Using a simple model of photodissociated atomic hydrogen on a galactic scale, it is possible to derive total hydrogen volume densities. These densities, obtained through a combination of atomic hydrogen, far-ultraviolet and metallicity data, provide an independent probe of the combined atomic and molecular hydrogen gas in galactic discs. We present

We present a method for speckle holography that is optimized for crowded fields. Its two key features are an iterative improvement of the instantaneous point spread functions (PSFs) extracted from each speckle frame and the (optional) simultaneous use of multiple reference stars. In this way, high signal-to-noise ratio and accuracy can be achieved

Aims: The aim of the project is to contribute to the characterisation of the spectral energy distribution of the coolest brown dwarfs discovered to date, the Y dwarfs. Methods: We obtained z-band far-red imaging for six Y dwarfs and a T9 + Y0 binary with the OSIRIS (Optical System for Imaging and low Resolution Integrated Spectroscopy) instrument

The launch of NASA's Wide-field Infrared Survey Explorer (WISE) in late 2009 ushered in an new era in the study of the solar neighborhood. Its mid-infrared capabilities have proven critical to the discovery of the bulk of the coolest (\teff 1000 K) brown dwarfs, including the new Y dwarfs, and its all-sky coverage lends itself to searches for high

Aims: We aim to study the dust ejected by main-belt comet (MBC) (300163) 2006 VW139 to obtain information on the ejection mechanism and the spectral properties of the object. This will help to see if they are compatible with those of "normal" comets. Methods: Broad-band images in the g and r band as well as a low-resolution spectrum in the 0.35-0.9

This paper is the second of two papers devoted to the study of the evolution of the cosmological horizons (particle and event horizons). Specifically, in this paper we consider a general accelerated universe with countably infinitely many constant state equations, and we obtain simple expressions in terms of their respective recession velocities