What sparks the radio loud phase of nearby quasars? (2017)
Keywords :
galaxies active - quasars: spectra - quasars: radio continuum
Abstract:To better constrain the hypotheses proposed to explain why only a few quasars are radio loud (R_L_), we compare the characteristics of 1958 nearby (z<=0.3) SDSS (Sloan Digital Sky Survey) quasars, covered by the FIRST (Faint Images of the Radio Sky at Twenty Centimeters) and NVSS (NRAO VLA Sky Survey) radio surveys. Only 22 per cent are R_L_ with log(L_1.4GHz_)>=22.5W/Hz, the majority being compact (C), weak radio sources (WRS), with log(L_1.4GHz_)<24.5W/Hz. 15 per cent of the RL quasars have extended radio morphologies: 3 per cent have a core and a jet (J), 2 per cent have a core with one lobe (L), and 10 per cent have a core with two lobes (T), the majority being powerful radio sources (PRS), with log(L_1.4GHz_)>=24.5W/Hz. In general, RL
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From the SDSS DR7 quasar catalog (CDS Cat. VII/260) we selected those
with redshift z<=0.3 and excluded those with H_{beta}_ line less than
1000km/s as well as spectra with deficient pixels or dubious emission
features, acquiring a final sample of 1958 quasars. SDSS spectra were
used to derive line widths of the H{beta} and OIII emission lines, the
5100-Angstrom continuum luminosity as well as the Eddington ratio
L_bol_/L_Edd_. Both visual inspection and an automated algorithm
were employed to classify and quantify the environment of the QSOs in
terms of neighboring galaxies as well as the visual morphology of the
QSOs on gri composite SDSS images. Images from the 1.4-GHz radio
surveys NVSS and FIRST, centered on the QSOs and corresponding to at
least 1Mpc in size (at the QSO redshift) were inspected, and the
radio morphology and 1.4-GHz flux densities recorded, accepting radio
detections within 2-arcsec of the QSO down to a minimum flux density
of 1.0mJy in NVSS and 0.5mJy in FIRST (approximately the 3-sigma
levels in these surveys, and two times lower than the lower flux
limits of the NVSS and FIRST source catalogues). From the integrated
flux density we derived the 1.4-GHz radio luminosity, and for extended
radio sources we catalogued the largest linear size (LLS). Three new
giant radio quasars larger than 1Mpc in projected size were found,
including the largest such example presently known, with an LLS of
4.45Mpc (H_0_=67.7, {Omega}_m_=0.309, {Omega}_{Lambda}_=0.691).
Heinz Andernach, heinz(at)astro.ugto.mx
