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Sökning: L773:1538 4357 OR L773:0004 637X > Populärvet., debatt m.m.

  • Resultat 1-8 av 8
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1.
  • Laursen, Peter, 1975-, et al. (författare)
  • Intergalactic Transmission and its Impact on the Ly{\alpha} Line
  • 2011
  • Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 728:1, s. 52-
  • Tidskriftsartikel (populärvet., debatt m.m.)abstract
    • We study the intergalactic transmission of radiation in the vicinity of the Ly{\alpha} wavelength. Simulating sightlines through the intergalactic medium (IGM) in detailed cosmological hydrosimulations, the impact of the IGM on the shape of the line profile from Ly{\alpha} emitting galaxies at redshifts 2.5 to 6.5 is investigated. In particular we show that taking into account the correlation of the density and velocity fields of the IGM with the galaxies, the blue part of the spectrum may be appreciably reduced, even at relatively low redshifts. This may in some cases provide an alternative to the often-invoked outflow scenario, although it is concluded that this model is still a plausible explanation of the many asymmetric Ly{\alpha} profiles observed. Applying the calculated wavelength dependent transmission to simulated spectra from Ly{\alpha} emitting galaxies, we derive the fraction of photons that are lost in the IGM, in addition to what is absorbed internally in the galaxies due to dust. Moreover, by comparing the calculated transmission of radiation blueward of the Ly{\alpha} line, the total optical depth to Thomson scattering of cosmic microwave background, with corresponding observations, we are able to constrain the epoch when the Universe was reionized to z <~ 8.5.
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2.
  • Chevalier, Roger, et al. (författare)
  • Shock Breakout Emission from a Type Ib/c Supernova: XRT 080109/SN 2008D
  • 2008
  • Ingår i: The Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 683:2, s. L135-L138
  • Tidskriftsartikel (populärvet., debatt m.m.)abstract
    • The X-ray transient 080109, associated with SN 2008D, can be attributed to the shock breakout emission from a normal Type Ib/c supernova. If the observed emission is interpreted as thermal emission, the temperature and radiated energy are close to expectations, considering that scattering dominates absorption processes so that spectrum formation occurs deep within the photosphere. The X-ray emission observed at ~10 days is attributed to inverse Compton scattering of photospheric photons with relativistic electrons produced in the interaction of the supernova with the progenitor wind. A simple model for the optical/ultraviolet emission from shock breakout is developed and applied to SN 1987A, SN 1999ex, SN 2008D, and SN 2006aj, all of which have optical emission observed at t~1 day. The emission from the first three can plausibly be attributed to shock breakout emission. The photospheric temperature is most sensitive to the radius of the progenitor star core and the radii in these cases are in line with expectations from stellar evolution. The early optical/ultraviolet observations of SN 2006aj cannot be accommodated by a nonrelativistic shock breakout model in a straightforward way.
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3.
  • Fathi, Kambiz, et al. (författare)
  • Streaming Motions toward the Supermassive Black Hole in NGC 1097
  • 2006
  • Ingår i: AstrophysicalJournal Letters. - : American Astronomical Society. ; 641:L25
  • Tidskriftsartikel (populärvet., debatt m.m.)abstract
    • We have used GMOS-IFU and high-resolution HST-ACS observations to map, in unprecedented detail, the gas velocity field and structure within the 0.7 kpc circumnuclear ring of the SBb LINER/Seyfert 1 galaxy NGC 1097. We find clear evidence of radial streaming motions associated with spiral structures leading to the unresolved (<3.5 pc) nucleus, which we interpret as part of the fueling chain by which gas is transported to the nuclear starburst and supermassive black hole.
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4.
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5.
  • Storchi-Bergmann, Thaisa, et al. (författare)
  • {Nuclear spirals as feeding channels to the Supermassive Black Hole: the case of the galaxy NGC6951
  • 2007
  • Ingår i: Astropjysical Journals. - : American Astronomical Society. ; 670:959
  • Tidskriftsartikel (populärvet., debatt m.m.)abstract
    • We report the discovery of gas streaming motions along nuclear spiral arms toward the LINER nucleus of the galaxy NGC 6951. The observations, obtained using the GMOS integral field spectrograph on the Gemini North telescope, yielded maps of the flux distributions and gas kinematics in the Hα, [N ii] λ6584, and [S ii] λλ6717, 6731 emission lines of the inner 7x15 arcsec of the galaxy. This region includes a circumnuclear star‐forming ring with a radius of 500 pc, a nuclear spiral inside the ring, and the LINER nucleus. The kinematics of the ionized gas is dominated by rotation, but subtraction of a kinematic model of a rotating exponential disk reveals deviations from circular rotation within the nuclear ring that can be attributed to (1) streaming motions along the nuclear spiral arms and (2) a bipolar outflow that seems to be associated with a nuclear jet. On the basis of the observed streaming velocities and geometry of the spiral arms, we estimate a mass inflow rate of ionized gas of ≈ 3x10^-4 M yr−1, which is on the order of the accretion rate necessary to power the LINER nucleus of NGC 6951. Similar streaming motions toward the nucleus of another galaxy with a LINER nucleus, NGC 1097, have been reported by our group in a previous paper. Taken together, these results support a scenario in which nuclear spirals are channels through which matter is transferred from galactic scales to the nuclear region to feed the supermassive black hole.
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6.
  • Thöne, Christian, et al. (författare)
  • Spatially Resolved Properties of the GRB 060505 Host: Implications for the Nature of the Progenitor
  • 2008
  • Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 676:1151
  • Tidskriftsartikel (populärvet., debatt m.m.)abstract
    • GRB 060505 was the first well-observed nearby possible long-duration gamma-ray burst (GRB) that had no associated supernova. Here we present spatially resolved spectra of the host galaxy of GRB 060505, an Sbc spiral, at redshift z=0.0889. The GRB occurred inside a star-forming region in the northern spiral arm at 6.5 kpc from the center. From the position of the emission lines, we determine a maximum rotational velocity for the galaxy of v~212 km s-1, corresponding to a mass of 1.14×1011 Msolar within 11 kpc from the center. By fitting single-age spectral synthesis models to the stellar continuum, we derive a very young age for the GRB site, confirmed by photometric and Hα line measurements, of around ~6 Myr, which corresponds to the lifetime of a 32 Msolar star. The metallicity derived from several emission-line measurements varies throughout the galaxy and is lowest at the GRB site. Using the Two Degree Field Galaxy Redshift Survey we can locate the host galaxy in its large-scale (~Mpc) environment. The galaxy lies in the foreground of a filamentary overdensity, extending southwest from the galaxy cluster Abell 3837 at z=0.0896. The properties of the GRB site are similar to those found for other long-duration GRB host galaxies with high specific star formation rate and low metallicity, which is an indication that GRB 060505 originated from a young, massive star that died without making a supernova.
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7.
  • Collet, Remo, et al. (författare)
  • The Chemical Compositions of the Extreme Halo Stars HE 0107-5240 and HE 1327-2326 Inferred from Three-dimensional Hydrodynamical Model Atmospheres
  • 2006
  • Ingår i: Astrophysical Journal. - 0004-637X. ; 664:2, s. L121-L124
  • Tidskriftsartikel (populärvet., debatt m.m.)abstract
    • We investigate the impact of realistic three-dimensional (3D) hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper-iron-poor stars HE 0107-5240 and HE 1327-2326. We derive the chemical compositions of the two stars by means of a detailed 3D analysis of spectral lines under the assumption of local thermodynamic equilibrium (LTE). The lower temperatures of the line-forming regions of the hydrodynamical models cause changes in the predicted spectral line strengths. In particular, we find the 3D abundances of C, N, and O to be lower by about -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundances of iron peak elements are also decreased but by smaller factors (about -0.2 dex). We caution, however, that the neglected non-LTE effects might actually be substantial for these metals. We finally discuss possible implications for studies of early Galactic chemical evolution.
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8.
  • J.G., Cohen, et al. (författare)
  • Abundances In Very Metal-Poor Dwarf Stars
  • 2004
  • Ingår i: Astrophysical Journal. - 0004-637X. ; 603:2, s. 1107-1135
  • Tidskriftsartikel (populärvet., debatt m.m.)abstract
    • We discuss the detailed composition of 28 extremely metal-poor (EMP) dwarfs, 22 of which are from the Hamburg/ESO Survey (HES), based on Keck echelle spectra. Our sample has a median [Fe/H] of -2.7 dex, extends to -3.5 dex, and is somewhat less metal-poor than was expected from [Fe/H](HK, HES) determined from low-resolution spectra. Our analysis supports the existence of a sharp decline in the distribution of halo stars with metallicity below [Fe/H]=-3.0 dex. So far no additional turnoff stars with [Fe/H]<-3.5 have been identified in our follow-up efforts. For the best-observed elements between Mg and Ni, we find that the abundance ratios appear to have reached a plateau, i.e., [X/Fe] is approximately constant as a function of [Fe/H], except for Cr, Mn, and Co, which show trends of abundance ratios varying with [Fe/H]. These abundance ratios at low metallicity correspond approximately to the yield expected from Type II supernovae (SNe) with a narrow range in mass and explosion parameters; high-mass Type II SN progenitors are required. The dispersion of [X/Fe] about this plateau level is surprisingly small and is still dominated by measurement errors rather than intrinsic scatter. These results place strong constraints on the characteristics of the contributing SNe. The dispersion in neutron-capture elements and the abundance trends for Cr, Mn, and Co are consistent with previous studies of evolved EMP stars. We find halo-like enhancements for the α-elements Mg, Ca, and Ti, but solar Si/Fe ratios for these dwarfs. This contrasts with studies of EMP giant stars, which show Si enhancements similar to other α-elements. Sc/Fe is another case where the results from EMP dwarfs and from EMP giants disagree; our Sc/Fe ratios are enhanced compared to the solar value by ~0.2 dex. Although this conflicts with the solar Sc/Fe values seen in EMP giants, we note that α-like Sc/Fe ratios have been claimed for dwarfs at higher metallicity. Two dwarfs in the sample are carbon stars, while two others have significant C enhancements, all with 12C/13C ~7 and with C/N between 10 and 150. Three of these C-rich stars have large enhancements of the heavy neutron capture elements, including lead, which implies a strong s-process contribution, presumably from binary mass transfer; the fourth shows no excess of Sr or Ba.
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  • Resultat 1-8 av 8

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