Halos Of Spiral Galaxies Research Articles

On the generalized Schuster density law

A special case of the generalized Schuster density law for stellar systems with spherical symmetry is discussed; here the exponent in the denominator is equal to i/2 where i is a positive integer. Special attention is paid to the situation 2 ? i ? 5 since then the mass distributions in almost all approximately spherical stellar systems and subsystems known to exist - e. g. dark coronae of galaxies, bulges and halos of spiral galaxies, as well as the systems with the classical Schuster density law - are included. With certain improvements one can also obtain more ample variants including the density continuously attaining zero at a finite radius, somewhat different descriptions of the mass distribution, as well as generalizations towards axial symmetry. It is shown among others, that a spheroid with this mass distribution (i=4) yields the same total mass as the exponential disc and that the mass distribution proposed by King belongs asymptotically to the generalized Schuster density law (i=3).

Diffuse Ionized Gas in Halos of Spiral Galaxies

AbstractOver the last couple of years Diffuse Ionized Gas (DIG) has been identified as an important constituent of the interstellar medium (ISM) in the halos of spiral galaxies. Imaging in and spectroscopy of optical emission lines allow us to study the distribution and excitation of this gas with a spatial resolution not achievable for other phases of the ISM in external galaxies. Its origin and ionization is under debate and give important constraints for models of the ISM in general and on the large scale exchange of matter between disk and halo in particular. This review summarizes more recent observational results and compares them with model predictions. The data available now demonstrate that the presence of DIG in the disk-halo interface of spiral galaxies is related to star formation processes in the underlying disk. While photoionization by OB stars in the disk seems a viable source for the power required to ionize the DIG, additional processes are needed to explain some of the spectral features. The observed correlation with properties of the non-thermal radio continuum indicate that magnetic fields and cosmic rays could play a role for the physics of this medium.

Topological pumping of magnetic fields by galactic fountains

Abstract Topological pumping of the large-scale galactic magnetic field by the galactic fountain flow can be an important effect responsible for μG-strength magnetic fields in gaseous halos of spiral galaxies. The physical nature of this effect is discussed in application to spiral galaxies. In particular, we consider how topological pumping affects galactic dynamos and discuss specific properties of the large-scale galactic magnetic fields embedded in a turbulent flow of interstellar gas.

GALACTIC MAGNETISM: Recent Developments and Perspectives

▪ Abstract We discuss current observational and theoretical knowledge of magnetic fields, especially the large-scale structure in the disks and halos of spiral galaxies. Among other topics, we consider the enhancement of global magnetic fields in the interarm regions, magnetic spiral arms, and representations as superpositions of azimuthal modes, emphasizing a number of unresolved questions. It is argued that a turbulent hydromagnetic dynamo of some kind and an inverse cascade of magnetic energy gives the most plausible explanation for the regular galactic magnetic fields. Primordial theory is found to be unsatisfactory, and fields of cosmological origin may not even be able to provide a seed field for a dynamo. Although dynamo theory has its own problems, the general form of the dynamo equations appears quite robust. Finally, detailed models of magnetic field generation in galaxies, allowing for factors such as spiral structure, starbursts, galactic winds, and fountains, are discussed and confronted with observations.

The Highly Flattened Dark Matter Halo of NGC 4244

In a previous paper (Olling 1995, \aj, 110, 591; astro-ph/9505002) a method was developed to determine the shapes of dark matter halos of spiral galaxies from an accurate determination of the rotation curve, the flaring of the gas layer and the velocity dispersion in the HI. Here this method is applied to the almost edge-on Scd galaxy NGC 4244 for which the necessary parameters are determined in the accompanying paper (AJ, Aug. 1996; astro-ph/9605110). The observed flaring of the HI beyond the optical disk puts significant constraints on the shape of the dark matter halo, which are almost independent of the stellar mass-to-light ratio. NGC 4244's dark matter halo is found to be highly flattened with a shortest-to-longest axis ratio of 0.2 (-0.1)(+0.3). If the dark matter is disk-like, the data presented in this paper imply that the vertical velocity dispersion of the dark matter must be 10% - 30% larger than the measured tangential dispersion in the HI. Alternatively, the measured flaring curve is consistent with a round halo if the gaseous velocity dispersion ellipsoid is anisotropic. In that case the vertical dispersion of the gas is 50 - 70% of the measured tangential velocity dispersion.

Can the Tully-Fisher Relation Be the Result of Initial Conditions?

We use Monte Carlo realizations of halo formation histories and a spherical accretion model to calculate the expected scatter in the velocity dispersions of galactic halos of a given mass due to differences in their formation times. Assuming that the rotational velocity of a spiral galaxy is determined by the velocity dispersion of its halo and that its luminosity is related to its total baryonic mass, this scatter translates to a minimum intrinsic scatter in the Tully-Fisher relation. For popular cosmological models we find that the scatter due to variations in formation histories is by itself greater than allowed by observations. Unless halos of spiral galaxies formed at high redshift (z>1) and did not later accrete any significant amount of mass, the Tully-Fisher relation is not likely to be the direct result of cosmological initial conditions but rather a consequence of a subsequent feedback process.

On the Usage of Flaring Gas Layers to Determine the Shape of Dark Matter Halos

I present a new method of deriving the shape of the dark matter (DM) halos of spiral galaxies. The method relies on the comparison of model predictions with high spectral and spatial resolution HI observations of the gas layer. The potential arising from the {\em total} mass distribution of the galaxy is used in the calculation of the vertical distribution of the gas. I developed a new algorithm to calculate the force field of an arbitrary, azimuthally symmetric, density distribution. This algorithm is used to calculate the forces due to the radially truncated stellar disk as well as of the flaring gas layer. I use a simple two-parameter family of disk-halo models which have essentially the same observed equatorial rotation curve but different vertical forces. This mass model is composed of a stellar disk with constant M/L, and a DM-halo with a given axial ratio. I approximate the radial force due to the gaseous disk, and iteratively determine the vertical force due to the global distribution of the gas. The thickness of the gaseous disk is sensitive to both the flattening of the DM-halo and the self-gravity of the gas, but not to the particular choice of disk-halo decomposition. I show that the determination of the thickness of the gas layer is not restricted to edge-on galaxies, but can be measured for moderately inclined systems as well.

Satellites as Probes of the Halos of Spiral Galaxies

Satellites as Probes of the Halos of Spiral Galaxies

Brown dwarfs, slow accretion, deuterium and nuclear flashes

We are celebrating Willy Fowler and Nuclear Astrophysics, but I want to discuss a topic where the onset of nuclear reactions marks failure andnot success*). The outer halos of spiral galaxies like ours contain a lot of dark matter which may or may not be baryonic. If it is, one thing that is excluded (because the luminosity would be too great) is ordinary stars with mass M large enough to burn hydrogen, yet small enough (M 0.08 M® and yet unable to burn hydrogen so that they are cool enough to make up the halo dark matter. However, ifM grows sufficiently the hydrogen starts burning in a flash and continues to burn and the star has too small a mass to light ratio to be a good dark matter candidate. Just what this critical value of Mmin is depends, indirectly, on nuclear reactions of deuterium and that reminds me of Willy Fowler.

A measurement of Omega(0) from the internal dynamics of spiral galaxies

view Abstract Citations (3) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Measurement of Omega 0 from the Internal Dynamics of Spiral Galaxies Salucci, Paolo ; Persic, Massimo ; Borgani, Stefano Abstract The mean mass density of the universe is a primary target of observational cosmology. We devise a method to estimate this quantity by extending to spiral disks the argument that the peculiar motions of galaxies are generated by the overdensities of matter related to galaxy clustering. We obtain the density excess associated with the dark halos of spiral galaxies by considering, rather than the motion of a companion galaxy bound in an unknown orbit (as in earlier work of Davis & Peebles), the motion of test bodies rotating in the disk plane of spiral galaxies. This is done by means of a proper visible/dark mass decomposition of galaxy rotation curves. On scales R ~ 3-50 kpc (for a Hubble constant value of 50 km s^-1^ Mpc^-1^), we find that the mass excess around galaxies scales with R like the excess in number of spiral galaxies. On the assumption that light traces mass, we estimate the value of the cosmological density parameter to be {OMEGA}_0_ = 0.35 +/- 0.15. If the universe is flat, galaxy clustering is enhanced over matter clustering by a biasing factor of about 3. Publication: The Astrophysical Journal Pub Date: March 1993 DOI: 10.1086/172378 Bibcode: 1993ApJ...405..459S Keywords: Cosmology; Density Measurement; Galactic Clusters; Mass Distribution; Spiral Galaxies; Dark Matter; Galactic Mass; Galactic Rotation; Radial Velocity; Astrophysics; COSMOLOGY: OBSERVATIONS; GALAXIES: CLUSTERING; GALAXIES: KINEMATICS AND DYNAMICS; GALAXIES: SPIRAL full text sources ADS |

DARK MATTER WITH VARIABLE MASSES

String effective theories contain a dilaton scalar field which couples to gravity, matter and radiation. In general, particle masses will have different dilaton couplings. We can always choose a conformal frame in which baryons have constant masses while (nonbaryonic) dark matter have variable masses, in the context of a scalar-tensor gravity theory. We are interested in the phenomenology of this scenario. Dark matter with variable masses could have a measurable effect on the dynamical motion of the halo of spiral galaxies, which may affect cold dark matter models of galaxy formation. As a consequence of variable masses, the energy-momentum tensor is not conserved; there is a dissipative effect, due to the dilaton coupling, associated with a “dark entropy” production. In particular, if axions had variable masses they could be diluted away, thus opening the “axion window.” Assuming that dark matter with variable masses dominates the cosmological evolution during the matter era, it will affect the primordial nucleosynthesis predictions on the abundances of light elements. Furthermore, the dilaton also couples to radiation in the form of a variable gauge coupling. Experimental bounds will constrain the parameters of this model.

THE MASS OF SPIRAL GALAXY HALOS

ABSTRACT A brief discussion of previous and current work on the determination of the mass distribution of spiral galaxy halos is presented.

Scalar-tensor theories of gravity with Phi -dependent masses

The authors study new physical phenomena and constraints in generalized scalar-tensor theories of gravity with Phi -dependent masses. They investigate a scenario (which can arise in string theories) with two types of Phi -dependent masses which could correspond to visible and dark matter sectors. The parameters of this theory are constrained from post-Newtonian bounds, primordial nucleosynthesis and the age of the universe. They present a perfect fluid formalism for the dark matter sector with variable masses and find an entropy increase effect during the matter era and, in principle, a measurable effect on the motion of the halo of spiral galaxies. For the case of string effective theories, the constancy of gauge couplings provides new bounds which are orders of magnitude stronger than the previous ones.

Elongated disks and the scatter in the Tully-Fisher relation

It is often speculated that the dark halos of spiral galaxies are triaxial or prolate, with the long axis pointing in the plane of the disk. This would produce a potential that is elongated in the plane of the disk. The signatures produced by such deviations from axisymmetry are explored. Observable effects in the photometry and kinematics of the stars and gas in the disk are found, if substructure like bars and spiral arms can be ignored. These effects include ellipticity gradients and position angle twists in disks, velocity gradients along the apparent minor axis of the disk, differences between kinematically determined inclinations, and systematic residuals in circular orbits fits to the velocity field

Evolution of dust grains through a hot gaseous halo

The evolution of bare spherical dust grains in the halo of spiral galaxies is analyzed. Two different grain compounds, graphites and astronomical silicates, are considered. The detailed mass and luminosity distributions for the Milky Way and NGC 3198 (considered as representative of the Sb and Sc types, respectively) are used to evaluate the range of possible evolutionary tracks. Aside from radiative and gravitational forces, the effects of drag and sputtering from a gaseous halo are included. A simple isothermal and hydrostatic density structure, with temperatures in the range 3 × 10 5 -10 6 K, has been used for this gaseous halo

Magnetic Fields in Disks and Halos of Spiral Galaxies

Spiral galaxies host interstellar magnetic fields of 4-15 μG total strength. A significant fraction of the field lines shows large-scale structures. At face-on or moderately inclined view, the field lines run generally parallel to the spiral arms, either with uniform direction with respect to azimuthal angle (axisymmetric spiral, ASS), with one reversal along azimuthal angle (bisymmetric spiral, BSS), or with spiral orientation without dominating direction.At edge-on view, the field is concentrated in a thin disk, often surrounded by a thick radio disk with field lines mostly parallel to the plane, similar to the quadrupole-type dynamo field. Radio polarization data from NGC891 indicate that the thermal gas seen in Hα is responsible for Faraday depolarization. The required scaleheight of the field of ~4 kpc is comparable to the value expected in case of energy equipartition between magnetic fields and cosmic rays. The interacting edge-on galaxy NGC 4631 shows a much larger radio halo with field lines perpendicular to the disk, possibly driven by a strong galactic wind or the result of a dipole-type halo field.Field lines bending out of the plane are also visible in face-on galaxies as regions with high rotation measures and low star-formation activity. The resemblance to the phenomenon of the solar corona suggests to call them “galactic coronal holes”.

Magnetic Fields in the Disk-Halo Interface

A review is given of large-scale magnetic fields in disks and halos of spiral galaxies. A particular attention is given to vertical field structures, and we discuss their origin and implication on their interaction with halo gas. We point out that the disk-halo magnetic interface plays an important role in circulation of interstellar gas in galaxies, in particular a large-scale circulartion from the galactic center to outer disk regions.

Can galactic halos be made of baryons?

Several arguments are presented indicating that the apparently non-luminous matter forming massive halos of spiral galaxies is not baryonic. There are difficulties with a halo dominated by gas, snowballs, dust and rocks, jupiters, low mass stars, dead stars and neutron stars. Also, halos may not be composed of black holes unless they are either extremely efficiently accreting or primordial. Consequently, it appears that a significant fraction of the universe may be in the form of massive neutrinos, gravitinos, monopoles, etc.

The role of magnetic fields in extragalactic astronomy

The evidence is reviewed for a universal magnetic field of strength 10−9–10−8G; it has been extended to include the diffuse fields of galactic clusters and the extensive magnetic halos of spiral galaxies. Some likely effects of the universal fieldB 0 are as follows: (1) As suggested previously,B 0 is coupled to protogalaxies and evolves into magnetic structures which depend on the angle between the field and the gas rotational axis. These provide the blueprints for the various types of the Hubble sequence, (ii) The relatively few ‘grand-design’ spiral galaxies result from tidal interactioon (M51-type), but ‘normal’ spirals form as a result of the spiral oblique field) magnetic blueprint acting on sheared gravitational instabilities (Goldreich and Lynden-Bell). (iii) The model explains the prevalent warped galactic disks and perhaps their flat H1 rotation curves. (iv) A variety of puzzling H1 concentrations may have hydromagnetic explanations; they include the high-velocity clouds, streamers, rings and central systems. (v) Clusters of galaxies are known to have diffuse magnetic fields, and these are likely to explain the absence of spiral galaxies and the nature of the intracluster gas. (vi) Spiral galaxies are now known to have extensive magnetic halos. These appear explicable only in terms of the universal magnetic field model.

Halos of spiral galaxies - Photometry and mass-to-light ratios

view Abstract Citations (57) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Halos of spiral galaxies - Photometry and mass-to-light ratios Spinrad, H. ; Ostriker, J. P. ; Stone, R. P. S. ; Chiu, L. -T. G. ; Bruzual A., G. Abstract Photographic and photoelectric photometry of three representative edge-on spiral galaxies, NGC 4594 (Sa), NGC 4565 (Sb), and NGC 253 (Sc), shows that each has an extensive faint spheroidal component qualitatively similar to that of an elliptical galaxy. However, their exterior surface brightnesses decline more rapidly, as power laws, with the inverse 2.5 or 3rd power of distance, and the ellipticity tends to decrease with increasing radius. The total spheroidal luminosity is comparable to or greater than the disk luminosity in all cases. Radio and optical rotation curves for two of the galaxies (NGC 4594 and 4565) indicate that the local mass-to-light ratio increases sharply with radius from the square root of 10 in the central regions to 10 to the 2.5 power to 1000 for NGC 4565 at 50 kpc. The integral mass-to-light ratios are probably less than about 0.01. Color information is inconclusive; no clear evidence for a population variation with distance has been found other than the well-known trend toward bluer B-V color due to decreasing metallicity with radius. In the Sc galaxy NGC 253 one can say that any color gradient is small to a distance of about 10 kpc. Most of the mass appears to reside in a component having a very large value of local M/L ratio and, at present, a totally unknown nature. Publication: The Astrophysical Journal Pub Date: October 1978 DOI: 10.1086/156467 Bibcode: 1978ApJ...225...56S Keywords: Astronomical Photometry; Galactic Structure; Halos; Mass To Light Ratios; Spiral Galaxies; Astronomical Photography; Brightness; Electrophotometry; Elliptical Galaxies; Galactic Rotation; Hubble Diagram; Isophotes; Astrophysics full text sources ADS | data products NED (6)