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|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.
|Water-Vapor Maser Survey for Active Galactic Nuclei: A Megamaser in NGC 6926|
We made a survey of water-vapor maser emission for 93 AGNs with theNobeyama 45-m and Mopra 22-m telescopes from 1999 to 2002. A megamaserwas detected in a Seyfert 2 galaxy, NGC 6926, at a distance of 80Mpc, in2002 June. [Greenhill et al. (2003a) have also reported a detection ofthe megamaser at the close date.] The peak flux density was 110mJy, andthe total isotropic luminosity was 340 Lȯ. The masershows triply peaked spectrum, suggesting an edge-on disk. A narrow-linefeature of the maser components at VLSR = 6001 kms-1 was strongly variable with a time scale of a few tens ofdays, and the variation should be of intrinsic origin. We also showed apossibility of variability of water-vapor maser emission of a megamaserpreviously detected in a Seyfert/ultraluminous FIR galaxy, NGC 6240.
|The Remnants of Intergalactic Supernovae|
Intergalactic Type Ia supernovae (SNe) have been discovered recently inrich galaxy clusters. These SNe are likely the descendants of anintergalactic stellar population, which has been discovered in recentyears through a variety of tracers. We estimate the observationalsignatures of the associated SN remnants (SNRs) in the unusualintracluster medium (ICM) environment. We find that if Type Ia SNe stillhave a circumstellar medium (CSM) at the time of explosion, then theirremnants are visible in the optical for ~102-103yr, with properties similar to young galactic SNRs. In contrast withgalactic SNRs, in which the ejecta from the explosion interacts with theinterstellar medium (ISM), intracluster SNRs become undetectable in theoptical band once their ejecta passes beyond the CSM and enters the hotand tenuous ICM. If Type Ia SNe have a CSM, there should be ~150 youngSNRs in the nearby Virgo Cluster, with an Hα luminosity of~1035 ergs s-1 and an angular size of ~0.1". Weinvestigate the possibility that members of this SNR population may haverecently been detected but incorrectly identified as intergalactic H IIregions. Alternatively, if optical intergalactic SNRs do not exist inVirgo, this will constitute evidence that Type Ia SNe are devoid of aCSM, with implications for progenitor scenarios. Regardless of thepresence of a CSM, about 10 older SNRs per square degree should bedetectable in Virgo in the radio band, with fluxes of order 0.1 mJy at 1GHz. Their angular sizes (~1"), morphologies, and lack of opticalassociation with distant galaxies can distinguish them from the muchmore numerous background population. Their detection would provide anaccurate measurement of the intracluster SN rate. Deep pointedobservations toward the site of SN 1980I, a possibly intergalactic TypeIa event in Virgo, could test for the existence of a CSM by comparisonto our predictions for the early-time development of intergalactic SNRs.
|Kinematics of the local universe . XII. 21-cm line measurements of 586 galaxies with the new Nançay receiver|
This paper presents 586 new 21-cm neutral hydrogen line measurementscarried out with the FORT receiver of the meridian transit Nançayradiotelescope in the period July 2000-March 2003. This observationalprogramme is part of a larger project aiming at collecting an exhaustiveand magnitude-complete HI extragalactic catalogue for Tully-Fisherapplications. It is associated with the building of the MIGALEspectroscopic archive and database.Tables 2, 3 and HI-profiles and corresponding comments are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (22.214.171.124) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/373, or directly atour web site http://klun.obs-nancay.fr
|Ages and metallicities of Hickson compact group galaxies|
Hickson compact groups (HCGs) constitute an interesting extreme in therange of environments in which galaxies are located, as the spacedensity of galaxies in these small groups are otherwise only found inthe centres of much larger clusters. The work presented here uses Lickindices to make a comparison of ages and chemical compositions ofgalaxies in HCGs with those in other environments (clusters, loosegroups and the field). The metallicity and relative abundance of`α-elements' show strong correlations with galaxy age and centralvelocity dispersion, with similar trends found in all environments.However, we show that the previously reported correlation betweenα-element abundance ratios and velocity dispersion disappears whena full account is taken of the abundance ratio pattern in thecalibration stars. This correlation is thus found to be an artefact ofincomplete calibration to the Lick system.Variations are seen in the ranges and average values of age, metallicityand α-element abundance ratios for galaxies in differentenvironments. Age distributions support the hierarchical formationprediction that field galaxies are on average younger than their clustercounterparts. However, the ages of HCG galaxies are shown to be moresimilar to those of cluster galaxies than those in the field, contraryto the expectations of current hierarchical models. A trend for lowervelocity dispersion galaxies to be younger was also seen. This is againinconsistent with hierarchical collapse models, but is qualitativelyconsistent with the latest N-body smoothed particle hydrodynamics modelsbased on monolithic collapse in which star formation continues for manyGyr in low-mass haloes.
|Discovery of Intergalactic H II Regions|
We have discovered a number of very small isolated H II regions 20-30kpc from their nearest galaxy. The H II regions appear as tiny emissionline dots (ELdots) in narrow band images obtained by the NOAO Survey forIonization in Neutral Gas Galaxies (SINGG). We have spectroscopicconfirmation of 5 isolated H II regions in 3 systems. The Hαluminosities of the H II regions are equivalent to the ionizing flux ofonly 1 large or a few small OB stars each. These stars appear to haveformed in situ and represent atypical star formation in the low densityenvironment of galaxy outskirts. In situ star formation in theintergalactic medium offers an alternative to galactic wind models toexplain metal enrichment. In interacting systems (2 out of 3), isolatedH II regions could be a starting point for tidal dwarf galaxies.
|The Relation between Galaxy Activity and the Dynamics of Compact Groups of Galaxies|
Using a sample of 91 galaxies distributed over 27 compact groups (CGs)of galaxies, we define an index that allows us to quantify their levelof activity due to an active galactic nucleus (AGN) or star formation.By combining the mean activity index with the mean morphological type ofthe galaxies in a group, we are able to quantify the evolutionary stateof the groups. We find that they span an evolutionary sequence thatcorrelates with the spatial configuration of the galaxies in the CG. Wedistinguish three main configuration types: A, B, and C. Type A CGs showpredominantly low velocity dispersions and are rich in late-type spiralsthat show active star formation or harbor an AGN. Type B groups haveintermediate velocity dispersions and contain a large fraction ofinteracting or merging galaxies. Type C comprises CGs with high velocitydispersions, which are dominated by elliptical galaxies that show noactivity. We suggest that evolution proceeds A==>B==>C. Mappingthe groups with different evolution levels in a diagram of radius versusvelocity dispersion does not reveal the pattern expected based on theconventional fast merger model for CGs, which predicts a direct relationbetween these two parameters. Instead, we observe a trend contrary toexpectation: the evolutionary state of a group increases with velocitydispersion. This trend seems to be related to the masses of thestructures in which CGs are embedded. In general, the evolutionary stateof a group increases with the mass of the structure. This suggestseither that galaxies evolve more rapidly in massive structures or thatthe formation of CGs embedded in massive structures predated theformation of CGs associated with lower mass systems. Our observationsare consistent with the structure formation predicted by the CDM model(or ΛCDM), only if the formation of galaxies is a biased process.
|An IRAS High Resolution Image Restoration (HIRES) Atlas of All Interacting Galaxies in the IRAS Revised Bright Galaxy Sample|
The importance of far-infrared observations for our understanding ofextreme activity in interacting and merging galaxies has beenillustrated by many studies. Even though two decades have passed sinceits launch, the most complete all-sky survey to date from which far-IRselected galaxy samples can be chosen is still that of the InfraredAstronomical Satellite (IRAS). However, the spatial resolution of theIRAS all-sky survey is insufficient to resolve the emission fromindividual galaxies in most interacting galaxy pairs, and hence previousstudies of their far-IR properties have had to concentrate either onglobal system properties or on the properties of very widely separatedand weakly interacting pairs. Using the HIRES image reconstructiontechnique, it is possible to achieve a spatial resolution ranging from30" to 1.5m (depending on wavelength and detector coverage), whichis a fourfold improvement over the normal resolution of IRAS. This issufficient to resolve the far-IR emission from the individual galaxiesin many interacting systems detected by IRAS, which is very importantfor meaningful comparisons with single, isolated galaxies. We presenthigh-resolution 12, 25, 60, and 100 μm images of 106 interactinggalaxy systems contained in the IRAS Revised Bright Galaxy Sample (RBGS,Sanders et al.), a complete sample of all galaxies having a 60 μmflux density greater than 5.24 Jy. These systems were selected to haveat least two distinguishable galaxies separated by less than threeaverage galactic diameters, and thus we have excluded very widelyseparated systems and very advanced mergers. Additionally, some systemshave been included that are more than three galactic diameters apart,yet have separations less than 4' and are thus likely to suffer fromconfusion in the RBGS. The new complete survey has the same propertiesas the prototype survey of Surace et al. We find no increased tendencyfor infrared-bright galaxies to be associated with other infrared-brightgalaxies among the widely separated pairs studied here. We find smallenhancements in far-IR activity in multiple galaxy systems relative toRBGS noninteracting galaxies with the same blue luminosity distribution.We also find no differences in infrared activity (as measured byinfrared color and luminosity) between late- and early-type spiralgalaxies.
|Intergalactic H II Regions Discovered in SINGG|
A number of very small isolated H II regions have been discovered atprojected distances up to 30 kpc from their nearest galaxy. These H IIregions appear as tiny emission-line objects in narrowband imagesobtained by the NOAO Survey for Ionization in Neutral Gas Galaxies(SINGG). We present spectroscopic confirmation of four isolated H IIregions in two systems; both systems have tidal H I features. Theresults are consistent with stars forming in interactive debris as aresult of cloud-cloud collisions. The Hα luminosities of theisolated H II regions are equivalent to the ionizing flux of only a fewO stars each. They are most likely ionized by stars formed in situ andrepresent atypical star formation in the low-density environment of theouter parts of galaxies. A small but finite intergalactic star formationrate will enrich and ionize the surrounding medium. In one system, NGC1533, we calculate a star formation rate of 1.5×10-3Msolar yr-1, resulting in a metal enrichment of~1×10-3 solar for the continuous formation of stars.Such systems may have been more common in the past and a similarenrichment level is measured for the ``metallicity floor'' in dampedLyα absorption systems.
|Radio emission from AGN detected by the VLA FIRST survey|
Using the most recent (April 2003) version of the VLA FIRST survey radiocatalog, we have searched for radio emission from >2800 AGN takenfrom the most recent (2001) version of the Veron-Cetty and Veron AGNcatalog. These AGN lie in the 9033 square degrees of sky alreadycovered by the VLA FIRST survey. Our work has resulted in positivedetection of radio emission from 775 AGN of which 214 are new detectionsat radio wavelengths.Tables 3 and 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/35
|COLA. II. Radio and Spectroscopic Diagnostics of Nuclear Activity in Galaxies|
We present optical spectroscopic observations of 93 galaxies taken fromthe infrared-selected COLA (compact objects in low-power AGNs) sample.These are all galaxies for which we have previously obtainedlow-resolution radio observations and high-resolution (<0.05")Australian Long Baseline Array snapshots. The sample spans the range offar-IR luminosities from normal galaxies to luminous infrared galaxiesand contains a significant number of galaxies involved in galaxy-galaxyinteractions. Of the galaxies observed, 78 (84%) exhibit emission linesindicating that they are either AGNs or actively forming stars(starburst galaxies). Using a newly developed, theoretically based,optical emission line scheme to classify the spectra, we find that 15%of the emission-line galaxies are Seyfert galaxies, 77% are starbursts,and the rest are either borderline AGN/starburst or show ambiguouscharacteristics. We find little evidence for an increase in the fractionof AGNs in the sample as a function of far-IR (FIR) luminosity, incontrast to previous studies, but our sample covers only a small rangein infrared luminosity(1010.5Lsolar<=LFIR<=1011.7 Lsolar), and thus a weak trend may be masked. Instead,as the infrared luminosity increases, so does the fraction of metal-richstarbursts, objects that on more traditional diagnostic diagrams wouldhave been classified as weak, low-ionization, narrow emission lineregions. As a whole the Seyfert galaxies exhibit a small, butstatistically significant, radio excess on the radio-FIR correlationcompared to the galaxies classified as starbursts. Compact (<0.05")radio cores are detected in 55% of the Seyfert galaxies, and thesegalaxies exhibit a significantly larger radio excess than the Seyfertgalaxies in which compact cores were not detected. Our results indicatethat there may be two distinct populations of Seyfert galaxies,``radio-excess'' Seyfert galaxies, which exhibit extended radiostructures and compact radio cores, and ``radio-quiet'' Seyfertgalaxies, in which the majority of the radio emission can be attributedto star formation in the host galaxy. No significant difference is seenbetween the IR and optical spectroscopic properties of Seyfert galaxieswith and without radio cores.
|Dynamical Effects of Interactions and the Tully-Fisher Relation for Hickson Compact Groups|
We investigate the properties of the B-band Tully-Fisher (T-F) relationfor 25 compact group galaxies, using Vmax derived fromtwo-dimensional velocity maps. Our main result is that the majority ofthe Hickson Compact Group galaxies lie on the T-F relation. However,about 20% of the galaxies, including the lowest-mass systems, havehigher B luminosities for a given mass, or alternatively, a mass that istoo low for their luminosities. We favor a scenario in which outliershave been brightened because of either enhanced star formation ormerging. Alternatively, the T-F outliers may have undergone truncationof their dark halo due to interactions. It is possible that in somecases both effects contribute. The fact that the B-band T-F relation issimilar for compact group and field galaxies tells us that thesegalaxies show common mass-to-size relations and that the halos ofcompact group galaxies have not been significantly stripped insideR25. We find that 75% of the compact group galaxies studied(22 out of 29) have highly peculiar velocity fields. Nevertheless, acareful choice of inclination, position angle, and center, obtained fromthe velocity field, and an average of the velocities over a large sectorof the galaxy enabled the determination of fairly well-behaved rotationcurves for the galaxies. However, two of the compact group galaxies HCG91a and HCG 96a, which are the most massive members in M51-like pairs,have very asymmetric rotation curves, with one arm rising and the otherone falling, indicating most probably a recent perturbation by the smallclose companions.
|Redshift-Distance Survey of Early-Type Galaxies: Spectroscopic Data|
We present central velocity dispersions and Mg2 line indicesfor an all-sky sample of ~1178 elliptical and S0 galaxies, of which 984had no previous measures. This sample contains the largest set ofhomogeneous spectroscopic data for a uniform sample of ellipticalgalaxies in the nearby universe. These galaxies were observed as part ofthe ENEAR project, designed to study the peculiar motions and internalproperties of the local early-type galaxies. Using 523 repeatedobservations of 317 galaxies obtained during different runs, the dataare brought to a common zero point. These multiple observations, takenduring the many runs and different instrumental setups employed for thisproject, are used to derive statistical corrections to the data and arefound to be relatively small, typically <~5% of the velocitydispersion and 0.01 mag in the Mg2 line strength. Typicalerrors are about 8% in velocity dispersion and 0.01 mag inMg2, in good agreement with values published elsewhere.
|The Seyfert Population in the Local Universe|
The magnitude-limited catalog of the Southern Sky Redshift Survey(SSRS2) is used to characterize the properties of galaxies hostingactive galactic nuclei (AGNs). Using emission-line ratios, we identify atotal of 162 (3%) Seyfert galaxies out of the parent sample with 5399galaxies. The sample contains 121 Seyfert 2 galaxies and 41 Seyfert 1galaxies. The SSRS2 Seyfert galaxies are predominantly in spirals oftypes Sb and earlier or in galaxies with perturbed appearance as theresult of strong interactions or mergers. Seyfert galaxies in thissample are twice as common in barred hosts as the non-Seyfert galaxies.By assigning galaxies to groups using a percolation algorithm, we findthat the Seyfert galaxies in the SSRS2 are more likely to be found inbinary systems when compared with galaxies in the SSRS2 parent sample.However, there is no statistically significant difference between theSeyfert and SSRS2 parent sample when systems with more than two galaxiesare considered. The analysis of the present sample suggests that thereis a stronger correlation between the presence of the AGN phenomenonwith internal properties of galaxies (morphology, presence of bar,luminosity) than with environmental effects (local galaxy density, groupvelocity dispersion, nearest neighbor distance).Partly based on observations at European Southern Observatory (ESO),under the ESO-ON agreement to operate the 1.52 m telescope.
|Redshift-Distance Survey of Early-Type Galaxies: Circular-Aperture Photometry|
We present R-band CCD photometry for 1332 early-type galaxies, observedas part of the ENEAR survey of peculiar motions using early-typegalaxies in the nearby universe. Circular apertures are used to tracethe surface brightness profiles, which are then fitted by atwo-component bulge-disk model. From the fits, we obtain the structuralparameters required to estimate galaxy distances using theDn-σ and fundamental plane relations. We find thatabout 12% of the galaxies are well represented by a pure r1/4law, while 87% are best fitted by a two-component model. There are 356repeated observations of 257 galaxies obtained during different runsthat are used to derive statistical corrections and bring the data to acommon system. We also use these repeated observations to estimate ourinternal errors. The accuracy of our measurements are tested by thecomparison of 354 galaxies in common with other authors. Typical errorsin our measurements are 0.011 dex for logDn, 0.064 dex forlogre, 0.086 mag arcsec-2 for<μe>, and 0.09 for mRC,comparable to those estimated by other authors. The photometric datareported here represent one of the largest high-quality and uniformall-sky samples currently available for early-type galaxies in thenearby universe, especially suitable for peculiar motion studies.Based on observations at Cerro Tololo Inter-American Observatory (CTIO),National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation (NSF);European Southern Observatory (ESO); Fred Lawrence Whipple Observatory(FLWO); and the MDM Observatory on Kitt Peak.
|Optical and Near-Infrared Imaging of the IRAS 1 Jy Sample of Ultraluminous Infrared Galaxies. II. The Analysis|
An R and K' atlas of the IRAS 1 Jy sample of 118ultraluminous infrared galaxies (ULIGs) was presented in a companionpaper (our Paper I). The present paper discusses the results from theanalysis of these images supplemented with new spectroscopic dataobtained at Keck. All but one object in the 1 Jy sample show signs of astrong tidal interaction/merger. Multiple mergers involving more thantwo galaxies are seen in no more than five of the 118 (<5%) systems.None of the 1 Jy sources is in the first-approach stage of theinteraction, and most (56%) of them harbor a single disturbed nucleusand are therefore in the later stages of a merger. Seyfert galaxies(especially those of type 1), warm ULIGs(f25/f60>0.2), and the more luminous systems(>1012.5 Lsolar) all show a strong tendency to beadvanced mergers with a single nucleus. The individual galaxies in thebinary systems of the 1 Jy sample show a broad distribution in hostmagnitudes (luminosities) with a mean of -21.02+/-0.76 mag(0.85+0.86-0.43 L*) at R and-23.98+/-1.25 mag (0.90+1.94-0.61 L*)at K', and a R- or K'-band luminosity ratiogenerally less than ~4. Single-nucleus ULIGs also show a broaddistribution in host magnitudes (luminosities) with an average of-21.77+/-0.92 mag (1.69+2.25-0.97 L*)at R and -25.03+/-0.94 mag (2.36+3.24-1.38) atK'. These distributions overlap considerably with those ofquasars. The same statement applies to R-K' colors in ULIG and quasarhosts. An analysis of the surface brightness profiles of the hostgalaxies in single-nucleus sources reveals that about 73% of the R andK' surface brightness profiles are fitted adequately by anelliptical-like R1/4 law. These elliptical-like 1 Jy systemshave luminosity and R-band axial ratio distributions that are similar tothose of normal (inactive) intermediate-luminosity ellipticals andfollow with some scatter the same μe-rerelation, giving credence to the idea that some of these objects mayeventually become intermediate-luminosity elliptical galaxies if theyget rid of their excess gas or transform this gas into stars. Theseelliptical-like hosts are most common among merger remnants with Seyfert1 nuclei (83%), Seyfert 2 optical characteristics (69%), or mid-infrared(ISO) AGN signatures (80%). The mean half-light radius of these ULIGs is4.80+/-1.37 kpc at R and 3.48+/-1.39 kpc at K', typical ofintermediate-luminosity ellipticals. These values are in excellentagreement with recent quasar measurements obtained at H with the HubbleSpace Telescope (HST), but they are systematically lower than other HSTmeasurements derived at R. The reason for this discrepancy between thetwo quasar data sets is not known. In general, the results from thepresent study are consistent with the merger-driven evolutionarysequence ``cool ULIGs-->warm ULIGs-->quasars.'' However, manyexceptions appear to exist to this simple picture (e.g., 46% of the 41advanced mergers show no obvious signs of Seyfert activity). Thisunderlines the importance of using a large homogeneous sample like the 1Jy sample to draw statistically meaningful conclusions; the problems ofsmall sample size and/or inhomogeneous selection criteria have plaguedmany studies of luminous infrared galaxies in the past.
|Optical and Near-Infrared Imaging of the IRAS 1 Jy Sample of Ultraluminous Infrared Galaxies. I. The Atlas|
An imaging survey of the IRAS 1 Jy sample of 118 ultraluminous infraredgalaxies was conducted at optical (R) and near-infrared (K')wavelengths using the University of Hawaii 2.2 m telescope. The methodsof observation and data reduction are described. An R and K'atlas of the entire sample is presented along with some of the basicastrometric and photometric parameters derived from these images. A moredetailed analysis of these data is presented in a companion paper.
|First Results from the COLA Project: The Radio-Far-Infrared Correlation and Compact Radio Cores in Southern COLA Galaxies|
We present the first results from the COLA (compact objects in low-powerAGNs) project, which aims to determine the relationship between onefacet of AGN activity, the compact radio core, and star formation in thecircumnuclear region of the host galaxy. This will be accomplished bythe comparison of the multiwavelength properties of a sample of AGNswith compact radio cores to those of a sample of AGNs without compactcores and a matched sample of galaxies without AGNs. In this paper wediscuss the selection criteria for our galaxy samples and present theinitial radio observations of the 107 southern(δ<0deg) galaxies in our sample. Low-resolution ATCAobservations at 4.8, 2.5, and 1.4 GHz and high-resolution,single-baseline snapshots at 2.3 GHz with the Australian Long BaselineArray (LBA) are presented. We find that for the majority of the galaxiesin our sample, the radio luminosity is correlated with the far-infrared(FIR) luminosity. However, a small number of galaxies exhibit a radioexcess causing them to depart from the FIR-radio correlation. Compactradio cores are detected at fluxes greater than 1.5 mJy in nine of the105 galaxies observed with the LBA. The majority (8/9) of these galaxiesexhibit a radio excess, and 50% (7/14) of the galaxies that lie abovethe radio-FIR correlation by more than 1 σ have compact radiocores. The emission from the compact cores is too weak to account forthis radio excess, implying that there are radio structures associatedwith the compact cores that extend farther than the 0.05" resolution(corresponding to a linear scale 11-22 pc) of the LBA. There is noevidence that the radio luminosity of the compact cores is correlatedwith the FIR galaxy luminosity, indicating that the core contributeslittle to the overall FIR emission of the galaxy. The galaxies withcompact cores tend to be classified optically as AGNs, with two-thirds(6/9) exhibiting Seyfert-like optical emission line ratios, and theremaining galaxies classified either as composite objects (2/9) orstarburst (1/9). The galaxies classified optically as AGNs also exhibitthe largest radio excesses, and we therefore conclude that a large radioexcess on the radio-FIR correlation is a strong indication of an AGNwith a compact radio core.
|X-ray luminosities of galaxies in groups|
We have derived the X-ray luminosities of a sample of galaxies ingroups, making careful allowance for contaminating intragroup emission.The LX:LB and LX:LFIRrelations of spiral galaxies in groups appear to be indistinguishablefrom those in other environments, however the elliptical galaxies fallinto two distinct classes. The first class is central-dominant groupgalaxies, which are very X-ray luminous and may be the focus of groupcooling flows. All other early-type galaxies in groups belong to thesecond class, which populates an almost constant band ofLX/LB over the range9.8
|ISOCAM-CVF 5-12 MICRON SPECTROSCOPY OF ULTRALUMINOUS INFRARED GALAXIES|
We present low-resolution mid-infrared (MIR) spectra of 16 ultraluminousinfrared galaxies (ULIRGs) obtained with the circular variable filter(CVF) spectroscopy mode of ISOCAM on board the Infrared SpaceObservatory (ISO). Our sample completes previous ISO spectroscopy ofultra- and hyperluminous infrared galaxies toward higher luminosities.The combined samples cover an infrared luminosity range of~1012-1013.1 Lsolar. To discriminateactive galactic nucleus (AGN) and starburst activity, we use theAGN-related MIR continuum and the starburst-related 6.2, 7.7, 8.6, and11.3 μm MIR emission bands attributed to aromatic carbonaceousmaterial. For about half of the high-luminosity ULIRGs studied here,strong aromatic emission bands suggest starburst dominance. Otherspectra are dominated by a strong AGN-related continuum with weaksuperposed emission features of uncertain nature. Our sample containsone unusual example, IRAS F00183-7111, of an AGN that is highly obscuredeven in the MIR. An improved method to characterize quantitatively therelative contribution of star formation and AGN activity to the MIRemission of ULIRGs is presented. The ULIRG spectra are fitted by asuperposition of a starburst and an AGN spectrum, both of which may beobscured at different levels. Models in which starburst and AGNobscuration differ are significantly more successful than models with asingle extinction. Previous results based on a simpler line-to-continuummeasure of aromatic emission strength are confirmed, further supportingthe robustness of the aromatic emission feature as a diagnostic of ULIRGpower sources. As dominant sources of the bolometric luminosity,starbursts prevail at the lower end and AGNs at the higher end of thisrange. The transition between mostly starburst and mostly AGN poweredoccurs at ~1012.4-1012.5 Lsolar, andindividual luminous starbursts are found up to ~1012.65Lsolar. Based on observations with the Infrared SpaceObservatory (ISO), an ESA project with instruments funded by ESA MemberStates (especially the PI countries: France, Germany, the Netherlands,and the United Kingdom) with the participation of ISAS and NASA.
|Where is the neutral atomic gas in Hickson groups?|
We have analyzed the total HI contents of 72 Hickson compact groups ofgalaxies (HCGs) and the detailed spatial distributions and kinematics ofHI within a subset of 16 groups using the high angular resolutionobservations obtained with the VLA in order to investigate a possibleevolutionary scenario for these densest systems in the present daygalaxy hierarchy. For the more homogeneous subsample of 48 groups, wefound a mean HI deficiency of Def_HI = 0.40 +/- 0.07, which correspondsto 40% of the expected HI for the optical luminosities and morphologicaltypes of the member galaxies. The individual galaxies show largerdegrees of deficiency than the groups globally, Def_HI = 0.62 +/- 0.09(24% of the expected HI), due in most cases to efficient gas strippingfrom individual galaxies into the group environment visible in the VLAmaps. The degree of deficiency is found to be similar to the centralgalaxies of Virgo and Coma cluster, and Coma I group, in spite of thesignificantly different characteristics (number of galaxies, velocitydispersion) of these environments. It does not seem plausible that asignificant amount of extended HI has been missed by the observations.Hence phase transformation of the atomic gas should explain the HIdeficiency. The groups richer in early type galaxies or more compactwith larger velocity dispersions show a weak tendency to be more HIdeficient. The detection rate of HCGs at X-ray wavelengths is larger forHI deficient groups, although the hot gas distribution and hence itsorigin is only known for a few cases. In the evolutionary scenario wepropose, the amount of detected HI would decrease further withevolution, by continuous tidal stripping and/or heating. The H_2 contentalso tends to be lower than expected for the galaxies in HI deficientgroups, this may suggest that the HI stripping by frequent tidalinteraction breaks the balance between the disruption of molecularclouds by star formation and the replenishment from the ambient HI. Thiswork is partially based on observations made with the VLA operated bythe National Radio Astronomy Observatory, a facility of the NationalScience Foundation operated under cooperative agreement by AssociatedUniversities, Inc., ALFOSC, which is owned by the Instituto deAstrofísica de Andalucía (IAA, CSIC) and operated at theNordic Optical Telescope (NOT) under agreement between IAA and the NBIfAof the Astronomical Observatory of Copenhagen, and 1.5 m telescope ofthe Observatorio de Sierra Nevada, Granada, Spain, which is operated bythe IAA (CSIC).
|XMM-Newton first-light observations of the Hickson galaxy group 16|
This paper presents the XMM-Newton first-light observations of theHickson-16 compact group of galaxies. Groups are possibly the oldestlarge-scale structures in the Universe, pre-dating clusters of galaxies,and are highly evolved. This group of small galaxies, at a redshift of0.0132 (or 80 Mpc) is exceptional in the having the highestconcentration of starburst or AGN activity in the nearby Universe. So itis a veritable laboratory for the study of the relationship betweengalaxy interactions and nuclear activity. Previous optical emission linestudies indicated a strong ionising continuum in the galaxies, but itsorigin, whether from starbursts, or AGN, was unclear. Combined imagingand spectroscopy with the EPIC X-ray CCDs unequivocally reveals aheavily obscured AGN and a separately identified thermal (starburst)plasma, in NGC 835, NGC 833, & NGC 839. NGC 838 shows only starburstthermal emission. Starbursts and AGN can evidently coexist in members ofthis highly evolved system of merged and merging galaxies, implying ahigh probability for the formation of AGN as well as starbursts inpost-merger galaxies.
|Cold gas and star formation in a merging galaxy sequence|
We explore the evolution of the cold gas (molecular and neutralhydrogen) and star formation activity during galaxy interactions, usinga merging galaxy sequence comprising both pre- and post-mergercandidates. Data for this study come from the literature, but aresupplemented by some new radio observations presented here. First, weconfirm that the ratio of far-infrared luminosity to molecular hydrogenmass (LFIRM(H2); star formation efficiency)increases close to nuclear coalescence. After the merging of the twonuclei there is evidence that the star formation efficiency declinesagain to values typical of ellipticals. This trend can be attributed toM(H2) depletion arising from interaction induced starformation. However, there is significant scatter, likely to arise fromdifferences in the interaction details (e.g., disc-to-bulge ratio,geometry) of individual systems. Secondly, we find that the centralmolecular hydrogen surface density, ΣH2,increases close to the final stages of the merging of the two nuclei.Such a trend, indicating gas inflows caused by gravitationalinstabilities during the interaction, is also predicted by numericalsimulations. Furthermore, there is evidence for a decreasing fraction ofcold gas mass from early interacting systems to merger remnants,attributed to neutral hydrogen conversion into other forms (e.g., stars,hot gas) and molecular hydrogen depletion resulting from ongoing starformation. The evolution of the total-radio to blue-band luminosityratio, reflecting the total (disc and nucleus) star formation activity,is also investigated. Although this ratio is on average higher than thatfor isolated spirals, we find a marginal increase along the mergingsequence, attributed to the relative insensitivity of disc starformation to interactions. However, a similar result is also obtainedfor the nuclear radio emission, although galaxy interactions arebelieved to significantly affect the activity (star formation, AGN) inthe central galaxy regions. Nevertheless, the nuclear-radio to blue-bandluminosity ratio is significantly elevated compared with that forisolated spirals. Finally, we find that the FIR-radio flux ratiodistribution of interacting galaxies is consistent with star formationbeing the main energizing source.
|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.
|Evidence for Multiple Mergers among Ultraluminous Infrared Galaxies: Remnants of Compact Groups?|
In a large sample of ultraluminous infrared galaxies (ULIRGs) imagedwith the Hubble Space Telescope, we have identified a significantsubsample that shows evidence for multiple mergers. The evidence is seenamong two classes of ULIRGs: (1) those with multiple remnant nuclei intheir core, sometimes accompanied by a complex system of tidal tails,and (2) those that are in fact dense groupings of interacting(soon-to-merge) galaxies. We conservatively estimate that, in theredshift range 0.05
|A Dynamical Study of Galaxies in the Hickson Compact Groups|
To investigate dynamical properties of spiral galaxies in the Hicksoncompact groups (HCGs), we present rotation curves of 30 galaxies in 20HCGs. We found as follows: (1) There is no significant relation betweendynamical peculiarity and morphological peculiarity in HCG spiralgalaxies. (2) There is no significant relation between the dynamicalproperties and the frequency distribution of nuclear activities in HCGspiral galaxies. (3) There are no significant correlations between thedynamical properties of HCG spiral galaxies and any group properties(i.e., size, velocity dispersion, galaxy number density, and crossingtime). (4) Asymmetric and peculiar rotation curves are more frequentlyseen in the HCG spiral galaxies than in field spiral galaxies or incluster ones. However, this tendency is more obviously seen in late-typeHCG spiral galaxies. These results suggest that the dynamical propertiesof HCG spiral galaxies do not strongly correlate with the morphology,the nuclear activity, and the group properties. Our results also suggestthat more frequent galaxy collisions occur in the HCGs than in the fieldand in the clusters.
|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.
|Optical Spectroscopy of the IRAS 1 JY Sample of Ultraluminous Infrared Galaxies|
This paper discusses the optical spectroscopic properties of the IRAS 1Jy sample (f_60>1 Jy) of ultraluminous infrared galaxies (ULIGs:L_IR>10^12 L_solar H_0=75 km s^-1 Mpc^-1 and q_0=0). One hundredeight of the 118 1 Jy ULIGs have been observed at Deltalambda=8.3Å resolution over the wavelength range ~4500-8900 Å. Thesedata are combined with large, previously published sets of opticalspectroscopic data of lower luminosity infrared galaxies to look forsystematic trends with infrared luminosity over the luminosity rangeL_IR~10^10.5-10^13 L_solar. As found in previous studies, the fractionof Seyfert galaxies among luminous infrared galaxies increases abruptlyabove L_IR~10^12.3 L_solar-about 50% of the galaxies withL_IR>10^12.3 L_solar present Seyfert characteristics. Many of theoptical and infrared spectroscopic properties of the Seyfert galaxiesare consistent with the presence of a genuine active galactic nucleus(AGN). About 30% of these galaxies are Seyfert 1 galaxies withbroad-line regions similar to those of optical quasars. Publishednear-infrared spectroscopy also suggests that many of the Seyfert 2galaxies (especially those with warm IRAS 25-60 μm colors) are infact obscured Seyfert 1 galaxies with broad (>~2000 km s^-1)recombination lines at 2 mum, where dust obscuration is less important.The percentage of Seyfert 1 ULIGs increases with infrared luminosity,contrary to the predictions of the standard unification model forSeyfert galaxies. Comparisons of the broad-line luminosities of opticaland obscured Seyfert 1 ULIGs with those of optically selected quasars ofcomparable bolometric luminosity suggest that the dominant energy sourcein most of these ULIGs is the same as in optical quasars, namely massaccretion onto a supermassive black hole, rather than a starburst. Theseresults are consistent with recently published ISO, ASCA, and VLBI data.On the other hand, there is no unambiguous optical or near-infraredspectroscopic evidence for AGNs in ULIGs optically classified as H IIregion galaxies (~30% of the whole sample) or as LINERs (~40%). Theapparent lack of energetically important AGNs in these objects supportsthe results from recent mid-infrared spectroscopy with ISO.Photoionization by hot stars from recent starbursts appears to be thedominant source of ionization in the objects with H II region-likespectra, while both hot stars and shocks may contribute to theionization in ULIGs with LINER-like spectra. The weaker Hβ and Mg Ib stellar absorption features, larger Hα emission equivalentwidths, and bluer optical continuum colors in objects with higherinfrared luminosities suggest that the starbursts took place morerecently (<~few times 10^7 yr) and/or are more important (~10% of thegalaxy mass) in ULIGs than in their lower luminosity counterparts. Asfound in optically selected starbursts, the emission-line gas in ULIGsis dustier than the stellar populations that are producing the opticalcontinuum. The color excess derived from the Balmer-line ratio does notsignificantly depend on the infrared luminosity, optical spectral type,or IRAS 25-60 μm color of the luminous infrared galaxies. Theseresults suggest that the optical method used to determine the colorexcess in infrared galaxies underestimates the amount of dust in thedustier objects.
|On the Influence of the Environment on the Star Formation Rates of a Sample of Galaxies in Nearby Compact Groups|
We present the results of a study of the star formation rates (SFRs) ofa sample of disk galaxies in nearby compact groups compared with theSFRs of a sample of field galaxies. For this purpose, Hαluminosities and equivalent widths were derived for the galaxies of oursample. A direct comparison of the equivalent widths and Hαluminosities, normalized to the B luminosities and estimated area of thegalaxies of both samples, yields the result that the median values ofthese quantities are almost identical for both samples, although thedistributions for the compact-group sample are broader around the meanvalue than was found in the field galaxy sample. This result can beexplained by assuming that although interactions between galaxies incompact groups can alter the SFRs, the median value of the normalizedSFRs is preserved, being almost indistinguishable from the correspondingvalue for field galaxies. Measuring the global L_Hα/L_B of thegroups, including early-type galaxies, we find that most of the groupsthat show the highest level of L_Hα/L_B with respect to a set ofsynthetic groups built out of field galaxies show tidal features in atleast one of their members. Finally, we have explored the relationshipbetween the ratio L_Hα/L_B and several relevant dynamicalparameters of the groups: velocity dispersion, crossing time, radius,and the mass-to-luminosity ratio, finding no clear correlation. Thissuggests that the exact dynamical state of a group does not control theSFR of the group as a whole. Our results are compatible with a scenariofor compact groups of galaxies in which the dark matter of the group isarranged in a common halo, therefore preventing a fast collapse of thegalaxies.
|Mid-Infrared and Optical Spectroscopy of Ultraluminous Infrared Galaxies: A Comparison|
New tools from Infrared Space Observatory (ISO) mid-infraredspectroscopy have recently become available to determine the powersources of dust-obscured ultraluminous infrared galaxies (ULIRGs). Wecompare ISO classifications-starburst or active galactic nucleus(AGN)-with classifications from optical spectroscopy and withoptical/near-infrared searches for hidden broad-line regions. Theagreement between mid-infrared and optical classification is excellentif optical LINER spectra are assigned to the starburst group. Thestarburst nature of ULIRG LINERs strongly supports the suggestion thatLINER spectra in infrared-selected galaxies, rather than being anexpression of the AGN phenomenon, are due to shocks that are probablyrelated to galactic superwinds. Differences between ISO and opticalclassification provide clues on the evolution of ULIRGs and on theconfiguration of obscuring dust. We find few ISO AGNs with optical H IIor LINER identification, suggesting that highly obscured AGNs exist butare not typical for the ULIRG phenomenon in general. Rather, our resultsindicate that strong AGN activity, once triggered, quickly breaks theobscuring screen at least in certain directions, thus becomingdetectable over a wide wavelength range.
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