Search

Start Over Content Level Research
641. AGN automatic photometric classification
ID:
ivo://CDS.VizieR/J/MNRAS/437/968
Title:
AGN automatic photometric classification
Short Name:
J/MNRAS/437/968
Date:
21 Oct 2021
Publisher:
CDS
Description:
In this paper, we discuss an application of machine-learning-based methods to the identification of candidate active galactic nucleus (AGN) from optical survey data and to the automatic classification ofAGNs in broad classes. We applied four different machine-learning algorithms, namely the Multi Layer Perceptron, trained, respectively, with the Conjugate Gradient, the Scaled Conjugate Gradient, the Quasi Newton learning rules and the Support Vector Machines, Q4 to tackle the problem of the classification of emission line galaxies in different classes, mainly AGNs versus non-AGNs, obtained using optical photometry in place of the diagnostics based on line intensity ratios which are classically used in the literature. Using the same photometric features, we discuss also the behaviour of the classifiers on finer AGN classification tasks, namely Seyfert I versus Seyfert II, and Seyfert versus LINER. Furthermore, we describe the algorithms employed, the samples of spectroscopically classified galaxies used to train the algorithms, the procedure followed to select the photometric parameters and the performances of our methods in terms of multiple statistical indicators. The results of the experiments show that the application of self-adaptive data mining algorithms trained on spectroscopic data sets and applied to carefully chosen photometric parameters represents a viable alternative to the classical methods that employ time-consuming spectroscopic observations.
642. AGN candidates from 2MASS/ROSAT catalogs
ID:
ivo://CDS.VizieR/J/MNRAS/405/2062
Title:
AGN candidates from 2MASS/ROSAT catalogs
Short Name:
J/MNRAS/405/2062
Date:
21 Oct 2021
Publisher:
CDS
Description:
We report on the near-infrared-selected active galactic nuclei (AGN) candidates extracted from Two Micron All Sky Survey (2MASS)/ROSAT catalogues and discuss their properties. First, near-infrared counterparts of an X-ray source in ROSAT catalogues [namely bright source catalogue (BSC, Cat. IX/10) and faint source catalogue (FSC, Cat. IX/29)] were extracted by positional cross-identification of <=30arcsec. As these counterparts would contain many mis-identifications, we further imposed near-infrared colour selection criteria and extracted reliable AGN candidates (BSC: 5273, FSC: 10071). Of the 5273 (10071) candidates in the BSC (FSC), 2053 (1008) are known AGNs. Near-infrared and X-ray properties of candidates show similar properties with known AGNs and are consistent with the previous studies. We also searched for counterparts in other wavelengths (i.e. optical, near-infrared and radio) and investigated properties in multiwavelength. No significant difference between known AGNs and unclassified sources could be seen. However, some unclassified sources in the FSC showed slightly different properties compared with known AGNs. Consequently, it is highly probable that we could extract reliable AGN candidates, though candidates in the FSC might be spurious.
643. 231 AGN candidates from the 2FGL catalog
ID:
ivo://CDS.VizieR/J/ApJ/782/41
Title:
231 AGN candidates from the 2FGL catalog
Short Name:
J/ApJ/782/41
Date:
21 Oct 2021
Publisher:
CDS
Description:
The second Fermi-LAT source catalog (2FGL) is the deepest all-sky survey available in the gamma-ray band. It contains 1873 sources, of which 576 remain unassociated. Machine-learning algorithms can be trained on the gamma-ray properties of known active galactic nuclei (AGNs) to find objects with AGN-like properties in the unassociated sample. This analysis finds 231 high-confidence AGN candidates, with increased robustness provided by intersecting two complementary algorithms. A method to estimate the performance of the classification algorithm is also presented, that takes into account the differences between associated and unassociated gamma-ray sources. Follow-up observations targeting AGN candidates, or studies of multiwavelength archival data, will reduce the number of unassociated gamma-ray sources and contribute to a more complete characterization of the population of gamma-ray emitting AGNs.
644. AGN candidates from the WISE, 2MASS, RASS (W2R)
ID:
ivo://CDS.VizieR/J/ApJ/751/52
Title:
AGN candidates from the WISE, 2MASS, RASS (W2R)
Short Name:
J/ApJ/751/52
Date:
21 Oct 2021
Publisher:
CDS
Description:
We have developed the "S_IX_" statistic to identify bright, highly likely active galactic nucleus (AGN) candidates solely on the basis of Wide-field Infrared Survey Explorer (WISE), Two Micron All-Sky Survey (2MASS), and ROSAT all-sky survey (RASS) data. This statistic was optimized with data from the preliminary WISE survey and the Sloan Digital Sky Survey, and tested with Lick 3m Kast spectroscopy. We find that sources with S_IX_<0 have a >~95% likelihood of being an AGN (defined in this paper as a Seyfert 1, quasar, or blazar). This statistic was then applied to the full WISE/2MASS/RASS dataset, including the final WISE data release, to yield the "W2R" sample of 4316 sources with S_IX_<0. Only 2209 of these sources are currently in the Veron-Cetty and Veron (VCV) catalog of spectroscopically confirmed AGNs, indicating that the W2R sample contains nearly 2000 new, relatively bright (J<~16) AGNs. We utilize the W2R sample to quantify biases and incompleteness in the VCV catalog. We find that it is highly complete for bright (J<14), northern AGNs, but the completeness drops below 50% for fainter, southern samples and for sources near the Galactic plane. This approach also led to the spectroscopic identification of 10 new AGNs in the Kepler field, more than doubling the number of AGNs being monitored by Kepler. The W2R sample contains better than 1 bright AGN every 10 deg^2^, permitting construction of AGN samples in any sufficiently large region of sky.
645. AGN candidates in the COSMOS field
ID:
ivo://CDS.VizieR/J/ApJS/172/383
Title:
AGN candidates in the COSMOS field
Short Name:
J/ApJS/172/383
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present spectroscopic redshifts for the first 466 X-ray and radio-selected AGN targets in the 2deg^2^ COSMOS field. Spectra were obtained with the IMACS instrument on the Magellan (Baade) telescope, using the nod-and-shuffle technique. We identify a variety of type 1 and type 2 AGNs, as well as red galaxies with no emission lines. Our redshift yield is 72% down to i_AB_=24, although the yield is >90% for i_AB_<22. We expect the completeness to increase as the survey continues. When our survey is complete and additional redshifts from the zCOSMOS project are included, we anticipate ~1100 AGNs with redshifts over the entire COSMOS field. Our redshift survey is consistent with an obscured AGN population that peaks at z~0.7, although further work is necessary to disentangle the selection effects.
646. AGN catalog from the AKARI NEP Wide field
ID:
ivo://CDS.VizieR/J/A+A/651/A108
Title:
AGN catalog from the AKARI NEP Wide field
Short Name:
J/A+A/651/A108
Date:
22 Feb 2022
Publisher:
CDS
Description:
The North Ecliptic Pole (NEP) field provides a unique set of panchromatic data, well suited for active galactic nuclei (AGN) studies. Selection of AGN candidates is often based on mid-infrared (MIR) measurements. Such method, despite its effectiveness, strongly reduces a catalog volume due to the MIR detection condition. Modern machine learning techniques can solve this problem by finding similar selection criteria using only optical and near-infrared (NIR) data. Aims of this work were to create a reliable AGN candidates catalog from the NEP field using a combination of optical SUBARU/HSC and NIR AKARI/IRC data and, consequently, to develop an efficient alternative for the MIR-based AKARI/IRC selection technique. A set of supervised machine learning algorithms was tested in order to perform an efficient AGN selection. Best of the models were formed into a majority voting scheme, which used the most popular classification result to produce the final AGN catalog. Additional analysis of catalog properties was performed in form of the spectral energy distribution (SED) fitting via the CIGALE software. The obtained catalog of 465 AGN candidates (out of 33119 objects) is characterized by 73% purity and 64% completeness. This new classification shows consistency with the MIR-based selection. Moreover, 76% of the obtained catalog can be found only with the new method due to the lack of MIR detection for most of the new AGN candidates. Training data, codes and final catalog are available via the github repository. Final AGN candidates catalog is also available via the CDS service.
647. AGN central masses and broad-line region sizes
ID:
ivo://CDS.VizieR/J/ApJ/613/682
Title:
AGN central masses and broad-line region sizes
Short Name:
J/ApJ/613/682
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present improved black hole masses for 35 active galactic nuclei (AGNs) based on a complete and consistent reanalysis of broad emission-line reverberation-mapping data. From objects with multiple line measurements, we find that the highest precision measure of the virial product c{tau}{Delta}V^2^/G, where {tau} is the emission-line lag relative to continuum variations and {Delta}V is the emission-line width, is obtained by using the cross-correlation function centroid (as opposed to the cross-correlation function peak) for the time delay and the line dispersion (as opposed to FWHM) for the line width and by measuring the line width in the variable part of the spectrum.
648. AGN contrib. of interac. galaxies
ID:
ivo://CDS.VizieR/J/MNRAS/499/4325
Title:
AGN contrib. of interac. galaxies
Short Name:
J/MNRAS/499/4325
Date:
21 Oct 2021
Publisher:
CDS
Description:
Emission from active galactic nuclei (AGNs) is known to play an important role in the evolution of many galaxies including luminous and ultraluminous systems (U/LIRGs), as well as merging systems. However, the extent, duration, and exact effects of its influence are still imperfectly understood, as are the conditions that lead to it dominating the IR emission, and then fading again after coalescence. To assess the impact of AGNs on interacting systems, we present a Spectral Energy Distribution (SED) analysis of a sample of 188 nearby galaxies We gather and systematically re-reduce archival broad-band imaging mosaics from the ultraviolet to the far-infrared using data from GALEX, SDSS, 2MASS, IRAS, WISE, Spitzer, and Herschel. We use spectroscopy from Spitzer/IRS to obtain fluxes from fine-structure lines that trace star formation and AGN activity. Utilizing the SED modelling and fitting tool CIGALE, we derive the physical conditions of the interstellar medium, both in star-forming regions and in nuclear regions dominated by the AGN in these galaxies. We investigate how the star formation rates (SFRs) and the fractional AGN contributions (fAGN) depend on stellar mass, galaxy type, and merger stage. We find that luminous galaxies more massive than about 10^10M* are likely to deviate significantly from the conventional galaxy main-sequence relation. Interestingly, infrared AGN luminosity and stellar mass in this set of objects are much tighter than SFR and stellar mass. We find that buried AGNs may occupy a locus between bright starbursts and pure AGNs in the fAGN-[NeV]/[NeII] plane. We identify a modest correlation between fAGN and mergers in their later stages.
649. AGN core shift measurements
ID:
ivo://CDS.VizieR/J/MNRAS/485/1822
Title:
AGN core shift measurements
Short Name:
J/MNRAS/485/1822
Date:
21 Oct 2021
Publisher:
CDS
Description:
The apparent position of jet base (core) in radio-loud active galactic nuclei changes with frequency because of synchrotron self-absorption. Studying this 'core shift' effect enables us to reconstruct properties of the jet regions close to the central engine. We report here results from core shift measurements in AGNs observed with global VLBI at 2 and 8GHz at epochs from 1994 to 2016. Our sample contains 40 objects observed at least 10 times during that period. The core shift is determined using a new automatic procedure introduced to minimize possible biases. The resulting multiple epoch measurements of the core position are employed for examining temporal variability of the core shift. We argue that the core shift variability is a common phenomenon, as established for 33 of 40 AGNs we study. Our analysis shows that the typical offsets between the core positions at 2 and 8GHz are about 0.5mas and they vary in time. Typical variability of the individual core positions is about 0.3mas. The measurements show a strong dependence between the core position and its flux density, suggesting that changes in both are likely related to the nuclear flares injecting denser plasma into the flow. We determine that density of emitting relativistic particles significantly increases during these flares, while relative magnetic field changes less and in the opposite direction.
650. AGN data and absorption-line measurements
ID:
ivo://CDS.VizieR/J/A+A/607/A48
Title:
AGN data and absorption-line measurements
Short Name:
J/A+A/607/A48
Date:
21 Oct 2021
Publisher:
CDS
Description:
The Milky Way is surrounded by large amounts of diffuse gaseous matter that connects the stellar body of our Galaxy with its large-scale Local Group (LG) environment. To characterize the absorption properties of this circumgalactic medium (CGM) and its relation to the LG we present the so-far largest survey of metal absorption in Galactic high-velocity clouds (HVCs) using archival ultraviolet (UV) spectra of extragalactic background sources. The UV data are obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) and are supplemented by 21cm radio observations of neutral hydrogen. Along 270 sightlines we measure metal absorption in the lines of SiII, SiIII, CII, and CIV and associated HI 21cm emission in HVCs in the velocity range |v_LSR_|=100-500km/s. With this unprecedented large HVC sample we were able to improve the statistics on HVC covering fractions, ionization conditions, small-scale structure, CGM mass, and inflow rate. For the first time, we determine robustly the angular two point correlation function of the high-velocity absorbers, systematically analyze antipodal sightlines on the celestial sphere, and compare the HVC absorption characteristics with that of Damped Lyman alpha absorbers (DLAs) and constrained cosmological simulations of the LG (CLUES project). The overall sky-covering fraction of high-velocity absorption is 77+/-6 percent for the most sensitive ion in our survey, SiIII, and for column densities log N(SiIII)>12.1. This value is 4-5 times higher than the covering fraction of 21 cm neutral hydrogen emission at log N(HI)>18.7 along the same lines of sight, demonstrating that the Milky Way's CGM is multi-phase and predominantly ionized. The measured equivalent-width ratios of SiII, SiIII, CII, and CIV are inhomogeneously distributed on large and small angular scales, suggesting a complex spatial distribution of multi-phase gas that surrounds the neutral 21cm HVCs. We estimate that the total mass and accretion rate of the neutral and ionized CGM traced by HVCs is M_HVC_>3.0x10^9^M_{sun}_ and dM_HVC_/dt>6.1M_{sun}_/yr, where the Magellanic Stream (MS) contributes with more than 90 percent to this mass/mass-flow. If seen from an external vantage point, the Milky Way disk plus CGM would appear as a DLA that would exhibit for most viewing angles an extraordinary large velocity spread of dv=400-800km/s, a result of the complex kinematics of the Milky Way CGM that is dominated by the presence of the MS. We detect a velocity dipole of high-velocity absorption at low/high galactic latitudes that we associate with LG gas that streams to the LG barycenter. This scenario is supported by the gas kinematics predicted from the LG simulations. Our study confirms previous results, indicating that the Milky Way CGM contains sufficient gaseous material to feed the Milky Way disk over the next Gyr at a rate of a few solar masses per year, if the CGM gas can actually reach the MW disk. We demonstrate that the CGM is composed of discrete gaseous structures that exhibit a large-scale kinematics together with small-scale variations in physical conditions. The MS clearly dominates both the cross section and mass flow of high-velocity gas in the Milky Way's CGM. The possible presence of high-velocity LG gas underlines the important role of the local cosmological environment in the large-scale gas-circulation processes in and around the Milky Way.

Limit your search

more »

more »

more »

more »

more »

more »

more »