Halos Of Spiral Galaxies Research Articles

F(T) theory solutions for traversable wormhole existence and neutron stars mass limits problems

In this paper, some Schwarzschild-like solutions in the framework of [Formula: see text] theory where [Formula: see text] means the scalar tensor are reconstructed and analyzed in the context of some open problems like traversable wormholes and neutron starts mass limits. Like some authors, we search for the potentiality of the presence of traversable wormholes in the outer/inner regions halo of galaxies particularly the Milky Way galaxy by considering diagonal tetrads which support the Morris–Thorne metric. In such description where attention is attached to the energy density of dark matter present in the halo of spiral galaxies, a new dark matter density emerges in this work. The reconstructed density profile reveals the existence of traversable wormholes not only in the inner of the Milky Way galaxy but also in the both outer and inner of another galaxy smaller than the Milky Way galaxy. The use of Einasto density profile in this work makes it predict traversable wormholes in both outer and inner of the Milky Way galaxy. In the second part of this work, the non-diagonal tetrads describing the Schwarzschild metric are considered to solve the [Formula: see text] motion equations, where solutions describing neutron stars are provided. Several numerical analyses based on neutron stars mass are done and the obtained results give the best fit with observational data in the framework of neutron stars mass limit.

Dependence of stellar substructures in M31 type galaxy on satellite morphology in galaxy mergers

Stellar streams and shells are observed in halos of spiral galaxies. In this paper, we investigated the formation of these structures due to mergers between the host spiral galaxy and its dwarf satellite galaxies. We run the N-body simulations with two morphological models of a dwarf galaxy for different initial positions. One model is a spheroidal dwarf, dSph, and the other is a dwarf with a disk. We found that both models form stellar shells and streams and, in the case of the progenitor with a disk, streams are more prominent. After several pericentric passages, there is a possibility of formation of several streams. The remnant of the progenitor is more likely to disrupt later into the merger in the case of spiral progenitor than in the case of dSph.

Which Milky Way masses are consistent with the slightly declining 5–25 kpc rotation curve?

Context. Discoveries of extended rotation curves have suggested that spiral galaxy halos contain dark matter. This has led to many studies that estimated the total mass of the Galaxy, mostly using the Navarro, Frenk, and White (NFW) density profile. Aims. We determine the effect that the choice of the dark matter profile has on the predicted values of extrapolated total masses. Methods. We considered a recently reported Milky Way (MW) rotation curve, first because of its unprecedented accuracy, and second because the Galactic disk appears to be least affected by past major mergers that have fully reshaped the initial disk. Results. We find that the use of an NFW profile (or its generalized form, gNFW) to calculate the dark-matter contribution to the MW rotation curve generates apparently inconsistent results such as an increase in baryonic mass that leads to an increase in dark matter mass. Furthermore, we find that NFW and gNFW profiles narrow the total mass range, leading to a possible methodological bias particularly against low MW masses. Using the Einasto profile, which is better suited to represent cold dark matter halos, we finally found that the slightly decreasing rotation curve of the MW favors a total mass that can be as low as 2.6 × 1011 M⊙, disregarding any other dynamical tracers farther out in the MW. This is inconsistent with values higher than 18 × 1011 M⊙ for any type of cold dark matter halo profiles under the assumption that stars and gas do not affect the predicted dark matter distribution in the MW. Conclusions. This methodological paper encourages the use of the Einasto profile to characterize rotation curves with the aim of evaluating their total masses.

CHANG-ES

Context. The magnetic field in spiral galaxies is known to have a large-scale spiral structure along the galactic disk and is observed as X-shaped in the halo of some galaxies. While the disk field can be well explained by dynamo action, the three-dimensional structure of the halo field and its physical nature are still unclear. Aims. As first steps towards understanding the halo fields, we want to clarify whether or not the observed X-shaped field is a wide-spread pattern in the halos of spiral galaxies. We also aim to investigate whether these halo fields are simply turbulent fields ordered by compression or shear (anisotropic turbulent fields), or have a large-scale regular structure. Methods. Analysis of the Faraday rotation in the halo is used as a tool to distinguish anisotropic turbulent fields from large-scale magnetic fields. However, this has been challenging until recently because of the faint halo emission in linear polarization. Our sensitive VLA broadband observations in C-band and L-band of 35 spiral galaxies seen edge-on (called CHANG-ES) allowed us to perform rotation measure synthesis (RM synthesis) in their halos and to analyze the results. We further accomplished a stacking of the observed polarization maps of 28 CHANG-ES galaxies in C-band. Results. Though the stacked edge-on galaxies were of different Hubble type, and had differing star formation activity and interaction activity, the stacked image clearly reveals an X-shaped structure of the apparent magnetic field. We detected a large-scale (coherent) halo field in all 16 galaxies that have extended polarized emission in their halos. We detected large-scale field reversals in all of their halos. In six galaxies, these are along lines that are approximately perpendicular to the galactic midplane (vertical RMTL) with about 2 kpc separation. Only in NGC 3044 and possibly in NGC 3448 did we observe vertical giant magnetic ropes (GMR) similar to those detected recently in NGC 4631. Conclusions. The observed X-shaped structure of the halo field seems to be an underlying feature of spiral galaxies. It can be regarded as the two-dimensional projection of the regular magnetic field which we found to have scales of typically 1 kpc or larger observed over several kiloparsecs. The ordered magnetic field extends far out in the halo and beyond. We detected large-scale magnetic field reversals in the halo that may indicate that GMR are more or less tightly wound. With these discoveries, we hope to stimulate model simulations for the halo magnetic field that should also explain the determined asymmetry of the polarized intensity (PI).

Two-integral distribution functions in axisymmetric galaxies: Implications for dark matter searches

We address the problem of reconstructing the phase-space distribution function for an extended collisionless system, with known density profile and in equilibrium within an axisymmetric gravitational potential. Assuming that it depends on only two integrals of motion, namely the energy and the component of the angular momentum along the axis of symmetry $L_z$, there is a one-to-one correspondence between the density profile and the component of the distribution function that is even in $L_z$, as well as between the weighted azimuthal velocity profile and the odd component. This inversion procedure was originally proposed by Lynden-Bell and later refined in its numerical implementation by Hunter & Qian; after overcoming a technical difficulty, we apply it here for the first time in presence of a strongly flattened component, as a novel approach of extracting the phase-space distribution function for dark matter particles in the halo of spiral galaxies. We compare results obtained for realistic axisymmetric models to those in the spherical symmetric limit as assumed in previous analyses, showing the rather severe shortcomings in the latter. We then apply the scheme to the Milky Way and discuss the implications for the direct dark matter searches. In particular, we reinterpret the null results of the Xenon1T experiment for spin-(in)dependent interactions and make predictions for the annual modulation of the signal for a set of axisymmetric models, including a self-consistently defined co-rotating halo.

A fresh view of magnetic fields and cosmic ray electrons in halos of spiral galaxies

AbstractRecent numerical models of the multiphase ISM underline the importance of cosmic rays and magnetic fields for the physics of the ISM in disc galaxies. Observations of properties of the ISM in galactic halos constrain models of the expected exchange of matter between the star-forming disc and the environment (circumgalactic medium, CGM). We present new observational evidence from radio-continuum polarization studies of edge-on galaxies on magnetic field strength and structure as well as cosmic ray electron transport in galactic halos. The findings are discussed in the context of the disk-halo interaction of the interstellar medium. In addition, it is also briefly demonstrated how recent LOFAR observations of edge-on galaxies further constrain the extent of magnetic fields in galactic halos.

Astronomical Signatures of Dark Matter

Several independent astronomical observations in different wavelength bands reveal the existence of much larger quantities of matter than what we would deduce from assuming a solar mass to light ratio. They are very high velocities of individual galaxies within clusters of galaxies, higher than expected rotation rates of stars in the outer regions of galaxies, 21 cm line studies indicative of increasing mass to light ratios with radius in the halos of spiral galaxies, hot gaseous X-ray emitting halos around many elliptical galaxies, and clusters of galaxies requiring a much larger component of unseen mass for the hot gas to be bound. The level of gravitational attraction needed for the spatial distribution of galaxies to evolve from the small perturbations implied by the very slightly anisotropic cosmic microwave background radiation to its current web-like configuration requires much more mass than is observed across the entire electromagnetic spectrum. Distorted shapes of galaxies and other features created by gravitational lensing in the images of many astronomical objects require an amount of dark matter consistent with other estimates. The unambiguous detection of dark matter and more recently evidence for dark energy has positioned astronomy at the frontier of fundamental physics as it was in the 17th century.

The Multiwavelength Picture of the Disk-Halo Interaction

The emission properties of the components of the multiphase interstellar medium (ISM) require observations at very different wavelengths covering the range from X-rays for the hot phase to radio emission for the non-thermal component. This contribution gives an overview of the observational evidence for the presence of a multiphase ISM in halos of spiral galaxies. It is shown that the presence of halo gas correlates with the starformation rate in the disk. Recent radiocontinuum observations for NGC 253 are discussed with regard to the propagation of cosmic rays into the halos.

The Ghost of a Dwarf Galaxy: Fossils of the Hierarchical Formation of the Nearby Spiral Galaxy NGC 5907

We present an extragalactic perspective of an extended stellar tidal stream wrapping around the edge-on spiral galaxy NGC 5907. Our deep images reveal for the first time a large-scale complex of arcing loops that is an excellent example of how low-mass satellite accretion can produce an interwoven, rosette-like structure of debris dispersed in the halo of its host galaxy. The existence of this structure, which has probably formed and survived for several gigayears, confirms that halos of spiral galaxies in the local universe may still contain a significant number of galactic fossils from their hierarchical formation. To examine the validity of the external accretion scenario, we present N-body simulations of the tidal disruption of a dwarf galaxy-like system in a disk galaxy plus dark halo potential that demonstrate that most of the tidal features observed in NGC 5907 can be explained by a single accretion event. Unfortunately, with no kinematic data and only the projected geometry of the stream as a constraint, the parameters of our model are considerably degenerate and, for now, must be considered illustrative only. Interestingly, NGC 5907 has long been considered a prototypical example of a warped spiral in relative isolation. The presence of an extended tidal stream challenges this picture and suggests that the gravitational perturbations induced by the stream progenitor must be considered as a possible cause for the warp. The detection of an old, complex tidal stream in a nearby galaxy with rather modest instrumentation points to the viability of surveys to find extragalactic tidal substructures around spiral galaxies in the Local Volume (<15 Mpc), with the prospect of obtaining a census with enough statistical significance to be compared with cosmological simulations.

Rotation of Galaxy Dark Matter Halos

AbstractThe degree to which outer dark matter halos of spiral galaxies rotate with the disk is sensitive to their accretion history and may be probed with associated satellite galaxies. We use the Steward Observatory Bok telescope to measure the sense of rotation of nearby isolated spirals and combine these data with those of their associated satellites (drawn from SDSS) to directly test predictions from numerical simulations. We aim to constrain models of galaxy formation by measuring the projected component of the halo angular momentum that is aligned with that of spiral galaxy disks, Jz. We find the mean bulk rotation of the ensemble satellite system to be co-rotating with the disk with a velocity of 22 ± 13 km/s, in general agreement with previous observational studies and suggesting that galaxy disks could be formed by halo baryons collapsing by a factor of ≈10. We also find a prograde satellite fraction of 51% and Jz, of the satellite system to be positively correlated with the disk, albeit at low significance (2655 ± 2232 kpc km/s).

Halos of Spiral Galaxies. I. The Tip of the Red Giant Branch as a Distance Indicator

We have imaged the halo populations of a sample of nearby spiral galaxies using the Wide Field Planetary Camera 2 on broad the Hubble Space Telescope with the aim of studying the stellar population properties and relating them to those of the host galaxies. In four galaxies, the red-giant branch is sufficiently well populated to measure the magnitude of the tip of the red-giant branch (TRGB), a well-known distance indicator. Using both the Sobel edge-detection technique and maximum-likelihood analysis to measure the $I$-band magnitude of the red giant branch tip, we determine distances to four nearby galaxies: NGC 253, NGC 4244, and NGC 4945, NGC 4258. For the first three galaxies, the TRGB distance determined here is more direct, and likely to be more accurate, than previous distance estimates. In the case of NGC 4258, our TRGB distance is in good agreement with the the geometrical maser distance, supporting the the Large Magellanic Cloud distance modulus $(m-M)_0 = 18.50$ that is generally adopted in recent estimates of the Hubble constant.

Low-column density gas clumps in the halo of the Milky Way

We report on the detection of low-column density neutral hydrogen clumps in the halo of the Milky Way. Using high-resolution (FWHM ∼ 7 km s -1 ) optical spectra obtained with the VLT/UVES spectrograph we detect narrow interstellar absorption by Ca II and Na I at high radial velocities near u LSR -150 km s -1 toward the quasar PKS 1448-232 (/ = 335.4, b = +31.7). Follow-up Hi 21 cm observations with the VIA unveil the presence of a complex of small neutral hydrogen clumps with H 1 column densities <8 × 10 18 cm -2 . The measured H line widths imply that the gas is relatively cold with temperatures T ≤ 900 K. Although the line of sight towards PKS 1448-232 does not pass immediately through a known large high-velocity cloud (HVC). the sky position and the measured radial velocities suggest that these clumps arc associated with HVC cloud complex L. An inspection of other UVES quasar spectra shows that weak. narrow Ca II absorption at high velocities is a common phenomenon, even in directions where high-velocity H 1 21 cm emission is not detected. This suggests that the Milky Way halo contains a large number of high-velocity neutral gas clumps with low H I column densities. If such clumps are typical for halos of spiral galaxies, they should contribute significantly to the population of strong Mg II absorbers and Lyman-Limit Systems (LLS) seen in the circumgalactic environment of other galaxies.

Thick disks and halos of spiral galaxies M 81, NGC 55 and NGC 300

By using images from the HST/WFPC2/ACS archive, we have analyzed the spatial distribution of the AGB and RGB stars along the galactocentric radius of nearby spiral galaxies M~81, NGC 300 and NGC 55. Using color-magnitude diagrams and stellar luminosity functions, we gauge the stellar contents of the surroundings of three galaxies. The red giant population (RGB) identified at large galactocentric radii yields a distance of $3.85\pm0.08$ Mpc for M~81, $2.12\pm0.10$ Mpc for NGC 55, and $2.00\pm0.13$ Mpc for NGC 300, and a mean stellar metallicity of $-$0.65, $-$1.25, and $-$0.87. We find that there are two number density gradients of RGB stars along the radius, which correspond to the thick disk and halo components of the galaxies. We confirm the presence of metallicity gradient of evolved stars at these galaxies, based on the systematic changes of the color distribution of red giant stars. These results imply that thick disk might be a general feature of the spiral galaxies, and endorse a further investigation of the outer stellar edges of nearby spirals, which is critical in constraining the origin and evolution of galaxies.

Vector field and rotational curves in dark galactic halos

We study equations of a non-gauge vector field in a spherically symmetric static metric. The constant vector field with a scale arrangement of components, the temporal component about the Planck mass mPl and the radial component about M suppressed with respect to the Planck mass, serves as a source of metric reproducing flat rotation curves in dark halos of spiral galaxies, so that the velocity of rotation v0 is determined by the hierarchy of scales: , and M ∼ 1012 GeV. A natural estimate of Milgrom's acceleration about the Hubble rate is obtained.

Microlensing Candidates in M87 and the Virgo Cluster with theHubble Space Telescope

The position of the giant elliptical galaxy M87 at the center of the Virgo Cluster means that the inferred column density of dark matter associated with both the cluster halo and the galaxy halo is quite large. This system is thus an important laboratory for studying massive dark objects in elliptical galaxies and galaxy clusters by gravitational microlensing, strongly complementing the studies of spiral galaxy halos performed in the Local Group. We have performed a microlensing survey of M87 with the WFPC2 instrument on the Hubble Space Telescope. Over a period of 30 days, with images taken once daily, we discover seven variable sources. Four are variable stars of some sort, two are consistent with classical novae, and one exhibits an excellent microlensing light curve, although with a very blue color, implying the somewhat disfavored possibility of a horizontal-branch source being lensed. On the basis of sensitivity calculations from artificial stars and from artificial light curves, we estimate the expected microlensing rate. We find that the detection of one event is consistent with a dark halo with a 20% contribution of microlensing objects for both M87 and the Virgo Cluster, similar to the value found from observations in the Local Group. Further work is required to test the hypothesized microlensing component to the cluster.

High Velocity Gas in the Halos of Spiral Galaxies

Recent, high sensitivity, HI observations of nearby spiral galaxies show that their thin ‘cold’ disks are surrounded by thick layers (halos) of neutral gas with anomalous kinematics. We present results for three galaxies viewed at different inclination angles: NGC 891 (edge-on), NGC 2403 (i=60°), and NGC 6946 (almost face-on). These studies show the presence of halo gas up to distances of 10-15 kpc from the plane. Such gas has a mean rotation 25-50 km s−1 lower than that of the gas in the plane, and some complexes are detected at very high velocities, up to 200-300 km s−1. The nature and origin of this halo gas are poorly understood. It can either be the result of a galactic fountain or of accretion from the intergalactic medium. It is probably analogous to some of the High Velocity Clouds (HVCs) of the Milky Way.

The Origin of Angular Momentum in Dark Matter Halos

We propose a new explanation for the origin of angular momentum in galaxies and their dark halos, in which the halos obtain their spin through the cumulative acquisition of angular momentum from satellite accretion. In our model, the buildup of angular momentum is a random walk process associated with the mass assembly history of the halo's major progenitor. We assume no correlation between the angular momenta of accreted objects. The main role of tidal torques in this approach is to produce the random tangential velocities of merging satellites. Using the extended Press-Schechter approximation, we calculate the growth of mass, angular momentum, and spin parameter λ for many halos. Our random walk model reproduces the key features of the angular momentum of halos found in ΛCDM N-body simulations: a lognormal distribution in λ with an average of ≈ 0.045 and dispersion σλ = 0.56, independent of mass and redshift. The evolution of the spin parameter in individual halos in this model is quite different from the steady increase with time of angular momentum in the tidal torque picture. We find both in N-body simulations and in our random walk model that the value of λ changes significantly with time for a halo's major progenitor. It typically has a sharp increase due to major mergers and a steady decline during periods of gradual accretion of small satellites. The model predicts that, on average, the λ of ~1012 M☉ halos that had major mergers after redshift z = 3 should be substantially larger than the λ of those that did not. Perhaps surprisingly, this suggests that halos that host later forming elliptical galaxies should rotate faster than halos of spiral galaxies.

Bipolar hypershell models of the extended hot interstellar medium in spiral galaxies

We simulated the million degree interstellar medium and its soft X-ray images in the disk and halo of spiral galaxies using the bipolar hypershell (BHS) model. In this model dumbbell- or hourglass-shaped expanding shells of several kpc radii are produced by a sudden energy release in the central region. We then applied our model to a mini-sample of starburst galaxies seen under different inclinations, namely the nearly edge-on galaxies NGC 253, NGC 3079 and M 82, the highly inclined galaxies NGC 4258 and NGC 1808 as well as the nearly face-on galaxy M 83. For all galaxies, our results reproduce the X-ray characteristics observed in the 0.1-2.4 keV ROSAT energy band: the bipolar hypershell morphology, the spectral energy distribution of the diffuse disk and halo emission as well as absorption gaps in the diffuse X-ray emission caused by a shadowing of soft X-rays due to cold intervening gas in the disks of the galaxies. In general, the required total energy for the starburst is estimated to be of the order of 1055 ergs, corresponding to the overall kinetic energy generated in ~104 type-II supernova explosions. The expansion velocity of the shells is estimated to be ~200 km s-1 , which is necessary to heat the gas to ~0.2 keV (2.3 million K), and the age to be of the order of 3 107 years. In the case of the very nearby, nearly edge-on galaxy NGC 253 all characteristics of the BHS model can be studied with high spatial resolution. Using the property that the shell morphology is sensitive to the ambient density distribution, we propose using soft X-ray data to probe the gas distributions in the disk, halo and intergalactic space in general. The application of our model to images at higher spatial and spectral resolution, as provided by Chandra and XMM, will help us to further disentangle the ISM density distributions and will lead to a better understanding of the disk halo interface.

Extraplanar Dust in Spiral Galaxies: Observations and Implications

Recent optical and submillimeter observations have begun to probe the existence of dust grains in the halos of spiral galaxies. I review our own work in this area which employs high-resolution optical images of edge-on spiral galaxies to trace high-z dust in absorption against the background stellar light of the galaxies. We have found that a substantial fraction of such galaxies (>50%) show extensive webs of dust-bearing clouds to heights z>2 kpc. Extraplanar dust in galaxies is statistically correlated with extraplanar diffuse ionized gas, though there is no evidence for a direct, physical relationship between these two phases of the high-z interstellar medium. The dense high-z clouds individually have masses estimated to be >10^5} to 10^6 solar masses. The detailed properties of the observed dust structures suggest the clouds seen in our images may represent the dense phase of a multiphase ISM at high-z. Such dense clouds can have an important effect on the observed light distribution in spiral galaxies. I discuss the effects such high-z dust can have on quantitative measures of the vertical structure of stars and ionized gas in edge-on systems.

Do All Spiral Galaxies Have a Massive Dark Halo?

We have investigated the masses and extents of dark halos of spiral galaxies by two methods. First, we have reanalyzed HI outer rotation curves so far obtained considering the velocity uncertainties. We confirmed that many HI rotation curves indicate the existence of dark matter to some extent. However, we also found that only few rotation curves provide direct evidences for halos extended beyond 10 disk scale length. On the other hand, recent HI observations revealed that several galaxies have declining rotation curves that are approximated by Keplerian in the outermost regions. Considering the velocity uncertainties in the outer rotation curves, we found that these declining rotation curves are not rare compared to flat rotation curves. If these declining rotation curves indeed trace the mass truncation, these results indicate that some dark halos have moderate masses that are comparable or slightly larger than disk masses.