195 research outputs found
Abundant dust found in intergalactic space
Galactic dust constitutes approximately half of the elements more massive
than helium produced in stellar nucleosynthesis. Notwithstanding the formation
of dust grains in the dense, cool atmospheres of late-type stars, there still
remain huge uncertainties concerning the origin and fate of galactic stardust.
In this paper, we identify the intergalactic medium (i.e. the region between
gravitationally-bound galaxies) as a major sink for galactic dust. We discover
a systematic shift in the colour of background galaxies viewed through the
intergalactic medium of the nearby M81 group. This reddening coincides with
atomic, neutral gas previously detected between the group members. The
dust-to-HI mass ratio is high (1/20) compared to that of the solar neighborhood
(1/120) suggesting that the dust originates from the centre of one or more of
the galaxies in the group. Indeed, M82, which is known to be ejecting dust and
gas in a starburst-driven superwind, is cited as the probable main source.Comment: 5 pages, 3 figures, 1 table. ApJ Letters in pres
Modelling the dust content of spiral galaxies: More dust mass vs. enhanced dust grain emissivity
We present detailed modelling of the spectral energy distribution (SED) of
the spiral galaxies NGC 891, NGC 4013, and NGC 5907 in the far-infrared (FIR)
and sub-millimeter (submm) wavelengths. The model takes into account the
emission of the diffuse dust component, which is heated by the UV and optical
radiation fields produced by the stars, as well as the emission produced
locally in star forming HII complexes. The radiative transfer simulations of
Xilouris et al. (1999) in the optical bands are used to constrain the stellar
and dust geometrical parameters, as well as the total amount of dust. We find
that the submm emission predicted by our model can not account for the observed
fluxes at these wavelengths. We examine two cases, one having more dust
embedded in a second thin disk and another allowing for an enhanced
submillimeter emissivity of the dust grains. We argue that both cases can
equally well reproduce the observed SED. The case of having more dust embedded
in a second disk though, is not supported by the near-infrared observations and
thus more realistic distributions of the dust (i.e., in spiral arms and clumps)
have to be examined in order to better fit the surface brightness of each
galaxy.Comment: To appear in the proceedings of: "The Spectral Energy Distribution of
Gas-Rich Galaxies: Confronting Models with Data", Heidelberg, 4-8 Oct. 2004,
eds. C.C. Popescu and R.J. Tuffs, AIP Conf. Ser., in pres
Is the Galactic submillimeter dust emissivity underestimated?
We present detailed modeling of the spectral energy distribution (SED) of the
spiral galaxies NGC 891, NGC 4013, and NGC 5907 in the far-infrared and submm
wavelengths. The model takes into account the emission produced by the diffuse
dust and the star forming HII complexes. The dust mass is constrained by
radiative transfer simulations in the optical (Xilouris et al. 1999). We find
that the submm emission predicted by our model cannot account for the observed
fluxes. Two scenarios may account for the "missing" submm flux. In the first
scenario (Popescu et al. 2000), additional dust (to that derived from the
optical, and associated with young stars) is embedded in the galaxy in the form
of a thin disk and gives rise to additional submm emission. The other scenario
investigates whether the average submm emissivity of the dust grains is higher
than the values widely used in Galactic environments. In this case, the dust
mass is equal to that derived from the optical observations, and the submm
emissivity is treated as a free parameter calculated by fitting our model to
the observed SED. We find the submm emissivity value to be ~3 times that often
used for our Galaxy. While both scenarios reproduce the observed 850 micron
surface brightness, the extra embedded dust model is not supported by the near
infrared observations. We, thus, find that the enhanced dust submm emissivity
scenario is the most plausible. [abridged]Comment: 12 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
Depolarization of Pulsar Radio Emission
We show that intensity dependent depolarization of single pulses (e.g.,
Xiluoris et al. 1994) may be due to the nonlinear decay of the "upper" ordinary
mode into an unpolarized extraordinary mode and a backward propagating wave.
The decay occurs in the innermost parts of the pulsar magnetosphere for
obliquely propagating O waves.Comment: 6 pages, 1 postscript figur
Arecibo H I absorption measurements of pulsars and the electron density at intermediate longitudes in the first galactic quadrant
We have used the Arecibo telescope to measure the H i absorption spectra of eight pulsars. We show how kinematic distance measurements depend on the values of the Galactic constants R_0 and θ_0 , and we select our preferred current values from the literature. We then derive kinematic distances for the low-latitude pulsars in our sample and electron densities along their lines of sight. We combine these measurements with all others in the inner Galactic plane visible from Arecibo to study the electron density in this region. The electron density in the interarm range 48° ‹l ‹70° is 0.017^(+0.012)_(-0.007)(68% c.l.) cm^(−3). This is 0.75^(+0.49)_(-0.22(68% c.l) of the value calculated by the Galactic electron density model of Cordes & Lazio. The model agrees more closely with electron density measurements toward Arecibo pulsars lying closer to the Galactic center, at 30° ‹l ‹ 48° . Our analysis leads to the best current estimate of the distance of the relativistic binary pulsar B1913+16:d = 9.0 ± 3 kpc. We use the high-latitude pulsars to search for small-scale structure in the interstellar hydrogen observed in absorption over multiple epochs. PSR B0301+19 exhibited significant changes in its absorption spectrum over 22 yr, indicating H i structure on a ~500 AU scale
On the Excess Dispersion in the Polarization Position Angle of Pulsar Radio Emission
The polarization position angles (PA) of pulsar radio emission occupy a
distribution that can be much wider than what is expected from the average
linear polarization and the off-pulse instrumental noise. Contrary to our
limited understanding of the emission mechanism, the excess dispersion in PA
implies that pulsar PAs vary in a random fashion. An eigenvalue analysis of the
measured Stokes parameters is developed to determine the origin of the excess
PA dispersion. The analysis is applied to sensitive, well-calibrated
polarization observations of PSR B1929+10 and PSR B2020+28. The analysis
clarifies the origin of polarization fluctuations in the emission and reveals
that the excess PA dispersion is caused by the isotropic inflation of the data
point cluster formed by the measured Stokes parameters. The inflation of the
cluster is not consistent with random fluctuations in PA, as might be expected
from random changes in the orientation of the magnetic field lines in the
emission region or from stochastic Faraday rotation in either the pulsar
magnetosphere or the interstellar medium. The inflation of the cluster, and
thus the excess PA dispersion, is attributed to randomly polarized radiation in
the received pulsar signal. The analysis also indicates that orthogonal
polarization modes (OPM) occur where the radio emission is heavily modulated.
In fact, OPM may only occur where the modulation index exceeds a critical value
of about 0.3.Comment: Accepted for publication in Ap
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