22 research outputs found
Constraining the evolution of young radio-loud AGN
GPS and CSS radio sources are the objects of choice to investigate the
evolution of young radio-loud AGN. Previous investigations, mainly based on
number counts and source size distributions, indicate that GPS/CSS sources
decrease significantly in radio power when evolving into old, extended objects.
We suggest this is preceded by a period of increase in radio luminosity, which
lasts as long as the radio source is confined within the core-radius of its
host galaxy. We have selected a sample of nearby compact radio sources,
unbiased by radio spectrum, to determine their radio luminosity function, size
distribution, dynamical ages, and emission line properties in a complete and
homogeneous way. First results indicate that the large majority of objects
(>80%) exhibit classical GPS/CSS radio spectra, and show structures consistent
with the being compact double, or compact symmetric objects. This sample
provides and ideal basis to further test and constrain possible evolution
scenarios, and to investigate the relation between radio spectra and
morphologies, orientation and Doppler boosting in samples of young radio-loud
AGN, in an unbiased way.Comment: LaTeX, 8 pages, 3 figs: Accepted by Publications of the Astronomical
Society of Australia, as part of the proceedings of the 3rd GPS/CSS workshop,
eds. T. Tzioumis et a
Nanoscopic compartmentalization of membrane protein motion at the axon initial segment
The axon initial segment (AIS) is enriched in specific adaptor, cytoskeletal, and transmembrane molecules. During AIS establishment, a membrane diffusion barrier is formed between the axonal and somatodendritic domains. Recently, an axonal periodic pattern of actin, spectrin, and ankyrin forming 190-nm-spaced, ring-like structures has been discovered. However, whether this structure is related to the diffusion barrier function is unclear. Here, we performed single-particle tracking time-course experiments on hippocampal neurons during AIS development. We analyzed the mobility of lipid-anchored molecules by high-speed single-particle tracking and correlated positions of membrane molecules with the nanoscopic organization of the AIS cytoskeleton. We observe a strong reduction in mobility early in AIS development. Membrane protein motion in the AIS plasma membrane is confined to a repetitive pattern of ∼190-nm-spaced segments along the AIS axis as early as day in vitro 4, and this pattern alternates with actin rings. Mathematical modeling shows that diffusion barriers between the segments significantly reduce lateral diffusion along the axon
The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. I. The Images
We present images from a space-VLBI survey using the facilities of the VLBI
Space Observatory Programme (VSOP), drawing our sample from the well-studied
Pearson-Readhead survey of extragalactic radio sources. Our survey has taken
advantage of long space-VLBI baselines and large arrays of ground antennas,
such as the Very Long Baseline Array and European VLBI Network, to obtain high
resolution images of 27 active galactic nuclei, and to measure the core
brightness temperatures of these sources more accurately than is possible from
the ground. A detailed analysis of the source properties is given in
accompanying papers. We have also performed an extensive series of simulations
to investigate the errors in VSOP images caused by the relatively large holes
in the (u,v) plane when sources are observed near the orbit normal direction.
We find that while the nominal dynamic range (defined as the ratio of map peak
to off-source error) often exceeds 1000:1, the true dynamic range (map peak to
on-source error) is only about 30:1 for relatively complex core-jet sources.
For sources dominated by a strong point source, this value rises to
approximately 100:1. We find the true dynamic range to be a relatively weak
function of the difference in position angle (PA) between the jet PA and (u,v)
coverage major axis PA. For low signal-to-noise regions typically located down
the jet away from the core, large errors can occur, causing spurious features
in VSOP images that should be interpreted with caution.Comment: 26 pages, 16 figures. Accepted for publication in the Astrophysical
Journal. A version with higher resolution figures (7 Mb) can be found at
http://sgra.jpl.nasa.gov/html_lister/pr_papers/map_paper.p
Observations of HI Absorbing Gas in Compact Radio Sources at Cosmological Redshifts
We present an overview of the occurrence and properties of atomic gas
associated with compact radio sources at redshifts up to z=0.85. Searches for
HI 21cm absorption were made with the Westerbork Synthesis Radio Telescope at
UHF-high frequencies (725-1200 MHz). Detections were obtained for 19 of the 57
sources with usable spectra (33%). We have found a large range in line depths,
from tau=0.16 to tau<=0.001. There is a substantial variety of line profiles,
including Gaussians of less than 10km/s, to more typically 150km/s, as well as
irregular and multi-peaked absorption profiles, sometimes spanning several
hundred km/s. Assuming uniform coverage of the entire radio source, we obtain
column depths of atomic gas between 1e19 and 3.3e21(Tsp/100K)(1/f)cm^(-2).
There is evidence for significant gas motions, but in contrast to earlier
results at low redshift, there are many sources in which the HI velocity is
substantially negative (up to v=-1420km/s) with respect to the optical
redshift, suggesting that in these sources the atomic gas, rather than falling
into the centre, may be be flowing out, interacting with the jets, or rotating
around the nucleus.Comment: 10 pages, accepted for publication in A&
What are the megahertz peaked-spectrum sources?
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record [MNRAS (July 1, 2016) 459: 2455-2471. First published online April 7, 2016] is available online at: doi: 10.1093/mnras/stw799Megahertz peaked-spectrum (MPS) sources have spectra that peak at frequencies below 1 GHz in the observer's frame and are believed to be radio-loud active galactic nuclei (AGN). We recently presented a new method to search for high-redshift AGN by identifying unusually compact MPS sources. In this paper, we present European VLBI Network (EVN) observations of 11 MPS sources which we use to determine their sizes and investigate the nature of the sources with ~10 mas resolution. Of the 11 sources, we detect nine with the EVN. Combining the EVN observations with spectral and redshift information, we show that the detected sources are all AGN with linear sizes smaller than 1.1 kpc and are likely young. This shows that low-frequency colour-colour diagrams are an easy and efficient way of selecting small AGN and explains our high detection fraction (82%) in comparison to comparable surveys. Finally we argue that the detected sources are all likely compact symmetric objects and that none of the sources are blazars.Peer reviewe
Fundamental galaxy parameters for radio-loud AGN and the black hole - radio power connection
We have determined the central velocity dispersion and surface brightness
profiles for a sample of powerful radio galaxies in the redshift range
0.06<z<0.31, which were selected on the basis of their young radio source. The
optical hosts follow the fundamental plane of elliptical galaxies, showing that
young radio sources reside in normal ellipticals, as do other types of radio
galaxies. As young radio sources are relatively straightforward to select and
the contributions of the AGN light to the optical spectra are minimal, these
objects can readily be used to study the evolution of the fundamental plane of
elliptical galaxies out to z=1, independently of optical selection effects. The
black hole masses of the objects in our sample have been determined using the
tight empirical relation of M_bh with central velocity dispersion, and for
literature samples of classical radio galaxies and optically selected
ellipticals. Only the optically selected in-active galaxies are found to
exhibit a correlation between M_bh and radio luminosity. In contrast, the radio
powers of the AGN in the samples do not correlate with M_bh at all, with
objects at a given black hole mass ranging over 7 orders of magnitude in radio
power. We have been able to tie in the population of powerful radio sources
with its parent population of in-active elliptical galaxies: the local black
hole mass function has been determined, which was combined with the fraction of
radio-loud black holes as function of M_bh, as determined from the optically
selected galaxy sample, to derive the local volume-density of radio galaxies
and the distribution of their black hole masses. These are shown to be
consistent with the local radio luminosity function and the distribution of
black hole masses in the radio selected samples [ABBREVIATED]Comment: LaTeX, 13 pages, 10 figures: submitted to MNRA
Multi-frequency study of the B3 VLA sample
We present 74-MHz data of 365 B3 VLA sources, which were obtained
as a by-product of the observations of Tschager et al. (2003, A&A, 402, 171) who
observed a large area of sky with the VLA in A-array. Apart from the
lowest-frequency observations of B3 VLA sources performed so far
contributing to our on-going multi-frequency study of this survey,
these A-array data also provide the first morphological information
at this low frequency. The most intriguing result is the discovery
of two new Giant Radio Galaxy candidates, B3 1232+397B at a redshift
of 3.22 and B3 1419+419 at a redshift of 0.367. This means that
B3 1232+397B would be the most distant GRG known to date. At the same
time, these two objects are two new examples of sources with
recurrent activity. Our results give a foretaste of the impact
which extremely low-frequency radio continuum observations will have for
the study of source evolution