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Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| X-ray surveys and wide-field optical/near-infrared imaging with JDEM
I briefly describe a few important scientific issues that could beaddressed effectively via the combination of data from JDEM and X-raymissions. The topics covered are largely focused on active galacticnuclei (AGN) and include (1) the selection of AGN via X-ray emission andoptical variability, (2) nuclear outbursts in galaxies due to transientfueling of their supermassive black holes, (3) moderate-luminosity AGNat high redshift (z > 4) found via application of“dropout” techniques to X-ray sources, and (4) thehost-galaxy morphologies of X-ray selected AGN. I also describe thesubstantial challenges to obtaining wide-field X-ray data withsufficient sensitivity to complement JDEM properly.
| Effects of a Black Hole's Gravitational Field on the Luminosity of a Star during a Close Encounter
To complement hydrodynamic studies of the tidal disruption of the starby a massive black hole, we present the study of stellar luminosity andits variations produced by the strong gravitational field of the blackhole during a close encounter. By simulating the relativistically movingstar and its emitted light and taking into account general relativisticeffects on particle and light trajectories, our results show that theblack hole's gravity alone induces apparent stellar luminosityvariations on typical timescales of a few rg/c[=(5s)mbh/(106Msolar)] to a few 100rg/c[~(10minutes)mbh/(106Msolar)], whererg=Gmbh/c2. We discern different caseswith respect to the strength of tidal interaction and focus on two: (1)a star encountering a giant black hole traces spacetime almost as apoint particle, so the apparent luminosity variations are dominated byclearly recognizable general relativistic effects, and (2) in a closeencounter of a star with a black hole of similar size, the stellardebris is spread around the black hole by processes in whichhydrodynamics plays an important role. We discuss limitations andresults of our approach.
| On the alignment between binary spiral galaxies
We show some significance against the null hypothesis of randominteractions of binary spiral galaxies, and in favour of the alternativethat more interactions than expected occur for axes either nearlyparallel (spins being parallel or anti-parallel) or nearly orthogonal.We discuss this in the context of similar prior studies, using adifferent statistical focus in such a way that we are able toincorporate additional data.
| Chandra observations of five X-ray transient galactic nuclei
We report on exploratory Chandra observations of five galactic nucleithat were found to be X-ray bright during the ROSAT All-Sky Survey (withLX>~ 1043 erg s-1) but subsequentlyexhibited a dramatic decline in X-ray luminosity. Very little is knownabout the post-outburst X-ray properties of these enigmatic sources. Inall five cases Chandra detects an X-ray source positionally coincidentwith the nucleus of the host galaxy. The spectrum of the brightestsource (IC 3599) appears consistent with a steep power law (Γ~3.6). The other sources have too few counts to extract individual,well-determined spectra, but their X-ray spectra appear flatter(Γ~ 2) on average. The Chandra fluxes are~102-103 fainter than was observed during theoutburst (up to 12 yr previously). That all post-outburst X-rayobservations have seen a similarly low X-ray luminosities is consistentwith these sources having `switched' to a persistent low-luminositystate. Unfortunately the relative dearth of long-term monitoring andother data mean that the physical mechanism responsible for thisspectacular behaviour is still highly unconstrained.
| Determination of the Thickness of Non-Edge-on Disk Galaxies
We propose a method to determine the thickness of non-edge-on diskgalaxies from their observed structure of spiral arms, based on thesolution of the truly three-dimensional Poisson's equation for alogarithmic disturbance of density and under the condition where theself-consistency of the density wave theory is no longer valid. Fromtheir measured number of arms, pitch angle and location of the innermostpoint of the spiral arms, we derive and present the thicknesses of 34spiral galaxies.
| Follow-Up Chandra Observations of Three Candidate Tidal Disruption Events
Large-amplitude, high-luminosity soft X-ray flares were detected by theROSAT All-Sky Survey in several galaxies with no evidence of Seyfertactivity in their ground-based optical spectra. These flares had theproperties predicted for a tidal disruption of a star by a centralsupermassive black hole. We report Chandra observations of three ofthese galaxies taken a decade after their flares that reveal weaknuclear X-ray sources that are from 240 to 6000 times fainter than theirluminosities at peak, supporting the theory that these were specialevents and not ongoing active galactic nucleus (AGN) variability. Thedecline of RX J1624.9+7554 by a factor of 6000 is consistent with the(t-tD)-5/3 decay predicted for the fallback phaseof a tidal disruption event, but only if ROSAT was lucky enough to catchthe event exactly at its peak in 1990 October. RX J1242.6-1119A hasdeclined by a factor of 240, also consistent with(t-tD)-5/3. In the H II galaxy NGC 5905 we findonly resolved, soft X-ray emission that is undoubtedly associated withstarburst activity. When accounting for the starburst component, theROSAT observations of NGC 5905, as well as the Chandra upper limit onits nuclear flux, are consistent with a (t-tD)-5/3decay by at least a factor of 1000. Although we found weak Seyfert 2emission lines in Hubble Space Telescope spectra of NGC 5905, indicatingthat a low-luminosity AGN was present prior to the X-ray flare, we favora tidal disruption explanation for the flare itself.
| Chandra Observations of the Narrow-Line Seyfert 1 Galaxy RX J2217.9-5941
We report the results of two Chandra ACIS-S observations from 2003February and August of the highly X-ray variable narrow-line Seyfert 1galaxy RX J2217.9-5941. Observations spanning the time from the ROSATAll-Sky Survey (RASS) through an ASCA observation in 1998 indicateapparently monotonically decreasing flux by a factor of 30. The Chandraobservations reveal increased emission over that seen with ASCA,supporting a persistent variability rather than an X-ray outburst event.However, the cause of the strong X-ray variability remains unclear. OurChandra observations confirm the steep soft X-ray spectrum in the0.2-2.0 keV band found during the RASS observation(αX=2.7). The spectral shape of the source appears tobe variable, with the spectrum becoming softer when the source becomesfainter. Best-fitting models to the data include an absorbed brokenpower law, a blackbody plus a power law, and a power law with partialcovering absorption. The latter model suggests a variable partialcovering absorber in the line of sight, which can explain in part thevariability seen in RX J2217.9-5941. We suggest that there might be apopulation of narrow-line Seyfert 1 galaxies that are, at least attimes, highly absorbed.
| Structure and kinematics of candidatedouble-barred galaxies
Results of optical and NIR spectral and photometric observations of asample of candidate double-barred galaxies are presented. Velocityfields and velocity dispersion maps of stars and ionized gas, continuumand emission-line images were constructed from integral-fieldspectroscopy observations carried out at the 6 m telescope (BTA) of SAORAS, with the MPFS spectrograph and the scanning Fabry-PerotInterferometer. NGC 2681 was also observed with thelong-slit spectrograph of the BTA. Optical and NIR images were obtainedat the BTA and at the 2.1 m telescope (OAN, México).High-resolution images were retrieved from the HST data archive.Morphological and kinematic features of all 13 sample objects aredescribed in detail. Attention is focused on the interpretation ofobserved non-circular motions of gas and stars in circumnuclear (onekiloparsec-scale) regions. We have shown first of all that these motionsare caused by the gravitational potential of a large-scale bar.NGC 3368 and NGC 3786 have nuclearbars only, their isophotal twist at larger radii being connected withthe bright spiral arms. Three cases of inner polar disks in our sample(NGC 2681, NGC 3368 andNGC 5850) are considered. We found ionized-gascounter-rotation in the central kiloparsec of the lenticular galaxyNGC 3945. Seven galaxies (NGC 470,NGC 2273, NGC 2681, NGC3945, NGC 5566, NGC5905, and NGC 6951) have inner mini-disksnested in large-scale bars. Minispiral structures occur often in thesenuclear disks. It is interesting that the majority of the observed,morphological and kinematical, features in the sample galaxies can beexplained without the secondary bar hypothesis. Thus we suggest that adynamically independent secondary bar is a rarer phenomenon than followsfrom isophotal analysis of the images only.Based on observations carried out at the 6 m telescope of the SpecialAstrophysical Observatory of the Russian Academy of Sciences, operatedunder the financial support of the Science Department of Russia(registration number 01-43), at the 2.1 m telescope of the ObservatorioAstronónico Nacional, San Pedro Martir, México, and fromthe data archive of the NASA/ESA Hubble Space Telescope at the SpaceTelescope Science Institute. STScI is operated by the association ofUniversities for Research in Astronomy, Inc. under NASA contract NAS5-26555.Tables 1 to 6 and Figures 2-13 and 15-18 are only available inelectronic form at http://www.edpsciences.org
| 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
| Giant-Amplitude X-ray Flares as Probes of the Black Hole Region of Nearby Galaxies
Not Available
| Vertical Scale Parameter Estimates for 48 Non-edge-on Spiral Galaxies
In the first paper of this series, we directly studied the mathematicalforms, symmetry of spiral structure, and the projection of galacticdiscs on the images, and measured the pitch angles of the spiral armsand inclination angles of the galactic discs for 60 spiral galaxies. Inthis second paper, we estimate the vertical scale parameters of 48non-edge-on spiral galaxies based on the method proposed by Peng et al.and on the results given in Paper I. As we know, for edge-on discgalaxies we can obtain the vertical scale parameter from the photometry,once a mathematical form is specified for the vertical lightdistribution. For non-edge-on galaxies, some other methods have to beused. The statistical result was that the vertical scale parameter iscomparable for edge-on and non-edge-on galaxies, although it is obtainedfrom two very different methods.
| Follow-Up Hubble Space Telescope/Space Telescope Imaging Spectroscopy of Three Candidate Tidal Disruption Events
Large-amplitude, high-luminosity, soft X-ray flares were detected by theROSAT All-Sky Survey in several galaxies with no evidence for Seyfertactivity in their ground-based optical spectra. These flares had theproperties predicted for a tidal disruption event by a centralsupermassive black hole: soft X-ray spectrum, timescale of months, andlarge X-ray luminosity (1042-1044 ergss-1). In order to evaluate the alternative hypothesis thatthese flares could have been some form of extreme active galacticnucleus variability, we obtained follow-up optical spectroscopy of threeof the flaring galaxies a decade later with the Space Telescope ImagingSpectrograph and a narrow slit to search for or place stringent limitson the presence of any persistent Seyfert-like emission in their nuclei.Two of the galaxies, RX J1624.9+7554 and RX J1242.6-1119, show neitherevidence for emission lines nor a nonstellar continuum in their HubbleSpace Telescope nuclear spectra, consistent with their ground-basedclassification as inactive galaxies. They remain the most convincing ashosts of tidal disruption events. NGC 5905, previously known as astarburst H II galaxy due to its strong emission lines, has in its inner0.1" a nucleus with narrow emission line ratios requiring a Seyfert 2classification. This weak Seyfert 2 nucleus in NGC 5905, which wasmasked by the many surrounding H II regions in ground-based spectra,requires a low level of prior nonstellar photoionization, thus raisingsome uncertainty about the nature of its X-ray flare, which may or maynot have been a tidal disruption event. The absence of both broademission lines and nuclear X-ray absorption in NGC 5905 alsocharacterizes it as a true Seyfert 2 galaxy, yet one that has varied bymore than a factor of 100 in X-rays.On the basis of observations with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. These observations are associated withproposal 9177.
| X-Ray-Luminous Galaxies. I. Chandra Observations of IRAS 00317-2142
We present Chandra observations of the enigmatic galaxy IRAS 00317-2142,which is classified as a star-forming galaxy on the basis of theionization level of its emission lines. However, a weak broad Hαwing and a high X-ray luminosity give away the presence of an activenucleus. The Chandra image reveals a nuclear point source(L(2-10keV)~6×1041ergss-1),contributing over 80% of the galaxy X-ray counts in the 0.3-8 keV band.This is surrounded by some fainter nebulosity extending up to 6 kpc. Thenucleus does not show evidence for short-term variability. However, wedetect long-term variations between the ROSAT, ASCA, and Chandra epochs.Indeed, the source has decreased its flux by over a factor of 25 in aperiod of about 10 yr. The nuclear X-ray spectrum is well represented bya power law with a photon index ofΓ=1.91+0.17-0.15, while the extendedemission is represented by a Raymond-Smith component with a temperatureof ~0.6 keV. We find no evidence for the presence of an Fe line. Thenucleus is absorbed by an intrinsic column density ofNH~8×1020cm-2. Thus, the Chandraobservations suggest that at least the X-ray emission is dominated by atype 1 active galactic nucleus (AGN). Then the observed optical spectrumcan be explained by the masking of the nucleus by the powerfulstar-forming component. These, together with previous X-ray observationsof galaxies with no clear signs of AGN activity in the optical (e.g.,NGC 6240), cast doubt on the optical classification scheme and haveimplications for the nature of the ``normal'' galaxies detected in deepChandra X-ray surveys.
| Frequency of X-ray transients in galactic nuclei
Not Available
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| Bar strengths in spiral galaxies estimated from 2MASS images
Non-axisymmetric forces are presented for a sample of 107 spiralgalaxies, of which 31 are barred (SB) and 53 show nuclear activity. As adata base we use JHK images from the 2 Micron All-sky Survey, and thenon-axisymmetries are characterized by the ratio of the tangential forceto the mean axisymmetric radial force field, following Buta & Block.Bar strengths have an important role in many extragalactic problems andtherefore it is important to verify that the different numerical methodsapplied for calculating the forces give mutually consistent results. Weapply both direct Cartesian integration and a polar grid integrationutilizing a limited number of azimuthal Fourier components of density.We find that the bar strength is independent of the method used toevaluate the gravitational potential. However, because of thedistance-dependent smoothing by Fourier decomposition, the polar methodis more suitable for weak and noisy images. The largest source ofuncertainty in the derived bar strength appears to be the uncertainty inthe vertical scaleheight, which is difficult to measure directly formost galaxies. On the other hand, the derived bar strength is ratherinsensitive to the possible gradient in the vertical scaleheight of thedisc or to the exact model of the vertical density distribution,provided that the same effective vertical dispersion is assumed in allmodels. In comparison with the pioneering study by Buta & Block, thebar strength estimate is improved here by taking into account thedependence of the vertical scaleheight on the Hubble type: we find thatfor thin discs bar strengths are stronger than for thick discs by anamount that may correspond to as much as one bar strength class. Weconfirm the previous result by Buta and co-workers showing that thedispersion in bar strength is large among all the de Vaucouleurs opticalbar classes. In the near-infrared 40 per cent of the galaxies in oursample have bars (showing constant phases in the m= 2 Fourier amplitudesin the bar region), while in the optical band one-third of these barsare obscured by dust. Significant non-axisymmetric forces can also beinduced by the spiral arms, generally in the outer parts of the galacticdiscs, which may have important implications on galaxy evolution.Possible biases of the selected sample are also studied: we find thatthe number of bars identified drops rapidly when the inclination of thegalactic disc is larger than 50°. A similar bias is found in theThird Reference Catalogue of Bright Galaxies, which might be of interestwhen comparing bar frequencies at high and low redshifts.
| Two-Dimensional Spectroscopy of Double-Barred Galaxies
We describe the results of our spectroscopy for a sample of barredgalaxies whose inner regions exhibit an isophotal twist commonly calleda secondary bar. The line-of-sight velocity fields of the ionized gasand stars and the light-of-sight velocity dispersion fields of the starswere constructed from two-dimensional spectroscopy with the 6m SpecialAstrophysical Observatory telescope. We detected various types ofnon-circular motions of ionized gas: radial flows within large-scalebars, counter-rotation of the gas and stars at the center of NGC 3945, apolar gaseous disk in NGC 5850, etc. Our analysis of the optical andnear-infrared images (both ground-based and those from the Hubble SpaceTelescope) revealed circumnuclear minispirals in five objects. Thepresence of an inner (secondary) bar in the galaxy images is shown tohave no effect on the circumnuclear kinematics of the gas and stars.Thus, contrary to popular belief, the secondary bar is not a dynamicallydecoupled galactic structure. We conclude that the so-calleddouble-barred galaxies are not a separate type of galaxies but are acombination of objects with distinctly different morphology of theircircumnuclear regions.
| The Giant X-Ray Flare of NGC 5905: Tidal Disruption of a Star, a Brown Dwarf, or a Planet?
We model the 1990 giant X-ray flare of the quiescent galaxy NGC 5905 asthe tidal disruption of a star by a supermassive black hole. From theobserved rapid decline of the luminosity, over a timescale of a fewyears, we argue that the flare was powered by the fallback of debrisrather than subsequent accretion via a thin disk. The fallback modelallows constraints to be set on the black hole mass and the mass ofdebris. The latter must be very much less than a solar mass to explainthe very low luminosity of the flare. The observations can be explainedeither as the partial stripping of the outer layers of a low-massmain-sequence star or as the disruption of a brown dwarf or a giantplanet. We find that the X-ray emission in the flare must haveoriginated within a small patch rather than over the entire torus ofcircularized material surrounding the black hole. We suggest that thepatch corresponds to the ``bright spot'' where the stream of returningdebris impacts the torus. Interestingly, although the peak luminosity ofthe flare was highly sub-Eddington, the peak flux from the bright spotwas close to the Eddington limit. We speculate on the implications ofthis result for observations of other flare events.
| Large-Amplitude X-Ray Outbursts from Galactic Nuclei: A Systematic Survey using ROSAT Archival Data
In recent years, luminous X-ray outbursts with variability amplitudes ashigh as ~400 have been serendipitously detected from a small number ofactive and inactive galaxies. These outbursts may result from the tidaldisruptions of stars by supermassive black holes, as well as accretiondisk instabilities. In order to place the first reliable constraints onthe rate of such outbursts in the universe and to test the stellar tidaldisruption hypothesis, we have performed a systematic and completesurvey for them by cross-correlating ROSAT All-Sky Survey (RASS) andpointed Position Sensitive Proportional Counter data. We have detectedfive galaxies that were in outburst during the RASS, three of which showno signs of nuclear activity; these objects had been reported onindividually in previous studies. After making reasonable correctionsfor the complicated selection effects, we conclude that the rate oflarge-amplitude X-ray outbursts from inactive galaxies in the localuniverse is ~9.1×10-6 galaxy-1yr-1. This rate is consistent with the predicted rate ofstellar tidal disruption events in such galaxies. When only the twoactive galaxies are considered, we find a rate for active galaxies of~8.5×10-4 galaxy-1 yr-1. In orderto place tighter constraints on these rates, additional outbursts mustbe detected.
| 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.
| The Giant X-Ray Flare of NGC 5905: Tidal Disruption of a Star, a Brown Dwarf, or a Planet?
We model the 1990 giant X-ray flare of the quiescent galaxy NGC 5905 asthe tidal disruption of a star by a supermassive black hole. From theobserved rapid decline of the luminosity, over a timescale of a fewyears, we argue that the flare was powered by the fallback of debrisrather than subsequent accretion via a thin disk. The fallback modelallows constraints to be set on the black hole mass and the mass ofdebris. The latter must be very much less than a solar mass to explainthe very low luminosity of the flare. The observations can be explainedeither as the partial stripping of the outer layers of a low-mass mainsequence star or as the disruption of a brown dwarf or a giant planet.We find that the X-ray emission in the flare must have originated withina small patch rather than over the entire torus of circularized materialsurrounding the black hole. We suggest that the patch corresponds to the``bright spot'' where the stream of returning debris impacts the torus.Interestingly, although the peak luminosity of the flare was highlysub-Eddington, the peak flux from the bright spot was close to theEddington limit. We speculate on the implications of this result forobservations of other flare events.
| Follow-Up STIS Spectroscopy of Three Candidate Tidal Disruption Events
Dormant, supermassive black holes, suspected to be present in thecenters of normal galaxies, should reveal themselves by a UV/X-ray flarewhen they tidally disrupt a star and some fraction of the tidal debrisis accreted. Such an event is very rare in the nucleus of a galaxy(10-4 /yr); however, the ROSAT All-Sky Survey in 1990-91 wasan ideal experiment to detect these flares since it sampled hundreds ofthousands of galaxies in the soft X-ray band. Several flares weredetected by ROSAT in galaxies with no evidence for AGN activity in theirground-based optical spectra. These large amplitude X-ray flares had theproperties predicted for a tidal disruption event: a soft X-rayspectrum, time-scale of months, and a large X-ray luminosity(1042 to 1044 erg/s). In order to evaluate thealternative hypothesis that the flares could have been some form ofextreme AGN variability, we obtained follow-up optical spectroscopy ofthree of these flaring galaxies a decade later with the HST STIS and anarrow slit to search for or place stringent limits on the presence ofany permanent Seyfert-like emission in the their nuclei. Two of thegalaxies, RXJ1624.9+7554 and RXJ1242.6-1119, show no evidence foremission lines or non-stellar continuum in their HST nuclear spectra,consistent with their ground-based classification as inactive galaxies.NGC 5905, previously known as a starburst HII galaxy due to it strongemission lines, has in its inner 0.1 arcseconds a nucleus with narrowemission-line ratios consistent with a Seyfert 2 galaxy in thediagnostic diagrams of Veilleux and Osterbrock. The weak Seyfert nucleusin NGC 5905, which was masked by the many surrounding H II regions inground-based spectra, raises questions about the nature of its X-rayflare.
| Supernovae in isolated galaxies, in pairs and in groups of galaxies
In order to investigate the influence of environment on supernova (SN)production, we have performed a statistical investigation of the SNediscovered in isolated galaxies, in pairs and in groups of galaxies. 22SNe in 18 isolated galaxies, 48 SNe in 40 galaxy members of 37 pairs and211 SNe in 170 galaxy members of 116 groups have been selected andstudied. We found that the radial distributions of core-collapse SNe ingalaxies located in different environments are similar, and consistentwith those reported by Bartunov, Makarova & Tsvetkov. SNe discoveredin pairs do not favour a particular direction with respect to thecompanion galaxy. Also, the azimuthal distributions inside the hostmembers of galaxy groups are consistent with being isotropics. The factthat SNe are more frequent in the brighter components of the pairs andgroups is expected from the dependence of the SN rates on the galaxyluminosity. There is an indication that the SN rate is higher in galaxypairs compared with that in groups. This can be related to the enhancedstar formation rate in strongly interacting systems. It is concludedthat, with the possible exception of strongly interacting systems, theparent galaxy environment has no direct influence on SN production.
| A complete sample of Seyfert galaxies selected at 0.25 keV
We have used the ROSAT Bright Source Catalogue to extract a completesample of sources selected in the band from 0.1-0.4keV. This0.25keV-selected sample is composed of 54 Seyfert galaxies, 25 BLLacertae objects, 4 clusters and 27 Galactic stars or binaries.Seyfert-type galaxies with `ultrasoft' X-ray spectra can very often beclassed optically as narrow-line Seyfert 1s (NLS1s). Such objects arereadily detected in 0.25keV surveys; the sample reported here contains20 NLS1s, corresponding to a 40 per cent fraction of the Seyferts.Optical spectra of the Seyfert galaxies were gathered for correlativeanalysis, which confirmed the well-known relations between X-ray slopeand optical spectral properties (e.g. [Oiii]/Hβ ratio; Feiistrength, Hβ width). The various intercorrelations are most likelydriven, fundamentally, by the shape of the photoionizing continuum inSeyfert nuclei. We argue that a steep X-ray spectrum is a betterindicator of an `extreme' set of physical properties in Seyfert galaxiesthan is the narrowness of the optical Hβ line. The correlationstudies were also used to isolate a number of Seyfert galaxies withapparently `anomalous' properties. Of particular interest are the sixobjects with relatively weak permitted line emission (Hβ and Feii)and weak optical continua. Such objects are rare in most surveys, buttwo of these (IC 3599 and NGC 5905) are known to be transient activegalactic nuclei in which the X-ray flux has faded by factors ~100. Ifthe other four objects also turn out to be transient, this woulddemonstrate that 0.25keV surveys provide an efficient way of finding aninteresting class of object. Finally, the luminosity function of the0.25keV-selected Seyfert galaxies was determined and broken down intosubsamples to investigate the relative space densities of Seyferts whenseparated on the basis of either X-ray slope or Hβ linewidth.
| High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample - II. The properties of compact radio components
We discuss the properties of compact nuclear radio components in Seyfertgalaxies from the extended 12-μm AGN sample of Rush et al. Our mainresults can be summarized as follows. Type 1 and type 2 Seyferts producecompact radio components which are indistinguishable in strength andaspect, indicating that their central engines are alike, as proposed bythe unification model. Infrared IRAS fluxes are more closely correlatedwith low-resolution radio fluxes than high-resolution radio fluxes,suggesting that they are dominated by kiloparsec-scale, extranuclearemission regions; extranuclear emission may be stronger in type 2Seyferts. Early-type Seyfert galaxies tend to have stronger nuclearradio emission than late-type Seyfert galaxies. V-shaped extendedemission-line regions, indicative of `ionization cones', are usuallyfound in sources with large, collimated radio outflows. Hidden broadlines are most likely to be found in sources with powerful nuclear radiosources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm fluxdensities have well-matched properties.
| A Method of Obtaining the Pitch Angle of Spiral Arms and the Inclination of Galactic Discs
We investigate the mathematical form, the symmetry of spiral structureand the projected images of galactic discs. The measured pitch angles ofspiral arms and inclination angles of galactic discs for 60 spiralgalaxies are presented. The global spiral structure is emphasized in thestudy. It is found that, except for small-scale distortions, the spiralarms of those galaxies that were classified as AC 12 in the armclassification system of Elmegreen & Elmegreen, can be representedby the logarithmic spiral form.
| A Dust-penetrated Classification Scheme for Bars as Inferred from Their Gravitational Force Fields
The division of galaxies into ``barred'' (SB) and ``normal'' (S) spiralsis a fundamental aspect of the Hubble galaxy classification system. This``tuning fork'' view was revised by de Vaucouleurs, whose classificationvolume recognized apparent ``bar strength'' (SA, SAB, SB) as acontinuous property of galaxies called the ``family.'' However, the SA,SAB, and SB families are purely visual judgments that can have littlebearing on the actual bar strength in a given galaxy. Until veryrecently, published bar judgments were based exclusively on blue lightimages, where internal extinction or star formation can either mask abar completely or give the false impression of a bar in a nonbarredgalaxy. Near-infrared camera arrays, which principally trace the oldstellar population in both normal and barred galaxies, now facilitate aquantification of bar strength in terms of their gravitationalpotentials and force fields. In this paper, we show that the maximumvalue, Qb, of the ratio of the tangential force to the meanaxisymmetric radial force in a barred disk galaxy is a quantitativemeasure of the strength of a bar. Qb does not measure barellipticity or bar shape but rather depends on the actual forcing due tothe bar embedded in its disk. We show that a wide range of true barstrengths characterizes the category ``SB,'' while the de Vaucouleurscategory ``SAB'' corresponds to a narrower range of bar strengths. Wepresent Qb values for 36 galaxies, and we incorporate our barclasses into a dust-penetrated classification system for spiralgalaxies.
| The Frequency of Active and Quiescent Galaxies with Companions: Implications for the Feeding of the Nucleus
We analyze the idea that nuclear activity, either active galactic nuclei(AGNs) or star formation, can be triggered by interactions by studyingthe percentage of active, H II, and quiescent galaxies with companions.Our sample was selected from the Palomar survey and avoids selectionbiases faced by previous studies. This sample was split into fivedifferent groups, Seyfert galaxies, LINERs, transition galaxies, H IIgalaxies, and absorption-line galaxies. The comparison between the localgalaxy density distributions of the different groups showed that in mostcases there is no statistically significant difference among galaxies ofdifferent activity types, with the exception that absorption-linegalaxies are seen in higher density environments, since most of them arein the Virgo Cluster. The comparison of the percentage of galaxies withnearby companions showed that there is a higher percentage of LINERs,transition galaxies, and absorption-line galaxies with companions thanSeyfert and H II galaxies. However, we find that when we consider onlygalaxies of similar morphological types (elliptical or spiral), there isno difference in the percentage of galaxies with companions amongdifferent activity types, indicating that the former result was due tothe morphology-density effect. In addition, only small differences arefound when we consider galaxies with similar Hα luminosities. Thecomparison between H II galaxies of different Hα luminositiesshows that there is a significantly higher percentage of galaxies withcompanions among H II galaxies with L(Hα)>1039 ergss-1 than among those with L(Hα)<=1039ergs s-1, indicating that interactions increase the amount ofcircumnuclear star formation, in agreement with previous results. Thefact that we find that galaxies of different activity types have thesame percentage of companions suggests that interactions betweengalaxies is not a necessary condition to trigger the nuclear activity inAGNs. We compare our results with previous ones and discuss theirimplications.
| Deep-Imaging Observations of a Candidate of an Absorbed QSO at z=0.653, AX J131831+3341
The results of deep-imaging observations of a candidate of an absorbedQSO at z=0.653, AX J131831+3341, are presented. AX J131831+3341 wasfound during the course of optical follow-up observations of the ASCALarge Sky Survey, and has an X-ray luminosity of 1045 ergs-1 (2-10 keV), which corresponds to those of QSOs. Itsoptical spectrum shows no significant broad Hβ emission line,suggesting that the object is an absorbed QSO. Deep R and V band imagesreveal the presence of a point-like nucleus and an asymmetric extendedcomponent. The nuclear component has a blue color, and the opticalmagnitude is much fainter than that expected from the observed X-rayflux for typical type-1 AGNs. These photometric properties and thepresence of broad Mg II 2800 { Angstroms} emission can be explainedsimultaneously if the observed nuclear light is dominated by scatterednuclear light, though there is a possibility that the nuclear componentis a slightly absorbed nucleus if its intrinsic X-ray to optical fluxratio is the largest among X-ray selected AGNs. The size of the extendedcomponent, which is thought to be the host galaxy of the QSO, is largerthan those of normal disk galaxies at z=0-0.75, and the absolutemagnitude is similar to those of the brightest host galaxies of QSOs atredshifts smaller than 0.5. The V-R and R-I colors of the component areconsistent with a 1 Gyr-old stellar population model without absorption.
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