tone down the language. This is a remarkable piece of history about a physicist facing dire consequences at the hands of the authorities. A particular passage in the letter has captured my attention. Galileo wrote “This (scientific knowledge) applies especially to those sciences about which one can read only very small phrases and scattered conclusions in the Scripture as is particularly the case for astronomy, of which it contains such a small portion that one does not even find in it the names of all the planets.” I will link this statement back to Galileo’s observations of the planet Neptune that occurred in December 1612 to January 1613 just on one year before he wrote his inflammatory letter. We may be able to glean new insights into his thinking at the time.
Number of posts found: 201