arXiv subjects

T. Worrakitpoonpon

Publications and source records attributed to T. Worrakitpoonpon.

4 recordsLinked to original sources

arXiv2026

In-depth analysis of bar formation mechanisms of disk galaxies in halos of different concentrations

We use $N$-body simulations to investigate the distinct bar formation processes in disks residing in halos of various concentrations. In a highly concentrated halo, the bar development is limited by the dominant multi-arm modes as a result of the swing amplification in the early stage. Only after the multi-arm modes decay, the bar growth proceeds mechanically owing to the particle trapping in continuation of that bar seed. In this scheme, the corotation resonance of the bar modes does not come into play at all, justified by a low amount of disk-halo angular momentum transfer and a modestly decreasing bar pattern speed. On the other hand, although reducing the halo concentration suggests the reduction of the preferred swing-amplified modes to be bi-symmetric, the bar formation in a lowly concentrated halo does not involve the swing amplification at all. Rather, the fast-growing linearly unstable bar modes of a single uniform frequency is solely the governing factor, attributed to a mild shearing. The bar modes trigger the corotation resonance since the beginning and such resonance is maintained until the end, which leads to a high amount of angular momentum transfer and a fast slowdown. For the intermediate halo concentration, the kinematical analyses of multiple non-axisymmetric modes suggests that the linear modes, the swing amplification, and the particle trapping are all present in the evolution chronology. To specify bars formed in the different halo concentrations, full analyses of the isophotal shape, the radial Fourier amplitude, and the resonance diagram can be of use.

astro-ph.GA↗

arXiv2023

Investigating scaling relations in X-ray reverberating AGN using symbolic regression

Symbolic regression (SR) is a regression analysis based on genetic algorithms to search for mathematical expressions that best fit a given data set, by allowing the expressions themselves to mutate. We use the SR to analyze the parameter relations of the X-ray reverberating Active Galactic Nuclei (AGN) where the soft Fe-L lags were observed by XMM-Newton. Firstly, we revisit the lag-mass scaling relations by using the SR to derive all possible mathematical expressions and test them in terms of accuracy, simplicity and robustness. We find that the correlation between the lags, $τ$, and the black hole mass, $M_{\rm BH}$, is certain, but the relation should be written in the form of $\log (τ) = α+ β(\log{(M_{\rm BH}/M_{\odot})})^γ$, where $1 \lesssim γ\lesssim 2$. Moreover, incorporating more parameters such as the reflection fraction ($RF$) and the Eddington ratio ($λ_{\rm Edd}$) to the lag-mass scaling relation is made possible by the SR. It reveals that $α$, rather than being a constant, can be $-2.15 + 0.02RF$ or $0.03(RF + λ_{\rm Edd})$, with the fine-tuned different $β$ and $γ$. These further support the relativistic disc-reflection framework in which such functional dependencies can be straightforwardly explained. Furthermore, we derive their host-galaxy mass, $M_{\ast}$, by fitting the spectral energy distribution (SED). We find that the SR model supports a non-linear $M_{\rm BH}$--$M_{\ast}$ relationship, while $\log (M_{\rm BH}/M_{\ast})$ varies between $-5.4$ and $-1.5$, with an average value of $\sim -3.7$. No significant correlation between $M_{\ast}$ and $λ_{\rm Edd}$ is confirmed in these samples.

astro-ph.HE↗

arXiv2023

Dynamical analysis of Maclaurin disk with velocity dispersion and its influence on bar formation

We investigate the influence of Toomre's $Q$ parameter on the bar-forming dynamics of Maclaurin disk using $N$-body simulations. According to the Toomre's criterion, local velocity dispersion parametrized by $Q\geq 1$ is required to suppress the local axisymmetric instability but, in turn, it deviates particle orbits from nearly circular limit in which particle natural frequencies are calculated. We resolve this by including the effect of velocity dispersion, as the pressure potential, into the effective potential with the gravitational potential. With this formulation, circular orbit approximation is retrieved. The effective potential hypothesis can describe the $Q$-dependences of angular and epicyclic motions of the bar-forming processes and the established bars reasonably well provided that $Q\geq 1$. This indicates the influence of initial $Q$ that is imprinted in the entire disk dynamics, not only that $Q$ serves as the stability indicator. In addition, we perform the stability test for the disk-in-halo systems. With the presence of halo, disks are more susceptible to the bar formation as seen by the elevated critical $Q$ than that for the isolated disk. This is attributed to the differential rotation that builds the unstable non-axisymmetric spiral modes more efficiently which are the ingredients of bar instability.

astro-ph.GA↗

arXiv2015

Angular momentum generation in cold gravitational collapse

During the violent relaxation of a self-gravitating system a significant fraction of its mass may be ejected. If the time varying gravitational field also breaks spherical symmetry this mass can potentially carry angular momentum. Thus starting initial configurations with zero angular momentum can in principle lead to a bound virialized system with non-zero angular momentum. We explore here, using numerical simulations, how much angular momentum can be generated in a virialized structure in this way, starting from configurations of cold particles which are very close to spherically symmetric. For initial configurations in which spherical symmetry is broken only by the Poissonian fluctuations associated with the finite particle number $N$, with $N$ in range $10^3$ to $10^5$, we find that the relaxed structures have standard "spin" parameters $λ\sim 10^{-3}$, and decreasing slowly with $N$. For slightly ellipsoidal initial conditions, in which the finite-$N$ fluctuations break the residual reflection symmetries, we observe values $λ\sim 10^{-2}$, of the same order of magnitude as those reported for elliptical galaxies. The net angular momentum vector is typically aligned close to normal to the major semi-axis of the triaxial relaxed structure, and also with that of the ejected mass. This simple mechanism may provide an alternative, or complement, to "tidal torque theory" for understanding the origin of angular momentum in astrophysical structures.

astro-ph.GA↗