64 research outputs found
A Sample of 669 Ultra Steep Spectrum Radio Sources to Find High Redshift Radio Galaxies
Since radio sources with Ultra Steep Spectra (USS; alpha <~ -1.30; S ~
nu^alpha) are efficient tracers of high redshift radio galaxies (HzRGs), we
have defined three samples of such USS sources using the recently completed
WENSS, TEXAS, MRC, NVSS and PMN radio-surveys. Our combined sample contains 669
sources with S_1400 > 10 mJy and covers virtually the entire sky outside the
Galactic plane (|b|>15). For our 2 largest samples, covering declination > -35,
we selected only sources with angular sizes Theta < 1'. For 410 sources, we
present radio-maps with 0.3" to ~5" resolution from VLA and ATCA observations
or from the FIRST survey, which allows the optical identification of these
radio sources. We find that the spectral index distribution of 143,000 sources
from the WENSS and NVSS consists of a steep spectrum galaxy and a flat spectrum
quasar population, with the relative contribution of flat spectrum sources
doubling from S_1400 >0.1 Jy to S_1400 >2.5 Jy. The identification fraction of
our USS sources on the POSS (R <~ 20) is as low as 15%, independent of spectral
index alpha < -1.30. We further show that 85% of the USS sources that can be
identified with an X-ray source are probably contained in galaxy clusters, and
that alpha < -1.6 sources are excellent Galactic pulsar candidates, because the
percentage of these sources is four times higher in the Galactic plane. Our
sample has been constructed to start an intensive campaign to obtain a large
sample of high redshift objects (z>3) that is selected in a way that does not
suffer from dust extinction or any other optical bias [abridged].Comment: 28 Pages, 12 PostScript figures. Accepted for publication in
Astronomy & Astrophysics Supplements. Appendices B, C & D available on
http://www.strw.LeidenUniv.nl/~debreuck/paper
On the evolution of young radio-loud AGN
This paper describes an investigation of the early evolution of extragalactic
radio sources using samples of faint and bright Gigahertz Peaked Spectrum (GPS)
and Compact Steep Spectrum (CSS) radio galaxies. Correlations found between
their peak frequency, peak flux density and angular size provide strong
evidence that synchrotron self absorption is the cause of the spectral
turnovers, and indicate that young radio sources evolve in a self-similar way.
In addition, the data seem to suggest that the sources are in equipartition
while they evolve. If GPS sources evolve to large size radio sources, their
redshift dependent birth-functions should be the same. Therefore, since the
lifetimes of radio sources are thought to be short compared to the Hubble time,
the observed difference in redshift distribution between GPS and large size
sources must be due to a difference in slope of their luminosity functions. We
argue that this slope is strongly affected by the luminosity evolution of the
individual sources. A scenario for the luminosity evolution is proposed in
which GPS sources increase in luminosity and large scale radio sources decrease
in luminosity with time. This evolution scenario is expected for a ram-pressure
confined radio source in a surrounding medium with a King profile density. In
the inner parts of the King profile, the density of the medium is constant and
the radio source builds up its luminosity, but after it grows large enough the
density of th e surrounding medium declines and the luminosity of the radio
source decreases. A comparison of the local luminosity function (LLF) of GPS
galaxies with that of extended sources is a good test for this evolution
scenario [abridged].Comment: LaTeX, 11 pages, 8 figures; Accepted by MNRAS. Related papers may be
found at http://www.ast.cam.ac.uk/~snellen . Valuable comments of referee
incorporated. More discussion on simulation
Observing the Seyfert 2 nucleus of NGC 1068 with the VLT Interferometer
Dusty tori have been suggested to play a crucial role in determining the
physical characteristics of active galactic nuclei (AGN), but investigation of
their properties has stalled for lack of high resolution mid-IR imaging.
Recently, a long-awaited breakthrough in this field was achieved: NGC 1068, a
nearby AGN, was the first extragalactic object to be observed with a mid-IR
interferometer, thereby obtaining the needed angular resolution to study the
alleged torus.
In this proceeding, first the field of AGN research is briefly reviewed, with
an emphasis on models of dusty tori. Second, the general properties of the key
object NGC 1068 are discussed. Third, the MIDI mid-IR interferometric data set
is presented together with a first attempt to interpret this data in the
context of tori models. Fourth, preliminary MIDI interferometric spectra of the
nucleus of the nearby starbursting galaxy Circinus are presented. Finally, we
briefly discuss the prospects of ESA's Darwin mission for observing nearby and
distant AGN. This mission will allow detailed mapping of tori of low luminosity
AGN such as NGC 1068 up to redshifts of 1 - 2 and more luminous AGN up to
redshift of 10 and beyond (abridged).Comment: 10 pages, 3 figures, published in Proceedings of SPIE Volume 5491
"New Frontiers in Stellar Interferometry
Neutral hydrogen gas, past and future star-formation in galaxies in and around the 'Sausage' merging galaxy cluster
CIZA J2242.8+5301 (, nicknamed 'Sausage') is an extremely massive
( ), merging cluster with shock waves
towards its outskirts, which was found to host numerous emission-line galaxies.
We performed extremely deep Westerbork Synthesis Radio Telescope HI
observations of the 'Sausage' cluster to investigate the effect of the merger
and the shocks on the gas reservoirs fuelling present and future star formation
(SF) in cluster members. By using spectral stacking, we find that the
emission-line galaxies in the 'Sausage' cluster have, on average, as much HI
gas as field galaxies (when accounting for the fact cluster galaxies are more
massive than the field galaxies), contrary to previous studies. Since the
cluster galaxies are more massive than the field spirals, they may have been
able to retain their gas during the cluster merger. The large HI reservoirs are
expected to be consumed within Gyr by the vigorous SF and AGN
activity and/or driven out by the out-flows we observe. We find that the
star-formation rate in a large fraction of H emission-line cluster
galaxies correlates well with the radio broad band emission, tracing supernova
remnant emission. This suggests that the cluster galaxies, all located in
post-shock regions, may have been undergoing sustained SFR for at least 100
Myr. This fully supports the interpretation proposed by Stroe et al. (2015) and
Sobral et al. (2015) that gas-rich cluster galaxies have been triggered to form
stars by the passage of the shock.Comment: Accepted to MNRAS, 14 pages, 9 figures, 7 table
Imaging and Spectroscopy of Ultra Steep Spectrum Radio Sources
We present a sample of 40 Ultra Steep Spectrum (USS, ,
) radio sources selected from the Westerbork in
the Southern Hemisphere (WISH) catalog. The USS sources have been imaged in
K--band at the Cerro Tololo Inter-American Observatory (CTIO) and with the Very
Large Telescope at Cerro Paranal. We also present VLT, Keck and Willian
Herschel Telescope(WHT) optical spectroscopy of 14 targets selection from 4
different USS samples. For 12 sources, we have been able to determine the
redshifts, including 4 new radio galaxies at z > 3. We find that most of our
USS sources have predominantly small (6'') radio sizes and faint magnitudes
(K~18). The mean K-band counterpart magnitude is =18.6. The expected
redshift distribution estimated using the Hubble K-z diagram has a mean of
2.13, which is higher than the predicted redshift obtained
for the SUMSS-NVSS sample and the expected redshift obtained in the 6C
survey. The compact USS sample analyzed here may contain a higher fraction of
galaxies which are high redshift and/or are heavily obscured by dust. Using the
74, 352 and 1400 MHz flux densities of a sub-sample, we construct a radio
colour-colour diagram. We find that all but one of our USS sources have a
strong tendency to flatten below 352 MHz. We also find that the highest
redshift source from this paper (at z=3.84) does not show evidence for spectral
flattening down to 151 MHz. This suggests that very low frequency selected USS
samples will likely be more efficient to find high redshift galaxies.Comment: Accepted for publication in MNRAS, 14 pages including 44 PostScript
figures and 4 tables. Version with all figures available from
http://www.eso.org/~cdebreuc/papers.htm
VLT Spectroscopy of the z=4.11 Radio Galaxy TN J1338-1942
We present optical, infrared and radio data of the z=4.11 radio galaxy TN
J1338-1942, including an intermediate resolution spectrum obtained with FORS1on
the VLT Antu telescope. TN J1338-1942 was the first z>4 radio galaxy to be
discovered in the southern hemisphere and is one of the most luminous Ly-alpha
objects in its class. The Ly-alpha and rest-frame optical emission appear
co-spatial with the brightest radio hotspot of this very asymmetric radio
source, suggesting extremely strong interaction with dense ambient clouds. The
VLT observations are sufficiently sensitive to detect the continuum flux both
blue-ward and red-ward of the Ly-alpha emission, allowing us to measure the
Ly-alpha forest continuum break (Ly-alpha 'discontinuity', D_A) and the Lyman
limit. We measure a D_A=0.37+-0.1, which is ~0.2 lower than the values found
for quasars at this redshift. We interpret this difference as possibly due to a
bias towards large D_A introduced in high--redshift quasar samples that are
selected on the basis of specific optical colors. If such a bias would exist in
optically selected quasars, - and even in samples of Lyman break galaxies -,
then the space density of both classes of object will be underestimated.
Furthermore, the average HI column density along cosmological lines of sight as
determined using quasar absorption lines would be overestimated. Because of
their radio-based selection, we argue that z>4 radio galaxies are excellent
objects for investigating D_A statistics. [abridged]Comment: 6 Pages, including 4 PostScript figures. Accepted for publication in
the Astronomy & Astrophysics Letter
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