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Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| The Pattern Speeds of 38 Barred Galaxies
We estimate the pattern speeds of 38 barred galaxies by simulationmodeling. We construct the gravitational potentials of the galaxies fromnear-IR photometry by assuming that the mass-to-light ratio (M/L) isconstant in the H band and a single pattern speed dominates in thestellar disk. We use the response of gaseous and stellar particle disksto a rigidly rotating potential to determine the pattern speed. If ourassumptions are correct, then the pattern speed depends on themorphological type: the average value of the ratio of the corotationresonance radius to the bar radius, ℛ, increases from about 1.1 intype SB0/a to 1.4 in SBb and 1.7 in SBc. Within the error estimates, allthe bars in galaxies of type SBab or earlier are fast rotators, havingℛ<=1.4, whereas late-type galaxies include both fast and slowrotators.
| The Vertical Stellar Kinematics in Face-On Barred Galaxies: Estimating the Ages of Bars
In order to perform a detailed study of the stellar kinematics in thevertical axis of bars, we obtained high signal-to-noise spectra alongthe major and minor axes of the bars in a sample of 14 face-on galaxiesand used them to determine the line-of-sight stellar velocitydistribution, parameterized as a Gauss-Hermite series. With these data,we developed a diagnostic tool that allows one to distinguish betweenrecently formed and evolved bars, as well as to estimate their ages,assuming that bars form in vertically thin disks that are recognizableby low values for the vertical velocity dispersion σz.Through N-body realizations of bar unstable disk galaxies we were alsoable to check the timescales involved in the processes that give bars animportant vertical structure. We show that σz inevolved bars is roughly 100 km s-1, which translates to aheight scale of about 1.4 kpc, giving support to scenarios in whichbulges form through disk material. Furthermore, the bars in ournumerical simulations have values for σz generallysmaller than 50 km s-1, even after evolving for 2 Gyr,suggesting that a slow process is responsible for making bars asvertically thick as we observe. We verify theoretically that theSpitzer-Schwarzschild mechanism is quantitatively able to explain theseobservations if we assume that giant molecular clouds are twice asconcentrated along the bar as in the rest of the disk.
| The Distribution of Bar and Spiral Arm Strengths in Disk Galaxies
The distribution of bar strengths in disk galaxies is a fundamentalproperty of the galaxy population that has only begun to be explored. Wehave applied the bar-spiral separation method of Buta and coworkers toderive the distribution of maximum relative gravitational bar torques,Qb, for 147 spiral galaxies in the statistically well-definedOhio State University Bright Galaxy Survey (OSUBGS) sample. Our goal isto examine the properties of bars as independently as possible of theirassociated spirals. We find that the distribution of bar strengthdeclines smoothly with increasing Qb, with more than 40% ofthe sample having Qb<=0.1. In the context of recurrent barformation, this suggests that strongly barred states are relativelyshort-lived compared to weakly barred or nonbarred states. We do notfind compelling evidence for a bimodal distribution of bar strengths.Instead, the distribution is fairly smooth in the range0.0<=Qb<0.8. Our analysis also provides a first look atspiral strengths Qs in the OSUBGS sample, based on the sametorque indicator. We are able to verify a possible weak correlationbetween Qs and Qb, in the sense that galaxies withthe strongest bars tend to also have strong spirals.
| A sample of X-ray emitting normal galaxies from the BMW-HRI Catalogue
We obtained a sample of 143 normal galaxies with X-ray luminosity in therange 1038{-}1043 erg s-1 from thecross-correlation of the ROSAT HRI Brera Multi-scale Wavelet (BMW-HRI)Catalogue with the Lyon-Meudon Extragalactic Database (LEDA). We findthat the average X-ray properties of this sample are in good agreementwith those of other samples of galaxies in the literature. We selected acomplete flux limited serendipitous sample of 32 galaxies from which wederived the log N-log S distribution of normal galaxies in the fluxrange 1.1{-} 110 × 10-14 erg cm-2s-1. The resulting distribution is consistent with theEuclidean -1.5 slope. Comparisons with other samples, such as theExtended Medium Sensitivity Survey, the ROSAT All Sky Survey, theXMM-Newton/2dF survey, and the Chandra Deep Field Survey indicate thatthe log N -log S distribution of normal galaxies is consistent with aEuclidean slope over a flux range of about 6 decades.
| Spectro-morphology of galaxies: A multi-wavelength (UV-R) classification method
We present a quantitative method to classify galaxies, based onmulti-wavelength data and constructed from the properties of nearbygalaxies. Our objective is to define an classification method that canbe used for low and high redshift objects. We estimate the concentrationof light (C) at the galaxy center and the 180° rotational asymmetry(A), computed at several wavelengths, from ultraviolet (UV) to I-band.The variation of the indices of concentration and asymmetry with thewavelength reflects the proportion and the distribution of young and oldstellar populations in galaxies. In general C is found to decrease, andA to increase from optical to UV: the patchy appearance of a galaxy inthe UV with no bulge is often very different from its counterpart atoptical wavelengths, with a prominent bulge and a more regular disk. Wequantify the variation of C and A with wavelength. In this way we areable to distinguish five types of galaxies that we callspectro-morphological types: compact, ringed, spiral, irregular andcentral-starburst galaxies, which can be differentiated by thedistribution of their stellar populations. We discuss in detail themorphology of the galaxies of the sample, and describe the morphologicalcharacteristics of each spectro-morphological type. We applyspectro-morphology to three objects at a redshift z1 in the HubbleDeep Field North, which gives encouraging results for applications tolarge samples of high-redshift galaxies. This method of morphologicalclassification could be used to study the evolution of the morphologywith redshift and is expected to put observational constraints onscenarios of galaxy evolution.
| 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.
| Secular Evolution and the Formation of Pseudobulges in Disk Galaxies
The Universe is in transition. At early times, galactic evolution wasdominated by hierarchical clustering and merging, processes that areviolent and rapid. In the far future, evolution will mostly be secularthe slow rearrangement of energy and mass that results from interactionsinvolving collective phenomena such as bars, oval disks, spiralstructure, and triaxial dark halos. Both processes are important now.This review discusses internal secular evolution, concentrating on oneimportant consequence, the buildup of dense central components in diskgalaxies that look like classical, merger-built bulges but that weremade slowly out of disk gas. We call these pseudobulges.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Deprojecting spiral galaxies using Fourier analysis. Application to the Ohio sample
We use two new methods developed recently (Barberàet al.\cite{bar03}, A&A, 415, 849), as well as information obtained fromthe literature, to calculate the orientation parameters of the spiralgalaxies in the Ohio State University Bright Galaxy Survey. We comparethe results of these methods with data from the literature, and find ingeneral good agreement. We provide a homogeneous set of mean orientationparameters which can be used to approximately deproject the disks of thegalaxies and facilitate a number of statistical studies of galaxyproperties.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/421/595
| Double-barred galaxies. I. A catalog of barred galaxies with stellar secondary bars and inner disks
I present a catalog of 67 barred galaxies which contain distinct,elliptical stellar structures inside their bars. Fifty of these aredouble-barred galaxies: a small-scale, inner or secondary bar isembedded within a large-scale, outer or primary bar. I providehomogenized measurements of the sizes, ellipticities, and orientationsof both inner and outer bars, along with global parameters for thegalaxies. The other 17 are classified as inner-disk galaxies, where alarge-scale bar harbors an inner elliptical structure which is alignedwith the galaxy's outer disk. Four of the double-barred galaxies alsopossess inner disks, located in between the inner and outer bars. Whilethe inner-disk classification is ad-hoc - and undoubtedly includes someinner bars with chance alignments (five such probable cases areidentified) - there is good evidence that inner disks form astatistically distinct population, and that at least some are indeeddisks rather than bars. In addition, I list 36 galaxies which may bedouble-barred, but for which current observations are ambiguous orincomplete, and another 23 galaxies which have been previously suggestedas potentially being double-barred, but which are probably not. Falsedouble-bar identifications are usually due to features such as nuclearrings and spirals being misclassified as bars; I provide someillustrated examples of how this can happen.A detailed statistical analysis of the general population of double-barand inner-disk galaxies, as represented by this catalog, will bepresented in a companion paper.Tables \ref{tab:measured} and \ref{tab:deproj} are only available inelectronic form at http://www.edpsciences.org
| Astronomical seeing at the Magdalena Ridge Observatory
Not Available
| The Internal Ultraviolet-Optical Color Dispersion: Quantifying the Morphological K-Correction
We present a quantitative measure of the internal color dispersionwithin galaxies, which quantifies differences in galaxy morphology as afunction of observed wavelength. We apply this statistic to a sample oflocal galaxies with archival images at 1500 and 2500 Å from theUltraviolet Imaging Telescope and ground-based B-band observations inorder to investigate how the internal dispersion between these colorsrelates to global galaxy properties (e.g., luminosity, color, andmorphological type). In general, the dispersion in the internal galaxycolors correlates with transformations in the galaxy morphology as afunction of wavelength; i.e., our internal color dispersion statisticquantifies the morphological K-correction. Mid-type spiral galaxiesexhibit the highest dispersion in their ultraviolet-optical internalcolors, which stems from differences in the stellar content thatconstitute the bulge, disk, and spiral-arm components. Irregular andlate-type spiral galaxies show moderate internal color dispersion,although with lower values relative to the mid-type spirals. Thisimplies that young stars generally dominate the ultraviolet-opticalgalaxy colors, modulo variations in the dust, gas, and stellardistributions. Elliptical, lenticular, and early-type spiral galaxiesgenerally have low or negligible internal color dispersion, whichindicates that the stars contributing to the ultraviolet-opticalemission have a very homogeneous distribution. We discuss theapplication of the internal color dispersion to high-redshift galaxiesin deep Hubble Space Telescope images. By simulating the appearance ofthe local galaxy sample at cosmological distances, many of the galaxieshave luminosities that are sufficiently bright at rest-frame opticalwavelengths to be detected within the limits of the currently deepestnear-infrared surveys, even with no evolution. Under the assumption thatthe luminosity and color evolution of the local galaxies conform withthe measured values of high-redshift objects, we show that galaxies'intrinsic internal color dispersion remains measurable out to z~3.
| When Is a Bulge Not a Bulge? Inner Disks Masquerading as Bulges in NGC 2787 and NGC 3945
We present a detailed morphological, photometric, and kinematic analysisof two barred S0 galaxies with large, luminous inner disks inside theirbars. We show that these structures, in addition to being geometricallydisklike, have exponential profiles (scale lengths ~300-500 pc) distinctfrom the central, nonexponential bulges. We also find them to bekinematically disklike. The inner disk in NGC 2787 has a luminosityroughly twice that of the bulge; but in NGC 3945, the inner disk isalmost 10 times more luminous than the bulge, which itself is extremelysmall (half-light radius ~100 pc, in a galaxy with an outer ring ofradius ~14 kpc) and has only ~5% of the total luminosity-a bulge/totalratio much more typical of an Sc galaxy. We estimate that at least 20%of (barred) S0 galaxies may have similar structures, which means thattheir bulge/disk ratios may be significantly overestimated. These innerdisks dominate the central light of their galaxies; they are at least anorder of magnitude larger than typical ``nuclear disks'' found inelliptical and early-type spiral galaxies. Consequently, they mustaffect the dynamics of the bars in which they reside.
| Infrared Emission of Normal Galaxies from 2.5 to 12 Micron: Infrared Space Observatory Spectra, Near-Infrared Continuum, and Mid-Infrared Emission Features
We present ISOPHOT spectra of the regions 2.5-4.9 μm and 5.8-11.6μm for a sample of 45 disk galaxies from the US Infrared SpaceObservatory Key Project on Normal Galaxies. The galaxies were selectedto span the range in global properties of normal, star-forming diskgalaxies in the local universe. The spectra can be decomposed into threespectral components: (1) continuum emission from stellar photospheres,which dominates the near-infrared (NIR; 2.5-4.9 μm) spectral region;(2) a weak NIR excess continuum, which has a color temperature of~103 K, carries a luminosity of a few percent of the totalfar-infrared (FIR) dust luminosity LFIR and most likelyarises from the interstellar medium (ISM); and (3) the well-known broademission features at 6.2, 7.7, 8.6, and 11.3 μm, which are generallyattributed to aromatic carbon particles. These aromatic features inemission (AFEs) dominate the mid-infrared (MIR; 5.8-11.6 μm) part ofthe spectrum and resemble the so-called type A spectra observed in manynonstellar sources and the diffuse ISM in our own Galaxy. The fewnotable exceptions include NGC 4418, where a dust continuum replaces theAFEs in MIR, and NGC 1569, where the AFEs are weak and the strongestemission feature is [S IV] 10.51 μm. The relative strengths of theAFEs vary by 15%-25% among the galaxies. However, little correlation isseen between these variations and either IRAS 60 μm/100 μm fluxdensity ratio R(60/100) or the FIR/blue luminosity ratioLFIR/LB, two widely used indicators of the currentstar formation activity, suggesting that the observed variations are nota consequence of the radiation field differences among the galaxies. Wedemonstrate that the NIR excess continuum and AFE emission arecorrelated, suggesting that they are produced by similar mechanisms andsimilar (or the same) material. On the other hand, as the current starformation activity increases, the overall strengths of the AFEs and theNIR excess continuum drop significantly with respect to that of the FIRemission from large dust grains. In particular, the summed luminosity ofthe AFEs falls from ~0.2 LFIR for the most ``IR-quiescent''galaxies to ~0.1 LFIR for the most ``IR-active'' galaxies.This is likely a consequence of the preferential destruction in intenseradiation fields of the small carriers responsible for the NIR/AFEemission.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries, France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.
| Chandra X-Ray Observations of NGC 1316 (Fornax A)
We report the results of the Chandra ACIS subarcsecond resolution X-rayobservation of the archetypal merger radio galaxy NGC 1316 (Fornax A).We confirm the presence of fine substructures in the hot interstellarmedium (ISM). Some of these are likely to result from interaction withthe radio jets, while others may be related to a complex interminglingof different phases of the ISM. We detect a low-luminosity X-ray activegalactic nucleus (AGN) with LX=5×1039 ergss-1 (in 0.3-8 keV) and a Γ=1.7 power-law energyspectrum. We also detect 81 point sources within the 25th magnitudeisophotal ellipse of NGC 1316 (LX in the range of2×1037 to 2×1039 ergs s-1),with hard (kT~5 keV) X-ray spectra, typical of X-ray binaries, and aspatial radial distribution consistent with that of the optical (i.e.,stellar) surface brightness. We derive the X-ray luminosity function(XLF) of these sources, correcting for the incompleteness at the faintend caused by the presence of the diffuse emission from the hot ISM inthe central regions of NGC 1316 and by the widening of the Chandrapoint-spread functions at increasing distance from the aim point. Withthese corrections, the XLF is well reproduced by a single-unbroken-powerlaw with a slope of -1.3 down to our threshold luminosity of~3×1037 ergs s-1. The hot ISM hastemperatures in the 0.5-0.6 keV range, its surface brightnessdistribution is more centrally concentrated than that of the pointsources, and its temperature appears to decrease at larger radii. Theseproperties suggest that the ISM may be subject to partial winds. Takinginto account the spectral complexity of the ISM, and the presence ofunresolved low luminosity X-ray sources (which can be inferred from thespectra), we constrain the metal abundance of the hot ISM to beZ=0.25-1.3 Zsolar (90% confidence).
| Companions of Bright Barred Shapley-Ames Galaxies
Companion galaxy environment for a subset of 78 bright and nearby barredgalaxies from the Shapley-Ames Catalog is presented. Among the spiralbarred galaxies, there are Seyfert galaxies, galaxies with circumnuclearstructures, galaxies not associated with any large-scale galaxy cloudstructure, galaxies with peculiar disk morphology (crooked arms), andgalaxies with normal disk morphology; the list includes all Hubbletypes. The companion galaxy list includes the number of companiongalaxies within 20 diameters, their Hubble type, and projectedseparation distance. In addition, the companion environment was searchedfor four known active spiral galaxies, three of them are Seyfertgalaxies, namely, NGC 1068, NGC 1097, and NGC 5548, and one is astarburst galaxy, M82. Among the results obtained, it is noted that theonly spiral barred galaxy classified as Seyfert 1 in our list has nocompanions within a projected distance of 20 diameters; six out of 10Seyfert 2 bar galaxies have no companions within 10 diameters, six outof 10 Seyfert 2 galaxies have one or more companions at projectedseparation distances between 10 and 20 diameters; six out of 12 galaxieswith circumnuclear structures have two or more companions within 20diameters.
| A Search for H I in Five Elliptical Galaxies with Fine Structure
We report on VLA H I spectral line observations of five early-typegalaxies classified as optically peculiar because of the presence ofjets, ripples, or other optical fine structure. We detect H I within theprimary beam (30' half-power beamwidth) in four of the five systems.However, in only one case is this gas associated with the targetedelliptical galaxy. In the other cases the H I is associated with anearby gas-rich disk or dwarf galaxy. The one H I detection is for NGC7626, where we tentatively detect an H I cloud lying between 20 and 40kpc southwest of the galaxy center. Its origin is unclear. Our failureto detect obvious tidal H I features suggests that if thesefine-structure elliptical galaxies are remnants of disk galaxy mergers,either the progenitors were gas-poor or they are well evolved and anygaseous tidal features have dispersed and/or been converted into otherphases. Our targeted systems all reside in groups or clusters, and itseems likely that tidal H I is shorter lived in these environments thansuggested by studies of more isolated merger remnants.
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| Dust-induced Systematic Errors in Ultraviolet-derived Star Formation Rates
Rest-frame far-ultraviolet (FUV) luminosities form the ``backbone'' ofour understanding of star formation (SF) at all cosmic epochs. Theseluminosities are typically corrected for dust by assuming that the tightrelationship between the UV spectral slopes (β) and the FUVattenuations (AFUV) of starburst galaxies applies to allstar-forming galaxies. Data from seven independent UV experimentsdemonstrate that quiescent, ``normal'' star-forming galaxies deviatesubstantially from the starburst galaxy β-AFUVcorrelation in the sense that normal galaxies are redder thanstarbursts. Spatially resolved data for the Large Magellanic Cloudsuggest that dust geometry and properties, coupled with a smallcontribution from older stellar populations, cause deviations from thestarburst galaxy β-AFUV correlation. Folding in data forstarbursts and ultraluminous infrared galaxies, it is clear that neitherrest-frame UV/optical colors nor UV/Hα colors help significantlyin constraining the UV attenuation. These results argue that theestimation of SF rates from rest-frame UV and optical data alone issubject to large (factors of at least a few) systematic uncertaintiesbecause of dust, which cannot be reliably corrected for using onlyUV/optical diagnostics.
| The Visibility of Galactic Bars and Spiral Structure at High Redshifts
We investigate the visibility of galactic bars and spiral structure inthe distant universe by artificially redshifting 101 B-band CCD imagesof local spiral galaxies from the Ohio State University Bright SpiralGalaxy Survey. These local galaxy images represent a much fairerstatistical baseline than the galaxy atlas images presented by Frei etal. in 1995, the most commonly used calibration sample for morphologicalwork at high redshifts. Our artificially redshifted images correspond toHubble Space Telescope I814-band observations of the localgalaxy sample seen at z=0.7, with integration times matching those ofboth the very deep northern Hubble Deep Field (HDF) data and the muchshallower HDF flanking field observations. The expected visibility ofgalactic bars is probed in two ways: (1) using traditional visualclassification and (2) by charting the changing shape of the galaxydistribution in ``Hubble space,'' a quantitative two-parameterdescription of galactic structure that maps closely onto Hubble'soriginal tuning fork. Both analyses suggest that over two-thirds ofstrongly barred luminous local spirals (i.e., objects classified as SBin the Third Reference Catalogue) would still be classified as stronglybarred at z=0.7 in the HDF data. Under the same conditions, most weaklybarred spirals (classified SAB in the Third Reference Catalogue) wouldbe classified as regular spirals. The corresponding visibility of spiralstructure is assessed visually, by comparing luminosity classificationsfor the artificially redshifted sample with the corresponding luminosityclassifications from the Revised Shapley-Ames Catalog. We find that forexposure times similar to that of the HDF, spiral structure should bedetectable in most luminous (MB~M*) low-inclination spiralgalaxies at z=0.7 in which it is present. However, obvious spiralstructure is only detectable in ~30% of comparable galaxies in the HDFflanking field data using the Wide Field Planetary Camera 2. Our studyof artificially redshifted local galaxy images suggests that, whenviewed at similar resolution, noise level, and redshift-correctedwavelength, barred spirals are less common at z~0.7 than they are atz=0.0, although more data are needed to definitively rule out thepossibility that cosmic variance is responsible for much of this effect.
| An Ultraviolet/Optical Atlas of Bright Galaxies
We present wide-field imagery and photometry of 43 selected nearbygalaxies of all morphological types at ultraviolet and opticalwavelengths. The ultraviolet (UV) images, in two broad bands at 1500 and2500 Å, were obtained using the Ultraviolet Imaging Telescope(UIT) during the Astro-1 Spacelab mission. The UV images have ~3"resolution, and the comparison sets of ground-based CCD images (in oneor more of B, V, R, and Hα) have pixel scales and fields of viewclosely matching the UV frames. The atlas consists of multiband imagesand plots of UV/optical surface brightness and color profiles. Otherassociated parameters, such as integrated photometry and half-lightradii, are tabulated. In an appendix, we discuss the sensitivity ofdifferent wavebands to a galaxy's star formation history in the form of``history weighting functions'' and emphasize the importance of UVobservations as probes of evolution during the past 10-1000 Myr. We findthat UV galaxy morphologies are usually significantly different fromvisible band morphologies as a consequence of spatially inhomogeneousstellar populations. Differences are quite pronounced for systems in themiddle range of Hubble types, Sa through Sc, but less so for ellipticalsor late-type disks. Normal ellipticals and large spiral bulges arefainter and more compact in the UV. However, they typically exhibitsmooth UV profiles with far-UV/optical color gradients which are largerthan any at optical/IR wavelengths. The far-UV light in these cases isprobably produced by extreme horizontal branch stars and theirdescendants in the dominant, low-mass, metal-rich population. The coolstars in the large bulges of Sa and Sb spirals fade in the UV while hotOB stars in their disks brighten, such that their Hubble classificationsbecome significantly later. In the far-UV, early-type spirals oftenappear as peculiar, ringlike systems. In some spiral disks, UV-brightstructures closely outline the spiral pattern; in others, the disks canbe much more fragmented and chaotic than at optical wavelengths.Contributions by bright active galactic nuclei (AGNs) to the integratedUV light in our sample range from less than 10% to nearly 100%. A numberof systems have unusual UV-bright structures in their inner disks,including rings, compact knots, and starburst nuclei, which could easilydominate the UV light in high-redshift analogs. A significant butvariable fraction of the far-UV light in spiral disks is diffuse ratherthan closely concentrated to star-forming regions. Dust in normal spiraldisks does not control UV morphologies, even in some highly inclineddisk systems. The heaviest extinction is apparently confined to thinlayers and the immediate vicinity of young H II complexes; the UV lightemerges from thicker star distributions, regions evacuated of dust byphotodestruction or winds, or by virtue of strong dust clumpiness. Onlyin cases where the dust layers are disturbed does dust appear to be amajor factor in UV morphology. The UV-bright plume of M82 indicates thatdust scattering of UV photons can be important in some cases. In acompanion paper, we discuss far-UV data from the Astro-2 mission andoptical comparisons for another 35 galaxies, emphasizing face-onspirals.
| Far-Infrared Spectroscopy of Normal Galaxies: Physical Conditions in the Interstellar Medium
The most important cooling lines of the neutral interstellar medium(ISM) lie in the far-infrared (FIR). We present measurements by theInfrared Space Observatory Long Wavelength Spectrometer of seven linesfrom neutral and ionized ISM of 60 normal, star-forming galaxies. Thegalaxy sample spans a range in properties such as morphology, FIR colors(indicating dust temperature), and FIR/blue ratios (indicating starformation activity and optical depth). In two-thirds of the galaxies inthis sample, the [C II] line flux is proportional to FIR dust continuum.The other one-third show a smooth decline inL[CII]/LFIR with increasing Fν(60μm)/Fν(100 μm) and LFIR/LB,spanning a range of a factor of more than 50. Two galaxies at the warmand active extreme of the range haveL[CII]/LFIR<2×10-4 (3 σupper limit). This is due to increased positive grain charge in thewarmer and more active galaxies, which leads to less efficient heatingby photoelectrons from dust grains. The ratio of the two principalphotodissociation region (PDR) cooling linesL[OI]/L[CII] shows a tight correlation withFν(60 μm)/Fν(100 μm), indicating thatboth gas and dust temperatures increase together. We derive atheoretical scaling between [N II] (122 μm) and [C II] from ionizedgas and use it to separate [C II] emission from neutral PDRs and ionizedgas. Comparison of PDR models of Kaufman et al. with observed ratios of(1) L[OI]/L[CII] and(L[CII]+L[OI])/LFIR and (2)L[OI]/LFIR and Fν(60μm)/Fν(100 μm) yields far-UV flux G0 andgas density n. The G0 and n values estimated from the twomethods agree to better than a factor of 2 and 1.5, respectively, inmore than half the sources. The derived G0 and n correlatewith each other, and G0 increases with n asG0~nα, where α~1.4 . We interpret thiscorrelation as arising from Strömgren sphere scalings if much ofthe line and continuum luminosity arises near star-forming regions. Thehigh values of PDR surface temperature (270-900 K) and pressure(6×104-1.5×107 K cm-3)derived also support the view that a significant part of grain and gasheating in the galaxies occurs very close to star-forming regions. Thedifferences in G0 and n from galaxy to galaxy may be due todifferences in the physical properties of the star-forming clouds.Galaxies with higher G0 and n have larger and/or denserstar-forming clouds.
| A Comparison of Ultraviolet Imaging Telescope Far-Ultraviolet and Hα Star Formation Rates
We have used archival ultraviolet (UV) imaging of 50 nearby star-forminggalaxies obtained with the Ultraviolet Imaging Telescope (UIT) to deriveintegrated near-UV and far-UV magnitudes, and have combined these datawith Hα, far-infrared, and thermal radio continuum measurements toexplore the consistency of UV and Hα star formation rates (SFRs).In agreement with previous studies, we find that the UV and HαSFRs are qualitatively consistent, even before corrections forextinction are applied. The uncorrected UV SFRs are systematically lowerby a factor of 1.5 (with a factor of 2 scatter) among luminous galaxieswith SFR>~1 Msolar yr-1, indicating a highereffective attenuation of the far-UV radiation. Among less luminousgalaxies there is no significant offset between the Hα and far-UVSFR scales. This behavior is consistent with that of higher redshiftsamples observed by Sullivan et al., Glazebrook et al., and Yan et al.for comparable ranges of galaxy luminosities and absolute SFRs.Far-infrared and thermal radio continuum data available for a subset ofour sample allow us to estimate the attenuation in the UV and atHα independently. The UV and Hα attenuations appear to becorrelated, and confirm systematically higher attenuations in the UV.Although the galaxies in our sample show modest levels of attenuation(with median values of 0.9 mag at Hα and 1.4 mag at 1550 Å),the range across the sample is large, ~4 mag for Hα and >~5 magin the far-UV (1550 Å). This indicates that the application of asingle characteristic extinction correction to Hα or UV SFRs isonly realistic for large, well-defined and well-studied galaxy samples,and that extinction bias may be important for UV oremission-line-selected samples of star-forming galaxies.
| Ultraviolet Signposts of Resonant Dynamics in the Starburst-ringed SAB Galaxy M94 (NGC 4736)
The dynamic orchestration of star-birth activity in the starburst-ringedgalaxy M94 (NGC 4736) is investigated using images from the UltravioletImaging Telescope (UIT; far-ultraviolet [FUV] band), Hubble SpaceTelescope (HST; near-ultraviolet [NUV] band), Kitt Peak 0.9 m telescope(Hα, R, and I bands), and Palomar 5 m telescope (B band), alongwith spectra from the International Ultraviolet Explorer (IUE) and theLick 1 m telescope. The wide-field UIT image shows FUV emission from (1)an elongated nucleus, (2) a diffuse inner disk, where Hα isobserved in absorption, (3) a bright inner ring of H II regions at theperimeter of the inner disk (R=48"=1.1 kpc), and (4) two 500 pc sizeknots of hot stars exterior to the ring on diametrically opposite sidesof the nucleus (R=130"=2.9 kpc). The HST Faint Object Camera imageresolves the NUV emission from the nuclear region into a bright core anda faint 20" long ``minibar'' at a position angle of 30°. Optical andIUE spectroscopy of the nucleus and diffuse inner disk indicates a~107-108 yr old stellar population from low-levelstar-birth activity blended with some LINER activity. Analysis of theHα-, FUV-, NUV-, B-, R-, and I-band emissions, along with otherobserved tracers of stars and gas in M94, indicates that most of thestar formation is being orchestrated via ring-bar dynamics, involvingthe nuclear minibar, inner ring, oval disk, and outer ring. The innerstarburst ring and bisymmetric knots at intermediate radius, inparticular, argue for bar-mediated resonances as the primary drivers ofevolution in M94 at the present epoch. Similar processes may begoverning the evolution of the ``core-dominated'' galaxies that havebeen observed at high redshift. The gravitationally lensed ``PretzelGalaxy'' (0024+1654) at a redshift of ~1.5 provides an importantprecedent in this regard.
| Atomic and molecular gas in the merger galaxy NGC 1316 (Fornax A) and its environment
We present and interpret observations of atomic and molecular gas towardthe southern elliptical galaxy NGC 1316 (Fornax A), a strong double-loberadio source with a disturbed optical morphology that includes numerousshells and loops. The 12CO(1-0), 12CO(2-1), and Hiobservations were made with SEST and the VLA. CO emission correspondingto a total molecular hydrogen mass of ~ 5 x 108Msun was detected towards the central position as well asnorthwest and southeast of the nucleus in the regions of the dustpatches. The origin of that gas is likely external and due to accretionof one or several small gas-rich galaxies. Hi was not detected in thecentral region of NGC 1316, but ~ 2 x 107 Msun ofatomic gas was found towards the giant Hii region discovered bySchweizer (1980) located 6{farcm }7 (or 36.2 kpc) from the nucleus. Hiwas also found at three other locations in the outer part of NGC 1316.The Hi distributions and kinematics of the two nearby spiral companionsof NGC 1316, NGC 1317 (a barred galaxy to the north) and NGC 1310 (tothe west) could be studied. Both galaxies have unusually small Hi disksthat may have been affected by ram-pressure stripping.
| The neutral hydrogen content of Fornax cluster galaxies
We present a new set of deep H I observations of member galaxies of theFornax cluster. We detected 35 cluster galaxies in HI. The resulting sample, the most comprehensive to date, is used toinvestigate the distribution of neutral hydrogen in the clustergalaxies. We compare the H I content of the detected cluster galaxieswith that of field galaxies by measuring H I mass-to-light ratios andthe H I deficiency parameter of Solanes et al. (\cite{Sol96}). The meanH I mass-to-light ratio of the cluster galaxies is 0.68+/- 0.15,significantly lower than for a sample of H I-selected field galaxies(1.15+/- 0.10), although not as low as in the Virgocluster (0.45+/- 0.03). In addition, the H I content of twocluster galaxies (NGC 1316C and NGC1326B) appears to have been affected by interactions. The meanH I deficiency for the cluster is 0.38+/-0.09 (for galaxy types T=1-6),significantly greater than for the field sample (0.05+/-0.03). Boththese tests show that Fornax cluster galaxies are HI-deficient compared to field galaxies. The kinematics of the clustergalaxies suggests that the H I deficiency may be caused by ram-pressurestripping of galaxies on orbits that pass close to the cluster core. Wealso derive the most complete B-band Tully - Fisher relation of inclinedspiral galaxies in Fornax. A subcluster in theSouth-West of the main cluster contributes considerably to the scatter.The scatter for galaxies in the main cluster alone is 0.50 mag, which isslightly larger than the intrinsic scatter of 0.4 mag. We use the Tully- Fisher relation to derive a distance modulus ofFornax relative to the Virgocluster of -0.38+/- 0.14 mag. The galaxies in the subclusterare (1.0+/-0.5) mag brighter than the galaxies of the main cluster,indicating that they are situated in the foreground. With their meanvelocity 95 km s-1 higher than that of the main cluster weconclude that the subcluster is falling into the main Fornaxcluster.
| ROSAT-HRI observations of six southern galaxy pairs
We present the detailed analysis of the X-ray data for 6 pairs, isolatedor in poor groups, observed at high resolution with the ROSAT HRI . Inall cases, the stronger X-ray source is associated with the brighterearly-type member and is extended. The extent varies from galactic togroup scale, from 3 (RR 210b) to 182 kpc( RR 22a). The fainter membersare detected only in two pairs, RR 210 and RR 259. Except for one case,no significant substructures have been detected in the X-ray maps,possibly also as a consequence of the poor statistics. The core radii ofthe X-ray surface brightness profiles are in the range 1-3 kpc. Thedistribution of the luminosities of galaxies in pairs encompasses a verywide range of both luminosities and LX / LBratios, in spite of the very small number of objects studied so far. Ourdata provide no evidence that pair membership affects the X-rayproperties of galaxies. Observation are discussed in the context of thepair/group evolution.
| 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.
| Circumnuclear Star Formation in the Early-Type Resonance Ring Barred Spiral Galaxy NGC 1326
We present multiband WFPC2 images of the nuclear ring of NGC 1326, anearly-type southern barred spiral in the nearby Fornax galaxy cluster.The ring is a typical example of a 1 kpc-sized star-forming ring locatedin the region of the inner Lindblad resonances with the weak primarybar. The images reveal nearly 1000 point sources in and around the ring.Those sources brighter than MV=-9 are probably massive youngclusters, while the fainter sources may include a mix of extremelyluminous stars and fainter clusters. From an analysis of reddening-freeparameters and two-color plots, we find evidence for a spread in ages ofring clusters, from less than 5 Myr to at least 200 Myr. The olderclusters still lie within the nuclear ring, with no evidence ofmigration of the ring being found over this time period. The luminosityfunction reveals no clusters having an (uncorrected) absolute magnitudeMV brighter than -11, and even after correction forextinction the most luminous cluster identified has onlyM0V=-12.6. The ring seems to lack the ``super-starclusters'' (SSCs) seen in starburst systems and in other nuclear rings,and the analysis suggests that SSCs are not a universal property ofthese rings. Complex dust structure is found inside the ring, south ofthe nucleus, and extinction is especially severe on the west side of thering. An Hα image reveals hundreds of H II regions and emissioncomplexes in the nuclear ring, but there is little correlation betweenthese H II regions and the observed continuum sources, most likely owingto reddening and age differences in the ring. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy (AURA), Inc., under NASA Contract NAS 5-26555.
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