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Low-Luminosity Active Galaxies and Their Central Black Holes
Central black hole masses for 117 spiral galaxies representingmorphological stages S0/a through Sc and taken from the largespectroscopic survey of Ho et al. are derived using Ks-banddata from the Two Micron All Sky Survey. Black hole masses are foundusing a calibrated black hole-Ks bulge luminosity relation,while bulge luminosities are measured by means of a two-dimensionalbulge-disk decomposition routine. The black hole masses are correlatedagainst a variety of parameters representing properties of the nucleusand host galaxy. Nuclear properties such as line width (FWHM [N II]), aswell as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [NII]/Hα, and [S II]/Hα), show a very high degree ofcorrelation with black hole mass. The excellent correlation with linewidth supports the view that the emission-line gas is in virialequilibrium with either the black hole or bulge potential. The very goodemission-line ratio correlations may indicate a change in ionizingcontinuum shape with black hole mass in the sense that more massiveblack holes generate harder spectra. Apart from theinclination-corrected rotational velocity, no excellent correlations arefound between black hole mass and host galaxy properties. Significantdifferences are found between the distributions of black hole masses inearly-, mid-, and late-type spiral galaxies (subsamples A, B, and C) inthe sense that early-type galaxies have preferentially larger centralblack holes, consistent with observations that Seyfert galaxies arefound preferentially in early-type systems. The line width distributionsshow a marked difference among subsamples A, B, and C in the sense thatearlier type galaxies have larger line widths. There are also cleardifferences in line ratios between subsamples A+B and C that likely arerelated to the level of ionization in the gas. Finally, aKs-band Simien & de Vaucouleurs diagram shows excellentagreement with the original B-band relation, although there is a largedispersion at a given morphological stage.
| The structure of galactic disks. Studying late-type spiral galaxies using SDSS
Using imaging data from the SDSS survey, we present the g' and r' radialstellar light distribution of a complete sample of ~90 face-on tointermediate inclined, nearby, late-type (Sb-Sdm) spiral galaxies. Thesurface brightness profiles are reliable (1 σ uncertainty lessthan 0.2 mag) down to μ27 mag/''. Only ~10% of all galaxies havea normal/standard purely exponential disk down to our noise limit. Thesurface brightness distribution of the rest of the galaxies is betterdescribed as a broken exponential. About 60% of the galaxies have abreak in the exponential profile between 1.5-4.5 times thescalelength followed by a downbending, steeper outer region. Another~30% shows also a clear break between 4.0-6.0 times thescalelength but followed by an upbending, shallower outer region. A fewgalaxies have even a more complex surface brightness distribution. Theshape of the profiles correlates with Hubble type. Downbending breaksare more frequent in later Hubble types while the fraction of upbendingbreaks rises towards earlier types. No clear relation is found betweenthe environment, as characterised by the number of neighbours, and theshape of the profiles of the galaxies.
| 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.
| The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies
LINER nuclei, which are present in many nearby galactic bulges, may bethe manifestation of low-rate or low-radiative-efficiency accretion ontosupermassive central black holes. However, it has been unclear whetherthe compact UV nuclear sources present in many LINERs are clusters ofmassive stars, rather than being directly related to the accretionprocess. We have used the Hubble Space Telescope to monitor the UVvariability of a sample of 17 galaxies with LINER nuclei and compactnuclear UV sources. Fifteen of the 17 galaxies were observed more thanonce, with two to five epochs per galaxy, spanning up to a year. Wedetect significant variability in most of the sample, with peak-to-peakamplitudes from a few percent to 50%. In most cases, correlatedvariations are seen in two independent bands (F250W and F330W).Comparison to previous UV measurements indicates, for many objects,long-term variations by factors of a few over decade timescales.Variability is detected in LINERs with and without detected compactradio cores, in LINERs that have broad Hα wings detected in theiroptical spectra (``LINER 1s''), and in those that do not (``LINER 2s'').This variability demonstrates the existence of a nonstellar component inthe UV continuum of all types and sets a lower limit to the luminosityof this component. Interestingly, all the LINERs that have detectedradio cores have variable UV nuclei, as one would expect from bona fideactive galactic nuclei. We note a trend in the UV color (F250W/F330W)with spectral type-LINER 1s tend to be bluer than LINER 2s. This trendmay indicate a link between the shape of the nonstellar continuum andthe presence or the visibility of a broad-line region. In one target,the poststarburst galaxy NGC 4736, we detect variability in a previouslynoted UV source that is offset by 2.5" (~60 pc in projection) from thenucleus. This may be the nearest example of a binary active nucleus andof the process leading to black hole merging.Based on observations with the Hubble Space Telescope, which is operatedby AURA, Inc., under NASA contract NAS 5-26555.
| A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei
Using the Green Bank Telescope, we have conducted a survey for 1.3 cmwater maser emission toward the nuclei of nearby active galaxies, themost sensitive large survey for H2O masers to date. Among 145galaxies observed, maser emission was newly detected in 11 sources andconfirmed in one other. Our survey targeted nearby (v<12,000 kms-1), mainly type 2 active galactic nuclei (AGNs) north ofδ=-20deg and includes a few additional sources as well.We find that more than one-third of Seyfert 2 galaxies have strong maseremission, although the detection rate declines beyond v~5000 kms-1 because of sensitivity limits. Two of the masersdiscovered during this survey are found in unexpected hosts: NGC 4151(Seyfert 1.5) and NGC 2782 (starburst). We discuss the possiblerelations between the large X-ray column to NGC 4151 and a possiblehidden AGN in NGC 2782 to the detected masers. Four of the masersdiscovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, havehigh-velocity lines symmetrically spaced about the systemic velocity, alikely signature of molecular gas in a nuclear accretion disk. The masersource in NGC 6323, in particular, reveals the classic spectrum of a``disk maser'' represented by three distinct groups of Dopplercomponents. Future single-dish and VLBI observations of these fourgalaxies could provide a measurement of the distance to each galaxy andof the Hubble constant, independent of standard candle calibrations.
| Companions to Isolated Elliptical Galaxies: Revisiting the Bothun-Sullivan Sample
We investigate the number of physical companion galaxies for a sample ofrelatively isolated elliptical galaxies. The NASA/IPAC ExtragalacticDatabase (NED) has been used to reinvestigate the incidence of satellitegalaxies for a sample of 34 elliptical galaxies, first investigated byBothun & Sullivan using a visual inspection of Palomar Sky Surveyprints out to a projected search radius of 75 kpc. We have repeatedtheir original investigation using data cataloged in NED. Nine of theseelliptical galaxies appear to be members of galaxy clusters; theremaining sample of 25 galaxies reveals an average of +1.0+/-0.5apparent companions per galaxy within a projected search radius of 75kpc, in excess of two equal-area comparison regions displaced by 150-300kpc. This is significantly larger than the +0.12+/-0.42companions/galaxy found by Bothun & Sullivan for the identicalsample. Making use of published radial velocities, mostly availablesince the completion of the Bothun-Sullivan study, identifies thephysical companions and gives a somewhat lower estimate of +0.4companions per elliptical galaxy. This is still 3 times larger than theoriginal statistical study, but given the incomplete and heterogeneousnature of the survey redshifts in NED, it still yields a firm lowerlimit on the number (and identity) of physical companions. An expansionof the search radius out to 300 kpc, again restricted to sampling onlythose objects with known redshifts in NED, gives another lower limit of4.5 physical companions per galaxy. (Excluding five elliptical galaxiesin the Fornax Cluster, this average drops to 3.5 companions perelliptical.) These physical companions are individually identified andlisted, and the ensemble-averaged radial density distribution of theseassociated galaxies is presented. For the ensemble, the radial densitydistribution is found to have a falloff consistent withρ~R-0.5 out to approximately 150 kpc. For non-FornaxCluster companions the falloff continues out to the 300 kpc limit of thesurvey. The velocity dispersion of these companions is found to reach amaximum of 350 km s-1 at around 120 kpc, after which theyfall at a rate consistent with Keplerian falloff. This falloff may thenindicate the detection of a cut-off in the mass-density distribution inthe elliptical galaxies' dark matter halo at ~100 kpc.
| Nuclear Properties of a Sample of Nearby Spiral Galaxies from Hubble Space Telescope STIS Imaging
We present surface photometry for the central regions of a sample of 48spiral galaxies (mostly unbarred and barred of type Sbc or Sc) observedwith the Space Telescope Imaging Spectrograph on board the Hubble SpaceTelescope. Surface brightness profiles (SBPs) were derived and modeledwith a Nuker law. We also analyzed archival Wide Field Planetary Camera2 images with a larger field of view, which are available for 18galaxies in our sample. We modeled the extracted bulge SBPs with anexponential, an r1/4, or an rn profile. Inagreement with previous studies, we find that bulges of Sbc galaxiesfall into two categories: bulges well described by an exponentialprofile and those well described by an r1/4 profile. Only onegalaxy requires the use of a more general Sérsic profile toproperly describe the bulge. Nuclear photometrically distinct componentsare found in ~55% of the galaxies. For those that we classify as starclusters on the basis of their resolved extent, we find absolutemagnitudes that are brighter on average than those previously identifiedin spiral galaxies. This might be due to a bias in our sample towardstar-forming galaxies, combined with a trend for star-forming galaxiesto host brighter central clusters.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Inner-truncated Disks in Galaxies
We present an analysis of the disk brightness profiles of 218 spiral andlenticular galaxies. At least 28% of disk galaxies exhibit innertruncations in these profiles. There are no significant trends oftruncation incidence with Hubble type, but the incidence among barredsystems is 49%, more than 4 times that for nonbarred galaxies. However,not all barred systems have inner truncations, and not allinner-truncated systems are currently barred. Truncations represent areal dearth of disk stars in the inner regions and are not an artifactof our selection or fitting procedures nor the result of obscuration bydust. Disk surface brightness profiles in the outer regions are wellrepresented by simple exponentials for both truncated and nontruncateddisks. However, truncated and nontruncated systems have systematicallydifferent slopes and central surface brightness parameters for theirdisk brightness distributions. Truncation radii do not appear tocorrelate well with the sizes or brightnesses of the bulges. Thissuggests that the low angular momentum material apparently missing fromthe inner disk was not simply consumed in forming the bulge population.Disk parameters and the statistics of bar orientations in our sampleindicate that the missing stars of the inner disk have not simply beenredistributed azimuthally into bar structures. The sharpness of thebrightness truncations and their locations with respect to othergalactic structures suggest that resonances associated with diskkinematics, or tidal interactions with the mass of bulge stars, might beresponsible for this phenomenon.
| Radio emission from AGN detected by the VLA FIRST survey
Using the most recent (April 2003) version of the VLA FIRST survey radiocatalog, we have searched for radio emission from >2800 AGN takenfrom the most recent (2001) version of the Veron-Cetty and Veron AGNcatalog. These AGN lie in the 9033 square degrees of sky alreadycovered by the VLA FIRST survey. Our work has resulted in positivedetection of radio emission from 775 AGN of which 214 are new detectionsat radio wavelengths.Tables 3 and 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/35
| The size of the broad-line regions in dwarf active galaxies
Using the empirical relation between black hole mass and bulge velocitydispersion established for active and quiescent galaxies, we derive thesizes of the broad-line regions (BLRs) for 22 dwarf active galaxies.These sizes are compared with those of Seyfert 1 galaxies and quasars.We find that the size of the BLRs of Seyfert 1s and quasars is wellscaled with the luminosity of the Hα line with a slope of ~=0.5,and dwarf active galaxies show larger BLRs than the values predicted bythe BLR size-luminosity relation for Seyfert 1 galaxies andquasi-stellar objects (QSOs). The results suggest that the BLRs of dwarfactive galactic nuclei (AGN) have lower ionization or/and lower densitythan those of Seyfert 1 galaxies and QSOs. Photoionization calculationsshow that the large BLRs are consistent with observed emission linespectra.
| On the Nature of Low-Luminosity Narrow-Line Active Galactic Nuclei
There is clear observational evidence that some narrow-line (type 2)active galactic nuclei (AGNs) have a hidden broad-line region (BLR) andare thus intrinsically broad-line (type 1) AGNs. Does this AGNunification apply for all type 2 AGNs? Indirect arguments suggest thatsome ``true'' type 2 AGNs, i.e., AGNs having no obscured BLR, do exist,but it is not clear why the BLR is missing in these AGNs. Here we pointout a possible natural explanation. The observed radius-luminosityrelation for the BLR implies an increasing line width with decreasingluminosity for a given black hole mass (MBH). In addition,there appears to be an upper limit to the observed width of broademission lines in AGNs of Δvmax~25,000 kms-1, which may reflect a physical limit above which the BLRmay not be able to survive. Thus, at a low enough luminosity the BLRradius shrinks below the Δvmax radius, leaving noregion where the BLR can exist, although the AGN may remain otherwise``normal.'' The implied minimum bolometric luminosity required tosustain a BLR with Δv<25,000 km s-1 isLmin~1041.8(MBH/108Msolar)2.All AGNs with L
| A Search for ``Dwarf'' Seyfert Nuclei. VI. Properties of Emission-Line Nuclei in Nearby Galaxies
We use the database from Paper III to quantify the global and nuclearproperties of emission-line nuclei in the Palomar spectroscopic surveyof nearby galaxies. We show that the host galaxies of Seyferts, LINERs,and transition objects share remarkably similar large-scale propertiesand local environments. The distinguishing traits emerge on nuclearscales. Compared with LINERs, Seyfert nuclei are an order of magnitudemore luminous and exhibit higher electron densities and internalextinction. We suggest that Seyfert galaxies possess characteristicallymore gas-rich circumnuclear regions and hence a more abundant fuelreservoir and plausibly higher accretion rates. The differences betweenthe ionization states of the narrow emission-line regions of Seyfertsand LINERs can be partly explained by the differences in their nebularproperties. Transition-type objects are consistent with being composite(LINER/H II) systems. With very few exceptions, the stellar populationwithin the central few hundred parsecs of the host galaxies is uniformlyold, a finding that presents a serious challenge to starburst orpost-starburst models for these objects. Seyferts and LINERs havevirtually indistinguishable velocity fields as inferred from their linewidths and line asymmetries. Transition nuclei tend to have narrowerlines and more ambiguous evidence for line asymmetries. All threeclasses of objects obey a strong correlation between line width and lineluminosity. We argue that the angular momentum content of circumnucleargas may be an important factor in determining whether a nucleus becomesactive. Finally, we discuss some possible complications for theunification model of Seyfert galaxies posed by our observations.
| An Atlas of Hubble Space Telescope Spectra and Images of Nearby Spiral Galaxies
We have observed 54 nearby spiral galaxies with the Space TelescopeImaging Spectrograph (STIS) on the Hubble Space Telescope to obtainoptical long-slit spectra of nuclear gas disks and STIS optical (~Rband) images of the central 5''×5'' of thegalaxies. These spectra are being used to determine the velocity fieldof nuclear disks and hence to detect the presence of central massiveblack holes. Here we present the spectra for the successfulobservations. Dust obscuration can be significant at opticalwavelengths, and so we also combine the STIS images with archivalNear-Infrared Camera and Multi-Object Spectrometer H-band images toproduce color maps to investigate the morphology of gas and dust in thecentral regions. We find a great variety in the different morphologies,from smooth distributions to well-defined nuclear spirals and dustlanes.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| 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.
| Compact groups in the UZC galaxy sample
Applying an automatic neighbour search algorithm to the 3D UZC galaxycatalogue (Falco et al. \cite{Falco}) we have identified 291 compactgroups (CGs) with radial velocity between 1000 and 10 000 kms-1. The sample is analysed to investigate whether Tripletsdisplay kinematical and morphological characteristics similar to higherorder CGs (Multiplets). It is found that Triplets constitute lowvelocity dispersion structures, have a gas-rich galaxy population andare typically retrieved in sparse environments. Conversely Multipletsshow higher velocity dispersion, include few gas-rich members and aregenerally embedded structures. Evidence hence emerges indicating thatTriplets and Multiplets, though sharing a common scale, correspond todifferent galaxy systems. Triplets are typically field structures whilstMultiplets are mainly subclumps (either temporarily projected orcollapsing) within larger structures. Simulations show that selectioneffects can only partially account for differences, but significantcontamination of Triplets by field galaxy interlopers could eventuallyinduce the observed dependences on multiplicity. Tables 1 and 2 are onlyavailable in electronic at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/35
| Star formation in the warped outer pseudoring of the spiral galaxy NGC 3642
NGC 3642 was classified as a spiral galaxy with three rings and no bar.We have performed an HI and optical study of this nearly face-on galaxy.We find that the nuclear ring might in fact be part of an innerone-armed spiral, that could be driving nuclear accretion and feedingthe central activity in the inner kpc. The inner ring is faint, and theouter ring is a rather ill-defined pseudoring. Furthermore, the sizeratio of the rings is such that they cannot be due to a single patternspeed linking them together. The outer pseudoring is peculiar, since itlies in the faint outer parts of the disk, where star formation is stillgoing on at 1.4 times the optical radius. Higher HI column densities areassociated with these regions and the atomic gas layer is warped. Theseperturbations affect only the outer disk, since the kinematics withinthe main body conforms well to an ordinary differentially rotating disk.We propose here that both nuclear activity and star formation in thewarped outer parts might be linked to the fact that NGC 3642 is locatedin a rich environment, where its close neighbors show clear signs ofmerging. Our suggestion is that NGC 3642 has captured recently alow-mass, gas-rich dwarf, and star formation was triggered in thisinfalling external gas that produced also a pronounced warp in thegaseous disk.
| Structure and Kinematics of NGC 615 and Its Nuclear Star Formation History
The results of a complex photometric and spectral investigation of theregular Sb galaxy NGC 615 are presented. The observations were made withthe 6 m, 1 m, and 0.6 m telescopes of the Special AstrophysicalObservatory RAS (Nizhnij Arkhyz, Russia) and the 1.5 m ESO telescope (LaSilla, Chile). The analysis of the radial brightness profiles revealsthe existence of at least two decoupled exponential components; theparameters of the outer component are typical for the global disks of Sbgalaxies, and the inner component seen in the radius range 9"-30"(1.0-3.5 kpc) is distinguished by compactness and high surfacebrightness. A combination of two-dimensional velocity fields for starsand ionized gas in the central part of the galaxy and of the long-slitvelocity profiles along the major axis up to the optical border of NGC615 provides some interesting findings. In the very center, R<3" (0.3kpc), stars and gas rotate together, and the dynamical and photometricmajor axes are turned with respect to the global line of nodes; weconclude that an inclined circumnuclear disk is detected. In the bulge,R=3"-6" (0.35-0.75 kpc), the stellar velocity dispersion is more than100 km s-1, the stars rotate twice as slowly as the ionizedgas, and a counterrotating gaseous component is observed. In the innerexponential component the stars rotate together with the ionized gas,and the stellar velocity dispersion drops to about 50 km s-1.As the photometric major axis of this component is turned with respectto the line of nodes, we call it ``an oval inner disk.'' The mean age ofthe stellar populations in the nucleus and in the inner disk beyond itsH II regions is 5 Gyr, while the bulge is much older. Both thechemically decoupled nucleus of NGC 615 and the inner disk probablyformed the bulk of their more luminous stars in a secondary burst ofstar formation, perhaps provoked by the close passage of another galaxysome gigayears ago. Based on observations collected with the 6 m, 1 m,and 0.6 m telescopes of the Special Astrophysical Observatory (SAO) ofthe Russian Academy of Sciences (RAS) and with the 1.52 m ESO telescopeat La Silla.
| Environment status of blue compact galaxies and trigger of star formation
The work studies of the environment of low-mass galaxies with activestar formation (SF) and a possible trigger of SF bursts due togravitational interaction. Following the study by Taylor et al. (1995),we extend the search for possible disturbing galaxies of various massesto a much larger sample of 86 BCGs from the sky region of the SecondByurakan survey (SBS). The BCG magnitudes and radial velocities arerevised and up-dated. The sample under study is separated by thecriteria: EW([O III]λ5007) > 45 Å andVh < 6,000 kms and should be representative of alllow-mass galaxies which experience SF bursts. We argue that the moderatetidal disturbers should be taken into account, and incorporate therespective range of distances in the search for disturbing neighbours.The majority of the neighbours in the vicinity of the studied BCGs arefound through the study of their environment among UZC (Falco et al.\cite{Falco99}) galaxies, and the follow-up careful search of thefainter galaxies in the NED database. For the remaining BCGs, theneighbouring galaxies are found based on the results of the SAO 6mtelescope spectroscopy. By studing the data on the radial velocities ofgalaxies in the vicinity of BCGs we found: 1) 33 of the studied BCGs(~38.5%) are associated with significantly brighter galaxies (Δ B>= 1.5m); 2) 23 BCGs (~26.5%) have neighbours either ofcomparable or significantly lower brightness; 3) 14 of the studied BCGs(16\%) with no evident associated galaxy are either certain, orprobable, mergers. Summarizing, we conclude that in ~80% (or more) BCGsfrom the studied sample, the SF bursts are triggered either by tidalaction of various strengths from other galaxies, or due to mergers oflow-mass galaxies. We briefly discuss the implications of our mainconclusion for evolutionary links of BCGs to other types of low-massgalaxies. Part of our sample falls into the volume belonging to theLocal Supercluster. Therefore we formulate the results separately on the``Local Supercluster volume'' and ``general field region''. The totalfractions of BCGs likely triggered by interaction with other galaxy arerespectively, ~84.5% and 80% for the nearby volume and for the generalfield. The fractions of BCGs with significantly brighter disturbers inthese two groups are seemingly different (~54±14% vs.~31.5±7%, respectively). Among the so called ``isolated'' BCGs(that is, without a bright neighbouring galaxy) in both the LocalSupercluster volume and in general field, ~43±10% are probablydisturbed by dwarf galaxies and ~26±8% have a merger morphology.In the Appendix we present the results of the spectroscopy with the SAO6m telescope of 27 galaxies in an attempt to find possible disturbinggalaxies in the vicinity of some of the sample BCGs. Tables 2, 3 and A.1with their notes and Figs. A.1-A.3 are only available in electronic format http://www.edpsciences.org
| 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.
| Radio Sources in Low-Luminosity Active Galactic Nuclei. I. VLA Detections of Compact, Flat-Spectrum Cores
We report a high-resolution (0.2"), 15 GHz survey of a sample of 48low-luminosity active galactic nuclei with the Very LargeArray.5 Compact radio emission has beendetected above a flux density of 1.1 mJy in 57% (17 of 30) oflow-ionization nuclear emission-line region (LINER) nuclei andlow-luminosity Seyfert galaxies. The 2 cm radio power is significantlycorrelated with the emission-line ([O I] λ6300) luminosity. Usingradio fluxes at other frequencies from the literature, we find that atleast 15 of the 18 detected radio cores have a flat to inverted spectrum(α>=-0.3, Sν~να). While thepresent observations are consistent with the radio emission originatingin star-forming regions (the brightness temperatures are>=102.5-4.5 K), higher resolution radio observations of 10of the detected sources, reported in an accompanying paper, show thatthe cores are very compact (<~1 pc), of high brightness temperature(Tb>~108 K), and probably synchrotronself-absorbed, ruling out a starburst origin. Thus, our results suggestthat at least 50% of low-luminosity Seyfert galaxies and LINERs in thesample are accretion powered, with the radio emission presumably comingfrom jets or advection-dominated accretion flows. We have detected only1 of 18 ``transition'' (i.e., LINER+H II) nuclei observed, indicatingthat their radio cores are significantly weaker than those of ``pure''LINERs. Compact 2 cm radio cores are found in both type 1 (i.e., withbroad Hα) and type 2 (without broad Hα) nuclei. There isweak evidence, limited in significance by small numbers, thatlow-luminosity active galactic nuclei with compact radio cores exhibitradio ejecta preferentially aligned along the rotation axis of thegalaxy disk. If this result were confirmed by a larger sample, it wouldlend support to the idea that the misalignment of accretion disks withthe galaxy stellar disk in more luminous Seyfert galaxies is a result ofradiation-pressure-induced warping of their accretion disks.
| X-ray Emission Mechanisms of LINERs
We present here an analysis of the X-ray properties of a sample of LINERgalaxies observed with the ROSAT PSPC and HRI instruments. A spatialanalysis shows that the bulk of the X-ray emission is consistent witharising from a point source; some extended emission appears at weakemission levels. The X-ray spectra are formally best described by apowerlaw with photon index Γ_x ~ -2 or thermal emission from aRaymond-Smith plasma with highly subsolar abundances (Z <= 0.1).Several emission mechanisms that might contribute to the observed X-rayspectra are discussed. In particular, we take the very subsolarabundances derived from Raymond-Smith fits as an indication of a morecomplex emission mechanism, like the presence of a second hard componentor plasma out of equilibrium.
| At the Heart of Barred Galaxies
Not Available
| 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.
| Polarized Broad-Line Emission from Low-Luminosity Active Galactic Nuclei
In order to determine whether unified models of active galactic nucleiapply to low-luminosity objects, we have undertaken aspectropolarimetric survey of LINERs and Seyfert nuclei at the KeckObservatory. The 14 objects observed have a median Hα luminosityof 8x1039 ergs s-1, well below the typical valueof ~1041 ergs s-1 for Markarian Seyfert nuclei.Polarized broad Hα emission is detected in three LINERs: NGC 315,NGC 1052, and NGC 4261. Each of these is an elliptical galaxy with adouble-sided radio jet, and the emission-line polarization in each caseis oriented roughly perpendicular to the jet axis, as expected for theobscuring torus model. NGC 4261 and NGC 315 are known to contain dustycircumnuclear disks, which may be the outer extensions of the obscuringtori. The detection of polarized broad-line emission suggests that theseobjects are nearby, low-luminosity analogs of obscured quasars residingin narrow-line radio galaxies. The nuclear continuum of thelow-luminosity Seyfert 1 galaxy NGC 4395 is polarized at p=0.67%,possibly the result of an electron-scattering region near the nucleus.Continuum polarization is detected in other objects, with a median levelof p=0.36% over 5100-6100 Å, but in most cases this is likely tobe the result of transmission through foreground dust. The lack ofsignificant broad-line polarization in most type 1 LINERs is consistentwith the hypothesis that we view the broad-line regions of these objectsdirectly, rather than in scattered light.
| Small-Scale Anisotropy of Cosmic Rays above 10^19 eV Observed with the Akeno Giant Air Shower Array
With the Akeno Giant Air Shower Array, 581 cosmic rays above 10^19 eV,47 above 4x10^19 eV, and seven above 10^20 eV were observed until 1998August. The arrival direction distribution of these extremely highenergy cosmic rays has been studied. While no significant large-scaleanisotropy is found on the celestial sphere, some interesting clustersof cosmic rays are observed. Above 4x10^19 eV, there are one triplet andthree doublets within a separation angle of 2.5d, and the probability ofobserving these clusters by a chance coincidence under an isotropicdistribution is smaller than 1%. The triplet is especially observedagainst expected 0.05 events. The cos(theta_GC) distribution expectedfrom the dark matter halo model fits the data as well as an isotropicdistribution above 2x10^19 and 4x10^19 eV, but the fit with the darkmatter halo model is poorer than the isotropic distribution above 10^19eV. The arrival direction distribution of seven 10^20 eV cosmic rays isconsistent with that of lower energy cosmic rays and is uniform. Threeof the seven are members of doublets above about 4x10^19 eV.
| Soft X-ray properties of LINERs
Not Available
| AGNs with composite spectra. II. Additional data
In a previous paper \cite[(Véron et al. 1997)]{ver97} wepresented medium resolution (3.4 Angstroms FWHM) spectroscopicobservations of 15 ``transition objects'', selected for having anambiguous location in the \cite[Veilleux & Osterbrock (1987)]{vei87}diagnostic diagrams, and showed that most of them were in fact``composite'', this being due to the simultaneous presence on the slitof both a Seyfert or Liner nucleus and a H Ii region. Here, we reportnew spectroscopic observations of 53 emission-line galaxies with a``transition'' spectrum, bringing up to 61 the total number of observedobjects in an unbiased sample of 88 ``transition objects''. Almost allof the observed galaxies have a ``composite" nature, confirming thefinding that true ``transition'' spectra may not exist at all. Byeliminating ``composite objects'' from the diagnostic diagrams, a clearseparation between the different classes of nuclear emission-lineregions (Seyfert 2s, Liners and H Ii regions) becomes apparent; byrestricting the volume occupied by the different line-emitting regionsin the 3-dimensional diagnostic diagrams, we are also restricting therange of possible physical parameters in these regions. There seems tobe no continuity between Seyfert 2s and Liners, the two classesoccupying distinct volumes in the 3-dimensional space defined by lambda6300/Hα ii, lambda 6583/Hα , and lambda 6300/Hα .Based on observations collected at the Observatoire de Haute-Provence(CNRS), France, and Hubble Space Telescope (HST) data obtained from theSpace Telescope European Coordinating Facility (ST-ECF) archive. Tables5 and 6 are also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| X-ray properties of LINERs
We present an investigation of the X-ray properties of 13 LINERs basedon {ROSAT} all-sky survey and pointed PSPC and HRI observations. Severalsources are studied for the first time in X-rays. The X-ray spectra arebest described by a powerlaw with photon index {Gamma_x } ~ -2.0 orthermal emission from gas with very subsolar abundances. Theluminosities range between log L_x = 37.7 (NGC 404) and 40.8 (NGC 4450).No X-ray variability on the timescale of hours/days is detected. This isin line with the suggestion that LINERs may accrete in theadvection-dominated mode. On a longer term, one of the objects, NGC2768, turns out to be slightly variable. Some sources appear to beextended at weak emission levels whereas the bulk of the X-ray emissionis consistent with arising from a point source within the PSPCinstrumental resolution. L_x/L_B ratios are derived and emissionmechanisms that potentially contribute to the observed X-rayluminosities are discussed. We also examine the presence of second X-raysources near the target sources both in terms of instrumental effectsand in terms of `real' sources within the LINER galaxies.
| Discovery of a Nuclear X-Ray Source in NGC 7331: Evidence for a Massive Black Hole
Using ROSAT, we have made the first deep X-ray observations of NGC 7331,discovering a nuclear X-ray source coincident with the previouslydetermined radio and optical nuclear centers. The positions,luminosities, and fluxes of X-ray sources in the field are compared withpreviously identified radio and optical sources. The nucleus of NGC 7331has been analyzed to discern any new evidence supporting the presence ofa massive black hole, and comparisons are made with other low-ionizationnuclear emission-line region (LINER) galaxies. The flux ratio of coreradio to soft X-ray emission in NGC 7331 is lower than in other LINERsincluded in our sample.
| Extensive Spiral Structure and Corotation Resonance
Spiral density wave theories demand that grand-design spiral structurebe bounded, at most, between the inner and outer Lindblad resonances ofthe spiral pattern. The corotation resonance lies between the outer andthe inner Lindblad resonances. The locations of the resonances are atradii whose ratios to each other are rather independent of the shape ofthe rotation curve. The measured ratio of outer to inner extent ofspiral structure for a given spiral galaxy can be compared to thestandard ratio of corotation to inner Lindblad resonance radius. In thecase that the measured ratio far exceeds the standard ratio, it islikely that the corotation resonance is within the bright optical disk.Studying such galaxies can teach us how the action of resonances sculptsthe appearance of spiral disks. This paper reports observations of 140disk galaxies, leading to resonance ratio tests for 109 qualified spiralgalaxies. It lists candidates that have a good chance of having thecorotation resonance radius within the bright optical disk.
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