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171. Classification of Fermi blazar cand. from the 4FGL
ID:
ivo://CDS.VizieR/J/ApJ/887/134
Title:
Classification of Fermi blazar cand. from the 4FGL
Short Name:
J/ApJ/887/134
Date:
21 Oct 2021
Publisher:
CDS
Description:
The recently published fourth Fermi Large Area Telescope source catalog (4FGL) reports 5065 gamma-ray sources in terms of direct observational gamma-ray properties. Among the sources, the largest population is the active galactic nuclei (AGNs), which consists of 3137 blazars, 42 radio galaxies, and 28 other AGNs. The blazar sample comprises 694 flat-spectrum radio quasars (FSRQs), 1131 BL Lac-type objects (BL Lacs), and 1312 blazar candidates of an unknown type (BCUs). The classification of blazars is difficult using optical spectroscopy given the limited knowledge with respect to their intrinsic properties, and the limited availability of astronomical observations. To overcome these challenges, machine-learning algorithms are being investigated as alternative approaches. Using the 4FGL catalog, a sample of 3137 Fermi blazars with 23 parameters is systematically selected. Three established supervised machine-learning algorithms (random forests (RFs), support vector machines (SVMs), artificial neural networks (ANNs)) are employed to general predictive models to classify the BCUs. We analyze the results for all of the different combinations of parameters. Interestingly, a previously reported trend the use of more parameters leading to higher accuracy is not found. Considering the least number of parameters used, combinations of eight, 12 or 10 parameters in the SVM, ANN, or RF generated models achieve the highest accuracy (Accuracy ~91.8%, or ~92.9%). Using the combined classification results from the optimal combinations of parameters, 724 BL Lac type candidates and 332 FSRQ type candidates are predicted; however, 256 remain without a clear prediction.
172. 3C295 LOFAR, MERLIN and VLA images
ID:
ivo://CDS.VizieR/J/A+A/658/A10
Title:
3C295 LOFAR, MERLIN and VLA images
Short Name:
J/A+A/658/A10
Date:
02 Feb 2022 13:43:44
Publisher:
CDS
Description:
3C295 is a bright, compact steep spectrum source with a well-studied integrated radio spectral energy distribution (SED) from 132MHz to 15GHz. However, spatially resolved spectral studies have been limited due to a lack of high resolution images at low radio frequencies. These frequencies are crucial for measuring absorption processes, and anchoring the overall spectral modelling of the radio SED. In this paper, we use International LOw-Frequency ARray (LOFAR) Telescope (ILT) observations of 3C295 to study its spatially resolved spectral properties with sub-arcsecond resolution at 132MHz. Combining our new 132MHz observation with archival data at 1.6GHz, 4.8GHz, and 15GHz, we are able to carryout a resolved radio spectral analysis. The spectral properties of the hotspots provides evidence for low frequency flattening. In contrast, the spectral shape across the lobes is consistent with a Jaffe-Perola spectral ageing model. Usingthe integrated spectral information for each component, we then fitted low-frequency absorption models to the hotspots,finding that both free-free absorption and synchrotron self-absorption models provide a better fit to the data than a standard power law. Although we can say there is low-frequency absorption present in the hot spots of 3C295, future observations with the Low Band Antenna of the ILT at 55MHz may allow us to distinguish the type of absorption.
173. Clustering of high-redshift QSOs from SDSS
ID:
ivo://CDS.VizieR/J/AJ/133/2222
Title:
Clustering of high-redshift QSOs from SDSS
Short Name:
J/AJ/133/2222
Date:
21 Oct 2021
Publisher:
CDS
Description:
We study the two-point correlation function of a uniformly selected sample of 4426 luminous optical quasars with redshift 2.9<=z<=5.4 selected over 4041deg^2^ from the Fifth Data Release of the Sloan Digital Sky Survey. We fit a power-law to the projected correlation function w_p_(r_p_) to marginalize over redshift-space distortions and redshift errors.
174. Color excess of SDSS quasars with DLA systems
ID:
ivo://CDS.VizieR/J/A+A/478/701
Title:
Color excess of SDSS quasars with DLA systems
Short Name:
J/A+A/478/701
Date:
21 Oct 2021
Publisher:
CDS
Description:
We analyzed the spectroscopic and photometric database of the 5th data release of the Sloan Digital Sky Survey (SDSS) to search for evidence of the quasar reddening produced by dust embedded in intervening damped Ly{alpha} (DLA) systems. From a list of 5164 quasars in the interval of emission redshift 2.25<=z_e_<=3.5 and SDSS spectra with signal-to-noise ratio SNR>~4, we built up an "absorption sample" of 248 QSOs with a single DLA system in the interval of absorption redshift 2.2<z_a_<=3.5 and a "pool" of 1959 control QSOs without DLA systems or strong metal systems. For each QSO of the absorption sample we extracted from the pool a subset of control QSOs that are closest in redshift and magnitude.
175. Column densities of CGM absorption lines
ID:
ivo://CDS.VizieR/J/ApJ/883/78
Title:
Column densities of CGM absorption lines
Short Name:
J/ApJ/883/78
Date:
21 Oct 2021
Publisher:
CDS
Description:
We investigate the geometric distribution of gas metallicities in the circumgalactic medium (CGM) around 47, z<0.7 galaxies from the "Multiphase Galaxy Halos" Survey. Using a combination of quasar spectra from Hubble Space Telescope (HST)/COS and from Keck/HIRES or Very Large Telescope/UVES, we measure column densities of, or determine limits on, CGM absorption lines. We then use a Markov Chain Monte Carlo approach with Cloudy to estimate the metallicity of cool (T~10^4^K) CGM gas. We also use HST images to determine host-galaxy inclination and quasar-galaxy azimuthal angles. Our sample spans a HI column density range of 13.8cm^-2^<logN_HI_<19.9cm^-2^. We find (1) while the metallicity distribution appears bimodal, a Hartigan dip test cannot rule out a unimodal distribution (0.4{sigma}). (2) CGM metallicities are independent of halo mass, spanning three orders of magnitude at a fixed halo mass. (3) The CGM metallicity does not depend on the galaxy azimuthal and inclination angles regardless of HI column density, impact parameter, and galaxy color. (4) The ionization parameter does not depend on azimuthal angle. We suggest that the partial Lyman limit metallicity bimodality is not driven by a spatial azimuthal bimodality. Our results are consistent with simulations where the CGM is complex and outflowing, accreting, and recycled gas are well-homogenized at z<0.7. The presence of low-metallicity gas at all orientations suggests that cold streams of accreting filaments are not necessarily aligned with the galaxy plane at low redshifts or intergalactic transfer may dominate. Finally, our results support simulations showing that strong metal absorption can mask the presence of low-metallicity gas in integrated line-of-sight CGM metallicities.
176. Compact bright radio-loud AGNs. III. VLBA 43GHz
ID:
ivo://CDS.VizieR/J/ApJS/247/57
Title:
Compact bright radio-loud AGNs. III. VLBA 43GHz
Short Name:
J/ApJS/247/57
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present the results from the 43GHz Very Long Baseline Array (VLBA) observations of 124 compact radio-loud active galactic nuclei (AGNs) that were conducted between 2014 November and 2016 May. The typical dimensions of the restoring beam in each image are about 0.5masx0.2mas. The highest resolution of 0.2mas corresponds to a physical size of 0.02pc for the lowest redshift source in the sample. The 43GHz very long baseline interferometry (VLBI) images of 97 AGNs are presented for the first time. We study the source compactness on milliarcsecond and submilliarcsecond scales, and suggest that 95 sources in our sample are suitable for future space VLBI observations. By analyzing our data supplemented with other VLBA AGN surveys from the literature, we find that the core brightness temperature increases with increasing frequency below a break frequency ~7GHz, and decreases between ~7 and 240GHz but increases again above 240GHz in the rest frame of the sources. This indicates that the synchrotron opacity changes from optically thick to thin. We also find a strong statistical correlation between radio and {gamma}-ray flux densities. Our correlation is tighter than those in the literature derived from lower-frequency VLBI data, suggesting that the {gamma}-ray emission is produced more cospatially with the 43GHz VLBA core emission. This correlation can also be extrapolated to the unbeamed AGN population, implying that a universal {gamma}-ray production mechanism might be at work for all types of AGNs.
177. Compact steep spectrum new sample
ID:
ivo://CDS.VizieR/J/MNRAS/408/2261
Title:
Compact steep spectrum new sample
Short Name:
J/MNRAS/408/2261
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present a new sample of compact steep spectrum (CSS) sources with radio luminosity below 10^26^W/Hz at 1.4GHz; these are called low-luminosity compact (LLC) objects. The sources have been selected from the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey and observed with the multi-element radio linked interferometer network (MERLIN) at the L and C bands. The main criterion used for selection was the luminosity of the objects, and approximately one-third of the CSS sources from the new sample have a value of radio luminosity comparable to Fanaroff-Riley type 1 sources (FR Is). About 80 per cent of the sources have been resolved and about 30 per cent have weak extended emission and disturbed structures when compared with the observations of higher-luminosity CSS sources. We have studied the correlation between radio power and linear size, and the redshift with a larger sample that also included published samples of compact objects and large-scale FR IIs and FR Is.
178. Comparison of Radio-loud and Quiet Quasars
ID:
ivo://CDS.VizieR/J/ApJ/511/612
Title:
Comparison of Radio-loud and Quiet Quasars
Short Name:
J/ApJ/511/612
Date:
21 Oct 2021
Publisher:
CDS
Description:
We have made radio observations of 87 optically selected quasars at 5 GHz with the VLA in order to measure the radio power for these objects and hence determine how the fraction of radio-loud quasars varies with redshift and optical luminosity. The sample has been selected from the recently completed Edinburgh Quasar Survey and covers a redshift range of 0.3{<=}z{<=}1.5 and an optical absolute magnitude range of (-26.5){<=}M_B_{<=}(-23.5) (h=1/2, q_o_=1/2). We have also matched other existing surveys with the Faint Images of the Radio Sky at Twenty Centimeters and NRAO VLA Sky Survey radio catalogs and combined these data so that the optical luminosity-redshift plane is now far better sampled than before. We have fitted a model to the probability of a quasar being radio-loud as a function of absolute magnitude and redshift, and from this model we infer the radio-loud and radio-quiet optical luminosity functions. The radio-loud optical luminosity function is featureless and flatter than the radio-quiet one. It evolves at a marginally slower rate if quasars evolve by density evolution, but the difference in the rate of evolutions of the two different classes is much less than was previously thought.
179. Compilation of known QSOs for the Gaia mission
ID:
ivo://CDS.VizieR/J/other/RAA/19.29
Title:
Compilation of known QSOs for the Gaia mission
Short Name:
J/other/RAA/19.2
Date:
21 Oct 2021
Publisher:
CDS
Description:
Quasars are essential for astrometric in the sense that they are spatial stationary because of their large distance from the Sun. The European Space Agency (ESA) space astrometric satellite Gaia is scanning the whole sky with unprecedented accuracy up to a few uas level. However, Gaia's two fields of view observations strategy may introduce a parallax bias in the Gaia catalog. Since it presents no significant parallax, quasar is perfect nature object to detect such bias. More importantly, quasars can be used to construct a Celestial Reference Frame in the optical wavelengths in Gaia mission. In this paper, we compile the most reliable quasars existing in literatures.The final compilation (designated as Known Quasars Catalog for Gaia mission, KQCG) contains 1842076 objects, among of them, 797632 objects are found in Gaia DR1 after cross-identifications. This catalog will be very useful in Gaia mission.
180. Composite HST Spectrum of Quasars
ID:
ivo://CDS.VizieR/J/ApJ/475/469
Title:
Composite HST Spectrum of Quasars
Short Name:
J/ApJ/475/469
Date:
21 Oct 2021
Publisher:
CDS
Description:
We construct a composite quasar spectrum from 284 HST FOS spectra of 101 quasars with redshifts z>0.33. The spectrum covers the wavelengths between 350 and 3000A in the rest frame, with a peak S/N level of ~130 per A at ~1200A. Since ~90% of the sample quasars have redshift z<1.5, the spectrum is suitable for studying the wavelength region shortward of Ly{alpha} without large effects from intervening Ly{alpha} forest absorption. Data in the waveband between 350 and 600A are mainly from the spectra of z>1.5 quasars, for which significant corrections for the accumulated Lyman-series line and continuum absorption have been applied. There is a significant steepening of the continuum slope around 1050A. The continuum between 1050 and 2200A can be modeled as a power law f_{nu} proportional to {nu}_{alpha}_ with {alpha}=-0.99+/-0.05. For the full sample the power-law index in the extreme ultraviolet (EUV) between 350 and 1050A is {alpha}=-1.96+/-0.15. For the radio-loud subsample (60 objects), the EUV spectral index is {alpha}~=-2.2, while for the radio-quiet subsample (41 objects) it is {alpha}~=-1.8. The continuum flux in the wavelengths near the Lyman limit shows a depression of ~10%. The break in the power-law index and the slight depression of the continuum near the Lyman limit are features expected in Comptonized accretion-disk spectra. Comptonization produces a power-law tail in the wavelength band shortward of 1000A and smears out the Lyman-limit edge of the intrinsic accretion-disk spectrum. In the EUV waveband, we detect several possible emission features, including one around 690A that may be O III + N III produced by the Bowen fluorescence effect. Comparing our composite spectrum with one made at higher redshifts by Francis et al. (1991ApJ...373..465F), we find that the equivalent widths of Ly{alpha} and high-ionization emission lines are larger in our sample, reflecting a known luminosity dependence. The equivalent widths of low-ionization lines do not exhibit such a dependence, suggesting that the quasar EUV continuum above ~50eV is steeper at higher luminosity. Radio-quiet quasars appear to show a slightly harder continuum and lower ionization levels in their emission lines.