Kyle Oman
Assoc. Professor & Royal Society Dorothy Hodgkin Fellow
✨Recruiting✨
I am open to taking on a scholarship-funded PhD student to begin in Autumn 2025.
➡️Details here⬅️
I hope to offer one or more undergraduate summer projects in 2025.
➡️Details here⬅️
I’m an Associate Professor and Royal Society Dorothy Hodgkin fellow at the Institute for Computational Cosmology and the Centre for Extragalactic Astronomy in Durham, UK. In my main line of research, I work at the interface of cosmological hydrodynamical galaxy formation simulations and large 21‑cm radio surveys to study the dark matter content and structure of galaxies. I also occasionally work on galaxy formation and evolution (especially of satellites in groups and clusters), weak gravitational lensing, and Galactic archaeology. I like to do a bit of data visualisation when I can find the time, and try to make useful contributions to open-source code. I have a public tool for making mock 21‑cm observations of simulated galaxies:
selected publications
2023
- The many reasons that the rotation curves of low-mass galaxies can fail as tracers of their matter distributionsEleanor R. Downing, and Kyle A. Oman(2023) MNRAS 522 3318
It is routinely assumed that galaxy rotation curves are equal to their circular velocity curves (modulo some corrections) such that they are good dynamical mass tracers. We take a visualization- driven approach to exploring the limits of the validity of this assumption for a sample of 33 low-mass galaxies (60 vmax/km s-1 120) from the APOSTLE suite of cosmological hydrodynamical simulations. Only three of these have rotation curves nearly equal to their circular velocity curves at z = 0, the rest are undergoing a wide variety of dynamical perturbations. We use our visualizations to guide an assessment of how many galaxies are likely to be strongly perturbed by processes in several categories: mergers/interactions (affecting 6/33 galaxies), bulk radial gas inflows (19/33), vertical gas outflows (15/33), distortions driven by a non-spherical DM halo (17/33), warps (8/33), and winds due to motion through the intergalactic medium (5/33). Most galaxies fall into more than one of these categories; only 5/33 are not in any of them. The sum of these effects leads to an underestimation of the low-velocity slope of the baryonic Tully-Fisher relation (α ∼3.1 instead of α ∼3.9, where Mbar ∝ vα) that is difficult to avoid, and could plausibly be the source of a significant portion of the observed diversity in low-mass galaxy rotation curve shapes.
@article{2023MNRAS.522.3318D, author = {{Downing}, Eleanor R. and {Oman}, Kyle A.}, title = {{The many reasons that the rotation curves of low-mass galaxies can fail as tracers of their matter distributions}}, journal = {\mnras}, keywords = {galaxies: dwarf, galaxies: kinematics and dynamics, dark matter, Astrophysics - Astrophysics of Galaxies}, year = {2023}, month = jul, volume = {522}, number = {3}, pages = {3318-3336}, doi = {10.1093/mnras/stad868}, archiveprefix = {arXiv}, eprint = {2301.05242}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2023MNRAS.522.3318D}, adsnote = {Provided by the SAO/NASA Astrophysics Data System}, } - The diversity of rotation curves of simulated galaxies with cusps and coresFinn A. Roper, Kyle A. Oman, Carlos S. Frenk, and 3 more authors(2023) MNRAS 521 1316
We use ΛCDM cosmological hydrodynamical simulations to explore the kinematics of gaseous discs in late-type dwarf galaxies. We create high-resolution 21-cm ’observations’ of simulated dwarfs produced in two variations of the EAGLE galaxy formation model: one where supernova-driven gas flows redistribute dark matter and form constant-density central ’cores’, and another where the central ’cusps’ survive intact. We ’observe’ each galaxy along multiple sightlines and derive a rotation curve for each observation using a conventional tilted- ring approach to model the gas kinematics. We find that the modelling process introduces systematic discrepancies between the recovered rotation curve and the actual circular velocity curve driven primarily by (i) non-circular gas orbits within the discs; (ii) the finite thickness of gaseous discs, which leads to overlap of different radii in projection; and (iii) departures from dynamical equilibrium. Dwarfs with dark matter cusps often appear to have a core, whilst the inverse error is less common. These effects naturally reproduce an observed trend which other models struggle to explain: late-type dwarfs with more steeply rising rotation curves appear to be dark matter- dominated in the inner regions, whereas the opposite seems to hold in galaxies with core-like rotation curves. We conclude that if similar effects affect the rotation curves of observed dwarfs, a late-type dwarf population in which all galaxies have sizeable dark matter cores is most likely incompatible with current measurements.
@article{2023MNRAS.521.1316R, author = {{Roper}, Finn A. and {Oman}, Kyle A. and {Frenk}, Carlos S. and {Ben{\'\i}tez-Llambay}, Alejandro and {Navarro}, Julio F. and {Santos-Santos}, Isabel M. E.}, title = {{The diversity of rotation curves of simulated galaxies with cusps and cores}}, journal = {\mnras}, keywords = {galaxies: dwarf, galaxies: kinematics and dynamics, dark matter, Astrophysics - Astrophysics of Galaxies}, year = {2023}, month = may, volume = {521}, number = {1}, pages = {1316-1336}, doi = {10.1093/mnras/stad549}, archiveprefix = {arXiv}, eprint = {2203.16652}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2023MNRAS.521.1316R}, adsnote = {Provided by the SAO/NASA Astrophysics Data System}, }
2021
- A homogeneous measurement of the delay between the onsets of gas stripping and star formation quenching in satellite galaxies of groups and clustersKyle A. Oman, Yannick M. Bahé, Julia Healy, and 3 more authors(2021) MNRAS 501 5073
We combine orbital information from N-body simulations with an analytic model for star formation quenching and SDSS observations to infer the differential effect of the group/cluster environment on star formation in satellite galaxies. We also consider a model for gas stripping, using the same input supplemented with HI fluxes from the ALFALFA survey. The models are motivated by and tested on the Hydrangea cosmological hydrodynamical simulation suite. We recover the characteristic times when satellite galaxies are stripped and quenched. Stripping in massive (Mvir ∼1014.5 M⊙) clusters typically occurs at or just before the first pericentric passage. Lower mass (∼1013.5 M⊙) groups strip their satellites on a significantly longer (by ∼3 Gyr) time-scale. Quenching occurs later: Balmer emission lines typically fade ∼3.5 Gyr (5.5 Gyr) after first pericentre in clusters (groups), followed a few hundred Myr later by reddenning in (g-r) colour. These ’delay time-scales’ are remarkably constant across the entire satellite stellar mass range probed (∼109.5-1011 M⊙), a feature closely tied to our treatment of ’group pre-processing’. The lowest mass groups in our sample (∼1012.5 M⊙) strip and quench their satellites extremely inefficiently: typical time-scales may approach the age of the Universe. Our measurements are qualitatively consistent with the ’delayed-then-rapid’ quenching scenario advocated for by several other studies, but we find significantly longer delay times. Our combination of a homogeneous analysis and input catalogues yields new insight into the sequence of events leading to quenching across wide intervals in host and satellite mass.
@article{2021MNRAS.501.5073O, author = {{Oman}, Kyle A. and {Bah{\'e}}, Yannick M. and {Healy}, Julia and {Hess}, Kelley M. and {Hudson}, Michael J. and {Verheijen}, Marc A. W.}, title = {{A homogeneous measurement of the delay between the onsets of gas stripping and star formation quenching in satellite galaxies of groups and clusters}}, journal = {\mnras}, keywords = {galaxies: clusters: general, galaxies: evolution, galaxies: groups: general, Astrophysics - Astrophysics of Galaxies}, year = {2021}, month = mar, volume = {501}, number = {4}, pages = {5073-5095}, doi = {10.1093/mnras/staa3845}, archiveprefix = {arXiv}, eprint = {2009.00667}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2021MNRAS.501.5073O}, adsnote = {Provided by the SAO/NASA Astrophysics Data System}, }
2020
- Baryonic clues to the puzzling diversity of dwarf galaxy rotation curvesIsabel M. E. Santos-Santos, Julio F. Navarro, Andrew Robertson, and 7 more authors(2020) MNRAS 495 58
We use a compilation of disc galaxy rotation curves to assess the role of the luminous component (’baryons’) in the rotation curve diversity problem. As in earlier work, we find that rotation curve shape correlates with baryonic surface density: high surface density galaxies have rapidly rising rotation curves consistent with cuspy cold dark matter haloes; slowly rising rotation curves (characteristic of galaxies with inner mass deficits or ’cores’) occur only in low surface density galaxies. The correlation, however, seems too weak to be the main driver of the diversity. In addition, dwarf galaxies exhibit a clear trend, from ’cuspy’ systems where baryons are unimportant in the inner mass budget to ’cored’ galaxies where baryons actually dominate. This trend constrains the various scenarios proposed to explain the diversity, such as (I) baryonic inflows and outflows during galaxy formation; (II) dark matter self- interactions; (III) variations in the baryonic mass structure coupled to rotation velocities through the ’mass discrepancy- acceleration relation’ (MDAR); or (IV) non-circular motions in gaseous discs. Together with analytical modelling and cosmological hydrodynamical simulations, our analysis shows that each of these scenarios has promising features, but none seems to fully account for the observed diversity. The MDAR, in particular, is inconsistent with the observed trend between rotation curve shape and baryonic importance; either the trend is caused by systematic errors in the data or the MDAR does not apply. The origin of the dwarf galaxy rotation curve diversity and its relation to the structure of cold dark matter haloes remains an open issue.
@article{2020MNRAS.495...58S, author = {{Santos-Santos}, Isabel M. E. and {Navarro}, Julio F. and {Robertson}, Andrew and {Ben{\'\i}tez-Llambay}, Alejandro and {Oman}, Kyle A. and {Lovell}, Mark R. and {Frenk}, Carlos S. and {Ludlow}, Aaron D. and {Fattahi}, Azadeh and {Ritz}, Adam}, title = {{Baryonic clues to the puzzling diversity of dwarf galaxy rotation curves}}, journal = {\mnras}, keywords = {galaxies: dwarf, galaxies: evolution, galaxies: formation, galaxies: haloes, dark matter, cosmology: theory, Astrophysics - Astrophysics of Galaxies}, year = {2020}, month = jun, volume = {495}, number = {1}, pages = {58-77}, doi = {10.1093/mnras/staa1072}, archiveprefix = {arXiv}, eprint = {1911.09116}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2020MNRAS.495...58S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System}, }
2019
- Non-circular motions and the diversity of dwarf galaxy rotation curves(2019) MNRAS 482 821
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}, eprint = {1706.07478}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2019MNRAS.482..821O}, adsnote = {Provided by the SAO/NASA Astrophysics Data System}, }
2015
- The unexpected diversity of dwarf galaxy rotation curves(2015) MNRAS 452 3650
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}, eprint = {1504.01437}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2015MNRAS.452.3650O}, adsnote = {Provided by the SAO/NASA Astrophysics Data System}, }