Dark Clusters Research Articles

Gravitational Lensing and Dark Structures

We examine whether a cosmologically significant distribution of dark galaxy group or cluster-sized objects can have an optical depth for multiple imaging of distant background sources which is comparable to that from known galaxies while at the same time producing angular splittings of the same order of magnitude. Our purpose is to explore whether such objects could realistically account for some of the observed lenses. Modeling such systems as isothermal spheres with core radii, and assuming a Schechter-type distribution function, we find that independent of the cosmology (open, flat matter dominated, or flat cosmological constant dominated) an allowed parameter range exists which is comparable in velocity dispersion to that for known compact groups of galaxies, although the preferred core radii are somewhat smaller than that normally assumed for compact groups. Dark cluster-sized objects, on the other hand, cannot reproduce the observed lensing characteristics. If the one known Dark cluster were a good representative of such a distribution, most such objects would not produce multiple images. We also present a result for the angular splitting due to an isothermal sphere lens with non-zero core radius, extending earlier work of Hinshaw and Krauss (1987). Our results are expressed as contour plots for fixed lensing probabilities, and angular splittings.

Massive dark clusters and the heating problem of galactic discs

Massive dark clusters and the heating problem of galactic discs

In vitro fertilization and cortical granule distribution of bovine oocytes having heterogeneous ooplasm with dark clusters.

In vitro maturation, fertilization and subsequent development of oocytes with homogeneous (category 1), or heterogeneous ooplasm (category 2) were investigated. No significant differences were observed in the nuclear maturation and total fertilization rates between the two categories. However, category 2 oocytes showed a higher normal fertilization rate due to their lower incidence of polyspermy as compared to category 1 oocytes. Electron microscopic study revealed that all category 2 oocytes had cortical granules lined up next to the plasma membrane, and that some category 1 oocytes still had small clusters of cortical granules after maturation. Although the proportion of cleaved zygotes was higher in category 2, the percentages of cleaved zygotes that developed to the blastocyst stage did not differ between the two categories. These results demonstrate that oocytes with heterogeneous ooplasm have a higher capacity for normal fertilization due to the reduction in polyspermy. This can be attributed to the normal distribution of cortical granules in category 2 oocytes after maturation.

Deep Imaging of AX J2019+112: The Luminosity of a “Dark Cluster”

We detect a distant cluster of galaxies centered on the QSO lens and luminous X-ray source AX J2019+112, a.k.a. the Dark Cluster. Using deep V and I Keck images and wide-field Ks imaging from the New Technology Telescope (NTT), a tight red sequence of galaxies is identified within a radius of 0.2 h-1 Mpc of the known z = 1.01 elliptical lensing galaxy. The sequence, which includes the central elliptical galaxy, has a slope in good agreement with the model predictions of Kodama et al. for z ~ 1. We estimate the integrated rest-frame luminosity of the cluster to be LV ≥ 3.2 × 1011 h-2 L☉ (after accounting for significant extinction at the low latitude of this field), more than an order of magnitude higher than previous estimates. The central region of the cluster is deconvolved using the technique of Magain, Courbin, & Sohy, revealing a thick central arc coincident with an extended radio source. All the observed lensing features are readily explained by differential magnification of a radio-loud active galactic nucleus by a shallow elliptical potential. The QSO must lie just outside the diamond caustic, producing two images; the arc is a highly magnified image formed from a region close to the center of the host galaxy, projecting inside the caustic. The mass-to-light ratio within an aperture of 0.4 h-1 Mpc is MX/LV = 224h(M/LV)☉, using the X-ray temperature. The strong lens model yields a compatible value, M/LV = 372h(M/LV)☉, whereas an independent weak-lensing analysis sets an upper limit of M/LV < 520h(M/LV)☉, typical of massive clusters.

Gamma-Ray Astronomy and Baryonic Dark Matter

Recently, Dixon et al. have re-analyzed the EGRET data, finding a statistically significant diffuse $\gamma$-ray emission from the galactic halo. We show that this emission can naturally be explained within a previously-proposed model for baryonic dark matter, in which $\gamma$-rays are produced through the interaction of high-energy cosmic-ray protons with cold $H_2$ clouds clumped into dark clusters - these dark clusters supposedly populate the outer galactic halo and can show up in microlensing observations. Our estimate for the halo $\gamma$-ray flux turns out to be in remarkably good agreement with the discovery by Dixon et al. We also address future prospects to test our predictions.

An instrumental source for the dark pixel clusters in the Polar VIS and UVI Experiments

The controversy over the observation of dark pixels by the DE‐1 and Polar imagers has centered around the small‐comet theory rather than an instrumental source to account for the dark pixels. To check for instrumental effects, we simulated the Polar VIS and UVI response to uniform illumination and calculated the rate of dark pixel clusters. The simulation includes the finite size and exponential amplitude of light pulses from the experiments' image intensifiers and the removal of contamination by penetrating radiation. The dark pixel cluster size distributions from the computer simulations reproduce the distributions observed in both of the Polar imagers. (The VIS distributions were produced independently from one day of raw data kindly provided by Frank and Sigwarth and from a world‐wide‐web catalog of atmospheric hole events prepared by Frank and Sigwarth from their own Polar data. These two sets of VIS distributions are in agreement with each other and with the computer simulations.) These instrumental effects fully explain the dark pixel clusters without the need for invoking a geophysical phenomena.

Small‐comet “atmospheric holes” are instrument noise

Frank and Sigwarth claim that dark pixel clusters observed by the VIS Earth Camera are atmospheric holes created by small comets. We have examined their catalog, which contains about 700,000 of these “atmospheric holes”, for consistency with this small‐comet hypothesis and with instrument noise. A geometrical requirement of the small‐comet hypothesis is that the number of pixels in a typical cluster must vary by a factor &gt; 100 with spacecraft altitude because of the inverse‐square law of the apparent cluster area versus distance. We find no systematic variation of duster size with spacecraft altitude. The Iowa catalog data are consistent with instrument noise because neither the size distribution nor the event rate of dark pixel clusters depend on altitude. At altitudes outside of the radiation belts during the one day of available raw data, more than 75% of the dark pixel dusters result from the process that Frank and Sigwarth employ to remove bright pixels caused by energetic particles. This data processing also causes additional meaningless dark pixel clusters to occur in the dark sky or over the dark Earth.

Comparison of dark pixels observed by VIS and UVI in dayglow images

Frank and Sigwarth [1997a] claim that the dark pixels observed in dayglow images obtained by the Earth sensor of the Visible Imaging System (VIS) are due to bombardment of Earth by 20 to 40 ton cosmic snowballs. We have independently studied the VIS data and compared the dark pixels from the VIS images to those obtained from the overlapping images from the Ultraviolet Imager (UVI). We find the occurrence distributions of the dark pixels, single and multiple, from VIS and UVI are nearly identical. The distributions also do not show any altitude dependence. A search for evidence of spacecraft “wobble” motion, whose presence would indicate that the source is external to the camera, has found that pairs of dark pixel clusters are uniformly distributed in orientation and no preference is observed in the wobble direction. Instrument artifacts as the source of the dark pixels is the most likely explanation for these results. The conclusion of this study is that neither VIS nor UVI provide any scientific evidence that the dark pixels are geophysical.

Microlensing Halo Models with Abundant Brown Dwarfs

All previous attempts to understand the microlensing results toward the Large Magellanic Cloud (LMC) have assumed homogeneous present-day mass functions (PDMFs) for the lensing populations. Here, we present an investigation into the microlensing characteristics of halos with spatially varying PDMFs and anisotropic velocity dispersion tensors. One attractive possibility—suggested by baryonic dark cluster formation in pregalactic and protogalactic cooling flows—is that the inner halo is dominated by stellar mass objects, whereas low-mass brown dwarfs become more prevalent on moving outward. The contribution to the microlensing rate must be dominated by dark remnants (~0.5 M☉) to recover the observed timescales of the microlensing experiments. But, even though stellar remnants control the rate, they do not dominate the mass of the baryonic halo, and so the well-known enrichment and mass budget problems are much less severe. Using a simple Ansatz for the spatial variation of the PDMF, models are constructed in which the contribution of brown dwarfs to the mass of the baryonic halo is ~55% and to the total halo is ~30%. An unusual property of the models is that they predict that the average timescale of events toward M31 is shorter than the average timescale toward the LMC. This is because the longer line of sight toward M31 probes more of the far halo in which brown dwarfs are the most common constituent.

Stellar Gas Flows into a Dark Cluster Potential at the Galactic Center

The evidence for the presence of a concentration of dark matter at the Galactic center is now very compelling. There is no question that the stellar and gas kinematics within 0.01 pc is dominated by under-luminous matter in the form of either a massive black hole, a highly condensed distribution of stellar remnants, or a more exotic source of gravity. The unique, compact radio source Sgr A* appears to be coincident with the center of this region, but its size (less than 3x10^14 cm at lambda=1.35cm) is still significantly smaller than the current limiting volume enclosing this mass. Sgr A* may be the black hole, if the dark matter distribution is point-like. If not, we are left with a puzzle regarding its nature, and a question of why this source should be so unique and lie only at the Galactic center. Here, we examine an alternative to the black hole paradigm---that the gravitating matter is a condensed cluster of stellar remnants---and study the properties of the Galactic center wind flowing through this region. Some of this gas is trapped in the cluster potential, and we study in detail whether this hot, magnetized gas is in the proper physical state to produce Sgr A*'s spectrum. We find that at least for the Galactic center environment, the temperature of the trapped gas never attains the value required for significant GHz emission. In addition, continuum (mostly bremsstrahlung) emission at higher frequencies is below the current measurements and upper limits for this source. We conclude that the cluster potential is too shallow for the trapped Galactic center wind to account for Sgr A*'s spectrum, which instead appears to be produced only within an environment that has a steep-gradient potential like that generated by a black hole.

Halo Dark Clusters of Brown Dwarfs and Molecular Clouds

The discovery of Massive Astrophysical Compact Halo Objects (MACHOs) in microlensing experiments makes it compelling to understand their physical nature, as well as their formation mechanism. Within the present uncertainties, brown dwarfs are a viable candidate for MACHOs, and the present paper deals with this option. According to a recently proposed scenario, brown dwarfs are clumped along with cold molecular clouds into dark clusters -- in several respects similar to globular clusters -- which form in the outer part of the galactic halo. Here, we analyze the dynamics of these dark clusters and we address the possibility that a sizable fraction of MACHOs can be binary brown dwarfs. Moreover, we point out that Ly-$\alpha$ absorption systems naturally fit within the present picture.

Binary brown dwarfs in the Galactic halo?

Microlensing events towards the Large Magellanic Cloud entail that a sizable fraction of dark matter is in the form of MACHOs (Massive Astrophysical Compact Halo Objects), presumably located in the halo of the Galaxy. Within the present uncertainties, brown dwarfs are a viable candidate for MACHOs. Various reasons strongly suggest that a large amount of MACHOs should actually consist of binary brown dwarfs. Yet, this circumstance looks in flat contradiction with the fact that MACHOs have been detected as unresolved objects so far. We show that such an apparent paradox does not exist within a model in which MACHOs are clumped into dark clusters along with cold molecular clouds, since dynamical friction on these clouds makes binary brown dwarfs very close. Moreover, we argue that future microlensing experiments with a more accurate photometric observation can resolve binary brown dwarfs.

Binary brown dwarfs in the Galactic halo?

A B STR A CT Microlensing events towards the Large Magellanic Cloud imply that a sizeable fraction of dark matter is in the form of MACHOs (Massive Astrophysical Compact Halo Objects), presumably located in the halo of the Galaxy. Within the present uncertainties, brown dwarfs are viable candidates for MACHOs. Various considerations strongly suggest that a large number of MACHOs should actually consist of binary brown dwarfs. Yet this circumstance appears to be in flat contradiction with the fact that MACHOs have been detected as unresolved objects so far. We show that such an apparent paradox does not exist within a model in which MACHOs are clumped into dark clusters along with cold molecular clouds, since dynamical friction on these clouds makes binary brown dwarfs very close. Moreover, we argue that future microlensing experiments with more accurate photometric observations can resolve binary brown dwarfs.

Does the UVI on polar detect cosmic snowballs?

If 20 to 40 ton cosmic snowballs pelt Earth as claimed by Frank and Sigwarth [1997a], dark pixels will be produced in the 130.4 nm images of dayglow obtained by the Ultraviolet Imager (UVI) on the Polar spacecraft. Examination of the UVI images has revealed that dayglow images are indeed spotted with single and multiple dark pixels. But is a snowball the only explanation for these dark pixels? To learn more about the dark pixels, we have examined the calibration images obtained from the same camera just before the instrument was launched. We find that dark pixels similar to those in dayglow images also exist in calibration images. This strongly indicates that the source of the dark pixels is instrumental. For further verification, a statistical analysis found the dark pixels from dayglow and calibration images have nearly identically shaped occurrence patterns. We have also looked for evidence of spacecraft “wobble” which demonstrates that the source of a bright or dark feature in the images is external to the camera, but found none for dark pixels. Finally, we studied the bright streaks that frequently appear in UVI images, sometimes comet‐like in appearance. These trails are ionization tracks produced by cosmic rays or other penetrating energetic particles interacting with our camera. We conclude that the source of the dark pixels in dayglow images is internal to the camera system and there is no scientific evidence from UVI that snowballs pelt Earth.

Dynamical Constraints on Alternatives to Supermassive Black Holes in Galactic Nuclei

The compelling dynamical evidence for massive dark objects in galactic nuclei does not uniquely imply massive black holes (BHs). To argue convincingly that these objects are BHs we must rule out alternatives to a BH, and the alternative to a point mass is a cluster of some sort of nonluminous objects, such as a cluster of brown dwarfs or stellar remnants. We use simple physical considerations to derive the maximum possible lifetime of a dark cluster that may consist of any plausible form of nonluminous gravitating objects?from brown dwarfs and very low mass objects of cosmic composition to white dwarfs, neutron stars, and black holes. The lower this limit relative to the galaxy age, the more implausible the cluster hypothesis is, thus arguing for a point mass. A cluster with a lifetime much shorter than 10 Gyr is unacceptable, since observing it at the present epoch would be highly improbable. Since the goal is to rule out a dark cluster by showing that its lifetime must be very short, we make the most generous assumptions possible under the observational constraints to allow for its survival. We find that the lifetime of such a hypothetical cluster must be much shorter than the galaxy age only in the cases of NGC 4258 and our Galaxy, thus strongly arguing for a point mass. In all other galaxies, the case of a massive BH, although compelling, is not yet watertight. We also note that there are two exotic alternatives to a massive BH that cannot be ruled out even in the cases of NGC 4258 and the Galaxy: clusters of elementary particles (e.g., bosons), and clusters of very low mass (0.04 M?) BHs. We point out, however, serious difficulties with these alternatives, and argue that they are highly implausible.

On the Stringent Constraint on Massive Dark Clusters in the Galactic Halo

This Letter revises the recent claims of a dynamical constraint on massive dark clusters from the Galactic globular cluster luminosity function (GCLF). We point out that this argument is invalid for two reasons. The first point is that with a proper sample of globular clusters, the GCLF presents a significant trend with Galactocentric position, and second, the disruption timescale of globular clusters by encounters with these massive objects depends strongly on many uncertain parameters. For the usual halo model of Cadwell & Ostriker, it may be smaller by only a factor of 2 for distant globular clusters. Therefore, the existence of massive dark clusters in the halo could be possible. This provides a scenario in which the formation of dark clusters and globular clusters may have a similar origin.

How one galaxy can be a cluster

One would expect a galaxy cluster to be a cluster of galaxies. Now a strange object has been discovered whose X-ray emission betrays it as a massive cluster, but which only contains one obvious galaxy at optical wavelengths. Odder still, this dark cluster is rich in heavy elements, presumably produced by stars - but what stars?

A dark cluster of galaxies at redshift z = 1

The abundance of metals in the hot, gaseous X-ray haloes of galaxy clusters depends crucially on the evolution of the constituent galaxies and their associated stellar populations. The metal abundances in X-ray clusters at high redshifts should therefore provide important insights into the nature and epoch of galaxyformation. Here we report the detection of an extended X-ray source in the direction of the lensed quasi-stellar object MG2016+112 (refs 1, 2). Although deep optical searches have failed to reveal a galaxy cluster at the lens position3,4, the X-ray emission is consistent with thermal bremsstrahlung radiation from a hot metal-rich, diffuse gaseous halo, as observed in nearby galaxy clusters. This is the most distant galaxy cluster discovered in X-rays so far. Furthermore, the mass of the cluster derived from this emission is consistent with that implied by lensing models of the system5. Given that the cluster apparently comprises few galaxies, yet contains a large amount of iron, a new type of astronomical object is implied by our results. A revision of theoretical models of the metal enrichment process in galaxy clusters may therefore be required.

GAMMA-RAY BURSTS FROM HALO NEUTRON STARS CROSSING DARK CLUSTERS

Two classes of neutron stars may exist in the galactic halo: high velocity neutron stars originating from the disk and injected in the halo and neutron stars originating from globular clusters (via type II supernovae and/or accretion induced collapse of white dwarfs). Moreover, the halo dark matter is likely in the form of dark clusters made of Massive Astrophysical Compact Halo Objects (MACHOs) and cold molecular clouds, expected to be formed in the outer part of the galaxy. We suggest that halo neutron stars may emit gamma-ray bursts crossing dark clusters. This assumption allows us to explain all the observed properties of gamma-ray bursts (rate, isotropy, cumulative peak flux distribution), including the different spectral properties of the two classes of short and long bursts. Several methods to test this model, independently on observations of gamma-ray bursts, are discussed.

Infection of Adventitious Roots of Agrostis palustris by Idriella bolleyi1

AbstractSeven isolates of Idriella bolleyi obtained from adventitious roots of Agrostis palustris displaying symptoms of thinning, low vigor, and death of turf in a random, nondescript pattern were evaluated for their ability to infect adventitious roots of A. palustris under high and low temperature regimes and to affect plant growth and symptom expression. All isolates of I. bolleyi infected roots under high and low temperature regimes and all but one isolate decreased shoot and root dry weight. Isolate IB‐2 decreased dry weight under low temperatures, but not under high temperatures. Stolon growth of root‐inoculated plants increased or remained unchanged among plants subjected to high temperature; stolon growth decreased on plants subjected to low temperature. Most isolates of I. bolleyi decreased the chlorophyll content of leaves of plants subjected to high temperature; chlorophyll content of leaves of inoculated plants subjected to low temperature remained unchanged. Hyphal growth was observed on inoculated adventitious roots and was accompanied by dark clusters of fungal cells and chlamydospores on the epidermis and cessation structures within the cortex. Vascular tissues were not penetrated by I. bolleyi. It was concluded that I. bolleyi can infect adventitious roots of A. palustris as a minor root pathogen and alter root and shoot growth.