2019
Formation of Massive Black Holes in pre-Galactic Gas Clouds (Nature, January 2019)
The formation mechanism(s) for the most massive black holes in our Universe are unknown. In this work we found that dark matter haloes which accrete extremely rapidly induce signbificant dynamical heating. This combined with the low efficiency of molecular hydrogen cooling in the early Universe suppresses the formation of 'normal' metal free stars. Instead, high accretion rates onto a now rapidly accreting central core result in the formation of super-massive star. The super massive star then quickly collapses into a massive black hole. This work was published in Nature, John Regan & Turlough Downes were co-authors.
PDF
Link: https://www.nature.com/articles/s41586-019-0873-4 (closed access)
Link: https://arxiv.org/abs/1901.07563 (open access)
The impact of neutral particles on the dynamics of filaments in fusion energy reactors (Plasma Physics and Controlled Fusion, February, 2019)
Filaments are observed to form during plasma instabilities in so-called Tokamak fusion reactors. These instabilities cause sudden transport of energy out of the fusion plasma and towards plasma-facing elements of the reactor. This can damage the reactor itself and an understanding of these filaments is critical to managing the plasma instabilities. This study, in collaboration with colleagues from the Culham Science Centre, York University and the National Centre for Plasma Science and Technology, presents calculations showing the impact of the presence of electrically neutral particles on these filaments. Prof Downes is an author on this work.
Link: https://arxiv.org/abs/1806.07319
2018
An absolute sodium abundance for a cloud-free ‘hot Saturn’ exoplanet (Nature, May 2018)
Ernst de-Mooji was a co-author on this work.
Dust in turbulent proto-planetary disks (The Astrophysical Journal, October 2018)
Astronomers observe that proto-planetary disks (in which all exoplanets are born) contain lots of dust. Indeed, it is this dust which ultimately forms exoplanets. In this work, in collaboration with colleagues at the Niels Bohr Institute in Copenhagen, the effects of electrically charged dust grains on the dynamics of the disk are studied. The model used makes use of the complex interplay between magnetic fields and the gas and dust in the disk to find out how the dust can accumulate in certain regions, making planet formation more likely there. This explains how the dust self-organises in these disks, as is observed by telescopes. Prof Downes is an author on this work.
Link: https://arxiv.org/abs/1808.07660
A new computational method for predicting the behaviour of solar plasma (Astronomy & Astrophysics, July 2018)
Surprisingly, very important parts of our Sun's surface are actually rather cool. In these regions, the plasma that is the Sun's atmosphere can become only weakly ionised. Specialised computational techniques are needed to study and predict the behaviour of these regions of the Sun. This work, in collaboration with colleagues from the Institute of Astrophysics in Tenerife and the University of La Laguna in Tenerife, presents a novel and extremely efficient technique for simulating these difficult plasmas. Prof Downes is an author on this work and the work is partly based on techniques invented at Dublin City University about a decade ago.
Link: https://arxiv.org/abs/1803.04891
Rise of the First SuperMassive Stars (Monthly Notices of the Royal Astronomical Society, August 2018)
Super-Massive stars can form in the early Universe if the accretion rates onto the stars are sufficently high (~ 0.1 Msolar/yr). We found here that in dark matter haloes exposed to a mild Lyman-Werner background that SMSs can form and reach masses of a few tens of thousands of solar masses before collapsing into a massive black hole. John Regan & Turlough Downes were co-authors on this work.
Link: https://arxiv.org/pdf/1803.04527.pdf
Link: https://arxiv.org/pdf/1708.07772.pdf
Gravitational Wave Signals from the First Massive Black Holes (Monthly Notices of the Royal Astronomical Society, September 2018)
Gravitational waves from the merger of black hole binaries were recently discovered by the LIGO observatory. While LIGO is sensitive to the merger of stellar mass black holes from the local Universe, LISA will be sensitive to the merger of much more massive black holes in the early Universe. In this work we make predictions for LISA for detecting the mergers of black hole seeds from the early Universe shortly after the formation of the first massive black holes.
John Regan was an author on this work.
Link: https://arxiv.org/pdf/1805.06901.pdf
The growth of small black holes in the early Universe (Monthly Notices of the Royal Astronomical Society, November 2018)
Stellar mass black holes form from the end point of massive stars. Stars in the early universe are known as Population III stars as they formed from material in the early Universe which has less elements than the present day universe. These stars are thought to have been more massive than the stars in the present day universe and hence the black holes which formed from their collapse are also thought to be more massive. In this work we found that small black holes forming in the local universe grow very inefficiently and do not increase their initial masses. These black holes are therefore unlikely to be the massive black holes we observe today and are instead likely to form a population of smaller black holes much like those observed with LIGO. John Regan & Turlough Downes were co-authors on this paper.
Link: https://arxiv.org/pdf/1804.06477.pdf
2017
Rapid formation of massive black holes in close proximity to embryonic protogalaxies (Nature Astronomy, February 2017)
In order to form a massive black hole seed an intermediary step may be the formation of a super-massive star (SMS). SMSs can be formed when there is rapid accretion onto a central proto-star. In this case the star does not contract and instead stays 'puffed-up'. A SMS continues to accrete throughout it's lifetime eventually reaching masses up to 100,000 solar masses. Due it's large mass the star collapses directly into a black hole. In this work we showed that the necessary conditions to achieve the high accrete rates necessary for SMS formation may exist in the early Universe when two galaxies are formed in close proximity. The intense radiation from star formation in the first galaxy steralises the neighbouring galaxy preventing normal star formation and leading inevitably to the formation of a SMS. John Regan was the lead author on this work.
The effect of cosmic rays on proto-planetary disks (Monthly Notices of the Royal Astronomical Society, November 2017)
Cosmic rays are high energy particles are a form of ionising radiation ubiquitous in our Galaxy. It had been believed, though, that cosmic rays were not present in proto-planetary disks because the disks were shielded by outflows and winds from the forming star around which these winds orbit. However, in this work, we carefully study the energetic particles produced by the forming star itself - stellar cosmic rays. We find that these can cause significant ionisation of the proto-planetary disk out as far as 2 AU from the star itself. This has implications for planet formation as well as, indeed, star formation itself since the level of ionisation of the disk is a critical parameter in determining whether or not the disk is turbulent.
Link: https://arxiv.org/abs/1707.07522
Formation of Massive Black Holes in pre-Galactic Gas Clouds (Nature, January 2019)
The formation mechanism(s) for the most massive black holes in our Universe are unknown. In this work we found that dark matter haloes which accrete extremely rapidly induce signbificant dynamical heating. This combined with the low efficiency of molecular hydrogen cooling in the early Universe suppresses the formation of 'normal' metal free stars. Instead, high accretion rates onto a now rapidly accreting central core result in the formation of super-massive star. The super massive star then quickly collapses into a massive black hole. This work was published in Nature, John Regan & Turlough Downes were co-authors.
Link: https://www.nature.com/articles/s41586-019-0873-4 (closed access)
Link: https://arxiv.org/abs/1901.07563 (open access)
The impact of neutral particles on the dynamics of filaments in fusion energy reactors (Plasma Physics and Controlled Fusion, February, 2019)
Filaments are observed to form during plasma instabilities in so-called Tokamak fusion reactors. These instabilities cause sudden transport of energy out of the fusion plasma and towards plasma-facing elements of the reactor. This can damage the reactor itself and an understanding of these filaments is critical to managing the plasma instabilities. This study, in collaboration with colleagues from the Culham Science Centre, York University and the National Centre for Plasma Science and Technology, presents calculations showing the impact of the presence of electrically neutral particles on these filaments. Prof Downes is an author on this work.
Link: https://arxiv.org/abs/1806.07319
2018
An absolute sodium abundance for a cloud-free ‘hot Saturn’ exoplanet (Nature, May 2018)
Ernst de-Mooji was a co-author on this work.
Dust in turbulent proto-planetary disks (The Astrophysical Journal, October 2018)
Astronomers observe that proto-planetary disks (in which all exoplanets are born) contain lots of dust. Indeed, it is this dust which ultimately forms exoplanets. In this work, in collaboration with colleagues at the Niels Bohr Institute in Copenhagen, the effects of electrically charged dust grains on the dynamics of the disk are studied. The model used makes use of the complex interplay between magnetic fields and the gas and dust in the disk to find out how the dust can accumulate in certain regions, making planet formation more likely there. This explains how the dust self-organises in these disks, as is observed by telescopes. Prof Downes is an author on this work.
Link: https://arxiv.org/abs/1808.07660
A new computational method for predicting the behaviour of solar plasma (Astronomy & Astrophysics, July 2018)
Surprisingly, very important parts of our Sun's surface are actually rather cool. In these regions, the plasma that is the Sun's atmosphere can become only weakly ionised. Specialised computational techniques are needed to study and predict the behaviour of these regions of the Sun. This work, in collaboration with colleagues from the Institute of Astrophysics in Tenerife and the University of La Laguna in Tenerife, presents a novel and extremely efficient technique for simulating these difficult plasmas. Prof Downes is an author on this work and the work is partly based on techniques invented at Dublin City University about a decade ago.
Link: https://arxiv.org/abs/1803.04891
Rise of the First SuperMassive Stars (Monthly Notices of the Royal Astronomical Society, August 2018)
Super-Massive stars can form in the early Universe if the accretion rates onto the stars are sufficently high (~ 0.1 Msolar/yr). We found here that in dark matter haloes exposed to a mild Lyman-Werner background that SMSs can form and reach masses of a few tens of thousands of solar masses before collapsing into a massive black hole. John Regan & Turlough Downes were co-authors on this work.
Link: https://arxiv.org/pdf/1803.04527.pdf
Link: https://arxiv.org/pdf/1708.07772.pdf
Gravitational Wave Signals from the First Massive Black Holes (Monthly Notices of the Royal Astronomical Society, September 2018)
Gravitational waves from the merger of black hole binaries were recently discovered by the LIGO observatory. While LIGO is sensitive to the merger of stellar mass black holes from the local Universe, LISA will be sensitive to the merger of much more massive black holes in the early Universe. In this work we make predictions for LISA for detecting the mergers of black hole seeds from the early Universe shortly after the formation of the first massive black holes.
John Regan was an author on this work.
Link: https://arxiv.org/pdf/1805.06901.pdf
The growth of small black holes in the early Universe (Monthly Notices of the Royal Astronomical Society, November 2018)
Stellar mass black holes form from the end point of massive stars. Stars in the early universe are known as Population III stars as they formed from material in the early Universe which has less elements than the present day universe. These stars are thought to have been more massive than the stars in the present day universe and hence the black holes which formed from their collapse are also thought to be more massive. In this work we found that small black holes forming in the local universe grow very inefficiently and do not increase their initial masses. These black holes are therefore unlikely to be the massive black holes we observe today and are instead likely to form a population of smaller black holes much like those observed with LIGO. John Regan & Turlough Downes were co-authors on this paper.
Link: https://arxiv.org/pdf/1804.06477.pdf
2017
Rapid formation of massive black holes in close proximity to embryonic protogalaxies (Nature Astronomy, February 2017)
In order to form a massive black hole seed an intermediary step may be the formation of a super-massive star (SMS). SMSs can be formed when there is rapid accretion onto a central proto-star. In this case the star does not contract and instead stays 'puffed-up'. A SMS continues to accrete throughout it's lifetime eventually reaching masses up to 100,000 solar masses. Due it's large mass the star collapses directly into a black hole. In this work we showed that the necessary conditions to achieve the high accrete rates necessary for SMS formation may exist in the early Universe when two galaxies are formed in close proximity. The intense radiation from star formation in the first galaxy steralises the neighbouring galaxy preventing normal star formation and leading inevitably to the formation of a SMS. John Regan was the lead author on this work.
The effect of cosmic rays on proto-planetary disks (Monthly Notices of the Royal Astronomical Society, November 2017)
Cosmic rays are high energy particles are a form of ionising radiation ubiquitous in our Galaxy. It had been believed, though, that cosmic rays were not present in proto-planetary disks because the disks were shielded by outflows and winds from the forming star around which these winds orbit. However, in this work, we carefully study the energetic particles produced by the forming star itself - stellar cosmic rays. We find that these can cause significant ionisation of the proto-planetary disk out as far as 2 AU from the star itself. This has implications for planet formation as well as, indeed, star formation itself since the level of ionisation of the disk is a critical parameter in determining whether or not the disk is turbulent.
Link: https://arxiv.org/abs/1707.07522