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| On the Hydrodynamic Interplay Between a Young Nuclear Starburst and a Central Supermassive Black Hole
We present one-dimensional numerical simulations, which consider theeffects of radiative cooling and gravity on the hydrodynamics of thematter reinserted by stellar winds and supernovae within young nuclearstarbursts (NSBs) with a central supermassive black hole (SMBH). Thesimulations confirm our previous semi-analytic results for low-energeticstarbursts, evolving in a quasi-adiabatic regime, and extend them tomore powerful starbursts evolving in the catastrophic cooling regime.The simulations show a bimodal hydrodynamic solution in all cases. Theypresent a quasi-stationary accretion flow onto the black hole, definedby the matter reinserted by massive stars within the stagnation volumeand a stationary starburst wind, driven by the high thermal pressureacquired in the region between the stagnation and the starburst radii.In the catastrophic cooling regime, the stagnation radius rapidlyapproaches the surface of the starburst region, as one considers moremassive starbursts. This leads to larger accretion rates onto the SMBHand concurrently to powerful winds able to inhibit interstellar matterfrom approaching the NSB. Our self-consistent model thus establishes adirect physical link between the SMBH accretion rate and the nuclearstar formation activity of the host galaxy and provides a good upperlimit to the accretion rate onto the central black hole.
| Bulges of Nearby Galaxies with Spitzer: Scaling Relations in Pseudobulges and Classical Bulges
We investigate scaling relations of bulges using bulge-diskdecompositions at 3.6 ?m and present bulge classifications for 173E-Sd galaxies within 20 Mpc. Pseudobulges and classical bulges areidentified using Sérsic index, Hubble Space Telescope morphology,and star formation activity (traced by 8 ?m emission). In the near-IRpseudobulges have nb < 2 and classical bulges havenb >2, as found in the optical. Sérsic index andmorphology are essentially equivalent properties for bulgeclassification purposes. We confirm, using a much more robust sample,that the Sérsic index of pseudobulges is uncorrelated with otherbulge structural properties, unlike for classical bulges and ellipticalgalaxies. Also, the half-light radius of pseudobulges is not correlatedwith any other bulge property. We also find a new correlation betweensurface brightness and pseudobulge luminosity; pseudobulges become moreluminous as they become more dense. Classical bulges follow thewell-known scaling relations between surface brightness, luminosity, andhalf-light radius that are established by elliptical galaxies. We showthat those pseudobulges (as indicated by Sérsic index and nuclearmorphology) that have low specific star formation rates are very similarto models of galaxies in which both a pseudobulge and classical bulgeexist. Therefore, pseudobulge identification that relies only onstructural indicators is incomplete. Our results, especially those onscaling relations, imply that pseudobulges are very different types ofobjects than elliptical galaxies.
| Star Formation Efficiency in the Barred Spiral Galaxy NGC 4303
We present new 12CO (J = 1 – 0) observations of thebarred galaxy NGC 4303 using the Nobeyama 45 m telescope (NRO45) and theCombined Array for Research in Millimeter-wave Astronomy (CARMA). TheH? images of barred spiral galaxies often show active starformation in spiral arms, but less so in bars. We quantify thedifference by measuring star formation rate (SFR) and star formationefficiency (SFE) at a scale where local star formation is spatiallyresolved. Our CO map covers the central 2farcm3 region of the galaxy;the combination of NRO45 and CARMA provides a high fidelity image,enabling accurate measurements of molecular gas surface density. We findthat SFR and SFE are twice as high in the spiral arms as in the bar. Wediscuss this difference in the context of the Kennicutt-Schmidt (KS)law, which indicates a constant SFR at a given gas surface density. TheKS law breaks down at our native resolution (~250 pc), and substantialsmoothing (to 500 pc) is necessary to reproduce the KS law, althoughwith greater scatter.
| Chemical Abundances for 855 Giants in the Globular Cluster Omega Centauri (NGC 5139)
We present elemental abundances for 855 red giant branch (RGB) stars inthe globular cluster Omega Centauri (? Cen) from spectra obtainedwith the Blanco 4 m telescope and Hydra multifiber spectrograph. Thesample includes nearly all RGB stars brighter than V = 13.5 and spans? Cen's full metallicity range. The heavy ? elements (Si,Ca, and Ti) are generally enhanced by ~+0.3 dex and exhibit ametallicity-dependent morphology that may be attributed to mass andmetallicity-dependent Type II supernova (SN) yields. The heavy ?and Fe-peak abundances suggest minimal contributions from Type Ia SNe.The light elements (O, Na, and Al) exhibit >0.5 dex abundancedispersions at all metallicities, and a majority of stars with[Fe/H]> - 1.6 have [O/Fe], [Na/Fe], and [Al/Fe] abundancessimilar to those in monometallic globular clusters, as well as O-Na,O-Al anticorrelations and the Na-Al correlation in all but the mostmetal-rich stars. A combination of pollution from intermediate-massasymptotic giant branch stars and in situ mixing may explain the lightelement abundance patterns. A large fraction (27%) of ? Cen starsare O-poor ([O/Fe] < 0) and are preferentially located within 5'-10'of the cluster center. The O-poor giants are spatially similar, locatedin the same metallicity range, and are present in nearly equalproportions to blue main-sequence stars. This suggests that the O-poorgiants and blue main-sequence stars may share a common origin. [La/Fe]increases sharply at [Fe/H] >~ -1.6, and the [La/Eu] ratiosindicate that the increase is due to almost pure s-process production.
| An Extragalactic 12CO J = 3-2 Survey with the Heinrich Hertz Telescope
We present results of a 12CO J = 3-2 survey of 125 nearbygalaxies obtained with the 10 m Heinrich Hertz Telescope, with the aimto characterize the properties of warm and dense molecular gas in alarge variety of environments. With an angular resolution of 22'',12CO 3-2 emission was detected in 114 targets. Based on 61galaxies observed with equal beam sizes the 12CO 3-2/1-0integrated line intensity ratio R 31 is found to vary from0.2 to 1.9, with an average value of 0.81. No correlations are found forR 31 to Hubble-type and far-infrared luminosity. Possibleindications for a correlation with inclination angle and the 60?m/100 ?m color temperature of the dust are not significant.Higher R 31 ratios than in "normal" galaxies, hinting atenhanced molecular excitation, may be found in galaxies hosting activegalactic nuclei. Even higher average values are determined for galaxieswith bars or starbursts, the latter being identified by the ratio ofinfrared luminosity versus isophotal area, log [(L FIR/Lsun)/(D 2 25/kpc2)] >7.25. (U)LIRGs are found to have the highest averaged R 31value. This may be a consequence of particularly vigorous star formationactivity, triggered by galaxy interaction and merger events. The nuclearCO luminosities are slightly sublinearly correlated with the global FIRluminosity in both the 12CO J = 3-2 and the 1-0 lines. Theslope of the log-log plots rises with compactness of the respectivegalaxy sub-sample, indicating a higher average density and a largerfraction of thermalized gas in distant luminous galaxies. While linearor sublinear correlations for the 12CO J = 3-2 line can beexplained, if the bulk of the observed J = 3-2 emission originates fromthe molecular gas with densities below the critical one, the case of the12CO J = 1-0 line with its small critical density remains apuzzle.
| Arm and Interarm Star Formation in Spiral Galaxies
We investigate the relationship between spiral arms and star formationin the grand-design spirals NGC 5194 and NGC 628 and in the flocculentspiral NGC 6946. Filtered maps of near-IR (3.6 ?m) emission allow usto identify "arm regions" that should correspond to regions of stellarmass density enhancements. The two grand-design spirals show a cleartwo-armed structure, while NGC 6946 is more complex. We examine thesearm and interarm regions, looking at maps that trace recent starformation—far-ultraviolet (GALEX NGS) and 24 ?m emission(Spitzer SINGS)—and cold gas-CO (HERACLES) and H I (THINGS). Wefind the star formation tracers and CO more concentrated in the spiralarms than the stellar 3.6 ?m flux. If we define the spiral arms asthe 25% highest pixels in the filtered 3.6 ?m images, we find thatthe majority (60%) of star formation tracers occur in the interarmregions; this result persists qualitatively even when considering thepotential impact of finite data resolution and diffuse interarm 24 ?memission. Even with a generous definition of the arms (45% highestpixels), interarm regions still contribute at least 30% to theintegrated star formation rate (SFR) tracers. We look for evidence thatspiral arms trigger star or cloud formation using the ratios of SFR(traced by a combination of FUV and 24 ?m emission) to H2(traced by CO) and H2 to H I. Any enhancement ofSFR/M(H2) in the arm region is very small (less than 10%) andthe grand-design spirals show no enhancement compared to the flocculenttarget. Arm regions do show a weak enhancement in H2/H Icompared to the interarm regions, but at a fixed gas surface densitythere is little clear enhancement in the H2/H I ratio in thearm regions. Thus, it seems that spiral arms may only act to concentratethe gas to higher densities in the arms.
| The Chandra M101 Megasecond: Diffuse Emission
Because M101 is nearly face-on, it provides an excellent laboratory inwhich to study the distribution of X-ray-emitting gas in a typicallate-type spiral galaxy. We obtained a Chandra observation with acumulative exposure of roughly 1 Ms to study the diffuse X-ray emissionin M101. The bulk of the X-ray emission is correlated with the starformation traced by the far-UV (FUV) emission. The global FUV/X-raycorrelation is nonlinear (the X-ray surface brightness is roughlyproportional to the square root of the FUV surface brightness) and thesmall-scale correlation is poor, probably due to the delay between theFUV emission and the X-ray production in star-forming regions. The X-rayemission contains only minor contributions from unresolved stars(lsim3%), unresolved X-ray point sources (lsim4%), and individualsupernova remnants (~3%). The global spectrum of the diffuse emissioncan be reasonably well fitted with a three-component thermal model, butthe fitted temperatures are not unique; many distributions of emissionmeasure can produce the same temperatures when observed with the currentCCD energy resolution. The spectrum of the diffuse emission depends onthe environment; regions with higher X-ray surface brightnesses haverelatively stronger hard components, but there is no significantevidence that the temperatures of the emitting components increase withsurface brightness.
| An observational estimate for the mean secular evolution rate in spiral galaxies
We have observationally quantified the effect of gravitational torqueson stars in disc galaxies due to the stellar distribution itself andexplored whether these torques are efficient at transporting angularmomentum within a Hubble time. We derive instantaneous torque maps for asample of 24 spiral galaxies, based on stellar mass maps that werederived using the pixel-by-pixel mass-to-light estimator by Zibetti, Rixand Charlot. In conjunction with an estimate of the rotation velocity,the mass maps allow us to determine the torque-induced instantaneousangular momentum flow across different radii, resulting from the overallstellar distributions for each galaxy in the sample. By stacking thesample, which effectively replaces a time average by an ensembleaverage, we find that the torques due to the stellar disc act totransport angular momentum outwards over much of the disc (within threedisc scalelengths). The strength of the ensemble-averaged gravitationaltorques within one disc scalelength has a time-scale of 4 Gyr forangular momentum redistribution.The individual torque profiles show that only a third of our sampleexhibit torques strong enough to redistribute angular momentum within aHubble time, mostly those with strong bars. However, advective angularmomentum transport is another source of angular momentum redistribution,especially in the presence of long-lived spiral arms, but is notaccessible to direct observations. The torque-driven angular momentumredistribution is thus observed to be effective, either in one-third ofdisc galaxies or in most disc galaxies one third of the time and shouldlead to either changes in the mass density profile or the orbitalshapes.We use a set of self-consistent disc, bulge and halo simulated isolateddisc galaxies with realistic cold gas fractions to verify that thetorques exerted by the stellar distribution, such as spiral arms or abar, exceed those of the gas and halo, as assumed in the analysis of theobservations.This study is the first to observationally determine the strength oftorque-driven angular momentum flow of stars for a sample of spiralgalaxies, providing an important empirical constraint on secularevolution.
| Large Late-Time Asphericities in Three Type IIP Supernovae
Type II-plateau supernovae (SNe IIP) are the results of the explosionsof red supergiants and are the most common subclass of core-collapsesupernovae. Past observations have shown that the outer layers of theejecta of SNe IIP are largely spherical, but the degree of asphericityincreases toward the core. We present evidence for high degrees ofasphericity in the inner cores of three recent SNe IIP (SNe 2006my,2006ov, and 2007aa), as revealed by late-time opticalspectropolarimetry. The three objects were all selected to have very lowinterstellar polarization (ISP), which minimizes the uncertainties inISP removal and allows us to use the continuum polarization as a tracerof asphericity. The three objects have intrinsic continuum polarizationsin the range of 0.83%-1.56% in observations taken after the end of thephotometric plateau, with the polarization dropping to almost zero atthe wavelengths of strong emission lines. Our observations of SN 2007aaat earlier times, taken on the photometric plateau, show contrastinglysmaller continuum polarizations (~0.1%). The late-time H? and [OI] line profiles of SN 2006ov provide further evidence for asphericitiesin the inner ejecta. Such high core polarizations in very ordinarycore-collapse supernovae provide further evidence that essentially allcore-collapse supernova explosions are highly aspherical, even if theouter parts of the ejecta show only small deviations from sphericalsymmetry.
| On the Curvature of Dust Lanes in Galactic Bars
We test the theoretical prediction that the straightest dust lanes inbars are found in strongly barred galaxies, or more specifically, thatthe degree of curvature of the dust lanes is inversely proportional tothe strength of the bar. The test uses archival images of barredgalaxies for which a reliable nonaxisymmetric torque parameter (Qb) and the radius at which Q b has been measured(r(Q b)) have been published in the literature. Our resultsconfirm the theoretical prediction but show a large spread that cannotbe accounted for by measurement errors. We simulate 238 galaxies withdifferent bar and bulge parameters in order to investigate the origin ofthe spread in the dust lane curvature versus Q b relation.From these simulations, we conclude that the spread is greatly reducedwhen describing the bar strength as a linear combination of the barparameters Q b and the quotient of the major and minor axesof the bar, a/b. Thus, we conclude that the dust lane curvature ispredominantly determined by the parameters of the bar.
| On the Progenitor of the Type II-Plateau SN 2008cn in NGC 4603
A trend is emerging regarding the progenitor stars that give rise to themost common core-collapse supernovae (SNe), those of Type II-Plateau(II-P): they generally appear to be red supergiants with a limited rangeof initial masses, ~8-16 M sun. Here, we consider anotherexample, SN 2008cn, in the nearly face-on spiral galaxy NGC 4603. Evenwith limited photometric data, it appears that SN 2008cn is not a normalSN II-P, but is of the high-luminosity subclass. Through comparison ofpre- and post-explosion images obtained with the Wide Field andPlanetary Camera 2 on board the Hubble Space Telescope, we have isolateda supergiant star prior to explosion at nearly the same position as theSN. We provide evidence that this supergiant may well be the progenitorof the SN, although this identification is not entirely unambiguous.This is exacerbated by the distance to the host galaxy, 33.3 Mpc, makingSN 2008cn the most distant SN II-P yet for which an attempt has beenmade to identify a progenitor star in pre-SN images. The progenitorcandidate has a more yellow color ([V – I]0 = 0.98 magand T eff = 5200 ± 300 K) than generally would beexpected and, if a single star, would require that it exploded during a"blue loop" evolutionary phase, which is theoretically not expected tooccur. Nonetheless, we estimate an initial mass of M ini = 15± 2 M sun for this star, which is within the expectedmass range for SN II-P progenitors. The yellower color could also arisefrom the blend of two or more stars, such as a red supergiant and abrighter, blue supergiant. Such a red supergiant hidden in this blendcould instead be the progenitor and would also have an initial masswithin the expected progenitor mass range. Furthermore, the yellowsupergiant could be in a massive, interacting binary system, analogousto the possible yellow supergiant progenitor of the high-luminosity SNII-P 2004et. Finally, if the yellow supergiant is not the progenitor, oris not a stellar blend or binary containing the progenitor, then weconstrain any undetected progenitor star to be a red supergiant with Mini lsim 11 M sun, considering a physically morerealistic scenario of explosion at the model endpoint luminosity for arotating star.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 NAS 05-26555. Based in part on datagathered with the 6.5 m Magellan telescopes, located at Las CampanasObservatory, Chile.
| The Fundamental Plane of Accretion onto Black Holes with Dynamical Masses
Black hole accretion and jet production are areas of intensive study inastrophysics. Recent work has found a relation between radio luminosity,X-ray luminosity, and black hole mass. With the assumption that radioand X-ray luminosities are suitable proxies for jet power and accretionpower, respectively, a broad fundamental connection between accretionand jet production is implied. In an effort to refine these links andenhance their power, we have explored the above relations exclusivelyamong black holes with direct, dynamical mass-measurements. Thisapproach not only eliminates systematic errors incurred through the useof secondary mass measurements, but also effectively restricts the rangeof distances considered to a volume-limited sample. Further, we haveexclusively used archival data from the Chandra X-ray Observatory tobest isolate nuclear sources. We find log LR = (4.80 ±0.24) + (0.78 ± 0.27)log M BH + (0.67 ±0.12)log LX , in broad agreement with prior efforts. Owing tothe nature of our sample, the plane can be turned into an effective masspredictor. When the full sample is considered, masses are predicted lessaccurately than with the well-known M-σ relation. If obscuredactive galactic nuclei are excluded, the plane is potentially a betterpredictor than other scaling measures.
| H I in nearby low-luminosity QSO host galaxies
We searched for 21 cm H I emission in a sample of 27 previously COdetected nearby galaxies hosting low-luminosity quasi - stellar objects(QSOs). In this paper we investigate the relationship between the H Iand infrared properties of these host galaxies, compare the atomic andmolecular gas content and look for connections to the optical and FIRproperties. The single dish observations have been made with theEffelsberg 100-m telescope with a beam size of 9.5´. The sampleobjects have been drawn from a wide-angle survey for optically brightQSOs (HES), which have declinations δ >-30° and redshiftsup to z = 0.06. 12 host galaxies from the sample have been detected inthe H I 21 cm emission line. Eight of them have a spiral geometry,whereas the other four are bulge dominated and probably of ellipticaltype (E/S0). Three of the objects seem to be in a phase ofmerging/interaction. The neutral atomic gas masses range from 1.1× 10^9 Msun up to 3.8 × 10^10 Msun.The median H I gas mass in the whole sample is of the order of 11.4× 10^9 Msun, which is a factor of two higher than the HI content of our galaxy. We find no strong correlation between H I massand IR luminosity. The objects agree well within the expectations fromthe Tully-Fisher relation. In the color-color diagram we find allsources in the estimated locations. With the non-detected sources weclearly sample an upper envelope of this mass distribution.
| Comparisons of the radial distributions of core-collapse supernovae with those of young and old stellar populations
We present observational constraints on the nature of core-collapse (CC)supernovae (SNe) through an investigation into their radialdistributions with respect to those of young and old stellar populationswithin their host galaxies, as traced by Hα emission and R-bandlight, respectively. We discuss results and the implications they haveon the nature of SN progenitors, for a sample of 177 CC SNe.We find that the radial positions of the overall CC population closelyfollow the radial distribution of Hα emission, implying that bothare excellent tracers of star formation within galaxies. Within thisoverall distribution, we find that there is a central deficit of SNIIwhich is offset by a central excess of SNIb/c. This implies a strongmetallicity dependence on the relative production of the two types, withSNIb/c arising from higher metallicity progenitors than SNII. Separatingthe SNIb/c into individual classes, we find that a trend emerges interms of progenitor metallicity going from SNII through SNIb to SNIc,with the latter arising from the highest metallicity progenitors.Based on observations made with the Isaac Newton Telescope operated onthe island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Institute deAstrofísica de Canarias, and on observations made with theLiverpool Telescope operated on the island of La Palma by Liverpool JohnMoores University in the Spanish Observatorio del Roque de los Muchachosof the Instituto de Astrofísica de Canarias with financialsupport from the UK Science and Technology Facilities Council.E-mail: anderson@das.uchile.cl
| Strategies for spectroscopy on extremely large telescopes - III. Remapping switched fibre systems
We explore the use of remapping techniques to improve the efficiency ofhighly multiplexed fibre systems for astronomical spectroscopy. This isparticularly important for the implementation of diverse fieldspectroscopy (DFS) using highly multiplexed monolithic fibre systems(MFS). DFS allows arbitrary distributions of target regions to beaddressed to optimize observing efficiency when observing complex,clumpy structures such as protoclusters which will be increasinglyaccessible to extremely large telescopes. We show how the adoption ofvarious types of remapping between the input and output of an MFS canallow contiguous regions of spatial elements to be selected using onlysimple switch arrays. Finally, we show how this compares in efficiencywith integral-field and multi-object spectroscopy by simulations usingartificial and real catalogues of objects. With the adoption of thesemapping strategies, DFS outperforms other techniques when addressing arange of realistic target distributions. These techniques are alsoapplicable to biomedical science and were in fact inspired by it.
| Comparison of integrated photometric parameters of extragalactic star formation complexes and open star clusters in the Milky Way
A comparative analysis of the evolution of integrated photometricparameters for young star formation complexes in spiral and irregulargalaxies and open star clusters in our Galaxy is performed.Extragalactic star formation complexes seen as giant H II regions andopen star clusters in the Milky Way are shown to represent a singleevolutionary sequence of objects at different stages of their evolutionwhen the extinction is properly taken into account.
| The Herschel Reference Survey
The Herschel Reference Survey is a Herschel guaranteed time key projectand will be a benchmark study of dust in the nearby universe. The surveywill complement a number of other Herschel key projects including largecosmological surveys that trace dust in the distant universe. We willuse Herschel to produce images of a statistically-complete sample of 323galaxies at 250, 350, and 500 ?m. The sample is volume-limited,containing sources with distances between 15 and 25 Mpc and flux limitsin the band to minimize the selection effects associated with dust andwith young high-mass stars and to introduce a selection in stellar mass.The sample spans the whole range of morphological types (ellipticals tolate-type spirals) and environments (from the field to the center of theVirgo Cluster) and as such will be useful for other purposes than ourown. We plan to use the survey to investigate (i) the dust content ofgalaxies as a function of Hubble type, stellar mass, and environment;(ii) the connection between the dust content and composition and theother phases of the interstellar medium; and (iii) the origin andevolution of dust in galaxies. In this article, we describe the goals ofthe survey, the details of the sample and some of the auxiliaryobserving programs that we have started to collect complementary data.We also use the available multifrequency data to carry out an analysisof the statistical properties of the sample.
| 12CO(J = 1 – 0) On-the-Fly Mapping Survey of the Virgo Cluster Spirals. I. Data and Atlas
We have performed an On-The-Fly (OTF) mapping survey of12CO(J = 1-0) emission in 28 Virgo cluster spiral galaxiesusing the Five College Radio Astronomy Observatory (FCRAO) 14 mtelescope. This survey aims to characterize the CO distribution,kinematics, and luminosity of a large sample of galaxies covering thefull extents of stellar disks, rather than sampling only the inner disksor the major axis as was done by many previous single dish andinterferometric CO surveys. CO emission is detected in 20 galaxies amongthe 28 Virgo spirals observed. An atlas consisting of global measures,radial measures, and maps is presented for each detected galaxy. A notesummarizing the CO data is also presented along with relevantinformation from the literature. The CO properties derived from our OTFobservations are presented and compared with the results from the FCRAOExtragalactic CO Survey by Young et al. which utilizedposition-switching observations along the major axis and a model fittingmethod. We find that our OTF-derived CO properties agree well with theYoung et al. results in many cases, but the Young et al.measurements are larger by a factor of 1.4-2.4 for seven (out of 18)cases. We will explore further the possible causes for the discrepancyin the analysis paper currently under preparation.
| AGN/starburst connection in action: the half million second RGS spectrum of NGC 1365
Context: High-resolution X-ray observations in the imaging and spectraldomain have recently opened a new window on active galactic nuclei (AGN)feedback onto the circumnuclear gas. Spectral diagnostics, as well asthe remarkable morphological coincidence between [O iii] and X-rays,point to AGN photoionisation as the dominant ionisation mechanism onscales as large as a few kpc. Aims: In this paper we extend thesestudies to the nearby Seyfert 2 galaxy NGC 1365,known to host a circumnuclear ring of intense star formation at≃1.3 kpc from the nucleus. The main scope of this investigation isto study the connection between nuclear activity and star formation innearby AGN. Methods: We present a deep (≃5.8 days) 0.3-2 keVhigh-resolution spectrum of NGC 1365, collected withthe reflection grating spectrometer (RGS) on board XMM-Newton. Results:The spectrum is dominated by strong recombination lines of He- andH-like transitions from carbon to silicon, as well as by L transitionsfrom Fexvii. The continuum is strong, especially in the 10 to 20 Årange. Formal fits require two optically thin, collisionally ionisedplasma components, with temperatures ≃300 and ≃640 eV.However, they leave the bulk of the forbidden components of theHe-α Ovii and Nvi triplets unaccounted for. These features can beexplained as being produced by photoionised gas. NGC1365 is therefore the first obscured AGN, whosehigh-resolution X-ray spectrum requires both collisional ionisation andphotoionisation. Conclusions: The relative weakness of photoionisationdoes not stem from the intrinsic weakness of its AGN, whose X-rayluminosity is ≳ 1042 erg s-1. We suggest thatit may instead come from the line-of-sight from the active nucleus tothe NLR being blocked by optically thick matter in the broad lineregion, at the same time responsible for the large observed variation ofthe column density obscuring the X-ray active nucleus. Alternatively,NGC 1365 could host a remarkably luminous nuclearstarburst when compared to the AGN accretion power.
| Formation of Massive Galaxies at High Redshift: Cold Streams, Clumpy Disks, and Compact Spheroids
We present a simple theoretical framework for massive galaxies at highredshift, where the main assembly and star formation occurred, andreport on the first cosmological simulations that reveal clumpy disksconsistent with our analysis. The evolution is governed by the interplaybetween smooth and clumpy cold streams, disk instability, and bulgeformation. Intense, relatively smooth streams maintain an unstable densegas-rich disk. Instability with high turbulence and giant clumps, each afew percent of the disk mass, is self-regulated by gravitationalinteractions within the disk. The clumps migrate into a bulge in lsim10dynamical times, or lsim0.5 Gyr. The cosmological streams replenish thedraining disk and prolong the clumpy phase to several Gigayears in asteady state, with comparable masses in disk, bulge, and dark matterwithin the disk radius. The clumps form stars in dense subclumpsfollowing the overall accretion rate, ~100 M sunyr–1, and each clump converts into stars in ~0.5 Gyr.While the clumps coalesce dissipatively to a compact bulge, thestar-forming disk is extended because the incoming streams keep theouter disk dense and susceptible to instability and because of angularmomentum transport. Passive spheroid-dominated galaxies form when thestreams are more clumpy: the external clumps merge into a massive bulgeand stir up disk turbulence that stabilize the disk and suppress in situclump and star formation. We predict a bimodality in galaxy type by z ~3, involving giant-clump star-forming disks and spheroid-dominatedgalaxies of suppressed star formation. After z ~ 1, the disks tend to bestabilized by the dominant stellar disks and bulges. Most of the high-zmassive disks are likely to end up as today's early-type galaxies.
| Fractal Dimension of Galaxy Isophotes
In this paper we investigate the use of the fractal dimension of galaxyisophotes in galaxy classification. We have applied two differentmethods for determining fractal dimensions to the isophotes ofelliptical and spiral galaxies derived from CCD images. We conclude thatfractal dimension alone is not a reliable tool but that combined withother parameters in a neural net algorithm the fractal dimension couldbe of use. In particular, we have used three parameters to segregate theellipticals and lenticulars from the spiral galaxies in our sample.These three parameters are the correlation fractal dimension Dcorr, the difference between the correlation fractaldimension and the capacity fractal dimension D corr – Dcap, and, thirdly, the B – V color of the galaxy.
| New H2O masers in Seyfert and FIR bright galaxies. III. The southern sample
Context: A relationship between the water maser detection rate and farinfrared (FIR) flux densities was established as a result of two 22 GHzmaser surveys in a complete sample of galaxies ({Dec>-30°) with{100 μ m} flux densities of >50 Jy and >30 Jy. Aims: Weattempted to discover new maser sources and investigate the galaxieshosting the maser spots by extending previous surveys to southerngalaxies with particular emphasis on the study of their nuclear regions.Methods: A sample of 12 galaxies with {Dec<-30° and S100μ m>50 Jy was observed with the 70-m telescope of theCanberra deep space communication complex (CDSCC) at Tidbinbilla(Australia) in a search for water maser emission. The average 3σnoise level of the survey was 15 mJy for a {0.42 km s-1}channel, corresponding to a detection threshold of ˜ 0.1Lȯ for the isotropic maser luminosity at a distance of25 Mpc. Results: Two new detections are reported: a kilomaser with anisotropic luminosity L{H2O}˜5Lȯ in NGC 3620 and a maser with about twice thisluminosity in the merger system NGC 3256. The detections have beenfollowed-up by continuum and spectral line interferometric observationswith the Australia Telescope Compact Array (ATCA). In NGC 3256, afraction (about a third) of the maser emission originates in two hotspots associated with star formation activity, which are offset from thegalactic nuclei of the system. The remaining emission may originate inweaker centres of maser activity distributed over the central 50''. ForNGC 3620, the water maser is coincident with the nuclear region of thegalaxy. Our continuum observations indicate that the nature of thenuclear emission is probably linked to particularly intense starformation. Including the historical detection in NGC 4945, the watermaser detection rate in the southern sample is 15% (3/20), consistentwith the northern sample. The high rate of maser detections in thecomplete all-sky FIR sample (23%, 15/65) confirms the existence of alink between overall FIR flux density and maser phenomena. A relationbetween H2O and OH masers in the FIR sample is also discussed.
| A Search for "Dwarf" Seyfert Nuclei. VII. A Catalog of Central Stellar Velocity Dispersions of Nearby Galaxies
We present new central stellar velocity dispersion measurements for 428galaxies in the Palomar spectroscopic survey of bright, northerngalaxies. Of these, 142 have no previously published measurements, mostbeing relatively late-type systems with low velocity dispersions(lsim100 km s–1). We provide updates to a number ofliterature dispersions with large uncertainties. Our measurements arebased on a direct pixel-fitting technique that can accommodate compositestellar populations by calculating an optimal linear combination ofinput stellar templates. The original Palomar survey data were takenunder conditions that are not ideally suited for deriving stellarvelocity dispersions for galaxies with a wide range of Hubble types. Wedescribe an effective strategy to circumvent this complication anddemonstrate that we can still obtain reliable velocity dispersions forthis sample of well-studied nearby galaxies.
| The Imperial IRAS-FSC Redshift Catalogue
We present a new catalogue, the Imperial IRAS-FSC Redshift Catalogue(IIFSCz), of 60303 galaxies selected at 60?m from the IRAS FaintSource Catalogue (FSC). The IIFSCz consists of accurate position,optical, near-infrared and/or radio identifications, spectroscopicredshift (if available) or photometric redshift (if possible), predictedfar-infrared (FIR) and submillimetre (submm) fluxes ranging from 12 to1380?m based upon the best-fitting infrared template. About 55 percent of the galaxies in the IIFSCz have spectroscopic redshifts, and afurther 20 per cent have photometric redshifts obtained through eitherthe training set or the template-fitting method. For S(60) > 0.36 Jy,the 90 per cent completeness limit of the FSC, 90 per cent of thesources have either spectroscopic or photometric redshifts. Scientificapplications of the IIFSCz include validation of current and forthcominginfrared and submm/mm surveys such as AKARI, Planck and Herschel,follow-up studies of rare source populations, large-scale structure andgalaxy bias, local multiwavelength luminosity functions and sourcecounts. The catalogue is publicly available athttp://astro.imperial.ac.uk/~mrr/fss/.
| Pattern Corotation Radii from Potential-Density Phase-Shifts for 153 OSUBGS Sample Galaxies
The potential-density phase-shift method is an effective new tool forinvestigating the structure and evolution of galaxies. In this paper, weapply the method to 153 galaxies in the Ohio State University BrightGalaxy Survey (OSUBGS) to study the general relationship between patterncorotation radii and the morphology of spiral galaxies. The analysis isbased on near-infrared H-band images that have been deprojected anddecomposed assuming a spherical bulge. We find that multiple patternspeeds are common in disk galaxies. By selecting those corotation radiiclose to or slightly larger than the bar radius as being the barcorotation (CR) radius, we find that the average and standard deviationof the ratio \cal R = r(CR)/r(bar), is 1.20 ± 0.52 for 101galaxies having well-defined bars. There is an indication that thisratio depends weakly on galaxy type in the sense that the average rangesfrom 1.03 ± 0.37 for 65 galaxies of type Sbc and earlier, to 1.50± 0.63 for 36 galaxies of type Sc and later. Our bar corotationradii are on average smaller than those estimated fromsingle-pattern-speed numerical simulations, most likely because thesesimulations tend to find the pattern speed which generates a densityresponse in the gas that best matches the morphology of the outer spiralstructure. Although we find CR radii in most of the sample galaxies thatsatisfy conventional ideas about the extent of bars, we also considerthe alternative interpretation that in many cases the bar CR is actuallyinside the bar and that the bar ends close to its outer Lindbladresonance instead of its CR. These "superfast" bars are the mostcontroversial finding from our study. We see evidence in the phase-shiftdistributions for ongoing decoupling of patterns, which hints at theformation pathways of nested patterns, and which in turn further hintsat the longevity of the density wave patterns in galaxies. We alsoexamine how uncertainties in the orientation parameters of galaxies andin the shapes of bulges affect our results.
| NGC 2770: A Supernova Ib Factory?
NGC 2770 has been the host of three supernovae (SNe) of Type Ib duringthe last ten years, SN 1999eh, SN 2007uy, and SN 2008D. SN 2008Dattracted special attention due to the serendipitous discovery of anassociated X-ray transient. In this paper, we study the properties ofNGC 2770 and specifically the three SN sites to investigate whether thisgalaxy is in any way peculiar to cause a high frequency of SNe Ib. Wemodel the global spectral energy distribution of the galaxy frombroadband data and derive a star formation and SN rate comparable to thevalues of the Milky Way. We further study the galaxy using longslitspectroscopy covering the major axis and the three SN sites. From thespectroscopic study we find subsolar metallicities for the SN sites, ahigh extinction and a moderate star formation rate. In a high-resolutionspectrum, we also detect diffuse interstellar bands in the line of sighttoward SN 2008D. A comparison of NGC 2770 to the global properties of agalaxy sample with high SN occurrence (>= 3 SN in the last 100 years)suggests that NGC 2770 is not particularly destined to produce such anenhancement of observed SNe. Its properties are also very different fromgamma-ray burst host galaxies. Statistical considerations on SN Ibdetection rates give a probability of ~1.5% to find a galaxy with threeIb SNe detected in ten years. The high number of rare Ib SNe in thisgalaxy is therefore likely to be a coincidence rather than specialproperties of the galaxy itself. NGC 2770 has a small irregularcompanion, NGC 2770B, which is highly star-forming, has a very low massand one of the lowest metallicities detected in the nearby universe asderived from longslit spectroscopy. In the most metal poor part, we evendetect Wolf-Rayet (WR) features, which is at odds with most currentmodels of WR stars which require high metallicities.Based on observations with the Nordic Optical Telescope, ESO proposal080.D-0526, the GALEX and NED databases.
| The M-σ and M-L Relations in Galactic Bulges, and Determinations of Their Intrinsic Scatter
We derive improved versions of the relations between supermassive blackhole mass (M BH) and host-galaxy bulge velocity dispersion(σ) and luminosity (L; the M-σ and M-L relations), based on49 M BH measurements and 19 upper limits. Particularattention is paid to recovery of the intrinsic scatter(epsilon0) in both relations. We find log(M BH/Msun) = α + βlog(σ/200 kms–1) with (α, β, epsilon0) =(8.12 ± 0.08, 4.24 ± 0.41, 0.44 ± 0.06) for allgalaxies and (α, β, epsilon0) = (8.23 ±0.08, 3.96 ± 0.42, 0.31 ± 0.06) for ellipticals. Theresults for ellipticals are consistent with previous studies, but theintrinsic scatter recovered for spirals is significantly larger. Thescatter inferred reinforces the need for its consideration whencalculating local black hole mass function based on the M-σrelation, and further implies that there may be substantial selectionbias in studies of the evolution of the M-σ relation. We estimatethe M-L relationship as log(M BH/M sun) = α+ βlog(LV /1011 L sun,V ) of(α, β, epsilon0) = (8.95 ± 0.11, 1.11± 0.18, 0.38 ± 0.09); using only early-type galaxies.These results appear to be insensitive to a wide range of assumptionsabout the measurement errors and the distribution of intrinsic scatter.We show that culling the sample according to the resolution of the blackhole's sphere of influence biases the relations to larger mean masses,larger slopes, and incorrect intrinsic residuals.
| Determining Star Formation Timescale and Pattern Speed in Nearby Spiral Galaxies
We present a revised method for simultaneous determination of thepattern speed (ΩP) and star formation timescale (tSF) of spiral galaxies, which is originally proposed in ourprevious work. As this method utilizes offsets between molecular andyoung-stellar arms, we refer to it as the "Offset Method." Details ofthe method, its application, and results for CO and Hα images of13 nearby spiral galaxies are described here. CO data are from ourobservations with the Nobeyama Millimeter Array for two galaxies, andfrom the BIMA SONG for the rest. Out of 13 galaxies, we were able toderive ΩP and t SF for five galaxies. Wecategorize them as "C" galaxies as their offsets are clear. Our findingsfrom these galaxies are as follows. (1) The corotation radius calculatedby the derived ΩP is close to the edge of the CO data,and is about half of the optical radius for three galaxies. (2) Thederived t SF is roughly consistent with the free-fall time oftypical molecular clouds, which indicates that the gravitationalinstability is the dominant mechanism triggering star formation inspiral arms. (3) The t SF is found to be almost independentof the surface density of molecular gas, metallicity, or spiral armstrengths. The number of "C" galaxies and the quality of CO data,however, are not enough to confirm these relationships. We also findthat two other galaxies show no offsets between CO and Hα,although their arms are clearly traced, and categorize them as "N"galaxies. The presence of a bar could account for this feature, sincethese two galaxies are both barred. With one galaxy excluded from ouranalysis due to its poor rotation curve, offsets of the remaining fivegalaxies are found to be ambiguous. Either their dependence on therotational frequency cannot be explained by our picture, or the numberor quality of data is not sufficient for the analysis. We categorizethem as "A" galaxies. The possible reasons for this ambiguity are (1)the density wave is weaker, and/or (2) observational resolution andsensitivity are not enough to detect the spiral arms and their offsetsclearly. The former is supported by our finding that the arm strengthsof "A" galaxies are slightly weaker than that of "C" galaxies.
| Molecular Hydrogen Deficiency in H I-poor Galaxies and its Implications for Star Formation
We use a sample of 47 homogeneous and high-sensitivity CO images takenfrom the Nobeyama and BIMA surveys to demonstrate that, contrary tocommon belief, a significant number (~40%) of H I-deficient nearbyspiral galaxies are also depleted in molecular hydrogen. While H Ideficiency by itself is not a sufficient condition for molecular gasdepletion, we find that H2 reduction is associated with theremoval of H I inside the galaxy optical disk. Those H I-deficientgalaxies with normal H2 content have lost H I mainly fromoutside their optical disks, where the H2 content is low inall galaxies. This finding is consistent with theoretical models inwhich the molecular fraction in a galaxy is determined primarily by itsgas column density. Our result is supported by indirect evidence thatmolecular deficient galaxies form stars at a lower rate or have dimmerfar infrared fluxes than gas rich galaxies, as expected if the starformation rate is determined by the molecular hydrogen content. Ourresult is consistent with a scenario in which, when the atomic gascolumn density is lowered inside the optical disk below the criticalvalue required to form molecular hydrogen and stars, spirals becomequiescent and passive evolving systems. We speculate that this processwould act on the timescale set by the gas depletion rate and might be afirst step for the transition between the blue and red sequence observedin the color-magnitude diagram.
| Bulges of Nearby Galaxies with Spitzer: The Growth of Pseudobulges in Disk Galaxies and its Connection to Outer Disks
We study star formation rates (SFRs) and stellar masses in bulges ofnearby disk galaxies. For this we construct a new SFR indicator thatlinearly combines data from the Spitzer Space Telescope and the GalaxyEvolution Explorer. All bulges are found to be forming starsirrespective of bulge type (pseudobulge or classical bulge). Atpresent-day SFR the median pseudobulge could have grown the present-daystellar mass in 8 Gyr. Classical bulges have the lowest specific SFRimplying a growth times that are longer than a Hubble time, and thus thepresent-day SFR does not likely play a major role in the evolution ofclassical bulges. In almost all galaxies in our sample the specific SFR(SFR per unit stellar mass) of the bulge is higher than that of theouter disk. This suggests that almost all galaxies are increasing theirB/T through internal star formation. The SFR in pseudobulges correlateswith their structure. More massive pseudobulges have higher SFR density,this is consistent with that stellar mass being formed by moderate,extended star formation. Bulges in late-type galaxies have similar SFRsas pseudobulges in intermediate-type galaxies, and are similar in radialsize. However, they are deficient in mass; thus, they have much shortergrowth times, ~2 Gyr. We identify a class of bulges that have nuclearmorphology similar to pseudobulges, significantly lower specific SFRthan pseudobulges, and are closer to classical bulges in structuralparameter correlations. These are possibly composite objects, evolvedpseudobulges or classical bulges experiencing transient, enhancednuclear star formation. Our results are consistent with a scenario inwhich bulge growth via internal star formation is a natural, and nearubiquitous phenomenon in disk galaxies. Those galaxies with largeclassical bulges are not affected by the in situ bulge growth, likelybecause the majority of their stellar mass comes from some otherphenomenon. Yet, those galaxies without a classical bulge, over longperiods of extended star formation are able to growth a pseudobulge.Though cold accretion is not ruled out, for pseudobulge galaxies anaddition of stellar mass from mergers or accretion is not required toexplain the bulge mass. In this sense, galaxies with pseudobulges mayvery well be bulgeless (or "quasi-bulgeless") galaxies, and galaxieswith classical bulges are galaxies in which both internal evolution andhierarchical merging are responsible for the bulge mass by fractionsthat vary from galaxy to galaxy.
| Galaxies Correlating with Ultra-High Energy Cosmic Rays
The Pierre Auger Observatory reported that 20 of the 27 highest energycosmic rays have arrival directions within 3fdg2 of a nearby galaxy inthe Veron-Cetty and Veron (VCV) Catalog of Quasars and Active GalacticNuclei (12th ed.), with a 1% probability that this would be due tochance if the cosmic ray directions were isotropic. In this paper, weexamine the correlated galaxies to gain insight into the possibleultra-high energy cosmic ray (UHECR) sources. We find that 14 of the 21correlated VCV galaxies are active galactic nuclei (AGNs) and wedetermine their bolometric luminosities. The remaining seven areprimarily star-forming galaxies. The bolometric luminosities of thecorrelated AGNs are all greater than 5 × 1042 ergs-1. This may explain the absence of UHECRs from the Virgoregion in spite of the large number of VCV galaxies in Virgo, since mostof the VCV galaxies in the Virgo region are low-luminosity AGNs.Interestingly, the bolometric luminosities of most of the AGNs aresignificantly lower than that required to satisfy the minimum conditionfor UHECR acceleration in a continuous jet. If a UHECR-AGN correlationis substantiated with further statistics, our results lend support tothe recently proposed "giant AGN flare" mechanism for UHECRacceleration.
| Bulge n and B/T in High-Mass Galaxies: Constraints on the Origin of Bulges in Hierarchical Models
We use the bulge Sérsic index n and bulge-to-total mass ratio(B/T) to explore the fundamental question of how bulges form. We performtwo-dimensional bulge-disk-bar decomposition on H-band images of 143bright, high-mass (M sstarf >= 1.0 × 1010M sun) low-to-moderately inclined (i < 70°) spirals.Our results are as follows. (1) Our H-band bar fraction (~58%) isconsistent with that from ellipse fits. (2) 70% of the stellar mass isin disks, 10% in bars, and 20% in bulges. (3) A large fraction (~69%) ofbright spirals have B/T<= 0.2, and ~76% have low n <= 2 bulges.These bulges exist in barred and unbarred galaxies across a wide rangeof Hubble types. (4) About 65% (68%) of bright spirals with n <= 2(B/T <= 0.2) bulges host bars, suggesting a possible link betweenbars and bulges. (5) We compare the results with predictions from a setof ΛCDM models. In the models, a high-mass spiral can have abulge with a present-day low B/T<= 0.2 only if it did not undergo amajor merger since z <= 2. The predicted fraction (~ 1.6%) ofhigh-mass spirals, which have undergone a major merger since z <= 4and host a bulge with a present-day low B/T <= 0.2, is a factor ofover 30 smaller than the observed fraction (~66%) of high-mass spiralswith B/T <= 0.2. Thus, contrary to common perception, bulges builtvia major mergers since z <= 4 seriously fail to account for thebulges present in ~66% of high mass spirals. Most of these present-daylow B/T <= 0.2 bulges are likely to have been built by a combinationof minor mergers and/or secular processes since z <= 4.
| Understanding the H2/HI ratio in galaxies
We revisit the mass ratio ηgalaxy between molecularhydrogen (H2) and atomic hydrogen (HI) in different galaxiesfrom a phenomenological and theoretical viewpoint. First, the localH2 mass function (MF) is estimated from the local COluminosity function (LF) of the FCRAO Extragalactic CO Survey, adoptinga variable CO-to-H2 conversion fitted to nearby observations.This implies an average H2 density and in the local Universe.Secondly, we investigate the correlations between ηgalaxyand global galaxy properties in a sample of 245 local galaxies. Based onthese correlations we introduce four phenomenological models forηgalaxy, which we apply to estimate H2 massesfor each HI galaxy in the HIPASS catalogue. The resulting H2MFs (one for each model for ηgalaxy) are compared to thereference H2 MF derived from the CO LF, thus allowing us todetermine the Bayesian evidence of each model and to identify a clearbest model, in which, for spiral galaxies, ηgalaxynegatively correlates with both galaxy Hubble type and total gas mass.Thirdly, we derive a theoretical model for ηgalaxy forregular galaxies based on an expression for their axially symmetricpressure profile dictating the degree of molecularization. This model isquantitatively similar to the best phenomenological one at redshift z =0, and hence represents a consistent generalization while providing aphysical explanation for the dependence of ηgalaxy onglobal galaxy properties. Applying the best phenomenological model forηgalaxy to the HIPASS sample, we derive the firstintegral cold gas MF (HI + H2 + helium) of the localuniverse.
| The hard X-ray view of bright infrared galaxies
Aims. The synthesis of the cosmic X-ray background (CXB) requires alarge population of Compton-thick active galactic nuclei that have notbeen detected so far. We probe whether bright infrared galaxies couldharbor a population of Compton-thick nuclei and if they could contributesignificantly. Methods: We analyzed 112 Ms of INTEGRAL observationsobtained on 613 galaxies from the IRAS Revised Bright Galaxy Sample. Wederived the average hard X-ray (18-80 keV) emission of Seyfert andvarious non Seyfert galaxy subsamples to estimate their relativecontribution to the locally emitted CXB. Results: The Seyfert 1 & 2are detected at hard X-rays. None of the other galaxy subsamples weredetected. ULIRGs are at least 5 times under-luminous at hard X-rays whencompared to Seyferts. The upper limit obtained for the average nonSeyfert galaxies is as low as 7 × 10-13 ergs-1 cm-2. On average, these galaxies do notcontain active nuclei brighter than 1041 erg s-1at hard X-rays. The total hard X-ray flux detected from the sample is4.9 × 10-9 erg s-1 cm-2 (about 1%of the CXB), and 64% of this originates in absorbed active nuclei. Localnon-Seyfert galaxies contribute for less than 7% and do not harbor theCompton-thick nuclei assumed to synthesize the locally emitted CXB.
| VISIR Observations of Local Seyfert Nuclei and the Mid-infrared - Hard X-ray Correlation
High angular resolution mid-infrared observations with the VISIRinstrument at the Very Large Telescope have allowed the distribution ofdust around local active galactic nuclei (AGN) to be studied. Theobservational results support the unified scenario for AGN and bringconstraints on the properties of its key component, a dusty torusobscuring the view onto the AGN when viewed close to the equatorialplane.
| Detection of Star Streams and Turbulence in Nearby Galaxies: Power Spectrum Analysis of Spitzer Images
Fourier transform power spectra of azimuthal scans in 33 galaxies imagedwith the Infrared Array Camera on the Spitzer Space Telescope show anapproximate power-law structure over a wide range of wavenumbers with agradual steepening from 3.6 or 4.5 μm to 8.0 μm, in the order ofan increasing contribution from gas and dust. At radii with active starformation, the average of the slopes of the 8 μm power spectra atintermediate spatial frequencies is about the same for all galaxies,independent of spiral arm morphology. These power spectra arereminiscent of turbulence, although the 8 μm slopes, averaging–2.0, are slightly steeper than expected (–1.7).Reconstructed images using only these intermediate spatial frequenciesat 4.5 μm show aging star complexes distorted by shear. These shearedfeatures illustrate the transition from a hierarchical structure duringstar formation into azimuthal star streams like the Pleiades movinggroup in the Solar neighborhood. This is the first time that young starstreams have been observed in spiral galaxies other than the Milky Way.
| The Integrated Polarization of Spiral Galaxy Disks
We present integrated polarization properties of nearby spiral galaxiesat 4.8 GHz, and models for the integrated polarization of spiral galaxydisks as a function of inclination. Spiral galaxies in our sample haveobserved integrated fractional polarization in the range lsim1%-17.6%.At inclinations less than 50°, the fractional polarization dependsmostly on the ratio of random to regular magnetic field strength. Athigher inclinations, Faraday depolarization associated with the regularmagnetic field becomes more important. The observed degree ofpolarization is lower (<4%) for more luminous galaxies, in particularthose with L 4.8 > 2 × 1021 WHz–1. The polarization angle of the integrated emissionis aligned with the apparent minor axis of the disk for galaxies withouta bar. In our axially symmetric models, the polarization angle of theintegrated emission is independent of wavelength. Simulateddistributions of fractional polarization for randomly oriented spiralgalaxies at 4.8 GHz and 1.4 GHz are presented. We conclude thatpolarization measurements, e.g., with the Square Kilometre Array, ofunresolved spiral galaxies allow statistical studies of the magneticfield in disk galaxies using large samples in the local universe and athigh redshift. As these galaxies behave as idealized background sourceswithout internal Faraday rotation, they can be used to detectlarge-scale magnetic fields in the intergalactic medium.
| Chandra Observations of Nuclear X-Ray Emission from Low Surface Brightness Galaxies
We present Chandra detections of X-ray emission from the active galacticnuclei (AGN) in two giant low surface brightness (LSB) galaxies, UGC2936 and UGC 1455. Their X-ray luminosities are 1.8 ×1042 ergs s–1 and 1.1 ×1040 ergs s–1 respectively. Of the twogalaxies, UGC 2936 is radio loud. Together with another LSB galaxy UGC6614 (XMM-Newton archival data) both appear to lie above the X-ray-radiofundamental plane, and their AGN have black hole masses that are lowcompared to similar galaxies lying on the correlation. However, thebulges in these galaxies are well developed, and we detect diffuse X-rayemission from four of the eight galaxies in our sample. Our resultssuggest that the bulges of giant LSB galaxies evolve independently oftheir halo-dominated disks which are low in star formation and diskdynamics. The centers follow an evolutionary path similar to that ofbulge-dominated normal galaxies on the Hubble sequence, but the LSBdisks remain unevolved. Thus, the bulge and disk evolution are decoupledand so whatever star formation processes produced the bulges did notaffect the disks.
| Study of the stellar line-strength indices and kinematics along bars. I. Bar age and metallicity gradients
Aims. This is the first paper of a series that aims to understand theformation and evolution of bars in early-type spirals and theirinfluence in the evolution of the galaxy. Methods: Optical long-slitspectra along the bar major-axis of a sample of 20 galaxies areanalyzed. Velocity and velocity dispersion profiles along the bar arepresented. Line-strength indices in the bar region are also measured toderive stellar mean-age and metallicity distributions along the barsusing stellar population models. Results: We obtain mean ages,metallicities and chemical abundances along the bar of 20 galaxies withmorphological types from SB0 to SBbc. The main result is that we find alarge variation in age and metallicity along the bar in ~45% of oursample. We find three different types of bars according to theirmetallicity and age distribution along the radius: 1) Bars with negativemetallicity gradients. They show a mean young/intermediate population(<2 Gyr), and have amongst the lowest stellar maximum centralvelocity dispersion of the sample; 2) bars with null metallicitygradients. These galaxies do not show any gradient in their metallicitydistribution along the bar and have negative age gradients (i.e. youngerpopulations at the bar end); 3) bars with positive metallicitygradients, i.e. more metal rich at the bar ends. These galaxies arepredominantly those with higher velocity dispersion and an older meanpopulation. We found no significant correlation between the age andmetallicity distribution, and bar/galaxy parameters such as the AGNpresence, size or the bar strength. From the kinematics, we find thatall the galaxies show a disk-like central component. Conclusions: Theresults from the metallicity and age gradients indicate that mostgalaxies with high central stellar velocity dispersion host bars thatcould have been formed more than 3 Gyr ago, while galaxies with lowercentral velocity dispersions show a wider distribution in theirpopulation and age gradients. A few bars show characteristics compatiblewith having been formed less than <2 Gyr ago. However, we do not havea definite answer to explain the observed gradients and these resultsplace strong constrains on models of bar formation and evolution. Thedistribution of mean stellar population parameters in the bar withrespect to σ is similar to that found in bulges, indicating aclose link in the evolution of both components. The disk-like centralcomponents also show the important role played by bars in the secularevolution of the central structure.Based on observationsobtained at Siding Spring Observatory (RSAA, ANU, Australia) and the INTtelescope at the ING, La Palma, Spain.Appendices are only available in electronic form at http://www.aanda.org
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