Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  




Upload your image

DSS Images   Other Images

Related articles

RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.

Hubble's top 10.
Not Available

Explaining the Color Distributions of Globular Cluster Systems in Elliptical Galaxies
The colors of globular clusters in most large elliptical galaxies arebimodal. This is generally taken as evidence for the presence of twocluster subpopulations that have different geneses. However, here wefind that, because of the nonlinear nature of the metallicity-to-colortransformation, a coeval group of old clusters with a unimodalmetallicity spread can exhibit color bimodality. The models of clustercolors indicate that horizontal-branch stars are the main drivers behindthe empirical nonlinearity. We show that the scenario gives simple andcohesive explanations for all the key observations and could simplifytheories of elliptical galaxy formation.

A quasar in every galaxy ?
Not Available

Astronomie gamma : le ciel revele aux tres hautes energies.
Not Available

X-ray spectroscopy of cooling clusters
Electronic Article Available from Elsevier Science.

Environmental Effects on Late-Type Galaxies in Nearby Clusters
The transformations that take place in late-type galaxies in theenvironment of rich clusters of galaxies at z=0 are reviewed. From thehandful of late-type galaxies that inhabit local clusters, whether theywere formed in situ and survived as such, avoiding transformation oreven destruction, or if they are newcomers that have recently fallen infrom outside, we can learn an important lesson on the latest stages ofgalaxy evolution. We start by reviewing the observational scenario,covering the broadest possible stretch of the electromagnetic spectrum,from the gas tracers (radio and optical) to the star formation tracers(UV and optical), the old star tracers (near-IR), and the dust (far-IR).Strong emphasis is given to the three nearby, well-studied clustersVirgo, A1367, and Coma, which are representative of differentevolutionary stages, from unrelaxed and spiral-rich (Virgo) to relaxedand spiral-poor (Coma). We continue by providing a review of models ofgalaxy interactions that are relevant to clusters of galaxies.Prototypes of various mechanisms and processes are discussed, and theirtypical timescales are given in an appendix. Observations indicate thepresence of healthy late-type galaxies falling into nearby clustersindividually or as part of massive groups. More rare are infallinggalaxies belonging to compact groups, where significant preprocessingmight take place. Once they have entered the cluster, they lose theirgas and quench their star formation activity, becoming anemic.Observations and theory agree in indicating that the interaction withthe intergalactic medium is responsible for the gas depletion. However,this process cannot be the origin of the cluster lenticular galaxypopulation. Physical and statistical properties of S0 galaxies in nearbyclusters and at higher redshift indicate that they originate from spiralgalaxies that have been transformed by gravitational interactions.

Milliarcsecond-Scale Spectral Properties and Jet Motions in M 87
We have combined high resolution VLBI Space Observatory Programme (VSOP)data at 1.6 and 4.8GHz with Very Long Baseline Array (VLBA) data athigher frequencies and with similar resolutions to study the spectralproperties of the core of M 87 with milliarcsec resolution. The VSOPdata allows a more accurate measurement of the turnover frequency, andhence more reliable determination of associated physical parameters ofthe source. Comparison of the images with previously published imagesyields no evidence for significant motions of components in theparsec-scale jet. In addition, the brightness temperatures obtained frommodelfits to the core are well below the inverse Compton limit,suggesting that the radio emission we are observing is not stronglyDoppler boosted.

Ten Milliparsec-Scale Structure of the Nucleus Region in Centaurus A
We present the results of a VLBI Space Observatory Programme (VSOP)observation of the subparsec structure in Cen A at 4.9GHz. Theobservation produced an image of the subparsec jet components with aresolution of three-times better than images from previous VLBImonitoring campaigns at 8.4GHz, and twice better than the previous 22GHzstudies. Owing to its proximity, our Cen A space-VLBI image is one ofthe highest spatial-resolution images of an AGN ever made -- 0.01pc perbeam -- comparable only to the recent 43GHz VLBI images of M87. Theelongated core region is resolved into several components of over 10milliarcsec long (0.2pc), including a compact component of brightnesstemperature 2.2 × 1010 K. A counterjet was detected: ifwe assume jet-counterjet symmetry, a relatively slow jet speed, and alarge viewing angle, as derived from previous observations, the imageallows us to investigate the distribution of ionized gas around thecore, which is opaque at this frequency due to free-free absorption. Wealso analyzed the jet geometry in terms of collimation. Assuming thestrongest component to be the core, the jet opening angle at ˜ 5000rS from the core is estimated to be ˜ 12°, with thecollimation of the jet to ˜ 3° continuing out to ˜ 20000rS. This result is consistent with previous studies of thejet in M87, which favor MHD disk outflow models. Future space VLBIobservations at higher frequencies will probably be able to image thecollimation region, within 1000 rS of the center of Cen A,together with the accretion disk itself.

Stellar kinematics and populations of early-type galaxies with the SAURON and OASIS integral-field spectrographs
We summarise the results and achievements of integral-field spectroscopyof early-type galaxies, observed as part of a survey using both theSAURON and OASIS spectrographs. From the perspective of integral-fieldspectroscopy, these otherwise smooth and featureless objects show awealth of structure, both in their stellar kinematics and populations.We focus on the stellar content, and examine properties on bothkiloparsec scales with SAURON, and scales of 100’s of parsecs withOASIS. These complementary studies reveal two types of kinematicallydistinct components (KDCs), differing primarily in their intrinsicsizes. In previous studies, KDCs and their host galaxies have generallybeen found to be unremarkable in other aspects. We show that large KDCs,typical of the well-studied cases, indeed show little or no agedifferences with their host galaxy. The KDCs detected with the higherspatial-resolution of OASIS are intrinsically smaller and include, incontrast, a significant fraction of young stars. We speculate on therelationship between KDCs and their host galaxies, and the implicationsfor young populations in early-type galaxies.

The source of mass accreted by the central black hole in cooling flow clusters
This paper reports the study of the cold-feedback heating in coolingflow clusters. In the cold-feedback model the mass accreted by thecentral black hole originates in non-linear over-dense blobs of gasresiding in an extended region (r ≲ 5–30 kpc); these blobsare originally hot, but then cool faster than their environment and sinktoward the center. The intra-cluster medium (ICM) entropy profile mustbe shallow for the blobs to reach the center as cold blobs. I build atoy model to explore the role of the entropy profile and the populationof dense blobs in the cold-feedback mechanism. The mass accretion rateby the central black hole is determined by the cooling time of the ICM,the entropy profile, and the presence of inhomogeneities. The massaccretion rate determines the energy injected by the black hole back tothe ICM. These active galactic nucleus (AGN) outbursts not only heat theICM, but also change the entropy profile in the cluster and causeinhomogeneities that are the seeds of future dense blobs. Therefore, inaddition to the ICM temperature (or energy), the ICM entropy profile andICM inhomogeneities are also ingredients in the feedback mechanism.

Scalar potential model of redshift and discrete redshift
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.

RCoronae Borealis at the 2003 light minimum.
Not Available

Bulk motion of ultrarelativistic conical blazar jets
Allowing for the conical shape of ultrarelativistic blazar jets withopening angles of a few degrees on parsec-scales, we show that theirbulk Lorentz factors and viewing angles can be much larger than thevalues usually inferred by combining their flux-variability andproper-motion measurements. This is in accord with our earlier findingthat such ultrarelativistic (Lorentz factor, Γ > 30) conicaljets can reconcile the relatively slow apparent motions of Very LongBaseline Interferometry (VLBI) knots in TeV blazars with the extremelyfast flows implied by their rapid γ-ray variability. This jetgeometry also implies that de-projected jet opening angles willtypically be significantly underestimated from VLBI measurements. Inaddition, de-projected jet lengths will be considerably overestimated ifhigh Lorentz factors and significant opening angles are not taken intoaccount.

Magnetic jets from swirling discs
A broad swathe of astrophysical phenomena, ranging from tubularplanetary nebulae through Herbig-Haro objects, radio galaxy and quasaremissions to gamma-ray bursts and perhaps high-energy cosmic rays, maybe driven by magnetically dominated jets emanating from accretion discs.We give a self-contained account of the analytic theory ofnon-relativistic magnetically dominated jets wound up by a swirling discand making a magnetic cavity in a background medium of any prescribedpressure, p(z). We solve the time-dependent problem for any specifieddistribution of magnetic flux P(R, 0) emerging from the disc at z = 0,with any specified disc angular velocity Ωd(R). Thephysics required to do this involves only the freezing of the lines offorce to the conducting medium and the principle of minimum energy.In a constant pressure environment, the magnetically dominated cavity ishighly collimated and advances along the axis at a constant speedclosely related to the maximum circular velocity of the accretion disc.Even within the cavity the field is strongly concentrated towards theaxis. The twist in the jet field/<|Bz|> is close to and thewidth of the jet decreases upwards. By contrast, when the backgroundpressure falls off with height with powers approaching z-4,the head of the jet accelerates strongly and the twist of the jet ismuch smaller. The width increases to give an almost conical magneticcavity with apex at the source. Such a regime may be responsible forsome of the longest strongly collimated jets. When the backgroundpressure falls off faster than z-4, there are no quasi-staticconfigurations of well-twisted fields and the pressure confinement isreplaced by a dynamic effective pressure or a relativistic expansion. Inthe regimes with rapid acceleration, the outgoing and incoming fieldslinking the twist back to the source are almost anti-parallel so thereis a possibility that magnetic reconnections may break up the jet into aseries of magnetic `smoke-rings' travelling out along the axis.

The SAURON project - VI. Line strength maps of 48 elliptical and lenticular galaxies
We present absorption line strength maps of 48 representative ellipticaland lenticular galaxies obtained as part of a survey of nearby galaxiesusing our custom-built integral-field spectrograph, SAURON, operating onthe William Herschel Telescope. Using high-quality spectra, spatiallybinned to a constant signal-to-noise ratio, we measure four key age,metallicity and abundance ratio sensitive indices from the Lick/IDSsystem over a two-dimensional field extending up to approximately oneeffective radius. A discussion of calibrations and offsets is given,along with a description of error estimation and nebular emissioncorrection. We modify the classical Fe5270 index to define a new index,Fe5270S, which maximizes the useable spatial coverage ofSAURON. Maps of Hβ, Fe5015, Mgb and Fe5270S arepresented for each galaxy. We use the maps to compute average linestrengths integrated over circular apertures of one-eighth effectiveradius, and compare the resulting relations of index versus velocitydispersion with previous long-slit work. The metal line strength mapsshow generally negative gradients with increasing radius roughlyconsistent with the morphology of the light profiles. Remarkabledeviations from this general trend exist, particularly the Mgb isoindexcontours appear to be flatter than the isophotes of the surfacebrightness for about 40 per cent of our galaxies without significantdust features. Generally, these galaxies exhibit significant rotation.We infer from this that the fast-rotating component features a highermetallicity and/or an increased Mg/Fe ratio as compared to the galaxy asa whole. The Hβ maps are typically flat or show a mild positiveoutwards radial gradient, while a few galaxies show strong central peaksand/or elevated overall Hβ strength likely connected to recent starformation activity. For the most prominent post-starburst galaxies, eventhe metal line strength maps show a reversed gradient.

The infrared jet in Centaurus A: multiwavelength constraints on emission mechanisms and particle acceleration
We report on Spitzer and Gemini observations of the jet of Centaurus Ain the infrared, which we combine with radio, ultraviolet and X-raydata. Spitzer detects jet emission from about 2 arcmin from the nucleus,the emission becoming particularly bright after the jet flare point at~3.4 arcmin. Where X-ray and infrared emission are seen together thebroad-band data strongly support a synchrotron origin for the X-rays.The jet flare point is marked by a broad, diffuse region of X-rays whichmay be associated with a shock: we discuss possible physical mechanismsfor this. The infrared jet persists after the flare point regionalthough X-ray emission is absent; it is plausible that here we areseeing the effects of particle acceleration followed by downstreamadvection with synchrotron losses. Gemini data probe the inner regionsof the jet, putting limits on the mid-infrared flux of jet knots.

General relativistic magnetohydrodynamic simulations of the jet formation and large-scale propagation from black hole accretion systems
The formation and large-scale propagation of Poynting-dominated jetsproduced by accreting, rapidly rotating black hole systems are studiedby numerically integrating the general relativistic magnetohydrodynamicequations of motion to follow the self-consistent interaction betweenaccretion discs and black holes. This study extends previous similarwork by studying jets till t~ 104GM/c3 out to r~104GM/c2, by which the jet is superfastmagnetosonic and moves at a lab-frame bulk Lorentz factor of Γ~ 10with a maximum terminal Lorentz factor of Γ&infy;<~103. The radial structure of the Poynting-dominated jet ispiece-wise self-similar, and fits to flow quantities along the fieldline are provided. Beyond the Alfvén surface at r~10-100GM/c2, the jet becomes marginally unstable to (atleast) current-driven instabilities. Such instabilities drive shocks inthe jet that limit the efficiency of magnetic acceleration andcollimation. These instabilities also induce jet substructure with 3<~Γ<~ 15. The jet is shown to only marginally satisfy thenecessary and sufficient conditions for kink instability, so this mayexplain how astrophysical jets can extend to large distances withoutcompletely disrupting. At large distance, the jet angular structure isGaussian-like (or uniform within the core with sharp exponential wings)with a half-opening angle of ~5° and there is an extended componentout to ~ 27°. Unlike in some hydrodynamic simulations, theenvironment is found to play a negligible role in jet structure,acceleration, and collimation as long as the ambient pressure of thesurrounding medium is small compared to the magnetic pressure in thejet.

The dipole anisotropy of the 2 Micron All-Sky Redshift Survey
We estimate the acceleration on the Local Group (LG) from the 2 MicronAll-Sky Redshift Survey (2MRS). The sample used includes about 23200galaxies with extinction-corrected magnitudes brighter thanKs= 11.25 and it allows us to calculate the flux-weighteddipole. The near-infrared flux-weighted dipoles are very robust becausethey closely approximate a mass-weighted dipole, bypassing the effectsof redshift distortions and require no preferred reference frame. Thisis combined with the redshift information to determine the change indipole with distance. The misalignment angle between the LG and thecosmic microwave background (CMB) dipole drops to 12°+/- 7° ataround 50h-1Mpc, but then increases at larger distances,reaching 21°+/- 8° at around 130h-1Mpc. Exclusion ofthe galaxies Maffei 1, Maffei 2, Dwingeloo 1, IC342 and M87 brings theresultant flux dipole to 14°+/- 7° away from the CMB velocitydipole. In both cases, the dipole seemingly converges by60h-1Mpc. Assuming convergence, the comparison of the 2MRSflux dipole and the CMB dipole provides a value for the combination ofthe mass density and luminosity bias parametersΩ0.6m/bL= 0.40 +/- 0.09.

Magnetic fields in the centre of the Perseus cluster
We present Very Long Baseline Array (VLBA) observations of the nucleusof NGC1275, the central, dominant galaxy in the Perseus cluster ofgalaxies. These are the first observations to resolve the linearlypolarized emission from 3C84, and from them we determine a Faradayrotation measure (RM) ranging from 6500 to 7500radm-2 acrossthe tip of the bright southern jet component. At 22GHz some polarizationis also detected from the central pc of 3C84, indicating the presence ofeven more extreme RMs that depolarize the core at lower frequencies. Thenature of the Faraday screen is most consistent with being produced bymagnetic fields associated with the optical filaments of ionized gas inthe Perseus cluster.

Planetary nebulae as tracers of galaxy stellar populations
We address the general problem of the luminosity-specific planetarynebula (PN) number, better known as the `α' ratio, given byα=NPN/Lgal, and its relationship with theage and metallicity of the parent stellar population. Our analysisrelies on population synthesis models that account for simple stellarpopulations (SSPs), and more elaborate galaxy models covering the fullstar formation range of the different Hubble morphological types. Thistheoretical framework is compared with the updated census of the PNpopulation in Local Group (LG) galaxies and external ellipticals in theLeo group, and the Virgo and Fornax clusters.The main conclusions of our study can be summarized as follows. (i)According to the post-asymptotic giant branch (AGB) stellar core mass,PN lifetime in a SSP is constrained by three relevant regimes, driven bythe nuclear (Mcore>~ 0.57Msolar), dynamical(0.57Msolar>~Mcore>~ 0.55Msolar)and transition (0.55Msolar>~Mcore>~0.52Msolar) time-scales. The lower limit for Mcorealso sets the minimum mass for stars to reach the AGB thermal-pulsingphase and experience the PN event. (ii) Mass loss is the crucialmechanism to constrain the value of α, through the definition ofthe initial-to-final mass relation (IFMR). The Reimers mass-lossparametrization, calibrated on Pop II stars of Galactic globularclusters, poorly reproduces the observed value of α in late-typegalaxies, while a better fit is obtained using the empirical IFMRderived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds10000yr, which suggests that Mcore<~ 0.65Msolarcores dominate, throughout. (iv) The relative PN deficiency inelliptical galaxies, and the observed trend of α with galaxyoptical colours, support the presence of a prevailing fraction oflow-mass cores (Mcore<~ 0.55Msolar) in the PNdistribution and a reduced visibility time-scale for the nebulae as aconsequence of the increased AGB transition time. The stellar componentwith Mcore<~ 0.52Msolar, which overrides the PNphase, could provide an enhanced contribution to hotter HB and post-HBevolution, as directly observed in M 32 and the bulge of M 31. Thisimplies that the most UV-enhanced ellipticals should also display thelowest values of α, as confirmed by the Virgo cluster early-typegalaxy population. (v) Any blue-straggler population, invoked asprogenitor of the Mcore>~ 0.7Msolar PNe inorder to preserve the constancy of the bright luminosity-functioncut-off magnitude in ellipticals, must be confined to a small fraction(a few per cent at most) of the whole galaxy PN population.

The influence of radio galaxy activity on X-ray absorption lines from the intracluster medium
We present an investigation of the X-ray absorption features predictedby hydrodynamic simulations of radio galaxies interacting with theintracluster medium (ICM) of their host galaxy clusters. We show howthese absorption lines can be used as a new diagnostic for the radiogalaxy-ICM interactions. Such interactions have been observed innumerous systems by ROSAT, Chandra and XMM-Newton, and understandingthem has implications for active galactic nucleus (AGN) feedback andgalaxy formation. Starting from the hydrodynamic simulations ofReynolds, Heinz & Begelman, we calculate the properties of thehighly ionized iron and oxygen lines (seen in absorption against thecentral AGN), predicting line shapes, equivalent widths, columndensities and velocity shifts. The main effect of the jet on theabsorption lines is a reduction of the line strength from that of thequiescent ICM and the introduction of some velocity structure in theline profile. We investigate whether these features are detectable withcurrent as well as future high-resolution X-ray spectrometers. Weconclude that the Chandra transmission gratings have insufficientsensitivity to detect these features with high significance, andcertainly would not allow a study of the dynamics of the interaction viaabsorption signatures. Constellation-X, on the other hand, will allowsuperb constraints to be derived. We can also use this analysis toassess the idea that radio galaxy induced ICM outflows give rise to theresonant oxygen X-ray absorption lines that have been claimed asevidence for the warm-hot intergalactic medium (WHIM). We show thatthese detached, high-velocity oxygen absorption lines cannot result froma radio galaxy-ICM interaction, thereby strengthening the WHIMinterpretation.

Gemini/GMOS spectra of globular clusters in the Virgo giant elliptical NGC 4649
NGC 4649 (M60) is one of a handful of giant Virgo ellipticals. We haveobtained Gemini/GMOS (Gemini North Multi-Object Spectrograph) spectrafor 38 globular clusters (GCs) associated with this galaxy. Applying themulti-index χ2 minimization technique of Proctor andSansom with the single stellar population models of Thomas, Maraston andKorn, we derive ages, metallicities and α-element abundanceratios. We find several young (2-3 Gyr old) supersolar metallicity GCs,while the majority are old (>10 Gyr), spanning a range ofmetallicities from solar to [Z/H]=-2. At least two of these young GCsare at large projected radii of 17-20 kpc. The galaxy itself shows noobvious signs of a recent starburst, interaction or merger. A trend ofdecreasing α-element ratio with increasing metallicity is found.

Constraints on alternatives to supermassive black holes
Observations of the centres of galaxies continue to evolve, and it isuseful to take a fresh look at the constraints that exist onalternatives to supermassive black holes at their centres. We discussconstraints complementary to those of Maoz and demonstrate that anextremely wide range of other possibilities can be excluded. Inparticular, we present the new argument that for the velocitydispersions inferred for many galactic nuclei, even binaries made ofpoint masses cannot stave off core collapse because hard binaries are sotight that they merge via emission of gravitational radiation beforethey can engage in three-body or four-body interactions. We also showthat under these conditions core collapse leads inevitably to runawaygrowth of a central black hole with a significant fraction of theinitial mass, regardless of the masses of the individual stars. Forclusters of non-interacting low-mass objects (from low-mass stars toelementary particles), the relaxation of stars and compact objects thatpass inside the dark region will be accelerated by interactions with thedark mass. If the dark region is instead a self-supported object, suchas a fermion ball, then if stellar-mass black holes exist they willcollide with the object, settle, and consume it. The net result is thatthe keyhole through which alternatives to supermassive black holes mustpass is substantially smaller and more contrived than it was even a fewyears ago.

Enhanced mass-to-light ratios in ultracompact dwarf galaxies through tidal interaction with the centre of the host galaxy
A recent study of ultracompact dwarf galaxies (UCDs) in the Virgocluster revealed that some of them show faint envelopes and havemeasured mass-to-light ratios (M/Ls) of 5 and larger, which cannot beexplained by simple population-synthesis models. It is believed thatthis proves that some of the UCDs must possess a dark matter halo andmay therefore be the stripped nuclei of dwarf ellipticals rather thanmerged star cluster complexes.Using an efficient N-body method, we investigate if a close passage of aUCD through the central region of the host galaxy is able to enhance themeasured M/L by tidal forces leaving the satellite slightly out ofvirial equilibrium and thereby leading to an overestimation of itsvirial mass.We find this to be possible and discuss the general problem of measuringdynamical masses for objects that are probably interacting with theirhosts.

Imaging optically-thin hotspots near the black hole horizon of Sgr A* at radio and near-infrared wavelengths
Submilliarcsecond astrometry and imaging of the black hole Sgr A* at theGalactic Centre may become possible in the near future at infrared andsubmillimetre wavelengths. Motivated by the observations of short-terminfrared and X-ray variability of Sgr A*, in a previous paper, wecomputed the expected images and light curves, including polarization,associated with a compact emission region orbiting the central blackhole. We extend this work, using a more realistic hotspot model andincluding the effects of opacity in the underlying accretion flow. Wefind that at infrared wavelengths, the qualitative features identifiedby our earlier work are present, namely it is possible to extract theblack hole mass and spin from spot images and light curves of theobserved flux and polarization. At radio wavelengths, disc opacityproduces significant departures from the infrared behaviour, but thereare still generic signatures of the black hole properties. Detailedcomparison of these results with future data can be used to test generalrelativity and to improve existing models for the accretion flow in theimmediate vicinity of the black hole.

On the origin and excitation of the extended nebula surrounding NGC1275
We use line-of-sight velocity information on the filamentaryemission-line nebula of NGC1275 to infer a dynamical model of thenebula's flow through the surrounding intracluster gas. We detectoutflowing gas and flow patterns that match simulations of buoyantlyrising bubbles from which we deduce that some of the nebula filamentshave been drawn out of NGC1275. We find a radial gradient of the ratio[NII]λ6584/Hα which may be due to a variation inmetallicity, interactions with the surrounding intracluster medium or ahardening of the excitation mechanism. We find no preferred spatialcorrelation of stellar clusters within the filaments and there is anotable lack of [OIII]λ5007 emission, therefore it is unlikelythat the filaments are ionized by stellar ultraviolet.

Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
We introduce and discuss the properties of a theoretical (B-K)(J-K)integrated colour diagram for single-age, single-metallicity stellarpopulations. We show how this combination of integrated colours is ableto largely disentangle the well-known age-metallicity degeneracy whenthe age of the population is greater than ~300Myr, and thus providesvaluable estimates of both age and metallicity of unresolved stellarsystems. We discuss in detail the effect on this colour-colour diagramof α-enhanced metal abundance ratios (typical of the oldestpopulations in the Galaxy), the presence of blue horizontal branch starsunaccounted for in the theoretical calibration and of statistical colourfluctuations in low-mass stellar systems. In the case of populationswith multiple stellar generations, the luminosity-weighted mean ageobtained from this diagram is shown to be heavily biased towards theyoungest stellar components. We then apply this method to several datasets for which optical and near-infrared photometry are available in theliterature. We find that Large Magellanic Cloud and M31 clusters havecolours which are consistent with the predictions of the models, butthese do not provide a sensitive test due to the fluctuations which arepredicted by our modelling of the Poisson statistics in such low-masssystems. For the two Local Group dwarf galaxies NGC 185 and 6822, themean ages derived from the integrated colours are consistent with thestar formation histories inferred independently from photometricobservations of their resolved stellar populations.The methods developed here are applied to samples of nearby early-typegalaxies with high-quality aperture photometry in the literature. Asample of bright field and Virgo cluster elliptical galaxies is found toexhibit a range of luminosity-weighted mean ages from 3 to 14Gyr, with amean of ~8Gyr, independent of environment, and mean metallicities at orjust above the solar value. Colour gradients are found in all of thegalaxies studied, in the sense that central regions are redder. Apartfrom two radio galaxies, where the extreme central colours are clearlydriven by the active galactic nucleus, and one galaxy which also shows aradial age gradient, these colour changes appear consistent withmetallicity changes at a constant mean age. Finally, aperture data forfive Virgo early-type dwarf galaxies show that these galaxies appear tobe shifted to lower mean metallicities and lower mean ages (range1-6Gyr) than their higher luminosity counterparts.

The central kinematics of NGC 1399 measured with 14 pc resolution
We present near-infrared (NIR) adaptive optics-assisted spectroscopicobservations of the CO(Δμ= 2) absorption bands towards thecentre of the giant elliptical galaxy NGC 1399. The observations weremade with NAOS-CONICA (on the European Southern Observatory's Very LargeTelescope) and have a full width at half-maximum resolution of0.15arcsec (14pc). Kinematic analysis of the observations reveals adecoupled core and strongly non-Gaussian line-of-sight velocity profilesin the central 0.2arcsec (19pc). NIR imaging also indicates anasymmetric elongation of the central isophotes in the same region.We use spherical orbit-superposition models to interpret the kinematics,using a set of orthogonal `eigen-velocity profiles' that allow us to fitmodels directly to spectra. The models require a central black hole ofmass 1.2+0.5-0.6× 109Msolar, with a strongly tangentially biased orbitdistribution in the inner 40pc.

The SAURON project - V. Integral-field emission-line kinematics of 48 elliptical and lenticular galaxies
We present the emission-line fluxes and kinematics of 48 representativeelliptical and lenticular galaxies obtained with our custom-builtintegral-field spectrograph, SAURON, operating on the William HerschelTelescope. Hβ, [OIII]λλ4959,5007 and[NI]λλ5198,5200 emission lines were measured using a newprocedure that simultaneously fits both the stellar spectrum and theemission lines. Using this technique we can detect emission lines downto an equivalent width of 0.1 Å set by the current limitations indescribing galaxy spectra with synthetic and real stellar templates,rather than by the quality of our spectra. Gas velocities and velocitydispersions are typically accurate to within 14 and 20 kms-1, respectively, and at worse to within 25 and 40 kms-1. The errors on the flux of the [OIII] and Hβ linesare on average 10 and 20 per cent, respectively, and never exceed 30 percent. Emission is clearly detected in 75 per cent of our samplegalaxies, and comes in a variety of resolved spatial distributions andkinematic behaviours. A mild dependence on the Hubble type and galacticenvironment is observed, with higher detection rates in lenticulargalaxies and field objects. More significant is the fact that only 55per cent of the galaxies in the Virgo cluster exhibit clearly detectedemission. The ionized-gas kinematics is rarely consistent with simplecoplanar circular motions. However, the gas almost never displayscompletely irregular kinematics, generally showing coherent motions withsmooth variations in angular momentum. In the majority of the cases, thegas kinematics is decoupled from the stellar kinematics, and in half ofthe objects this decoupling implies a recent acquisition of gaseousmaterial. Over the entire sample however, the distribution of the meanmisalignment values between stellar and gaseous angular momenta isinconsistent with a purely external origin. The distribution ofkinematic misalignment values is found to be strongly dependent on theapparent flattening and the level of rotational support of galaxies,with flatter, fast rotating objects hosting preferentially corotatinggaseous and stellar systems. In a third of the cases, the distributionand kinematics of the gas underscore the presence of non-axisymmetricperturbations of the gravitational potential. Consistent with previousstudies, the presence of dust features is always accompanied by gasemission while the converse is not always true. A considerable range ofvalues for the [OIII]/Hβ ratio is found both across the sample andwithin single galaxies. Despite the limitations of this ratio as anemission-line diagnostic, this finding suggests either that a variety ofmechanisms is responsible for the gas excitation in E and S0 galaxies orthat the metallicity of the interstellar material is quiteheterogeneous.

Submit a new article

Related links

Submit a new link

Member of following groups:

Observation and Astrometry data

Right ascension:12h30m48.00s
Apparent magnitude:8.6

Catalogs and designations:
Proper Names   (Edit)
MessierM 87
NGC 2000.0NGC 4486
J/AJ/90/1681VCC 1316

→ Request more catalogs and designations from VizieR