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|Dwarf galaxies in the dynamically evolved NGC 1407 Group|
The NGC 1407 Group stands out among nearby structures by its propertiesthat suggest it is massive and evolved. It shares properties withentities that have been called fossil groups: the 1.4mdifferential between the dominant elliptical galaxy and the secondbrightest galaxy comes close to satisfying the definition that has beenused to define the fossil class. There are few intermediate-luminositygalaxies, but a large number of dwarfs in the group. We estimate thereare 250 group members to the depth of our survey. The slope of the faintend of the luminosity function (reaching MR = -12) is α= -1.35. Velocities for 35 galaxies demonstrate that this group with onedominant galaxy has a mass of 7 × 1013Msolarand M/LR = 340Msolar/Lsolar. Twogalaxies in close proximity to NGC 1407 have very large blueshifts. Themost notable is the second brightest galaxy, NGC 1400, with a velocityof -1072 km s-1 with respect to the group mean. We report thedetection of X-ray emission from this galaxy and from the group.
|Eridanus - a supergroup in the local Universe?|
We examine a possible supergroup in the direction of the Eridanusconstellation using 6dF Galaxy Survey second data release (6dFGS DR2)positions and velocities together with Two-Micron All-Sky Survey andHyper-Lyon-Meudon Extragalactic DAtabase photometry. We perform afriends-of-friends analysis to determine which galaxies are associatedwith each substructure before examining the properties of theconstituent galaxies. The overall structure is made up of threeindividual groups that are likely to merge to form a cluster of mass ~7× 1013Msolar. We conclude that thisstructure is a supergroup. We also examine the colours, morphologies andluminosities of the galaxies in the region with respect to their localprojected surface density. We find that the colours of the galaxiesredden with increasing density, the median luminosities are brighterwith increasing environmental density and the morphologies of thegalaxies show a strong morphology-density relation. The colours andluminosities of the galaxies in the supergroup are already similar tothose of galaxies in clusters; however, the supergroup contains morelate-type galaxies, consistent with its lower projected surface density.Due to the velocity dispersion of the groups in the supergroup, whichare lower than those of clusters, we conclude that the properties of theconstituent galaxies are likely to be a result of merging orstrangulation processes in groups outlying this structure.
|Low-Mass X-Ray Binaries in Six Elliptical Galaxies: Connection to Globular Clusters|
We present a systematic study of the low-mass X-ray binary (LMXB)populations of six elliptical galaxies, aimed at investigating thedetected LMXB-globular cluster (GC) connection. We utilize Chandraarchival data to identify X-ray point sources and HST archival datasupplemented by ground observations to identify 6173 GCs. Afterscreening and cross-matching, we associate 209 LMXBs with red GC (RGCs)and 76 LMXBs with blue GCs (BGCs), while we find no optical GCcounterpart for 258 LMXBs. This is the largest GC-LMXB sample studied sofar. We confirm previous reports suggesting that the fraction of GCsassociated with LMXBs is ~3 times larger in RGCs than in BGCs,indicating that metallicity is a primary factor in the GC LMXBformation. We find that GCs located near the galaxy center have a higherprobability of harboring LMXBs than those in the outskirts, suggestingthat there must be another parameter (in addition to metallicity)governing LMXB formation in GCs. This second parameter, dependent on thegalactocentric distance, may be a distance dependent encounter rate. Wefind no significant differences in the shape of X-ray luminosityfunction, LX/LV distribution, X-ray spectra amongRGC, BGC, and field LMXBs. The similarity of the X-ray spectra isinconsistent with the irradiation-induced stellar wind model prediction.The similarity of the X-ray luminosity functions (XLFs) of GC LMXBs andfield LMXBs indicates that there is no significant difference in thefraction of black hole binaries present. We cannot either prove orreject the hypothesis that all LMXBs were formed in GCs.
|A Chandra View of Dark Matter in Early-Type Galaxies|
We present a Chandra study of mass profiles in seven ellipticalgalaxies, of which three have galaxy-scale and four have group-scalehalos, demarcated at 1013 Msolar. These representthe best available data for nearby objects with comparable X-rayluminosities. We measure approximately flat mass-to-light (M/L) profileswithin an optical half-light radius (Reff), rising by anorder of magnitude at ~10 Reff, which confirms the presenceof dark matter (DM). The data indicate hydrostatic equilibrium, which isalso supported by agreement with studies of stellar kinematics inelliptical galaxies. The data are well fitted by a model comprising anNFW DM profile and a baryonic component following the optical light. Thedistribution of DM halo concentration parameters (c) versusMvir agrees with ΛCDM predictions and our observationsof bright groups. Concentrations are slightly higher than expected,which is most likely a selection effect. Omitting the stellar massdrastically increases c, possibly explaining large concentrations foundby some past observers. The stellar M/LK agree withpopulation synthesis models, assuming a Kroupa IMF. Allowing adiabaticcompression (AC) of the DM halo by baryons made M/L more discrepant,casting some doubt on AC. Our best-fitting models imply total baryonfractions ~0.04-0.09, consistent with models of galaxy formationincorporating strong feedback. The groups exhibit positive temperaturegradients, consistent with the ``universal'' profiles found in othergroups and clusters, whereas the galaxies have negative gradients,suggesting a change in the evolutionary history of the systems aroundMvir~=1013 Msolar.
|A Chandra Survey of Early-Type Galaxies. I. Metal Enrichment in the Interstellar Medium|
We present a Chandra study of the emission-weighted metal abundances in28 early-type galaxies, spanning ~3 orders of magnitude in X-rayluminosity (LX). We report constraints for Fe, O, Ne, Mg, Si,S, and Ni. We find no evidence of the very subsolar Fe abundance(ZFe) historically reported, confirming a trend in recentobservations of bright galaxies and groups, nor do we find anycorrelation between ZFe and luminosity. Excepting one case,the ISM is single-phase, indicating that multitemperature fits foundwith ASCA reflected temperature gradients that we resolve with Chandra.We find no evidence that ZFe (ISM) is substantially lowerthan the stellar metallicity estimated from simple stellar populationmodels. In general, these quantities are similar, which is inconsistentwith galactic wind models and recent hierarchical chemical enrichmentsimulations. Our abundance ratio constraints imply that 66%+/-11% of theISM Fe was produced in SNe Ia, similar to the solar neighborhood,indicating similar enrichment histories for elliptical galaxies and theMilky Way. Although these values are sensitive to the considerablesystematic uncertainty in the supernova yields, they are in agreementwith observations of more massive systems. This indicates considerablehomology in the enrichment process operating from cluster scales tolow-to-intermediate-LX galaxies. The data uniformly exhibitlow ZO/ZMg ratios, which have been reported insome clusters, groups, and galaxies. This is inconsistent with standardSN II metal yield calculations and may indicate an additional source ofenrichment, such as Population III hypernovae.
|Scaling Mass Profiles around Elliptical Galaxies Observed with Chandra and XMM-Newton|
We investigated the dynamical structure of 53 elliptical galaxies usingthe Chandra archival X-ray data. In X-ray-luminous galaxies, temperatureincreases with radius and gas density is systematically higher at theoptical outskirts, indicating the presence of a significant amount ofthe group-scale hot gas. In contrast, X-ray-dim galaxies show a flat ordeclining temperature profile against radius and the gas density isrelatively lower at the optical outskirts. Thus, it is found thatX-ray-bright and faint elliptical galaxies are clearly distinguished bythe temperature and gas density profile. The mass profile is well scaledby a virial radius r200 rather than an optical half-radiusre, is quite similar at (0.001-0.03)r200 betweenX-ray-luminous and dim galaxies, and smoothly connects to those profilesof clusters of galaxies. At the inner region of(0.001-0.01)r200 or (0.1-1)re, the mass profilewell traces a stellar mass with a constant mass-to-light ratio ofM/LB=3-10 Msolar/Lsolar. TheM/LB ratio of X-ray-bright galaxies rises up steeply beyond0.01r200 and thus requires a presence of massive dark matterhalo. From the deprojection analysis combined with the XMM-Newton data,we found that X-ray-dim galaxies NGC 3923, NGC 720, and IC 1459 alsohave a high M/LB ratio of 20-30 at 20 kpc, comparable to thatof X-ray-luminous galaxies. Therefore, dark matter is indicated to becommon in elliptical galaxies; their dark matter distribution, as wellas that of galaxy clusters, almost follows the NFW profile.
|Radio Continuum and Far-infrared Emission from the Galaxies in the Eridanus Group|
The Eridanus galaxies follow the well-known radio-FIR correlation. Themajority (70%) of these galaxies have their star formation rates belowthat of the Milky Way. The galaxies that have a significant excess ofradio emission are identified as low luminosity AGNs based on theirradio morphologies obtained from the GMRT observations. There are nopowerful AGNs (L20 cm>1023WHz-1) in the group. The twomost far-infrared and radio luminous galaxies in the group have opticaland HI morphologies suggestive of recent tidal interactions. TheEridanus group also has two far-infrared luminous but radio-deficientgalaxies. It is believed that these galaxies are observed within a fewMyr of the onset of an intense star formation episode after beingquiescent for at least a 100 Myr. The upper end of the radio luminositydistribution of the Eridanus galaxies (L20 cm1022WHz-1) isconsistent with that of the field galaxies, other groups, and late-typegalaxies in nearby clusters.
|GMRT HI Observations of the Eridanus Group of Galaxies I.|
The GMRT HI 21cm-line observations of galaxies in the Eridanus group arepresented. The Eridanus group, at a distance of ~23 Mpc, is a loosegroup of ~200 galaxies. The group extends to more than 10 Mpc inprojection. The velocity dispersion of the galaxies in the group is ~240km s-1. The galaxies are clustered into different sub-groups. Theoverall population mix of the group is 30% (E + S0) and 70% (Sp + Irr).The observations of 57 Eridanus galaxies were carried out with the GMRTfor ~ 200 h. HI emission was detected from 31 galaxies. The channel rmsof ~1 mJy beam-1 was achieved for most of the image-cubes made with 4 hof data. The corresponding HI column density sensitivity (3σ) is~1 × 1020 cm-2 for a velocity-width of ~ 13.4 km s-1.The 3σ detection lss surface densities, HI disk parameters and HIrotation curves are presented. The velocity fields are analysedseparately for the approaching and the receding sides of the galaxies.These data will be used to study the HI and the radio continuumproperties, the Tully-Fisher relations, the dark matter halos, and thekinematical and HI lopsidedness in galaxies.
|Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis|
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
|Chandra Study of X-Ray Point Sources in the Early-Type Galaxy NGC 4552 (M89)|
We present a Chandra ACIS study of the early-type galaxy NGC 4552. Wedetect 47 X-ray point sources, most of which are likely low-mass X-raybinaries (LMXBs), within four effective radii (Re). Thebrightest X-ray source coincides with the optical, UV, and radio centerof the galaxy and shows variability on >1 hr timescales, indicatingthe possible existence of a low-luminosity active galactic nucleus(AGN). The 46 off-center sources and the unresolved point sourcescontribute about 29% and 20% to the total luminosity of the galaxy,respectively. We find that after correcting for the incompleteness atthe low-luminosity end, the observed cumulative X-ray luminosityfunction (XLF) of the off-center sources is best fitted by a brokenpower-law model with a break atLb=4.4+2.0-1.4×1038ergs s-1. We identified 210 globular cluster (GC) candidatesin a HST WFPC2 optical image of the galaxy's central region. Of the 25off-center LMXBs that fall within the WFPC2 field of view, 10 sourcesare coincident with a GC. Thus, the fraction of the GCs hosting brightLMXBs and the fraction of the LMXBs associated with GCs are 4.8% and40%, respectively. In the V and I bands, the GCs hosting bright LMXBsare typically 1-2 mag brighter than the GCs with no detected LMXBs.There are about 1.9+/-0.4 times as many LMXBs in the red, metal-rich GCsas there are in the blue, metal-poor ones. We find no obvious differencebetween the luminosity distributions of LMXBs in GCs and in the field,but the cumulative spectrum of the LMXBs in GCs tends to be softer thanthat of the LMXBs in field. We detected three X-ray sources that haveisotropic luminosities larger than 1039 ergs s-1.Only one of these is located in the joint Chandra-HST field and is foundto be associated with a GC. By studying its ACIS spectra we infer thatthe this may be a candidate black hole system with a mass of 15-135Msolar. One of the other sources with a luminosity brighterthan 1039 ergs s-1 reveals temporal variations inbrightness on timescales greater than 1 hr.
|The Birthplace of Low-Mass X-Ray Binaries: Field Versus Globular Cluster Populations|
Recent Chandra studies of low-mass X-ray binaries (LMXBs) withinearly-type galaxies have found that LMXBs are commonly located withinglobular clusters of the galaxies. However, whether all LMXBs are formedwithin globular clusters has remained an open question. If all LMXBsformed within globular clusters, the summed X-ray luminosity of theLMXBs in a galaxy should be directly proportional to the number ofglobular clusters in the galaxy regardless of where the LMXBs currentlyreside. We have compared these two quantities over the same angular areafor a sample of 12 elliptical and S0 galaxies observed with Chandra andfound that the correlation between the two quantities is weaker thanexpected if all LMXBs formed within globular clusters. This indicatesthat a significant number of the LMXBs were formed in the field andnaturally accounts for the spread in field-to-cluster fractions of LMXBsfrom galaxy to galaxy. We also find that the ``pollution'' of globularcluster LMXBs into the field has been minimal within ellipticalgalaxies, but there is evidence that roughly half of the LMXBsoriginally in the globular clusters of S0 galaxies in our sample haveescaped into the field. This is likely due to higher globular clusterdisruption rates in S0s, resulting from stronger gravitational shockscaused by the passage of globular clusters through the disks of S0galaxies that are absent in elliptical galaxies.
|Thermal Evolution of Supernova Iron in Elliptical Galaxies|
Interpretations of the spatial distribution, abundance ratios, andglobal masses of metals in the hot gas of galaxy clusters in terms ofsupernova enrichment have been problematical. For example, the abundanceof iron and other elements occasionally declines toward the center justwhere the stellar and supernova densities are highest. Also, the mass ofgas-phase iron per unit stellar mass or light is lower in ellipticalgalaxies and groups than in rich galaxy clusters. We discusshypothetical scenarios in which these abundance anomalies can resultfrom the preferential buoyant separation of metals. However, in this andall previous attempts to explain these metallicity observations it hasbeen assumed that all metals created by supernovae are present in eithervisible stars or the hot gas. We discuss here the possibility that someof the iron expelled into the hot gas by Type Ia supernovae may haveradiatively cooled, avoiding detection by X-ray and optical observers.Hydrodynamic models of Type Ia explosions in the hot gas insideelliptical galaxies create a gas of nearly pure iron that is severaltimes hotter than the local interstellar gas. We describe the subsequentthermal evolution of the iron-rich gas as it radiates and thermallymixes with the surrounding gas. There is a critical time by which theiron ions must mix into the ambient gas to avoid rapid radiativecooling. We find that successful mixing is possible if the iron ionsdiffuse with large mean free paths, as in an unmagnetized plasma.However, the Larmor radii of the iron ions are exceptionally small inmicrogauss fields, so the field geometry must be highly tangled orradial to allow the iron to mix by diffusion faster than it cools byradiative losses. The possibility that some of the supernova iron coolscannot be easily discounted.
|Nuclear Accretion in Galaxies of the Local Universe: Clues from Chandra Observations|
In order to find an explanation for the radiative quiescence ofsupermassive black holes in the local universe, the most accurateestimates for a sample of nearby galaxies are collected for the mass ofa central black hole (MBH), the nuclear X-ray luminosityLX,nuc, and the circumnuclear hot gas density andtemperature, by using Chandra data. The nuclear X-ray luminosityLX,nuc varies by ~3 orders of magnitude and does not show arelationship with MBH or with the Bondi mass accretion rateM˙B LX,nuc is always much lower than expectedif M˙B ends in a standard accretion disk with highradiative efficiency (this instead can be the case of the active nucleusof Cen A). Radiatively inefficient accretion as in the standardadvection-dominated accretion flow (ADAF) modeling may explain the lowluminosities of a few cases; for others, the predicted luminosity isstill too high, and, in terms of Eddington-scaled quantities, it isincreasingly higher than that observed for increasingM˙B. Variants of the simple radiatively inefficientscenario including outflow and convection may reproduce the low emissionlevels observed, since the amount of matter actually accreted is reducedconsiderably. However, the most promising scenario includes feedbackfrom accretion on the surrounding gas; this has the important advantagesof naturally explaining the observed lack of relationship amongLX,nuc, MBH, and M˙B, and evadingthe problem of the fate of the material accumulating in the centralgalactic regions over cosmological times.
|On the Nature of X-Ray Sources in Early-Type Galaxies|
We show that the observed relationship between the fraction of low-massX-ray binaries (LMXBs) found in globular clusters (GCs) and theGC-specific frequency for early-type galaxies is consistent with an LMXBformation model in which the field population of LMXBs is formed in situvia primordial binary formation. The suggestion that a significantfraction of the field LMXB population in early-type galaxies was formedin GCs is not required by the data. Finally, we discuss observationalstudies that will test this model more thoroughly.
|X-ray properties of NGC 300. I. Global properties of X-ray point sources and their optical counterparts|
We present X-ray properties of NGC 300 point sources, extracted from 66ks of XMM-Newton data taken in 2000 December and 2001 January. A totalof 163 sources were detected in the energy range of 0.3-6 keV. We reporton the global properties of the sources detected inside theD25 optical disk, such as the hardness ratio and X-rayfluxes, and on the properties of their optical counterparts found in B,V, and R images from the 2.2 m MPG/ESO telescope. Furthermore, wecross-correlate the X-ray sources with SIMBAD, the USNO-A2.0 catalog,and radio catalogues.
|A catalogue of ultraluminous X-ray sources in external galaxies|
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125
|The Group Evolution Multiwavelength Study (GEMS): bimodal luminosity functions in galaxy groups|
We present B- and R-band luminosity functions (LFs) for a sample of 25nearby groups of galaxies. We find that the LFs of the groups with lowX-ray luminosity (LX < 1041.7 ergs-1) are significantly different from those of the X-raybrighter groups, showing a prominent dip around MB=-18. Whileboth categories show lack of late-type galaxies in their centralregions, X-ray dim groups also show a more marked concentration ofoptical luminosity towards the centre. A toy simulation shows that inthe low velocity dispersion environment, as in the X-ray dim group,dynamical friction would facilitate more rapid merging, thus depletingintermediate-luminosity galaxies to form a few giant central galaxies,resulting in the prominent dip seen in our LFs. We suggest that X-raydim (or low velocity dispersion) groups are the present sites of rapiddynamical evolution rather than their X-ray bright counterparts, and maybe the modern precursors of fossil groups. We predict that these groupsof low velocity dispersion would harbour younger stellar populationsthan groups or clusters with higher velocity dispersion.
|The isolated elliptical NGC 4555 observed with Chandra|
We present analysis of a Chandra observation of the elliptical galaxyNGC 4555. The galaxy lies in a very low density environment, eitherisolated from all galaxies of similar mass or on the outskirts of agroup. Despite this, NGC 4555 has a large gaseous halo, extending to~60kpc. We find the mean gas temperature to be ~0.95keV and the Feabundance to be ~0.5Zsolar. We model the surface brightness,temperature and abundance distribution of the halo and use these resultsto estimate parameters such as the entropy and cooling time of the gas,and the total gravitational mass of the galaxy. In contrast to recentresults showing that moderate luminosity ellipticals contain relativelysmall quantities of dark matter, our results show that NGC 4555 has amassive dark halo and large mass-to-light ratio(56.8+34.2-35.8Msolar/LBsolarat 50kpc, 42.7+14.6-21.2 at 5re,1σ errors). We discuss this disparity and consider possiblemechanisms by which galaxies might reduce their dark matter content.
|The GEMS project: X-ray analysis and statistical properties of the group sample|
The Group Evolution Multiwavelength Study (GEMS) involves amultiwavelength study of a sample of 60 galaxy groups, chosen to span awide range of group properties. Substantial ROSAT Position SensitiveProportional Counter (PSPC) observations, available for all of thesegroups, are used to characterize the state of the intergalactic mediumin each. We present the results of a uniform analysis of these ROSATdata and a statistical investigation of the relationship between X-rayand optical properties across the sample. Our analysis improves inseveral respects on previous work: (i) we distinguish between systems inwhich the hot gas is a group-scale medium and those in which it appearsto be just a hot halo associated with a central galaxy; (ii) weextrapolate X-ray luminosities to a fixed overdensity radius(r500) using fitted surface brightness models, in order toavoid biases arising from the fact that cooler systems are detectable tosmaller radii, and (iii) optical properties have been rederived in auniform manner from the NASA Extragalactic Database, rather than relyingon the data in the disparate collection of group catalogues from whichour systems are drawn.The steepening of the LX-TX relation in the groupregime reported previously is not seen in our sample, which fits well onto the cluster trend, albeit with large non-statistical scatter. Anumber of biases affect the fitting of regression lines under thesecircumstances, and until the impact of these has been thoroughlyinvestigated it seems best to regard the slope of the groupLX-TX relation as being poorly determined. Asignificant problem in comparing the properties of groups and clustersis the derivation of system radii, to allow different systems to becompared within regions having the same overdensity. We find evidencethat group velocity dispersion (σv) provides a veryunreliable measure of system mass (and hence radius), with a number ofgroups having remarkably low values of σv, given thatthey appear from their X-ray properties to be collapsed systems. Weconfirm that the surface brightness profiles of groups are significantlyflatter than those of clusters - the maximum value of theβfit parameter for our sample is 0.58, lower than thetypical value of 0.67 seen in clusters - however, we find no significanttendency within our sample for cooler groups to show flatter profiles.This result is inconsistent with simple universal pre-heating models.The morphology of the galaxies in the GEMS groups is correlated to theirX-ray properties in a number of ways: we confirm the very strongrelationship between X-ray emission and a dominant early-type centralgalaxy, which has been noted since the early X-ray studies of groups,and also find that spiral fraction is correlated with the temperature ofthe hot gas and hence the depth of the gravitational potential. A classof spiral-rich groups with little or no X-ray emission probablycorresponds to groups that have not yet fully collapsed.
|Dynamical friction in flattened systems: a numerical test of Binney's approach|
We carry out a set of self-consistent N-body calculations to investigatehow important the velocity anisotropy in non-spherical dark matterhaloes is for dynamical friction. For this purpose, we allow satellitegalaxies to orbit within flattened and live dark matter haloes (DMHs)and compare the resulting orbit evolution with a semi-analytic code.This code solves the equation of motion of the same satellite orbitswith mass loss and assumes the same DMH, but either employsChandrasekhar's dynamical friction formula, which does not incorporatethe velocity anisotropy, or Binney's description of dynamical frictionin anisotropic systems. In the numerical and the two semi-analyticmodels, the satellites are given different initial orbital inclinationsand orbital eccentricities, whereas the parent galaxy is composed of aDMH with aspect ratio qh= 0.6.We find that Binney's approach successfully describes the overallsatellite decay and orbital inclination decrease for the whole set oforbits, with an averaged discrepancy of less than 4 per cent in orbitalradius during the first three orbits. If Chandrasekhar's expression isused instead, the discrepancy increases to 20 per cent. Binney'streatment therefore appears to provide a significantly improvedtreatment of dynamical friction in anisotropic systems.The velocity anisotropy of the DMH velocity distribution function leadsto a significant decrease with time of the inclination of non-polarsatellite orbits. But, at the same time, it reduces the difference indecay times between polar and coplanar orbits evident in a flattened DMHwhen the anisotropic DMH velocity distribution function is not takeninto account explicitly. Our N-body calculations furthermore indicatethat polar orbits survive about 1.6 times longer than coplanar orbitsand that the orbital eccentricity e remains close to its initial valueif satellites decay slowly towards the galaxy centre. However, orbits ofrapidly decaying satellites modelled with the semi-analytic code show astrong orbital circularization () not present in the N-bodycomputations.
|The SAI Catalog of Supernovae and Radial Distributions of Supernovae of Various Types in Galaxies|
We describe the Sternberg Astronomical Institute (SAI) catalog ofsupernovae. We show that the radial distributions of type-Ia, type-Ibc,and type-II supernovae differ in the central parts of spiral galaxiesand are similar in their outer regions, while the radial distribution oftype-Ia supernovae in elliptical galaxies differs from that in spiraland lenticular galaxies. We give a list of the supernovae that arefarthest from the galactic centers, estimate their relative explosionrate, and discuss their possible origins.
|Cold Dust in Early-Type Galaxies. I. Observations|
We describe far-infrared observations of early-type galaxies selectedfrom the Infrared Space Observatory (ISO) archive. This ratherinhomogeneous sample includes 39 giant elliptical galaxies and 14 S0 (orlater) galaxies. These galaxies were observed with the array photometerPHOT on-board the ISO satellite using a variety of different observingmodes-sparse maps, mini-maps, oversampled maps, and singlepointings-each of which requires different and often rather elaboratephotometric reduction procedures. The ISO background data agree wellwith the COBE-DIRBE results to which we have renormalized ourcalibrations. As a further check, the ISO fluxes from galaxies at 60 and100 μm agree very well with those previously observed with IRAS atthese wavelengths. The spatial resolution of ISO is several timesgreater than that of IRAS, and the ISO observations extend out to 200μm, which views a significantly greater mass of colder dust notassessable to IRAS. Most of the galaxies are essentially point sourcesat ISO resolution, but a few are clearly extended at FIR wavelengthswith image sizes that increase with FIR wavelength. The integratedfar-infrared luminosities do not correlate with optical luminosities,suggesting that the dust may have an external, merger-related origin. Ingeneral, the far-infrared spectral energy distributions can be modeledwith dust at two temperatures, ~43 and ~20 K, which probably representlimits of a continuous range of temperatures. The colder dust componentdominates the total mass of dust, 106-107Msolar, which is typically more than 10 times larger than thedust masses previously estimated for the same galaxies using IRASobservations. For S0 galaxies we find that the optically normalizedfar-infrared luminosity LFIR/LB correlatesstrongly with the mid-infrared luminosityL15μm/LB, but that correlation is weaker forelliptical galaxies.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and United Kingdom) and with the participation of ISAS andNASA.
|A Chandra View of the Normal S0 Galaxy NGC 1332. II. Solar Abundances in the Hot Gas and Implications for Supernova Enrichment|
Using a new Chandra ACIS-S3 observation of the normal, isolated,moderate-LX lenticular galaxy NGC 1332, we resolve theemission into ~75 point sources and a significant diffuse component. Wepresent a detailed analysis of the spectral properties of the diffuseemission, constraining both the temperature profile and the metalabundances in the hot gas. The characteristics of the point-sourcepopulation and the spatial properties of the diffuse emission arediscussed in two companion papers. The diffuse component comprises hotgas with an ~isothermal temperature profile (~0.5 keV) and emission fromunresolved point sources. In contrast with the cool cores of many groupsand clusters, we find a small central temperature peak. We obtainemission-weighted abundance constraints within 20 kpc for several keyelements: Fe, O, Ne, Mg, and Si. The measured iron abundance(ZFe=1.1 in solar units; >0.53 at 99% confidence) stronglyexcludes the very subsolar values often historically reported forearly-type galaxies. This continues, in a lower LX system, atrend in recent observations of bright galaxies and groups. Theabundance ratios, with respect to Fe, of the other elements were alsofound to be ~solar, with the exception of ZO/ZFe,which was significantly lower (<0.4), as seen in several brightgalaxies, groups, and clusters. Such a low O abundance is not predictedby simple models of ISM enrichment by Type Ia and Type II supernovae(SNe) and may indicate a significant contribution from primordialhypernovae. Revisiting Chandra observations of themoderate-LX, isolated elliptical galaxy NGC 720, we obtainsimilar abundance constraints(ZFe=0.71+0.40-0.21, 90% confidence;ZO/ZFe=0.23+/-0.21). Adopting standard SNe Ia andSNe II metal yield models, our abundance ratio constraints imply that73%+/-5% and 85%+/-6% of the Fe enrichment in NGC 1332 and NGC 720,respectively, arises from SNe Ia. Although these results are sensitiveto the considerable systematic uncertainty in the SNe yields, they arein good agreement with observations of more massive systems. These twocases of moderate-LX early-type galaxies reveal a consistentpattern of metal enrichment from cluster scales to moderateLX/LB galaxies.
|A Chandra View of the Normal S0 Galaxy NGC 1332. I. An Unbroken, Steep Power-Law Luminosity Function for the Low-Mass X-Ray Binary Population|
Chandra ACIS -S3 observations of the nearby S0 galaxy NGC 1332 resolvemuch of the X-ray emission into 73 point sources, of which 37 lie withinthe D25 isophote. The remaining galaxy emission compriseshot, diffuse gas and unresolved sources and is discussed in twocompanion papers. The point-source X-ray luminosity function (XLF) showsthe characteristic break seen in other early-type galaxies at~2×1038 ergs s-1. After applying correctionsfor detection incompleteness at low luminosities due to source confusionand contamination from diffuse galactic emission, the break vanishes andthe data are well described as a single power law. This result castsfurther doubt on there being a ``universal'' XLF break in early-typegalaxies, marking the division between neutron star and black holesystems. The logarithmic slope of the differential XLF (dN/dL),β=2.7+/-0.5, is marginally (~2.5 σ) steeper than has beenfound for analogous completeness-corrected fits of other early-typegalaxies but closely matches the behavior seen at high luminosities inthese systems. Two of the sources within D25 areultraluminous X-ray sources (ULXs), although neither haveLX>2×1039 ergs s-1. Theabsence of very luminous ULXs in early-type galaxies suggests a break inthe XLF slope at ~(1-2)×1039 ergs s-1,although the data were not of sufficient quality to constrain such afeature in NGC 1332. The sources have a spatial distribution consistentwith the optical light and display a range of characteristics that areconsistent with an LMXB population. The general spectral characteristicsof the individual sources, as well as the composite source spectra, arein good agreement with observations of other early-type galaxies,although a small number of highly absorbed sources are seen. Two sourceshave very soft spectra, two show strong variability, indicating compactbinary nature, and one source shows evidence of an extended radialprofile. We do not detect a central source in NGC 1332, but we find afaint [LX=(2+/-1)×1038 ergs s-1]point source coincident with the center of the companion dwarf galaxyNGC 1331.
|The Lack of Very Ultraluminous X-Ray Sources in Early-Type Galaxies|
We have searched for ultraluminous X-ray sources (ULXs) in a sample of28 elliptical and S0 galaxies observed with Chandra. We find that thenumber of X-ray sources detected at a flux level that would correspondto a 0.3-10 keV X-ray luminosity of ~2×1039 ergss-1 or greater (for which we have used the designation veryultraluminous X-ray sources [VULXs]) at the distance of each galaxy isequal to the number of expected foreground/background objects. Inaddition, the VULXs are uniformly distributed over the Chandra field ofview rather than distributed like the optical light of the galaxies,strengthening the argument that the high-flux sources are unassociatedwith the galaxies. We have also taken the VULX candidate list of Colbertand Ptak and determined the spatial distribution of VULXs in early-typegalaxies and late-type galaxies separately. While the spiral galaxyVULXs are clearly concentrated toward the centers of the galaxies, theearly-type galaxy VULXs are distributed randomly over the ROSAT HRIfield of view, again indicating that they are not associated with thegalaxies themselves. We conclude that with the exception of two rarehigh-luminosity objects within globular clusters of the ellipticalgalaxy NGC 1399, VULXs are generally not found within old stellarsystems. However, we do find a significant population of sources withluminosities of (1-2)×1039 ergs s-1 thatreside within the sample galaxies that can be explained by accretiononto 10-20 Msolar black holes. Given our results, we proposethat ULXs be defined as X-ray sources with LX(0.3-10keV)>2×1039 ergs s-1.
|The 1000 Brightest HIPASS Galaxies: H I Properties|
We present the HIPASS Bright Galaxy Catalog (BGC), which contains the1000 H I brightest galaxies in the southern sky as obtained from the H IParkes All-Sky Survey (HIPASS). The selection of the brightest sourcesis based on their H I peak flux density (Speak>~116 mJy)as measured from the spatially integrated HIPASS spectrum. The derived HI masses range from ~107 to 4×1010Msolar. While the BGC (z<0.03) is complete inSpeak, only a subset of ~500 sources can be consideredcomplete in integrated H I flux density (FHI>~25 Jy kms-1). The HIPASS BGC contains a total of 158 new redshifts.These belong to 91 new sources for which no optical or infraredcounterparts have previously been cataloged, an additional 51 galaxiesfor which no redshifts were previously known, and 16 galaxies for whichthe cataloged optical velocities disagree. Of the 91 newly cataloged BGCsources, only four are definite H I clouds: while three are likelyMagellanic debris with velocities around 400 km s-1, one is atidal cloud associated with the NGC 2442 galaxy group. The remaining 87new BGC sources, the majority of which lie in the zone of avoidance,appear to be galaxies. We identified optical counterparts to all but oneof the 30 new galaxies at Galactic latitudes |b|>10deg.Therefore, the BGC yields no evidence for a population of``free-floating'' intergalactic H I clouds without associated opticalcounterparts. HIPASS provides a clear view of the local large-scalestructure. The dominant features in the sky distribution of the BGC arethe Supergalactic Plane and the Local Void. In addition, one can clearlysee the Centaurus Wall, which connects via the Hydra and Antlia Clustersto the Puppis Filament. Some previously hardly noticable galaxy groupsstand out quite distinctly in the H I sky distribution. Several newstructures, including some not behind the Milky Way, are seen for thefirst time.
|X-ray scaling properties of early-type galaxies|
We present an analysis of 39 X-ray luminous early-type galaxies observedwith the ROSAT PSPC. Using multicomponent spectral and spatial fits tothese data, we have measured halo abundance, temperature, luminosity andsurface brightness profile. We compare these measurements to similarresults from galaxy groups and clusters, fitting a number of relationscommonly used in the study of these larger objects. In particular, wefind that the σ-TX relation for our sample is similarto that reported for clusters, consistent with βspec= 1,and that the LX-TX relation has a steep slope(gradient 4.8 +/- 0.7) comparable with that found for galaxy groups.Assuming isothermality, we construct three-dimensional models of ourgalaxies, allowing us to measure gas entropy. We find no correlationbetween gas entropy and system mass, but do find a trend forlow-temperature systems to have reduced gas fractions. We conclude thatthe galaxies in our sample are likely to have developed their haloesthrough galaxy winds, influenced by their surrounding environment.
|A Survey for H2O Megamasers. III. Monitoring Water Vapor Masers in Active Galaxies|
We present single-dish monitoring of the spectra of 13 extragalacticwater megamasers taken over a period of 9 years and a single epoch ofsensitive spectra for seven others. The primary motivation is a searchfor drifting line velocities analogous to those of the systemic featuresin NGC 4258, which are known to result from centripetal acceleration ofgas in an edge-on, subparsec molecular disk. We detect a velocity driftanalogous to that in NGC 4258 in only one source, NGC 2639. Another, themaser source in NGC 1052, exhibits erratic changes in its broad maserprofile over time. Narrow maser features in all of the other diskgalaxies discussed here either remain essentially constant in velocityover the monitoring period or are sufficiently weak or variable inintensity that individual features cannot be traced reliably from oneepoch to the next. In the context of a circumnuclear, molecular diskmodel, our results suggest that either (a) the maser lines seen aresystemic features subject to a much smaller acceleration than present inNGC 4258, presumably because the gas is farther from the nuclear blackhole, or (b) we are detecting ``satellite'' lines for which theacceleration is in the plane of the sky.Our data include the first K-band science observations taken with thenew 100 m Green Bank Telescope (GBT). The GBT data were taken duringtesting and commissioning of several new components and so are subjectto some limitations; nevertheless, they are in most cases the mostsensitive H2O spectra ever taken for each source and cover800 MHz (~=10,800 km s-1) of bandwidth. Many new maserfeatures are detected in these observations. Our data also include atentative and a clear detection of the megamaser in NGC 6240 at epochs ayear and a few months, respectively, prior to the detections reported byHagiwara et al. and Nakai et al.We also report a search for water vapor masers toward the nuclei of 58highly inclined (i>80deg), nearby galaxies. These sourceswere selected to investigate the tendency that H2O megamasersfavor inclined galaxies. None were detected, confirming that megamasersare associated exclusively with active galactic nuclei.
|An X-Ray Atlas of Groups of Galaxies|
A search was conducted for a hot intragroup medium in 109 low-redshiftgalaxy groups observed with the ROSAT PSPC. Evidence for diffuse,extended X-ray emission is found in at least 61 groups. Approximatelyone-third of these detections have not been previously reported in theliterature. Most of the groups are detected out to less than half of thevirial radius with ROSAT. Although some spiral-rich groups do contain anintragroup medium, diffuse emission is restricted to groups that containat least one early-type galaxy.
|Redshift-Distance Survey of Early-Type Galaxies: Circular-Aperture Photometry|
We present R-band CCD photometry for 1332 early-type galaxies, observedas part of the ENEAR survey of peculiar motions using early-typegalaxies in the nearby universe. Circular apertures are used to tracethe surface brightness profiles, which are then fitted by atwo-component bulge-disk model. From the fits, we obtain the structuralparameters required to estimate galaxy distances using theDn-σ and fundamental plane relations. We find thatabout 12% of the galaxies are well represented by a pure r1/4law, while 87% are best fitted by a two-component model. There are 356repeated observations of 257 galaxies obtained during different runsthat are used to derive statistical corrections and bring the data to acommon system. We also use these repeated observations to estimate ourinternal errors. The accuracy of our measurements are tested by thecomparison of 354 galaxies in common with other authors. Typical errorsin our measurements are 0.011 dex for logDn, 0.064 dex forlogre, 0.086 mag arcsec-2 for<μe>, and 0.09 for mRC,comparable to those estimated by other authors. The photometric datareported here represent one of the largest high-quality and uniformall-sky samples currently available for early-type galaxies in thenearby universe, especially suitable for peculiar motion studies.Based on observations at Cerro Tololo Inter-American Observatory (CTIO),National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation (NSF);European Southern Observatory (ESO); Fred Lawrence Whipple Observatory(FLWO); and the MDM Observatory on Kitt Peak.
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