Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Programs for laser-AO assisted integral-field spectrometers on ionized flows
An AO-assisted integral-field spectrograph is becoming the mostefficient tool with which to explore ionized gas outflows. It maps faintspectral lines that diagnose cloud dust content, gas pressure,excitation mechanism, and chemical abundances. Coupled with recentimprovements in photoionization models, the total mass hence flowenergetics can be estimated. Establishing a consistent dynamicalframework requires linking multi-frequency datasets to track the energyflow through its optimal-contrast emission in the various ISM phases. Ishow HST results on AGN, starburst nuclei, and Galactic Herbig-HaroObjects that need complementary 3D spectra at comparable spatialresolution to come soon from laser-guided AO + integral-fieldspectrographs at the William-Herschel and SOAR telescopes.
| Multiwavelength Star Formation Indicators: Observations
We present a compilation of multiwavelength data on different starformation indicators for a sample of nearby star forming galaxies. Herewe discuss the observations, reductions and measurements of ultravioletimages obtained with STIS on board the Hubble Space Telescope (HST),ground-based Hα, and VLA 8.46 GHz radio images. These observationsare complemented with infrared fluxes, as well as large-apertureoptical, radio, and ultraviolet data from the literature. This databasewill be used in a forthcoming paper to compare star formation rates atdifferent wave bands. We also present spectral energy distributions(SEDs) for those galaxies with at least one far-infrared measurementsfrom ISO, longward of 100 μm. These SEDs are divided in two groups,those that are dominated by the far-infrared emission, and those forwhich the contribution from the far-infrared and optical emission iscomparable. These SEDs are useful tools to study the properties ofhigh-redshift galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| 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.
| Chandra Observations of Gas Stripping in the Elliptical Galaxy NGC 4552 in the Virgo Cluster
We use a 54.4 ks Chandra observation to study ram pressure stripping inNGC 4552 (M89), an elliptical galaxy in the Virgo Cluster. Chandraimages in the 0.5-2 keV band show a sharp leading edge in the surfacebrightness 3.1 kpc north of the galaxy center, a cool(kT=0.51+0.09-0.06 keV) tail with mean densityne~(5.4+/-1.7)×10-3 cm-3extending ~10 kpc to the south of the galaxy, and two 3-4 kpc horns ofemission extending southward away from the leading edge. These are allfeatures characteristic of supersonic ram pressure stripping of galaxygas, due to NGC 4552's motion through the surrounding Virgo ICM. Fittingthe surface brightness profile and spectra across the leading edge, wefind the galaxy gas inside the edge is cooler(kT=0.43+0.03-0.02 keV) and denser(ne~0.010 cm-3) than the surrounding Virgo ICM[kT=2.2+0.7-0.4 keV andne=(3.0+/-0.3)×10-4 cm-3]. Theresulting pressure ratio between the free-streaming ICM and cluster gasat the stagnation point is ~7.6+3.4-2.0 for galaxygas metallicities of 0.5+0.5-0.3Zsolar, which suggests that NGC 4552 is moving supersonicallythrough the cluster with a velocity v~1680+390-220km s-1 (Mach 2.2+0.5-0.3) at an angleξ~35deg+/-7deg toward us with respect to theplane of the sky.
| Ultraviolet-to-Far-Infrared Properties of Local Star-forming Galaxies
We present the results of a multiwavelength study of nearby galaxiesaimed at understanding the relation between the ultraviolet andfar-infrared emission in star-forming galaxies. The data set comprisesnew ultraviolet (from HST STIS), ground-based Hα, and radiocontinuum observations, together with archival infrared data (from IRASand ISO). The local galaxies are used as benchmarks for comparison ofthe infrared-to-ultraviolet properties with two populations ofhigh-redshift galaxies: the submillimeter star-forming galaxies detectedby SCUBA and the ultraviolet-selected Lyman break galaxies (LBGs). Inaddition, the long wavelength baseline covered by the present dataenables us to compare the star formation rates (SFRs) derived from theobserved ultraviolet, Hα, infrared, and radio luminosities and togauge the impact of dust opacity in the local galaxies. We also derive anew calibration for the nonthermal part of the radio SFR estimator,based on the comparison of 1.4 GHz measurements with a new estimator ofthe bolometric luminosity of the star-forming regions. We find that moreactively star-forming galaxies show higher dust opacities, which is inline with previous results. We find that the local star-forming galaxieshave a lower Fλ(205 μm)/Fλ(UV)ratio by 2-3 orders of magnitude than the submillimeter-selectedgalaxies and may have a similar or somewhat higherFλ(205 μm)/Fλ(UV) ratio thanLBGs. The Fλ(205 μm)/Fλ(UV) ratioof the local galaxy population may be influenced by the cool dustemission in the far-infrared heated by nonionizing stellar populations,which may be reduced or absent in the LBGs.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Discovery of a 500 Parsec Shell in the Nucleus of Centaurus A
Spitzer Space Telescope mid-infrared images of the radio galaxyCentaurus A reveal a shell-like, bipolar, structure 500 pc to the northand south of the nucleus. This shell is seen in 5.8, 8.0, and 24 μmbroadband images. Such a remarkable shell has not been previouslydetected in a radio galaxy and, if confirmed, would be the firstextragalactic nuclear shell detected at mid-infrared wavelengths.Assuming that it is a coherent expanding structure, we estimate that theshell is a few million years old and has a mass on the order of amillion solar masses. A conservative estimate for the mechanical energyin the wind-driven bubble is 1053 ergs. The shell could havebeen created by a small, few-thousand solar mass, nuclear burst of starformation. Alternatively, the bolometric luminosity of the activenucleus is sufficiently large that it could power the shell. Constraintson the shell's velocity are lacking. However, if the shell is moving at1000 km s-1, then the required mechanical energy would be 100times larger.
| Late-Time Radio Observations of 68 Type Ibc Supernovae: Strong Constraints on Off-Axis Gamma-Ray Bursts
We present late-time radio observations of 68 local Type Ibc supernovae,including six events with broad optical absorption lines(``hypernovae''). None of these objects exhibit radio emissionattributable to off-axis gamma-ray burst jets spreading into our line ofsight. Comparison with our afterglow models reveals the followingconclusions. (1) Less than ~10% of Type Ibc supernovae are associatedwith typical gamma-ray bursts initially directed away from our line ofsight; this places an empirical constraint on the GRB beaming factor of<~104, corresponding toan average jet opening angle, θj>~0.8d. (2) Thisholds in particular for the broad-lined supernovae (SNe 1997dq, 1997ef,1998ey, 2002ap, 2002bl, and 2003jd), which have been argued to host GRBjets. Our observations reveal no evidence for typical (or evensubenergetic) GRBs and rule out the scenario in which every broad-linedSN harbors a GRB at the 84% confidence level. Their large photosphericvelocities and asymmetric ejecta (inferred from spectropolarimetry andnebular spectroscopy) appear to be characteristic of the nonrelativisticSN explosion and do not necessarily imply the existence of associatedGRB jets.
| Discovery of Water Maser Emission in Eight AGNs with 70 m Antennas of NASA's Deep Space Network
We report the discovery of water maser emission in eight active galacticnuclei (AGNs) with the 70 m NASA Deep Space Network (DSN) antennas atTidbinbilla, Australia, and Robledo, Spain. The positions of the newlydiscovered masers, measured with the VLA, are consistent with theoptical positions of the host nuclei to within 1 σ (0.3" radio and1.3" optical) and most likely mark the locations of the embedded centralengines. The spectra of two sources, NGC 3393 and NGC 5495, display thecharacteristic spectral signature of emission from an edge-on accretiondisk, with orbital velocities of ~600 and ~400 km s-1,respectively. In a survey with DSN facilities of 630 AGNs selected fromthe NASA Extragalactic Database, we have discovered a total of 15 watermaser sources. The resulting incidence rate of maser emission amongnearby (vsys<7000 km s-1) Seyfert 1.8-2.0 andLINER systems is ~10% for a typical rms noise level of ~14 mJy over 1.3km s-1 spectral channels. As a result of this work, thenumber of nearby AGNs (vsys<7000 km s-1)observed with <20 mJy rms noise has increased from 130 to 449.
| X-Ray Emission from Megamaser Galaxy IC 2560
An observation of the H2O megamaser galaxy IC 2560 with theChandra X-Ray Observatory reveals a complex spectrum composed of softX-ray emission due to multitemperature thermal plasma and a hardcontinuum with strong emission lines. The continuum is most likely aCompton reflection (reprocessing) of primary emission that is completelyabsorbed at least up to 7 keV. The lines can be identified withfluorescence from Si, S, and Fe in the lowest ionization stages. Theequivalent widths of the Si and S lines are broadly compatible withthose anticipated for reprocessing by optically thick cold plasma ofsolar abundances, while the large equivalent width of the Fe linerequires some overabundance of iron. A contribution to the line from atransmitted component cannot be ruled out, but the limits on thestrength of the Compton shoulder make it less likely. From thebolometric luminosity of the nuclear region, we infer that the sourceradiates at 1%-10% of its Eddington luminosity for an adopted centralmass of 3×106 Msolar. The overall spectrumis consistent with the hypotheses that the central engines powering thedetected megamasers in accretion disks are obscured from direct view bythe associated accretion disk material itself and that there is acorrelation between the occurrence of megamaser emission andCompton-thick absorption columns. For the 11 known galaxies with bothcolumn density measurements and maser emission believed to arise fromaccretion disks, eight AGNs are Compton thick.
| A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei
Seyfert galaxies commonly host compact jets spanning 10-100 pc scales,but larger structures are resolved out in long-baseline aperturesynthesis surveys. Previous, targeted studies showed thatkiloparsec-scale radio structures (KSRs) may be a common feature ofSeyfert and LINER galaxies, and the origin of KSRs may be starbursts oractive galactic nuclei (AGNs). We report a new Very Large Array surveyof a complete sample of Seyfert and LINER galaxies. Out of all of thesurveyed radio-quiet sources, we find that 44% (19 out of 43) showextended radio structures at least 1 kpc in total extent that do notmatch the morphology of the disk or its associated star-forming regions.The detection rate is a lower limit owing to the combined effects ofprojection and resolution. The infrared colors of the KSR host galaxiesare unremarkable compared to other Seyfert galaxies, and the large-scaleoutflows orient randomly with respect to the host galaxy axes. The KSRSeyfert galaxies instead stand out by deviating significantly from thefar-infrared-radio correlation for star-forming galaxies, with tendencytoward radio excess, and they are more likely to have a relativelyluminous, compact radio source in the nucleus; these results argue thatKSRs are powered by the AGNs rather than starbursts. The high detectionrate indicates that Seyfert galaxies generate radio outflows over asignificant fraction of their lifetime, which is much longer than thedynamical timescale of an AGN-powered jet but is comparable instead tothe buoyancy timescale. The likely explanation is that the KSRsoriginate from jet plasma that has been decelerated by interaction withthe nuclear interstellar medium (ISM). Based on a simple ram pressureargument, the kinetic power of the jet on kiloparsec scales is about 3orders of magnitude weaker than the power of the jet on 10-100 pcscales. This result is consistent with the interaction model, in whichcase virtually all of the jet power must be lost to the ISM within theinner kiloparsec.
| 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.
| H2O Maser Emission in the Starburst Galaxy NGC 253
We report Very Large Array CnB- and A-configuration observations of the22 GHz H2O maser line toward the starburst galaxy NGC 253. Wedetect a broad, highly blueshifted (VLSR=120.5 kms-1) H2O line coincident with the continuum sourceTH4, which is believed to be a supernova remnant. The H2Omaser emission arises from an area whose size is smaller than 0.9 pc. Aweaker feature at VLSR=60.0 km s-1 is also seen atthe same position. We also detect the extended 1.3 cm continuum alongthe galactic disk of NGC 253, as well as the two central continuumsources TH2 and TH4.
| Seyfert Galaxies and the Hard X-Ray Background: Artificial Chandra Observations of z=0.3 Active Galaxies
Deep X-ray surveys have resolved much of the X-ray background radiationbelow 2 keV into discrete sources, but the background above 8 keVremains largely unresolved. The obscured (type 2) active galactic nuclei(AGNs) that are expected to dominate the hard X-ray background have notyet been detected in sufficient numbers to account for the observedbackground flux. However, deep X-ray surveys have revealed large numbersof faint quiescent and starburst galaxies at moderate redshifts. Inhopes of recovering the missing AGN population, it has been suggestedthat the defining optical spectral features of low-luminosity Seyfertnuclei at large distances may be overwhelmed by their host galaxies,causing them to appear optically quiescent in deep surveys. We test thispossibility by artificially redshifting a sample of 23 nearby,well-studied active galaxies to z=0.3, testing them for X-ray AGNsignatures, and comparing them to the objects detected in deep X-raysurveys. We find that these redshifted galaxies have propertiesconsistent with the deep-field normal and optically bright, X-ray-faintgalaxy (OBXF) populations, supporting the hypothesis that the numbers ofAGNs in deep X-ray surveys are being underestimated and suggesting thatOBXFs should not be ruled out as candidate AGN hosts that couldcontribute to the hard X-ray background source population.
| On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies
We investigate the relation between X-ray nuclear emission, opticalemission line luminosities and black hole masses for a sample of 47Seyfert galaxies. The sample, which has been selected from the Palomaroptical spectroscopic survey of nearby galaxies (Ho et al. 1997a, ApJS,112, 315), covers a wide range of nuclear powers, from L2-10keV ~ 1043 erg/s down to very low luminosities(L2-10 keV ~ 1038 erg/s). Best available data fromChandra, XMM-Newton and, in a few cases, ASCA observations have beenconsidered. Thanks to the good spatial resolution available from theseobservations and a proper modeling of the various spectral components,it has been possible to obtain accurate nuclear X-ray luminosities notcontaminated by off-nuclear sources and/or diffuse emission. X-rayluminosities have then been corrected taking into account the likelycandidate Compton thick sources, which are a high fraction (>30%)among type 2 Seyferts in our sample. The main result of this study isthat we confirm strong linear correlations between 2-10 keV,[OIII]λ5007, Hα luminosities which show the same slope asquasars and luminous Seyfert galaxies, independent of the level ofnuclear activity displayed. Moreover, despite the wide range ofEddington ratios (L/L_Edd) tested here (six orders of magnitude, from0.1 down to ~10-7), no correlation is found between the X-rayor optical emission line luminosities and the black hole mass. Ourresults suggest that Seyfert nuclei in our sample are consistent withbeing a scaled-down version of more luminous AGN.
| Extragalactic H_2O masers and X-ray absorbing column densities
Having conducted a search for the λ 1.3 cm (22 GHz) water vaporline towards galaxies with nuclear activity, large nuclear columndensities or high infrared luminosities, we present H2O spectra for NGC2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and400 Lȯ. The H2O maser in UGC 5101 is by far the mostluminous yet found in an ultraluminous infrared galaxy. NGC 3393 revealsthe classic spectrum of a "disk maser", represented by three distinctgroups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below1000 km s-1. Based on the literature and archive data, X-rayabsorbing column densities are compiled for the 64 galaxies withreported maser sources beyond the Magellanic Clouds. For NGC 2782 andNGC 5728, we present Chandra archive data that indicate the presence ofan active galactic nucleus in both galaxies. Modeling the hard nuclearX-ray emission, NGC 2782 is best fit by a high energy reflectionspectrum with NH 1024 cm-2. ForNGC 5728, partial absorption with a power law spectrum indicatesNH 8 × 1023 cm-2. Thecorrelation between absorbing column and H2O emission is analyzed. Thereis a striking difference between kilo- and megamasers with megamasersbeing associated with higher column densities. All kilomasers (L_H_2O< 10 Lȯ) except NGC 2273 and NGC 5194 areCompton-thin, i.e. their absorbing columns are <1024cm-2. Among the H{2}O megamasers, 50% arise fromCompton-thick and 85% from heavily obscured (>1023cm-2) active galactic nuclei. These values are not larger butconsistent with those from samples of Seyfert 2 galaxies not selected onthe basis of maser emission. The similarity in column densities can beexplained by small deviations in position between maser spots andnuclear X-ray source and a high degree of clumpiness in thecircumnuclear interstellar medium.
| X-ray spectral survey with XMM-Newton of a complete sample of nearby Seyfert galaxies
Results obtained from an X-ray spectral survey of nearby Seyfertgalaxies using XMM-Newton are reported. The sample was opticallyselected, well defined, complete in B magnitude, and distance limited:it consists of the nearest (D 22 Mpc) 27 Seyfert galaxies (9 oftype 1, 18 of type 2) taken from the Ho et al. (1997a, ApJS, 112, 315)sample. This is one of the largest atlases of hard X-ray spectra oflow-luminosity active galaxies ever assembled. All nuclear sourcesexcept two Seyfert 2s are detected between 2 and 10 keV, half for thefirst time ever, and average spectra are obtained for all of them.Nuclear luminosities reach values down to 1038 ergs-1. The shape of the distribution of X-ray parameters isaffected by the presence of Compton-thick objects (30% among type2s). The latter have been identified either directly from their intenseFeK line and flat X-ray spectra, or indirectly with flux diagnosticdiagrams which use isotropic indicators. After taking into account thesehighly absorbed sources, we find that (i) the intrinsic X-ray spectralproperties (i.e., spectral shapes and luminosities above 2 keV) areconsistent between type 1 and type 2 Seyferts, as expected from "unifiedmodels"; (ii) Seyfert galaxies as a whole are distributed fairlycontinuously over the entire range of N_H, between 1020 and1025 cm-2; and (iii) while Seyfert 1s tend to havelower NH and Seyfert 2s tend to have the highest, we find 30%and 10% exceptions, respectively. Overall the sample is of sufficientquality to well represent the average intrinsic X-ray spectralproperties of nearby active galactic nuclei, including a proper estimateof the distribution of their absorbing columns. Finally, we concludethat, with the exception of a few cases, the present study agrees withpredictions of unified models of Seyfert galaxies, and extends theirvalidity down to very low luminosities.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
| The Diffuse Ionized Gas in the large telescopes era
In this workshop we summarize the ``state of the art'' of the DiffuseIonized Gas. We present all the possible situations which can produceionization outside an H II region, as well as some of the observationsthat can be performed with the GTC instrumentation and how relevant theycan be in the undestanding of the ionization mechanisms of the DIG.
| Some astronomical niches with 3D spectroscopy
An overview of some of the most interesting results obtained with theuse of 3D spectrometers working in 4m-class telescopes is given with thepurpose of taking advantage of those experiences in the definition ofscientific programs for telescopes of larger diameter as the GTC.
| Water-Vapor Maser Survey for Active Galactic Nuclei: A Megamaser in NGC 6926
We made a survey of water-vapor maser emission for 93 AGNs with theNobeyama 45-m and Mopra 22-m telescopes from 1999 to 2002. A megamaserwas detected in a Seyfert 2 galaxy, NGC 6926, at a distance of 80Mpc, in2002 June. [Greenhill et al. (2003a) have also reported a detection ofthe megamaser at the close date.] The peak flux density was 110mJy, andthe total isotropic luminosity was 340 Lȯ. The masershows triply peaked spectrum, suggesting an edge-on disk. A narrow-linefeature of the maser components at VLSR = 6001 kms-1 was strongly variable with a time scale of a few tens ofdays, and the variation should be of intrinsic origin. We also showed apossibility of variability of water-vapor maser emission of a megamaserpreviously detected in a Seyfert/ultraluminous FIR galaxy, NGC 6240.
| The dynamics and high-energy emission of conductive gas clouds in supernova-driven galactic superwinds
Superwinds from starburst galaxies are multiphase outflows that sweep upand incorporate ambient galactic disc and halo gas. The interaction ofthis denser material with the more diffuse hot wind gas is thought togive rise to the OVI emission and absorption in the far ultraviolet(FUV) and the soft thermal X-ray emission observed in superwinds. Inthis paper, we present high-resolution hydrodynamical models of warmionized clouds embedded in a superwind, and compare the OVI and softX-ray properties to the existing observational data. These modelsinclude thermal conduction, which we show plays an important role inshaping both the dynamics and radiative properties of the resultingwind/cloud interaction. Heat conduction stabilizes the cloud byinhibiting the growth of Kelvin-Helmholtz and Rayleigh-Taylorinstabilities, and also generates a shock wave at the cloud's surfacethat compresses the cloud. This dynamical behaviour influences theobservable properties. We find that while OVI emission and absorptionalways arises in cloud material at the periphery of the cloud, most ofthe soft X-ray arises in the region between the wind bow shock and thecloud surface, and probes either wind or cloud material depending on thestrength of conduction and the relative abundances of the wind withrespect to the cloud. In general, only a small fraction (<~1 percent) of the wind mechanical energy intersecting a cloud is radiatedaway at ultraviolet (UV) and X-ray wavelengths, with more wind energygoing into accelerating the cloud. Clouds in relatively slow cool windsradiate a larger fraction of their energy, which are inconsistent withobservational constraints. Models with heat conduction at Spitzer-levelsare found to produce observational properties closer to those observedin superwinds than models with no thermal conduction, in particular, interms of the OVI to X-ray luminosity ratio, but cloud life times areuncomfortably short (<~1 Myr) compared to the dynamical ages of realwinds. We experimented with reducing the thermal conductivity for oneset of model parameters, and found that even when we reduced conductionby a factor of 25 that the simulations retained the beneficialhydrodynamical stability and low OVI to X-ray luminosity ratio found inthe Spitzer-level conductive models, while also having reducedevaporation rates. Although more work is required to simulate clouds forlonger times and to investigate cloud acceleration and thermalconduction at sub-Spitzer levels in a wider range of models, we concludethat thermal conduction can no longer be ignored in superwinds.
| Submillimetre photometry of X-ray absorbed quasi-stellar objects: their formation and evolutionary status
We present an analysis of the submillimetre/X-ray properties of 19 X-rayabsorbed, Compton-thin quasi-stellar objects (QSOs) selected to haveluminosities and redshifts that represent the peak of cosmic QSOactivity, i.e. ~L* objects at 1 < z < 3. Of these, we present newdata for 11 objects not previously observed at submillimetre wavelengthsand additional data for a further three. The detection rate is 42 percent, much higher than typically reported for samples of QSOs. Detectionstatistics show (at the 3-4σ level) that this sample of absorbedQSOs has a higher submillimetre output than a matched sample ofunabsorbed QSOs. We argue that the far-infrared luminosity is producedby massive star formation. In this case, the correlation found betweenfar-infrared luminosity and redshift can be interpreted as cosmologicalevolution of the star formation rate in the QSO host galaxies. Becausethe submillimetre luminous phase is confined to z > 1.5, the highstar formation rates are consistent with a scenario in which the QSOsevolve to become local luminous elliptical galaxies.Combining these results with previously published data for X-rayunabsorbed QSOs and submillimetre-selected galaxies, we propose thefollowing evolutionary sequence: (i) the forming galaxy is initiallyfar-infrared luminous but X-ray weak similar to the sources discoveredby the Submillimetre Common-User Bolometer Array (SCUBA); (ii) as theblack hole and spheroid grow with time, a point is reached when thecentral QSO becomes powerful enough to terminate the star formation andeject the bulk of the fuel supply (the Compton-thin absorbed QSO phase); (iii) this transition is followed by a period of unobscured QSOactivity, which subsequently declines to leave a quiescent spheroidalgalaxy.
| Infrared mergers and infrared quasi-stellar objects with galactic winds - III. Mrk 231: an exploding young quasi-stellar object with composite outflow/broad absorption lines (and multiple expanding superbubbles)
We present a study of outflow (OF) and broad absorption line (BAL)systems in Mrk 231, and in similar infrared (IR) quasi-stellar objects(QSOs). This study is based mainly on one-dimensional andtwo-dimensional spectroscopy (obtained at La Palma/William HerschelTelescope, Hubble Space Telescope, International Ultraviolet Explorer,European Southern Observatory/New Technology Telescope, Kitt PeakNational Observatory, Apache Point Observatory and Complejo AstronomicoEl Leoncito observatories) plus Hubble Space Telescope images. For Mrk231, we report evidence that the extreme nuclear OF process has at leastthree main components on different scales, which are probably associatedwith: (i) the radio jet, at parsec scale; (ii) the extreme starburst atparsec and kiloparsec scale. This OF has generated at least fourconcentric expanding superbubbles and the BAL systems.Specifically, inside and very close to the nucleus the two-dimensionalspectra show the presence of an OF emission bump in the blendHα+[NII], with a peak at the same velocity of the main BAL-Isystem (VEjectionBAL-I~-4700 km s-1). This bumpwas more clearly detected in the area located at 0.6-1.5 arcsec(490-1220 pc), to the south-west of the nucleus core, showing a strongand broad peak. In addition, in the same direction [at position angle(PA) ~-120°, i.e. close to the PA of the small-scale radio jet] at1.7-2.5 arcsec, we also detected multiple narrow emission-linecomponents, with `greatly' enhanced [NII]/Hα ratio (very similarto the spectra of jets bow shocks). These results suggest that the BAL-Isystem is generated in OF clouds associated with the parsec-scale jet.The Hubble Space Telescope images show four (or possibly five) nuclearsuperbubbles or shells with radii r~ 2.9, 1.5, 1.0, 0.6 and 0.2 kpc. Forthese bubbles, the two-dimensional Hα velocity field map andtwo-dimensional spectra show the following. (i) At the border of themore extended bubble (S1), a clear expansion of the shell withblueshifted velocities (with circular shape and at a radius r~ 5.0arcsec). This bubble shows a rupture arc - to the south - suggestingthat the bubble is in the blowout phase. The axis of this rupture orejection (at PA ~ 00°) is coincident with the axis of theintermediate and large-scale structures detected at radio wavelengths.(ii) In addition, in the three more external bubbles (S1, S2, S3), thetwo-dimensional William Herschel Telescope spectra show multipleemission-line components with OF velocities, of S1, S2 and S3 =[-(650 - 420) +/- 30], [-500+/- 30] and [-230 +/- 30] km s-1. (iii) In the wholecircumnuclear region (1.8 < r < 5 arcsec), the [NII]/Hα and[SII]/Hα narrow emission-line ratios show high values (>0.8),which are consistent with low-ionization nuclear emission-line region/OFprocesses associated with fast velocity shocks. Therefore, we suggestthat these giant bubbles are associated with the large-scale nuclear OFcomponent, which is generated - at least in part - by the extremenuclear starburst: giant supernova/hypernova explosions.The variability of the short-lived BAL-III NaI D system was studied,covering almost all the period in which this system appeared (between~1984 and 2004). We have found that the BAL-III light curve is clearlyasymmetric with a steep increase, a clear maximum and an exponentialfall (similar to the shape of a supernova light curve). The origin ofthis BAL-III system is discussed, mainly in the framework of an extremeexplosive event, probably associated with giant supernova/hypernovaexplosions.Finally, the IR colour diagram and the ultraviolet BAL systems of IR +GW/OF + FeII QSOs are analysed. This study shows two new BAL IR QSOs andsuggests/confirms that these objects could be nearby young BAL QSOs,similar to those detected recently at z~ 6.0. We propose that the phaseof young QSOs is associated with accretion of a large amount of gas (bythe supermassive black hole) + extreme starbursts + extreme compositeOFs/BALs.
| On the nature of bulges in general and of box/peanut bulges in particular: input from N-body simulations
Objects designated as bulges in disc galaxies do not form a homogeneousclass. I distinguish three types: the classical bulges, the propertiesof which are similar to those of ellipticals and which form by collapseor merging; boxy and peanut bulges, which are seen in near-edge-ongalaxies and which are in fact just a part of the bar seen edge-on; and,finally, disc-like bulges, which result from the inflow of (mainly) gasto the centre-most parts, and subsequent star formation. I make adetailed comparison of the properties of boxy and peanut bulges withthose of N-body bars seen edge-on, and answer previously voicedobjections about the links between the two. I also present and analysesimulations where a boxy/peanut feature is present at the same time as aclassical spheroidal bulge, and compare them with observations. Finally,I propose a nomenclature that can help to distinguish between the threetypes of bulges and avoid considerable confusion.
| Dust in spiral galaxies: global properties
We present and analyse high-quality Submillimetre Common-User BolometerArray (SCUBA) 850- and 450-μm images of 14 local spiral galaxies,including the detection of dust well out into the extended disc in manycases. We use these data in conjunction with published far-infrared fluxdensities from IRAS and ISO, and millimetre-wave measurements fromground-based facilities to deduce the global properties of the dust inthese galaxies, in particular temperature and mass. We find that simpletwo-temperature greybody models of fixed dust emissivity index β= 2and with typical temperatures of 25 < Twarm < 40 K and10 < Tcold < 20 K provide good fits to the overallspectral energy distributions. The dust mass in the cold componentcorrelates with the mass in atomic hydrogen and the mass in the warmcomponent correlates with the mass in molecular hydrogen. These resultsthus fit the simple picture in which the cold dust is heatedpredominantly by the interstellar radiation field, while the hot dust isheated predominantly by OB stars in more active regions, although weargue that there is some mixing. The mean gas-to-dust mass ratio is 120+/- 60, very similar to that found within our own galaxy and roughly afactor of 10 lower than that derived from IRAS data alone. Thegas-to-dust mass ratios in the warm, molecular component are on averagehigher than those in the cold, atomic component. We compare ourmodelling results with similar results for more luminous spiral galaxiesselected at far-infrared wavelengths by the SCUBA Local Universe GalaxySurvey. We find that whilst the total dust mass distributions of the twosamples are indistinguishable, they have significantly different dusttemperature distributions in both the warm and cold components. Wesuggest that this difference might be related to the level of starformation activity in these systems, with the more active galaxieshaving more intense interstellar radiation fields and higher dusttemperatures.
| An Analytic Model of Galactic Winds and Mass Outflows
Galactic winds and mass outflows are observed both in nearby starburstgalaxies and in high-redshift star-forming galaxies. We develop a simpleanalytic model to understand the observed superwind phenomenon with adiscussion of the model uncertainties. Our model is built upon the modelof McKee & Ostriker for the interstellar medium. It allows one topredict how properties of a superwind, such as wind velocity and massoutflow rate, are related to properties of its star-forming host galaxy,such as size, gas density and star formation rate. The model predicts athreshold of star formation rate density for the generation ofobservable galactic winds. Galaxies with more concentrated starformation activities produce superwinds with higher velocities. Thepredicted mass outflow rates are comparable to (or slightly larger than)the corresponding star formation rates. We apply our model to both localstarburst galaxies and high-redshift Lyman break galaxies, and find itspredictions to be in good agreement with current observations. Our modelis simple and so can be easily incorporated into numerical simulationsand semi-analytical models of galaxy formation.
| 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.
| Mega-Masers and Galaxies
In the Galaxy, microwave radiation can be amplified in the interstellarmedium in the immediate neighborhood of young stellar objects, orcircumstellar envelopes around evolved stars, resulting in cosmic maseremission. Cosmic masers exist because, in contrast to terrestrialconditions, the interstellar gas density is very low so that levelpopulation in molecules is typically not in thermal equilibrium, andsometimes inverted. In the nuclear regions of external galaxies, thereexist very powerful OH ( 18 cm) and H2O ( 1.35 cm) cosmicmasers with line luminosities of 102 104Lȯ, 106 times more luminous than typicalGalactic maser sources. These are the "mega-masers," found inhigh-density molecular gas located within parsecs of active galacticnuclei in the case of H2O mega-masers, or within the central100 pc of nuclear star-burst regions in the case of OH mega-masers.H2O mega-masers are most frequently found in galactic nucleiwith Seyfert2 or LINER spectral characteristics, in spiral and someelliptical galaxies. OH mega-masers are found in ultra-luminous IRgalaxies (ULIRG) with the warmest IR colors, and importantly, the OHluminosity is observed to increase with the IR luminosity:LOH L1.2IR. Because of the extremelyhigh-surface brightness, H2O mega-maser emission can bemapped at sub-milli-arc-second resolution by Very Long BaselineInterferometry (VLBI), providing a powerful tool to probe spatial andkinematic distributions of molecular gas in distant galactic nuclei atscales below one parsec. An excellent example is the active galaxy, NGC4258, in which mapping of the H2O mega-maser emission hasprovided the first direct evidence in an active galactic nucleus for theexistence of a thin Keplerian accretion disk with turbulence, as well ashighly compelling evidence for the existence of a massive black hole.The NGC 4258 mega-maser has also provided a geometric distancedetermination of extremely high precision. H2O mega-maseremission is also found to arise from postshocked gas from the impact ofnuclear jets or outflows on the surrounding molecular clouds.High-resolution observations have shown that OH mega-masers originatefrom the molecular gas medium in 100-pc scale nuclear star-burstregions. It is proposed that such extreme star-burst regions, withextensive high-density gas bathed in a very high far-IR radiation field,are conducive to the formation of a very large number of OH masersources that collectively produce the OH mega-maser emission. In theearly Universe, galaxies or mergers could go through a very luminousphase, powered by intensive star-bursts and AGN formation, and couldhave extremely large OH and H2O maser luminosities, possiblyproducing giga-masers. With the increasing sensitivity of new telescopesand receivers, surveys and high-resolution studies of mega-masers andgiga-masers will be very important tracers and high-resolution probes ofactive galactic nuclei, dust embedded star-bursts in the earliestgalaxies and galaxy mergers in the epoch of very active star formationat z 2 and beyond. Distance determination of giga-masers at z 1 2can provide on independent measure of how fast the universe isexpanding.
| 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.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
INHABITED SKY
