The Melbourne Astro group has weekly colloquia, group meetings, and journal clubs. Normal times and places can be found on the calendar here.
A list of recent colloquia, with slides in some cases, can be found here
The Melbourne Astro group has weekly colloquia, group meetings, and journal clubs. Normal times and places can be found on the calendar here.
A list of recent colloquia, with slides in some cases, can be found here
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: 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
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
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
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
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
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
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
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
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
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
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
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
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
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
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: email@example.com
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: firstname.lastname@example.org
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.
Tuesday 08 Dec 2020 @ 12:00 p.m., Zoom colloquium
Mahsa Rahimi, University of Melbourne (Completion Seminar) Email: email@example.com
Wednesday December 2nd 2020 @12pm, Zoom Colloquium
Dr Sam Vaughan The University of Sydney
Nearby galaxies can be divided into two broad categories: those which have blue colours, disc-like morphologies and are forming stars; and those which are red, have spheroidal morphologies and have ceased their star formation. Explaining why this is the case is a key challenge of galaxy evolution theories, but a comprehensive theory of what causes galaxies to quench their star formation is still missing. I will discuss a toy model of quenching which successfully reproduces a number of important differences between quiescent and star-forming galaxies at redshift 0, including their different mass-metallicity relations and mass-size planes. This model was born out of my studies of stellar metallicities in the SAMI galaxy survey, and I will also give a brief summary of my metallicity measurements and some of the powerful statistical tools I’ve used in my analysis.
Wednesday November 25th 2020 @12pm, Zoom Colloquium
Dr Neil Goeckner-Wald KIPAC
Precision measurements of the anisotropies in the Cosmic Microwave Background (CMB) have become one of the cornerstones of modern cosmology. One major objective of current CMB experiments is the discovery of a stochastic background of gravitational waves generically produced by theories of cosmic inflation. Such a signal could be detected as an excess of odd-parity polarization in the CMB at degree angular scales. The South Pole Observatory is a coordinated effort between the South Pole Telescope (SPT) and the BICEP/Keck collaborations that will use the synergies of the two experiments to search for this signal in the presence of galactic and gravitational lensing foregrounds with unprecedented sensitivity. In this talk I will discuss some recent and upcoming results from both projects focusing on the search for inflationary gravitational waves in the CMB.
Wednesday November 18th 2020 @12pm, Zoom Colloquium
Prof Chris Tinney UNSW
Breakthrough discoveries in astronomy invariably come through one of two routes – applying established techniques to new classes of objects discovered from new classes of surveys, or pushing established techniques to new levels of precision to make available previously unexplored observational phase space. Examples of the former include the discovery of T and Y-class brown dwarfs from surveys like 2MASS or WISE, or higher and higher redshift QSOs from larger and larger surveys.
Opening new phase space through higher measurement precision has been particularly prominent lately – exoplanets being first discovered by increasing precise Doppler measurements, more exoplanets being discovered by increasing precise photometry from space, or gravitation waves being confirmed by increasingly precise strain meters.
The Veloce instrument on the AAT seeks to push the boundaries of increasingly precise Doppler velocity measurement at a fraction of the cost of competing instruments. I’ll describe what it takes to make your instrument more precise, at lower cost, than other leading brands.
Wednesday November 11th 2020 @12pm, Zoom Colloquium
Dr Jordan Mirocha McGill University
A relatively simple model has emerged in recent years that can explain the bulk properties of high redshift galaxies: star formation is fueled by the inflow of pristine material from the intergalactic medium, and proceeds with an efficiency that depends strongly on the mass of a galaxy’s parent dark matter halo but not obviously on cosmic time. We generally interpret this mass dependence as a signature of stellar feedback despite the fact that stellar feedback models also predict time evolution in the star formation efficiency. In this talk, I’ll focus on this apparent tension, and show that to remain in agreement with observations, competing feedback scenarios require qualitatively different assumptions about the properties of dust and the duty cycle of star formation in galaxies. As a result, I’ll discuss the prospects for distinguishing models based on the ‘dustiness’ and ‘burstiness’ of galaxies found in upcoming galaxy surveys with JWST and ALMA, and potentially via constraints on reionization from future 21-cm experiments.
Wednesday October 21st 2020 @12pm, Zoom Colloquium
Poojan Agrawal Swinburne University
Recent observations of galaxies and star clusters have highlighted the need for systematic studies dedicated to exploring the impact of uncertain parameters of stellar evolution on the properties of stellar populations. While the use of fitting formulae to stellar tracks remains a popular choice for modelling stellar evolution in population synthesis codes, they are not adaptable to changes in the stellar tracks. In this talk, I will present results from an alternative approach, METISSE, which uses interpolation between sets of pre-computed stellar tracks to approximate evolution parameters for a population of stars. It can readily make use of stellar models computed with different stellar evolution codes and can compare their predictions for populations of stars. Using METISSE with the data from two different stellar evolution codes, I will show how different physical ingredients used in the evolution of massive stars, such as the treatment of their radiation dominated envelopes, can lead to differences in their evolutionary properties. These differences are important as they can help us account for observations of the stellar populations and the formation of gravitational wave progenitors.
Wednesday October 14th 2020 @12pm, Zoom Colloquium
Dorota Bayer Swinburne University
While a direct detection of the dark-matter particle remains very challenging, the nature of dark matter can potentially be constrained indirectly — by comparing the properties of substructure in galactic haloes with predictions from the phenomenological dark-matter models, such as the cold, warm or hot dark matter. Whereas these models are practically indistinguishable with respect to the predicted characteristics of high-mass substructure, the critical difference lies in the abundance and statistical properties of low-mass galactic substructure. Galaxy-galaxy strong gravitational lensing provides a unique opportunity to search for gravitational signatures of such low-mass substructure in lens galaxies beyond the Local Group. In this talk, I will present a novel approach to observationally constrain the statistical nature of low-mass sub-galactic structure in the inner regions of massive elliptical lens galaxies, based on the power spectrum of surface-brightness anomalies measured in highly-magnified galaxy-scale Einstein rings and gravitational arcs. A future comparison of these results with the predictions from hydrodynamical simulations might either verify the CDM paradigm or require its substantial revision.
Wednesday September 30th 2020 @12pm, Zoom Colloquium
Dr Lilian Garratt-Smithson University of Western Australia
Simulations are now allowing us to probe the scales of the CGM (circumgalactic medium) around galaxies in order to look at the influence of realistic galaxy formation processes. However, it is apparent that the properties of the multi-phase CGM are not yet converged in simulations (e.g. Van de Voort et al., 2019; Hummels et al., 2019), hence their reliability to make predictions is still in question. It is also clear the CGM plays a key role in the evolution of a galaxy; it is a supply of gas for star formation and a key site for feedback-generated outflows, along with the recycling of baryons (for a recent review see Tumlinson et al., 2017).
During this talk I will discuss the results of my recent paper (Garratt-Smithson et al. 2020 – available on astro-ph) and in particular the significant mass of HI seen in the CGM of EAGLE galaxies between redshift 2 and 0. I will explore the physical conditions of this HI, and discuss whether or not we can constrain the fraction existing in the cold neutral medium. I will also discuss the physical processes we can constrain by studying the detailed properties of this CGM HI gas; in particular I link my results back to AGN and stellar feedback, along with how this is modelled in simulations. Finally, I will discuss my current projects, which aim to explore the CGM in greater detail using a mix of idealised and cosmological zoom simulations.
Wednesday September 16th 2020 @12pm, Zoom Colloquium
Pol Gurri Perez Swinburne University
Weak gravitational lensing provides an observational avenue to determine the relation between the halo mass and stellar mass of a galaxy. While we expect two galaxies with the same stellar mass to have different halos, at the moment, existing weak lensing studies are only sensitive to an average halo mass. In this talk, I will present an end-to-end methodology to measure the effects of weak lensing on individual galaxy-galaxy systems exploiting their kinematic information. I will present the results of analysing 21 weakly lensed systems and present ways to overcome weak lensing limitations and be sensitive to the dispersion in halo masses.
Wednesday September 9th 2020 @12pm, Zoom Colloquium
Dr Rob Bassett Swinburne University
Understanding the sources responsible for driving reionization has been a major goal in astrophysics for many years. One critical measurement required is the ionizing (or Lyman continuum, LyC) escape fraction from observed galaxy samples. A major difficulty arises from the level of transmission of LyC through the intergalactic medium (IGM), an unknown (but essential) quantity in the calculation of LyC escape from individual sources. The typical method is to assume an average transmission value based on consideration of HI column density distribution functions, but is this appropriate? In general, observational surveys at high redshift are strongly biased towards the brightest objects as these are the easiest to detect. Given the fact that LyC emission is remarkably faint, we should expect to only detect those galaxies with the highest emergent LyC flux. This, in turn, suggests that detections of LyC in surveys will be biased towards IGM sightlines with higher than average transmission of ionizing photons. In this talk I discuss the quantification of this IGM transmission bias for LyC detections and explore the implications when considering the recovered LyC escape values from current surveys. Careful consideration of such biases will be critical in understanding how LyC escape depends on galaxy properties, which ultimately colours our understanding of how reionization proceeds.
Wednesday August 26th 2020 @12pm, Zoom Colloquium
Dr Colin Jacobs Swinburne University
Neural Networks are finding increasing use in many areas of astronomy, but often act as “black boxes”. Many techniques exist to probe in the internals of neural networks but not all are relevant to scientists. In this talk I discuss some of the techniques developed in computer vision to investigate what neural networks are learning, and discuss some of their benefits and problems when applied to astronomy. I introduce a simple technique to probe what neural networks have learned and apply it to networks trained to find strong gravitational lenses.
Wednesday July 22nd 2020 @12pm, Zoom Colloquium
Melanie Hampel Monash University
The quest for the origin of the elements in the universe combines different fields of physics and astronomy, from the smallest scales of nuclear reactions to large scales of giant stars. To understand the chemical history of our universe the abundances of elements heavier than iron are observed in the photospheres of old stars. The vast majority of heavy elements are formed by the slow (s) and rapid (r) neutron-capture processes. However, some observations of heavy-element abundance patterns of old stars are incompatible with either of these processes or even a combination of both.
I will show that these puzzling heavy-element patterns can be explained as the result of a separate neutron-capture process operating at neutron densities intermediate to the s and r process: the i process. Comparing theoretical predictions of i-process nucleosynthesis with the observed abundance patterns gives us new insights into uncertain phases of stellar evolution and will ultimately help us understand the origin of the elements in our universe.
Wednesday June 17th 2020 @12pm, Zoom Colloquium
Nikhil Sarin Monash University
The first neutron star merger observed with gravitational waves and in electromagnetic radiation confirmed that binary neutron star mergers are the progenitors of at least some short gamma-ray bursts. The multi-messenger observations have been used to a probe a lot of fundamental physics, however, despite the wealth of observations the fate of the remnant of GW170817 is still uncertain. I will give an overview of binary neutron star mergers focussing on the nature of the remnant from observations of short gamma-ray bursts and theoretical considerations. I will discuss the implications of these observations on the nuclear equation of state, neutron star dynamics and gamma-ray bursts.
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