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&