Extragalactic Objects Research Articles

Periodicity of quasar and galaxy redshift

Context. An approach for studying the large-scale structure of the Universe lies in the detection and analysis of periodicity of redshift values of extragalactic objects, galaxies, and quasi stellar objects (QSO), in particular. Moreover, the hypothesis of the existence of multiple periodicities in the redshift distributions deserves exploration. The task is compounded by the presence of confounding effects and measurement noise. Aims. Studies of periodicity detection in redshift values of extragalactic objects obtained from the Sloan Digital Sky Survey (SDSS) have been conducted in the past, largely based on the Fourier transform. The present study aims to revisit the same thing using the singular value decomposition (SVD) as the primary tool. Methods. Periodicity detection and the determination of the fundamental period have been performed using a standard spectral approach as well as a SVD-based approach for a variety of simulated datasets. The analysis of the quasar redshift data from DR12 and galaxy redshift dataset of DR10 from SDSS data has been carried out. Results. A wide range of periodicities are observed in different redshift ranges of the quasar datasets. For redshifts greater than 0.03, a period length of 0.2094 was determined while periodicities of 0.1210 and 0.0654 were obtained for redshift ranges (0.03, 1) and (3, 4), respectively. Twin periodicities of 0.1153 and 0.0807 were obtained for the redshift range (1, 3). Determining the ranges to be examined has been done based on histogram computation; the binwidths of which have been obtained by employing a kernel density estimation. The redshift sequence for the galaxy samples exhibits a somewhat different nature, but still contains periodic components. Twin periodicities of 0.0056 and 0.0079 were observed for a redshift range greater than 0.03. Conclusions. Galaxy and quasar redshift values form sequences, which are not only discrete in amplitude but also contain periodic components. The superiority of the singular value decomposition method over the spectral estimation approach, in redshift periodicity analysis especially from the point of view of robustness, is demonstrated through simulations. The existence of periodicity for quasar and galaxy families is thus firmly established, lending further support to the Hoyle-Narlikar variable mass theory.

WISE J044232.92+322734.9: A star-forming galaxy at redshift 1.1 seen through a Galactic dust clump?

Context. Physically unassociated background or foreground objects seen towards submillimetre sources are potential contaminants of both the studies of young stellar objects embedded in Galactic dust clumps and multiwavelength counterparts of submillimetre galaxies (SMGs). Aims. We aim to search for and characterise the properties of a potential extragalactic object seen in projection towards a Galactic dust clump. Methods. We employed the near-infrared (3.4 μm and 4.6 μm) and mid-infrared (12 μm and 22 μm) data from the Wide-field Infrared Survey Explorer (WISE) and the submillimetre data from the Planck satellite. Results. We uncovered a source, namely the WISE source J044232.92+322734.9 (hereafter J044232.92), which is detected in the W1–W3 bands of WISE, but undetected at 22 μm (W4), and whose WISE infrared (IR) colours suggest that it is a star-forming galaxy (SFG). This source is seen in projection towards the Planck-detected dust clump PGCC G169.20-8.96, which likely belongs to the Taurus-Auriga cloud complex, at a distance of 140 pc. We used the MAGPHYS+photo-z spectral energy distribution (SED) code to derive the photometric redshift and physical properties of J044232.92. The redshift was derived to be zphot = 1.132−0.165+0.280, while, for example, the stellar mass, IR (8–1000 μm) luminosity, and star formation rate were derived to be M⋆ = 4.6−2.5+4.7 × 1011 M⊙, LIR = 2.8−1.5+5.7 × 1012 L⊙, and SFR = 191−146+580 M⊙ yr−1 (or 281−155+569 M⊙ yr−1 when estimated from the IR luminosity). The derived value of LIR suggests that J044232.92 could be an ultraluminous IR galaxy, and we found that it is consistent with a main sequence SFG at a redshift of 1.132. Conclusions. The estimated physical properties of J044232.92 are comparable to those of SMGs, except that the derived stellar mass of J044232.92 appears somewhat higher (by a factor of 4–5) than the average stellar masses of SMGs. However, the stellar mass difference could just reflect the poorly sampled SED in the ultraviolet, optical, and near-IR regimes. Indeed, the SED of J044232.92 could not be well constrained using the currently available data (WISE only), and hence the derived redshift of the source and its physical properties should be taken as preliminary estimates. Further observations, in particular high-resolution (sub-)millimetre and radio continuum imaging, are needed to better constrain the redshift and physical properties of J044232.92 and to see if the source really is a galaxy seen through a Galactic dust clump, in particular an SMG population member at z ∼ 1.1.

CHESS: An innovative concept for high-resolution, far-UV spectroscopy

The space ultraviolet (UV) is a critical astronomical observing window, where a multitude of atomic, ionic, and molecular signatures provide crucial insight into planetary, interstellar, stellar, intergalactic, and extragalactic objects. The next generation of large space telescopes require highly sensitive, moderate-to-high resolution UV spectrograph. However, sensitive observations in the UV are difficult, as UV optical performance and imaging efficiencies have lagged behind counterparts in the visible and infrared regimes. This has historically resulted in simple, low-bounce instruments to increase sensitivity. In this study, we present the design, fabrication, and calibration of a simple, high resolution, high throughput far-UV spectrograph - the Colorado High-resolution Echelle Stellar Spectrograph (CHESS). CHESS is a sounding rocket payload to demonstrate the instrument design for the next-generation UV space telescopes. We present tests and results on the performance of several state-of-the-art diffraction grating and detector technologies for far-UV astronomical applications that were flown aboard the first two iterations of CHESS. The CHESS spectrograph was used to study the atomic-to-molecular transitions within translucent cloud regions in the interstellar medium (ISM) through absorption spectroscopy. The first two flights looked at the sightlines towards alpha Virgo and epsilon Persei, and flight results are presented.

What did the seahorse swallow? APEX 170 GHz observations of the chemical conditions in the Seahorse infrared dark cloud

Context.Infrared dark clouds (IRDCs) are useful target sources for the studies of molecular cloud substructure evolution and early stages of star formation. Determining the chemical composition of IRDCs helps to constrain the initial conditions and timescales (via chemical clocks) of star formation in these often filamentary, dense interstellar clouds.Aims.We aim to determine the fractional abundances of multiple different molecular species in the filamentary IRDC G304.74+01.32, nicknamed the Seahorse IRDC, and to search for relationships between the abundances and potential evolutionary trends.Methods.We used the Atacama Pathfinder EXperiment (APEX) telescope to observe spectral lines occurring at about 170 GHz frequency towards 14 positions along the full extent of the Seahorse filament. The sample is composed of five clumps that appear dark in the mid-IR, eight clumps that are associated with mid-IR sources, and one clump that is already hosting an H IIregion and is, hence, likely to be in the most advanced stage of evolution of all the target sources. We also employed our previous 870μm dust continuum imaging data of the Seahorse.Results.Six spectral line transitions were detected (≥3σ) altogether, namely, SO(NJ= 44−33), H13CN(J= 2−1), H13CO+(J= 2−1), SiO(J= 4−3), HN13C(J= 2−1), and C2H(N= 2−1). While SO, H13CO+, and HN13C were detected in every source, the detection rates for C2H and H13CN were 92.9 and 85.7%, respectively. Only one source (SMM 3) showed detectable SiO emission (7.1% detection rate). Three clumps (SMM 5, 6, and 7) showed the SO, H13CN, H13CO+, HN13C, and C2H lines in absorption. Of the detected species, C2H was found to be the most abundant one with respect to H2(a few times 10−9on average), while HN13C was found to be the least abundant species (a few times 10−11). We found three positive correlations among the derived molecular abundances, of which those between C2H and HN13C and HN13C and H13CO+are the most significant (correlation coefficientr≃ 0.9). The statistically most significant evolutionary trends we uncovered are the drops in the C2H abundance and in the [HN13C]∕[H13CN] ratio as the clump evolves from an IR dark stage to an IR bright stage and then to an H IIregion.Conclusions.The absorption lines detected towards SMM 6 and SMM 7 could arise from continuum radiation from an embedded young stellar object and an extragalactic object seen along the line of sight. However, the cause of absorption lines in the IR dark clump SMM 5 remains unclear. The correlations we found between the different molecular abundances can be understood as arising from the gas-phase electron (ionisation degree) and atomic carbon abundances. With the exception of H13CN and H13CO+, the fractional abundances of the detected molecules in the Seahorse are relatively low compared to those in other IRDC sources. The [C2H] evolutionary indicator we found is in agreement with previous studies, and can be explained by the conversion of C2H to other species (e.g. CO) when the clump temperature rises, especially after the ignition of a hot molecular core in the clump. The decrease of [HN13C]∕[H13CN] as the clump evolves is also likely to reflect the increase in the clump temperature, which leads to an enhanced formation of HCN and its13C isotopologue. Both single-dish and high-resolution interferometric imaging of molecular line emission (or absorption) of the Seahorse filament are required to understand the large-scale spatial distribution of the gas and to search for possible hot, high-mass star-forming cores in the cloud.

Blasts from the Past: Supernova Shock Breakouts among X-Ray Transients in the XMM-Newton Archive

Abstract The first electromagnetic signal from a supernova (SN) is released when the shock crosses the progenitor surface. This shock breakout (SBO) emission provides constraints on progenitor and explosion properties. Observationally, SBOs appear as minute- to hour-long extragalactic X-ray transients. They are challenging to detect and only one SBO has been observed to date. Here, we search the XMM-Newton archive and find 12 new SN SBO candidates. We identify host galaxies to nine of these at estimated redshifts of 0.1–1. The SBO candidates have energies of ∼1046 erg, timescales of 30–3000 s, and temperatures of 0.1–1 keV. They are all consistent with being SN SBOs, but some may be misidentified Galactic foreground sources or other extragalactic objects. SBOs from blue supergiants agree well with most of the candidates. However, a few could be SBOs from Wolf–Rayet stars surrounded by dense circumstellar media, whereas two are more naturally explained as SBOs from red supergiants. The observations tentatively support non-spherical SBOs and are in agreement with asymmetries predicted by recent three-dimensional SN explosion simulations. eROSITA may detect ∼2 SBOs per year, which could be detected in live analyses and promptly followed up.

The Gaia reference frame for bright sources examined using VLBI observations of radio stars (Corrigendum)

Positions and proper motions of Gaia sources are expressed in a reference frame that ideally should be non-rotating relative to distant extragalactic objects, coincident with the International Celestial Reference System (ICRS), and consistent across all magnitudes. For sources fainter than 16th magnitude this is achieved thanks to Gaia's direct observations of quasars. At brighter magnitudes it is difficult to validate the quality of the reference frame due to the scarcity of comparison data. This paper examines the use of VLBI observations of radio stars to determine the spin and orientation of the bright reference frame of Gaia. Simultaneous estimation of the six spin and orientation parameters makes optimal use of VLBI data and makes it possible to include even single-epoch VLBI observations in the solution. The method is applied to Gaia Data Release 2 (DR2) using published VLBI data for 41 radio stars. Results for the 26 best-fitting sources indicate that the bright reference frame of Gaia DR2 is rotating relative to the faint quasars at a rate of about 0.1 mas/yr, significant at 2-sigma level. This supports a similar conclusion based on a comparison with stellar positions in the Hipparcos frame. The accuracy is currently limited by the small number of radio sources used, by uncertainties in the Gaia DR2 proper motions, and by the astrophysical nature of the radio stars. While the origin of the indicated rotation is understood and can be avoided in future data releases, it remains important to validate the bright reference frame of Gaia by independent observations. This can be achieved using VLBI astrometry, which may require re-observing the old sample of radio stars as well as measuring new objects. The unique historical value of positional measurements is stressed and VLBI observers are urged to ensure that relevant positional information is preserved for the future.

The study of the angular and spatial distribution of radio-selected AGNs and star-forming galaxies in the ELAIS N1 field

ABSTRACT The cosmic evolution of bias of different source populations with underlying dark matter density field in post-reionization era can shed light on large-scale structures. Studying the angular and spatial distribution of different compact sources using deep radio catalogue at low frequency is essential to understand the matter distribution of the present Universe. Here, we investigate the relationship of luminous matter with their host dark matter haloes by measuring the angular and spatial clustering of sources (two-point statistics), using deep radio observation of ELAIS N1 (EN1) field with upgraded Giant Metrewave Radio Telescope (uGMRT) at 300–500 MHz. We also analyse the 612 MHz GMRT archival data of the same field to understand the cosmic evolution of clustering of different source populations. We classify the sources as star-forming galaxies (SFGs) and active galactic nuclei (AGNs) based on their radio luminosity. We find that the spatial clustering length and bias to the dark matter density field of SFGs are smaller than AGNs at both frequencies. This proves that AGNs are mainly hosted by massive haloes and hence strongly clustered. However, a small decrease in the bias for both kind of sources at higher frequency indicates that we are most likely tracing the faint objects residing in less-massive haloes at higher frequencies. Our results are in excellent agreement with previous findings at radio and multifrequency surveys. However, comparison with SKADS simulation suggests that the halo mass for different populations used in the simulation is systematically lower. This work quantifies the spatial distribution of extragalactic compact objects in EN1 field and bridges the gap between shallow and deep surveys.

An Active Flexure Compensation Method for LAMOST spectrograph based on BP-Neural Network

The Large sky AreaMulti-Object fiber Spectroscopic Telescope (LAMOST) is a Chinese national scientific research facility operated by National Astronomical Observatories, Chinese Academy of Sciences (NAOC). The LAMOST survey for the Milky Way Galaxy and extra-galactic objects has been carried out for several years. The accuracies in measuring radial velocity are expected to be 5 km s−1 for the low resolution spectroscopic survey (R = 1800), and 1 km s−1 for the medium resolution mode. The stability of spectrograph is the main factor affecting the accuracies in measuring radial velocity, so an Active Flexure Compensation Method (AFCM) based on Back Propagation Neural Network (BPNN) is proposed in this paper. It utilizes a deep BP (4-layer, 5-layer etc.) model of thermal-induced flexure to periodically predict and apply flexure corrections by commanding the corresponding tilt and tip motions to the camera. The spectrograph camera system is adjusted so that the positions of these spots match those in a reference image. The simulated calibration of this compensation method analytically illustrates its performance on LAMOST spectrograph.

Probabilistic fibre-to-target assignment algorithm for multi-object spectroscopic surveys

Context. Several new multi-object spectrographs are currently planned or under construction that are capable of observing thousands of Galactic and extragalactic objects simultaneously. Aims. In this paper we present a probabilistic fibre-to-target assignment algorithm that takes spectrograph targeting constraints into account and is capable of dealing with multiple concurrent surveys. We present this algorithm using the 4-m Multi-Object Spectroscopic Telescope (4MOST) as an example. Methods. The key idea of the proposed algorithm is to assign probabilities to fibre-target pairs. The assignment of probabilities takes the fibre positioner’s capabilities and constraints into account. Additionally, these probabilities include requirements from surveys and take the required exposure time, number density variation, and angular clustering of targets across each survey into account. The main advantage of a probabilistic approach is that it allows for accurate and easy computation of the target selection function for the different surveys, which involves determining the probability of observing a target, given an input catalogue. Results. The probabilistic fibre-to-target assignment allows us to achieve maximally uniform completeness within a single field of view. The proposed algorithm maximises the fraction of successfully observed targets whilst minimising the selection bias as a function of exposure time. In the case of several concurrent surveys, the algorithm maximally satisfies the scientific requirements of each survey and no specific survey is penalised or prioritised. Conclusions. The algorithm presented is a proposed solution for the 4MOST project that allows for an unbiased targeting of many simultaneous surveys. With some modifications, the algorithm may also be applied to other multi-object spectroscopic surveys.

AGILE Observations of Two Repeating Fast Radio Bursts with Low Intrinsic Dispersion Measures

Abstract We focus on two repeating fast radio bursts (FRBs) recently detected by the CHIME/FRB experiment in 2018–2019 (Source 1: 180916.J0158+65, and Source 2: 181030.J1054+73). These sources have low excess dispersion measures ( and , respectively), implying relatively small maximal distances. They were repeatedly observed by AGILE in the MeV–GeV energy range. We do not detect prompt emission simultaneously with these repeating events. This search is particularly significant for the submillisecond and millisecond integrations obtainable by AGILE. The sources are constrained to emit a MeV-fluence in the millisecond range below corresponding to an isotropic energy near erg for a distance of 150 Mpc (applicable to Source 1). We also searched for γ-ray emission for time intervals up to 100 days, obtaining 3σ upper limits (ULs) for the average isotropic luminosity above 50 MeV, (5–10) . For a source distance near 100 kpc (possibly applicable to Source 2), our ULs imply , and 2 . Our results are significant in constraining the high-energy emission of underlying sources such as magnetars, or other phenomena related to extragalactic compact objects, and show the prompt emission to be lower than the peak of the 2004 magnetar outburst of SGR 1806-20 for source distances less than about 100 Mpc.

Successful application of PSF-R techniques to the case of the globular cluster NGC 6121 (M 4)

Context. Precise photometric and astrometric measurements on astronomical images require an accurate knowledge of the point spread function (PSF). When the PSF cannot be modelled directly from the image, PSF-reconstruction techniques become the only viable solution. So far, however, their performance on real observations has rarely been quantified. Aims. In this Letter, we test the performance of a novel hybrid technique, called PRIME, on Adaptive Optics-assisted SPHERE/ZIMPOL observations of the Galactic globular cluster NGC 6121. Methods. PRIME couples PSF-reconstruction techniques, based on control-loop data and direct image fitting performed on the only bright point-like source available in the field of view of the ZIMPOL exposures, with the aim of building the PSF model. Results. By exploiting this model, the magnitudes and positions of the stars in the field can be measured with an unprecedented precision, which surpasses that obtained by more standard methods by at least a factor of four for on-axis stars and by up to a factor of two on fainter, off-axis stars. Conclusions. Our results demonstrate the power of PRIME in recovering precise magnitudes and positions when the information directly coming from astronomical images is limited to only a few point-like sources and, thus, paving the way for a proper analysis of future Extremely Large Telescope observations of sparse stellar fields or individual extragalactic objects.

A parallax distance to 3C 273 through spectroastrometry and reverberation mapping

Distance measurements for extragalactic objects are a fundamental problem in astronomy and cosmology. In the era of precision cosmology, we urgently need better measurements of cosmological distances to observationally test the increasing $H_{0}$ tension of the Hubble constant measured from different tools. Using spectroastrometry, GRAVITY at The Very Large Telescope Interferometer successfully revealed the structure, kinematics and angular sizes of the broad-line region (BLR) of 3C 273 with an unprecedentedly high spatial resolution. Fortunately, reverberation mapping (RM) of active galactic nuclei (AGNs) reliably provides linear sizes of their BLRs. Here we report a joint analysis of spectroastrometry and RM observations to measure AGN distances. We apply this analysis to 3C 273 observed by both GRAVITY and an RM campaign,and find an angular distance of $551.5_{-78.7}^{+97.3}\, {\rm Mpc}$ and $H_{0}=71.5_{-10.6}^{+11.9}\,{\rm km\,s^{-1}\,Mpc^{-1}}$. Advantages of the analysis are 1) its pure geometrical measurements and 2) it simultaneously yields mass of the central black hole in the BLR. Moreover, we can conveniently repeat measurements of selected AGNs to efficiently reduce the statistical and systematic errors. Future observations of a reasonably sized sample ($\sim 30$ AGNs) will provide distances of the AGNs and hence a new way of measuring $H_{0}$ with a high precision $\left(\lesssim 3\%\right)$ to test the $H_{0}$ tension.

Precise Measurements of the Speed of Light with High-redshift Quasars: Ultra-compact Radio Structure and Strong Gravitational Lensing

Abstract Although the speed of light has been measured with very high precision, most of these measurements were carried out on Earth or in our close cosmic surroundings. In this Letter, we propose an original idea to combine the observations of ultra-compact structure in radio quasars and strong gravitational lensing with quasars acting as background sources to estimate the speed of light. The method will provide precise measurements of the speed of light using extragalactic objects at different redshifts. We evaluate if current or future missions such as the Large Synoptic Survey Telescope (LSST) and Dark Energy Survey (DES) can be sensitive enough to detect any variation of c. Our results show that strongly lensed quasars observed by LSST would produce robust constraints on Δc/c at the level of 10−4, if the compact radio structure measurements are available.

Characterization of unresolved and unclassified sources detected in radio continuum surveys of the Galactic plane

ABSTRACT The continuum emission from 1 to 2 GHz of The H i/OH/Recombination line survey of the inner Milky Way (THOR) at ≲18 arcsec resolution covers ∼132 deg2 of the Galactic plane and detects 10 387 sources. Similarly, the first data release of the Global View of Star Formation in the Milky Way (GLOSTAR) survey covers ∼16 deg2 of the Galactic plane from 4 to 8 GHz at 18 arcsec resolution and detects 1575 sources. However, a large fraction of the unresolved discrete sources detected in these radio continuum surveys of the Galactic plane remain unclassified. Here, we study the Euclidean-normalized differential source counts of unclassified and unresolved sources detected in these surveys and compare them with simulated extragalactic radio source populations and previously established source counts. We find that the differential source counts for THOR and GLOSTAR surveys are in excellent agreement with both simulation and previous observations. We also estimate the angular two-point correlation function of unclassified and unresolved sources detected in THOR survey. We find a higher clustering amplitude in comparison with the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey up to the angular separation of 5°. The decrease in angular correlation with increasing flux cut and the excellent agreement of clustering pattern of sources above 1 mJy with high-z samples (z > 0.5) of the FIRST survey indicates that these sources might be high-z extragalactic compact objects. The similar pattern of one-point and two-point statistics of unclassified and compact sources with extragalactic surveys and simulations confirms the extragalactic origin of these sources.

The Gaia reference frame for bright sources examined using VLBI observations of radio stars

Context. Positions and proper motions of Gaia sources are expressed in a reference frame that ideally should be non-rotating relative to distant extragalactic objects, coincident with the International Celestial Reference System (ICRS), and consistent across all magnitudes. For sources fainter than 16th magnitude, this is achieved through Gaia’s direct observations of quasars. At brighter magnitudes, it is difficult to validate the quality of the reference frame because comparison data are scarce. Aims. The aim of this paper is to examine the use of very long baseline interferometry (VLBI) observations of radio stars to determine the spin and orientation of the bright reference frame of current and future Gaia data releases. Methods. Simultaneous estimation of the six spin and orientation parameters makes optimal use of VLBI data and makes it possible to include even single-epoch VLBI observations in the solution. The method is applied to Gaia Data Release 2 (DR2) using published VLBI data for 41 radio stars. Results. The VLBI data for the best-fitting 26 sources indicate that the bright reference frame of Gaia DR2 rotates relative to the faint quasars at a rate of about 0.1 mas yr−1, which is significant at the 2σ level. This supports a similar conclusion based on a comparison with stellar positions in the HIPPARCOS frame. The accuracy is currently limited because only a few radio sources are included in the solution, by uncertainties in the Gaia DR2 proper motions, and by issues related to the astrophysical nature of the radio stars. Conclusions. While the origin of the indicated rotation is understood and can be avoided in future data releases, it remains important to validate the bright reference frame of Gaia by independent observations. This can be achieved using VLBI astrometry, which may require re-observing the old sample of radio stars as well as measuring new objects. The unique historical value of positional measurements is stressed and VLBI observers are urged to ensure that relevant positional information is preserved for the future.

KiDS-SQuaD

Context.The KiDS Strongly lensed QUAsar Detection project (KiDS-SQuaD) is aimed at finding as many previously undiscovered gravitational lensed quasars as possible in the Kilo Degree Survey. This is the second paper of this series where we present a new, automatic object-classification method based on the machine learning technique.Aims.The main goal of this paper is to build a catalogue of bright extragalactic objects (galaxies and quasars) from the KiDS Data Release 4, with minimum stellar contamination and preserving the completeness as much as possible. We show here that this catalogue represents the perfect starting point to search for reliable gravitationally lensed quasar candidates.Methods.After testing some of the most used machine learning algorithms, decision-tree-based classifiers, we decided to use CatBoost, which was specifically trained with the aim of creating a sample of extragalactic sources that is as clean of stars as possible. We discuss the input data, define the training sample for the classifier, give quantitative estimates of its performances, and finally describe the validation results withGaiaDR2, AllWISE, and GAMA catalogues.Results.We built and made available to the scientific community the KiDS Bright EXtraGalactic Objects catalogue (KiDS-BEXGO), specifically created to find gravitational lenses but applicable to a wide number of scientific purposes. The KiDS-BEXGO catalogue is made of ≈6 million sources classified as quasars (≈200 000) and galaxies (≈5.7 M) up tor < 22m. To demonstrate the potential of the catalogue in the search for strongly lensed quasars, we selected ≈950 “Multiplets”: close pairs of quasars or galaxies surrounded by at least one quasar. We present cutouts and coordinates of the 12 most reliable gravitationally lensed quasar candidates. We showed that employing a machine learning method decreases the stellar contaminants within the gravitationally lensed candidates, comparing the current results to the previous ones, presented in the first paper from this series.Conclusions.Our work presents the first comprehensive identification of bright extragalactic objects in KiDS DR4 data, which is, for us, the first necessary step towards finding strong gravitational lenses in wide-sky photometric surveys, but has also many other more general astrophysical applications.

High-energy gamma-ray sources in the VVV survey – I. The blazars

ABSTRACT There is a lack of blazar detections in the galactic plane area, even in the most recent blazar catalogues, mainly due to incompleteness resulting from interstellar dust and stellar contamination of our Galaxy. The VISTA Variables in the Vía Láctea survey (VVV) aims to map variable sources towards the galactic plane in the near-infrared (IR). There are three catalogued and one candidate blazars in the VVV area, which are also detected by Fermi-LAT in the gamma-ray band and by the Wide-field Infrared Survey Explorer (WISE) at mid-IR wavelengths. We study the physical properties of these blazars in order to use them as a reference that would allow us to find counterparts of other unidentified gamma-ray sources in the future. All four sources have VVV counterparts, and two of them, 5BZUJ1731−3003 and 5BZQJ1802−3940 are also associated with WISE sources. They present very different properties compared with stellar or extragalactic objects. Their VVV Ks light curves show significant variability, and are characterized using a machine learning analysis. In particular, the blazar 5BZQJ1802−3940, a flat spectrum radio quasar, exhibits the smallest uncertainties in the Fermi-LAT position and it is a bright WISE source. Its near-IR properties in the colour–magnitude and colour–colour diagrams are very different from the stars and extragalactic objects in the field. It has the highest amplitude in the Ks differential light curve and fractional variability amplitude. This pilot study will allow us to search for and identify other blazars hidden behind the Galactic plane using the VVV survey data base.

INFRARED COUNTERPARTS OF X-RAY GALAXIES

Population studies of the extragalactic objects are a major part of the universe large-scale structure study. Apart from radio, infrared, and visible wavelength bands, observations and further identification of extragalactic objects such as galaxies, quasars, blazers, liners, and active star burst regions are also conducted in the X-ray and gamma bands. In this paper we make identification and cross-correlate of the infrared and X-ray observational data, build a distribution of a selected sample sources by types and attempted to analyze types of the extragalactic objects at distances up to z = 0.1 using observational data of relevant space observatories. Data from a leading X-ray space observatory XMM-Newton were used to compile the largest catalog of X-ray sources. Current version of XMM SSC (Serendipitous Source Catalog) contains more than half a million sources. In our previous works we selected and analyzed a sample of 5021 X-ray galaxies observed by XMM-Newton. Identification and classification of these sources is essential next step of the study. In this study we used infrared apparent magnitudes from WISE catalog of AGN candidates. In 2010 space telescope WISE performed full sky survey in four infrared bands and detected 747 million sources. WISE catalog of AGN candidates amounts 4 million of possible extragalactic sources. We built infrared color-color diagram for our sample of X-ray galaxies and assessed their types using WISE telescope data. In this study we also analyzed large scale structure of the universe (distances up to z=0.1). This analysis revealed Coma galaxy cluster and SDSS Sloan Great Wall. In the further studies we are planning to investigate the distribution of different types of X-ray galaxies within the large-scale structures of the Universe.

Quasar and galaxy classification in Gaia Data Release 2

ABSTRACT We construct a supervised classifier based on Gaussian Mixture Models to probabilistically classify objects in Gaia data release 2 (GDR2) using only photometric and astrometric data in that release. The model is trained empirically to classify objects into three classes – star, quasar, galaxy – for G ≥ 14.5 mag down to the Gaia magnitude limit of G = 21.0 mag. Galaxies and quasars are identified for the training set by a cross-match to objects with spectroscopic classifications from the Sloan Digital Sky Survey. Stars are defined directly from GDR2. When allowing for the expectation that quasars are 500 times rarer than stars, and galaxies 7500 times rarer than stars (the class imbalance problem), samples classified with a threshold probability of 0.5 are predicted to have purities of 0.43 for quasars and 0.28 for galaxies, and completenesses of 0.58 and 0.72, respectively. The purities can be increased up to 0.60 by adopting a higher threshold. Not accounting for this expected low frequency of extragalactic objects (the class prior) would give both erroneously optimistic performance predictions and severely impure samples. Applying our model to all 1.20 billion objects in GDR2 with the required features, we classify 2.3 million objects as quasars and 0.37 million objects as galaxies (with individual probabilities above 0.5). The small number of galaxies is due to the strong bias of the satellite detection algorithm and on-ground data selection against extended objects. We infer the true number of quasars and galaxies – as these classes are defined by our training set – to be 690 000 and 110 000, respectively (±50 per cent). The aim of this work is to see how well extragalactic objects can be classified using only GDR2 data. Better classifications should be possible with the low resolution spectroscopy (BP/RP) planned for GDR3.

The OTELO survey

Context. The evolution of galaxies through cosmic time is studied observationally by means of extragalactic surveys. The usefulness of these surveys is greatly improved by increasing the cosmological volume, in either depth or area, and by observing the same targets in different wavelength ranges. A multi-wavelength approach using different observational techniques can compensate for observational biases. Aims. The OTELO survey aims to provide the deepest narrow-band survey to date in terms of minimum detectable flux and emission line equivalent width in order to detect the faintest extragalactic emission line systems. In this way, OTELO data will complements other broad-band, narrow-band, and spectroscopic surveys. Methods. The red tunable filter of the OSIRIS instrument on the 10.4 m Gran Telescopio Canarias (GTC) is used to scan a spectral window centred at 9175 Å, which is free from strong sky emission lines, with a sampling interval of 6 Å and a bandwidth of 12 Å in the most deeply explored EGS region. Careful data reduction using improved techniques for sky ring subtraction, accurate astrometry, photometric calibration, and source extraction enables us to compile the OTELO catalogue. This catalogue is complemented with ancillary data ranging from deep X-ray to far-infrared, including high resolution HST images, which allow us to segregate the different types of targets, derive precise photometric redshifts, and obtain the morphological classification of the extragalactic objects detected. Results. The OTELO multi-wavelength catalogue contains 11 237 entries and is 50% complete at AB magnitude 26.38. Of these sources, 6600 have photometric redshifts with an uncertainty δ zphot better than 0.2 (1+zphot). A total of 4336 of these sources correspond to preliminary emission line candidates, which are complemented by 81 candidate stars and 483 sources that qualify as absorption line systems. The OTELO survey results will be released to the public on the second half of 2019.