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A post-mortem investigation of the Type IIb supernova 2001ig
We present images taken with the Gemini Multi-Object Spectrograph onGemini-South, in excellent (<0.5 arcsec) seeing, of supernova (SN)2001ig in NGC 7424, ~1000 d after explosion. A point source seen at thesite of the SN is shown to have colours inconsistent with being an HIIregion or a SN 1993J-like remnant, but can be matched to a late-Bthrough late-F supergiant with AV < 1. We believe thisobject is the massive binary companion responsible for periodicmodulation in mass-loss material around the Wolf-Rayet progenitor whichgave rise to significant structure in the SN radio light curve.
| An ultraluminous X-ray microquasar in NGC5408?
We studied the radio source associated with the ultraluminous X-raysource in NGC5408 (LX ~ 1040ergs-1).The radio spectrum is steep (index ~ -1), consistent with optically thinsynchrotron emission, not with flat-spectrum core emission. Its fluxdensity (~0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same inthe March 2000 and December 2004 observations, suggesting steadyemission rather than a transient outburst. However, it is orders ofmagnitude higher than expected from steady jets in stellar-massmicroquasar. Based on its radio flux and spectral index, we suggest thatthe radio source is either an unusually bright supernova remnant, or,more likely, a radio lobe powered by a jet from the black hole (BH).Moreover, there is speculative evidence that the source is marginallyresolved with a radius ~30 pc. A faint HII region of similar sizeappears to coincide with the radio and X-ray sources, but its ionizationmechanism remains unclear. Using a self-similar solution for theexpansion of a jet-powered electron-positron plasma bubble, in theminimum-energy approximation, we show that the observed flux and(speculative) size are consistent with an average jet power ~ 7 ×1038ergs-1 ~ 0.1LX ~0.1LEdd, an age ~105 yr, a current velocity ofexpansion ~80 km s-1. We briefly discuss the importance ofthis source as a key to understand the balance between luminosity andjet power in accreting BHs.
| The Survey for Ionization in Neutral Gas Galaxies. I. Description and Initial Results
We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG),a census of star formation in H I-selected galaxies. The survey consistsof Hα and R-band imaging of a sample of 468 galaxies selected fromthe H I Parkes All Sky Survey (HIPASS). The sample spans three decadesin H I mass and is free of many of the biases that affect otherstar-forming galaxy samples. We present the criteria for sampleselection, list the entire sample, discuss our observational techniques,and describe the data reduction and calibration methods. This paperfocuses on 93 SINGG targets whose observations have been fully reducedand analyzed to date. The majority of these show a single emission linegalaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs ina single field. All of the targets in this sample are detected inHα, indicating that dormant (non-star-forming) galaxies withMHI>~3×107 Msolar are veryrare. A database of the measured global properties of the ELGs ispresented. The ELG sample spans 4 orders of magnitude in luminosity(Hα and R band), and Hα surface brightness, nearly 3 ordersof magnitude in R surface brightness and nearly 2 orders of magnitude inHα equivalent width (EW). The surface brightness distribution ofour sample is broader than that of the Sloan Digital Sky Survey (SDSS)spectroscopic sample, the EW distribution is broader than prism-selectedsamples, and the morphologies found include all common types ofstar-forming galaxies (e.g., irregular, spiral, blue compact dwarf,starbursts, merging and colliding systems, and even residual starformation in S0 and Sa spirals). Thus, SINGG presents a superior censusof star formation in the local universe suitable for further studiesranging from the analysis of H II regions to determination of the localcosmic star formation rate density.
| Masses of Star Clusters in the Nuclei of Bulgeless Spiral Galaxies
In the last decade star clusters have been found in the centers ofspiral galaxies across all Hubble types. We here present a spectroscopicstudy of the exceptionally bright (106-108Lsolar) but compact (re~5 pc) nuclear starclusters in very late type spirals with the Ultraviolet and VisualEchelle Spectrograph at the VLT. We find that the velocity dispersionsof the nine clusters in our sample range from 13 to 34 kms-1. Using photometric data from the Hubble Space TelescopeWFPC2 and spherically symmetric dynamical models, we determine massesbetween 8×105 and 6×107Msolar. The mass-to-light ratios range from 0.2 to 1.5 in theI band. This indicates a young mean age for most clusters, in agreementwith previous studies. Given their high masses and small sizes, we findthat nuclear clusters are among the objects with the highest meansurface density known (up to 105 Msolarpc-2). From their dynamical properties we infer that, ratherthan small bulges, the closest structural kin of nuclear clusters appearto be massive compact star clusters. This includes such differentobjects as globular clusters, ``super star clusters,'' ultracompactdwarf galaxies (UCDs), and the nuclei of dwarf elliptical galaxies. Itis a challenge to explain why, despite the widely different currentenvironments, all different types of massive star clusters share verysimilar and structural properties. A possible explanation links UCDs andmassive globular clusters to nuclear star clusters through stripping ofnucleated dwarf galaxies in a merger event. The extreme properties ofthis type of cluster would then be a consequence of the clusters'location in the centers of their respective host galaxies.
| Discovery of a Solitary Dwarf Galaxy in the APPLES Survey
During the APPLES parallel campaign, the Hubble Space Telescope (HST)Advanced Camera for Surveys has resolved a distant stellar system, whichappears to be an isolated dwarf galaxy. It is characterized by acircularly symmetric distribution of stars with an integrated magnitudemF775W=20.13+/-0.02, a central surface brightnessμF775W~=21.33+/-0.18 mag arcsec-2, and ahalf-light radius of ~=1.8". The ACS and VLT spectra show no evidence ofionized gas and appear to be dominated by a 3 Gyr old stellarpopulation. The OB spectral type derived for two resolved stars in thegrism data and the systemic radial velocity (Vhel~=670 kms-1) measured from the VLT data give a fiducial distance of~=9+/-2 Mpc. These findings, with the support of the spatial morphology,would classify the system among the dwarf spheroidal (dSph) galaxies.Following IAU rules, we have named this newly discovered galaxy APPLES1. An intriguing peculiarity of APPLES 1 is that the properties (age andmetallicity) of the stellar content so far detected are similar to thoseof dSph galaxies in the Local Group, where star formation is thought tobe driven by galaxy interactions and mergers. Yet, APPLES 1 seems not tobe associated with a major group or cluster of galaxies. Therefore,APPLES 1 could be the first example of a field dSph galaxy withself-sustained and regulated star formation, and therefore would make aninteresting test case for studies of the formation and evolution ofunperturbed dSph galaxies.
| Extragalactic binaries as core-collapse supernova progenitors
Binary star systems are likely the progenitors of many core-collapse(Type II, Type Ib/c) supernovae (SNe). We present observationalinvestigations using ground-based and Hubble Space Telescope opticalimaging and radio monitoring of SNe and their environments, which eitherindicate or attempt to constrain the possible binary nature of the SNprogenitors. For example, from radio observations with the Very LargeArray of the Type II-linear SN 1979C in M100 we conclude that theprogenitor was possibly in a massive, highly eccentric binary, similarto the VV Cephei systems. The Type IIb SN 1993J in M81 is presumed tohave a massive progenitor in an interacting binary system, and fromHubble imaging we cannot yet constrain the nature of the presumedmassive, blue companion. We will present additional results for otherType Ib/c and II SNe.
| Modulations in the radio light curve of the Type IIb supernova 2001ig: evidence for a Wolf-Rayet binary progenitor?
We describe the radio evolution of supernova (SN) 2001ig in NGC 7424,from 700 d of multifrequency monitoring with the Australia TelescopeCompact Array (ATCA) and the Very Large Array (VLA). We find thatdeviations of the radio light curves at each frequency from the standard`minishell' model are consistent with density modulations in thecircumstellar medium (CSM), which seem to recur with a period near 150d. One possibility is that these are due to enhanced mass loss fromthermal pulses in an asymptotic giant branch star progenitor. A morelikely scenario, however, is that the progenitor was a Wolf-Rayet (WR)star, whose stellar wind collided with that from a massive hot companionon an eccentric 100-d orbit, leading to a regular build-up of CSMmaterial on the required time and spatial scales. Recent observations of`dusty pinwheels' in WR binary systems lend credibility to this model.Since such binary systems are also thought to provide the necessaryconditions for envelope stripping which would cause the WR star toappear as a Type Ib/c SN event rather than a Type II, these radioobservations of SN 2001ig may provide the key to linking Type Ib/c SNeto Type IIb events, and even to some types of gamma-ray bursts.
| A Hubble Space Telescope Census of Nuclear Star Clusters in Late-Type Spiral Galaxies. II. Cluster Sizes and Structural Parameter Correlations
We investigate the structural properties of nuclear star clusters inlate-type spiral galaxies. More specifically, we fit analytical modelsto Hubble Space Telescope images of 39 nuclear clusters in order todetermine their effective radii after correction for the instrumentalpoint-spread function. We use the results of this analysis to comparethe luminosities and sizes of nuclear star clusters to those of otherellipsoidal stellar systems, in particular the Milky Way globularclusters. Our nuclear clusters have a median effective radius ofre=3.5 pc, with 50% of the sample falling in the range2.4pc<=re<=5.0pc. This narrow size distribution isstatistically indistinguishable from that of Galactic globular clusters,even though the nuclear clusters are, on average, 4 mag brighter thanthe old globular clusters. We discuss some possible interpretations ofthis result. From a comparison of nuclear cluster luminosities withvarious properties of their host galaxies, we confirm that more luminousgalaxies harbor more luminous nuclear clusters. It remains unclearwhether this correlation mainly reflects the influence of galaxy size,mass, and/or star formation rate. Since the brighter galaxies in oursample typically have stellar disks with a higher central surfacebrightness, nuclear cluster luminosity also correlates with thisproperty of their hosts. On the other hand, we find no evidence for acorrelation between the presence of a nuclear star cluster and thepresence of a large-scale stellar bar.
| The structure and environment of young stellar clusters in spiral galaxies
A search for stellar clusters has been carried out in 18 nearby spiralgalaxies, using archive images from the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. All of the galaxies have previouslybeen imaged from the ground in UBVI. A catalogue of structuralparameters, photometry and comments based on visual inspection of theclusters is compiled and used to investigate correlations betweencluster structure, environment, age and mass. Least-squares fits to thedata suggest correlations between both the full-width at half-maximum(FWHM) and half-light radius (Reff) of the clusters and theirmasses (M) at about the 3σ level. Although both relations show alarge scatter, the fits have substantially shallower slopes than for aconstant-density relation (size ∝ M1/3). However, manyof the youngest clusters have extended halos which make theReff determinations uncertain. There is no evidence forgalaxy-to-galaxy variations in the mean cluster sizes. In particular,the mean sizes do not appear to depend on the host galaxy star formationrate surface density. Many of the youngest objects (age <107 years) are located in strongly crowded regions, and about1/3-1/2 of them are double or multiple sources. The HST images are alsoused to check the nature of cluster candidates identified in a previousground-based survey. The contamination rate in the ground-based sampleis generally less than about 20%, but some cluster identificationsremain ambiguous because of crowding even with HST imaging, especiallyfor the youngest objects.Full Tables \ref{tab:all}-\ref{tab:hstphot}, and \ref{tab:gb} are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/537Based on observations obtained with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| Annual report of the director for fiscal year 2001-2002.
Not Available
| X-Ray Emission from a Sample of Young Supernovae
When a massive star produces a powerful stellar wind prior to itssupernova event, theory predicts that the collision of the explodedstellar ejecta with the wind leads to a reverse shock, creating softX-ray emission. To understand the frequency at which luminous youngX-ray supernovae occur, we used ROSAT to observe a complete sample ofnearby supernovae (vhelio<1700 km s-1) thatoccurred in the period 1985.5 through 1994.3, which included eight TypeIa supernovae and 19 Type Ib and Type II events. Three supernovae aredetected in this time frame, SN 1987A (LMC), SN 1993J (NGC 3031), and apreviously unreported source, SN 1992ad, a Type II supernova in NGC4411b. No supernova had 0.5-2 keV luminosities exceeding2×1039 ergs s-1, so at the 95% confidencelevel, the probability of an individual supernova exceeding thisluminosity limit is less than 12%. Two of these supernovae hadluminosities brighter than 6×1038 ergs s-1and at the 95% confidence level, the probability of a supernova beingdetected above this luminosity is in the range 8.7%-51%. It is unlikelyfor young supernovae to be a large component of the IntermediateLuminosity X-Ray Object (IXO or ULX) class, where the luminosity exceeds2×1039 ergs s-1. The rate of successfuldetections appears to increase for sensitivities in the 1037ergs s-1 range, especially when obtained close to the time ofthe event.
| Supernova 2001ig in NGC 7424
IAUC 7994 available at Central Bureau for Astronomical Telegrams.
| Supernova 2001ig in NGC 7424
IAUC 7913 available at Central Bureau for Astronomical Telegrams.
| Supernova 2001ig in NGC 7424
IAUC 7804 available at Central Bureau for Astronomical Telegrams.
| Supernova 2001ig in NGC 7424
IAUC 7793 available at Central Bureau for Astronomical Telegrams.
| The Luminosity Function of Star Clusters in Spiral Galaxies
Star clusters in six nearby spiral galaxies are examined using archiveimages from the Wide Field Planetary Camera 2 (WFPC2) on board theHubble Space Telescope (HST). The galaxies have previously been studiedfrom the ground and some of them are known to possess rich populationsof ``young massive clusters.'' Comparison with the HST images indicatesa success rate of ~75% for the ground-based cluster detections, withtypical contaminants being blends or loose groupings of several stars incrowded regions. The luminosity functions (LFs) of cluster candidatesidentified on the HST images are analyzed and compared with existingdata for the Milky Way and the LMC. The LFs are well approximated bypower laws of the form dN(L)/dL~Lα, with slopes in therange -2.4<~α<~-2.0. The steeper slopes tend to be foundamong fits covering brighter magnitude intervals, although direct hintsof a variation in the LF slope with magnitude are seen only at lowsignificance in two galaxies. The surface density of star clusters at areference magnitude of MV=-8,Σ-8cl, scales with the mean star formationrate (SFR) per unit area, ΣSFR. Assuming that the LFcan be generally expressed asdN(L)/dL=cAΣγSFRLα,where A is the galaxy area, γ~1.0-1.4, α=-2.4, and thenormalization constant c is determined from the WFPC2 data analyzedhere, the maximum cluster luminosity expected in a galaxy from randomsampling of the LF is estimated as a function of ΣSFRand A. The predictions agree well with existing observations of galaxiesspanning a wide range of ΣSFR values, suggesting thatsampling statistics play an important role in determining the maximumobserved luminosities of star clusters in galaxies. Based onobservations obtained 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.
| A Hubble Space Telescope Census of Nuclear Star Clusters in Late-Type Spiral Galaxies. I. Observations and Image Analysis
We present new Hubble Space Telescope I-band images of a sample of 77nearby late-type spiral galaxies with low inclination. The main purposeof this catalog is to study the frequency and properties of nuclear starclusters. In 59 galaxies of our sample, we have identified a distinct,compact (but resolved), and dominant source at or very close to thephotocenter. In many cases, these clusters are the only prominent sourcewithin a few kiloparsecs from the galaxy nucleus. We present surfacebrightness profiles, derived from elliptical isophote fits, of allgalaxies for which the fit was successful. We use the fitted isophotesat radii larger than 2" to check whether the location of the clustercoincides with the photocenter of the galaxy and confirm that in nearlyall cases, we are truly dealing with ``nuclear'' star clusters. Fromanalytical fits to the surface brightness profiles, we derive thecluster luminosities after subtraction of the light contribution fromthe underlying galaxy disk and/or bulge. Based on observations made withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS 5-26555. Theseobservations are associated with proposal 8599.
| Supernova 2001ig in NGC 7424.
Not Available
| Supernova 2001ig in NGC 7424.
Not Available
| Supernova 2001ig in NGC 7424
IAUC 7777 available at Central Bureau for Astronomical Telegrams.
| Supernova 2001ig in NGC 7424
IAUC 7772 available at Central Bureau for Astronomical Telegrams.
| Supernova 2001ig in NGC 7424
IAUC 7772 available at Central Bureau for Astronomical Telegrams.
| 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.
| Young massive star clusters in nearby spiral galaxies. III. Correlations between cluster populations and host galaxy properties
We present an analysis of correlations between integrated properties ofgalaxies and their populations of young massive star clusters. Data for21 nearby galaxies presented by Larsen & Richtler (\cite{lr99}) areused together with literature data for 10 additional galaxies, spanninga range in specific U-band cluster luminosity T_L(U) from 0 to 15. Wefind that T_L(U) correlates with several observable host galaxyparameters, in particular the ratio of Far-Infrared (FIR) tomBox{B-band} flux and the surface brightness. Taking the FIRluminosity as an indicator of the star formation rate (SFR), it is foundthat T_L(U) correlates very well with the SFR per unit area. A similarcorrelation is seen between T_L(U) and the atomic hydrogen surfacedensity. The cluster formation efficiency seems to depend on the SFR ina continuous way, rather than being related to any particularly violentmode of star formation. We discuss fundamental features of possiblescenarios for cluster formation. One possibility is that the correlationbetween T_L(U) and SFR is due to a common controlling parameter, mostprobably the high density of the ISM. Another scenario conceives a highT_L(U) as resulting from the energy input from many massive stars incase of a high SFR. Based on observations made with the Nordic OpticalTelescope, operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio delRoque de los Muchachos of the Instituto de Astrofisica de Canarias, andwith the Danish 1.5-m telescope at ESO, La Silla, Chile.
| At the Heart of Barred Galaxies
Not Available
| The QDOT all-sky IRAS galaxy redshift survey
We describe the construction of the QDOT survey, which is publiclyavailable from an anonymous FTP account. The catalogue consists ofinfrared properties and redshifts of an all-sky sample of 2387 IRASgalaxies brighter than the IRAS PSC 60-μm completeness limit(S_60>0.6Jy), sparsely sampled at a rate of one-in-six. At |b|>10deg, after removing a small number of Galactic sources, the redshiftcompleteness is better than 98per cent (2086/2127). New redshifts for1401 IRAS sources were obtained to complete the catalogue; themeasurement and reduction of these are described, and the new redshiftstabulated here. We also tabulate all sources at |b|>10 deg with noredshift so far, and sources with conflicting alternative redshiftseither from our own work, or from published velocities. A list of 95ultraluminous galaxies (i.e. with L_60μm>10^12 L_solar) is alsoprovided. Of these, ~20per cent are AGN of some kind; the broad-lineobjects typically show strong Feii emission. Since the publication ofthe first QDOT papers, there have been several hundred velocity changes:some velocities are new, some QDOT velocities have been replaced by moreaccurate values, and some errors have been corrected. We also present anew analysis of the accuracy and linearity of IRAS 60-μm fluxes. Wefind that the flux uncertainties are well described by a combination of0.05-Jy fixed size uncertainty and 8per cent fractional uncertainty.This is not enough to cause the large Malmquist-type errors in the rateof evolution postulated by Fisher et al. We do, however, find marginalevidence for non-linearity in the PSC 60-μm flux scale, in the sensethat faint sources may have fluxes overestimated by about 5per centcompared with bright sources. We update some of the previous scientificanalyses to assess the changes. The main new results are as follows. (1)The luminosity function is very well determined overall but is uncertainby a factor of several at the very highest luminosities(L_60μm>5x10^12L_solar), as this is where the remainingunidentified objects are almost certainly concentrated. (2) Thebest-fitting rate of evolution is somewhat lower than our previousestimate; expressed as pure density evolution with density varying as(1+z)^p, we find p=5.6+/-2.3. Making a rough correction for the possible(but very uncertain) non-linearity of fluxes, we find p=4.5+/-2.3. (3)The dipole amplitude decreases a little, and the implied value of thedensity parameter, assuming that IRAS galaxies trace the mass, isΩ=0.9(+0.45, -0.25). (4) Finally, the estimate of density varianceon large scales changes negligibly, still indicating a significantdiscrepancy from the predictions of simple cold dark matter cosmogonies.
| Spiral patterns with straight arm segments
The phenomenon of `rows', which are straight geometrical segments in thespiral arms of galaxies, is studied. The Whirlpool nebula, Messier 51(NGC 5194) in Canes Venatici, is considered to be an example of a giantgrand design galaxy. Optical photographs, Hα, ultraviolet andfar-ultraviolet images, CO, 21-cm and synchrotron emission maps, and aK_s-band mosaic of M51 are studied. With this observational material,multiple rows can be recognized in the spiral arms of the galaxy. Therows comprise a major part of the arms. The lengths of the rows increasealmost linearly with distance from the centre. They intersect oneanother at an (average) angle ~2π/3. A possible physical explanationof the phenomenon of rows is discussed on the basis of the assumptionthat the formation of straight arm segments might be due to thegas-dynamical effect of stability of flat shock fronts, and the tendencyof a slightly curved shock front to become flat. A quantitativeflattening criterion enables an explanation of the geometricalproperties of the arm patterns found in M51 and also in M101. A brieflist of spirals with rows is given.
| Young massive star clusters in nearby galaxies. II. Software tools, data reductions and cluster sizes
A detailed description is given of the data analysis leading to thediscovery of young massive star clusters (YMCs) in a sample of 21 nearbygalaxies. A new useful tool, ishape, for the derivation of intrinsicshape parameters of compact objects is presented, and some test resultsare shown. Completeness tests for the cluster samples are discussed, andishape is used to estimate cluster sizes. Half-light radii of 0-20 pcare derived for clusters in 2 of the most cluster-rich galaxies,NGC 1313 and NGC 5236, which iswithin the range spanned by globular clusters in the Milky Way and byYMCs in the LMC and some starburst galaxies. Photometric data for allclusters, along with positions and estimated half-light radii, aretabulated. Based on observations made with the Nordic Optical Telescope,operated on the island of La Palma jointly by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias, and with theDanish 1.5-m telescope at ESO, La Silla, Chile. Table \ref{tab:clusters}is also available in electronic form at the CDS via anonymous ftp(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Young massive star clusters in nearby galaxies . I. Identification and general properties of the cluster systems
Using ground-based UBVRIHα CCD photometry we have been carryingout a search for young massive star clusters (YMCs) in a sampleconsisting of 21 nearby spiral galaxies. We find a large varietyconcerning the richness of the cluster systems, with some galaxiescontaining no YMCs at all and others hosting very large numbers of YMCs.Examples of galaxies with poor cluster systems are NGC 300 and NGC 4395,while the richest cluster systems are found in the galaxies NGC 5236 (M83), NGC 2997 and NGC 1313. The age distributions of clusters in thesegalaxies show no obvious peaks, indicating that massive clusters areformed as an ongoing process rather than in bursts. This is in contrastto what is observed in starbursts and merger galaxies. The radialdistributions of clusters follow the Hα surface brightnesses. Forthe galaxies in our sample there is no correlation between themorphological type and the presence of YMCs. Based on observations madewith the Nordic Optical Telescope, operated on the island of La Palmajointly by Denmark, Finland, Iceland, Norway, and Sweden, in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias, and with the Danish 1.5-m telescope at ESO, La Silla,Chile.
| Very Wide Galaxy Pairs of the Northern and Southern Sky
We present highly accurate observations of the 21 cm line of hydrogen ingalaxies made at the Arecibo and Parkes Observatories. The galaxiesobserved have been identified, through rigorous selection criteriaapplied to the CfA and SSRS catalogs, as being members of pairs withprojected separations of up to 1.5 Mpc (H0 = 75 km s-1 Mpc-1). Theseobservations form the completion of the Chengalur-Nordgren galaxy pairsample with data previously published by Chengalur, Nordgren andcolleagues. The new selection criteria used in this paper are anextension to larger projected separations of the criteria usedpreviously. Forty-nine new galaxies are observed, while H I is detectedin 41 of them. With the addition of these galaxies, the completed samplehas highly accurate H I velocities for a total of 219 galaxies.
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