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We report on optical spectroscopy obtained at the 10.4 m Gran Telescopio Canarias of 28 Fermi γ-ray sources that completes the study of a sample of 60 targets of unidentified objects for which the detection of an X-ray and/or radio source inside the 3FGL error box is available. The observations aim to characterize the nature and measure the redshift of these sources. For all optical counterparts, the observations allow us to establish their active galactic nucleus nature. In particular, we found 24 BL Lac objects, one quasar, one narrow line Seyfert 1, and two objects showing spectral features typical of Seyfert 2 galaxies. For most of these, we determine a spectroscopic redshift, while for five we can set lower limits based on the lack of stellar features from the host galaxy. The global properties of the full sample are briefly discussed. [ABSTRACT FROM AUTHOR]

We report on the detection of a stellar halo around NGC 1560, a 109M⊙ spiral galaxy member of the Maffei group. We obtained deep images in the r- and i-bands using the 10.4 m Gran Telescopio Canarias in a field centered at ∼3.7 arcmin (projected distance of 3.5 kpc) from the center of this galaxy. The luminosity function and the color–magnitude diagram show a clear excess of stars with respect to the expected foreground level at magnitudes fainter than the red giant branch tip at the distance of NGC 1560. The colors of the halo stars imply a metallicity of Z ∼ Z⊙/50, while their counts correspond to a stellar mass of ∼107M⊙ in the sampled region. Assuming a power-law profile for the surface mass density of the halo, our data suggest a total stellar mass of 108M⊙ for the halo of NGC 1560. [ABSTRACT FROM AUTHOR]

We present optical spectroscopy secured at the 10 m Gran Telescopio Canarias of the counterparts of 20 extragalactic γ-ray sources detected by the Fermi satellite. The observations allow us to investigate the nature of these sources and to determine their redshift. We find that all optical counterparts have a spectrum that is consistent with a BL Lac object nature. We are able to determine the redshift for 11 objects and set spectroscopic redshift limits for five targets. The optical spectrum is found featureless for only four sources. In the latter cases, we can set lower limits on the redshift based on the assumption that they are hosted by a typical massive elliptical galaxy whose spectrum is diluted by the nonthermal continuum. The observations allow us to unveil the nature of these gamma-ray sources and provide a sanity check of a tool to discover the counterparts of γ-ray emitters/blazars based on their multiwavelength emission. [ABSTRACT FROM AUTHOR]

We present optical spectra with high signal-to-noise ratio of 10 BL Lac objects detected at GeV energies by the Fermi satellite (3FGL catalog), which previous observations suggested are at relatively high redshift. The new observations, obtained at the 10 m Gran Telescopio Canarias, allowed us to find the redshift for J0814.5+2943 (z = 0.703), and we can set a spectroscopic lower limit for J0008.0+4713 (z > 1.659) and J1107.7+0222 (z > 1.0735) on the basis of Mg ii intervening absorption features. In addition we confirm the redshifts for J0505.5+0416 (z = 0.423) and J1450+5200 (z > 2.470). Finally we contradict the previous z estimates for five objects (J0049.7+0237, J0243.5+7119, J0802.0+1005, J1109.4+2411, and J2116.1+3339). [ABSTRACT FROM AUTHOR]

We report results of a spectroscopic campaign carried out at the 10 m Gran Telescopio Canarias for a sample of 22 BL Lac objects detected (or candidates) at TeV energies, aiming to determine or constrain their redshift. This is of fundamental importance for the interpretation of their emission models and for population studies and is also mandatory for studying the interaction of high-energy photons with the extragalactic background light using TeV sources. Optical spectra with high signal-to-noise ratios in the range 4250–10000 Å were obtained to search for faint emission or absorption lines from both the host galaxy and the nucleus. We determine a new redshift for PKS 1424+240 (z = 0.604) and a tentative one for 1ES 0033+595 (z = 0.467). We are able to set new spectroscopic redshift lower limits for three other sources on the basis of Mg ii and Ca ii intervening absorption features: BZB J1243+3627 (z > 0.483), BZB J1540+8155 (z > 0.672), and BZB 0J2323+4210 (z > 0.267). We confirm previous redshift estimates for four blazars: S3 0218+357 (z = 0.944), 1ES 1215+303 (z = 0.129), W Comae (z = 0.102), and MS 1221.8+2452 (z = 0.218). For the remaining targets, in seven cases (S2 0109+22, 3C 66A, VER J0521+211, S4 0954+65, BZB J1120+4214, S3 1227+25, BZB J2323+4210), we do not validate the proposed redshift. Finally, for all sources of still-unknown redshift, we set a lower limit based on the minimum equivalent width of absorption features expected from the host galaxy. [ABSTRACT FROM AUTHOR]