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|Chandra and ROSAT Observations of NGC 5044: Profile of Dark Halos in Galaxy Groups|
We combined spatially resolved Chandra data with the ROSAT data toconstrain the dark-matter distribution in the galaxy group NGC 5044.Within 250 h-150kpc, the total mass is found to be 1.6 × 1013 Mȯ, 12% of whichconstitutes baryonic mass. Within the inner central regions, the totalmass profile exhibits a double structure, typical for groups containinga cD galaxy. Following previous studies of mostly rich galaxy clusters,we studied in detail the nature of such a double structure, whereby weinferred likely interface between the cD galaxy and the surroundinggalaxies. For this interesting group, we determined for the first timethe galaxy-group interface, which is around 7.5kpc from the peak of theX-ray emission. The total mass internal to this interface radius isfound to be 7.1 × 1010 Mȯ. Beyondthis radius, the total mass profile becomes DM-dominated and thecorresponding DM profile is reasonably fitted with the NFW model,yielding results consistent with the observed scatter expected for CDMhalos. A power-law fit to the DM mass profile gives α = 1.88± 0.32, a slope that is within the observed range, but issignificantly larger than that of low surface brightness galaxies andself-interacting DM halos.
|HI content in galaxies in loose groups|
Gas deficiency in cluster spirals is well known and ram-pressurestripping is considered the main gas removal mechanism. In some compactgroups too gas deficiency is reported. However, gas deficiency in loosegroups is not yet well established. Lower dispersion of the membervelocities and the lower density of the intragroup medium in small loosegroups favour tidal stripping as the main gas removal process in them.Recent releases of data from the HI Parkes All-Sky Survey (HIPASS) andcatalogues of nearby loose groups with associated diffuse X-ray emissionhave allowed us to test this notion. In this paper, we address thefollowing questions: (i) do galaxies in groups with diffuse X-rayemission statistically have lower gas content compared to the ones ingroups without diffuse X-ray emission? (ii) does HI deficiency vary withthe X-ray luminosity, LX, of the loose group in a systematicway? We find that (i) galaxies in groups with diffuse X-ray emission, onaverage, are HI deficient, and have lost more gas compared to those ingroups without X-ray emission; the latter are found not to havesignificant HI deficiency; (ii) no systematic dependence of the HIdeficiency with LX is found. Ram-pressure-assisted tidalstripping and evaporation by thermal conduction are the two possiblemechanisms to account for this excess gas loss.
|Globular clusters, satellite galaxies and stellar haloes from early dark matter peaks|
The Milky Way contains several distinct old stellar components thatprovide a fossil record of its formation. We can understand theirspatial distribution and kinematics in a hierarchical formation scenarioby associating the protogalactic fragments envisaged by Searle &Zinn (1978) with the rare peaks able to cool gas in the cold dark matterdensity field collapsing at redshift z > 10. We use hierarchicalstructure formation simulations to explore the kinematics and spatialdistribution of these early star-forming structures in galaxy haloestoday. Most of the protogalaxies rapidly merge, their stellar contentsand dark matter becoming smoothly distributed and forming the innerGalactic halo. The metal-poor globular clusters and old halo starsbecome tracers of this early evolutionary phase, centrally biased andnaturally reproducing the observed steep fall off with radius. The mostoutlying peaks fall in late and survive to the present day as satellitegalaxies. The observed radial velocity dispersion profile and the localradial velocity anisotropy of Milky Way halo stars are successfullyreproduced in this model. If this epoch of structure formation coincideswith a suppression of further cooling into lower sigma peaks then we canreproduce the rarity, kinematics and spatial distribution of satellitegalaxies as suggested by Bullock, Kravtsov & Weinberg (2000).Reionization at z= 12 +/- 2 provides a natural solution to the missingsatellites problem. Measuring the distribution of globular clusters andhalo light on scales from galaxies to clusters could be used toconstrain global versus local reionization models. If reionizationoccurs contemporary, our model predicts a constant frequency of blueglobulars relative to the host halo mass, except for dwarf galaxieswhere the average relative frequencies become smaller.
|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.
|The Two-dimensional XMM-Newton Group Survey: z < 0.012 Groups|
We present the results of the two-dimensional XMM-Newton Group Survey(2dXGS), an archival study of nearby galaxy groups. In this paper weconsider 11 nearby systems (z<0.012) in Mulchaey et al., which span abroad range in X-ray luminosity from 1040 to 1043ergs s-1. We measure the iron abundance and temperaturedistribution in these systems and derive pressure and entropy maps. Wefind statistically significant evidence for structure in the entropy andpressure of the gas component of seven groups on the 10%-20% level. TheXMM-Newton data for the three groups with best statistics also suggestpatchy metallicity distributions within the central 20-50 kpc of thebrightest group galaxy, probed with 2-10 kpc resolution. This providesinsights into the processes associated with thermalization of thestellar mass loss. Analysis of the global properties of the groupsreveals a subclass of X-ray-faint groups, which are characterized byboth higher entropy and lower pressure. We suggest that the mergerhistory of the central elliptical is responsible for both the source andthe observed thermodynamical properties of the hot gas of theX-ray-faint groups.
|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.
|Nearby early-type galaxies with ionized gas. II. Line-strength indices for 18 additional galaxies|
We previously presented a data-set of line-strength indices for 50early-type galaxies in the nearby Universe. The galaxy sample is biasedtoward galaxies showing emission lines, located in environmentscorresponding to a broad range of local galaxy densities, althoughpredominantly in low density environments. The present addendum enlargesthe above data-set of line-strength indices by analyzing 18 additionalearly-type galaxies (three galaxies, NGC 3607, NGC 5077 and NGC 5898were presented in the previous set). We measured 25 line-strengthindices, defined by the Lick IDS "standard" system (Trager et al. 1998,ApJS, 116, 1; Worthey & Ottaviani 1997, ApJS, 111, 377), for 7luminosity weighted apertures and 4 gradients of each galaxy. Thisaddendum presents the line-strength data-set and compares it with theavailable data in the literature.
|Star-forming accretion flows and the low-luminosity nuclei of giant elliptical galaxies|
The luminosities of the centres of nearby elliptical galaxies are verylow compared to models of thin disc accretion on to their black holes atthe Bondi rate, typically a few hundredths to a few tenths of a solarmass per year. This has motivated models of inefficiently radiatedaccretion that invoke weak electron-ion thermal coupling, and/orinhibited accretion rates due to convection or outflows. Here we pointout that, even if such processes are operating, a significant fractionof the accreting gas is prevented from reaching the central black holebecause it condenses into stars in a gravitationally unstable disc. Starformation occurs inside the Bondi radius (typically ~100 pc in giantellipticals), but still relatively far from the black hole in terms ofSchwarzschild radii. Star formation depletes and heats the gas disc,eventually leading to a marginally stable, but much reduced, accretionflow to the black hole. We predict the presence of cold (~100 K), dustygas discs, containing clustered Hα emission and occasional Type IIsupernovae, both resulting from the presence of massive stars. Starformation accounts for several features of the M87 system: a thin disc,traced by Hα emission, is observed on scales of about 100 pc, withfeatures reminiscent of spiral arms and dust lanes; the star formationrate inferred from the intensity of Hα emission is consistent withthe Bondi accretion rate of the system. Star formation may thereforehelp to suppress accretion on to the central engines of massiveellipticals. We also discuss some implications for the fuelling of theGalactic Centre and quasars.
|On the dynamics of the satellite galaxies in NGC 5044|
The NGC 5044 galaxy group is dominated by a luminous elliptical galaxythat is surrounded by ~160 dwarf satellites. The projected numberdensity profile of this dwarf population deviates within ~1/3 of thevirial radius from a projected Navarro, Frenk and White (NFW) profile,which is assumed to approximate the underlying total matterdistribution. By means of a semi-analytic model, we demonstrate that theinterplay between gravitation, dynamical friction and tidal mass lossand destruction can explain the observed number density profile. We useonly two parameters in our models: the total to stellar mass fraction ofthe satellite haloes and the disruption efficiency. The disruptionefficiency is expressed by a minimum radius. If the tidal radius of agalaxy (halo) falls below this radius, it is assumed to becomeunobservable. The preferred parameters are an initial total to stellarmass fraction of ~20 and a disruption radius of 4 kpc. In that model,about 20 per cent of all the satellites are totally disrupted on theirorbits within the group environment. Dynamical friction is lessimportant in shaping the inner slope of the number density profilebecause the reduction in mass by tidal forces lowers the impact of thefriction term. The main destruction mechanism is tide. In the preferredmodel, the total B-band luminosity of all disrupted galaxies is abouttwice the observed luminosity of the central elliptical galaxy,indicating that a significant fraction of stars are scattered into theintragroup medium. Dwarf galaxy satellites closer to the centre of theNGC 5044 group may exhibit optical evidence of partial tidal disruption.If dynamical friction forces the satellite to merge with the centralelliptical, the angular momentum of the satellite tends to be removed atthe apocentre passage. Afterwards, the satellite drops radially towardsthe centre.
|AWM 4 - an isothermal cluster observed with XMM-Newton|
We present an analysis of an XMM-Newton observation of the poor clusterAWM 4. The cluster is relaxed and its X-ray halo is regular with noapparent substructure. Azimuthally averaged radial spectral profilessuggest that the cluster is isothermal to a radius of at least 160 kpc,with no evidence of a central cooling region. Spectral mapping showssome significant temperature and abundance substructure, but no evidenceof strong cooling in the cluster core. Abundance increases in the core,but not to the extent expected, and we find some indication of gasmixing. Modelling the three-dimensional properties of the system, weshow that ongoing heating by an active galactic nuclei (AGN) in thedominant elliptical, NGC 6051, is likely to be responsible for the lackof cooling. We also compare AWM 4 to MKW 4, a cluster of similar massobserved recently with XMM-Newton. While the two systems have similargravitational mass profiles, MKW 4 has a cool core and a somewhatsteeper gas density profile, which leads to a lower core entropy. AWM 4has a considerably larger gas fraction at 0.1R200, and weshow that these differences result from the difference in mass betweenthe two dominant galaxies and the activity cycles of their AGN. Weestimate the energy required to raise the temperature profile of MKW 4to match that of AWM 4 to be 9 × 1058 erg or 3 ×1043 erg s-1 for 100 Myr, comparable to the likelypower output of the AGN in AWM 4.
|Evidence for radio-source heating of groups|
We report evidence that the gas properties of X-ray groups containingradio galaxies differ from those of radio-quiet groups. For awell-studied sample of ROSAT-observed groups, we found that more thanhalf of the elliptical-dominated groups can be considered `radio-loud',and that radio-loud groups are likely to be hotter at a given X-rayluminosity than radio-quiet groups. We tested three different models forthe origin of the effect and conclude that radio-source heating is themost likely explanation. We found several examples of groups where thereis strong evidence from Chandra or XMM-Newton images for interactionsbetween the radio source and the group gas. A variety of radio-sourceheating processes are important, including shock-heating by youngsources and gentler heating by larger sources. The heating effects canbe longer-lasting than the radio emission. We show that the sample ofX-ray groups used in our study is not significantly biased in thefraction of radio-loud groups that it contains. This allows us toconclude that the energy per particle that low-power radio galaxies caninject over the group lifetime is comparable to the requirements ofstructure formation models.
|The low-luminosity galaxy population in the NGC5044 Group|
We present multicolour imaging for a sample of 33 dwarf andintermediate-luminosity galaxies in the field of the NGC5044 Group,complemented with mid-resolution spectroscopy for a subsample of 13objects. With these data, a revised membership and morphologicalclassification is made for the galaxies in the sample. We were able toconfirm all but one of the `definite members' included in thespectroscopic subsample, galaxies which were originally classified basedon morphological criteria. An important fraction of background galaxies,however, is probably present among `likely' and `possible' members.The presence of a nucleus could be detected in just five out of the ninegalaxies originally classified as dE,N, confirming the intrinsicdifficulty of photographic-plate morphological classification for thiskind of object. Our deep surface photometry provided clear evidence fordisc structure in at least three galaxies previously catalogued as dE ordS0. Their transition-type properties are also evident from thecolour-magnitude diagram, where they lie near the late-type galaxylocus, suggesting an evolutionary connection between a parentdisc-galaxy population and at least some present-day dEs.Six new dSph candidates were also found, most of them at small projecteddistances from NGC5044, the central galaxy of the group.The NGC5044 Group appears clearly defined in redshift space, with a meanheliocentric radial velocity of = 2461 +/- 84kms-1 (z= 0.0082), and a moderate dispersion ofσvr= 431 km s-1. Our kinematicaldata show no luminosity segregation for early-type galaxies: both dwarfand bright E/S0 systems show very similar velocity distributions(σvr~ 290 km s-1). This is incontrast to late-type galaxies, which seem to display a broaderdistribution (σvr~ 680 km s-1).
|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.
|A Fundamental Plane Relation for the X-Ray Gas in Normal Elliptical Galaxies|
We report on the discovery of a new correlation between globalparameters of the hot interstellar gas in elliptical galaxies. Wereanalyze archival Chandra data for 30 normal early-type systems,removing the contributions of resolved and unresolved point sources toreveal the X-ray morphology of the hot gas. We determine the half-lightradius, RX, and the mean surface brightness, IX,from the gas surface brightness profiles. A spectral analysis determinesthe temperature, TX, of the gas within 3 optical effectiveradii. We find that the galaxies lie on an X-ray gas fundamental plane(XGFP) of the formTX~R0.28XI0.22X.This is close to, but distinct from, a simple luminosity-temperaturerelation. The intrinsic width of the XGFP is only 0.07 dex, nearlyidentical to that of the stellar (optical) fundamental plane (SFP). Thisis surprising since X-ray gas masses are typically ~10-2 ofthe stellar masses. We show that the XGFP is not a simple consequence ofthe virial theorem or hydrostatic equilibrium and that it is essentiallyindependent of the SFP. The XGFP thus represents a genuinely newconstraint on the hydrodynamical evolution of elliptical galaxies.
|Chandra Observation of the Cluster of Galaxies MS 0839.9+2938 at z = 0.194: The Central Excess Iron and Type Ia Supernova Enrichment|
We present the Chandra study of the intermediate-redshift (z=0.194)cluster of galaxies MS 0839.9+2938. By performing both projected anddeprojected spectral analyses, we find that the gas temperature isapproximately constant at about 4 keV within 130-444h-170 kpc. In the inner regions, the gastemperature decreases toward the center, reaching <~3 keV in thecentral 37 h-170 kpc. This implies that the lowerand upper limits of the mass deposit rate are 9-34 and 96-126Msolar yr-1, respectively, within 74h-170 kpc, where the gas is significantly colder.Along with the temperature drop, we detect a significant inward ironabundance increase from about 0.4 Zsolar in the outer regionsto ~=1 Zsolar within the central 37h-170 kpc. Thus, MS 0839.9+2938 is the clustershowing the most significant central iron excess at z>~0.2. We arguethat most of the excess iron should have been contributed by SNe Ia.Using the observed SN Ia rate and stellar mass loss rate, we estimatethat the time needed to enrich the central region with excess iron is6.4-7.9 Gyr, which is similar to those found for nearby clusters.Coinciding with the optical extension of the cD galaxy (up to about 30h-170 kpc), the observed X-ray surface brightnessprofile exhibits an excess beyond the distribution expected by eitherthe β model or the Navarro-Frenk-White (NFW) model and can be wellfitted with an empirical two-β model that leads to a relativelyflatter mass profile in the innermost region.
|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.
|The Ages of Elliptical Galaxies from Mid-Infrared Emission|
The mid-infrared (10-20 μm) luminosity of elliptical galaxies isdominated by the integrated emission from circumstellar dust in redgiant stars. As a single stellar population evolves, the rate of dustymass loss from red giant stars decreases with time, so the mid-infraredluminosity should also decline with stellar age. To seek such acorrelation, we have used archival Infrared Space Observatory (ISO)observations to determine surface brightness profiles and central fluxesat 15 μm in 17 early-type galaxies for which stellar ages have beendetermined from optical spectral indices. The radial surface brightnessdistributions at 15 μm generally follow the stellar de Vaucouleursprofile, as expected. We find that the surface brightness ratioμ15μm/μIband is systematically higher inelliptical galaxies with ages <~5 Gyr and in galaxies that exhibitevidence of recent mergers. Within the accuracy of our observations,μ15μm/μIband shows no age dependence forages >~5 Gyr. The corresponding flux ratiosF15μm/FIband within apertures scaled to theeffective radius (Re/8) are proportional to theμ15μm/μIband ratios at larger galacticradii, indicating that no 15 μm emission is detected from centraldust clouds visible in optical images in some of our sample galaxies.Emission at 15 μm is observed in noncentral massive clouds of dustand cold gas in NGC 1316, an elliptical galaxy that is thought to havehad a recent merger. Recent Spitzer Space Telescope data also indicatethe presence of polycyclic aromatic hydrocarbon (PAH) emission at 8μm. Several ellipticals have extended regions of 15 μm emissionthat have no obvious counterparts at other frequencies.
|Formation and evolution of dwarf elliptical galaxies. I. Structural and kinematical properties|
This paper is the first in a series in which we present the results ofan ESO Large Program on the kinematics and internal dynamics of dwarfelliptical galaxies (dEs). We obtained deep major and minor axis spectraof 15 dEs and broad-band imaging of 22 dEs. Here, we investigate therelations between the parameters that quantify the structure (B-bandluminosity L_B, half-light radius R_e, and mean surface brightnesswithin the half-light radius Ie = LB / 2 πR_e^2) and internal dynamics (velocity dispersion σ) of dEs. Weconfront predictions of the currently popular theories for dE formationand evolution with the observed position of dEs in log LB vs.log σ, log LB vs. log R_e, log LB vs. logI_e, and log Re vs. log Ie diagrams and in the(log σ,log R_e,log I_e) parameter space in which bright andintermediate-luminosity elliptical galaxies and bulges of spirals definea Fundamental Plane (FP). In order to achieve statistical significanceand to cover a parameter interval that is large enough for reliableinferences to be made, we merge the data set presented in this paperwith two other recently published, equally large data sets. We show thatthe dE sequences in the various univariate diagrams are disjunct fromthose traced by bright and intermediate-luminosity elliptical galaxiesand bulges of spirals. It appears that semi-analytical models (SAMs)that incorporate quiescent star formation with an essentiallyz-independent star-formation efficiency, combined with post-mergerstarbursts and the dynamical response after supernova-driven gas-loss,are able to reproduce the position of the dEs in the various univariatediagrams. SAMs with star-formation efficiencies that rise as a functionof redshift are excluded since they leave the observed sequences tracedby dEs virtually unpopulated. dEs tend to lie above the FP and the FPresidual declines as a function of luminosity. Again, models that takeinto account the response after supernova-driven mass-loss correctlypredict the position of dEs in the (log σ,log R_e,log I_e)parameter space as well as the trend of the FP residual as a function ofluminosity. While these findings are clearly a success for thehierarchical-merging picture of galaxy formation, they do notnecessarily invalidate the alternative “harassment”scenario, which posits that dEs stem from perturbed and strippedlate-type disk galaxies that entered clusters and groups of galaxiesabout 5 Gyr ago.
|Massive elliptical galaxies in X-rays: The role of late gas accretion|
We present a new chemical evolution model meant to be a first step inthe self-consistent study of both optical and X-ray properties ofelliptical galaxies. Detailed cooling and heating processes in theinterstellar medium (ISM) are taken into account using a mono-phaseone-zone treatment which allows a more reliable modelling of thegalactic wind regime with respect to previous work. The modelsuccessfully reproduces simultaneously the mass-metallicity, thecolour-magnitude, the LX - LB and theLX - T relations, as well as the observed trend of the[Mg/Fe] ratio as a function of σ, by adopting the prescriptions ofPipino & Matteucci (2004) for the gas infall and star formationtimescales. We found that a late secondary accretion of gas from theenvironment plays a fundamental role in driving the LX -LB and LX - T relations and can explain theirlarge observational scatter. The iron discrepancy, namely the too highpredicted iron abundance in X-ray haloes of ellipticals compared toobservations, still persists. On the other hand, we predict [O/Fe] inthe ISM which is in good agreement with the most recent observations. Wesuggest possible mechanisms acting on a galactic scale which may solvethe iron discrepancy. In particular, mixing of gas driven by AGNs maypreserve the gas mass (and thus the X-ray luminosity) while diluting theiron abundance. New predictions for the amounts of iron, oxygen andenergy ejected into the intracluster medium (ICM) are presented and weconclude that type Ia supernovae (SNe Ia) play a fundamental role in theICM enrichment. SNe Ia activity, in fact, may power a galactic windlasting for a considerable amount of the galactic lifetime, even in thecase for which the efficiency of energy transfer into the ISM per SN Iaevent is less than unity.
|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 discovery of new galaxy members in the NGC 5044 and 1052 groups|
We present the results of neutral hydrogen (HI) observations of the NGC5044 and NGC 1052 groups, as part of a GEMS (Group EvolutionMultiwavelength Study) investigation into the formation and evolution ofgalaxies in nearby groups. Two new group members have been discoveredduring a wide-field HI imaging survey conducted using the ATNF Parkestelescope. These results, as well as those from follow-up HI synthesisand optical imaging, are presented here. J1320 - 1427, a new member ofthe NGC 5044 group, has an HI mass of and , with a radial velocity of v=2750 kms-1. The optical galaxy is characterized by tworegions of star formation, surrounded by an extended, diffuse halo.J0249-0806, the new member of the NGC 1052 group, has and v= 1450kms-1. The optical image reveals a low-surface-brightnessgalaxy. We interpret both of these galaxies as irregular type, withJ0249 - 0806 possibly undergoing first infall into the NGC 1052 group.
|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.
|Ages and metallicities of Hickson compact group galaxies|
Hickson compact groups (HCGs) constitute an interesting extreme in therange of environments in which galaxies are located, as the spacedensity of galaxies in these small groups are otherwise only found inthe centres of much larger clusters. The work presented here uses Lickindices to make a comparison of ages and chemical compositions ofgalaxies in HCGs with those in other environments (clusters, loosegroups and the field). The metallicity and relative abundance of`α-elements' show strong correlations with galaxy age and centralvelocity dispersion, with similar trends found in all environments.However, we show that the previously reported correlation betweenα-element abundance ratios and velocity dispersion disappears whena full account is taken of the abundance ratio pattern in thecalibration stars. This correlation is thus found to be an artefact ofincomplete calibration to the Lick system.Variations are seen in the ranges and average values of age, metallicityand α-element abundance ratios for galaxies in differentenvironments. Age distributions support the hierarchical formationprediction that field galaxies are on average younger than their clustercounterparts. However, the ages of HCG galaxies are shown to be moresimilar to those of cluster galaxies than those in the field, contraryto the expectations of current hierarchical models. A trend for lowervelocity dispersion galaxies to be younger was also seen. This is againinconsistent with hierarchical collapse models, but is qualitativelyconsistent with the latest N-body smoothed particle hydrodynamics modelsbased on monolithic collapse in which star formation continues for manyGyr in low-mass haloes.
|Scaling relations in early-type galaxies belonging to groups|
We present a photometric analysis of a large sample of early-typegalaxies in 16 nearby groups, imaged with the Wide-Field Camera on theIsaac Newton Telescope. Using a two-dimensional surface brightnessdecomposition routine, we fit Sersic (r1/n) and exponentialmodels to their bulge and disc components, respectively. Dividing thegalaxies into three subsamples according to the X-ray luminosities oftheir parent groups, we compare their photometric properties. Galaxiesin X-ray luminous groups tend to be larger and more luminous than thosein groups with undetected or low X-ray luminosities, but no significantdifferences in n are seen. Both normal and dwarf elliptical galaxies inthe central regions of groups are found to have cuspier profiles thantheir counterparts in group outskirts.Structural differences between dwarf and normal elliptical galaxies areapparent in terms of an offset between their `photometric planes' in thespace of n, re and μ0. Dwarf ellipticals arefound to populate a surface, with remarkably low scatter, in this spacewith significant curvature, somewhat similar to the surfaces of constantentropy proposed by Màrquez et al. Normal ellipticals are offsetfrom this distribution in a direction of higher specific entropy. Thismay indicate that the two populations are distinguished by the action ofgalaxy merging on larger galaxies.
|Spatial distribution of galaxies in the Puppis region|
We determine the spatial distribution of the galaxies located behind thepart of the zone of avoidance of the Milky Way defined by 220°
|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.
|Absence of Dwarf Galaxies at High Redshifts: Evidence from a Galaxy Group|
The galaxy group NGC 5044 consists of a luminous giant elliptical galaxysurrounded by a cluster of ~160 low-luminosity and dwarf galaxies,mostly of early type. The cumulative projected radial distribution ofdwarf galaxies in the NGC 5044 group, unlike distributions of moreluminous galaxies in rich clusters, does not follow a projected darkmatter (NFW) profile. A deficiency or absence of low-luminosity galaxiesis apparent in NGC 5044 within about 350 kpc. Most of the dwarf galaxiesin NGC 5044 entered the virial radius at redshiftsz<~2.5(af/0.25), where af=1/(1+zf)is the epoch of group formation, and very few entered during redshiftsz>~2.5(af/0.25). The peculiar, non-NFW shape of theprojected cumulative dwarf galaxy distribution in NGC 5044 within 350kpc resembles the characteristic cumulative distribution of darksubhalos that are also known to be relatively young. Dynamical frictionis unlikely to explain the apparent lack of group member galaxies atsmall radii in NGC 5044.
|Time-dependent Circulation Flows: Iron Enrichment in Cooling Flows with Heated Return Flows|
We describe a new type of dynamical model for hot gas in galaxy groupsand clusters in which gas moves simultaneously in both radialdirections. The observational motivations for this type of flow arecompelling. X-ray spectra indicate that little or no gas is cooling tolow temperatures. Bubbles of hot gas typically appear in Chandra X-rayimages and XMM-Newton X-ray spectra within ~50 kpc of the centralelliptical galaxy. These bubbles must be buoyant. Furthermore, theelemental composition and total mass of gas-phase iron observed within~100 kpc of the center can be understood as the accumulated outflow ofmost or all of the iron produced by Type Ia supernovae in the centralgalaxy over time. This gaseous iron has been circulating for manygigayears, unable to cool. As dense inflowing gas cools, it produces apositive central temperature gradient, a characteristic feature ofnormal cooling flows. This gas dominates the local X-ray spectrum butshares the total available volume with centrally heated, outflowing gas.Circulating flows eventually cool catastrophically if the outflowing gastransports mass but no heat; to maintain the circulation both mass andenergy must be supplied to the inflowing gas over a large volume,extending to the cooling radius. The rapid radial recirculation of gaswithin ~50 kpc results in a flat core in the gas iron abundance, similarto many group and cluster observations. We believe the circulation flowsdescribed here are the first gasdynamic, long-term evolutionary modelsthat are in good agreement with all essential features observed in thehot gas: little or no gas cools as required by XMM spectra, the gastemperature increases outward near the center, and the gaseous ironabundance is about solar near the center and decreases outward.
|XMM-Newton Observations of NGC 507: Supersolar Metal Abundances in the Hot Interstellar Medium|
We present the results of the X-ray XMM-Newton observations of NGC 507,a dominant elliptical galaxy in a small group of galaxies, and reportsupersolar metal abundances of both Fe and α-elements in the hotinterstellar medium (ISM) of this galaxy. These results are robust inthat we considered all possible systematic effects in our analysis. Wefind ZFe=2-3 times solar inside the D25 ellipse ofNGC 507. This is the highest ZFe reported so far for the hothalo of an elliptical galaxy; this high iron abundance is fullyconsistent with the predictions of stellar evolution models, whichinclude the yield of both Type II and Type Ia supernovae (SNe). Ouranalysis shows that abundance measurements are critically dependent onthe selection of the proper emission model. The spatially resolved,high-quality XMM-Newton spectra provide enough statistics to formallyrequire at least three emission components in each of four circumnuclearconcentric shells (within 5' or 100 kpc): two soft thermal componentsindicating a range of temperatures in the hot ISM plus a hardercomponent, consistent with the integrated output of low-mass X-raybinaries (LMXBs) in NGC 507. The two-component (thermal+LMXB) modelcustomarily used in past studies yields a much lower ZFe,consistent with previous reports of subsolar metal abundances. Thismodel, however, gives a significantly worse fit to the data (F-testprobability<0.0001). The abundance of α-elements (mostaccurately determined by Si) is also found to be supersolar. Theα-element-to-Fe abundance ratio is close to the solar ratio,suggesting that ~70% of the iron mass in the hot ISM originated fromType Ia SNe. The α-element-to-Fe abundance ratio remains constantout to at least 100 kpc, indicating that Types II and Ia SN ejecta arewell mixed on a scale much larger than the extent of the stellar body.
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