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Late-type galaxies observed with SAURON: two-dimensional stellar and emission-line kinematics of 18 spirals
We present the stellar and gas kinematics of a sample of 18 nearbylate-type spiral galaxies (Hubble types ranging from Sb to Sd), observedwith the integral-field spectrograph SAURON at the 4.2-m WilliamHerschel Telescope. SAURON covers the spectral range 4800-5380Å,allowing us to measure the Hβ, Fe, Mgb absorption features and theemission in the Hβ line and the [OIII]λλ4959,5007Å and [NI]λλ5198, 5200Å doublets over a 33× 41-arcsec2 field of view. The maps cover the nuclearregion of these late-type galaxies and in all cases include the entirebulge. In many cases the stellar kinematics suggests the presence of acold inner region, as visible from a central drop in the stellarvelocity dispersion. The ionized gas is almost ubiquitous and behaves ina complicated fashion: the gas velocity fields often display morefeatures than the stellar ones, including wiggles in the zero-velocitylines, irregular distributions, ring-like structures. The line ratio[OIII]/Hβ often takes on low values over most of the field,probably indicating a wide-spread star formation.
| Hubble Space Telescope STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble Type
We study the nuclear star clusters (NCs) in spiral galaxies of variousHubble types using spectra obtained with the STIS on board the HubbleSpace Telescope (HST). We observed the nuclear clusters in 40 galaxies,selected from two previous HST WFPC2 imaging surveys. At a spatialresolution of ~0.2" the spectra provide a better separation of clusterlight from underlying galaxy light than is possible with ground-basedspectra. Approximately half of the spectra have a sufficiently highsignal-to-noise ratio for detailed stellar population analysis. For theother half we only measure the continuum slope, as quantified by the B-Vcolor. To infer the star formation history, metallicity, and dustextinction, we fit weighted superpositions of single-age stellarpopulation templates to the high signal-to-noise ratio spectra. We usethe results to determine the luminosity-weighted age, mass-to-lightratio, and masses of the clusters. Approximately half of the sampleclusters contain a population younger than 1 Gyr. Theluminosity-weighted ages range from 10 Myr to 10 Gyr. The stellarpopulations of NCs are generally best fit as a mixture of populations ofdifferent ages. This indicates that NCs did not form in a single event,but that instead they had additional star formation long after theoldest stars formed. On average, the sample clusters in late-typespirals have a younger luminosity-weighted mean age than those inearly-type spirals (L=8.37+/-0.25 vs.9.23+/-0.21). The average mass-weighted ages are older by ~0.7 dex,indicating that there often is an underlying older population that doesnot contribute much light but does contain most of the mass. The averagecluster masses are smaller in late-type spirals than in early-typespirals (logM=6.25+/-0.21 vs. 7.63+/-0.24) and exceed the masses typicalof globular clusters. The cluster mass correlates loosely with totalgalaxy luminosity. It correlates more strongly with both the Hubble typeof the host galaxy and the luminosity of its bulge. The lattercorrelation has the same slope as the well-known correlation betweensupermassive black hole mass and bulge luminosity. The properties ofboth nuclear clusters and black holes in the centers of spiral galaxiesare therefore intimately connected to the properties of the host galaxy,and in particular its bulge component. Plausible formation scenarioshave to account for this. We discuss various possible selection biasesin our results, but conclude that none of them can explain thedifferences seen between clusters in early- and late-type spirals. Theinability to infer spectroscopically the populations of faint clustersdoes introduce a bias toward younger ages, but not necessarily towardhigher masses.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555. These observations areassociated with proposals 9070 and 9783.
| Objective Classification of Spiral Galaxies Having Extended Rotation Curves Beyond the Optical Radius
We carry out an objective classification of four samples of spiralgalaxies having extended rotation curves beyond the optical radius. Amultivariate statistical analysis (viz., principal component analysis[PCA]) shows that about 96% of the total variation is due to twocomponents, one being the combination of absolute blue magnitude andmaximum rotational velocity beyond the optical region and the otherbeing the central density of the halo. On the basis of PCA a fundamentalplane has been constructed that reduces the scatter in the Tully-Fisherrelation up to a maximum of 16%. A multiple stepwise regression analysisof the variation of the overall shape of the rotation curves shows thatit is mainly determined by the central surface brightness, while theshape purely in the outer part of the galaxy (beyond the optical radius)is mainly determined by the size of the galactic disk.
| The AMIGA sample of isolated galaxies. II. Morphological refinement
We present a refinement of the optical morphologies for galaxies in theCatalog of Isolated Galaxies that forms the basis of the AMIGA (Analysisof the interstellar Medium of Isolated GAlaxies) project. Uniformreclassification using the digitized POSS II data benefited from thehigh resolution and dynamic range of that sky survey. Comparison withindependent classifications made for an SDSS overlap sample of more than200 galaxies confirms the reliability of the early vs. late-typediscrimination and the accuracy of spiral subtypes within Δ T =1-2. CCD images taken at the Observatorio de Sierra Nevada were alsoused to solve ambiguities in early versus late-type classifications. Aconsiderable number of galaxies in the catalog (n = 193) are flagged forthe presence of nearby companions or signs of distortion likely due tointeraction. This most isolated sample of galaxies in the local Universeis dominated by two populations: 1) 82% are spirals (Sa-Sd) with thebulk being luminous systems with small bulges (63% between types Sb-Sc)and 2) a significant population of early-type E-S0 galaxies (14%). Mostof the types later than Sd are low luminosity galaxies concentrated inthe local supercluster where isolation is difficult to evaluate. Thelate-type spiral majority of the sample spans a luminosity rangeMB-corr = -18 to -22 mag. Few of the E/S0 population are moreluminous than -21.0 marking the absence of the often-sought superL* merger (e.g. fossil elliptical) population. The rarity ofhigh luminosity systems results in a fainter derived M* forthis population compared to the spiral optical luminosity function(OLF). The E-S0 population is from 0.2 to 0.6 mag fainter depending onhow the sample is defined. This marks the AMIGA sample as unique amongsamples that compare early and late-type OLFs separately. In othersamples, which always involve galaxies in higher density environments,M^*_E/S0 is almost always 0.3-0.5 mag brighter than M^*_S, presumablyreflecting a stronger correlation between M* andenvironmental density for early-type galaxies.
| The Baryonic Tully-Fisher Relation of Galaxies with Extended Rotation Curves and the Stellar Mass of Rotating Galaxies
I investigate the baryonic Tully-Fisher relation for a sample ofgalaxies with extended 21 cm rotation curves spanning the range 20 kms-1<~Vf<=300 km s-1. A variety ofscalings of the stellar mass-to-light ratio Υ* areconsidered. For each prescription for Υ*, I give fitsof the form Md=AVxf.Presumably, the prescription that comes closest to the correct valuewill minimize the scatter in the relation. The fit with minimum scatterhas A=50 Msolar km-4 s4 andx=4. This relation holds over five decades in mass. Galaxy color,stellar fraction, and Υ* are correlated with eachother and with Md, in the sense that more massivegalaxies tend to be more evolved. There is a systematic dependence ofthe degree of maximality of disks on surface brightness. High surfacebrightness galaxies typically have Υ*~3/4 of themaximum disk value, while low surface brightness galaxies typicallyattain ~1/4 of this amount.
| The Molecular Interstellar Medium of Dwarf Galaxies on Kiloparsec Scales: A New Survey for CO in Northern, IRAS-detected Dwarf Galaxies
We present a new survey for CO in dwarf galaxies using the ARO Kitt Peak12 m telescope. This survey consists of observations of the centralregions of 121 northern dwarfs with IRAS detections and no known COemission. We detect CO in 28 of these galaxies and marginally detectanother 16, increasing by about 50% the number of such galaxies known tohave significant CO emission. The galaxies we detect are comparable instellar and dynamical mass to the Large Magellanic Cloud, althoughsomewhat brighter in CO and fainter in the far-IR. Within dwarfs, wefind that the CO luminosity LCO is most strongly correlatedwith the K-band and the far-infrared luminosities. There are also strongcorrelations with the radio continuum (RC) and B-band luminosities andlinear diameter. Conversely, we find that far-IR dust temperature is apoor predictor of CO emission within the dwarfs alone, although a goodpredictor of normalized CO content among a larger sample of galaxies. Wesuggest that LCO and LK correlate well because thestellar component of a galaxy dominates the midplane gravitational fieldand thus sets the pressure and density of the atomic gas, which controlthe formation of H2 from H I. We compare our sample with moremassive galaxies and find that dwarfs and large galaxies obey the samerelationship between CO and the 1.4 GHz RC surface brightness. Thisrelationship is well described by a Schmidt law withΣRC~Σ1.3CO. Therefore,dwarf galaxies and large spirals exhibit the same relationship betweenmolecular gas and star formation rate (SFR). We find that this result isrobust to moderate changes in the RC-to-SFR and CO-to-H2conversion factors. Our data appear to be inconsistent with large (orderof magnitude) variations in the CO-to-H2 conversion factor inthe star-forming molecular gas.
| Light and Motion in the Local Volume
Using high-quality data on 149 galaxies within 10 Mpc, I find nocorrelation between luminosity and peculiar velocity at all. There is nounequivocal sign on scales of 1-2 Mpc of the expected gravitationaleffect of the brightest galaxies, in particular infall toward groups, orof infall toward the supergalactic plane on any scale. Either darkmatter is not distributed in the same way as luminous matter in thisregion, or peculiar velocities are not due to fluctuations in mass. Thesensitivity of peculiar velocity studies to the background model ishighlighted.
| Mass Modeling of Disk Galaxies: Degeneracies, Constraints, and Adiabatic Contraction
This paper addresses available constraints on mass models fitted torotation curves. Mass models of disk galaxies have well-knowndegeneracies that prevent a unique mass decomposition. The most notableis due to the unknown value of the stellar mass-to-light ratio (thedisk-halo degeneracy); even with this known, degeneracies between thehalo parameters themselves may prevent an unambiguous determination ofthe shape of the dark halo profile, which includes the inner densityslope of the dark matter halo. The latter is often referred to as the``cusp-core degeneracy.'' We explore constraints on the disk and haloparameters and apply these to four mock and six observed disk galaxieswith high resolution and extended rotation curves. Our full set ofconstraints consists of mass-to-light (M/L) ratios from stellarpopulation synthesis models based on B-R colors, constraints on haloparameters from N-body simulations, and constraining the halo virialvelocity to be less than the maximum observed velocity. Theseconstraints are only partially successful in lifting the cusp-coredegeneracy. The effect of adiabatic contraction of the halo by the diskis to steepen cores into cusps and reduce the best-fit haloconcentration and M/L values (often significantly). We also discuss theeffect of disk thickness, halo flattening, distance errors, and rotationcurve error values on mass modeling. Increasing the imposed minimumrotation curve error from typically low, underestimated values to morerealistic estimates decreases the χ2 substantially andmakes distinguishing between a cuspy or cored halo profile even moredifficult. In spite of the degeneracies and uncertainties present, ourconstrained mass modeling favors submaximal disks (i.e., a dominanthalo) at 2.2 disk scale lengths, withVdisk/Vtot<~0.6. This result holds for both theunbarred and weakly barred galaxies in our sample.
| Revealing the Supernova Remnant Population of M33 with Chandra
We present results of a search for supernova remnants (SNRs) in archivalChandra images of M33. We have identified X-ray SNRs by comparing thelist of Chandra X-ray sources in M33 with tabulations of SNR candidatesidentified from (1) elevated [S II]/Hα ratios in the optical and(2) radio spectral indices. In addition, we have searched for opticalcounterparts to soft sources in the Chandra images and X-ray SNRcandidates identified in the XMM-Newton survey of M33. Of the 98optically known SNRs in M33, 22 have been detected at >3 σlevel in the soft band (0.35-1.1 keV). At least four of these SNRcandidates are spatially extended based on a comparison of the data tosimulated images of point sources. Aside from the optically matchingSNRs, we have found one soft X-ray source in M33 that exhibits nooptical emission and is coincident with a known radio source. The radiospectral index of this source is consistent with particle accelerationin shocks, leading us to suggest that it is a nonradiative SNR. We havealso found new optical counterparts to two soft X-ray SNRs in M33. Thesecounterparts exhibit enhanced [S II]/Hα ratios characteristic ofradiative shocks. Pending confirmation from optical spectroscopy, theidentification of these two optical counterparts increases the totalnumber of known optically emitting SNRs in M33 to 100. This brings thetotal number of identified SNRs with X-ray counterparts, including thoseexclusively detected by the XMM-Newton survey of M33, to 37 SNRs. Wefind that while there are a similar number of confirmed X-ray SNRs inM33 and the LMC with X-ray luminosities in excess of 1035ergs s-1, nearly 40% of the LMC SNRs are brighter than1036 ergs s-1, while only 13% of the M33 sampleexceed this luminosity. Including X-ray SNR candidates from theXMM-Newton survey (objects lacking optical counterparts) increases thefraction of M33 SNRs brighter than 1036 ergs s-1to 22%, still only half the LMC fraction. The differences in luminositydistributions cannot be fully explained by uncertainty in spectral modelparameters and are not fully accounted for by abundance differencesbetween the galaxies.
| Cores of dark matter haloes correlate with stellar scalelengths
We investigate in detail the mass distribution obtained by means ofhigh-resolution rotation curves of 25 galaxies of differentmorphological types. The dark matter contribution to the circularrotation velocity is well-described by resorting to a dark component,the density of which shows an inner core, i.e. a central constantdensity region. We find a very strong correlation between the coreradius size RC and the stellar exponential scalelengthRD: RC~=13[RD/(5kpc)]1.05kpc, and between RCand the galaxy dynamical mass at this distance,Mdyn(RC). These relationships would not beexpected if the core radii were the product of an incorrectdecomposition procedure, or the biased result of wrong or misunderstoodobservational data. The very strong correlation between the dark andluminous scalelengths found here seems to hold also for different Hubbletypes and opens new scenarios for the nature of the dark matter ingalaxies.
| Dynamical friction for dark halo satellites: effects of tidal mass loss and growing host potential
Motivated by observations of inner halo satellite remnants like the Sgrstream and ω Centauri, we develop fully analytical models to studythe orbital decay and tidal mass loss of satellites on eccentric orbitsin an isothermal potential of a host galaxy halo. The orbital decay rateis often severely overestimated if applying Chandrasekhar's formulawithout correcting for (i) the evaporation and tidal loss of thesatellite, and (ii) the contraction of satellite orbits due to adiabaticgrowth of the host galaxy potential over the Hubble time. As a satellitemigrates inwards, the increasing halo density affects the dynamicalfriction in two opposite ways: (1) it boosts the number of haloparticles swept in the gravitational `wake' of the satellite, henceincreasing the drag on the satellite, and (2) it boosts the tide which`peels off' the satellite, and reduces the amplitude of the wake. Thesecompeting processes can be modelled analytically for a satellite withthe help of an empirical formula for the mass-loss history. Theanalytical model agrees with more traditional numerical simulations oftidal mass loss and dynamical friction. Rapid mass loss due toincreasing tides at smaller and smaller radius makes it less likely forstreams or remnants of infalling satellites to intrude into the innerhalo (like the Sgr stream and ω Centauri) than to stay in theouter halo (like the Magellanic stream), hence any intermediate-masscentral black holes of the satellites are also probably `hung up' atlarge distances as well. It is difficult for the black holes of thesatellites to come close enough to merge into the supermassive blackhole in the centre of the host potential unless the satellites startedwith (i) pericentres much smaller than the typical distances topresent-day observed satellites, and (ii) central density much higherthan in the often seen finite-density cores of observed satellites.
| Observational Constraints on the Physical Parameters of Dark Matter Halos
After looking at the difference in the mass distribution between massivespiral and dwarf irregular (dIrr) and low surface brightness (LSB)galaxies, the central Dark Matter (DM) concentration (flat vs cuspy) indwarf and LSB galaxies, derived from observations, will be examined. Wewill then present what kind of observational constraints can be put onthe total mass and total extent of DM halos from the studies ofindividual galaxies, small groups, satellites' dynamics and tidal tailsof interacting systems. Finally, we will discuss how limits on thephysical parameters of DM halos could be set by deriving extendedrotation curves beyond the HI radius (r > rHI), usingeither Lyα absorption or Hα emission observations.
| The visible matter -- dark matter coupling
In the inner parts of spiral galaxies, of high or low surfacebrightness, there is a close correlation between rotation curve shapeand light distribution. For any feature in the luminosity profile thereis a corresponding feature in the rotation curve and vice versa. Thisimplies that the gravitational potential is strongly correlated with thedistribution of luminosity: either the luminous mass dominates or thereis a close coupling between luminous and dark matter. In a similar way,the declining rotation curves observed in the outer parts of highluminosity systems are a clear signature of the stellar disk whicheither dominates or traces the distribution of mass.The notion that the baryons are dynamically important in the centres ofgalaxies, including LSBs, undermines the whole controversy over thecusps in CDM halos and the comparison with the observations. If thebaryons dominate in the central regions of all spirals, including LSBs,how can the CDM profiles be compared with the observations?Alternatively, if the baryons do not dominate but simply trace the DMdistribution, why, in systems of comparable luminosity, are some DMhalos cuspy (like the light) and others (also like the light) are not?
| 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
| Inner-truncated Disks in Galaxies
We present an analysis of the disk brightness profiles of 218 spiral andlenticular galaxies. At least 28% of disk galaxies exhibit innertruncations in these profiles. There are no significant trends oftruncation incidence with Hubble type, but the incidence among barredsystems is 49%, more than 4 times that for nonbarred galaxies. However,not all barred systems have inner truncations, and not allinner-truncated systems are currently barred. Truncations represent areal dearth of disk stars in the inner regions and are not an artifactof our selection or fitting procedures nor the result of obscuration bydust. Disk surface brightness profiles in the outer regions are wellrepresented by simple exponentials for both truncated and nontruncateddisks. However, truncated and nontruncated systems have systematicallydifferent slopes and central surface brightness parameters for theirdisk brightness distributions. Truncation radii do not appear tocorrelate well with the sizes or brightnesses of the bulges. Thissuggests that the low angular momentum material apparently missing fromthe inner disk was not simply consumed in forming the bulge population.Disk parameters and the statistics of bar orientations in our sampleindicate that the missing stars of the inner disk have not simply beenredistributed azimuthally into bar structures. The sharpness of thebrightness truncations and their locations with respect to othergalactic structures suggest that resonances associated with diskkinematics, or tidal interactions with the mass of bulge stars, might beresponsible for this phenomenon.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| A Hubble Space Telescope Census of Nuclear Star Clusters in Late-Type Spiral Galaxies. II. Cluster Sizes and Structural Parameter Correlations
We investigate the structural properties of nuclear star clusters inlate-type spiral galaxies. More specifically, we fit analytical modelsto Hubble Space Telescope images of 39 nuclear clusters in order todetermine their effective radii after correction for the instrumentalpoint-spread function. We use the results of this analysis to comparethe luminosities and sizes of nuclear star clusters to those of otherellipsoidal stellar systems, in particular the Milky Way globularclusters. Our nuclear clusters have a median effective radius ofre=3.5 pc, with 50% of the sample falling in the range2.4pc<=re<=5.0pc. This narrow size distribution isstatistically indistinguishable from that of Galactic globular clusters,even though the nuclear clusters are, on average, 4 mag brighter thanthe old globular clusters. We discuss some possible interpretations ofthis result. From a comparison of nuclear cluster luminosities withvarious properties of their host galaxies, we confirm that more luminousgalaxies harbor more luminous nuclear clusters. It remains unclearwhether this correlation mainly reflects the influence of galaxy size,mass, and/or star formation rate. Since the brighter galaxies in oursample typically have stellar disks with a higher central surfacebrightness, nuclear cluster luminosity also correlates with thisproperty of their hosts. On the other hand, we find no evidence for acorrelation between the presence of a nuclear star cluster and thepresence of a large-scale stellar bar.
| The structure and environment of young stellar clusters in spiral galaxies
A search for stellar clusters has been carried out in 18 nearby spiralgalaxies, using archive images from the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. All of the galaxies have previouslybeen imaged from the ground in UBVI. A catalogue of structuralparameters, photometry and comments based on visual inspection of theclusters is compiled and used to investigate correlations betweencluster structure, environment, age and mass. Least-squares fits to thedata suggest correlations between both the full-width at half-maximum(FWHM) and half-light radius (Reff) of the clusters and theirmasses (M) at about the 3σ level. Although both relations show alarge scatter, the fits have substantially shallower slopes than for aconstant-density relation (size ∝ M1/3). However, manyof the youngest clusters have extended halos which make theReff determinations uncertain. There is no evidence forgalaxy-to-galaxy variations in the mean cluster sizes. In particular,the mean sizes do not appear to depend on the host galaxy star formationrate surface density. Many of the youngest objects (age <107 years) are located in strongly crowded regions, and about1/3-1/2 of them are double or multiple sources. The HST images are alsoused to check the nature of cluster candidates identified in a previousground-based survey. The contamination rate in the ground-based sampleis generally less than about 20%, but some cluster identificationsremain ambiguous because of crowding even with HST imaging, especiallyfor the youngest objects.Full Tables \ref{tab:all}-\ref{tab:hstphot}, and \ref{tab:gb} are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/537Based on observations obtained with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.
| 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
| The Hα galaxy survey. I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxies
We discuss the selection and observations of a large sample of nearbygalaxies, which we are using to quantify the star formation activity inthe local Universe. The sample consists of 334 galaxies across allHubble types from S0/a to Im and with recession velocities of between 0and 3000 km s-1. The basic data for each galaxy are narrowband H\alpha +[NII] and R-band imaging, from which we derive starformation rates, H\alpha +[NII] equivalent widths and surfacebrightnesses, and R-band total magnitudes. A strong correlation is foundbetween total star formation rate and Hubble type, with the strongeststar formation in isolated galaxies occurring in Sc and Sbc types. Moresurprisingly, no significant trend is found between H\alpha +[NII]equivalent width and galaxy R-band luminosity. More detailed analyses ofthe data set presented here will be described in subsequent papers.Based on observations made with the Jacobus Kapteyn Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.The full version of Table \ref{tab3} is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/23 Reduced image datafor this survey can be downloaded fromhttp://www.astro.livjm.ac.uk/HaGS/
| 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.
| High-Resolution Measurements of the Dark Matter Halo of NGC 2976: Evidence for a Shallow Density Profile
We have obtained two-dimensional velocity fields of the dwarf spiralgalaxy NGC 2976 in Hα and CO. The high spatial (~75 pc) andspectral (13 and 2 km s-1, respectively) resolution of theseobservations, along with our multicolor optical and near-infraredimaging, allows us to measure the shape of the density profile of thedark matter halo with good precision. We find that the total (baryonicplus dark matter) mass distribution of NGC 2976 follows aρtot~r-0.27+/-0.09 power law out to a radiusof 1.8 kpc, assuming that the observed radial motions provide nosupport. The density profile attributed to the dark halo is evenshallower, consistent with a nearly constant density of dark matter overthe entire observed region. A maximal disk fit yields an upper limit tothe K-band stellar mass-to-light ratio (M*/LK) of0.09+0.15-0.08Msolar/LsolarK(including systematic uncertainties), with the caveat that forM*/LK>0.19Msolar/LsolarKthe dark matter density increases with radius, which is unphysical.Assuming0.10Msolar/LsolarK<~M*/LK<=0.19Msolar/LsolarK,the dark matter density profile lies betweenρDM~r-0.17 andρDM~r-0.01. Therefore, independent of anyassumptions about the stellar disk or the functional form of the densityprofile, NGC 2976 does not contain a cuspy dark matter halo. We alsoinvestigate some of the systematic effects that can hamper rotationcurve studies and show that (1) long-slit rotation curves are far morevulnerable to systematic errors than two-dimensional velocity fields,(2) NGC 2976 contains radial motions that are as large as 90% of therotational velocities at small radii, and (3) the Hα and COvelocity fields of NGC 2976 agree within their uncertainties, with atypical scatter between the two velocities of 5.3 km s-1 atany position in the galaxy.Based on observations carried out at the WIYN Observatory. The WIYNObservatory is a joint facility of the University of Wisconsin-Madison,Indiana University, Yale University, and the National Optical AstronomyObservatory.
| The Kinematic State of the Local Volume
The kinematics of galaxies within 10 Mpc of the Milky Way isinvestigated using published distances and radial velocities. Withrespect to the average Hubble flow (isotropic or simple anisotropic),there is no systematic relation between peculiar velocity dispersion andabsolute magnitude over a range of 10 mag; neither is there any apparentvariation with galaxy type or between field and cluster members. Thereare several possible explanations for the lack of variation, though allhave difficulties: either there is no relationship between light andmass on these scales, the peculiar velocities are not produced bygravitational interaction, or the background dynamical picture is wrongin some systematic way. The extremely cold local flow of 40-60 kms-1 dispersion reported by some authors is shown to be anartifact of sparse data, a velocity dispersion of over 100 kms-1 being closer to the actual value. Galaxies with a high(positive) radial velocity have been selected against in studies of thisvolume, biasing numerical results.
| The Contribution of H I-rich Galaxies to the Damped Lyα Absorber Population at z = 0
We present a study of the expected properties of the low-redshift dampedLyα absorber population determined from a sample of H I-selectedgalaxies in the local universe. Because of a tight correlation betweenthe H I mass and H I cross section, which we demonstrate spans allgalaxy types, we can use our H I-selected sample to predict theproperties of the absorption-line systems. We use measurements of thenumber density and H I cross section of galaxies to show that the totalH I cross section at column densities sufficient to produce dampedLyα absorption is consistent with no evolution of the absorberpopulation. We also find that the dN/dz distribution is dominated bygalaxies with H I masses near 109 Msolar. However,because of the large dispersion in the correlation between H I mass andstellar luminosity, we find that the distribution of dN/dz as a functionof LJ is fairly flat. In addition, we examine the line widthsof the H I-selected galaxies and show that there may be evolution in thekinematics of H I-rich galaxies, but it is not necessary for the higherredshift population to contain a greater proportion of high-massgalaxies than we find locally.
| A Search for ``Dwarf'' Seyfert Nuclei. VI. Properties of Emission-Line Nuclei in Nearby Galaxies
We use the database from Paper III to quantify the global and nuclearproperties of emission-line nuclei in the Palomar spectroscopic surveyof nearby galaxies. We show that the host galaxies of Seyferts, LINERs,and transition objects share remarkably similar large-scale propertiesand local environments. The distinguishing traits emerge on nuclearscales. Compared with LINERs, Seyfert nuclei are an order of magnitudemore luminous and exhibit higher electron densities and internalextinction. We suggest that Seyfert galaxies possess characteristicallymore gas-rich circumnuclear regions and hence a more abundant fuelreservoir and plausibly higher accretion rates. The differences betweenthe ionization states of the narrow emission-line regions of Seyfertsand LINERs can be partly explained by the differences in their nebularproperties. Transition-type objects are consistent with being composite(LINER/H II) systems. With very few exceptions, the stellar populationwithin the central few hundred parsecs of the host galaxies is uniformlyold, a finding that presents a serious challenge to starburst orpost-starburst models for these objects. Seyferts and LINERs havevirtually indistinguishable velocity fields as inferred from their linewidths and line asymmetries. Transition nuclei tend to have narrowerlines and more ambiguous evidence for line asymmetries. All threeclasses of objects obey a strong correlation between line width and lineluminosity. We argue that the angular momentum content of circumnucleargas may be an important factor in determining whether a nucleus becomesactive. Finally, we discuss some possible complications for theunification model of Seyfert galaxies posed by our observations.
| Searching for Bulges at the End of the Hubble Sequence
We investigate the stellar disk properties of a sample of 19 nearbyspiral galaxies with low inclination and late Hubble type (Scd orlater). We combine our high-resolution Hubble Space Telescope I-bandobservations with existing ground-based optical images to obtain surfacebrightness profiles that cover a high dynamic range of galactic radii.Most of these galaxies contain a nuclear star cluster, as discussed in aseparate paper. The main goal of the present work is to constrain theproperties of stellar bulges at these extremely late Hubble types. Wefind that the surface brightness profiles of the latest-type spiralgalaxies are complex, with a wide range in shapes. We have sorted oursample in a sequence, starting with ``pure'' disk galaxies(approximately 30% of the sample). These galaxies have exponentialstellar disks that extend inward to within a few tens of parsecs fromthe nucleus, where the light from the nuclear cluster starts todominate. They appear to be truly bulgeless systems. Progressing alongthe sequence, the galaxies show increasingly prominent deviations from asimple exponential disk model on kiloparsec scales. Traditionally, suchdeviations have prompted ``bulge-disk'' decompositions. Indeed, thesurface brightness profiles of these galaxies are generally well fittedby adding a second (exponential) bulge component. However, we find thatmost surface brightness profiles can be fitted equally well (or better)with a single Sérsic-type R1/n profile over the entireradial range of the galaxy without requiring a separate ``bulge''component. We warn in a general sense against identification of bulgessolely on the basis of single-band surface brightness profiles. Despitethe narrow range of Hubble types in our sample, the surface brightnessprofiles are far from uniform. The differences between the variousgalaxies appear unrelated to their Hubble types, thus questioning theusefulness of the Hubble sequence for the subcategorization of thelatest-type spiral galaxies. A number of galaxies show central excessemission on spatial scales of a few hundred parsecs that cannot beattributed to the nuclear cluster, the Sérsic-type description ofthe stellar disk, or what one would generally consider to be a bulgecomponent. The origin of this light component remains unclear.Based in part 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. These observations are associatedwith proposal 8599.
| Revised positions for CIG galaxies
We present revised positions for the 1051 galaxies belonging to theKarachentseva Catalog of Isolated Galaxies (CIG). New positions werecalculated by applying SExtractor to the Digitized Sky Survey CIG fieldswith a spatial resolution of 1 arcsper 2. We visually checked theresults and for 118 galaxies had to recompute the assigned positions dueto complex morphologies (e.g. distorted isophotes, undefined nuclei,knotty galaxies) or the presence of bright stars. We found differencesbetween older and newer positions of up to 38 arcsec with a mean valueof 2 arcsper 96 relative to SIMBAD and up to 38 arcsec and 2 arcsper 42respectively relative to UZC. Based on star positions from the APMcatalog we determined that the DSS astrometry of five CIG fields has amean offset in (alpha , delta ) of (-0 arcsper 90, 0 arcsper 93) with adispersion of 0 arcsper 4. These results have been confirmed using the2MASS All-Sky Catalog of Point Sources. The intrinsic errors of ourmethod combined with the astrometric ones are of the order of 0 arcsper5.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/411/391
| Galaxy classification using fractal signature
Fractal geometry is becoming increasingly important in the study ofimage characteristics. For recognition of regions and objects in naturalscenes, there is always a need for features that are invariant and theyprovide a good set of descriptive values for the region. There are manyfractal features that can be generated from an image. In this paper,fractal signatures of nearby galaxies are studied with the aim ofclassifying them. The fractal signature over a range of scales proved tobe an efficient feature set with good discriminating power. Classifierswere designed using nearest neighbour method and neural networktechnique. Using the nearest distance approach, classification rate wasfound to be 92%. By the neural network method it has been found toincrease to 95%.
| 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
| Modified Newtonian Dynamics as an Alternative to Dark Matter
Modified Newtonian dynamics (MOND) is an empirically motivatedmodification of Newtonian gravity or inertia suggested by Milgrom as analternative to cosmic dark matter. The basic idea is that ataccelerations below ao ~ 10-8 cm/s2 ~cHo/6 the effective gravitational attraction approaches√(gnao), where gn is the usualNewtonian acceleration. This simple algorithm yields flat rotationcurves for spiral galaxies and a mass-rotation velocity relation of theform M ∝ V4 that forms the basis for the observedluminosity-rotation velocity relation-the Tully-Fisher law. We reviewthe phenomenological success of MOND on scales ranging from dwarfspheroidal galaxies to superclusters and demonstrate that the evidencefor dark matter can be equally well interpreted as evidence for MOND. Wediscuss the possible physical basis for an acceleration-basedmodification of Newtonian dynamics as well as the extention of MOND tocosmology and structure formation.
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