Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Comment regarding the functional form of the Schmidt law
Star formation rates on the galactic scale are describedphenomenologically by two distinct relationships, as emphasized recentlyby Elmegreen [Elmegreen, B.G., 2002. ApJ, 577, 206. astro-ph/0207114.].The first of these is the Schmidt law, which is a power-law relationbetween the star formation rate SFR and the column density Σ. Theother relationship is that there is a cutoff in the gas density belowwhich star formation shuts off. The purpose of this paper is to arguethat: (1) these two relationships can be accommodated by a singlefunctional form of the Schmidt law, (2) this functional form ismotivated by the hypothesis that star formation is a criticalphenomenon, and that as a corollary, (3) the existence of a sharp cutoffmay thus be an emergent property of galaxies, as was argued by Seiden[Seiden, P.E.,1983. ApJ, 266, 555.], as opposed to the classical viewthat this cutoff is due to an instability criterion.
| Scalar potential model of redshift and discrete redshift
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.
| The star formation history and evolution of the circumnuclear region of M100
Context. Star-forming nuclear rings in barred galaxies are common innearby spirals, and their detailed study can lead to important insightsinto the evolution of galaxies, their bars and their central regions. Wepresent integral field spectroscopic observations obtained with SAURONof the bar and circumnuclear region of the barred spiral galaxy M100,complemented by new Spitzer Space Telescope imaging of the region.Aims. We use these data to enhance our understanding of the formation,evolution and current properties of the bar and ring.Methods. We derive the kinematics of the gas and the stars and quantifycircular and non-circular motions using kinemetry. We analyse this inconjunction with the optical and infrared morphology, and our previouslypublished dynamical modelling. By comparing line indices to simplestellar population models we estimate the ages and metallicities of thestellar populations present within the region, especially in and aroundthe ring.Results. The stellar and gaseous velocity fields are remarkably similar,and we confirm that the velocity fields show strong evidence fornon-circular motions due to the bar and the associated density wave.These are strongest just outside the nuclear ring, where our kinemetricanalysis indicates inflow across the spiral armlets and into the ringregion. The line strength maps all indicate the presence of a youngerpopulation within this ring, but detailed modelling of the linestrengths shows that in addition to this young population, old stars arepresent. These old stars must have been formed in an event of massivestar formation which produced the bulk of the mass, and which ended some3 Gyr ago, a constraint set by the age of the stars in the bar and thenucleus. Our best-fitting model is one in which the current starformation is but the latest of a series of relatively short bursts ofstar formation which have occurred for the last 500 Myr or so. A clearbipolar azimuthal age gradient is seen within the ring, with theyoungest stars occurring near where the bar dust lanes connect with thering.Conclusions. Our kinematic and morphological results all confirm thepicture in which the nuclear ring in M100, considered typical, is fed bygas flowing in from the disc under the action of the bar, is slowed downnear a pair of resonances, and forms significant amounts of massivestars. Detailed stellar population modelling shows how the underlyingbulge and disc were put in place a number of Gyr ago, and that thenuclear ring has been forming stars since about 500 Myr ago in a stablesuccession of bursts. This confirms that nuclear rings of this kind canform under the influence of a resonant structure set up by a bar, andproves that they are stable features of a galaxy rather than one-offstarburst events.
| Hαkinematics of the SINGS nearby galaxies survey - I*
This is the first part of an Hαkinematics follow-up survey of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample. The data for28galaxies are presented. The observations were done on three differenttelescopes with Fabry-Perot of New Technology for the Observatoire dumont Megantic (FaNTOmM), an integral field photon-counting spectrometer,installed in the respective focal reducer of each telescope. The datareduction was done through a newly built pipeline with the aim ofproducing the most homogenous data set possible. Adaptive spatialbinning was applied to the data cubes in order to get a constantsignal-to-noise ratio across the field of view. Radial velocity andmonochromatic maps were generated using a new algorithm, and thekinematical parameters were derived using tilted-ring models.
| A Virgo high-resolution Hα kinematical survey - II. The Atlas
A catalogue of ionized gas velocity fields for a sample of 30 spiral andirregular galaxies of the Virgo cluster has been obtained by using 3Doptical data. The aim of this survey is to study the influence ofhigh-density environments on the gaseous kinematics of local clustergalaxies. Observations of the Hα line by means of Fabry-Perotinterferometry have been performed at the Canada-France-HawaiiTelescope, European Southern Observatory 3.6-m telescope, Observatoirede Haute-Provence 1.93-m telescope and Observatoire du montMégantic telescope at angular and spectral samplings from 0.4 to1.6arcsec and 7 to 16kms-1. A recently developed, automaticand adaptive spatial binning technique is used to reach a nearlyconstant signal-to-noise ratio (S/N) over the whole field of view,allowing us to keep a high spatial resolution in high-S/N regions andextend the detection of signal in low-S/N regions. This paper is part ofa series and presents the integrated emission-line and velocity maps ofthe galaxies. Both Hα morphologies and kinematics exhibit signs ofperturbations in the form of, for example, external filaments, inner andnuclear spiral- and ring-like structures, inner kinematical twists,kinematical decoupling of a nuclear spiral, streaming motions alongspiral arms and misalignment between kinematical and photometricorientation axes.
| Gas and stellar dynamics in NGC 1068: probing the galactic gravitational potential
We present SAURON integral field spectrography of the central 1.5 kpc ofthe nearby Seyfert 2 galaxy NGC 1068, encompassing the well-knownnear-infrared (NIR) inner bar observed in the K band. We havesuccessively disentangled the respective contributions of the ionizedgas and stars, thus deriving their two-dimensional distribution andkinematics. The [OIII] and Hβ emission lines exhibit a verydifferent spatial distribution and kinematics, the latter followinginner spiral arms with clumps associated with star formation. Stronginward streaming motions are observed in both the Hβ and [OIII]kinematics. The stellar kinematics also exhibit clear signatures of anon-axisymmetric tumbling potential, with a twist in both the velocityand Gauss-Hermite h3 fields. We re-examined the long-slitdata of Shapiro, Gerssen & van der Marel using a pPXF: a strongdecoupling of the Gauss-Hermite term h3 is revealed, and thecentral decrease of Gauss-Hermite term h4 hinted in theSAURON data is confirmed. These data also suggest that NGC 1068 is agood candidate for a so-called σ drop. We confirm the possiblepresence of two separate pattern speeds applying the Tremaine-Weinbergmethod to the Fabry-Perot Hα map. We also examine the stellarkinematics of bars formed in N-body+smoothed particle hydrodynamics(SPH) simulations built from axisymmetric initial conditionsapproximating the luminosity distribution of NGC 1068. The resultingvelocity, dispersion and higher order Gauss-Hermite moments successfullyreproduce a number of properties observed in the two-dimensionalkinematics of NGC 1068 and the long-slit data, showing that thekinematic signature of the NIR bar is imprinted in the stellarkinematics. The remaining differences between the models and theobserved properties are likely mostly due to the exclusion of starformation and the lack of the primary large-scale oval/bar in thesimulations. These models nevertheless suggest that the inner bar coulddrive a significant amount of gas down to a scale of ~ 300 pc. Thiswould be consistent with the interpretation of the σ drop in NGC1068 being the result of central gas accretion followed by an episode ofstar formation.
| Supernovae 2006U, 2006V, 2006W, 2006X. (Lick Observatory Supernova Search)
Not Available
| Scale Heights of Non-Edge-on Spiral Galaxies
We present a method of calculating the scale height of non-edge-onspiral galaxies, together with a formula for errors. The method is basedon solving Poisson's equation for a logarithmic disturbance of matterdensity in spiral galaxies. We show that the spiral arms can not extendto inside the ``forbidden radius'' r0, due to the effect ofthe finite thickness of the disk. The method is tested by re-calculatingthe scale heights of 71 northern spiral galaxies previously calculatedby Ma, Peng & Gu. Our results differ from theirs by less than 9%. Wealso present the scale heights of a further 23 non-edge-on spiralgalaxies.
| Supernova 2006X in NGC 4321
CBET 393 available at Central Bureau for Astronomical Telegrams.
| Supernova 2006X in NGC 4321
CBET 393 available at Central Bureau for Astronomical Telegrams.
| Chandra Observation of Supernova 2006X in M100
No X-ray source is detected at the position of the Type Ia SN 2006X inM100 (IAUC #8667, CBET #393, ATel #726) in a 37.9 ks Chandra ACIS-SX-ray observation from 2006-02-18.72UT. The 3-sigma upper limit to the(0.2-10 keV band) count rate is 4.5E-04 cts/s, corresponding to anunabsorbed X-ray flux and luminosity of 4E-15 ergs/cm/cm/s and 1E38ergs/s, respectively, for an assumed thermal plasma spectrum with atemperature of 10 keV, an absorbing foreground column density of 2.39E20cm**-2 (Dickey & Lockman 1990, ARAA 28, 215) and a distance of 17.1Mpc (Freedman et al.
| Supernova 2006X in NGC 4321 (M100)
We report the lack of detection of radio emission near the position ofthe type-Ia supernova 2006X (IAUC 8667, CBET 393) with the Very LargeArray: Upper limits (3 sigma) to any radio flux densitywere established on 2006 Feb. 09.37 UT of <0.162 mJy (SpectralLuminosity < 5.6 x 10^25 erg/s/Hz) at 22.460 GHz (wavelength 1.3 cm)and <0.047 mJy (Spectral Luminosity < 1.5 x 10^25 erg/s/Hz) at8.460 GHz (wavelength 3.5 cm), among the deepest and earliest limits yetobtained for radio emission from a type Ia supernova.
| Radio Observations of Type Ia SN2006X
Alicia Soderberg (Caltech) reports: I observed the Type Ia supernova2006X in NGC 4321 (IAUC 8667, CBET 393) with the Very Large Array on2006 February 9.34 UT (approximately 2 days after optical discovery).The 2.5 hour observation at 8.46 GHz reveals no radio source coincidentwith the optical SN position. I place a limit on the radio flux densityof 45 microJy (3 sigma). At a distance of 16 Mpc this limit correspondsto a luminosity of 1.4 x 10^25 erg/s/Hz.
| Swift Observations of Supernova 2006X in M100
S. Immler, Goddard Space Flight Center, NASA and Universities SpaceResearch Association, P. Brown, Pennsylvania State University, andPeter Milne, U. of Arizona, on behalf of the Swift-satellite team, report on Swift Ultraviolet/Optical Telescope (UVOT; wavelength range 170-650 nm) and X-Ray Telescope (XRT; 0.2-10 keV) observations of theType Ia SN 2006X (CBET #393) obtained on Feb 8.33 UT.
| Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity Corrections
With this paper we continue the preparation for a forthcoming summaryreport of our experiment with the HST to determine the Hubble constantusing Type Ia supernovae as standard candles. Two problems areaddressed. (1) We examine the need for, and determine the value of, thecorrections to the apparent magnitudes of our program Cepheids in the 11previous calibration papers due to sensitivity drifts and chargetransfer effects of the HST WFPC2 camera over the life time of theexperiment from 1992 to 2001. (2) The corrected apparent magnitudes areapplied to all our previous photometric data from which revised distancemoduli are calculated for the eight program galaxies that are parents tothe calibrator Ia supernovae. Two different Cepheid P-L relations areused; one for the Galaxy and one for the LMC. These differ both in slopeand zero point at a fixed period. The procedures for determining theabsorption and reddening corrections for each Cepheid are discussed.Corrections for the effects of metallicity differences between theprogram galaxies and the two adopted P-L relations are derived andapplied. The distance moduli derived here for the eight supernovaeprogram galaxies, and for 29 others, average 0.20 mag fainter (moredistant) than those derived by Gibson et al. and Freedman et al. intheir 2000 and 2001 summary papers for reasons discussed in this paper.The effect on the Hubble constant is the subject of our forthcomingsummary paper.
| Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies
Mid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions.
| Determination of the Hubble Constant, the Intrinsic Scatter of Luminosities of Type Ia Supernovae, and Evidence for Nonstandard Dust in Other Galaxies
A sample of 109 Type Ia supernovae (SNe Ia) with recession velocity<~30,000 km s-1 is compiled from published SN Ia lightcurves to explore the expansion rate of the local universe. Based on thecolor parameter ΔC12 and the decline rateΔm15, we found that the average absorption-to-reddeningratios for SN Ia host galaxies are RUBVI=4.37+/-0.25,3.33+/-0.11, 2.30+/-0.11, and 1.18+/-0.11, which are systematicallylower than the standard values in the Galaxy. We investigated thecorrelations of the intrinsic luminosity with light-curve decline rate,color index, and SN environmental parameters. In particular, we foundthat SNe Ia in E/S0 galaxies close to the central region are brighterthan those in the outer region, which may suggest a possible metallicityeffect on SN luminosity. The dependence of SN luminosity on galacticenvironment disappears after corrections for the extinction andΔC12. The Hubble diagrams constructed using 73 Hubbleflow SNe Ia yield a 1 σ scatter of <~0.12 mag in BVI bands and~0.16 mag in U band. The luminosity difference between normal SNe Ia andpeculiar objects (including SN 1991bg-like and SN 1991T-like events) hasnow been reduced to within 0.15 mag via ΔC12correction. We use the same precepts to correct the nearby SNe Ia withCepheid distances and found that the fully corrected absolute magnitudesof SNe Ia are MB=-19.33+/-0.06 andMV=-19.27+/-0.05. We deduced a value for the Hubble constantof H0=72+/-6 (total) km s-1 Mpc-1.
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Star Formation in Isolated Disk Galaxies. II. Schmidt Laws and Efficiency of Gravitational Collapse
We model gravitational instability in a wide range of isolated diskgalaxies, using GADGET, a three-dimensional, smoothed particlehydrodynamics code. The model galaxies include a dark matter halo and adisk of stars and isothermal gas. Absorbing sink particles are used todirectly measure the mass of gravitationally collapsing gas. Below thedensity at which they are inserted, the collapsing gas is fullyresolved. We make the assumption that stars and molecular gas formwithin the sink particle once it is created and that the star formationrate is the gravitational collapse rate times a constant efficiencyfactor. In our models, the derived star formation rate declinesexponentially with time, and radial profiles of atomic and molecular gasand star formation rate reproduce observed behavior. We derive from ourmodels and discuss both the global and local Schmidt laws for starformation: power-law relations between surface densities of gas and starformation rate. The global Schmidt law observed in disk galaxies isquantitatively reproduced by our models. We find that the surfacedensity of star formation rate directly correlates with the strength oflocal gravitational instability. The local Schmidt laws of individualgalaxies in our models show clear evidence of star formation thresholds.The variations in both the slope and the normalization of the localSchmidt laws cover the observed range. The averaged values agree wellwith the observed average and with the global law. Our results suggestthat the nonlinear development of gravitational instability determinesthe local and global Schmidt laws and the star formation thresholds. Wederive from our models the quantitative dependence of the global starformation efficiency on the initial gravitational instability ofgalaxies. The more unstable a galaxy is, the quicker and moreefficiently its gas collapses gravitationally and forms stars.
| Radio and Optical Follow-up Observations of a Uniform Radio Transient Search: Implications for Gamma-Ray Bursts and Supernovae
We present the first full characterization of the transient radio skyvia radio and optical follow-up observations of all the possible radiotransients we have discovered in a survey covering ~1/17 of the sky. Thetwo confirmed radio transients turn out to be an optically obscuredradio supernova (SN) in the nearby galaxy NGC 4216, the first such eventto be discovered by a wide-field radio survey, and a source notassociated with a bright host galaxy. We speculate that this secondsource may be a flare from a peculiar radio-loud AGN, or a burst from anunusual Galactic compact object, but its nature merits further study. Weplace an upper limit of 65 radio transients above 6 mJy over the entiresky (95% confidence level). The implications are as follows. First, wederive a limit on the typical beaming of GRBs; we findf-1b>~60, ~5 times higher than our earlierresults[f-1b≡(θ2jet/2)-1].Second, our results impose an upper limit on the rate of events thateject >~1051 ergs in unconfined relativistic ejecta,whether or not accompanied by detectable emission in wavebands otherthan the radio. Our estimated rate, n˙<=1000 yr-1Gpc-1, is about 2 orders of magnitude smaller than the rateof core-collapse SNe (and Type Ib/c events in particular), indicatingthat only a minority of such events eject significant amounts ofrelativistic material, which are required by fireball models oflong-soft GRBs. Finally, we show that wider and/or deeper radiovariability surveys are expected to detect numerous orphan radio GRBafterglows and illustrate the great potential of new radio instrumentsto revolutionize the study of nearby SNe.
| A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals
The scale lengths of the old stars and ionized gas distributions arecompared for similar samples of Virgo Cluster members and field spiralgalaxies via Hα and broad R-band surface photometry. While theR-band and Hα scale lengths are, on average, comparable for thecombined sample, we find significant differences between the field andcluster samples. While the Hα scale lengths of the field galaxiesare a factor of 1.14+/-0.07 longer, on average, than their R-band scalelengths, the Hα scale lengths of Virgo Cluster members are, onaverage, 20% smaller than their R-band scale lengths. Furthermore, inVirgo, the scale length ratios are correlated with the size of thestar-forming disk: galaxies with smaller overall Hα extents alsoshow steeper radial falloff of star formation activity. At the sametime, we find no strong trends in scale length ratio as a function ofother galaxy properties, including galaxy luminosity, inclination,morphological type, central R-band light concentration, or bar type. Ourresults for Hα emission are similar to other results for dustemission, suggesting that Hα and dust have similar distributions.The environmental dependence of the Hα scale length placesadditional constraints on the evolutionary process(es) that cause gasdepletion and a suppression of the star formation rate in clusters ofgalaxies.
| Thermal and non-thermal components of the interstellar medium at sub-kiloparsec scales in galaxies
Aims. We present new radio continuum observations of ten BIMA SONGgalaxies, taken at 1.4 GHz with the Very Large Array. These observationsallow us to extend the study of the relationships between the radiocontinuum (RC) and CO emission to 22 CO luminous galaxies for whichsingle dish CO images have been added to interferometric data. NewSpitzer infrared (IR) images of six of these galaxies have beenreleased. The analysis of these high resolution images allowed us toprobe the RC-IR-CO correlations down to linear scales of a few hundredpc. Methods: .We compare the point-by-point RC, CO and mid-IRintensities across entire galaxy disks, producing radial profiles andspatially resolved images of the RC/CO and RC/mid-IR ratios.Results: .For the 22 galaxies analysed, the RC-CO correlation on scalesfrom ~10 kpc down to ~100 pc is nearly linear and has a scatter of afactor of two, i.e. comparable to that of the global correlations. Thereis no evidence for any severe degradation of the scatter below the kpcscale. This also applies to the six galaxies for which high-resolutionmid-IR data are available. In the case of NGC 5194,we find that the non-thermal radio spectral index is correlated with theRC/FIR ratio. Conclusions: .The scatter of the point-by-pointcorrelations does not increase significantly with spatial resolution. Wethus conclude that we have not yet probed the physical scales at whichthe correlations break down. However, we observe local deviations fromthe correlations in regions with a high star formation rate, such as thespiral arms, where we observe a flat radio spectrum and a low RC/FIRratio. In the intra-arm regions and in the peripheral regions of thedisk, the RC/FIR is generally higher and it is characterized by asteepening of the radio spectrum.
| Toward a clean sample of ultra-luminous X-ray sources
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
| Abundance segregation in Virgo spiral galaxies
Aims.To derive O/H, N/O, and S/O abundances of ion{H}{ii} regionslocated in nine Virgo spiral galaxies, as well as to verify what is thecause of the abundance segregation found. Methods: .We employedphotoionization models to reproduce observed emission-line intensitiesof ion{H}{ii} regions located in the galaxies considered. The abundancegradients obtained were interpreted using a grid of chemical evolutionmodels. Results: .Our models indicate that galaxies near to thecore of the Virgo cluster are overabundant in O/H, N/O, and S/O by about0.25 dex in comparison to the ones at the periphery. With one exception,models with upper stellar mass limit of Mu = 30-40Mȯ and age of the ionizing star cluster ranging from 1.5to 2.5 Myr were able to reproduce the observational data. Chemicalmodels indicate the collapse time-scale for inner regions of galaxiesnear to the Virgo core is larger than the one in galaxies located at theintermediate and peripheral positions in the cluster, what can be due tothe dense environment existing in Virgo Cluster core.
| The extragalactic Cepheid bias: a new test using the period-luminosity-color relation
We use the Period-Luminosity-Color relation (PLC) for Cepheids to testfor the existence of a bias in extragalactic distances derived from theclassical Period-Luminosity (PL) relation. We calculate the parametersof the PLC using several galaxies observed with the Hubble SpaceTelescope and show that this calculation must be conducted with a PLCwritten in a form where the parameters are independent. The coefficientsthus obtained are similar to those derived from theoretical models.Calibrating with a few unbiased galaxies, we apply this PLC to allgalaxies of the Hubble Space Telescope Key Program (HSTKP) and comparethe distance moduli with those published by the HSTKP team. The newdistance moduli are larger (more exactly, the larger the distance thelarger the difference), consistent with a bias. Further, the bias trendthat is observed is the same previously obtained from two independentmethods based either on the local Hubble law or on a theoretical modelof the bias. The results are quite stable but when we force the PLCrelation closer to the classical PL relation by using unrealisticparameters, the agreement with HSTKP distance moduli is retrieved. Thisalso suggests that the PL relation leads to biased distance moduli. Thenew distance moduli reduce the scatter in the calibration of theabsolute magnitude of supernovae SNIa at their maximum. This may alsosuggest that the relation between the amplitude at maximum and the decayof the light curve Δ m15 may not be as strong asbelieved.
| A radio census of nuclear activity in nearby galaxies
In order to determine the incidence of black hole accretion-drivennuclear activity in nearby galaxies, as manifested by their radioemission, we have carried out a high-resolution Multi-ElementRadio-Linked Interferometer Network (MERLIN) survey of LINERs andcomposite LINER/Hii galaxies from a complete magnitude-limited sample ofbright nearby galaxies (Palomar sample) with unknown arcsecond-scaleradio properties. There are fifteen radio detections, of which three arenew subarcsecond-scale radio core detections, all being candidate AGN.The detected galaxies supplement the already known low-luminosity AGN -low-luminosity Seyferts, LINERs and composite LINER/Hii galaxies - inthe Palomar sample. Combining all radio-detected Seyferts, LINERs andcomposite LINER/Hii galaxies (LTS sources), we obtain an overall radiodetection rate of 54% (22% of all bright nearby galaxies) and weestimate that at least ~50% (~20% of all bright nearby galaxies) aretrue AGN. The radio powers of the LTS galaxies allow the construction ofa local radio luminosity function. By comparing the luminosity functionwith those of selected moderate-redshift AGN, selected from the 2dF/NVSSsurvey, we find that LTS sources naturally extend the RLF of powerfulAGN down to powers of about 10 times that of Sgr A*.
| 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.
| Galaxies, Clusters of Galaxies, and the Magnetic Field Interchange
Not Available
| Virgo High-Resolution CO Survey: V. Circumnuclear Elliptical Ring in NGC 4569
We present high-resolution (1''.8 ‑ 4''.5) CO data of the Virgospiral galaxy NGC 4569, obtained using the Nobeyama Millimeter Array. Wefound that the molecular gas is highly concentrated in the circumnuclearregion with two off-center peaks. A CO image with the highest angularresolution of 2''.0 × 1''.8 shows that six blobs likely form acircumnuclear elliptical ring (CER) with a semimajor axis of 720pc. TheCER shows a strongly twisted velocity field, and the position-velocitydiagram shows significant forbidden velocity components. These kineticfeatures are understood as being non-circular motion due to abar-potential. We found that the CER coincides with the Hα brightcentral core and that the mass ratio of the molecular gas to thedynamical mass is about 18%. These results support a gaseous inflowscenario induced by a weak bar potential and self-gravity of the gas.
| The Schmidt Law at High Molecular Densities
We combined Hα and recent high-resolution12CO(J=1‑0) data to consider the quantitative relationbetween the gas mass and the star-formation rate, or the so-calledSchmidt law in nearby spiral galaxies at regions of high moleculardensity. The relation between the gas quantity and the star-formationrate has not been previously studied for high-density regions, but usinghigh-resolution CO data obtained at the Nobeyama Millimeter Array, wefound that the Schmidt law is valid at densities as high as 103Modotpc-2 for sample spiral galaxies, which is anorder of magnitude denser than what has been known to be the maximumdensity at which the empirical law holds for non-starburst galaxies.Furthermore, we obtained a Schmidt law index of N = 1.33 ± 0.09and a roughly constant star-formation efficiency over the entire disk,even within several hundred parsecs of the nucleus. These results implythat the physics of star formation does not change in the centralregions of spiral galaxies. Comparisons with starburst galaxies are alsogiven. We find a possible discontinuity in the Schmidt law betweennormal and starburst galaxies.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
