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Razieh Emami

Publications and source records attributed to Razieh Emami.

At least 37 records · Page 2Linked to original sources

Diverse dark matter haloes in Two-field Fuzzy Dark Matter

Fuzzy dark matter (FDM) is a compelling candidate for dark matter, offering a natural explanation for the structure of diffuse low-mass haloes. However, the canonical FDM model with a mass of $10^{-22}~{\rm eV}$ encounters challenges in reproducing the observed diversity of dwarf galaxies, except for possibly scenarios where strong galactic feedback is invoked. The introduction of multiple-field FDM can provide a potential resolution to this diversity issue. The theoretical plausibility of this dark matter model is also enhanced by the fact that multiple axion species with logarithmically-distributed mass spectrum exist as a generic prediction of string theory. In this paper, we consider the axiverse hypothesis and investigate non-linear structure formation in the two-field fuzzy dark matter (2FDM) model. Our cosmological simulation with an unprecedented resolution and self-consistent initial conditions reveals the diverse structures of dark matter haloes in the 2FDM model for the first time. Depending on the formation time and local tidal activities, late-time haloes can host solitons of nested cores or solitons of one dominant species.

astro-ph.CO↗

Origin of the ring ellipticity in the black hole images of M87*

We investigate the origin of the elliptical ring structure observed in the images of the supermassive black hole M87*, aiming to disentangle contributions from gravitational, astrophysical, and imaging effects. Leveraging the enhanced capabilities of the Event Horizon Telescope (EHT) 2018 array, including improved $(u,v)$-coverage from the Greenland Telescope, we measure the ring's ellipticity using five independent imaging methods, obtaining a consistent average value of $τ= 0.08_{-0.02}^{+0.03}$ with a position angle $ξ= 50.1_{-7.6}^{+6.2}$ degrees. To interpret this measurement, we compare against General Relativistic Magnetohydrodynamic (GRMHD) simulations spanning a wide range of physical parameters including thermal or non-thermal electron distribution function, spins, and ion-to-electron temperature ratios in both low and high-density regions. We find no statistically significant correlation between spin and ellipticity in GRMHD images. Instead, we identify a correlation between ellipticity and the fraction of non-ring emission, particularly in non-thermal models and models with higher jet emission. These results indicate that the ellipticity measured from the \m87 emission structure is consistent with that expected from simulations of turbulent accretion flows around black holes, where it is dominated by astrophysical effects rather than gravitational ones. Future high-resolution imaging, including space very long baseline interferometry and long-term monitoring, will be essential to isolate gravitational signatures from astrophysical effects.

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Big, Dusty Galaxies in Blue Jay: Insights into the Relationship Between Morphology and Dust Attenuation at Cosmic Noon

The dust attenuation of galaxies is highly diverse and closely linked to stellar population properties and the star dust geometry, yet its relationship to galaxy morphology remains poorly understood. We present a study of 141 galaxies ($9<\log(\rm M_{\star}/\rm M_{\odot})<11.5$) at $1.7<z<3.5$ from the Blue Jay survey combining deep JWST/NIRCam imaging and $R\sim1000$ JWST/NIRSpec spectra. Using \texttt{Prospector} to perform a joint analysis of these data with non-parametric star-formation histories and a two-component dust model with flexible attenuation laws, we constrain stellar and nebular properties. We find that the shape and strength of the attenuation law vary systematically with optical dust attenuation ($A_V$), stellar mass, and star formation rate (SFR). $A_V$ correlates strongly with stellar mass for starbursts, star-forming galaxies and quiescent galaxies. The inclusion of morphological information tightens these correlations: attenuation correlates more strongly with stellar mass and SFR surface densities than with the global quantities. The Balmer decrement-derived nebular attenuation for 67 of these galaxies shows consistent trends with the stellar continuum attenuation. We detect a wavelength-dependent size gradient: massive galaxies ($\rm M_{\star}\gtrsim 10^{10}~M_{\odot}$) appear $\sim30\%$ larger in the rest-optical than in the rest-NIR, driven by central dust attenuation that flattens optical light profiles. Lower-mass systems exhibit more diverse size ratios, consistent with either inside-out growth or central starbursts. These results demonstrate that dust attenuation significantly alters observed galaxy structure and highlight the necessity of flexible attenuation models for accurate physical and morphological inference at cosmic noon.

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Unravelling the role of merger histories in the population of In situ stars: linking IllustrisTNG cosmological simulation to H3 survey

We undertake a comprehensive investigation into the distribution of in~situ stars within Milky Way-like galaxies, leveraging TNG50 simulations and comparing their predictions with data from the H3 survey. Our analysis reveals that 28% of galaxies demonstrate reasonable agreement with H3, while only 12% exhibit excellent alignment in their profiles, regardless of the specific spatial cut employed to define in~situ stars. To uncover the underlying factors contributing to deviations between TNG50 and H3 distributions, we scrutinise correlation coefficients among internal drivers(e.g., virial radius, star formation rate [SFR]) and merger-related parameters (such as the effective mass-ratio, mean distance, average redshift, total number of mergers, average spin-ratio and maximum spin alignment between merging galaxies). Notably, we identify significant correlations between deviations from observational data and key parameters such as the median slope of virial radius, mean SFR values, and the rate of SFR change across different redshift scans. Furthermore, positive correlations emerge between deviations from observational data and parameters related to galaxy mergers. We validate these correlations using the Random Forest Regression method. Our findings underscore the invaluable insights provided by the H3 survey in unravelling the cosmic history of galaxies akin to the Milky Way, thereby advancing our understanding of galactic evolution and shedding light on the formation and evolution of Milky Way-like galaxies in cosmological simulations.

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Novel Polarimetric Analysis of Near Horizon Flaring Episodes in M87* in Millimeter Wavelength

Recent multi-wavelength observations of M87* \citep{2024A&A...692A.140A} revealed a high-energy $γ$-ray flare without a corresponding millimeter counterpart. We present a theoretical polarimetric study to evaluate the presence and nature of a potential millimeter flare in M87*, using a suite of general relativistic magnetohydrodynamical simulations with varying black hole (BH) spins and magnetic field configurations. We find that the emergence of a millimeter flare is strongly influenced by both spin and magnetic structure, with limited sensitivity to the electron distribution (thermal vs. non-thermal). We model the intensity light curve with a damped random walk (DRW) and compare the characteristic timescale ($τ$) with recent SMA observations, finding that the simulated $τ$ exceeds observed values by over an order of magnitude. In a flaring case with BH spin a=+0.5, we identify a distinct millimeter flare followed by an order-of-magnitude flux drop. All Stokes parameters show variability near the flare, including a sign reversal in the electric vector position angle. While most $β_m$ modes remain stable, the $EB$-correlation phase is highly sensitive to both the flare peak and decay. We examine polarimetric signatures in photon sub-rings, focusing on modes ns=0 and ns=1. The ns=0 signal closely matches the full image, while ns=1 reveals distinct behaviors, highlighting the potential of space VLBI to isolate sub-ring features. Finally, we analyze the magnetic and velocity field evolution during the flare, finding that magnetic reconnection weakens during the flux decay, and the clockwise velocity flow transitions into an outflow-dominated regime. These results suggest that transient radio variability near flares encodes key information about black hole spin and magnetic field structure, offering a novel probe into the physics of active galactic nuclei.

astro-ph.HE↗

AGN Feedback in Quiescent Galaxies at Cosmic Noon Traced by Ionized Gas Emission

We analyze ionized gas emission lines in deep rest-frame optical spectra of 16 quiescent galaxies at redshift $1.7<z<3.5$ observed with JWST/NIRSpec by the Blue Jay survey. Robust detection of emission lines in $75\%$ of the sample indicates the presence of ongoing ionizing sources in this passive population. The H$α$ line luminosities confirm that the population is quiescent, with star formation rates that are at least ten times lower than the main sequence of star formation. The quiescent sample is clearly separate from the star-forming population in line diagnostic diagrams, and occupies a region usually populated by active galactic nuclei (AGN). Analysis of the observed line ratios, equivalent widths, and velocity dispersions leads us to conclude that in most cases the gas is ionized by AGN activity, despite the lack of X-ray detections. A subset of the sample also hosts ionized and/or neutral outflows. Our results show, for the first time using a representative sample, that low luminosity AGN are extremely common among quiescent galaxies at high redshift. These low luminosity AGN may play a key role in quenching star formation and in maintaining massive galaxies quiescent from Cosmic Noon to $z\sim0$.

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Modeling Cosmic Rays at AGN Jet-Driven Shock Fronts

Active Galactic Nuclei (AGN) feedback is a key physical mechanism proposed to regulate star formation, primarily in massive galaxies. In particular, cosmic rays associated with AGN jets have the potential to efficiently suppress cooling flows and quench star formation. The locus of cosmic ray production and their coupling to gas play a crucial role in the overall self-regulation process. To investigate this in detail, we conduct high-resolution, non-cosmological MHD simulations of a massive $10^{14} {\rm M_\odot}$ halo using the FIRE-2 (Feedback In Realistic Environments) stellar feedback model. We explore a variety of AGN jet feedback scenarios with cosmic rays, examining different values for the cosmic ray energy fraction in jets, cosmic ray coupling sites (in the black hole vicinity versus at the large-scale jet-driven shock front), and jet precession parameters. Our findings indicate that when cosmic rays are injected near the black hole, they efficiently inhibit black hole accretion by suppressing the density before the jet propagates out to large radii. As a result, this leads to episodic black hole accretion, with the jet not having sufficient energy flux to reach large radii and impact cooling flows. Conversely, if the cosmic rays are injected at the jet-driven shock front, not only does the jet sustain a higher overall energy flux for an extended period, but it also disperses cosmic rays out to larger radii, more effectively suppressing the cooling flow. Furthermore, the period and angle of jet precession can influence the position of shock fronts. We identify an optimal range of jet precession periods ($\sim$ tens of Myr) that generates shocks at the inner circumgalactic medium, where cooling flows are most severe. We report that this specific configuration offers the most effective scenario for cosmic rays at the shock front to suppress the cooling flow and star formation.

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The putative center in NGC 1052

Many active galaxies harbor powerful relativistic jets, however, the detailed mechanisms of their formation and acceleration remain poorly understood. To investigate the area of jet acceleration and collimation with the highest available angular resolution, we study the innermost region of the bipolar jet in the nearby low-ionization nuclear emission-line region (LINER) galaxy NGC 1052. We combined observations of NGC 1052 taken with VLBA, GMVA, and EHT over one week in the spring of 2017. For the first time, NGC 1052 was detected with the EHT, providing a size of the central region in-between both jet bases of 250 RS (Schwarzschild radii) perpendicular to the jet axes. This size estimate supports previous studies of the jets expansion profile which suggest two breaks of the profile at around 300 RS and 10000 RS distances to the core. Furthermore, we estimated the magnetic field to be 1.25 Gauss at a distance of 22 μas from the central engine by fitting a synchrotron-self absorption spectrum to the innermost emission feature, which shows a spectral turn-over at about 130 GHz. Assuming a purely poloidal magnetic field, this implies an upper limit on the magnetic field strength at the event horizon of 26000 Gauss, which is consistent with previous measurements. The complex, low-brightness, double-sided jet structure in NGC 1052 makes it a challenge to detect the source at millimeter (mm) wavelengths. However, our first EHT observations have demonstrated that detection is possible up to at least 230 GHz. This study offers a glimpse through the dense surrounding torus and into the innermost central region, where the jets are formed. This has enabled us to finally resolve this region and provide improved constraints on its expansion and magnetic field strength.

astro-ph.HE↗

A multi-frequency study of sub-parsec jets with the Event Horizon Telescope

The 2017 observing campaign of the Event Horizon Telescope (EHT) delivered the first very long baseline interferometry (VLBI) images at the observing frequency of 230 GHz, leading to a number of unique studies on black holes and relativistic jets from active galactic nuclei (AGN). In total, eighteen sources were observed: the main science targets, Sgr A* and M87 along with various calibrators. We investigated the morphology of the sixteen AGN in the EHT 2017 data set, focusing on the properties of the VLBI cores: size, flux density, and brightness temperature. We studied their dependence on the observing frequency in order to compare it with the Blandford-Königl (BK) jet model. We modeled the source structure of seven AGN in the EHT 2017 data set using linearly polarized circular Gaussian components and collected results for the other nine AGN from dedicated EHT publications, complemented by lower frequency data in the 2-86 GHz range. Then, we studied the dependences of the VLBI core flux density, size, and brightness temperature on the frequency measured in the AGN host frame. We compared the observations with the BK jet model and estimated the magnetic field strength dependence on the distance from the central black hole. Our results indicate a deviation from the standard BK model, particularly in the decrease of the brightness temperature with the observing frequency. Either bulk acceleration of the jet material, energy transfer from the magnetic field to the particles, or both are required to explain the observations.

astro-ph.HE↗

Rapid Quenching of Galaxies at Cosmic Noon

The existence of massive quiescent galaxies at high redshift seems to require rapid quenching, but it is unclear whether all quiescent galaxies have gone through this phase and what physical mechanisms are involved. To study rapid quenching, we use rest-frame colors to select 12 young quiescent galaxies at $z \sim 1.5$. From spectral energy distribution fitting, we find that they all experienced intense starbursts prior to rapid quenching. We confirm this with deep Magellan/FIRE spectroscopic observations for a subset of seven galaxies. Broad emission lines are detected for two galaxies and are most likely caused by AGN activity. The other five galaxies do not show any emission features, suggesting that gas has already been removed or depleted. Most of the rapidly quenched galaxies are more compact than normal quiescent galaxies, providing evidence for a central starburst in the recent past. We estimate an average transition time of $300\,\rm Myr$ for the rapid quenching phase. Approximately $4\%$ of quiescent galaxies at $z=1.5$ have gone through rapid quenching; this fraction increases to $23\%$ at $z=2.2$. We identify analogs in the TNG100 simulation and find that rapid quenching for these galaxies is driven by AGN, and for half of the cases, gas-rich major mergers seem to trigger the starburst. We conclude that these massive quiescent galaxies are not just rapidly quenched but also rapidly formed through a major starburst. We speculate that mergers drive gas inflow towards the central regions and grow supermassive black holes, leading to rapid quenching by AGN feedback.

astro-ph.GA↗

Widespread rapid quenching at cosmic noon revealed by JWST deep spectroscopy

Massive quiescent galaxies in the young universe are expected to be quenched rapidly, but it is unclear whether they all experience starbursts before quenching and what physical mechanism drives rapid quenching. We study 14 massive quiescent galaxies ($\log(M_\star/M_\odot) > 10$) at $z\sim2$ selected from a representative sample of the Blue Jay survey. We reconstruct their star formation histories by fitting spectral energy distribution models to the JWST/NIRSpec $R\sim1000$ spectra. We find that massive quiescent galaxies can be split into three categories with roughly equal numbers of galaxies according to their SFHs: 1) Relatively old galaxies quenched at early epochs; 2) Galaxies that are rapidly and recently quenched after a flat or bursty formation history (depending on the assumed prior); 3) Galaxies that are rapidly and recently quenched after a major starburst. Most recently quenched galaxies show neutral gas outflows, probed by blueshifted $\rm Na\,I\,D$ absorption, and ionized gas emission, with line ratios consistent with active galactic nucleus (AGN) diagnostics. This suggests that AGN activity drives multi-phase gas outflows, leading to rapid quenching. By tracing back the SFHs of the entire sample, we predict the number density of massive quiescent galaxies at $z=4-6$: $n=(1.5-6.0)\times10^{-5}\,\rm Mpc^{-3}$. The two old massive quiescent galaxies in our sample appear to have extremely early formation and quenching ($z\gtrsim6$), possibly descendants of early post-starbursts at $z>3$. These galaxies still show neutral gas reservoirs and weak H$α$ emission, perhaps because the ejective AGN feedback that caused rapid quenching has weakened over time.

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First Very Long Baseline Interferometry Detections at 870μm

The first very long baseline interferometry (VLBI) detections at 870$μ$m wavelength (345$\,$GHz frequency) are reported, achieving the highest diffraction-limited angular resolution yet obtained from the surface of the Earth, and the highest-frequency example of the VLBI technique to date. These include strong detections for multiple sources observed on inter-continental baselines between telescopes in Chile, Hawaii, and Spain, obtained during observations in October 2018. The longest-baseline detections approach 11$\,$G$λ$ corresponding to an angular resolution, or fringe spacing, of 19$μ$as. The Allan deviation of the visibility phase at 870$μ$m is comparable to that at 1.3$\,$mm on the relevant integration time scales between 2 and 100$\,$s. The detections confirm that the sensitivity and signal chain stability of stations in the Event Horizon Telescope (EHT) array are suitable for VLBI observations at 870$μ$m. Operation at this short wavelength, combined with anticipated enhancements of the EHT, will lead to a unique high angular resolution instrument for black hole studies, capable of resolving the event horizons of supermassive black holes in both space and time.

astro-ph.IM↗

Dramatic Drop in the X-Ray Polarization of Swift J1727.8$-$1613 in the Soft Spectral State

Black-hole X-ray binaries exhibit different spectral and timing properties in different accretion states. The X-ray outburst of a recently discovered and extraordinarily bright source, Swift$~$J1727.8$-$1613, has enabled the first investigation of how the X-ray polarization properties of a source evolve with spectral state. The 2$-$8 keV polarization degree was previously measured by the Imaging X-ray Polarimetry Explorer (IXPE) to be $\approx$ 4% in the hard and hard intermediate states. Here we present new IXPE results taken in the soft state, with the X-ray flux dominated by the thermal accretion-disk emission. We find that the polarization degree has dropped dramatically to $\lesssim$ 1%. This result indicates that the measured X-ray polarization is largely sensitive to the accretion state and the polarization fraction is significantly higher in the hard state when the X-ray emission is dominated by up-scattered radiation in the X-ray corona. The combined polarization measurements in the soft and hard states disfavor a very high or low inclination of the system.

astro-ph.HE↗

Star Formation Shut Down by Multiphase Gas Outflow in a Galaxy at a Redshift of 2.45

Large-scale outflows driven by supermassive black holes are thought to play a fundamental role in suppressing star formation in massive galaxies. However, direct observational evidence for this hypothesis is still lacking, particularly in the young universe where star formation quenching is remarkably rapid, thus requiring effective removal of gas as opposed to slow gas heating. While outflows of ionized gas are commonly detected in massive distant galaxies, the amount of ejected mass is too small to be able to suppress star formation. Gas ejection is expected to be more efficient in the neutral and molecular phases, but at high redshift these have only been observed in starbursts and quasars. Here we report JWST spectroscopy of a massive galaxy experiencing rapid quenching at redshift z=2.445. We detect a weak outflow of ionized gas and a powerful outflow of neutral gas, with a mass outflow rate that is sufficient to quench the star formation. Neither X-ray or radio activity are detected; however, the presence of a supermassive black hole is suggested by the properties of the ionized gas emission lines. We thus conclude that supermassive black holes are able to rapidly suppress star formation in massive galaxies by efficiently ejecting neutral gas.

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Cue: A Fast and Flexible Photoionization Emulator for Modeling Nebular Emission Powered By Almost Any Ionizing Source

The complex physics governing nebular emission in galaxies, particularly in the early universe, often defy simple low-dimensional models. This has proven to be a significant barrier in understanding the (often diverse) ionizing sources powering this emission. We present Cue, a highly flexible tool for interpreting nebular emission across a wide range of abundances and ionizing conditions of galaxies at different redshifts. Unlike typical nebular models used to interpret extragalactic nebular emission, our model does not require a specific ionizing spectrum as a source, instead approximating the ionizing spectrum with a 4-part piece-wise power-law. We train a neural net emulator based on the CLOUDY photoionization modeling code and make self-consistent nebular continuum and line emission predictions. Along with the flexible ionizing spectra, we allow freedom in [O/H], [N/O], [C/O], gas density, and total ionizing photon budget. This flexibility allows us to either marginalize over or directly measure the incident ionizing radiation, thereby directly interrogating the source of the ionizing photons in distant galaxies via their nebular emission. Our emulator demonstrates a high accuracy, with $\sim$1% uncertainty in predicting the nebular continuum and $\sim$5% uncertainty in the emission lines. Mock tests suggest Cue is well-calibrated and produces useful constraints on the ionizing spectra when $S/N (\mathrm{H}_α) \gtrsim 10$, and furthermore capable of distinguishing between the ionizing spectra predicted by single and binary stellar models. The compute efficiency of neural networks facilitates future applications of Cue for rapid modeling of the nebular emission in large samples and Monte Carlo sampling techniques.

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Magnetic flux plays an important role during a BHXRB outburst in radiative 2T GRMHD simulations

Black hole (BH) X-ray binaries cycle through different spectral states of accretion over the course of months to years. Although fluctuations in the BH mass accretion rate are generally recognized as the most important component of state transitions, it is becoming increasingly evident that magnetic fields play a similarly important role. In this article, we present the first radiative two-temperature (2T) general relativistic magnetohydrodynamics (GRMHD) simulations in which an accretion disk transitions from a quiescent state at an accretion rate of $\dot{M} \sim 10^{-10} \dot{M}_{\rm Edd}$ to a hard-intermediate state at an accretion rate of $\dot{M} \sim 10^{-2} \dot{M}_{\rm Edd}$. This huge parameter space in mass accretion rate is bridged by artificially rescaling the gas density scale of the simulations. We present two jetted BH models with varying degrees of magnetic flux saturation. We demonstrate that in `Standard and Normal Evolution' models, which are unsaturated with magnetic flux, the hot torus collapses into a thin and cold accretion disk when $\dot{M} \gtrsim 5\times 10^{-3} \dot{M}_{\rm Edd}$. On the other hand, in `Magnetically Arrested Disk' models, which are fully saturated with vertical magnetic flux, the plasma remains mostly hot with substructures that condense into cold clumps of gas when $\dot{M} \gtrsim 1 \times 10^{-2} \dot{M}_{\rm Edd}$. This suggests that the spectral signatures observed during state transitions are closely tied to the level of magnetic flux saturation.

astro-ph.HE↗

JWST Reveals Widespread AGN-Driven Neutral Gas Outflows in Massive z ~ 2 Galaxies

We use deep JWST/NIRSpec R~1000 slit spectra of 113 galaxies at 1.7 < z < 3.5, selected from the mass-complete Blue Jay survey, to investigate the prevalence and typical properties of neutral gas outflows at cosmic noon. We detect excess Na I D absorption (beyond the stellar contribution) in 46% of massive galaxies ($\log$ M$_*$/M$_\odot >$ 10), with similar incidence rates in star-forming and quenching systems. Half of the absorption profiles are blueshifted by at least 100 km/s, providing unambiguous evidence for neutral gas outflows. Galaxies with strong Na I D absorption are distinguished by enhanced emission line ratios consistent with AGN ionization. We conservatively measure mass outflow rates of 3 - 100 $M_\odot$ yr$^{-1}$; comparable to or exceeding ionized gas outflow rates measured for galaxies at similar stellar mass and redshift. The outflows from the quenching systems (log(sSFR)[yr$^{-1}$] $\lesssim$ -10) have mass loading factors of 4 - 360, and the energy and momentum outflow rates exceed the expected injection rates from supernova explosions, suggesting that these galaxies could possibly be caught in a rapid blowout phase powered by the AGN. Our findings suggest that AGN-driven ejection of cold gas may be a dominant mechanism for fast quenching of star formation at z~2.

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Gas Morphology of Milky Way-like Galaxies in the TNG50 Simulation: Signals of Twisting and Stretching

We present an in-depth analysis of gas morphologies for a sample of 25 Milky Way-like galaxies from the IllustrisTNG TNG50 simulation. We constrain the morphology of cold, warm, hot gas, and gas particles as a whole using a Local Shell Iterative Method (LSIM) and explore its observational implications by computing the hard-to-soft X-ray ratio, which ranges between $10^{-3}$-$10^{-2}$ in the inner $\sim 50 \rm kpc$ of the distribution and $10^{-5}$-$10^{-4}$ at the outer portion of the hot gas distribution. We group galaxies into three main categories: simple, stretched, and twisted. These categories are based on the radial reorientation of the principal axes of the reduced inertia tensor. We find that a vast majority ($77\%$) of the galaxies in our sample exhibit twisting patterns in their radial profiles. Additionally, we present detailed comparisons between 1) the gaseous distributions belonging to individual temperature regimes, 2) the cold gas distributions and stellar distributions, and 3) the gaseous distributions and dark matter (DM) halos. We find a strong correlation between the morphological properties of the cold gas and stellar distributions. Furthermore, we find a correlation between gaseous distributions with DM halo that increases with gas temperature, implying that we may use the warm-hot gaseous morphology as a tracer to probe the DM morphology. Finally, we show gaseous distributions exhibit significantly more prolate morphologies than the stellar distributions and DM halos, which we hypothesize is due to stellar and AGN feedback.

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