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Possible Supernova in NGC 4656
IAUC 8498 available at Central Bureau for Astronomical Telegrams.

Possible Supernova in NGC 4656
IAUC 8497 available at Central Bureau for Astronomical Telegrams.

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.

Star Formation Properties of a Large Sample of Irregular Galaxies
We present the results of Hα imaging of a large sample ofirregular galaxies. Our sample includes 94 galaxies with morphologicalclassifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems.The sample spans a large range in galactic parameters, includingintegrated absolute magnitude (MV of -9 to -19), averagesurface brightness (20-27 mag arcsec-2), current starformation activity (0-1.3 Msolar yr-1kpc-2), and relative gas content(0.02-5Msolar/LB). The Hα images were usedto measure the integrated star formation rates, determine the extents ofstar formation in the disks, and compare azimuthally averaged radialprofiles of current star formation to older starlight. The integratedstar formation rates of Im galaxies normalized to the physical size ofthe galaxy span a range of a factor of 104 with 10% Imgalaxies and one Sm system having no measurable star formation at thepresent time. The BCDs fall, on average, at the high star formation rateend of the range. We find no correlation between star formation activityand proximity to other cataloged galaxies. Two galaxies located in voidsare similar in properties to the Sm group in our sample. The H IIregions in these galaxies are most often found within the Holmbergradius RH, although in a few systems H II regions are tracedas far as 1.7RH. Similarly, most of the star formation isfound within three disk scale lengths RD, but in somegalaxies H II regions are traced as far as 6RD. A comparisonof Hα surface photometry with V-band surface photometry shows thatthe two approximately follow each other with radius in Sm galaxies, butin most BCDs there is an excess of Hα emission in the centers thatdrops with radius. In approximately half of the Im galaxies Hα andV correspond well, and in the rest there are small to large differencesin the relative rate of falloff with radius. The cases with stronggradients in the LHα/LV ratios and with highcentral star formation rate densities, which include most of the BCDs,require a significant fraction of their gas to migrate to the center inthe last gigayear. We discuss possible torques that could have causedthis without leaving an obvious signature, including dark matter barsand past interactions or mergers with small galaxies or H I clouds.There is now a substantial amount of evidence for these processes amongmany surveys of BCDs. We note that such gas migration will also increasethe local pressure and possibly enhance the formation of massive denseclusters but conclude that the star formation process itself does notappear to differ much among BCD, Im, and Sm types. In particular, thereis evidence in the distribution function for Hα surface brightnessthat the turbulent Mach numbers are all about the same in these systems.This follows from the Hα distribution functions corrected forexponential disk gradients, which are log-normal with a nearly constantdispersion. Thus, the influence of shock-triggered star formation isapparently no greater in BCDs than in Im and Sm types.

Cold dust in a selected sample of nearby galaxies. I. The interacting galaxy NGC 4631
We have observed the continuum emission of the interacting galaxy NGC4631 at λλ 870 μm and 1.23 mm using theHeinrich-Hertz-Telescope on Mt. Graham and the IRAM 30-m telescope onPico Veleta. We have obtained fully sampled maps which cover the opticalemission out to a radius of about 7' at both wavelengths. For a detailedanalysis, we carefully subtracted the line contributions and synchrotronand free-free emission from the data, which added up to 6% at 1.23 mmand 10% at 0.87 mm. We combined the flux densities with FIR data toobtain dust spectra and calculate dust temperatures, absorption crosssections, and masses. Assuming a ``standard'' dust model, which consistsof two populations of big grains at moderate and warm temperatures, weobtained temperatures of 18 K and 50 K for the both components. However,such a model suffers from an excess of the radiation at λ 1.23mm, and the dust absorption cross section seems to be enhanced by afactor 3 compared to previous results and theoretiexpectations. At largegalactocentric radii, where the galaxy shows disturbances as a result ofgravitational interaction, this effect seems to be even stronger. Somepossibilities to resolve these problems are discussed. The data could beexplained by a very cold dust component at a temperature of 4-6 K, anincreased abundance of very small grains, or a component of grains withunusual optical properties. We favour the latter possibility, since thefirst two lead to inconsistencies

Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.

Extraplanar Emission-Line Gas in Edge-On Spiral Galaxies. I. Deep Emission-Line Imaging
The extraplanar diffuse ionized gas (eDIG) in 17 nearby, edge-on diskgalaxies is studied using deep Taurus Tunable Filter Hα and [N II]λ6583 images and conventional interference filter Hα+[N II]λλ6548, 6583 images that reach flux levels generally below~1×10-17 ergs s-1 cm-2arcsec-2. [N II] λ6583/Hα excitation maps areavailable for 10 of these objects. All but one galaxy in the sampleexhibit eDIG. The contribution of the eDIG to the total Hαluminosity is relatively constant, on the order of 12%+/-4%. TheHα scale height of the eDIG derived from a two-exponential fit tothe vertical emission profile ranges from 0.4 to 17.9 kpc, with anaverage of 4.3 kpc. This average value is noticeably larger than theeDIG scale height measured in our Galaxy and other galaxies. Thisdifference in scale height is probably due in part to the lower fluxlimits of our observations. The ionized mass of the extraplanarcomponent inferred by assuming a constant filling factor of 0.2 and aconstant path length through the disk of 5 kpc ranges from1.4×107 to 2.4×108 Msolar,with an average value of 1.2×108 Msolar.Under these same assumptions, the recombination rate required to keepthe eDIG ionized ranges from 0.44×106 to13×106 s-1 cm-2 of the disk, orabout 10%-325% of the Galactic value. A quantitative analysis of thetopology of the eDIG confirms that several galaxies in the sample have ahighly structured eDIG morphology. The distribution of the eDIG emissionis often correlated with the locations of the H II regions in the disk,supporting the hypothesis that the predominant source of ionization ofthe eDIG is photoionization from OB stars located in the H II regions. Astrong correlation is found between the IR (or far-IR) luminosities perunit disk area (basically a measure of the star formation rate per unitdisk area) and the extraplanar ionized mass, further providing supportfor a strong connection between the disk and eDIG components in thesegalaxies. The excitation maps confirm that the [N II]/Hα ratiosare systematically higher in the eDIG than in the disk. Althoughphotoionization by disk OB stars is generally able to explain theseelevated [N II]/Hα ratios, a secondary source of ionizationappears to be needed when one also takes into account other line ratios;more detail is given in a companion paper (our Paper II).

A Survey for H2O Megamasers. III. Monitoring Water Vapor Masers in Active Galaxies
We present single-dish monitoring of the spectra of 13 extragalacticwater megamasers taken over a period of 9 years and a single epoch ofsensitive spectra for seven others. The primary motivation is a searchfor drifting line velocities analogous to those of the systemic featuresin NGC 4258, which are known to result from centripetal acceleration ofgas in an edge-on, subparsec molecular disk. We detect a velocity driftanalogous to that in NGC 4258 in only one source, NGC 2639. Another, themaser source in NGC 1052, exhibits erratic changes in its broad maserprofile over time. Narrow maser features in all of the other diskgalaxies discussed here either remain essentially constant in velocityover the monitoring period or are sufficiently weak or variable inintensity that individual features cannot be traced reliably from oneepoch to the next. In the context of a circumnuclear, molecular diskmodel, our results suggest that either (a) the maser lines seen aresystemic features subject to a much smaller acceleration than present inNGC 4258, presumably because the gas is farther from the nuclear blackhole, or (b) we are detecting ``satellite'' lines for which theacceleration is in the plane of the sky.Our data include the first K-band science observations taken with thenew 100 m Green Bank Telescope (GBT). The GBT data were taken duringtesting and commissioning of several new components and so are subjectto some limitations; nevertheless, they are in most cases the mostsensitive H2O spectra ever taken for each source and cover800 MHz (~=10,800 km s-1) of bandwidth. Many new maserfeatures are detected in these observations. Our data also include atentative and a clear detection of the megamaser in NGC 6240 at epochs ayear and a few months, respectively, prior to the detections reported byHagiwara et al. and Nakai et al.We also report a search for water vapor masers toward the nuclei of 58highly inclined (i>80deg), nearby galaxies. These sourceswere selected to investigate the tendency that H2O megamasersfavor inclined galaxies. None were detected, confirming that megamasersare associated exclusively with active galactic nuclei.

Probing O VI Emission in the Halos of Edge-on Spiral Galaxies
We have used the Far Ultraviolet Spectroscopic Explorer to search for OVI λλ1031.926, 1037.617 emission in the halos of theedge-on spiral galaxies NGC 4631 and NGC 891. In NGC 4631, we detected OVI in emission toward a soft X-ray bubble above a region containingnumerous Hα arcs and filaments. The line-of-sight component of themotion of the O VI gas appears to match the underlying disk rotation.The observed O VI luminosities can account for 0.2%-2% of the totalenergy input from supernovae (assuming a full O VI- emitting halo) andyield mass flux cooling rates between 0.48 and 2.8 Msolaryr-1 depending on the model used in the derivations. On thebasis of these findings, we believe it is likely that we are seeingcooling, galactic fountain gas. No emission was detected from the haloof NGC 891, a galaxy in a direction with considerably high foregroundGalactic extinction.

The 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 Origin of the Dust Arch in the Halo of NGC 4631: An Expanding Superbubble?
We study the nature and the origin of the dust arch in the halo of theedge-on galaxy NGC 4631 detected by Neininger & Dumke. We present COobservations made using the new on-the-fly mapping mode with the FiveCollege Radio Astronomy Observatory 14 m telescope and find no evidencefor CO emission associated with the dust arch. Our examination ofpreviously published H I data shows that, if previous assumptions aboutthe dust temperature and gas/dust ratio are correct, then there must bemolecular gas associated with the arch, below our detection threshold.If this is true, then the molecular mass associated with the dust archis between 1.5×108 Msolar and9.7×108 Msolar, and likely toward the lowend of the range. A consequence of this is that the maximum allowedvalue for the CO-to-H2 conversion factor is 6.5 times theGalactic value, but most likely closer to the Galactic value. Thekinematics of the H I apparently associated with the dust arch revealsthat the gas here is not part of an expanding shell or outflow but isinstead two separate features (a tidal arm and a plume of H I stickingout into the halo) that are seen projected together and appear as ashell. Thus there is no connection between the dust ``arch'' and the hotX-ray-emitting gas that appears to surround the galaxy.

A search for Low Surface Brightness galaxies in the near-infrared. I. Selection of the sample
A sample of about 3800 Low Surface Brightness (LSB) galaxies wasselected using the all-sky near-infrared (J, H and Ks-band)2MASS survey. The selected objects have a mean central surfacebrightness within a 5'' radius around their centre fainter than 18 magarcsec-2 in the Ks band, making them the lowestsurface brightness galaxies detected by 2MASS. A description is given ofthe relevant properties of the 2MASS survey and the LSB galaxy selectionprocedure, as well as of basic photometric properties of the selectedobjects. The latter properties are compared to those of other samples ofgalaxies, of both LSBs and ``classical'' high surface brightness (HSB)objects, which were selected in the optical. The 2MASS LSBs have aBT_c-KT colour which is on average 0.9 mag bluerthan that of HSBs from the NGC. The 2MASS sample does not appear tocontain a significant population of red objects.All tables and Figs. 2a-c are only available in electronic form athttp://www.edpsciences.org

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 ( or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5

Internal Variation of Electron Density in Galactic and Extragalactic HII Regions
Not Available

ROSAT Blank Field Sources. I. Sample Selection and Archival Data
We have identified a population of ``blank field sources'' (or``blanks'') among the ROSAT bright unidentified X-ray sources with faintoptical counterparts. The extreme X-ray over optical flux ratio ofblanks is not compatible with the main classes of X-ray emitters exceptfor extreme BL Lacertae objects. From the analysis of ROSAT archivaldata we found no indication of variability, and evidence for only threesources, out of 16, needing absorption in excess of the Galactic value.We also found evidence for an extended nature for only one of the fiveblanks with a serendipitous HRI detection; this source (1WGAJ1226.9+3332) was confirmed as a z=0.89 cluster of galaxies. Palomarimages reveal the presence of a red (O-E>= 2) counterpart in theX-ray error circle for six blanks. The identification process brought tothe discovery of another high-z cluster of galaxies, one (possiblyextreme) BL Lac, two ultraluminous X-ray sources in nearby galaxies, andtwo apparently normal type 1 active galactic nuclei (AGNs). These AGNs,together with four more AGN-like objects, seem to form a well-definedgroup: they present unabsorbed X-ray spectra but red Palomarcounterparts. We discuss the possible explanations for the discrepancybetween the X-ray and optical data, among which are a suppressed bigblue bump emission, an extreme dust-to-gas ratio (~40-60 times theGalactic ratio), a high-redshift (z>=3.5) quasar nature, an atypicaldust grain size distribution, and a dusty warm absorber. These AGN-likeblanks seem to be the bright (and easier to study) analogs of thesources that are found in deep Chandra observations. Three more blanksstill have an unknown nature.

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.

An Infrared Space Observatory Atlas of Bright Spiral Galaxies
In this first paper in a series we present an atlas of infrared imagesand photometry from 1.2 to 180 μm for a sample of bright spiralgalaxies. The atlas galaxies are an optically selected,magnitude-limited sample of 77 spiral and S0 galaxies chosen from theRevised Shapley-Ames Catalog (RSA). The sample is a representativesample of spiral galaxies and includes Seyfert galaxies, LINERs,interacting galaxies, and peculiar galaxies. Using the Infrared SpaceObservatory (ISO), we have obtained 12 μm images and photometry at60, 100, and 180 μm for the galaxies. In addition to its imagingcapabilities, ISO provides substantially better angular resolution thanis available in the IRAS survey, and this permits discrimination betweeninfrared activity in the central regions and global infrared emission inthe disks of these galaxies. These ISO data have been supplemented withJHK imaging using ground-based telescopes. The atlas includes 2 and 12μm images. Following an analysis of the properties of the galaxies,we have compared the mid-infrared and far-infrared ISO photometry withIRAS photometry. The systematic differences we find between the IRASFaint Source Catalog and ISO measurements are directly related to thespatial extent of the ISO fluxes, and we discuss the reliability of IRASFaint Source Catalog total flux densities and flux ratios for nearbygalaxies. In our analysis of the 12 μm morphological features we findthat most but not all galaxies have bright nuclear emission. We find 12μm structures such as rings, spiral arm fragments, knotted spiralarms, and bright sources in the disks that are sometimes brighter thanthe nuclei at mid-infrared wavelengths. These features, which arepresumably associated with extranuclear star formation, are common inthe disks of Sb and later galaxies but are relatively unimportant inS0-Sab galaxies. Based on observations with the Infrared SpaceObservatory (ISO), an ESA project with instruments funded by ESA MemberStates (especially the PI countries: France, Germany, Netherlands, andUnited Kingdom) and with the participation of ISAS and NASA.

The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.

The Westerbork HI survey of spiral and irregular galaxies. II. R-band surface photometry of late-type dwarf galaxies
R-band surface photometry is presented for 171 late-type dwarf andirregular galaxies. For a subsample of 46 galaxies B-band photometry ispresented as well. We present surface brightness profiles as well asisophotal and photometric parameters including magnitudes, diameters andcentral surface brightnesses. Absolute photometry is accurate to 0.1 magor better for 77% of the sample. For over 85% of the galaxies the radialsurface brightness profiles are consistent with published data withinthe measured photometric uncertainty. For most of the galaxies in thesample H I data have been obtained with the Westerbork Synthesis RadioTelescope. The galaxies in our sample are part of the WHISP project(Westerbork H I Survey of Spiral and Irregular Galaxies), which aims atmapping about 500 nearby spiral and irregular galaxies in H I. Theavailability of H I data makes this data set useful for a wide range ofstudies of the structure, dark matter content and kinematics oflate-type dwarf galaxies. Based on observations made with INT operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias. The tables in Appendix A are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/863. Thefigures in Appendix B are only available in electronic formhttp://www.edpsciences.org

The SCUBA Local Universe Galaxy Survey - II. 450-μm data: evidence for cold dust in bright IRAS galaxies
This is the second in a series of papers presenting results from theSCUBA Local Universe Galaxy Survey. In our first paper we provided850-μm flux densities for 104 galaxies selected from the IRAS BrightGalaxy Sample and we found that the 60-, 100-μm (IRAS) and 850-μm(SCUBA) fluxes could be adequately fitted by emission from dust at asingle temperature. In this paper we present 450-μm data for thegalaxies. With the new data, the spectral energy distributions of thegalaxies can no longer be fitted with an isothermal dust model - twotemperature components are now required. Using our 450-μm data andfluxes from the literature, we find that the 450/850-μm flux ratiofor the galaxies is remarkably constant, and this holds from objects inwhich the star formation rate is similar to our own Galaxy, toultraluminous infrared galaxies (ULIRGs) such as Arp 220. The onlypossible explanation for this is if the dust emissivity index for all ofthe galaxies is ~2 and the cold dust component has a similar temperaturein all galaxies [formmu3](Tc~20-21K). The 60-μmluminosities of the galaxies were found to depend on both the dust massand the relative amount of energy in the warm component, with a tendencyfor the temperature effects to dominate at the highest L60.The dust masses estimated using the new temperatures are higher by afactor of ~2 than those determined previously using a singletemperature. This brings the gas-to-dust ratios of the IRAS galaxiesinto agreement with those of the Milky Way and other spiral galaxieswhich have been intensively studied in the submm.

The rise and rise of the deep sky image
Presidential Address to the British Astronomical Association, 2000October 25

Far-Ultraviolet Imagery of the Edge-on Spiral Galaxy NGC 4631
Far-ultraviolet (FUV) imagery of the edge-on, Sc/SBd galaxy NGC 4631reveals very strong FUV emission, resulting from active star formation,uniformly distributed along the galactic midplane. Multiband imagery, HI and H II position-velocity curves, and extinction considerations allimply that the emission is from the outer edges of the visible galaxy.The overall FUV morphology of this edge-on disk system is remarkablysimilar to those of the ``chain galaxies'' evident at high redshift,thus suggesting a similar interpretation for at least some of thosedistant objects. FUV, U, B, and V magnitudes, measured for 48star-forming regions, along with corresponding Hα and Hβmeasurements are used to construct diagnostic color-color diagrams.Although there are significant exceptions, most of the star-formingregions are less massive and older than 30 Doradus. Comparison with theexpectations from two star formation models yields ages of 2.7 to 10 Myrfor the instantaneous burst (IB) model and star formation cutoff ages of0 to 9 Myr for the continuous star formation (CSF) model. Interpreted interms of the IB model the photometry implies a total created mass in the48 star-forming regions of 2.5×107 Msolar.When viewed as resulting from constant star formation the photometryimplies a star formation rate of 0.33 Msolar yr-1.These results are compared to those derived from FIR and radioobservations. Corrections for FUV emission reprocessed by interstellargrains are estimated. A large ring, ~3 kpc in diameter, of 14star-forming regions is concentrically located with an expanding H Ishell toward the eastern end of the galaxy. Our observations imply thatthe shell may have been generated primarily by supernovae arising from5.3×104 OB stars in a massive star-forming regionbeginning about 20 Myr ago, and that the presently observed FUV brightemission is due to second generation stars.

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.

A ROSAT High Resolution Imager survey of bright nearby galaxies
We use the extensive public archive of ROSAT High Resolution Imager(HRI) observations to carry out a statistical investigation of the X-rayproperties of nearby galaxies. Specifically we focus on the sample of486 bright (BT<=12.5) northern galaxies studied by Ho,Filippenko and Sargent (HFS) in the context of their exploration of theoptical spectroscopic properties of nearby galactic nuclei. Over 20percent of HFS galaxies are encompassed in ROSAT HRI fields of reasonable(>=10ks) exposure. The X-ray sources detected within the opticalextent of each galaxy are categorized as either nuclear or non-nuclear,depending on whether the source is positioned within or outside a25-arcsec radius circle centred on the optical nucleus. A nuclear X-raysource is detected in over 70per cent of the galaxies harbouring eithera Seyfert or LINER nucleus, compared with a detection rate of only~40per cent in less active systems. The correlation of the Hαluminosity with nuclear X-ray luminosity previously observed in QSOs andbright Seyfert 1 galaxies appears to extend down into the regime ofultra-low luminosity(LX~1038-1040ergs-1) activegalactic nuclei (AGN). The inferred accretion rates for this sample oflow-luminosity AGN are significantly sub-Eddington. In total, 142non-nuclear sources were detected. In combination with published datafor M31, this leads to a luminosity distribution (normalized to anoptical blue luminosity of 1010Lsolar) for thediscrete X-ray source population in spiral galaxies of the formdN/dL38 = (1.0 ± 0.2)L-1.838where L38 is the X-ray luminosity in units of1038ergs-1. The implied LXLBratio is ~1.1×1039ergs-1(1010Lsolar)-1. The nature ofthe substantial number of `superluminous' non-nuclear objects detectedin the survey is discussed.

Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.

Interferometric 12CO Observations of the Central Disk of NGC 4631: An Energetic Molecular Outflow
We present interferometric observations of CO J=1-0 emission in thecentral regions of the edge-on galaxy NGC 4631, known for its extendedgaseous halo and its tidal interactions. Previous single-dishobservations revealed that almost all of the CO emission arises from acentral ring or barlike structure of length ~4 kpc. We confirm thisstructure at higher resolution and find that it is bent at the center,reflecting the overall bend in this galaxy apparent from optical images.The kinematic evidence favors a rigidly rotating ring over a bar. Thegaseous halo emission in several tracers is concentrated above and belowthis molecular structure. To the north of an emission peak at theeastern end of the structure is an extraplanar feature showingfilamentary and shell-like properties we interpret as an energeticmolecular outflow. The energies involved are difficult to estimate butare probably of order 1054 ergs or more. The CO concentrationin the disk below this structure coincides with a bright H II regioncomplex, a peak of radio emission, and the brightest X-ray feature inthe inner disk of the galaxy seen in a ROSAT HRI map, all suggestingintense star formation. A filament of radio continuum emission may alsohave a footprint in this region of the disk. The origin of the outflowis unclear.

The Neutral Hydrogen Distribution in Merging Galaxies: Differences between Stellar and Gaseous Tidal Morphologies
As part of several H I synthesis-mapping studies of merging galaxies, wehave mapped the tidal gas in the three disk-disk merger systems Arp 157(NGC 520), Arp 220, and Arp 299 (NGC 3690). These systems differ fromthe majority of the mergers mapped in H I in that their stellar andgaseous tidal features do not coincide. In particular, they exhibitlarge stellar tidal features with little if any accompanying neutral gasand large gas-rich tidal features with little if any accompanyingstarlight. On smaller scales, there are striking anticorrelations inwhich the gaseous and stellar tidal features appear to cross. We exploreseveral possible causes for these differences, including dustobscuration, ram pressure stripping, and ionization effects. No singleexplanation can account for all of the observed differences. The factthat each of these systems shows evidence for a starburst-drivensuperwind expanding in the direction of the most strikinganticorrelations leads us to suggest that the superwind is primarilyresponsible for the observed differences, either by sweeping thefeatures clear of gas via ram pressure or by excavating a clearsightline toward the starburst and allowing UV photons to ionize regionsof the tails. If this suggestion is correct, only systems hosting agalactic superwind and experiencing a high-inclination encountergeometry (such that tidal gas is lifted high above the starburstregions) should exhibit such extreme differences between their H I andoptical tidal morphologies.

X-ray observations of the starburst galaxy NGC 253 --- II. Extended emission from hot gas in the nuclear area, disk, and halo
Spatial and spectral analysis of deep ROSAT HRI and PSPC observations ofthe near edge-on starburst galaxy NGC 253 reveal diffuse soft X-rayemission, which contributes 80% to its total X-ray luminosity(LX = 5 1039 erg s-1, corrected forforeground absorption). The nuclear area, disk, and halo contribution tothe luminosity is about equal. The starburst nucleus itself is highlyabsorbed and not visible in the ROSAT band. The emission from thenuclear area stems from a heavily absorbed source with an extent of 250pc (FWHM) about 100 pc above the nucleus along the SE minor axis("nuclear source", X34), and the "X-ray plume". The nuclear source isbest described as having a thermal bremsstrahlung spectrum with atemperature of T = 1.2 keV (NH = 3 1021cm-2) and LXexgal = 3 1038erg s-1 (corrected for Galactic foreground absorption). Thespectrum of the hollow-cone shaped plume (opening angle of 32̂ andextent of ~ 700 pc along the SE minor axis) is best modeled by acomposite of a thermal bremsstrahlung (NH = 3 1020cm-2, T = 1.2 keV, LXexgal = 4.61038 erg s-1) and a thin thermal plasma (Galacticforeground absorption, T = 0.33 keV, LXexgal = 41038 erg s-1). The diffuse nuclear emissioncomponents trace interactions between the galactic super-wind emitted bythe starburst nucleus, and the dense interstellar medium of the disk.Diffuse emission from the disk is heavily absorbed and follows thespiral structure. It can be described by a thin thermal plasma spectrum(T = 0.7 keV, intrinsic luminosity LXintr = 1.21039 erg s-1), and most likely reflects a mixtureof sources (X-ray binaries, supernova remnants, and emission from H IIregions) and the hot interstellar medium. The surface brightness profilereveals a bright inner and a fainter outer component along the majoraxis with extents of ∓3.4 kpc and ∓7.5 kpc. We analysedthe total halo emission separated into two geometrical areas; the"corona" (scale height ~ 1 kpc) and the "outer halo". The coronalemission (T = 0.2 keV, LXintr = 7.81038 erg s-1) is only detected from the near sideof the disk (in the SE), emission from the back (in the NW) is shadowedby the intervening interstellar medium unambiguously determining theorientation of NGC 253 in space. In the NW we see the near edge of thedisk is seen, but the far component of the halo, and vice versa in theSE. The emission in the outer halo can be traced to projected distancesfrom the disk of 9 kpc, and shows a horn-like structure. Luminositiesare higher (10 and 5 1038 erg s-1, respectively)and spectra harder in the NW halo than in the SE. The emission in thecorona and outer halo is most likely caused by a strong galactic windemanating from the starburst nucleus. As an additional contribution tothe coronal emission floating on the disk like a spectacle-glass, wepropose hot gas fueled from galactic fountains originating within theboiling star-forming disk. A two temperature thermal plasma model withtemperatures of 0.13 and 0.62 keV or a thin thermal plasma model withtemperature of 0.15 keV and Gaussian components above ~0.7 keV andGalactic foreground absorption are needed to arrive at acceptable fitsfor the NW halo. This may be explained by starburst-driven super-windsor by effects of a non- equilibrium cooling function in a plasmaexpanding in fountains or winds. We compare our results to observationsat other wavelengths and from other galaxies.

Multicolor photometry and spectrophotometry of star-forming complexes in spiral and irregular galaxies for analyses of star-formation parameters. Data and reductions
Intermediate-band photometric measurements of star-forming complexes(SFCs) in six spiral and irregular galaxies are presented, and added topreviously published multicolor data to form a master database. The oldand new data have been reduced to a standard photometric system, and theaccuracy of various color-index measurements compared. A total of 928measurements of 569 SFCs in 49 galaxies are considered. The observedcolors of SFCs-giant HII regions-in external galaxies are compared withtheoretical (U_B)(B_V), LCI(U_B), and LCI(B_V) diagrams for a wide rangeof star-formation parameters (the observed colors were reduced to asingle photometric system and corrected for extinction). The presence ofobservational selection effects in the data sample is demonstrated. Thearea occupied by theoretical evolutionary tracks is consistent with theobserved distribution of colors for star-forming complexes.

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.

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Observation and Astrometry data

Constellation:Canes Venatici
Right ascension:12h43m58.20s
Aparent dimensions:9.12′ × 1.202′

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NGC 2000.0NGC 4656

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