Deep spectra of the planetary nebula IC 418 were obtained in the wavelength range 4190-5850 A. The data made it possible to map an extended halo about the source, evaluate the variation in halo intensity with radius, and model the results in terms of a radially symmetric dust scatter envelope. It is suggested that the nebula is in a relatively

We present new [S II] images of the Herbig-Haro (HH) 30 jet and counterjet observed in 2006, 2007, and 2010 that, combined with previous data, allowed us to measure with improved accuracy the positions and proper motions of the jet and counterjet knots. Our results show that the motion of the knots is essentially ballistic, with the exception of

We report the detection of a radial velocity companion to SDSS J091709.55+463821.8, the lowest mass white dwarf currently known, with M~0.17 Msolar. The radial velocity of the white dwarf shows variations with a semi-amplitude of 148.8+/-6.9 km s-1 and a period of 7.5936+/-0.0024 hr, which implies a companion mass of M>=0.28 Msolar. The lack of

Nuclear rings are characterized by their high star formation rates, and are believed to be the products of gas inflow towards the inner regions of spiral galaxies. We present first results from a statistical survey of these rings, aimed to provide an insight into their triggering mechanisms and star formation properties. We gathered photometric

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The cosmic star formation rate is observed to drop sharply after redshift z= 2. We use two large, cosmological, smoothed particle hydrodynamics simulations to investigate how this decline is related to the evolution of gas accretion and to outflows driven by active galactic nuclei (AGN). We find that the drop in the star formation rate follows a