arXiv subjects

Zixuan Peng

Publications and source records attributed to Zixuan Peng.

11 recordsLinked to original sources

arXiv2026

Astronomical Advantages of a Boost Mission to Facilitate HST Science into the 2030s: Imaging the Circumgalactic Medium of Galaxies

We present the case for imaging ultraviolet line emission from highly ionized metals and HI Lya in the circumgalactic medium of galaxies, should the Hubble Space Telescope receive an orbital boost. Hubble can uniquely probe emission lines with ionization potentials between 13 and 200 electron-volts (Lya, CIV, OVI, NeVIII, etc). Spatial mapping of the diffuse material traced by these transitions is critical to constraining the physics of feedback and the energetic exchange between galaxies and their circumgalactic environments, as well as basic morphologies of the dominant mass component. Deep high-resolution mapping of these features will not be possible with any other observatory, existing or planned, until HWO is launched, which leaves HST as a critical observatory to test key science drivers for HWO. If HST receives an orbital boost, it can (a) provide the first statistical constraints on the spatial distribution of warm-hot CGM and (b) provide important avenues for science case development, as well as target/pointing selection, for HWO's upcoming spectroscopic facilities.

astro-ph.IM↗

arXiv2026

An Improved Upper Bound for the Dirichlet Spectrum in Diophantine Approximation

We study the continuous part of the Dirichlet spectrum $\mathbb{D}$ and improve the best previously published upper bound for the ray-origin constant $\delta$. Building on and refining V. A. Ivanov's approach, we introduce a Cantor-type set $F_4^*$ defined by certain restrictions on partial quotients. For its thickness, we prove $\tau(\log(F_4^*))>1$, and apply sum-set results for Cantor sets to prove that the set $F_4^* \cdot F_4^*$ is an interval. Finally, we establish a new upper bound $\delta\le \frac{111(397+\sqrt{26565})}{65522}\approx0.94866$.

math.NT↗

arXiv2025

Modeling Emission-Line Surface Brightness in a Multiphase Galactic Wind: An O VI Case Study

We present a fast and robust analytic framework for predicting surface brightness (SB) of emission lines in galactic winds as a function of radius up to $\sim 100$ kpc out in the circum-galactic medium. We model multiphase structure in galactic winds by capturing emission from both the volume-filling hot phase (T $\sim 10^{6-7}$ K) and turbulent radiative mixing layers that host intermediate temperature gas at the boundaries of cold clouds (T $\sim 10^4$ K). Our multiphase framework makes significantly different predictions of emission signatures compared to traditional single-phase models and explains the paucity of OVI SB measurements in the literature. After accounting for ram pressure equilibrium between the cold clouds and hot wind in supersonic outflows, non-equilibrium ionization effects, and energy budgets other than mechanical energy from core-collapse supernovae, our OVI SB predictions qualitatively match observational results. Our framework provides constraints on the optimal galactic wind properties that facilitate OVI emission observations, including star formation rate surface density, hot phase mass loading factor, and thermalization efficiency factor. These constraints are consistent with existing observations and can help inform future target selections.

astro-ph.GA↗

arXiv2025

When Stars Mimic Monsters: Spectral Evidence for an $\eta$ Carinae-like Giant Eruption in SBS 0335$-$052 E

SBS 0335$-$052 E is an extremely low-metallicity ($Z\sim0.04\,Z_{\odot}$) blue compact dwarf galaxy. An active galactic nucleus has been proposed to explain the broad H$\alpha$ emission and near-infrared (NIR) time variability in super star clusters 1 and 2 (SSCs 1&2). However, Peng et al. discovered broad wings in the forbidden [O III] $\lambda5007$ emission (up to $\sim5\,000\,\rm{km\,s^{-1}}$), challenging the broad-line region interpretation. We present new KCWI/KCRM integral-field spectroscopy to directly compare spectra across multiple SSCs. The nebula surrounding SSCs 1&2 shows unique features. The Ly$\beta$-pumped O I $\lambda8446$ emission constrains $\tau_{\rm\,Ly\alpha}\sim10^8$. Multiple ionization states of iron are detected from Fe$^{+}$ to Fe$^{+4}$. Stellar photoionization models can reproduce the [Fe III]/[Fe II] and [Fe IV]/[Fe III] line ratios at high density ($n_e\sim10^6\,\rm{cm^{-3}}$), but they fail to account for most of the [Fe V] emission. The broad H$\alpha$ wings exhibit an exponential profile; the asymmetric wings extend from $\sim-5\,000\,\rm{km\,s^{-1}}$ to $\sim10\,000\,\rm{km\,s^{-1}}$. Thomson scattering in a radially expanding medium provides a good fit with $v_w\sim200\,\rm{km\,s^{-1}}$, optical depth $\tau_e\sim10$, and an outer to inner radius of 10. Enhanced N/O and potentially depleted Fe/O ratios are consistent with CNO-cycled ejecta from massive stars and with dust formation, respectively. We propose that mass loss from a massive star interacting with its circumstellar medium drives a shock that powers the NIR variability, the luminous X-ray point source, and the [Fe V] emission. If confirmed, the proposed stellar eruption would be a distant example of an $\eta$ Carinae-like giant eruption, and the first in an ultra-low metallicity environment.

astro-ph.GA↗

arXiv2025

Extended Enriched Gas in a Multi-Galaxy Merger at Redshift 6.7

Recent JWST observations have uncovered high-redshift galaxies characterized by multiple star-forming clumps, many of which appear to be undergoing mergers. Such mergers, especially those of two galaxies with equivalent masses, play a critical role in driving galaxy evolution and regulating the chemical composition of their environments. Here, we report a major merger of at least five galaxies, dubbed JWST's Quintet (JQ), at redshift 6.7, discovered in the JWST GOODS-South field. This system resides in a small area $\sim4.5''\times4.5''$ ($24.6\times24.6$ pkpc$^2$), containing over 17 galaxy-size clumps with a total stellar mass of $10^{10}\ M_\odot$. The JQ system has a total star formation rate of 240 -- 270 $M_\odot$ yr$^{-1}$, placing it $\sim1$ dex above the median star formation rate-mass main sequence at this epoch. The high mass and star formation rate of the JQ galaxies are consistent with the star formation history of those unexpected massive quiescent galaxies observed at redshift 4-5, offering a plausible evolutionary pathway for the formation of such galaxies. We also detect a large [O III]+H$\beta$ emitting gaseous halo surrounding and connecting four galaxies in the JQ, suggesting the existence of heavy elements in the surrounding medium -- inner part of its circumgalactic medium (CGM). This provides direct evidence for the metal enrichment of galaxies' environments through merger-induced tidal stripping, just 800 Myr after the Big Bang.

astro-ph.GA↗

arXiv2024

Physical Origins of Outflowing Cold Clouds in Local Star-forming Dwarf Galaxies

We study the physical origins of outflowing cold clouds in a sample of 14 low-redshift dwarf ($M_{\ast} \lesssim 10^{10}$ $M_{\odot}$) galaxies from the COS Legacy Archive Spectroscopic SurveY (CLASSY) using Keck/ESI data. Outflows are traced by broad (FWHM ~ 260 $\rm{km}$ $\rm{s^{-1}}$) and very-broad (VB; FWHM ~ 1200 $\rm{km}$ $\rm{s^{-1}}$) velocity components in strong emission lines like [O III] $\lambda 5007$ and $\rm{H}\alpha$. The maximum velocities ($v_{\rm{max}}$) of broad components correlate positively with SFR, unlike the anti-correlation observed for VB components, and are consistent with superbubble models. In contrast, supernova-driven galactic wind models better reproduce the $v_{\rm{max}}$ of VB components. Direct radiative cooling from a hot wind significantly underestimates the luminosities of both broad and VB components. A multi-phase wind model with turbulent radiative mixing reduces this discrepancy to at least one dex for most VB components. Stellar photoionization likely provides additional energy since broad components lie in the starburst locus of excitation diagnostic diagrams. We propose a novel interpretation of outflow origins in star-forming dwarf galaxies$-$broad components trace expanding superbubble shells, while VB components originate from galactic winds. One-zone photoionization models fail to explain the low-ionization lines ([S II] and [O I]) of broad components near the maximal starburst regime, which two-zone photoionization models with density-bounded channels instead reproduce. These two-zone models indicate anisotropic leakage of Lyman continuum photons through low-density channels formed by expanding superbubbles. Our study highlights extreme outflows ($v_{\rm{max}} \gtrsim 1000$ $\rm{km}$ $\rm{s^{-1}}$) in 9 out of 14 star-forming dwarf galaxies, comparable to AGN-driven winds.

astro-ph.GA↗

arXiv2024

Resolving the Mechanical and Radiative Feedback in J1044+0353 with KCWI Spectral Mapping

We present integral field spectroscopy toward and around J1044+0353, a rapidly growing, low-metallicity galaxy which produces extreme [O III] line emission. A new map of the O32 flux ratio reveals a density-bounded ionization cone emerging from the starburst. The interaction of the hydrogen ionizing radiation, produced by the very young starburst, with a cavity previously carved out by a galactic outflow, whose apex lies well outside the starburst region, determines the pathway for global Lyman continuum (LyC) escape. In the region within a few hundred parsecs of the young starburst, we demonstrate that superbubble breakthrough and blowout contribute distinct components to the [O III] line profile, broad and very-broad emission line wings, respectively. We draw attention to the large [O III] luminosity of the broad component and argue that this emission comes from photoionized, superbubble shells rather than a galactic wind as is often assumed. The spatially resolved H eII 4686 nebula appears to be photoionized by young star clusters. Stellar wind emission from these stars is likely the source of line wings detected on the He II line profile. This broader He II component indicates slow stellar winds, consistent with an increase in stellar rotation (and a decrease in effective escape speed) at the metallicity of J1044+0353. At least in J1044+0353, the recent star formation history plays a critical role in generating a global pathway for LyC escape, and the anisotropic escape would likely be missed by direct observations of the LyC.

astro-ph.GA↗

arXiv2023

Using KCWI to Explore the Chemical Inhomogeneities and Evolution of J1044+0353

J1044+0353 is considered a local analog of the young galaxies that ionized the intergalactic medium at high-redshift due to its low mass, low metallicity, high specific star formation rate, and strong high-ionization emission lines. We use integral field spectroscopy to trace the propagation of the starburst across this small galaxy using Balmer emission- and absorption-line equivalent widths and find a post-starburst population (~ 15 - 20 Myr) roughly one kpc east of the much younger, compact starburst (~ 3 - 4 Myr). Using the direct electron temperature method to map the O/H abundance ratio, we find similar metallicity (1 to 3 sigma) between the starburst and post-starburst regions but with a significant dispersion of about 0.3 dex within the latter. We also map the Doppler shift and width of the strong emission lines. Over scales several times the size of the galaxy, we discover a velocity gradient parallel to the galaxy's minor axis. The steepest gradients (~ 30 $\mathrm{km \ s^{-1} \ kpc^{-1}}$) appear to emanate from the oldest stellar association. We identify the velocity gradient as an outflow viewed edge-on based on the increased line width and skew in a biconical region. We discuss how this outflow and the gas inflow necessary to trigger the starburst affect the chemical evolution of J1044+0353. We conclude that the stellar associations driving the galactic outflow are spatially offset from the youngest association, and a chemical evolution model with a metal-enriched wind requires a more realistic inflow rate than a homogeneous chemical evolution model.

astro-ph.GA↗

arXiv2021

Efficient Speech Emotion Recognition Using Multi-Scale CNN and Attention

Emotion recognition from speech is a challenging task. Re-cent advances in deep learning have led bi-directional recur-rent neural network (Bi-RNN) and attention mechanism as astandard method for speech emotion recognition, extractingand attending multi-modal features - audio and text, and thenfusing them for downstream emotion classification tasks. Inthis paper, we propose a simple yet efficient neural networkarchitecture to exploit both acoustic and lexical informationfrom speech. The proposed framework using multi-scale con-volutional layers (MSCNN) to obtain both audio and text hid-den representations. Then, a statistical pooling unit (SPU)is used to further extract the features in each modality. Be-sides, an attention module can be built on top of the MSCNN-SPU (audio) and MSCNN (text) to further improve the perfor-mance. Extensive experiments show that the proposed modeloutperforms previous state-of-the-art methods on IEMOCAPdataset with four emotion categories (i.e., angry, happy, sadand neutral) in both weighted accuracy (WA) and unweightedaccuracy (UA), with an improvement of 5.0% and 5.2% respectively under the ASR setting.

cs.SD↗

arXiv2019

Active Learning for Segmentation Based on Bayesian Sample Queries

Segmentation of anatomical structures is a fundamental image analysis task for many applications in the medical field. Deep learning methods have been shown to perform well, but for this purpose large numbers of manual annotations are needed in the first place, which necessitate prohibitive levels of resources that are often unavailable. In an active learning framework of selecting informed samples for manual labeling, expert clinician time for manual annotation can be optimally utilized, enabling the establishment of large labeled datasets for machine learning. In this paper, we propose a novel method that combines representativeness with uncertainty in order to estimate ideal samples to be annotated, iteratively from a given dataset. Our novel representativeness metric is based on Bayesian sampling, by using information-maximizing autoencoders. We conduct experiments on a shoulder magnetic resonance imaging (MRI) dataset for the segmentation of four musculoskeletal tissue classes. Quantitative results show that the annotation of representative samples selected by our proposed querying method yields an improved segmentation performance at each active learning iteration, compared to a baseline method that also employs uncertainty and representativeness metrics. For instance, with only 10% of the dataset annotated, our method reaches within 5% of Dice score expected from the upper bound scenario of all the dataset given as annotated (an impractical scenario due to resource constraints), and this gap drops down to a mere 2% when less than a fifth of the dataset samples are annotated. Such active learning approach to selecting samples to annotate enables an optimal use of the expert clinician time, being often the bottleneck in realizing machine learning solutions in medicine.

cs.CV↗

arXiv2018

Active Learning for Segmentation by Optimizing Content Information for Maximal Entropy

Segmentation is essential for medical image analysis tasks such as intervention planning, therapy guidance, diagnosis, treatment decisions. Deep learning is becoming increasingly prominent for segmentation, where the lack of annotations, however, often becomes the main limitation. Due to privacy concerns and ethical considerations, most medical datasets are created, curated, and allow access only locally. Furthermore, current deep learning methods are often suboptimal in translating anatomical knowledge between different medical imaging modalities. Active learning can be used to select an informed set of image samples to request for manual annotation, in order to best utilize the limited annotation time of clinical experts for optimal outcomes, which we focus on in this work. Our contributions herein are two fold: (1) we enforce domain-representativeness of selected samples using a proposed penalization scheme to maximize information at the network abstraction layer, and (2) we propose a Borda-count based sample querying scheme for selecting samples for segmentation. Comparative experiments with baseline approaches show that the samples queried with our proposed method, where both above contributions are combined, result in significantly improved segmentation performance for this active learning task.

cs.CV↗