Category Archives: Paper

Two new papers: How do powerful supermassive black holes impact upon molecular gas?

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Two new papers have been published from the Quasar Feedback Survey, which is a project led by Chris Harrison. The two papers, led by PhD Students Aishwarya Girdhar and Stephen Molyneux, use data from the ALMA and APEX facilities to establish how rapidly growing supermassive black holes (which we call quasars) are able to influence the molecular gas in their host galaxies. Emission lines associated with Carbon Monoxide (CO) are used to trace the molecular gas.

In the study of Girdhar et al., four quasars are selected from the survey to map in lots of detail, the distribution and motions of the molecular gas. These quasars were selected because they are known to have large bubbles of radio emission extending well beyond the main host galaxy disks. These are thought to be inflated by radio jets expanding from the central black hole. Molecular gas appears to be being lifted by these expanding radio bubbles in two of the targets investigated (see upper panels in the figure). This may be making it more difficult for the galaxy to form stars in the future (molecular gas is the fuel for forming new stars). In all four targets, another impact of the powerful, growing black holes was revealed: extreme velocities were observed close to the central regions (see lower panels in the figure), likely due to “outflowing” gas driven by the jets or powerful radiation in these regions.

In the study of Molyneux et al., 17 quasars were studies from the survey. Using information of multiple different CO emission lines (this time with no spatial information), it was investigated if the molecular gas across the whole galaxies is “excited” due to the powerful quasars and jets in these systems. In short, the conditions of the molecular gas were found to be consistent with that expected if there was no additional excitation (beyond what is typically expected for these sort of highly star-forming host galaxies). The conclusion of these two studies, is that the impact of the quasars is not to instantly affect the whole gas reservoir, but instead, more localised and longer term affects (driving outflows, lifting the gas) may make it more difficult for the host galaxies to form stars in the future. This is consistent with expectations from theoretical models and simulations, as recently demonstrated by another one of our PhD students, Samuel Ward, in Ward et al. (2022).

Hidden supermassive black holes reveal their secrets through radio signals

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An artist’s impression of a red quasar. Red quasars are enshrouded by gas and dust, which may get blown away by outflows from the supermassive black hole, eventually revealing a typical blue quasar.
Credit: S. Munro & L. Klindt, Licence: Attribution (CC BY 4.0)

A team of international astronomers, led by Newcastle University, have used new data from the Dark Energy Spectroscopic Instrument (DESI), which is conducting a five year survey of large scale structure in the universe that will include optical spectra for ~3 million quasars; extremely bright galaxies powered by supermassive black holes. They found that quasars that contained more dust, and therefore appeared redder, were more likely to have stronger radio emission compared to the quasars that had very little-to-no dust, appearing very blue.

Photograph of the Mayall Telescope, which hosts the Dark Energy Spectroscopic Instrument, at Kitt Peak National Observatory. Credit: Marilyn Sargent, photographer. Copyright: © 2018 The Regents of the University of California, Lawrence Berkeley National Laboratory

Almost every known galaxy contains a supermassive black hole, which are black holes with a mass millions to billions that of our Sun, at its centre, including our own Milky Way. In some galaxies there is lots of material in the centre, feeding and growing this supermassive black hole, making it very energetic and “active”. The most powerful type of these active galaxies are called “quasars”, which are some of the brightest objects in the Universe. Most quasars appear very blue, due to the bright disc of matter that orbits and feeds the central supermassive black hole which is very bright in optical and ultraviolet wavelengths. However, astronomers have found that a significant fraction of these quasars appear very red, although the nature of these objects is still not well understood.

In order to understand the physics of these red quasars, “spectroscopic” measurements are required, which can be used to analyse the quasar light at different wavelengths. The “shape” of the quasar’s spectrum can indicate the amount of dust present surrounding the central region. Observing the radio emission from quasars can also tell you about the energetics of the central supermassive black hole; whether it is launching powerful “winds” or “jets” that might shape the surrounding galaxy.

Images of DESI quasars, going from blue (typical) quasars on the left to the red quasars on the right, taken from the Legacy Survey Viewer. The redder quasars are more likely to have strong radio emission compared to the bluer quasars. Credit: V. Fawcett, using images from https://www.legacysurvey.org/viewer

This new study, led by Dr Victoria Fawcett of Newcastle University, and previously Durham University, uses spectroscopic observations from DESI to measure the amount of dust (reddening) in a sample of ~35,000 quasars and link this to the observed radio emission. They find that DESI is capable of observing much more extreme red (dusty) quasars compared to similar/previous spectroscopic surveys, such as the Sloan Digital Sky Survey (SDSS). They also find that redder quasars are much more likely to have strong radio emission compared to typical blue quasars (see link to movie at the end of the article).

This reddening-radio connection is likely due to powerful outflows of gas driven away from the supermassive black hole, which slam into the surrounding dust, causing shocks and radio emission. These outflows will eventually blow away all the dust and gas in the central region of the galaxy, revealing a blue quasar and resulting in weaker radio emission. This is consistent with the emerging picture that red quasars are a younger, “blow-out” phase in the evolution of galaxies. Red quasars may therefore be extremely important for understanding how galaxies evolve over time.

Paper: https://academic.oup.com/mnras/article/525/4/5575/7273139?login=true

Paper: Testing a Novel Approach to Measuring the Intrinsic Alignment of Galaxies

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Charlie MacMahon has just submitted his first paper as first author for publication, and it is already available as a pre-print on the arXiv! In it, he and his supervisor Danielle Leonard investigate a novel method for measuring intrinsic alignment.

A simplistic diagram showing how intrinsic alignment influences our measurements of galaxy shape and why this affects weak lensing measurements.

Intrinsic alignment refers to a phenomenon in which galaxies near to each other will align with local, large-scale gravitational fields in a way that causes their shapes to become statistically correlated. These correlations can bias weak lensing measurements, but are themselves also interesting tracers of galaxy evolution and underlying cosmology.

Taking the difference of the shape estimates at the two different measurement scales allows us to recover the difference in the intrinsic alignment signal.

Charlie’s paper looks at measuring intrinsic alignment using two different estimators of galaxy shape, which are individually sensitive to different parts of galaxies, because it’s expected that the outer regions of a galaxy should be more aligned with local fields than the inner regions. This method allows a portion of the intrinsic alignment signal to be recovered in a way that is more robust to uncertainty in galaxy redshift than other methods.

In the paper, the method is applied for the first time to real galaxy shape data from the Dark Energy Survey Year 1, and various assumptions of the method are tested to help develop a framework of necessary considerations and steps for using this method. The paper shows the difficulty of working with observational data and the importance of rigorous testing for contamination and systematics.

Constraining power of the method for different combinations of shape estimator parameters, when fitting a model of intrinsic alignment to mock Rubin Observatory data.

Taking what is learnt from this first application to data, forecasts are conducted using a mock catalogue of Rubin Observatory data (part of the next generation of lensing surveys) and a contemporary model for intrinsic alignment. With this data, the performance of the method is evaluated in various contexts, and requirements are then placed on the shape estimators to ensure robustness to systematics and a strong measurement are achieved. Overall, the paper proposes a clear path for the continued development of the method, and demonstrates promise for its use with future Rubin Observatory data.

Hidden supermassive black holes brought to life by galaxies on collision course

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A new study, led by Sean Dougherty who carried out this work whilst an MPhil student in our group, was publicly released today. The study found that supermassive black holes obscured by dust are more likely to grow and release tremendous amounts of energy, when they are inside galaxies that are expected to collide with a neighbouring galaxy. It is published in Monthly Notices of the Royal Astronomical Society and is available on the arXiv. 

A press release was produced to celebrate the results, shared by both the Royal Astronomical Society and Newcastle University. 

This study presents a new statistical method to overcome the previous limitations of measuring accurate distances of galaxies and supermassive black holes. It applies a statistical approach to determine galaxy distances using images at different wavelengths (i.e., ‘photometric redshifts’) and removes the need for spectroscopic distance measurements for individual galaxies, which are only available for a small fraction of galaxies.

They applied this new method to hundreds of thousands of galaxies in the distant universe (looking at galaxies formed 2 to 6 billion years after the Big Bang) to better understand the so-called ‘cosmic noon’, a time when most of the Universe’s galaxy and black hole growth is expected to have taken place.

Using this new method, Sean investigated the number of growing supermassive black holes (called active galactic nuclei), in galaxies which are in close pairs with other galaxies.  This was compared to the number found in galaxies without close pairs i.e., ‘isolated’ galaxies.  In agreement with previous work, it is found that the fraction of galaxies containing active galactic nuclei identified with X-rays is the same for both galaxy pairs and isolated galaxies. However, those which are hidden in the X-rays due to obscuring dust, and are only seen in infrared light, are twice as common in the close galaxy pairs.  

The excess (or ‘enhancement’) of obscured active galactic nuclei (growing super massive black hole) found in galaxy pairs, compared to isolated galaxies, as a function of separation to another galaxy. The results show that there is a boost in the number by a factor of ~2 for the closest galaxy pairs.

The difficulty in finding these black holes and in establishing precise distance measurements explains why this result has previously been challenging to pin down for these distant `cosmic noon’ galaxies.  

The expectation is that these close galaxy pairs are on the route to colliding, and eventually merging. This process helps drive gas down onto the black holes. On its journey this gas releases a tremendous amount of energy and heats up the surrounding dust, which glows in the infrared.

Special Sonification Edition in Nature Astronomy and Panellist for UN webinar

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November 2022: This month Nature Astronomy published a special issue of their journal, featuring four articles on sonification of astronomical data, co-ordinated by the Audio Universe team, led by Dr. Chris Harrison. Furthermore, Chris Harrison will be one of five expert panellists in a webinar on sonification in space sciences run by the United Nations Office for Outer Space Affairs (UNOOSA).

The Nature Astronomy special issue is introduced by the editor with a commentary entitled “Hearing is Believing“. The special issue is a result of the Audible Workshop, that was organised and chaired by the Audio Universe team (including Chris Harrison, Anita Zanella and Nic Bonne), that took place in the Lorentz Centre (Leiden) last year. This workshop brought together 50 researchers from a variety of backgrounds including astronomy, sound perception, sound design and education. The workshop discussed the current status of astronomy sonification projects as well as the current challenges facing progress in this area and ideas for future plans. Nature Astronomy took this opportunity to allow sonification to be included as a figure for the first time. Hopefully opening up the door for many more published sonifications in this journal and others. Some of the outcomes of the workshop are summarised in the articles below:

(1) A meeting report led by Chris Harrison. Link to main article: https://www.nature.com/articles/s41550-021-01582-y. Link to a preprint version on arxiv: https://arxiv.org/abs/2206.13542

(2) A review of almost 100 sonification projects in astronomy, led by Anita Zanella and Chris Harrison. Link to main article: https://www.nature.com/articles/s41550-022-01721-z. Link to a preprint version on arxiv: https://arxiv.org/abs/2206.13536

Partly due to this work, and due to the wider Audio Universe project, UNOOSA contacted Chris Harrison to act as a consultant on a policy recommendation document they are working on the topic of sonification and accessibility in space sciences. As part of this UNSOOA Space for Persons with Disabilities project they are running a public webinar on the 17th of November, and Chris is one of the five expert panellists taking part.

Paper: Making (dark matter) waves – how wave interference can help model cold dark matter

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Alex’s first first-author paper is now available on arXiv! An art piece Alex made related to this work also placed 2nd in the Art of Science competition hosted by the SAgE faculty at Newcastle. You can see more images and descriptions in Alex’s twitter thread on the paper (and the associated art piece).

This paper models the dark matter field as a single wavefunction, rather than traditional fluid variables or collisionless particles. We show explicitly how the complex phenomenology of multi-streaming (caused by collisionless particles flowing through each other) is encoded in the interference and oscillations of the wavefunction in a simple toy model. This wave model avoids the infinite density spikes which occur when evolving classical cold dark matter collapse.

Evolution of the position and density of a set of cold dark matter particles under the Zel’dovich approximation.

This paper demonstrates how the oscillations in the wavefunction can be “unwoven” to recover a set of wavefunctions corresponding to the set of classical streams in the Zel’dovich approximation. In the multi-stream region, where the dark matter cannot be described by a perfect fluid, we demonstrate how to separate the wavefunction into an “average part”, which describes the classical fluid behaviour, and an oscillatory “hidden part” which is responsible for producing beyond perfect fluid quantities such as velocity dispersion.

Comparison between the Zel’dovich approximation for cold dark matter and evolution of a wavefunction under the free Schrödinger equation. The wavefunction avoids the infinite density spikes (caustics) seen in classical cold dark matter, and introduces small scale interference to decorate the classical density.
Splitting of the wavefunction into three parts, each corresponding to classical dark matter trajectories.

The dense caustics formed by cold dark matter are replaced with diffraction caustics in the simple wave model, which provide classification of these caustics, and certain universal features related to the wave nature of the model. Such features are akin to those found in truly wavelike models of dark matter, such as fuzzy dark matter and ultralight axions.

The wave field corresponding to a cusp caustic. The scaling of the peak height and fringe widths are universal features, classified by catastrophe theory.

Paper: Quasar-driven outflows do not cause rapid in-situ quenching of star formation

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Jan Scholtz, former PhD student of Chris Harrison, has published his paper re-assessing the insitu impact of ionised outflows (driven by three z~2.5 quasars) on the star formation in their host galaxies. The paper has been accepted by MNRAS and is available on the arXiv:2106.05277.

The paper uses spatially-resolved measurements of the dust distribution (using sub-mm interferometric data from ALMA) and the ionised gas properties (using integral field spectroscopic data from SINFONI). The three quasars under investigation were of particular interest due to previous claims (using only SINFONI data) of strong evidence that the star formation was suppressed at the location of the galaxy-wide ionised outflows. However, the new evidence from ALMA suggests that dusty-star formation is still ongoing at the locations of the outflows, in at least two of these targets. Nonetheless, compared to regular star-forming galaxies at the same redshift, and with the same mass, their star formation rates appear to be low. This might mean the impact by the quasar driven outflows on the host galaxy, is not a rapid shut down of star formation, but star formation could be suppressed on longer timescales by the cumulative effect of quasar episodes during the growth of these massive black holes. This all adds to helping solve the mystery of how quasars change the life of galaxies!

Quasar Feedback Survey Launched

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I am delighted to introduce our Quasar Feedback Survey (https://blogs.ncl.ac.uk/quasarfeedbacksurvey/) a multi-wavelength study of how quasars interact with their host galaxies. With this first paper (https://arxiv.org/abs/2103.00014) we use high-resolution VLA imaging to discover that hidden radio AGN are prevalent, and demonstrate a connection between the radio properties and the ionised gas kinematics. This all shows the importance of studying the radio emission, even in “radio quiet”, quasar hosts to understand feedback. With the launch of our website we have also released data products from this paper and our earlier pilot papers on sub-sets of the targets. These include data tables, radio images, data cubes etc.

Example radio images from our sample showing the diversity of radio morphologies

Paper: Signal-to-noise in gravitational-wave detection

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Common-spectrum process versus cross-correlation for gravitational-wave searches using pulsar timing arrays, by Joseph D. Romano, Jeffrey S. Hazboun, Xavier Siemens, and Anne M. Archibald

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.