Searcharxiv⌕ Search

arXiv subjects

Vadim Semenov

Publications and source records attributed to Vadim Semenov.

11 recordsLinked to original sources
arXiv2025

Rising from the Ashes II: The Bar-driven Abundance Bimodality of the Milky Way

The Milky Way hosts at least two modes in its present day distribution of Fe and alpha-elements. The exact cause of this bimodality is disputed, but one class of explanations involves the merger between the Milky Way and a relatively massive satellite (Gaia-Sausage-Enceladus) at z~2. However, reproducing this bimodality in simulations is not straightforward, with conflicting results on the prevalence, morphology, and mechanism behind multimodality. We present a case study of a galaxy in the Illustris TNG50 simulation which undergoes sequential phases of starburst, brief quiescence, and then rejuvenation. This scenario results in a pronounced abundance bimodality after a post-processing adjustment of the [alpha/Fe] of old stars designed to mimic a higher star formation efficiency in dense gas. The high- and low-alpha sequences are separated in time by the brief quiescent period, which is not associated with a merger but by the formation of a bar followed by AGN activity. This galaxy indicates a novel scenario in which the alpha-bimodality in the Milky Way is caused by the formation of the bar via AGN-induced quenching. In addition to a stellar age gap in the Milky Way, we predict that abundance bimodalities should be more common in barred as opposed to unbarred galaxies.

astro-ph.GA↗
arXiv2025

DyGEnc: Encoding a Sequence of Textual Scene Graphs to Reason and Answer Questions in Dynamic Scenes

The analysis of events in dynamic environments poses a fundamental challenge in the development of intelligent agents and robots capable of interacting with humans. Current approaches predominantly utilize visual models. However, these methods often capture information implicitly from images, lacking interpretable spatial-temporal object representations. To address this issue we introduce DyGEnc - a novel method for Encoding a Dynamic Graph. This method integrates compressed spatial-temporal structural observation representation with the cognitive capabilities of large language models. The purpose of this integration is to enable advanced question answering based on a sequence of textual scene graphs. Extended evaluations on the STAR and AGQA datasets indicate that DyGEnc outperforms existing visual methods by a large margin of 15-25% in addressing queries regarding the history of human-to-object interactions. Furthermore, the proposed method can be seamlessly extended to process raw input images utilizing foundational models for extracting explicit textual scene graphs, as substantiated by the results of a robotic experiment conducted with a wheeled manipulator platform. We hope that these findings will contribute to the implementation of robust and compressed graph-based robotic memory for long-horizon reasoning. Code is available at github.com/linukc/DyGEnc.

cs.CV↗
arXiv2024

The Discrete Gauss Image Problem

We study the Discrete Gauss Image Problem, a generalization of Aleksandrov's classical question on the existence of convex bodies with prescribed integral curvature. We introduce a combinatorial problem called the Assignment Problem and show its equivalence to the Discrete Gauss Image Problem. We establish sufficient (and nearly necessary) geometric conditions on measures that solve both problems. Additionally, we provide new discrete interpretations of some classical concepts related to Aleksandrov's integral curtvature, such as, for example, connecting Aleksandrov relation to Hall's Marriage Theorem.

math.MG↗
arXiv2024

The Gauss Image Problem with Weak Aleksandrov Condition

We introduce a relaxation of the Aleksandrov condition for the Gauss Image Problem. This weaker condition turns out to be a necessary condition for two measures to be related by a convex body. We provide several properties of the new condition. A solution to the Gauss Image Problem is obtained for the case when one of the measures is assumed to be discrete and the another measure is assumed to be absolutely continuous, under the new relaxed assumption.

math.MG↗
arXiv2024

Entrainment of Hot Gas into Cold Streams: The Origin of Excessive Star-formation Rates at Cosmic Noon

We explore the evolution of cold streams from the cosmic web that feed galaxies through their shock-heated circumgalactic medium (CGM) at cosmic noon, $z\simeq 1-5$. In addition to the hydrodynamical instabilities and radiative cooling that we have incorporated in earlier works, we embed the stream and the hot CGM in the gravitational potential of the host dark-matter halo, deriving equilibrium profiles for both. Self-gravity within the stream is tentatively ignored. We find that the cold streams gradually entrain a large mass of initially hot CGM gas that cools in the mixing layer and condenses onto the stream. This entrainment, combined with the acceleration down the gravitational potential well, typically triples the inward cold inflow rate into the central galaxy, compared to the original rate at the virial radius, which makes the entrained gas the dominant source of gas supply to the galaxy. The potential sources for the hot gas to be entrained are recycled enriched gas that has been previously ejected from the galaxy, and fresh virial-shock-heated gas that has accumulated in the CGM. This can naturally elevate the star formation rate in the galaxy by a factor of $\sim 3$ compared to the gas accretion rate onto the halo, thus explaining the otherwise puzzling observed excess of star formation at cosmic noon. When accounting for self-shielding of dense gas from the UV background, we find that the energy radiated from the streams, originating predominantly from the cooling of the entrained gas, is consistent with observed Lyman-$α$ blobs around galaxies.

astro-ph.GA↗
arXiv2023

Tilted Dark Halos are Common, Long-Lived, and can Warp Galactic Disks

In the $Λ$-CDM paradigm, the dark halo governs the gravitational potential within which a galaxy can form and evolve. In this Letter we show that the present-day inner ($r<50\text{ kpc}$) dark halo can be significantly misaligned with the stellar disk. To this end, we use the TNG50 run from the cosmological magneto-hydrodynamic IllustrisTNG simulation suite. Such "tilted" dark halos can arise from a variety of processes including major mergers, massive fly-bys, or interactions with satellite companions. Furthermore, we show that tilted dark halos: (1) are well traced by tilted stellar halos, (2) can maintain their tilt for $>$ 5 Gyr in isolated evolution, and (3) can generate warps in the outer disks that are stable over many Gyr. A tilted dark halo holds clues to important events in the formation history of a galaxy, and could help explain the abundance of warped disks in galaxy observations, including the Milky Way.

astro-ph.GA↗
arXiv2023

The Uniqueness of the Gauss Image Measure

We show that if the Gauss Image Measure of submeasure $λ$ via convex body $K$ agrees with the Gauss Image Measure of $λ$ via convex body $L$, then the radial Gauss Image maps of their duals, are equal to each other almost everywhere as multivalued maps with respect to $λ$. As an application of this result, we establish that, in this case, dual bodies, $K^*$ and $L^*$, are equal up to a dilation on each rectifiable path connected component of the support of $λ$. Additionally, we provide many previously unknown properties of the radial Gauss Image map, most notably its variational Lipschitz behavior, establish some measure theory concepts for multivalued maps and, as a supplement, show how the main uniqueness statement neatly follows from the Hopf Theorem under additional smooth assumptions on $K$ and $L$.

math.MG↗
arXiv2018

The AGORA High-Resolution Galaxy Simulations Comparison Project. II: Isolated Disk Test

Using an isolated Milky Way-mass galaxy simulation, we compare results from 9 state-of-the-art gravito-hydrodynamics codes widely used in the numerical community. We utilize the infrastructure we have built for the AGORA High-resolution Galaxy Simulations Comparison Project. This includes the common disk initial conditions, common physics models (e.g., radiative cooling and UV background by the standardized package Grackle) and common analysis toolkit yt, all of which are publicly available. Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production are carefully constrained across code platforms. With numerical accuracy that resolves the disk scale height, we find that the codes overall agree well with one another in many dimensions including: gas and stellar surface densities, rotation curves, velocity dispersions, density and temperature distribution functions, disk vertical heights, stellar clumps, star formation rates, and Kennicutt-Schmidt relations. Quantities such as velocity dispersions are very robust (agreement within a few tens of percent at all radii) while measures like newly-formed stellar clump mass functions show more significant variation (difference by up to a factor of ~3). Systematic differences exist, for example, between mesh-based and particle-based codes in the low density region, and between more diffusive and less diffusive schemes in the high density tail of the density distribution. Yet intrinsic code differences are generally small compared to the variations in numerical implementations of the common subgrid physics such as supernova feedback. Our experiment reassures that, if adequately designed in accordance with our proposed common parameters, results of a modern high-resolution galaxy formation simulation are more sensitive to input physics than to intrinsic differences in numerical schemes.

astro-ph.GA↗
arXiv2016

On conservative sequences and their application to ergodic multiplier problems

The conservative sequence of a set $A$ under a transformation $T$ is the set of all $n \in \mathbb{Z}$ such that $T^n A \cap A \not = \varnothing$. By studying these sequences, we prove that given any countable collection of nonsingular transformations with no finite invariant measure $\{T_i\}$, there exists a rank-one transformation $S$ such that $T_i \times S$ is not ergodic for all $i$. Moreover, $S$ can be chosen to be rigid or have infinite ergodic index. We establish similar results for $\mathbb{Z}^d$ actions and flows. Then, we find sufficient conditions on rank-one transformations $T$ that guarantee the existence of a rank-one transformation $S$ such that $T \times S$ is ergodic, or, alternatively, conditions that guarantee that $T \times S$ is conservative but not ergodic. In particular, the infinite Chacón transformation satisfies both conditions. Finally, for a given ergodic transformation $T$, we study the Baire categories of the sets $E(T)$, $\bar{E}C(T)$ and $\bar{C}(T)$ of transformations $S$ such that $T \times S$ is ergodic, ergodic but not conservative, and conservative, respectively.

math.DS↗
arXiv2013

Dark matter halo formation in the multicomponent dark matter models

This work investigates a set of cosmological collisionless N-body simulations with featured power spectra of initial perturbations in the context of the core-cusp and satellites problems. On the studied power spectra some scales of fluctuations were suppressed. Such spectral features can be caused by multicomponent dark matter. The density profiles innermost resolved slopes $α\equiv d \log(ρ) /d \log(r) $ of the five largest haloes were measured and its dependence on the parameters of the suppression was traced. In a certain range of the parameters the slopes flatten from initial value of about -1.2 to -0.6 or even to -0.2 in one of the cases. This qualitatively demonstrates that (i) profiles shape depends on initial power spectrum and (ii) this effect may be responsible for the solution of the core-cusp problem. The suppression of some part of the initial power spectrum also leads to the decrease of the number of massive subhaloes.

astro-ph.CO↗