Stellar Matter Research Articles

Galaxy stellar and total mass estimation using machine learning

ABSTRACT Conventional galaxy mass estimation methods suffer from model assumptions and degeneracies. Machine learning (ML), which reduces the reliance on such assumptions, can be used to determine how well present-day observations can yield predictions for the distributions of stellar and dark matter. In this work, we use a general sample of galaxies from the TNG100 simulation to investigate the ability of multibranch convolutional neural network (CNN) based ML methods to predict the central (i.e. within 1−2 effective radii) stellar and total masses, and the stellar mass-to-light ratio (M*/L). These models take galaxy images and spatially resolved mean velocity and velocity dispersion maps as inputs. Such CNN-based models can, in general, break the degeneracy between baryonic and dark matter in the sense that the model can make reliable predictions on the individual contributions of each component. For example, with r-band images and two galaxy kinematic maps as inputs, our model predicting M*/L has a prediction uncertainty of 0.04 dex. Moreover, to investigate which (global) features significantly contribute to the correct predictions of the properties above, we utilize a gradient-boosting machine. We find that galaxy luminosity dominates the prediction of all masses in the central regions, with stellar velocity dispersion coming next. We also investigate the main contributing features when predicting stellar and dark matter mass fractions (f*, fDM) and the dark matter mass MDM, and discuss the underlying astrophysics.

Two-dimensional kinematics and dynamical modelling of the ‘Jackpot’ gravitational lens from deep MUSE observations

ABSTRACT We present results from the first spatially resolved kinematic and dynamical modelling analysis of the unique SDSSJ0946+1006 (‘Jackpot’) triple-source lens system, where a single massive foreground $z\, =\, 0.222$ galaxy multiple-images three background sources at different redshifts. Deep integral field unit spectroscopic data were obtained using the MUSE instrument on the VLT, which, compared to previous single-slit observations, provides full azimuthal area coverage, high sensitivity (5 h integration) and high angular resolution (0.5 arcsec full width at half-maximum). To account for the strong continuum contributions from the $z\, =\, 0.609$ source, a multiple-component stellar template fitting technique is adopted to fit to the spectra of both the lens galaxy and the bright lensed background arc simultaneously. Through this, we robustly measure the first and second moments of the 2D stellar kinematics out to about 10 kpc from the centre of the lens, as well as resolving the inner profile inwards to ∼1 kpc. The 2D kinematic maps show a steep velocity dispersion gradient and a clear rotational component. We constrain the characteristic properties of the stellar and dark matter (DM) mass components with a sufficiently flexible parametrised dynamical model and an imposed lensing mass and find a DM density slope of $\gamma \, =\, 1.73\substack{+0.17 \\ -0.26}$, i.e. significantly steeper than an unmodified NFW profile ($\gamma \, =\, 1$) and consistent with a contracted DM halo. Our fitted models have a lensing-equivalent density slope of $\eta \, =\, 0.96\pm 0.02$, and thus we confirm most pure lensing results in finding a near isothermal profile for this galaxy.

Understanding shape and centroid deviations in 39 strong lensing galaxy clusters in various dynamical states

Through observational tests of strong lensing galaxy clusters, we can test simulation derived structure predictions that follow from Λ Cold Dark Matter (ΛCDM) cosmology. The shape and centroid deviations between the total matter distribution, stellar matter distributions, and hot intracluster gas distribution serve as an observational test of these theoretical structure predictions. We measure the position angles, ellipticities, and locations/centroids of the brightest cluster galaxy (BCG), intracluster light (ICL). the hot intracluster medium (ICM), and the core lensing mass for a sample of strong lensing galaxy clusters from the SDSS Giant Arcs Survey (SGAS). We utilize HST WFC3/1R imaging data to measure the shapes/centroids of the ICL and BCG distributions and use Chandra ACIS-I X-ray data to measure the shapes/centroids of the ICM. Additionally, we measure the concentration parameter (c) and asymmetry parameter (A) to incorporate cluster dynamical state into our analysis. Using this multicomponent approach, we evaluate the different components in terms of their ability to trace out the DM halo of clusters in various dynamical states.

Ultra-low-mass and small-radius white dwarfs made of heavy elements

Seven possible ultra-low-mass and small-radius white dwarfs have been recently identified, with masses ranging from ∼0.02 M⊙ to ∼0.08 M⊙ and radii ranging from ∼ 4,270 km to 10670 km. The mass–radius measurements of these white dwarfs pose challenges to traditional white dwarf models, assuming they are mostly made of nuclei lighter than 56Fe. In this work, we consider the possibility that those white dwarfs are made of heavier elements. Due to the small charge-to-mass ratios in heavy elements, the electron number density in white dwarf matter is effectively reduced, which reduces the pressure with additional contributions of lattice energy and electron polarization corrections. This consequently leads to white dwarfs with much smaller masses and radii, which coincide with the seven ultra-low-mass and small-radius white dwarfs. The mass of the most massive white dwarfs is effectively reduced and could possibly account for the sub-Chandrasekhar progenitors in underluminous Type Ia supernovae. The corresponding equation of state and matter contents of dense stellar matter with and without reaching the cold-catalyzed ground state are presented, which are obtained using the latest Atomic Mass Evaluation (AME 2020). Further observations are necessary to unveil the actual matter contents in those white dwarfs via, e.g., spectroscopy, asteroseismology, and the discoveries of other ultra-low-mass and small-radius white dwarfs.

Some new dark energy star models in Rastall gravity via Tolman–Kuchowicz potentials

In this paper, we are interested in modeling a new relativistic dark energy star by enforcing the static and anisotropic spherically symmetric fluid distribution in the realm of the Rastall theory of gravity. To do so, we have assumed a linear dark energy equation of state coupled with baryonic stellar matter: ρde=βρ, which has been used as a seed solution along with Tolman–Kuchowicz potentials to obtain the analytical solutions of the Rastall field equations. The stability of various physical attributes like matter variables, energy conditions, stability analysis, mass–radius relation, compactness and surface redshift have been examined corresponding to the observational data of the compact star PSRJ1614−2230 (R=9.69 km). Based on the evident results, for the small values of the Rastall parameter α, our proposed solutions are viable for a new stable relativistic dark energy star model in the realm of Rastall gravity, which comparatively seems to be more interesting than the results given in previous study (Saklany et al., 2022).

COSMOS2020: The galaxy stellar mass function

Context. How galaxies form, assemble, and cease their star formation is a central question within the modern landscape of galaxy evolution studies. These processes are indelibly imprinted on the galaxy stellar mass function (SMF), and its measurement and understanding is key to uncovering a unified theory of galaxy evolution. Aims. We present constraints on the shape and evolution of the galaxy SMF, the quiescent galaxy fraction, and the cosmic stellar mass density across 90% of the history of the Universe from z = 7.5 → 0.2 as a means to study the physical processes that underpin galaxy evolution. Methods. The COSMOS survey is an ideal laboratory for studying representative galaxy samples. Now equipped with deeper and more homogeneous near-infrared coverage exploited by the COSMOS2020 catalog, we leverage the large 1.27 deg2 effective area to improve sample statistics and understand spatial variations (cosmic variance) – particularly for rare, massive galaxies – and push to higher redshifts with greater confidence and mass completeness than previous studies. We divide the total stellar mass function into star-forming and quiescent subsamples through NUVrJ color-color selection. The measurements are then fit with single- and double-component Schechter functions to infer the intrinsic galaxy stellar mass function, the evolution of its key parameters, and the cosmic stellar mass density out to z = 7.5. Finally, we compare our measurements to predictions from state-of-the-art cosmological simulations and theoretical dark matter halo mass functions. Results. We find a smooth, monotonic evolution in the galaxy stellar mass function since z = 7.5, in general agreement with previous studies. The number density of star-forming systems have undergone remarkably consistent growth spanning four decades in stellar mass from z = 7.5 → 2 whereupon high-mass systems become predominantly quiescent (“downsizing”). Meanwhile, the assembly and growth of low-mass quiescent systems only occurred recently, and rapidly. An excess of massive systems at z ≈ 2.5 − 5.5 with strikingly red colors, with some being newly identified, increase the observed number densities to the point where the SMF cannot be reconciled with a Schechter function. Conclusions. Systematics including cosmic variance and/or active galactic nuclei contamination are unlikely to fully explain this excess, and so we speculate that they may be dust-obscured populations similar to those found in far infrared surveys. Furthermore, we find a sustained agreement from z ≈ 3 − 6 between the stellar and dark matter halo mass functions for the most massive systems, suggesting that star formation in massive halos may be more efficient at early times.

Hybrid stars built with density-dependent models

ABSTRACT Using a density-dependent quark model and a relativistic model within the mean-field approximation for hadrons with density-dependent meson-baryon couplings, we construct, for the first time, models that describe hybrid neutron stars consisting of nucleons and exotic baryons (hyperons and Δ-resonances). We do the study using a Maxwell construction. The quark-hadron phase transition in the stellar matter is determined through the structure, composition, and properties of the hybrid neutron star matter. The macroscopic properties of the star are determined, and the results for these particular models are found to be compatible with recent observational astrophysical data.

Improved Galactic diffuse emission model strengthens the case for a Millisecond Pulsar explanation of the Fermi GeV excess

After more than a decade since its discovery, the Galactic center gamma-ray excess - discovered with the Fermi Large Area Telescope - remains puzzling. While the spectrum of the signal can be explained by either dark matter or an unresolved population of millisecond pulsars, the spatial morphology of this excess seems to hold the key to separate the two theories. In this contribution, we present the results of a recent study in which we use bleeding edge models for interstellar gas, inverse Compton emission, and stellar mass models to reanalyze the Galactic center excess. We find that the spatial morphology of the excess is highly correlated with stellar matter in the Galactic bulge, providing strong support for the millisecond pulsar hypothesis.

Recent astrophysical observations reproduced by a short-range correlated van der Waals-type model?

ABSTRACT We perform an improvement in a van der Waals-type model by including its effects of short-range correlations (SRCs). Attractive and repulsive parts of the nucleon–nucleon interaction are assumed to be density-dependent functions, more specifically, we adopt the Carnahan–Starling (CS) method for the latter, and a suitable expression for the former in order to reproduce the structure of the Clausius (C) real gas model. The parametrizations of the resulting model, named as Clausius-–Carnahan–Starling (CCS)-SRC model, are shown to be capable of reproducing the flow constraint at the high-density regime of symmetric nuclear matter for incompressibility values inside the range of K0 = (240 ± 20) MeV. In the context of stellar matter, our findings point out a good agreement of the CCS-SRC model with recent astrophysical observational data, namely, mass–radius contours and dimensionless tidal deformability regions and values, coming from gravitational waves data related to the GW170817 and GW190425 events, and from the NASA’s Neutron star Interior Composition Explorer mission. Furthermore, the values for the symmetry energy slope of the model (L0) are in agreement with a recent range found for this quantity, claimed to be consistent with results reported by the updated lead radius experiment (PREX-2) collaboration. In this case, higher values of L0 are favoured, while the opposite scenario does not allow simultaneous compatibility between the model and the astrophysical data.

Pulsar glitch in a strangeon star model – III. The recovery

ABSTRACT Strangeon star model has passed various observational tests, such as the massive pulsars and the tidal deformability during binary mergers. Pulsar glitch, as a useful probe for studying the interior structure of pulsars, has also been studied in strangeon star model in our previous papers, including the recovery coefficient, the waiting time of glitches, and glitch activity. In this paper, the recovery process of a glitch is described in the strangeon star model, based on the starquake picture established before (in Paper I). After the starquake, the inner motion of the stellar matter would reduce the tangential pressure in the cracked places at the equatorial plane. The recovery (increase) of the tangential pressure would be achieved by a viscous flow towards the cracked places at equatorial plane, which leads to the exponential recovery of the spin frequency. A uniform viscous flow can reproduce the single exponential decay observed in some glitches, and the viscous time-scale τ and the depth h of the cracking place below the surface can be fitted by the recovery data. It is found that h increases with glitch size Δν/ν, which is expected in the glitch scenario of strangeon stars. The magnitude of the recovery predicted in this recovery model is also consistent with that derived from observations. The single exponential decay reproduced by a uniform viscous flow can be generalized to two or more exponentials by the multicomponent of viscous flows.

Chandrasekhar Mass Limit of White Dwarfs in Modified Gravity

We investigate the Chandrasekhar mass limit of white dwarfs in various models of f(R) gravity. Two equations of state for stellar matter are used: the simple relativistic polytropic equation with polytropic index n=3 and the realistic Chandrasekhar equation of state. For calculations, it is convenient to use the equivalent scalar–tensor theory in the Einstein frame and then to return to the Jordan frame picture. For white dwarfs, we can neglect terms containing relativistic effects from General Relativity and we consider the reduced system of equations. Its solution for any model of f(R)=R+βRm (m≥2, β>0) gravity leads to the conclusion that the stellar mass decreases in comparison with standard General Relativity. For realistic equations of state, we find that there is a value of the central density for which the mass of a white dwarf peaks. Therefore, in frames of modified gravity, there is a lower limit on the radius of stable white dwarfs, and this minimal radius is greater than in General Relativity. We also investigate the behavior of the Chandrasekhar mass limit in f(R) gravity.

Exotic baryons in hot neutron stars

ABSTRACT We study the nuclear isentropic equation of state for a stellar matter composed of nucleons, hyperons, and Δ-resonances. We investigate different snapshots of the evolution of a neutron star, from its birth as a lepton-rich proto-neutron star in the aftermath of a supernova explosion to a lepton-poor regime when the star starts cooling to a catalysed configuration. We use a relativistic model within the mean-field approximation to describe the hot stellar matter and adopt density-dependent couplings adjusted by the DDME2 parameterization. We use baryon–meson couplings for the spin-1/2 baryonic octet and spin-3/2 decuplet determined in a unified manner relying on SU(6) and SU(3) symmetry arguments. We observe that Λ is the dominant exotic particle in the star at different entropies for both neutrino-free and neutrino-trapped stellar matter. For a fixed entropy, the inclusion of new particles (hyperons and/or delta resonances) in the stellar matter decreases the temperature. Also, an increase in entropy per baryon (1–2) with decreasing lepton number density (0.4–0.2) leads to an increase in stellar radii and a decrease in its mass due to neutrino diffusion. In the neutrino transparent matter, the radii decrease from entropy per baryon 2 to $T\, =\, 0$ without a significant change in stellar mass.

Relativistic stellar modeling with perfect fluid core and anisotropic envelope fluid

We investigate the effect of density perturbations and local anisotropy on the stability of stellar matter structures in general relativity using the concept of cracking. Adopting a core-envelope model of a super-dense star, we examine the properties and stability conditions by introducing anisotropic pressure to the envelope region. Furthermore, we propose self-bound compact stars with an anisotropic envelope as a potential progenitor for starquakes. We show how the difference between sound propagation in radial and tangential directions would be used to identify potentially stable regions within a configuration. Due to an increase in the anisotropic parameter, strain energy accumulates in the envelope region and becomes a potential candidate for building-up quake like situation. This stress-energy stored in the envelope region that would be released during a starquake of a self-bound compact star is computed as a function of the magnitude of anisotropy at the core-envelope boundary. Numerical studies for spherically asymmetric compact stars indicate that the stress energy can be as high as $$10^{50}$$ erg if the tangential pressure is slightly more significant than the radial pressure. It is happened to be of the same order as the energy associated with giant $$\gamma $$ -ray bursts. Thus, the present study will be useful for the correlation studies between starquakes and GRBs.

Priorities in gravitational waveforms for future space-borne detectors: vacuum accuracy or environment?

ABSTRACT In preparation for future space-borne gravitational-wave (GW) detectors, should the modelling effort focus on high-precision vacuum templates or on the astrophysical environment of the sources? We perform a systematic comparison of the phase contributions caused by (1) known environmental effects in both gaseous and stellar matter backgrounds, or (2) high-order post-Newtonian (PN) terms in the evolution of mHz GW sources during the inspiral stage of massive binaries. We use the accuracy of currently available analytical waveform models as a benchmark value, finding the following trends: the largest unmodelled phase contributions are likely environmental rather than PN for binaries lighter than ∼107/(1 + z)2 M⊙, where z is the redshift. Binaries heavier than ∼108/(1 + z) M⊙ do not require more accurate inspiral waveforms due to low signal-to-noise ratios (SNRs). For high-SNR sources, environmental phase contributions are relevant at low redshift, while high-order vacuum templates are required at z ≳ 4. Led by these findings, we argue that including environmental effects in waveform models should be prioritized in order to maximize the science yield of future mHz detectors.

Dynamics of Molecular Gas in the Central Region of the Quasar I Zwicky 1

We present a study of the molecular gas distribution and kinematics in the cicumnuclear region (radii ≲2 kpc) of the z ≈ 0.061 quasar I Zwicky 1 using a collection of available Atacama Large Millimeter/submillimeter Array observations of the carbon monoxide (CO) emission. With an angular resolution of ∼0.″36 (corresponding to ∼400 pc), the host-galaxy substructures including the nuclear molecular gas disk, spiral arms, and a compact bar-like component are resolved. We analyzed the gas kinematics based on the CO image cube and obtained the rotation curve and radial distribution of velocity dispersion. The velocity dispersion is about 30 km s−1 in the outer CO disk region and rises up to ≳100 km s−1 at radius ≲1 kpc, suggesting that the central region of the disk is dynamically hot. We constrain the CO-to-H2 conversion factor, α CO, by modeling the cold gas disk dynamics. We find that, with prior knowledge about the stellar and dark matter components, the α CO value in the circumnuclear region of this quasar host galaxy is , which is between the value reported in ultraluminous infrared galaxies and in the Milky Way. The central 1 kpc region of this quasar host galaxy has significant star formation activity, which can be identified as a nuclear starburst. We further investigate the high-velocity dispersion in the central region. We find that the interstellar medium (ISM) turbulent pressure derived from the gas velocity dispersion is in equilibrium with the weight of the ISM. This argues against extra power from active galactic nuclei feedback that significantly affects the kinematics of the cold molecular gas.

On the edge: the relation between stellar and dark matter haloes of Milky Way-mass galaxies

ABSTRACTWe investigate the build-up of the accreted stellar and dark matter haloes of Milky Way-like galaxies in APOSTLE suite of cosmological hydrodynamics simulations. We show that the stellar halo is made up primarily of stars stripped from a small number of massive dwarfs, most of which are disrupted by the present day. The dark matter halo, on the other hand, is made up primarily of small unresolved subhaloes (≲106 M⊙) and a ‘smooth’ component consisting of particles which were never bound to a subhalo. Despite these differences, the massive dwarfs that make up the majority of the stellar halo also contribute a significant fraction of the dark matter. The stars and dark matter stripped from these dwarfs are related through their kinematics and this leaves imprints in the phase-space structure of the haloes. We examine the relation between the location of features, such as caustics, in the phase space of the stars and dark halo properties. We show that the ‘edge’ of the stellar halo is a probe of dark matter halo mass and assembly history. The edges of Milky Way-mass galaxies should be visible at a surface brightness of 31–36 mag arcsec−2.

Neutron star versus neutral star: On the 90th anniversary of Landau's publication in astrophysics

AbstractIn the late age of developing quantum mechanics, Lev Landau made great efforts to understand the nature of matter, even stellar matter, by applying the quantum theory. Ninety years ago, he published his idea of “neutron” star, which burst upon him during his visit over Europe in the previous year. The key point that motivated Landau to write the paper is to make a state with lower energy for “gigantic nucleus”, avoiding extremely high kinematic energy of electron due to the new Fermi–Dirac statistics at that time. Landau had no alternative but to neutralize by “combining a proton and an electron” before the discovery of neutron. However, our understanding of the nature has fundamentally improved today, and another way (strangeonization) could also embody neutralization and thus a low‐energy state that Landau had in mind, which could further make unprecedented opportunities in this multi‐messenger era of astronomy. Strangeon matter in “old” physics may impact dramatically today's physics, from compact stars initiated by Landau, to cosmic rays and dark matter. In this essay, we are making briefly the origin and development of neutron star concept to reform radically, to remember Landau's substantial contribution to astrophysics and to recall those peculiar memories.

Scattering‐state solutions to the Dirac spinors with negative‐energy eigenstates in a rotating spheroid

AbstractBased on the work of Fu et al. (2020), we derive the rest seven scattering‐state solutions to the Dirac equation when and establish a relation between differential scattering cross‐section, , and stellar matter density, , using the long‐wave approximation. It is found that the sensitivity of average scattering cross‐sections to the change in the stellar matter density is proportional to . We find that the average scattering amplitudes , as well as average scattering cross‐sections , are independent of the masses of particles, , for four scattering states , i = 0, 1, 2 and 3, while and depend on , for the rest four scattering states, , i = 0, 1, 2 and 3. By measuring the scattering cross‐section, we can determine the density of the ellipsoid and obtain its gravitational properties. This work will be useful in understanding the properties of anti‐Dirac spinors and the physical effects in a rotating spheroid.

The generalized polytropic model for the Sun-like stars

The Eddington method based on simultaneous consideration of gas and light pressures with a homogeneous сhemical composition of stellar matter was generalized for the case of model with a spatially inhomogeneous chemical composition. As a result, it was obtained the equation of state, which is expressed by a generalized polytrope with index n=3. As an example, it was solved the equilibrium equation for the Sun both using the standard polytropic equation of state and generalized polytrope. The coordinate dependence of the Sun characteristics was calculated within two models. Obtained results are compared with the results of numerical calculations for the Sun based on the system of Schwarzschild equations for the standard model. It was shown that the standard polytropic model is applicable only for the Sun of zero-age. The Sun characteristics calculated with help of generalized equation of state are close to the results of numerical calculations based on Schwarzschild equations. It was concluded that the standard polytropic model is applicable for the stars of zero-age main sequence, and the generalized model – for the stars of finite age, in which thermonuclear reactions have already created a significant spatially inhomogeneity of chemical composition inside of the core.

Imagine the World, New Insight into Creation, Gravity and Evolution

Since the publication of Einstein’s theory of general relativity, a variety of interpretations have been suggested. In 1937, Paul Dirac presented his Large Number Hypothesis (LNH) based on the large difference between gravitational and electromagnetic forces. As a consequence, the energy is proportional to the radius squared and the gravitational constant is inversely proportional to the radius of the universe. The energy increases during expansion, Dirac used the term “additive creation”. The objective is to prove the validity of Dirac’s claims. A new theory, CBU (Continuously Breeding Universe), has been developed. The universe is considered a black hole originating from the single fluctuation of a positron-electron pair. The expansion is driven by the formation of new pairs. The negative gravitational potential energy balances the increase in matter energy. Due to G ~ 1/r, the Planck length and Planck time tP depend on the curvature of space. The Schrödinger solution of an initial positron-electron fluctuation contains a parameter equal to the Planck length. The CBU theory postulates that the primordial universe undergoes a transition from a black hole to a photon-filled universe. After the transition, one half of the energy is bound to numerous “small” black holes, the seeds for galaxies, while the other half propagates as CMB (Cosmic Microwave Background) radiation. The CMB photons are due to e+ -e- annihilations. Characteristically, the CMB photons are pairwise entangled, the radiation loses wave energy but compensates by increasing the photon numbers. According to a new model for black holes, a continuous inflow of matter prevents the black holes from becoming singularities. An energy gap between the event horizon and the inner photon sphere is the source of real matter from a QED vacuum foam. There is evidence that all stellar matter originates from a proton-antiproton outflow from the galaxy central black hole.