As part of our ongoing strategy to grow astronomy/astrophysics at Newcastle University we are delighted to announce that we are advertising for applicants for an observational/theoretical astrophysics lectureship to commence by September 2021. The application deadline is May 19th. For details visit: https://jobs.ncl.ac.uk/job/Newcastle-LecturerSenior-Lecturer-in-Astrophysics/652248901/
The position is for observational or theoretical astrophysicists. Beyond this, there are no specified areas of research expertise. However, the applicants should consider how they compliment current areas of expertise at Newcastle which includes: compact objects; galaxies/AGN; cosmology; and MHD simulations of the ISM/stellar interiors. In addition to a strong research track record, applicants should have a genuine interest and commitment to developing the role of under-represented groups in Physics, and an interest in establishing innovative, evidence-based programmes that will target these groups at all levels.
Alex has published their first article for the astrobites collaboration! This article is about constraining the mass of fuzzy dark matter by tracking the imprint of the star formation rate of the early universe through the 21-cm line, based on the work of Nebrin, Ghara, and Mellema (2019). You can read the full astrobites article here.
This is an announcement about the Lady Bertha Jeffreys PhD studentship currently being advertised to attract the best students to our School. We are inviting students to apply for the Lady Bertha Jeffreys PhD studentship. This is a prestigious studentship from the estate of Lady Bertha Jeffreys Bequest and funds from the estate of her husband Sir Harold Jeffreys. Both Anne and Chris are offering projects for this studentship. More details available here: https://www.ncl.ac.uk/postgraduate/funding/sources/ukeustudents/msp029.html
Our Ph.D. student, Aishwarya Girdhar was featured as the “Physicist of the week” by The Working Group for Equal Opportunities (AKC) of the German Physical Society (DPG) in the 52nd week of 2020. Aish is a second-year Ph.D. student at the European Southern Observatory (ESO), in Garching, Munich, and is jointly supervised by Dr. Chris Harrison at Newcastle University and Dr. Vincenzo Mainieri at the European Southern Observatory.
Alex will begin writing for Astrobites starting in 2021. The Astrobites collaboration is a group of astronomy and astrophysics graduate students around the world who write daily summaries of recent astrophysics research, accessible to the undergraduate level. These “daily summary posts” have made up the backbone of Astrobites over the last 10 years, and in more recent years they have begun to also write about things beyond daily summaries, including series of posts about DEI problems in astronomy, mental health in academia, what the day-to-day life in astronomy looks like, and application processes and career advice.
Chris Harrison will be taking on a new PhD student in October 2021.
This PhD project is about using cutting-edge astronomical observations to understand the underlying physics of how supermassive black holes impact on the evolution of galaxies. When supermassive black holes grow to become known as “active galactic nuclei” (AGN), it is believed that they drive outflows of gas and consequently regulate the level of star formation inside their host galaxies. However, key questions remain that will be addressed in this studentship: “Through which physical processes do AGN inject energy over galaxy-wide scales?” and “Is there any observational evidence for impact upon star formation by AGN?”.
The student will join the international team of the “Quasar Feedback Survey”, which includes observers and theorists tackling these questions. The student will perform analyses on our multi-wavelength observations of galaxies that host the most powerful AGN (called quasars), including radio observations and integral field spectroscopy. They will extract key physical quantities from the data that will then be compared to theoretical predictions. Due to the extensive data available there is plenty of flexibility in the project depending on which aspects are most appealing to the student. Some programming experience will be required for the project and previous astrophysics education would be desirable.
For more information and to find out how to apply visit findaphd.com
Pulsar timing arrays are trying to detect gravitational waves with periods of years by sensing their effect on the arrival time of pulses from pulsars all over the sky. A gravitational wave passing over the Earth compresses space in some directions and expands it in others, and this should produce a correlated pattern of delays between multiple pulsars. At these frequencies, the signal we expect to detect is a mixture of gravitational waves from all the supermassive black hole binaries in the Universe; this should look like random noise with a power-law spectrum, strongest at the lowest frequencies. Recently, NANOGrav detected hints of such a spectrum. It detected these hints not in the cross-correlations between pulsars, but as a common spectrum of noise in the autocorrelations of individual pulsars. By contrast, LIGO’s attempt to detect a stochastic gravitational-wave background is expected to find evidence in the cross-correlations first. (And of course we will find cross-correlations much more convincing evidence for a gravitational-wave origin than auto-correlations – there are many fewer alternative sources of cross-correlated noise.) So why did NANOGrav find evidence first in the autocorrelations? This paper uses a simple toy model to explain why.
The gravitational-wave signal, with a “red” spectrum where the lowest frequencies dominate, must be distinguished from pulsar intrinsic noise, which is largely the same at all frequencies. If the non-gravitational noise dominated even at low frequencies, as it does for LIGO, then the cross-correlations provide greater averaging and a detection will happen first in the cross-correlations. If the gravitational-wave signal is stronger than the noise at low frequencies, as it is for current pulsar timing arrays, then what limits our ability to characterize it is the fact that it is itself a noise process – its own intrinsic variance limits our ability to measure it well enough to believe it is real. In this case the cross-correlations no longer have independent noise, and averaging them provides much less benefit. In this situation, the paper shows, autocorrelations have a higher signal-to-noise.
PINT: A Modern Software Package for Pulsar Timing by Jing Luo, Scott Ransom, Paul Demorest, Paul S. Ray, Anne Archibald, Matthew Kerr, Ross J. Jennings, Matteo Bachetti, Rutger van Haasteren, Chloe A. Champagne, Jonathan Colen, Camryn Phillips, Josef Zimmerman, Kevin Stovall, Michael T. Lam, and Fredrick A. Jenet.
The defining characteristic of pulsars — that they emit regular pulses of radio waves or other radiation — also provides one of the most powerful tools for studying them and their environments: we can measure the arrival time of these pulses at our telescopes quite precisely. These arrival times carry information about the rotation of the pulsar (affected by the energy loss to a particle wind and by interaction of the crust with the internal superfluid), the orbit of the pulsar if it’s in a binary system (including tidal and relativistic effects on the orbit, and relativistic time dilation and light bending), the location of the pulsar in relation to the Earth’s orbit (including its distance), the interstellar medium (varying as our line of sight sweeps through it), and possibly even gravitational waves passing over the Earth. But to extract the interesting science from these measurements, we need a tool that models how all these physical processes affect the observed arrival times. PINT is such a tool.
The difference between observed arrival times and arrival times modelled with PINT. The parameters of the model, including position on the sky, pulsar spin period, and pulsar orbital parameters, have been adjusted to match the observed data as well as possible, and the discrepancies are within about 20 microseconds, and are broadly consistent with the uncertainties on the measurements themselves.
Of the tools that exist to do this task, TEMPO has a proud history of scientific discovery but was written in FORTRAN in the 1980s and was never designed to operate with sub-microsecond accuracy. The tool TEMPO2 was meant to be its successor, written in the 2000s, but much of TEMPO2’s code is simply original TEMPO code converted to C++, and the package can be difficult to work with. PINT is intended as a largely independent development project, and indeed the process of developing PINT has revealed at least one long-standing error (now fixed) in TEMPO2’s computations. PINT is also written to be a flexible python library, well documented and well tested, so that its data structures and tools can be applied to tasks that do not fit naturally into traditional pulsar timing.
PINT is an open-source project under active development; you can find it on GitHub. It has extensive, if still in-progress, documentation as well.
Primordial gravitational waves, thought to be sourced by inflation in the very early universe, should imprint themselves on the polarisation signal of the cosmic microwave background (CMB). Direct detection of the “B-mode” polarisation characteristic of these gravitational waves would provide strong evidence for an inflationary period in the early universe, as well as providing insight into the mechanisms driving it. The strength of the tensor perturbations which correspond to gravitational waves is parametrised by the “tensor to scalar ratio”, r.
Detecting true B mode signal from the CMB is difficult, as the primordial B mode signal is dominated by foreground elements, such as thermal dust emission and synchrotron emission within our own galaxy. These galactic foregrounds can be dealt with by masking—cutting the portions of the sky where the galactic plane is out of the signal—though this has the disadvantage of throwing away true signal. The other approach is to model the foreground elements statistically, with some dependence on frequency.
The standard technique is to characterise the foreground elements’ spectral energy density (a function of frequency) by an amplitude and a spectral index. These spectral indices are usually considered to be constant across the sky, which, given that they are empirically fit parameters, rather than motivated by underlying physics, is not well justified. For small sky fraction surveys, the difference is thought to be negligible, but for future CMB experiments, this assumption should be relaxed. This paper is a proof of concept for an extension beyond assuming a constant spectral index.
Allowing the spectral indices of dust and synchrotron emission to vary across the sky, we propagate forward the effect on the total observed power spectrum in a very general way using a moment expansion. A highly simplified case of this model is then tested against simulations of varying degrees of realism, to see whether this kind of expansion tightens the constraints on r compared to not using the moment expansion. The result is not possible to significantly detect the effects of this variation on the foreground multi-frequency power spectra for Simon’s-Observatory-like sensitivities, at least using the strong simplifications we used to test against simulations. Nevertheless the methodology set up in this paper, as well as potential extensions, will be useful for analysis of ground based CMB experiments such as Simon’s Observatory and CMB-Stage 4.
Remote outreach talk to the Newcastle Astronomical Society, given by Alex Gough
The Skeleton of Our Universe
The goal of this talk is to introduce the topic of my research, understanding the largest structures in the universe, to the members of the Newcastle Astronomical Society. This begins by setting the stage for where cosmology takes place, and winding back the cosmic clock to the early universe and the cosmic microwave background (CMB). From there, understanding that the very early universe is nearly the same everywhere, with only 10 parts per million deviation from the mean density, it becomes an obvious scientific question to understand how those tiny fluctuations grow into the rich structure of galaxies we see today. Understanding this growth, and the role dark matter has to play in it, is the focus of my research.
After touring through the history of the universe, we take a detour into understanding how these huge distances and times are actually measured. This detour provides a link from my work in cosmology to the stellar physics and observations that members of an astronomical society are more familiar with. It also provides a nice opportunity to look at beautiful space pictures.
The end point of the talk is the 6 numbers one needs to measure to construct the universe. These are based on the 6 parameters in ΛCDM (the standard cosmological model), slightly modified to make them more accessible to this general audience. These break down into:
2 numbers from the early universe: the amplitude and scale dependence of the fluctuations in the CMB
2 numbers for the “pie recipe” of the universe: how much dark energy and dark matter do we have
2 timescales for the universe: the age of the universe, and the time you have to wait for the first stars to form.
