Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| The SAURON project - VI. Line strength maps of 48 elliptical and lenticular galaxies
We present absorption line strength maps of 48 representative ellipticaland lenticular galaxies obtained as part of a survey of nearby galaxiesusing our custom-built integral-field spectrograph, SAURON, operating onthe William Herschel Telescope. Using high-quality spectra, spatiallybinned to a constant signal-to-noise ratio, we measure four key age,metallicity and abundance ratio sensitive indices from the Lick/IDSsystem over a two-dimensional field extending up to approximately oneeffective radius. A discussion of calibrations and offsets is given,along with a description of error estimation and nebular emissioncorrection. We modify the classical Fe5270 index to define a new index,Fe5270S, which maximizes the useable spatial coverage ofSAURON. Maps of Hβ, Fe5015, Mgb and Fe5270S arepresented for each galaxy. We use the maps to compute average linestrengths integrated over circular apertures of one-eighth effectiveradius, and compare the resulting relations of index versus velocitydispersion with previous long-slit work. The metal line strength mapsshow generally negative gradients with increasing radius roughlyconsistent with the morphology of the light profiles. Remarkabledeviations from this general trend exist, particularly the Mgb isoindexcontours appear to be flatter than the isophotes of the surfacebrightness for about 40 per cent of our galaxies without significantdust features. Generally, these galaxies exhibit significant rotation.We infer from this that the fast-rotating component features a highermetallicity and/or an increased Mg/Fe ratio as compared to the galaxy asa whole. The Hβ maps are typically flat or show a mild positiveoutwards radial gradient, while a few galaxies show strong central peaksand/or elevated overall Hβ strength likely connected to recent starformation activity. For the most prominent post-starburst galaxies, eventhe metal line strength maps show a reversed gradient.
| Gemini/GMOS spectra of globular clusters in the Leo group elliptical NGC 3379
The Leo group elliptical NGC 3379 is one of the few normal ellipticalgalaxies close enough to make possible observations of resolved stellarpopulations, deep globular cluster (GC) photometry and highsignal-to-noise ratio GC spectra. We have obtained Gemini/GMOS spectrafor 22 GCs associated with NGC 3379. We derive ages, metallicities andα-element abundance ratios from simple stellar population modelsusing the recent multi-index χ2 minimization method ofProctor & Sansom. All of these GCs are found to be consistent withold ages, i.e. >~10Gyr, with a wide range of metallicities. This iscomparable to the ages and metallicities that Gregg et al. found acouple of years ago for resolved stellar populations in the outerregions of this elliptical. A trend of decreasing α-elementabundance ratio with increasing metallicity is indicated.The projected velocity dispersion of the GC system is consistent withbeing constant with radius. Non-parametric, isotropic models require asignificant increase in the mass-to-light ratio at large radii. Thisresult is in contrast to that of Romanowsky et al., who recently found adecrease in the velocity dispersion profile as determined from planetarynebulae (PN). Our constant dispersion requires a normal-sized dark halo,although without anisotropic models we cannot rigorously determine thedark halo mass.A two-sided χ2 test over all radii gives a 2σdifference between the mass profile derived from our GCs compared to thePN-derived mass model of Romanowsky et al. However, if we restrict ouranalysis to radii beyond one effective radius and test if the GCvelocity dispersion is consistently higher, we determine a > 3σdifference between the mass models, and hence we favour the conclusionthat NGC 3379 does indeed have dark matter at large radii in its halo.
| The SAURON project - V. Integral-field emission-line kinematics of 48 elliptical and lenticular galaxies
We present the emission-line fluxes and kinematics of 48 representativeelliptical and lenticular galaxies obtained with our custom-builtintegral-field spectrograph, SAURON, operating on the William HerschelTelescope. Hβ, [OIII]λλ4959,5007 and[NI]λλ5198,5200 emission lines were measured using a newprocedure that simultaneously fits both the stellar spectrum and theemission lines. Using this technique we can detect emission lines downto an equivalent width of 0.1 Å set by the current limitations indescribing galaxy spectra with synthetic and real stellar templates,rather than by the quality of our spectra. Gas velocities and velocitydispersions are typically accurate to within 14 and 20 kms-1, respectively, and at worse to within 25 and 40 kms-1. The errors on the flux of the [OIII] and Hβ linesare on average 10 and 20 per cent, respectively, and never exceed 30 percent. Emission is clearly detected in 75 per cent of our samplegalaxies, and comes in a variety of resolved spatial distributions andkinematic behaviours. A mild dependence on the Hubble type and galacticenvironment is observed, with higher detection rates in lenticulargalaxies and field objects. More significant is the fact that only 55per cent of the galaxies in the Virgo cluster exhibit clearly detectedemission. The ionized-gas kinematics is rarely consistent with simplecoplanar circular motions. However, the gas almost never displayscompletely irregular kinematics, generally showing coherent motions withsmooth variations in angular momentum. In the majority of the cases, thegas kinematics is decoupled from the stellar kinematics, and in half ofthe objects this decoupling implies a recent acquisition of gaseousmaterial. Over the entire sample however, the distribution of the meanmisalignment values between stellar and gaseous angular momenta isinconsistent with a purely external origin. The distribution ofkinematic misalignment values is found to be strongly dependent on theapparent flattening and the level of rotational support of galaxies,with flatter, fast rotating objects hosting preferentially corotatinggaseous and stellar systems. In a third of the cases, the distributionand kinematics of the gas underscore the presence of non-axisymmetricperturbations of the gravitational potential. Consistent with previousstudies, the presence of dust features is always accompanied by gasemission while the converse is not always true. A considerable range ofvalues for the [OIII]/Hβ ratio is found both across the sample andwithin single galaxies. Despite the limitations of this ratio as anemission-line diagnostic, this finding suggests either that a variety ofmechanisms is responsible for the gas excitation in E and S0 galaxies orthat the metallicity of the interstellar material is quiteheterogeneous.
| The SAURON project - IV. The mass-to-light ratio, the virial mass estimator and the Fundamental Plane of elliptical and lenticular galaxies
We investigate the well-known correlations between the dynamicalmass-to-light ratio (M/L) and other global observables of elliptical (E)and lenticular (S0) galaxies. We construct two-integral Jeans andthree-integral Schwarzschild dynamical models for a sample of 25 E/S0galaxies with SAURON integral-field stellar kinematics to about oneeffective (half-light) radius Re. They have well-calibratedI-band Hubble Space Telescope WFPC2 and large-field ground-basedphotometry, accurate surface brightness fluctuation distances, and theirobserved kinematics is consistent with an axisymmetric intrinsic shape.All these factors result in an unprecedented accuracy in the M/Lmeasurements. We find a tight correlation of the form (M/L) = (3.80 +/-0.14) ×(σe/200kms-1)0.84+/-0.07 betweenthe M/L (in the I band) measured from the dynamical models and theluminosity-weighted second moment σe of the LOSVDwithin Re. The observed rms scatter in M/L for our sample is18 per cent, while the inferred intrinsic scatter is ~13 per cent. The(M/L)-σe relation can be included in the remarkableseries of tight correlations between σe and othergalaxy global observables. The comparison of the observed correlationswith the predictions of the Fundamental Plane (FP), and with simplevirial estimates, shows that the `tilt' of the FP of early-typegalaxies, describing the deviation of the FP from the virial relation,is almost exclusively due to a real M/L variation, while structural andorbital non-homology have a negligible effect. When the photometricparameters are determined in the `classic' way, using growth curves, andthe σe is measured in a large aperture, the virial massappears to be a reliable estimator of the mass in the central regions ofgalaxies, and can be safely used where more `expensive' models are notfeasible (e.g. in high-redshift studies). In this case the best-fittingvirial relation has the form (M/L)vir= (5.0 +/- 0.1)×Reσ2e/(LG), in reasonableagreement with simple theoretical predictions. We find no differencebetween the M/L of the galaxies in clusters and in the field. Thecomparison of the dynamical M/L with the (M/L)pop inferredfrom the analysis of the stellar population, indicates a median darkmatter fraction in early-type galaxies of ~30 per cent of the total massinside one Re, in broad agreement with previous studies, andit also shows that the stellar initial mass function varies little amongdifferent galaxies. Our results suggest a variation in M/L at constant(M/L)pop, which seems to be linked to the galaxy dynamics. Wespeculate that fast-rotating galaxies have lower dark matter fractionsthan the slow-rotating and generally more-massive ones. If correct, thiswould suggest a connection between the galaxy assembly history and thedark matter halo structure. The tightness of our correlation providessome evidence against cuspy nuclear dark matter profiles in galaxies.
| The Cool ISM in S0 Galaxies. II. A Survey of Atomic Gas
The place of lenticular galaxies within the range of types of galaxiesremains unclear. We previously reported the mass of molecular hydrogenfor a volume-limited sample of lenticular galaxies, where we saw thatthe amount of gas was less than that predicted by the return of stellarmass to the interstellar medium. Here we report observations of atomichydrogen (H I) for the same sample. Detections in several galaxies makemore compelling the case presented in our earlier paper that the mass ofcool gas in S0 galaxies cuts off at ~10% of what is expected fromcurrent models of gas return from stellar evolution. The molecular andatomic phases of the gas in our sample galaxies appear to be separateand distinct, both spatially and in velocity space. We propose that themolecular gas arises mostly from the stellar mass returned to thegalaxy, while the atomic hydrogen is mainly accumulated from externalsources (infall, captured dwarfs, etc.). While this proposal fits mostof the observations, it makes the presence of the upper mass cutoff evenmore mysterious.
| Stellar Populations in Nearby Lenticular Galaxies
We have obtained two-dimensional spectral data for a sample of 58 nearbyS0 galaxies with the Multi-Pupil Fiber/Field Spectrograph of the 6 mtelescope of the Special Astrophysical Observatory of the RussianAcademy of Sciences. The Lick indices Hβ, Mg b, and arecalculated separately for the nuclei and for the bulges taken as therings between R=4'' and 7", and the luminosity-weighted ages,metallicities, and Mg/Fe ratios of the stellar populations are estimatedby comparing the data to single stellar population (SSP) models. Fourtypes of galaxy environments are considered: clusters, centers ofgroups, other places in groups, and the field. The nuclei are found tobe on average slightly younger than the bulges in any type ofenvironment, and the bulges of S0 galaxies in sparse environments areyounger than those in dense environments. The effect can be partlyattributed to the well-known age correlation with the stellar velocitydispersion in early-type galaxies (in our sample the galaxies in sparseenvironments are on average less massive than those in denseenvironments), but for the most massive S0 galaxies, withσ*=170-220 km s-1, the age dependence on theenvironment is still significant at the confidence level of 1.5 σ.Based on observations collected with the 6 m telescope (BTA) at theSpecial Astrophysical Observatory (SAO) of the Russian Academy ofSciences (RAS).
| The ACS Virgo Cluster Survey. XI. The Nature of Diffuse Star Clusters in Early-Type Galaxies
We use HST ACS imaging of 100 early-type galaxies in the ACS VirgoCluster Survey to investigate the nature of diffuse star clusters(DSCs). Compared to globular clusters (GCs), these star clusters havelow luminosities (MV>-8) and a broad distribution of sizes(320 magarcsec-2). The median colors of diffuse star cluster systems(1.1
| The ACS Virgo Cluster Survey. IX. The Color Distributions of Globular Cluster Systems in Early-Type Galaxies
We present the color distributions of globular cluster (GC) systems for100 early-type galaxies observed in the ACS Virgo Cluster Survey, thedeepest and most homogeneous survey of this kind to date. On average,galaxies at all luminosities in our study (-22
| The Spatial Distribution of Faint Fuzzy Star Clusters in NGC 5195
We present a study of a faint fuzzy star cluster system in the nearbySB0 galaxy NGC 5195 interacting with the famous spiral galaxy NGC 5194(M51), based on HST ACS BVI mosaic images taken by the Hubble HeritageTeam. We have found about 50 faint fuzzy star clusters around NGC 5195that are larger than typical globular clusters with effective radiireff>7 pc and that are red with (V-I)>1.0. They aremostly fainter than MV~-8.3 mag. From the comparison of BVIphotometry of these clusters with the simple stellar population models,we find that they are as massive as ~105 Msolarand older than 1 Gyr. Strikingly, most of these clusters are found to bescattered in an elongated region almost perpendicular to the northernspiral arm of NGC 5194, and the center of the region is slightly northof the NGC 5195 center, while normal compact red clusters of NGC 5195are located around the bright optical body of the host galaxy. This isin contrast to the cases of NGC 1023 and NGC 3384 where the spatialdistribution of faint fuzzy clusters shows a ring structure around thehost galaxy. We suggest that at least some faint fuzzy clusters areexperiencing tidal interactions with the companion galaxy NGC 5194 andmust be associated with the tidal debris in the western halo of NGC5195.
| Structural parameters from ground-based observations of newly discovered globular clusters in NGC 5128
We have investigated a number of globular cluster candidates from arecent wide-field study by Harris et al. (2004a, AJ, 128, 712) of thegiant elliptical galaxy NGC 5128. We used theMagellan I telescope + MagIC camera under excellent seeing conditions(0.3 arcsec-0.6 arcsec) and obtained very high resolution images for asample of 44 candidates. Of these, 15 appear to be bonafide globularclusters in NGC 5128 while the rest are eitherforeground stars or background galaxies. We also serendipitouslydiscovered 18 new cluster candidates in the same fields. Our imagesallow us to study the light profiles of the likely clusters, all ofwhich are well resolved. This is the first ground-based study ofstructural parameters for globular clusters outside the Local Group. Wecompare the psf-deconvolved profiles with King models and derivestructural parameters, ellipticities and surface brightnesses. Wecompare the derived structural properties with those of otherwell-studied globular cluster systems. In general, our clusters aresimilar in size, ellipticity, core radius and central surface brightnessto their counterparts in other galaxies, in particular those inNGC 5128 observed with HST by Harris et al. (2002,AJ, 124, 1435). However, our clusters extend to higher ellipticities andlarger half-light radii than their Galactic counterparts, as do theHarris et al. sample. Combining our results with those of Harris et al.fills in the gaps previously existing in rh - MVparameter space and indicates that any substantial difference betweenpresumed distinct cluster types in this diagram, including for examplethe Faint Fuzzies of Larsen & Brodie (2000, AJ, 120, 2938) and the"extended, luminous" M 31 clusters of Huxor et al. (2005, MNRAS, 360,1007) is now removed and that clusters form a continuum in this diagram.Indeed, this continuum now extends to the realm of the Ultra CompactDwarfs. The metal-rich clusters in our sample have half-light radii thatare almost twice as large in the mean as their metal-poor counterparts,at odds with the generally accepted trend. The possibility exists thatthis result could be due in part to contamination by backgroundgalaxies. We have carried out additional analysis to quantify thiscontamination. This shows that, although galaxies cannot be easily toldapart from clusters in some of the structural diagrams, the combinationof excellent image quality and Washington photometry should limit thecontamination to roughly 10% of the population of cluster candidates.Finally, our discovery of a substantial number of new cluster candidatesin the relatively distant regions of the NGC 5128halo suggests that current values of the total number of globularclusters may be underestimates.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
| A new population of extended, luminous star clusters in the halo of M31
We present three new clusters discovered in the halo of M31 which,although having globular-like colours and luminosities, have unusuallylarge half-light radii, ~30 pc. They lie at projected galactocentricdistances of ~15 to ~35 kpc. These objects begin to fill the gap inparameter space between globular clusters and dwarf spheroidals, and areunlike any clusters found in the Milky Way, or elsewhere to date.Colour-magnitude diagrams, integrated photometric properties and derivedKing profile fit parameters are given, and we discuss possible originsof these clusters and their relationships to other populations.
| The stellar populations of low-luminosity active galactic nuclei - III. Spatially resolved spectral properties
In a recently completed survey of the stellar population properties oflow-ionization nuclear emission-line regions (LINERs) and LINER/HIItransition objects (TOs), we have identified a numerous class ofgalactic nuclei which stand out because of their conspicuous108-9 yr populations, traced by high-order Balmer absorptionlines and other stellar indices. These objects are called `young-TOs',because they all have TO-like emission-line ratios. In this paper weextend this previous work, which concentrated on the nuclear properties,by investigating the radial variations of spectral properties inlow-luminosity active galactic nuclei (LLAGNs). Our analysis is based onhigh signal-to-noise ratio (S/N) long-slit spectra in the 3500-5500Å interval for a sample of 47 galaxies. The data probe distancesof typically up to 850 pc from the nucleus with a resolution of ~100 pc(~1 arcsec) and S/N ~ 30. Stellar population gradients are mapped by theradial profiles of absorption-line equivalent widths and continuumcolours along the slit. These variations are further analysed by meansof a decomposition of each spectrum in terms of template galaxiesrepresentative of very young (<=107 yr), intermediate age(108-9 yr) and old (1010 yr) stellar populations.This study reveals that young-TOs also differ from old-TOs andold-LINERs in terms of the spatial distributions of their stellarpopulations and dust. Specifically, our main findings are as follows.(i) Significant stellar population gradients are found almostexclusively in young-TOs. (ii) The intermediate age population ofyoung-TOs, although heavily concentrated in the nucleus, reachesdistances of up to a few hundred pc from the nucleus. Nevertheless, thehalf width at half-maximum of its brightness profile is more typically100 pc or less. (iii) Objects with predominantly old stellar populationspresent spatially homogeneous spectra, be they LINERs or TOs. (iv)Young-TOs have much more dust in their central regions than otherLLAGNs. (v) The B-band luminosities of the central <~1 Gyr populationin young-TOs are within an order of magnitude of MB=-15,implying masses of the order of ~107-108Msolar. This population was 10-100 times more luminous in itsformation epoch, at which time young massive stars would have completelyoutshone any active nucleus, unless the AGN too was brighter in thepast.
| Chemically Decoupled Nuclei in Five Lenticular Galaxies from SAURON Data
We analyze data from the SAURON integral-field spectrograph of theWilliam Herschel 4-m telescope for five lenticular galaxies in which wepreviously found chemically decoupled nuclei from observations with theMultipupil Fiber Spectrograph of the 6-m Special AstrophysicalObservatory telescope. In a larger field of view, we confirmed thepresence of peaks of the equivalent width of the Mg Ib λ5175absorption line in the nuclei of all five galaxies. However, thestructure of the chemically decoupled regions turned out to be highlyvaried even in such a small sample: from compact unresolved knots todisks with an extent of several hundred parsecs and, in one case, atriaxial compact minibar-type structure. We confirmed the presence of aninner gaseous polar ring in NGC 7280 and found it in NGC 7332. In theirouter parts, the planes of these polar rings are warped toward the planeof stellar rotation in such a way that the gas counterrotates withrespect to the stars. This behavior of the gas in a triaxial potentialwas predicted by several theoretical models.
| Supermassive Black Holes: Relation to Dark Halos
Estimates of the masses of supermassive black holes (M bh ) in thenuclei of disk galaxies with known rotation curves are compared withestimates of the rotational velocities V m and the“indicative” masses of the galaxies M i . Although there isa correlation between M bh and V m or M i , it is appreciably weakerthan the correlation with the central velocity dispersion. The values ofM bh for early-type galaxies (S0-Sab), which have more massive bulges,are, on average, higher than the values for late-type galaxies with thesame rotational velocities. We conclude that the black-hole masses aredetermined primarily by the properties of the bulge and not therotational velocity or the mass of the galaxy.
| On the Relevance of the Tremaine-Weinberg Method Applied to an Hα Velocity Field: Pattern Speed Determination in M100 (NGC 4321)
The relevance of the Tremaine-Weinberg (TW) method is tested formeasuring bar, spiral, and inner structure pattern speeds using agaseous velocity field. The TW method is applied to various simulatedbarred galaxies in order to demonstrate its validity in seven differentconfigurations, including star formation and/or dark matter halo. Thereliability of the different physical processes involved and of thevarious observational parameters is also tested. The simulations showthat the TW method could be applied to gaseous velocity fields to get agood estimate of the bar pattern speed, under the condition that regionsof shocks are avoided and measurements are confined to regions where thegaseous bar is well formed. We successfully apply the TW method to theHα velocity field of the Virgo Cluster galaxy M100 (NGC 4321) andderive pattern speeds of 55+/-5 km s-1 kpc-1 forthe nuclear structure, 30+/-2 km s-1 kpc-1 for thebar, and 20+/-1 km s-1 kpc-1 for the spiralpattern, in full agreement with published determinations using the samemethod or alternative ones.
| Star Cluster Survival in Star Cluster Complexes under Extreme Residual Gas Expulsion
After the stars of a new, embedded star cluster have formed, they blowthe remaining gas out of the cluster. Especially winds of massive starsand definitely the onset of the first supernovae can remove the residualgas from a cluster. This leads to a very violent mass loss and leavesthe cluster out of dynamical equilibrium. Standard models predict thatwithin the cluster volume the star formation efficiency (SFE) has to beabout 33% for sudden (within 1 crossing time of the cluster) gasexpulsion to retain some of the stars in a bound cluster. If theefficiency is lower, the stars of the cluster mostly disperse. Recentobservations reveal that in strong starbursts star clusters do not formin isolation, but in complexes containing dozens and up to severalhundred star clusters, i.e., in superclusters. By carrying out numericalexperiments for such objects placed at distances >=10 kpc from thecenter of the galaxy, we demonstrate that under these conditions (i.e.,the deeper potential of the star cluster complex and the merging processof the star clusters within these superclusters) the SFEs can be as lowas 20% and still leave a gravitationally bound stellar population. Suchan object resembles the outer Milky Way globular clusters and the faintfuzzy star clusters recently discovered in NGC 1023.
| Close Binaries as the Progenitors of the Brightest Planetary Nebulae
We investigate the possible progenitors of the planetary nebulae (PNs)that populate the top 0.5 mag of the [O III] λ5007 planetarynebula luminosity function (PNLF). We show that the absolute luminosityof the PNLF cutoff demands that the central stars of these most luminousPNs be >~0.6 Msolar and that such high-mass PN cores mustexist in every galaxy. We also use the bolometric luminosity-specific PNnumber density to show that in early-type galaxies, [O III]-bright PNsare relatively rare, with only ~10% of stars evolving to these brightmagnitudes. We demonstrate that the combination of these two factsimplies that either all early-type systems contain a small, smoothlydistributed component of young (<~1 Gyr old) stars or that anothermechanism exists for creating high core mass PNs. We argue that binarystar evolution is this second mechanism and demonstrate that bluestragglers have the appropriate core properties and number density toexplain the observations. We discuss the implications of thisalternative mode of stellar evolution and speculate on how coalescedbinaries might affect the use of PNs for measuring a galaxy's starformation history and chemical evolution.
| Faint Fuzzies and the Formation of Lenticular Galaxies
We investigate the dynamical state of a new class of extended starclusters known as ``faint fuzzies,'' which were discovered in two nearbyS0 galaxies, NGC 1023 and NGC 3384. It is shown that the faint fuzziesof NGC 1023 lie in a fast-rotating ringlike structure within thegalactic disk with mean radius of 5 kpc, rotational velocity of 200 kms-1, and velocity dispersion of 115 km s-1. Wepropose a scenario for the origin of faint fuzzies that is connected tothe origin of S0 galaxies as a result of galaxy-galaxy interactions indense environments. As is apparent in the Cartwheel galaxy and confirmedby numerical simulations, the passage of a small galaxy through, orclose to, the center of a disk galaxy can form a ring of clumpy starformation with a radius comparable to the faint fuzzy ring radius in NGC1023. In this case, the faint fuzzies are signposts for thetransformation of spiral galaxies into lenticulars via suchinteractions.
| Origin of Radio Emission from Nearby Low-Luminosity Active Galactic Nuclei
We use the observational data in radio, optical, and X-ray wave bandsfor a sample of active galactic nuclei (AGNs) with measured black holemasses to explore the origin of radio emission from nearbylow-luminosity active galactic nuclei (LLAGNs). The maximal luminosityof an advection-dominated accretion flow (ADAF) can be calculated for agiven black hole mass, as there is a critical accretion rate above whichthe ADAF is no longer present. We find that the radio luminosities arehigher than the maximal luminosities expected from the ADAF model formost sources in this sample. This implies that the radio emission ispredominantly from the jets in these sources. The radio emission from asmall fraction of the sources (15/60; referred to as radio-weak sources)in this sample can be explained by the ADAF model. However, comparingthe observed multiband emission data with the spectra calculated for theADAF or adiabatic inflow-outflow solution (ADIOS) cases, we find thatneither ADAF nor ADIOS models can reproduce the observed multibandemission simultaneously, with reasonable magnetic field strengths, forthese radio-weak sources. A variety of other possibilities arediscussed, and we suggest that the radio emission is probably dominatedby jet emission even in these radio-weak LLAGNs.
| The Centers of Early-Type Galaxies with Hubble Space Telescope. V. New WFPC2 Photometry
We present observations of 77 early-type galaxies imaged with the PC1CCD of the Hubble Space Telescope (HST) WFPC2. ``Nuker-law'' parametricfits to the surface brightness profiles are used to classify the centralstructure into ``core'' or ``power-law'' forms. Core galaxies aretypically rounder than power-law galaxies. Nearly all power-law galaxieswith central ellipticities ɛ>=0.3 have stellar disks,implying that disks are present in power-law galaxies withɛ<0.3 but are not visible because of unfavorable geometry. Afew low-luminosity flattened core galaxies also have disks; these may betransition forms from power-law galaxies to more luminous core galaxies,which lack disks. Several core galaxies have strong isophote twistsinterior to their break radii, although power-law galaxies have interiortwists of similar physical significance when the photometricperturbations implied by the twists are evaluated. Central colorgradients are typically consistent with the envelope gradients; coregalaxies have somewhat weaker color gradients than power-law galaxies.Nuclei are found in 29% of the core galaxies and 60% of the power-lawgalaxies. Nuclei are typically bluer than the surrounding galaxy. Whilesome nuclei are associated with active galactic nuclei (AGNs), just asmany are not; conversely, not all galaxies known to have a low-level AGNexhibit detectable nuclei in the broadband filters. NGC 4073 and 4382are found to have central minima in their intrinsic starlightdistributions; NGC 4382 resembles the double nucleus of M31. In general,the peak brightness location is coincident with the photocenter of thecore to a typical physical scale of <1 pc. Five galaxies, however,have centers significantly displaced from their surrounding cores; thesemay be unresolved asymmetric double nuclei. Finally, as noted byprevious authors, central dust is visible in about half of the galaxies.The presence and strength of dust correlates with nuclear emission;thus, dust may outline gas that is falling into the central black hole.The prevalence of dust and its morphology suggest that dust clouds form,settle to the center, and disappear repeatedly on ~108 yrtimescales. We discuss the hypothesis that cores are created by thedecay of a massive black hole binary formed in a merger. Apart fromtheir brightness profiles, there are no strong differences between coregalaxies and power-law galaxies that demand this scenario; however, therounder shapes of core, their lack of disks, and their reduced colorgradients may be consistent with it.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555. These observations are associated withGO and GTO proposals 5236, 5446, 5454, 5512, 5943, 5990, 5999, 6099,6386, 6554, 6587, 6633, 7468, 8683, and 9107.
| The Westerbork HI survey of spiral and irregular galaxies. III. HI observations of early-type disk galaxies
We present Hi observations of 68 early-type disk galaxies from the WHISPsurvey. They have morphological types between S0 and Sab and absoluteB-band magnitudes between -14 and -22. These galaxies form the massive,high surface-brightness extreme of the disk galaxy population, few ofwhich have been imaged in Hi before. The Hi properties of the galaxiesin our sample span a large range; the average values of MHI/LB and DH I/D25 are comparableto the ones found in later-type spirals, but the dispersions around themean are larger. No significant differences are found between the S0/S0aand the Sa/Sab galaxies. Our early-type disk galaxies follow the same Himass-diameter relation as later-type spiral galaxies, but theireffective Hi surface densities are slightly lower than those found inlater-type systems. In some galaxies, distinct rings of Hi emissioncoincide with regions of enhanced star formation, even though theaverage gas densities are far below the threshold of star formationderived by Kennicutt (1989, ApJ, 344, 685). Apparently, additionalmechanisms, as yet unknown, regulate star formation at low surfacedensities. Many of the galaxies in our sample have lopsided gasmorphologies; in most cases this can be linked to recent or ongoinginteractions or merger events. Asymmetries are rare in quiescentgalaxies. Kinematic lopsidedness is rare, both in interacting andisolated systems. In the appendix, we present an atlas of the Hiobservations: for all galaxies we show Hi surface density maps, globalprofiles, velocity fields and radial surface density profiles.
| Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles
We present a study of the optical brightness profiles of early typegalaxies, using a number of samples of radio galaxies and opticallyselected elliptical galaxies. For the radio galaxy samples - B2 ofFanaroff-Riley type I and 3C of Fanaroff-Riley type II - we determined anumber of parameters that describe a "Nuker-law" profile, which werecompared with those already known for the optically selected objects. Wefind that radio active galaxies are always of the "core" type (i.e. aninner Nuker law slope γ < 0.3). However, there are core-typegalaxies which harbor no significant radio source and which areindistinguishable from the radio active galaxies. We do not find anyradio detected galaxy with a power law profile (γ > 0.5). Thisdifference is not due to any effect with absolute magnitude, since in aregion of overlap in magnitude the dichotomy between radio active andradio quiescent galaxies remains. We speculate that core-type objectsrepresent the galaxies that have been, are, or may become, radio activeat some stage in their lives; active and non-active core-type galaxiesare therefore identical in all respects except their eventualradio-activity: on HST scales we do not find any relationship betweenboxiness and radio-activity. There is a fundamental plane, defined bythe parameters of the core (break radius rb and breakbrightness μ_b), which is seen in the strong correlation betweenrb and μ_b. The break radius is also linearly proportionalto the optical Luminosity in the I band. Moreover, for the few galaxieswith an independently measured black hole mass, the break radius turnsout to be tightly correlated with MBH. The black hole masscorrelates even better with the combination of fundamental planeparameters rb and μ_b, which represents the centralvelocity dispersion.
| Radio sources in low-luminosity active galactic nuclei. IV. Radio luminosity function, importance of jet power, and radio properties of the complete Palomar sample
We present the completed results of a high resolution radio imagingsurvey of all ( 200) low-luminosity active galactic nuclei (LLAGNs) andAGNs in the Palomar Spectroscopic Sample of all ( 488) bright northerngalaxies. The high incidences of pc-scale radio nuclei, with impliedbrightness temperatures ≳107 K, and sub-parsec jetsargue for accreting black holes in ≳50% of all LINERs andlow-luminosity Seyferts; there is no evidence against all LLAGNs beingmini-AGNs. The detected parsec-scale radio nuclei are preferentiallyfound in massive ellipticals and in type 1 nuclei (i.e. nuclei withbroad Hα emission). The radio luminosity function (RLF) of PalomarSample LLAGNs and AGNs extends three orders of magnitude below, and iscontinuous with, that of “classical” AGNs. We find marginalevidence for a low-luminosity turnover in the RLF; nevertheless LLAGNsare responsible for a significant fraction of present day massaccretion. Adopting a model of a relativistic jet from Falcke &Biermann, we show that the accretion power output in LLAGNs is dominatedby the kinetic power in the observed jets rather than the radiatedbolometric luminosity. The Palomar LLAGNs and AGNs follow the samescaling between jet kinetic power and narrow line region (NLR)luminosity as the parsec to kilo-parsec jets in powerful radio galaxies.Eddington ratios {l_Edd} (=L_Emitted/L_Eddington) of≤10-1{-}10-5 are implied in jet models of theradio emission. We find evidence that, in analogy to Galactic black holecandidates, LINERs are in a “low/hard” state (gas poornuclei, low Eddington ratio, ability to launch collimated jets) whilelow-luminosity Seyferts are in a “high” state (gas richnuclei, higher Eddington ratio, less likely to launch collimated jets).In addition to dominating the radiated bolometric luminosity of thenucleus, the radio jets are energetically more significant thansupernovae in the host galaxies, and are potentially able to depositsufficient energy into the innermost parsecs to significantly slow thegas supply to the accretion disk.
| Photometric properties and origin of bulges in SB0 galaxies
We have derived the photometric parameters for the structural componentsof a sample of fourteen SB0 galaxies by applying a parametricphotometric decomposition to their observed I-band surface brightnessdistribution. We find that SB0 bulges are similar to bulges of theearly-type unbarred spirals, i.e. they have nearly exponential surfacebrightness profiles (< n>=1.48±0.16) and their effectiveradii are strongly coupled to the scale lengths of their surroundingdiscs (< r_e/h>=0.20±0.01). The photometric analysis alonedoes not allow us to differentiate SB0 bulges from unbarred S0 ones.However, three sample bulges have disc properties typical ofpseudobulges. The bulges of NGC 1308 and NGC 4340 rotate faster thanbulges of unbarred galaxies and models of isotropic oblate spheroidswith equal ellipticity. The bulge of IC 874 has a velocity dispersionlower than expected from the Faber-Jackson correlation and thefundamental plane of the elliptical galaxies and S0 bulges. Theremaining sample bulges are classical bulges, and are kinematicallysimilar to lower-luminosity ellipticals. In particular, they follow theFaber-Jackson correlation, lie on the fundamental plane and those forwhich stellar kinematics are available rotate as fast as the bulges ofunbarred galaxies.
| Globular clusters and the formation of the outer Galactic halo
Globular clusters in the outer halo (Rgc > 15kpc) arefound to be systematically fainter than those at smaller Galactocentricdistances. Within the outer halo the compact clusters with half-lightradii Rh < 10pc are only found at Rgc <40kpc, while on the other hand the larger clusters with Rh> 10pc are encountered at all Galactocentric distances. Among thecompact clusters with Rh < 10pc that have Rgc> 15kpc, there are two objects with surprisingly high metallicities.One of these is Terzan 7, which is a companion of the Sagittarius dwarf.The other is Palomar 1. The data on these two objects suggests that theymight have had similar evolutionary histories. It is also noted that,with one exception, luminous globular clusters in the outer halo are allcompact whereas faint ones may have any radius. This also holds forglobular clusters in the Large Magellanic Cloud, Small Magellanic Cloudand Fornax dwarf. The lone exception is the large luminous globular NGC2419. Possibly this object is not a normal globular cluster, but thestripped core of a former dwarf spheroidal. In this respect it mayresemble ω Centauri.
| The SAURON project - III. Integral-field absorption-line kinematics of 48 elliptical and lenticular galaxies
We present the stellar kinematics of 48 representative elliptical andlenticular galaxies obtained with our custom-built integral-fieldspectrograph SAURON operating on the William Herschel Telescope. Thedata were homogeneously processed through a dedicated reduction andanalysis pipeline. All resulting SAURON data cubes were spatially binnedto a constant minimum signal-to-noise ratio. We have measured thestellar kinematics with an optimized (penalized pixel-fitting) routinewhich fits the spectra in pixel space, via the use of optimal templates,and prevents the presence of emission lines to affect the measurements.We have thus generated maps of the mean stellar velocity V, the velocitydispersion σ, and the Gauss-Hermite moments h3 andh4 of the line-of-sight velocity distributions. The mapsextend to approximately one effective radius. Many objects displaykinematic twists, kinematically decoupled components, central stellardiscs, and other peculiarities, the nature of which will be discussed infuture papers of this series.
| Dynamical friction for dark halo satellites: effects of tidal mass loss and growing host potential
Motivated by observations of inner halo satellite remnants like the Sgrstream and ω Centauri, we develop fully analytical models to studythe orbital decay and tidal mass loss of satellites on eccentric orbitsin an isothermal potential of a host galaxy halo. The orbital decay rateis often severely overestimated if applying Chandrasekhar's formulawithout correcting for (i) the evaporation and tidal loss of thesatellite, and (ii) the contraction of satellite orbits due to adiabaticgrowth of the host galaxy potential over the Hubble time. As a satellitemigrates inwards, the increasing halo density affects the dynamicalfriction in two opposite ways: (1) it boosts the number of haloparticles swept in the gravitational `wake' of the satellite, henceincreasing the drag on the satellite, and (2) it boosts the tide which`peels off' the satellite, and reduces the amplitude of the wake. Thesecompeting processes can be modelled analytically for a satellite withthe help of an empirical formula for the mass-loss history. Theanalytical model agrees with more traditional numerical simulations oftidal mass loss and dynamical friction. Rapid mass loss due toincreasing tides at smaller and smaller radius makes it less likely forstreams or remnants of infalling satellites to intrude into the innerhalo (like the Sgr stream and ω Centauri) than to stay in theouter halo (like the Magellanic stream), hence any intermediate-masscentral black holes of the satellites are also probably `hung up' atlarge distances as well. It is difficult for the black holes of thesatellites to come close enough to merge into the supermassive blackhole in the centre of the host potential unless the satellites startedwith (i) pericentres much smaller than the typical distances topresent-day observed satellites, and (ii) central density much higherthan in the often seen finite-density cores of observed satellites.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
Каталог и designations:
|
INHABITED SKY
