Galactic Parameters Research Articles

A study of a homogeneous sample of optically selected active galactic nuclei. 3: Optical observations

We present CCD observations in BVR optical bands of the homogeneous sample of 42 Seyfert 1 galaxies, that we also observed in near-IR JHK bands (Zitelli et al. 1993; Danese et al. 1992). We have applied to these data the same analysis procedure used for the infrared images, with the aim of separating the galaxian and nuclear fluxes, as well as investigating the main characteristics of the hosts. Nuclear fluxes have been estimated with typical global errors of 0.15 mag, while 0.3 mag errors are on average associated to host galaxy magnitudes. It is shown that in half of the host galaxies the bulge contribute more than 40% to the total observed fluxes even within small apertures. In the sample galaxies disks are dominating over the bulge in the large majority of the cases, suggesting that a significant fraction of Seyfert galaxies could be late-type spirals. Moreover the galactic morphological parameters are in the ranges of typical spiral galaxies. The statistics of galactic colors show that the host galaxies tend to be bluer than the normal ones. This fact added to redder colors found in the IR-bands strengthens the suggestion that in Seyfert galaxies the star formation is on average enhanced.

On the central surface brightness of galactic disks

Abstract In the framework of the Lin-Pringle model of the galactic disk evolution, the observed approximate constancy of the central surface brightness Io of an exponential disk is discussed. The disk is assumed to be embedded in a massive halo of the cold dark matter. We suppose that a protogalaxy acquired its angular momentum from tidal torques by interaction with surroundings at early stages of its formation. A self-similar solution for the stellar component evolution and the condition of total disk angular momentum conservation allow to estimate the disk central surface density (and luminosity) as a function of halo parameters. These parameters are determined by the amplitude and spectral index (n) of the density fluctuations from which the galaxy has been formed. It is shown that if n Io-2 at the galactic scale (CDM-model), I 0 is constant and independent of other galactic parameters. The predicted values of I 0 are in a good agreement with the observed ones if galaxies have been formed at rather high peaks of cosmological density fluctuations.

Galactic disk kinematics traced by low-altitude stars of 55 km/s mean transverse velocity within 500 PC from the sun

The Galactic disk velocity field in a 500 pc radius solar neighborhood is mapped on the basis of a set of 75,000 stars at an altitude lower than 300 pc above the Galactic plane and with a 55 km/s mean transverse velocity. An age range of 5 Gyr is inferred from the ellipsoidal distribution of the velocities in the Galactic plane. A circular motion with an expansion of 21 +/-12 km/s kpc was observed, suggesting a nonflat rotation cure, even when the young Gould's Belt stars are removed. A velocity gradient of 0.3 km/s pc delineates the shock front of large known clouds in the Pleiades, the Hyades, the Perseus, and the Scorpio-Ophiucus regions, suggesting that the Galactic differential rotation parameters describing stellar streaming cannot be segregated from the interstellar medium velocity. A transverse velocity dispersion increase of 3 km/s over a 50-pc cell size is noticeable in the Pleiades only. An 8 +/-4 km/s heating process of the local stellar disk during an encounter with dust clouds near an 18 +/-4 km/s transverse velocity is suggested.

The Cosmic Far Ultraviolet Background

Recent progress in understanding the cosmic far-UV background is reviewed. The correlation between the far-UV background and Galactic parameters is examined, and the dust-scattered Galactic radiation is considered as the primary source of the far-UV background. The spectrum of the far-UV background is discussed along with the cosmic background from 912 to 1200 A. The extragalactic background and related exotica are addressed.

Galactic structure parameters from radial velocities of young stars

Radial velocities of OB stars are utilised in order to determine the values of some galactic structure parameters in the solar neighbourhood.

A survey of proper motion stars. IX - The galactic halo's metallicity gradient

view Abstract Citations (66) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Survey of Proper Motion Stars. IX. The Galactic Halo's Metallicity Gradient Carney, Bruce W. ; Aguilar, Luis ; Latham, David W. ; Laird, John B. Abstract Using data already presented for a survey of proper motion stars and the Bahcall, Schmidt, and Soneira (1983) model of the Galaxy, Galactic orbital parameters are computed, including planar and three-dimensional eccentricities, apo- and perigalacticon distances, and maximum distances reached above/below the plane, based on extreme values for R and the absolute value of Z over 15 azimuthal periods. The orbital data are used to bin the survey's stars by apogalacticon and maximum Z distances. In an attempt to isolate a halo population sample, analyses are restricted to those stars that lag behind the local standard of the rest's circular orbital velocity by 50, 100, 150, and 200 km/s. The mean metallicities of the stars in a variety of Rapo and Zmax bins are compared . Publication: The Astronomical Journal Pub Date: January 1990 DOI: 10.1086/115320 Bibcode: 1990AJ.....99..201C Keywords: Halos; Metallic Stars; Milky Way Galaxy; Sky Surveys (Astronomy); Stellar Motions; Heavy Elements; Kinetic Energy; Stellar Mass; Stellar Orbits; Astrophysics; GALAXIES: THE GALAXY; STARS: POPULATION II; STARS: HIGH-VELOCITY; STARS: PROPER-MOTION full text sources ADS | data products SIMBAD (744)

A self-regulated star formation rate as a function of global galactic parameters

A sample of active dwarf galaxies and a sample of Sc and irregular galaxies are analyzed based on the assumption that the stellar formation rate is self-regulated in such a way that it maintains the critical pressure P(max) near the gas pressure P(g) of the interstellar medium, where P(max) is the pressure of the marginal state of stability for the transition warm gas to small clouds. The stellar formation rate Psi(PP) consistent with the condition P(max) = P(g), can be expressed as a simple analytical function of global galactic parameters. When it is assumed that the dust temperature, the gas metallicity, and the galactic diameter remain constant, it is found that Psi(PP) is reduced to the law of Schmidt with an exponent value of 1.43. Specifically, for the Galaxy, it is fonud that Psi(PP) = about 10 solar masses/yr, which is reasonably consistent with previous values. 17 refs.

Determining mass-to-light ratios in elliptical galaxies

If the endstate of cooling hot gas in elliptical galaxies is a population of optically dark, low-mass stars near the galactic cores, the mass-to-light ratio could be expected to vary significantly with projected radius. No strong variation in M/L is observed. To investigate the sensitivity and reliability of observational mass-to-light determinations for a variety of galactic parameters, model galaxies having de Vaucouleurs profiles (but with central cores and outer cutoffs), variable velocity ellipsoid structure, and extended dark halos are constructed. Spurious radial variations in M/L can occur when none are present if the properties of the galactic models are processed similar to observational data. Conversely, when a population of diffuse dark stellar matter is added near the galactic cores, large gradients in M/L can escape detection. However, the magnitude of the central velocity dispersion and its variation with projected radius within the effective radius both suggest that a component of dark stars is unlikely to be more massive than about 30 times the core mass of luminous stars. This restriction is important in establishing the initial mass function of stars in elliptical galaxies and the history of winds and cooling inflows in the interstellar medium. 35 references.

Self-Regulating Star Formation in Isolated Galaxies

AbstractRecent investigation (Parravano, 1987) shows that the diffuse phases of the ISM condense mainly by the transition from warm gas to small cool clouds (WG → SC). In this work we introduce the new hypothesis that the star formation rate (SFR) in isolated galaxies is self-regulated in such a way that it maintains Pmax close to the ISM gas pressure. Here Pmax is the gas pressure at the marginal state of stability for the transition WG → SC. This hypothesis leads to a relation between global galactic parameters which appears to be applicable to various morphological groups of isolated galaxies.

A model of static cosmic-ray-supported galactic coronae

The distributions of gas density and of cosmic-ray proton energy density are calculated for a static cosmic-ray-supported gaseous halo. The transport equation used for the cosmic rays includes convection at the Alfven speed, diffusion, and losses due to wave generation by the cosmic-ray streaming. The wave amplitudes are used to determine the diffusion coefficient used in the cosmic-ray transport equation. The requirement that the cosmic-ray energy flux out of the disk equals the energy production rate per unit area of the disk is not inconsistent with the gas number density at the halo base being about 0.001/cu cm, as required by photoionization models for the production of the C IV and Si IV features observed in the halo gas. The sensitivity of the halo density to the various local galactic parameters is discussed. The implications of recent gamma-ray observations for the model are considered. 39 references.

The significance of deep star counts for models of the Galaxy and the surface density of faint quasars

Models of the number–apparent magnitude distribution for galactic stars are constructed, and compared with recently available data. The uncertainties in both the galactic structure parameters and the star counts are such that several different parameter combinations can be fitted equally well to the data. However, highly flattened spheroids (axial ratio less than ∼ 0.5), and spheroids with a density decrease slower than about r−3 are difficult to reconcile with existing observations. For |$20\lesssim m_B\lesssim 24$|⁠, the number–colour diagram shows that almost all stars are either G type or M type, independent of apparent magnitude. These may be identified with disc M and spheroid G stars. Their distribution in the colour–apparent magnitude diagram is as expected from the forms of the disc and spheroid colour–absolute magnitude relations, and the differing stellar number density variations between the disc and spheroid components of the Galaxy. An upper limit of 200 ± 50 quasars per square degree with |$B-V\lesssim 0.35$| and |$20\leqslant m_B\leqslant 23$| is derived.

The universe at faint magnitudes. I - Models for the galaxy and the predicted star counts

A detailed model is constructed for the disk and spheroid components of the Galaxy from which the distribution of visible stars and mass in the Galaxy is calculated. The application of star counts to the determination of galactic structure parameters is demonstrated. The possibility of detecting a halo component with the aid of star counts is also investigated quantitatively.

Galactic spiral shocks - Vertical structure, thermal phase effects, and self-gravity

view Abstract Citations (28) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Galactic spiral shocks - Vertical structure, thermal phase effects, and self-gravity Tubbs, A. D. Abstract By use of two-dimensional, time-dependent, hydrodynamical calculations, the steady-state vertical gas structure of galactic shocks has been determined. A variety of energy densities of gas, magnetic field, and relativistic cosmic rays was assumed. The midplane shock structure is remarkably similar to that found in one-dimensional calculations, with no significant decrease in shock strength and no sudden vertical motion of the postshock gas. The shocks are straight and nearly perpendicular to the galactic plane for almost any realistic galactic parameters. Two-phase calculations, utilizing a time-dependent model of the heating and cooling of the interstellar medium, show no evidence for so-called accretion fronts. The clouds or high-density regions which form in these calculations are shown to form only in the midplane of the galaxy. Publication: The Astrophysical Journal Pub Date: August 1980 DOI: 10.1086/158174 Bibcode: 1980ApJ...239..882T Keywords: Density Wave Model; Gravitational Effects; Interstellar Gas; Shock Waves; Spiral Galaxies; Stellar Motions; Temperature Effects; Disk Galaxies; Galactic Structure; Gas Density; Gas Heating; Gas Temperature; Isothermal Processes; Vertical Distribution; Astrophysics full text sources ADS |

Energy-dependent diffusion of cosmic rays in the dynamical halo model

We use a simple Runge-Kutta method to obtain exact numerical results for the energy-dependent diffusion of galactic cosmic-ray nuclei in the dynamical halo model. A new observational limit on the galactic modulation parameter q=VD/K is obtained from the demodulated interplanetary proton observations. The numericals results are compared with the approximate analytic solutions of Jokipii and Jones.

Evidence for the deficiency of short cosmic ray pathlengths and a physically realistic explanation ? The no-near-sources model

From our recent observations of the charge and energy spectra of cosmic-ray nuclei we have constructed secondary-to-primary charge ratios at the two ends of the charge spectrum. These ratios are found to be inconsistent with thead hoc leaky-box model of cosmic-ray propagation which leads to an exponential pathlength distribution. Models for which the pathlength distribution function is deficient in short pathlengths provide a more consistent picture. Several of these models are investigated, bothad hoc and physical. The physical model considered here is one for which detailed galactic propagation parameters and boundary conditions are used and for which there exists no near sources of cosmic rays over a time interval corresponding to a few times the cosmic-ray age.

On the formation of spiral and elliptical galaxies

The key factor distinguishing an elliptical from a spiral galaxy is proposed to be the amount of gas left over at the point of maximum collapse of the protogalaxy. The gas left depends on the ratio of the star formation time scale tau/sub s/ to the collapse time of the protogalaxy tau/sub c/. Observed properties of ellipticals imply that they formed out of relatively larger density perturbations at recombination, giving them a relatively small tau/sub s//tau/sub c/ (if a rho/sup 2/ star formation law is adopted); and thus they are expected to essentially complete their star formation by the time of maximum collapse. Observations of spiral galaxies and our Galaxy imply that they formed out of less dense density perturbations giving them a larger tau/sub s//tau/sub c/. In the latter, star formation is not nearly completed by the time of maximum collapse. A detailed numerical model including both stars and gas was used to follow the collapse of a spiral galaxy. The gas and stars collapse together until they hit the plane, whereupon the gas forms a disk and the stars relax to form a spheroidal halo. The competing effects of dissipation and turbulent viscosity are analyzed using a modelmore » of cloud formation in the early protogalaxy. The gas disk does not hit the plane at its equilibrium position, and both radial and nonaxisymmetric oscillations of the disk are expected before equilibrium is reached. ''Zeroth'' order Maclaurin spheroid models for equilibrium spiral and elliptical galaxies are presented, which relate many useful galactic parameters to the original size and angular momentum. Monte Carlo calculations suggest that the tidal interaction picture of Peebles is capable of explaining the angular momentum of our Galaxy and the observed distribution of ellipticities in elliptical galaxies. This also explains why ellipticals are found preferentially in large clusters: because protogalaxies arising from larger initial density fluctuations cluster more easily. (AIP)« less

Cosmic Ray Streaming--III SELF-CONSISTENT SOLUTIONS

Self-consistent solutions are presented, in which cosmic rays are confined by Alfvén waves which are dissipated through a cascade involving sound waves. There are solutions in which Fermi acceleration of particles by the Alfvén waves can reduce or even invert the adiabatic energy losses associated with motion down density gradients. For galactic parameters, such acceleration does not occur, but a very high degree of confinement is predicted, with particles of energy less than some tens of GeV being restricted to stream less quickly than |$\sim 50\,\text{km s}^{-1}$| (twice the Alfvén speed).

Local Density Gradient and Galactic Parameters Determined from Nearby Stars

Local Density Gradient and Galactic Parameters Determined from Nearby Stars

Empirical models of cosmic ray propagation in the Galaxy

The homogeneous and flat diffusion models of the propagation of cosmic rays through the Galaxy are considered. It is shown that, with the usual choice of galactic parameters, in particular, those pertaining to its gas distribution, and subject to the restriction of not too heavy nuclei (nuclei no heavier than iron), both models are equivalent from the point of view of the description of the element composition of cosmic rays. This conclusion is justifiable both for galactic and metagalactic theories of the occurrence of cosmic rays. A connection is established between the effective path length of the matter which nuclei encounter in the Galaxy and the parameters of the diffusion model with an inhomogeneous distribution of an interstellar gas. Initial results allow the interpretation of a relationship of cosmic ray element composition to their energies. Restrictions which must be placed on such a relationship are shown from the different data on cosmic rays at the Earth.

The Third and Fourth Moments of the Local Stellar Velocity Distribution

AbstractThe velocity moments through the fourth order were calculated for a subset of 870 astrophysically selected stars from Gliese’s catalog. An essential part of this program was the determination of the covariances of estimates of the moments; both sampling and observational error contributions were taken into account in evaluating the total covariance matrix. In applications to the study of galactic structure, the Gliese stars are regarded as tracers of the galactic potential. The moments found are consistent with conventional assumptions about symmetries of the galaxy. Certain galactic parameters, such as the curvature in the law of rotation and the asymmetric drift, were studied with the help of the values obtained for the moments.