We present the detection of a sample of galaxies which are extremely faint in the optical and near-infrared but bright at mid-infrared wavelengths. This population of galaxies, missed by the deepest HST surveys such as CANDELS or Hubble Frontier Fields, are considerably bright in IRAC. The bulk of the sample (65%) is located in a 2σ region around

The quest to find relic galaxies is ongoing, i.e. galaxies that seem to remain untouched from the primeval Universe. These galaxies are usually massive (> 8×10^{10} M_{⊙}), with very small sizes (effective radius 2 Kpc) and with old (> 10 Gyr) stellar populations. Observationally, it is not well tested whether these objects live in galaxy

The young σ Orionis cluster in the Ori OB 1b association is one of the most important clusters for understanding the stellar and substellar formation and evolution. In this work, we used 197 low-resolution optical spectra of 167 stars. We determined spectral types and measured Li I and Hα equivalent widths (EWs). We used Gaia DR2 astrometry

We report the study of an “Einstein Cross” configuration first identified in a set of HST images by Cerny et al. Deep spectroscopic observations obtained at the Spanish 10.4 m Gran Telescopio Canarias telescope, allowed us to demonstrate the lens nature of the system, that consists of a Lyman-break galaxy (LBG), not a quasi-stellar object as is

The criteria for spectral classification of B-type stars were established almost 80 years ago from low-resolution spectra on photographic plates. Since Hipparcos measured accurate distances for nearby stars, it has been clear that, although sound in general terms, the current set of MK standard stars in the B range has many inconsistencies, with

The mechanisms producing fast variability of the γ-ray emission in active galactic nuclei (AGNs) are under debate. The MAGIC telescopes detected a fast, very-high-energy (VHE, E > 100 GeV) γ-ray flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of (1.5 ± 0.3) × 10-10 photons cm-2 s-1 and halving time of 26 ± 8 min. The MAGIC