Galactic Observations Research Articles

Galactic densities, substructure and the initial power spectrum

Although the currently favored cold dark matter plus cosmological constant model for structure formation assumes an n = 1 scale-invariant initial power spectrum, most inflation models produce at least mild deviations from n = 1. Because the lever arm from the CMB normalization to galaxy scales is long, even a small “tilt” can have important implications for galactic observations. Here we calculate the COBS-normalized power spectra for several well-motivated models of inflation and compute implications for the substructure content and central densities of galaxy halos. Using an analytic model, normalized against N-body simulations, we show that while halos in the standard ( n = 1) model are overdense by a factor of ∼ 6 compared to observations, several of our example inflation+LCDM models predict halo densities well within the range of observations, which prefer models with n ∼ 0.85. We go on to use a semi-analytic model (also normalized against N-body simulations) to follow the merger histories of galaxy-sized halos and track the orbital decay, disruption, and evolution of the merging substructure. Models with n ∼ 0.85 predict a factor of ∼ 3 fewer subhalos at a fixed circular velocity than the standard n = 1 case. Although this level of reduction does not resolve the “dwarf satellite problem’, it does imply that the level of feedback required to match the observed number of dwarfs is sensitive to the initial power spectrum. Finally, the fraction of galaxy-halo mass that is bound up in substructure is consistent with limits imposed by multiply imaged quasars for all models considered: f sat > 0.01 even for an effective tilt of n ∼ 0.8. We conclude that, at their current level, lensing constraints of this kind do not provide an interesting probe of the primordial power spectrum.

The gravitational wave background from cosmological compact binaries

We use a population synthesis approach to characterize, as a function of cosmic time, the extragalactic close binary population descended from stars of low to intermediate initial mass. The unresolved gravitational wave (GW) background due to these systems is calculated for the 0.1‐10 mHz frequency band of the planned Laser Interferometer Space Antenna (LISA). This background is found to be dominated by emission from close white dwarf‐white dwarf (WD‐ WD) pairs. The spectral shape can be understood in terms of some simple analytic arguments. To quantify the astrophysical uncertainties, we construct a range of evolutionary models which produce populations consistent with Galactic observations of close WD‐WD binaries. The models differ in binary evolution prescriptions as well as initial parameter distributions and cosmic star formation histories. We compare the resulting background spectra, the shapes of which are found to be insensitive to the model chosen, and different to those found recently by Schneider et al. From this set of models, we constrain the amplitude of the extragalactic

Between simple and chemodynamical models of galaxies

At first sight, the World of chemical and chemodynamical models might seem to be composed of two camps. But in reality, it is more like a spectrum of possible approaches and tools for modelling and interpreting galactic observations and understanding the evolution of galaxies. In Table I below I roughly sketch the geography of that World, by arranging a selection of studies with respect to the complexity of the local physics and the global complexity indicated by the spatial dimensions. Simplified though this map is, it gives an overview of the modelling tools available to us today.

Twenty years of Galactic observations in searching for bursts of collapse neutrinos with the Baksan underground scintillation telescope

The results of twenty-year-long Galactic observations in neutrino radiation are summarized. Except for the recording of a neutrino signal from the supernova SN 1987A, no Galactic bursts of collapse neutrinos have been detected. An upper bound on the mean frequency of gravitational collapses in our Galaxy was obtained, f collapse (at 90% confidence) <0.13 yr−1.

Spiral Magnetohydrodynamic Density Waves with a Tangential Shear Force

Evidenced by synchrotron radio-continuum emissions from spiral galaxies, magnetic fields with estimated strengths of several microgauss or stronger are embedded in the interstellar medium (ISM) of spiral galaxies and play important dynamic and diagnostic roles in large-scale galactic density-wave spiral structures as well as in the formation of tight circumnuclear spirals in the central regions where magnetic field strengths may be higher than several tens of microgauss. In this paper, we examine several physical effects of the tangential shear force on spiral magnetohydrodynamic (MHD) density waves in a thin magnetized gas sheet with differential rotation. This tangential shear force in the azimuthal direction, mainly caused by the disk differential rotation, is of inertial type and operates only for nonaxisymmetric MHD density wave perturbations. We focus on shear-related processes of interest: (1) The short-, long-, and open-MHD density waves may become unstable near corotation. (2) Fast magnetoacoustic density waves may become overstable (oscillatory growth) outside the Lindblad resonances. (3) Slow MHD density waves may become unstable near corotation. We discuss relevant theoretical works and conceptual applications of these results to multiwavelength galactic observations, including spiral galaxies, barred spiral galaxies, and tight circumnuclear spirals in central regions of spiral galaxies. In terms of the angular momentum transport, spiral MHD density waves driven by the tangential shear force should also find important applications in magnetized accretion disks in broader astrophysical contexts.

Workshop 2: Cosmic Microwave Background, Lensing and Large-scale Structure

Papers presented in the second workshop are briefly reviewed. The 1+3 covariant approach to microwave background anisotropies and structure formation was well represented. Other topics included gravitational lensing and caustics, alternative approaches to galactic observations, conserved quantities in perturbation theory and primordial black holes.

The Bright Side of Dark Matter

We show that it is not possible in the absence of dark matter to construct a four-dimensional metric that explains galactic observations. In particular, by working with an effective potential it is shown that a metric which is constructed to fit flat rotation curves in spiral galaxies leads to the wrong sign for the bending of light i.e. repulsion instead of attraction. Hence, without dark matter the motion of particles on galactic scales cannot be explained in terms of geodesic motion on a four- dimensional metric. This reveals a new bright side to dark matter: it is indispensable if we wish to retain the cherished equivalence principle.

Attitude Control of Space Platform Based Tethered Satellite System

Using an order N Lagrangian formulation, the paper studies attitude control of a rigid platform supporting a flexible tether connected to a rigid satellite. The system, in an arbitrary orbit, is free to undergo three-dimensional motion in both rigid and flexible degrees of freedom. As can be expected, the governing equations of motion, in general, are highly nonlinear, nonautonomous, and coupled, and are amenable only to numerical integration. The control is achieved through time dependent offset of the tether attachment point, as determined through the Liapunov method, thus providing regulated amount of tether tension induced damping moment. Results suggest that the controller is quite successful in stabilizing the platform about its nominal equilibrium position in a few orbits, even in the presence of relatively large disturbances. Furthermore, extensive parametric study suggests that the controller is quite versatile in imparting any desired orientation to the platform. This would enable the system to undertake diverse missions aimed at communications, launch and retrieval of spacecraft, monitoring the Earth's environment, planetary and galactic observations, etc.

Cosmological implications of two conflicting deuterium abundances

Constraints on big bang nucleosynthesis (BBN) and on cosmological parameters from conflicting deuterium observations in different high redshift QSO systems are discussed. The high deuterium observations by Carswell et al., Songaila et al., and Rugers and Hogan are consistent with ${}^{4}$He and ${}^{7}$Li observations and standard BBN ${(N}_{\ensuremath{\nu}}$ =3) and allows ${N}_{\ensuremath{\nu}}&lt;~3.6$ at 95% C.L., but are inconsistent with local observations of D and ${}^{3}$He in the context of conventional theories of stellar and galactic evolution. In contrast, the low deuterium observations by Tytler, Fan, and Burles and Burles and Tytler are consistent with the constraints from local galactic observations, but require ${N}_{\ensuremath{\nu}}=1.9\ifmmode\pm\else\textpm\fi{}0.3$ at 68% C.L., excluding standard BBN at 99.9% C.L., unless the systematic uncertainties in the ${}^{4}$He observations have been underestimated by a large amount. The high and low primordial deuterium abundances imply, respectively, ${\ensuremath{\Omega}}_{B}{h}^{2}=0.005$--$0.01$ and ${\ensuremath{\Omega}}_{B}{h}^{2}=0.02$--$0.03$ at 95% C.L. When combined with the high baryon fraction inferred from x-ray observations of rich clusters, the corresponding total mass densities (for $50&lt;~{H}_{0}&lt;~90)$ are ${\ensuremath{\Omega}}_{M}=0.05$--$0.20$ and ${\ensuremath{\Omega}}_{M}=0.2$--$0.7$, respectively (95% C.L.). The range of ${\ensuremath{\Omega}}_{M}$ corresponding to high D is in conflict with dynamical constraints $({\ensuremath{\Omega}}_{\mathrm{m}}&gt;~0.2$--$0.3)$ and with the shape parameter constraint $(\ensuremath{\Gamma}={\ensuremath{\Omega}}_{M}h=0.25\ifmmode\pm\else\textpm\fi{}0.05)$ from large scale structure formation in CDM and $\ensuremath{\Lambda}$CDM models.

Theory of Exploring the Dark Halo with Microlensing. I. Power-Law Models

The detection of microlensing has opened the way for the development of new methods in galactic astronomy. This series of papers investigates what microlensing can teach us about the structure and shape of the dark halo. In this paper we present formulas for the microlensing rate, optical depth and event duration distributions for a simple set of axisymmetric disk-halo models. The halos are based on the power--law which have simple velocity distributions. Using these models, we show that there is a large uncertainty in the predicted microlensing rate because of uncertainty in the halo parameters. For example, models which reproduce the measured galactic observables to within their errors still differ in microlensing rate towards the Magellanic Clouds by more than a factor of ten. We find that while the more easily computed optical depth correlates well with microlensing rate, the ratio of optical depth to rate can vary by a factor of two (or greater if the disk is maximal). Comparison of microlensing rates towards the Large and Small Magellanic Clouds (LMC and SMC) and M31 can be used to aid determinations of the halo flattening and rotation curve slope. For example, the ratio of microlensing rates towards the LMC and SMC is $\sim 0.7-0.8$ for E0 halos and $\sim 1.0 - 1.2$ for E7 halos (c.f. Sackett \& Gould 1993). Once the flattening has been established, the ratio of microlensing rates towards M31 and the LMC may help to distinguish between models with rising, flat or falling rotation curves. Comparison of rates along LMC and galactic bulge lines-of-sight gives useful information on the halo core radius, although this may not be so easy to extract in practice. Maximal disk models provide substantially smaller halo optical depths, shorter event durations and even larger model uncertainties.

Can galactic observations be explained by a relativistic gravity theory?

We consider the possibility of an alternative gravity theory explaining the dynamics of galactic systems without dark matter. From very general assumptions about the structure of a relativistic gravity theory we derive a general expression for the metric to order $(v/c)^2$. This allows us to compare the predictions of the theory with various experimental data: the Newtonian limit, light deflection and retardation, rotation of galaxies and gravitational lensing. Our general conclusion is that the possibility for any gravity theory to explain the behaviour of galaxies without dark matter is rather improbable.

Isotropic and semi-isotropic observations in cosmology

We investigate the geometric consequences for dust universes of isotropies within the set of idealized galactic observations---area distance, number counts, proper motions and image distortion. Anisotropy in the Hubble parameter is shown to indicate non-zero proper motions. If the proper motions and distortion are assumed to be zero (semi-isotropic observations), then the vorticity vanishes, the Hubble and deceleration parameters are isotropic, and Einstein's equations strongly constrain the area distance and the number counts. These constraints allow limited anisotropy, and in principle they provide indirect tests for the presence of proper motions and distortion. If, further, we assume isotropy of the area distance and number counts (isotropic observations), then we prove that Einstein's equations force the spacetime geometry to be isotropic. With the Copernican principle, this leads to an observational proof of the standard FLRW model.

IntegratedBVRIJHK photometry of galactic globular clusters

We present preliminary BVRI photometric data of 22 galactic globular clusters of the Southern hemisphere. The observations were carried out at the ESO Observatory, during two observing runs in August 1988 (VRI) and March 1989 (BVRI). In both periods half of the total number of nights was used to perform near-infrared (JHK) photometry of the same clusters: the goal of this quasi-contemporary photometry was to obtain a homogeneus sample of galactic clusters photometric observations, on a wavelength range as wide as possible.

Ambiguities in the identification of giant molecular cloud complexes from longitude-velocity diagrams

Techniques which use longitude-velocity diagrams to identify molecular cloud complexes in the disk of the Galaxy are investigated by means of model Galactic disks generated from N-body cloud-particle simulations. A procedure similar to the method used to reduce the low-level emission in Galactic l-v diagrams is employed to isolate complexes of emission in the model l-v diagram (LVCs) from the 'background'clouds. The LVCs produced in this manner yield a size-line-width relationship with a slope of 0.58 and a mass spectrum with a slope of 1.55, consistent with Galactic observations. It is demonstrated that associations identified as LVCs are often chance superpositions of clouds spread out along the line of sight in the disk of the model system. This indicates that the l-v diagram cannot be used to unambiguously determine the location of molecular cloud complexes in the model Galactic disk. The modeling results also indicate that the existence of a size-line-width relationship is not a reliable indicator of the physical nature of cloud complexes, in particular, whether the complexes are gravitationally bound objects.

The planning and analysis of galactic disk tracer observations

A systematic analysis is conducted of the Bahcall (1984) method for the determination of galactic disk potential, and hence missing mass, on the basis of tracer observations at less than 1 kpc. A mathematical characterization of observational effectiveness is used to determine the optimal observation strategy and optical binning of gathered data. Demonstrations are undertaken for the desirability of (1) increasing observational scale height when this parameter is lower than 1; (2) the primacy of velocity over distance measurements; (3) the uncertainty of distance errors unless the mean distance error is reduced to less 0.06/sq rt (N/500), where N is the number of tracers; (4) tracer data should be divided into about four bins, rather than the 10 used by Bahcall; (5) survey effectiveness deteriorates with tracer deviation from the isothermal; and (6) while a few hundred tracers suffice for establishing the presence of missing matter, many more are needed to ascertain the missing component's velocity dispersion. 14 refs.

Decaying-Particle Universe Models with Nonzero Cosmological Constant and the Galactic Number Count Observation

Decaying-particle universe models with nonzero cosmological constant A are considered in connection with the recent observational [Galaxy number count N-redshift zl relation and it is shown that the best-fitted values of A and the density parameter f.1 depend on the decay time td: For Hotd =0.05, we get .G,=0.494 and Ac2/(3Ho2) = -0.046. If the model is required to be fiat, the fitted values are f.1=0.689 and Ac2/(3Ho2)=0.311 for Hotd=0.05. The cosmic ages are found to be of the order of or larger than 0.6/Ho in the fitted models. Moreover the nonlinear growth of the density perturbations is considered and the evolution of the baryon density is examined in several fitted models.

Observational and dynamical properties of small universes

It is possible that we live in a “small universe”, that is, we might have seen round the universe many times since decoupling so that there are many images of each galaxy. We estimate the number of images per galaxy in different small universes, and review how apparent isotropy of galactic observations and backgroung radiation measurements can naturally occur in such universes. Furthermore interesting physical effects may result; in particular, in some cases the dynamical behaviour of such space-times is necessarily like that of a Friedmann universe, so that inhomogeneous small universes may both look like and evolve like the standard isotropic and spatially homogeneous world models.

Astrophysical applications of Fabry-Perot multiple beam interferometry by Fabry, his colleagues and followers

In this review, astronomical applications of Fabry-Perot fringes to Doppler-Fizeau shift measurements are described. Interest of spectral and spatial restoration is shown by modern developments and significant results in the galactic and extragalactic observations.

The mass and light distribution of the galaxy: a three-component model

The galaxy is represented schematically by a three-component model: a disc having the form of a modified exponential distribution, a spheroidal (bulge + nucleus) component and a dark halo component which, following the nomenclature of Einasto, we call the corona. The shapes of these components, chosen on the basis of observations of other galaxies, are consistent with imperfect knowledge of the Galaxy; values of the adjustable parameters are chosen by a least square minimization technique to best fit the most accurate kinematical and dynamical galactic observations. The local radius, circular velocity and escape velocity are found to be (R⊙, V⊙, Vesc) = (9.05 ± 0.33 kpc, 247 ± 13 km/s, 550 ± 24) quite close to the values determined from observations directly. The masses in the three components are (MD, MSp, MC) = (0.78 ± 0.13, 0.81 ± 0.09, 20.3) × 1011 M⊙ for a model with coronal radius of 335 kpc. If the quite uncertain coronal radius is reduced to 100 kpc the model is essentially unchanged except that then MC = 6.65 × 1011 M⊙. The disc and spheroidal components have in either case luminosities (in the visual band of (LD, LSp) = (2.0, 0.2) × 1010 L⊙. The galaxy is a normal giant spiral of type Sb-Sc similar to NGC 4565.

Spectral classification of O and B0 supergiants in the Magellanic Clouds

view Abstract Citations (292) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectral classification of O and B0 supergiants in the Magellanic Clouds. Walborn, N. R. Abstract A spectroscopic study has been made of the brightest known O, OB + WN, and B0 supergiants in the Magellanic Clouds, based upon wide high-resolution classification spectrograms. Among the interesting LMC objects are found the third known O3 supergiant and a superluminous late-O nitrogen supergiant. Also noteworthy are four peculiar Of stars with pronounced low-excitation envelopes, which have no exact counterparts identified in the Galaxy. An example of the numerous compact multiple systems among these stars is shown in which a W-R component has been identified by interference-filter photography. In the SMC, further evidence for large metal deficiencies is provided by two luminous B0 supergiants, which have Si/He line ratios similar to those in galactic stars of luminosity class IV. Possible evidence for a similar but smaller effect in the LMC is given in terms of a systematic discrepancy between the Si/He luminosity classes and derived absolute magnitudes. The absolute magnitudes of the normal Of stars in both Clouds confirm the supergiant nature of such objects, previously inferred from galactic observations. Twelve of the most interesting spectrograms are reproduced. Publication: The Astrophysical Journal Pub Date: July 1977 DOI: 10.1086/155334 Bibcode: 1977ApJ...215...53W Keywords: B Stars; Magellanic Clouds; O Stars; Stellar Spectra; Supergiant Stars; Hot Stars; Spectrograms; Stellar Magnitude; Stellar Spectrophotometry; Tables (Data); Astrophysics full text sources ADS | data products SIMBAD (34)