Optical Spectroscopy with Very Large Telescope: Instrumental Development and Data Interpretation. The case of the Rocky-Planets Finder ESPRESSO and Spectral Characterization of BL Lac Objects.

This PhD thesis is focused on one of the main research area in modern Astrophysics: the spectroscopy. Within the current framework of observational astronomy, I carried out the Thesis aiming at the figure of Instrument Scientist who acts as a junction between astronomer and design engineers joining the science and technology in order to contribute to a more consolidated and optimised design of new instrumentations for large experiments. For this reason, in this Thesis I will present two main parts of my PhD activities that have characterised my entire doctoral training. In particular, I will first introduce ESPRESSO, the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations, which will be a new generation extra stable spectrograph at the European Southern Observatory (ESO) Very Large Telescope (VLT). ESPRESSO will be a spectrograph mainly optimised for finding rocky planet orbiting main sequence G stars in the habitable zone through the Radial Velocity (RV) method. I will outline my contribution to ESPRESSO in Chapter 2 where I illustrate simulations on the expected performances of the detectors used by a main part of the instrument: the Front End Unit. This subsystem is responsible to stabilise the field and pupil of ESPRESSO through active optics and to collect the light from the four telescope and inject it through fiber feeding the spectrograph. I also outline the design of the Exposure Meter of the instrument which is in charge to continuously monitoring the exposure in order to evaluate a critical value, called Mean Time of Exposure (MTE), which is used in order to correct the measured Radial Velocity for the relative Earth motion at the time of observation. For the rst time, the exposure will be monitored chromatically allowing a better characterisation of the behaviour of the MTE and of the overall observation simultaneously. In Chapter 3 I focus on pure astrophysical research based on spectroscopy of a particular class of Active Galatic Nuclei: the BL Lacertae Objects. These objects are characterised by a strong non-thermal emission which arises from the accreting nucleus. For this reason, spectral features (when present) are extremely diluted making the determination of their redshifts a challenging task. With the adoption of FORS2 spectrograph, in low resolution, at 10mt class VLT I completed a survey of these objects started few years ago and I derived from the complete sample a conspicuous number of properties of the class, including the characterisation of their line-of-sight Mg II absorber systems. Moreover, I will outline the results obtained from observation of a small subsample of strong FERMI gamma-rays (and in certain cases VHE emitters) source with ESO X-SHOOTER which combines medium resolution (R 4000) and large spectral range. In particular, for the source PKS 0048-097 the determination of its redshift for the first time allowed to deeply investigate its Spectral Energy Distribution and the close environment of the object. The spectra of the other sources in the sample are also illustrated and explicated for particular interesting objects

Spectroscopy of BL Lac objects of extraordinary luminosity

Aims. We aim to determine the redshift (or stringent lower limits) of a number of bright BL Lacs objects. Methods. We secured medium resolution optical and near-infrared spectra of 4 bright BL Lac objects of unknown redshift using the spectrograph X-Shooter at the ESO-VLT. Results. In spite of the high quality of the spectra and the extended spectral range of the observations we have not detected intrisic spectral features for these sources. However we are able to provide strigent lower limits to their redshift. In particular, for the two TeV sources PG 1553+113 and H 1722+119 we infer z > 0.30 and z > 0.35 respectively. We also detect an intervening Ca II absorption doublet in the spectrum of MH 2136-428 that is ascribed to the the halo of a nearby giant elliptical galaxy at \sim 100 kpc of projected distance. Conclusions. Under the hypothesis that all BL Lacs are hosted by luminous bulge dominated galaxies, the present state of art spectroscopic observations of bright BL Lacs indicate that these objects are likely sources with extremely beamed nuclear emission . We present simulations to show under which circustances it will be possible to probe this hypothesis from the detection of very weak absorptions using the next generation of extremely large optical telescopes.Comment: Accepted for publication in A&

On the redshift of the very high energy {\gamma}-ray BL Lac object S2 0109+22

The determination of the redshift of extragalactic gamma ray sources is of fundamental importance for the interpretation of their emission models and extragalactic population studies, AGN classification schemes, and to study the interaction with the extragalactic background light. Here we focus on the BL Lac S2 0109+22, recently detected at very high energies. We obtained a high signal-to-noise optical spectrum at Gran Telescopio Canarias for the source over the spectral range 4000-10000 Angstrom, to search for spectral features of the host galaxy and/or of the nuclear source and it was compared with previous observations in order to confirm and/or dispute its redshift determination. We find the S2 0109+22 optical spectrum is well represented by a power law continuum without any absorption or emission feature of Equivalent Width >0.1 Angstrom. Based on the new optical spectrum we are able to set a redshift lower limit z>0.35, assuming the source is hosted by a massive elliptical galaxy typical for this class of sources. The redshift z=0.265 proposed in the literature for this object is most probably referred to a r=18.3 galaxy at 15" from S2 0109. This galaxy could belong to a group of faint galaxies located at z~0.26.Comment: 5 pages, 5 figures; accepted on MNRA

Spectroscopy of 10 gamma-ray BL Lac objects at high redshift

We present high S/N optical spectra of 10 BL Lac objects detected at GeV energies by Fermi satellite (3FGL catalog), for which previous observations suggested that they 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 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 for 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).Comment: 10 pages, 2 figures, 4 tables; accepted for publication in Ap

ESO VLT Optical Spectroscopy of BL Lac Objects IV. New spectra and properties of the full sample

We present the last chapter of a spectroscopy program aimed at deriving the redshift or a lower limit to the redshift of BL Lac objects using medium resolution spectroscopy. Here we report new spectra for 33 BL Lac object candidates obtained in 2008-2009 confirming the BL Lac nature of 25 sources and for 5 objects we obtained new redshifts. These new observations are combined with our previous data in order to construct a homogeneous sample of \sim 70 BL Lacs with high quality spectroscopy. All these spectra can be accessed at the website http://www.oapd.inaf.it/zbllac/. The average spectrum, beaming properties of the full sample, discussion on intervening systems and future perspectives are addressed.Comment: 25 pages, 13 Figures. Accepted for publication in the Astronomical Journa