Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Multiwavelength Star Formation Indicators: Observations
We present a compilation of multiwavelength data on different starformation indicators for a sample of nearby star forming galaxies. Herewe discuss the observations, reductions and measurements of ultravioletimages obtained with STIS on board the Hubble Space Telescope (HST),ground-based Hα, and VLA 8.46 GHz radio images. These observationsare complemented with infrared fluxes, as well as large-apertureoptical, radio, and ultraviolet data from the literature. This databasewill be used in a forthcoming paper to compare star formation rates atdifferent wave bands. We also present spectral energy distributions(SEDs) for those galaxies with at least one far-infrared measurementsfrom ISO, longward of 100 μm. These SEDs are divided in two groups,those that are dominated by the far-infrared emission, and those forwhich the contribution from the far-infrared and optical emission iscomparable. These SEDs are useful tools to study the properties ofhigh-redshift galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| On the Lengths, Colors, and Ages of 18 Face-on Bars
Along with a brief analysis we present data obtained from BVRI andKs images of a sample of 19 galaxies (18 barred and 1unbarred), which will be further explored in a future paper. We measuredthe lengths and colors of the bars, created color maps, and estimatedglobal color gradients. Applying a method developed in a companionpaper, we could distinguish for seven galaxies in our sample those whosebars have been recently formed from the ones with already evolved bars.We estimated an average difference in the optical colors between youngand evolved bars that may be translated to an age difference of theorder of 10 Gyr, meaning that bars may be, at least in some cases,long-standing structures. Moreover, our results show that, on average,evolved bars are longer than young bars. This seems to indicate that,during its evolution, a bar grows longer by capturing stars from thedisk, in agreement with recent numerical and analytical results.Although the statistical significance of these results is low, andfurther studies are needed to confirm them, we discuss the implicationsfrom our results on the possibility of bars being a recurrentphenomenon. We also present isophotal contours for all our images aswell as radial profiles of relevant photometric and geometricparameters.
| Ultraviolet-to-Far-Infrared Properties of Local Star-forming Galaxies
We present the results of a multiwavelength study of nearby galaxiesaimed at understanding the relation between the ultraviolet andfar-infrared emission in star-forming galaxies. The data set comprisesnew ultraviolet (from HST STIS), ground-based Hα, and radiocontinuum observations, together with archival infrared data (from IRASand ISO). The local galaxies are used as benchmarks for comparison ofthe infrared-to-ultraviolet properties with two populations ofhigh-redshift galaxies: the submillimeter star-forming galaxies detectedby SCUBA and the ultraviolet-selected Lyman break galaxies (LBGs). Inaddition, the long wavelength baseline covered by the present dataenables us to compare the star formation rates (SFRs) derived from theobserved ultraviolet, Hα, infrared, and radio luminosities and togauge the impact of dust opacity in the local galaxies. We also derive anew calibration for the nonthermal part of the radio SFR estimator,based on the comparison of 1.4 GHz measurements with a new estimator ofthe bolometric luminosity of the star-forming regions. We find that moreactively star-forming galaxies show higher dust opacities, which is inline with previous results. We find that the local star-forming galaxieshave a lower Fλ(205 μm)/Fλ(UV)ratio by 2-3 orders of magnitude than the submillimeter-selectedgalaxies and may have a similar or somewhat higherFλ(205 μm)/Fλ(UV) ratio thanLBGs. The Fλ(205 μm)/Fλ(UV) ratioof the local galaxy population may be influenced by the cool dustemission in the far-infrared heated by nonionizing stellar populations,which may be reduced or absent in the LBGs.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Late-Time Spectroscopy of SN 2002cx: The Prototype of a New Subclass of Type Ia Supernovae
We present Keck optical spectra of SN 2002cx, the most peculiar knownType Ia supernova (SN Ia), taken 227 and 277 days past maximum light.Astonishingly, the spectra are not dominated by the forbidden emissionlines of iron that are a hallmark of thermonuclear SNe in the nebularphase. Instead, we identify numerous P Cygni profiles of Fe II at verylow expansion velocities of ~700 km s-1, which are withoutprecedent in SNe Ia. We also report the tentative identification oflow-velocity O I in these spectra, suggesting the presence of unburnedmaterial near the center of the exploding white dwarf. SN 2002cx is theprototype of a new subclass of SNe Ia, with spectral characteristicsthat may be consistent with recent pure deflagration models ofChandrasekhar-mass thermonuclear SNe. These are distinct from themajority of SNe Ia, for which an alternative explosion mechanism, suchas a delayed detonation, may be required.
| Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| BHαBAR: big Hα kinematical sample of barred spiral galaxies - I. Fabry-Perot observations of 21 galaxies
We present the Hα gas kinematics of 21 representative barredspiral galaxies belonging to the BHαBAR sample. The galaxies wereobserved with FaNTOmM, a Fabry-Perot integral-field spectrometer, onthree different telescopes. The three-dimensional data cubes wereprocessed through a robust pipeline with the aim of providing the mosthomogeneous and accurate data set possible useful for further analysis.The data cubes were spatially binned to a constant signal-to-noiseratio, typically around 7. Maps of the monochromatic Hα emissionline and of the velocity field were generated and the kinematicalparameters were derived for the whole sample using tilted-ring models.The photometrical and kinematical parameters (position angle of themajor axis, inclination, systemic velocity and kinematical centre) arein relative good agreement, except perhaps for the later-type spirals.
| Supernovae 2005cc, 2005cd, 2005ce, 2005cf
IAUC 8534 available at Central Bureau for Astronomical Telegrams.
| Supernova 2005cc in NGC 5383
CBET 154 available at Central Bureau for Astronomical Telegrams.
| Type Determination for SN 2005cc
The Nearby Supernova Factory reports on three spectra (range 340-1000nm) of SN 2005cc in NGC 5383 (CBET #154) obtained May 21.4 UT, May 24.4UT and May 26.4 UT with the SuperNova Integral Field Spectrograph on theUniversity of Hawaii 2.2-m telescope. The first spectrum shows a redcontinuum with very weak narrow absorption features unlike any otherknown supernova spectrum. The spectrum obtained May 24.4 UT showsoverall increased flux and deeper narrow absorption features similarlyto that of SN 2002cx at -4 ( Li et al.
| The Central Region of Barred Galaxies: Molecular Environment, Starbursts, and Secular Evolution
Stellar bars drive gas into the circumnuclear (CN) region of galaxies.To investigate the fate of the CN gas and star formation (SF), we studya sample of barred nonstarbursts and starbursts with high-resolution CO,optical, Hα, radio continuum, Brγ, and HST data, and findthe following. (1) The inner kiloparsec of bars differs markedly fromthe outer disk. It hosts molecular gas surface densitiesΣgas-m of 500-3500 Msolar pc-2,gas mass fractions of 10%-30%, and epicyclic frequencies of several100-1000 km s-1 kpc-1. Consequently, in the CNregion gravitational instabilities can only grow at high gas densitiesand on short timescales, explaining in part why powerful starburstsreside there. (2) Across the sample, we find bar pattern speeds withupper limits of 43-115 km s-1 pc-1 and outer innerLindblad resonance radii of >500 pc. (3) Barred starbursts andnonstarbursts have CN SF rates of 3-11 and 0.1-2 Msolaryr-1, despite similar CN gas masses. TheΣgas-m value in the starbursts is larger (1000-3500Msolar pc-2) and close to the Toomre criticaldensity over a large region. (4) Molecular gas makes up 10%-30% of theCN dynamical mass and fuels large CN SF rates in the starbursts,building young, massive, high-V/σ components. Implications forsecular evolution along the Hubble sequence are discussed.
| The Vertical Stellar Kinematics in Face-On Barred Galaxies: Estimating the Ages of Bars
In order to perform a detailed study of the stellar kinematics in thevertical axis of bars, we obtained high signal-to-noise spectra alongthe major and minor axes of the bars in a sample of 14 face-on galaxiesand used them to determine the line-of-sight stellar velocitydistribution, parameterized as a Gauss-Hermite series. With these data,we developed a diagnostic tool that allows one to distinguish betweenrecently formed and evolved bars, as well as to estimate their ages,assuming that bars form in vertically thin disks that are recognizableby low values for the vertical velocity dispersion σz.Through N-body realizations of bar unstable disk galaxies we were alsoable to check the timescales involved in the processes that give bars animportant vertical structure. We show that σz inevolved bars is roughly 100 km s-1, which translates to aheight scale of about 1.4 kpc, giving support to scenarios in whichbulges form through disk material. Furthermore, the bars in ournumerical simulations have values for σz generallysmaller than 50 km s-1, even after evolving for 2 Gyr,suggesting that a slow process is responsible for making bars asvertically thick as we observe. We verify theoretically that theSpitzer-Schwarzschild mechanism is quantitatively able to explain theseobservations if we assume that giant molecular clouds are twice asconcentrated along the bar as in the rest of the disk.
| NGC 300: An Extremely Faint, Outer Stellar Disk Observed to 10 Scale Lengths
We have used the Gemini Multi-Object Spectrograph (GMOS) on the GeminiSouth 8 m telescope in exceptional conditions (0.6" FWHM seeing) toobserve the outer stellar disk of the Sculptor Group galaxy NGC 300 attwo locations. At our point-source detection threshold ofr'=27.0 (3 σ) mag, we trace the stellar disk out to aradius of 24', or 2.2R25, where R25 is the 25 magarcsec-2 isophotal radius. This corresponds to about 10 scalelengths in this low-luminosity spiral galaxy (MB=-18.6), orabout 14.4 kpc at a Cepheid distance of 2.0+/-0.07 Mpc. The backgroundgalaxy counts are derived in the outermost field, and these are within10% of the mean survey counts from both Hubble Deep Fields. Theluminosity profile is well described by a nucleus plus a simpleexponential profile out to 10 optical scale lengths. We reach aneffective surface brightness of μr'=30.5 magarcsec-2 (2 σ) at 55% completeness, which doubles theknown radial extent of the optical disk. These levels are exceedinglyfaint in the sense that the equivalent surface brightness in B or V isabout 32 mag arcsec-2. We find no evidence for truncation ofthe stellar disk. Only star counts can be used to reliably trace thedisk to such faint levels, since surface photometry is ultimatelylimited by nonstellar sources of radiation. In the Appendix, we derivethe expected surface brightness of one such source: dust scattering ofstarlight in the outer disk.
| CO in the Bipolar Radio Continuum Galaxy NGC 3367
CO(1-0) emission has been imaged at 2" resolution in the central 10 kpcof the barred spiral galaxy NGC 3367. This galaxy has bipolarsynchrotron lobes out to a radius of 6 kpc, straddling the compactnucleus. The peak molecular emission is in a source of radius 2" (425pc) centered on the galaxy nucleus. The molecular gas mass is~3×108 Msolar in this peak and~5.9×108 Msolar within a radius of 4.5" (950pc). The very large gas masses in the central source imply extinctionssufficiently high to completely obscure optical emission lines (e.g.,broad-line region) associated with the nuclear radio source. Theobserved Balmer lines probably originate in the narrow-line region a fewhundred parsecs from the nucleus. The CO emission in the central regionis elongated northeast-southwest, very similar to the position angle ofthe large-scale synchrotron lobes. This elongation is likely due to thenonaxisymmetric gravitational potential of the stellar bar. We inferthat the northeast radio continuum lobe is on the far side of the galaxyand the southwest lobe is on the near side. The central mass ofmolecular gas is of sufficient mass to power the active galactic nucleusaccretion luminosity for over 108 yr at 3 Msolaryr-1.
| A molecular face-on view of the Galactic Centre region
We present a method to derive positions of molecular clouds along thelines of sight from a quantitative comparison between 2.6-mm CO emissionlines and 18-cm OH absorption lines, and apply it to the centralkiloparsecs of the Milky Way. With some simple but justifiableassumptions, we derive a face-on distribution of the CO brightness andcorresponding radial velocity in the Galactic Centre without any help ofkinematical models. The derived face-on distribution of the gas iselongated and inclined so that the Galactic-eastern (positive longitude)side is closer to us. The gas distribution is dominated by a bar-likecentral condensation, whose apparent size is 500 × 200 pc. A ridgefeature is seen to stretch from one end of the central condensation,though its elongated morphology might be artificial. The velocity fieldshows clear signs of non-circular motion in the central condensation.The `expanding molecular ring' feature corresponds to the peripheralregion surrounding the central condensation, with the Galactic-easternend being closer to us. These characteristics agree with a picture inwhich the kinematics of the gas in the central kiloparsec of the Galaxyis under the strong influence of a barred potential. The face-ondistribution of the in situ pressure of the molecular gas is derivedfrom the CO multiline analysis. The derived pressure is found to behighest in the central 100 pc. In this region, the gas is accumulatingand is forming stars.
| Dark Matter in Galaxies: Observational overview
I review the observational side of the present state of the debate aboutthe dark matter in galaxies, with emphasis on the core/cusp problem inlow surface brightness galaxies, and the question of maximum /sub-maximum disks in spiral galaxies. Some remarks are made about thedwarf spheroidals around the Milky Way, and about elliptical galaxies.
| 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.
| Starbursts in barred spiral galaxies. VI. HI observations and the K-band Tully-Fisher relation
This paper reports a study of the effect of a bar on the neutralhydrogen (HI) content of starburst and Seyfert galaxies. We also makecomparisons with a sample of ``normal'' galaxies and investigate howwell starburst and Seyfert galaxies follow the fundamental scalingTully-Fisher (TF) relation defined for normal galaxies. 111 Markarian(Mrk) IRAS galaxies were observed with the Nançay radiotelescope,and HI data were obtained for 80 galaxies, of which 64 are newdetections. We determined the (20 and 50%) linewidths, the maximumvelocity of rotation and total HI flux for each galaxy. Thesemeasurements are complemented by data from the literature to form asample of Mrk IRAS (74% starburst, 23% Seyfert and 3% unknown) galaxiescontaining 105 unbarred and 113 barred ones. Barred galaxies have lowertotal and bias-corrected HI masses than unbarred galaxies, and this istrue for both Mrk IRAS and normal galaxies. This robust result suggeststhat bars funnel the HI gas toward the center of the galaxy where itbecomes molecular before forming new stars. The Mrk IRAS galaxies havehigher bias-corrected HI masses than normal galaxies. They also showsignificant departures from the TF relation, both in the B and K bands.The most deviant points from the TF relation tend to have a strongfar-infrared luminosity and a low oxygen abundance. These resultssuggest that a fraction of our Mrk IRAS galaxies are still in theprocess of formation, and that their neutral HI gas, partly of externalorigin, has not yet reached a stationary state.Based on observations obtained at the large radiotelescope ofObservatoire de Nançay, operated by Observatoire de Paris.Tables 5 and 6 are only (and Table 4 also) available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/515
| Exploring spiral galaxy potentials with hydrodynamical simulations
We study how well the complex gas velocity fields induced by massivespiral arms are modelled by the hydrodynamical simulations that we usedrecently to constrain the dark matter fraction in nearby spiralgalaxies. More specifically, we explore the dependence of the positionsand amplitudes of features in the gas flow on the temperature of theinterstellar medium (assumed to behave as a one-component isothermalfluid), the non-axisymmetric disc contribution to the galacticpotential, the pattern speed Ωp, and finally thenumerical resolution of the simulation. We argue that, afterconstraining the pattern speed reasonably well by matching thesimulations to the observed spiral arm morphology, the amplitude of thenon-axisymmetric perturbation (the disc fraction) is left as the primaryparameter determining the gas dynamics. However, owing to thesensitivity of the positions of the shocks to modelling parameters, onehas to be cautious when quantitatively comparing the simulations toobservations. In particular, we show that a global least-squaresanalysis is not the optimal method for distinguishing different models,as it tends to slightly favour low disc fraction models. Nevertheless,we conclude that, given observational data of reasonably high spatialresolution and an accurate shock-resolving hydro-code, this methodtightly constrains the dark matter content within spiral galaxies. Wefurther argue that, even if the perturbations induced by spiral arms areweaker than those of strong bars, they are better suited for this kindof analysis because the spiral arms extend to larger radii where effectslike inflows due to numerical viscosity and morphological dependence ongas sound speed are less of a concern than they are in the centres ofdiscs.
| The PDS versus Markarian starburst galaxies: comparing strong and weak IRAS emitter at 12 and 25 μm in the nearby Universe
The characteristics of the starburst galaxies from the Pico dos Diassurvey (PDS) are compared with those of the nearby ultraviolet (UV)bright Markarian starburst galaxies, having the same limit in redshift(vh < 7500 km s-1) and absolute B magnitude(MB < -18). An important difference is found: theMarkarian galaxies are generally undetected at 12 and 25 μm in IRAS.This is consistent with the UV excess shown by these galaxies andsuggests that the youngest star-forming regions dominating thesegalaxies are relatively free of dust.The far-infrared selection criteria for the PDS are shown to introduce astrong bias towards massive (luminous) and large size late-type spiralgalaxies. This is contrary to the Markarian galaxies, which are found tobe remarkably rich in smaller size early-type galaxies. These resultssuggest that only late-type spirals with a large and massive disc arestrong emitters at 12 and 25 μm in IRAS in the nearby Universe.The Markarian and PDS starburst galaxies are shown to share the sameenvironment. This rules out an explanation of the differences observedin terms of external parameters. These differences may be explained byassuming two different levels of evolution, the Markarian being lessevolved than the PDS galaxies. This interpretation is fully consistentwith the disc formation hypothesis proposed by Coziol et al. to explainthe special properties of the Markarian SBNG.
| What determines the strength and the slowdown rate of bars?
Isolated barred galaxies evolve by redistributing their angularmomentum, which, emitted by material in the inner disc at resonance withthe bar, can be absorbed by resonant material in the outer disc, or inthe halo. The amount of angular momentum that can be emitted/absorbed ata given resonance depends on the distribution function of theemitting/absorbing material. It thus depends not only on the amount ofmaterial on resonant orbits, but also on the velocity dispersion of thatmaterial. As it loses angular momentum, the bar becomes stronger and italso rotates slower. Thus the strength of the bar and the decrease ofits pattern speed with time are set by the amount of angular momentumexchanged within the galaxy, which, in turn, is regulated by the massdistribution and the velocity dispersion of the material in the disc andspheroidal components. Correlations between the pattern speed of thebar, its strength and the angular momentum absorbed by the spheroid(halo plus bulge) argue strongly that it is the amount of angularmomentum exchanged that determines the strength and the slowdown rate ofthe bar. The decrease of the bar pattern speed with time should not beused to set constraints on the halo-to-disc mass ratio, since it dependsalso on the velocity dispersion of the halo and disc material.
| Circumnuclear Dust in Nearby Active and Inactive Galaxies. II. Bars, Nuclear Spirals, and the Fueling of Active Galactic Nuclei
We present a detailed study of the relation between circumnuclear dustmorphology, host-galaxy properties, and nuclear activity in nearbygalaxies. We use our sample of 123 nearby galaxies withvisible-near-infrared color maps from the Hubble Space Telescope tocreate well-matched, ``paired'' samples of 28 active and 28 inactivegalaxies, as well as 19 barred and 19 unbarred galaxies, that have thesame host-galaxy properties. Comparison of the barred and unbarredgalaxies shows that grand-design nuclear dust spirals are found only ingalaxies with a large-scale bar. These nuclear dust spirals, which arepresent in approximately one-third of all barred galaxies, also appearto be connected to the dust lanes along the leading edges of thelarge-scale bars. Grand-design nuclear spirals are more common thaninner rings, which are present in only a small minority of the barredgalaxies. Tightly wound nuclear dust spirals, in contrast, show a strongtendency to avoid galaxies with large-scale bars. Comparison of theactive galactic nuclei (AGNs)and inactive samples shows that nucleardust spirals, which may trace shocks and angular momentum dissipation inthe interstellar medium, occur with comparable frequency in both activeand inactive galaxies. The only difference between the active andinactive galaxies is that several inactive galaxies appear to completelylack dust structure in their circumnuclear region, while none of theAGNs lack this structure. The comparable frequency of nuclear spirals inactive and inactive galaxies, combined with previous work that finds nosignificant difference in the frequency of bars or interactions betweenwell-matched active and inactive galaxies, suggests that no universalfueling mechanism for low-luminosity AGNs operates at spatial scalesgreater than a ~100 pc radius from the galactic nuclei. The similaritiesof the circumnuclear environments of active and inactive galaxiessuggest that the lifetime of nuclear activity is less than thecharacteristic inflow time from these spatial scales. Anorder-of-magnitude estimate of this inflow time is the dynamicaltimescale. This sets an upper limit of several million years to thelifetime of an individual episode of nuclear activity.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| 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.
| Do bulges of early- and late-type spirals have different morphology?
We study HST/NICMOS H-band images of bulges of two equal-sized samplesof early- (TRC3 <= 3) and late-type spiral (mainly Sbc-Sc)galaxies matched in outer disk axis ratio. We find that bulges oflate-type spirals are more elongated than their counterparts inearly-type spirals. Using a KS-test we find that the two distributionsare different at the 98.4% confidence level. We conclude that the twodata sets are different, i.e. late-type galaxies have a broaderellipticity distribution and contain more elongated features in theinner regions. We discuss the possibility that these would correspond tobars at a later evolutionary stage, i.e. secularly evolved bars.Consequent implications are raised, and we discuss relevant questionsregarding the formation and structure of bulges. Are bulges ofearly-type and late-type spirals different? Are their formationscenarios different? Can we talk about bulges in the same way fordifferent types of galaxies?
| The cold gas properties of Markarian galaxies
A sample of 61 Markarian galaxies detected in the CO line was compiled.Using available HI, element H2, optical and radio continuumdata, the analysis of the gas kinematics and the star formationproperties for this sample of galaxies was performed. The mainconclusion can be summarized as follows: (1) The HI and CO line widthsare well correlated. Interaction between galaxies has no influence onthe CO line broadening. A rapidly rotating nuclear disk in the galaxymight lead to the CO line broadening with less influence on the HI line.(2) The atomic and molecular gas surface densities are well correlatedwith the blue, FIR and radio continuum surface brightness; however, thecorrelation for molecular component is stronger.\ (3) In general, thegalaxies with UV-excess (Markarian galaxies) do not differ in their starformation properties from the non-UV galaxies.
| Morphology, photometry and kinematics of N -body bars - I. Three models with different halo central concentrations
We discuss the morphology, photometry and kinematics of the bars whichhave formed in three N -body simulations. These have initially the samedisc and the same halo-to-disc mass ratio, but their haloes have verydifferent central concentrations. The third model includes a bulge. Thebar in the model with the centrally concentrated halo (model MH) is muchstronger, longer and thinner than the bar in the model with the lesscentrally concentrated halo (model MD). Its shape, when viewed side-on,evolves from boxy to peanut and then to `X'-shaped, as opposed to thatof model MD, which stays boxy. The projected density profiles obtainedfrom cuts along the bar major axis, for both the face-on and the edge-onviews, show a flat part, as opposed to those of model MD which arefalling rapidly. A Fourier analysis of the face-on density distributionof model MH shows very large , 4, 6 and 8 components. Contrary to this,for model MD the components and 8 are negligible. The velocity field ofmodel MH shows strong deviations from axial symmetry, and in particularhas wavy isovelocities near the end of the bar when viewed along the barminor axis. When viewed edge-on, it shows cylindrical rotation, whichthe MD model does not. The properties of the bar of the model with abulge and a non-centrally concentrated halo (MDB) are intermediatebetween those of the bars of the other two models. All three modelsexhibit a lot of inflow of the disc material during their evolution, sothat by the end of the simulations the disc dominates over the halo inthe inner parts, even for model MH, for which the halo and disccontributions were initially comparable in that region.
| Molecular Gas in Candidate Double-barred Galaxies. I. The Diverse Morphology and Dynamics of NGC 2273 and NGC 5728
Double bars have been proposed as a means of transporting molecular gaspast inner Lindblad resonances into the nuclear regions, where it canfuel active or starburst nuclei. Thus far, the existence of double barshas been determined predominantly through analysis of near-infraredimages, which can tell us little about the dynamics and inflow rates ofthese systems. We have observed two double-bar galaxy candidates (NGC2273 and NGC 5728) in 12CO J=1-0 with the Owens Valley RadioObservatory Millimeter Array. Despite the similarity in thenear-infrared images of these galaxies, we see rather different nuclearmorphologies in the CO maps. NGC 2273 shows evidence of a nuclear gasbar, aligned with the nuclear stellar bar seen in the near-infraredimages. Both the nuclear gaseous and the stellar bars are misalignedfrom the large-scale bar by ~90°, which also allows the possibilitythat both are the result of stars and gas populating the x2orbits of the primary bar. Estimates using dynamical friction argumentsand star formation rates suggest significant gas inflow rates along thenuclear bar of NGC 2273. Conversely, NGC 5728 does not show any evidencefor a nuclear molecular bar but shows an arc of CO clumps that peaksjust to the southwest of the dynamical center and curves to thesoutheast, where it follows the dust lane to the south. Models ofdouble-barred galaxies suggest that these galaxies should contain largeamounts of molecular gas in their nuclei. Our calculations suggest thatboth galaxies contain sufficient amounts of gas in their nuclei, butonly NGC 2273 shows evidence for a nuclear gas bar. This may be theresult of past episodes of star formation exhausting and dispersing thenuclear gas of NGC 5728, but it is more likely evidence that NGC 5728has undergone a minor merger event.
| Discovery and Implications of a New Large-Scale Stellar Bar in NGC 5248
For decades, the grand-design SAB spiral galaxy NGC 5248 has beenpostulated to host a short bar of semimajor axis 22" (1.6 kpc). Fromdynamical and morphological arguments, however, we argue that its spiralstructure is being driven by a large-scale bar whose corotation radiuslies at ~115" (8.6 kpc). Our estimate is based partially on a deepR-band image, which reveals that the feature previously thought to be aninclined disk is in fact an extended stellar bar. The bar is embeddedwithin a fainter outer disk visible out to a radius of 230" (17.2 kpc).The bar has a deprojected ellipticity of 0.44 and a semimajor axis of95" (7.1 kpc). The classical grand-design spirals of NGC 5248, prominentin B, R, and K light, lie on the leading edge of the large-scale stellarbar and are accompanied by concave dust lanes out to at least 70". Theoffset between the dust and young stars is consistent with ourunderstanding of gas flows in barred galaxies, where shocks along theleading edges of a moderately strong bar compress the gas to formmassive young stars. While in many strongly barred galaxies, opticalspiral arms are prominent outside the bar but not within it, NGC 5248illustrates how intense star formation along a moderately strong bar canlead to conspicuous open spiral arms within the bar itself. NGC 5248also provides a clear example of how a large-scale stellar bar embeddedwithin a faint outer optical disk can be misidentified as an inclineddisk when imaging studies lack the sensitivity to detect the actualouter disk. We discuss the implications for the estimated bar fractionat higher redshifts.
| Nested and Single Bars in Seyfert and Non-Seyfert Galaxies
We analyze the observed properties of nested and single stellar barsystems in disk galaxies. The 112 galaxies in our sample comprise thelargest matched Seyfert versus non-Seyfert galaxy sample of nearbygalaxies with complete near-infrared or optical imaging sensitive tolength scales ranging from tens of parsecs to tens of kiloparsecs. Thepresence of bars is deduced by fitting ellipses to isophotes in HubbleSpace Telescope (HST) H-band images up to 10" radius and in ground-basednear-infrared and optical images outside the H-band images. This is aconservative approach that is likely to result in an underestimate ofthe true bar fraction. We find that a significant fraction of the samplegalaxies, 17%+/-4%, have more than one bar, and that 28%+/-5% of barredgalaxies have nested bars. The bar fractions appear to be stableaccording to reasonable changes in our adopted bar criteria. For thenested bars, we detect a clear division in length between thelarge-scale (primary) bars and small-scale (secondary) bars, in bothabsolute and normalized (to the size of the galaxy) length. We arguethat this bimodal distribution can be understood within the framework ofdisk resonances, specifically the inner Lindblad resonances (ILRs),which are located where the gravitational potential of the innermostgalaxy switches effectively from three-dimensional to two-dimensional.This conclusion is further strengthened by the observed distribution ofthe sizes of nuclear rings which are dynamically associated with theILRs. While primary bar sizes are found to correlate with the hostgalaxy sizes, no such correlation is observed for the secondary bars.Moreover, we find that secondary bars differ morphologically from singlebars. Our matched Seyfert and non-Seyfert samples show a statisticallysignificant excess of bars among the Seyfert galaxies at practically alllength scales. We confirm our previous results that bars are moreabundant in Seyfert hosts than in non-Seyfert galaxies and that Seyfertgalaxies always show a preponderance of ``thick'' bars compared to thebars in non-Seyfert galaxies. Finally, no correlation is observedbetween the presence of a bar and that of companion galaxies, evenrelatively bright ones. Overall, since star formation and dustextinction can be significant even in the H band, the stellar dynamicsof the central kiloparsec cannot always be revealed reliably by the useof near-infrared surface photometry alone.
| Molecular Gas and Star Formation in Bars of Nearby Spiral Galaxies
We compare the distribution of molecular gas and star formation activityin the bar region of six spirals (NGC 2903, 3627, 4321, 5457, 6946, andIC 342) from the BIMA Survey of Nearby Galaxies. The molecular gas,traced using the CO (J=1-0) emission line, is brightest along theleading edge of the stellar bar in the bar dust lanes. The starformation activity, traced using the Hα emission line, is offsettoward the leading side of the CO emission. A cross-correlation analysisshows that (1) the H II regions are offset 0-800 pc on the leading sideof the CO emission, (2) the largest offsets are found in the strongestbars, and (3) there is a wide range in offsets in a single bar with nosystematic pattern as a function of the galactocentric radius. TheCO-Hα offset constrains how stars may form depending on the gasflow. We examine possible star formation scenarios in context of the twomain classes of bar gas-flow simulations, the N-body/sticky particle andhydrodynamic models. Though both model gas flows are generallyconsistent with the observed offsets, we suggest the inclusion of atwo-phase or multiphase medium to improve the agreement between modelsand observations.
| 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.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
