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On the Spiral Structure of NGC 5248: An Analytic Approach
An improved nonlinear asymptotic theory of spiral density waves isdeveloped. It extends the theory developed by Yuan & Kuo to a globaltheory, not just one limited to the vicinity of the resonance. It canthen be applied to the recent high-resolution observations of CO and HSTNICMOS of nearby spiral galaxies. As an example, we apply the theory tothe nearby grand-design spiral galaxy NGC 5248, which has excellent COobservations in the central regions and exhibits a continuous opticalspiral pattern from about 7 kpc to about 100 pc. Using the improvednonlinear asymptotic theory, we confirm the earlier results that theinner two-arm spiral structure of NGC 5248 is driven by a slowlyrotating bar potential at the outer inner Lindblad resonance and is acontinuation of the outer optical spiral arms. Using the new theory, wecalculate the streamlines, surface density, and line-of-sight velocitycontours of the bar-driven flow in the central gas disk, and theseresults are in good agreement with the observations.
| Star Formation in Satellite Galaxies
We present narrowband observations of the Hα emission in a sampleof 31 satellites orbiting isolated giant spiral galaxies. The samplestudied spans the range -19 mag
| A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals
The scale lengths of the old stars and ionized gas distributions arecompared for similar samples of Virgo Cluster members and field spiralgalaxies via Hα and broad R-band surface photometry. While theR-band and Hα scale lengths are, on average, comparable for thecombined sample, we find significant differences between the field andcluster samples. While the Hα scale lengths of the field galaxiesare a factor of 1.14+/-0.07 longer, on average, than their R-band scalelengths, the Hα scale lengths of Virgo Cluster members are, onaverage, 20% smaller than their R-band scale lengths. Furthermore, inVirgo, the scale length ratios are correlated with the size of thestar-forming disk: galaxies with smaller overall Hα extents alsoshow steeper radial falloff of star formation activity. At the sametime, we find no strong trends in scale length ratio as a function ofother galaxy properties, including galaxy luminosity, inclination,morphological type, central R-band light concentration, or bar type. Ourresults for Hα emission are similar to other results for dustemission, suggesting that Hα and dust have similar distributions.The environmental dependence of the Hα scale length placesadditional constraints on the evolutionary process(es) that cause gasdepletion and a suppression of the star formation rate in clusters ofgalaxies.
| Thermal and non-thermal components of the interstellar medium at sub-kiloparsec scales in galaxies
Aims. We present new radio continuum observations of ten BIMA SONGgalaxies, taken at 1.4 GHz with the Very Large Array. These observationsallow us to extend the study of the relationships between the radiocontinuum (RC) and CO emission to 22 CO luminous galaxies for whichsingle dish CO images have been added to interferometric data. NewSpitzer infrared (IR) images of six of these galaxies have beenreleased. The analysis of these high resolution images allowed us toprobe the RC-IR-CO correlations down to linear scales of a few hundredpc. Methods: .We compare the point-by-point RC, CO and mid-IRintensities across entire galaxy disks, producing radial profiles andspatially resolved images of the RC/CO and RC/mid-IR ratios.Results: .For the 22 galaxies analysed, the RC-CO correlation on scalesfrom ~10 kpc down to ~100 pc is nearly linear and has a scatter of afactor of two, i.e. comparable to that of the global correlations. Thereis no evidence for any severe degradation of the scatter below the kpcscale. This also applies to the six galaxies for which high-resolutionmid-IR data are available. In the case of NGC 5194,we find that the non-thermal radio spectral index is correlated with theRC/FIR ratio. Conclusions: .The scatter of the point-by-pointcorrelations does not increase significantly with spatial resolution. Wethus conclude that we have not yet probed the physical scales at whichthe correlations break down. However, we observe local deviations fromthe correlations in regions with a high star formation rate, such as thespiral arms, where we observe a flat radio spectrum and a low RC/FIRratio. In the intra-arm regions and in the peripheral regions of thedisk, the RC/FIR is generally higher and it is characterized by asteepening of the radio spectrum.
| The structure of galactic disks. Studying late-type spiral galaxies using SDSS
Using imaging data from the SDSS survey, we present the g' and r' radialstellar light distribution of a complete sample of ~90 face-on tointermediate inclined, nearby, late-type (Sb-Sdm) spiral galaxies. Thesurface brightness profiles are reliable (1 σ uncertainty lessthan 0.2 mag) down to μ27 mag/''. Only ~10% of all galaxies havea normal/standard purely exponential disk down to our noise limit. Thesurface brightness distribution of the rest of the galaxies is betterdescribed as a broken exponential. About 60% of the galaxies have abreak in the exponential profile between 1.5-4.5 times thescalelength followed by a downbending, steeper outer region. Another~30% shows also a clear break between 4.0-6.0 times thescalelength but followed by an upbending, shallower outer region. A fewgalaxies have even a more complex surface brightness distribution. Theshape of the profiles correlates with Hubble type. Downbending breaksare more frequent in later Hubble types while the fraction of upbendingbreaks rises towards earlier types. No clear relation is found betweenthe environment, as characterised by the number of neighbours, and theshape of the profiles of the galaxies.
| Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| The Schmidt Law at High Molecular Densities
We combined Hα and recent high-resolution12CO(J=1‑0) data to consider the quantitative relationbetween the gas mass and the star-formation rate, or the so-calledSchmidt law in nearby spiral galaxies at regions of high moleculardensity. The relation between the gas quantity and the star-formationrate has not been previously studied for high-density regions, but usinghigh-resolution CO data obtained at the Nobeyama Millimeter Array, wefound that the Schmidt law is valid at densities as high as 103Modotpc-2 for sample spiral galaxies, which is anorder of magnitude denser than what has been known to be the maximumdensity at which the empirical law holds for non-starburst galaxies.Furthermore, we obtained a Schmidt law index of N = 1.33 ± 0.09and a roughly constant star-formation efficiency over the entire disk,even within several hundred parsecs of the nucleus. These results implythat the physics of star formation does not change in the centralregions of spiral galaxies. Comparisons with starburst galaxies are alsogiven. We find a possible discontinuity in the Schmidt law betweennormal and starburst galaxies.
| Halos of Spiral Galaxies. II. Halo Metallicity-Luminosity Relation
Using the Hubble Space Telescope, we have resolved individual red giantbranch stars in the halos of eight nearby spiral galaxies. The fieldslie at projected distances between 2 and 13 kpc along the galaxies'minor axes. The data set allows a first look at the systematic trends inhalo stellar populations. We have found that bright galaxies tend tohave broad red giant branch star color distributions with redder meancolors, suggesting that the heavy-element abundance spread increaseswith the parent galaxy luminosity. The mean metallicity of the stellarhalo, estimated using the mean colors of red giant branch stars,correlates with the parent galaxy luminosity. The metallicity of theMilky Way halo falls nearly 1 dex below this luminosity-metallicityrelation, suggesting that the halo of the Galaxy is more the exceptionthan the rule for spiral galaxies; i.e., massive spirals with metal-poorhalos are unusual. The luminosity-halo stellar abundance relation isconsistent with the scaling relation expected for stellar systemsembedded in dominant halos, suggesting that the bulk of the halo stellarpopulation may have formed in situ.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Secular Evolution via Bar-driven Gas Inflow: Results from BIMA SONG
We present an analysis of the molecular gas distributions in the 29barred and 15 unbarred spirals in the BIMA CO (J=1-0) Survey of NearbyGalaxies (SONG). For galaxies that are bright in CO, we confirm theconclusion by Sakamoto et al. that barred spirals have higher moleculargas concentrations in the central kiloparsec. The SONG sample alsoincludes 27 galaxies below the CO brightness limit used by Sakamoto etal. Even in these less CO-bright galaxies we show that high central gasconcentrations are more common in barred galaxies, consistent withradial inflow driven by the bar. However, there is a significantpopulation of early-type (Sa-Sbc) barred spirals (6 of 19) that have nomolecular gas detected in the nuclear region and have very little out tothe bar corotation radius. This suggests that in barred galaxies withgas-deficient nuclear regions, the bar has already driven most of thegas within the bar corotation radius to the nuclear region, where it hasbeen consumed by star formation. The median mass of nuclear moleculargas is over 4 times higher in early-type bars than in late-type (Sc-Sdm)bars. Since previous work has shown that the gas consumption rate is anorder of magnitude higher in early-type bars, this implies that theearly types have significantly higher bar-driven inflows. The loweraccretion rates in late-type bars can probably be attributed to theknown differences in bar structure between early and late types. Despitethe evidence for bar-driven inflows in both early and late Hubble-typespirals, the data indicate that it is highly unlikely for a late-typegalaxy to evolve into an early type via bar-induced gas inflow.Nonetheless, secular evolutionary processes are undoubtedly present, andpseudobulges are inevitable; evidence for pseudobulges is likely to beclearest in early-type galaxies because of their high gas inflow ratesand higher star formation activity.
| 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.
| Nuclear Properties of Nearby Spiral Galaxies from Hubble Space Telescope NICMOS Imaging and STIS Spectroscopy
We investigate the central regions of 23 spiral galaxies using SpaceTelescope Imaging Spectrograph (STIS) spectroscopy and archivalNear-Infrared Camera and Multi-Object Spectrometer (NICMOS) imaging. Thesample is taken from our program to determine the masses of centralmassive black holes (MBHs) in 54 nearby spiral galaxies. Stars arelikely to contribute significantly to any dynamical central massconcentration that we find in our MBH program, and this paper is part ofa series to investigate the nuclear properties of these galaxies. We usethe Nuker law to fit surface brightness profiles, derived from theNICMOS images, to look for nuclear star clusters and find possibleextended sources in three of the 23 galaxies studied (13%). The factthat this fraction is lower than that inferred from optical Hubble SpaceTelescope studies is probably due to the greater spatial resolution ofthose studies. Using R-H and J-H colors and equivalent widths ofHα emission (from the STIS spectra), we investigate the nature ofthe stellar population with evolutionary models. Under the assumption ofhot stars ionizing the gas, as opposed to a weak active galactic nucleus(AGN), we find that there are young stellar populations (~10-20 Myr);however, these data do not allow us to determine what percentage of thetotal nuclear stellar population they form. In addition, in an attemptto find any unknown AGN, we use [N II] and [S II] line flux ratios(relative to Hα) and find tentative evidence for weak AGNs in NGC1300 and NGC 4536.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555.
| The molecular connection to the FIR-radio continuum correlation in galaxies
We have studied the relationships between the radio continuum (RC) andCO emission for a set of galaxies selected from the BIMA Survey ofNearby Galaxies. We find that the global CO-RC correlation is as tightas the global FIR-RC correlation for the 24 galaxies studied. Within 9galaxies with ~6´´ CO and RC data available, the CO and RCemission is as tightly correlated as its global value; the radiallyaveraged correlation is nearly linear, extends over four order ofmagnitude and holds down to the smallest linear resolution of theobservations, which is ~100 pc. We define q_CO/RC as the log of theratio of the CO to RC flux as a way to characterize the CO-RCcorrelation. Combining 6´´ pixel-by-pixel comparisons acrossall sources yields an average small-scale correlation of q_CO/RC = 1.1± 0.28; that is, the spatially resolved correlation has adispersion that is less than a factor of 2. There are however systematicvariations in the CO/RC ratio; the strongest organized structures inq_CO/RC tend to be found along spiral arms and on size scales muchlarger than the resolution of the observations. We do not measure anysystematic trend in CO/RC ratio as a function of radius in galaxies. Theconstancy of the CO/RC ratio stands in contrast to the previouslymeasured decrease in the FIR/RC ratio as a function of radius ingalaxies. We suggest that the excellent correlation between the CO, RCand FIR emission in galaxies is a consequence of regulation byhydrostatic pressure; this model links all three emissions withoutinvoking an explicit dependence on a star formation scenario.
| Structure and star formation in disk galaxies. III. Nuclear and circumnuclear Hα emission
From Hα images of a carefully selected sample of 57 relativelylarge, Northern spiral galaxies with low inclination, we study thedistribution of the Hα emission in the circumnuclear and nuclearregions. At a resolution of around 100 parsec, we find that the nuclearHα emission in the sample galaxies is often peaked, andsignificantly more often so among AGN host galaxies. The circumnuclearHα emission, within a radius of two kpc, is often patchy inlate-type, and absent or in the form of a nuclear ring in early-typegalaxies. There is no clear correlation of nuclear or circumnuclearHα morphology with the presence or absence of a bar in the hostgalaxy, except for the nuclear rings which occur in barred hosts. Thepresence or absence of close bright companion galaxies does not affectthe circumnuclear Hα morphology, but their presence does correlatewith a higher fraction of nuclear Hα peaks. Nuclear rings occur inat least 21% (±5%) of spiral galaxies, and occur predominantly ingalaxies also hosting an AGN. Only two of our 12 nuclear rings occur ina galaxy which is neither an AGN nor a starburst host. We confirm thatweaker bars host larger nuclear rings. The implications of these resultson our understanding of the occurrence and morphology of massive starformation, as well as non-stellar activity, in the central regions ofgalaxies are discussed.
| 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.
| Determination of the Thickness of Non-Edge-on Disk Galaxies
We propose a method to determine the thickness of non-edge-on diskgalaxies from their observed structure of spiral arms, based on thesolution of the truly three-dimensional Poisson's equation for alogarithmic disturbance of density and under the condition where theself-consistency of the density wave theory is no longer valid. Fromtheir measured number of arms, pitch angle and location of the innermostpoint of the spiral arms, we derive and present the thicknesses of 34spiral galaxies.
| 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.
| The Properties of Satellite Galaxies in External Systems. II. Photometry and Colors
In this second paper dedicated to the study of satellite galaxies wepresent broadband photometry in the B, V, R, and I filters of 49satellite galaxies orbiting giant isolated spiral galaxies. Firstanalysis of the properties of these objects are presented by means ofcolor-color and color-magnitude diagrams for early- and late-typesatellites. Although we find differences in the slope of the V-I versusMv color magnitude diagram, as a whole, the relations are inagreement with the trends known to date for galaxies of similarmagnitudes in nearby clusters of galaxies. Comparison with the relationsfound for satellites in the Local Group allows us to sample better thebright end of the luminosity function of satellite galaxies and extendsfor brighter objects the validity of the color-magnitude relation foundfor dwarf galaxies in the Local Group. Most of the E/S0 galaxies in oursample show a negative color gradient with values similar to those knownfor early-type galaxies in other environments.
| Studies of Extragalactic Formaldehyde and Radio Recombination Lines
We present the most sensitive and extensive survey yet performed ofextragalactic H2CO 6 cm (4.829 GHz) emission/absorption.Sixty-two sources were observed with the C-band system of the AreciboTelescope to a 1 σ rms noise level of ~0.3 mJy. We report a newdetection of H2CO 6 cm absorption toward NGC 520 and theconfirmation of H2CO 6 cm absorption toward several sources.We report confirmation of H2CO 6 cm emission toward the OHmegamasers Arp 220, IC 860, and IRAS 15107+0724. At present these arethe only extragalactic H2CO 6 cm emitters independentlyconfirmed. A characterization of the properties of formaldehydeabsorbers and emitters based on infrared properties of the galaxies isdiscussed. We also conducted a simultaneous survey of the H110αhydrogen recombination line toward a sample of 53 objects. We report thedetection of H110α toward the giant extragalactic H II region NGC604 in M33.
| The Role of Pressure in Giant Molecular Cloud Formation
We examine the hypothesis that hydrostatic pressure alone determines theratio of atomic to molecular gas averaged over a particular radius indisk galaxies. The hypothesis implies that the transition radius, thelocation where the ratio is unity, should always occur at the same valueof stellar surface density in all galaxies. We examine data for 28galaxies and find that the stellar surface density at the transitionradius is indeed constant to within 40% at a value of 120Msolar pc-2. If the hypothesis can be confirmed atall radii within a large range of galaxy types and metallicities,combining it with the observed relation between the star formation rateand H2 surface density may enable us to derive a physicallymotivated star formation prescription with wide applicability.
| On Determining Extinction from Reddening
The influence of shifts in effective wavelengths on ratios of total toselective extinction is examined, primarily to determine how to evaluatethe Galactic extinction of extragalactic bodies in a way that minimizessystematic errors. In the process, a new procedure is developed forevaluating the Galactic or extragalactic extinction of any source in anyfilter from any index of reddening. The amount of dust along a sightlineis quantified by the optical depth at 1 μm, which has the advantageof being roughly equal numerically to E(B-V). The optical depth can bederived iteratively from a color excess using an appropriate spectralenergy distribution (SED) for the reddening probe, and a monochromaticlaw of reddening which delivers a value of AV/E(B-V)characteristic of the obscuring medium when applied to the spectrum of areference source for which this ratio is known. Knowledge of the opticaldepth then facilitates the determination of the extinction of any sourcein any filter without concern as to the shape of the spectrum of theprobe. The ratio of total to selective extinction for stars and galaxiesis synthesized for a variety of filter combinations in order to examinevariations with type, tilt, optical depth, and redshift. For thispurpose, representative integrated SEDs spanning the space ultravioletto the near-infrared are constructed for galaxy types E, Sab, Sbc, Scd,and Im, all at well-defined inclinations. In addition, an algorithm toadjust the shapes of the SEDs for tilt is developed. Along the mainsequence, the classical ratio of total to selective extinction,AV/E(B-V), increases by 23% from O5 to M6. At late types,there are differences as high as 17% between evolved and unevolvedstars. Along the Hubble sequence, AV/E(B-V) decreases by 5%from E to Im. The value for elliptical galaxies falls near the locus forthe main sequence, not the giant branch. Correlated against B-I,AV/E(B-V) for star-forming galaxies is systematically lowerthan for stars of the same color by up to 5%. It increases much morerapidly with tilt than with the optical depth of Galactic dust, althoughneither dependence is strong. For both stars and galaxies,AV/E(B-V) varies dramatically with the redshift. Changes of16% for a Type Ia supernovae and 22% for a Cepheid are seen out toz=0.4. For elliptical galaxies, a variation of 30% can be expected outto z=1, the precise form of which being dependent upon the ultravioletexcess. Even infrared ratios of total to selective extinction, such asAH/E(B-V), change significantly with color and redshiftbecause of differential shifts in the effective wavelengths of B and V.As a gauge of reddening, E(V-I) is greatly preferable to E(B-V), becauseit is much less sensitive to color and redshift, yet more sensitive tothe optical depth of dust. A demonstration is given on how to quantifyupper limits to Galactic extinction which should be placed on studies ofhigh-redshift supernovae, to reduce the redshift dependence ofextinction corrections to a range that is insignificant compared withresiduals supporting accelerated universal expansion. When the newtechnique for evaluating extinction corrections is applied to Cepheidsin M31, distances for fields at different radii become less dispersed,confirming that the period-luminosity relation is not very sensitive tometallicity. However, the discrepancy between the Cepheid and maserdistances to NGC 4258 cannot be attributed to the method of handling theextinction.Submission of the final version of this paper was held off until thetechniques described herein could be made available through the WorldWide Web. The York Extinction Solver (YES) can be accessed at http://cadcwww.hia.nrc.ca/yes.
| Nuclear Properties of a Sample of Nearby Spiral Galaxies from Hubble Space Telescope STIS Imaging
We present surface photometry for the central regions of a sample of 48spiral galaxies (mostly unbarred and barred of type Sbc or Sc) observedwith the Space Telescope Imaging Spectrograph on board the Hubble SpaceTelescope. Surface brightness profiles (SBPs) were derived and modeledwith a Nuker law. We also analyzed archival Wide Field Planetary Camera2 images with a larger field of view, which are available for 18galaxies in our sample. We modeled the extracted bulge SBPs with anexponential, an r1/4, or an rn profile. Inagreement with previous studies, we find that bulges of Sbc galaxiesfall into two categories: bulges well described by an exponentialprofile and those well described by an r1/4 profile. Only onegalaxy requires the use of a more general Sérsic profile toproperly describe the bulge. Nuclear photometrically distinct componentsare found in ~55% of the galaxies. For those that we classify as starclusters on the basis of their resolved extent, we find absolutemagnitudes that are brighter on average than those previously identifiedin spiral galaxies. This might be due to a bias in our sample towardstar-forming galaxies, combined with a trend for star-forming galaxiesto host brighter central clusters.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| A New Nonparametric Approach to Galaxy Morphological Classification
We present two new nonparametric methods for quantifying galaxymorphology: the relative distribution of the galaxy pixel flux values(the Gini coefficient or G) and the second-order moment of the brightest20% of the galaxy's flux (M20). We test the robustness of Gand M20 to decreasing signal-to-noise ratio (S/N) and spatialresolution and find that both measures are reliable to within 10% forimages with average S/N per pixel greater than 2 and resolutions betterthan 1000 and 500 pc, respectively. We have measured G andM20, as well as concentration (C), asymmetry (A), andclumpiness (S) in the rest-frame near-ultraviolet/optical wavelengthsfor 148 bright local ``normal'' Hubble-type galaxies (E-Sd) galaxies, 22dwarf irregulars, and 73 0.05
| The nuclear ring in the unbarred galaxy NGC 278: Result of a minor merger?
We present fully sampled high angular resolution two-dimensionalkinematics in the Hα spectral line, optical and near-infraredimaging, as well as 21 cm atomic hydrogen data of the spiral galaxy NGC278. This is a small non-barred galaxy, which has a bright star forminginner region of about 2 kpc in diameter, reminiscent of nuclear ringsseen mainly in barred galaxies. The Hα kinematics show a disturbedvelocity field, which may be partly the result of spiral density wavestreaming motions. The 21 cm data trace the atomic hydrogen well outsidethe optical disk. The H I is not abundant but clearly shows disturbedmorphology and kinematics. We postulate that the current structure ofNGC 278 is a result of a recent interaction with a small gas-richgalaxy, which is now dispersed into the outer disk of NGC 278.Non-axisymmetries set up in the disk by this minor merger may well bethe cause of the intense star formation in the inner region, which canbe interpreted as a rare example of a nuclear ring in a non-barredgalaxy. Rather than being induced by a bar, this nuclear ring would thenbe the direct result of an interaction event in the recent history ofthe galaxy.
| Are galactic disks dynamically influenced by dust?
Dynamically cold components are well known to destabilize hotter, evenmuch more massive components. E.g. stellar disks can become unstable bya small admixture of cold gas or proto-planetary disks might bedestabilized by a small fraction of dust. In this paper we studied thedynamical influence of a cold dust component on the gaseous phase in thecentral regions of galactic disks. We performed two-dimensionalhydrodynamical simulations for flat multi-component disks embedded in acombined static stellar and dark matter potential. The pressure-freedust component is coupled to the gas by a drag force depending on theirvelocity difference.It turned out that the most unstable regions are those with either a lowor near to minimum Toomre parameter or with rigid rotation, i.e. thecentral area. In that regions the dust-free disks become most unstablefor high azimuthal modes (m 8), whereas in dusty disks all modeshave a similar amplitude resulting in a patchy appearance. Thestructures in the dust have a larger contrast between arm and inter-armregions than those of the gas. The dust peaks are frequently correlatedwith peaks of the gas distribution, but they do not necessarily coincidewith them. Therefore, a large scatter in the dust-to-gas ratios isexpected. The appearance of the dust is more cellular (i.e. sometimesconnecting different spiral features), whereas the gas is organized in amulti-armed spiral structure.We found that an admixture of 2% dust (relative to the mass of the gas)destabilizes gaseous disks substantially, whereas dust-to-gas ratiosbelow 1% have no influence on the evolution of the gaseous disk. For ahigh dust-to-gas ratio of 10% the instabilities reach a saturation levelalready after 30 Myr. The stability of the gaseous disks also stronglydepends on their Toomre parameter. But even in hot gaseous disks adestabilizing influence of the dust component has been found.
| Minor-axis velocity gradients in disk galaxies
We present the ionized-gas kinematics and photometry of a sample of 4spiral galaxies which are characterized by a zero-velocity plateau alongthe major axis and a velocity gradient along the minor axis,respectively. By combining these new kinematical data with thoseavailable in the literature for the ionized-gas component of the S0s andspirals listed in the Revised Shapley-Ames Catalog of Bright Galaxies werealized that about 50% of unbarred galaxies show a remarkable gasvelocity gradient along the optical minor axis. This fraction rises toabout 60% if we include unbarred galaxies with an irregular velocityprofile along the minor axis. This phenomenon is observed all along theHubble sequence of disk galaxies, and it is particularly frequent inearly-type spirals. Since minor-axis velocity gradients are unexpectedif the gas is moving onto circular orbits in a disk coplanar to thestellar one, we conclude that non-circular and off-plane gas motions arenot rare in the inner regions of disk galaxies.Based on observations carried out at the European Southern Observatoryin La Silla (Chile) (ESO 69.B-0706 and 70.B-0338), with the MultipleMirror Telescope which is a joint facility of the SmithsonianInstitution and the University of Arizona, and with the ItalianTelescopio Nazionale Galileo (AOT-5, 3-18) at the Observatorio del Roquede los Muchachos in La Palma (Spain).Table 1 is only available in electronic form athttp://www.edpsciences.org. Table 5 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/507
| Deprojecting spiral galaxies using Fourier analysis. Application to the Frei sample
We present two methods that can be used to deproject spirals, based onFourier analysis of their images, and discuss their potential andrestrictions. Our methods perform particularly well for galaxies moreinclined than 50° or for non-barred galaxies moreinclined than 35°. They are fast and straightforward touse, and thus ideal for large samples of galaxies. Moreover, they arevery robust for low resolutions and thus are appropriate for samples ofcosmological interest. The relevant software is available from us uponrequest. We use these methods to determine the values of the positionand inclination angles for a sample of 79 spiral galaxies contained inthe Frei et al. (\cite{frei96}) sample. We compare our results with thevalues found in the literature, based on other methods. We findstatistically very good agreementTable 7 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/415/849
| Structure and star formation in disc galaxies - I. Sample selection and near-infrared imaging
We present near-infrared imaging of a sample of 57 relatively large,northern spiral galaxies with low inclination. After describing theselection criteria and some of the basic properties of the sample, wegive a detailed description of the data collection and reductionprocedures. The Ksλ= 2.2-μm images cover most ofthe disc for all galaxies, with a field of view of at least 4.2 arcmin.The spatial resolution is better than 1 arcsec for most images. We fitbulge and exponential disc components to radial profiles of the lightdistribution. We then derive the basic parameters of these components,and the bulge/disc ratio, and explore correlations of these parameterswith several galaxy parameters.
| 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.
| Vertical Scale Parameter Estimates for 48 Non-edge-on Spiral Galaxies
In the first paper of this series, we directly studied the mathematicalforms, symmetry of spiral structure, and the projection of galacticdiscs on the images, and measured the pitch angles of the spiral armsand inclination angles of the galactic discs for 60 spiral galaxies. Inthis second paper, we estimate the vertical scale parameters of 48non-edge-on spiral galaxies based on the method proposed by Peng et al.and on the results given in Paper I. As we know, for edge-on discgalaxies we can obtain the vertical scale parameter from the photometry,once a mathematical form is specified for the vertical lightdistribution. For non-edge-on galaxies, some other methods have to beused. The statistical result was that the vertical scale parameter iscomparable for edge-on and non-edge-on galaxies, although it is obtainedfrom two very different methods.
| The Relationship between Stellar Light Distributions of Galaxies and Their Formation Histories
A major problem in extragalactic astronomy is the inability todistinguish in a robust, physical, and model-independent way how galaxypopulations are physically related to each other and to their formationhistories. A similar, but distinct, and also long-standing question iswhether the structural appearances of galaxies, as seen through theirstellar light distributions, contain enough physical information tooffer this classification. We argue through the use of 240 images ofnearby galaxies that three model-independent parameters measured on asingle galaxy image reveal its major ongoing and past formation modesand can be used as a robust classification system. These parametersquantitatively measure: the concentration (C), asymmetry (A), andclumpiness (S) of a galaxy's stellar light distribution. When combinedinto a three-dimensional ``CAS'' volume all major classes of galaxies invarious phases of evolution are cleanly distinguished. We argue thatthese three parameters correlate with important modes of galaxyevolution: star formation and major merging activity. This is arguedthrough the strong correlation of Hα equivalent width andbroadband colors with the clumpiness parameter S, the uniquely largeasymmetries of 66 galaxies undergoing mergers, and the correlation ofbulge to total light ratios, and stellar masses, with the concentrationindex. As an obvious goal is to use this system at high redshifts totrace evolution, we demonstrate that these parameters can be measured,within a reasonable and quantifiable uncertainty with available data outto z~3 using the Hubble Space Telescope GOODS ACS and Hubble Deep Fieldimages.
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