Insights into binary black hole formation from gravitational waves

Wed 01 Nov, 2017 @12PM, Level 7
Dr. Simon Stevenson, OzGrav postdoctoral fellow
Swinburne University
Email: spstevenson[at]swin.edu.au

Abstract
In its first observing run (O1), Advanced LIGO detected gravitational waves from two binary black hole mergers, GW150914 and GW151226, along with a statistically less significant candidate LVT151012. The Advanced LIGO detectors were joined by the Advanced Virgo detector in the recently concluded second observing run (O2). Another two binary black hole mergers, GW170104 and GW170814, have been announced so far from this analysis, with the analysis of the data ongoing. These observations confirmed the existence of merging stellar mass black hole binaries originating in low metallicity environments. How to form merging binary black holes remains an open question theoretically. There are many proposed mechanisms, falling broadly into two families: 1) Isolated binary evolution and 2) dynamical formation in dense stellar environments. We show that comparing the distribution of observed black hole masses and spins to those predicted from population models will allow us to gain insight into how binary black holes form. We examine the measurements of the effective spin parameters for the 5 binary black hole observations published thus far and show that the data already exhibit a mild (2.7 sigma) preference for an isotropic distribution of spins over spins aligned with the binary orbital angular momentum. Alternatively, dimensionless black hole spin magnitudes in these systems could be intrinsically small (average spin magnitude < 0.2), at odds with measurements of black hole spin magnitudes in high mass X-ray binaries. Both of these conclusions have implications for the formation of binary black holes, and the formation of heavy black holes in supernovae.

Probing Fundamental Physics with Strong Gravitational Lensing

Wed 25 Oct, 2017 @12PM, Level 7
Dr. Thomas Collett, Dennis Sciama Fellow
Institute of Cosmology & Gravitation (ICG), Univ. of Portsmouth
Email: thomas.collett[at]port.ac.uk

Abstract
Local measurements of the expansion rate are in tension with those inferred from observations of the distant Universe. Is this the first sign of new physics or merely a sign of systematic errors within individual probes? This key question remains unsolved, because there are only a handful of established probes. Here I will talk about how strong gravitational lensing offers a new window on precision cosmology, shining a new light on the dark Universe.

I will present strong lensing constraints on the expansion rate of the Universe and the equation of state of dark energy. I will also show how lensing combined with stellar dynamics yields the most precise test to date of the validity of General Relativity on extragalactic scales.

How standard is the standard candle?

Wed 11 Oct, 2017 @12PM, Level 7
Prof. Jeremy Mould, Professor
Swinburne University
Email: jmould[at]swin.edu.au

Abstract
Over 25 years ago New York Times science writer Dennis Overbye wrote a book oddly titled Lonely Hearts of the Cosmos, narrating the saga of the measurement of distances in the expanding universe, beginning with Edwin Hubble’s first measurements and culminating in a controversy in the scientific literature between Allan Sandage, who had taken up Hubble’s mantle, and Texas astronomer, Gerard de Vaucouleurs. Stranger still, another NYT writer later reported that the film rights had been taken up, and that Tom Hanks had been invited to play Sandage. No movie was made, however, and the controversy was cleared up a few years later by NASA’s Hubble Space Telescope. The age of the expanding universe is now known to an accuracy of a few percent.

There is now a new urgency for increased accuracy in measurements of the Hubble Constant, the misnomer for the current epoch value of the accelerating expansion rate. Stellar Populations and the Distance Scale, sometimes called the distance ladder, as it ascends from the geometric diameter of the Earth’s orbit, has been joined by a rival ladder, descending from the surface of last scattering of the cosmic background radiation. They were expected to meet in the middle, but there seems to be a gap. Or is there? If there is, the standard model of cosmology, the Nobel prize winning ΛCDM model, may be lacking something important. Is it another neutrino, curvature, or a dark matter-neutrino interaction? Right now, the jury is out, and the race for better data is on. Volume simulations indicate that sample variance is not an issue. Are supernovae really standard candles? The OzDES survey is addressing this issue.

The Hubble Constant from Baryon Acoustic Oscillations (BAO) is a standard ruler, rather than a standard candle like supernovae. The ruler is simply the sound horizon at recombination. Simulations tell us that the linear regime for the evolution of structure pertains, and the ruler is unchanged in length in comoving coordinates to a fraction of a percent over the life of the Universe. The Taipan project is expected to achieve 1% accuracy in H0 (1σ) from redshifts of over a million galaxies in the next 5 years. Between the high redshift of BAO and the low redshift of the Local Group, the Hubble Constant can be measured using gravitational lensing with no reference to stars. The H0liCow collaboration has results in agreement with the stellar pops distance scale. A convincing next step would be two independent stellar populations distance scales (Pop I & Pop II) agreeing to 1%.

HI and metal absorption lines during the Epoch of Reionization

Wed 04 Oct, 2017 @12PM, Level 7
Luz Ángela Garcia, PhD student
Swinburne University
Email: lgarcia[at]swin.edu.au

Abstract
In this work, we study the epoch of Reionization (EoR) with metal absorption lines in quasar spectra at high redshift, using high resolution hydrodynamical simulations (an improved version of GADGET-3). For this purpose, we set up the physical conditions of the intergalactic medium (IGM) at the redshift of the EoR, and we post-process the simulations to implement a uniform UV ionizing background for quasars and galaxies (Haardt-Madau 2012), the metal ions with CLOUDY 8.1 and HI self-shielding prescription (Rahmati et al. 2013). We use Voigt profile fitting to compute the column densities of the ions from the synthetic spectra and obtain a statistical distribution of the absorbers. This procedure allows us to study the evolution of the state of the IGM at high redshift, compute the cosmological mass density of CIV and HI and other ions. Our simulations produce absorbers properties that are in good agreement with observations in the literature, especially for the high ionization species.

Furthermore, we are able to reproduce an observed example of an LAE galaxy-CIV absorber pair at z=5.7, proving a physical insight into such systems beyond the limit of current observations. Finally, we vary of the uniform UVB at z~6, and compare directly with observations of different metal ions, in order to constrain the ionizing background at the tail of Reionization.

Stochastic gravitational waves in the reach of aLIGO?

Wed 27 Sep, 2017 @12PM, Level 7

Prof. Csaba Balazs, Professor
Monash University
Email: csaba.balazs[at]monash.edu

Abstract
Based on a scalar singlet extension of the Standard Model of elementary particles, I present a scenario that features stochastic gravitational waves potentially observable by aLIGO.

The many lives of AGN II: the formation and evolution of radio jets and their impact on galaxy evolution

Wed 20 Sep, 2017 @12PM, Level 7
Prof. Darren Croton, Professor
Swinburne University
Email: dcroton[at]astro.swin.edu.au

Abstract
In this talk i describe new efforts to model radio AGN in a cosmological context using the SAGE semi-analytic galaxy model and a number of large cosmological N-body simulations. Our new method tracks the physical properties of radio jets in massive galaxies, including the evolution of radio lobes and their impact on the surrounding gas. Unlike the previous efforts of Croton et al. 2006, we now self consistently track the cooling-heating cycle that significantly shapes the life and death of many types of galaxies. Adding jet physics to SAGE adds new physical properties to the model output, which in turn allows us to make more detailed predictions for the AGN and galaxy populations, and build customised AGN-focused mock survey catalogues for comparison with observations.

Shining a light on planetary processes

Wed 13 Sep, 2017 @12PM, level 7
Dr. Helen Brand, Scientist – Powder Diffraction
Australian Synchrotron

Email: helen.brand[at]synchrotron.org.au

Abstract

Jarosites and related minerals are of great importance to a range of mineral processing and research applications. They are used in the removal of iron species from smelting processes; they occur in metal bioleaching systems, and in the desulphurisation of coal; they are present in acid mine drainage environments.

There has been a recent resurgence in interest in jarosite and associated minerals since their detection on Mars by the MER rover Opportunity. In this context, the presence of jarosite has been recognised as a likely indicator of liquid water at the surface of Mars in the past and it is hoped that their study will provide insight into the environmental history of Mars.

Acid sulfate soils cover large areas of the Australian coastline and are likely to be a major constituent of the Martian environment. The oxidation of acid sulfate soils, coupled with potential release of heavy metals and acidic groundwaters, can have serious consequences for fragile ecosystems. Understanding these sediments will provide insight into the biogeochemical processes that affect the lifetimes of transient mineral species on Earth, and may be used to better understand soil acidification, contaminant mobility at sites affected by acid and metalliferous drainage, and even constrain past weathering and putative biosignatures on Mars.

Knowledge of the behaviour of jarosite minerals under the actual conditions that they are found in is crucial to understanding their potential environmental impacts on both Earth and Mars. To this end, we are engaged in a program to study the formation, stability and alteration of jarosite minerals using a complementary suite of in situ synchrotron and neutron techniques.

After GW150914: gravitational-wave astronomy in the era of routine detection

Wed 30 Aug, 2017 @12PM, level 7
Dr. Eric Thrane, Senior Lecturer
Monash University

Email: eric.thrane[a]monash.edu

Abstract

The first detection of gravitational waves from merging black holes (dubbed GW150914) ushered in the era of observational gravitational-wave astronomy. Since this seminal discovery, Laser Interferometer Gravitational-wave Observatory (LIGO) has announced the detection of three additional confirmed merger events—four if we include the marginal LVT151012. As the sensitivity of our detector network improves, detections will become routine. Indeed, at design sensitivity, LIGO may be detecting several events every week. This glut of gravitational waves presents us with new opportunities. I discuss a few of the research directions that have emerged in the wake of LIGO’s first detections including observationally-driven analysis of binary black hole formation channels, measurements of gravitational-wave memory, and tests of the famed no-hair theorem.

Modelling thermonuclear supernovae: how to blow up a white dwarf star

Wed 09 Aug, 2017 @12PM, level 7
Dr. Stuart Sim, Lecturer/Associate Investigator (SkyMapper)
Queen’s University Belfast, UK

Email: s.sim[at]qub.ac.uk

Abstract

Aside from being spectacular displays in their own right, Type Ia supernova explosions have a key role in measuring the expansion history of the Universe and synthesizing the iron group elements. But what is their origin? That Type Ia supernovae arise from exploding white dwarfs is relatively well-established but the manner in which the explosion is ignited and how this can be determined from what we observe remain hotly debated issues.
I will discuss the theoretical modelling of Type Ia supernovae with particular focus on how radiative transfer simulations can be used to test explosion scenarios. I will argue that understanding the diversity of thermonuclear supernovae requires us to investigate a variety of different progenitor scenarios. Specifically, I will present recent results from our work on both Chandrasekhar mass white dwarf explosion scenarios and sub-Chandrasekhar mass models.

Galaxies at Cosmic Dawn: Exploring the First Billion Years with Hubble and Spitzer – Implications for JWST

Wed 02 Aug, 2017 @12PM, level 7
Prof. Garth Illingworth, Professor
University of California Santa Cruz
Email: gedi[at]ucolick.org

Abstract

Hubble has revolutionized the discovery and study of very distant galaxies through its deep imaging surveys. Together the HST WFC3/IR and ACS cameras have opened up the exploration of the universe in the first billion years after the Big Bang. I will discuss what we have learned about the earliest galaxies during the reionization epoch at z>6 from the remarkable HST and Spitzer imaging surveys (e.g., HUDF/XDF, GOODS, HUDF09/12 and CANDELS), as well as surveys of galaxy clusters like the Frontier Fields (HFF). Lensing clusters provide extraordinary opportunities for characterizing the faintest earliest galaxies, but also present extraordinary challenges. Together these surveys have reliably established the volume density of galaxies in the first billion years down to extremely faint levels around -14.5 mag. The results from deep UV luminosity functions from Hubble, combined with the recent results from Planck, indicate that galaxies dominate the UV ionizing flux that reionized the universe. Some of the greatest surprises have come from the discovery of very luminous galaxies at z~8-11, around 400-650 million years after the Big Bang. Spectroscopic followup of these very rare, bright galaxies has confirmed redshifts from z~7 to z~11, and revealed, surprisingly, strong Lyα emission near the peak of reionization when the HI fraction in the IGM is high. The small sizes of galaxies at high redshifts, from analysis of the HFF cluster samples, reveal objects that, remarkably, are as small as globular clusters and dwarf galaxies. The recent confirmation of a z=11.1 galaxy, just 400 million years after the Big Bang, by a combination of Hubble and Spitzer data, pushed Hubble into JWST territory, far beyond what we ever expected Hubble could do. Twenty years of astonishing progress with Hubble and Spitzer leave me looking to JWST to provide even more remarkable exploration of the realm of the first galaxies at “Cosmic Sunrise”. The latest results on the sizes of distant galaxies, on the star formation rate density at z~10 and from Planck indicating that reionization began around z~10 together have significant implications for the detectability of the “first galaxies” with JWST.