- ID:
- ivo://CDS.VizieR/J/ApJS/220/7
- Title:
- Abundance of X-shaped radio sources. I. VLA survey
- Short Name:
- J/ApJS/220/7
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Cheung (2007, J/AJ/133/2097) identified a sample of 100 candidate X-shaped radio galaxies using the NRAO FIRST survey; these are small-axial-ratio extended radio sources with off-axis emission. Here, we present radio images of 52 of these sources that have been made from archival Very Large Array data with resolution of about 1". Fifty-one of the 52 were observed at 1.4GHz, 7 were observed at 1.4 and 5GHz, and 1 was observed only at 5GHz. We also present overlays of the Sloan Digital Sky Survey red images for 48 of the sources, and DSS-II overlays for the remainder. Optical counterparts have been identified for most sources, but there remain a few empty fields. Our higher resolution VLA images along with FIRST survey images of the sources in the sample reveal that extended extragalactic radio sources with small axial ratios are largely (60%) cases of double radio sources with twin lobes that have off-axis extensions, usually with inversion-symmetric structure. The available radio images indicate that at most 20% of sources might be genuine X-shaped radio sources that could have formed by a restarting of beams in a new direction following an interruption and axis flip. The remaining 20% are in neither of these categories.
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- ID:
- ivo://CDS.VizieR/J/ApJ/799/72
- Title:
- Binary AGNs from the VLA Stripe 82 survey
- Short Name:
- J/ApJ/799/72
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Galaxy mergers play an important role in the growth of galaxies and their supermassive black holes. Simulations suggest that tidal interactions could enhance black hole accretion, which can be tested by the fraction of binary active galactic nuclei (AGNs) among galaxy mergers. However, determining the fraction requires a statistical sample of binaries. We have identified kiloparsec-scale binary AGNs directly from high-resolution radio imaging. Inside the 92deg^2^ covered by the high-resolution Very Large Array survey of the Sloan Digital Sky Survey (SDSS) Stripe 82 field, we identified 22 grade A and 30 grade B candidates of binary radio AGNs with angular separations less than 5" (10kpc at z=0.1). Eight of the candidates have optical spectra for both components from the SDSS spectroscopic surveys and our Keck program. Two grade B candidates are projected pairs, but the remaining six candidates are all compelling cases of binary AGNs based on either emission line ratios or the excess in radio power compared to the H{alpha}-traced star formation rate. Only two of the six binaries were previously discovered by an optical spectroscopic search. Based on these results, we estimate that ~60% of our binary candidates would be confirmed once we obtain complete spectroscopic information. We conclude that wide-area high-resolution radio surveys offer an efficient method to identify large samples of binary AGNs. These radio-selected binary AGNs complement binaries identified at other wavelengths and are useful for understanding the triggering mechanisms of black hole accretion.
- ID:
- ivo://CDS.VizieR/J/ApJS/197/24
- Title:
- Chandra large-scale extragalactic jets. I.
- Short Name:
- J/ApJS/197/24
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In this paper, we report the first stages of an investigation into the X-ray properties of extragalactic jets (XJET project). Our approach is to subject all sources for which X-ray emission has been detected by Chandra to uniform reduction procedures. Using Chandra archival data for 106 such sources, we measure X-ray fluxes in three bands and compare these to radio fluxes. We discuss the sample, the reduction methods, and present first results for the ratio of X-ray to radio flux for jet knots and hotspots. In particular, we apply statistical tests to various distributions of key observational parameters to evaluate differences between the different classes of sources.
- ID:
- ivo://CDS.VizieR/J/ApJ/744/84
- Title:
- Combined sample of radio-loud AGNs at 408MHz
- Short Name:
- J/ApJ/744/84
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The work in this paper aims at determining the evolution and possible co-evolution of radio-loud active galactic nuclei (AGNs) and their cores via their radio luminosity functions (i.e., total and core RLFs, respectively). Using a large combined sample of 1063 radio-loud AGNs selected at low radio frequency, we investigate the RLF (radio luminosity function) at 408MHz of steep-spectrum radio sources. Our results support a luminosity-dependent evolution. Using core flux density data of the complete sample 3CRR (Laing et al., 1983, Cat. J/MNRAS/204/151; see also Cat. VIII/1), we investigate the core RLF at 5.0GHz. Based on the combined sample with incomplete core flux data, we also estimate the core RLF using a modified factor of completeness. Both results are consistent and show that the comoving number density of radio cores displays a persistent decline with redshift, implying a negative density evolution. We find that the core RLF is obviously different from the total RLF at the 408 MHz band which is mainly contributed by extended lobes, implying that the cores and extended lobes could not be co-evolving at radio emission.
- ID:
- ivo://CDS.VizieR/J/ApJ/710/764
- Title:
- Extended radio emission in MOJAVE blazars
- Short Name:
- J/ApJ/710/764
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of a study on the kiloparsec-scale radio emission in the complete flux density limited MOJAVE sample, comprising 135 radio-loud active galactic nuclei. New 1.4GHz Very Large Array (VLA) radio images of six quasars and previously unpublished images of 21 blazars are presented, along with an analysis of the high-resolution (VLA A-array) 1.4GHz emission for the entire sample. While extended emission is detected in the majority of the sources, about 7% of the sources exhibit only radio core emission. We expect more sensitive radio observations, however, to detect faint emission in these sources, as we have detected in the erstwhile "core-only" source, 1548+056. The kiloparsec-scale radio morphology varies widely across the sample. Many BL Lac objects exhibit extended radio power and kiloparsec-scale morphology typical of powerful FRII jets, while a substantial number of quasars possess radio powers intermediate between FRIs and FRIIs.
- ID:
- ivo://CDS.VizieR/J/ApJ/836/174
- Title:
- Extragalactic peaked-spectrum radio sources
- Short Name:
- J/ApJ/836/174
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a sample of 1483 sources that display spectral peaks between 72MHz and 1.4GHz, selected from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. The GLEAM survey is the widest fractional bandwidth all-sky survey to date, ideal for identifying peaked-spectrum sources at low radio frequencies. Our peaked-spectrum sources are the low-frequency analogs of gigahertz-peaked spectrum (GPS) and compact-steep spectrum (CSS) sources, which have been hypothesized to be the precursors to massive radio galaxies. Our sample more than doubles the number of known peaked-spectrum candidates, and 95% of our sample have a newly characterized spectral peak. We highlight that some GPS sources peaking above 5GHz have had multiple epochs of nuclear activity, and we demonstrate the possibility of identifying high-redshift (z>2) galaxies via steep optically thin spectral indices and low observed peak frequencies. The distribution of the optically thick spectral indices of our sample is consistent with past GPS/CSS samples but with a large dispersion, suggesting that the spectral peak is a product of an inhomogeneous environment that is individualistic. We find no dependence of observed peak frequency with redshift, consistent with the peaked-spectrum sample comprising both local CSS sources and high-redshift GPS sources. The 5GHz luminosity distribution lacks the brightest GPS and CSS sources of previous samples, implying that a convolution of source evolution and redshift influences the type of peaked-spectrum sources identified below 1GHz.
- ID:
- ivo://CDS.VizieR/J/ApJ/722/L7
- Title:
- Fermi/LAT detected MOJAVE AGNs
- Short Name:
- J/ApJ/722/L7
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report the detection of a non-zero time delay between radio emission measured by the VLBA at 15.4GHz and {gamma}-ray radiation ({gamma}-ray leads radio) registered by the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope for a sample of 183 radio and {gamma}-ray bright active galactic nuclei. For the correlation analysis, we used 0.1-100GeV {gamma}-ray photon fluxes, taken from monthly binned measurements from the first Fermi LAT catalog (1FGL; Abdo et al. 2010, Cat. J/ApJS/188/405), and 15.4GHz radio flux densities from the MOJAVE VLBA program (Lister et al. 2009, Cat. J/AJ/137/3718). The correlation is most pronounced if the core flux density is used, strongly indicating that the {gamma}-ray emission is generated within the compact region of the 15GHz VLBA core. Determining the Pearson's r and Kendall's {tau} correlation coefficients for different time lags, we find that for the majority of sources the radio/{gamma}-ray delay ranges from 1 to 8 months in the observer's frame and peaks at approximately 1.2 months in the source's frame. We interpret the primary source of the time delay to be synchrotron opacity in the nuclear region.
- ID:
- ivo://CDS.VizieR/J/ApJ/740/59
- Title:
- 1.4GHz and X-ray sources in 12 clusters of galaxies
- Short Name:
- J/ApJ/740/59
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using Chandra imaging spectroscopy and Very Large Array (VLA) L-band radio maps, we have identified radio sources at P_1.4GHz_>=5x10^23^W/Hz and X-ray point sources (XPSs) at L_0.3-8keV_>=5x10^42^erg/s in L>L* galaxies in 12 high-redshift (0.4<z<1.2) clusters of galaxies. The radio galaxies and XPSs in this cluster sample, chosen to be consistent with Coma Cluster progenitors at these redshifts, are compared to those found at low-z analyzed in Hart et al. (Paper I, 2009ApJ...705..854H). Within a projected radius of 1Mpc of the cluster cores, we find 17 cluster radio galaxies (11 with secure redshifts, including one luminous FR II radio source at z=0.826, and six more with host galaxy colors similar to cluster ellipticals). Within this same projected radius, we identify seven spectroscopically confirmed cluster XPSs, all with cluster red-sequence (CRS) host galaxy colors. Consistent with the results from Martini et al. (2009ApJ...701...66M), we estimate a minimum X-ray active fraction of 1.4%+/-0.8% for cluster red-sequence galaxies in high-z clusters, corresponding to an approximate 10-fold increase from 0.15%+/-0.15% at low-z. Although complete redshift information is lacking for several XPSs in z>0.4 cluster fields, the increased numbers and luminosities of the CRS radio galaxies and XPSs suggest a substantial (9-10-fold) increase in the heat injected into high-redshift clusters by AGNs compared to the present epoch.
- ID:
- ivo://CDS.VizieR/J/ApJ/831/168
- Title:
- 6GHz JVLA observations of low-z SDSS quasars
- Short Name:
- J/ApJ/831/168
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We discuss 6GHz JVLA observations covering a volume-limited sample of 178 low-redshift (0.2<z<0.3) optically selected quasi-stellar objects (QSOs). Our 176 radio detections fall into two clear categories: (1) about 20% are radio-loud QSOs (RLQs) with spectral luminosities of L_6_>~10^23.2^W/Hz that are primarily generated in the active galactic nucleus (AGN) responsible for the excess optical luminosity that defines a bona fide QSO; and (2) the remaining 80% that are radio-quiet QSOs (RQQs) that have 10^21^<~L_6_<~10^23.2^W/Hz and radio sizes <~10kpc, and we suggest that the bulk of their radio emission is powered by star formation in their host galaxies. "Radio-silent" QSOs (L_6_<~10^21^W/Hz) are rare, so most RQQ host galaxies form stars faster than the Milky Way; they are not "red and dead" ellipticals. Earlier radio observations did not have the luminosity sensitivity of L_6_<~10^21^W/Hz that is needed to distinguish between such RLQs and RQQs. Strong, generally double-sided radio emission spanning >>10kpc was found to be associated with 13 of the 18 RLQ cores with peak flux densities of S_p_>5mJy/beam (log(L)>~24). The radio luminosity function of optically selected QSOs and the extended radio emission associated with RLQs are both inconsistent with simple "unified" models that invoke relativistic beaming from randomly oriented QSOs to explain the difference between RLQs and RQQs. Some intrinsic property of the AGNs or their host galaxies must also determine whether or not a QSO appears radio-loud.
- ID:
- ivo://CDS.VizieR/J/AJ/154/169
- Title:
- GMRT observations of head-tail radio galaxies
- Short Name:
- J/AJ/154/169
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results from a study of seven large known head-tail radio galaxies based on observations using the Giant Metrewave Radio Telescope at 240 and 610 MHz. These observations are used to study the radio morphologies and distribution of the spectral indices across the sources. The overall morphology of the radio tails of these sources is suggestive of random motions of the optical host around the cluster potential. The presence of multiple bends and wiggles in several head-tail sources is possibly due to the precessing radio jets. We find steepening of the spectral index along the radio tails. The prevailing equipartition magnetic field also decreases along the radio tails of these sources. These steepening trends are attributed to the synchrotron aging of plasma toward the ends of the tails. The dynamical ages of these sample sources have been estimated to be ~10^8^ yr, which is a factor of six more than the age estimates from the radiative losses due to synchrotron cooling.
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