arXiv subjects

Andrew J. Fox

Publications and source records attributed to Andrew J. Fox.

At least 37 records · Page 2Linked to original sources

arXiv2025

Ultraviolet Properties of Multi-phase Gas Toward the Inner Galaxy

We present a systematic study of the multi-phase interstellar gas in the Inner Galaxy using HST/STIS absorption spectroscopy of 16 massive stars located at spectroscopic distances between 1.3 and 10 kpc in the region $-30^\circ\lesssim l \lesssim+30^\circ$ and $-15^\circ\lesssim b \lesssim+15^\circ$. These sight lines probe gas above and below the Sagittarius Carina, Scutum Crux-Centaurus, Norma, and Near 3 kpc spiral arms in a range of $z$-height from 0 to 1.5 kpc. Along the 16 sight lines, we measure velocity centroids for 800 UV absorption-line components across multiple gas phases (molecular CO, neutral, low ion, and high ion). We find that 619/800 components have velocities that are consistent with a simple model of co-rotation with the disk, indicating that multi-phase gas with disk-like kinematics extends at least 1 kpc into the halo. We present a database of absorption-line parameters that can be used for kinematic modeling of gas flows into and out of the Galactic disk.

astro-ph.GA↗

arXiv2025

The UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Large Director's Discretionary Program with Hubble. I. Goals, Design, and Initial Results

Specifically selected to leverage the unique ultraviolet capabilities of the Hubble Space Telescope, the Hubble Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) is a Director's Discretionary program of approximately 1000 orbits - the largest ever executed - that produced a UV spectroscopic library of O and B stars in nearby low metallicity galaxies and accreting low mass stars in the Milky Way. Observations from ULLYSES combined with archival spectra uniformly sample the fundamental astrophysical parameter space for each mass regime, including spectral type, luminosity class, and metallicity for massive stars, and the mass, age, and disk accretion rate for low-mass stars. The ULLYSES spectral library of massive stars will be critical to characterize how massive stars evolve at different metallicities; to advance our understanding of the production of ionizing photons, and thus of galaxy evolution and the re-ionization of the Universe; and to provide the templates necessary for the synthesis of integrated stellar populations. The massive star spectra are also transforming our understanding of the interstellar and circumgalactic media of low metallicity galaxies. On the low-mass end, UV spectra of T Tauri stars contain a plethora of diagnostics of accretion, winds, and the warm disk surface. These diagnostics are crucial for evaluating disk evolution and provide important input to assess atmospheric escape of planets and to interpret powerful probes of disk chemistry, as observed with ALMA and JWST. In this paper we motivate the design of the program, describe the observing strategy and target selection, and present initial results.

astro-ph.SR↗

arXiv2025

The Distance to the Magellanic Stream: Constraints from Optical Absorption along Stellar Sightlines

The Magellanic Stream (MS) is a large tail of neutral and ionized gas originating from tidal and hydrodynamical interactions between the Magellanic Clouds as they orbit the Milky Way (MW). It carries a significant gas reservoir that could impact the future evolution of the MW. Despite its importance, no direct observational constraints on the Stream's distance have been previously published. In this study, we analyze Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph (VLT/UVES) spectra of five blue horizontal branch (BHB) stars in the MW halo located at distances ranging from 13 to 56 kpc near two regions of the Stream (near Stream longitudes of -79 and -98 degrees), with the aim of detecting Ca II and Na I absorption. No Ca II or Na I absorption is detected at Stream velocities in any of the individual spectra, or in higher signal-to-noise stacks of the spectra. The resulting limits on the Ca II absorption are significantly lower than the Ca II columns measured in the Stream along extragalactic sightlines. These non-detections establish a firm lower distance limit of 42 kpc for one region of the Stream. For the other region, we set a firm lower limit of 20 kpc and a tentative lower limit of 55 kpc from the most distant star, but deeper spectra are needed to confirm this. Our results provide the first observational constraints on the gaseous Stream's distance.

astro-ph.GA↗

arXiv2025

DIISC -- VI (COS-DIISC): UV Metal Absorption Relative to the H I disk of Galaxies

As part of the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) survey, we present the UV metal absorption features in the Circumgalactic Medium (CGM) near the H I gas disk ($<$4.5$R_\mathrm{HI}$) of 31 nearby galaxies through quasar absorption line spectroscopy. Of the ions under study, Si III $\lambda1206$ was most frequently detected (18 of 31 sight lines), while C II $\lambda1334$ and Si II $\lambda1260$ were detected in 17 and 15 of 31 sight lines, respectively. Many components were consistent with photoionization equilibrium models, most of the cold and cool gas phase clouds were found to have lengths smaller than 2 kpc. Sight lines with smaller impact parameters ($ρ$) normalized by the galaxy's virial radius ($R_\mathrm{vir}$) and H I radius ($R_\mathrm{HI}$) tend to have more components and larger rest-frame equivalent widths ($W_r$) than those that probe the CGM at larger radii. In particular, we find that the location of metals are better traced by $ρ$ / $R_\mathrm{HI}$ rather than the traditional $ρ$ / $R_\mathrm{vir}$. Larger covering fractions are found closer to galaxies, with a radial decline that depends on the $W_r$ limit used. Our results provide new insights into the spatial distribution of metals around the H I disks of low-redshift galaxies.

astro-ph.GA↗

arXiv2025

The Gaseous Blowout of the 30 Doradus Starburst Region in the LMC

Widespread galactic winds emanate from the Large Magellanic Cloud (LMC), with the 30 Doradus starburst region generating the fastest and most concentrated gas flows. We report on the gas distribution, kinematics, and ionization conditions of the near-side outflow along 8 down-the-barrel sightlines using UV absorption-line observations from the HST's ULLYSES program for this region along with H I 21-cm observations from the GASS and GASKAP surveys. We find that within 1.7 degrees from the center of 30 Doradus, the wind reaches maximum speeds of $100-150\,\text{km}\,\text{s}^{-1}$ from the LMC's disk. The total integrated column densities of low-ions (O I, Si II, and Fe II) in the blueshifted wind, up to $v_{\rm LSR}=150\,\text{km}\,\text{s}^{-1}$, are highest near the center and decline radially outward. We estimate an outflow mass of $M_{\rm outflow,\,Si II}\approx(5.7-8.6)\,\times 10^{5} M_{\odot}$, outflow rate of $\dot{M}_{\rm outflow}\gtrsim0.02 M_{\odot}\,\text{yr}^{-1}$, and mass loading factor of $η\gtrsim0.10$ within 0.52 degrees from the center of 30 Doradus. The observed ion ratios$-$together with photoionization modeling$-$reveal that this wind is roughly $40-97\%$ photoionized. The metallicities and dust depletion patterns of the high-velocity absorbers at $v_{\rm LSR}\approx+120\,\text{km}\,\text{s}^{-1}$ can be explained by either a foreground Milky Way (MW) halo cloud or an outflow from the LMC. For the high-ions, Si IV and C IV are broader and kinematically offset from the low-ions, suggesting turbulent mixing layers (TMLs) existing in the wind. Finally, our hydrodynamical simulations of the Magellanic Clouds (MCs) and MW system suggest that the Magellanic Corona can protect the LMC winds from the ram-pressure forces exerted by the MW's halo.

astro-ph.GA↗

arXiv2024

The Truncated Circumgalactic Medium of the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is the nearest massive galaxy to the Milky Way. Its circumgalactic medium is complex and multi-phase, containing both stripped HI structures like the Magellanic Stream and Bridge, and a diffuse warm corona seen in high-ion absorption. We analyze 28 AGN sightlines passing within 35 kpc of the LMC with archival HST/COS spectra to characterize the cool (T\approx10^4$ K) gas in the LMC CGM, via new measurements of UV absorption in six low ions (OI, FeII, SiII, AlII, SII, and NiII) and one intermediate ion (SiIII). We show that a declining column-density profile is present in all seven ions, with the low-ion profiles having a steeper slope than the high-ion profiles in CIV and SiIV reported by Krishnarao et al. 2022. Crucially, absorption at the LMC systemic velocity is only detected (in all ions) out to 17 kpc. Beyond this distance, the gas has a lower velocity and is associated with the Magellanic Stream. These results demonstrate that the LMC's CGM is composed of two distinct components: a compact inner halo extending to 17 kpc, and a more extended stripped region associated with the Stream. The compactness and truncation of the LMC's inner CGM agree with recent simulations of ram-pressure stripping of the LMC by the Milky Way's extended corona.

astro-ph.GA↗

arXiv2024

DIISC Survey: Deciphering the Interplay Between the Interstellar Medium, Stars, and the Circumgalactic Medium Survey

We present the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) Survey. This survey is designed to investigate the correlations in properties between the circumgalactic medium (CGM), the interstellar medium (ISM), stellar distributions, and young star-forming regions. The galaxies were chosen to have a QSO sightline within 3.5 times the HI radii probing the disk-CGM interface. The sample contains 34 low-redshift galaxies with a median stellar mass of 10$^{10.45}~\rm M_{\odot}$ probed at a median impact parameter of $ρ=55~kpc$. The survey combines ultraviolet spectroscopic data from the Cosmic Origins Spectrograph aboard the Hubble Space Telescope with HI 21 cm hyperfine transition imaging with the Very Large Array (VLA), ultraviolet imaging from Galaxy Evolution Explorer (GALEX), and optical imaging and spectroscopy with the MMT and Vatican Advanced Technology Telescope. We describe the specific goals of the survey, data reduction, high-level data products, and some early results. We present the discovery of a strong inverse correlation, at a confidence level of 99.99%, between Lyman $α$ equivalent width, $\rm W_{Lyα}$, and impact parameter normalized by the HI radius ($ρ/R_{HI}$). We find $ρ/R_{HI}$ to be a better empirical predictor of Lyman $α$ equivalent width than virial radius normalized impact parameter ($ρ/R_{vir}$) or parameterizations combining $ρ,~R_{vir}$, stellar mass, and star formation rate. We conclude that the strong anticorrelation between the Lyman $α$ equivalent width and $ρ/R_{HI}$ indicates that the neutral gas distribution of the CGM is more closely connected to the galaxy's gas disk rather than its stellar and dark matter content.

astro-ph.GA↗

arXiv2024

The COS CGM Compendium V: The Dichotomy of OVI Associated with Low- and High-Metallicity Cool Gas at z < 1

We analyze the \ovi\ content and kinematics for 126 {\hi}-selected absorbers at $0.14 \lesssim z \lesssim 0.73$ for which the metallicities of their cool photoionized phase have been determined. We separate the absorbers into 100 strong {\lya} forest systems (SLFSs with $15 \la$\,{\colden}\,$< 16.2$) and 26 partial Lyman Limit systems (pLLSs with $16.2\le$\,{\colden}\,$\le 17.2$). The sample is drawn from the COS CGM Compendium (CCC) and has \ovi\ coverage in $\sn \geq 8$ {\it HST}/COS G130M/G160M QSO spectra, yielding a $2σ$ completeness level of {\coldenovi}$\,\geq 13.6$. The \ovi\ detection rates differ substantially between low-metallicity (LM; {\xh} $\leq -1.4$) and high-metallicity (HM; {\xh} $> -1.4$) SLFSs, with 20\% and 60\% detection rates, respectively. The \ovi\ detection frequency for the HM and LM pLLSs is, however, similar at $\sim$60\%. The SLFSs and pLLSs without detected \ovi\ are consistent with the absorbing gas being in a single phase, while those with \ovi\ trace multiphase gas. We show that the \ovi\ velocity widths and column densities have different distributions in LM and HM gas. We find a strong correlation between \ovi\ column density and metallicity. The strongest (\coldenovi$\,\ga 14$) and broadest {\ovi} absorbers are nearly always associated with HM absorbers, while weaker \ovi\ absorbers are found in both LM and HM absorbers. From comparisons with galaxy-selected and blind \ovi\ surveys, we conclude absorbers with \coldenovi$\,\ga 14$ most likely arise in the circumgalactic medium (CGM) of star-forming galaxies. Absorbers with weak \ovi\ likely trace the extended CGM or intergalactic medium (IGM), while those without \ovi\ likely originate in the IGM.

astro-ph.GA↗

arXiv2024

Data-driven Koopman operator predictions of turbulent dynamics in models of shear flows

The Koopman operator enables the analysis of nonlinear dynamical systems through a linear perspective by describing time evolution in the infinite-dimensional space of observables. Here this formalism is applied to shear flows, specifically the minimal flow unit plane Couette flow and a nine-dimensional model of turbulent shear flow between infinite parallel free-slip walls under a sinusoidal body force (Moehlis et al., New J. Phys. 6, 56 (2004)). We identify a finite set of observables and approximate the Koopman operator using neural networks, following the method developed by Constante-Amores et al., Chaos, 34(4), 043119 (2024). Then the time evolution is determined with a method here denoted as "Projected Koopman Dynamics". Using a high-dimensional approximate Koopman operator directly for the plane Couette system is computationally infeasible due to the high state space dimension of direct numerical simulations (DNS). However, the long-term dynamics of the dissipative Navier-Stokes equations are expected to live on a lower-dimensional invariant manifold. Motivated by this fact, we perform dimension reduction on the DNS data with a variant of the IRMAE-WD (implicit rank-minimizing autoencoder-weight decay) autoencoder (Zeng et al., MLST 5(2), 025053 (2024)) before applying the Koopman approach. We compare the performance of data-driven state space (neural ordinary differential equation) and Projected Koopman Dynamics approaches in manifold coordinates for plane Couette flow. Projected Koopman Dynamics effectively captures the chaotic behavior of Couette flow and the nine-dimensional model, excelling in both short-term tracking and long-term statistical analysis, outperforming other frameworks applied to these shear flows.

physics.flu-dyn↗

arXiv2024

Properties of the Magellanic Corona

We characterize the Magellanic Corona, the warm gaseous halo around the Large Magellanic Cloud (LMC). The Corona is a key ingredient in the formation of the Magellanic Stream (Lucchini et al. 2020, 2021) and has recently been observed in high-ion absorption around the LMC. In this work we present a suite of high-resolution hydrodynamical simulations to constrain its total mass and temperature prior to the infall of the Magellanic Clouds to our Galaxy. We find that the LMC is able to host a stable Corona before and during its approach to the MW through to the present day. With a Magellanic Corona of $>2\times10^9$ M$_\odot$ at $3\times10^5$ K, our simulations can reproduce the observed total mass of the neutral and ionized components of the Trailing Stream, size of the LMC disk, ionization fractions along the Stream, morphology of the neutral gas, and on-sky extent of the ionized gas. The Corona plays an integral role in the survival, morphology, and composition of the Magellanic Clouds and the Trailing Stream.

astro-ph.GA↗

arXiv2024

CLusters in the Uv as EngineS (CLUES). II. Sub-kpc scale outflows driven by stellar feedback

We analyze the far-ultraviolet spectroscopy of 20 young and massive star clusters (YSCs) in 11 nearby star-forming galaxies. We probe the interstellar gas intervening along the line of sight, detecting several metal absorption lines of a wide range of ionization potentials, from 6.0 eV to 77.5 eV. Multiple-component Voigt fits to the absorption lines are used to study the kinematics of the gas. We find that nearly all targets in the sample feature gas outflowing from 30 up to 190 km per second, often both in the neutral and ionized phase. The outflow velocities correlate with the underlying stellar population properties directly linked to the feedback: the mass of the YSCs, the photon production rate and the instantaneous mechanical luminosity produced by stellar winds and SNe. We detect a neutral inflow in 4 targets, which we interpret as likely not associated with the star cluster but tracing larger scale gas kinematics. A comparison between the outflows energy and that produced by the associated young stellar populations suggests an average coupling efficiency of 10 per cent with a broad scatter. Our results extend the relation found in previous works between galactic outflows and the host galaxy star-formation rate to smaller scales, pointing towards the key role that clustered star formation and feedback play in regulating galaxy growth.

astro-ph.GA↗

arXiv2024

α-element enhancements in the Magellanic Interstellar Medium: evidence for recent star formation

Important questions on the chemical composition of the neutral ISM in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are still open. It is usually assumed that their metallicity is uniform and equal to that measured in hot stars and HII regions, but direct measurements on the neutral ISM have not been performed until now. Deriving the metallicity from the observed metal abundances is not straightforward because they also depend on the depletion of metals into dust as well as nucleosynthesis effects such as $α$-element enhancement. We collect literature column densities of TiII, NiII, CrII, FeII, MnII, SiII, CuII, MgII, SII, PII, ZnII, and OI in the neutral ISM towards 32 and 22 hot stars in the LMC and SMC. We measure the metallicity, dust depletion, and $α$-element enhancements in the neutral ISM in the LMC and SMC. We find $α$-element enhancements in the neutral ISM in most systems, on average 0.26 dex (0.35 dex) for the LMC (SMC), and Mn under-abundance in the SMC (on average $-0.35$ dex). These are higher than for stars at similar metallicities. The observed $α$-element enhancements and Mn under-abundance are likely due to bursts of star formation, more recently than ~1 Gyr ago, that enriched the ISM from core-collapse supernovae. We find total neutral ISM metallicities that are mostly consistent with hot stars metallicity, on average [M/H]$_{\rm tot} = -0.33$ ($-0.83$), in the LMC (SMC). In six systems, however, we find significantly lower metallicities, two out of 32 in the LMC (with ~16% solar) and four out of 22 in the SMC (3 and 10% solar), two of which are in the outskirts of the SMC near the Magellanic Bridge, a region known for having a lower metallicity. With the exception of lines of sight towards the Magellanic Bridge, the neutral gas in the LMC and SMC appears fairly well mixed in terms of metallicity. [abridged]

astro-ph.GA↗

arXiv2023

Autoencoders for discovering manifold dimension and coordinates in data from complex dynamical systems

While many phenomena in physics and engineering are formally high-dimensional, their long-time dynamics often live on a lower-dimensional manifold. The present work introduces an autoencoder framework that combines implicit regularization with internal linear layers and $L_2$ regularization (weight decay) to automatically estimate the underlying dimensionality of a data set, produce an orthogonal manifold coordinate system, and provide the mapping functions between the ambient space and manifold space, allowing for out-of-sample projections. We validate our framework's ability to estimate the manifold dimension for a series of datasets from dynamical systems of varying complexities and compare to other state-of-the-art estimators. We analyze the training dynamics of the network to glean insight into the mechanism of low-rank learning and find that collectively each of the implicit regularizing layers compound the low-rank representation and even self-correct during training. Analysis of gradient descent dynamics for this architecture in the linear case reveals the role of the internal linear layers in leading to faster decay of a "collective weight variable" incorporating all layers, and the role of weight decay in breaking degeneracies and thus driving convergence along directions in which no decay would occur in its absence. We show that this framework can be naturally extended for applications of state-space modeling and forecasting by generating a data-driven dynamic model of a spatiotemporally chaotic partial differential equation using only the manifold coordinates. Finally, we demonstrate that our framework is robust to hyperparameter choices.

cs.LG↗

arXiv2023

The Metallicities of Five Small High-Velocity Clouds

High-velocity clouds (HVCs) are multi-phase gas structures whose velocities (|v_LSR|>100 km/s) are too high to be explained by Galactic disk rotation. While large HVCs are well characterized, compact and small HVCs (with HI angular sizes of a few degrees) are poorly understood. Possible origins for such small clouds include Milky Way halo gas or fragments of the Magellanic System, but neither their origin nor their connection to the Milky Way halo has been confirmed. We use new Hubble Space Telescope/Cosmic Origins Spectrograph UV spectra and Green Bank Telescope HI spectra to measure the metallicities of five small HVCs in the southern Galactic sky projected near the Magellanic System. We build a set of distance-dependent Cloudy photoionization models for each cloud and calculate their ionization-corrected metallicities. All five small HVCs have oxygen metallicities <0.17 Z_sun, indicating they do not originate in the disk of the Milky Way. Two of the five have metallicities of 0.16-0.17 Z_sun, similar to the Magellanic Stream, suggesting these clouds are fragments of the Magellanic System. The remaining three clouds have much lower metallicities of 0.02-0.04 Z_sun. While the origin of these low-metallicity clouds is unclear, they could be gaseous mini-halos or gas stripped from dwarf galaxies by ram pressure or tidal interactions. These results suggest that small HVCs do not all reside in the inner Milky Way halo or the Magellanic System, but instead can trace more distant structures.

astro-ph.GA↗

arXiv2023

DIISC-III: Signatures of Stellar Disk Growth in Nearby Galaxies

We explore the growth of the stellar disks in 14 nearby spiral galaxies as part of the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) survey. We study the radial distribution of specific star formation rates (sSFR) and investigate the ratio of the difference in the outer and inner sSFR ($Δ_{sSFR}~={\rm sSFR}_{out}-{\rm sSFR}_{in}$) of the disk and the total sSFR, $Δ_{sSFR}$/sSFR to quantify disk growth. We find $Δ_{sSFR}$/sSFR and the HI gas fraction to show a mild correlation of Spearman's $ρ=0.30$, indicating that star formation and disk growth are likely to proceed outward in galactic disks with high HI gas fractions. The HI gas fractions and $Δ_{sSFR}$/sSFR of the galaxies also increase with the distance to the nearest L$_\star$ neighbor, suggesting that galaxies are likely to sustain their ISM cold gas and exhibit inside-out growth in isolated environments. However, the HI content in their circumgalactic medium, probed by the Ly$α$ equivalent width (W$_{Lyα}$) excess, is observed to be suppressed in isolated environments, apparent from the strong anti-correlation between the W$_{Lyα}$ excess and the distance to the 5$^{\rm th}$ nearest L$_\star$ neighbor (Spearman's $ρ=-0.62$). As expected, W$_{Lyα}$ is also found to be suppressed in cluster galaxies. We find no relation between the W$_{Lyα}$ excess of the detected CGM absorber and $Δ_{sSFR}$/sSFR implying that the enhancement and suppression of the circumgalactic HI gas does not affect the direction in which star formation proceeds in a galactic disk or vice-versa.

astro-ph.GA↗

arXiv2023

MUSE-ALMA Haloes IX: Morphologies and Stellar Properties of Gas-rich Galaxies

Understanding how galaxies interact with the circumgalactic medium (CGM) requires determining how galaxies morphological and stellar properties correlate with their CGM properties. We report an analysis of 66 well-imaged galaxies detected in HST and VLT MUSE observations and determined to be within $\pm$500 km s$^{-1}$ of the redshifts of strong intervening quasar absorbers at $0.2 \lesssim z \lesssim 1.4$ with H I column densities $N_{\rm H I}$ $>$ $10^{18}$ $\rm cm^{-2}$. We present the geometrical properties (Sérsic indices, effective radii, axis ratios, and position angles) of these galaxies determined using GALFIT. Using these properties along with star formation rates (SFRs, estimated using the H$α$ or [O II] luminosity) and stellar masses ($M_{*}$ estimated from spectral energy distribution fits), we examine correlations among various stellar and CGM properties. Our main findings are as follows: (1) SFR correlates well with $M_{*}$, and most absorption-selected galaxies are consistent with the star formation main sequence (SFMS) of the global population. (2) More massive absorber counterparts are more centrally concentrated and are larger in size. (3) Galaxy sizes and normalized impact parameters correlate negatively with $N_{\rm H I}$, consistent with higher $N_{\rm H I}$ absorption arising in smaller galaxies, and closer to galaxy centers. (4) Absorption and emission metallicities correlate with $M_{*}$ and sSFR, implying metal-poor absorbers arise in galaxies with low past star formation and faster current gas consumption rates. (5) SFR surface densities of absorption-selected galaxies are higher than predicted by the Kennicutt-Schmidt relation for local galaxies, suggesting a higher star formation efficiency in the absorption-selected galaxies.

astro-ph.GA↗

arXiv2023

The Signature of the Northern Galactic Center Region in Low-Velocity UV Absorption

The Galactic Center (GC) is surrounded by plasma lobes that extend up to ~14 kpc above and below the plane. Until now, UV absorption studies of these lobes have only focused on high-velocity components (|v_LSR| > 100 km/s) because low- and intermediate-velocity (LIV) components (|v_LSR| <100 km/s) are blended with foreground interstellar medium. To overcome this difficulty, we present a differential experiment to compare the LIV absorption between different structures within the GC region, including the Fermi Bubbles (FBs; seen in gamma-rays), the eROSITA Bubbles (eBs; seen in X-rays), and the Loop I North Polar Spur (LNPS) association, an X-ray and radio feature within the northern eB. We use far-UV spectra from Hubble Space Telescope to measure LIV Si IV absorption in 61 AGN sight lines, of which 21 pass through the FBs, 53 pass through the eBs, and 18 pass through the LNPS. We also compare our measurements to those in the literature from sight lines covering the disk-halo interface and CGM. We find that the FBs and eBs have enhancements in measured columns of 0.22-0.29 dex in log. We also remove the contribution of a modeled disk and CGM component from the measured Si IV columns and find that the northern eB still retains a Si IV enhancement of 0.62 dex in log. A similar enhancement is not seen in the southern eB. Since the LNPS model-subtracted residuals show an enhancement compared to the rest of the northern eB of 0.69 dex, the northern eB enhancement may be caused by the LNPS.

astro-ph.GA↗

arXiv2023

Predicting extreme events in a data-driven model of turbulent shear flow using an atlas of charts

Dynamical systems with extreme events are difficult to capture with data-driven modeling, due to the relative scarcity of data within extreme events compared to the typical dynamics of the system, and the strong dependence of the long-time occurrence of extreme events on short-time conditions.A recently developed technique [Floryan, D. & Graham, M. D. Data-driven discovery of intrinsic dynamics. Nat Mach Intell $\textbf{4}$, 1113-1120 (2022)], here denoted as $\textit{Charts and Atlases for Nonlinear Data-Driven Dynamics on Manifolds}$, or CANDyMan, overcomes these difficulties by decomposing the time series into separate charts based on data similarity, learning dynamical models on each chart via individual time-mapping neural networks, then stitching the charts together to create a single atlas to yield a global dynamical model. We apply CANDyMan to a nine-dimensional model of turbulent shear flow between infinite parallel free-slip walls under a sinusoidal body force [Moehlis, J., Faisst, H. & Eckhardt, B. A low-dimensional model for turbulent shear flows. New J Phys $\textbf{6}$, 56 (2004)], which undergoes extreme events in the form of intermittent quasi-laminarization and long-time full laminarization. We demonstrate that the CANDyMan method allows the trained dynamical models to more accurately forecast the evolution of the model coefficients, reducing the error in the predictions as the model evolves forward in time. The technique exhibits more accurate predictions of extreme events, capturing the frequency of quasi-laminarization events and predicting the time until full laminarization more accurately than a single neural network.

physics.flu-dyn↗