Australia Telescope National Facility

S&A Colloquium: Jean Francois Nezan (IETR – INSA, France)
2023-06-21 15:00 - 16:00 (Australia/Sydney) | NSW
Dataflow models for the design and programming of computing systems in astronomy

Abstract

Projects in astronomy like the Square Kilometer Array (SKA) or the European Extremely Large Telescope are among the world’s largest Big Data projects and the largest international computing collaborations, with unique computing challenges in Signal Processing and Machine Learning (SPML) still to be solved. The challenge in terms of computing, data transport and storage capacity is to design a processing chain ranging from the acquisition of raw data from the sensors to the production and the analysis of multidimensional images of the sky with Worldwide Distributed Calculations. In that context, a new generation of Low-Power High-Performance Computing Systems has to replace general-purpose High-Performance Computing (HPC) systems to meet the challenge of climate change, including the reuse and upgrades of computing systems already operational in a recycling approach. State of the art Programming Models and their Development Frameworks are lagging behind in offering support for efficient use of resources, high service availability and quality and cost competitiveness. This presentation will discuss how dataflow models associated with platform- and component-based designs can help to tame complexity during the design and the operating phases of big projects in astronomy, assessing the performance both in time and energy of a complex scientific workflow on a not-yet-existing computing infrastructure.
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S&A Colloquium: Ashley Ruiter (UNSW, Canberra)
2023-06-28 15:00 - 16:00
Mergers of white dwarfs and the transients they make

Abstract

For the last decade, mergers of double white dwarfs have been considered to be a likely, possibly even favoured, progenitor scenario for Type Ia supernovae. Though the Milky Way harbours ~100 million double white dwarf binaries, only a small fraction of these will actually merge in a Hubble time, let alone possess the “right” physical properties to lead to a Type Ia supernova if they do even merge. A different outcome of a white dwarf merger is the creation of another type of stellar transient: a hydrogen-deficient carbon (HdC) star. We know of ~130 of such ‘dusty’ HdC stars in the Galaxy – these are the R Coronae Borealis (RCB) stars, and they are thought to be the end result of a merger of a carbon-oxygen white dwarf and a helium-rich white dwarf, though their formation is not completely understood. In 2021, another type of hydrogen-deficient carbon star was found in great numbers: 27 ‘dustless’ HdC stars were spectroscopically confirmed in our own Milky Way, increasing the known number of this class from 4 (discovered >50 yrs ago) to 31. In 2022 we were granted time on the high-resolution (R~80,000) Veloce echelle spectrograph with the aim of disentangling the difference in evolutionary history between the long-known ‘dusty’ RCB stars and the newly-discovered plethora of ‘dustless’ HdC stars – both classes of objects presumed to be the result of double white dwarf mergers. Though we were mostly fully clouded out and we await new opportunities for obtaining and analysing observational data, I will discuss how different formation channels of white dwarf mergers may be plausibly linked to various transient phenomena such as Type Ia supernovae, dusty RCB stars, and dustless HdC stars.
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S&A Colloquium: Ophelie Renaud (IETR-INSA, France)
2023-07-12 15:00 - 16:00
Designing and Programming Astronomy Computing Systems for Optimal Power Efficiency and Performance

Abstract

In this presentation, we address the challenging problem of deploying complex astronomical applications on efficient target architectures. The allocation of resources in such scenarios significantly impacts performance outcomes. To automate and expedite this process, we propose a method that leverages the Synchronous Dataflow (SDF) model and the Scaling up of Cluster of Actor on the Processing Element (SCAPE) technique. Our approach, implemented in the PREESM rapid prototyping tool, facilitates automated deployment on CPU, GPU, and FPGA architectures. We showcase the advanced implementation of CPU-based resource allocation in our tool, which serves as a foundation for other target architectures. By providing a fast and easy resolution to this NP-hard problem, our method greatly enhances the efficiency of deploying astronomical applications.
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S&A Colloquium: Darren Croton (Swinburne)
2023-07-19 15:00 - 16:00
How to model the Universe in N easy steps

Abstract

Much progress has been made in recent years in our understanding of the co-evolution of galaxies and supermassive black holes, and their connection to the underlying large-scale dark matter structure. In this talk I will discuss simulation and modelling techniques that bridge theories of galaxy and black hole formation with the properties of observed galaxy populations. In addition, I will discuss a number of open questions important for extra-galactic astronomy and cosmology, and explain how future large-scale surveys and galaxy formation models may jointly address them.
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S&A Colloquium: Shriharsh Tendulkar (NCRA, India)
2023-08-02 15:00 - 16:00
Multi-scale and multi-wavelength searches for FRBs (and other wavelengths)

Abstract

Fulfilling the true promise of fast radio bursts (FRBs) as cosmological probes, understanding their origins, and their association with other transients necessitates our understanding of the FRB emission mechanism, their formation channel(s), and their local environments. Most emission mechanisms expect that the prompt radio emission represents a small fraction of the total energy released — a larger fraction of the energy should be released at higher frequency wavebands: optical, UV, X-ray and in some cases, gravitational waves (GW) and neutrinos, either as prompt emission or as a longer-lived afterglow. Given that the observed rates of FRBs are two to three orders of magnitude higher than the sky rates of any other extragalactic transient, it is clear that most FRBs will not have detectable multi-wavelength counterparts. The X-ray bursts aligned with the FRB-like radio emission from SGR 1935+2154 highlight the need to be alert to the brightest and nearest FRBs since those will be the most likely to have detectable multi-wavelength counterparts. Theoretical expectations also highlight the need to search for radio transients not just in the millisecond timescale, but also at second timescales, an area of phase space that is badly ignored. In this talk, I will discuss results from CHIME/FRB in trying to constrain counterparts of FRBs, FRB-GRB connections, and FRB-GW connections and our efforts to increase the phase space of radio transients by searching for not-so-fast radio bursts. For these science cases, I will also discuss two new projects: The proposed All Sky Transient Radio Array (ASTRA), an ultra-wide field of view telescope to detect the brightest and nearest radio transients and Daksha, a proposed all-sky X-ray mission with 2-satellites in antipodal orbits to increase the detection rate of GRBs and electromagnetic counterparts of GW events.
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S&A Colloquium: Barnali Das (CSIRO)
2023-08-09 15:00 - 16:00
Coherent radio emission from hot magnetic stars

Abstract

Hot magnetic stars are the perfect laboratories for understanding stellar magnetospheric phenomena due to the fact that their fields are extremely stable throughout their lifetimes, and also have very simple topologies (dipolar in most cases). Their strong stellar winds interact with the magnetic fields giving rise to co-rotating magnetospheres extending up to several tens of stellar radii. The magnetosphere manifests itself via emission over a wide range of the electromagnetic spectrum, starting from radio to X-ray. These emissions convey different kinds of information, and together, they act as probes for wind-magnetic field interplay. Among the different emissions that these magnetospheres produce, the latest discovered, and consequently, the least understood one is the coherent radio emission, also referred to as auroral radio emission. This is produced by electron cyclotron maser emission (ECME, the same mechanism that drives auroral radio emission from magnetic ultracool dwarfs and planets). In my talk, I will take you on our journey towards demystifying this phenomenon, starting with some serendipitous discoveries, followed by a systematic sub-GHz survey, which ironically led to the discoveries of many more unexplained characteristics, including discovery of radio flares, in addition to providing solutions to existing problems. I will present the results that we have obtained so far regarding ECME incidence, wideband properties and stability, and potential scenarios to explain the observations. I will end with my future plan to obtain more definitive insights about magnetospheric operation and the role that ATNF instruments can play in achieving that goal.
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S&A Colloquium: Virginia Kilborn (Swinburne)
2023-08-16 15:00 - 16:00
HI surveys from next generation radio telescopes

Abstract

Hydrogen is the most common element in the Universe. Powering star formation, it plays a crucial role in the formation and evolution of galaxies. Observations of Neutral Hydrogen (HI) in the radio wavelength allow us to survey for galaxies in the nearby universe, and next generation surveys from telescopes such as the Australian Square Kilometre Array Pathfinder (ASKAP), meerKAT and FAST are expected to provide redshifts, kinematics and gas content of around hundreds of thousands of galaxies, and probe the gaseous environment surrounding – and inbetween – galaxies. In this talk I’ll provide an overview of HI science, what we’ve learnt to date, and touch on some of the potential future results of these new telescopes.
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S&A Colloquium: Jennifer West (NSCC)
2023-08-23 15:00 - 16:00
Progress on understanding the Galactic magnetic field

Abstract

The pursuit of understanding the structure and origin of the Milky Way’s magnetic field is a central science question driving several radio polarization surveys, including the Polarization Sky Survey of the Universe’s Magnetism (POSSUM) and The Global Magneto-Ionic Medium Survey (GMIMS). I will give a brief overview of the progress we have made towards unravelling the answers to these questions.
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S&A Colloquium: Sarah Pearce (SKAO)
2023-08-30 15:00 - 16:00
The next generation of radio astronomy: progress on construction of the SKA Observatory

Abstract

The SKA Observatory is now starting construction of its two telescopes, SKA-Low and SKA-Mid, on site in Western Australia and South Africa. This talk will provide the latest status on construction, exploring the current and planned work at each site. It will also look at establishment of the Observatory worldwide, with a particular focus on Australia and the partnerships with CSIRO.
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S&A Colloquium: Hyein Yoon (University of Sydney)
2023-09-06 15:00 - 16:00
FLASH – New searches for Extragalactic HI 21-cm absorption at 0.4 < z < 1.0

Abstract

FLASH (the First Large Absorption Survey in HI) is a wide-field radio survey searching for 21-cm line absorption at 0.4 < z < 1.0. The goal of the survey is to understand the evolution of the neutral gas in galaxies and its role in AGN accretion and feedback. In this talk, I will highlight a set of new HI 21-cm absorption lines detected in the ASKAP Pilot Survey Phase 1 and 2. Both associated systems (where the HI gas is at the same redshift as the background radio source) and intervening systems (where the HI gas is in the foreground along the line-of-sight) will be discussed. Additionally, I will report an update on the FLASH Full Survey, which will continue for the next five years.
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S&A Colloquium: Luke Davies (UWA)
2023-09-13 15:00 - 16:00
A Universal Murder Mystery – What is killing galaxies?

Abstract

The Universe is dying. All across the cosmos for the last 10 billion years galaxies have been continuously killed, and we still don’t know why. Today the Universe is littered with dead galaxies, and their distribution is correlated with location (environment). This suggests that location is one of the prime suspects in this galactic whodunit. However, previous observations and techniques have restricted us to only measuring galaxy environments in the nearby Universe, inhibiting our ability to identify when, where and why they are killed. In addition, galaxy location might not be the only suspect in town. Monsters are hiding at the centres of the most massive galaxies, ready to lead to their untimely demise. In this talk I will discuss various programs using large multi-wavelength surveys, such as GAMA and DEVILS, to explore the smoking gun factors that have led to the death of galaxies over the last 10billion years.
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S&A Colloquium: Anais Moller (Swinburne)
2023-09-20 15:00 - 16:00
Transients in the era of large scale surveys

Abstract

Vera C. Rubin Observatory will open a new era for optical astronomy by imaging the Southern Sky at unprecedented depths. I will introduce Fink, which will process Rubin’s millions transient detections every night. Fink not only processes immense datasets in real-time, but selects the most promising candidates for a breadth of science using machine learning and multi-wavelength and messenger survey data. n To make a leap to this new era, I will review the contributions of machine learning classification for transient science and cosmology in the Dark Energy Survey. I will then present results from Fink’s processing of the Zwicky Transient Survey, as a precursor of Rubin. These results include selection methods and analyses of supernovae, kilonovae and satellite glints. I will conclude with the potential of connecting Rubin’s data with other observing facilities for multi-wavelength and messenger astronomy and our current efforts with Australian facilities.
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S&A Colloquium: Chayan Chatterjee (UWA)
2023-09-27 15:00 - 16:00
Enabling rapid discovery of gravitational waves using machine learning

Abstract

Gravitational waves are ripples in spacetime curvature created by some of the most energetic events in the universe like the collisions of compact astrophysical objects like black holes and neutron stars. These ripples propagate through space at the speed of light and are detected by several km long laser interferometers called LIGO and Virgo, located in the US and Italy. The simultaneous observation of gravitational waves and prompt electromagnetic emissions from the compact object mergers can help reveal properties of extreme matter and gravity during and immediately after the coalescence. However, such simultaneous observations rely on rapid detection and sky localization of gravitational waves, often requiring alerts to be sent out before merger.

In this seminar, I will describe different machine learning models that I have developed to solve challenging problems in rapid gravitational wave discovery – pre- and post-merger sky localization, and waveform extraction from real detector data. I will demonstrate the accuracy and feasibility of these methods on simulated data as well as on real gravitational wave events detected during the first three observation runs of LIGO and Virgo. Finally, I will talk about the future scope of this research work and highlight some of the other areas in gravitational wave astronomy where these deep learning techniques can be applied.
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S&A Colloquium: Chris M. Moeckel (UC Berkeley)
2023-10-25 15:00 - 16:00
Radio observations of the giant planets: from the ground and up-close

Abstract

The vertical distribution and evolution of trace gases in planetary atmospheres are key observables for understanding the dynamics of the atmosphere. Radio observations, such as observations from the Microwave Radiometer (MWR) on board of Juno and observations from the earth-based telescopes such as the Very Large Array (VLA) are our current best tools to understand the sub-cloud structure on the giant planets. However, inverting radio observations to recover the atmospheric structure is non-trivia. Based on the commonalities between different atmospheric model frameworks, we show surprising results of what we learned about Jupiter’s atmosphere and address the implications for the atmosphere of giant planets in general.
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S&A Colloquium: Fernando Camilo (SARAO)
2023-11-01 15:00 - 16:00
MeerKAT at 5

Abstract

The MeerKAT radio telescope in South Africa was inaugurated 5 years ago. In this talk I’ll provide a broad overview of science results to date. Among other examples, I’ll show: how an exquisitely sensitive continuum image is constraining the star formation history of the universe, putting it into tension with UV+IR-derived evolution; the discovery of OH megamasers at redshift > 0.5; how the study of neutral hydrogen down to column densities of ~5e17/cm^2 is enabling Local Group science outside the Local Group, and revealing how gas is shaped and removed from galaxies in cluster environments. Come see how MeerKAT combined with a pulsar timing backend developed in Australia is enabling unprecedented tomographic mapping of the Local Bubble and constraints on a stochastic background of gravitational waves. I’ll also cover the discovery and study of numerous extragalactic and Galactic transients – including still mysterious sources with rotation periods up to tens of minutes – done in part jointly with the MWA and ASKAP.
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S&A Colloquium: Yuzhe (Robert) Song (Swinburne)
2023-11-08 15:00 - 16:00
Gamma-ray Emission from Nearby Stellar Flares and Undetected Pulsars

Abstract

The continuing survey of the gamma-ray sky by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope enables us to carry out population studies in gamma-rays. We developed novel stacking techniques to analyse Fermi-LAT data beyond the limit of point source sensitivity, which subsequently have been applied to various types of astrophysical sources.

So far, less than 10% of known pulsars are detected in gamma-rays. Stacking analysis of pulsars off the Galactic plane shows a significant detection, with a pulsar-like spectral index and a characteristic flux well below the Fermi-LAT point source sensitivity. Using COMPAS, a rapid binary population synthesis code, we provide a best fit model of distributions of pulsar properties at birth that can describe our observations.

Young ultra-cool dwarfs are extremely magnetically active and produce highly energetic flares, yet the Sun is the only isolated main sequence star detected in gamma-rays. We present a temporal analysis of red dwarf TVLM 513-46546 and reported periodicity in gamma-rays. Stacking Fermi-LAT data during flare events identified from optical and X-ray flare surveys reports an upper limit of flare gamma-ray emission. It likely originates from decay of neutral pions generated from the relativistic protons interacting within the stellar atmosphere. If confirmed, this could be catastrophic on exoplanet atmospheres and significantly reduces the habitability on these worlds.
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S&A Colloquium: Joshua Pritchard (University of Sydney)
2023-11-15 15:00 - 16:00
Searches for Stellar Radio Activity in Circular Polarisation

Abstract

Transient radio bursts produced in the coronae of stellar atmospheres and within planetary magnetospheres often feature a high degree of circular polarisation. These events reveal the presence of strong magnetic fields, and trace particle acceleration driven by magnetic reconnection, space weather, and auroral current systems. The detection of stellar radio emission has historically been challenged by the overwhelmingly higher surface density of extra-Galactic sources that dominate the radio sky, which produce a large number of false positive associations to optically identified stars. Our knowledge of the radio star population has therefore been primarily driven by targeted studies of the most active, nearby systems, impacting the inference of population statistics and discovery of new regimes of stellar radio activity.

In this talk I will present a body of work that exploits the circular polarisation of magnetically driven stellar radio emission to identify large samples of radio stars in widefield surveys. I will describe the application of this technique to two widefield surveys with the Australian Square Kilometre Array Pathfinder (ASKAP—the Rapid ASKAP Continuum Survey (RACS) and the Variables and Slow Transients (VAST) Pilot Survey—which resulted in 229 detections of a sample of 76 radio stars. Through repeat sampling of this group our observations place constraints on the statistical fraction of the M-dwarf population that produce radio bursts and provide a forecast of expected detection rates in future surveys with ASKAP and the Square Kilometre Array. Finally, I will discuss the utility of circular polarisation searches to uncover novel regimes of stellar radio activity. I will highlight the discovery of rotationally modulated auroral radio pulses from an old, slowly rotating M9.5 ultracool dwarf in a two year monitoring campaign with ASKAP and the Australia Telescope Compact Array. This object is the oldest and slowest rotator among known aurorally active ultracool dwarfs and displays a rich variety of radio burst features, forming an important probe of the dynamo mechanism responsible for strong magnetism in ageing cool stars.
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S&A Colloquium: Prajval Shastri (ICRAR / RRI)
2023-11-22 15:00 - 16:00
Different Angles on Accreting Supermassive Black Holes

Abstract

Supermassive black holes appear to inhabit the centres of most galaxies and co-evolve with them, thus playing a significant role in regulating galaxy assembly. We are able to spot these black holes to very high redshifts when they accrete matter, which is also what causes them to impact their environments out to spatial scales that are well beyond their gravitational sphere of influence. In this talk I will discuss the understanding that has emerged from multiple-frequencies studies of the systematics of accreting supermassive black holes.

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S&A Colloquium: Amit Seta (ANU)
2023-11-29 15:00 - 16:00
Magnetic fields in the Milky Way from pulsar observations

Abstract

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 rotation and dispersion measure. 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. We then correlate existing pulsar observations from the Australia Telescope National Facility with regions of enhanced thermal electron density probed by CO, Zeeman, neutral hydrogen 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 provides a good estimate of the magnetic field on Galactic scales, but might break down on sub-kpc scales.
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S&A Colloquium: Dipanjan Mitra (NCRA, TIFR)
2023-12-06 15:00 - 16:00
Rotating vector model of pulsars

Abstract

More than Fifty years have passed since the discovery of pulsars, yet the physical mechanism of their coherent radio emission is unclear. The linear polarization position angle (PPA) tracks observed in normal pulsars (periods > 0.1 sec) can be explained by the rotating vector model (RVM), which forms the basis for understanding pulsar coherent radio emission physics. In several pulsars only one RVM like PPA track is seen while in other pulsars two parallel orthogonal tracks are seen, and this variation can be understood as radiation coming out from the pulsar magnetosphere to mainly consist of extraordinary or ordinary waves polarized perpendicular or parallel to the planes of pulsar dipolar magnetic ﬁeld. However there also exists an equally large sample of pulsars where the PPA traverses shows a complex non-RVM pattern. This diverse behaviour is in conflict with applying a unique emission mechanism to the pulsar population, and hence the fundamental question of whether the outgoing waves are excited by maser or curvature radiation (also known as antenna mechanism) had remained unresolved. In this talk I will argue that high quality single pulse polarimetric observations can be used to distinguish the emission mechanism. I will showcase examples of how highly linearly polarized signals can unravel the underlying RVM, particularly in pulsars with PPAs showing complex patterns. This feature strongly favours the coherent curvature radiation mechanism to be universally applicable in the normal pulsar population.
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S&A Colloquium: Ron Ekers (CSIRO)
2023-12-13 15:00 - 16:00
History of radio astronomy

Abstract

It was 90 years ago when Karl Jansky announced his discovery of radio noise from the Milky Way Galaxy at the April 1933 meeting of URSI in Washington and Radio Astronomy was born. I was asked to give a General Lecture on the entire history of radio astronomy at the URSI General Assembly in Sapporo, Japan on 23 Aug 2023. I will give a slightly abbreviated version of this lecture as a S&A colloquium.

Instead of just presenting a historical review of all the discoveries I will explore some of the circumstances leading up to the discoveries, including some stories not generally known, but which provide the background and context. These details are often excised from the standard scientific narrative but are essential to understand the roles played by serendipity, prediction, and technology. There is “nothing fortuitous” in so-called serendipitous discoveries. As Pasteur famously quoted “In the field of observation, chance favors only the prepared mind.”

While many discoveries are serendipitous, they depend on the development of new technology. So, it is the telescopes, the instruments connected to the telescopes, and the data analysis that leads to most new discoveries. The scientific discoveries for which facilities become famous are rarely those predicted from the science goals for which the telescopes were built. Building the next generation of radio telescopes to continue our 90 years of innovation and discovery will set new challenges.
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S&A Colloquium: Joe Callingham (ASTRON/Leiden University)
2023-12-20 15:00 - 16:00
Radio stars and Exoplanets

Abstract

One key question that astronomy is attempting to answer is whether there are habitable planets around stars other than our Sun. While we have entered an era where identifying nearby exoplanets has become standard, discerning whether the environmental conditions dictated by the host stars are suitable for life has proved far more elusive. The detection of low-frequency radio emission from an M dwarf or an exoplanet provides a direct probe of extrasolar space weather and the planet’s magnetic field – information crucial for assessing the potential habitability of the planet. In this talk, I will outline our LOFAR survey of stellar systems, with a focus on our recent detection of strong, highly circularly polarised low-frequency radio emission associated with nearby stars – the expected signpost of star-exoplanet interactions. I will discuss how our survey represents the most comprehensive observations of stellar systems at low frequencies, and the implications of this new population in understanding the magnetosphere of M dwarfs and exoplanetary magnetic fields.

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S&A Colloquium: Kana Morokuma-Matsui (University of Tokyo)
2024-01-17 15:00 - 16:00
ALMA CO surveys of the Fornax cluster and the Eridanus supergroup

Abstract

The evolution of galaxies is affected by their surrounding environment. The star formation (SF) in galaxies in galaxy clusters is known to be less active compared to those in the normal field. In order to understand the SF quenching processes in cluster galaxies, we conducted CO(J=1-0) mapping survey of 64 Fornax cluster galaxies and 103 Eridanus supergroup galaxies using the Morita array of ALMA in cycles 5 and 7, respectively. The Fornax cluster is the 2nd nearest galaxy cluster from the Milky Way at a distance of 20 Mpc. The Eridanus supergroup is located near the Fornax cluster and is an ensemble of galaxy groups that are expected to be a single cluster in the future. For the Fornax cluster, the achieved beam size and sensitivity are 15′′ × 8′′ and ~12 mJy/beam1 at the velocity resolution of ∼10 km/s, respectively. CO emission is detected from 23 out of the 64 galaxies. We find the low SF activity in the cluster galaxies is attributed to the low molecular gas contents of galaxies. The SF efficiency from molecular gas, a ratio of SF rate over molecular gas mass, in the Fornax galaxies is comparable to that of field galaxies. Combined with archival HI data, we also find that the atomic-gas reservoirs are more heavily reduced than the molecular-gas reservoirs in the Fornax galaxies with low SF activity. In this colloquium, I will present the preliminary results for the Eridanus supergroup galaxies.

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S&A Colloquium: Tomoki Morokuma
2024-01-24 15:00 - 16:00
Time Domain Astronomy with Subaru Hyper Suprime-Cam and A Serendipitously Discovered Active Galactic Nucleus with Large Brightness Decline

Abstract

Time Domain Astronomy has been getting popular especially in optical astronomy, mainly using wide-field imaging instruments. Hyper Suprime-Cam (HSC) on the 8.2-m Subaru telescope currently has the best survey capability in terms of survey speed. We have been conducting transient surveys with Subaru/HSC in various time scales from minutes to years since its first light in 2014. I will summarize several main results from our transient surveys. In the latter part of my talk, I focus on one distant active galactic nucleus (AGN) showing a very large decline in brightness over 20 years. This object is interpreted as a rare phenomena in an AGN where its accretion activity has been almost shut down.

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S&A Colloquium: Hamsa Padmanabhan (University of Geneva)
2024-02-07 15:00 - 16:00
Deciphering Cosmic Dawn: A Conquest of the Final Frontier

Abstract

I will begin with a broad-based introduction to some of the big open questions in cosmology, and illustrate how upcoming experiments are well-poised to help answer them, especially at the Cosmic Dawn — the birth of the first galaxies in our Universe — widely believed to be the ‘final frontier’ of cosmological surveys today. This period — about a hundred million years after the Big Bang — is primarily accessible due to radiation from hydrogen, the most abundant element in the Universe, which emits at a wavelength of 21 cm, in the radio band. An exquisite investigation of the Cosmic Dawn will soon become possible with an emerging technique called intensity mapping (IM), which measures the integrated 21 cm emission from all sources, using large arrays of radio telescopes. A particular advantage of IM is that it provides a tomographic, or three-dimensional picture of the Universe, unlocking significantly more information than we presently have from galaxy surveys. I will overview the latest advances in research related to the evolution of hydrogen over 12 billion years of cosmic time, involving a novel data-driven framework to interpret current and future observations. This allows us to fully utilize our present knowledge of astrophysics in order to develop cosmological forecasts from IM. Apart from offering key insights into the nature of the first galaxies, this opens up the exciting possibility of testing theories of fundamental physics from the Cosmic Dawn.

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S&A Colloquium: Rudrani Chowdhury (University of Hong Kong)
2024-02-14 15:00 - 16:00
Detecting last light from first stars

Abstract

First generation metal-free stars, referred to as population III (Pop III) stars, are believed to be the first objects to form out of the pristine gas in the very early Universe. Pop III stars have different structures from the current generation of stars and are important for generating heavy elements and shaping subsequent star formation. However, it is very challenging to directly detect Pop III stars given their high redshifts and short life-times. In this work, we propose a novel signature for detecting Pop III stars through their tidal disruption events (TDEs) by massive black holes. We model the emission properties and calculate the expected rates for these unique TDEs in the early Universe at z ~10. We find that Pop III star TDEs have much higher mass fallback rates compared to normal TDEs in the local universe and are therefore rather luminous, rendering them feasible for detection. They also have very long observed flare evolution timescale, making it more likely to detect such TDEs during their rising phase. We further demonstrate that a large fraction of the TDE emissions are redshifted to infrared wavelengths and can be detected by the James Webb Space Telescope and the Nancy Grace Roman Space Telescope. Lastly, the TDE rate sensitively depends on the black hole mass function in the early Universe. We find a promising Pop III star TDE detection rate of up to a few tens per year using the Nancy Grace Roman Space Telescope.

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S&A Colloquium: Stanley Owocki (University of Delaware)
2024-02-28 15:00 - 16:00
Stellar-wind-fed magnetospheres of magnetic massive stars

Abstract

A sub-population (~9%) of hot (OB) stars exhibit strong (B~100-10,000 G), large-scale (often predominantly dipolar) magnetic fields that channel their stellar wind outflows into circumstellar magnetospheres. For young, rapidly rotating B-stars that have not yet been spun down by wind-magnetic braking, wind material can be trapped between the Kepler co-rotation radius (RK) and the Alfven radius (RA), forming then a “Centrifugal Magnetosphere” (CM), with density set at the critical level for “Centrifugal Breakout” (CBO) against the confining magnetic tension. This talk discusses how such CBO controls both the onset and strength of observed H-alpha emission, while the energetics of the associated CBO-driven magnetic reconnection match well the observed scalings of a non-thermal, circularly polarized radio emission from such stars .

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S&A Colloquium: Nivedita Mahesh (California Institute of Technology)
2024-03-06 15:00 - 16:00
The Frontier in Low Frequency Radio Astrophysics from the Farside of the Moon

Abstract

The Farside Array for Radio Science Investigations of the Dark ages and Exoplanets (FARSIDE) is a NASA Probe-class concept to place a low-frequency radio interferometric array on the far side of the Moon. FARSIDE will look for radio emissions from magnetospheres of stars and exoplanets, which affect exoplanet habitability and serve as a pathfinder for studying the Dark Ages of the Universe. The FARSIDE architecture employs a novel design consisting of 128 pairs of antennas and receiver nodes distributed over a 10 km × 10 km area in a four-arm spiral configuration that will operate from 200 kHz to 40 MHz. I will present the planned mission architecture, array layout, antenna packaging, and deployment trade study. In addition, this talk will cover the projected performance and sensitivity of the array for the two primary sciences cases. Given the novel deployment strategy for the FARSIDE array, we have developed a forward modeling pipeline to simulate its polarization performance. This lunar array concept is developed a lot on the heritage of a similar low frequency telescope on ground – the OVRO Long Wavelength Array. And I will briefly discuss the current status and pipeline development efforts of the newly upgraded OVRO-LWA. The talk will end with an overview of one of humanities’ next “great” observatories – FarView – a NASA Innovative Advanced Concepts. Farview will be an observatory built in-situ on the lunar farside using materials from regolith.

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S&A Colloquium: Magda Arnaboldi (ESO)
2024-03-13 15:00 - 16:00
The evolution of the Giant Andromeda galaxy mapped using planetary nebulae in its disc and inner halo

Abstract

In this talk on the Andromeda (M31) galaxy, I will present the results from a homogenous, extended survey of Planetary Nebulae covering the entire disc and inner halo of M31 out to 50 kpc radius. Taken jointly with observational results from previous extensive investigations, these findings support a recent (~2.5 Gyr), massive (1:4 mass ratio) accretion event from a satellite infalling along the Giant stellar stream, in a nearly radial orbit. This tumultuous recent past of Andromeda is very different from our own Milky Way galaxy, that experienced a rather quiescent evolution in the last 8 Gyrs. I will present the evidence for a younger thin and older thicker disc component in M31 and present the age-velocity dispersion relation at the corresponding radial distance of the solar neighbourhood in M31. Then I explore the chemical composition of these discs using Oxygen and Argon element abundance. The two kinematically distinct discs in Andromeda are also chemically distinct, 1) with the thin disc reaching higher Argon abundances than the thicker disc, 2) the thicker disc having a positive Oxygen and null Argon radial gradient, 3) while the thin disc presents a negative radial gradient in both Oxygen and Argon. The latter gradient is consistent with the negative Oxygen gradient previously measured for the HII regions. I then use the log(O/Ar) vs. (12 + log(Ar/H) ) distribution of stars with different ages to constraint the chemical evolution of the parent stellar populations in the thin and thicker M31 discs. The distributions in this plane show that the chemical and also the structural properties of the thin and thicker discs in M31 are remarkably different from those determined for the Milky Way. I then investigate the stellar populations and kinematics of the inner halo substructures – NE and W shelves, Giant stellar stream and G1 clump – and compare them with the detailed predictions from simulations of a major merger event in Andromeda. The goal is to understand whether it is a wet or a dry merger, and to further constrain the pre-merger M31 disc population and that of the satellite as well. As part of the forward look, I shall present the potential information from the combined use of Oxygen and Argon to constrain the early phases of star formation in z>4.0 galaxies.

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S&A Colloquium: Anthony J. Beasley (NRAO)
2024-03-20 15:00 - 16:00
Near and Long-term Trends and Opportunities in US Radio Astronomy

Abstract

In this talk I will discuss the ambitious scientific program underway in the United States over the next decade to explore the radio Universe, in concert with numerous powerful new instruments around the world, and facilities at other wavelengths. The scientific opportunities and technical/operational challenges being faced by ground-based radio astronomy in the United States currently are significant; half a decade beyond the 2020 Decadal Survey process, factors both internal and external to the astronomy community have slowed the development of many U.S. facilities. NRAO is exploring several new technologies and operational approaches to optimize the scientific return from our existing and planned instruments; these will be described. As the field of radio astronomy approaches its centenary, we continue to seek new scientific horizons.

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