255 research outputs found
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
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
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&
The Inner Scale Length of Spiral Galaxy Rotation Curves
We use the tapering effect of Halpha/[NII] rotation curves of spiral galaxies
first noted by Goad & Roberts (1981) to investigate the internal extinction in
disks. The scale length of exponential fits to the inner part of rotation
curves depends strongly on the disk axial ratio. Preliminary modelling of the
effect implies substantial opacity of the central parts of disks at a
wavelength of 0.66 \mu. In addition, the average kinematic scale length of
rotation curves, when corrected to face--on perspective, has a nearly constant
value of about 1.7 h^{-1} kpc, for all luminosity classes. The interpretation
of that effect, as the result of the increasing dominance of the baryonic mass
in the inner parts of galaxies, yields a mean baryonic mass--to--light ratio in
the I band \Upsilon_I= 2.7h M_sun/L_{sun,I}, within the inner 1.7 h^{-1} kpc of
disks.Comment: 11 pages, including one table and one figure, AAStex scheduled to
appear in the Astrophysical Journal (Letters
Radiation Transfer Models in Galaxies
The dust in galaxies makes radiation transport calculations in them
absolutely necessary. It is not only common practice in Astrophysics, but also
wisdom, to try to make as simple models as possible to simulate physical
systems. For spiral galaxies, however, this turned out to be catastrophic. For
years, the major question of the opacity of spiral galaxies kept the community
divided, because the models were too simple. A spiral galaxy appears, to first
order, to have exponential distributions of stars and dust, which cannot be
approximated with uniform distributions. We will review the radiative transfer
methods used in galaxies and we will comment on their pluses and minuses. We
will also present some of the main results of the application of one of the
methods to the observations.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
Dust properties of external galaxies; NGC 891 revisited
We compare 850um SCUBA images of NGC 891 with the corresponding V-band
optical depth predicted from radiation transfer simulations. These two tracers
of dust show a very similar distribution along the minor axis and a reasonable
agreement along the major axis. Assuming that the grains responsible for
optical extinction are also the source of 850um emission we derive a
submillimeter emissivity (emission efficiency) for dust in the NGC 891 disk.
This quantity is found to be a factor of 2-3 higher than the generally-accepted
(but highly uncertain) values adopted for the Milky Way. It should be stated,
however, that if a substantial fraction of dust in NGC 891 is clumped, the
emissivity in the two galaxies may be quite similar. We use our newly-acquired
emissivity to convert our 850um images into detailed maps of dust mass and,
utilizing 21cm and CO-emission data for NGC 891, derive the gas-to-dust ratio
along the disk. We compute an average ratio of 260 -- a value consistent with
the Milky Way and external spirals within the uncertainties in deriving both
the dust mass and the quantity of molecular gas. The bulk of dust in NGC 891
appears to be closely associated with the molecular gas phase although it may
start to follow the distribution of atomic hydrogen at radii >9 kpc (i.e. >0.5
R_25). Using the optical depth of the NGC 891 disk, we quantify how light
emitted at high redshift is attenuated by dust residing in foreground spirals.
For B-band observations of galaxies typically found in the Hubble Deep Field,
the amount of light lost is expected to be small (~ 5%). This value depends
critically on the maximum radial extent of cold dust in spiral disks (which is
poorly known). It may also represent a lower limit if galaxies expel dust over
time into the intergalactic medium.Comment: 22 pages, 7 figures, A&A accepte
Are spiral galaxies optically thin or thick?
The opacity of spiral galaxies is examined by modelling the dust and stellar
content of individual galaxies. The model is applied to five late-type spiral
galaxies (NGC 4013, IC 2531, UGC 1082, NGC 5529 and NGC 5907). Having analyzed
a total of seven galaxies thus far, the five galaxies mentioned above plus UGC
2048 and NGC 891 presented in (Xilouris et al. 1997, 1998), we are able to draw
some general conclusions, the most significant of which are: 1) The face-on
central optical depth is less than one in all optical bands indicating that
typical spiral galaxies like the ones that we have modelled would be completely
transparent if they were to be seen face-on. 2) The dust scaleheight is about
half that of the stars, which means that the dust is more concentrated near the
plane of the disk. 3) The dust scalelength is about 1.4 times larger than that
of the stars and the dust is more radially extended than the stars. 4) The dust
mass is found to be about an order of a magnitude more than previously measured
using the IRAS fluxes, indicating the existence of a cold dust component. The
gas-to-dust mass ratio calculated is close to the value derived for our Galaxy.
5) The derived extinction law matches quite well the Galactic extinction law,
indicating a universal dust behaviour.Comment: 13 pages. Accepted for publication in A&
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