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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.
| Discovery Among the Disks
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| The Classification of Galaxies: Early History and Ongoing Developments
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
| Infrared Spectral Energy Distributions of Nearby Galaxies
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities.
| Nuclear spirals in galaxies: gas response to an asymmetric potential - I. Linear theory
Nuclear spirals can provide a wealth of information about the nuclearpotential in disc galaxies. They are unlikely to form in nuclei withsolid-body rotation, yet they are present in a majority of galacticcentres. Their morphology varies depending on whether a central massiveblack hole (MBH) is present in or absent from the galaxy. In this paperI consider predictions of the linear theory for waves induced in gas byan asymmetric gravitational potential, which are applicable to thenuclear spirals observed in galaxies. The generation and propagation ofwaves are governed by dynamical resonances, and inclusion of an MBH canmove or even create resonances, greatly altering the extent and shape ofthe nuclear spiral. I will use predictions of the linear theorypresented here as a guideline when interpreting hydrodynamical models inthe second paper of this series. I also comment on modifications thatself-gravity in gas imposes on the induced waves.
| 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.
| Nuclear Stellar Populations in the Infrared Space Observatory Atlas of Bright Spiral Galaxies
To understand the nuclear stellar populations and star formationhistories of the nuclei of spiral galaxies, we have obtained K-bandnuclear spectra for 41 galaxies and H-band spectra for 20 galaxies inthe Infrared Space Observatory's Atlas of Bright Spiral Galaxies. In thevast majority of the subsample (80%), the near-infrared spectra suggestthat evolved red stars completely dominate the nuclear stellarpopulations and that hot young stars are virtually nonexistent. Thesignatures of recent star formation activity are only found in 20% ofthe subsample, even though older red stars still dominate the stellarpopulations in these galaxies. Given the dominance of evolved stars inmost galaxy nuclei and the nature of the emission lines in the galaxieswhere they were detected, we suggest that nuclear star formationproceeds in the form of instantaneous bursts. The stars produced bythese bursts comprise only ~2% of the total nuclear stellar mass inthese galaxies, but we demonstrate how the nuclear stellar populationsof normal spiral galaxies can be built up through a series of thesebursts. The bursts were detected only in Sbc galaxies and later, andboth bars and interactions appeared to be sufficient, but not necessary,triggers for the nuclear star formation activity. The vast majority ofgalaxies with nuclear star formation were classified as H II galaxies.With one exception, LINERs and transition objects were dominated byolder red stars, which suggested that star formation was not responsiblefor generating these galaxies' optical line emission.
| Science with Tunable Filters
In just two years, the OSIRIS tunable filter spectrograph on the 10 mGTC will herald a new era in spectrophotometric imaging, from 350 nm to1 μ m. Like its forebear, the TTF at the Anglo-Australian Telescope(AAT), OSIRIS will offer a wide variety of observing modes linked tocharge shuffling in order to achieve exquisite differential imaging. Insome respects, simply repeating the many science cases conducted at theAAT will lead to advances in a number of fields. This is all butguaranteed by the better apparatus, observing conditions and largeraperture. However, the expected improvement in sensitivity suggests manynew avenues: large surveys of extended sources, absorption line imaging,time series and coronographic imaging, to name a few. OSIRIS willprovide some of the deepest photometric ``diffuse light'' images todate, much better than what can be achieved with an integral fieldspectrograph, and over a much wider field of view.
| SINGS: The SIRTF Nearby Galaxies Survey
The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging andspectroscopic survey of 75 nearby galaxies. Its primary goal is tocharacterize the infrared emission of galaxies and their principalinfrared-emitting components, across a broad range of galaxy propertiesand star formation environments. SINGS will provide new insights intothe physical processes connecting star formation to the interstellarmedium properties of galaxies and provide a vital foundation forunderstanding infrared observations of the distant universe andultraluminous and active galaxies. The galaxy sample and observingstrategy have been designed to maximize the scientific and archivalvalue of the data set for the SIRTF user community at large. The SIRTFimages and spectra will be supplemented by a comprehensivemultiwavelength library of ancillary and complementary observations,including radio continuum, H I, CO, submillimeter, BVRIJHK, Hα,Paα, ultraviolet, and X-ray data. This paper describes the mainastrophysical issues to be addressed by SINGS, the galaxy sample and theobserving strategy, and the SIRTF and other ancillary data products.
| Taurus Tunable Filter - Seven Years of Observing
The Taurus Tunable Filter (TTF) has now been in regular use for sevenyears on the Anglo-Australian Telescope (AAT). The instrument was alsoused for three years (1996-1999) on the William Herschel Telescope(WHT). We present a brief review of the different applications in orderto illustrate the versatility of tunable filters in optical/IRspectrophotometric imaging. Tunable filters are now either planned orunder development for 6-10m class telescopes which ensures their use foryears to come.
| Star cluster formation and evolution in nearby starburst galaxies - II. Initial conditions
We use the ages, masses and metallicities of the rich young star clustersystems in the nearby starburst galaxies NGC 3310 and 6745 to derivetheir cluster formation histories and subsequent evolution. We furtherexpand our analysis of the systematic uncertainties involved in the useof broad-band observations to derive these parameters (Paper I) byexamining the effects of a priori assumptions on the individual clustermetallicities. The age (and metallicity) distributions of both theclusters in the circumnuclear ring in NGC 3310 and of those outside thering are statistically indistinguishable, but there is a clear andsignificant excess of higher-mass clusters in the ring compared to thenon-ring cluster sample. It is likely that the physical conditions inthe starburst ring may be conducive for the formation of higher-massstar clusters, on average, than in the relatively more quiescentenvironment of the main galactic disc. For the NGC6745 cluster system wederive a median age of ~10 Myr. NGC6745 contains a significantpopulation of high-mass `super star clusters', with masses in the range6.5 <~ log(Mcl/Msolar) <~ 8.0. Thisdetection supports the scenario that such objects form preferentially inthe extreme environments of interacting galaxies. The age of the clusterpopulations in both NGC3310 and 6745 is significantly lower than theirrespective characteristic cluster disruption time-scales, respectivelylog(tdis4/yr) = 8.05 and 7.75, for 104Msolar clusters. This allows us to obtain an independentestimate of the initial cluster mass function slope, α=2.04(+/-0.23)+0.13-0.43 for NGC3310, and1.96(+/-0.15) +/- 0.19 for NGC6745, respectively, for massesMcl>~ 105 Msolar andMcl>~ 4 × 105 Msolar. Thesemass function slopes are consistent with those of other young starcluster systems in interacting and starburst galaxies.
| The Formation of Nuclear Rings in Barred Spiral Galaxies
Although nuclear rings of gas and star formation are common in barredspiral galaxies, current theories of why and how they form do notprovide the level of detail needed to quantify the effect that theserings can have on the fueling of active galactic nuclei and on theevolution of their host galaxy. In this paper we use detailed modelingto show that existence of nuclear rings is directly related to theexistence of the orbit family whose major axis is perpendicular to themajor axis of the bar (x2). We explore a large range ofbarred galaxy potentials, and for each potential we use atwo-dimensional hydrodynamic simulation to determine whether and at whatradius a nuclear ring forms. We compare the results of the hydrodynamicsimulations to numerical integrations of periodic orbits in a barredpotential and show that the rings only form when a minimum number ofx2 orbits exist. Because the rings migrate inward with timeas they accumulate gas, the radius at which a nuclear ring is seen doesnot give direct information on the shape of the rotation curve. We alsoshow that the common assumption that nuclear rings are related to aninner Lindblad resonance is incorrect. In fact, we show that there is noresonance at the inner Lindblad resonance in barred galaxies. We alsocompare the predictions of this theory to Hubble Space Telescopeobservations and show that it correctly predicts the observed gas andstar formation morphology of nuclear rings.
| Spectral and photometric evolution of young stellar populations: The impact of gaseous emission at various metallicities
We include gaseous continuum and line emission into our GALEV models forthe spectral and photometric evolution of Simple Stellar Populations(SSPs) for various metallicities in the range 0.02 <=Z/Zsun <= 2.5. This allows to extend them to significantlyyounger ages than before. They now cover the age range from 4 Myr allthrough 14 Gyr. We point out the very important contributions of gaseousemission to broad band fluxes and their strong metallicity dependenceduring very early evolutionary stages of star clusters, galaxies orsubgalactic fragments with vigorous ongoing star formation.Emission-line contributions are commonly seen in these activelystar-forming regions. Models without gaseous emission cannot explaintheir observed colors at all, or lead to wrong age estimates. We useup-to-date Lyman continuum (=Lyc) emission rates and decided to userecent empirical determinations of emission line ratios relative toHbeta for subsolar metallicities. We justify this approachfor all situations where no or not enough spectral information isavailable to determine all the parameters required by photoionizationmodels. The effects of gaseous line and continuum emission on broad bandfluxes are shown for different metallicities and as a function of age.In addition to the many filter systems already included in our earliermodels, we here also include the HST NICMOS and Advanced Camera forSurveys (=ACS) filter systems.Data files are provided in the electronic version at CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/401/1063, and athttp://www.uni-sw.gwdg.de/~galev/panders/
| Optical Spectroscopy of Metal-rich H II Regions and Circumnuclear Hot Spots in M83 and NGC 3351
We present optical spectroscopy of a sample of metal-rich extragalacticH II regions in the spiral galaxies M83 (NGC 5236), NGC 3351, and NGC6384. The metal abundance, estimated from semiempirical methods usingstrong emission lines, is found to be above solar for most of theobjects. The sample includes a number of circumnuclear H II regions (hotspots), which are found in general to have spectral properties similarto the H II regions located in the disks. Different age estimators[equivalent width of the Hβ emission line W(Hβ), Balmer lineabsorption profiles, UV spectra] provide consistently young ages (4-6Myr) for the hot spots. We have detected the Wolf-Rayet (W-R) bumpfeature at 4650 Å in five of the objects, and in fewer casespossibly the WC features in the red at 5696 and 5808 Å. Sixadditional objects showing W-R features are drawn from our previous workon extragalactic H II regions. From the measured luminosity of thestellar He II λ4686 line we estimate a small number of WN stars(from one or two to about 30). We have compared the measured intensitiesand equivalent widths of the W-R bump to the predictions of recentevolutionary models for massive stellar populations. By assuminginstantaneous bursts of star formation the ages derived from W(Hβ),between 3 and 6 Myr, are in agreement with the age span predicted by themodels. The observed strength of the W-R bump is in agreement with thepredictions for only half of the objects, the remainder showing lowerI(4650)/I(Hβ) and W(4650) than the models of the appropriatemetallicities. Our favored explanation is related to the finite numberof stars formed in the clusters and to stochastic effects likely to beat work in star-forming regions of the size considered here. The He Iλ5876 recombination line is used as an indicator of theequivalent effective temperature of the ionizing clusters, Teff. We have found that this temperature is not affected by thepresence of W-R stars. From a comparison with published photoionizationmodels based on synthetic cluster spectral energy distributions we findsome evidence for an overestimation of the number of He ionizing photonsin the model fluxes. In general, however, the massive star diagnosticsconsidered in our work are in agreement with the predictions of recentevolutionary models calculated with a Salpeter initial mass function anda high upper mass limit. We find no compelling evidence for a depletionof massive stars (M>40-50 Msolar) in the initial massfunction of metal-rich clusters, contrary to our previous conclusionsbased on older evolutionary models.
| An Infrared Space Observatory Atlas of Bright Spiral Galaxies
In this first paper in a series we present an atlas of infrared imagesand photometry from 1.2 to 180 μm for a sample of bright spiralgalaxies. The atlas galaxies are an optically selected,magnitude-limited sample of 77 spiral and S0 galaxies chosen from theRevised Shapley-Ames Catalog (RSA). The sample is a representativesample of spiral galaxies and includes Seyfert galaxies, LINERs,interacting galaxies, and peculiar galaxies. Using the Infrared SpaceObservatory (ISO), we have obtained 12 μm images and photometry at60, 100, and 180 μm for the galaxies. In addition to its imagingcapabilities, ISO provides substantially better angular resolution thanis available in the IRAS survey, and this permits discrimination betweeninfrared activity in the central regions and global infrared emission inthe disks of these galaxies. These ISO data have been supplemented withJHK imaging using ground-based telescopes. The atlas includes 2 and 12μm images. Following an analysis of the properties of the galaxies,we have compared the mid-infrared and far-infrared ISO photometry withIRAS photometry. The systematic differences we find between the IRASFaint Source Catalog and ISO measurements are directly related to thespatial extent of the ISO fluxes, and we discuss the reliability of IRASFaint Source Catalog total flux densities and flux ratios for nearbygalaxies. In our analysis of the 12 μm morphological features we findthat most but not all galaxies have bright nuclear emission. We find 12μm structures such as rings, spiral arm fragments, knotted spiralarms, and bright sources in the disks that are sometimes brighter thanthe nuclei at mid-infrared wavelengths. These features, which arepresumably associated with extranuclear star formation, are common inthe disks of Sb and later galaxies but are relatively unimportant inS0-Sab galaxies. Based on observations with the Infrared SpaceObservatory (ISO), an ESA project with instruments funded by ESA MemberStates (especially the PI countries: France, Germany, Netherlands, andUnited Kingdom) and with the participation of ISAS and NASA.
| NGC 4314. IV. Photometry of Star Clusters with the Hubble Space Telescope: History of Star Formation in the Vicinity of a Nuclear Ring
Using Hubble Space Telescope (HST) WFPC2 images, we have obtained U, B,V, I, and Hα photometry for 76 star clusters in the nuclearstar-forming ring of the barred spiral galaxy NGC 4314. These clustersare likely associated with an inner inner Lindblad resonance (IILR). Theblue colors and Hα emission for most of these clusters imply veryyoung ages of 1-15 Myr. Age estimates based on several reddening-freeparameters indicate that the present epoch of star formation has lastedat least 30 Myr. By estimating the masses of stars in the clusters andcomparing with the Hα luminosity, we conclude that a significantfraction of ongoing star formation in the nuclear ring of NGC 4314occurs in clusters. The cluster masses identify these as young openclusters, not young globular clusters. Farther out in the galaxy, justexterior to the ring of young stars, previous ground-based observationsrevealed two symmetric stellar spiral arms that may be associated withan outer inner Lindblad resonance (OILR). With our HST data, we haverevealed part of this structure and its colors in more detail. Thespiral arm colors are consistent with stellar ages between 40 and 200Myr. The age difference between the inner ring of young stars (IILR) andthe larger oval-like feature containing the blue arms (OILR) supports aninterpretation of the morphology of the nuclear region of NGC 4314 thatrequires a reservoir of gas that becomes more compact over time. Wespeculate that as the gas distribution becomes more centrallyconcentrated, it interacts with these two resonances. Each resonancetriggers star formation, resulting in two distinct epochs of starformation. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| 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.
| Far-Ultraviolet and Hα Imaging of Nearby Spiral Galaxies: The OB Stellar Population in the Diffuse Ionized Gas
We have compared Hα and far-ultraviolet (FUV) images of 10 nearbyspirals, with the goal of understanding the contribution of field OBstars to the ionization of the diffuse ionized gas (DIG) in spiralgalaxies. The FUV images were obtained by the Ultraviolet ImagingTelescope (UIT), and the Hα images were obtained using variousground-based telescopes. In all of the galaxies, theFHα/FUIT flux ratio is lower in the DIG thanin the H II regions. This is likely an indication that the mean spectraltype for OB stars in the field is later than that in H II regions.Comparison of the NLyc/LUIT ratio with models ofevolving stellar populations shows that the stellar population in theDIG is consistent with either an older single-burst population or asteady state model with constant star formation and an initial massfunction (IMF) slope steeper than α=2.35. The steady state modelis probably a more realistic representation of the stellar populationoutside of H II regions. The steep IMF slope simulates the steeppresent-day mass function slope expected for field OB stars and does notnecessarily indicate that the IMF slope is actually steeper thanα=2.35. We compared the FHα/FUIT ratioin the DIG of these galaxies with that in M33, in which the field OBstellar population has previously been investigated using Hubble SpaceTelescope images. If the mean spectral types of stars in H II regionsand in the DIG are the same as in M33 and the difference in extinctionbetween DIG and H II regions is constant among galaxies, then theanalysis suggests that field stars are important sources of ionizationin most galaxies and may be the dominant source in some galaxies. TheFHα/FUIT ratio is correlated with Hαsurface brightness in both DIG and H II regions, although there is alarge scatter in faint H II regions, which may be due to undersamplingthe IMF in regions with a low total mass of stars formed. TheFHα/FUIT ratio is often highest in thecenters of galaxies and in the spiral arms, which is also where the DIGis brightest. This can be explained if the extinction is greater inthese regions or if the fraction of DIG ionized by leakage is lower inthe interarm regions.
| 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.
| Quantitative Morphology of Galaxies Observed in the Ultraviolet
We present a quantitative study of the far-ultraviolet (FUV) and opticalmorphology in 32 nearby galaxies and estimate the ``morphologicalk-correction'' expected if these objects were observed unevolved at highredshift. Using the common indices of central concentration (C) androtational asymmetry (A) to quantify morphology, we considerindependently two phenomena that give rise to this k-correction.Bandshifting, the decrease in the rest-frame wavelength of lightobserved through optical filters, is explored by measuring these indicesin several passbands for each galaxy, and it is found to be the primarydriver of changes in C and A. In general, the optical trend found fordecreasing C and increasing A when going to shorter wavelengths extendsto the FUV. However, the patchy nature of recent star formation inlate-type galaxies, which is accentuated in the FUV, results in poorquantitative correspondence between morphologies determined in theoptical and FUV. We then artificially redshift our FUV images into theHubble Deep Field (HDF) filters to simulate various cosmologicaldistance effects, such as surface brightness dimming and loss of spatialresolution. Hubble types of many galaxies in our sample are not readilyidentifiable at redshifts beyond z~1, and the galaxies themselves aredifficult to detect beyond z~3. Because only features of the highestsurface brightness remain visible at cosmological distances, the changein C and A between simulated high-z galaxies and their unredshiftedcounterparts depends on whether their irregular features are primarilybright or faint. Our simulations suggest that k-corrections alone areindeed capable of producing the peculiar morphologies observed at highredshift; for example, several spiral galaxies have C and A indicestypical of irregular or peculiar HDF objects viewed at z>=2. Webriefly discuss some elements of a scheme to classify rest-frame UVimages, mergers, protogalaxies, and other objects for which classicalHubble types do not adequately encompass the existing morphology.
| An Ultraviolet through Infrared Look at Star Formation and Super Star Clusters in Two Circumnuclear Starburst Rings
We present broadband (U, V, I, and H) and narrowband (Hα+[N II]and Paα) images of the circumnuclear starburst rings in two nearbyspiral galaxies, NGC 1512 and NGC 5248, obtained with the WFPC2 andNICMOS cameras on HST. Combined with previously published ultraviolet(UV) HST images at 2300 Å, these data provide a particularly widewavelength range with which to study the properties of the stellarpopulations, the gas, and the dust in the rings. The young star clustersand the line-emitting gas have different spatial distributions, withsome large (50 pc scale) line-emitting regions that have littleassociated continuum emission, but a Paα equivalent widthindicating an embedded stellar population a few megayears old. Theobserved Hα/Paα intensity ratios suggest the gas is mixedwith dust, making it effective at completely obscuring some of the youngclusters. We identify the major (about 500 in each galaxy) compactcontinuum sources (super star clusters and individual stars) and analyzetheir spectral energy distributions (SEDs) from 0.2 to 1.6 μm byfitting them with a grid of spectral synthesis models with a range ofages and dust extinction. Most of the visible clusters are only mildlyreddened, with AV=0 to 1 mag, suggesting that the processesthat clear out the gas and dust of the stellar birth clouds areefficient and fast. The patchiness of the dust distribution makes itdifficult to reliably estimate the star formation rate, based on UVcontinuum slope or hydrogen emission-line ratios, in starbursts such asthese. The cluster SEDs are consistent with a range in ages, from 1 to300 Myr, but with only a minority older than a few tens of megayears. Wepoint out an age bias, the result of the steep luminosity function ofthe clusters combined with the fading of clusters as they age, whichcauses young clusters to be overrepresented at any luminosity.Accounting for this bias, the fraction of old clusters is consistentwith continuous star formation in the rings over the past ~300 Myr.Because of the uncertainties in dating the clusters, we cannot rule outepisodic, ~20 Myr long bursts of star formation, but the presence ofUV-bright rings in about 10% of spiral galaxies argues against thispossibility. Although most of the observed SEDs are well fitted by arange of models, some of the brightest young clusters have excessemission in the IR that is not predicted by the models and may bethermal reradiation by circumstellar dust. The cluster mass functionsfollow a power-law distribution with index -2, similar to that recentlyderived for the starburst in the merging Antennae galaxies, andextending to ~105 Msolar. The lack of a mass scalemeans that subsequent evolution of the mass function is required, ifsome of the SSCs are to evolve into globular clusters. The clusters arespatially unresolved or marginally resolved, corresponding to V-bandGaussian radii of less than a few parsecs, at an assumed distance of 10Mpc. In NGC 5248, we report a previously unknown, 60 pc radius, inneremission-line ring, and in NGC 1512, a peculiar compact (0.1" diameter)source with an Hα+[N II] equivalent width of ~7000 Å, whichmay be a so-called Balmer-dominated supernova remnant. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy (AURA), Inc., under NASAcontract NAS 5-26555.
| A Comparison between PAα and Hα Emission: The Relation between Mean H II Region Reddening, Local Gas Density, and Metallicity
We measure reddenings to H II regions in NGC 1512, 2903, 4449, and 6946,and M51 from Hubble Space Telescope (HST) Paα and Hα images.Extinctions range from AV~5 to 0 depending upon the galaxy.For the galaxies with HST images in both lines, NGC 1512, NGC 2903, andM51, the Paα and Hα emission are almost identical inmorphology, which implies that little emission from bright H II regionsis hidden from view by regions of comparatively high extinction. Thescatter in the measured extinctions in each galaxy is only +/-0.5 mag.We compare the reddenings we measure in five galaxies using thePaα-to-Hα ratios to those measured previously from theBalmer decrement in the Large Megallanic Cloud and as a function ofradius in M101 and M51. We find that luminosity-weighted meanextinctions of these ensembles of H II regions are correlated with gassurface density and metallicity. The correlation is consistent with themean extinction depending on dust density, where the dust-to-gas massratio scales with the metallicity. This trend is expected if H IIregions tend to be located near the midplane of a gas disk and emergefrom their parent molecular clouds soon after birth. In environmentswith gas densities below a few hundred solar masses per square parsec,star formation rates estimated from integrated line fluxes and meanextinctions are likely to be fairly accurate. 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, Inc., under NASA contract NAS 5-26555.
| 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.
| Comparing Galaxy Morphology at Ultraviolet and Optical Wavelengths
We have undertaken an imaging survey of 34 nearby galaxies infar-ultraviolet (FUV, ~1500 Å) and optical (UBVRI) passbands tocharacterize galaxy morphology as a function of wavelength. This sample,which includes a range of classical Hubble types from elliptical toirregular, with emphasis on spirals at low inclination angle, provides avaluable database for comparison with images of high-z galaxies whoseFUV light is redshifted into the optical and near-infrared bands.Ultraviolet data are from the Ultraviolet Imaging Telescope (UIT)Astro-2 mission. We present images and surface brightness profiles foreach galaxy, and we discuss the wavelength dependence of morphology fordifferent Hubble types in the context of understanding high-z objects.In general, the dominance of young stars in the FUV produces the patchyappearance of a morphological type later than that inferred from opticalimages. Prominent rings and circumnuclear star formation regions areclearly evident in FUV images of spirals, while bulges, bars, and old,red stellar disks are faint to invisible at these short wavelengths.However, the magnitude of the change in apparent morphology ranges fromdramatic in early-type spirals with prominent optical bulges to slightin late-type spirals and irregulars, in which young stars dominate boththe UV and optical emission. Starburst galaxies with centrallyconcentrated, symmetric bursts display an apparent ``E/S0'' structure inthe FUV, while starbursts associated with rings or mergers produce apeculiar morphology. We briefly discuss the inadequacy of the opticallydefined Hubble sequence in describing FUV galaxy images and estimatingmorphological k-corrections, and we suggest some directions for futureresearch with this data set.
| 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.
| A UV through IR Look at Star Formation and Super Star Clusters in Two Circumnuclear Starburst Rings
We present broad-band (U, V, I, and H) and narrow-band (Hα +[N II]and Paα ) images of the circumnuclear starburst rings in twonearby spiral galaxies, NGC 1512 and NGC 5248, obtained with the WFPC2and NICMOS cameras on HST. Combined with previously published HST imagesat 2300 Å, these data provide a particularly wide wavelength rangewith which to study the properties of the stellar populations, the gas,and the dust in the rings. We examine the morphology, the line emission,the ionizing photon budget, and the extinction of the gas in the H IIregions. We identify the major (about 500 in each galaxy) compactcontinuum sources (super star clusters and individual stars), measuretheir spectral energy distributions from 0.2 μ m to 1.6 μ m, andcompare them to spectral synthesis models to determine ages andextinctions. Most of the luminous sources (MV≈ -10 to-12 mag) are super star clusters that are blue, a few million years old,and not highly reddened. The starburst in each ring thus appears be abrief and well-synchronized event.
| Galaxy Interactions: The HI Signature
Invited review in Session 3: Tidal Interactions.
| Neutral Hydrogen and Dark Matter in Spiral Galaxies
The first part presents a brief review of the main HI properties ofisolated, normal spiral galaxies and of the phenomena which seem tocharacterize and dominate their internal metabolism. In the second partattention is drawn to all those processes, such as tidal interactions,accretion and mergers, that depend on the galaxy environment and mayplay a significant role in galaxy formation and evolution. In the thirdpart the observational evidence for the dark matter component of spiralgalaxies is discussed.
| Star formation in bar environments regions. II. Physical properties, age and abundances of H II
The nebular properties (electronic density, extinction, age, O/Habundances) of H ii regions found along the bars of the sample of barredspiral galaxies studied by Martin & Friedli (1997) are examined.From line ratio diagnostic diagrams, it is showed that regions locatedalong the major axis of the bars have a normal photoionization spectrum,that is, line ratios reproductible from nebular conditions and ionizingstar radiation field normally encountered in extragalactic H ii regions.There is an indication, however, that their degree of ionization mightbe somewhat different. Another ionization mechanism (high-velocityshocks or hard UV radiation) is clearly present for regions found nearbythe centers of the galaxies. The electronic density of the regions alongthe bars is very close to that of disc regions (< N_e> ~ 80 cm(-3)). On average, bar and disc regions have a similar visual extinction(A_V ~ 1 mag) with exceptions for some regions located near the bar dustlanes of the earlier types of galaxies in our sample. Although theaverage Hα equivalent width of bar H ii regions ( ~ 250 Å)is half that of disc regions, this disparity could be due touncertainties in the galactic continuum and does not translate into asignificant age difference. The oxygen abundance distribution was alsoinvestigated in the bar of these galaxies. The O/H scatter was found tobe very small (<0.1 dex) indicating that mixing of the chemicalcomposition by gas flows is very efficient in a barred structure.Observations reported in this paper were obtained at the Multiple MirrorTelescope Observatory, a facility operated jointly by the University ofArizona and the Smithsonian Institution Physical properties, age andabundances of H ii
| Discovery of a Double Circumnuclear Ring and Minibar in the Starburst Galaxy M83
Our (J-K) color observations of the central regions of the starburstgalaxy M83 reveal a double circumnuclear ring. The main dust lanesspiral into the outer nuclear ring at a radius of 150 pc. The two ringsmay coincide with two inner Lindblad resonances. The main hot spotsoccur in an arc that is between the rings. A dust bar that is offset 90deg from the primary stellar bar connects the outer nuclear ring to theinner nuclear ring at a radius of 50 pc and may provide the path for gasto flow to the central starburst.
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