Galaxies in clusters: the observational characteristics of bow shocks, wakes and tails

The dynamical signatures of the interaction between galaxies in clusters and the intracluster medium (ICM) can potentially yield significant information about the structure and dynamical history of clusters. To develop our understanding of this phenomenon we present results from numerical modelling of the galaxy–ICM interaction, as the galaxy moves through the cluster. The simulations have been performed for a broad range of ICM temperatures ( kT _cl=1, 4 and 8 keV), representative of poor clusters or groups through to rich clusters.
There are several dynamical features that can be identified in these simulations. For supersonic galaxy motion, a leading bow shock is present, and also a weak gravitationally focused wake or tail behind the galaxy (analogous to Bondi–Hoyle accretion). For galaxies with higher mass replenishment rates and a denser interstellar medium (ISM), the dominant feature is a dense ram-pressure stripped tail. In line with other simulations, we find that the ICM/galaxy–ISM interaction can result in complex time-dependent dynamics, with ram-pressure stripping occurring in an episodic manner.
In order to facilitate this comparison between the observational consequences of dynamical studies and X-ray observations we have calculated synthetic X-ray flux and hardness maps from these simulations. These calculations predict that the ram-pressure stripped tail will usually be the most visible feature, though in nearby galaxies the bow shock preceding the galaxy should also be apparent in deeper X-ray observations. We briefly discuss these results and compare them with X-ray observations of galaxies where there is evidence of such interactions.     

Binary brown dwarfs in the Galactic halo?

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.     

Recovering the intrinsic shape of early-type galaxies

We investigate how well the intrinsic shape of early-type galaxies can be recovered when both photometric and two-dimensional stellar kinematic observations are available. We simulate these observations with galaxy models that are representative of observed oblate fast-rotator to triaxial slow-rotator early-type galaxies. By fitting realistic triaxial dynamical models to these simulated observations, we recover the intrinsic shape (and mass-to-light ratio), without making additional (ad hoc) assumptions on the orientation.
For (near) axisymmetric galaxies, the dynamical modelling can strongly exclude triaxiality, but the regular kinematics do not further tighten the constraint on the intrinsic flattening significantly, so that the inclination is nearly unconstrained above the photometric lower limit even with two-dimensional stellar kinematics. Triaxial galaxies can have additional complexity in both the observed photometry and kinematics, such as twists and (central) kinematically decoupled components, which allows the intrinsic shape to be accurately recovered. For galaxies that are very round or show no significant rotation, recovery of the shape is degenerate, unless additional constraints such as from a thin disc are available.     

The evolution of binary fractions in globular clusters

We study the evolution of binary stars in globular clusters using a new Monte Carlo approach combining a population synthesis code ( startrack ) and a simple treatment of dynamical interactions in the dense cluster core using a new tool for computing three- and four-body interactions ( fewbody ). We find that the combination of stellar evolution and dynamical interactions (binary–single and binary–binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster such as 47 Tuc, assuming an initial binary fraction of 100 per cent, is only ∼ 5–10 per cent. We show that this is in good agreement with recent Hubble Space Telescope observations of close binaries in the core of 47 Tuc, provided that a realistic distribution of binary periods is used to interpret the results. Our findings also have important consequences for the dynamical modelling of globular clusters, suggesting that 'realistic models' should incorporate much larger initial binary fractions than has usually been the case in the past.     

The infrared counterpart of the Z source GX 5–1

We have obtained UK Infrared Telescope infrared observations of the field of the bright Galactic Z source GX 5–1. From an astrometric plate solution tied to Tycho–ACT standards we have obtained accurate positions for the stars in our field which, combined with an accurate radio position, have allowed us to identify the probable infrared counterpart of GX 5–1. Narrow-band photometry marginally suggests excess Br γ emission in the counterpart, supporting its association with an accretion-disc source. No significant variability is observed in a limited number of observations. We compare the H and K magnitudes with those of other Z sources, and briefly discuss possible sources of infrared emission in these systems.     

A new dynamical model for the Uranian satellites

We have developed a new dynamical model of the main Uranian satellites, based on numerical integration and fitted to astrometric observations. Old observations, as well as modern and Voyager observations have been included. This model has provided ephemerides that have already been used for predicting the mutual events during the PHE-URA campaign. It is updated here to improve the prediction of these events. We also tried to assess the real accuracy of our ephemerides by checking the distance differences of the Uranian satellites, using simultaneously our former and new model. It appears that both solutions are very close to each other (within few tens of kilometers), and most probably accurate at the level of few hundred of kilometers. Using new available meridian observations of the Uranian satellites, we have checked the Uranian ephemeris accuracy using DE406. An error of more than 0.1 arcsec on the Uranian position is observed.     

Dense molecular gas in a sample of LIRGs and ULIRGs: The low-redshift connection to the huge high-redshift starbursts and AGNs

The sample of nearby LIRGs and ULIRGs for which dense molecular gas tracers have been measured is building up, allowing for the study of the physical and chemical properties of the gas in the variety of objects in which the most intense star formation and/or AGN activity in the local universe is taking place. This characterisation is essential to understand the processes involved, discard others and help to interpret the powerful starbursts and AGNs at high redshift that are currently being discovered and that will routinely be mapped by ALMA. We have studied the properties of the dense molecular gas in a sample of 17 nearby LIRGs and ULIRGs through millimeter observations of several molecules (HCO⁺, HCN, CN, HNC and CS) that trace different physical and chemical conditions of the dense gas in these extreme objects. In this paper we present the results of our HCO⁺ and HCN observations. We conclude that the very large range of measured line luminosity ratios for these two molecules severely questions the use of a unique molecular tracer to derive the dense gas mass in these galaxies.     

XMM–Newton observations of soft gamma-ray repeaters

All the confirmed Soft Gamma-ray Repeaters have been observed with the EPIC instrument on the XMM–Newton satellite. We review the results obtained in these observations, providing the most accurate spectra on the persistent X-ray emission in the 1–10 keV range for these objects, and discuss them in the context of the magnetar interpretation.      

A MERLIN neutral hydrogen absorption study of the luminous infrared merger NGC 6240

We present neutral hydrogen absorption observations of the luminous infrared merger NGC 6240 using MERLIN with a resolution of 0.2 arcsec. Broad absorption (a few hundred km s⁻¹) has been found against two compact radio sources within the central kpc providing dynamical information about the neutral gas components in front of these sources. A narrow absorption component is also detected superimposed upon this broad absorption and additionally against some of the extended L -band continuum. From these results we deduce that the broad component is a result of absorption by a highly disturbed disc-like structure of neutral gas aligned along the position angle of the two compact radio sources, similar to the model previously proposed by Tacconi et al. at the end of the last century based on spectral CO emission data. The narrow component is likely to arise from absorption by less disturbed neutral gas at much larger scales within the system.
Continuum observations presented here at 1.4 and 5 GHz support the view that NGC 6240 contains a double nucleus resulting from a galactic merger event and show these as two compact radio sources separated by 1.52 arcsec. We have also applied luminosity and morphological considerations to the continuum results to determine the most feasible source of radio emission for this luminous merger galaxy. We conclude that the most likely source of the radio flux found in NGC 6240 is a combination of starburst emission from radio luminous supernova remnants, similar to those found in Arp 220, and emission from a weak AGN probably triggered by a merger event.     

Modelling the formation of individual galaxies: A morphology problem for CDM?

We use a semi-analytic model of halo formation to study the dynamical history of giant field galaxies like the Milky Way. We find that in a concordance LCDM cosmology, most isolated disk galaxies have remained undisturbed for 8–10 Gyr, such that the age of the Milky Way's thin disk is unremarkable. Many systems also have older disk components which have been thickened by minor mergers, consistent with recent observations of nearby field galaxies. We do have a considerable problem, however, reproducing the morphological mix of nearby galaxies. In our fiducial model, most systems have disk-to-bulge mass ratios of order 1, and look like S0s rather than spirals. This result depends mainly on merger statistics, and is unchanged for most reasonable choices of our model parameters. We discuss two possible solutions to this morphology problem in LCDM. This revised version was published online in August 2006 with corrections to the Cover Date.     

Chandra High-Resolution Camera observations of the luminous X-ray source in the starburst galaxy M82

We analyse Chandra High Resolution Camera observations of the starburst galaxy M82, concentrating on the most luminous X-ray source. We find a position for the source of         (J2000) with a 1 σ radial error of 0.7 arcsec. The accurate X-ray position shows that the luminous source is neither at the dynamical centre of M82 nor coincident with any suggested radio AGN candidate. The source is highly variable between observations, which suggests that it is a compact object and not a supernova or remnant. There is no significant short-term variability within the observations. Dynamical friction and the off-centre position place an upper bound of 10⁵–10⁶ M_⊙ on the mass of the object, depending on its age. The X-ray luminosity suggests a compact object mass of at least 500 M_⊙. Thus the luminous source in M82 may represent a new class of compact object with a mass intermediate between those of stellar-mass black hole candidates and supermassive black holes.     

Expected Number Counts of Radio Galaxy Clusters

1 INTRODUCTION Over the past years, diffuse radio halos have been detected in a few tens of nearby, richclusters. They often extend to a distance of 1 Mpc from the cluster centers, and have regularshape, low surface brightness and steep radio spectrum. Some clusters also contain peculiarradio structures called radio relics. Both radio halos and relics are believed to arise from themerging of sub-cluster structures (Buote 2001). The first radio halo, Coma C, was detected about 30 years a…     

Photometric and spectroscopic investigation of the oscillating Algol type binary: EW Boo

We obtained the physical and geometrical parameters of the EW Boo system, which exhibits short period and small amplitude pulsations as well as brightness variations due to orbital motion of components. Towards this end we carried out photometric observations at Ankara University Kreiken Observatory (AUKO) as well as spectroscopic observations at TUBITAK National Observatory (TNO). The light and radial velocity curves obtained from these observations have been simultaneously analyzed with PHOEBE and the absolute parameters of the system along with the geometric parameters of the components have been determined. Using model light curves of EW Boo, light curve regions in which the pulsations are active have been determined and as a result of analyses performed in the frequency region, characteristic parameters of pulsations have been obtained. We find that the results are compatible with current parameters of similar systems in the literature. The evolutionary status of the components is propounded and discussed.     

Modelling gaseous and stellar kinematics in the disc galaxies NGC 772, 3898 and 7782

We present V -band surface photometry and major-axis kinematics of stars and ionized gas of three early-type spiral galaxies, namely NGC 772, 3898 and 7782. For each galaxy we present a self-consistent Jeans model for the stellar kinematics, adopting the light distribution of bulge and disc derived by means of a two-dimensional parametric photometric decomposition. This allows us to investigate the presence of non-circular gas motions, and derive the mass distribution of luminous and dark matter in these objects.
NGC 772 and 7782 have apparently normal kinematics with the ionized gas tracing the gravitational equilibrium circular speed. This is not true in the innermost region (| r |≲8 arcsec) of NGC 3898, where the ionized gas is rotating more slowly than the circular velocity predicted by dynamical modelling. This phenomenon is common in the bulge-dominated galaxies for which dynamical modelling enables us to make the direct comparison between the gas velocity and the circular speed, and it poses questions about the reliability of galaxy mass distributions derived by the direct decomposition of the observed ionized-gas rotation curve into the contributions of luminous and dark matter.     

Using XMM-Newton to measure the spectrum of the Vela pulsar and its phase variation

We report on our analysis of two XMM-Newton observations of the Vela pulsar performed in December 2000 (total exposure time: 96.5 ks). We succeeded in resolving the pulsar spectrum from the surrounding bright nebular emission taking advantage both of the accurate calibration of the EPIC point spread function and of the Chandra/HRC surface brightness map of the nebula. This made it possible to assess to pulsar spectral shape disentangling its thermal and non-thermal components. Exploiting the photon harvest, we have also been able to perform a phase-resolved study of the pulsar emission.      

-ray wakes as probes of galaxy cluster dynamics

If a galaxy resides in a cluster, then its passage through the pervasive intracluster medium will produce a detectable signature in the X-ray emission from the cluster. Such features have now been detected in a number of systems. The simplest kinematic information that can be extracted from this signature is the galaxy's direction of motion on the plane of the sky. This paper explores the constraints on cluster dynamics that could be derived from such information. In particular, we show that it is possible to define a projected anisotropy parameter, B ( r ), which is directly analogous to the usual orbital anisotropy parameter. We describe an estimator for this quantity, ( R ), which can be derived in a robust and straightforward manner. We present a simple dynamical model for a cluster consisting of a Michie distribution function of galaxies orbiting in a truncated singular isothermal sphere potential. Using this model, we demonstrate the ambiguity between the distribution of mass and the distribution of galaxy orbits when interpreting the traditional measures of cluster kinematics (the projected density of galaxies and their line-of-sight velocity dispersion). As an example, we show how two very different dynamical models can fit the kinematic properties of the Coma cluster. We demonstrate that the measurement of using a relatively small sample of wake directions ( N _wake≈50) would provide an effective mechanism for lifting this degeneracy. Thus, by combining X-ray measurements of wake directions with number counts and line-of-sight velocities derived from optical data, it will prove possible to measure both the orbit distribution and the form of the gravitational potential in clusters of galaxies. The requisite X-ray observations lie within reach of the soon-to-be-launched AXAF satellite.     

Dynamical Evolution of Gamma-Ray Burst Remnants with Evolving Radiative Efficiency

In previous works, a generic dynamical model has been suggested by Huang et al., which is shown to be correct for both adiabatic and radiative blast-waves, in both ultra-relativistic and non-relativistic phases. In deriving their equations, Huang et al. have assumed that the radiative efficiency of the fireball is constant. They then applied their model directly to realistic cases where the radiative efficiency evolves with time. In this paper, we abandon the above assumption and re-derive a more accurate dynamical equation for gamma-ray burst remnants. Numerical results show that the model presented by Huang et al. is accurate enough in general cases.     

Disc-like objects in hierarchical hydrodynamical simulations: comparison with observations

We present results from a careful and detailed analysis of the structural and dynamical properties of a sample of 29 disc-like objects identified at z =0 in three AP3M–SPH fully consistent cosmological simulations. These simulations are realizations of a CDM hierarchical model, in which an inefficient Schmidt-law-like algorithm to model the stellar formation process has been implemented. We focus on properties that can be constrained with available data from observations of spiral galaxies, namely the bulge and disc structural parameters and the rotation curves. Comparison with data from Broeils, de Jong and Courteau gives satisfactory agreement, in contrast with previous findings using other codes. This suggests that the stellar formation implementation we have used has succeeded in forming compact bulges that stabilize disc-like structures in the violent phases of their assembly, while in the quiescent phases the gas has cooled and collapsed in accord with the Fall & Efstathiou standard model of disc formation.     

The expected detection of dust emission from high-redshift Lyman α galaxies

Recent results have shown that a substantial fraction of high-redshift Lyman α (Lyα) galaxies contain considerable amounts of dust. This implies that Lyα galaxies are not primordial, as has been thought in the past. However, this dust has not been directly detected in emission; rather it has been inferred based on extinction estimates from rest-frame ultraviolet (UV) and optical observations. This can be tricky, as both dust and old stars redden galactic spectra at the wavelengths used to infer dust. Measuring dust emission directly from these galaxies is thus a more accurate way to estimate the total dust mass, giving us real physical information on the stellar populations and interstellar medium enrichment. New generation instruments, such as the Atacama Large Millimeter Array and Sub-Millimeter Array, should be able to detect dust emission from some of these galaxies in the submillimeter. Using measurements of the UV spectral slopes, we derive far-infrared flux predictions for of a sample of  23 z ≥ 4  Lyα galaxies. We find that in only a few hours, we can detect dust emission from 39 ± 22 per cent of our Lyα galaxies. Comparing these results to those found from a sample of 21 Lyman break galaxies (LBGs), we find that LBGs are on average 60 per cent more likely to be detected than Lyα galaxies, implying that they are more dusty, and thus indicating an evolutionary difference between these objects. These observations will provide better constraints on dust in these galaxies than those derived from their UV and optical fluxes alone. Undeniable proof of dust in these galaxies could explain the larger than expected Lyα equivalent widths seen in many Lyα galaxies today.