409 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
Calculating the submillimeter emissivity of dust grains in spiral galaxies
We use the radiation transfer simulation of Xilouris et al. (1999) to
constrain the quantity of dust in three nearby spiral galaxies (NGC 4013, NGC
5907 and NGC 891). The predicted visual optical depth from the model is
compared with the thermal continuum radiation detected from these galaxies at
850 microns. This comparison yields the emissivity of dust grains in the
submillimeter waveband which is a factor 4 higher than the benchmark,
semi-empirical model of Draine & Lee (1984). Our estimates are more closely
aligned with recent measurements carried out in the laboratory on amorphous
carbon and silicate particulates. A comparison between the distribution of 850
microns surface brightness and the intensity levels in the ^{12}CO(1-0) and 21
cm lines underlines the spatial association between dust detected in the
submillimeter waveband and molecular gas clouds. We suggest that the relatively
high emissivity values that we derive may be attributable to amorphous, fluffy
grains situated in denser gas environments.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
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
The extent of dust in NGC 891 from Herschel/SPIRE images
We analyse Herschel/SPIRE images of the edge-on spiral galaxy NGC 891 at 250,
350 and 500 micron. Using a 3D radiative transfer model we confirm that the
dust has a radial fall-off similar to the stellar disk. The dust disk shows a
break at about 12 kpc from the center, where the profile becomes steeper.
Beyond this break, emission can be traced up to 90% of the optical disk in the
NE side. On the SW, we confirm dust emission associated with the extended,
asymmetric HI disk, previously detected by the Infrared Space Observatory
(ISO). This emission is marginally consistent with the large diffuse dust disk
inferred from radiative transfer fits to optical images. No excess emission is
found above the plane beyond that of the thin, unresolved, disk.Comment: Letter accepted for publication in A&A; final version after
shortening and language editin
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
X-ray selected Infrared Excess AGN in the Chandra Deep Fields: a moderate fraction of Compton-thick sources
We examine the properties of the X-ray detected, Infrared Excess AGN or Dust
Obscured Galaxies (DOGs) in the Chandra Deep Fields (CDF). We find 26 X-ray
selected sources which obey the 24 micron to R-band flux ratio criterion
f_24/f_R>1000. These are at a median redshift of 2.3 while their IR
luminosities are above 10^12 solar. Their X-ray luminosities are all above a
few times 10^42 erg s-1 in the 2-10 keV band unambiguously arguing that these
host AGN. Nevertheless, their IR Spectral Energy Distributions are split
between AGN (Mrk231) and star-forming templates (Arp220). Our primary goal is
to examine their individual X-ray spectra in order to assess whether this X-ray
detected DOG population contains heavily obscured or even Compton-thick
sources. The X-ray spectroscopy reveals a mixed bag of objects. We find that
four out of the 12 sources with adequate photon statistics and hence reliable
X-ray spectra, show evidence for a hard X-ray spectral index (~1) or
harder,consistent with a Compton-thick spectrum. In total 12 out of the 26 DOGs
show evidence for flat spectral indices. However, owing to the limited photon
statistics we cannot differentiate whether these are flat because they are
reflection-dominated or because they show moderate amounts of absorption. Seven
DOGs show relatively steep spectra (>1.4) indicative of small column densities.
All the above suggest a fraction of Compton-thick sources that does not exceed
5%. The average X-ray spectrum of all 26 DOGs is hard (~1.1) or even harder
(~0.6) when we exclude the brightest sources. These spectral indices are well
in agreement with the stacked spectrum of X-ray undetected sources (~0.8 in the
CDFN). This could suggest (but not necessarily prove) that X-ray undetected
DOGs, in a similar fashion to the X-ray detected ones presented here, are
hosting a moderate fraction of Compton-thick sources.Comment: 16 pages To appear in A&
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