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|X-ray spectroscopy of cooling clusters|
Electronic Article Available from Elsevier Science.
|The Expanding Radio Lobe of 3C 84 Revealed by VSOP Observations|
The expansion and inner proper motions of a young radio lobe associatedwith the bright radio source 3C 84 in the Seyfert galaxy NGC 1275 havebeen detected by VSOP observations. These proper motions are consistentwith the evolution scenario of classical double radio sources. Theapparent expansion velocity is (0.50 ± 0.09) c. The age of theradio lobe is estimated to be (45.7 ± 8.9)yr in 2001, which is inagreement with the age previously estimated by Romney et al. (1982). Thetotal flux density at 5GHz increased at the end of the 1950's, markedseveral peaks in the middle of the 1980's, and is presently in a decayphase. The decay of the total flux density can naturally be explained byan adiabatic cooling due to expansion of the radio lobe, and previouslymeasured spectral indices suggest that the emission comes from thesurface of the radio lobe.
|Ten Milliparsec-Scale Structure of the Nucleus Region in Centaurus A|
We present the results of a VLBI Space Observatory Programme (VSOP)observation of the subparsec structure in Cen A at 4.9GHz. Theobservation produced an image of the subparsec jet components with aresolution of three-times better than images from previous VLBImonitoring campaigns at 8.4GHz, and twice better than the previous 22GHzstudies. Owing to its proximity, our Cen A space-VLBI image is one ofthe highest spatial-resolution images of an AGN ever made -- 0.01pc perbeam -- comparable only to the recent 43GHz VLBI images of M87. Theelongated core region is resolved into several components of over 10milliarcsec long (0.2pc), including a compact component of brightnesstemperature 2.2 × 1010 K. A counterjet was detected: ifwe assume jet-counterjet symmetry, a relatively slow jet speed, and alarge viewing angle, as derived from previous observations, the imageallows us to investigate the distribution of ionized gas around thecore, which is opaque at this frequency due to free-free absorption. Wealso analyzed the jet geometry in terms of collimation. Assuming thestrongest component to be the core, the jet opening angle at 5000rS from the core is estimated to be 12°, with thecollimation of the jet to 3° continuing out to 20000rS. This result is consistent with previous studies of thejet in M87, which favor MHD disk outflow models. Future space VLBIobservations at higher frequencies will probably be able to image thecollimation region, within 1000 rS of the center of Cen A,together with the accretion disk itself.
|Integral field spectroscopy with GEMINI: Extragalactic star cluster in NGC1275|
Studies of globular cluster systems play a critical role in ourunderstanding of galaxy formation. Imaging with the Hubble SpaceTelescope has revealed that young star clusters are formed copiously ingalaxy mergers, strengthening theories in which giant ellipticalgalaxies are formed by the merger of spirals [e.g. Whitmore, B.C.,Schweizer, F., Leitherer, C., Borne, K., Robert, C., 1993. AstronomicalJournal. 106, 1354; Miller, B.W., Whitmore, B.C., Schweizer, F., Fall,S.M., 1997. Astronomical Journal. 114, 2381; Zepf, S.E., Ashman, K.M.,English, J., Freeman, K.C., Sharples, R.M., 1999. Astronomical Journal.118, 752; Ashman, K.M., Zepf, S.E., 1992. Astrophysical Journal. 384,50]. However, the formation and evolution of globular cluster systems isstill not well understood. Ages and metallicities of the clusters areuncertain either because of degeneracy in the broad-band colors or dueto variable reddening. Also, the luminosity function of the youngclusters, which depends critically on the metallicities and ages of theclusters, appears to be single power-laws while the luminosity functionof old clusters has a well-defined break. Either there is significantdynamical evolution of the cluster systems or metallicity affects themass function of forming clusters. Spectroscopy of these clusters areneeded to improve the metallicity and age measurements and to study thekinematics of young cluster systems. Therefore, we have obtained GMOSIFU data of 4 clusters in NGC1275. We will present preliminary resultslike metallicities, ages, and velocities of the star clusters from IFUspectroscopy.
|15 years of very long baseline interferometry observations of two compact radio sources in Messier 82|
We present the results of a second epoch of 18-cm global very longbaseline interferometry (VLBI) observations, taken on 2001 February 23,of the central kiloparsec of the nearby starburst galaxy Messier 82.These observations further investigate the structural and flux evolutionof the most compact radio sources in the central region of M82. The twomost compact radio objects in M82 have been investigated (41.95+575 and43.31+592). Using this recent epoch of data in comparison with ourprevious global VLBI observations and two earlier epochs of EuropeanVLBI network observations, we measure expansion velocities in the rangeof 1500-2000 kms-1 for 41.95+575 and 9000-11000kms-1 for 43.31+592 using various independent methods. Ineach case, the measured remnant expansion velocities are significantlylarger than the canonical expansion velocity (500 kms-1) ofsupernova remnants (SNRs) within M82 predicted from theoretical models.In this paper, we discuss the implications of these measured expansionvelocities with respect to the high-density environment that the SNRsare expected to reside in within the centre of the M82 starburst.
|Magnetic fields in the centre of the Perseus cluster|
We present Very Long Baseline Array (VLBA) observations of the nucleusof NGC1275, the central, dominant galaxy in the Perseus cluster ofgalaxies. These are the first observations to resolve the linearlypolarized emission from 3C84, and from them we determine a Faradayrotation measure (RM) ranging from 6500 to 7500radm-2 acrossthe tip of the bright southern jet component. At 22GHz some polarizationis also detected from the central pc of 3C84, indicating the presence ofeven more extreme RMs that depolarize the core at lower frequencies. Thenature of the Faraday screen is most consistent with being produced bymagnetic fields associated with the optical filaments of ionized gas inthe Perseus cluster.
|The hot, warm and cold gas in Arp 227 - an evolving poor group|
Arp 227 represents a prototypical example of an interacting mixed pairof galaxies located in a low-density environment. We investigate the gasproperties of the pair in the X-ray, Hα, HI and CO bands. We alsodetect two additional members of the group in HI which indicates thatthe pair constitutes the dominant members of a loose group.The HI distribution shows a tail of gas that connects the spiral member,NGC 470, to the lenticular, NGC 474, showing that the two main membersare currently undergoing interaction. The Hα emission reveals thepresence of secondary components at the centre of NGC 470, superposed onthe main component tracing the rotation of the galaxy. This latter mapsa nearly unperturbed velocity field. The dominant, nearly unperturbedtrend of the kinematics is confirmed by CO observations, althoughrestricted to the centre of the galaxy. The X-ray luminosity of NGC 470is comparable with that of a `normal' spiral galaxy. NGC 474 on theother hand is very gas-poor and has not been detected in Hα. ItsX-ray luminosity is consistent with the low end of the expected emissionfrom discrete sources.Arp 227 as a loose group shows several signatures of galaxy-galaxyinteraction. Our observations suggest the presence of signatures ofinteraction in the overall kinematics of the spiral companion. Theongoing interaction is clearly visible only in the outer HI halo of NGC470. While the large shell system of NGC 474 could be associated with anaccretion event, the secondary components in the Hα profile in thecentre of NGC 470 could be due to the interaction with the companion.The low X-ray luminosity of NGC 470 seems to be a characteristic ofdynamically young systems. All the above evidence suggest that Arp 227is an evolving group in the early phase of its evolution and that itsdrivers are the accretion of faint galaxies and the ongoing large-scaleinteraction between NGC 470 and 474.
|The influence of radio galaxy activity on X-ray absorption lines from the intracluster medium|
We present an investigation of the X-ray absorption features predictedby hydrodynamic simulations of radio galaxies interacting with theintracluster medium (ICM) of their host galaxy clusters. We show howthese absorption lines can be used as a new diagnostic for the radiogalaxy-ICM interactions. Such interactions have been observed innumerous systems by ROSAT, Chandra and XMM-Newton, and understandingthem has implications for active galactic nucleus (AGN) feedback andgalaxy formation. Starting from the hydrodynamic simulations ofReynolds, Heinz & Begelman, we calculate the properties of thehighly ionized iron and oxygen lines (seen in absorption against thecentral AGN), predicting line shapes, equivalent widths, columndensities and velocity shifts. The main effect of the jet on theabsorption lines is a reduction of the line strength from that of thequiescent ICM and the introduction of some velocity structure in theline profile. We investigate whether these features are detectable withcurrent as well as future high-resolution X-ray spectrometers. Weconclude that the Chandra transmission gratings have insufficientsensitivity to detect these features with high significance, andcertainly would not allow a study of the dynamics of the interaction viaabsorption signatures. Constellation-X, on the other hand, will allowsuperb constraints to be derived. We can also use this analysis toassess the idea that radio galaxy induced ICM outflows give rise to theresonant oxygen X-ray absorption lines that have been claimed asevidence for the warm-hot intergalactic medium (WHIM). We show thatthese detached, high-velocity oxygen absorption lines cannot result froma radio galaxy-ICM interaction, thereby strengthening the WHIMinterpretation.
|Near-infrared study of CIZA J1324.7-5736, the second richest cluster of galaxies in the Great Attractor|
We present the result of a deep near-infrared survey of the newlyidentified X-ray luminous cluster of galaxies CIZA J1324.7-5736 in theGreat Attractor (GA) region. In a 35 × 35 arcmin2region, 111 galaxy candidates with rKs20 >arcsec are identified. Comparison of the extinction-correctedKs-band luminosity function of CIZA J1324.7-5736 with thoseof nearby clusters indicates that the richness class of CIZAJ1324.7-5736 is almost the same as, or richer than, the Pavo, Centaurusand Hydra clusters but poorer than the Coma, Perseus and Norma clusters.CIZA J1324.7-5736 is possibly the second richest cluster in the GAregion following the Norma cluster. The position of CIZA J1324.7-5736[in the (l, b, v) space] is close to the Centaurus-Crux cluster and theagglomeration of galaxies detected by the Parkes HI survey. CIZAJ1324.7-5736, together with the Centaurus-Crux cluster and the HI galaxyagglomeration, is most likely to be one of the richest localconcentrations in the GA overdensity of galaxies.
|The ultraluminous X-ray sources in the high-velocity system of NGC1275|
We report the results of a study of X-ray point sources coincident withthe high-velocity system (HVS) projected in front of NGC1275. A verydeep X-ray image of the core of the Perseus cluster, made with theChandra X-ray Observatory, has been used. We find a population ofultraluminous X-ray sources [ULXs seven sources with LX(0.5 -7.0 keV) > 7 × 1039ergs-1]. As with theULX populations in the Antennae and Cartwheel galaxies, those in the HVSare associated with a region of very active star formation. Severalsources have possible optical counterparts found on the Hubble SpaceTelescope (HST) images, although the X-ray brightest one does not.Absorbed power-law models fit the X-ray spectra, with most having aphoton index between 2 and 3.
|On the origin and excitation of the extended nebula surrounding NGC1275|
We use line-of-sight velocity information on the filamentaryemission-line nebula of NGC1275 to infer a dynamical model of thenebula's flow through the surrounding intracluster gas. We detectoutflowing gas and flow patterns that match simulations of buoyantlyrising bubbles from which we deduce that some of the nebula filamentshave been drawn out of NGC1275. We find a radial gradient of the ratio[NII]λ6584/Hα which may be due to a variation inmetallicity, interactions with the surrounding intracluster medium or ahardening of the excitation mechanism. We find no preferred spatialcorrelation of stellar clusters within the filaments and there is anotable lack of [OIII]λ5007 emission, therefore it is unlikelythat the filaments are ionized by stellar ultraviolet.
|The radio properties of the cD galaxy of Abell 2390|
We present multifrequency, multi-epoch radio imaging of the complexradio source B2151+174 in the core of the cluster, Abell 2390 (z~=0.23). From new and literature data, we conclude that the Faranoff-Rileytype II (FRII)-powerful radio source is the combination of a compact,core-dominated `medium-sized symmetric object' (MSO) with a moreextended, steeper spectrum mini-halo. B2151+174 is unusual in a numberof important aspects. (i) It is one of the most compact and flatspectrum sources in a cluster core known; (ii) it shows a complex,compact twin-jet structure in a north-south orientation; (iii) theorientation of the jets is 45° misaligned with apparent structure(ionization cones and dust disc) of the host galaxy on larger scales.Since the twin-jet of the MSO has its northern half with an apparent`twist', it might be that precession of the central supermassive blackhole explains this misalignment. B2151+174 may be an example of theearly stage (103-104yr duration) of a `bubble'being blown into the intracluster medium where the plasma has yet toexpand.
|Precession of the super-massive black hole in NGC 1275 (3C 84)?|
The X-ray holes at the centre of the Perseus cluster of galaxies are notall at the same position angle with respect to the centre of thecluster. This configuration would result if the jet inflating thebubbles is precessing, or moving around, and the bubbles detach atdifferent times. The orientations which best fit the observed traveldirections are an inclination of the precession axis to the line ofsight of 120° and an opening angle of 50°. From the time-scalesfor the bubbles seen in the cluster, the precession time-scale,τprec, is around 3.3 × 107yr. Thebubbles rising up through different parts of the cluster may haveinteracted with the central cool gas, forming the whorl of cool gasobserved in the temperature structure of the cluster. The dynamics ofbubbles rising in fluids is discussed. The conditions present in thecluster are such that oscillatory motion, observed for bubbles rising influids on Earth, should take place. However, the time-scale for thismotion is longer than that taken for the bubbles to evolve intospherical-cap bubbles, which do not undergo a path instability, so suchmotion is not expected to occur.
|A very deep Chandra observation of the Perseus cluster: shocks, ripples and conduction|
We present the first results from a very deep Chandra X-ray observationof the core of the Perseus cluster of galaxies. A pressure map reveals aclear thick band of high pressure around the inner radio bubbles. Thegas in the band must be expanding outwards and the sharp front to it isidentified as a shock front, yet we see no temperature jump across it;indeed, there is more soft emission behind the shock than in front ofit. We conclude that in this inner region either thermal conductionoperates efficiently or the co-existing relativistic plasma seen as theradio mini-halo is mediating the shock. If common, isothermal shocks incluster cores mean that we cannot diagnose the expansion speed of radiobubbles from temperature measurements alone. They can at times expandmore rapidly than currently assumed without producing significantregions of hot gas. Bubbles may also be significantly more energetic.The pressure ripples found in earlier images are identified asisothermal sound waves. A simple estimate based on their amplitudeconfirms that they can be an effective distributed heat source able tobalance radiative cooling. We see multiphase gas with about109Msolar at a temperature of about 0.5 keV. Much,but not all, of this X-ray emitting cooler gas is spatially associatedwith the optical filamentary nebula around the central galaxy, NGC1275.A residual cooling flow of about 50Msolaryr-1 maybe taking place. A channel is found in the pressure map along the pathof the bubbles, with indications found of outer bubbles. The channelconnects in the south (S) with a curious cold front.
|Supernovae 2005my, 2005mz, 2005na|
IAUC 8655 available at Central Bureau for Astronomical Telegrams.
|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.
|A Comprehensive Search for Gamma-Ray Lines in the First Year of Data from the INTEGRAL Spectrometer|
We have carried out an extensive search for gamma-ray lines in the firstyear of public data from the spectrometer (SPI) on the INTEGRAL mission.INTEGRAL has spent a large fraction of its observing time in theGalactic plane with particular concentration in the Galactic center (GC)region (~3 Ms in the first year). Hence the most sensitive searchregions are in the Galactic plane and center. The phase space of thesearch spans the energy range 20-8000 keV and line widths from 0 to 1000keV (FWHM). It includes both diffuse and pointlike emission. We havesearched for variable emission on timescales down to ~1000 s. Diffuseemission has been searched for on a range of different spatial scalesfrom ~20° (the approximate field of view of the spectrometer) up tothe entire Galactic plane. Our search procedures were verified by therecovery of the known gamma-ray lines at 511 and 1809 keV at theappropriate intensities and significances. We find no evidence for anypreviously unknown gamma-ray lines. The upper limits range from afew×10-5 to a few×10-2 cm-2s-1 depending on line width, energy, and exposure; regions ofstrong instrumental background lines were excluded from the search.Comparison is made between our results and various prior predictions ofastrophysical lines.
|A Compact Supermassive Binary Black Hole System|
We report on the discovery of a supermassive binary black hole system inthe radio galaxy 0402+379, with a projected separation between the twoblack holes of just 7.3 pc. This is the closest black hole pair yetfound by more than 2 orders of magnitude. These results are based onrecent multifrequency observations using the Very Long Baseline Array(VLBA), which reveal two compact, variable, flat-spectrum, active nucleiwithin the elliptical host galaxy of 0402+379. Multiepoch observationsfrom the VLBA also provide constraints on the total mass and dynamics ofthe system. Low spectral resolution spectroscopy using the Hobby-EberlyTelescope indicates two velocity systems with a combined mass of the twoblack holes of ~1.5×108 Msolar. The twonuclei appear stationary, while the jets emanating from the weaker ofthe two nuclei appear to move out and terminate in bright hot spots. Thediscovery of this system has implications for the number of close binaryblack holes that might be sources of gravitational radiation. Green BankTelescope observations at 22 GHz to search for water masers in thisinteresting system are also presented.
|Discovery of an Extreme MeV Blazar with the Swift Burst Alert Telescope|
The Burst Alert Telescope (BAT) on board Swift detected bright emissionfrom 15-195 keV from the source SWIFT J0746.3+2548 (J0746 in thefollowing), identified with the optically faint (R~19), z=2.979 quasarSDSS J074625.87+244901.2. Here we present Swift and multiwavelengthobservations of this source. The X-ray emission from J0746 is variableon timescales of hours to weeks in 0.5-8 keV and of a few months in15-195 keV, but there is no accompanying spectral variability in the0.5-8 keV band. There is a suggestion that the BAT spectrum, initiallyvery hard (photon index Γ~0.7), steepened to Γ~1.3 in a fewmonths, together with a decrease of the 15-195 keV flux by a factor ~2.The 0.5-8 keV continuum is well described by a power law withΓ~1.3 and spectral flattening below 1 keV. The latter can bedescribed with a column density in excess of the Galactic value withintrinsic column density NzH~1022cm-2, or with a flatter power law, implying a sharp(ΔΓ>~1) break across 16 keV in the quasar's rest frame.The spectral energy distribution of J0746 is double-humped, with thefirst component peaking at IR wavelengths and the second component atMeV energies. These properties suggest that J0746 is a blazar with highgamma-ray luminosity and low peak energy (MeV), stretching the blazarsequence to an extreme.
|Polarization Observations of the Anomalous Microwave Emission in the Perseus Molecular Complex with the COSMOSOMAS Experiment|
The anomalous microwave emission detected in the Perseus molecularcomplex by Watson et al. has been observed at 11 GHz through dualorthogonal polarizations with the COSMOSOMAS experiment. Stokes U and Qmaps were obtained at a resolution of ~0.9d for a 30°×30°region including the Perseus molecular complex. Faint polarized emissionhas been measured; we find Q=-0.2%+/-1.0% andU=-3.4+1.8-1.4%, both at the 95% confidence level,with a systematic uncertainty estimated to be lower than 1% determinedfrom tests of the instrumental performance using unpolarized sources inour map as null hypothesis. The resulting total polarization level isΠ=3.4+1.5-1.9%. These are the first constraintson the polarization properties of an anomalous microwave emissionsource. The low level of polarization seems to indicate that theparticles responsible for this emission in the Perseus molecular complexare not significantly aligned in a common direction over the wholeregion, as a consequence of either a high structural symmetry in theemitting particle or a low-intensity magnetic field. Our weak detectionis fully consistent with predictions from electric dipole emission andresonance relaxation at this frequency.
|A New Probe of Dense Gas at High Redshift: Detection of HCO+ (5-4) Line Emission in APM 08279+5255|
We report the detection of HCO+ (5-4) emission from the broadabsorption line quasar APM 08279+5255 at z=3.911 based on observationsconducted with the IRAM Plateau de Bure Interferometer. This representsthe first detection of this molecular ion at such a high redshift. Theinferred line luminosity, uncorrected for lensing, isL'HCO+=(3.5+/-0.6)×1010K km s-1 pc2. The HCO+ J=5-4 sourceposition coincides within the errors with that reported from previousHCN J=5-4 and high-J CO line observations of this quasar. TheHCO+ line profile central velocity and width are consistentwith those derived from HCN. This result suggests that HCO+(5-4) emission comes roughly from the same circumnuclear region probedby HCN. However, the HCN (5-4)/HCO+ (5-4) intensity ratiomeasured in APM 08279+5255 is significantly larger than that predictedby simple radiative transfer models, which assume collisional excitationand equal molecular abundances. This could imply that the[HCN]/[HCO+] abundance ratio is particularly large in thissource, or that the J=5 rotational levels are predominantly excited byinfrared fluorescent radiation.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by the Institut National des Sciencesde l'Univers du CNRS (France), the Max-Planck-Gesellschaft (Germany),and the Instituto Geográfico Nacional (Spain).
|Possible Molecular Spiral Arms in the Protoplanetary Disk of AB Aurigae|
The circumstellar dust disk of the Herbig Ae star AB Aur has been foundto exhibit complex spiral-like structures in the near-IR image obtainedwith the Subaru Telescope. We present maps of the disk in both 12CO (3-2) and dust continuum at 345 GHz with the SubmillimeterArray at an angular resolution of 1.0"×0.7" (144×100 AU).The continuum emission traces a dust disk with a central depression anda maximum overall dimension of 450 AU (FWHM). This dust disk exhibitsseveral distinct peaks that appear to coincide with bright features inthe near-IR image, in particular the brightest inner spiral arm. The COemission traces a rotating gas disk of size 530×330 AU with adeprojected maximum velocity of 2.8 km s-1 at 450 AU. Incontrast with the dust disk, the gas disk exhibits an intensity peak atthe stellar position. Furthermore, the CO emission in several velocitychannels traces the innermost spiral arm seen in the near-IR. We comparethe observed spatial-kinematic structure of the CO emission to a simplemodel of a disk in Keplerian rotation and find that only the emissiontracing the main spiral arm clearly lies outside the confines of ourmodel. This emission has a net outward radial motion compared with theradial velocity predicted by the model at the location of the mainspiral arms. The disk of AB Aur is therefore quite different from theKeplerian disks seen around many Herbig Ae stars. The spiral-likestructures of the disk with non-Keplerian motions we revealed in12CO (3-2), together with the central depression of the dustdisk, could be explained to be driven by the possible existence of agiant planet forming in the disk.
|Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)|
We present Spitzer 3.6-160 μm images, Spitzer mid-infrared spectra,and JCMT SCUBA 850 μm images of the Sombrero Galaxy (NGC 4594), an Sagalaxy with a 109 Msolar low-luminosity activegalactic nucleus (AGN). The brightest infrared sources in the galaxy arethe nucleus and the dust ring. The spectral energy distribution of theAGN demonstrates that, while the environment around the AGN is aprominent source of mid-infrared emission, it is a relatively weaksource of far-infrared emission, as had been inferred for AGNs inprevious research. The weak nuclear 160 μm emission and thenegligible polycyclic aromatic hydrocarbon emission from the nucleusalso implies that the nucleus is a site of only weak star formationactivity and the nucleus contains relatively little cool interstellargas needed to fuel such activity. We propose that this galaxy may berepresentative of a subset of low-ionization nuclear emission regiongalaxies that are in a quiescent AGN phase because of the lack of gasneeded to fuel circumnuclear star formation and Seyfert-like AGNactivity. Surprisingly, the AGN is the predominant source of 850 μmemission. We examine the possible emission mechanisms that could giverise to the 850 μm emission and find that neither thermal dustemission, CO line emission, bremsstrahlung emission, nor the synchrotronemission observed at radio wavelengths can adequately explain themeasured 850 μm flux density by themselves. The remainingpossibilities for the source of the 850 μm emission include acombination of known emission mechanisms, synchrotron emission that isself-absorbed at wavelengths longer than 850 μm, or unidentifiedspectral lines in the 850 μm band.
|Very Large Array H53α and H92α Line Observations of the Central Region of NGC 253|
We present new VLA observations toward NGC 253 of the recombination lineH53α (43 GHz) at an angular resolution of 1.5"×1.0". Thefree-free emission at 43 GHz is estimated to be ~140 mJy, implying astar formation rate of 2 Msolar yr-1 in thenuclear region of this starburst galaxy. A reanalysis is made forpreviously reported H92α observations carried out with angularresolution of 1.5"×1.0" and 0.36"×0.21". Based on the lineand continuum emission models used for the 1.5"×1.0" angularresolution observations, the RRLs H53α and H92α are tracersof the high-density (~105 cm-3) and low-density(~103 cm-3) thermally ionized gas components inNGC 253, respectively. The velocity fields observed in the H53αand H92α lines (1.5"×1.0") are consistent. The velocitygradient in the central ~18 pc of the NE component, as observed in boththe H53α and H92α lines, is in the opposite direction to thevelocity gradient determined from the CO observations. The enclosedvirial mass, as deduced from the H53α velocity gradient over theNE component, is ~5×106 Msolar in thecentral ~18 pc region. The H92α line observations at high angularresolution (0.36"×0.21") reveal a larger velocity gradient, alonga P.A.~-45deg on the NE component, of ~110 km s-1arcsec-1. The dynamical mass estimated using the high angularresolution H92α data (~7×106 Msolar)supports the existence of an accreted massive object in the nuclearregion of NGC 253.
|VLA Observations of Carbon 91α Recombination Line Emission in W49 North|
We have detected C91α (8.5891 GHz) emission toward fourultracompact H II regions (UCHs; W49G, J, L, and C) in the W49 Northmassive star-forming region with the Very Large Array (VLA) at 3"resolution. No carbon line emission was detected toward UCHs W49F, A, O,S, and Q at this frequency to a 3 σ level of 2 mJy. We alsoobserved the same region in the C75α line (15.3 GHz) with nodetection at a 3 σ level of 6 mJy with a 1.7" beam. Detection ofline emission toward these sources add supporting data to the 2005result of Roshi et al. that many UCHs have an associatedphotodissociation region (PDR). The similarity of the LSR velocities ofcarbon recombination lines and H2CO absorption toward UCHs inW49 North suggests that the PDRs reside in the dense interface zonesurrounding these H II regions. Combining the observed carbon lineparameters at 8.6 GHz with the upper limits on line emission at 15.3GHz, we obtain constraints on the physical properties of the PDRsassociated with W49G and J. The upper limit on the number density ofhydrogen molecule obtained from carbon line models is~5×106 cm-3.
|TeV Gamma-Ray Observations of the Perseus and Abell 2029 Galaxy Clusters|
Galaxy clusters might be sources of TeV gamma rays emitted byhigh-energy protons and electrons accelerated by large-scale structureformation shocks, galactic winds, or active galactic nuclei.Furthermore, gamma rays may be produced in dark matter particleannihilation processes at the cluster cores. We report on observationsof the galaxy clusters Perseus and A2029 using the 10 m Whipple Cerenkovtelescope during the 2003-2004 and 2004-2005 observing seasons. We applya two-dimensional analysis technique to scrutinize the clusters for TeVemission. In this paper we first determine flux upper limits on TeVgamma-ray emission from point sources within the clusters. Second, wederive upper limits on the extended cluster emission. We subsequentlycompare the flux upper limits with EGRET upper limits at 100 MeV andtheoretical models. Assuming that the gamma-ray surface brightnessprofile mimics that of the thermal X-ray emission and that the spectrumof cluster cosmic rays extends all the way from thermal energies tomulti-TeV energies with a differential spectral index of -2.1, ourresults imply that the cosmic-ray proton energy density is less than7.9% of the thermal energy density for the Perseus Cluster.
|O VI Observations of Galaxy Clusters: Evidence for Modest Cooling Flows|
A prediction of the galaxy-cluster cooling flow model is that as gascools from the ambient cluster temperature, emission lines are producedin gas at subsequently decreasing temperatures. Gas passing through105.5 K emits in the lines of O VI λλ1032,1035, and here we report a FUSE study of these lines in three coolingflow clusters, Abell 426, Abell 1795, and AWM 7. No emission wasdetected from AWM 7, but O VI is detected from the centers of Abell 426and Abell 1795, and possibly to the south of the center in Abell 1795,where X-ray and optical emission line filaments lie. In Abell 426 theseline luminosities imply a cooling rate of 32+/-6 Msolaryr-1 within the central r=6.2 kpc region, while for Abell1795 the central cooling rate is 26+/-7 Msolaryr-1 (within r=22 kpc), and about 42+/-9 Msolaryr-1 including the southern pointing. Including otherstudies, three of six clusters have O VI emission, and they also havestar formation as well as emission lines from 104 K gas.These observations are generally consistent with the cooling flow model,but at a rate closer to 30 Msolar yr-1 than to theoriginally suggested values of 102-10 3Msolar yr-1.
|Chandra and XMM-Newton Observations of a Sample of Low-Redshift FR I and FR II Radio Galaxy Nuclei|
We present spectral results from Chandra and XMM-Newton observations ofa sample of 22 low-redshift (z<0.1) radio galaxies and considerwhether the core emission originates from the base of a relativisticjet, or an accretion flow, or contains contributions from both. We findcorrelations between the unabsorbed X-ray, radio, and optical fluxes andluminosities of FR I-type radio-galaxy cores, implying a common originin the form of a jet. On the other hand, we find that the X-ray spectraof FR II-type radio galaxy cores are dominated by absorbed emission,with NH>~1023 atoms cm-2, which islikely to originate in an accretion flow. We discuss several models thatmay account for the different nuclear properties of FR I- and FR II-typecores and also demonstrate that both heavily obscured, accretion-relatedand unobscured, jet-related components may be present in all radiogalaxy nuclei. Any absorbed, accretion-related components in FR I-typegalaxies have low radiative efficiencies.
|The Detection of 3He+ in a Planetary Nebula Using the VLA|
We used the VLA to search for 3He+ emission fromtwo Galactic planetary nebulae (PNe): NGC 6572 and J320. Standardstellar models predict that the 3He/H abundance ratios forPNe should be 1-2 orders of magnitude higher than the primordial value(3He/H~10-5 by number) determined from Galactic HII region abundances and confirmed by WMAP cosmic microwave backgroundresults. Chemical evolution models suggest that fewer than 5% of all PNeenrich the interstellar medium (ISM) with 3He at the level ofstandard stellar models. Our target PNe are therefore anomalous in thatthey were selected from a sample deliberately biased to contain objectswith properties that maximized the likelihood of a 3Hedetection by the VLA. We have detected the 8.665 GHz hyperfine3He+ transition in J320 at the 4 σ level.The 3He/H abundance ratio is 1.9×10-3 withroughly a factor of 2 uncertainty. For NGC 6572 we find an upper limitof 3He/H<~10-3. This detection of3He in J320 makes it the second PN known to have ananomalously high 3He abundance, confirming that at least somelow-mass stars produce significant amounts of 3He thatsurvives to the PN stage and enriches the ISM.
|A Large Millimeter Flare on the RS CVn Binary σ Geminorum|
The flaring behavior of active RS CVn binaries has been studiedextensively at centimeter wavelengths, but very little has beenpublished concerning RS CVn flares in the millimeter region, where thenonthermal gyrosynchrotron emission is much more likely to be opticallythin. We present 3 mm (99 GHz) observations of a large flare on theactive binary σ Gem using the Owens Valley Millimeter Array. Two 3hr observations in 2004 February showed only 3 σ upper limits of 3mJy, while the third observation on 2004 April 22 showed a large flareoutburst that filled the 6 hr observation and reached a peak fluxdensity of 90 mJy. The strongest brightening was preceded by at least a4 hr period of significantly elevated emission at 20-40 mJy. Theseobservations demonstrate the feasibility of detailed multifrequencystudies of nonthermal electron distributions in active star coronae withthe next generation of millimeter and centimeter radio arrays.
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