Stacy Kim

Stacy Kim

Research Fellow

Academic and research departments

Astrophysics Research Group.


Areas of specialism

near-field cosmology; dark matter; computational astrophysics


Research interests


Matthew D. A. Orkney, Justin Read, Martin P. Rey, Imran Nasim, Andrew Pontzen, Oscar Agertz, Stacy Y. Kim, Maxime Delorme, Walter Dehnen (2021)EDGE: two routes to dark matter core formation in ultra-faint dwarfs, In: Monthly notices of the Royal Astronomical Society504(3)pp. 3509-3522 Oxford Univ Press

In the standard Lambda cold dark matter paradigm, pure dark matter simulations predict dwarf galaxies should inhabit dark matter haloes with a centrally diverging density 'cusp'. This is in conflict with observations that typically favour a constant density 'core'. We investigate this 'cusp-core problem' in 'ultra-faint' dwarf galaxies simulated as part of the 'Engineering Dwarfs at Galaxy formation's Edge' project. We find, similarly to previous work, that gravitational potential fluctuations within the central region of the simulated dwarfs kinematically heat the dark matter particles, lowering the dwarfs' central dark matter density. However, these fluctuations are not exclusively caused by gas inflow/outflow, but also by impulsive heating from minor mergers. We use the genetic modification approach on one of our dwarf's initial conditions to show how a delayed assembly history leads to more late minor mergers and, correspondingly, more dark matter heating. This provides a mechanism by which even ultra-faint dwarfs (M-star < 10(5) M-circle dot), in which star formation was fully quenched at high redshift, can have their central dark matter density lowered over time. In contrast, we find that late major mergers can regenerate a central dark matter cusp, if the merging galaxy had sufficiently little star formation. The combination of these effects leads us to predict significant stochasticity in the central dark matter density slopes of the smallest dwarfs, driven by their unique star formation and mass assembly histories.

Martin P. Rey, Andrew Pontzen, Oscar Agertz, Matthew D. A. Orkney, Justin Read, Amelie Saintonge, Stacy Y. Kim, Payel Das (2022)EDGE: What shapes the relationship between H i and stellar observables in faint dwarf galaxies?, In: Monthly notices of the Royal Astronomical Society511(4)pp. 5672-5681 Oxford Univ Press

We show how the interplay between feedback and mass-growth histories introduces scatter in the relationship between stellar and neutral gas properties of field faint dwarf galaxies (M-*less than or similar to 10(6) M-circle dot). Across a suite of cosmological, high-resolution zoomed simulations, we find that dwarf galaxies of stellar masses 10(5)

Matthew D A Orkney, Justin I Read, Oscar Agertz, Andrew Pontzen, Martin P Rey, Alex Goater, Ethan Taylor, Stacy Y Kim, Maxime Delorme (2022)EDGE: the puzzling ellipticity of Eridanus II’s star cluster and its implications for dark matter at the heart of an ultra-faint dwarf, In: Monthly notices of the Royal Astronomical Society515(1)pp. 185-200 Oxford University Press

ABSTRACT The Eridanus II (EriII) ‘ultra-faint’ dwarf has a large (15 pc) and low-mass (4.3 × 103 M⊙) star cluster (SC) offset from its centre by 23 ± 3 pc in projection. Its size and offset are naturally explained if EriII has a central dark matter core, but such a core may be challenging to explain in a ΛCDM cosmology. In this paper, we revisit the survival and evolution of EriII’s SC, focusing for the first time on its puzzlingly large ellipticity ($0.31^{+0.05}_{-0.06}$). We perform a suite of 960 direct N-body simulations of SCs, orbiting within a range of spherical background potentials fit to ultra-faint dwarf (UFD) galaxy simulations. We find only two scenarios that come close to explaining EriII’s SC. In the first scenario, EriII has a low-density dark matter core (of size ${\sim}70\, \text{pc}$ and density $\lesssim 2\times 10^8\, \text{M}_{\odot }\, \text{kpc}^{-3}$). In this model, the high ellipticity of EriII’s SC is set at birth, with the lack of tidal forces in the core allowing its ellipticity to remain frozen for long times. In the second scenario, EriII’s SC orbits in a partial core, with its high ellipticity owing to its imminent tidal destruction. However, this latter model struggles to reproduce the large size of EriII’s SC, and it predicts substantial tidal tails around EriII’s SC that should have already been seen in the data. This leads us to favour the cored model. We discuss potential caveats to these findings, and the implications of the cored model for galaxy formation and the nature of dark matter.

R Shearman, G Lorusso, A Boso, PATRICK HENRY REGAN, S Nishimura, Z Y Xu, A Jungclaus, Y Shimizu, G S Simpson, P-A Söderström, H Watanabe, F Browne, P Doornenbal, G Gey, H S Jung, B Meyer, T Sumikama, J Taprogge, Zs Vajta, J Wu, H Baba, G Benzoni, K Y Chae, F C L Crespi, N Fukuda, R Gernhäuser, N Inabe, T Isobe, T Kajino, D Kameda, G D Kim, STACY YEONCHI KIM, I Kajouharov, F G Kondev, T Kubo, N Kurz, Y K Kwon, G J Lane, ZHONG LI, A Montaner-Pizá, K Moschner, F Naqvi, M Niikura, H Nishibata, A Odahara, R Orlandi, Z Patel, Zsolt Podolyak, H Sakurai, H Schaffner, P Schury, S Shibagaki, K Steiger, H Suzuki, H Takeda, A Wendt, A Yagi, K Yoshinaga (2020)Determination of beta-delayed neutron emission probability limits of rhodium isotopes by gamma-ray spectroscopy, In: Journal of physics. Conference series1643(1) IOP Publishing

The decay of five neutron-heavy rhodium isotopes were studied at the Radioactive Isotope Beam Factory (RIBF) Facility at the RIKEN Nishina Center after relativistic fission of 238U beam on a thick beryllium target. Previously unknown associated gamma-ray decay energies are reported for each nuclide, and through evaluating the intensity of the 2+ → 0+ E2 transition in the even-even palladium daughter nuclei, 120,122,124Pd, from the beta-tagged gamma-ray spectra an upper or lower limit of beta-delayed neutron emission is deduced for each nuclei. A general, expected trend of increasing Pn is observed in the direction of the neutron drip line.

J Taprogge, A Jungclaus, H Grawe, S Nishimura, P Doornenbal, G Lorusso, GS Simpson, P-A Soederstroem, T Sumikama, ZY Xu, H Baba, F Browne, N Fukuda, R Gernhaeuser, G Gey, N Inabe, T Isobe, HS Jung, D Kameda, GD Kim, Y-K Kim, I Kojouharov, T Kubo, N Kurz, YK Kwon, Z Li, H Sakurai, H Schaffner, K Steiger, H Suzuki, H Takeda, Z Vajta, H Watanabe, J Wu, A Yagi, K Yoshinaga, G Benzoni, S Boenig, KY Chae, L Coraggio, A Covello, J-M Daugas, F Drouet, A Gadea, A Gargano, S Ilieva, FG Kondev, T Kroell, GJ Lane, A Montaner-Piza, K Moschner, D Mucher, F Naqvi, M Niikura, H Nishibata, A Odahara, R Orlandi, Z Patel, Z Podolyak, A Wendt (2015)beta decay of Cd-129 and excited states in In-129, In: PHYSICAL REVIEW C91(5)ARTN 0pp. ?-? AMER PHYSICAL SOC
M Holl, S Lindberg, A Heinz, Y Kondo, T Nakamura, J. A Tostevin, H Wang, T Nilsson, N. L Achouri, H. Al Falou, L Atar, T Aumann, H Baba, K Boretzky, C Caesar, D Calvet, H Chae, N Chiga, A Corsi, H. L Crawford, F Delaunay, A Delbart, Q Deshayes, P. Díaz Fernández, Z Dombrádi, C. A Douma, Z Elekes, P Fallon, I Gašparić, J. -M Gheller, J Gibelin, A Gillibert, M. N Harakeh, A Hirayama, C. R Hoffman, A Horvat, Á Horváth, J. W Hwang, T Isobe, J Kahlbow, N Kalantar-Nayestanaki, S Kawase, S Kim, K Kisamori, T Kobayashi, D Körper, S Koyama, I Kuti, V Lapoux, F. M Marqués, S Masuoka, J Mayer, K Miki, T Murakami, M Najafi, K Nakano, N Nakatsuka, A Obertelli, F. de Oliveira Santos, N. A Orr, H Otsu, T Ozaki, V Panin, S Paschalis, A Revel, D Rossi, A. T Saito, T. Y Saito, M Sasano, H Sato, Y Satou, H Scheit, F Schindler, P Schrock, M Shikata, Y Shimizu, H Simon, D Sohler, O Sorlin, L Stuhl, S Takeuchi, M Tanaka, M Thoennessen, H Törnqvist, Y Togano, T Tomai, J Tscheuschner, J Tsubota, T Uesaka, Z Yang, M Yasuda, K Yoneda (2022)Border of the Island of Inversion: Unbound states in $^{29}$Ne, In: Physical Review C105034301 American Physical Society

The nucleus $^{29}$Ne is situated at the border of the island of inversion. Despite significant efforts, no bound low-lying intruder $f_{7/2}$-state, which would place $^{29}$Ne firmly inside the island of inversion, has yet been observed. Here, the first investigation of unbound states of $^{29}$Ne is reported. The states were populated in $^{30}\mathrm{Ne}(p,pn)$ and $^{30}\mathrm{Na}(p,2p)$ reactions at a beam energy of around $230$ MeV/nucleon, and analyzed in terms of their resonance properties, partial cross sections and momentum distributions. The momentum distributions are compared to calculations using the eikonal, direct reaction model, allowing $\ell$-assignments for the observed states. The lowest-lying resonance at an excitation energy of 1.48(4) MeV shows clear signs of a significant $\ell$=3-component, giving first evidence for $f_{7/2}$ single particle strength in $^{29}$Ne. The excitation energies and strengths of the observed states are compared to shell-model calculations using the sdpf-u-mix interaction

A. Revel, O. Sorlin, F.M Marques, Y. Kondo, J. Kahlbow, T. Nakamura, N.A Orr, F. Nowacki, Jeffrey Tostevin, C.X Yuan, N.L Achouri, H. Al Falou, L. Atar, T. Aumann, H. Baba, K. Boretzky, C. Caesar, D. Calvet, H. Chae, N. Chiga, A. Corsi, H.L Crawford, F. Delaunay, A. Delbart, Q. Deshayes, Z. Dombradi, C.A Douma, Z. Elekes, P. Fallon, I. Gasparic, J.M Gheller, J. Gibelin, A. Gillibert, M.N Harakeh, W. He, A. Hirayama, C.R Hoffman, M. Holl, A. Horvat, A. Horvath, J.W Hwang, T. Isobe, N. Kalantar-Nayestanaki, S. Kawase, S. Kim, K. Kisamori, T. Kobayashi, D. Korper, S. Koyama, I. Kuti, V. Lapoux, S. Lindberg, S. Masuoka, J. Mayer, K. Miki, T. Murakami, M. Najafi, K. Nakano, N. Nakatsuka, T. Nilsson, A. Obertelli, F. de Oliviera Santos, H. Otsu, T. Ozaki, V. Panin, S. Paschalis, D. Rossi, A.T Saito, T. Saito, M. Sasano, H. Sato, Y. Satou, H. Scheit, F. Schindler, P. Schrock, M. Shikata, Y. Shimizu, H. Simon, D. Sohler, L. Stuhl, S. Takeuchi, M. Tanaka, M. Thoennsen, H. Tornqvist, Y. Togano, T. Tomai, J. Tscheuschner, J. Tsubota, T. Uesaka, Z. Yang, M. Yasuda, K. Yoneda (2020)Extending the Southern Shore of the Island of Inversion to 28F, In: Physical Review Letters124(15) American Physical Society

Detailed spectroscopy of the neutron-unbound nucleus 28 F has been performed for the first time following proton/neutron removal from 29 Ne / 29 F beams at energies around 230 MeV / nucleon . The invariant-mass spectra were reconstructed for both the 27 F ( * ) + n and 26 F ( * ) + 2 n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the 28 F ground state, with S n ( 28 F ) = − 199 ( 6 )     keV , while analysis of the 2 n decay channel allowed a considerably improved S n ( 27 F ) = 1620 ( 60 )     keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of 28 F . Importantly, in the case of the ground state, the reconstructed 27 F + n momentum distribution following neutron removal from 29 F indicates that it arises mainly from the 1 p 3 / 2 neutron intruder configuration. This demonstrates that the island of inversion around N = 20 includes 28 F , and most probably 29 F , and suggests that 28 O is not doubly magic.

N Kobayashi, T Nakamura, Y Kondo, JA Tostevin, N Aoi, H Baba, R Barthelemy, MA Famiano, N Fukuda, N Inabe, M Ishihara, R Kanungo, S Kim, T Kubo, GS Lee, HS Lee, M Matsushita, T Motobayashi, T Ohnishi, NA Orr, H Otsu, T Sako, H Sakurai, Y Satou, T Sumikama, H Takeda, S Takeuchi, R Tanaka, Y Togano, K Yoneda (2016)One-neutron removal from Ne29: Defining the lower limits of the island of inversion, In: Physical Review C: Nuclear Physics93014613 American Physical Society

Background: Very neutron-rich isotopes, including Ne30-32, in the vicinity of N=20 are known to exhibit ground states dominated by fp-shell intruder configurations: the “island of inversion.” Systematics for the Ne-isotopic chain suggest that such configurations may be in strong competition with normal shell-model configurations in the ground state of Ne29. Purpose: A determination of the structure of Ne29 is thus important to delineate the extent of the island of inversion and better understand structural evolution in neutron-rich Ne isotopes. This is accomplished here through a combined investigation of nuclear and Coulomb-induced one-neutron removal reactions. Method: Cross sections for one-neutron removal on carbon and lead targets and the parallel momentum distribution of the Ne28 residues from the carbon target are measured at around 240 MeV/nucleon. The measurements are compared with reaction calculations combined with spectroscopic information from SDPF-M shell-model wave functions. Results: The deduced width of the inclusive parallel momentum distribution, 98(12) MeV/c (FWHM), suggests that the ground state of Ne29 has a spin parity of 3/2-. Detailed comparisons of the measured inclusive and partial cross sections of the two targets and the parallel momentum distribution of the carbon target with reaction calculations, combined with spectroscopic information from large-scale shell-model calculations, are all consistent with a 3/2- spin-parity assignment. Conclusions: The results indicate that Ne29 lies within the island of inversion and that the ground state of Ne29 is dominated by a Ne28(0+1)xp3/2 neutron configuration. Combined with recently measured interaction cross sections, it is concluded that Ne29 may exhibit a moderately developed halo-like distribution

N Kobayashi, T Nakamura, Y Kondo, JA Tostevin, Y Utsuno, N Aoi, H Baba, R Bartelemy, MA Famiano, N Fukuda, N Inabe, M Ishihara, R Kunungo, S Kim, T Kim, GS Kubo, HS Lee, M Lee, T Matsushita, T Motobayashi, NA Ohnishi, H Orr, T Otsu, T Otsuka, H Sako, Y Sakurai, T Satou, H Sumikama, S Takeda, R Takeuchi, Y Tanaka, K Togano, Yoneda (2014)Observation of a p-Wave One-Neutron Halo Configuration in 37Mg, In: Physical Review Letters112242501

Cross sections of 1n-removal reactions from the neutron-rich nucleus Mg37 on C and Pb targets and the parallel momentum distributions of the Mg37 residues from the C target have been measured at 240  MeV/nucleon. A combined analysis of these distinct nuclear- and Coulomb-dominated reaction data shows that the Mg37 ground state has a small 1n separation energy of 0.22+0.12−0.09  MeV and an appreciable p-wave neutron single-particle strength. These results confirm that Mg37 lies near the edge of the “island of inversion” and has a sizable p-wave neutron halo component, the heaviest such system identified to date.

Additional publications