Galactic System Research Articles

Division VII: Galactic System: (Systeme Galactique)

The scientific interests of Division VII are concerned with the Galactic system. Its two constituent commissions are Commission 33 (Structure and Dynamics of the Galactic System) and Commission 37 (Star Clusters and Associations). Please refer to the reports of these two commissions for the scientific highlights and lists of major conferences in this area.Study of the Galactic system involves a wide range of astrophysics and astronomical techniques, and the fields covered by many other commissions are therefore of very significant interest to Division VII. In particular, astronomers with interests in the Galactic system could consider reading also the reports of Commissions 5, 7, 24-30, 34-36, 40, 44 and 45, among others.

ASCA results on SS 433: a galactic astrophysical jet system

The ASCA observations of SS 433 revealed two important properties of SS 433. The X-ray emission from the central point source exhibited multi-temperature thermal spectra which have two velocity components corresponding to the two anti-parallel jets outflowing at 0.26 c. The detailed analysis of the spectra revealed that the kinetic luminosity of SS 433 far exceeds the Eddington luminosity for any stellar size objects. The X-ray lobes, which are distributed at a very large scale (tens of parsecs) were found to have non-thermal spectra, suggesting presence of acceleration of energetic particles. There are still essential problems awaiting to be addressed. For example, identification of the compact object underlying the jets requires sensitive and detailed spectroscopy which will be made possible with ASTRO-E.

ASCA results on SS 433: a galactic astrophysical jet system

ASCA results on SS 433: a galactic astrophysical jet system

Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy

HIRES on the Keck I telescope has been used to measure the first radial velocities for stars belonging to eleven, heavily-reddened globular clusters in the direction of the inner Galaxy. The question of kinematic substructuring among the Galactic globular cluster system is investigated using an updated catalog of globular cluster distances, metallicities and velocities. It is found that the population of metal-rich globular clusters shows significant rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc of the Galactic center, the measured rotation velocity and line-of-sight velocity dispersion are similar to those of bulge field stars. We investigate claims that the metal-rich clusters are associated with the central Galactic bar by comparing the kinematics of the innermost clusters to that of the atomic hydrogen in the inner Galaxy. The longitude-velocity diagram of both metal-rich and metal-poor clusters bears a remarkable similarity to that of the gas, including the same non-circular motions which have traditionally been interpreted as evidence for a Galactic bar, or, alternatively, a non-axisymmetric bulge. However, uncertainties in the existing three-dimensional Galactocentric positions for most of the clusters do not yet allow an unambiguous discrimination between the competing scenarios of membership in a rigidly rotating bar, or in a bulge which is an oblate isotropic rotator. We conclude that the majority of metal-rich clusters within the central 4 kpc of the Galaxy are probably associated with the bulge/bar, and not the thick disk. (ABRIDGED)

ITAL]HUBBLE SPACE TELESCOPE[/ITAL] Imaging of Globular Clusters in the Edge-on Spiral Galaxies NGC 4565 and NGC 5907

We present a study of the globular cluster systems of two edge-on spiral galaxies, NGC 4565 and NGC 5907, from WFPC2 images in the F450W and F814W filters. The globular cluster systems of both galaxies appear to be similar to the Galactic globular cluster system. In particular, we derive total numbers of globular clusters of NGC(4565) = 204 ± 38 and NGC(5907) = 170 ± 41 (where the first are statistical, the second potential systematic errors) for NGC 4565 and NGC 5907, respectively. This determination is based on a comparison with the Milky Way system, for which we adopt a total number of globular clusters of 180 ± 20. The specific frequency of both galaxies is SN 0.6, indistinguishable from the value for the Milky Way. The similarity in the globular cluster systems of the two galaxies is noteworthy, since they have significantly different thick disks and bulge-to-disk ratios. This would suggest that these two components do not play a major role in the building up of a globular cluster system around late-type galaxies.

Investigating the Metal Line Systems at [CLC][ITAL]z[/ITAL][/CLC] = 1.9 toward J2233−606 in the Hubble Deep Field South

The combination of Space Telescope Imaging Spectrograph (STIS) and optical spectroscopy with STIS imaging of the field surrounding J2233-606 affords a unique opportunity to study the physical nature of the quasar absorption-line systems. We present an analysis of the ionization state, chemical abundances, and kinematic characteristics of two metal line systems at z = 1.92 and 1.94 toward J2233-606. We focus on these two systems because (1) the observations provide full coverage of the Lyman series, and hence an accurate determination of their H I column densities, and (2) they exhibit many metal line transitions that allow for a measurement of their ionization state and chemical abundances. Line profile fits of the Lyman series for the two systems indicate log N(H I) = 17.15 ± 0.02 cm-2 evenly distributed between two components for the z = 1.92 system and log N(H I) = 16.33 ± 0.04 cm-2 for the z = 1.94 system. By comparing observed ionic ratios of C and Si against calculations performed with the CLOUDY software package, we find the ionization state is high and well constrained in both systems. Applying ionization corrections to the measured ionic column densities, we determine that the systems exhibit significantly different metallicities: ≈1/50 solar and less than 1/200 solar for the two components of z = 1.92 and ≈40% solar at z = 1.94. The most significant uncertainties regarding these measurements are associated with the assumptions inherent to the CLOUDY simulations. The properties of the z = 1.92 system are consistent with a low-metallicity galaxy (e.g., a dwarf galaxy) as well as absorption by large-scale structure of the intergalactic medium. On the other hand, the high metallicity of the z = 1.94 system suggests significant star formation and is therefore assumed associated with a galactic system. Furthermore, we predict it is the more likely to be observed with the STIS imaging and follow-up observations. If a galaxy is identified, our results provide direct measurements on the properties of the interstellar medium for a z ≈ 2 galaxy.

Deep [ITAL]Hubble Space Telescope[/ITAL] Observations of Blue Star Clusters in NGC 3597

We have analyzed HST/WFPC2 images of NGC 3597 and find ~700 compact objects surrounding the galaxy with an average (B-R)_0~0.6. We propose that the majority of these objects are young globular clusters. They have a spread in colors that is consistent with that expected for a population of young clusters with a common age and spread induced by photometric errors and reddening within NGC 3597. If these objects were similar to the Galactic globular cluster system seen at a younger age, we would predict a turnover in the luminosity function at B~23. However, we find that the luminosity function for the clusters is increasing to the limits of our photometry (B~27).

The Modified Newtonian Dynamics Predicts an Absolute Maximum to the Acceleration Produced by “Dark Halos”

We have recently discovered that the modified Newtonian dynamics (MOND) implies some universal upper bound on the acceleration that can be produced by a "dark halo," which, in a Newtonian analysis, is assumed to account for the effects of MOND. Not surprisingly, the limit is on the order of the acceleration constant of the theory. This can be contrasted directly with the results of structure-formation simulations. The new limit is substantial and different from earlier MOND acceleration limits (discussed in connection with the MOND explanation of the Freeman law for galaxy disks and the Fish law for elliptical galaxies): it pertains to the "halo" and not to the observed galaxy; it is absolute and independent of further physical assumptions on the nature of the galactic system; and it applies at all radii, whereas the other limits apply only to the mean acceleration in the system.

Substructure in Dark Halos: Orbital Eccentricities and Dynamical Friction

The virialized regions of galaxies and clusters contain significant amounts of substructure; clusters have hundreds to thousands of galaxies, and satellite systems and globular clusters orbit the halos of individual galaxies. These orbits can decay owing to dynamical friction. Depending on their orbits and their masses, the substructures either merge, are disrupted, or survive to the present day. We examine the distributions of eccentricities of orbits within mass distributions similar to those we see for galaxies and clusters. A comprehensive understanding of these orbital properties is essential to calculate the rates of physical processes relevant to the formation and evolution of galaxies and clusters. We derive the orbital eccentricity distributions for a number of spherical potentials. These distributions depend strongly on the velocity anisotropy, but only slightly on the shape of the potential. The eccentricity distributions in the case of an isotropic distribution function are strongly skewed toward high eccentricities, with a median value of typically ~0.6, corresponding to an apocenter-to-pericenter ratio of 4.0. We also present high-resolution N-body simulations of the orbital decay of satellite systems on eccentric orbits in an isothermal halo. The dynamical friction timescales are found to decrease with increasing orbital eccentricity because of the dominating deceleration at the orbit's pericenter. The orbital eccentricity stays remarkably constant throughout the decay; although the eccentricity decreases near pericenter, it increases again near apocenter, such that there is no net circularization. We briefly discuss several applications for our derived distributions of orbital eccentricities and the resulting decay rates from dynamical friction. We compare the theoretical eccentricity distributions to those of globular clusters and galactic satellites for which all six phase-space coordinates (and therewith their orbits) have been determined. We find that the globular clusters are consistent with a close-to-isotropic velocity distribution, and they show large orbital eccentricities because of this (not in spite of this, as has been previously asserted). In addition, we find that the limited data on the Galactic system of satellites appears to be different and warrants further investigation as a clue to the formation and evolution of our Milky Way and its halo substructure.

Evolution of the mass function of the Galactic globular cluster system

In this work we investigate the evolution of the mass function of the Galactic globular cluster system (GCMF) taking into account the effects of stellar evolution, two‐body relaxation, disc shocking and dynamical friction on the evolution of individual globular clusters. We have adopted a lognormal initial GCMF and considered a wide range of initial values for the dispersion, σ, and the mean value, 〈log M〉. We have studied in detail the dependence on the initial conditions of the final values of σ, 〈log M〉, the fraction of the initial number of clusters surviving after one Hubble time and the difference between the properties of the GCMF of clusters closer to the Galactic Centre and those of clusters located in the outer regions of the Galaxy. In most of the cases considered, evolutionary processes alter significantly the initial population of globular clusters and the disruption of a significant number of globular clusters leads to a flattening in the spatial distribution of clusters in the central regions of the Galaxy. The initial lognormal shape of the GCMF is preserved in most cases and if a power‐law in M is adopted for the initial GCMF, evolutionary processes tend to modify it into a lognormal GCMF. The difference between initial and final values of σ and 〈log M〉 as well as the difference between the final values of these parameters for inner and outer clusters can be positive or negative depending on initial conditions. A significant effect of evolutionary processes does not necessarily give rise to a strong trend of 〈log M〉 with the galactocentric distance. The existence of a particular initial GCMF able to keep its initial shape and parameters unaltered during the entire evolution through a subtle balance between disruption of clusters and evolution of the masses of those which survive, suggested by Vesperini, is confirmed.

The Ages and Abundances of a Large Sample of M87 Globular Clusters

A subset of 150 globular clusters (GCs) in M87 has been selected for abundance and age determinations from the 1997 sample of Cohen & Ryzhov. This has been done solely on the basis of the signal-to-noise ratios of the spectra. Indices that measure the strength of the strongest spectral features were determined for the M87 GCs and from new data for twelve galactic GCs. Combining the new and existing data for the galactic GCs and comparing the (U-R) colors and the line indices gave qualitative indications for the ages and abundances of the M87 GC system. Quantitative results, which confirm and extend the qualitative ones, were obtained by applying the Worthey models for the integrated light of stellar systems of a single age, calibrated by observations of galactic globular clusters, to deduce abundances and ages for the objects in our sample. We find that the M87 GCs span a wide range in metallicity, from very metal-poor to somewhat above solar metallicity. The mean [Fe/H] of -0.95 dex is higher than that of the galactic GC system, and there is a metal-rich tail that reaches to higher [Fe/H] than one finds among the galactic GCs. Excluding the very metal-rich tail, there is marginal evidence for a bimodal distribution over the single one at the 89% significance level. The two subpopulations in this model are located at -1.3 and -0.7 dex and contain 40% and 60% of the total, respectively. The dispersion in [Fe/H] for each of the model subpopulations is σ = 0.3 dex. The mean metallicity of the M87 GC system is about a factor of 4 lower than that of the M87 stellar halo at a fixed projected radius R. The metallicity inferred from the X-ray studies is similar to that of the M87 stellar halo, not to that of the M87 GC system. We infer the relative abundances of Na, Mg, and Fe in the M87 GCs from the strength of their spectral features. The behavior of these elements between the metal-rich and metal-poor M87 GCs is similar to that shown by the galactic GCs and by halo stars in the Galaxy. The pattern of chemical evolution in these disparate old stellar systems is, as far as we can tell, identical. Superposed on a very large dispersion in abundance at all R, there is a small but real radial gradient in the mean abundance of the M87 GCs with R, but no detectable change in the Hβ index with R. We obtain a median age for the M87 GC system of 13 Gyr, similar to that found for the galactic GCs. The dispersion about that value (σ = 2 Gyr) is small.

The ASCA Observation Campaign of SS433

Here we present a very short review of the ASCA observation campaign of the enigmatic galactic jet system SS433. The campaign started in 1994 just after the launch, and ended in 1996. Various phases of the 162.5-day precession and 13-day orbital motion were sampled. With ASCA, the Doppler-shifted pairs from various ion species from Si to Ni were resolved for the first time (Kotani et al. 1994). The Doppler-shift parameters were determined with an accuracy comparable to optical spectroscopy (Kawai 1995). No velocity gradient was found between the X-ray emission region of the jet and the optical. The distance between them was constrained to be less than 1015 cm. Line intensity ratios of Fe XXVI/Fe XXV give the base temperature of the jet to be 20 keV (Kotani et al. 1996). The variation of the apparent base temperature of the jet can be explained in terms of the partial occultation of the jet by a precessing accretion disk (Kotani et al. 1997a). From the variation, the disk radius and the disk height in unit of the X-ray jet length were estimated to be 0.23 ± 0.10 and 0.0232 ± 0.0049, respectively. (These are an improved version of the values in Kotani et al. (1997a).) SS433 is also known as an eclipsing binary. Because the emission from each jet with ASCA, it is possible to know how much of which jet is occulted by the companion star during an eclipse. Relative size of the companion star gives Roche lobe size and thus mass ratio MX/MC = 0.22+0.09-0.16 (Kotani 1997b). With the help of Doppler modulation, compact star mass is constrained. However, the values of Doppler modulation reported from optical observations largely scatters. D'Odorico et al. (1991) reported 112 km s−1 and this gives MX = 0.68+0.43-0.53 M⊙, i.e., a white dwarf, while Fabrika and Bychkova (1990) reported 175 km s−1, which gives 2.6+1.6-2.0 M⊙. (This error includes systematic errors of the X-ray data, and will be reduced in future analysis.) On the other hand, the absolute size of the system were determined with a satisfactory precision. For example, the X-ray jet length was determined to be 2×1013 cm, ten times larger than previous estimations (Kotani et al. 1997c). Other physical parameters of the jet can be derived from the X-ray jet length. Mass outflow rate and the kinetic luminosity of both jet were determined to be 8 × 10−6 M⊙ yr−1 and 1.6 × 1040 erg s−1 (Kotani et al. 1997d), implicating a highly super critical accretion. Most of these values are first precise measurements and/or “radical” revisions of previous estimations. The new picture of SS433 drawn here is far stormy and highly energetic.

The Galactic population of pulsars

We review our knowledge of the population of normal and millisecond radio pulsars in the disk of the Galaxy and in the Galactic globular cluster system. There are about 150 000 active normal pulsars in the Galaxy with luminosity exceeding 1 mJy kpc 2, a new one being born about once every 125 years with mean velocity of 400–500 km s −1. The total Galactic population of millisecond pulsars is similar in size, although the rate of formation of these is probably one every 10 5 years. There are probably about 10 000 pulsars in the globular cluster system, of which 90% are millisecond pulsars. The remaining 10% are much younger, apparently normal pulsars, and must be formed at least as frequently as the millisecond pulsars.

Evolution of the Galactic globular cluster system

We study the dynamical evolution of disc and halo globular clusters in the Milky Way using a series of Fokker-Planck calculations combined with parametric statistical models. Our sample of 111 clusters with velocity data is predicted to descend from an initial population of 250 clusters, implying more than a factor of two decrease in population size due to evolution. Approximately 200 of these clusters are in a halo component and 50 in a disc component. The estimated initial halo population follows a coreless R-3.38 density profile in good agreement with current estimates for the distribution of halo field stars. The observed core in the present-day distribution of halo clusters results from the rapid evaporation of clusters in the inner regions of the Galaxy. The initial halo population is also predicted to have a radially biased orbit distribution in rough agreement with the observed kinematics of halo field stars. The isotropy of the present-day halo cluster distribution results from the evaporation of clusters on elongated orbits. Similarly, the initial disc component has a nearly isotropic initial distribution that becomes more tangentially biased with time. However, the inferred initial characteristics of the disc component do not match the kinematics of the rapidly rotating thin or thick disc stellar populations. These characteristics may be more indicative of the flattened halo component discussed by Zinno Detailed examination of cluster evolution confirms the importance of disc heating. Clusters on low-inclination orbits experience the strongest disc heating because of optimal matches in resonant frequencies. Disc heating on high­ inclination orbits is weaker but still dominates over spheroidal heating. Evaporation times depend weakly on initial concentration, density and height of oscillation above the disc.

Interstellar scintillation of radiosources as a probe for investigations of the local interstellar medium

Abstract Based upon numerous observational data of variability in extragalactic radiosources of the Northern and Southern sky at low frequencies the possibility of obtaining information on ionized component structure in the local interstellar medium (LISM) is investigated. Isophot maps of scintillation index based upon these data were constructed in the galactic coordinate system on 80 MHz and 160 MHz for 1 month and 1 year time intervals of source variability. In the direction where scintillation indicates increase, regions of interaction of interstellar wind with large-scale structure of the LISM are located. Regions of decreased scintillation indices are observed towards the third galactic quadrant (near l = 240°), there is a gas-free tunnel. Long-term observations of the sample from over 50 extragalactic radiosources using the radiotelescope DKR-1000 at a frequency of 102 MHz are used to justify the method of determing the distribution character of the LISM ionized component.

Magnetohydrodynamic density waves in a galactic disk system of stars and gas

We study galactic magnetohydrodynamic (MHD) density waves in a composite system consisting of a stellar disk and a magnetized thermal gaseous disk. Perturbations in the two disks are coupled through gravitational interaction. In the tight-winding regime, dispersion relations for MHD density waves are derived under two different approximations for the stellar disk. This investigation clarifies the interrelation between spiral structures in the stellar disk and spiral synchrotron radio structures in the magnetized thermal gaseous disk.

Voyager Measurements of the Mass Composition of Cosmic‐Ray Ne, Mg, Si and S Nuclei

We report new measurements of the mass composition of cosmic-ray Ne, Mg, Si, and S nuclei made on the Voyager spacecraft. These measurements have ~4 times the statistical accuracy of previously published measurements covering these four charges. With the new cosmic-ray source mass fractions of these elements that we obtain, only the isotope 22Ne shows a cosmic-ray source abundance that is significantly different from the solar abundances. The limits of ±15% that we set on the cosmic-ray source-to-solar abundance ratios of 25Mg/24Mg and 26Mg/24Mg as well as the heavier Si isotopes place severe limits on the models that have been proposed to explain compositional differences between galactic cosmic rays and solar system abundances. The only two statistically significant isotopic differences between the cosmic-ray source and the solar system in the charge range Z = 6-16 now appear to be the underabundance of 14N and the overabundance of 22Ne. This suggests to us that the helium-burning process in which 14N is turned into 22Ne plays an important role in at least some of the sources of those particles ultimately accelerated as cosmic rays.

The capture and escape of stars

The shape of galaxies depends on their orbital populations. These populations change through capture into and escape from resonance. Capture problems fall into distinct cases depending upon the shape of the potential well. To visualise the effective potential well for orbital capture, a diagrammatic approach to the resonant perturbation theory of Born is presented. These diagrams we call equiaction sections. To illustrate their use, we present examples drawn from both galactic and Solar System dynamics. The probability of capture for generic shapes of the potential well is calculated. A number of predictions are made. First, there are barred galaxies that possess two outer rings of gas and stars (type R1R2). We show how to relate changes in the pattern speed and amplitude of the bar to the strength of the two rings. Secondly, under certain conditions, small disturbances can lead to dramatic changes in orbital shape. This can be exploited as a mechanism to pump counter-rotating stars and gas into the nuclei of disk galaxies. Tidal resonant forcing of highly inclined orbits around a central mass causes a substantial increase in the likelihood of collision. Thirdly, the angular momentum of a potential well is changed by the passage of stars across or capture into the well. This can lead to the creation of holes, notches and high velocity tails in the stellar distribution function, whose form we explicitly calculate.

On the evolution of the Galactic globular cluster system

In this paper we address the issue of the origin of some observational properties of the galactic globular cluster system. After a preliminary study of some general properties of the main evolutionary processes, we investigate the evolution of systems of globular clusters located in a model of the Milky Way starting from different initial conditions. We study the role of the evolutionary processes in changing the spatial distribution and mass function of the cluster system, in establishing and/or preserving some of the observed correlations and trends between internal properties of globular clusters and between internal properties and location inside the host galaxy and we provide an estimate for the rates of core collapse and disruption of globular clusters. The initial mass function and spatial distribution of the cluster system evolve quite significantly in one Hubble time and the evolution is toward a final state similar to the observed one. If the mass function is initially taken to be a log-normal distribution similar to the one currently observed in our galaxy, its shape is not significantly altered during the entire simulation even though a significant number of clusters are disrupted before one Hubble time, which suggests that the present mass function might represent a sort of 'quasi-equilibrium' distribution.

Abundances for globular cluster giants. I. Homogeneous metallicities for 24 clusters

We have obtained high-res, high S/N ratio CCD echelle spectra of 10 bright red giants in 3 GCs (47Tuc, NGC6752 and NGC6397) roughly spanning the range of metallicities of the galactic GC system; they reveal no evidence of star to star variation of [Fe/H] in these 3 GCs. A large set of high quality literature data (EWs from high-res CCD spectra) was re-analyzed in a self-consistent way to integrate our data and derive new [Fe/H] for more than 160 bright red giants in 24 GCs. This set was used to define a new [Fe/H] scale for GCs based on high quality, direct spectroscopic data, on updated model atmospheres from the grid of Kurucz (1992) and on a careful fine abundance analysis (using a common set of atomic and atmospheric parameters for all stars). Given the high internal homogeneity, our new scale supersedes the discrepancies of previous attempts. The internal uncertainty in [Fe/H] is very small: 0.06 dex (24 GCs) on average, that can be interpreted as the mean precision of cluster ranking. Compared to our system, metallicities on the widely used Zinn and West's scale are about 0.10dex higher for [Fe/H]$>-1$, 0.23dex lower for $-1<[Fe/H]<-1.9$ and 0.11dex too high for [Fe/H]$<-1.9$: the non-linearity is significant at 3$\sigma$ level. A quadratic transformation corrects older values to the new scale in the range of our calibrating GCs ($-2.24\le[Fe/H]_{ZW}\le-0.51$). A minor disagreement is found at low [Fe/H] between the metallicity scale based on field and cluster RR Lyrae's (via a new calibration of the $\Delta$S index) and our new GCs metallicities, that could be ascribed to non-linearity in the [Fe/H]$-\Delta$S relationship. The impact of new metallicities on major astrophysical problems is exemplified through the Oosterhoff effect in the classical pair M3 and M15.