17,872 research outputs found
The radio spectra of reddened 2MASS QSOs: evidence for young radio jets
Multifrequency radio continuum observations (1.4-22 GHz) of a sample of
reddened QSOs are presented. We find a high incidence (13/16) of radio spectral
properties, such as low frequency turnovers, high frequency spectral breaks or
steep power-law slopes, similar to those observed in powerful compact steep
spectrum (CSS) and gigahertz-peaked spectrum (GPS) sources. The radio data are
consistent with relatively young radio jets with synchotron ages <1e6-1e7yr.
This calculation is limited by the lack of high resolution (milli-arcsec) radio
observations. For the one source in the sample that such data are available a
much younger radio age is determined, <2e3yr, similar to those of GPS/CSS
sources. These findings are consistent with claims that reddened QSOs are young
systems captured at the first stages of the growth of their supermassive black
holes. It also suggests that expanding radio lobes may be an important feedback
mode at the early stages of the evolution of AGN.Comment: 9 pages, to appear in MNRA
Vanishing lines in generalized Adams spectral sequences are generic
We show that in a generalized Adams spectral sequence, the presence of a
vanishing line of fixed slope (at some term of the spectral sequence, with some
intercept) is a generic property.Comment: 11 pages. Published copy, also available at
http://www.maths.warwick.ac.uk/gt/GTVol3/paper7.abs.htm
Galaxy evolution across the optical emission-line diagnostic diagrams?
The discovery of the M-sigma relation, the local galaxy bimodality, and the
link between black-hole and host-galaxy properties, have raised the question
whether AGN play a role in galaxy evolution. Several theoretical models
implement AGN feedback to explain the observed galaxy luminosity function, and
possibly the color and morphological transformation of spiral galaxies into
passive ellipticals. To understand the importance of AGN feedback, a study of
the AGN populations in the radio-optical domain is crucial. A mass sequence
linking star-forming galaxies and AGN has been already noted in previous works,
and it is now investigated as possible evolutionary sequence. We observed a
sample of 119 intermediate-redshift (0.04<z<0.4) SDSS-FIRST radio emitters with
the Effelsberg 100-m telescope at 4.85 and 10.45 GHz and obtained spectral
indices. We find indications of spectral index flattening in high-metallicity
star-forming galaxies, composite galaxies, and Seyferts. This "flattening
sequence" along the [NII]-based emission-line diagnostic diagram is consistent
with the hardening of galaxy ionizing field, due to nuclear activity. After
combining our data with FIRST measurements at 1.4 GHz, we find that the
three-point radio spectra of Seyferts and LINERs show substantial differences,
attributable to small radio core components and larger (arcsecond sized)
jet/lobe components, respectively. A visual inspection of FIRST images seems to
confirm this hypothesis. Galaxies along this sequence are hypothesized to be
transitioning from the active star-forming galaxies (blue cloud) to the passive
elliptical galaxies (red sequence). This supports the suggestion that AGN play
a role in shutting down star-formation, and allow the transition from one
galaxy class to the other.Comment: 20 pages, 19 figures, accepted for publication in A&
Phenotypic analysis of host-parasite interactions in lambs infected with Teladorsagia circumcincta
The Phoenix Deep Survey: spectroscopic catalog
The Phoenix Deep Survey is a multi-wavelength survey based on deep 1.4 GHz
radio imaging, reaching well into the sub-100 microJy level. One of the aims of
this survey is to characterize the sub-mJy radio population, exploring its
nature and evolution. In this paper we present the catalog and results of the
spectroscopic observations aimed at characterizing the optically ``bright''
(R<~ 21.5 mag) counterparts of faint radio sources. Out of 371 sources with
redshift determination, 21% have absorption lines only, 11% show AGN
signatures, 32% are star-forming galaxies, 34% show narrow emission lines that
do not allow detailed spectral classification (due to poor signal-to-noise
ratio and/or lack of diagnostic emission lines) and the remaining 2% are
identified with stars. For the star-forming galaxies with a Balmer decrement
measurement we find a median extinction of A(Ha)=1.9 mag, higher than that of
optically selected samples. This is a result of the radio selection, which is
not biased against dusty systems. Using the available spectroscopic
information, we estimate the radio luminosity function of star-forming galaxies
in two independent redshift bins at z~0.1 and 0.3 respectively. We find direct
evidence for strong luminosity evolution of these systems consistent with L(1.4
GHz) ~ (1+z)^(2.7).Comment: 39 pages, 12 figures. References added, and minor changes to reflect
published versio
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