Contents
Images
Upload your image
DSS Images Other Images
Related articles
Boxy/peanut `bulges': comparing the structure of galaxies with the underlying families of periodic orbits
The vertical profiles of disc galaxies are built by the material trappedaround stable periodic orbits, which form their `skeletons'. Therefore,knowledge of the stability of the main families of periodic orbits inappropriate 3D models enables one to predict possible morphologies foredge-on disc galaxies. In a pilot survey we compare the orbitalstructures that lead to the appearance of `peanut'- and `X'-likefeatures with the edge-on profiles of three disc galaxies (IC 2531, NGC4013 and UGC 2048). The subtraction from the images of a modelrepresenting the axisymmetric component of the galaxies reveals thecontribution of the non-axisymmetric terms. We find a directcorrespondence between the orbital profiles of 3D bars in models and theobserved main morphological features of the residuals. We also apply asimple unsharp masking technique in order to study the sharpest featuresof the images. Our basic conclusion is that the morphology of the boxy`bulges' of these galaxies can be explained by considering disc materialtrapped around stable 3D periodic orbits. In most models, thesebuilding-block periodic orbits are bifurcated from the planar centralfamily of a non-axisymmetric component, usually a bar, at low-ordervertical resonances. In such a case, the boxy `bulges' are parts of barsseen edge-on. For the three galaxies we study, the families associatedwith the `peanut' or `X'-shape morphology are probably bifurcations atthe vertical 2/1 or 4/1 resonance.
| On the origin of warps and the role of the intergalactic medium
There is still no consensus as to what causes galactic discs to becomewarped. Successful models should account for the frequent occurrence ofwarps in quite isolated galaxies, their amplitude as well as theobserved azimuthal and vertical distributions of the HI layer.Intergalactic accretion flows and intergalactic magnetic fields may bendthe outer parts of spiral galaxies. In this paper we consider theviability of these non-gravitational torques to take the gas off theplane. We show that magnetically generated warps are clearly flawedbecause they would wrap up into a spiral in less than two or threegalactic rotations. The inclusion of any magnetic diffusivity to dilutethe wrapping effect causes the amplitude of the warp to damp. We alsoconsider the observational consequences of the accretion of anintergalactic plane-parallel flow at infinity. We have computed theamplitude and warp asymmetry in the accretion model, for a disc embeddedin a flattened dark matter halo, including self-consistently thecontribution of the modes with azimuthal wavenumbers m= 0 and m= 1.Since the m= 0 component, giving a U-shaped profile, is not negligiblecompared to the m= 1 component, this model predicts quite asymmetricwarps, maximum gas displacements on the two sides in the ratio 3 : 2 forthe preferred Galactic parameters, and the presence of a fraction ~3.5per cent of U-shaped warps, at least. The azimuthal dependence of themoment transfer by the ram pressure would produce a strong asymmetry inthe thickness of the HI layer and asymmetric density distributions in z,in conflict with observational data for the warp in our Galaxy and inexternal galaxies. The amount of accretion that is required to explainthe Galactic warp would give gas scaleheights in the far outer disc thatare too small. We conclude that accretion of a flow with no net angularmomentum cannot be the main and only cause of warps.
| Radial Profiles of Star Formation in the Far Outer Regions of Galaxy Disks
Star formation in galaxies is triggered by a combination of processes,including gravitational instabilities, spiral wave shocks, stellarcompression, and turbulence compression. Some of these persist in thefar outer regions, where the column density is far below the thresholdfor instabilities, making the outer disk cutoff somewhat gradual. Weshow that in a galaxy with a single exponential gas profile the starformation rate can have a double exponential, with a shallow one in theinner part and a steep one in the outer part. Such double exponentialshave been observed recently in the broadband intensity profiles ofspiral and dwarf irregular galaxies. The break radius in our modeloccurs slightly outside the threshold for instabilities, provided theMach number for compressive motions remains of order unity to largeradii. The ratio of the break radius to the inner exponential scalelength increases for higher surface brightness disks because theunstable part extends farther out. This is also in agreement withobservations. Galaxies with extended outer gas disks that fall moreslowly than a single exponential, such as 1/R, can have their starformation rate scale approximately as a single exponential with radius,even out to 10 disk scale lengths. Hα profiles should drop muchfaster than the star formation rate as a result of the rapidlydecreasing ambient density.
| 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.
| 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.
| A radio census of nuclear activity in nearby galaxies
In order to determine the incidence of black hole accretion-drivennuclear activity in nearby galaxies, as manifested by their radioemission, we have carried out a high-resolution Multi-ElementRadio-Linked Interferometer Network (MERLIN) survey of LINERs andcomposite LINER/Hii galaxies from a complete magnitude-limited sample ofbright nearby galaxies (Palomar sample) with unknown arcsecond-scaleradio properties. There are fifteen radio detections, of which three arenew subarcsecond-scale radio core detections, all being candidate AGN.The detected galaxies supplement the already known low-luminosity AGN -low-luminosity Seyferts, LINERs and composite LINER/Hii galaxies - inthe Palomar sample. Combining all radio-detected Seyferts, LINERs andcomposite LINER/Hii galaxies (LTS sources), we obtain an overall radiodetection rate of 54% (22% of all bright nearby galaxies) and weestimate that at least ~50% (~20% of all bright nearby galaxies) aretrue AGN. The radio powers of the LTS galaxies allow the construction ofa local radio luminosity function. By comparing the luminosity functionwith those of selected moderate-redshift AGN, selected from the 2dF/NVSSsurvey, we find that LTS sources naturally extend the RLF of powerfulAGN down to powers of about 10 times that of Sgr A*.
| 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.
| A Chandra Snapshot Survey of Infrared-bright LINERs: A Possible Link Between Star Formation, Active Galactic Nucleus Fueling, and Mass Accretion
We present results from a high-resolution X-ray imaging study of nearbyLINERs observed by ACIS on board Chandra. This study complements andextends previous X-ray studies of LINERs, focusing on the underexploredpopulation of nearby dust-enshrouded infrared-bright LINERs. The sampleconsists of 15 IR-bright LINERs (LFIR/LB>3),with distances that range from 11 to 26 Mpc. Combining our sample withprevious Chandra studies, we find that ~51% (28/55) of the LINERsdisplay compact hard X-ray cores. The nuclear 2-10 keV luminosities ofthe galaxies in this expanded sample range from ~2×1038to ~2×1044 ergs s-1. We find that the mostextreme IR-faint LINERs are exclusively active galactic nuclei (AGNs).The fraction of LINERs containing AGNs appears to decrease with IRbrightness and increase again at the highest values ofLFIR/LB. We find that of the 24 LINERs showingcompact nuclear hard X-ray cores in the expanded sample that wereobserved at Hα wavelengths, only eight actually show evidence of abroad line. Similarly, of the 14 LINERs showing compact nuclear hardX-ray cores with corresponding radio observations, only eight display acompact flat spectrum radio core. These findings emphasize the need forhigh-resolution X-ray imaging observations in the study of IR-brightLINERs. Finally, we find an intriguing trend in the Eddington ratioversus LFIR and LFIR/LB for theAGN-LINERs in the expanded sample that extends over 7 orders ofmagnitude in L/LEdd. This correlation may imply a linkbetween black hole growth, as measured by the Eddington ratio, and thestar formation rate, as measured by the far-IR luminosity andIR-brightness ratio. If the far-IR luminosity is an indicator of themolecular gas content in our sample of LINERs, our results may furtherindicate that the mass accretion rate scales with the host galaxy's fuelsupply. We discuss the potential implications of our results in theframework of black hole growth and AGN fueling in low-luminosity AGNs.
| Is the Galactic submillimeter dust emissivity underestimated?
We present detailed modeling of the spectral energy distribution (SED)of the spiral galaxies NGC 891, NGC 4013 and NGC 5907 in thefar-infrared (FIR) and sub-millimeter (submm) wavelengths. The modeltakes into account the emission of the diffuse dust component, which isheated by the UV and optical radiation field produced by the stars, aswell as the emission produced locally in star forming HII complexes.Radiative transfer simulations in the optical bands are used toconstrain the stellar and dust geometrical parameters and the dust mass.We find that the submm emission predicted by our model cannot accountfor the observed fluxes at these wavelengths. Two scenarios that couldaccount for the "missing" submm flux are examined. In the first scenariodust additional to that derived from the optical wavelengths is embeddedin the galaxy in the form of a thin disk. This additional dust disk,which is not detectable in the optical and which is associated with theyoung stellar population, gives rise to additional submm emission, andmakes the total flux match the observed values. The other scenarioexamines the possibility that the average emissivity at submmwavelengths of the dust grains found both in a diffuse component and indenser environments (e.g. molecular gas clouds) is higher than thevalues widely used in Galactic environments. This enhanced emissivityreproduces the observed FIR and submm fluxes with the dust mass equal tothat derived from the optical observations. In the second scenario, wetreat the submm emissivity as a free parameter and calculate its nominalvalue by fitting our model to the observed SED. We find a dustemissivity which is ~ 3 times the often-used values for our Galaxy. Bothscenarios can equally well reproduce the observed 850 μm surfacebrightness for all three galaxies. However, we argue that the scenarioof having more dust embedded in a second disk is not supported by thenear infrared observations. At 2.16 μm, the model images with asecond dust disk reveal a prominent dust lane which is not present inthe observations. Thus, the enhanced emissivity at submm wavelengths isa real possibility and the Galactic submillimeter dust emissivity may beunderestimated.
| Dynamical friction in flattened systems: a numerical test of Binney's approach
We carry out a set of self-consistent N-body calculations to investigatehow important the velocity anisotropy in non-spherical dark matterhaloes is for dynamical friction. For this purpose, we allow satellitegalaxies to orbit within flattened and live dark matter haloes (DMHs)and compare the resulting orbit evolution with a semi-analytic code.This code solves the equation of motion of the same satellite orbitswith mass loss and assumes the same DMH, but either employsChandrasekhar's dynamical friction formula, which does not incorporatethe velocity anisotropy, or Binney's description of dynamical frictionin anisotropic systems. In the numerical and the two semi-analyticmodels, the satellites are given different initial orbital inclinationsand orbital eccentricities, whereas the parent galaxy is composed of aDMH with aspect ratio qh= 0.6.We find that Binney's approach successfully describes the overallsatellite decay and orbital inclination decrease for the whole set oforbits, with an averaged discrepancy of less than 4 per cent in orbitalradius during the first three orbits. If Chandrasekhar's expression isused instead, the discrepancy increases to 20 per cent. Binney'streatment therefore appears to provide a significantly improvedtreatment of dynamical friction in anisotropic systems.The velocity anisotropy of the DMH velocity distribution function leadsto a significant decrease with time of the inclination of non-polarsatellite orbits. But, at the same time, it reduces the difference indecay times between polar and coplanar orbits evident in a flattened DMHwhen the anisotropic DMH velocity distribution function is not takeninto account explicitly. Our N-body calculations furthermore indicatethat polar orbits survive about 1.6 times longer than coplanar orbitsand that the orbital eccentricity e remains close to its initial valueif satellites decay slowly towards the galaxy centre. However, orbits ofrapidly decaying satellites modelled with the semi-analytic code show astrong orbital circularization () not present in the N-bodycomputations.
| Molecular Gas in the Edge-On Galaxy NGC 4013
Our OVRO observations at 300 pc resolution of the molecular gas disk inthe edge-on spiral galaxy NGC 4013 show no evidence for extraplanarmaterial at our sensitivity limit. The observed molecular gas kinematicsare in agreement with gas motion in a barred potential.
| A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei
Using the Green Bank Telescope, we have conducted a survey for 1.3 cmwater maser emission toward the nuclei of nearby active galaxies, themost sensitive large survey for H2O masers to date. Among 145galaxies observed, maser emission was newly detected in 11 sources andconfirmed in one other. Our survey targeted nearby (v<12,000 kms-1), mainly type 2 active galactic nuclei (AGNs) north ofδ=-20deg and includes a few additional sources as well.We find that more than one-third of Seyfert 2 galaxies have strong maseremission, although the detection rate declines beyond v~5000 kms-1 because of sensitivity limits. Two of the masersdiscovered during this survey are found in unexpected hosts: NGC 4151(Seyfert 1.5) and NGC 2782 (starburst). We discuss the possiblerelations between the large X-ray column to NGC 4151 and a possiblehidden AGN in NGC 2782 to the detected masers. Four of the masersdiscovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, havehigh-velocity lines symmetrically spaced about the systemic velocity, alikely signature of molecular gas in a nuclear accretion disk. The masersource in NGC 6323, in particular, reveals the classic spectrum of a``disk maser'' represented by three distinct groups of Dopplercomponents. Future single-dish and VLBI observations of these fourgalaxies could provide a measurement of the distance to each galaxy andof the Hubble constant, independent of standard candle calibrations.
| The Globular Cluster Systems of Five Nearby Spiral Galaxies: New Insights from Hubble Space Telescope Imaging
We use available multifilter Hubble Space Telescope (HST) WFPC2 imagingof five (M81, M83, NGC 6946, M101, and M51, in order of distance)low-inclination, nearby spiral galaxies to study ancient star clusterpopulations. Combining rigorous selection criteria to rejectcontaminants (individual stars, background galaxies, and blends) withoptical photometry including the U bandpass, we unambiguously detectancient globular cluster (GC) systems in each galaxy. We presentluminosities, colors, and size (effective radius) measurements for ourcandidate GCs. These are used to estimate specific frequencies, toassess whether intrinsic color distributions are consistent with thepresence of both metal-poor and metal-rich GCs, and to compare relativesizes of ancient clusters between different galaxy systems. M81globulars have intrinsic color distributions that are very similar tothose in the Milky Way and M31, with ~40% of sample clusters havingcolors expected for a metal-rich population. The GC system in M51meanwhile, appears almost exclusively blue and metal-poor. This lack ofmetal-rich GCs associated with the M51 bulge indicates that the bulgeformation history of this Sbc galaxy may have differed significantlyfrom that of our own. Ancient clusters in M101 and possibly in NGC 6946,two of the three later type spirals in our sample, appear to haveluminosity distributions that continue to rise to our detection limit(MV~-6.0), well beyond the expected turnover(MV~-7.4) in the luminosity function. This is reminiscent ofthe situation in M33, a Local Group galaxy of similar Hubble type. Thefaint ancient cluster candidates in M101 and NGC 6946 have properties(colors and reff) similar to their more luminouscounterparts, and we suggest that these are either intermediate-age (3-9Gyr) disk clusters or the low-mass end of the original GC population.Potentially, these lower mass clusters were not destroyed because ofdifferent dynamical conditions relative to those present in earlier typegalaxies. If the faint, excess GC candidates are excluded, we find thatthe specific frequency (SN) of ancient clusters formed inlater type spirals is roughly constant, with SN=0.5+/-0.2. Ifwe consider only the blue, metal-poor clusters in the early-type spiralM81, this galaxy is also consistent with having formed a ``universal''specific frequency of halo GC population, with a value ofSN~0.6. By combining the results of this study withliterature values for other systems, we find that the total GC specificfrequencies in spirals appear to correlate best with Hubble type andbulge/total ratio, rather than with galaxy luminosity or galaxy mass.
| Star Formation Thresholds and Galaxy Edges: Why and Where
We study global star formation thresholds in the outer parts of galaxiesby investigating the stability of disk galaxies embedded in dark halos.The disks are self-gravitating, contain metals and dust, and are exposedto UV radiation. We find that the critical surface density for theexistence of a cold interstellar phase depends only weakly on theparameters of the model and coincides with the empirically derivedsurface density threshold for star formation. Furthermore, it is shownthat the drop in the thermal velocity dispersion associated with thetransition from the warm to the cold gas phase triggers gravitationalinstability on a wide range of scales. The presence of strong turbulencedoes not undermine this conclusion if the disk is self-gravitating.Models based on the hypothesis that the onset of thermal instabilitydetermines the star formation threshold in the outer parts of galaxiescan reproduce many observations, including the threshold radii, thecolumn densities, and the sizes of stellar disks as a function of diskscale length and mass. Finally, prescriptions are given for implementingstar formation thresholds in (semi-)analytic models andthree-dimensional hydrodynamical simulations of galaxy formation.
| The Mass Discrepancy-Acceleration Relation: Disk Mass and the Dark Matter Distribution
The mass discrepancy in disk galaxies is shown to be well correlatedwith acceleration, increasing systematically with decreasingacceleration below a critical scalea0~3700km2s-2kpc-1=1.2×10-10ms-2.For each galaxy, there is an optimal choice of stellar mass-to-lightratio that minimizes the scatter in this mass discrepancy-accelerationrelation. The same mass-to-light ratios also minimize the scatter in thebaryonic Tully-Fisher relation and are in excellent agreement with theexpectations of stellar population synthesis. Once the disk mass isdetermined in this fashion, the dark matter distribution is specified.The circular velocity attributable to the dark matter can be expressedas a simple equation that depends only on the observed distribution ofbaryonic mass. It is a challenge to understand how this very fine-tunedcoupling between mass and light comes about.
| The Formation of Dust Lanes: Implications for Galaxy Evolution
From a survey of edge-on disks, we find that disk galaxies show a sharp,mass-dependent transition in the structure of their dusty ISM. In moremassive, rapidly rotating disks withVc>120kms-1, we see the well-defined dust lanestraditionally associated with edge-on galaxies. However, in more slowlyrotating, lower mass galaxies with Vc<120kms-1,we find no dust lanes. Instead, the distribution of dust in thesegalaxies has a much larger scale height and thus appears more diffuse.Evidence suggests that the change in scale height is due primarily tochanges in the turbulent velocities supporting the gas layer rather thanto sharp changes in the gas surface density. A detailed analysis of oursample reveals that the decrease in the dust scale height is associatedwith the onset of disk instabilities, evaluated for a mixed star+gasdisk. Specifically, we find that all of the high-mass galaxies with dustlanes are gravitationally unstable and thus are prone to fragmentationand gravitational collapse along spiral arms. Empirically, our dataimply that turbulence has lower characteristic velocities in thepresence of disk instabilities, leading to smaller gas scale heights andthe appearance of narrow dust lanes. The drop in velocity dispersion maybe due either to a switch in the driving mechanism for turbulence fromsupernovae to gravitational instabilities or to a change in the responseof the ISM to supernovae after the ISM has collapsed to a dense layer.We hypothesize that the drop in gas scale height may lead to significantincreases in the star formation rate when disk instabilities arepresent. First, the collapse of the gas layer increases the typical gasdensity, reducing the star formation timescale. Second, the starformation efficiency increases because of lower turbulent velocities.These two effects can combine to produce a sharp increase in the starformation rate with little change in the gas surface density and maytherefore provide an explanation for the Kennicutt surface densitythreshold for star formation. Our data also suggest that star formationwill be systematically less efficient in low-mass disks withVc<120kms-1, since these galaxies are stableand lie entirely below the Kennicutt surface density threshold. In thesestable systems the effective nucleosynthetic yield is reduced becausethe star formation timescale becomes longer than the gas accretiontimescale, suppressing the metallicity. This effect can possibly producethe observed fall-off in metallicity at rotation speedsVc<120kms-1. Thus, infall provides an equallyplausible explanation of the mass-metallicity relation in disks asglobal outflows driven by supernova winds. The transitions in diskstability, dust structure, and/or star formation efficiency may also beresponsible for the observed changes in the slope of the Tully-Fisherrelation, in the sharp increase in the thickness of low-mass galaxydisks, and in the onset of bulges in galaxies withVc>~120kms-1. The latter observation lendssupport to theories in which bulges in late-type galaxies grow throughsecular evolution in response to disk instabilities. We include in thispaper relationships between the surface density and the vertical stellarvelocity dispersion as a function of galaxy rotation speed, which may beuseful constraints on galaxy formation models.
| 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 emissivity of dust grains in spiral galaxies
We use the radiation transfer simulation of Xilouris et al.(\cite{xilouris1999}) to constrain the quantity of dust in three nearbyspiral galaxies (NGC 4013, NGC 5907 and NGC 4565). The predicted visualoptical depth from the model is compared with the thermal continuumradiation detected from NGC 4013 and NGC 5907 at 850 μm and from NGC4565 at 1.2 mm. The former is based on SCUBA images of NGC 4013 and NGC5907, reduced and presented for the first time in this work. Thecomparison of visual optical depth and 850 μm (1.2 mm) emissionyields the emissivity of dust grains in the submillimeter (millimeter)waveband. We infer a value of 1.2 × 104 for theemissivity at 850 μm which is a factor 4 higher than the benchmark,semi-empirical model of Draine & Lee (\cite{draine1984}). At 1.2 mmour values are a factor 1.5 higher than this model. Our estimates aremore closely aligned with recent measurements carried out in thelaboratory on amorphous carbon and silicate particulates. A comparisonbetween the distribution of 850 μm (1.2 mm) surface brightness andthe intensity levels in the 12CO(1-0) and 21 cm linesunderlines the spatial association between dust detected in thesubmillimeter/millimeter waveband and molecular gas clouds. We suggestthat the relatively high emissivity values that we derive may beattributable to amorphous, fluffy grains situated in denser gasenvironments.
| Modelling the spectral energy distribution of galaxies. III. Attenuation of stellar light in spiral galaxies
We present new calculations of the attenuation of stellar light fromspiral galaxies using geometries for stars and dust which can reproducethe entire spectral energy distribution from the ultraviolet (UV) to theFar-infrared (FIR)/submillimeter (submm) and can also account for thesurface brightness distribution in both the optical/Near-infrared (NIR)and FIR/submm. The calculations are based on the model of Popescu et al.(\cite{Popescu2000}), which incorporates a dustless stellar bulge, adisk of old stars with associated diffuse dust, a thin disk of youngstars with associated diffuse dust, and a clumpy dust componentassociated with star-forming regions in the thin disk. The attenuations,which incorporate the effects of multiple anisotropic scattering, arederived separately for each stellar component, and presented in the formof easily accessible polynomial fits as a function of inclination, for agrid in optical depth and wavelength. The wavelength range considered isbetween 912 {Å} and 2.2 μm, sampled such that attenuation canbe conveniently calculated both for the standard optical bands and forthe bands covered by GALEX. The attenuation characteristics of theindividual stellar components show marked differences between eachother. A general formula is given for the calculation of compositeattenuation, valid for any combination of the bulge-to-disk ratio andamount of clumpiness. As an example, we show how the optical depthderived from the variation of attenuation with inclination depends onthe bulge-to-disk ratio. Finally, a recipe is given for aself-consistent determination of the optical depth from theHα/Hβ line ratio.Tables \ref{tab4}-\ref{tab6} and Appendix A are only available inelectronic form at http://www.edpsciences.org }
| Hubble Space Telescope observations of globular cluster systems along the Hubble sequence of spiral galaxies
We have studied the globular cluster systems of seven giant, edge-onspiral galaxies using Hubble Space Telescope imaging in V and I. Thegalaxy sample covers the Hubble types Sa to Sc, allowing us to study thevariation of the properties of globular cluster systems along the Hubblesequence. The photometry reaches ~1.5 mag beyond the turn-over magnitudeof the globular cluster luminosity function for each galaxy. Specificfrequencies of globular clusters (SN values) were evaluatedby comparing the numbers of globular clusters found in our WFPC2pointings with those in our Milky Way that would be detected in the samespatial region if placed at the distance of the target galaxies. Resultsfrom this method were found to be consistent with the more commonly usedmethod of constructing radial distribution functions of globularclusters. The SN values of spirals with B/T<~ 0.3 (i.e.spirals with a Hubble type later than about Sb) are consistent with avalue of SN= 0.55 +/- 0.25. We suggest that this populationof globular clusters represents a `universal', old halo population thatis present around each galaxy. Most galaxies in our sample haveSN values that are consistent with a scenario in whichglobular cluster systems are made up of (i) the aforementioned halopopulation plus (ii) a population that is associated with bulges, whichgrows approximately linearly with the mass of the bulge. Such scenariosinclude the `merger scenario' for the formation of elliptical galaxiesas well as the `multi-phase collapse' scenario, but it seemsinconsistent with the `secular evolution' scenario of Pfenniger &Norman, in which bulges are formed from disc stars by means of theredistribution of angular momentum through bar instabilities and/orminor perturbations. However, there is one bulge-dominated spiral galaxyin our sample (NGC7814) with a low SN value that isconsistent with those of the latest-type spirals. This means that the`secular evolution' scenario can still be viable for somebulge-dominated spirals. Thus, our results suggest that the formationhistories of galaxy bulges of early-type spirals can be significantlydifferent from one galaxy to another.
| Extraplanar Emission-Line Gas in Edge-On Spiral Galaxies. I. Deep Emission-Line Imaging
The extraplanar diffuse ionized gas (eDIG) in 17 nearby, edge-on diskgalaxies is studied using deep Taurus Tunable Filter Hα and [N II]λ6583 images and conventional interference filter Hα+[N II]λλ6548, 6583 images that reach flux levels generally below~1×10-17 ergs s-1 cm-2arcsec-2. [N II] λ6583/Hα excitation maps areavailable for 10 of these objects. All but one galaxy in the sampleexhibit eDIG. The contribution of the eDIG to the total Hαluminosity is relatively constant, on the order of 12%+/-4%. TheHα scale height of the eDIG derived from a two-exponential fit tothe vertical emission profile ranges from 0.4 to 17.9 kpc, with anaverage of 4.3 kpc. This average value is noticeably larger than theeDIG scale height measured in our Galaxy and other galaxies. Thisdifference in scale height is probably due in part to the lower fluxlimits of our observations. The ionized mass of the extraplanarcomponent inferred by assuming a constant filling factor of 0.2 and aconstant path length through the disk of 5 kpc ranges from1.4×107 to 2.4×108 Msolar,with an average value of 1.2×108 Msolar.Under these same assumptions, the recombination rate required to keepthe eDIG ionized ranges from 0.44×106 to13×106 s-1 cm-2 of the disk, orabout 10%-325% of the Galactic value. A quantitative analysis of thetopology of the eDIG confirms that several galaxies in the sample have ahighly structured eDIG morphology. The distribution of the eDIG emissionis often correlated with the locations of the H II regions in the disk,supporting the hypothesis that the predominant source of ionization ofthe eDIG is photoionization from OB stars located in the H II regions. Astrong correlation is found between the IR (or far-IR) luminosities perunit disk area (basically a measure of the star formation rate per unitdisk area) and the extraplanar ionized mass, further providing supportfor a strong connection between the disk and eDIG components in thesegalaxies. The excitation maps confirm that the [N II]/Hα ratiosare systematically higher in the eDIG than in the disk. Althoughphotoionization by disk OB stars is generally able to explain theseelevated [N II]/Hα ratios, a secondary source of ionizationappears to be needed when one also takes into account other line ratios;more detail is given in a companion paper (our Paper II).
| 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.
| Extraplanar Emission-Line Gas in Edge-on Spiral Galaxies. II. Optical Spectroscopy
The results from deep long-slit spectroscopy of nine edge-on spiralgalaxies with known extraplanar line emission are reported. Emissionfrom Hα, [N II] λλ6548, 6583, and [S II]λλ6716, 6731 is detected out to heights of a fewkiloparsecs in all of these galaxies. Several other fainter diagnosticlines such as [O I] λ6300, [O III] λλ4959, 5007,and He I λ5876 are also detected over a smaller scale. Therelative strengths, centroids, and widths of the various emission linesprovide constraints on the electron density, temperature, reddening,source(s) of ionization, and kinematics of the extraplanar gas. In allbut one galaxy, photoionization by massive OB stars alone hasdifficulties explaining all of the line ratios in the extraplanar gas.Hybrid models that combine photoionization by OB stars and anothersource of ionization such as photoionization by turbulent mixing layersor shocks provide a better fit to the data. The (upper limits on the)velocity gradients measured in these galaxies are consistent with thepredictions of the galactic fountain model to within the accuracy of themeasurements.
| 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 Hα survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. I. How common are gaseous halos among non-starburst galaxies?
In a series of two papers we present results of a new Hα imagingsurvey, aiming at the detection of extraplanar diffuse ionized gas inhalos of late-type spiral galaxies. We have investigated a sample of 74nearby edge-on spirals, covering the northern and southern hemisphere.In 30 galaxies we detected extraplanar diffuse emission at meandistances of |z| ~ 1-2 kpc. Individual filaments can be traced out to|z|<=6 kpc in a few cases. We find a good correlation between the FIRflux ratio (S60/S100) and the SFR per unit area(LFIR/D225), based on thedetections/non-detections. This is actually valid for starburst, normaland for quiescent galaxies. A minimal SFR per unit area for the lowestS60/S100 values, at which extended emission hasbeen detected, was derived, which amounts to dotEA25thres = (3.2+/-0.5)*E40ergs-1 kpc-2. There are galaxies where extraplanaremission was detected at smaller values ofLFIR/D225, however, only in combinationwith a significantly enhanced dust temperature. The results corroboratethe general view that the gaseous halos are a direct consequence of SFactivity in the underlying galactic disk.Based on observations collected at the European Southern Observatory,Chile (ESO No. 63.N-0070, ESO No. 64.N-0034, ESO No. 65.N.-0002).
| 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%.
| Disk Boundaries in Spiral Galaxies
We explore the hypothesis that the outer boundaries(“cutoffs”) of the stellar disks observed in many galaxiesare determined by the condition of local gravitational (Jeans) stabilityfor the gaseous protodisks at large galactocentric distances. The ratioof the surface density of the disk Σdisk to the critical value forJeans instability Σcrit is computed for a number of galaxies,assuming that the gas velocity dispersion in the forming diskcorresponded to its current thickness and that the disk itself is in aquasi-equilibrium state. The mean estimated stellar velocity dispersionin the vicinity of the cutoff (12 km/s) is close to the typical velocitydispersions of gaseous clouds in disk galaxies. At greater distances,such velocity dispersions should ensure gravitational stability of thedisk both at the present epoch and in the past. The cutoff radius of thedisk R cut is correlated with other disk parameters, and the ratioΣdisk/Σcrit at R cut is close to unity in most cases. Weconclude that the available observational data agree well with thehypothesis that stellar disk cutoffs are due to a rapid decrease in thestar-formation rate beyond R cut, where the gaseous disk has always beenstable.
| 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.
| The H I Line Width/Linear Diameter Relationship as an Independent Test of the Hubble Constant
The relationship between corrected H I line widths and linear diameters(LW/LD) for spiral galaxies is used as an independent check on the valueof the Hubble constant. After calibrating the Tully-Fisher (TF) relationin both the B and I bands, the B-band relation is used for galaxies ofmorphological/luminosity types Sc I, Sc I.2, Sc I.3, Sab, Sb, Sb I-II,and Sb II to derive the LW/LD relation. We find that for this sample thescatter in the LW/LD is smallest with a Hubble constant of 90-95 kms-1 Mpc-1. Lower values of the Hubble constantproduce a separation in the LW/LD relation that is a function ofmorphological type. Since a Hubble constant of 90-95 is significantlylarger than the final Key Project value of 72 km s-1Mpc-1, a comparison of TF, surface brightness fluctuation(SBF), and fundamental plane (FP) is made. This comparison indicatesthat the Key Project TF distances to 21 clusters may be too large. For asample of 11 clusters, the Key Project TF distances provide anunweighted mean Hubble constant of 77 km s-1Mpc-1, while a combination of the FP, SBF, and our TFdistances for the same 11 clusters gives H0=91 kms-1 Mpc-1. A more subtle result in our data is amorphological dichotomy in the Hubble constant. The data suggest that ScI galaxies follow a Hubble constant of 90-95 while Sb galaxies follow aHubble constant closer to 75 km s-1 Mpc-1.Possible explanations for this result are considered, but it is shownthat this Sb/Sc I Hubble flow discrepancy is also present in the VirgoCluster and is consistent with previous investigations that indicatethat some galaxies carry a component of age-related intrinsic redshift.
| 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.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
