Hasil untuk "Astrophysics"

Rodriguez L. F., Mirabel I. F.

Context. One of the most important discoveries by the James Webb Space Telescope (JWST) is the unexpected existence of very large quantities of so-called little red dots (LRDs) in the early Universe (z > 4). These are compact luminous red galaxies with intriguing physical properties, one of which is the absence of radio detections. Aims. We wish to know if LRDs give off radio emission produced by accreting intermediate-mass or supermassive black holes (IMBHs or SMBHs) or by frequent supernovae from a cluster of massive stars. Methods. Assuming that LRDs at high redshifts have not been detected at radio wavelengths because they reside at large distances and/or the observational capabilities are limited, we present images made from archive Very Large Array (VLA) radio observations of J1047+0739 and J1025+1402. These are two analog candidate LRDs in the Local Universe at redshifts z = 0.1−0.2. Results. The source J1047+0739 at z = 0.1682 is detected at 6.0 GHz in 2018 with the VLA-A as a compact source with a radius smaller than 0.2 arcsec (< 700 pc at d ≃ 750 Mpc). Its flux density was 117 ± 8 μJy and its in-band spectral index was −0.85 ± 0.24, which is typical of optically thin synchrotron emission. It was also detected at 5.0 GHz in 2010 with the VLA-C, showing a flux density of 130 ± 9 μJy. We also detect a compact source very near J1025+1402 (≃2″) with a flux density of 45 ± 10 μJy that might be tracing a kiloparsec-scale jet emanating from an IMBH or SMBH. Conclusions. The observed flux densities can be provided by either a radio luminous supernova or an accreting IMBH or SMBH. However, the lack of significant variation in the flux density over eight years favors the IMBH–SMBH hypothesis. Radio time monitoring of this and other Local Little Red Dots (LLRDs) might help us solve the mystery of the radio silence of its cosmological counterparts.

Abboudeh Georges, Hennebelle Patrick, Soler Juan D. et al.

Context. Turbulence plays an important role in shaping the interstellar medium, and it strongly influences star formation. Aims. We aim to identify the physical processes capable of sustaining H I turbulence in the solar neighborhood. Methods. We compare recent H I line-of-sight velocity observations within a volume of radius 70–500 pc centered on the Sun with a suite of 1 kpc numerical simulations that include two distinct turbulent drivers: (i) supernova (SN) feedback and (ii) imposed large-scale turbulent forcing. For each simulation, we constructed synthetic sky maps that closely mimic the observational one, allowing for a consistent comparison between the simulations and the observational data. Results. The H I observations show a median velocity dispersion of 11.1 km s−1 in the solar neighborhood. The SN-driven simulations systematically underpredict this value, yielding dispersions in the range 4.9–6.7 km s−1. We find that the simulations with strong enough large-scale forcing can reproduce not only the median observed velocity dispersion but also the observed velocity distribution.

Baoqiang Lao, Xiaolong Yang, Wenjun Xiao et al.

We present a catalog of 971 FR I radio galaxies (FR Is) identified from the Very Large Array Faint Images of the Radio Sky at Twenty centimeters (FIRST) survey. The identifications were made using a hybrid method that combines deep learning with ridgeline flux density distribution analysis. Among these sources, 845 are new discoveries. The catalog comprises sources characterized by edge-darkened double jets, an absence of significant bent morphology, and angular sizes ranging from 23″ to 159″. Optical and/or infrared counterparts have been identified for 813 FR Is. Among these, the host galaxies are predominantly (88.1%) red galaxies, with the remainder (11.9%) being blue galaxies; notably, most blue galaxies exhibit high radio power. The FR I sample spans a radio power range of 1.20 × 10 ^21 ≤ P _1400 ≤ 3.55 × 10 ^27 W Hz ^−1 at 1400 MHz and reaches redshifts up to z = 2.307. The host galaxies have r -band absolute magnitudes in the range of −24 ≲  M _r  ≲ −20 mag. For the 512 FR Is with estimates, the black hole masses fall within 10 ^7  ≲  M _BH  ≲ 7.94 × 10 ^9 M _⊙ . Based on optical emission-line ratios and mid-infrared colors, spectroscopic classification shows that 571 hosts are low-excitation radio galaxies and 59 are high-excitation radio galaxies.

Vincent Van Eylen, Richard Massey, Saeeda Awan et al.

Habitable Worlds Observatory (HWO) will be NASA's flagship space telescope of the 2040s, designed to search for life on other planets and to transform broad areas of astrophysics. NASA are seeking international partners, and the UK is well-placed to lead the design and construction of its imaging camera - which is likely to produce the mission's most visible public impact. Early participation in the mission would return investment to UK industry, and bring generational leadership for the UK in space science, space technology, and astrophysics.

Tim Neidig, Apiwit Kittiratpattana, Tom Reichert et al.

We present realistic estimates for the duration of the hadronic stage in central Au+Au reactions in the RHIC-BES energy regime. To this aim, we employ a full set of coupled rate equations to describe the time evolution of the system from chemical to kinetic freeze-out. Combined with the recently measured data by the STAR collaboration on K⁎/K ratios, we show that the previous estimates substantially underestimated the duration of this stage due to the omission of the regeneration of hadron resonances. We provide an improved relation between the K⁎/K ratio at chemical and kinetic freeze-out and the life time of the hadronic phase. The calculated improved life times are now in line with estimates from other methods and are relevant for the NA61 and STAR collaborations and for upcoming experiments at the FAIR facility.

Ahmad Nemer, Ivan Yu. Katkov, Joseph D. Gelfand et al.

Identifying the dominant ionizing sources in galaxies is essential for understanding their formation and evolution. Traditionally, spectra are classified based on their dominant ionizing source using strong emission lines and Baldwin, Phillips, & Terlevich (BPT) diagrams. The ionizing source is traditionally determined by the emission line ratios using the BPT diagrams. Low-ionization nuclear emission-line regions (LINERs) are a class of ionizing mechanisms that is observationally identified but with a poorly understood origin, unlike the case of star-forming regions and active galactic nuclei (AGN). LINERs, typically found in early-type galaxies, are often associated with low-luminosity AGN activity but may also be powered by aging stellar populations, particularly post-asymptotic giant branch (p-AGB) stars. In this study, we employ a machine-learning-based encoder, spender , to analyze the full MaNGA integral field unit spectra and identify key spectral features of LINERs. By examining the continuum and line emission of these spaxels, our approach aims to uncover hidden patterns and better understand the dominant ionizing sources. We show in this work that the neural-network-based encoder was able identify LINER sources from the stellar continuum alone. The characteristics of the stellar population underlying LINER regions are consistent with evolved low-mass stars, implying that the source driving LINER emission is probably p-AGB stars rather than AGN activity.

Ewan O’Sullivan, P. N. Appleton, B. A. Joshi et al.

We present Chandra and XMM-Newton X-ray observations of the compact group HCG 57, and optical integral field spectroscopy of the interacting galaxy pair HCG 57A/D. These two spiral galaxies recently suffered an off-axis collision with HCG 57D passing through the disk of A. We find evidence of a gas bridge linking the galaxies, containing ∼10 ^8 M _⊙ of hot, ∼1 keV thermal plasma and warm ionized gas radiating in H α , H β , [O  iii ] and [N  ii ] lines. The optical emission lines in the central regions of HCG 57D show excitation properties consistent with H  ii -regions, while the outer rim of HCG 57D parts of the bridge and the outer regions of HCG 57A show evidence of shocked gas consistent with shock velocities of 200–300 km s ^−1 . In contrast, the X-ray emitting gas requires a collision velocity of 650–750 km s ^−1 to explain the observed temperatures. These different shock velocities can be reconciled by considering the contributions of rotation to collision velocity in different parts of the disks, and the clumpy nature of the preshock medium in the galaxies, which likely lead to different shock velocities in different components of the turbulent postshocked gas. We examine the diffuse X-ray emission in the group members and their associated point sources, identifying X-ray active galactic nuclei in HCG 57A, B, and D. We also confirm the previously reported ∼1 keV intra-group medium and find it to be relaxed with a low central entropy (18.0 ± 1.7 keV cm ^2 within 20 kpc) but a long cooling time (5.9 ± 0.8 Gyr).

Doğa Veske, Albert Zhang, Zsuzsa Márka et al.

Coincident multimessenger observations of cosmic sources can offer numerous benefits, especially when used in the context of synergistic astrophysics. One significant advantage is enhancing the detection significance of separate detectors by correlating their data and assuming joint emission. We have formulated an approach for updating the Bayesian posterior probability of an astrophysical origin, namely $p_{\rm astro}$, relying on multimessenger coincidences assuming an emission model. The description is applicable to any combination of messengers. We demonstrated the formalism for the gravitational waves and high-energy neutrinos case. Applying our method to the public data of candidate coincident high-energy neutrinos with subthreshold gravitational-wave triggers, we found that in the case of highly energetic neutrino coincidences, $p_{\rm astro}$ can increase from approximately $\sim 0.1$ to $\sim 0.9$. The amount of improvement depends on the assumed joint emission model. If models are trusted, the marked improvement makes subthreshold detections much more confident. Moreover, the model dependency can also be used to test the consistency of different models. This work is a crucial step toward the goal of uniting all detectors on equal footing into a statistically integrated, Earth-sized observatory for comprehensive multimessenger astrophysics.

Ze WANG, Shufang SONG, Xu WANG et al.

The accurate prediction of aerothermal loads is the basis to guide hypersonic vehicle design. Under the background that classical aerothermal prediction methods are more and more difficult to meet the demand of efficient and accurate aerothermal prediction in engineering, data-driven aerothermal modeling prediction methods have gradually become a new paradigm of aerothermal prediction in recent years. Firstly, the relationship between the data-driven aerothermal modeling prediction method and the classical aerothermal prediction method was described. Then, from the modeling idea, the data-driven aerothermal modeling prediction methods were summarized into three categories: The dimensionality reduction modeling method of feature space, pointwise modeling method and physical information embedding modeling method were introduced and analyzed in detail. It is found that the data-driven aerothermal modeling prediction method is not only more accurate than the engineering algorithm, but also can effectively reduce the workload of test measurement and numerical calculation when combined with the sampling method, and the model given is more efficient and concise. Finally, the development trend of data-driven aerothermal modeling prediction methods was prospected. It is pointed out that the deep combination of data-driven technology and classical aerothermal prediction methods, aerothermal physical information embedding modeling methods and aerothermal prediction big models will be the key points of future research.

Hui-Ling Li, Miao Zhang, Yu-Meng Huang

Abstract In 2022, the Event Horizon Telescope (EHT) collaboration has reported the first observations of Sagittarius A*(SgrA*). Applying the EHT observational results, we find out constraints on non-singular Hayward parameter of regular dark energy black hole. Considering these constraints and different thin disk accretion, we present a detailed investigation into influence of different dark energy and Hayward parameters on shadows from non-singular Hayward black holes. In the first second-order attenuation function model, corresponding shadow radius and peak for observed intensity from direct image decrease with increasing dark energy parameter and Hayward parameter. However, for the lensing ring and photon ring, corresponding peak become bigger as dark energy parameter increase in case of fixed Hayward parameter. In the second third-order attenuation function model, significantly different from model 1, above two rings completely overlay on the direct image, resulting in two distinct peaks in the observed intensity. As increase of Hayward and dark energy parameters, the difference between the two peaks decreases, and shadows and observed intensity decrease. In the final inverse trigonometric function attenuation model, the result shows corresponding lensing ring as well as photon ring can be distinguished within the superposition region, and the superposition region becomes larger. With the increase of the dark energy parameter, the shadow radius exhibits a decreasing trend, while observed intensity increases. However, with the increase of the Hayward parameter, both decreases. Compared with the first two models, the shadow radius becomes smaller, but the observed intensity becomes larger, making the bright ring wider and brighter. Therefore, different accretion models and non-singular Hayward parameters can give rise to interesting and distinguish characteristic for the black hole shadow and rings.

Viola Gelli, Charlotte Mason, Christopher C. Hayward

The James Webb Space Telescope is unveiling a surprising lack of evolution in the number densities of ultraviolet (UV)-selected galaxies at redshift z ≳ 10. At the same time, observations and simulations are providing evidence for highly bursty star formation in high- z galaxies, resulting in significant scatter in their UV luminosities. Galaxies in low-mass dark matter halos are expected to experience most stochasticity due to their shallow potential wells. Here, we explore the impact of a mass-dependent stochasticity using a simple analytical model. We assume that scatter in the M _UV – M _h relation increases toward lower halo masses, following the decrease in halo escape velocity, ${\sigma }_{\mathrm{UV}}\sim {M}_{h}^{-1/3}$ , independent of redshift. Since low-mass halos are more dominant in the early universe, this model naturally predicts an increase in UV luminosity functions (LFs) at high redshifts compared to models without scatter. We make predictions for additional observables, which would be affected by stochasticity and could be used to constrain its amplitude, finding (i) galaxies are less clustered compared to the no-scatter scenario, with the difference increasing at higher- z ; (ii) assuming that star-bursting galaxies dominate the ionizing photon budget implies reionization starts earlier and is more gradual compared to the no-scatter case; (iii) at fixed UV magnitude, galaxies should exhibit wide ranges of UV slopes, nebular emission line strengths, and Balmer breaks. Comparing to observations, the mass-dependent stochasticity model successfully reproduces the observed LFs up to z ∼ 12. However, the model cannot match the observed z ∼ 14 LFs, implying additional physical processes enhance star formation efficiency in the earliest galaxies.

Fiorenzo Stoppa, Eric Cator, Gijs Nelemans

In astronomy, there is an opportunity to enhance the practice of validating models through statistical techniques, specifically to account for measurement error uncertainties. While models are commonly used to describe observations, there are instances where there is a lack of agreement between the two. This can occur when models are derived from incomplete theories, when a better-fitting model is not available or when measurement uncertainties are not correctly considered. However, with the application of specific tests that assess the consistency between observations and astrophysical models in a model-independent way, it is possible to address this issue. The consistency tests (ConTESTs) developed in this paper use a combination of non-parametric methods and distance measures to obtain a test statistic that evaluates the closeness of the astrophysical model to the observations. To draw conclusions on the consistency hypothesis, a simulation-based methodology is performed. In particular, we built two tests for density models and two for regression models to be used depending on the case at hand and the power of the test needed. We used ConTEST to examine synthetic examples in order to determine the effectiveness of the tests and provide guidance on using them while building a model. We also applied ConTEST to various astronomy cases, identifying which models were consistent and, if not, identifying the probable causes of rejection.

Melvyn Tyloo

Networks are widely used to model the interaction between individual dynamic systems. In many instances, the total number of units and interaction coupling are not fixed in time, and instead constantly evolve. In networks, this means that the number of nodes and edges both change over time. Various properties of coupled dynamic systems, such as their robustness against noise, essentially depend on the structure of the interaction network. Therefore, it is of considerable interest to predict how these properties are affected when the network grows as well as their relationship to the growth mechanism. Here, we focus on the time evolution of a network’s Kirchhoff index. We derive closed-form expressions for its variation in various scenarios, including the addition of both edges and nodes. For the latter case, we investigate the evolution where single nodes with one or two edges connecting to existing nodes are added recursively to a network. In both cases, we derive the relations between the properties of the nodes to which the new node connects along with the global evolution of network robustness. In particular, we show how different scalings of the Kirchhoff index can be obtained as a function of the number of nodes. We illustrate and confirm this theory via numerical simulations of randomly growing networks.

Saurya Das, Mitja Fridman, Gaetano Lambiase

Abstract A consistent theory of quantum gravity will require a fully quantum formulation of the classical equivalence principle. Such a formulation has been recently proposed in terms of the equality of the rest, inertial and gravitational mass operators, and for non-relativistic particles in a weak gravitational field. In this work, we propose a generalization to a fully relativistic formalism of the quantum equivalence principle, valid for all background space-times, as well as for massive bosons and fermions. The principle is trivially satisfied for massless particles. We show that if the equivalence principle is broken at the quantum level, it implies the modification of the standard Lorentz transformations in flat space-time and a corresponding modification of the metric in curved space-time by the different mass ratios. In other words, the observed geometry would effectively depend on the properties of the test particle. Testable predictions of potential violations of the quantum equivalence principle are proposed.

Sofia V. Forsblom, Juri Poutanen, Sergey S. Tsygankov et al.

The radiation from accreting X-ray pulsars was expected to be highly polarized, with some estimates for the polarization degree of up to 80%. However, phase-resolved and energy-resolved polarimetry of X-ray pulsars is required in order to test different models and to shed light on the emission processes and the geometry of the emission region. Here we present the first results of the observations of the accreting X-ray pulsar Vela X-1 performed with the Imaging X-ray Polarimetry Explorer. Vela X-1 is considered to be the archetypal example of a wind-accreting, high-mass X-ray binary system, consisting of a highly magnetized neutron star accreting matter from its supergiant stellar companion. The spectropolarimetric analysis of the phase-averaged data for Vela X-1 reveals a polarization degree (PD) of 2.3% ± 0.4% at the polarization angle (PA) of −47.°3 ± 5.°4. A low PD is consistent with the results obtained for other X-ray pulsars and is likely related to the inverse temperature structure of the neutron star atmosphere. The energy-resolved analysis shows the PD above 5 keV reaching 6%–10% and a ∼90° difference in the PA compared to the data in the 2–3 keV range. The phase-resolved spectropolarimetric analysis finds a PD in the range 0%–9% with the PA varying between −80° and 40°.

Run-ze DUAN, Xiao-lei ZHANG, Heng ZHANG et al.

Liquid atomization is widely used in industry, agriculture and other fields. During the atomization process, the liquid first forms a liquid film, and the liquid film becomes unstable in the gas-liquid boundary layer, resulting in the liquid film being broken. Therefore, the instability of the liquid film plays a very important role in the liquid atomization, and it is necessary to study the instability of the liquid film. In this paper, the instability of viscous planar liquid film in compressible gas was studied by numerical calculation method. Firstly, the velocity distributions of viscous planar liquid film and gas were derived, based on which the instability of viscous planar liquid film was investigated by spectral methods. It is found that in the sinuous and varicose modes, applying an electric field will accelerate the breakup of the plane liquid film. The sinuous mode is more unstable than the varicose mode, indicating that the sinuous mode plays a leading role in the instability of the plane liquid film. Increases of gas Mach number, electrical Euler number, gas Reynolds number, Weber number and momentum flux ratio accelerate the breakup of the liquid film. With the increase of the thickness ratio of gas boundary layer to liquid film and the liquid Reynolds number, the growth rate of the disturbance wave decreases, and the liquid film becomes more stable.

Z. Eker, F. Soydugan, V. Bakis et al.

Arbitrariness in the zero point of bolometric corrections is a nearly century-old paradigm leading to two more paradigms. "Bolometric corrections must always be negative," and "bolometric magnitude of a star ought to be brighter than its $V$ magnitude". Both were considered valid before IAU 2015 General Assembly Resolution B2, a revolutionary document that supersedes all three aforementioned paradigms. The purpose of this article is to initiate a new insight and a new understanding of the fundamental astrophysics and present new capabilities to obtain standard and more accurate stellar luminosities and gain more from accurate observations in the era after Gaia. The accuracy gained will aid in advancing stellar structure and evolution theories, and also Galactic and extragalactic research, observational cosmology and dark matter and dark energy searches.