- ID:
- ivo://CDS.VizieR/J/ApJ/689/108
- Title:
- MASIV survey. II. First four epochs
- Short Name:
- J/ApJ/689/108
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report on the variability of 443 flat-spectrum, compact radio sources monitored using the VLA for 3 days in four epochs at ~4 month intervals at 5GHz as part of the Micro-Arcsecond Scintillation-Induced Variability (MASIV) survey. Over half of these sources exhibited 2%-10% rms variations on timescales over 2 days. We analyzed the variations by two independent methods and find that the rms variability amplitudes of the sources correlate with the emission measure in the ionized interstellar medium along their respective lines of sight. We thus link the variations with interstellar scintillation of components of these sources, with some (unknown) fraction of the total flux density contained within a compact region of angular diameter in the range 10-50{mu}as. We also find that the variations decrease for high mean flux density sources and, most importantly, for high-redshift sources. The decrease in variability is probably due either to an increase in the apparent diameter of the source or to a decrease in the flux density of the compact fraction beyond z~2.
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- ID:
- ivo://CDS.VizieR/J/ApJ/767/14
- Title:
- MASIV survey III. Optical identifications
- Short Name:
- J/ApJ/767/14
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Intraday variability (IDV) of the radio emission from active galactic nuclei is now known to be predominantly due to interstellar scintillation (ISS). The MASIV (The Micro-Arcsecond Scintillation-Induced Variability) survey of 443 flat spectrum sources revealed that the IDV is related to the radio flux density and redshift. A study of the physical properties of these sources has been severely handicapped by the absence of reliable redshift measurements for many of these objects. This paper presents 79 new redshifts and a critical evaluation of 233 redshifts obtained from the literature. We classify spectroscopic identifications based on emission line properties, finding that 78% of the sources have broad emission lines and are mainly FSRQs. About 16% are weak lined objects, chiefly BL Lacs, and the remaining 6% are narrow line objects. The gross properties (redshift, spectroscopic class) of the MASIV sample are similar to those of other blazar surveys. However, the extreme compactness implied by ISS favors FSRQs and BL Lacs in the MASIV sample as these are the most compact object classes. We confirm that the level of IDV depends on the 5GHz flux density for all optical spectral types. We find that BL Lac objects tend to be more variable than broad line quasars. The level of ISS decreases substantially above a redshift of about two. The decrease is found to be generally consistent with ISS expected for beamed emission from a jet that is limited to a fixed maximum brightness temperature in the source rest frame.
- ID:
- ivo://CDS.VizieR/J/MNRAS/474/4396
- Title:
- MASIV Survey. IV. Radio AGNs variability
- Short Name:
- J/MNRAS/474/4396
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We investigate the relationship between 5GHz interstellar scintillation (ISS) and 15GHz intrinsic variability of compact, radio-selected active galactic nuclei (AGNs) drawn from the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory blazar monitoring program. We discover that the strongest scintillators at 5GHz (modulation index, m_5_>=0.02) all exhibit strong 15GHz intrinsic variability (m_15_>=0.1). This relationship can be attributed mainly to the mutual dependence of intrinsic variability and ISS amplitudes on radio core compactness at ~100{mu}s scales, and to a lesser extent, on their mutual dependences on source flux density, arcsec-scale core dominance and redshift. However, not all sources displaying strong intrinsic variations show high amplitude scintillation, since ISS is also strongly dependent on Galactic line-of-sight scattering properties. This observed relationship between intrinsic variability and ISS highlights the importance of optimizing the observing frequency, cadence, timespan and sky coverage of future radio variability surveys, such that these two effects can be better distinguished to study the underlying physics. For the full MASIV sample, we find that Fermi-detected gamma-ray loud sources exhibit significantly higher 5GHz ISS amplitudes than gamma-ray quiet sources. This relationship is weaker than the known correlation between gamma-ray loudness and the 15GHz variability amplitudes, most likely due to jet opacity effects.
- ID:
- ivo://CDS.VizieR/J/ApJ/872/148
- Title:
- 2MASX/NVSS galaxies brighter than K_20fe_=12.25
- Short Name:
- J/ApJ/872/148
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We identified 15658 NRAO VLA Sky Survey (NVSS) radio sources among the 55288 2 Micron All-Sky Survey eXtended (2MASX) galaxies brighter than k_20fe_=12.25 at {lambda}=2.16{mu}m and covering the {Omega}=7.016sr of sky defined by J2000 {delta}>-40{deg} and |b|>20{deg}. The complete sample of 15043 galaxies with 1.4GHz flux densities S>=2.45mJy contains a 99.9% spectroscopically complete subsample of 9517 galaxies with k_20fe_<=11.75. We used only radio and infrared data to quantitatively distinguish radio sources powered primarily by recent star formation from those powered by active galactic nuclei. The radio sources with log[L(W/Hz)]>19.3 that we used to derive the local spectral luminosity and power-density functions account for >99% of the total 1.4GHz spectral power densities U_SF_=(1.54+/-0.20)x10^19^W/Hz/Mpc^3^ and U_AGN_=(4.23+/-0.78)x10^19^W/Hz/Mpc^3^ in the universe today, and the spectroscopic subsample is large enough that the quoted errors are dominated by cosmic variance. The recent comoving star formation rate density indicated by USF is {psi}~0.015M_{sun}_/yr/Mpc^3^.
- ID:
- ivo://CDS.VizieR/J/ApJ/890/118
- Title:
- Megamaser Cosmology Project. XI. Z74-64 VLBI obs.
- Short Name:
- J/ApJ/890/118
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- As part of the survey component of the Megamaser Cosmology Project, we have discovered a disk megamaser system in the galaxy CGCG 074-064. Using the Green Bank Telescope and the Very Large Array, we have obtained spectral monitoring observations of this maser system at a monthly cadence over the course of two years. We find that the systemic maser features display line-of-sight accelerations of ~4.4km/s/yr that are nearly constant with velocity, while the high-velocity maser features show accelerations that are consistent with zero. We have also used the High-Sensitivity Array to make a high-sensitivity very long baseline interferometric map of the maser system in CGCG 074-064, which reveals that the masers reside in a thin, edge-on disk with a diameter of ~1.5mas (0.6pc). Fitting a three-dimensional warped disk model to the data, we measure a black hole mass of 2.42_-0.20_^+0.22^x10^7^M_{sun}_ and a geometric distance to the system of 87.6_-7.2_^+7.9^Mpc. Assuming a cosmic microwave background-frame recession velocity of 7308+/-150km/s, we constrain the Hubble constant to H_0_=81.0_-6.9_^+7.4^(stat.)+/-1.4(sys.)km/s/Mpc.
- ID:
- ivo://CDS.VizieR/J/ApJS/255/28
- Title:
- ~420MHz uGMRT obs. of MIR selected radio-loud AGN
- Short Name:
- J/ApJS/255/28
- Date:
- 11 Mar 2022
- Publisher:
- CDS
- Description:
- We present results from a spectroscopically blind search for associated and intervening HI 21cm and OH 18cm absorption lines toward 88 active galactic nuclei (AGN) at 2<z<5 using the uGMRT. The sample of AGN with 1.4GHz spectral luminosity in the range 10^27-29.3^W/Hz is selected using mid-infrared colors and closely resembles the distribution of the underlying quasar population. The search for associated or proximate absorption, defined to be within 3000km/s of the AGN redshift, led to one HI 21cm absorption detection (M1540-1453; z_abs_=2.1139). This is only the fourth known absorption at z>2. The detection rate (1.6_-1.4_^+3.8^%) suggests a low covering factor of the cold neutral medium (CNM; T~100K) associated with these powerful AGN. The intervening absorption line search, with a sensitivity to detect the CNM in damped Ly{alpha} systems (DLAs), has comoving absorption path lengths of {Delta}X=130.1 and 167.7 for HI and OH, respectively. The corresponding number of absorbers per unit comoving path length are <=0.014 and <=0.011, respectively. The former is at least 4.5 times lower than that of DLAs and consistent with the CNM cross section estimated using H_2_ and CI absorbers at z>2. Our AGN sample is optically fainter compared to the quasars used to search for DLAs in the past. In our optical spectra obtained using SALT and NOT, we detect five intervening (redshift path ~9.3) and two proximate DLAs. This is slightly excessive compared to the statistics based on optically selected quasars. The nondetection of HI 21cm absorption from these DLAs suggests a small CNM covering fraction around galaxies at z>2.
- ID:
- ivo://CDS.VizieR/J/A+A/570/A110
- Title:
- Mid-IR properties of OH maser galaxies
- Short Name:
- J/A+A/570/A110
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We compiled all 119 OH maser galaxies (110 out of them are megamasers, i.e., L_OH_>10L_{sun}_) published so far and cross-identified these OH masers with the Wide-Field Infrared Survey Explorer (WISE) catalog, to investigate the middle infrared (MIR) properties of OH maser galaxies. The WISE magnitude data at the 3.4, 4.6, 12 and 22{mu}m (W1 to W4) are collected for the OH maser sample and one control sample, which are non-detection sources. The color-color diagrams show that both OH megamaser (OHM) and non-OHM (ultra)luminous infrared galaxies ((U)LIRGs) are far away from the single blackbody model line and many of them can follow the path described by the power-law model. The active galaxy nuclei (AGN) fraction is about ~40% for both OHM and non-OHM (U)LIRGs, according to the AGN criteria W1-W2>=0.8. Among the Arecibo survey sample, OHM sources tend to have a lower luminosity at short MIR wavelengths (e.g., 3.4{mu}m and 4.6{mu}m) than that of non-OHM sources, which should come from the low OHM fraction among the survey sample with large 3.4{mu}m and 4.6{mu}m luminosity. The OHM fraction tends to increase with cooler MIR colors (larger F_22{mu}m_/F_3.4{mu}m_). These may be good for sample selection when searching OH megamasers, such as excluding extreme luminous sources at short MIR wavelengths, choosing sources with cooler MIR colors. In the case of the power-law model, we derived the spectral indices for our samples. For the Arecibo survey sample, OHM (U)LIRGs tend to have larger spectral index {alpha}_22-12_ than non-OHM sources, which agrees with previous results. One significant correlation exists between the WISE infrared luminosity at 22{mu}m and the color [W1]-[W4] for the Arecibo OHM hosts. These clues should provide suitable constraints on the sample selection for OH megamaser surveys by future advanced telescopes (e.g., FAST). In addition, the correlation of maser luminosity and the MIR luminosity of maser hosts tends to be non-significant, which may indirectly support the pumping of OHM emission that is dominated by the far infrared radiation, instead of the MIR radiation.
- ID:
- ivo://CDS.VizieR/J/ApJS/171/376
- Title:
- MOJAVE. III. VLA 1.4GHz images
- Short Name:
- J/ApJS/171/376
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The MOJAVE blazar sample consists of the 133 brightest, most compact radio-loud AGNs in the northern sky, and it is selected on the basis of VLBA 2cm correlated flux density exceeding 1.5Jy (2Jy for declinations south of 0) at any epoch between 1994 and 2003. Since 1994 we have been gathering VLBA data on the sample to measure superluminal jet speeds and to better understand the parsec-scale kinematics of AGN jets. We have obtained 1.4GHz VLA A configuration data on 57 of these sources to investigate whether the extended luminosity of blazars is correlated with parsec-scale jet speed and also to determine what other parsec-scale properties are related to extended morphology, such as optical emission line strength and gamma-ray emission. We present images and measurements of the kiloparsec scale emission from the VLA data, which will be used in subsequent statistical studies of the MOJAVE sample.
- ID:
- ivo://CDS.VizieR/J/ApJ/706/1253
- Title:
- MOJAVE VII. Blazar jet acceleration
- Short Name:
- J/ApJ/706/1253
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We discuss acceleration measurements for a large sample of extragalactic radio jets from the Monitoring Of Jets in Active Galactic Nuclei with VLBA Experiments (MOJAVE) program, which studies the parsec-scale jet structure and kinematics of a complete, flux-density-limited sample of active galactic nuclei (AGNs). Accelerations are measured from the apparent motion of individual jet features or "components" which may represent patterns in the jet flow. We find that significant accelerations are common both parallel and perpendicular to the observed component velocities.
- ID:
- ivo://CDS.VizieR/J/AJ/144/105
- Title:
- MOJAVE. VIII. Faraday rotation in AGN jets.
- Short Name:
- J/AJ/144/105
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report observations of Faraday rotation measures for a sample of 191 extragalactic radio jets observed within the MOJAVE program. Multifrequency Very Long Baseline Array observations were carried out over 12 epochs in 2006 at four frequencies between 8 and 15GHz. We detect parsec-scale Faraday rotation measures in 149 sources and find the quasars to have larger rotation measures on average than BL Lac objects. The median core rotation measures are significantly higher than in the jet components. This is especially true for quasars where we detect a significant negative correlation between the magnitude of the rotation measure and the de-projected distance from the core. We perform detailed simulations of the observational errors of total intensity, polarization, and Faraday rotation, and concentrate on the errors of transverse Faraday rotation measure gradients in unresolved jets. Our simulations show that the finite image restoring beam size has a significant effect on the observed rotation measure gradients, and spurious gradients can occur due to noise in the data if the jet is less than two beams wide in polarization. We detect significant transverse rotation measure gradients in four sources (0923+392, 1226+023, 2230+114, and 2251+158). In 1226+023 the rotation measure is for the first time seen to change sign from positive to negative over the transverse cuts, which supports the presence of a helical magnetic field in the jet. In this source we also detect variations in the jet rotation measure over a timescale of three months, which are difficult to explain with external Faraday screens and suggest internal Faraday rotation. By comparing fractional polarization changes in jet components between the four frequency bands to depolarization models, we find that an external purely random Faraday screen viewed through only a few lines of sight can explain most of our polarization observations, but in some sources, such as 1226+023 and 2251+158, internal Faraday rotation is needed.