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|Spitzer Space Telescope IRAC and MIPS Observations of the Interacting Galaxies IC 2163 and NGC 2207: Clumpy Emission|
IC 2163 and NGC 2207 are interacting galaxies that have been wellstudied at optical and radio wavelengths and simulated in numericalmodels to reproduce the observed kinematics and morphological features.Spitzer IRAC and MIPS observations reported here show over 200 brightclumps from young star complexes. The brightest IR clump is amorphologically peculiar region of star formation in the western arm ofNGC 2207. This clump, which dominates the Hα and radio continuumemission from both galaxies, accounts for ~12% of the total 24 μmflux. Nearly half of the clumps are regularly spaced along somefilamentary structure, whether in the starburst oval of IC 2163 or inthe thin spiral arms of NGC 2207. This regularity appears to influencethe clump luminosity function, making it peaked at a value nearly afactor of 10 above the completeness limit, particularly in the starburstoval. This is unlike the optical clusters inside the clumps, which havea luminosity function consistent with the usual power-law form. Thegiant IR clumps presumably formed by gravitational instabilities in thecompressed gas of the oval and the spiral arms, whereas the individualclusters formed by more chaotic processes, such as turbulencecompression, inside these larger scale structures.
|Late-Time Radio Observations of 68 Type Ibc Supernovae: Strong Constraints on Off-Axis Gamma-Ray Bursts|
We present late-time radio observations of 68 local Type Ibc supernovae,including six events with broad optical absorption lines(``hypernovae''). None of these objects exhibit radio emissionattributable to off-axis gamma-ray burst jets spreading into our line ofsight. Comparison with our afterglow models reveals the followingconclusions. (1) Less than ~10% of Type Ibc supernovae are associatedwith typical gamma-ray bursts initially directed away from our line ofsight; this places an empirical constraint on the GRB beaming factor of<~104, corresponding toan average jet opening angle, θj>~0.8d. (2) Thisholds in particular for the broad-lined supernovae (SNe 1997dq, 1997ef,1998ey, 2002ap, 2002bl, and 2003jd), which have been argued to host GRBjets. Our observations reveal no evidence for typical (or evensubenergetic) GRBs and rule out the scenario in which every broad-linedSN harbors a GRB at the 84% confidence level. Their large photosphericvelocities and asymmetric ejecta (inferred from spectropolarimetry andnebular spectroscopy) appear to be characteristic of the nonrelativisticSN explosion and do not necessarily imply the existence of associatedGRB jets.
|The grazing encounter between IC 2163 and NGC 2207: pushing the limits of observational modelling|
We present numerical hydrodynamical models of the collision between thegalaxies IC 2163 and NGC 2207. These models extend the results ofearlier work in which the galaxy discs were modelled one at a time. Weconfirm the general result that the collision is primarily planar, thatis, at moderate inclination relative to the two discs, and prograde forIC 2163, but retrograde for NGC 2207. We list 34 specific morphologicalor kinematic features on a variety of scales, found with multiwavebandobservations, which we use to constrain the models. The models are ableto reproduce most of these features, with a relative orbit in which thecompanion (IC 2163) disc first side-swipes the primary (NGC 2207) discon the west side, then moves around the edge of the primary disc to thenorth and to its current position on the east side. The models alsoprovide evidence that the dark matter halo of NGC 2207 has only moderateextent. For IC 2163, the prolonged prograde disturbance in the modelproduces a tidal tail, and an oval or ocular waveform very much like theobserved ones, including some fine structure. The retrograde disturbancein the model produces no strong waveforms within the primary galaxy.This suggests that the prominent spiral waves in NGC 2207 were presentbefore the collision, and models with waves imposed in the initialconditions confirm that they would not be disrupted by the collision.With an initial central hole in the gas disc of the primary, and imposedspirals, the model also reproduces the broad ring seen in HIobservations. Model gas disc kinematics compare well to the observed(HI) kinematics, providing further confirmation of its validity. Analgorithm for feedback heating from young stars is included, and thefeedback models suggest the occurrence of a moderate starburst in IC2163 about 250 Myr ago.We believe that this is now one of the best-modelled systems ofcolliding galaxies, though the model could still be improved byincluding full disc self-gravity. The confrontation between observationsand models of so many individual features provides one of the strongesttests of collision theory. The success of the models affirms thistheory, but the effort required to achieve this, and the sensitivity ofmodels to initial conditions, suggests that it will be difficult tomodel specific structures on scales smaller than about a kiloparsec inany collisional system.
|Bar-induced perturbation strengths of the galaxies in the Ohio State University Bright Galaxy Survey - I|
Bar-induced perturbation strengths are calculated for a well-definedmagnitude-limited sample of 180 spiral galaxies, based on the Ohio StateUniversity Bright Galaxy Survey. We use a gravitational torque method,the ratio of the maximal tangential force to the mean axisymmetricradial force, as a quantitative measure of the bar strength. Thegravitational potential is inferred from an H-band light distribution byassuming that the M/L ratio is constant throughout the disc. Galaxiesare deprojected using orientation parameters based on B-band images. Inorder to eliminate artificial stretching of the bulge, two-dimensionalbar-bulge-disc decomposition has been used to derive a reliable bulgemodel. This bulge model is subtracted from an image, the disc isdeprojected assuming it is thin, and then the bulge is added back byassuming that its mass distribution is spherically symmetric. We findthat removing the artificial bulge stretch is important especially forgalaxies having bars inside large bulges. We also find that the massesof the bulges can be significantly overestimated if bars are not takeninto account in the decomposition.Bars are identified using Fourier methods by requiring that the phasesof the main modes (m= 2, m= 4) are maintained nearly constant in the barregion. With such methods, bars are found in 65 per cent of the galaxiesin our sample, most of them being classified as SB-type systems in thenear-infrared by Eskridge and co-workers. We also suggest that as muchas ~70 per cent of the galaxies classified as SAB-types in thenear-infrared might actually be non-barred systems, many of them havingcentral ovals. It is also possible that a small fraction of the SAB-typegalaxies have weak non-classical bars with spiral-like morphologies.
|Spatial distribution of galaxies in the Puppis region|
We determine the spatial distribution of the galaxies located behind thepart of the zone of avoidance of the Milky Way defined by 220°
|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.
|Ongoing Mass Transfer in the Interacting Galaxy Pair NGC 1409/1410|
I present two-band Hubble Space Telescope (HST) STIS imaging and WIYNspectral mapping of ongoing mass transfer in the interacting galaxy pairNGC 1409/1410 (where NGC 1410 is the Seyfert galaxy also cataloged asIII Zw 55). Archival snapshot WFPC2 imaging from the survey by Malkanand coworkers showed a dust feature stretching between the galaxies,apparently being captured by NGC 1409. The new images allow estimates ofthe mass being transferred and the rate of transfer. An absorption lanetypically 0.25" (100 pc) wide, with a representative optical depthτB=0.2, cuts across the spiral structure of NGC 1410,crosses the 7 kpc projected space between the nuclei, wraps in front ofand, at the limits of detection, behind NGC 1409 and becomes a denser(τB=0.4) polar feature around the core of NGC 1409. Thecombination of extinction data in two passbands allows a crudethree-dimensional recovery of the dust structure, supporting thefront/back geometry derived from colors and extinction estimates. Thewhole feature contains on the order of 2×106Msolar in dust, implying about 3×108Msolar of gas and requiring a mass transfer rate averaging ~1Msolar yr-1, unless we are particularly unlucky inviewing angle. Curiously, this demonstrable case of mass transfer seemsto be independent of the occurrence of a Seyfert nucleus, since theSeyfert galaxy in this pair is the donor of the material. Likewise, therecipient shows no signs of recent star formation from incoming gas,although NGC 1410 has numerous luminous young star clusters andwidespread Hα emission.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
|The unexpected youth of globular clusters.|
|Classifications of the Host Galaxies of Supernovae, Set II|
Classifications on the DDO system are given for an additional 231 hostgalaxies of supernovae that have been discovered during the course ofthe Lick Observatory Supernova Search with the Katzman Automatic ImagingTelescope (KAIT). This brings the total number of hosts of supernovae(SNe) discovered (or independently rediscovered) by KAIT, which have sofar been classified on a homogeneous system, to 408. The probabilitythat SNe Ia and SNe II have a different distribution of host-galaxyHubble types is found to be 99.7%. A significant difference is alsofound between the distributions of the host galaxies of SNe Ia and ofSNe Ibc (defined here to include SNe Ib, Ib/c, and Ic). However, nosignificant difference is detected between the frequency distributionsof the host galaxies of SNe II and SNe IIn. This suggests that SNe IInare generally not SNe Ia embedded in circumstellar material that aremasquerading as SNe II. Furthermore, no significant difference is foundbetween the distribution of the Hubble types of the hosts of SNe Ibc andof SNe II. Additionally, SNe II-P and SNe II-L are found to occur amongsimilar stellar populations. The ratio of the number of SNe Ia-pec tonormal SNe Ia appears to be higher in early-type galaxies than it is ingalaxies of later morphological types. This suggests that the ancestorsof SNe Ia-pec may differ systematically in age or composition from theprogenitors of normal SNe Ia. Unexpectedly, five SNe of Types Ib/c, II,and IIn (all of which are thought to have massive progenitors) are foundin host galaxies that are nominally classified as types E and S0.However, in each case the galaxy classification is uncertain, or newlyinspected images show evidence suggesting a later classification. Amongthese five objects, NGC 3720, the host galaxy of SN 2002at, wasapparently misidentified in the Carnegie Atlas of Galaxies.
|A Search for Core-Collapse Supernova Progenitors in Hubble Space Telescope Images|
Identifying the massive progenitor stars that give rise to core-collapsesupernovae (SNe) is one of the main pursuits of supernova and stellarevolution studies. Using ground-based images of recent, nearby SNeobtained primarily with the Katzman Automatic Imaging Telescope,astrometry from the Two Micron All Sky Survey, and archival images fromthe Hubble Space Telescope, we have attempted the direct identificationof the progenitors of 16 Type II and Type Ib/c SNe. We may haveidentified the progenitors of the Type II SNe 1999br in NGC 4900, 1999evin NGC 4274, and 2001du in NGC 1365 as supergiant stars withM0V~-6 mag in all three cases. We may have alsoidentified the progenitors of the Type Ib SNe 2001B in IC 391 and 2001isin NGC 1961 as very luminous supergiants withM0V~-8 to -9 mag, and possibly the progenitor ofthe Type Ic SN 1999bu in NGC 3786 as a supergiant withM0V~-7.5 mag. Additionally, we have recovered atlate times SNe 1999dn in NGC 7714, 2000C in NGC 2415, and 2000ew in NGC3810, although none of these had detectable progenitors on pre-supernovaimages. In fact, for the remaining SNe only limits can be placed on theabsolute magnitude and color (when available) of the progenitor. Thedetected Type II progenitors and limits are consistent with redsupergiants as progenitor stars, although possibly not as red as we hadexpected. Our results for the Type Ib/c SNe do not strongly constraineither Wolf-Rayet stars or massive interacting binary systems asprogenitors. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the data archive of the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.
|Magnetohydrodynamic density waves in a composite disc system of interstellar medium and cosmic ray gas|
Multiwavelength observations from radio to soft X-ray bands oflarge-scale galactic spiral structures offer synthesized andcomprehensive views of nearby disc galaxies. In the presence of amassive dark matter halo, the density-wave dynamics on galactic scalesinvolves the stellar disc, the gas disc of the interstellar medium(ISM), the magnetic field and the cosmic ray gas (CRG). In this paper,we explore the dynamic and electromagnetic interplay between themagnetized ISM disc and the CRG disc so that structural and diagnosticfeatures of optical, infrared and synchrotron radio-continuum emissionsfrom a spiral galaxy can be understood physically. On time-scales ofgalactic density waves, cosmic rays collectively may be treated as arelativistically hot tenuous gas fluid that is tied to the large-scalemean magnetic field in transverse bulk motions but moves otherwisedifferently along the magnetic field relative to the ISM. For both fastand slow magnetohydrodynamic (MHD) density waves in a composite discsystem of magnetized ISM and CRG, the minute CRG mass-densityenhancement is phase shifted relative to the enhancement of the parallelmagnetic field. Owing to the extremely small number of cosmic rays, thelarge-scale magnetic field enhancement dominates in synchrotronradio-continuum emissions (as if the CRG is almost unperturbed) forspiral structural manifestations. In addition to the fast and slow MHDdensity waves, there also exists a suprathermal MHD wave mode by whichCRG adjusts itself with an effective suprathermal speed of sound closeto the speed of light c.
|ATCA HI observations of the peculiar galaxy IC 2554|
ATCA HI and radio continuum observations of the peculiar southern galaxyIC 2554 and its surroundings reveal typical signatures of an interactinggalaxy group. We detected a large HI cloud between IC 2554 and theelliptical galaxy NGC 3136B. The gas dynamics in IC 2554 itself, whichis sometimes described as a colliding pair, are surprisingly regular,whereas NGC 3136B was not detected. The HI cloud, which emerges from IC2554 as a large arc-shaped plume, has a size of ~30 kpc, larger thanthat of IC 2554. The total HI mass of the IC 2554 system is ~2 ×109 Msolar, one-third of which resides in the HIcloud. It is possible that tidal interaction between IC 2554 and NGC3136B caused this spectacular HI cloud, but the possibility of IC 2554being a merger remnant is also discussed. We also detected HI gas in thenearby galaxies ESO 092-G009 and RKK 1959 and an associated HI cloud,ATCA J1006-6710. Together they have an HI mass of ~4.6 ×108 Msolar. Another new HI source, ATCAJ1007-6659, with an HI mass of only ~2.2 × 107Msolar was detected roughly between IC 2554 and ESO 092-G009and corresponds to a face-on low surface brightness dwarf galaxy. Starformation is evident only in the galaxy IC 2554 with a rate of ~4Msolar yr-1.
|Supernova 2003H in NGC 2207|
IAUC 8049 available at Central Bureau for Astronomical Telegrams.
|Supernova 2003H in NGC 2207.|
M. Hamuy, Carnegie Observatories, reports on spectroscopic observationsmade (as detailed on IAUC 8045) by J. Maza on Jan. 9 and 10 UT: "Thespectrum of SN 2003H (cf. IAUC 8045, 8046) shows a red continuum with aprominent absorption at 575.1 nm; if this is He I 587.6-nm, theexpansion velocity is 9100 km/s. A more shallow blend of absorptionlines can be seen at 600-670 nm, and an isolated absorption is centeredat 690 nm. The Ca II triplet displays a prominent P-Cyg profile. Thisspectrum has some similarity to that of the type-Ib supernova 1984Lbefore maximum light, but the continuum is much more red. There is noevidence for strong Na I D interstellar lines, so dust extinction mustbe small and the red color is probably intrinsic to SN 2003H."
|Supernova 2003H in NGC 2207|
IAUC 8046 available at Central Bureau for Astronomical Telegrams.
|Supernovae 2003E, 2003F, 2003G, 2003H|
IAUC 8045 available at Central Bureau for Astronomical Telegrams.
|Interstellar Dust Grains|
This review surveys the observed properties of interstellar dust grains:the wavelength-dependent extinction of starlight, including absorptionfeatures, from UV to infrared; optical luminescence; infrared emission;microwave emission; optical, UV, and X-ray scattering by dust; andpolarization of starlight and of infrared emission. The relationshipbetween presolar grains in meteorites and the interstellar grainpopulation is discussed. Candidate grain materials and abundanceconstraints are considered. A dust model consisting of amorphoussilicate grains, graphite grains, and polycyclic aromatic hydrocarbonsis compared with observed emission and scattering. Some issuesconcerning evolution of interstellar dust are discussed.
|The IRAS Revised Bright Galaxy Sample|
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
|Massive Star Clusters in Ongoing Galaxy Interactions: Clues to Cluster Formation|
We present HST WFPC2 observations, supplemented by ground-based Hαdata, of the star-cluster populations in two pairs of interactinggalaxies selected for being in very different kinds of encounters seenat different stages. Dynamical information and n-body simulationsprovide the details of encounter geometry, mass ratio, and timing. InNGC 5752/4 we are seeing a weak encounter, well past closest approach,after about 2.5×108 yr. The large spiral NGC 5754 has anormal population of disk clusters, while the fainter companion NGC 5752exhibits a rich population of luminous clusters with a flatterluminosity function. The strong, ongoing encounter in NGC 6621/2, seenabout 1.0×108 yr past closest approach between roughlyequal-mass galaxies, has produced an extensive population of luminousclusters, particularly young and luminous in a small region between thetwo nuclei. This region is dynamically interesting, with such a strongperturbation in the velocity field that the rotation curve reversessign. From these results, in comparison with other strongly interactingsystems discussed in the literature, cluster formation requires athreshold level of perturbation, with stage of the interaction a lessimportant factor. The location of the most active star formation in NGC6621/2 draws attention to a possible role for the Toomre stabilitythreshold in shaping star formation in interacting galaxies. The richcluster populations in NGC 5752 and NGC 6621 show that direct contactbetween gas-rich galaxy disks is not a requirement to form luminousclusters and that they can be triggered by processes happening within asingle galaxy disk (albeit triggered by external perturbations).Based on observations 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.
|The Mysteries of Galaxy Spirals|
|Classifications of the Host Galaxies of Supernovae|
Classifications on the DDO system are given for the host galaxies of 177supernovae (SNe) that have been discovered since 1997 during the courseof the Lick Observatory Supernova Search with the Katzman AutomaticImaging Telescope. Whereas SNe Ia occur in all galaxy types, it isfound, at a high level of statistical confidence, that SNe Ib, Ic, andII are strongly concentrated in late-type galaxies. However, attentionis drawn to a possible exception provided by SN 2001I. This SN IInoccurred in the E2 galaxy UGC 2836, which was not expected to harbor amassive young supernova progenitor.
|Multiwavelength Structural Manifestations of Fast Magnetohydrodynamic Density Waves in Spiral Galaxies|
In a spiral galaxy, disks of stars and of magnetized interstellar medium(ISM) are dynamically coupled by gravity. The theory ofmagnetohydrodynamic (MHD) density waves in such a composite rotatingdisk system provides a basic framework for studying large-scale dynamicsand multiwavelength diagnostics of galactic structures. As swirllike,trailing spiral structures with broad, fuzzy arms (regular or irregular)manifest in neutral hydrogen H I disks that are usually larger thanoptical spiral patterns in disk galaxies, spiral MHD density waves in HI gas disks should persist within and beyond optical patterns. Sporadicdiffusions of relativistic cosmic-ray electrons beyond optical patternsmay cause radio continuum arms to occasionally extend across opticalspiral patterns and correlate with H I spiral arms at larger radii. Wehere report the first H I observations of the southern spiral galaxy NGC2997 to support the fast MHD density wave (FMDW) scenario and to confirmthe prior prediction that the isolated, polarized radio continuum armdiscovered in the southeast quadrant is indeed associated with a broadsegment of the H I arm. For multiwavelength observations of spiralgalaxies, we emphasize the important perspective that large-scalemagnetic fields of MHD density waves play the key role in organizingcorrelated spiral structures of the various underlying ISM components.Meanwhile, we briefly discuss the nature of circumnuclear structures ofNGC 2997 in reference to those of NGC 1097.
|A Search for H2O Maser Emission in Southern Active Galactic Nuclei and Star-forming Galaxies: Discovery of a Maser in the Edge-on Galaxy IRAS F01063-8034|
We report the cumulative results of five surveys for H2Omaser emission at 1.35 cm wavelength in 131 active galactic nuclei(AGNs) and star-forming galaxies, conducted at the Parkes Observatorybetween 1993 and 1998. We detected one new maser, in the edge-on galaxyIRAS F01063-8034, which exhibits a single ~0.1 Jy spectral feature at4282+/-6 km s-1 (heliocentric) with an unusually large54+/-16 km s-1 half-power full width. The centroid velocityof the emission increased to 4319.6+/-0.6 km s-1 (38+/-2 kms-1 width) over the 13 days between discovery andconfirmation of the detection. A similarly broad-line width and largechange in velocity has been noted for the maser in NGC 1052, wherein jetactivity excites the emission. Neither optical spectroscopy,radio-infrared correlations, nor infrared colors provide compellingevidence of unusual activity in the nucleus of IRAS F01063-8034. Sincethe galaxy appears to be outwardly normal at optical and infraredwavelengths, detection of an H2O maser therein is unique. Themaser emission is evidence that the galaxy harbors an AGN that isprobably obscured by the edge-on galactic disk. The detection highlightsthe possibility that undetected AGNs could be hidden in other relativelynearby galaxies. No other maser emission features have been identifiedat velocities between 3084 and 6181 km s-1.
|The H I Line Width/Linear Diameter Relationship as an Independent Test of the Hubble Constant|
The relationship between corrected H I line widths and linear diameters(LW/LD) for spiral galaxies is used as an independent check on the valueof the Hubble constant. After calibrating the Tully-Fisher (TF) relationin both the B and I bands, the B-band relation is used for galaxies ofmorphological/luminosity types Sc I, Sc I.2, Sc I.3, Sab, Sb, Sb I-II,and Sb II to derive the LW/LD relation. We find that for this sample thescatter in the LW/LD is smallest with a Hubble constant of 90-95 kms-1 Mpc-1. Lower values of the Hubble constantproduce a separation in the LW/LD relation that is a function ofmorphological type. Since a Hubble constant of 90-95 is significantlylarger than the final Key Project value of 72 km s-1Mpc-1, a comparison of TF, surface brightness fluctuation(SBF), and fundamental plane (FP) is made. This comparison indicatesthat the Key Project TF distances to 21 clusters may be too large. For asample of 11 clusters, the Key Project TF distances provide anunweighted mean Hubble constant of 77 km s-1Mpc-1, while a combination of the FP, SBF, and our TFdistances for the same 11 clusters gives H0=91 kms-1 Mpc-1. A more subtle result in our data is amorphological dichotomy in the Hubble constant. The data suggest that ScI galaxies follow a Hubble constant of 90-95 while Sb galaxies follow aHubble constant closer to 75 km s-1 Mpc-1.Possible explanations for this result are considered, but it is shownthat this Sb/Sc I Hubble flow discrepancy is also present in the VirgoCluster and is consistent with previous investigations that indicatethat some galaxies carry a component of age-related intrinsic redshift.
|Dusty Acoustic Turbulence in the Nuclear Disks of Two LINER Galaxies NGC 4450 and NGC 4736|
The structure of dust spirals in the nuclei of the SAab-type LINERgalaxies NGC 4450 and NGC 4736 is studied using archival Hubble SpaceTelescope Planetary Camera images. The spirals are typically onlyseveral hundredths of a magnitude fainter than the neighboring disks, sounsharp-mask techniques are used to highlight them. The ambientextinction is estimated to be less than 0.1 mag from the intensitydecrements of the dust features and from the spiral surface fillingfactor, which is about constant for all radii and sizes. The nucleardust spirals differ from main disk spirals in several respects: thenuclear spirals have no associated star formation, they are veryirregular with both trailing and leading components that often cross,they become darker as they approach the center, they completely fill theinner disks with a constant areal density, making the number of distinctspirals (the azimuthal wavenumber m) increase linearly with radius, andtheir number decreases with increasing arm width as a power law. Fouriertransform power spectra of the spirals, taken in the azimuthaldirection, show a power-law behavior with a slope of -5/3 over the rangeof frequencies where the power stands above the pixel noise. This is thesame slope as that found for the one-dimensional power spectra of H Iemission in the Large Magellanic Cloud and also the slope expected for athin turbulent disk. All of these properties suggest that the dustspirals are a manifestation of acoustic turbulence in the inner gasdisks of these galaxies. Such turbulence should dissipate orbital energyand transfer angular momentum outward, leading to a steady accretion ofgas toward the nucleus.
|Bar Galaxies and Their Environments|
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
|Circumnuclear Star Formation in the Spiral Galaxy NGC 3310|
The star-forming properties of the circumnuclear ring in the starburstspiral galaxy NGC 3310 have been studied in B, I, J, H, and K bandsusing images from KPNO and from the HST archives. The colors andmagnitudes of the star-forming regions indicate ages less than 10 Myrand masses of 104 to 105 Msolar for thelargest clumps. The luminosity distribution function of the diffusecircumnuclear clusters has a slope of about -2, which is typical ofresults both in circumnuclear rings and in the main disks of othergalaxies. There are 17 candidate super star clusters (SSCs), primarilyin the innermost southern spiral arm. The broad wavelength coverageallows a determination of reddening in the vicinity of the SSCs, whichappears to be small. The locations of the SSCs in the circumnuclear ringand in an inner spiral are coincident with radio continuum and emissionpeaks, and may be the result of a suspected cannibalization of a dwarfgalaxy in the last 10 Myr.
|CO Observations of the Interacting Galaxy Pair NGC 5394/95|
BIMA 12CO J=1-->0 observations are presented of the spiralgalaxies NGC 5394 and NGC 5395 that have undergone a recent, grazingencounter. In NGC 5394 approximately 80% of the CO emission detected byBIMA is concentrated in the central 800 pc (FWHM) starburst region, andthe rest is from a portion of the inner disk south and west of thecentral starburst. In an encounter simulation that reproduces some ofthe main features of this galaxy pair, a considerable amount of gas inNGC 5394 falls into the central region early in the collision. Theobserved total gas distribution in the disk of NGC 5394 is lopsided,with more H I, CO, and Hα emission coming from the western orsouthwestern side. The innermost western arm of NGC 5394 is seen in COand Hα emission, but the eastern inner-disk arm, which is verybright in the optical continuum, is not detected in CO or Hαemission. The NGC 5394 starburst region is similar in radio continuumluminosity and size to the M82 starburst and has a CO luminosity ~4times greater. A CO position-velocity diagram of the NGC 5394 nucleusreveals two separate velocity features very close to the center. Thismay indicate a nuclear ring or the ``twin peaks'' of an ILR or somedepletion of 12CO J=1-->0 at the nucleus. From acomparison of the radio continuum, Hα, 60 μm, and COluminosities, we estimate that the average extinction Av ofthe starburst nucleus is 3-4 mag, the star formation rate is ~6Msolar yr-1, and the conversion factorN(H2)/ICO in the starburst is a factor of 3-4below the standard value. Comparison of NGC 5394 with two other systemspreviously studied suggests that in prograde grazing encounters acentral starburst may not develop until near the end of the ocularphase. Very little of the CO emission from NGC 5395 found in previoussingle-dish observations is detected in the BIMA data; thus moleculargas in NGC 5395 does not appear to be strongly concentrated incompressed ridges.
|Supernovae in isolated galaxies, in pairs and in groups of galaxies|
In order to investigate the influence of environment on supernova (SN)production, we have performed a statistical investigation of the SNediscovered in isolated galaxies, in pairs and in groups of galaxies. 22SNe in 18 isolated galaxies, 48 SNe in 40 galaxy members of 37 pairs and211 SNe in 170 galaxy members of 116 groups have been selected andstudied. We found that the radial distributions of core-collapse SNe ingalaxies located in different environments are similar, and consistentwith those reported by Bartunov, Makarova & Tsvetkov. SNe discoveredin pairs do not favour a particular direction with respect to thecompanion galaxy. Also, the azimuthal distributions inside the hostmembers of galaxy groups are consistent with being isotropics. The factthat SNe are more frequent in the brighter components of the pairs andgroups is expected from the dependence of the SN rates on the galaxyluminosity. There is an indication that the SN rate is higher in galaxypairs compared with that in groups. This can be related to the enhancedstar formation rate in strongly interacting systems. It is concludedthat, with the possible exception of strongly interacting systems, theparent galaxy environment has no direct influence on SN production.
|H i observations of interacting galaxy pair NGC 4038/9|
We present the results of new radio interferometer Hi line observationsfor the merging galaxy pair NGC 4038/9 (`The Antennae'), obtained usingthe Australia Telescope Compact Array. The results improve substantiallywith respect to those of van der Hulst and show in detail the twomerging galactic discs and the two tidal tails produced by theirinteraction. The small edge-on spiral dwarf galaxy ESO 572-G045 is alsoseen near the tip of the southern tail, but distinct from it. It showsno signs of tidal interaction. The northern tidal tail of the Antennaeshows no Hi connection to the discs and has an extension towards thewest. The southern tidal tail is continuous, with a prominent Hiconcentration at its tip, roughly at the location of the tidal dwarfgalaxy observed optically by Mirabel, Dottori & Lutz. Clear velocitystructure is seen along the tidal tails and in the galactic discs. Radiocontinuum images at 20 and 13cm are also presented, showing the discs indetail.
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