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Start Over Subject active galactic nuclei
681. Radio Sources in Low-Luminosity AGNs. IV
ID:
ivo://CDS.VizieR/J/A+A/435/521
Title:
Radio Sources in Low-Luminosity AGNs. IV
Short Name:
J/A+A/435/521
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present the completed results of a high resolution radio imaging survey of all (~200) low-luminosity active galactic nuclei (LLAGNs) and AGNs in the Palomar Spectroscopic Sample of all (~488) bright northern galaxies. The high incidences of pc-scale radio nuclei, with implied brightness temperatures >~10^7K, and sub-parsec jets argue for accreting black holes in >=50% of all LINERs and low-luminosity Seyferts; there is no evidence against all LLAGNs being mini-AGNs. The detected parsec-scale radio nuclei are preferentially found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad Halpha emission). The radio luminosity function (RLF) of Palomar Sample LLAGNs and AGNs extends three orders of magnitude below, and is continuous with, that of 'classical' AGNs. We find marginal evidence for a low-luminosity turnover in the RLF; nevertheless LLAGNs are responsible for a significant fraction of present day mass accretion.
682. Radio spectral indices of active galaxies
ID:
ivo://CDS.VizieR/J/AN/341/703
Title:
Radio spectral indices of active galaxies
Short Name:
J/AN/341/703
Date:
21 Oct 2021
Publisher:
CDS
Description:
In this paper, we investigate radio properties of active galaxies taken from the reference Veron-Cetty & Veron (2010, Cat. VII/258) catalog. The galaxies are limited to magnitudes in the range of 12^m^-19^m^. We have cross-correlated the list with radio catalogs and selected those galaxies that have data on six or more radio fluxes at different wavelengths. As a result, we have 198 galaxies that satisfy these conditions. Using SDSS DR15, we have obtained 96 spectroscopic identifications of the 198 objects. After the classification, 85% of the 96 objects have changed their types. Available data on the classification of these objects and our classification showed that 56.7% of them are Seyfert galaxies. For all the objects, we have built radio spectra and estimated radio spectral indices. As a result, we obtain {alpha}>=-0.6089+/-0.056> ({alpha}_Seyfert_=-0.6013+/-0.027, {alpha}_LINER_=-0.5955+/-0.025, {alpha}_HII_=-0.6672+/-0.039, {alpha}_Comp._=-0.7128+/-0.043). We discuss the radio properties of active galaxies based on their radio spectral indices.
683. Radio variability of compact, flat-spectrum AGNs
ID:
ivo://CDS.VizieR/J/AJ/142/108
Title:
Radio variability of compact, flat-spectrum AGNs
Short Name:
J/AJ/142/108
Date:
21 Oct 2021
Publisher:
CDS
Description:
The 4.9GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey detected a drop in interstellar scintillation (ISS) for sources at redshifts z>~2, indicating an apparent increase in angular diameter or a decrease in flux density of the most compact components of these sources relative to their extended emission. This can result from intrinsic source size effects or scatter broadening in the intergalactic medium (IGM) in excess of the expected (1+z)^1/2^ angular diameter scaling of brightness temperature limited sources resulting from cosmological expansion. We report here 4.9GHz and 8.4GHz observations and data analysis for a sample of 140 compact, flat-spectrum sources which may allow us to determine the origin of this angular diameter-redshift relation by exploiting their different wavelength dependences. In addition to using ISS as a cosmological probe, the observations provide additional insight into source morphologies and the characteristics of ISS.
684. Radio variability to identifying changing jets
ID:
ivo://CDS.VizieR/J/A+A/654/A169
Title:
Radio variability to identifying changing jets
Short Name:
J/A+A/654/A169
Date:
16 Mar 2022 00:56:46
Publisher:
CDS
Description:
Supermassive black holes can launch highly relativistic jets with velocities reaching Lorentz factors of as high as {GAMMA}>50. How the jets accelerate to such high velocities and where along the jet they reach terminal velocity are open questions that are tightly linked to their structure as well as their launching and dissipation mechanisms. Changes in the beaming factor along the jets could potentially reveal jet acceleration, deceleration, or bending. We aim to (1) quantify the relativistic effects in multiple radio frequencies and (2) study possible jet velocity-viewing angle variations at parsec scales. We used the state-of-the-art code Magnetron to model light curves from the University of Michigan Radio Observatory and the Metsahovi Radio Observatory's monitoring programs in five frequencies covering about 25 years of observations in the 4.8 to 37GHz range for 61 sources. We supplement our data set with high-frequency radio observations in the 100-340GHz range from ALMA, CARMA, and SMA. For each frequency we estimate the Doppler factor which we use to quantify possible changes in the relativistic effects along the jets. The majority of our sources do not show any statistically significant difference in their Doppler factor across frequencies. This is consistent with constant velocity in a conical jet structure, as expected at parsec scales. However, our analysis reveals 17 sources where relativistic beaming changes as a function of frequency. In the majority of cases, the Doppler factor increases towards lower frequencies. Only 1253-053 shows the opposite behavior. By exploring their jet properties we find that the jet of 0420-014 is likely bent across the 4.8-340GHz range. For 0212+735, the jet is likely parabolic, and still accelerating in the 4.8-37GHz range. We discuss possible interpretations for the trends found in the remaining sources.
685. Radio-X-ray sources in the HDF(N) region.
ID:
ivo://CDS.VizieR/J/A+A/472/805
Title:
Radio-X-ray sources in the HDF(N) region.
Short Name:
J/A+A/472/805
Date:
21 Oct 2021
Publisher:
CDS
Description:
This catalogue lists the properties of 62 radio sources in the Hubble Deep Field (North) and Flanking Fields (HDFN) which were also detected by the Chandra X-ray satellite (see Alexander et al., 2003, Cat. <J/AJ/126/539>, file cdfn). This includes 55/92 sources brighter than 40uJy detected by MERLIN+VLA at 1.4GHz (see Muxlow et al., 2005, Cat. <J/MNRAS/358/1159>, Richards, 2000, Cat. <J/ApJ/533/611>); some were also detected by the VLA at 8.4GHz (Richards et al., 1998AJ....116.1039R) including 7 selected at this frequency. The cross-matching, derivation of properties and analysis are described fully in the paper. The radio data have a positional accuracy of 15 mas with respect to the ICRS (Muxlow et al., 2005, Cat. <J/MNRAS/358/1159>) but data using the GOODS HST ACS (Giavalisco et al., 2004, Cat. <II/261>) positions requires a shift of -342mas in Declination for alignment with the ICRS. We use measurements taken from the radio observations and from the literature to compile the flux densities, sizes and spectral/photon indices of the cross-matched radio+X-ray sources and their redshifts (where available), enabling us to derive the rest-frame luminosities. The 1.4-GHz detections are all resolved at 0.2-2arcsec resolution. The radio emission was classified using the source morphologies and radio spectral indices; optical and IR information was used as supporting evidence only. On this basis, starbursts outnumber radio AGN 3:1. The high-redshift starbursts have typical sizes of 5-10kpc and star formation rates of around 1000 Msun/yr, an order of magnitude more extended and intense than in the local universe. The X-ray sources are unresolved but their luminosity and spectral indices allows X-ray AGN and obscured (Type II) AGN to be identified (2004A&A...424..545P). In this way, we can distinguish between the origins of radio and X-ray emission from the same object. There is no obvious correlation between radio and X-ray luminosities nor spectral indices at z>1.3. About 70% of both the radio-selected AGN and the starburst samples were detected by Chandra. The X-ray luminosity indicates the presence of an AGN in at least half of the 45 cross-matched radio starbursts. Eleven of these are type-II AGN, of which 7 are at z>1.5. This distribution overlaps closely with the X-ray detected radio sources which were also detected by SCUBA. Almost all extended radio starbursts at z>1.3 host X-ray selected obscured AGN. The radio emission from most of these ultra-luminous objects is dominated by star formation although the highest redshift (z=4.424) source has a substantial AGN contribution. Star-formation appears to contribute less than 1/3 of their X-ray luminosity. Our results support the inferences from SCUBA and IR data, that at z>1.5, star formation is observably more extended and more copious, it is closely linked to AGN activity and it is triggered differently, compared with star formation at lower redshifts.
686. Random forests method to discover high-redshift QSOs
ID:
ivo://CDS.VizieR/J/AJ/162/72
Title:
Random forests method to discover high-redshift QSOs
Short Name:
J/AJ/162/72
Date:
14 Mar 2022 07:01:17
Publisher:
CDS
Description:
We present a method of selecting quasars up to redshift ~6 with random forests, a supervised machine-learning method, applied to Pan-STARRS1 and WISE data. We find that, thanks to the increasing set of known quasars, we can assemble a training set that enables supervised machine-learning algorithms to become a competitive alternative to other methods up to this redshift. We present a candidate set for the redshift range 4.8-6.3, which includes the region around z=5.5 where selecting quasars is difficult due to their photometric similarity to red and brown dwarfs. We demonstrate that, under our survey restrictions, we can reach a high completeness (66%{+/-}7% below redshift 5.6/83_-9_^+6^% above redshift 5.6) while maintaining a high selection efficiency (78_-8_^+10^%/94_-8_^+5^% ). Our selection efficiency is estimated via a novel method based on the different distributions of quasars and contaminants on the sky. The final catalog of 515 candidates includes 225 known quasars. We predict the candidate catalog to contain additional 148_-33_^+41^ new quasars below redshift 5.6 and 45_-8_^+5^ above, and we make the catalog publicly available. Spectroscopic follow-up observations of 37 candidates led us to discover 20 new high redshift quasars (18 at 4.6<~z<~5.5, 2z~5.7). These observations are consistent with our predictions on efficiency. We argue that random forests can lead to higher completeness because our candidate set contains a number of objects that would be rejected by common color cuts, including one of the newly discovered redshift 5.7 quasars.
687. RASS AGN sample
ID:
ivo://CDS.VizieR/J/A+AS/139/575
Title:
RASS AGN sample
Short Name:
J/A+AS/139/575
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present the first results of a program to identify the unknown bright active galactic nuclei (AGN) in the ROSAT All Sky Survey Bright Source Catalog (RASS-BSC). We have used logC+0.4R as an alternative expression for log(f_X_/f_opt_), where C is X-ray count rate and R stands for R magnitude. Then a high X-ray-to-optical flux ratio criterion has been used to select an AGN sample with 165 unidentified X-ray sources for optical spectroscopy. Those 165 X-ray sources have been identified in the following classes: 115 emission line AGN (QSOs and Seyferts), 2 BL Lac objects and 4 BL Lac candidates, 22 clusters of galaxies, 12 Galactic stars and 10 objects remain unidentified. This represent a success rate of about 73% for detecting AGN using our selection criteria. Plausibility is based upon the optical classification and X-ray characteristics of the sources.
688. RASS AGN sample. II.
ID:
ivo://CDS.VizieR/J/ApJ/590/73
Title:
RASS AGN sample. II.
Short Name:
J/ApJ/590/73
Date:
21 Oct 2021
Publisher:
CDS
Description:
This work studies the optical emission line properties of a sample of 155 low-redshift bright X-ray-selected ROSAT Seyfert 1-type active galactic nuclei for which adequate signal-to-noise ratio spectroscopic observations are available. We measured emission-line properties by performing multicomponent fits to the emission-line profiles, covering the effect of blended iron emission. We also obtained continuum parameters, including 250eV X-ray luminosities derived from the ROSAT database. In addition, the measured properties are gathered for a correlation analysis, which confirms the well-known relations between the strengths of Fe II, [O III] emission and the X-ray slope. We also detect striking correlations between H{beta} redshift (or blueshift) and flux ratios of Fe II to H{beta} broad component and [O III] to H{beta} narrow component. These trends are most likely driven by the Eddington ratio.
689. RASS-6dFGS catalogue
ID:
ivo://CDS.VizieR/J/MNRAS/401/1151
Title:
RASS-6dFGS catalogue
Short Name:
J/MNRAS/401/1151
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present a catalogue of 3405 X-ray sources from the ROSAT All Sky Survey (RASS) Bright Source Catalogue which fall within the area covered by the 6dF Galaxy Survey (6dFGS). The catalogue is count-rate limited at 0.05cts/s in the X-ray and covers the area of sky with {delta}<0{deg} and |b|>10{deg}. The RASS-6dFGS sample was one of the additional target catalogues of the 6dFGS and as a result we obtained optical spectra for 2224 (65 per cent) RASS sources. Of these, 1715 (77 per cent) have reliable redshifts with a median redshift of z=0.16 (excluding the Galactic sources). For the optically bright sources (b_J_<=17.5) in the observed sample, over 90 per cent have reliable redshifts. The catalogue mainly comprises quasi-stellar objects (QSOs) and active galaxies but also includes 238 Galactic sources. Of the sources with reliable redshifts the majority are type 1 active galactic nuclei (AGN, 69 per cent), while 12 per cent are type 2 AGN, 6 per cent absorption-line galaxies and 13 per cent are stars. We also identify a small number of optically faint, very low redshift, compact objects which fall outside the general trend in the b_J_-z plane. The RASS-6dFGS catalogue complements a number of Northern hemisphere samples, particularly the ROSAT Bright Source Catalogue-NRAO VLA Sky Survey (RBSC-NVSS) sample (Bauer et al., 2000, Cat. J/ApJS/129/547/), and furthermore, in the same region of sky (-40{deg}<{delta}<0{deg}) reveals an additional 561 sources that were not identified as part of that sample. We detect 918 sources (27 per cent) of the RASS-6dFGS sample in the radio using either the 1.4GHz NVSS or the 843MHz Sydney University Molonglo Sky Survey (SUMSS) catalogues and find that the detection rate changes with redshift. At redshifts larger than 1 virtually all of these sources have radio counterparts and with a median flux density of 1.15Jy, they are much stronger than the median flux density of 28.6mJy for the full sample. We attribute this to the fact that the X-ray flux of these objects is being boosted by a jet component, possibly Doppler boosted, that is only present in radio-loud AGN. The RASS-6dFGS sample provides a large set of homogeneous optical spectra ideal for future studies of X-ray emitting AGN.
690. RATAN-600 flux densities of 123 blazars
ID:
ivo://CDS.VizieR/J/MNRAS/450/2658
Title:
RATAN-600 flux densities of 123 blazars
Short Name:
J/MNRAS/450/2658
Date:
21 Oct 2021
Publisher:
CDS
Description:
We study the correlation between gamma-ray and radio band radiation for 123 blazars, using the Fermi-LAT first source catalog (1FGL) and the RATAN-600 data obtained at the same period of time (within a few months).