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The internal dynamics of the Local Group dwarf elliptical galaxies NGC 147, 185 and 205
We present three-integral dynamical models for the three Local Groupdwarf elliptical galaxies: NGC 147, 185 and 205. These models are fittedto the Two-Micron All-Sky Survey (2MASS) J-band surface-brightnessdistribution and the major-axis kinematics (mean streaming velocity andvelocity dispersion) and, in the case of NGC 205, also to the minor-axiskinematics. The kinematical information extends out to 2Re inthe case of NGC 205 and out to about 1Re in the case of NGC147 and 185. It is the first time models are constructed for the LocalGroup dwarf ellipticals (dEs) that allow for the presence of dark matterat large radii and that are constrained by kinematics out to at leastone half-light radius. The B-band mass-to-light ratios of all the threegalaxies are rather similar, (M/L)B ~3-4Msolar/Lsolar,B. Within the innertwo half-light radii, about 40-50 per cent of the mass is in the form ofdark matter, so dEs contain about as much dark matter as brightellipticals.Based on their appreciable apparent flattening, we modelled NGC 205 and147 as being viewed edge-on. For NGC 185, having a much rounderappearance on the sky, we produced models for different inclinations.NGC 205 and 147 have a relatively isotropic velocity dispersion tensorwithin the region where the internal dynamics are strongly constrainedby the data. Our estimated inclination for NGC 185 is i ~ 50°because in that case the model has an intrinsic flattening close to thepeak of the intrinsic shape distribution of dEs and it, like thebest-fitting models for NGC 147 and 205, is nearly isotropic. We alsoshow that the dynamical properties of the bright nucleus of NGC 205 arenot unlike those of a massive globular cluster.Based on observations collected at the Observatoire de Haute-Provence.E-mail: sven.derijcke@UGent.be (SDR) ‡Postdoctoral Fellow of the Fund for Scientific Research - Flanders(Belgium)(F.W.O).
| The dipole anisotropy of the 2 Micron All-Sky Redshift Survey
We estimate the acceleration on the Local Group (LG) from the 2 MicronAll-Sky Redshift Survey (2MRS). The sample used includes about 23200galaxies with extinction-corrected magnitudes brighter thanKs= 11.25 and it allows us to calculate the flux-weighteddipole. The near-infrared flux-weighted dipoles are very robust becausethey closely approximate a mass-weighted dipole, bypassing the effectsof redshift distortions and require no preferred reference frame. Thisis combined with the redshift information to determine the change indipole with distance. The misalignment angle between the LG and thecosmic microwave background (CMB) dipole drops to 12°+/- 7° ataround 50h-1Mpc, but then increases at larger distances,reaching 21°+/- 8° at around 130h-1Mpc. Exclusion ofthe galaxies Maffei 1, Maffei 2, Dwingeloo 1, IC342 and M87 brings theresultant flux dipole to 14°+/- 7° away from the CMB velocitydipole. In both cases, the dipole seemingly converges by60h-1Mpc. Assuming convergence, the comparison of the 2MRSflux dipole and the CMB dipole provides a value for the combination ofthe mass density and luminosity bias parametersΩ0.6m/bL= 0.40 +/- 0.09.
| Planetary nebulae as tracers of galaxy stellar populations
We address the general problem of the luminosity-specific planetarynebula (PN) number, better known as the `α' ratio, given byα=NPN/Lgal, and its relationship with theage and metallicity of the parent stellar population. Our analysisrelies on population synthesis models that account for simple stellarpopulations (SSPs), and more elaborate galaxy models covering the fullstar formation range of the different Hubble morphological types. Thistheoretical framework is compared with the updated census of the PNpopulation in Local Group (LG) galaxies and external ellipticals in theLeo group, and the Virgo and Fornax clusters.The main conclusions of our study can be summarized as follows. (i)According to the post-asymptotic giant branch (AGB) stellar core mass,PN lifetime in a SSP is constrained by three relevant regimes, driven bythe nuclear (Mcore>~ 0.57Msolar), dynamical(0.57Msolar>~Mcore>~ 0.55Msolar)and transition (0.55Msolar>~Mcore>~0.52Msolar) time-scales. The lower limit for Mcorealso sets the minimum mass for stars to reach the AGB thermal-pulsingphase and experience the PN event. (ii) Mass loss is the crucialmechanism to constrain the value of α, through the definition ofthe initial-to-final mass relation (IFMR). The Reimers mass-lossparametrization, calibrated on Pop II stars of Galactic globularclusters, poorly reproduces the observed value of α in late-typegalaxies, while a better fit is obtained using the empirical IFMRderived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds10000yr, which suggests that Mcore<~ 0.65Msolarcores dominate, throughout. (iv) The relative PN deficiency inelliptical galaxies, and the observed trend of α with galaxyoptical colours, support the presence of a prevailing fraction oflow-mass cores (Mcore<~ 0.55Msolar) in the PNdistribution and a reduced visibility time-scale for the nebulae as aconsequence of the increased AGB transition time. The stellar componentwith Mcore<~ 0.52Msolar, which overrides the PNphase, could provide an enhanced contribution to hotter HB and post-HBevolution, as directly observed in M 32 and the bulge of M 31. Thisimplies that the most UV-enhanced ellipticals should also display thelowest values of α, as confirmed by the Virgo cluster early-typegalaxy population. (v) Any blue-straggler population, invoked asprogenitor of the Mcore>~ 0.7Msolar PNe inorder to preserve the constancy of the bright luminosity-functioncut-off magnitude in ellipticals, must be confined to a small fraction(a few per cent at most) of the whole galaxy PN population.
| Morphological classification of nearby galaxies based on asymmetry and luminosity concentration
We investigate the behaviour of the asymmetry parameter A as amorphological parameter using a `volume-limited' sample of 349 galaxies(distance <=25Mpc,MV<=-18.5mag) and a largermagnitude-limited sample of 707 nearby galaxies. We confirm thecorrelation of A with morphological type. The late-type galaxies (Sdm,Sm and Im) have larger A than early-type galaxies, and they tend to havelarger A than spiral galaxies. We investigate the usefulness of the Aversus concentration index Cin diagram as a tool for theregular-irregular and early-late classification. The diagram is not veryuseful to the regular versus late-type irregular classification, asinferred previously, but it is found to be useful to the early-lateclassification.
| Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
We introduce and discuss the properties of a theoretical (B-K)(J-K)integrated colour diagram for single-age, single-metallicity stellarpopulations. We show how this combination of integrated colours is ableto largely disentangle the well-known age-metallicity degeneracy whenthe age of the population is greater than ~300Myr, and thus providesvaluable estimates of both age and metallicity of unresolved stellarsystems. We discuss in detail the effect on this colour-colour diagramof α-enhanced metal abundance ratios (typical of the oldestpopulations in the Galaxy), the presence of blue horizontal branch starsunaccounted for in the theoretical calibration and of statistical colourfluctuations in low-mass stellar systems. In the case of populationswith multiple stellar generations, the luminosity-weighted mean ageobtained from this diagram is shown to be heavily biased towards theyoungest stellar components. We then apply this method to several datasets for which optical and near-infrared photometry are available in theliterature. We find that Large Magellanic Cloud and M31 clusters havecolours which are consistent with the predictions of the models, butthese do not provide a sensitive test due to the fluctuations which arepredicted by our modelling of the Poisson statistics in such low-masssystems. For the two Local Group dwarf galaxies NGC 185 and 6822, themean ages derived from the integrated colours are consistent with thestar formation histories inferred independently from photometricobservations of their resolved stellar populations.The methods developed here are applied to samples of nearby early-typegalaxies with high-quality aperture photometry in the literature. Asample of bright field and Virgo cluster elliptical galaxies is found toexhibit a range of luminosity-weighted mean ages from 3 to 14Gyr, with amean of ~8Gyr, independent of environment, and mean metallicities at orjust above the solar value. Colour gradients are found in all of thegalaxies studied, in the sense that central regions are redder. Apartfrom two radio galaxies, where the extreme central colours are clearlydriven by the active galactic nucleus, and one galaxy which also shows aradial age gradient, these colour changes appear consistent withmetallicity changes at a constant mean age. Finally, aperture data forfive Virgo early-type dwarf galaxies show that these galaxies appear tobe shifted to lower mean metallicities and lower mean ages (range1-6Gyr) than their higher luminosity counterparts.
| Gas in early-type galaxies: cross-fuelling in late-type-early-type pairs?
We present 12CO (J= 1-0) and 12CO (J= 2-1)observations of eight early-type galaxies, forming part of a sample ofinteracting galaxies, each consisting of one late- and one early-typesystem. All of the early-type galaxies observed are undetected in CO tolow levels, allowing us to place tight constraints on their moleculargas content. Additionally, we present HI absorption data for one system.The implications for possible gas transfer from the late- to theearly-type galaxy during the interaction are discussed.
| The satellite distribution of M31
The spatial distribution of the Galactic satellite system plays animportant role in Galactic dynamics and cosmology, where its successfulreproduction is a key test of simulations of galaxy halo formation.Here, we examine its representative nature by conducting an analysis ofthe three-dimensional spatial distribution of the M31 subgroup ofgalaxies, the next closest system to our own. We begin by a discussionof distance estimates and incompleteness concerns, before revisiting thequestion of membership of the M31 subgroup. We constrain this byconsideration of the spatial and kinematic properties of the putativesatellites. Comparison of the distribution of M31 and Galacticsatellites relative to the galactic discs suggests that the Galacticsystem is probably modestly incomplete at low latitudes by ~=20 percent. We find that the radial distribution of satellites around M31 ismore extended than the Galactic subgroup; 50 per cent of the Galacticsatellites are found within ~100 kpc of the Galaxy, compared to ~200 kpcfor M31. We search for `ghostly streams' of satellites around M31, inthe same way others have done for the Galaxy, and find several,including some that contain many of the dwarf spheroidal satellites. Thelack of M31-centric kinematic data, however, means that we are unable toprobe whether these streams represent real physical associations.Finally, we find that the M31 satellites are asymmetrically distributedwith respect to our line of sight to this object, so that the majorityof its satellites are on its near side with respect to our line ofsight. We quantify this result in terms of the offset between M31 andthe centre of its satellite distribution, and find it to be significantat the ~ 3σ level. We discuss possible explanations for thisfinding, and suggest that many of the M31 satellites may have beenaccreted only relatively recently. Alternatively, this anisotropy may berelated to a similar result recently reported for the 2dFGRS, whichwould imply that the halo of M31 is not yet virialized. Until such timeas a satisfactory explanation for this finding is presented, however,our results warn against treating the M31 subgroup as complete, unbiasedand relaxed.
| Detection of a 60°-long Dwarf Galaxy Debris Stream
We report on a 60°-long stream of stars, extending from Ursa Majorto Sextans, in the Sloan Digital Sky Survey. The stream is approximately2° wide and is clearly distinct from the northern tidal arm of theSagittarius dwarf galaxy. The apparent width of the stream indicates aprogenitor with a size and mass similar to that of a dwarf galaxy. Thestream is about 21 kpc distant and appears to be oriented almostperpendicular to our line of sight. The visible portion of the streamdoes not pass near any known dwarf galaxies, although we cannot rule outthat the stream may form the inner part of a known dwarf galaxy's orbit.The most likely explanation is that the stream constitutes the remainsof a dwarf galaxy that has been completely disrupted at some point inthe past. We also briefly report on the discovery of a diminutiveGalactic satellite that lies near the projected path of the new streambut is unlikely to be related to it.
| Local Group Dwarf Galaxies and the Fundamental Manifold of Spheroids
The fundamental manifold (FM), an extension of the fundamental planeformalism, incorporates all spheroid-dominated stellar systems fromdwarf ellipticals up to the intracluster stellar populations of galaxyclusters by accounting for the continuous variation of the mass-to-lightratio within the effective radius re with scale. Here we findthat Local Group dwarf spheroidal and dwarf elliptical galaxies, whichprobe the FM relationship roughly one decade lower in re thanprevious work, lie on the extrapolation of the FM. When combined withthe earlier data, these Local Group dwarfs demonstrate the validity ofthe empirical manifold over nearly 4 orders of magnitude inre. The continuity of the galaxy locus on the manifold and,more specifically, the overlap on the FM of dwarf ellipticals like M32and dwarf spheroidals like Leo II, imply that dwarf spheroidals belongto the same family of spheroids as their more massive counterparts. Theonly significant outliers are Ursa Minor and Draco. We explore whetherthe deviation of these two galaxies from the manifold reflects abreakdown in the coherence of the empirical relationship at lowluminosities or rather the individual dynamical peculiarities of thesetwo objects. We discuss some implications of our results for how thelowest mass galaxies form.
| A Local Group Polar Ring Galaxy: NGC 6822
Star counts obtained from a 2° × 2° area centered on NGC6822 have revealed an optical image of this galaxy composed of twocomponents: in addition to the well-known H I disk with its youngstellar component, there is a spheroidal stellar structure as extensiveas its H I disk, but with its major axis at roughly right angles to it,that we traced to at least 36'. Radial velocities of over 100intermediate-age carbon stars found within this structure displaykinematics contrasting strongly with those of the H I disk. These Cstars belong to the spheroid. Although devoid of gas, the spheroidrotation is consistent with the I-band Tully-Fisher relation. Theorientation of the rotation axis that minimizes the stellar velocitydispersion coincides with the minor axis of the stellar populationellipsoid, lying very nearly in the plane of the H I disk. We concludethat the H I disk is a polar ring and that the spheroidal component isan erstwhile disk, a fossil remainder of a past close encounter episode.Based on observations obtained with MegaPrime/MegaCam, a joint projectof CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT),which is operated by the National Research Council (NRC) of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique (CNRS) of France, and the University of Hawaii.Based on observations acquired at the du Pont Telescope, from theObservatories of the Carnegie Institution of Washington.
| Masses of the local group and of the M81 group estimated from distortions in the local velocity field
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
| Weak redshift discretisation in the Local Group of galaxies?
We discuss the distribution of radial velocities of galaxies belongingto the Local Group. Two independent samples of galaxies as well asseveral methods of reduction from the heliocentric to the galactocentricradial velocities are explored. We applied the power spectrum analysisusing the Hann function as a weighting method, together with thejackknife error estimation. We performed a detailed analysis of thisapproach. The distribution of galaxy redshifts seems to be non-random.An excess of galaxies with radial velocities of 24 kms-1 and 36 km s-1 is detected, but theeffect is statistically weak. Only one peak for radial velocities of 24 km s-1 seems to be confirmed at the confidence levelof 95%.
| Neutral Hydrogen Clouds Near Early-Type Dwarf Galaxies of the Local Group
Parkes neutral hydrogen 21 cm line (H I) observations of thesurroundings of nine early-type Local Group dwarfs are presented. Wedetected numerous H I clouds in the general direction of those dwarfs,and these clouds are often offset from the optical center of thegalaxies. Although all the observed dwarfs, except Antlia, occupyphase-space regions where the high-velocity cloud (HVC) density is wellabove average, the measured offsets are smaller than one would expectfrom a fully random cloud distribution. Possible association is detectedfor 11 of the 16 investigated clouds, while for two galaxies, Sextansand Leo I, no H I was detected. The galaxies in which H I clouds werefound not to coincide with the optical yet have a significantprobability of being associated are the Sculptor dwarf, Tucana, LGS 3,Cetus, and Fornax. If the clouds are indeed associated, these galaxieshave H I masses of MHI=2×105,2×106, 7×105, 7×105,and 1×105 Msolar, respectively. However,neither ram pressure nor tidal stripping can easily explain the offsets.In some cases, large offsets are found where ram pressure should be theleast effective.
| The Anisotropic Distribution of M31 Satellite Galaxies: A Polar Great Plane of Early-type Companions
The highly anisotropic distribution and apparent alignment of theGalactic satellites in polar great planes begs the question of howcommon such distributions are. The satellite system of M31 is the onlynearby system for which we currently have sufficiently accuratedistances to study the three-dimensional satellite distribution. Wepresent the spatial distribution of the 15 currently known M31companions in a coordinate system centered on M31 and aligned with itsdisk. Through a detailed statistical analysis we show that the fullsatellite sample describes a plane that is inclined by -56° withrespect to the poles of M31 and has an rms height of 100 kpc. At 88% thestatistical significance of this plane is low, and it is unlikely tohave a physical meaning. We note that the great stellar stream foundnear Andromeda is inclined to this plane by 7°. Most of the M31satellites are found within <+/-40° of M31's disk; i.e., there islittle evidence for a Holmberg effect. If we confine our analysis toearly-type dwarfs, we find a best-fit polar plane within 5°-7°from the pole of M31. This polar great plane has a statisticalsignificance of 99.7% and includes all dSphs (except for And II), M32,NGC 147, and PegDIG. The rms distance of these galaxies from the polarplane is 16 kpc. The nearby spiral M33 has a distance of only ~3 kpcfrom this plane, which points toward the M81 group. We discuss theanisotropic distribution of M31's early-type companions in the frameworkof three scenarios, namely, as remnants of the breakup of a largerprogenitor, as a tracer of a prolate dark matter halo, and as a tracerof collapse along large-scale filaments. The first scenario requiresthat the breakup must have occurred at very early times and that thedwarfs continued to form stars thereafter to account for their stellarpopulation content and luminosity-metallicity relation. The thirdscenario seems to be plausible, especially when considering the apparentalignment of our potential satellite filament with several nearbygroups. The current data do not permit us to rule out any of thescenarios. Orbit information is needed to test the physical reality ofthe polar plane and of the different scenarios in more detail.
| Local Group Dwarf Elliptical Galaxies. I. Mapping the Dynamics of NGC 205 Beyond the Tidal Radius
NGC 205 is the nearest example of a dwarf elliptical galaxy and theprototype of this enigmatic galaxy class. Photometric evidence suggeststhat NGC 205, a close satellite of the M31 galaxy, is tidallyinteracting with its parent galaxy. We present stellar radial velocitymeasurements out to a projected radius of 20' (5 kpc) in NGC 205 basedon Keck DEIMOS multislit spectroscopic observations of 725 individualred giant branch stars. Our kinematic measurements extend from thecenter out to 6 times the effective radius of NGC 205, well past theexpected tidal radius. The contamination in our kinematic sample fromM31 field stars is estimated to be a few percent based on maximumlikelihood fits to the distribution of stars in position-velocity space.We measure a maximum major-axis rotation speed for the body of NGC 205of 11+/-5 km s-1 and note that this is based on observing adefinite turnover in the rotation curve; this is the first dE galaxy inwhich the maximum rotation velocity has been measured. Combined with thevelocity dispersion, we conclude that NGC 205 is supported by acombination of rotation and anisotropic velocity dispersion. At amajor-axis distance of 4.5 arcmin (1 kpc), the velocity profile of NGC205 turns over; stars beyond this radius are moving counter to therotation of the inner part of the galaxy. The turnover radius iscoincident with the onset of isophotal twisting and the estimated tidalradius, suggesting that the outer kinematics of NGC 205 is dominated bygravitational interactions with the nearby M31 galaxy. The motion ofstars beyond a radius of ~4.5 arcmin implies that NGC 205 is in aprograde encounter with its parent galaxy, M31.
| H I Detection of Two Dwarf S0 Galaxies in Nearby Groups: ESO 384-016 and NGC 59
A H I survey of 10 dE/dS0 galaxies in the nearby Sculptor and CentaurusA groups was made using the Australia Telescope Compact Array. Theobserved galaxies have accurate distances derived by Jerjen et al. usingthe surface brightness fluctuation technique. Their absolute magnitudesare in the range -9.5>MB>-15.3. Only two of the 10galaxies were detected at our detection limit (~1.0×106Msolar for the Centaurus group and ~5.3×105Msolar for the Sculptor group): the two dS0 galaxies ESO384-016 in the Centaurus A group and NGC 59 in the Sculptor group, withH I masses of (6.0+/-0.5)×106 and(1.4+/-0.1)×107 Msolar, respectively. Thosetwo detections were confirmed using the Green Bank Telescope. Thesesmall H I reservoirs could fuel future generations of low-level starformation and could explain the bluer colors seen at the center of thedetected galaxies. Similar to what is seen with the Virgo dwarfellipticalss, the two objects with H I appear to be on the outskirts ofthe groups.
| On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies
We investigate the relation between X-ray nuclear emission, opticalemission line luminosities and black hole masses for a sample of 47Seyfert galaxies. The sample, which has been selected from the Palomaroptical spectroscopic survey of nearby galaxies (Ho et al. 1997a, ApJS,112, 315), covers a wide range of nuclear powers, from L2-10keV ~ 1043 erg/s down to very low luminosities(L2-10 keV ~ 1038 erg/s). Best available data fromChandra, XMM-Newton and, in a few cases, ASCA observations have beenconsidered. Thanks to the good spatial resolution available from theseobservations and a proper modeling of the various spectral components,it has been possible to obtain accurate nuclear X-ray luminosities notcontaminated by off-nuclear sources and/or diffuse emission. X-rayluminosities have then been corrected taking into account the likelycandidate Compton thick sources, which are a high fraction (>30%)among type 2 Seyferts in our sample. The main result of this study isthat we confirm strong linear correlations between 2-10 keV,[OIII]λ5007, Hα luminosities which show the same slope asquasars and luminous Seyfert galaxies, independent of the level ofnuclear activity displayed. Moreover, despite the wide range ofEddington ratios (L/L_Edd) tested here (six orders of magnitude, from0.1 down to ~10-7), no correlation is found between the X-rayor optical emission line luminosities and the black hole mass. Ourresults suggest that Seyfert nuclei in our sample are consistent withbeing a scaled-down version of more luminous AGN.
| The evolved asymptotic giant branch stars in the central bar of the dwarf irregular galaxy NGC 6822
Images of {JHK} and {gi} obtained with the Canada-France-HawaiiTelescope are used to investigate the stellar contents of the resolvedasymptotic giant branch (AGB) population in the area of 3.6arcmin×6.3 arcmin on the central bar of the dwarf irregular galaxyNGC 6822. The upper envelope of near-infrared (J-K,K) and (H-K,K)color-magnitude diagrams (CMDs) shows four components of main-sequencestars, foreground stars, massive supergiants, and AGB stars. Thesecomponents are also noticeable in the bright part of (g-i,g) CMD. Weselect AGB stars in NGC 6822 from the (g-K,g) CMD, which has a longspectral wavelength base line in the color index. There are 141 C starswith a mean absolute magnitude of =-7.60±0.50, and with mean colors of <(J-K)0>=1.77±0.41, and <(H-K)0>=0.75±0.28. The number ratio of C stars toM-giants (C/M) is estimated as 0.27±0.03, while the northern partof the bar has a somewhat lower C/M ratio than the southern part. The(J-K) and (H-K) color distributions of AGB stars contain a main peak ofM-giant stars and a red tail of C stars. The broad color distributionsfor M-giants indicate that NGC 6822 has a wide range of intermediateages with a peak at log(t_yr)9.0. The MK luminosityfunction (LF) for C stars in the central bar of NGC 6822 is likely to bea Gaussian distribution, while the LF of C stars in the southern part isslightly skewed to the brighter side than in northern part. The meanbolometric magnitude of 141 C stars in NGC 6822 is =-4.36±0.54. The bolometric LF for M-giants extends upto M_bol=-6.5, while the one for C stars spans -5.8
| Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
| Near-IR photometry of asymptotic giant branch stars in the dwarf elliptical galaxy NGC 147
Near-infrared J, H and K' images were used to investigate the stellarcontents of the asymptotic giant branch (AGB) population in the nearbydwarf elliptical galaxy NGC 147. The obtained (K, J-K) and (K, H-K)color-magnitude diagrams contain stars of AGB and red giant branchpopulations, where the former consists of a group of bright blue stars,a dominant population of M giant and a red C star population. Weidentified 91 AGB C stars in NGC 147 with the mean absolute magnitudeand colors of < M_K> = -7.56, <(J-K)_0> = 1.81 and<(H-K)_0> = 0.74. The estimated number ratio of C stars to M giantstars (C/M) is 0.16±0.02. The estimated local C/M ratios of0.14± 0.02 for the inner region (r<70'') and 0.19± 0.03for the outer region (r>70'') indicate a weak radial gradient. Themean bolometric magnitude of 91 C stars in NGC 147 is < M_bol> =-4.32± 0.49. The bolometric luminosity function of M giant starsin NGC 147 extends up to M_bol = -5.8 mag, and that of only C starsspans -5.6
| Imaging resources for the GTC: the Local Group Census
The Local Group Census is a narrowband imaging survey aimed atcataloguing the emission-line populations in the galaxies of the LocalGroup. Data, which were obtained using the Wide Field Camera of the 2.5mIsaac Newton Telescope, are available to the whole astronomicalcommunity, resulting in a valuable imaging resource for follow-upspectroscopy with the GTC.
| A Dynamical Model for the Orbit of the Andromeda Galaxy M31 and the Origin of the Local Group of Galaxies
We propose a new model for the origin and evolution of the Local Groupof Galaxies (LGG) that naturally explains the formation of theMagellanic Clouds and their large orbital angular momenta around theGalaxy. The basic idea is that an off-center hydrodynamical collisionoccurred some 10Gyr ago between the primordial Andromeda galaxy (M31)and a similar Galaxy, and compressed the halo gas to form the LGG dwarfgalaxies, including the Magellanic Clouds. New-born dwarf galaxies canbe expected to locate on the orbital plane of these two massivegalaxies. We reexamined the two-dimensional sky distribution of the LGGmembers, and confirmed an early idea that they align along two similargreat circles. The planes of these circles are approximately normal tothe line joining the present position of the Sun and the galacticcenter. We made a distribution map of these objects, and found awell-defined plane of finite thickness. Thus we could determine theorbital elements of M31 relative to the Galaxy by reproducing thewell-studied dynamics of the LMC and the SMC around the Galaxy. Theexpected proper motion of M31 is (μl, μb) =(38 ± 16 μas yr-1, -49 ± 5 μasyr-1).
| The Local Group Census: searching for planetary nebulae in IC 1613, WLM and GR8
In the framework of the Local Group Census (LGC), a survey of the LocalGroup (LG) galaxies above Dec =-30° aimed at surveying thepopulations that have strong emission lines, we have searched forplanetary nebulae (PNe) in the low-metallicity dwarf irregular galaxiesIC 1613, WLM, GR 8. Two new candidate PNe have been found in IC 1613,one in WLM and none in GR 8. The observations presented in this paper,together with the previous results from the LGC, represent the firststep in the study of the PN population in low-metallicity, dwarfirregular galaxies of the Local Group. These observations will befollowed by deep spectroscopy to confirm the nature of these objects andto study their physical-chemical properties. We use the observed numberof PNe in each LG galaxy to estimate a lower limit to the mass of theintermediate-age population, which is compared with the star formationrate (SFR) of LG dwarf galaxies. These results are in agreement withthose from accurate star formation history (SFH) analyses for thesesmall galaxy systems.
| The HI content of Fornax dwarf elliptical galaxies: FCC032 and FCC336
We present HI 21-cm line observations, obtained with the AustraliaTelescope Compact Array, of two dwarf elliptical galaxies (dEs) in theFornax cluster: FCC032 and FCC336. The optical positions and velocitiesof these galaxies place them well within the Fornax cluster. FCC032 wasdetected at the 3σ significance level with a total HI flux densityof 0.66 +/- 0.22 Jy km s-1 or an HI mass of 5.0 +/- 1.7× 107h-275Msolar.Based on our deep Hα+[NII] narrow-band images, obtained with FORS2mounted on the Very Large Telescope, this dE was already known tocontain 600 -1800h-275Msolar of ionizedhydrogen (depending on the relative strengths of the Hα and [NII]emission lines). Hence, this is the first study of the complex,multiphase interstellar medium of a dE outside the Local Group. FCC336was detected at the same significance level: 0.37 +/- 0.10 Jy kms-1 or a total HI mass of 2.8 +/- 0.7 ×107h-275Msolar. Using acompilation of HI data of dwarf galaxies, we find that the observed highHI mass boundary of the distribution of dwarf irregulars, blue compactdwarfs and dwarf ellipticals in a logLB versuslogMHI diagram is in good agreement with a simple chemicalevolution model with continuous star formation. The existence of manygas-poor dEs (undetected at 21 cm) suggests that the environment (or,more particularly, a galaxy's orbit within a cluster) also plays acrucial role in determining the amount of gas in present-day dEs; forexample, FCC032 and FCC336 are located in the sparsely populatedoutskirts of the Fornax cluster. This is in agreement with HI surveys ofdEs in the Virgo cluster, and an Hα survey of the Fornax cluster,which also tend to place gas-rich dwarf galaxies in the clusterperiphery.
| The galaxy luminosity function from MR=-25 to MR=-9
Redshift surveys such as the Sloan Digital Sky Survey (SDSS) have givena very precise measurement of the galaxy luminosity function down toabout MR=-17 (~MB=-16). Fainter absolutemagnitudes cannot be probed because of the flux limit required forspectroscopy. Wide-field surveys of nearby groups using mosaic CCDs onlarge telescopes are able to reach much fainter absolute magnitudes,about MR=-10. These diffuse, spiral-rich groups are thoughtto be typical environments for galaxies, so their luminosity functionsshould be the same as the field luminosity function. The luminosityfunction of the groups at the bright end (MR < -17) islimited by Poisson statistics and is far less precise than that derivedfrom redshift surveys. Here we combine the results of the SDSS and thesurveys of nearby groups, and we supplement the results with studies ofLocal Group galaxies in order to determine the galaxy luminosityfunction over the entire range -25 < MR < -9. Theaverage logarithmic slope of the field luminosity function betweenMR=-19 and MR=-9 is α=-1.26, although asingle power law is a poor fit to the data over the entire magnituderange. We also determine the luminosity function of galaxy clusters anddemonstrate that it is different from the field luminosity function at ahigh level of significance; there are many more dwarf galaxies inclusters than in the field, due to a rise in the cluster luminosityfunction of α~-1.6 between MR=-17 andMR=-14.
| Distances and metallicities for 17 Local Group galaxies
We have obtained Johnson V and Gunni photometry for a large number ofLocal Group galaxies using the Isaac Newton Telescope Wide Field Camera(INT WFC). The majority of these galaxies are members of the M31subgroup and the observations are deep enough to study the top fewmagnitudes of the red giant branch in each system. We previouslymeasured the location of the tip of the red giant branch (TRGB) forAndromeda I, Andromeda II and M33 to within systematic uncertainties oftypically <0.05 mag. As the TRGB acts as a standard candle in old,metal-poor stellar populations, we were able to derive distances to eachof these galaxies. Here we derive TRGB distances to the giant spiralgalaxy M31 and 13 additional dwarf galaxies - NGC 205, 185, 147,Pegasus, WLM, LGS3, Cetus, Aquarius, And III, V, VI, VII and the newlydiscovered dwarf spheroidal And IX. The observations for each of thedwarf galaxies were intentionally taken in photometric conditions. Inaddition to the distances, we also self-consistently derive the medianmetallicity of each system from the colour of their red giant branches.This allows us to take into account the small metallicity variation ofthe absolute I magnitude of the TRGB. The homogeneous nature of our dataand the identical analysis applied to each of the 17 Local Groupgalaxies ensures that these estimates form a reliable set of distanceand metallicity determinations that are ideal for comparative studies ofLocal Group galaxy properties.
| Global Properties of Nearby Galaxies in Various Environments
We analyze a sample of the Local Volume that contains 451 galaxieswithin 10 Mpc. We compare the various global parameters of thesegalaxies with their tidal index that characterizes the local density ofthe environment. The closest correlation is observed between the densityof the galaxy’s environment and its morphological type. Theabundance of neutral hydrogen in the members of close groups was foundto be, on average, a factor of 3 lower than that in isolated galaxies.However, much of this difference is attributable to differentmorphological composition for the group members and field galaxies. Thetotal mass-to-luminosity ratio is virtually independent of the tidalindex of the galaxy, which indirectly indicates a low percentage oftidal systems among dwarf galaxies. All of the galaxies with three ormore companions in the Local Volume are shown to have masses above thethreshold value of 1010 M ȯ.
| The Classification of Galaxies: Early History and Ongoing Developments
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
| On the Accretion Origin of a Vast Extended Stellar Disk around the Andromeda Galaxy
We present the discovery of an inhomogenous, low surface brightness,extended disklike structure around the Andromeda galaxy (M31) based on alarge kinematic survey of more than 2800 stars with the Keck DEIMOSmultiobject spectrograph. The stellar structure spans radii from 15 kpcout to ~40 kpc, with detections out to R~70 kpc. The constituent starshave velocities close to the expected velocity of circular orbits in theplane of the M31 disk and typically have a velocity dispersion of ~30 kms-1. The color range on the upper red giant branch shows alarge spread indicative of a population with a significant range ofmetallicity. The mean metallicity of the population, measured from Ca IIequivalent widths, is [Fe/H]=-0.9+/-0.2. The morphology of the structureis irregular at large radii and shows a wealth of substructures thatmust be transitory in nature and are almost certainly tidal debris. Thepresence of these substructures indicates that the global entity wasformed by accretion. This extended disk follows smoothly on from thecentral parts of M31 disk out to ~40 kpc with an exponential density lawwith a scale length of 5.1+/-0.1 kpc, which is similar to that of thebright inner disk. However, the population possesses similar kinematicand abundance properties over the entire region where it is detected inthe survey. We estimate that the structure accounts for approximately10% of the total luminosity of the M31 disk, and given the huge scale,contains ~30% of the total disk angular momentum. This finding indicatesthat at least some galactic stellar disks are vastly larger thanpreviously thought and are formed, at least in their outer regions,primarily by accretion.
| Dynamical interaction of M 31 and M 32.
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