Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
An XMM-Newton observation of the massive edge-on Sb galaxy NGC 2613
We present an XMM-Newton observation of the massive edge-on Sb galaxyNGC 2613. We discover that this galaxy contains a deeply embedded activenucleus with a 0.3-10 keV luminosity of 3.3 ×1040ergs-1 and a line-of-sight absorption columnof 1.2 × 1023cm-2. Within the 25magarcsec-2 optical B-band isophote of the galaxy, we detectan additional four sources with an accumulated luminosity of 4.3 ×1039ergs-1. The bulk of the unresolved X-rayemission spatially follows the near-infrared (NIR) K-band surfacebrightness distribution; the luminosity ratioLX/LK ~ 8 × 10-4 is consistentwith that inferred from galactic discrete sources. This X-ray-NIRassociation and the compatibility of the X-ray spectral fit with theexpected spectrum of a population of discrete sources suggest thatlow-mass X-ray binaries (LMXBs) are the most likely emitters of theunresolved emission in the disc region. The remaining unresolvedemission is primarily due to extraplanar hot gas. The luminosity of thisgas is at least a factor of 10 less than that predicted by recentsimulations of intergalactic gas accretion by such a massive galaxy witha circular rotation speed Vc ~ 304 kms-1. Instead,we find that the extraplanar hot gas most likely represents discreteextensions away from the disc, including two `bubble-like' features oneither side of the nucleus. These extensions appear to correlate withradio continuum emission and, energetically, can be easily explained byoutflows from the galactic disc.
| Structure and kinematics of edge-on galaxy discs - V. The dynamics of stellar discs
In earlier papers in this series we determined the intrinsic stellardisc kinematics of 15 intermediate- to late-type edge-on spiral galaxiesusing a dynamical modelling technique. The sample covers a substantialrange in maximum rotation velocity and deprojected face-on surfacebrightness, and contains seven spirals with either a boxy orpeanut-shaped bulge. Here we discuss the structural, kinematical anddynamical properties. From the photometry we find that intrinsicallymore flattened discs tend to have a lower face-on central surfacebrightness and a larger dynamical mass-to-light ratio. This observationsuggests that, at a constant maximum rotational velocity, lower surfacebrightness discs have smaller vertical stellar velocity dispersions.Although the individual uncertainties are large, we find from thedynamical modelling that at least 12 discs are submaximal. The averagedisc contributes 53 +/- 4 per cent to the observed rotation at 2.2 discscalelengths (hR), with a 1σ scatter of 15 per cent.This percentage becomes somewhat lower when effects of finite discflattening and gravity by the dark halo and the gas are taken intoaccount. Since boxy and peanut-shaped bulges are probably associatedwith bars, the result suggests that at 2.2hR the submaximalnature of discs is independent of barredness. The possibility remainsthat very high surface brightness discs are maximal, as these discs areunderrepresented in our sample. We confirm that the radial stellar discvelocity dispersion is related to the galaxy maximum rotationalvelocity. The scatter in this σ versus vmax relationappears to correlate with the disc flattening, face-on central surfacebrightness and dynamical mass-to-light ratio. Low surface brightnessdiscs tend to be more flattened and have smaller stellar velocitydispersions. The findings are consistent with the observed correlationbetween disc flattening and dynamical mass-to-light ratio and cangenerally be reproduced by the simple collapse theory for disc galaxyformation. Finally, the disc mass Tully-Fisher relation is offset fromthe maximum-disc scaled stellar mass Tully-Fisher relation of the UrsaMajor cluster. This offset, -0.3 dex in mass, is naturally explained ifthe discs of the Ursa Major cluster spirals are submaximal.
| A serendipitous XMM-Newton observation of the intermediate polar WX Pyx
We briefly describe a serendipitous observation of the little-studiedintermediate polar WX Pyx using XMM-Newton. The X-ray spin period is1557.3 s, confirming the optical period published in 1996. An orbitalperiod of ~5.54 h is inferred from the separation of the spin-orbitsidelobe components. The soft and hard band spin-folded light curves areroughly sinusoidal in shape. The best-fit spectrum is consistent with abremsstrahlung temperature of ~18 keV. An upper limit of ~300 eV isassigned to the presence of Fe line emission. WX Pyx lies near TX and TVCol in the P_spin-P_orb plane.Research supported by contract number NAS8-39073 to SAO.
| Neutral hydrogen gas in 7 high-inclination spiral galaxies. I. The data
High-sensitivity interferometric H i line observations of a small sampleof seven galaxies with limiting column densities of a few times1019 cm-2 are presented. A tilted ring modelfitting routine was used to determine some global characteristics of theH i distribution and kinematics in the galaxy disks. 4 of the 7 galaxieshave low maximum rotation velocities of 125 km s-1,indicating that they are low-mass systems. Visual inspection shows thatat least one galaxy, NGC 4700, exhibits signs of extraplanar H iemission. An in-depth search for H i gas in the galaxy halos and thedetermination of halo gas properties, based on three-dimensionalmodeling, will follow in a separate publication. Companion galaxies weredetected in H i line emission near 3 of the 7 sample galaxies: NGC 1511,NGC 4565 and NGC 4700. One of these, NGC 1511, is found to be stronglyinteracting and is therefore not suitable for a study of the dependenceof its halo properties on the level of star formation activity in theunderlying disk. In the case of NGC 4700 the companion galaxy has novisible influence on its gas kinematics, while NGC 4565 might beaffected by its interaction with two small companions.Figures [see full text] and Appendix A are only available in electronicform at http://www.edpsciences.org
| HI in NGC 5433 and its environment: high-latitude emission in a small galaxy group
We present HI synthesis maps of the edge-on starburst NGC 5433 and itsenvironment, obtained with the Very Large Array in its C and Dconfigurations. The observations and spectral model residuals of themain disc emission in NGC 5433 reveal three extraplanar features. Weassociate two of these features with coherent extraplanar extensionsacross multiple spectral channels in our data, including a complete loopin position-velocity space. Interpreting the latter as an expandingshell we derive a corresponding input energy of 2 ×1054 erg, comparable to that for the largest supershellsfound in the Galaxy and those in other edge-on systems. NGC 5433 is in aricher environment than previously thought. We confirm that KUG 1359+326is a physical companion to NGC 5433 and find two new faint companions,both with Minnesota Automated Plate Scanner identifications, which welabel SIS-1 and SIS-2. Including the more distant IC 4357, NGC 5433 isthe dominant member of a group of at least five galaxies, spanning over750 kpc in a filamentary structure. A variety of evidence suggests thatinteractions are occurring in this group. While a number of underlyingmechanisms are consistent with the morphology of the high-latitudefeatures in NGC 5433, we argue that environmental effects may play arole in their generation.
| Spatial distribution of galaxies in the Puppis region
We determine the spatial distribution of the galaxies located behind thepart of the zone of avoidance of the Milky Way defined by 220°
| Stellar Velocity Dispersion and Mass Estimation for Galactic Disks
Available velocity dispersion estimates for the old stellar populationof galactic disks at galactocentric distances r=2L (where L is thephotometric radial scale length of the disk) are used to determine thethreshold local surface density of disks that are stable againstgravitational perturbations. The mass of the disk Mdcalculated under the assumption of its marginal stability is comparedwith the total mass Mt and luminosity LB of thegalaxy within r=4L. We corroborate the conclusion that a substantialfraction of the mass in galaxies is probably located in their darkhalos. The ratio of the radial velocity dispersion to the circularvelocity increases along the sequence of galactic color indices anddecreases from the early to late morphological types. For most of thegalaxies with large color indices (B-V)0 > 0.75, whichmainly belong to the S0 type, the velocity dispersion exceedssignificantly the threshold value required for the disk to be stable.The reverse situation is true for spiral galaxies: the ratiosMd/LB for these agree well with those expected forevolving stellar systems with the observed color indices. This suggeststhat the disks of spiral galaxies underwent no significant dynamicalheating after they reached a quasi-equilibrium stable state.
| Giant Metrewave Radio Telescope observations of NGC 3079
We present new observations at three frequencies (326, 615 and 1281 MHz)of the radio lobe spiral galaxy, NGC 3079, using the Giant MetrewaveRadio Telescope. These observations are consistent with previous dataobtained at other telescopes and reveal the structure of the nuclearradio lobes in exquisite detail. In addition, new features are observed,some with HI counterparts, showing broad-scale radio continuum emissionand extensions. The galaxy is surrounded by a radio halo that is atleast 4.8 kpc in height. Two giant radio extensions/loops are seen oneither side of the galaxy out to ~11 kpc from the major axis, onlyslightly offset from the direction of the smaller nuclear radio lobes.If these are associated with the nuclear outflow, then the galaxy hasexperienced episodic nuclear activity. Emission along the southern majoraxis suggests motion through a local intergalactic medium (not yetdetected), and it may be that NGC 3079 is itself creating this localintergalactic gas via outflows. We also present maps of the minimumenergy parameters for this galaxy, including cosmic ray energy density,electron diffusion length, magnetic field strength, particle lifetimeand power.
| The PDS versus Markarian starburst galaxies: comparing strong and weak IRAS emitter at 12 and 25 μm in the nearby Universe
The characteristics of the starburst galaxies from the Pico dos Diassurvey (PDS) are compared with those of the nearby ultraviolet (UV)bright Markarian starburst galaxies, having the same limit in redshift(vh < 7500 km s-1) and absolute B magnitude(MB < -18). An important difference is found: theMarkarian galaxies are generally undetected at 12 and 25 μm in IRAS.This is consistent with the UV excess shown by these galaxies andsuggests that the youngest star-forming regions dominating thesegalaxies are relatively free of dust.The far-infrared selection criteria for the PDS are shown to introduce astrong bias towards massive (luminous) and large size late-type spiralgalaxies. This is contrary to the Markarian galaxies, which are found tobe remarkably rich in smaller size early-type galaxies. These resultssuggest that only late-type spirals with a large and massive disc arestrong emitters at 12 and 25 μm in IRAS in the nearby Universe.The Markarian and PDS starburst galaxies are shown to share the sameenvironment. This rules out an explanation of the differences observedin terms of external parameters. These differences may be explained byassuming two different levels of evolution, the Markarian being lessevolved than the PDS galaxies. This interpretation is fully consistentwith the disc formation hypothesis proposed by Coziol et al. to explainthe special properties of the Markarian SBNG.
| High-Latitude H I in the Low Surface Brightness Galaxy UGC 7321
From the analysis of sensitive H I 21 cm line observations, we findevidence for vertically extended H I emission (z<~2.4 kpc) in the edge-on, lowsurface brightness spiral galaxy UGC 7321. Three-dimensional modelingsuggests that the H I disk of UGC 7321 is both warped and flared, butthat neither effect can fully reproduce the spatial distribution andkinematics of the highest z-height gas. We are able to model thehigh-latitude emission as an additional H I component in the form of a``thick disk'' or ``halo'' with a FWHM of ~3.3 kpc. We find tentativeevidence that the vertically extended gas declines in rotationalvelocity as a function of z, although we are unable to completely ruleout models with constant V(z). In spite of the low star formation rateof UGC 7321, energy from supernovae may be sufficient to sustain thishigh-latitude gas. However, alternative origins for this material, suchas slow, sustained infall, cannot yet be excluded.
| High-Latitude H I in NGC 2613: Buoyant Disk-Halo Outflow
We combine new VLA D configuration H I data of NGC 2613 with previoushigh-resolution data to show new disk-halo features in this galaxy. Theglobal H I distribution is modeled in detail using a technique that candisentangle the effects of inclination from scale height and can alsosolve for the average volume density distribution in and perpendicularto the disk. The model shows that the galaxy's inclination is on the lowend of the range given by Chaves & Irwin in a previous paper andthat the H I disk is thin (ze=188 pc), showing no evidencefor halo. Numerous discrete disk-halo features are observed, however,achieving z heights up to 28 kpc from midplane. One prominent feature inparticular, of mass 8×107 Msolar and height22 kpc, is seen on the advancing side of the galaxy at a projectedgalactocentric radius of 15.5 kpc. If this feature achieves such highlatitudes because of events in the disk alone, then input energies oforder ~1056 ergs are required. We have instead investigatedthe feasibility of such a large feature being produced via buoyancy(with drag) within a hot, preexisting X-ray corona. Reasonable plumedensities, temperatures, stall height (~11 kpc), outflow velocities, andages can indeed be achieved in this way. The advantage of this scenariois that the input energy need only be sufficient to produce blowout, acondition that requires a reduction of 3 orders of magnitude in energy.If this is correct, there should be an observable X-ray halo around NGC2613.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| The 2MASS Large Galaxy Atlas
We present the largest galaxies as seen in the near-infrared (1-2μm), imaged with the Two Micron All Sky Survey (2MASS), ranging inangular size from 1' to 1.5d. We highlight the 100 largest in thesample. The galaxies span all Hubble morphological types, includingelliptical galaxies, normal and barred spirals, and dwarf and peculiarclasses. The 2MASS Large Galaxy Atlas provides the necessary sensitivityand angular resolution to examine in detail morphologies in thenear-infrared, which may be radically different from those in theoptical. Internal structures such as spirals, bulges, warps, rings,bars, and star formation regions are resolved by 2MASS. In addition tolarge mosaic images, the atlas includes astrometric, photometric, andshape global measurements for each galaxy. A comparison of fundamentalmeasures (e.g., surface brightness, Hubble type) is carried out for thesample and compared with the Third Reference Catalogue. We furthershowcase NGC 253 and M51 (NGC 5194/5195) to demonstrate the quality anddepth of the data. The atlas represents the first uniform, all-sky,dust-penetrated view of galaxies of every type, as seen in thenear-infrared wavelength window that is most sensitive to the dominantmass component of galaxies. The images and catalogs are availablethrough the NASA/IPAC Extragalactic Database and Infrared ScienceArchive and are part of the 2MASS Extended Source Catalog.
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| A catalog of warps in spiral and lenticular galaxies in the Southern hemisphere
A catalog of optical warps of galaxies is presented. This can beconsidered complementary to that reported by Sánchez-Saavedra etal. (\cite{sanchez-saavedra}), with 42 galaxies in the northernhemisphere, and to that by Reshetnikov & Combes(\cite{reshetnikov99}), with 60 optical warps. The limits of the presentcatalog are: logr 25 > 0.60, B_t< 14.5, delta (2000) <0deg, -2.5 < t < 7. Therefore, lenticular galaxies havealso been considered. This catalog lists 150 warped galaxies out of asample of 276 edge-on galaxies and covers the whole southern hemisphere,except the Avoidance Zone. It is therefore very suitable for statisticalstudies of warps. It also provides a source guide for detailedparticular observations. We confirm the large frequency of warpedspirals: nearly all galaxies are warped. The frequency and warp angle donot present important differences for the different types of spirals.However, no lenticular warped galaxy has been found within the specifiedlimits. This finding constitutes an important restriction fortheoretical models.
| The Mysteries of Galaxy Spirals
Not Available
| The H I Line Width/Linear Diameter Relationship as an Independent Test of the Hubble Constant
The relationship between corrected H I line widths and linear diameters(LW/LD) for spiral galaxies is used as an independent check on the valueof the Hubble constant. After calibrating the Tully-Fisher (TF) relationin both the B and I bands, the B-band relation is used for galaxies ofmorphological/luminosity types Sc I, Sc I.2, Sc I.3, Sab, Sb, Sb I-II,and Sb II to derive the LW/LD relation. We find that for this sample thescatter in the LW/LD is smallest with a Hubble constant of 90-95 kms-1 Mpc-1. Lower values of the Hubble constantproduce a separation in the LW/LD relation that is a function ofmorphological type. Since a Hubble constant of 90-95 is significantlylarger than the final Key Project value of 72 km s-1Mpc-1, a comparison of TF, surface brightness fluctuation(SBF), and fundamental plane (FP) is made. This comparison indicatesthat the Key Project TF distances to 21 clusters may be too large. For asample of 11 clusters, the Key Project TF distances provide anunweighted mean Hubble constant of 77 km s-1Mpc-1, while a combination of the FP, SBF, and our TFdistances for the same 11 clusters gives H0=91 kms-1 Mpc-1. A more subtle result in our data is amorphological dichotomy in the Hubble constant. The data suggest that ScI galaxies follow a Hubble constant of 90-95 while Sb galaxies follow aHubble constant closer to 75 km s-1 Mpc-1.Possible explanations for this result are considered, but it is shownthat this Sb/Sc I Hubble flow discrepancy is also present in the VirgoCluster and is consistent with previous investigations that indicatethat some galaxies carry a component of age-related intrinsic redshift.
| Bar Galaxies and Their Environments
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
| High-Latitude H I in NGC 2613: Structure and Dynamics
Studies of edge-on spiral galaxies have revealed a number of cases inwhich kiloparsec-scale H I features extend from the disk into the haloregion. We have obtained VLA CnB array 21 cm H I observations of NGC2613, an edge-on spiral galaxy 26 Mpc away. We report that NGC 2613 isan extremely massive galaxy with a nearby companion galaxy (ESO495-G017). We present the first H I maps and masses for this companion.We have found three high-latitude features above the plane, symmetric inprojection with three below the plane in NGC 2613. All six features showa multiple velocity structure, with several ``spurs,'' as well as atrend to lag behind the disk gas with increasing distance from theplane. The radio continuum emission shows two large extensionsassociated with a strong continuum source in the disk. These are thehighest latitude discrete features clearly connected to a spiral galaxyyet detected in radio continuum. Moreover, two of the H I features showa strong correlation with these large continuum extensions, with thecontinuum arranged on the perimeter of the H I. We postulate that thesefeatures were formed by some mechanism internal to the galaxy, possiblyby spatially correlated supernovae or by the action of a preexistingrelativistic jet flowing through the plane of the galaxy.
| Homogenization of the Stellar Population along Late-Type Spiral Galaxies
We present a study of the broadband UBV color profiles for 257 Sbcbarred and nonbarred galaxies, using photoelectric aperture photometrydata from the literature. Using robust statistical methods, we haveestimated the color gradients of the galaxies, as well as the total andbulge mean colors. A comparative photometric study using CCD images wasdone. In our sample, the color gradients are negative (reddish inward)in approximately 59% of the objects, are almost null in 27%, and arepositive in 14%, considering only the face-on galaxies, which representapproximately 51% of the sample. The results do not change, essentially,when we include the edge-on galaxies. As a consequence of this study wehave also found that barred galaxies are overrepresented among theobjects having null or positive gradients, indicating that bars act as amechanism of homogenization of the stellar population. This effect ismore evident in the U-B color index, although it can also be detected inthe B-V color. A correlation between the total and bulge colors wasfound that is a consequence of an underlying correlation between thecolors of bulges and disks found by other authors. Moreover, the meantotal color is the same irrespective of the gradient regime, whilebulges are bluer in galaxies with null or positive gradients, whichindicates an increase of the star formation rate in the central regionsof these objects. We have also made a quantitative evaluation of theamount of extinction in the center of these galaxies. This was doneusing the Wide Field Planetary Camera 2 (WFPC2) and the Near InfraredCamera and Multi-Object Spectrometer (NICMOS) Hubble Space Telescope(HST) archival data, as well as CCD B, V, and I images. We show thatalthough the extinction in the V-band can reach values up to 2 mag inthe central region, it is unlikely that dust plays a fundamental role inglobal color gradients. We found no correlation between color and O/Habundance gradients. This result could suggest that the color gradientsare more sensitive to the age rather than to the metallicity of thestellar population. However, the absence of this correlation may becaused by dust extinction. We discuss this result by considering apicture in which bars are a relatively fast, recurrent phenomenon. Theseresults are not compatible with a pure classical monolithic scenario forbulge and disk formation. On the contrary, they favor a scenario inwhich both these components are evolving in a correlated process inwhich stellar bars play a crucial role. Based partly on observationsmade at the Pico dos Dias Observatory (PDO/LNA-CNPq), Brazil.
| The visible environment of galaxies with counterrotation
In this paper we present a statistical study of the environments of 49galaxies in which there is gas- or stellar-counterrotation. The numberof possible companions in the field (to apparent magnitude 22), theirsize and concentration were considered. All the statistical parameterswere analysed by means of Kolgomorov-Smirnov tests, using a controlsample of 43 galaxies without counterrotation. From our data, nosignificant differences between the counter-rotating and control samplesappear. This is different to Seyfert or radio-loud galaxies which lie inenvironments with a higher density of companions. On the contrary, if aweak tendency exists, for galaxies with gas counterrotation only, it isdiscovered in regions of space where the large scale density of galaxiesis smaller. Our results tend to disprove the hypothesis thatcounterrotation and polar rings derive from a recent interaction with asmall satellite or a galaxy of similar size. To a first approximation,they seem to follow the idea that all galaxies are born through a mergerprocess of smaller objects occurring very early in their life, or thatthey derive from a continuous, non-traumatic infall of gas that formedstars later. Whatever the special machinery is which producescounterrotation or polar rings instead of a co-planar, co-rotatingdistribution of gas and stars, it seems not to be connected to thepresent galaxy density of their environments.
| High-Resolution Radio Continuum Observations of Edge-on Spiral Galaxies
Radio continuum emission at 20 cm has been observed in a sample of 16edge-on galaxies, using the VLA in its A configuration. These galaxieswere observed recently at lower resolution by Irwin et al., who foundevidence for extraplanar disk-halo features in 15 of the 16 galaxies.Twelve of the galaxies are detected in the new high-resolutionobservations. Of these, only two, which were previously known Seyferts,show convincing evidence for AGNs. For six of the galaxies, we provideimproved positions for the galaxy nuclei. In four galaxies, radiocontinuum loops extending perpendicular to the major axis have beendiscovered, confirming previous conclusions from lower resolutionobservations that extraplanar emission is present, as well as providingfurther evidence that radio ``halos'' consist, at least in part, ofunderlying discrete features. For several galaxies, the resolution issufficient to reveal individual star-forming regions in the disk. Weexamine NGC 3556 in detail since this galaxy has been shown by King& Irwin to display extremely large H I supershells. The estimatedinput energy from supernovae in the brightest radio component of thisgalaxy is insufficient to drive the observed H I supershells. We explorepossible resolutions to this energy deficit.
| Box- and peanut-shaped bulges. I. Statistics
We present a classification for bulges of a complete sample of ~ 1350edge-on disk galaxies derived from the RC3 (Third Reference Catalogue ofBright Galaxies, de Vaucouleurs et al. \cite{rc3}). A visualclassification of the bulges using the Digitized Sky Survey (DSS) inthree types of b/p bulges or as an elliptical type is presented andsupported by CCD images. NIR observations reveal that dust extinctiondoes almost not influence the shape of bulges. There is no substantialdifference between the shape of bulges in the optical and in the NIR.Our analysis reveals that 45% of all bulges are box- and peanut-shaped(b/p). The frequency of b/p bulges for all morphological types from S0to Sd is > 40%. In particular, this is for the first time that such alarge frequency of b/p bulges is reported for galaxies as late as Sd.The fraction of the observed b/p bulges is large enough to explain theb/p bulges by bars. Partly based on observations collected at ESO/LaSilla (Chile), DSAZ/Calar Alto (Spain), and Lowell Observatory/Flagstaff(AZ/U.S.A.). Tables 6 and 7 are only available in electronic form at CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Radio jet-blown neutral hydrogen supershells in spiral galaxies?
Taking a clue from the pair of HI supershells found in the Scd galaxyNGC 3556 (M 108), we propose a new mechanism for the origin of HIsupershells in gas-rich massive galaxies. In this scenario, the twosupershells were inflated out of the neutral hydrogen disk due to thelocalised flaring of a pair of radio lobes formed by the jets ejectedfrom the nucleus during an active phase about ~ 107 yearsago, but have faded away by now. It is shown that the salient featuresof this supershell pair, such as their symmetrical locations about thegalactic centre, the anomalously large energy requirements, the largegalacto-centric distances, as well as the Z-symmetric hemisphericalshapes, find a more natural explanation in terms of this scenario, ascompared to the standard models which postulate either a massivestarburst, or the infall of external gas clouds. Other possibleimplications of this hypothesis are briefly discussed.
| High-Latitude Radio Emission in a Sample of Edge-on Spiral Galaxies
We have mapped 16 edge-on galaxies at 20 cm using the Very Large Arrayin its C configuration, and a subset of these galaxies in the Dconfiguration at 6 and/or 20 cm, in a search for extended (>~1 kpc)radio continuum emission above and below the plane. For five galaxies,we could form spectral index, energy, and magnetic field maps (assumingminimum energy). While the galaxies were partly chosen by radio fluxdensity, they span a variety of star formation rates (SFRs), and onlysix might be considered ``starburst'' galaxies. A range of Hubble typeand degree of isolation are also represented. The galaxies largely fallon the FIR-radio continuum correlation. They also display a correlationbetween IR surface brightness and warmth, extending the previouslyobserved relation of Lehnert & Heckman to galaxies with lower starformation rates. We find that all but one galaxy show evidence fornonthermal high-latitude radio continuum emission, suggesting thatcosmic-ray (CR) halos are common in star-forming galaxies. Of these,eight galaxies are new detections. The high-latitude emission is seenover a variety of spatial scales and in discrete and/or smooth features.In some cases, discrete features are seen on large scales, suggestingthat smooth radio halos may consist, in part, of discrete featurescombined with low spatial resolution. In general, the discrete featuresemanate from the disk, but estimates of CR diffusion lengths suggestthat diffusion alone is insufficient to transport the particles to thehigh latitudes seen (>15 kpc in one case). Thus CRs likely diffusethrough low-density regions and/or are assisted by other mechanisms(e.g., winds). We searched for correlations between the prevalence ofhigh-latitude radio emission and a number of other properties, includingthe global SFR, supernova input rate per unit star-forming area, E_A,and environment, and do not find clear correlations with any of theseproperties. A subset of the data allows, at best, for only a weakcorrelation with E_A. Our one nondetection (NGC 4517), however, occursat a threshold level consistent with that found by Dahlem, Lisenfeld,& Golla. The lack of a good correlation with star formationindicators could be the result of the different timescales for starformation processes compared with the duration of the radio emission.Correlations with other properties, such as environment, are moredifficult to assess. However, a few isolated galaxies display strongradio halos, indicating that an interaction is not necessary to producethe extraplanar emission.
| The kinematics of the bulge and the disc of NGC 7331
Results are presented of spectroscopic emission and absorption lineobservations along the major axis of the Sb galaxy NGC 7331. Thekinematics of the ionized gas and stellar component are derived, beingregular and symmetric with respect to the centre of the galaxy. Contraryto what may be expected, for R la 40'' the emission line gas appears torotate slower than the stars. A likely explanation for this phenomenonis an inclined and warped gas layer in those inner regions. In the bulgedominated region the absorption line profiles are asymmetric in thesense that they have a shallow extension towards the systemic velocity.No counterrotating stellar component is observed, which is contrary toprevious claims. As demonstrated, these claims might be based on a wronginterpretation of the employed analysis method. Outside the bulgedominated region the stellar radial velocities are in agreement with theneutral hydrogen dynamics and the stellar velocity dispersion decreasestowards larger radii. A detailed bulge/disc light decomposition has beenmade. This has been used to construct a kinematical model of NGC 7331from which model absorption line profiles were calculated. Theseprofiles have been compared with the observations and model parametershave been adjusted to obtain a good match. It appeared necessary tocombine a rapidly rotating disc having a radially decreasing velocitydispersion with a slowly rotating constant dispersion bulge. Then,simultaneously, the observed stellar radial velocities, the velocitydispersions and the observed asymmetry of the line profile could beexplained satisfactorily. An even better fit to the data can be achievedwhen the disc is relatively thinner and colder inside the bulge region.For the disc the stellar velocity dispersions and photometry result in amass-to-light ratio of 1.6 +/- 0.7 Msun/Lsun(I) .This value agrees with previous determinations for other discs usingobserved velocity dispersions. A rotation curve analysis allows thecalculation of the mass-to-light ratio of the bulge which amounts to 6.8in the I-band; considerably larger than the disc value. It appears thatthe mass distribution of NGC 7331 is completely dominated by thecombination of bulge and dark halo at all radii. Comparing welldetermined mass-to-light ratios of a number of bulges with disc values(M/L)_bulge is, on average, three times as large as (M/L)_disc in theI-band. For the B-band this ratio goes up to 7.2, a fact which shouldhave cosmological consequences.
| The Pico DOS Dias Survey Starburst Galaxies
We discuss the nature of the galaxies found in the Pico dos Dias Survey(PDS) for young stellar objects. The PDS galaxies were selected from theIRAS Point Source catalog. They have flux density of moderate or highquality at 12, 25, and 60 μm and spectral indices in the ranges -3.00<= alpha(25, 12) <= + 0.35 and -2.50 <= alpha(60, 25) <=+0.85. These criteria allowed the detection of 382 galaxies, which are amixture of starburst and Seyfert galaxies. Most of the PDS Seyfertgalaxies are included in the catalog of warm IRAS sources by de Grijp etal. The remaining galaxies constitute a homogeneous sample of luminous[log F (L_B/L_ȯ) = 9.9 +/- 0.4] starburst galaxies, 67% of whichwere not recognized as such before. The starburst nature of the PDSgalaxies is established by comparing their L_IR/L_B ratios and IRAScolors with a sample of emission-line galaxies from the literaturealready classified as starburst galaxies. The starburst galaxies show anexcess of FIR luminosity, and their IRAS colors are significantlydifferent from those of Seyfert galaxies-99% of the starburst galaxiesin our sample have a spectral index alpha(60, 25) < -1.9. As opposedto Seyfert galaxies, very few PDS starbursts are detected in X-rays. Inthe infrared, the starburst galaxies form a continuous sequence withnormal galaxies. But they generally can be distinguished from normalgalaxies by their spectral index alpha(60, 25) > -2.5. This colorcutoff also marks a change in the dominant morphologies of the galaxies:the normal IRAS galaxies are preferentially late-type spirals (Sb andlater), while the starbursts are more numerous among early-type spirals(earlier than Sbc). This preference of starbursts for early-type spiralsis not new, but a trait of the massive starburst nucleus galaxies(Coziol et al.). As in other starburst nucleus galaxy samples, the PDSstarbursts show no preference for barred galaxies. No difference isfound between the starbursts detected in the FIR and those detected onthe basis of UV excess. The PDS starburst galaxies represent the FIRluminous branch of the UV-bright starburst nucleus galaxies, with meanFIR luminosity log (L_IR/L_ȯ) = 10.3 +/- 0.5 and redshifts smallerthan 0.1. They form a complete sample limited in flux in the FIR at 2 x10^-10 ergs cm^-2 s^-1.
| Bulge-Disk Decomposition of 659 Spiral and Lenticular Galaxy Brightness Profiles
We present one of the largest homogeneous sets of spiral and lenticulargalaxy brightness profile decompositions completed to date. The 659galaxies in our sample have been fitted with a de Vaucouleurs law forthe bulge component and an inner-truncated exponential for the diskcomponent. Of the 659 galaxies in the sample, 620 were successfullyfitted with the chosen fitting functions. The fits are generally welldefined, with more than 90% having rms deviations from the observedprofile of less than 0.35 mag. We find no correlations of fittingquality, as measured by these rms residuals, with either morphologicaltype or inclination. Similarly, the estimated errors of the fittedcoefficients show no significant trends with type or inclination. Thesedecompositions form a useful basis for the study of the lightdistributions of spiral and lenticular galaxies. The object base issufficiently large that well-defined samples of galaxies can be selectedfrom it.
| Attenuation Effects in Spiral Galaxies: Multiwavelength Photometry and Disk Radiative Transfer Models
We present a quantitative investigation of the optical depth throughspiral disks using BVRIJHK colors of 15 highly inclined Sab-Sc spiralsand new models for radiative transfer in stellar disks. The modelsinclude exponential stellar and dust disks, exponential stellar bulgecomponents, multiple scattering, and both homogeneous and clumpy dustdistributions. Preliminary comparisons of the observed optical andnear-infrared (NIR) color gradients across galaxy dust lanes withpredictions from radiative transfer models with slab and sphericalshapes underscore the need for these realistic exponential diskgeometries. When compared with the extent of reddening predicted by thedisk models, the maximum optical and NIR color excesses in galaxy dustlanes imply central face-on optical depths of 0.5-2.0 in the V band. Forthese highly inclined systems, we find this inferred optical depth to belargely insensitive to the difference between clumpy and homogeneousdust distributions. Comparisons of galaxy color gradients to models withhigh central optical depths contradict the often-stated claim thatspiral disks are opaque out to D_25. Our derived optical depths showthat the predicted radial color changes caused by attenuation in face-ongalaxies are much smaller than the observed color gradients in suchsystems, which suggests that the observed changes are the result ofvariations in stellar content. The face-on optical depths also implythat, when viewed edge-on, galaxy dust lanes have optical depths greaterthan 1.0 even in the K band and, thus, the NIR mass-to-light ratiochanges across dust features.
| Catalogue of HI maps of galaxies. I.
A catalogue is presented of galaxies having large-scale observations inthe HI line. This catalogue collects from the literature the informationthat characterizes the observations in the 21-cm line and the way thatthese data were presented by means of maps, graphics and tables, forshowing the distribution and kinematics of the gas. It containsfurthermore a measure of the HI extension that is detected at the levelof the maximum sensitivity reached in the observations. This catalogueis intended as a guide for references on the HI maps published in theliterature from 1953 to 1995 and is the basis for the analysis of thedata presented in Paper II. The catalogue is only available inelectronic form at the CDS via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
