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|Hubble Space Telescope STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble Type|
We study the nuclear star clusters (NCs) in spiral galaxies of variousHubble types using spectra obtained with the STIS on board the HubbleSpace Telescope (HST). We observed the nuclear clusters in 40 galaxies,selected from two previous HST WFPC2 imaging surveys. At a spatialresolution of ~0.2" the spectra provide a better separation of clusterlight from underlying galaxy light than is possible with ground-basedspectra. Approximately half of the spectra have a sufficiently highsignal-to-noise ratio for detailed stellar population analysis. For theother half we only measure the continuum slope, as quantified by the B-Vcolor. To infer the star formation history, metallicity, and dustextinction, we fit weighted superpositions of single-age stellarpopulation templates to the high signal-to-noise ratio spectra. We usethe results to determine the luminosity-weighted age, mass-to-lightratio, and masses of the clusters. Approximately half of the sampleclusters contain a population younger than 1 Gyr. Theluminosity-weighted ages range from 10 Myr to 10 Gyr. The stellarpopulations of NCs are generally best fit as a mixture of populations ofdifferent ages. This indicates that NCs did not form in a single event,but that instead they had additional star formation long after theoldest stars formed. On average, the sample clusters in late-typespirals have a younger luminosity-weighted mean age than those inearly-type spirals (L=8.37+/-0.25 vs.9.23+/-0.21). The average mass-weighted ages are older by ~0.7 dex,indicating that there often is an underlying older population that doesnot contribute much light but does contain most of the mass. The averagecluster masses are smaller in late-type spirals than in early-typespirals (logM=6.25+/-0.21 vs. 7.63+/-0.24) and exceed the masses typicalof globular clusters. The cluster mass correlates loosely with totalgalaxy luminosity. It correlates more strongly with both the Hubble typeof the host galaxy and the luminosity of its bulge. The lattercorrelation has the same slope as the well-known correlation betweensupermassive black hole mass and bulge luminosity. The properties ofboth nuclear clusters and black holes in the centers of spiral galaxiesare therefore intimately connected to the properties of the host galaxy,and in particular its bulge component. Plausible formation scenarioshave to account for this. We discuss various possible selection biasesin our results, but conclude that none of them can explain thedifferences seen between clusters in early- and late-type spirals. Theinability to infer spectroscopically the populations of faint clustersdoes introduce a bias toward younger ages, but not necessarily towardhigher masses.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555. These observations areassociated with proposals 9070 and 9783.
|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 ISOPHOT 170 μm Serendipity Survey II. The catalog of optically identified galaxies%|
The ISOPHOT Serendipity Sky Survey strip-scanning measurements covering≈15% of the far-infrared (FIR) sky at 170 μm were searched forcompact sources associated with optically identified galaxies. CompactSerendipity Survey sources with a high signal-to-noise ratio in at leasttwo ISOPHOT C200 detector pixels were selected that have a positionalassociation with a galaxy identification in the NED and/or Simbaddatabases and a galaxy counterpart visible on the Digitized Sky Surveyplates. A catalog with 170 μm fluxes for more than 1900 galaxies hasbeen established, 200 of which were measured several times. The faintest170 μm fluxes reach values just below 0.5 Jy, while the brightest,already somewhat extended galaxies have fluxes up to ≈600 Jy. For thevast majority of listed galaxies, the 170 μm fluxes were measured forthe first time. While most of the galaxies are spirals, about 70 of thesources are classified as ellipticals or lenticulars. This is the onlycurrently available large-scale galaxy catalog containing a sufficientnumber of sources with 170 μm fluxes to allow further statisticalstudies of various FIR properties.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Members of the Consortium on the ISOPHOT Serendipity Survey (CISS) areMPIA Heidelberg, ESA ISO SOC Villafranca, AIP Potsdam, IPAC Pasadena,Imperial College London.Full Table 4 and Table 6 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/39
|Molecular gas in the central regions of the latest-type spiral galaxies|
Using the IRAM 30 >m telescope, we have surveyed an unbiased sampleof 47 nearby spiral galaxies of very late (Scd-Sm) Hubble-type foremission in the 12CO(1-0) and (2-1) lines. The sensitivity ofour data (a few mK) allows detection of about 60% of our sample in atleast one of the CO lines. The median detected H2 mass is1.4x 107 >msun within the central few kpc, assuming astandard conversion factor. We use the measured line intensities tocomplement existing studies of the molecular gas content of spiralgalaxies as a function of Hubble-type and to significantly improve thestatistical significance of such studies at the late end of the spiralsequence. We find that the latest-type spirals closely follow thecorrelation between molecular gas content and galaxy luminosityestablished for earlier Hubble types. The molecular gas in late-typegalaxies seems to be less centrally concentrated than in earlier types.We use Hubble Space Telescope optical images to correlate the moleculargas mass to the properties of the central galaxy disk and the compactstar cluster that occupies the nucleus of most late-type spirals. Thereis no clear correlation between the luminosity of the nuclear starcluster and the molecular gas mass, although the CO detection rate ishighest for the brightest clusters. It appears that the central surfacebrightness of the stellar disk is an important parameter for the amountof molecular gas at the galaxy center. Whether stellar bars play acritical role for the gas dynamics remains unclear, in part because ofuncertainties in the morphological classifications of our sample.
|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.
|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.
|A Redshift Survey for Galaxies behind the Milky Way near the Galactic Center|
The galaxy search at the region near the galactic center is still highlyincomplete. This paper presents results of our redshift measurements fora random sample of galaxies and galaxy candidates at l =~ 18(deg) to41(deg) and b =~ -17(deg) to 17(deg) , most of which were newlyidentified through our previous optical search. We have obtained 78 newradial velocities, and found two regions with enhanced density at aroundl =~ 32(deg) , cz =~ 1500 km s(-1) and at l =~ 30(deg) to 40(deg) , cz=~ 6000 to 7000 km s(-1) . Our data rule out the possible presence of anearby large void at the region cz < 6000 km s(-1) around l= 30(deg), which has been induced from the results of a systematic search forIRAS galaxies.
|Search and Redshift Survey for IRAS Galaxies behind the Milky Way and Structure of the Local Void|
This is the third and final paper of our systematic visual search forIRAS galaxies behind the Milky Way at |b| <= 15 deg. This paperpresents a catalog of 950 IRAS galaxies with 60 mu m flux densitieslarger than 0.6 Jy located between l = 0 deg and 150 deg, of which 293are newly identified by this search. We made a redshift survey for theidentified galaxies and obtained new redshift data of 171 galaxies. Wealso present newly measured redshifts of 27 IRAS galaxies between l =150 deg and 225 deg at |b| <= 15 deg. In this paper we studied thestructure of the Local void using IRAS galaxies and galaxies from theThird Reference Catalogue of Bright Galaxies in the region l = 30deg--120 deg and b = -50 deg to +30 deg. The center of the Local voidturned out to be located at l ~ 60 deg, b ~ -15 deg, and cz ~ 2500 kms-1, and the size is about 2500 km s-1 along the direction toward thecenter.
|A Search for Galaxies behind the Milky Way at Aquila and Sagittarius|
We searched for galaxies using Schmidt atlases in a Milky Way region ofabout 800 square degrees around l =~ 8(deg) to 43(deg) and b =~ -17(deg)to 17(deg) . The surveyed region is located between the northern Localvoid and the Ophiuchus void. We detected about 5300 galaxy candidates,which have galaxy-like images with sizes larger than 0.'1. Of these,only 30 have been given in galaxy catalogs and another 74 are associatedwith IRAS point sources containing 18 IRAS galaxies with knownredshifts. We made a catalog of detected objects in which the positionin (alpha , delta ) and (l, b), size, and features of image andmultiplicity are given. The sky distribution of detected objects showsthe presence of two clusters of galaxies and a group of nearby galaxieslocated around l=33(deg) , b=-15(deg) with cz=1500 km s(-1) .
|Large-Scale Structure at Low Galactic Latitude|
We have extended the CfA Redshift Survey to low galactic latitudes toinvestigate the relation between the Great Wall in the North GalacticCap and the Perseus-Pisces chain in the South Galactic Cap. We presentredshifts for 2020 galaxies in the Catalogue of Galaxies and of Clustersof Galaxies (Zwicky et al. 1961-68, CGCG) in the following regions: 4^h^<= α <= 8^h^, 17^h^ <= α <= 20^h^, 0^deg^ <=δ <= 45^deg^. In these regions, the redshift catalogue includes1664 galaxies with B(0) <= 15.5 (of which 820 are newly measured) andis 97% complete. We also include redshifts for an additional 356galaxies in these regions with B(O) > 15.5; of these, 148 werepreviously unmeasured. The CGCG samples the galaxy distribution down tob_II_ = 10^deg^. In this paper, we discuss the acquisition and reductionof the spectra, and we examine the qualitative features of the redshiftdistribution. The Great Wall and the Perseus-Pisces chain are not simplyconnected across the Zone of Avoidance. These structures, which at firstappear to be coherent on scales of ~100 h^-1^ Mpc or more, actually formthe boundaries of neighboring voids of considerably smaller scale,approximately 50h^-1^ Mpc. The structures delineated by ouroptically-selected sample are qualitatively similar to those detected bythe far-infrared-selected IRAS 1.2 Jansky Survey (Fisher et al. 1995).Although the IRAS survey probes more deeply into the Zone of Avoidance,our optically-selected survey provides better sampling of structures atb_II_ >= 10^deg^.
|Arm structure in normal spiral galaxies, 1: Multivariate data for 492 galaxies|
Multivariate data have been collected as part of an effort to develop anew classification system for spiral galaxies, one which is notnecessarily based on subjective morphological properties. A sample of492 moderately bright northern Sa and Sc spirals was chosen for futurestatistical analysis. New observations were made at 20 and 21 cm; thelatter data are described in detail here. Infrared Astronomy Satellite(IRAS) fluxes were obtained from archival data. Finally, new estimatesof arm pattern radomness and of local environmental harshness werecompiled for most sample objects.
|Galaxy structures in the Hercules region|
216 redshifts have been obtained in a region of 981 sq deg south of theHercules supercluster. 172 of these redshifts are of galaxies withmpg less than or equal to 15.1, 110 of which had no previousvelocity measurement. 44 new redshifts are of galaxies fainter thanmpg = 15.1. With these new data we have been able to define asample in a vast region (approximately 1700 sq deg) around Herculeslimited to mpg less than or equal to 15.1 with a velocitycompleteness of 81.5%. 189 galaxies have been morphologically classifiedso that all galaxies in the sample with known velocity now also haveknown morphology. The magnitude limited sample, including 556 galaxies,is then used to identify and describe galaxy structures in the region.We find that the overdense volume is small, that its overall appearanceis that of a coral branch floating in a sea of nothing and that earlyand late type galaxies defined different structures.
|General study of group membership. II - Determination of nearby groups|
We present a whole sky catalog of nearby groups of galaxies taken fromthe Lyon-Meudon Extragalactic Database. From the 78,000 objects in thedatabase, we extracted a sample of 6392 galaxies, complete up to thelimiting apparent magnitude B0 = 14.0. Moreover, in order to considersolely the galaxies of the local universe, all the selected galaxieshave a known recession velocity smaller than 5500 km/s. Two methods wereused in group construction: a Huchra-Geller (1982) derived percolationmethod and a Tully (1980) derived hierarchical method. Each method gaveus one catalog. These were then compared and synthesized to obtain asingle catalog containing the most reliable groups. There are 485 groupsof a least three members in the final catalog.
|A survey of small-scale extremes in extinction at low Galactic latitudes using IRAS galaxies|
Optical CCD and 21 cm H I data are combined here via the Tully-Fisherrelation to study the Galactic extinction in the V an I bands over anangular scale of a few arcmin at absolute Galactic latitudes between 2and 16 deg. Extinctions in V and I bands are found to be linearlycorrelated with A(I) about 0.58 A(V), with no noticeable environmentaldependence and consistent with existing results of Galactic reddening.There is a large scatter in the simple linear relation with csc /b/,confirming the existence of 'holes' and patches at low Galacticlatitudes. The mean correlation between Galactic extinction and H Icolumn density over a comparable angular scale is also found to beconsistent with those at high latitudes over large angular scales.Correlation is also found between optical extinction and the IRAS 100micron sky brightness, and this sky brightness is only slightly worsethan H I column density as an indicator for optical extinction.
|Identifying galaxies in the zone of avoidance|
A set of IR color criteria was applied to sources in the IRAS PointSource Catalog at low Galactic latitudes in order to select candidategalaxies. A total of 371 of the selected sources within a Galacticlatitude range (b) = 2-16 deg were searched for H I with the Arecibo 305m radio telescope. Twenty-six percent (97) were detected, including 36galaxies previously known only as IRAS sources. The H I spectra of thedetected galaxies are presented together with their observed redshiftsand velocity widths.
|Neutral hydrogen absorption by galaxies and implications for the soft X-ray background|
Results are presented from a search for neutral hydrogen absorption inthe 21-cm spectra of 59 radio continuum sources in the proximity ofspiral and lenticular galaxies using the Arecibo radio telescope. Fivegalaxies showed possible but uncertain absorption. For the 54 othersources, the column density of the neutral hydrogen in the interveningmaterial is well below 2 x 10 to the 19th/cu cm or the spin temperatureis at least one order of magnitude above the microwave backgroundtemperature. For these cases, subthermal effects are not hidingappreciable amounts of neutral hydrogen.
|Global properties of infrared bright galaxies|
Infrared flux densities of 182 galaxies, including 50 galaxies in theVirgo cluster, were analyzed using IRAS data for 12, 25, 60, and 100microns, and the results were compared with data listed in the PointSource Catalog (PSC, 1985). In addition, IR luminosities, L(IRs),colors, and warm dust masses were derived for these galaxies and werecompared with the interstellar gas masses and optical luminosities ofthe galaxies. It was found that, for galaxies whose optical diametermeasures between 5 and 8 arcmin, the PSC flux densities areunderestimated by a factor of 2 at 60 microns, and by a factor of 1.5 at100 microns. It was also found that, for 49 galaxies, the mass of warmdust correlated well with the H2 mass, and that L(IR) correlated withL(H-alpha), demonstrating that the L(IR) measures the rate of starformation in these galaxies.
|Uncertainties in 21 centimeter redshifts. I - Data|
High-precision data on the 21-cm redshifts, profile widths, and shapesfor 625 galaxies are presented. Each galaxy is listed in across-identification and morphology table. High-resolution spectra arealso given for each galaxy. Internal redshift consistency is roughly 1km/s for galaxies for which the S/N is above 15. No systematic effectshave been found which might influence the observed redshift quantizationat 72.5 km/s or its submultiples.
|A further study of the relation of the radio-far-infrared in galaxies. I - Observations and data processing|
The radio luminosities of 99 galaxies at 6.3 cm (and of 31 of them at2.8 cm) are determined on the basis of observations obtained with the100-m Effelsberg radio telescope during March-August 1984 and comparedto the IRAS color-corrected FIR luminosities, extending the survey of deJong et al. (1985). The data-reduction procedures are described, and theresults are presented in extensive tables and maps and brieflycharacterized. The correlation of radio to FIR luminosity is confirmedover about four decades in both parameters, and the dispersion of logP(6.3 cm)/L(FIR) is found to be about 0.2, which is significantlysmaller than the dispersion found by de Jong et al. The improvement isattributed to color correction, integration over the 100-60-micronrange, and the exclusion of ambiguous identifications.
|Normal supergiants and their imitations. I - Sodium as a mass indicator?|
Considering the possibility for a low-mass celestial body to attain highluminosity, characteristic for a normal supergiant, and mimicspectroscopic and photometric behavior of the latter, a method isproposed that could be possibly used to distinguish between supergiantsand their limitations. In this method, surface abundance of Na-23 wasused as a probable mass indicator, deriving the sodium-to-mass relationfor a sample of 34 well-observed stars. Special attention was paid tothe parameter-related errors and non-LTE effects for the Na I lines inthe visible. To account for the initial sodium content, all sodiumabundances were normalized, using the Na/Fe relation derived fromsolar-type dwarfs. The plot of the normalized sodium abundance againststellar mass has revealed a direct relationship between the twoparameters, suggesting a real interdependence. The classical Cepheid XCyg, believed to be a true intermediate-mass star, was well within thehighly scattered relation.
|Empirical properties of nearby groups of galaxies|
A new list of nearby groups was composed by Vennik (1984) on the basisof radial velocity data of galaxies using the hierarchical clusteringtechnique described by Materne (1978) and Tully (1980). This paperpresents a detailed analysis of empirical properties of groups on thebasis of that list. Various group selection criteria are compared, andthe main parameters of individual groups, possible selection effects,and relationships between various properties of the groups arediscussed. Synthesized groups are composed based on the luminosities ofthe groups and the dominating morphology.
|An Arecibo survey for extragalactic hydroxyl absorption. I - Presentation of results|
Hydroxyl absorption has been detected in a total of 24 galaxies;megamaser emission in six additional galaxies brings the total number ofdetections of extragalactic OH to 30. About 50 percent of theextragalactic absorption lines are asymmetrically skewed toward the red,indicating that the molecular disks could have an unusual velocity orexcitation structure. The hyperfine ratio for the 1667 and 1665 MHztransitions in most galaxies lies within the limits specified by LTEconditions.
|OH circumstellar masers in association with IRAS sources|
Because of their far-infrared colors, most sources of the IRAS circulars8 and 9 were suspected to be OH/IR objects, and observations have beenmade with Effelsberg and Arecibo radiotelescopes to confirm this nature.This study has been completed with the Nancay radiotelescope; four newobjects have been detected. A compilation of the results about thissample is presented. Using these results and an analysis of IRAS datafrom other types of infrared sources, the possibility of identifyingOH/IR stars by their (25-12) micron, (60-25) micron colors is discussed.
|Star formation in spiral galaxies|
The correlations between the FIR luminosity and the mass of gas inspiral galaxies are studied. It is shown that there is an almost linearrelationship between the two quantities. It is found that the specificluminosity of galaxies does not vary much between differentmorphological types. A correlation is found between the absolute face-onintegrated blue magnitude and the gas content of the galaxies. It isconcluded that not only is the current star forming activity constantper unit gas mass, but also that the ratio of mass in the form of gas tothe total mass, for spiral galaxies, is a constant to within a factor ofabout three.
|Identification of infrared sources in the IRAS circulars|
From IRAS (Infrared Astronomical Satellite) Circulars Nos. 4 to 15, 110infrared sources have been identified. Out of these 110 identifiedsources 99 are galaxies, nine are nebulae, one is a dark object, and oneis a non-stellar object. Thirty-five of these galaxies, have alreadybeen studied, mostly are spiral type. The characteristics of sixty-fourremaining galaxies, for which the type is not known or uncertain, arestudied by comparing their colors and luminosities with known galaxies.The galaxies observed by IRAS are mostly active galaxies.
|Properties of IRAS galaxies with B(0)T not greater than approximately 14.5|
The optical and infrared properties of 86 galaxies from IRAS circulars1-15, identified with optically bright galaxies in RC-2 and UGC havebeen studied. It is seen that B(0)T, the face-on integratedblue magnitude, is correlated with the far-infrared (FIR) flux. For asubsample of 61 galaxies for which distances are available, it is foundthat the color temperature of FIR emitting dust is correlated with theFIR luminosity, but not with the blue band luminosity. This along withthe observed ratio of L(FIR)/L(B) implies that the observed blueluminosity is unlikely to be associated with young star formationactivity. Associating FIR luminosity with young star formation activityin molecular clouds and the blue luminosity to the mass of the galaxy, avalue of 5-10 solar luminosity/solar mass is estimated for the meanratio of total FIR luminosity to the mass of the gas in these galaxies.
|CO emission from IRAS galaxies|
The results of a search for CO emission in 20 galaxies in the IRAScirculars are presented. The observations were carried out using the14-m millimeter telescope of the Five College Radio AstronomyObservatory (FCRAO) at a half-power beam width of 45 arcsec. CO wasdetected in 10 of the galaxies, including Arp 220 and NGC 6240. A roughcorrelation was found between CO and 10-micron luminosities, although awide scatter was observed in the relation. The very luminous far-IRgalaxies Arp 220 and NGC 6240 contain 3 x 10 to the 10th solar mass ofH2, or a factor of 3 more molecular gas than was found in the luminousspiral galaxies NGC and M51. It is shown that if the far-IR emissionoriginated in a region much smaller than 45 arcsec, the surface densityof the molecular hydrogen in Arp 220 may exceed that observed in any Scgalaxy.
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