Browsing by Author "Radovich, M."

Now showing 1 - 9 of 9
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    A random forest-based selection of optically variable AGN in the VST-COSMOS field
    (2021) De Cicco, D.; Bauer, F. E.; Paolillo, M.; Cavuoti, S.; Sanchez-Saez, P.; Brandt, W. N.; Pignata, G.; Vaccari, M.; Radovich, M.
    Context. The survey of the COSMOS field by the VLT Survey Telescope is an appealing testing ground for variability studies of active galactic nuclei (AGN). With 54 r-band visits over 3.3 yr and a single-visit depth of 24.6 r-band mag, the dataset is also particularly interesting in the context of performance forecasting for the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST).Aims. This work is the fifth in a series dedicated to the development of an automated, robust, and efficient methodology to identify optically variable AGN, aimed at deploying it on future LSST data.Methods. We test the performance of a random forest (RF) algorithm in selecting optically variable AGN candidates, investigating how the use of different AGN labeled sets (LSs) and features sets affects this performance. We define a heterogeneous AGN LS and choose a set of variability features and optical and near-infrared colors based on what can be extracted from LSST data.Results. We find that an AGN LS that includes only Type I sources allows for the selection of a highly pure (91%) sample of AGN candidates, obtaining a completeness with respect to spectroscopically confirmed AGN of 69% (vs. 59% in our previous work). The addition of colors to variability features mildly improves the performance of the RF classifier, while colors alone prove less effective than variability in selecting AGN as they return contaminated samples of candidates and fail to identify most host-dominated AGN. We observe that a bright (r less than or similar to 21 mag) AGN LS is able to retrieve candidate samples not affected by the magnitude cut, which is of great importance as faint AGN LSs for LSST-related studies will be hard to find and likely imbalanced. We estimate a sky density of 6.2x10(6) AGN for the LSST main survey down to our current magnitude limit.
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    A structure function analysis of VST-COSMOS AGN
    (2022) De Cicco, D.; Bauer, F. E.; Paolillo, M.; Sanchez-Saez, P.; Brandt, W. N.; Vagnetti, F.; Pignata, G.; Radovich, M.; Vaccari, M.
    Context. We present our sixth work in a series dedicated to variability studies of active galactic nuclei (AGN), based on the survey of the COSMOS field by the VLT Survey Telescope (VST). Its 54 r-band visits over 3.3 yr and single-visit depth of 24.6 r-band mag make this dataset a valuable scaled-down version that can help forecast the performance of the Rubin Observatory Legacy Survey of Space and Time (LSST).
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    Discovery of Two Einstein Crosses from Massive Post-blue Nugget Galaxies at z > 1 in KiDS*
    (2020) Napolitano, N. R.; Li, R.; Spiniello, C.; Tortora, C.; Sergeyev, A.; D'Ago, G.; Guo, X.; Xie, L.; Radovich, M.; Roy, N.; Koopmans, L. V. E.; Kuijken, K.; Bilicki, M.; Erben, T.; Getman, F.; Heymans, C.; Hildebrandt, H.; Moya, C.; Shan, H. Y.; Vernardos, G.; Wright, A. H.
    We report the discovery of two Einstein Crosses (ECs) in the footprint of the Kilo-Degree Survey (KiDS): KIDS J232940-340922 and KIDS J122456+005048. Using integral field spectroscopy from the Multi Unit Spectroscopic Explorer at the Very Large Telescope, we confirm their gravitational-lens nature. In both cases, the four spectra of the source clearly show a prominence of absorption features, hence revealing an evolved stellar population with little star formation. The lensing model of the two systems, assuming a singular isothermal ellipsoid (SIE) with external shear, shows that: (1) the two crosses, located at redshift z = 0.38 and 0.24, have Einstein radius R-E = 5.2 kpc and 5.4 kpc, respectively; (2) their projected dark matter fractions inside the half effective radius are 0.60 and 0.56 (Chabrier initial mass function); (3) the sources are ultra-compact galaxies, R-e similar to 0.9 kpc (at redshift, z(s) = 1.59) and R-e similar to 0.5 kpc (z(s) = 1.10), respectively. These results are unaffected by the underlying mass density assumption. Due to size, blue color, and absorption-dominated spectra, corroborated by low specific star formation rates derived from optical-near-infrared spectral energy distribution fitting, we argue that the two lensed sources in these ECs are blue nuggets migrating toward their quenching phase.
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    INSPIRE: INvestigating Stellar Population In RElics II. First data release (DR1)
    (2021) Spiniello, C.; Tortora, C.; D'Ago, G.; Coccato, L.; La Barbera, F.; Ferre-Mateu, A.; Pulsoni, C.; Arnaboldi, M.; Gallazzi, A.; Hunt, L.; Napolitano, N. R.; Radovich, M.; Scognamiglio, D.; Spavone, M.; Zibetti, S.
    Context. The INvestigating Stellar Population In RElics (INSPIRE) is an ongoing project targeting 52 ultra-compact massive galaxies at 0.1 < z < 0.5 with the X-shooter at VLT spectrograph (XSH). These objects are the ideal candidates to be `relics', massive red nuggets that have formed at high redshift (z > 2) through a short and intense star formation burst, and then have evolved passively and undisturbed until the present day. Relics provide a unique opportunity to study the mechanisms of star formation at high-z. Aims. INSPIRE is designed to spectroscopically confirm and fully characterise a large sample of relics, computing their number density in the redshift window 0:1 < z < 0:5 for the first time, thus providing a benchmark for cosmological galaxy formation simulations. In this paper, we present the INSPIRE Data Release (DR1), comprising 19 systems with observations completed in 2020. Methods. We use the methods already presented in the INSPIRE Pilot, but revisiting the 1D spectral extraction. For the 19 systems studied here, we obtain an estimate of the stellar velocity dispersion, fitting the two XSH arms (UVB and VIS) separately at their original spectral resolution to two spectra extracted in di fferent ways. We estimate [Mg /Fe] abundances via line-index strength and mass-weighted integrated stellar ages and metallicities with full spectral fitting on the combined (UVB +VIS) spectrum. Results. For each system, di fferent estimates of the velocity dispersion always agree within the errors. Spectroscopic ages are very old for 13 /19 galaxies, in agreement with the photometric ones, and metallicities are almost always (18 /19) super-solar, confirming the mass-metallicity relation. The [Mg /Fe] ratio is also larger than solar for the great majority of the galaxies, as expected. We find that ten objects formed more than 75% of their stellar mass (M-*) within 3 Gyr from the big bang and classify them as relics. Among these, we identify four galaxies that had already fully assembled their M-* by that time and are therefore `extreme relics' of the ancient Universe. Interestingly, relics, overall, have a larger [Mg /Fe] and a more metal-rich stellar population. They also have larger integrated velocity dispersion values compared to non-relics (both ultra-compact and normal-size) of similar stellar mass. Conclusions. The INSPIRE DR1 catalogue of ten known relics is the largest publicly available collection, augmenting the total number of confirmed relics by a factor of 3.3, and also enlarging the redshift window. The resulting lower limit for the number density of relics at 0.17 < z < 0.39 is rho similar to 9.1 x 10(-8) Mpc(-3).
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    INSPIRE: INvestigating Stellar Population In RElics III. Second data release (DR2): testing the systematics on the stellar velocity dispersion
    (2023) D'Ago, G.; Spiniello, C.; Coccato, L.; Tortora, C.; La Barbera, F.; Arnaboldi, M.; Bevacqua, D.; Ferre-Mateu, A.; Gallazzi, A.; Hartke, J.; Hunt, L. K.; Martin-Navarro, I.; Napolitano, N. R.; Pulsoni, C.; Radovich, M.; Saracco, P.; Scognamiglio, D.; Zibetti, S.
    Context. The project called INvestigating Stellar Population In RElics (INSPIRE) is based on VLT/X-shooter data from the homonymous on-going ESO Large Program. It targets 52 ultra-compact massive galaxies at 0.1 < z < 0.5 with the goal of constraining their kinematics and stellar population properties in great detail and of analysing their relic nature.
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    INSPIRE: INvestigating Stellar Population In RElics: I. Survey presentation and pilot study
    (2021) Spiniello, C.; Tortora, C.; D'Ago, G.; Coccato, L.; La Barbera, F.; Ferre-Mateu, A.; Napolitano, N. R.; Spavone, M.; Scognamiglio, D.; Arnaboldi, M.; Gallazzi, A.; Hunt, L.; Moehler, S.; Radovich, M.; Zibetti, S.
    Context. Massive elliptical galaxies are thought to form through a two-phase process. At early times (z> 2), an intense and fast starburst forms blue and disk-dominated galaxies. After quenching, the remaining structures become red, compact, and massive (i.e. red nuggets). Then, a time-extended second phase, which is dominated by mergers, causes structural evolution and size growth. Given the stochastic nature of mergers, a small fraction of red nuggets survive, without any interaction, massive and compact until today: these are relic galaxies. Since this fraction depends on the processes dominating the size growth, counting relics at low-z is a valuable way of disentangling between different galaxy evolution models.Aims. In this paper, we introduce the INvestigating Stellar Population In RElics (INSPIRE) Project, which aims to spectroscopically confirm and fully characterise a large number of relics at 0.1< z< 0.5. We focus here on the first results based on a pilot study targeting three systems, representative of the whole sample.Methods. For these three candidates, we extracted 1D optical spectra over an aperture of r=0.40 '', which comprises similar to 30% of the galaxies' light, and we obtained the line-of-sight integrated stellar velocity and velocity dispersion. We also inferred the stellar [alpha /Fe] abundance from line-index measurements and mass-weighted age and metallicity from full-spectral fitting with single stellar population models.Results. Two galaxies have large integrated stellar velocity dispersion values (sigma (star)similar to 250 km s(-1)), confirming their massive nature. They are populated by stars with super-solar metallicity and [alpha /Fe]. Both objects have formed >= 80% of their stellar mass within a short (similar to 0.5-1.0 Gyr) initial star formation episode occurred only similar to 1 Gyr after the Big Bang. The third galaxy has a more extended star formation history and a lower velocity dispersion. Thus we confirm two out of three candidates as relics.Conclusions. This paper is the first step towards assembling the final INSPIRE catalogue that will set stringent lower limits on the number density of relics at z< 0.5, thus constituting a benchmark for cosmological simulations, and their predictions on number densities, sizes, masses, and dynamical characteristics of these objects.
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    Nature versus nurture : relic nature and environment of the most massive passive galaxies at z < 0.5
    (2020) Tortora, C.; Napolitano, N. R.; Radovich, M.; Spiniello, C.; Hunt, L.; Roy, N.; Moscardini, L.; Scognamiglio, D.; Spavone, M.; D'Ago, Giuseppe; Cavuoti, S.; Longo, G.; Bellagamba, F.; Maturi, M.; Roncarelli, M.
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    Optically variable AGN in the three-year VST survey of the COSMOS field
    (2019) De Cicco, D.; Paolill, M.; Falocco, S.; Poulain, M.; Brandt, W. N.; Bauer, F. E.; Vagnetti, F.; Longo, G.; Grado, A.; Ragosta, F.; Botticella, M. T.; Pignata, G.; Vaccari, M.; Radovich, M.; Salvato, M.; Covone, G.; Napolitano, N. R.; Marchetti, L.; Schipani, P.
    Context. The analysis of the variability of active galactic nuclei (AGN) at different wavelengths and the study of possible correlations of different spectral windows are a current main field of inquiry. Optical variability has been largely used to identify AGN in multivisit surveys. The strength of a selection based on optical variability lies in the opportunity of analyzing data from surveys of large sky areas by ground-based telescopes. However, the effectiveness of optical variability selection with respect to other multiwavelength techniques has been poorly studied down to the depth that is expected from next-generation surveys.
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    Variability and transient search in the SUDARE–VOICE field: a new method to extract the light curves
    (2020) Liu, D.; Deng, W.; Fan, Z.; Fu, L.; Covone, G.; Vaccari, M.; Radovich, M.; Capaccioli, M.; De Cicco, D.; Grado, A.; Marchetti, L.; Napolitano, N.; Paolillo, M.; Pignata, Giuliano; Ragost, F.
    The VLT Survey Telescope (VST) Optical Imaging of the CDFS and ES1 Fields Survey, in synergy with the SUDARE survey, is a deep optical ugri imaging of the CDFS and ES1 fields using the VST. The observations for the CDFS field comprise about 4.38 deg2 down to r ∼ 26 mag. The total on-sky time spans over 4 yr in this field, distributed over four adjacent sub-fields. In this paper, we use the multiepoch r-band imaging data to measure the variability of the detected objects and search for transients. We perform careful astrometric and photometric calibrations and point spread function modelling. A new method, referring to as differential running-average photometry, is proposed to measure the light curves of the detected objects. With the method, the difference of PSFs between different epochs can be reduced, and the background fluctuations are also suppressed. Detailed uncertainty analysis and detrending corrections on the light curves are performed. We visually inspect the light curves to select variable objects, and present some objects with interesting light curves. Further investigation of these objects in combination with multiband data will be presented in our forthcoming paper.