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A new method for determining mass-to-light ratios of nearly face-on spiral galaxies
Context: . Aims: .This letter gives a new method for determiningmass-to-light ratios of nearly face-on spiral galaxies. Methods:.The method is based on the effective thickness of the galactic disk,the distribution of the vertical velocity dispersion, and the surfacebrightness of a spiral galaxy. Results: .As examples, the resultsof the determination of NGC 1566 and NGC 5247 in B-band are presented,and their mass-to-light ratios are 4.86 8.99 {M_ȯ L_ȯ-1} and 5.02 6.90 {M_ȯ L_ȯ -1}respective. Conclusions: .
| Constraining Dark Matter Halo Profiles and Galaxy Formation Models Using Spiral Arm Morphology. I. Method Outline
We investigate the use of spiral arm pitch angles as a probe of diskgalaxy mass profiles. We confirm our previous result that spiral armpitch angles (P) are well correlated with the rate of shear (S) in diskgalaxy rotation curves by using a much larger sample (51 galaxies) thanused previously (17 galaxies). We use this correlation to argue thatimaging data alone can provide a powerful probe of galactic massdistributions out to large look-back times. In contrast to previouswork, we show that observed spiral arm pitch angles are similar whenmeasured in the optical (at 0.4 μm) and the near-infrared (at 2.1μm) with a mean difference of 2.3d+/-2.7d. This is then used tostrengthen the known correlation between P and S using B-band images. Wethen use two example galaxies to demonstrate how an inferred shear ratecoupled with a bulge-disk decomposition model and a Tully-Fisher-derivedvelocity normalization can be used to place constraints on a galaxy'sbaryon fraction and dark matter halo profile. We show that ESO 582-G12,a galaxy with a high shear rate (slightly declining rotation curve) at~10 kpc, favors an adiabatically contracted halo, with high initial NFWconcentration (cvir>16) and a high fraction of halobaryons in the form of stars (~15%-40%). In contrast, IC 2522 has a lowshear rate (rising rotation curve) at ~10 kpc and favorsnonadiabatically contracted models with low NFW concentrations(cvir~=2-8) and a low stellar baryon fraction <10%.
| A new method to determine the thickness of non-edge-on disk galaxies
Aims.We present a new method to determine the thickness of non-edge-ondisk galaxies. This method allows us to investigate the mass-to-lightratio of the disk. Methods: .Our method is based on the comparisonof observations and theory of the distribution of the vertical velocitydispersion, which is obtained from the solution of three dimensionalPoisson equations and the galactic dynamical equation. Results:.As examples, the thickness and mass-to-light ratio of two diskgalaxies, NGC 1566 and NGC 5247, which have been extensively studied byspectroscopy, have been calculated. The calculated results areconsistent with observations and support the use of this method.However, due to the small sample size available, the results should beconfirmed on other samples of galaxies.
| Structure and kinematics of edge-on galaxy discs - V. The dynamics of stellar discs
In earlier papers in this series we determined the intrinsic stellardisc kinematics of 15 intermediate- to late-type edge-on spiral galaxiesusing a dynamical modelling technique. The sample covers a substantialrange in maximum rotation velocity and deprojected face-on surfacebrightness, and contains seven spirals with either a boxy orpeanut-shaped bulge. Here we discuss the structural, kinematical anddynamical properties. From the photometry we find that intrinsicallymore flattened discs tend to have a lower face-on central surfacebrightness and a larger dynamical mass-to-light ratio. This observationsuggests that, at a constant maximum rotational velocity, lower surfacebrightness discs have smaller vertical stellar velocity dispersions.Although the individual uncertainties are large, we find from thedynamical modelling that at least 12 discs are submaximal. The averagedisc contributes 53 +/- 4 per cent to the observed rotation at 2.2 discscalelengths (hR), with a 1σ scatter of 15 per cent.This percentage becomes somewhat lower when effects of finite discflattening and gravity by the dark halo and the gas are taken intoaccount. Since boxy and peanut-shaped bulges are probably associatedwith bars, the result suggests that at 2.2hR the submaximalnature of discs is independent of barredness. The possibility remainsthat very high surface brightness discs are maximal, as these discs areunderrepresented in our sample. We confirm that the radial stellar discvelocity dispersion is related to the galaxy maximum rotationalvelocity. The scatter in this σ versus vmax relationappears to correlate with the disc flattening, face-on central surfacebrightness and dynamical mass-to-light ratio. Low surface brightnessdiscs tend to be more flattened and have smaller stellar velocitydispersions. The findings are consistent with the observed correlationbetween disc flattening and dynamical mass-to-light ratio and cangenerally be reproduced by the simple collapse theory for disc galaxyformation. Finally, the disc mass Tully-Fisher relation is offset fromthe maximum-disc scaled stellar mass Tully-Fisher relation of the UrsaMajor cluster. This offset, -0.3 dex in mass, is naturally explained ifthe discs of the Ursa Major cluster spirals are submaximal.
| 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.
| 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.
| Stellar Velocity Dispersion and Mass Estimation for Galactic Disks
Available velocity dispersion estimates for the old stellar populationof galactic disks at galactocentric distances r=2L (where L is thephotometric radial scale length of the disk) are used to determine thethreshold local surface density of disks that are stable againstgravitational perturbations. The mass of the disk Mdcalculated under the assumption of its marginal stability is comparedwith the total mass Mt and luminosity LB of thegalaxy within r=4L. We corroborate the conclusion that a substantialfraction of the mass in galaxies is probably located in their darkhalos. The ratio of the radial velocity dispersion to the circularvelocity increases along the sequence of galactic color indices anddecreases from the early to late morphological types. For most of thegalaxies with large color indices (B-V)0 > 0.75, whichmainly belong to the S0 type, the velocity dispersion exceedssignificantly the threshold value required for the disk to be stable.The reverse situation is true for spiral galaxies: the ratiosMd/LB for these agree well with those expected forevolving stellar systems with the observed color indices. This suggeststhat the disks of spiral galaxies underwent no significant dynamicalheating after they reached a quasi-equilibrium stable state.
| 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.
| 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.
| 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
| 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.
| Simulations of normal spiral galaxies
Results from numerical simulations of normal isolated late-type spiralgalaxies are presented; specifically, the galaxy NGC 628 is used as atemplate. The method employs a TREESPH code including stellar particles,gas particles, cooling and heating of the gas, star formation accordingto a Jeans criterion and supernova feedback. A regular spiral disc canbe generated as an equilibrium situation of two opposing actions: on theone hand, cooling and dissipation of the gas; on the other hand, gasheating by the far-ultraviolet field of young stars and supernovamechanical forcing. The disc exhibits small- and medium-scale spiralstructure of which the multiplicity increases as a function of radius.The theory of swing amplification can explain, both qualitatively andquantitatively, the emerging spiral structure. In addition, swingamplification predicts that the existence of a grand-design m= 2 spiralis only possible if the disc is massive. The simulations show that thegalaxy is then unstable to bar formation, confirming the result ofOstriker & Peebles. The occurrence of this bar instability isfurther investigated. A general criterion is derived for the transitionbetween a stable and an unstable bar, depending on the disc masscontribution and the on-disc thickness. It seems that bar stabilitybarely depends on the presence of gas. A detailed quantitative analysisis made of the emerging spiral structure and a comparison is made withobservations. This demonstrates that the structure of the numericalisolated galaxies is not as strong and has a larger multiplicitycompared with the structure of some exemplary real galaxies. It isargued that the suggestion of Kormendy & Norman holds, i.e. that agrand design can only be generated by a central bar or by tidal forcesresulting from an encounter with another galaxy.
| The BIMA Survey of Nearby Galaxies (BIMA SONG). II. The CO Data
The BIMA Survey of Nearby Galaxies is a systematic imaging study of the3 mm CO J=1-0 molecular emission within the centers and disks of 44nearby spiral galaxies. The typical spatial resolution of the survey is6" or 360 pc at the average distance (12 Mpc) of the sample. Thevelocity resolution of the CO observations is 4 km s-1,though most maps are smoothed to 10 km s-1 resolution. For 33galaxies, multifield observations ensured that a region >~190"(=10 kpc) in diameter was imaged. For the remaining 11galaxies, which had smaller optical diameters and were on averagefarther away, single-pointing observations imaged a 100" diameter(=11 kpc) region. The sample was not chosen based on CO orinfrared brightness; instead, all spirals were included that met theselection criteria of vsolar<=2000 km s-1,δ>=-20deg, i<=70deg,D25<70', and BT<11.0. Thedetection rate was 41/44 sources or 93%; of the three nondetections, one(M81) is known to have CO emission at locations outside the survey fieldof view. Fully sampled single-dish CO data were incorporated into themaps for 24 galaxies; these single-dish data comprise the most extensivecollection of fully sampled, two-dimensional single-dish CO maps ofexternal galaxies to date. We also tabulate direct measurements of theglobal CO flux densities for these 24 sources. For the remaining 20sources, we collected sensitive single-dish spectra in order to evaluatethe large-scale flux recovery. We demonstrate that the measured ratiosof flux density recovered are a function of the signal-to-noise of theinterferometric data. We examine the degree of central peakedness of themolecular surface density distributions and show that the distributionsexhibit their brightest CO emission within the central 6" in only 20/44or 45% of the sample. We show that all three Local Group spiral galaxieshave CO morphologies that are represented in SONG, though the Milky WayCO luminosity is somewhat below the SONG average, and M31 and M33 arewell below average. This survey provides a unique public database ofintegrated intensity maps, channel maps, spectra, and velocity fields ofmolecular emission in nearby galaxies. It also lays the groundwork forextragalactic surveys by more powerful future millimeter-wavelengthinterferometers like CARMA and ALMA.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| An Atlas of Hubble Space Telescope Spectra and Images of Nearby Spiral Galaxies
We have observed 54 nearby spiral galaxies with the Space TelescopeImaging Spectrograph (STIS) on the Hubble Space Telescope to obtainoptical long-slit spectra of nuclear gas disks and STIS optical (~Rband) images of the central 5''×5'' of thegalaxies. These spectra are being used to determine the velocity fieldof nuclear disks and hence to detect the presence of central massiveblack holes. Here we present the spectra for the successfulobservations. Dust obscuration can be significant at opticalwavelengths, and so we also combine the STIS images with archivalNear-Infrared Camera and Multi-Object Spectrometer H-band images toproduce color maps to investigate the morphology of gas and dust in thecentral regions. We find a great variety in the different morphologies,from smooth distributions to well-defined nuclear spirals and dustlanes.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 NAS5-26555.
| 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
| Bar strengths in spiral galaxies estimated from 2MASS images
Non-axisymmetric forces are presented for a sample of 107 spiralgalaxies, of which 31 are barred (SB) and 53 show nuclear activity. As adata base we use JHK images from the 2 Micron All-sky Survey, and thenon-axisymmetries are characterized by the ratio of the tangential forceto the mean axisymmetric radial force field, following Buta & Block.Bar strengths have an important role in many extragalactic problems andtherefore it is important to verify that the different numerical methodsapplied for calculating the forces give mutually consistent results. Weapply both direct Cartesian integration and a polar grid integrationutilizing a limited number of azimuthal Fourier components of density.We find that the bar strength is independent of the method used toevaluate the gravitational potential. However, because of thedistance-dependent smoothing by Fourier decomposition, the polar methodis more suitable for weak and noisy images. The largest source ofuncertainty in the derived bar strength appears to be the uncertainty inthe vertical scaleheight, which is difficult to measure directly formost galaxies. On the other hand, the derived bar strength is ratherinsensitive to the possible gradient in the vertical scaleheight of thedisc or to the exact model of the vertical density distribution,provided that the same effective vertical dispersion is assumed in allmodels. In comparison with the pioneering study by Buta & Block, thebar strength estimate is improved here by taking into account thedependence of the vertical scaleheight on the Hubble type: we find thatfor thin discs bar strengths are stronger than for thick discs by anamount that may correspond to as much as one bar strength class. Weconfirm the previous result by Buta and co-workers showing that thedispersion in bar strength is large among all the de Vaucouleurs opticalbar classes. In the near-infrared 40 per cent of the galaxies in oursample have bars (showing constant phases in the m= 2 Fourier amplitudesin the bar region), while in the optical band one-third of these barsare obscured by dust. Significant non-axisymmetric forces can also beinduced by the spiral arms, generally in the outer parts of the galacticdiscs, which may have important implications on galaxy evolution.Possible biases of the selected sample are also studied: we find thatthe number of bars identified drops rapidly when the inclination of thegalactic disc is larger than 50°. A similar bias is found in theThird Reference Catalogue of Bright Galaxies, which might be of interestwhen comparing bar frequencies at high and low redshifts.
| Quarter-turn spirals just beyond the principal arms of galaxies
Observations in the optical show that grand design spirals consist of aset of principal arms and characteristic near-circular extensions thatcan be described as quarter-turn spirals. Arguments are presented infavour of the idea that the latter set of spirals is merely caused bythe response of the material of the disc to the gravitational potentialof the main spiral arms.
| Bar Galaxies and Their Environments
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
| 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.
| Radio continuum and CO emission in star-forming galaxies
We combine the radio continuum images from the NRAO VLA Sky Survey withthe CO-line observations from the extragalactic CO survey of the FiveCollege Radio Astronomy Observatory to study the relationship betweenmolecular gas and the star formation rate within the disks of 180 spiralgalaxies at 45\arcsec resolution. We find a tight correlation betweenthese quantities. On average, the ratio between the radio continuum andthe CO emission is constant, within a factor of 3, both inside the samegalaxy and from galaxy to galaxy. The mean star formation efficiencydeduced from the radio continuum corresponds to convert 3.5% of theavailable molecular gas into stars on a time scale of 108 yrand depends weakly on general galaxy properties, such as Hubble type ornuclear activity. A comparison is made with another similar analysisperformed using the Halpha luminosity as star formationindicator. The overall agreement we find between the two studiesreinforces the use of the radio luminosity as star formation rateindicator not only on global but also on local scales.
| Galaxies with Rows
The results of a search for galaxies with straight structural elements,usually spiral-arm rows (“rows” in the terminology ofVorontsov-Vel'yaminov), are reported. The list of galaxies that possess(or probably possess) such rows includes about 200 objects, of whichabout 70% are brighter than 14m. On the whole, galaxies with rows makeup 6 8% of all spiral galaxies with well-developed spiral patterns. Mostgalaxies with rows are gas-rich Sbc-Scd spirals. The fraction ofinteracting galaxies among them is appreciably higher than amonggalaxies without rows. Earlier conclusions that, as a rule, the lengthsof rows are similar to their galactocentric distances and that theangles between adjacent rows are concentrated near 120° areconfirmed. It is concluded that the rows must be transient hydrodynamicstructures that develop in normal galaxies.
| Probing for Dark Matter within Spiral Galaxy Disks
We explore the relative importance of the stellar mass density ascompared to the inner dark halo using the observed gas kinematicsthroughout the disk of the spiral galaxy NGC 4254 (M99). We performhydrodynamic simulations of the gas flow for a sequence of gravitationalpotentials in which we vary the stellar disk contribution to the totalpotential. This stellar portion of the potential was derived empiricallyfrom color-corrected K-band photometry reflecting the spiral arms in thestellar mass, while the halo was modeled as an isothermal sphere. Thesimulated gas density and the gas velocity field are then compared tothe observed stellar spiral arm morphology and to the Hα gaskinematics. We find that this method is a powerful tool to determine thecorotation radius of the spiral pattern and that it can be used to placean upper limit on the mass of the stellar disk. For the case of thegalaxy NGC 4254 we find RCR=7.5+/-1.1kpc, orRCR=2.1Rexp (K'). We also demonstrate that for amaximal disk the prominent spiral arms of the stellar componentoverpredict the noncircular gas motions unless an axisymmetric dark halocomponent contributes significantly (>~1/3) to the total potentialinside 2.2 K-band exponential disk scale lengths.
| The BIMA Survey of Nearby Galaxies. I. The Radial Distribution of CO Emission in Spiral Galaxies
We present the first results of the Berkeley-Illinois-MarylandAssociation Survey of Nearby Galaxies (BIMA SONG), an imaging survey ofthe CO J=1-0 emission in 44 nearby spiral galaxies at a typicalresolution of 6". BIMA SONG differs from previous high-resolution COsurveys in that (1) CO brightness was not an explicit selectioncriterion, (2) a larger area (200" diameter for most galaxies) of eachgalaxy was imaged, and (3) fully sampled single-dish CO data (55"resolution) were obtained for over half of the sample galaxies, so allof the CO flux is imaged in these galaxies. Here we present CO maps fora subsample of 15 BIMA SONG galaxies for which we have also obtainednear-infrared or optical broadband data. The CO maps display aremarkable variety of molecular gas morphologies, and, as expected, theCO surface brightness distributions show considerably more substructurethan the stellar light distributions, even when averaged over kiloparsecscales. The radial distribution of stellar light in galactic disks isgenerally characterized as an exponential. It is, therefore, of interestto investigate whether the molecular gas, which is the star-formingmedium, has a similar distribution. Though our low-resolutionsingle-dish radial profiles of CO emission can be described by simpleexponentials, this is not true for the emission at our full 6"resolution. The scale lengths of the CO disks are correlated with thescale lengths of the stellar disks, with a mean ratio of the scalelengths of about 1. There is, however, considerable intrinsic scatter inthe correlation. We also find that (1) there is also a weak correlationbetween the ratio of K-band to CO luminosity and Hubble type; (2) inhalf of the galaxies presented here, CO emission does not peak at thelocation of the stellar nucleus; (3) averaged over the inner kiloparsec,the CO emission in one-half of the galaxies exhibits an excess over thatexpected from an exponential disk, which is similar to the excess instellar light caused by the bulge stars; and (4) this excess CO emissionmay be due to an increase in the total molecular gas content in thebulge region, or alternatively, to an increase in the CO emissivitycaused by the increased pressure of the bulge region.
| The BIMA Survey of Nearby Galaxies (BIMA SONG)
BIMA SONG is a systematic imaging study of the 3 mm CO J = 1 --> 0molecular emission within the centres and discs of 44 nearby spiralgalaxies on size scales of a few hundred parsecs (6-9``). The overallgoal of the survey is to study the role of molecular gas in theevolution of spiral galaxies. To this end, BIMA SONG addresses 1) thedistribution and physical conditions of the molecular gas in galacticdiscs and its relation to star formation, 2) the effects of a stellarbar on the kinematics of molecular gas, including the possible inflow ofgas along a bar, and 3) the distribution and role of molecular gas inthe central few hundred parsecs of active and quiescent galaxies. Thesource list includes all (except M33 and M31) 44 galaxies of Hubbletypes Sa-Sd, with declinations δ > -20^°, visual magnitudesB < 11.0, velocities v_hel < 2000 km s^-1, and inclinations i <70^°. Beyond the specific scientific questions we will address, thissurvey will provide a unique database for astronomers who study galaxiesat all wavelengths.
| Arm and Interarm Star Formation in Spiral Galaxies
We present an outline of our study of the effects of star formation onthe different components of the interstellar medium in the discs ofspiral galaxies, both globally and as a function of arm and interarmenvironment. We are in the process of obtaining images of 57 spiralgalaxies at low inclinations, and analysing them to study thedistribution of recent massive star formation, old stars, young stars,gas and dust. We will dissect the images into arm and interarm regionsand compare and contrast the morphology and scale lengths within theseregions in H_α, HI, the near infrared, optical and (whereavailable) CO. Modelling will show how the scale lengths are affected bystar formation, how this differs between arms and interarms, and whetherthe Schmidt Law varies from the global values in the arm and interarmregions.
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