Centers Of Globular Clusters Research Articles

Can the known millisecond pulsars help in the detection of intermediate-mass black holes at the centers of globular clusters?

We consider the possibility of detecting intermediate-mass (103–104M⊙) black holes, whose existence at the centers of globular clusters is expected from optical and infrared observations, using precise pulse arrival timing for the millisecond pulsars in globular clusters known to date. For some of these pulsars closest to the cluster centers, we have calculated the expected delay times of pulses as they pass in the gravitational field of the central black hole. The detection of such a time delay by currently available instruments for the known pulsars is shown to be impossible at a black hole mass of 103M⊙ and very problematic at a black hole mass of 104M⊙. In addition, the signal delay will have a negligible effect on the pulsar periods and their first derivatives compared to the current accuracy of their measurements.

Upper limits on central black hole masses of globular clusters from radio emission and a possible black hole detection in the Ursa Minor dwarf galaxy

Abstract Intermediate-mass black holes (IMBHs) have been alternatively predicted to be quite common in the centres of globular clusters or nearly impossible to form and retain in the centres of globular clusters. As it has been recently shown that radio observations are currently the most sensitive observational technique for detecting such objects, we have obtained new deep radio observations of Omega Cen, and have reanalyzed older observations of M 15 in the hope of constraining the masses of possible black holes in their centres. In both cases, upper limits of about 100 μJy are found at GHz frequencies. We find that if the Bondi–Hoyle accretion rate truly represents the spherical accretion rate onto a black hole, then the masses of the black holes in the centres of these two galaxies are severely constrained – with mass limits of less than about 100 solar masses in both cases. If more realistic assumptions are made based on recent work showing the Bondi rate to be a severe overestimate, then the data for Omega Cen are marginally consistent with a black hole of about 1/1000 of the mass of the cluster (i.e. about 1000 M⊙). The data for M 15 are then only marginally consistent with previous reports of a ~2000 solar mass black hole, and we note that there is considerable hope for either detecting the black hole or improving this upper limit with current instrumentation. Finally, we discuss the possibility that the radio source near the core of the Ursa Minor dwarf spheroidal galaxy is a ~104-M⊙ black hole.

Radio emission as a test of the existence of intermediate-mass black holes in globular clusters and dwarf spheroidal galaxies

We take the established relation between black hole mass, X-ray luminosity, and radio luminosity and show that intermediate mass black holes, such as those predicted to exist at the centers of globular clusters, will be easily identifiable objects in deep radio observations. We show that the radio observations will be far more senstive than any possible X-ray observations. We also discuss the likely optical photometric and spectroscopic appearance of such systems in the event that radio detections are made.

Stellar kinematics in the centers of globular clusters

AbstractThe high central stellar densities in globular clusters provide a unique environment to study the fundamental dynamical process of two‐body relaxation. This process is the main driver of the dynamical evolution in the center of a globular cluster and has a profound effect on the structure of the cluster and on its stellar environment. We have obtained stellar absorption line spectra with STIS to measure the radial velocities of individual stars in the crowded center of the globular cluster M15. These data increase the number of stars with known radial velocities within the central arcsec by a factor of about three and significantly improve the constraints on the mass distribution in M15. The data provide the most detailed look of the central structure of any globular cluster and show that there is a compact dark central mass component. Similar studies using ground based facilities can be efficiently performed by employing Integral Field Units. We have started a project to better constrain the central mass density in the globular cluster M3 using the GMOS‐IFU on Gemini North. The data will also allow us to better understand the central rotation which is neither explained nor predicted by any globular cluster model. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An algorithm to “clean" close stellar companions

In our effort to study the post-main-sequence evolution of the globular cluster stars we developed some procedures which were designed to perform stellar photometry in moderately crowded fields by using a personal computer. This paper presents a description of an algorithm for cleaning close stellar companions which is included in our data processing. The main goal is to establish how the cleaning procedure biases the accuracy of the derived photometry. As a test for our data reduction, BV photographic photometry for nearly 500 stars, down to 0.6 arcmin from the center of globular cluster has been performed. In addition, we present a comparison of our photometric results with those obtained with the DAOPHOT package for the same cluster region. We also discuss the ability of three kinds of magnitudes - based on Gaussian fits, the use of synthetic apertures and the output of the DAOPHOT package - to reveal morphological features in the color-magnitude diagram.

Automatic analysis of crowded fields

There are many important 2D data reduction problems in astronomy that are not amenable to conventional automatic analysis. Typically these problem areas arise in fields where the number density of images is high and where the local sky background may vary rapidly. Within such regions the image number density becomes so high that the majority of images overlap, even at relatively high isophotes, and simple image parameter estimation algorithms become confused. Our goal has been to examine the potential for a fully automatic method that is both robust and efficient in terms of computer requirements, capable of dealing with complex multiple overlaps and able to generate the optimum estimates of image parameters. By applying the theory of maximum likelihood parameter estimation to this topic, we have been able to devise a coherent strategy for tackling the problem. This has led to the development of a fully automatic system capable of producing reliable results toward the centre of globular clusters and within the dense regions of nearby resolved galaxies.

Sources of gravitational waves

Violent events in the Universe, where gravity is relativistically strong, should produce copious amounts of gravitational radiation. Examples are the collapse of a normal star to form a neutron star or a black hole (supernova explosion); the pulsations of a newborn black hole; the rapid rotation of a young, deformed neutron star; quakes and pulsations of neutron stars; collisions between neutron stars and between black holes in the centres of globular clusters and in the nuclei of galaxies. Such phenomena typically are obscured from the sight of electromagnetic telescopes by thick layers of surrounding matter. However, gravitational waves should pass through the surrounding matter unimpeded. Detailed measurements of such gravitational waves - if they can be made - will give detailed information about the dynamical motions of matter and of space-time curvature in violent cosmic events.

Effects of seeing on the light distribution in the cores of elliptical galaxies

view Abstract Citations (137) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Effects of seeing on the light distribution in the cores of elliptical galaxies. Schweizer, F. Abstract The paper analyzes high-resolution profiles of giant elliptical galaxies published by King (1978). Smearing of the intrinsic light distribution by seeing is the probable cause for the low central surface brightness of these ellipticals; models are computed which indicate that the cores of most of them are probably not resolved, and the apparent core radii depend strongly on the seeing at the time of observation. To fit the presently unknown intrinsic light distributions in the cores of many giant ellipticals, King models of higher intrinsic concentration index than 2.5 may be needed, and future models must be convolved with the observed point-spread functions before comparison with observations. Parameters of ellipticals that depend on the intrinsic core radius, central surface brightness, or central projected velocity dispersion probably need revision. First indications are that mean densities in the cores of large ellipticals are similar to but higher than the mean densities observed under much better resolution at the centers of globular clusters in our galaxy. Publication: The Astrophysical Journal Pub Date: October 1979 DOI: 10.1086/157362 Bibcode: 1979ApJ...233...23S Keywords: Cores; Elliptical Galaxies; Galactic Nuclei; Galactic Structure; Light (Visible Radiation); Radiation Distribution; Astronomical Models; Brightness; High Resolution; Luminosity; Photometry; Surface Temperature; Astrophysics; Elliptical Galaxies:Nuclei; Elliptical Galaxies:Photometry; Elliptical Galaxies:Structure full text sources ADS | data products NED (17) SIMBAD (6) Related Materials (1) Erratum: 1980ApJ...236.1056S

The distribution and consumption rate of stars around a massive, collapsed object

We consider the steady-state distribution and consumption rate of stars orbiting a massive object at the center of a spherical, N-body stellar system. The distribution of stars is determined by the consumption of low angular momentum stars which pass within a small distance r/sub t/ of the central mass M and by the relaxation processes associated with gravitational stellar encounters. Our method employs an approximate, analytic analysis of the two-dimensional Fokker--Planck equation describing diffusion in energy E and angular momentum J. The basic results are the following: (1) Consumption of low angular momentum stars which have entered the ''loss-cone'' J > greater than or equal to r/sub t/, at which the root mean square angular momentum transferred to a star via stellar encounters in one orbital period equals J/sub min/. (2) The total consumption rate of stars by M is roughly the number of stars inside r/sub crit/ divided by the relaxation time at r/sub crit/. (3) A self-consistent solution can be found in which the distribution of stars is almost isotropic for high-J stars and varies only more » logarithmically with J for low-J stars. (4) The density of core stars has the following form: n(r) approx. =n (r/sub a/)(1+(r/sub a//r)/sup l/), wher the accretion radius r/sub a/ approx. GM/ >> r/sub t/, is the mean-squared velocity dispersion in the core outside r/sub a/, and l decreases slowly from approx. 1.75 at >> r/sub crit/ to approx. 1.60 at r approx. 10r/sub t/. Neglect of loss-cone effects gives a consumption rate too small by roughly the ratio r/sub t//r/sub crit/ and a constant exponent l = 1.75 for >> r/sub t/. These results are applied to massive black holes at the centers of globular clusters and galactic nuclei. (AIP) « less

On the Formation of Disk around a Compact Object by Two-Body Tidal Encounter

Fabian, Pringle & Rees recently argued that close binary systems may be formed by two-body tidal interaction, and that these may account for observed X-ray sources near the centres of globular clusters. In this note it is shown, by making use of Woolfson's results for capture processes, that through tidal interaction, disks can be formed around single compact objects which may emit thermal X-rays for |${10}^{5}\,\sim\,{10}^{6}$| yr.