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An ultraluminous X-ray microquasar in NGC5408?
We studied the radio source associated with the ultraluminous X-raysource in NGC5408 (LX ~ 1040ergs-1).The radio spectrum is steep (index ~ -1), consistent with optically thinsynchrotron emission, not with flat-spectrum core emission. Its fluxdensity (~0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same inthe March 2000 and December 2004 observations, suggesting steadyemission rather than a transient outburst. However, it is orders ofmagnitude higher than expected from steady jets in stellar-massmicroquasar. Based on its radio flux and spectral index, we suggest thatthe radio source is either an unusually bright supernova remnant, or,more likely, a radio lobe powered by a jet from the black hole (BH).Moreover, there is speculative evidence that the source is marginallyresolved with a radius ~30 pc. A faint HII region of similar sizeappears to coincide with the radio and X-ray sources, but its ionizationmechanism remains unclear. Using a self-similar solution for theexpansion of a jet-powered electron-positron plasma bubble, in theminimum-energy approximation, we show that the observed flux and(speculative) size are consistent with an average jet power ~ 7 ×1038ergs-1 ~ 0.1LX ~0.1LEdd, an age ~105 yr, a current velocity ofexpansion ~80 km s-1. We briefly discuss the importance ofthis source as a key to understand the balance between luminosity andjet power in accreting BHs.
| The Araucaria Project .
Results from a long-term observational project called the AraucariaProject are presented. Based on Wide Field optical monitoring of 8nearby galaxies, covering a large range of metallicities, more than 500Cepheids and a few hundred Blue Supergiant candidates were identified.From the analysis of Cepheid P-L relations of outstanding qualityderived from our data we conclude that the slope of these relations inthe I band and Wesenheit index are not dependent on metallicity.Comparing the I-band magnitudes of Cepheids of a period of ten days, ascomputed from our P-L relations, to the I-band magnitudes of the tip ofthe RGB, which is widely believed to be independent of populationeffects, we cannot see any obvious dependence of the zero point of theI-band P-L relation on metallicity. A preliminary analysis of IRfollow-up observations of sub-samples of the identified Cepheids invarious galaxies of the project show that the distances obtained fromthese data are systematically shorter by about of 0.1 mag than thosederived from the optical photometry. It is likely that this effect canbe attributed to the internal reddening in the program galaxies. Theselected Blue Supergiant candidates were observed spectroscopically with8m-class telescopes to determine their element abundances, and theirluminosities from the Flux-weighted Gravity-Luminosity Relationship.Results on this aspect of the Araucaria Project are presented in thereview of Kudritzki presented during this conference.
| Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies
Mid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions.
| The Disk and Extraplanar Environment of NGC 247
The stellar content of the spiral galaxy NGC 247 is investigated usingdeep visible and near-infrared images. The main-sequence turnoff (MSTO)in the inner 12 kpc of the disk corresponds to an age of ~6 Myr. A meanstar formation rate (SFR) of 0.1 Msolar yr-1during the past 16 Myr is computed from star counts. The color of thered supergiant plume does not change with radius, suggesting that themean metallicity of young stars does not vary by more than ~0.1 dex. Thenumber of bright main-sequence stars per local stellar mass densityclimbs toward larger radii out to a distance of 12 kpc; the scalelengths that characterize the radial distributions of young and oldstars in the disk thus differ. The density of bright main-sequence starswith respect to projected H I mass gradually drops with increasingradius. The population of very young stars disappears in the outer disk;the MSTO at galactocentric radii between 12 and 15 kpc corresponds to~16 Myr, while between 15 and 18 kpc the age is >=40 Myr. Red giantbranch (RGB) stars are resolved at a projected minor-axis galactocentricdistance of ~12 kpc. There is a broad spread in metallicity among theRGB stars, with a mean [M/H]~-1.2. The RGB tip occurs ati'=24.5+/-0.1, indicating that the distance modulus is27.9+/-0.1. Luminous AGB stars with an age ~3 Gyr are also seen in thisfield.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council of Canada (Canada), CONICYT (Chile), theAustralian Research Council (Australia), CNPq (Brazil), and CONICET(Argentina).This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| The Polycyclic Aromatic Hydrocarbon Emission Deficit in Low-Metallicity Galaxies-A Spitzer View
Archival observations of 18 starburst galaxies that span a wide range inmetallicity reveal for the first time a correlation between the ratio ofemission-line fluxes of [Fe II] at 26 μm and [Ne II] at 12.8 μmand the 7.7 μm PAH strength, with the [Fe II]/[Ne II] flux ratiodecreasing with increasing PAH strength. We also find a strongcorrelation between the [Fe II]/[Ne II] flux ratio and the host galaxymetallicity, with the flux ratio decreasing with increasing metallicity.Since [Fe II] emission has been linked primarily to supernova shocks, weattribute the high [Fe II]/[Ne II] ratios in low-metallicity galaxies toenhanced supernova activity. We consider this to be a dominant mechanismfor PAH destruction, rather than grain destruction in photoionizedregions surrounding young massive stars. We also consider whether theextreme youth of the low-metallicity galaxies is responsible for thelack of PAH emission.
| 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.
| Environment and luminosity of supernova remnants
The explosion of supernovae and the evolution of their remnants (SNRs)accelerate cosmic rays over a vast range of timescales. Magnetic fieldscan be investigated indirectly through one of the observationalsignatures of this acceleration, namely radio synchrotron emission. Withthe aim of better understanding the role of the magnetic field insupernova evolution, we explore the variation of SNR radio luminositieswith physical conditions in the surrounding interstellar medium. With adata set that comprises more than 90 individual SNRs in 10 galaxies, anda range of 3000 in ISM density and 104 in radio synchrotronluminosity, we find a significant correlation between the twoquantities. The observed trends support the hypothesis that adiabaticcompression of magnetic fields by itself is insufficient to explain theradio emission of the brighter and more luminous in SNRs.
| Objective Classification of Spiral Galaxies Having Extended Rotation Curves Beyond the Optical Radius
We carry out an objective classification of four samples of spiralgalaxies having extended rotation curves beyond the optical radius. Amultivariate statistical analysis (viz., principal component analysis[PCA]) shows that about 96% of the total variation is due to twocomponents, one being the combination of absolute blue magnitude andmaximum rotational velocity beyond the optical region and the otherbeing the central density of the halo. On the basis of PCA a fundamentalplane has been constructed that reduces the scatter in the Tully-Fisherrelation up to a maximum of 16%. A multiple stepwise regression analysisof the variation of the overall shape of the rotation curves shows thatit is mainly determined by the central surface brightness, while theshape purely in the outer part of the galaxy (beyond the optical radius)is mainly determined by the size of the galactic disk.
| Advanced Camera for Surveys Imaging of 25 Galaxies in Nearby Groups and in the Field
We present Hubble Space Telescope Advanced Camera for Surveys images andcolor-magnitude diagrams for 25 nearby galaxies with radial velocitiesVLG<500 km s-1. Distances are determined basedon the luminosities of stars at the tip of the red giant branch thatrange from 2 to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, arefound to be the nearest known representatives of blue compact dwarfobjects. Using high-quality data on distances and radial velocities of110 nearby field galaxies, we derive their mean Hubble ratio to be 68 kms-1 Mpc-1 with a standard deviation of 15 kms-1 Mpc-1. Peculiar velocities of most of thegalaxies, Vpec=VLG-68D, follow a Gaussiandistribution with σv=63 km s-1 but with atail toward high negative values. Our data display the known correlationbetween peculiar velocity and galaxy elevation above the LocalSupercluster plane. The small observed fraction of galaxies with highpeculiar velocities, Vpec<-500 km s-1, may beunderstood as objects associated with nearby groups (Coma I, Eridanus)outside the local volume.
| Dynamical mass estimates for two luminous star clusters in galactic merger remnants
We present high-dispersion spectra of two extremely massive starclusters in galactic merger remnants, obtained using the UVESspectrograph mounted on the ESO Very Large Telescope. One cluster, W30,is located in the ~500 Myr old merger remnant NGC 7252 and has avelocity dispersion and effective radius of σ=27.5±2.5 kms-1 and Reff=9.3±1.7 pc, respectively. Theother cluster, G114, located in the ~3 Gyr old merger remnant NGC 1316,is much more compact, Reff=4.08±0.55 pc, and has avelocity dispersion of σ=42.1±2.8 km s-1. Thesemeasurements allow an estimate of the virial mass of the two clusters,yielding Mdyn(W30)=1.59(±0.26)× 10^7Mȯ and Mdyn(G114)=1.64(±0.13)×10^7 Mȯ. Both clusters are extremely massive, being morethan three times heavier than the most massive globular clusters in theGalaxy. For both clusters we measure light-to-mass ratios, which whencompared to simple stellar population (SSP) models of the appropriateage, are consistent with a Kroupa-type stellar mass function. Usingmeasurements from the literature we find a strong age dependence on howwell SSP models (with underlying Kroupa or Salpeter-type stellar massfunctions) fit the light-to-mass ratio of clusters. Based on this resultwe suggest that the large scatter in the light-to-mass ratio of theyoungest clusters is not due to variations in the underlying stellarmass function, but instead to the rapidly changing internal dynamics ofyoung clusters. Based on sampling statistics we argue that while W30 andG114 are extremely massive, they are consistent with being the mostmassive clusters formed in a continuous power-law cluster massdistribution. Finally, based on the positions of old globular clusters,young massive clusters (YMCs), ultra-compact dwarf galaxies (UCDs) anddwarf-globular transition objects (DGTOs) in κ-space we concludethat 1) UCDs and DGTOs are consistent with the high mass end of starclusters and 2) YMCs occupy a much larger parameter space than oldglobular clusters, consistent with the idea of preferential disruptionof star clusters.
| A Search for Candidate Radio Supernova Remnants in the Nearby Irregular Starburst Galaxies NGC 4214 and NGC 4395
We present the results of a search for new candidate radio supernovaremnants (SNRs) in the nearby starburst irregular galaxies NGC 4214 andNGC 4395 using archived radio observations made with the Very LargeArray (VLA) at the wavelengths of 3.5 cm, 6 cm and 20 cm for NGC 4214and 6 cm and 20 cm for NGC 4395. These observations were analyzed aspart of our ongoing search for candidate radio SNRs in nearby galaxies:the goal of this search is to prepare a large sample of candidate radioSNRs for the purpose of a robust statistical study of the properties ofthese sources. Based on our analysis, we have confirmed the non-thermalnature of the discrete radio sources alpha and beta in NGC 4214 andclassify these sources as candidate radio SNRs based on their positionalcoincidences with HII regions in that galaxy. We have measured the fluxdensities of the two candidate radio SNRs at each wavelength andcalculated corresponding spectral indices: we have also measured fluxdensities of two other discrete radio sources in these galaxies - rho inNGC 4214 and #3 in NGC 4395 - which we suspect to be additionalcandidate radio SNRs based on their positional coincidences with otherHII regions in these galaxies. However, the radio data presentlyavailable for these sources cannot confirm such a classification andadditional observations are needed. We have also calculated the radioluminosities Lradio at the wavelength of 20 cm for these twocandidate radio SNRs as well as the corresponding values for the minimumtotal energy Emin required to power these radio sources viasynchrotron emission and the corresponding magnetic field strengthBmin. We have compared our mean calculated values for theseproperties with the mean values for populations of candidate radio SNRsin other starburst galaxies: while the values for Lradio andBmin are roughly comparable to the values seen in otherstarburst galaxies, the mean value for Emin is higher thanthe mean value of any other starburst galaxy. Finally, we include thesetwo candidate radio SNRs in a discussion of the Sigma-D relation forextragalactic candidate radio SNRs and find that these sources arelocated on the shallower end of the master Sigma-D relation for allextragalactic SNRs as derived by Urosevic et al. (2005).
| Measuring improved distances to nearby galaxies: Thae Araucaria project.
Not Available
| A comparative analysis of empirical calibrators for nebular metallicity
We present a new analysis of the main empirical calibrators of oxygenabundance for ionized gas nebulae. With that aim we have compiled anextensive sample of objects with emission-line data including thenear-infrared [SIII] lines and the weak auroral lines which allow forthe determination of the gas electron temperature. For all the objectsthe oxygen abundances have been derived in a homogenous way, using themost recent sets of atomic coefficients and taking into the account theeffect of particle density on the temperature of O+. Theresiduals between directly and empirically derived abundances as afunction of abundance have been studied. A grid of photoionizationmodels, covering the range of physical properties of the gas, has beenused to explain the origin of the uncertainties affecting each abundancecalibrator. The range of validity for each abundance parameter has beenidentified and its average uncertainty has been quantified.
| First Results from THINGS: The HI Nearby Galaxy Survey
We describe The HI Nearby Galaxy Survey (THINGS), the largestprogramever undertaken at the Very Large Array to perform 21-cm HIobservations of thehighest quality ( 7'', ≤ 5 km s^{-1}resolution) ofnearby galaxies. The goal of THINGS is to investigatekeycharacteristics related to galaxy morphology, star formation andmassdistribution across the Hubble sequence. A sample of 34 objectswithdistances between 3 and 10 Mpc will be observed, covering a widerangeof evolutionary stages and properties. Data from THINGSwillcomplement SINGS, the Spitzer Infrared Nearby Galaxy Survey. Forthe THINGS sample, high-quality observations at comparable resolutionwillthus be available from the X-ray regime through to the radio partofthe spectrum. THINGS data can be used to investigate issues such asthesmall-scale structure of the ISM, its three-dimensional structure,the(dark) matter distribution and processes leading to starformation. Todemonstrate the quality of the THINGS data products, wepresent someprelimary HI maps here of four galaxies from the THINGSsample.
| 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."
| Infrared Spectral Energy Distributions of Nearby Galaxies
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities.
| The Baryonic Tully-Fisher Relation of Galaxies with Extended Rotation Curves and the Stellar Mass of Rotating Galaxies
I investigate the baryonic Tully-Fisher relation for a sample ofgalaxies with extended 21 cm rotation curves spanning the range 20 kms-1<~Vf<=300 km s-1. A variety ofscalings of the stellar mass-to-light ratio Υ* areconsidered. For each prescription for Υ*, I give fitsof the form Md=AVxf.Presumably, the prescription that comes closest to the correct valuewill minimize the scatter in the relation. The fit with minimum scatterhas A=50 Msolar km-4 s4 andx=4. This relation holds over five decades in mass. Galaxy color,stellar fraction, and Υ* are correlated with eachother and with Md, in the sense that more massivegalaxies tend to be more evolved. There is a systematic dependence ofthe degree of maximality of disks on surface brightness. High surfacebrightness galaxies typically have Υ*~3/4 of themaximum disk value, while low surface brightness galaxies typicallyattain ~1/4 of this amount.
| Light and Motion in the Local Volume
Using high-quality data on 149 galaxies within 10 Mpc, I find nocorrelation between luminosity and peculiar velocity at all. There is nounequivocal sign on scales of 1-2 Mpc of the expected gravitationaleffect of the brightest galaxies, in particular infall toward groups, orof infall toward the supergalactic plane on any scale. Either darkmatter is not distributed in the same way as luminous matter in thisregion, or peculiar velocities are not due to fluctuations in mass. Thesensitivity of peculiar velocity studies to the background model ishighlighted.
| Masses of Star Clusters in the Nuclei of Bulgeless Spiral Galaxies
In the last decade star clusters have been found in the centers ofspiral galaxies across all Hubble types. We here present a spectroscopicstudy of the exceptionally bright (106-108Lsolar) but compact (re~5 pc) nuclear starclusters in very late type spirals with the Ultraviolet and VisualEchelle Spectrograph at the VLT. We find that the velocity dispersionsof the nine clusters in our sample range from 13 to 34 kms-1. Using photometric data from the Hubble Space TelescopeWFPC2 and spherically symmetric dynamical models, we determine massesbetween 8×105 and 6×107Msolar. The mass-to-light ratios range from 0.2 to 1.5 in theI band. This indicates a young mean age for most clusters, in agreementwith previous studies. Given their high masses and small sizes, we findthat nuclear clusters are among the objects with the highest meansurface density known (up to 105 Msolarpc-2). From their dynamical properties we infer that, ratherthan small bulges, the closest structural kin of nuclear clusters appearto be massive compact star clusters. This includes such differentobjects as globular clusters, ``super star clusters,'' ultracompactdwarf galaxies (UCDs), and the nuclei of dwarf elliptical galaxies. Itis a challenge to explain why, despite the widely different currentenvironments, all different types of massive star clusters share verysimilar and structural properties. A possible explanation links UCDs andmassive globular clusters to nuclear star clusters through stripping ofnucleated dwarf galaxies in a merger event. The extreme properties ofthis type of cluster would then be a consequence of the clusters'location in the centers of their respective host galaxies.
| Revealing the Supernova Remnant Population of M33 with Chandra
We present results of a search for supernova remnants (SNRs) in archivalChandra images of M33. We have identified X-ray SNRs by comparing thelist of Chandra X-ray sources in M33 with tabulations of SNR candidatesidentified from (1) elevated [S II]/Hα ratios in the optical and(2) radio spectral indices. In addition, we have searched for opticalcounterparts to soft sources in the Chandra images and X-ray SNRcandidates identified in the XMM-Newton survey of M33. Of the 98optically known SNRs in M33, 22 have been detected at >3 σlevel in the soft band (0.35-1.1 keV). At least four of these SNRcandidates are spatially extended based on a comparison of the data tosimulated images of point sources. Aside from the optically matchingSNRs, we have found one soft X-ray source in M33 that exhibits nooptical emission and is coincident with a known radio source. The radiospectral index of this source is consistent with particle accelerationin shocks, leading us to suggest that it is a nonradiative SNR. We havealso found new optical counterparts to two soft X-ray SNRs in M33. Thesecounterparts exhibit enhanced [S II]/Hα ratios characteristic ofradiative shocks. Pending confirmation from optical spectroscopy, theidentification of these two optical counterparts increases the totalnumber of known optically emitting SNRs in M33 to 100. This brings thetotal number of identified SNRs with X-ray counterparts, including thoseexclusively detected by the XMM-Newton survey of M33, to 37 SNRs. Wefind that while there are a similar number of confirmed X-ray SNRs inM33 and the LMC with X-ray luminosities in excess of 1035ergs s-1, nearly 40% of the LMC SNRs are brighter than1036 ergs s-1, while only 13% of the M33 sampleexceed this luminosity. Including X-ray SNR candidates from theXMM-Newton survey (objects lacking optical counterparts) increases thefraction of M33 SNRs brighter than 1036 ergs s-1to 22%, still only half the LMC fraction. The differences in luminositydistributions cannot be fully explained by uncertainty in spectral modelparameters and are not fully accounted for by abundance differencesbetween the galaxies.
| The Local Group and Other Neighboring Galaxy Groups
Over the last few years, rapid progress has been made in distancemeasurements for nearby galaxies based on the magnitude of stars on thetip of the red giant branch. Current CCD surveys with the Hubble SpaceTelescope (HST) and large ground-based telescopes bring ~10% accuratedistances for roughly a hundred galaxies within 5 Mpc. The new data ondistances to galaxies situated in (and around) the nearest groups-theLocal Group, M81 Group, Cen A/M83 Group, IC 342/Maffei Group, Sculptorfilament, and Canes Venatici cloud-allowed us to determine their totalmass from the radius of the zero-velocity surface, R0, whichseparates a group as bound against the homogeneous cosmic expansion. Thevalues of R0 for the virialized groups turn out to be closeeach other, in the range of 0.9-1.3 Mpc. As a result, the total massesof the groups are close to each other, as well, yielding total mass toblue luminosity ratios of 10-40 MsolarL-1solar. The new total mass estimates are 3-5times lower than old virial mass estimates of these groups. Becauseabout half of galaxies in the Local volume belong to such loose groups,the revision of the amount of dark matter (DM) leads to a low localdensity of matter, Ωm~=0.04, which is comparable withthe global baryonic fraction Ωb but much lower than theglobal density of matter, Ωm=0.27. To remove thediscrepancy between the global and local quantities ofΩm, we assume the existence of two different DMcomponents: (1) compact dark halos around individual galaxies and (2) anonbaryonic dark matter ``ocean'' with ΩDM1~=0.07 andΩDM2~=0.20, respectively.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| Discovery of a Solitary Dwarf Galaxy in the APPLES Survey
During the APPLES parallel campaign, the Hubble Space Telescope (HST)Advanced Camera for Surveys has resolved a distant stellar system, whichappears to be an isolated dwarf galaxy. It is characterized by acircularly symmetric distribution of stars with an integrated magnitudemF775W=20.13+/-0.02, a central surface brightnessμF775W~=21.33+/-0.18 mag arcsec-2, and ahalf-light radius of ~=1.8". The ACS and VLT spectra show no evidence ofionized gas and appear to be dominated by a 3 Gyr old stellarpopulation. The OB spectral type derived for two resolved stars in thegrism data and the systemic radial velocity (Vhel~=670 kms-1) measured from the VLT data give a fiducial distance of~=9+/-2 Mpc. These findings, with the support of the spatial morphology,would classify the system among the dwarf spheroidal (dSph) galaxies.Following IAU rules, we have named this newly discovered galaxy APPLES1. An intriguing peculiarity of APPLES 1 is that the properties (age andmetallicity) of the stellar content so far detected are similar to thoseof dSph galaxies in the Local Group, where star formation is thought tobe driven by galaxy interactions and mergers. Yet, APPLES 1 seems not tobe associated with a major group or cluster of galaxies. Therefore,APPLES 1 could be the first example of a field dSph galaxy withself-sustained and regulated star formation, and therefore would make aninteresting test case for studies of the formation and evolution ofunperturbed dSph galaxies.
| Dust properties of UV bright galaxies at z ~ 2
We investigate the properties of the extinction curve in the rest-frameUV for a sample of 34 UV-luminous galaxies at 2 < z < 2.5,selected from the FORS Deep Field (FDF) spectroscopic survey. A newparametric description of the rest-frame UV spectral energy distributionis adopted; its sensitivity to properties of the stellar populations orof dust attenuation is established with the use of models. The latterare computed by combining composite stellar population models andcalculations of radiative transfer of the stellar and scatteredradiation through the dusty interstellar medium (ISM) for a dust/starsconfiguration describing dust attenuation in local starbursts. In thefavoured configuration the stars are enveloped by a shell with atwo-phase, clumpy, dusty ISM. The distribution of the z 2UV-luminous FDF galaxies in several diagnostic diagrams shows that theirextinction curves range between those typical of the Small and LargeMagellanic Clouds (SMC and LMC, respectively). For the majority ofstrongly reddened objects having a UV continuum slope β > -0.4 asignificant 2175 Å absorption feature (or "UV bump") is inferred,indicating an LMC-like extinction curve. On the other hand, the UVcontinua of the least reddened objects are mostly consistent withSMC-like extinction curves, lacking a significant UV bump, as for thesample of local starbursts investigated by Calzetti and collaborators.Furthermore, the most opaque (β 0) and, thus (for ourmodels), dustiest UV-luminous FDF galaxies tend to be among the mostmetal-rich, most massive, and largest systems at z 2, indicating< Z > 0.5 {-} 1 Zȯ, < Mstars> 6 × 1010 Mȯ, and 4 kpc, respectively. The presence of the UVbump does not seem to depend on the total metallicity, as given by theequivalent width (EW) of the C IV doublet. Conversely, it seems to beassociated with a large average EW of the six most prominentinterstellar low-ionisation absorption lines falling in the FORSspectra. The average EW of these saturated lines offers a proxy for theISM topology. We interpret these results as the evidence for adifference in the properties of the dusty ISM among the most evolvedUV-luminous, massive galaxies at z 2.
| A survey for OB associations in the Sculptor Group spiral galaxy NGC 7793
We report on the results from application of an objective algorithm(PLC) to find OB associations, to B and V images of the Sculptor spiralgalaxy NGC 7793, which were obtained with the ESO VLT and FORSinstrument and basically cover the entire spatial extent of the galaxy.We detected 148 associations. Statistical tests show that less than 6 ofthese detections are caused by randomly concentrated blue stars. In thesize distribution, a sharp peak is observed at a value of about 35microradians, which corresponds to a linear diameter of 135 pc, assuminga distance of 3.91 Mpc to the galaxy. We also find 25 much largerobjects. A second application of the PLC technique shows that 20 of themare stellar complexes consisting of multiple sub-associations withtypical sizes on the order of 130 pc. A comparison of the sizedistribution of the detected OB associations in NGC 7793 with observeddistributions in other galaxies suggests that the conditions in twoSculptor Group galaxies (NGC 300 and NGC 7793) favour the formation oflarge associations. We provide a catalog giving coordinates and physicalparameters for all the associations and stellar complexes we have foundin our survey.
| The Σ - D relation for supernova remnants in nearby galaxies
This paper examines relations between the radio surface brightnessΣ and the diameter D (also known as Σ-D relations) for asample of extragalactic supernova remnants (SNRs) as constructed from acombination of published data and data from our own surveys. Our sampleof extragalactic SNRs is the largest ever devised for the purpose ofanalyzing Σ-D relations. The main results of this paper may besummarized as follows: (i) the empirical relations for SNRs in 10 of the11 nearby galaxies studied have the approximately trivial Σ∝D-2 form, therefore limiting their interpretation asphysically meaningful relations. In addition, these relations aresubject to selection effects rendering them even less useful. FurtherMonte Carlo simulations suggest that the effect of survey sensitivityhas the opposite effect of volume selection (e.g. Malmquist bias, avolume selection effect that shapes the Galactic sample) by tending toflatten the slopes toward a trivial relation. In this case, the trueslopes may be steeper than the observed slopes; (ii) compact M 82 SNRsappear to follow a uniquely different Σ-D relation in comparisonto the larger, older SNRs in the other 10 galaxies. Monte Carlosimulations suggest that the probability of this difference arising bychance is ≈1% to 10%, depending on what is assumed regarding theunderlying SNR population; (iii) three candidate hypernova remnants wereidentified in our sample of 11 nearby galaxies.
| Imaging and photometry of nearby dwarf galaxies. II. Southern dwarfs
We carried out CCD photometry in the Johnson-Cousins B and R bands of 23dwarf galaxies: SDIG, ESO 410-17, KK11, ESO 245-05, KKs3, KK27, KK38,KK40, IC 4662, KK244, KK246, KK247, KK248, KK249, KK253, KK255, KK256,KK257, KK258, KK259, UGCA 438, ESO 347-17, and UGCA 442. Both isolatedgalaxies and members of the Sculptor group and the NGC 1313 group wereobserved. The galaxy sample is characterized by a median distance of 9.3Mpc, and median absolute magnitude of -14.8 mag. The central surfacebrightnesses are in the range from 22.2 to 24.4 mag arcsec-2in B.Based on observations obtained with CTIO 1.5-m telescope, which isoperated by the Association of Universities for Research in AstronomyInc. (AURA), under a cooperative agreement with the National ScienceFoundation as part of the National Optical Astronomy Observatories.Tables 1 and 2, complete Figs. 1 and 2 are only available in electronicform at http://www.edpsciences.org
| The Parkes H I Survey of the Magellanic System
We present the first fully and uniformly sampled, spatially complete HIsurvey of the entire Magellanic System with high velocity resolution(Δ v = 1.0 km s-1), performed with the ParkesTelescope. Approximately 24 percent of the southern sky was covered bythis survey on a ≈5´ grid with an angular resolution of HPBW =14.1 arcmin. A fully automated data-reduction scheme was developed forthis survey to handle the large number of HI spectra(1.5×106). The individual Hanning smoothed andpolarization averaged spectra have an rms brightness temperature noiseof σ = 0.12 K. The final data-cubes have an rms noise ofσrms ≈ 0.05 K and an effective angular resolution of≈16´. In this paper we describe the survey parameters, thedata-reduction and the general distribution of the HI gas. The LargeMagellanic Cloud (LMC) and the Small Magellanic Cloud(SMC) are associated with huge gaseous features - theMagellanic Bridge, the InterfaceRegion, the Magellanic Stream, and theLeading Arm - with a total HI mass of M(HI) =4.87×108 Mȯ [d/55 kpc]2, ifall HI gas is at the same distance of 55 kpc. Approximately two thirdsof this HI gas is located close to the Magellanic Clouds(Magellanic Bridge and InterfaceRegion), and 25% of the HI gas is associated with theMagellanic Stream. The Leading Armhas a four times lower HI mass than the MagellanicStream, corresponding to 6% of the total HI mass of thegaseous features. We have analyzed the velocity field of the MagellanicClouds and their neighborhood introducing a LMC-standard-of-rest frame.The HI in the Magellanic Bridge shows low velocitiesrelative to the Magellanic Clouds suggesting an almost parallel motion,while the gas in the Interface Region hassignificantly higher relative velocities indicating that this gas isleaving the Magellanic Bridge building up a newsection of the Magellanic Stream. TheLeading Arm is connected to the MagellanicBridge close to an extended arm of the LMC.The clouds in the Magellanic Stream and theLeading Arm show significant differences, both in thecolumn density distribution and in the shapes of the line profiles. TheHI gas in the Magellanic Stream is more smoothlydistributed than the gas in the Leading Arm. Thesemorphological differences can be explained if the LeadingArm is at considerably lower z-heights and embedded in ahigher pressure ambient medium.The Parkes Telescope is part of the Australia Telescope which is fundedby the Commonwealth of Australia for operation as a National Facilitymanaged by CSIRO.
| A test for the origin of quasar redshifts
It is commonly accepted that quasar redshifts have a cosmologicalcharacter and that most of the quasars are at Gpc distances. However,there are some cases where several quasars with completely differentredshifts and a nearby active galaxy are aligned in a certain way oroccupy a very small patch on the sky, which is claimed by some authorsto be unlikely to happen by chance. Is there a small subset of quasarswith non-cosmological redshifts? For quasars apparently associated withgalaxies, we consider two scenarios for the origin of their redshift:(i) a change in the scale factor of the whole Universe (standard,cosmological scenario), and (ii) a velocity-induced Doppler shift of thespectrum of a nearby object (local, ejection scenario). We argue for asimple astrometric test which can distinguish between these two sourcesof quasar redshifts by constraining their proper motions.We give the predictions for the maximum possible proper motions of aquasar for the cosmological and local scenarios of the origin of theirredshifts. We apply these theoretical results to the Bukhmastovacatalogue, which contains more than 8000 close quasi-stellarobject-galaxy associations. In the standard interpretation of quasarredshifts, their typical proper motions are a fraction of μas, andbeyond the reach of planned astrometric missions such as GAIA and SIM.On the other hand, the quasars ejected from local active galactic nucleiat velocities close to the speed of light would have proper motions 5-6orders of magnitude larger, which would easily be measurable with futureastrometric missions, or even in some cases with HST, VLT and Kecktelescope. The distributions of proper motions for the cosmological andlocal scenarios are very well separated. Moreover, the divisioncorresponds nicely to the expected accuracy from GAIA and SIM.
| Observational Constraints on the Physical Parameters of Dark Matter Halos
After looking at the difference in the mass distribution between massivespiral and dwarf irregular (dIrr) and low surface brightness (LSB)galaxies, the central Dark Matter (DM) concentration (flat vs cuspy) indwarf and LSB galaxies, derived from observations, will be examined. Wewill then present what kind of observational constraints can be put onthe total mass and total extent of DM halos from the studies ofindividual galaxies, small groups, satellites' dynamics and tidal tailsof interacting systems. Finally, we will discuss how limits on thephysical parameters of DM halos could be set by deriving extendedrotation curves beyond the HI radius (r > rHI), usingeither Lyα absorption or Hα emission observations.
| UBVI Surface Photometry of the Spiral Galaxy NGC 300 in the Sculptor Group
We present UBVI surface photometry over a 20.5' × 20.5' area ofthe late-type spiral galaxy NGC 300. We have derived isophotal maps,surface brightness profiles, ellipticity profiles, position angleprofiles, and color profiles. By merging our I-band measurements withthose of Böker et al. based on Hubble Space Telescope observations,we have obtained combined I-band surface brightness profiles for theregion 0.02'' < r < 500'' and have decomposed the profiles intothree components: a nucleus, a bulge, and an exponential disk.
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