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VLT spectropolarimetry of two powerful radio galaxies at z ~ 1.4: ultraviolet continuum, emission-line properties and the nature of high-redshift dust
Deep spectropolarimetric observations, obtained with the Very LargeTelescope, are presented for two powerful radio galaxies, 0850-206 (z =1.3373) and 1303+091 (z = 1.4093). These observations cover therest-frame wavelength range ~1450-3750 Å. New radio observationsand continuum images of the same sources are also presented. Thesegalaxies are the first two observed from a complete sample of nine radiosources with redshifts in the range 1.3 <= z <= 1.5 (selected fromthe equatorial sample of powerful radio sources of Best, Röttgering& Lehnert), as part of a project aimed to investigate themulticomponent nature of the ultraviolet (UV) continuum in radiogalaxies and, in particular, any variations of the continuum propertieswith the radio source age.The larger radio source of the two, 0850-206, presents a high continuumfractional polarization, averaging 17 per cent across the observedwavelength range and reaching 24 per cent at rest-frame wavelengths of<~2000 Å. The smaller radio source, 1303+091, shows a lowercontinuum polarization, averaging 8 per cent and rising to 11 per centfor rest-frame wavelengths >~3000 Å. For both galaxies, theposition angle of the electric vector is generally constant withwavelength and within ~15° of perpendicular to the radio axis. Boththeir total flux spectra and polarized flux spectra reveal the2200-Ådust feature, and comparison with dust scattering modelssuggests that the composition of the dust in these galaxies is similarto that of Galactic dust. In 0850-206, scattered quasar radiationdominates the UV continuum emission, with the nebular continuumaccounting for no more than ~22 per cent and no requirement for anyadditional emission component such as emission from young stars. Incontrast, in 1303+091, unpolarized radiation could be a majorconstituent of the UV continuum emission, with starlight accounting forup to ~50 per cent and the nebular continuum accounting for ~11 percent.The emission-line properties of the galaxies are also studied from theirtotal intensity spectra. Comparison of the measured emission-line ratioswith both shock- and photoionization models shows that the nuclear andextended gas in these galaxies is mainly photoionized by the centralactive nucleus.
| The origin of massive O-type field stars. I. A search for clusters
We present a study aimed at clarifying the birthplace for 43 massiveO-type field stars. In this first paper we present the observationalpart: a search for stellar clusters near the target stars. We derivestellar density maps at two different resolving scales, viz. 0.25pc and 1.0 pc from NTT and TNG imaging and the 2MASS catalogue.These scales are typical for cluster sizes. The main result is that thelarge majority of the O-type field population are isolated stars: only12% (5 out of 43) of the O-type field stars is found to harbour asmall-scale stellar cluster. We review the literature and aim atcharacterizing the stellar field of each O-type field star with theemphasis on star formation and the presence of known young stellarclusters. An analysis of the result of this paper and a discussion ofthe O-type field population as products of a dynamical ejection event ispresented in an accompanying paper.Based on observations collected at the European Southern Observatory,Chile, and at the Italian Telescopio Nazionale Galileo (TNG) operated onthe island of La Palma by the Centro Galileo Galilei of the CNAA(Consorzio Nazionale per l'Astronomia e l'Astrofisica) at the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.Table 2 and Figs. 4 to 17 are available in electronic form athttp://www.edpsciences.org
| Evidence for Extremely High Dust Polarization Efficiency in NGC 3184
Recent studies have found the Type II-plateau supernova (SN) 1999gi tobe highly polarized (pmax=5.8%, where pmax is thehighest degree of polarization measured in the optical bandpass) andminimally reddened [E(B-V)=0.21+/-0.09 mag]. From multiple lines ofevidence, including the convincing fit of a ``Serkowski'' interstellarpolarization (ISP) curve to the continuum polarization shape, weconclude that the bulk of the observed polarization is likely due todust along the line of sight and is not intrinsic to SN 1999gi. Wepresent new spectropolarimetric observations of four distant Galacticstars close to the line of sight to SN 1999gi (two are within 0.02d) andfind that all are null to within 0.2%, effectively eliminating Galacticdust as the cause of the high polarization. The high ISP coupled withthe low reddening implies an extraordinarily high polarizationefficiency for the dust along this line of sight in NGC 3184:ISP/E(B-V)=31+22-9% mag -1. This isinconsistent with the empirical Galactic limit [ISP/E(B-V)<9%mag-1] and represents the highest polarization efficiency yetconfirmed for a single sight line in either the Milky Way or an externalgalaxy.
| Evidence for Asphericity in a Subluminous Type Ia Supernova: Spectropolarimetry of SN 1999by
We present polarization spectra near maximum light for the stronglysubluminous Type Ia supernova SN 1999by that show that the supernova isintrinsically polarized. SN 1999by has an observed, overall level ofpolarization of ~0.3%-0.8%, a rise of the polarization P redward of 6500Å, and a change in polarization across the Si II λ6150feature of about 0.4%. The presentation of the polarization at differentwavelengths in the Q-U plane is shown to be a powerful tool to determinethe overall geometry and the interstellar component. The distribution ofpoints with wavelength using this empirical Q-U plane method revealsthat SN 1999by has a well-defined axis of symmetry and suggests aninterstellar polarization (ISP) vector with PISP=0.3% andposition angle Θ=150deg with an error circle in the Q-Uplane of radius about 0.1%. Synthetic non-LTE spectra for axisymmetricconfigurations based on delayed-detonation models have been computedassuming ellipsoidal geometry. The input ejecta structure andcomposition are based on a Chandrasekhar mass delayed-detonation model.The parameters of the explosion are chosen to reproduce the timeevolution of IR spectra of SN 1999by without further adjustments.Spherical models are then mapped onto ellipsoidal geometries and theaxis ratio, viewing angle, and ISP adjusted to provide the bestagreement with the polarization spectra. Both flux and polarizationspectra can be reasonably well reproduced by models with an asphericityof ~20% observed equator-on. The general properties of the polarizationcan be understood as a consequence of the structure of subluminousmodels. Best fits are obtained for the theoretical models withPISP=0.25% and Θ=140deg, consistent with theempirical method. We discuss our results for this subluminous Type Ia inthe context of ``normally bright'' Type Ia supernovae. For normallybright Type Ia, the photosphere is near the inner iron-rich layers atmaximum light and the ubiquitous iron lines give a rapid variation tothe model polarization spectra. In subluminous models, the photospherenear maximum is in the silicon layers with fewer lines and a smootheroverall polarization spectrum, as observed for SN 1999by. Though dataare sparse, the low upper limits for polarization determined for manynormal events in contrast to the high polarization in SN 1999by maysuggest a relation between the asymmetry we observed and the mechanismthat produces a subluminous Type Ia. Among various mechanisms, rapidrotation of the progenitor white dwarf and/or an explosion during abinary white dwarf merger process are likely candidates to explain theasphericity in SN 1999by.
| Is It Round? Spectropolarimetry of the Type II-p Supernova 1999EM
We present the first multiepoch spectropolarimetry of a Type II plateausupernova (SN II-P), with optical observations of SN 1999em on days 7,40, 49, 159, and 163 after discovery. These data are used to probe thegeometry of the electron scattering atmosphere before, during, and afterthe plateau phase, which ended roughly 90 days after discovery. Weakcontinuum polarization with an unchanging polarization angle(θ~160deg) is detected at all epochs, with p~0.2% onday 7, p~0.3% on days 40 and 49, and p~0.5% in the final observations.Distinct polarization modulations across strong line features arepresent on days 40, 49, 159, and 163. Uncorrected for interstellarpolarization (which is believed to be quite small), polarization peaksare associated with strong P Cygni absorption troughs, and nearlycomplete depolarization is seen across the Hα emission profile.The temporal evolution of the continuum polarization and sharp changesacross lines indicate polarization intrinsic to SN 1999em. When modeledin terms of the oblate, electron scattering atmospheres of Höflich,the observed polarization suggests an asphericity of at least 7% duringthe period studied. The temporal polarization increase may indicategreater asphericity deeper into the ejecta. We discuss the implicationsof asphericity on the use of Type II-P supernovae as primaryextragalactic distance indicators through the expanding photospheremethod (EPM). If asphericity produces directionally dependent flux andpeculiar galaxy motions are characterized byσvrec=300 km s-1, it is shownthat the agreement between previous EPM measurements of SNe II anddistances to the host galaxies predicted by a linear Hubble lawrestricts mean SN II asphericity to values less than 30% (3 σ)during the photospheric phase.
| Polarization of Broad Absorption Line QSOS. I. A Spectropolarimetric Atlas
We present a spectropolarimetric survey of 36 broad absorption linequasi-stellar objects (BAL QSOs). The continuum, absorption trough, andemission-line polarization of BAL QSOs yield clues about theirstructure. We confirm that BAL QSOs are in general more highly polarizedthan non-BAL QSOs, consistent with a more equatorial viewing directionfor the former than the latter. We have identified two new highlypolarized QSOs in our sample (1232+1325 and 1333+2840). The polarizationrises weakly to the blue in most objects, perhaps owing to scatteringand absorption by dust particles. We find that a polarization increasein the BAL troughs is a general property of polarized BAL QSOs,indicating an excess of scattered light relative to direct light, andconsistent with the unification of BAL QSOs and non-BAL QSOs. We havealso discovered evidence of resonantly scattered photons in the red wingof the C IV broad emission lines of a few objects. In most cases, thebroad emission lines have lower polarization and a different positionangle than the continuum. The polarization characteristics oflow-ionization BAL QSOs are similar to those of high-ionization BALQSOs, suggesting a similar BAL wind geometry.
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Spectropolarimetry of 3CR 68.1: A Highly Inclined Quasar
We present Keck spectropolarimetry of the highly polarized radio-loudquasar 3CR 68.1 (z = 1.228, V = 19). The polarization increases from 5%in the red (4000 Angstroms rest frame) to greater than 10% in the blue(1900 Angstroms rest frame). The broad emission lines are polarized thesame as the continuum, which shows that 3CR 68.1 is not a blazar, as ithas sometimes been regarded in the past. We also present measurements ofthe emission lines and a strong, blueshifted, associated absorption linesystem as well as a detection at the emission-line redshift of Ca II Kabsorption, presumably from stars in the host galaxy. Neither a blazarnor a partially obscured radio-quiet QSO, 3CR 68.1 belongs to anobservationally rare class of highly polarized quasars. Taking intoaccount the other unusual properties of 3CR 68.1, such as its extremelyred spectral energy distribution and its extreme lobe dominance, weexplain our spectropolarimetric results in terms of unified models. Weargue that we have a dusty, highly inclined view of 3CR 68.1, withreddened scattered (polarized) quasar light diluted by even moredust-reddened quasar light reaching us directly from the nucleus.
| Catalogue of stars in the northern Milky Way having H-alpha in emission
Not Available
| Triggered Star Formation in a Massive Galaxy at Z = 3.8: 4C 41.17
We present deep spectropolarimetric observations obtained with the W. M.Keck Telescope of the very high redshift (z = 3.79786 +/- 0.0024) radiogalaxy 4C 41.17. We find that the bright, spatially extended rest-frameUV continuum emission from this galaxy, which is aligned with the radioaxis, is unpolarized (P2 sigma < 2.4%). This implies that scatteredAGN light, which is generally the dominant contributor to the rest-frameUV emission in z ~ 1 radio galaxies, is unlikely to be a major componentof the UV flux from 4C 41.17. The resulting total light spectrum showsabsorption lines and P Cygni-like features that are similar to thosedetected in the spectra of the recently discovered population of starforming galaxies at slightly lower (z ~ 2-3) redshifts. It may bepossible for a galactic outflow to contribute partially to theabsorption line profiles of the low ionization species; however, it isunlikely that the high-velocity wings of the high ionization lines aredominated by a galactic wind since the outflow mass implied by theabsorption line strengths is very large. The detection of the S V lambda1502 stellar photospheric absorption line, the shape of the blue wing ofthe Si IV profile, the unpolarized continuum emission, the inability ofany AGN-related processes to account for the UV continuum flux, and theoverall similarity of the UV continuum spectra of 4C 41.17 and thenearby star-forming region NGC 1741B1 strongly suggest that the UV lightfrom 4C 41.17 is dominated by young, hot stars. If all of the UVemission is due to starlight from a young population, the implied starformation rate is roughly 140-1100 h^{-2}50 Mȯ yr-1. Thedeep spectroscopy presented here combined with the morphology of thesystem at radio and optical wavelengths and the possibly comparable agesfor the radio source structure and the UV stellar population suggestthat star formation in 4C 41.17 was triggered by the expansion of theradio source into the ambient medium. Our current observations areconsistent with the hypothesis that 4C 41.17 is undergoing its majorepoch of star formation at z ~ 4, and that by z ~ 1 it will have evolvedto have spectral and morphological properties similar to those observedin known z ~ 1 powerful radio galaxies.
| On the Origin of the Ultraviolet Continuum Emission from the High-Redshift Radio Galaxy 3C 256
We report spectropolarimetric observations obtained with the W. M. KeckTelescope of the high-redshift (z = 1.824) aligned radio galaxy 3C 256.Our observations confirm that the spatially extended UV continuumemission from this galaxy is polarized (P_V_ ~ 10.9% +/- 0.9%) with theelectric vector perpendicular to the aligned radio and optical majoraxes (θ_V_ ~ 48.0^deg^ + 2.4^deg^). This strongly suggests that asignificant fraction of the rest frame UV continuum emission from thegalaxy is not starlight, but is instead scattered light from a powerfulactive galactic nucleus that is hidden from our direct view. The narrowemission lines, including Mg II, are unpolarized. The percentagepolarization of the continuum emission and the polarization positionangle are roughly constant as a function of wavelength. Although thepresent data do not permit us to discriminate between cool electrons anddust as the origin of the scattering, scattering by a population of hot(T ~> 10^7^ K) electrons cannot be the dominant process, since such apopulation would overproduce X-ray emission. A large population ofcooler electrons (T ~ 10^4^ K) could be responsible for both the lineemission and the scattered light, but would require that the dust-to-gasratio in the scattering cones is 10^-3^ times smaller than that in ourGalaxy and would imply that a large fraction of the baryonic mass in thegalaxy is in the ionized component of its interstellar medium. Dustscattering is more efficient, but would result in detectable extinctionof the emission-line spectrum unless the dust distribution is morehighly clumped than the line emitting gas. Finally, we detect a strong(W^rest^lambda_ ~ 12 A) broad (FWHM ~ 6500 km s^-1^)absorption line centered at λ_rest_ ~ 1477 A. We discuss severalpossibilities for its origin and conclude that the most likely candidateis absorption by a high-velocity broad absorption line cloud near thenucleus of 3C 256.
| Keck Spatially Resolved Spectropolarimetry of the Distant Radio Galaxy 3C 324
We present the results of spatially resolved optical spectropolarimetryof the distant (z = 1.2) radio galaxy 3C 324 obtained with the W. M.Keck telescope. The spatially integrated linear polarization is P ~ 11%with the electric vector oriented perpendicular to the UV continuumaxis. The polarization of the continuum and the position angle of theelectric vector are constant with wavelength. In addition, we find thatthe polarization is spatially extended, but it does not show strongvariations along the major axis of the UV continuum. We detect the broadpermitted emission line of Mg II λ2800 in the polarized spectrum.The polarization of the narrow forbidden emission line of [O II]λ3727 is small. Scattering of anisotropic radiation emitted by ahidden quasar nucleus is the favored mechanism to explain the observedproperties. The significant reddening required to model the observedtotal flux and polarized spectra, and to explain the small residual [OII] λ3727 line polarization, suggests that a substantial amountof dust is present. Scattering by hot (T_e_ > 10^6^ K) electrons canbe ruled out by the velocity width of the polarized Mg II λ2800line. The required masses of scattering particles and the presence ofdust in 3C 324 suggest that dust scattering is dominant in individualclouds, but a contribution by warm electron scattering cannot heexcluded. Despite the large uncertainties, our data suggest that afraction of the total UV flux is due to unpolarized radiation emitted insitu and whose nature is presently unclear. We detect a faint companiongalaxy along the radio axis of 3C 324 and find that its spectrum showsevidence for ongoing star formation. We discuss the implications of ourfindings and their relevance to the interpretation of the alignmenteffect, the unified model of radio-loud active galactic nuclei, and thestellar content of high-redshift radio galaxies.
| Magnetic fields around BOK globules: CCD polarimetry of CB 4
The small Bok globule CB 4 was probed using a CCD imaging polarimeter inorder to create a detailed map of the magnetic field associated withthis cloud. Stars as faint as 17th mag at V band were measuredpolarimetrically with uncertainties less than 1%. Sky transmissionvariations were minimized via a system of synchronous polaroid rotationand bidirectional charge shifting. In all, 80 stars behind the peripheryof the globule were accurately analyzed polarimetrically. Thelarge-scale (1-2 pc) magnetic field direction around CB 4 was found tobe very uniform (P.A. = 63.3 deg +/- 1.1 deg). Double-Gaussian fittingof the polarization position angle histogram gave a dispersion of 10 degabout the primary field direction. Possible field-line compression wasfound inward of approximately 0.2 pc from the cloud center. Noappreciable twisting of field lines was found. By plotting stellarseparations against differences of polarization angles, CB 4 was foundto have a magnetic field decorrelation length of approximately 0.1 pc,similar to the size of the visually opaque cire, but much smaller thanthe size of the bright optical rim or CO half-power contour ofapproximately 0.5 pc. The magnetic field decorrelation length may berelated to a characteristic transient clumping size, or perhaps even toclumps of a more permanent nature.
| An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
| The Hubble Space Telescope Northern-Hemisphere grid of stellar polarimetric standards
Results are presented from a comprehensive program of opticalpolarimetry of strongly polarized and null-polarization standard stars.Potential sources of systematic error either instrumental in origin ordue to absolute calibration are critically discussed. Although theproject emphasized the requirements of a spaceborne optical-UVobservatory (HST), the resulting grid of standards represents a distinctimprovement over previous such tabulations and offers an opportunity toreference future multiwavelength polarimetry to a common system.
| Detection of polarized ultraviolet emission from the high-redshift radio galaxy 3C 265
Significant polarization of the total emission of the high-redshiftradio galaxy 3C 265 has been detected. The polarization position angleis roughly perpendicular to the axis of the radio emission and to themajor axis of the UV emission of the galaxy in both B and R passbands.The observed high level of polarization and orientation of thepolarization position angle suggest that a substantial fraction of theobserved UV emission is scattered light. The data show no evidence offrequency dependence in the polarization between B and R.
| On the behaviour of the lines 2905 SI II and 2932 MG II in the spectra of B-A stars
Using Orion-2 observational data, it is shown that the ratio of theintensities of absorption lines 2905 (2904+2906) Si II and 2932(2928+2936) Mg II varies strongly when moving from B0 stars to A0 stars.It is shown that this phenomenon may be used as a sensitive indicatorfor determining the effective temperature of hot stars.
| New position determinations, and other data, for 1280 known Hα-emission stars in the milky way.
Not Available
| Discovery of interstellar circular polarization in the direction of the Crab nebula.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972MNRAS.159..191M&db_key=AST
| Luminous Stars in the Northern Milky Way
Not Available
| Die galaktischen Emissions-B-Sterne : (Spectralklassifikation, Photometrie, Entwicklung und Verteilung in der Milchstraszenebene)
Not Available
| Catalogue of stellar spectra classified in the Morgan-Keenan system
Not Available
| Polarization of the Crab Nebula.
Not Available
| Photometric, Polarization, and Spectrographic Observations of O and B Stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1956ApJS....2..389H&db_key=AST
| Studies in Galactic STRUCTURE.II.LUMINOSITY Classification for 1270 Blue Giant Stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1955ApJS....2...41M&db_key=AST
| Polarization of Stellar Radiation. III. The Polarization of 841 Stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1951ApJ...114..241H&db_key=AST
| A Finding List of O and B Stars of High Luminosity.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1951ApJ...113..141N&db_key=AST
| Supplement to the Mount Wilson Catalogue and Bibliography of Stars of Classes B and a whose Spectra have Bright Hydrogen Lines.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1943ApJ....98..153M&db_key=AST
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| Csillagkép: | Bika |
| Rektaszcenzió: | 05h36m23.02s |
| Deklináció: | +21°11'11.4" |
| Vizuális fényesség: | 8.996 |
| RA sajátmozgás: | 0.7 |
| Dec sajátmozgás: | -2.5 |
| B-T magnitude: | 9.199 |
| V-T magnitude: | 9.013 |
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