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Dust has been detected in high-redshift ($z>5$) galaxies but its origin is still being debated. Dust production in high-redshift galaxies could be dominated by stellar production or by accretion (dust growth) in the interstellar medium. Previous studies have shown that these two dust sources predict different grain size distributions, which lead to significantly different extinction curves. In this paper, we investigate how the difference in the extinction curves affects the dust attenuation properties of galaxies by performing radiative transfer calculations. To examine the major effects of the dust--stars distribution geometry, we adopt two representative cases in spherical symmetry: the well-mixed geometry (stars and dust are homogeneously mixed) and the two-layer geometry (young stars are more concentrated in the centre). In both cases, we confirm that the attenuation curve can be drastically steepened by scattering and by different optical depths between young and old stellar populations, and can be flattened by the existence of unobscured stellar populations. We can reproduce similar attenuation curves even with very different extinction curves. Thus, we conclude that it is difficult to distinguish the dust sources only with attenuation curves. However, if we include information on dust emission and plot the IRX (infrared excess)--$\beta$ (ultraviolet spectral slope) relation, different dust sources predict different positions in the IRX--$\beta$ diagram. A larger $\beta$ is preferred under a similar IRX if dust growth is the dominant dust source., Comment: 11 pages, 4 figures, accepted for publication in MNRAS

15 pages, 5 figures, 7 tables.-- ArXiv pre-print available at: http://arxiv.org/abs/0709.0813, We present characterization of the main properties of the night sky at the Calar Alto observatory for the time period between 2004 and 2007. We use optical spectrophotometric data, photometric-calibrated images taken in moonless observing periods, together with the observing conditions regularly monitored at the observatory, such as atmospheric extinction and seeing. We derive, for the first time, the typical moonless night-sky optical spectrum for the observatory. The spectrum shows a strong contamination by different pollution lines, in particular from mercury lines, whose contribution to the sky brightness in the different bands is of the order of ~0.09, ~0.16, and ~0.10 mag in B, V, and R, respectively. Regarding the strength of the sodium pollution line in comparison with the airglow emission, the observatory does not fulfill the IAU recommendations for a dark site. The zenith-corrected values of the moonless night-sky surface brightness are 22.39, 22.86, 22.01, 21.36, and 19.25 mag arcsec-2 in U, B, V, R and I, which indicates that Calar Alto is a particularly dark site for optical observations up to the I band. The fraction of astronomical useful nights at the observatory is ~70%, with ~30% of photometric nights. The typical extinction at the observatory is κ(V) ~ 0.15 mag in the winter season, with little dispersion. In summer the extinction has a wider range of values, although it does not reach the extreme peaks observed at other sites. The analysis of the winter and summer extinction curves indicates that the Rayleigh scattering is almost constant throughout the year. The rise of the extinction in the summer season is due to an enhancement of the aerosol extinction, most probably associated with an increase of dust in the atmosphere. The median seeing for the last two years (2005–2006) was ~0.90", being smaller in the summer (~0.87") than in the winter (~0.96"). We conclude in general that after 26 years of operations, Calar Alto is still a good astronomical site. Its main properties are similar in many aspects to those of other major observatories where 10 m–like telescopes are under operation or construction, thus being a natural candidate for future large aperture optical telescopes., SFS thanks the Spanish Plan Nacional de Astronomía program AYA2005-09413-C02-02, of the Spanish Ministry of Education and Science and the Plan Andaluz de Investigación of Junta de Andalucía as research group FQM322.

Polarization is an important tool to further the understanding of interstellar dust and the sources behind it. In this paper we describe our implementation of polarization that is due to scattering of light by spherical grains and electrons in the dust Monte Carlo radiative transfer code SKIRT. In contrast to the implementations of other Monte Carlo radiative transfer codes, ours uses co-moving reference frames that rely solely on the scattering processes. It fully supports the peel-off mechanism that is crucial for the efficient calculation of images in 3D Monte Carlo codes. We develop reproducible test cases that push the limits of our code. The results of our program are validated by comparison with analytically calculated solutions. Additionally, we compare results of our code to previously published results. We apply our method to models of dusty spiral galaxies at near-infrared and optical wavelengths. We calculate polarization degree maps and show them to contain signatures that trace characteristics of the dust arms independent of the inclination or rotation of the galaxy., 15 pages, 10 figures, accepted by Astronomy and Astrophysics

We present the discovery of a gravitationally lensed dust-reddened QSO at z = 2.517, identified in a survey for QSOs by infrared selection. Hubble Space Telescope imaging reveals a quadruply lensed system in a cusp configuration, with a maximum image separation of ~1.8\arcsec. We find that compared to the central image of the cusp, the neighboring brightest image is anomalous by a factor of ~ 7 - 10, which is the largest flux anomaly measured to date in a lensed QSO. Incorporating high-resolution Jansky Very Large Array radio imaging and sub-mm imaging with the Atacama Large (sub-)Millimetre Array, we conclude that a low-mass perturber is the most likely explanation for the anomaly. The optical through near-infrared spectrum reveals that the QSO is moderately reddened with E(B - V) = 0.7 - 0.9. We see an upturn in the ultraviolet spectrum due to ~ 1% of the intrinsic emission being leaked back into the line of sight, which suggests that the reddening is intrinsic and not due to the lens. The QSO may have an Eddington ratio as high as L/L_Edd ~ 0.2. Consistent with previous red QSO samples, this source exhibits outflows in its spectrum as well as morphological properties suggestive of it being in a merger-driven transitional phase. We find a host-galaxy stellar mass of log M_*/M_Sun = 11.4, which is higher than the local M_BH vs. M_* relation, but consistent with other high redshift QSOs. When de-magnified, this QSO is at the knee of the luminosity function, allowing for the detailed study of a more typical moderate-luminosity infrared-selected QSO at high redshift., Accepted for publication in ApJ; 29 pages, 18 figures, 8 tables. arXiv admin note: text overlap with arXiv:1807.05434