Aims:We introduce analytical response functions and their main properties as an important diagnostic tool that help understand Stokes profile formation physics and the meaning of well-known behaviors of standard inversion codes of the radiative transfer equation often used to measure solar magnetic fields. Methods: A Milne-Eddington model

We report on the discovery of a faint (MV~-10.6+/-0.2) dwarf spheroidal galaxy on deep F606W and F814W Hubble Space Telescope images of a Virgo intracluster field. The galaxy is easily resolved in our images, as our color magnitude diagram (CMD) extends >~1 magnitude beyond the tip of the red giant branch (RGB). Thus, it is the deepest CMD for a

Using the Millennium N-body Simulation we explore how the shape and angular momentum of galaxy dark matter haloes surrounding the largest cosmological voids are oriented. We find that the major and intermediate axes of the haloes tend to lie parallel to the surface of the voids, whereas the minor axis points preferentially in the radial direction

We have used the Hubble Space Telescope's Advanced Camera for Surveys (ACS) to detect and measure ~5300 stars in a single intracluster field in the Virgo Cluster. By performing F606W and F814W photometry on these stars, we have determined their metallicity distribution function and constrained the types of stars present in this portion of Virgo's

We present echelle spectrophotometry of the blue compact dwarf galaxy NGC 5253 obtained with the VLT UVES. We have measured the intensities of a large number of permitted and forbidden emission lines in four zones of the central part of the galaxy. We detect faint C II and O II recombination lines, the first time that these are unambiguously

We report the results of spectroscopic observations, obtained with the Gemini Multi-Object Spectrograph, of eight planetary nebulae (PNe) in the dwarf spheroidal (dSph) galaxy NGC 147, a companion of M31. The physico-chemical properties of the six brightest PNe were derived using both the empirical ICF method and photoionization modelling with