תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
GALEX : seeing starbirth, near and far.
Not Available
| 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.
| Nuclear embedded star clusters in NGC 7582*
We report on the discovery of several compact regions of mid-infraredemission in the star-forming circumnuclear disc of the starburst/Seyfert2 galaxy NGC 7582. The compact sources do not have counterparts in theoptical and near-infrared, suggesting that they are deeply embedded indust. We use the [NeII] 12.8-μm line emission to estimate theemission measure of the ionized gas, which in turn is used to assess thenumber of ionizing photons. Two of the brighter sources are found tohave ionizing fluxes of ~2.5 × 1052, whereas thefainter ones have ~1 × 1052 photon s-1.Comparing with a 1-Myr-old starburst, we derive stellar masses in therange 3-5 × 105Msolar and find that thenumber of O stars in each compact source is typically 0.6-1.6 ×103. We conclude that the compact mid-infrared sources arelikely to be young, embedded star clusters, of which only a few areknown so far. Our observation highlights the need for high-resolutionmid-infrared imaging to discover and study embedded star clusters in theproximity of active galactic nuclei.
| Revisiting the infrared spectra of active galactic nuclei with a new torus emission model
We describe improved modelling of the emission by dust in atoroidal-like structure heated by a central illuminating source withinactive galactic nuclei (AGNs). We have chosen a simple but realistictorus geometry, a flared disc, and a dust grain distribution functionincluding a full range of grain sizes. The optical depth within thetorus is computed in detail taking into account the differentsublimation temperatures of the silicate and graphite grains, whichsolves previously reported inconsistencies in the silicate emissionfeature in type 1 AGNs. We exploit this model to study the spectralenergy distributions (SEDs) of 58 extragalactic (both type 1 and type 2)sources using archival optical and infrared data. We find that both AGNand starburst contributions are often required to reproduce the observedSEDs, although in a few cases they are very well fitted by a pure AGNcomponent. The AGN contribution to the far-infrared luminosity is foundto be higher in type 1 sources, with all the type 2 requiring asubstantial contribution from a circumnuclear starburst. Our resultsappear in agreement with the AGN unified scheme, because thedistributions of key parameters of the torus models turn out to becompatible for type 1 and type 2 AGNs. Further support to theunification concept comes from comparison with medium-resolutioninfrared spectra of type 1 AGNs by the Spitzer observatory, showingevidence for a moderate silicate emission around 10 μm, which ourcode reproduces. From our analysis we infer accretion flows in the innernucleus of local AGNs characterized by high equatorial optical depths(AV~= 100), moderate sizes (Rmax < 100 pc) andvery high covering factors (f~= 80 per cent) on average.
| XMM-Newton observations of bright ROSAT selected active galactic nuclei with low intrinsic absorption
We present a sample of 21 ROSAT bright active galactic nuclei (AGNs),representing a range of spectral classes, and selected for follow-upsnapshot observations with XMM-Newton. The typical exposure was between5 and 10ks. The objects were primarily selected on the bases of X-raybrightness and not on hardness ratio; thus the sample cannot be strictlydefined as a `soft'sample. One of the main outcomes from the XMM-Newtonobservations was that all of the AGN, including 11 type 1.8-2 objects,required low levels of intrinsic absorption (NH<~1021cm-2). The low absorption in type 2 systems isa challenge to account for in the standard orientation-based unificationmodel, and we discuss possible physical and geometrical models whichcould elucidate the problem. Moreover, there does not appear to be anyrelation between the strength and shape of the soft excess, and thespectral classification of the AGN in this sample. We further identify anumber of AGN which deserve deeper observations or further analysis: forexample, the low-ionization nuclear emission regions (LINERs) NGC 5005and NGC 7331, where optically thin thermal and extended emission isdetected, and the narrow-line Seyfert 1 II Zw 177, which shows a broademission feature at ~ 5.8keV.
| 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.
| On the Fraction of X-Ray-obscured Quasars in the Local Universe
Recent wide-area hard X-ray and soft gamma-ray surveys have shown thatthe fraction of X-ray-obscured active galactic nuclei (AGNs) in thelocal universe significantly decreases with intrinsic luminosity. Inthis Letter we point out that two corrections have to be made to thesamples: (1) radio-loud AGNs have to be excluded, since their X-rayemission might be dominated by the jet component, and (2) Compton-thicksources have to be excluded too, since their hard X-ray and softgamma-ray emission are also strongly attenuated by Compton scattering.The soft gamma-ray-selected AGN samples obtained by Swift and INTEGRALprovide the best opportunity to study the fraction of obscured AGNs inthe local universe in the least biased way. We choose these samples tocheck if the corrections could alter the above result on the fraction ofobscured AGNs. We find that before the corrections both samples showsignificant anticorrelation between LX and NH,indicating an obvious decrease in the fraction of obscured AGNs withluminosity. However, after the corrections, we find only marginalevidence of anticorrelation (at the 98% confidence level) in the Swiftsample and no evidence at all in the INTEGRAL sample, which consists ofa comparable number of objects. We conclude that current samples onlyshow a marginal decrease in the fraction of obscured AGNs in the localuniverse and that much larger samples are required in order to reach amore robust conclusion.
| Constraining Dark Matter Halo Profiles and Galaxy Formation Models Using Spiral Arm Morphology. I. Method Outline
We investigate the use of spiral arm pitch angles as a probe of diskgalaxy mass profiles. We confirm our previous result that spiral armpitch angles (P) are well correlated with the rate of shear (S) in diskgalaxy rotation curves by using a much larger sample (51 galaxies) thanused previously (17 galaxies). We use this correlation to argue thatimaging data alone can provide a powerful probe of galactic massdistributions out to large look-back times. In contrast to previouswork, we show that observed spiral arm pitch angles are similar whenmeasured in the optical (at 0.4 μm) and the near-infrared (at 2.1μm) with a mean difference of 2.3d+/-2.7d. This is then used tostrengthen the known correlation between P and S using B-band images. Wethen use two example galaxies to demonstrate how an inferred shear ratecoupled with a bulge-disk decomposition model and a Tully-Fisher-derivedvelocity normalization can be used to place constraints on a galaxy'sbaryon fraction and dark matter halo profile. We show that ESO 582-G12,a galaxy with a high shear rate (slightly declining rotation curve) at~10 kpc, favors an adiabatically contracted halo, with high initial NFWconcentration (cvir>16) and a high fraction of halobaryons in the form of stars (~15%-40%). In contrast, IC 2522 has a lowshear rate (rising rotation curve) at ~10 kpc and favorsnonadiabatically contracted models with low NFW concentrations(cvir~=2-8) and a low stellar baryon fraction <10%.
| The XMM-Newton Examination of Energetics in the East Lobe of the Nearby Radio Galaxy Fornax A (NGC 1316)
An XMM-Newton observation of the east radio lobe of the nearby radiogalaxy Fornax A is reported. The diffuse hard X-ray emission associatedwith the east lobe, which was initially discovered by ASCA and ROSAT, isconfirmed with significant signal statistics, after strictly removing 59sources detected within the MOS field of view. Its X-ray spectrum isdescribed by a single power-law model, which is absorbed by a mediumwith a column density consistent with that toward the object. Thebest-fit X-ray photon index,ΓX=1.62+0.24-0.15, agrees withthe synchrotron radio index, ΓR=1.68+/-0.1, determinedfrom the radio spectrum between 29.9 MHz and 5 GHz. Hence, the inverseCompton interpretation for the diffuse X-rays is justified. The X-rayflux density in the east lobe is measured to be 90+/-21 nJy at 1 keV(including both statistical and systematic errors) with the index fixedat the radio value. This gives electron and magnetic energy densities of3.0+1.5-1.0×10-13 and6.1+5.7-3.5×10-14 ergscm-3, respectively. The latter corresponds to a magneticfield strength of 1.24+0.50-0.40 μG, which issmaller than the field estimated under the minimum energy condition,1.55 μG, although with a slightly large error. Reevaluation is alsomade of the ASCA result on the west lobe, to show that both lobes sharea similar physical condition in terms of energetics.
| Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations
We use very deep Spitzer MIPS 24 μm observations to examine thebolometric luminosities (Lbol) and UV extinction propertiesof more than 200 spectroscopically identified, optically selected(UnGR) z~2 galaxies, supplemented with near-IR-selected(``BzK'' and ``DRG'') and submillimeter galaxies at similar redshifts,in the GOODS-N field. Focusing on redshifts 1.51012 Lsolar, with a mean~=2×1011 Lsolar. Using24 μm observations as an independent probe of dust extinction, wefind that, as in the local universe, the obscurationLIR/L1600 is strongly dependent on Lboland ranges in value from <1 to ~1000 within the sample considered.However, the obscuration is generally ~10 times smaller at a givenLbol at z~2 than at z~0. We show that the values ofLIR and obscuration inferred from the UV spectral slopeβ generally agree well with the values inferred fromL5-8.5μm for Lbol<1012Lsolar. Using the specific SFRs of galaxies as a proxy forcold gas fraction, we find a wide range in the evolutionary state ofgalaxies at z~2, from galaxies that have just begun to form stars tothose that have already accumulated most of their stellar mass and areabout to become, or already are, passively evolving.Based, in part, on data obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and NASA and was made possibleby the generous financial support of the W. M. Keck Foundation. Alsobased in part on observations made with the Spitzer Space Telescope,which is operated by the Jet Propulsion Laboratory, California Instituteof Technology, under a contract with NASA.
| First Results from SAPAC: Toward a Three-dimensional Picture of the Fornax Cluster Core
A sophisticated surface brightness fluctuation (SBF) analysis packagehas been developed, designed to measure distances of early-type galaxiesby means of SBFs of unresolved stars. This suite of programs, calledSAPAC, is made readily available to the astronomical community forextensive testing, with the long-term goal of providing the necessarytools for systematic distance surveys of early-type galaxies usingmodern optical/near-IR telescopes equipped with wide-field cameras. Wediscuss the technical and scientific concepts of SAPAC and demonstrateits capabilities by analyzing deep B- and R-band CCD images of 10 dwarfelliptical galaxy candidates in the Fornax Cluster obtained with FORS1at the Very Large Telescope. All candidates are confirmed as clustermembers. We then turn our attention to the innermost region of theFornax Cluster. A total of 29 early-type galaxies closer than threecluster core radii (2deg) to the central galaxy NGC 1399 haveradial velocities and SBF distances. Their Hubble diagram exhibits apronounced S-shaped infall pattern, suggesting that Fornax is still inthe process of formation during the present epoch through a generalcollapse and possible accretion of distinct groups of galaxies. Fromfitting a model we estimate the cluster mass within 720 kpc projecteddistance of NGC 1399 to be 2.3+/-0.3×1014Msolar. The associated collapse time istcoll=2.9+1.6-0.9 Gyr. After cleansing our galaxy sample of afew kinematical outliers, the true distance of the Fornax Cluster coreis determined at 20.13+/-0.40 Mpc [(m-M)0=31.51+/-0.04 mag].Applying a bootstrap resampling technique on the distance distributionwith individual distance errors taken into account further reveals asmall intrinsic cluster depth of σint=0.74+0.52-0.74Mpc, in best agreement with the cluster's linear extension in the sky:σR.A.=σdecl.~0.5 Mpc. We conclude thatthe early-type galaxy population in the Fornax Cluster must be spatiallywell constrained, with no evidence of elongation along the line ofsight, in contrast to the Virgo Cluster. Moreover, we find marginalevidence for substructure, a result that is consistent with the youngevolutionary state of the cluster and the overall galaxy infall.Combining the kinematically defined cluster distance with the meancosmological velocity for the central cluster galaxy sample yields aHubble constant of H0=63+/-5 km s-1Mpc-1.Based on observations collected at the ESO Very Large Telescope, underprogram ESO 68.A-0176.
| Swift Observations of the Highly X-Ray Variable Narrow-Line Seyfert 1 Galaxy RX J0148.3-2758
We report on Swift observations of the narrow-line Seyfert 1 galaxy RXJ0148.3-2758. It was observed for 41.6 ks in 2005 May and for 15.8 ks in2005 December. On short as well as on long timescales, RX J0148.3-2758is a highly variable source. It doubles its X-ray flux within 18-25 ks.The observation of 2005 December 9, which had a flux 4 times lower thanduring the 2005 May observations, shows a significant hardening of theX-ray hardness ratio compared with the 2005 May and December 20 and 21observations. A detailed analysis of the X-ray spectra shows that weactually observe two spectral changes in RX J0148.3-2758: first, adecrease of the soft X-ray component between 2005 May and December 9,which is most likely due to an increase of the intrinsic absorbercolumn, and second, a decrease of the hard X-ray flux in the December 20and 21 observations. The soft X-ray spectral slopeαX,soft=2.58+0.15-0.12 during thehigh state in 2005 May agrees well with that measured by ROSAT(αX,soft=2.54+/-0.82). This soft X-ray spectrum issuperposed on a hard X-ray component withαX,hard=0.96+0.16-0.14, which isconsistent with the hard X-ray spectral slopeαX,hard=1.11+0.16-0.19 found byASCA. The soft X-ray slopeαX,soft=1.93+0.58-0.42 measuredduring the December 9 observation agrees well withαX,soft=2.03+0.23-0.20 measuredfrom the ASCA observation when RX J0148.3-2758 was also in a low state.In contrast to the strong X-ray variability, the analysis of the SwiftUV-Optical Telescope (UVOT) photometry from 2005 December of RXJ0148.3-2758 shows no significant variability in any of the six UVOTfilters. From the simultaneous X-ray and UV observations in 2005December we measured the X-ray loudness αox and foundit to vary between αox=1.5 and 1.8. Our Swiftobservations of RX J0148.3-2758 demonstrate the great potential that themultiwavelength observatory Swift has for active galactic nucleusscience.
| Spitzer IRS Spectra of a Large Sample of Seyfert Galaxies: A Variety of Infrared Spectral Energy Distributions in the Local Active Galactic Nucleus Population
We are conducting a large observing program with the Spitzer SpaceTelescope to determine the mid- to far-IR spectral energy distributionsof a well-defined sample of 87 nearby, 12 μm-selected Seyfertgalaxies. In this paper we present the results of Spitzer IRSlow-resolution spectroscopy of a statistically representative subsampleof 51 of the galaxies (59%), with an analysis of the continuum shapesand a comparison of the Seyfert types. We find that the spectra clearlydivide into groups based on their continuum shapes and spectralfeatures. The largest group (47% of the sample of 51) shows a very redcontinuum suggestive of cool dust and strong emission featuresattributed to polycyclic aromatic hydrocarbons. Sixteen objects (31%)have a power-law continuum with spectral indices of α5-20μm=-2.3 to -0.9 that flatten to α20-35μm=-1.1 to 0.0 at ~20 μm. Clear silicate emission featuresat 10 and 18 μm are found in two of these objects (Mrk 6 and Mrk335). A further 16% of the sample show power-law continua withunchanging slopes of α5-35 μm=-1.7 to -1.1. Twoobjects are dominated by a broad silicate absorption feature. One objectin the sample shows an unusual spectrum dominated by emission features,which is unlike any of the other spectra. Some spectral features areclearly related to a starburst contribution to the IR spectrum, whilethe mechanisms producing observed power-law continuum shapes, attributedto an active galactic nucleus (AGN) component, may be dust or nonthermalemission. The IR spectral types appear to be related to the Seyferttypes. Principal component analysis results suggest that the relativecontribution of starburst emission may be the dominant cause of variancein the observed spectra. The derived starburst component of eachspectrum, however, contributes <40% of the total flux density. Wecompare the IR emission with the optically thin radio emissionassociated with the AGN and find that Seyfert 1 galaxies have higherratios of IR to radio emission than Seyfert 2 galaxies, as predicted bythe unified model if the torus is optically thick in the mid-IR.However, smooth-density torus models predict a much larger differencebetween Seyfert types 1 and 2 than the factor of 2 difference observedin our sample; the observed factor of ~2 difference between the type 1and type 2 galaxies in their IR-to-radio ratios above 15 μm requiresthe standard smooth-density torus models to be optically thin at thesewavelengths. However, the resulting low torus opacity requires that thehigh observed columns detected in X-ray absorption be produced in gaswith a very low dust-to-gas ratio (perhaps within the dust sublimationregion). On the other hand, our observations may be consistent withclumpy torus models containing a steep radial distribution of opticallythick dense clumps. The selection of our sample at 12 μm, where thetorus may be optically thick, implies that there may beorientation-dependent biases in the sample; however, we do not find thatthe sample is biased toward Seyfert 2 galaxies with more luminouscentral engines, as would be expected. We find that the Seyfert 2galaxies typically show stronger starburst contributions than theSeyfert 1 galaxies in the sample, contrary to what is expected based onthe unified scheme for AGNs. This may be due to the selection effectthat only those Seyfert 2 galaxies with strong starburst contributionshad high enough integrated 12 μm flux densities to fall above theflux limit of the sample.
| Absolute Magnitude Distributions and Light Curves of Stripped-Envelope Supernovae
The absolute visual magnitudes of three Type IIb, 11 Type Ib, and 13Type Ic supernovae (collectively known as stripped-envelope supernovae)are studied by collecting data on the apparent magnitude, distance, andinterstellar extinction of each event. Weighted and unweighted meanabsolute magnitudes of the combined sample, as well as various subsetsof the sample, are reported. The limited sample size and theconsiderable uncertainties, especially those associated with extinctionin the host galaxies, prevent firm conclusions regarding differencesbetween the absolute magnitudes of supernovae of Types Ib and Ic, andregarding the existence of separate groups of overluminous andnormal-luminosity stripped-envelope supernovae. The spectroscopiccharacteristics of the events of the sample are considered. Three of thefour overluminous events are known to have had unusual spectra. Most butnot all of the normal-luminosity events have had typical spectra. Thelight curves of stripped-envelope supernovae are collected and compared.Because SN 1994I in M51 was very well observed, it often is regarded asthe prototypical Type Ic supernova, but it has the fastest light curvein the sample. Light curves are modeled by means of a simple analyticaltechnique that, combined with a constraint on E/M from spectroscopy,yields internally consistent values of ejected mass, kinetic energy, andnickel mass.
| 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.
| 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.
| Mid infrared properties of distant infrared luminous galaxies
We present evidence that the mid infrared (MIR, rest frame 5-30 μm)is a good tracer of the total infrared luminosity, L(IR)(=L[8{-}1000μm]), and star formation rate (SFR), of galaxies up to z 1.3. Weuse deep MIR images from the Infrared Space Observatory (ISO) and theSpitzer Space Telescope in the Northern field of the Great ObservatoriesOrigins Deep Survey (GOODS-N) together with VLA radio data to computethree independant estimates of L(IR). The L(IR, MIR) derived from theobserved 15 and/or 24 μm flux densities using a library of templateSEDs, and L(IR, radio), derived from the radio (1.4 and/or 8.5 GHz)using the radio-far infrared correlation, agree with a 1-σdispersion of 40%. We use the k-correction as a tool to probe differentparts of the MIR spectral energy distribution (SED) of galaxies as afunction of their redshift and find that on average distant galaxiespresent MIR SEDs very similar to local ones. However, in the redshiftrange z= 0.4-1.2, L(IR, 24 μm) is in better agreement with L(IR,radio) than L(IR, 15 μm) by 20%, suggesting that the warm dustcontinuum is a better tracer of the SFR than the broad emission featuresdue to polycyclic aromatic hydrocarbons (PAHs). We find marginalevidence for an evolution with redshift of the MIR SEDs: two thirds ofthe distant galaxies exhibit rest-frame MIR colors (L(12 μm)/L(7μm) and L(10 μm)/L(15 μm) luminosity ratios) below the medianvalue measured for local galaxies. Possible explanations are examinedbut these results are not sufficient to constrain the physics of theemitting regions. If confirmed through direct spectroscopy and if itgets amplified at higher redshifts, such an effect should be consideredwhen deriving cosmic star formation histories of dust-obscured galaxies.We compare three commonly used SED libraries which reproduce thecolor-luminosity correlations of local galaxies with our data anddiscuss possible refinements to the relative intensities of PAHs, warmdust continuum and silicate absorption.
| ISM properties in low-metallicity environments
We present new ISOCAM mid-infrared spectra of three starbursting nearbydwarf galaxies, NGC 1569, NGC 1140 and II Zw 40 and the 30 Dor region ofthe LMC and explore the properties of the ISM in low-metallicityenvironments, also using additional sources from the literature. Weanalyse the various components of the ISM probed by the mid-infraredobservations and compare them with other Galactic and extragalacticobjects. The MIR spectra of the low-metallicity starburst sources aredominated by the [ Ne iii] λ 15.56~μ m and [ S iv] λ10.51~μ m lines, as well as a steeply rising dust continuum. PAHbands are generaly faint, both locally and averaged over the fullgalaxy, in stark contrast to dustier starburst galaxies, where the PAHfeatures are very prominant and even dominate on global scales. Thehardness of the modeled interstellar radiation fields for the dwarfgalaxies increases as the presence of PAH band emission becomes lesspronounced. The [ Ne iii] /[ Ne ii] ratios averaged over the full galaxyare strikingly high, often >10. Thus, the hard radiation fields arepronounced and pervasive. We find a prominent correlation between thePAHs/VSGs and the [ Ne iii] /[ Ne ii] ratios for a wide range ofobjects, including the low metallicity galaxies as well as Galactic H iiregions and other metal-rich galaxies. This effect is consistent withthe hardness of the interstellar radiation field playing a major role inthe destruction of PAHs in the low metallicity ISM. We see a PAHs/VSGsand metallicity correlation, also found by Engelbracht et al. (2005,ApJ, 628, 29) for a larger survey. Combined effects of metallicity andradiation field seem to be playing important roles in the observedbehavior of PAHs in the low metallicity systems.
| Extragalactic Science with Tunable Filters
Tunable filters provide unique capabilities to carry out a wide array ofextragalactic projects. The emphasis of this review is on sciencerelating to starburst and active galaxies. Future avenues of researchwith 8-meter class telescopes equipped with tunable filters are alsodiscussed briefly.
| Aperture Effects on Star Formation Rate, Metallicity, and Reddening
We use 101 galaxies selected from the Nearby Field Galaxy Survey toinvestigate the effect of aperture size on the star formation rate,metallicity, and reddening determinations for galaxies. Our sampleincludes galaxies of all Hubble types except ellipticals with globalstar formation rates (SFRs) ranging from 0.01 to 100 Msolaryr-1, metallicities in the range 7.9<~log(O/H)+12<~9.0,and reddening of 0<~A(V)<~3.3. We compare the SFR, metallicity,and reddening derived from nuclear spectra to those derived fromintegrated spectra. For apertures capturing <20% of theB26 light, the differences between nuclear and globalmetallicity, extinction, and SFR are substantial. Late-type spiralgalaxies show the largest systematic difference (~0.14 dex), withnuclear metallicities greater than the global metallicities. The Sdm,Im, and Peculiar types have the largest scatter in nuclear/integratedmetallicities, indicating a large range in metallicity gradients forthese galaxy types, or clumpy metallicity distributions. We find littleevidence for systematic differences between nuclear and globalextinction estimates for any galaxy type. However, there is significantscatter between the nuclear and integrated extinction estimates fornuclear apertures containing <20% of the B26 flux. Wecalculate an ``expected'' SFR using our nuclear spectra and apply thecommonly used aperture correction method. The expected SFR overestimatesthe global value for early-type spirals, with large scatter for allHubble types, particularly late types. The differences between theexpected and global SFRs probably result from the assumption that thedistributions of the emission-line gas and the continuum are identical.The largest scatter (error) in the estimated SFR occurs when theaperture captures <20% of the B26 emission. We discuss theimplications of these results for metallicity-luminosity relations andstar formation history studies based on fiber spectra. To reducesystematic and random errors from aperture effects, we recommendselecting samples with fibers that capture >20% of the galaxy light.For the Sloan Digital Sky Survey and the Two-Degree Field GalaxyRedshift Survey, redshifts z>0.04 and 0.06 are required,respectively, to ensure a covering fraction >20% for galaxy sizessimilar to the average size, type, and luminosity observed in oursample. Higher luminosity samples and samples containing many late-typegalaxies require a larger minimum redshift to ensure that >20% of thegalaxy light is enclosed by the fiber.
| The lifetime of galactic bars: central mass concentrations and gravity torques
Bars in gas-rich spiral galaxies are short-lived. They drive gas inflowsthrough their gravity torques, and at the same time self-regulate theirstrength. Their robustness has been subject of debate, as it was thoughtthat only the resulting central mass concentrations (CMCs) wereweakening bars, and only relatively rare massive CMCs were able tocompletely destroy them. Through numerical simulations including gasdynamics, we find that with the gas parameters of normal spiralgalaxies, the CMC is not sufficient to fully dissolve the bar. Butanother overlooked mechanism, the transfer of angular momentum from theinfalling gas to the stellar bar, can also strongly weaken the bar. Inaddition, we show that gravity torques are correctly reproduced insimulations, and conclude that bars are transient features, withlifetime of 1-2 Gyr in typical Sb-Sc galaxies, because of the combinedeffects of CMCs and gravity torques, while most existing works hadfocussed on the CMC effects alone.
| How large are the bars in barred galaxies?
I present a study of the sizes (semimajor axes) of bars in discgalaxies, combining a detailed R-band study of 65 S0-Sb galaxies withthe B-band measurements of 70 Sb-Sd galaxies from Martin (1995). As hasbeen noted before with smaller samples, bars in early-type (S0-Sb)galaxies are clearly larger than bars in late-type (Sc-Sd) galaxies;this is true both for relative sizes (bar length as fraction ofisophotal radius R25 or exponential disc scalelength h) andabsolute sizes (kpc). S0-Sab bars extend to ~1-10 kpc (mean ~ 3.3 kpc),~0.2-0.8R25 (mean ~ 0.38R25) and ~0.5-2.5h (mean ~1.4h). Late-type bars extend to only ~0.5-3.5 kpc,~0.05-0.35R25 and 0.2-1.5h their mean sizes are ~1.5 kpc, ~0.14R25 and ~0.6h. Sb galaxies resemble earlier-type galaxiesin terms of bar size relative to h; their smallerR25-relative sizes may be a side effect of higher starformation, which increases R25 but not h. Sbc galaxies form atransition between the early- and late-type regimes. For S0-Sbcgalaxies, bar size correlates well with disc size (both R25and h); these correlations are stronger than the known correlation withMB. All correlations appear to be weaker or absent forlate-type galaxies; in particular, there seems to be no correlationbetween bar size and either h or MB for Sc-Sd galaxies.Because bar size scales with disc size and galaxy magnitude for mostHubble types, studies of bar evolution with redshift should selectsamples with similar distributions of disc size or magnitude(extrapolated to present-day values); otherwise, bar frequencies andsizes could be mis-estimated. Because early-type galaxies tend to havelarger bars, resolution-limited studies will preferentially find bars inearly-type galaxies (assuming no significant differential evolution inbar sizes). I show that the bars detected in Hubble Space Telescope(HST) near-infrared(IR) images at z~ 1 by Sheth et al. have absolutesizes consistent with those in bright, nearby S0-Sb galaxies. I alsocompare the sizes of real bars with those produced in simulations anddiscuss some possible implications for scenarios of secular evolutionalong the Hubble sequence. Simulations often produce bars as large as(or larger than) those seen in S0-Sb galaxies, but rarely any as smallas those in Sc-Sd galaxies.
| Bar-induced evolution of dark matter cusps
The evolution of a stellar bar transforms not only the galactic disc,but also the host dark matter halo. We present high-resolution, fullyself-consistent N-body simulations that clearly demonstrate that darkmatter halo central density cusps flatten as the bar torques the halo.This effect is independent of the bar formation mode and occurs even forrather short bars. The halo and bar evolution is mediated by resonantinteractions between orbits in the halo and the bar pattern speed, aspredicted by linear Hamiltonian perturbation theory. The bar lengthensand slows as it loses angular momentum, a process that occurs even inrather warm discs. We demonstrate that the bar and halo response can becritically underestimated for experiments that are unable to resolve therelevant resonant dynamics; this occurs when the phase space in theresonant region is undersampled or plagued by noise.
| Imprints of spiral arms in the oxygen distribution over the galactic disc
A theory for the oxygen abundance radial distribution formation in thegalactic disc of a spiral galaxy is developed. We take into account thatthe main sources of oxygen are Type II supernovae (SN II), theprogenitors of which are massive short-lived stars strongly concentratedin the spiral arms. Hence oxygen is the most sensitive indicator ofspiral arms' influence on galactic disc enrichment by heavy elements.Various models for the spiral density waves were analysed. We predictthat the imprints in the oxygen radial distribution will enable us todistinguish between different models for spiral patterns. Among otherparameters, the corotation radius happens to be one of the mostimportant.
| An atlas of calcium triplet spectra of active galaxies
We present a spectroscopic atlas of active galactic nuclei covering theregion around the λλ8498, 8542, 8662 calcium triplet(CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26Seyfert 1s, three starburst and six normal galaxies. The spectra pertainto the inner ~300 pc in radius, and thus sample the central kinematicsand stellar populations of active galaxies. The data are used to measurestellar velocity dispersions (σ*) with bothcross-correlation and direct fitting methods. These measurements arefound to be in good agreement with each other and with those in previousstudies for objects in common. The CaT equivalent width is alsomeasured. We find average values and sample dispersions ofWCaT of 4.6 +/- 2.0, 7.0 +/- 1.0 and 7.7 +/- 1.0 Å forSeyfert 1s, Seyfert 2s and normal galaxies, respectively. We furtherpresent an atlas of [SIII]λ9069 emission-line profiles for asubset of 40 galaxies. These data are analysed in a companion paperwhich addresses the connection between stellar and narrow-line regionkinematics, the behaviour of the CaT equivalent width as a function ofσ*, activity type and stellar population properties.
| On the transmission-dominated to reprocessing-dominated spectral state transitions in Seyfert 2 galaxies
We present Chandra and XMM-Newton observations of a small sample (11objects) of optically selected Seyfert 2 galaxies, for which ASCA andBeppoSAX had suggested Compton-thick obscuration of the active galacticnucleus (AGN). The main goal of this study is to estimate the rate oftransitions between `transmission-dominated' and`reprocessing-dominated' states. We discover one new transition in NGC4939, with a possible additional candidate in NGC 5643. This indicates atypical occurrence rate of at least ~0.02yr-1. Thesetransitions could be due to large changes of the obscuring gas columndensity, or to a transient dimming of the AGN activity, the latterscenario being supported by detailed analysis of the best-studiedevents. Independently of the ultimate mechanism, comparison of theobserved spectral dynamics with Monte Carlo simulations demonstratesthat the obscuring gas is largely inhomogeneous, with multiple absorbingcomponents possibly spread through the whole range of distances from thenucleus between a fraction of parsecs up to several hundred parsecs. Asa by-product of this study, we report the first measurement ever of thecolumn density covering the AGN in NGC 3393 (NH~= 4.4 ×1024cm-2), and the discovery of soft X-rayextended emission, apparently aligned along the host galaxy main axis inNGC 5005. The latter object most likely hosts an historicallymisclassified low-luminosity Compton-thin AGN.
| Supermassive Black Holes: Relation to Dark Halos
Estimates of the masses of supermassive black holes (M bh ) in thenuclei of disk galaxies with known rotation curves are compared withestimates of the rotational velocities V m and the“indicative” masses of the galaxies M i . Although there isa correlation between M bh and V m or M i , it is appreciably weakerthan the correlation with the central velocity dispersion. The values ofM bh for early-type galaxies (S0-Sab), which have more massive bulges,are, on average, higher than the values for late-type galaxies with thesame rotational velocities. We conclude that the black-hole masses aredetermined primarily by the properties of the bulge and not therotational velocity or the mass of the galaxy.
| Galactic Winds
Galactic winds are the primary mechanism by which energy and metals arerecycled in galaxies and are deposited into the intergalactic medium.New observations are revealing the ubiquity of this process,particularly at high redshift. We describe the physics behind thesewinds, discuss the observational evidence for them in nearbystar-forming and active galaxies and in the high-redshift universe, andconsider the implications of energetic winds for the formation andevolution of galaxies and the intergalactic medium. To inspire futureresearch, we conclude with a set of observational and theoreticalchallenges.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| The Swift/BAT High-Latitude Survey: First Results
We present preliminary results from the first 3 months of the SwiftBurst Alert Telescope (BAT) high Galactic latitude survey in the 14-195keV band. The survey reaches a flux of ~10-11 ergscm-2 s-1 and has ~2.7 arcmin (90% confidence)positional uncertainties for the faintest sources. This represents themost sensitive survey to date in this energy band. These data confirmthe conjectures that a high-energy-selected active galactic nucleus(AGN) sample would have very different properties from those selected inother bands and that it represents a ``true'' sample of the AGNpopulation. We have identified 86% of the 66 high-latitude sources.Twelve are Galactic-type sources, and 44 can be identified withpreviously known AGNs. All but five of the AGNs have archival X-rayspectra, enabling us to estimate the line-of-sight column densities andother spectral properties. Both of the z>0.11 objects are blazars.The median redshift of the others (excluding radio-loud objects) is0.012. We find that the column density distribution of these AGNs isbimodal, with 64% of the nonblazar sources having column densitiesNH>=1022 cm-2. None of the sourceswith logLX>43.5 (cgs units) show high column densities,and very few of the lower LX sources have low columndensities. Based on these data, we expect the final BAT catalog to have>200 AGNs and reach fluxes of less than ~10-11 ergscm-2 s-1 over the entire sky.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
קטלוגים וכינוים:
|
INHABITED SKY
