Contents
Images
Upload your image
DSS Images Other Images
Related articles
The multi-phase gaseous halos of star forming late-type galaxies. I. XMM-Newton observations of the hot ionized medium
This study presents first results from an X-ray mini-survey carried outwith XMM-Newton to investigate the diffuse Hot Ionized Medium in thehalos of nine nearby star-forming edge-on spiral galaxies. Diffusegaseous X-ray halos are detected in eight of our targets, covering awide range of star formation rates from quiescent to starburst cases.For four edge-on spiral galaxies, namely NGC 3044, NGC 3221, NGC 4634,and NGC 5775, we present the first published high resolution/sensitivitydetections of extended soft X-ray halos. EPIC X-ray contour mapsoverlaid onto Hα imaging data reveals that in all cases thepresence of X-ray halos is correlated with extraplanar Diffuse IonizedGas. Moreover, these halos are also associated with non-thermal cosmicray halos, as evidenced by radio continuum observations. SupplementalUV-data obtained with the OM-telescope at 210 nm show Diffuse IonizedGas to be well associated with UV emission originating in the underlyingdisk. Beside NGC 891, NGC 4634 is the second non-starburst galaxy with adiffuse soft X-ray halo (|z|≤ 4 kpc). In case of NGC 3877, for whichwe also present the first high resolution X-ray imaging data, no haloemission is detectable. EPIC pn spectra (0.3-12 keV) of the diffuseX-ray emission are extracted at different offset positions from thedisk, giving evidence to a significant decrease of gas temperatures,electron densities, and gas masses with increasing distance to theplane. A comparison between dynamical and radiative cooling time scalesimplies that the outflow in all targets is likely to be sustained. Wefind very strong indications that spatially correlated multi-phasegaseous halos are created by star forming activity in the disk plane. Ina forthcoming paper, we will present multi-frequency luminosityrelations and evaluate key parameters which might trigger the formationof multi-phase galaxy halos.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| A Study of Edge-On Galaxies with the Hubble Space Telescope Advanced Camera for Surveys. II. Vertical Distribution of the Resolved Stellar Population
We analyze the vertical distribution of the resolved stellar populationsin six low-mass (Vmax=67-131 km s-1), edge-on,spiral galaxies observed with the Hubble Space Telescope Advanced Camerafor Surveys. In each galaxy we find evidence for an extraplanar stellarcomponent extending up to 15 scale heights (3.5 kpc) above the plane,with a scale height typically twice that of two-dimensional fits toKs-band Two Micron All Sky Survey images. We analyze thevertical distribution as a function of stellar age by tracking changesin the color-magnitude diagram. The young stellar component(<~108 yr) is found to have a scale height larger than theyoung component in the Milky Way, suggesting that stars in theselow-mass galaxies form in a thicker disk. We also find that the scaleheight of a stellar population increases with age, with youngmain-sequence stars, intermediate-age asymptotic giant branch stars, andold red giant branch (RGB) stars having successively larger scaleheights in each galaxy. This systematic trend indicates that diskheating must play some role in producing the extraplanar stars. Weconstrain the rate of disk heating using the observed trend betweenscale height and stellar age and find that the observed heating ratesare dramatically smaller than in the Milky Way. The color distributionsof the RGB stars well above the midplane indicate that the extendedstellar components we see are moderately metal-poor, with peakmetallicities around [Fe/H]=-1 and with little or no metallicitygradient with height. The lack of metallicity gradient can be explainedif a majority of extraplanar RGB stars were formed at early times andare not dominated by a younger heated population. Our observationssuggest that, like the Milky Way, low-mass disk galaxies also havemultiple stellar components. In its structure, mean metallicity, and oldage, the RGB component in these galaxies seems analogous to the MilkyWay thick disk. However, without additional kinematic and abundancemeasurements, this association is only circumstantial, particularly inlight of the clear existence of some disk heating at intermediate ages.Finally, we find that the vertical dust distribution has a scale heightsomewhat larger than that of the main-sequence stars.
| A Study of Edge-On Galaxies with the Hubble Space Telescope Advanced Camera for Surveys. I. Initial Results
We present the initial results of a Hubble Space Telescope/AdvancedCamera for Surveys snapshot survey of 16 nearby, edge-on, late-typegalaxies covering a range in distance from 2 to 19 Mpc. The images ofthese galaxies show significant resolved stellar populations. We deriveF606W and F814W photometry for more than 1.2 million stars and presentcolor-magnitude diagrams that show a mixture of young, intermediate, andold stars in each galaxy. In one of the fields we serendipitously detectstars from the Large Magellanic Cloud. We also identify a candidateyoung dwarf galaxy lying ~2 kpc above the plane of NGC 4631. For thenearest six galaxies, we derive tip of the red giant branch distancesand demonstrate that these galaxies fall on the K-band Tully-Fisherrelation established in clusters. From the color of the red giantbranch, we also find evidence that these galaxies possess a metal-poorthick-disk or halo population.
| An IRAS High Resolution Image Restoration (HIRES) Atlas of All Interacting Galaxies in the IRAS Revised Bright Galaxy Sample
The importance of far-infrared observations for our understanding ofextreme activity in interacting and merging galaxies has beenillustrated by many studies. Even though two decades have passed sinceits launch, the most complete all-sky survey to date from which far-IRselected galaxy samples can be chosen is still that of the InfraredAstronomical Satellite (IRAS). However, the spatial resolution of theIRAS all-sky survey is insufficient to resolve the emission fromindividual galaxies in most interacting galaxy pairs, and hence previousstudies of their far-IR properties have had to concentrate either onglobal system properties or on the properties of very widely separatedand weakly interacting pairs. Using the HIRES image reconstructiontechnique, it is possible to achieve a spatial resolution ranging from30" to 1.5m (depending on wavelength and detector coverage), whichis a fourfold improvement over the normal resolution of IRAS. This issufficient to resolve the far-IR emission from the individual galaxiesin many interacting systems detected by IRAS, which is very importantfor meaningful comparisons with single, isolated galaxies. We presenthigh-resolution 12, 25, 60, and 100 μm images of 106 interactinggalaxy systems contained in the IRAS Revised Bright Galaxy Sample (RBGS,Sanders et al.), a complete sample of all galaxies having a 60 μmflux density greater than 5.24 Jy. These systems were selected to haveat least two distinguishable galaxies separated by less than threeaverage galactic diameters, and thus we have excluded very widelyseparated systems and very advanced mergers. Additionally, some systemshave been included that are more than three galactic diameters apart,yet have separations less than 4' and are thus likely to suffer fromconfusion in the RBGS. The new complete survey has the same propertiesas the prototype survey of Surace et al. We find no increased tendencyfor infrared-bright galaxies to be associated with other infrared-brightgalaxies among the widely separated pairs studied here. We find smallenhancements in far-IR activity in multiple galaxy systems relative toRBGS noninteracting galaxies with the same blue luminosity distribution.We also find no differences in infrared activity (as measured byinfrared color and luminosity) between late- and early-type spiralgalaxies.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| Properties of isolated disk galaxies
We present a new sample of northern isolated galaxies, which are definedby the physical criterion that they were not affected by other galaxiesin their evolution during the last few Gyr. To find them we used thelogarithmic ratio, f, between inner and tidal forces acting upon thecandidate galaxy by a possible perturber. The analysis of thedistribution of the f-values for the galaxies in the Coma cluster leadus to adopt the criterion f ≤ -4.5 for isolated galaxies. Thecandidates were chosen from the CfA catalog of galaxies within thevolume defined by cz ≤5000 km s-1, galactic latitudehigher than 40o and declination ≥-2.5o. Theselection of the sample, based on redshift values (when available),magnitudes and sizes of the candidate galaxies and possible perturberspresent in the same field is discussed. The final list of selectedisolated galaxies includes 203 objects from the initial 1706. The listcontains only truly isolated galaxies in the sense defined, but it is byno means complete, since all the galaxies with possible companions underthe f-criterion but with unknown redshift were discarded. We alsoselected a sample of perturbed galaxies comprised of all the diskgalaxies from the initial list with companions (with known redshift)satisfying f ≥ -2 and \Delta(cz) ≤500 km s-1; a totalof 130 objects. The statistical comparison of both samples showssignificant differences in morphology, sizes, masses, luminosities andcolor indices. Confirming previous results, we found that late spiral,Sc-type galaxies are, in particular, more frequent among isolatedgalaxies, whereas Lenticular galaxies are more abundant among perturbedgalaxies. Isolated systems appear to be smaller, less luminous and bluerthan interacting objects. We also found that bars are twice as frequentamong perturbed galaxies compared to isolated galaxies, in particularfor early Spirals and Lenticulars. The perturbed galaxies have higherLFIR/LB and Mmol/LB ratios,but the atomic gas content is similar for the two samples. The analysisof the luminosity-size and mass-luminosity relations shows similartrends for both families, the main difference being the almost totalabsence of big, bright and massive galaxies among the family of isolatedsystems, together with the almost total absence of small, faint and lowmass galaxies among the perturbed systems. All these aspects indicatethat the evolution induced by interactions with neighbors would proceedfrom late, small, faint and low mass Spirals to earlier, bigger, moreluminous and more massive spiral and lenticular galaxies, producing atthe same time a larger fraction of barred galaxies but preserving thesame relations between global parameters. The properties we found forour sample of isolated galaxies appear similar to those of high redshiftgalaxies, suggesting that the present-day isolated galaxies could bequietly evolved, unused building blocks surviving in low densityenvironments.Tables \ref{t1} and \ref{t2} are only available in electronic form athttp://www.edpsciences.org
| Cold dust in a selected sample of nearby galaxies. I. The interacting galaxy NGC 4631
We have observed the continuum emission of the interacting galaxy NGC4631 at λλ 870 μm and 1.23 mm using theHeinrich-Hertz-Telescope on Mt. Graham and the IRAM 30-m telescope onPico Veleta. We have obtained fully sampled maps which cover the opticalemission out to a radius of about 7' at both wavelengths. For a detailedanalysis, we carefully subtracted the line contributions and synchrotronand free-free emission from the data, which added up to 6% at 1.23 mmand 10% at 0.87 mm. We combined the flux densities with FIR data toobtain dust spectra and calculate dust temperatures, absorption crosssections, and masses. Assuming a ``standard'' dust model, which consistsof two populations of big grains at moderate and warm temperatures, weobtained temperatures of 18 K and 50 K for the both components. However,such a model suffers from an excess of the radiation at λ 1.23mm, and the dust absorption cross section seems to be enhanced by afactor 3 compared to previous results and theoretiexpectations. At largegalactocentric radii, where the galaxy shows disturbances as a result ofgravitational interaction, this effect seems to be even stronger. Somepossibilities to resolve these problems are discussed. The data could beexplained by a very cold dust component at a temperature of 4-6 K, anincreased abundance of very small grains, or a component of grains withunusual optical properties. We favour the latter possibility, since thefirst two lead to inconsistencies
| Probing O VI Emission in the Halos of Edge-on Spiral Galaxies
We have used the Far Ultraviolet Spectroscopic Explorer to search for OVI λλ1031.926, 1037.617 emission in the halos of theedge-on spiral galaxies NGC 4631 and NGC 891. In NGC 4631, we detected OVI in emission toward a soft X-ray bubble above a region containingnumerous Hα arcs and filaments. The line-of-sight component of themotion of the O VI gas appears to match the underlying disk rotation.The observed O VI luminosities can account for 0.2%-2% of the totalenergy input from supernovae (assuming a full O VI- emitting halo) andyield mass flux cooling rates between 0.48 and 2.8 Msolaryr-1 depending on the model used in the derivations. On thebasis of these findings, we believe it is likely that we are seeingcooling, galactic fountain gas. No emission was detected from the haloof NGC 891, a galaxy in a direction with considerably high foregroundGalactic extinction.
| The Origin of the Dust Arch in the Halo of NGC 4631: An Expanding Superbubble?
We study the nature and the origin of the dust arch in the halo of theedge-on galaxy NGC 4631 detected by Neininger & Dumke. We present COobservations made using the new on-the-fly mapping mode with the FiveCollege Radio Astronomy Observatory 14 m telescope and find no evidencefor CO emission associated with the dust arch. Our examination ofpreviously published H I data shows that, if previous assumptions aboutthe dust temperature and gas/dust ratio are correct, then there must bemolecular gas associated with the arch, below our detection threshold.If this is true, then the molecular mass associated with the dust archis between 1.5×108 Msolar and9.7×108 Msolar, and likely toward the lowend of the range. A consequence of this is that the maximum allowedvalue for the CO-to-H2 conversion factor is 6.5 times theGalactic value, but most likely closer to the Galactic value. Thekinematics of the H I apparently associated with the dust arch revealsthat the gas here is not part of an expanding shell or outflow but isinstead two separate features (a tidal arm and a plume of H I stickingout into the halo) that are seen projected together and appear as ashell. Thus there is no connection between the dust ``arch'' and the hotX-ray-emitting gas that appears to surround the galaxy.
| The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
| Kinematical data on early-type galaxies. VI.
We present the result of spectroscopic observations of a sample of 73galaxies, completing the database published in this series of articles.The sample contains mostly low-luminosity early-type objects, includingfour dwarfs of the Local Group (in particular, deep spectra of NGC 205),15 dEs or dS0s in the Virgo cluster, and UGC 05442, a spheroidal dwarfof the M 81 group. We have measured the central velocity dispersion forall but one object, and determined the major-axis rotation andvelocity-dispersion profiles for 59 objects. For the current sample ofdiffuse (or dwarf) elliptical galaxies, we have compared stellarrotation to velocity dispersion; the analysis suggests that theseobjects may be nearly rotationally flattened, and therefore thatanisotropy may be less important than previously thought. Based onobservations collected at the Observatoire de Haute-Provence. Table 1 isalso, and Tables 2 and 4 only, available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/384/371
| A catalogue and analysis of X-ray luminosities of early-type galaxies
We present a catalogue of X-ray luminosities for 401 early-typegalaxies, of which 136 are based on newly analysed ROSAT PSPC pointedobservations. The remaining luminosities are taken from the literatureand converted to a common energy band, spectral model and distancescale. Using this sample we fit the LX:LB relationfor early-type galaxies and find a best-fit slope for the catalogue of~2.2. We demonstrate the influence of group-dominant galaxies on the fitand present evidence that the relation is not well modelled by a singlepower-law fit. We also derive estimates of the contribution to galaxyX-ray luminosities from discrete-sources and conclude that they provideLdscr/LB~=29.5ergs-1LBsolar-1. Wecompare this result with luminosities from our catalogue. Lastly, weexamine the influence of environment on galaxy X-ray luminosity and onthe form of the LX:LB relation. We conclude thatalthough environment undoubtedly affects the X-ray properties ofindividual galaxies, particularly those in the centres of groups andclusters, it does not change the nature of whole populations.
| The SBF Survey of Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances
We report data for I-band surface brightness fluctuation (SBF)magnitudes, (V-I) colors, and distance moduli for 300 galaxies. Thesurvey contains E, S0, and early-type spiral galaxies in the proportionsof 49:42:9 and is essentially complete for E galaxies to Hubblevelocities of 2000 km s-1, with a substantial sampling of Egalaxies out to 4000 km s-1. The median error in distancemodulus is 0.22 mag. We also present two new results from the survey.(1) We compare the mean peculiar flow velocity (bulk flow) implied byour distances with predictions of typical cold dark matter transferfunctions as a function of scale, and we find very good agreement withcold, dark matter cosmologies if the transfer function scale parameterΓ and the power spectrum normalization σ8 arerelated by σ8Γ-0.5~2+/-0.5. Deriveddirectly from velocities, this result is independent of the distributionof galaxies or models for biasing. This modest bulk flow contradictsreports of large-scale, large-amplitude flows in the ~200 Mpc diametervolume surrounding our survey volume. (2) We present adistance-independent measure of absolute galaxy luminosity, N and showhow it correlates with galaxy properties such as color and velocitydispersion, demonstrating its utility for measuring galaxy distancesthrough large and unknown extinction. Observations in part from theMichigan-Dartmouth-MIT (MDM) Observatory.
| A Database of Cepheid Distance Moduli and Tip of the Red Giant Branch, Globular Cluster Luminosity Function, Planetary Nebula Luminosity Function, and Surface Brightness Fluctuation Data Useful for Distance Determinations
We present a compilation of Cepheid distance moduli and data for foursecondary distance indicators that employ stars in the old stellarpopulations: the planetary nebula luminosity function (PNLF), theglobular cluster luminosity function (GCLF), the tip of the red giantbranch (TRGB), and the surface brightness fluctuation (SBF) method. Thedatabase includes all data published as of 1999 July 15. The mainstrength of this compilation resides in the fact that all data are on aconsistent and homogeneous system: all Cepheid distances are derivedusing the same calibration of the period-luminosity relation, thetreatment of errors is consistent for all indicators, and measurementsthat are not considered reliable are excluded. As such, the database isideal for comparing any of the distance indicators considered, or forderiving a Cepheid calibration to any secondary distance indicator, suchas the Tully-Fisher relation, the Type Ia supernovae, or the fundamentalplane for elliptical galaxies. This task has already been undertaken byFerrarese et al., Sakai et al., Kelson et al., and Gibson et al.Specifically, the database includes (1) Cepheid distances, extinctions,and metallicities; (2) reddened apparent λ5007 Å magnitudesof the PNLF cutoff; (3) reddened apparent magnitudes and colors of theturnover of the GCLF (in both the V and B bands); (4) reddened apparentmagnitudes of the TRGB (in the I band) and V-I colors at 0.5 mag fainterthan the TRGB; and (5) reddened apparent surface brightness fluctuationmagnitudes measured in Kron-Cousin I, K', andKshort, and using the F814W filter with the Hubble SpaceTelescope (HST) WFPC2. In addition, for every galaxy in the database wegive reddening estimates from IRAS/DIRBE as well as H I maps, J2000coordinates, Hubble and T-type morphological classification, apparenttotal magnitude in B, and systemic velocity.
| Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.
| Interferometric 12CO Observations of the Central Disk of NGC 4631: An Energetic Molecular Outflow
We present interferometric observations of CO J=1-0 emission in thecentral regions of the edge-on galaxy NGC 4631, known for its extendedgaseous halo and its tidal interactions. Previous single-dishobservations revealed that almost all of the CO emission arises from acentral ring or barlike structure of length ~4 kpc. We confirm thisstructure at higher resolution and find that it is bent at the center,reflecting the overall bend in this galaxy apparent from optical images.The kinematic evidence favors a rigidly rotating ring over a bar. Thegaseous halo emission in several tracers is concentrated above and belowthis molecular structure. To the north of an emission peak at theeastern end of the structure is an extraplanar feature showingfilamentary and shell-like properties we interpret as an energeticmolecular outflow. The energies involved are difficult to estimate butare probably of order 1054 ergs or more. The CO concentrationin the disk below this structure coincides with a bright H II regioncomplex, a peak of radio emission, and the brightest X-ray feature inthe inner disk of the galaxy seen in a ROSAT HRI map, all suggestingintense star formation. A filament of radio continuum emission may alsohave a footprint in this region of the disk. The origin of the outflowis unclear.
| X-ray luminosities for a magnitude-limited sample of early-type galaxies from the ROSAT All-Sky Survey
For a magnitude-limited optical sample (B_T <= 13.5 mag) ofearly-type galaxies, we have derived X-ray luminosities from the ROSATAll-Sky Survey. The results are 101 detections and 192 useful upperlimits in the range from 10^36 to 10^44 erg s^-1. For most of thegalaxies no X-ray data have been available until now. On the basis ofthis sample with its full sky coverage, we find no galaxy with anunusually low flux from discrete emitters. Below log (L_B) ~ 9.2L_⊗ the X-ray emission is compatible with being entirely due todiscrete sources. Above log (L_B) ~ 11.2 L_osolar no galaxy with onlydiscrete emission is found. We further confirm earlier findings that L_xis strongly correlated with L_B. Over the entire data range the slope isfound to be 2.23 (+/- 0.12). We also find a luminosity dependence ofthis correlation. Below log L_x = 40.5 erg s^-1 it is consistent with aslope of 1, as expected from discrete emission. Above this value theslope is close to 2, as expected from gaseous emission. Comparing thedistribution of X-ray luminosities with the models of Ciotti et al.leads to the conclusion that the vast majority of early-type galaxiesare in the wind or outflow phase. Some of the galaxies may have alreadyexperienced the transition to the inflow phase. They show X-rayluminosities in excess of the value predicted by cooling flow modelswith the largest plausible standard supernova rates. A possibleexplanation for these super X-ray-luminous galaxies is suggested by thesmooth transition in the L_x--L_B plane from galaxies to clusters ofgalaxies. Gas connected to the group environment might cause the X-rayoverluminosity.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| Diffuse Ionized Gas in Edge-on Spiral Galaxies: Extraplanar and Outer Disk Hα Emission
We present Hα images of five edge-on galaxies: NGC 891, NGC 4631,NGC 4244, NGC 3003, and UGC 9242. We also analyze [S II]lambdalambda6717, 6731 and [O III] lambda5007 images of NGC 4631. Forseveral of these galaxies, these images are the most sensitive to date.We analyze the ionized gas content, with particular attention to thediffuse ionized gas (DIG). The DIG layer in NGC 891 is traced out to atleast 5 kpc from the midplane, confirming an earlier spectroscopicdetection. The DIG in four of these galaxies contributes 40%-50% of thetotal Hα luminosity, similar to the contribution in face-ongalaxies, but in NGC 891 the DIG contributes 80%-90%. This is likely dueto the higher dust content in the disk of NGC 891, which obscures the HII regions but may also reflect the extraordinary prominence of the DIGlayer in that galaxy. Our very deep image of UGC 9242 shows very lowsurface brightness emission, as low as 0.3 pc cm^-6, reaching 4-5 kpcabove the midplane. This galaxy also exhibits filaments near the brightHα nucleus, an indication of a starburst superwind. In NGC 4631 wesee a very large shell of emission extending 3.5 kpc into the halo. The[S II]/Hα and [O III]/Hα ratios in NGC 4631 are consistentwith the ratios seen in other galaxies, and with photoionization models.There is a region on the southeast side of disk where the [OIII]/(Hα+[N II]) ratio reaches over 1.0 in the DIG, whichcoincides with an H I supershell. We use our very deep images of NGC3003 and UGC 9242 to search for ionized gas in the outer disks as a testof the strength of the metagalactic ionizing radiation field. We find noouter disk emission down to our 1 sigma limit of 0.13 pc cm^-6 on scalesof 1.5 kpc in NGC 3003. Based on this limit we rule out a metagalacticionizing radiation field stronger than 11x10^-23 ergs cm^-2 s^-1 Hz^-1sr^-1. There is an indication of extended disk emission in UGC 9242,which would imply a stronger radiation field, but various concerns, mostimportantly flat-fielding uncertainties due to foreground stars in theimage, lead us to question whether this feature is real.
| The Nature of Accreting Black Holes in Nearby Galaxy Nuclei
We have found compact X-ray sources in the center of 21 (54%) of 39nearby face-on spiral and elliptical galaxies with available ROSAT HRIdata. ROSAT X-ray luminosities (0.2-2.4 keV) of these compact X-raysources are ~10^37-10^40 ergs s^-1 (with a mean of 3x10^39 ergs s^-1).The mean displacement between the location of the compact X-ray sourceand the optical photometric center of the galaxy is ~390 pc. The factthat compact nuclear sources were found in nearly all (five of six)galaxies with previous evidence for a black hole or an active galacticnucleus (AGN) indicates that at least some of the X-ray sources areaccreting supermassive black holes. ASCA spectra of six of the 21galaxies show the presence of a hard component with relatively steep(Gamma~2.5) spectral slope. A multicolor disk blackbody model fits thedata from the spiral galaxies well, suggesting that the X-ray object inthese galaxies may be similar to a black hole candidate in its soft(high) state. ASCA data from the elliptical galaxies indicate that hot(kT~0.7 keV) gas dominates the emission. The fact that (for both spiraland elliptical galaxies) the spectral slope is steeper than in normaltype 1 AGNs and that relatively low absorbing columns (N_H~10^21 cm^-2)were found to the power-law component indicates that these objects aresomehow geometrically and/or physically different from AGNs in normalactive galaxies. The X-ray sources in the spiral and elliptical galaxiesmay be black hole X-ray binaries, low-luminosity AGNs, or possibly youngX-ray luminous supernovae. Assuming the sources in the spiral galaxiesare accreting black holes in their soft state, we estimate black holemasses ~10^2-10^4 M_solar.
| Neutral Hydrogen and Dark Matter in Spiral Galaxies
The first part presents a brief review of the main HI properties ofisolated, normal spiral galaxies and of the phenomena which seem tocharacterize and dominate their internal metabolism. In the second partattention is drawn to all those processes, such as tidal interactions,accretion and mergers, that depend on the galaxy environment and mayplay a significant role in galaxy formation and evolution. In the thirdpart the observational evidence for the dark matter component of spiralgalaxies is discussed.
| A Search for Extraplanar Dust in Nearby Edge-on Spirals
We present high-resolution (0.6" to ~1.0") BV images of 12 edge-onspiral galaxies observed with the WIYN 3.5 m telescope. These imageswere obtained to search for extraplanar (|z|>0.4 kpc) absorbing duststructures similar to those previously found in NGC 891 (Howk &Savage). Many of these galaxies have been previously searched fordiffuse ionized gas at high z. Our imaged galaxies include a sample ofseven massive L_*-like spiral galaxies within D<~25 Mpc that haveinclinations i>~87^deg from the plane of the sky. We find that fiveof these seven systems show extraplanar dust, visible as highlystructured absorbing clouds against the background stellar light of thegalaxies. These dust structures lie at heights |z|>~0.4 kpc, whichshould be above most of the thin disk molecular material in thesegalaxies. The more prominent structures are estimated to have associatedgas masses >~10^5 M_solar the implied potential energies are>~10^52 ergs. All of the galaxies in our sample that show detectableHα emission at large z also show extraplanar dust structures. Noneof those galaxies for which extraplanar Hα searches were negativeshow evidence for extensive high-z dust. The existence of extraplanardust is a common property of massive spiral galaxies. We discuss severalmechanisms for shaping the observed dust features. We emphasize in thisdiscussion the possibility that these dusty clouds represent the densephase of a multiphase medium at high z in spiral galaxies. In a fewcases interactions with close galaxy companions could be responsible forthe high-z dust, either through dynamical stripping or triggered starformation. We can rule out warps as the source of the observed high-zdust. Flaring gas layers seem an unlikely source of the observedmaterial but cannot be ruled out at this time, except for those featuresthat clearly connect to energetic processes in the disk. The correlationbetween high-z dust and extraplanar Hα emission may simply suggestthat both trace the high-z interstellar medium in its various forms (orphases), the existence of which may ultimately be driven by vigorousstar formation in the underlying disk. The absorption produced by high-zdust and associated gas in spiral galaxies must be accounted for whenstudying extraplanar emission from spiral galaxies over much of theelectromagnetic spectrum.
| Central NGC 2146: A Firehose-Type Bending Instability in the Disk of Newly Formed Stars?
As the observations of highly flattened galaxies, including the MilkyWay, and gravitational N-body simulations show, the central parts ofthese systems at distances of, say, r < 0.5-0.7 kpc from the centerrotate slowly, and their local circular velocities of regular rotationbecome less than (or comparable to) the residual (random) velocities. Insuch a thin, practically nonrotating, pressure-supported central disk, atypical star moves along the bending, perpendicular to the equatorialplane layer, under the action of two forces acting in oppositedirections: the destabilizing centrifugal force, Fc, and the restoringgravitational attraction, Fg. Obviously, fierce instabilities of thebuckling kind developing perpendicular to the plane may not be avoidedif Fc > Fg. The latter condition is none other than the condition ofthe so-called firehose electromagnetic instability in collisionlessplasmas. The source of free energy in the instability is the intrinsicanisotropy of a velocity dispersion ("temperature"). It seems reasonablethat this is a natural mechanism for building a snake-shaped radiostructure, which has recently been observed by Zhao and coworkers in thecentral region of the spiral starburst galaxy NGC 2146 with the VeryLarge Array at an angular resolution of 2". In the current paper, inorder to investigate the dynamics of the central region, N-bodysimulations of the firehose-type bending instability are presented forthis galaxy. Use of concurrent computers has enabled us to make longsimulation runs using a sufficiently large number of particles in thedirect summation code, N ~ 20,000. In contrast to all previous N-bodysimulations of bending instabilities, we show how bending structures maybe longer lived in real starburst galaxies than in the computer models.The simulations clearly confirm the qualitative picture and, moreover,are in fair quantitative agreement with the theory. A theoreticalprediction is confirmed that the instability is driven by an excess ofplane kinetic energy of random motions of stars, when the ratio of thedispersion of radial velocities of stars in the plane cr to the velocitydispersion in the perpendicular direction cz is large enough, cr >~0.6cz, in other words, if the thickness of the stellar disk h ~ cz issmall enough. The extent to which our results on the stability of thedisk can have a bearing on observable spiral galaxies with a high starformation rate in the central parts is discussed.
| Groups of galaxies. III. Some empirical characteristics.
Not Available
| Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies
We present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.
| Gaseous Halos of Late-Type Spiral Galaxies
This article reviews the most recent observational results on disk-halointeractions in nearby galaxies. The implications of these results onour understanding of the structure of the interstellar medium (ISM) inour Galaxy and external spiral galaxies, with particular emphasis on thehalo ISM, are discussed. Disk-halo interactions occur only above thebrightest \hii\ regions, which are found almost exclusively in late-typespirals and dwarf and irregular galaxies. Circumnuclear starbursts arethe most energetic flavor of this phenomenon. The existence, the shapes,and the properties of gaseous halos depend on the level of energy inputinto the disk ISM per unit surface area, i.e. on the feedback of mass,momentum, and energy produced by vigorous star formation. Thus, currenttheoretical models of the ISM take into account that gaseous halos arenatural extensions of disk ISMs and are heated by the winds and ionizingradiation of massive stars, and by the shock waves of supernovae andtheir remnants. Observationally, this is reflected by the fact that allphases of the ISM known to exist in galaxy disks have also been detectedin halos. Disk-halo interactions are a very effective way ofredistributing energy, metals, and magnetic fields in galaxies and ofexpelling them into intergalactic space when outflows reach escapevelocit. (SECTION: Invited Review Articles)
| On the Origin and Evolution of Galactic Magnetic Fields
The existence of large-scale magnetic fields in galaxies is still achallenge for theoretical astrophysics. Are magnetic fields ofprimordial origin, produced somehow during the initial stages of cosmicevolution or are they intrinsically produced by the galaxies themselves?Especially observations of m G fields in high redshift (z = 2-3) dampedLyman alpha clouds, which are supposed to be the progenitors of diskgalaxies, raise questions about the origin of such strong fields onlyone or two Gigayears after the Big Bang. Recent observations of galacticmagnetic fields in nearby disk galaxies as well as in high redshiftobjects are reviewed and the role of electrodynamical coupling of thefields and the gas motions in different stages of galaxy evolution isemphasized. By presenting two different scenarios-action of a turbulentdynamo in axisymmetric differentially rotating disks and magnetic fieldamplification by non-axisymmetric dynamical processes (protogalacticcollapse and subsequent excitation of spiral arms and bars) - weillustrate the basic problems of magnetic field production andamplification in galactic systems. It is shown that origin andamplification via dynamical processes leads to appropriate time scalesand efficiencies to account for the strong magnetic fields in highredshift objects as well as the field structure in nearby disk galaxies.We describe the implications for galaxy formation if such strong fieldsexist in the epoch prior to galaxy formation. Finally we discuss ourconclusion that the origin and evolution of galactic magnetic fields canonly be understood by considering the time-varying velocity field of theconductor, the galactic interstellar medium in all stages of a galacticlifetime, in detail.
| Modeling the HI Supershell in the Edge-on-Galaxy NGC 4631 as an Energetic HVC Impact
High-resolution observations of the neutral hydrogen gas in the edge-ongalaxy NGC 4631 reveal an expanding, shell-like structure of diameter2.8 kpc and mass 1-2 x 10^8^ in the disk. The "supershell" is the mostenergetic and one of the largest known in a galactic disk. The formationof such a shell as a result of gas swept up by stellar winds andsupernova from an OB association would require energy input from ~>10^4^ OB stars. Instead, in this paper we use three-dimensionalhydrodynamical simulations to examine the possibility that the shell isformed by the oblique impact of a high velocity cloud (HVC) with thegalactic disk. Our results indicate that an HVC with initial radius 500pc and mass of 1.2 x 10^7^M_sun_ which impacts the disk at 200 km s^-1^evacuates a cavity of diameter 2.2 kpc and sweeps about 1.3 x 10^8^M_sun_ of ambient material into a thin shell which is structurally andkinematically similar to the observed shell. Clouds with differentcombinations of parameters but similar kinetic energies lead to similarresults. Thus, the properties of the impactor cannot be inferreduniquely from the observed properties of the shell. Moreover,three-dimensional magnetohydrodynamic models which include the effect ofan equipartition strength magnetic field in the galactic disk do notchange the structure of the shell substantially. We find that in orderto produce a structure like the observed shell, an oblique impact isnecessary. In addition, the synthetic position-velocity diagrams andchannel maps are sensitive to the assumed viewing angle, allowing us toconstrain the geometry of the impact. We find an excellent fit to theobservational data can be made for an impact trajectory inclined at anangle of 45^deg^ to the galactic plane and viewed at an angle 20^deg^away from the in-plane component of the velocity vector of the impactor.Our results strengthen the interpretation of the supershell as being theresult of an HVC impact.
| Long-slit spectra of extraplanar diffuse ionized gas in NGC4631.
We present long-slit optical spectroscopy and a Hα map of thehighly inclined interacting spiral galaxy NGC4631. This is the secondobject for which kinematics of the extraplanar diffuse ionized gas(eDIG) and variations of emission line ratios with position areinvestigated in an edge-on galaxy. We compare the measured line ratioswith photoionization models. Most of the eDIG detected at approx. 1kpcabove the disk is corotating within about 10km/s, with the exception ofa small extraplanar feature south-east of the nucleus. We findsignificant deviations of 70km/s from normal differential rotation ofthe disk in the south-eastern disturbed part of NGC4631. All thepeculiar velocities are consistent with the gravitational interactionNGC4631 is undergoing. [SII] and [NII] to Hα line intensity ratiosare significantly higher in the halo than in the disk. Line ratios inthe halo vary between 0.2 and 0.7 for both transitions. They can inprinciple be explained by photoionization in a diluted stellar radiationfield. However, the very high ratios require excitation parameters muchlower than for the eDIG in our Galaxy. As an inconsistency between thephotoionization models and our observations, the observed [SII]/[NII]ratio variation from the disk to the halo is not as strong as the modelspredict.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
