recruiting

PhD studentship

Application instructions

Gas dynamics and dark matter in dwarf galaxies

The lowest-mass, `dwarf’ galaxies have large dark-to-total matter ratios, making them excellent objects to study dark matter (Sawala et al. 2016). In this studentship, you will use a combination of galaxy formation simulations and 21-cm radio observations of atomic hydrogen gas in dwarf galaxies to discriminate between candidates for the particle making up the dark matter.

The mass distribution within its orbit dictates the orbital speed of a particle in a gravitational field. Measurements of the orbital speed of stars or gas in galaxies as a function of orbital radius – rotation curves – therefore let us map out the total mass distribution in a galaxy. Subtracting an estimate of the visible mass distribution leaves the dark matter distribution. It is a generic prediction of the standard $\Lambda$ cold dark matter cosmology that the centres of galaxies should have very high dark matter densities. The rotation curves of many dwarf galaxies, however, are consistent with much lower central densities. This is a long-standing discrepancy (Oman et al. 2015) that could signal a need for additional dark-sector physics, such as a scattering interaction between dark matter particles, but could also be a symptom of limitations in our astrophysical theories of galaxy formation or our ability to model observational data to produce rotation curves.

Left: A dark matter halo from the new Colibre galaxy formation simulations, with a zoom-in on the atomic gas of a dwarf galaxy embedded within it showing ordered rotation perturbed by bubbles inflated by supernova explosions. Upper right: The WALLABY survey at the Australian SKA Pathfinder is observing the atomic gas in half a million galaxies. Lower right: The MHONGOOSE survey combines very high sensitivity, spatial and spectral resolution in a survey atomic gas in 30 nearby galaxies.

Our group is working with the the brand-new Colibre galaxy formation model that is ideally suited to predict the gas dynamics in dwarf galaxies. We are also members of the WALLABY, MHONGOOSE and MIGHTEE-HI 21-cm radio surveys at pathfinder observatories for the forthcoming Square Kilometer Array Observatory. This is a powerful combination that will allow you to make detailed theoretical predictions and test them against new datasets during your studentship. You may choose to focus on theory by learning to run and analyse your own simulations, or you could focus on modelling observations (Oman et al. 2019) and developing new statistical methods. In either case, the question ‘what can dwarf galaxies reliably tell us about dark matter?’ will help guide your project.

This studentship is funded by the Royal Society and includes independent allowances for travel, equipment and skills development.

References

2019

Non-circular motions and the diversity of dwarf galaxy rotation curves

Kyle A. Oman, Antonino Marasco, Julio F. Navarro, and 3 more authors

(2019) MNRAS 482 821

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We use mock interferometric HI measurements and a conventional tilted- ring modelling procedure to estimate circular velocity curves of dwarf galaxy discs from the APOSTLE suite of Λ cold dark matter cosmological hydrodynamical simulations. The modelling yields a large diversity of rotation curves for an individual galaxy at fixed inclination, depending on the line-of-sight orientation. The diversity is driven by non-circular motions in the gas; in particular, by strong bisymmetric fluctuations in the azimuthal velocities that the tilted-ring model is ill-suited to account for and that are difficult to detect in model residuals. Large misestimates of the circular velocity arise when the kinematic major axis coincides with the extrema of the fluctuation pattern, in some cases mimicking the presence of kiloparsec- scale density ‘cores’, when none are actually present. The thickness of APOSTLE discs compounds this effect: more slowly rotating extra-planar gas systematically reduces the average line-of-sight speeds. The recovered rotation curves thus tend to underestimate the true circular velocity of APOSTLE galaxies in the inner regions. Non-circular motions provide an appealing explanation for the large apparent cores observed in galaxies such as DDO 47 and DDO 87, where the model residuals suggest that such motions might have affected estimates of the inner circular velocities. Although residuals from tilted-ring models in the simulations appear larger than in observed galaxies, our results suggest that non-circular motions should be carefully taken into account when considering the evidence for dark matter cores in individual galaxies.
@article{2019MNRAS.482..821O, author = {{Oman}, Kyle A. and {Marasco}, Antonino and {Navarro}, Julio F. and {Frenk}, Carlos S. and {Schaye}, Joop and {Ben{\'\i}tez-Llambay}, Alejandro}, title = {{Non-circular motions and the diversity of dwarf galaxy rotation curves}}, journal = {\mnras}, keywords = {ISM: kinematics and dynamics, galaxies: haloes, galaxies: structure, dark matter, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2019}, month = jan, volume = {482}, number = {1}, pages = {821-847}, doi = {10.1093/mnras/sty2687}, archiveprefix = {arXiv}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2019MNRAS.482..821O}, adsnote = {Provided by the SAO/NASA Astrophysics Data System}, }

2016

The APOSTLE simulations: solutions to the Local Group’s cosmic puzzles

Till Sawala, Carlos S. Frenk, Azadeh Fattahi, and 13 more authors

(2016) MNRAS 457 1931

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The Local Group galaxies offer some of the most discriminating tests of models of cosmic structure formation. For example, observations of the Milky Way (MW) and Andromeda satellite populations appear to be in disagreement with N-body simulations of the ‘lambda cold dark matter’ (ΛCDM) model: there are far fewer satellite galaxies than substructures in CDM haloes (the ‘missing satellites’ problem); dwarf galaxies seem to avoid the most massive substructures (the ‘too-big-to-fail’ problem); and the brightest satellites appear to orbit their host galaxies on a thin plane (the ‘planes of satellites’ problem). Here we present results from APOSTLE (A Project Of Simulating The Local Environment), a suite of cosmological hydrodynamic simulations of 12 volumes selected to match the kinematics of the Local Group (LG) members. Applying the EAGLE code to the LG environment, we find that our simulations match the observed abundance of LG galaxies, including the satellite galaxies of the MW and Andromeda. Due to changes to the structure of haloes and the evolution in the LG environment, the simulations reproduce the observed relation between stellar mass and velocity dispersion of individual dwarf spheroidal galaxies without necessitating the formation of cores in their dark matter profiles. Satellite systems form with a range of spatial anisotropies, including one similar to the MWs, confirming that such a configuration is not unexpected in ΛCDM. Finally, based on the observed velocity dispersion, size, and stellar mass, we provide estimates of the maximum circular velocity for the haloes of nine MW dwarf spheroidals.
@article{2016MNRAS.457.1931S, author = {{Sawala}, Till and {Frenk}, Carlos S. and {Fattahi}, Azadeh and {Navarro}, Julio F. and {Bower}, Richard G. and {Crain}, Robert A. and {Dalla Vecchia}, Claudio and {Furlong}, Michelle and {Helly}, John. C. and {Jenkins}, Adrian and {Oman}, Kyle A. and {Schaller}, Matthieu and {Schaye}, Joop and {Theuns}, Tom and {Trayford}, James and {White}, Simon D. M.}, title = {{The APOSTLE simulations: solutions to the Local Group's cosmic puzzles}}, journal = {\mnras}, keywords = {galaxies: evolution, galaxies: formation, cosmology: theory, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2016}, month = apr, volume = {457}, number = {2}, pages = {1931-1943}, doi = {10.1093/mnras/stw145}, archiveprefix = {arXiv}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2016MNRAS.457.1931S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System}, }

2015

The unexpected diversity of dwarf galaxy rotation curves

Kyle A. Oman, Julio F. Navarro, Azadeh Fattahi, and 9 more authors

(2015) MNRAS 452 3650

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We examine the circular velocity profiles of galaxies in Λ cold dark matter (CDM) cosmological hydrodynamical simulations from the EAGLE and LOCAL GROUPS projects and compare them with a compilation of observed rotation curves of galaxies spanning a wide range in mass. The shape of the circular velocity profiles of simulated galaxies varies systematically as a function of galaxy mass, but shows remarkably little variation at fixed maximum circular velocity. This is especially true for low-mass dark-matter-dominated systems, reflecting the expected similarity of the underlying CDM haloes. This is at odds with observed dwarf galaxies, which show a large diversity of rotation curve shapes, even at fixed maximum rotation speed. Some dwarfs have rotation curves that agree well with simulations, others do not. The latter are systems where the inferred mass enclosed in the inner regions is much lower than expected for CDM haloes and include many galaxies where previous work claims the presence of a constant density ‘core’. The ‘cusp versus core’ issue is thus better characterized as an ‘inner mass deficit’ problem than as a density slope mismatch. For several galaxies, the magnitude of this inner mass deficit is well in excess of that reported in recent simulations where cores result from baryon-induced fluctuations in the gravitational potential. We conclude that one or more of the following statements must be true: (i) the dark matter is more complex than envisaged by any current model; (ii) current simulations fail to reproduce the diversity in the effects of baryons on the inner regions of dwarf galaxies; and/or (iii) the mass profiles of ‘inner mass deficit’ galaxies inferred from kinematic data are incorrect.
@article{2015MNRAS.452.3650O, author = {{Oman}, Kyle A. and {Navarro}, Julio F. and {Fattahi}, Azadeh and {Frenk}, Carlos S. and {Sawala}, Till and {White}, Simon D. M. and {Bower}, Richard and {Crain}, Robert A. and {Furlong}, Michelle and {Schaller}, Matthieu and {Schaye}, Joop and {Theuns}, Tom}, title = {{The unexpected diversity of dwarf galaxy rotation curves}}, journal = {\mnras}, keywords = {galaxies: haloes, galaxies: structure, dark matter, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2015}, month = oct, volume = {452}, number = {4}, pages = {3650-3665}, doi = {10.1093/mnras/stv1504}, archiveprefix = {arXiv}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2015MNRAS.452.3650O}, adsnote = {Provided by the SAO/NASA Astrophysics Data System}, }