arXiv subjects

Yuchan Wang

Publications and source records attributed to Yuchan Wang.

5 recordsLinked to original sources

arXiv2025

Buried but not destroyed: the evolution from prompt cusps to NFW haloes

The internal structure of dark matter haloes encodes their assembly history and offers critical insight into the nature of dark matter and structure formation. Analytical studies and high-resolution simulations have recently predicted the formation of 'prompt cusps' - steep power-law density profiles that emerge rapidly from the monolithic collapse of primordial peaks. Yet, by $z=0$ most haloes are well described by Navarro-Frenk-White (NFW) density profiles, raising the question of how these early cusps are transformed in a cosmological setting. We address this problem using 64 zoom-in $N$-body simulations of eight haloes, each resimulated with eight different free-streaming wavenumbers to control the abundance of small-scale structure while keeping the large-scale environment fixed. To mitigate numerical discreteness effects, we classify artificial fragments and genuine subhaloes with a physically motivated procedure based on matching subhaloes to their progenitor peaks. At the population level, we demonstrate that haloes initially form prompt cusps, and their profiles subsequently transition towards the NFW form. Our study reveals three distinct pathways by which prompt cusps evolve: major mergers, accretion of artificial fragments, and interactions with large-scale filaments, each having a distinct impact on the inner density profile. In particular, we show that the original power-law cusp remains visible in the profile of particles associated with the primordial peak even when the total halo profile is already NFW-like. This work highlights the imprint of early collapse on present-day halo structure and provides new insights into the origin of the universality of the NFW profile.

astro-ph.CO↗

arXiv2021

Thermostatic Hyperthermia with Non-invasive Temperature Monitoring through Speed of Sound Imaging

Hyperthermia therapy (HT) is used to treat diseases through heating of high temperature usually in conjunction with some other medical therapeutics like chemotherapy and radiotherapy. In this study, we propose a promising thermostatic hyperthermia method that uses high-intensity focused ultrasound (HIFU) for clinical tumor treatment combined with diagnostic ultrasound image guidance and non-invasive temperature monitoring through the speed of sound (SOS) imaging. HIFU heating is realized by a ring ultrasound transducer array with 256 elements. The inner structure information of thigh tissue is obtained by B-mode ultrasound imaging. Since the relationship between the temperature and the SOS in the different human tissue is available, the temperature detection is converted to the SOS detection obtained by the full-wave inversion (FWI) method. Simulation results show that our model can achieve expected thermostatic hyperthermia on tumor target with 0.2 degree maximum temperature fluctuation for 5 hours. This study verifies the feasibility of the proposed thermostatic hyperthermia model. Furthermore, the temperature measurement can share the same ultrasound transducer array for HIFU heating and B-mode ultrasound imaging, which provides a guiding significance for clinical application.

physics.med-ph↗

arXiv2021

The obstructed jet in Mrk 231

Mrk~231 is the closest radio-quiet quasar known and one of the most luminous infrared galaxies in the local Universe. It is characterised by the co-existence of a radio jet and powerful multi-phase multi-scale outflows, making it an ideal laboratory to study active galactic nucleus (AGN) feedback. We analyse the multi-epoch very long baseline interferometry data of Mrk~231 and estimate the jet head advance speed to be $\lesssim0.013\ c$, suggesting a sub-relativistic jet flow. The jet position angle changes from $-113^\circ$ in the inner parsec to $-172^\circ$ at a projected distance of 25 parsec. The jet structure change might result from either a jet bending following the rotation of the circum-nuclear disc or the projection of a helical jet on the plane of the sky. In the large opening angle ($\sim60^\circ$) cone, the curved jet interacts with the interstellar medium and creates wide-aperture-angle shocks which subsequently dissipate a large portion of the jet power through radiation and contribute to powering the large-scale outflows. The low power and bent structure of the Mrk~231 jet, as well as extensive radiation dissipation, are consistent with the obstruction of the short-length jet by the host galaxy's environment.

astro-ph.GA↗

arXiv2020

Iterative removal of redshift space distortions from galaxy clustering

Observations of galaxy clustering are made in redshift space, which results in distortions to the underlying isotropic distribution of galaxies. These redshift-space distortions (RSD) not only degrade important features of the matter density field, such as the baryonic acoustic oscillation (BAO) peaks, but also pose challenges for the theoretical modelling of observational probes. Here we introduce an iterative nonlinear reconstruction algorithm to remove RSD effects from galaxy clustering measurements, and assess its performance by using mock galaxy catalogues. The new method is found to be able to recover the real-space galaxy correlation function with an accuracy of $\sim1\%$, and restore the quadrupole accurately to $0$, on scales $s\gtrsim20\Mpch$. It also leads to an improvement in the reconstruction of the initial density field, which could help to accurately locate the BAO peaks. An `internal calibration' scheme is proposed to determine the values of cosmological parameters as a part of the reconstruction process, and possibilities to break parameter degeneracies are discussed. RSD reconstruction can offer a potential way to simultaneously extract the cosmological parameters, initial density field, real-space galaxy positions and large-scale peculiar velocity field (of the real Universe), making it an alternative to standard perturbative approaches in galaxy clustering analysis, bypassing the need for RSD modelling.

astro-ph.CO↗

arXiv2020

Null controllability and inverse source problem for stochastic Grushin equation with boundary degeneracy and singularity

In this paper, we consider a null controllability and an inverse source problem for stochastic Grushin equation with boundary degeneracy and singularity. We construct two special weight functions to establish two Carleman estimates for the whole stochastic Grushin operator with singular potential by a weighted identity method. One is for the backward stochastic Grushin equation with singular weight function. We then apply it to prove the null controllability for stochastic Grushin equation for any $T$ and any degeneracy $γ>0$, when our control domain touches the degeneracy line $\{x=0\}$. In order to study the inverse source problem of determining two kinds of sources simultaneously, we prove the other Carleman estimate, which is for the forward stochastic Grushin equation with regular weight function. Based on this Carleman estimate, we obtain the uniqueness of the inverse source problem.

math.OC↗