588 research outputs found
The evolution of galaxy clustering since z=1 from the Calar Alto Deep Imaging Survey
We present results from an investigation of the clustering evolution of field
galaxies between a redshift of z~1 and the present epoch. The current analysis
relies on a sample of ~3600 galaxies from the Calar Alto Deep Imaging
Survey (CADIS). The redshift distribution extends to z~1.1, with formal
errors of sigma_z~0.02. Thus the amplitude of the three-dimensional correlation
function can be estimated by means of the projected correlation function
w(r_p). We developed a new method to overcome the influence of redshift errors
on w(r_p). We parametrise the evolution of the clustering strength with
redshift by a parameter q, the values of which give directly the deviation of
the evolution from the global Hubble flow. From a subsample of bright galaxies
we find q=-2.28+-0.31 for Omega_m=0.3, Omega_Lambda=0.7, that is a significant
growth of the clustering strength between z=1 and the present epoch. From
linear theory of dark matter clustering growth one would only expect q=-2.
Moreover, we establish that the measured clustering strength depends on galaxy
type: galaxies with early type SEDs (Hubble type: E0 to Sbc) are more strongly
clustered at redshifts z>0.2 than later types. The evolution of the amplitude
of the two-point correlation function for these ``old'' galaxies is much slower
(q=-0.85+-0.82 for Omega_m=0.3, Omega_Lambda=0.7).Comment: 15 pages, 9 figures, accepted for publication by A&
High-Fidelity VLA Imaging of the Radio Structure of 3C273
3C273, the nearest bright quasar, comprises a strong nuclear core and a
bright, one-sided jet extending ~ 23 arcseconds to the SW. The source has been
the subject of imaging campaigns in all wavebands. Extensive observations of
this source have been made with the Very Large Array and other telescopes as
part of a campaign to understand the jet emission mechanisms. Partial results
from the VLA radio campaign have been published, but to date, the complete set
of VLA imaging results has not been made available. We have utilized the VLA to
determine the radio structure of 3C273 in Stokes I, Q, and U, over the widest
possible frequency and resolution range. The VLA observed the source in all
four of its configurations, and with all eight of its frequency bands, spanning
73.8 MHz to 43 GHz. The data were taken in a pseudo-spectral line mode to
minimize the VLA's correlator errors, and were fully calibrated with subsequent
self-calibration techniques to maximise image fidelity. Images in Stokes
parameters I, Q, and U, spanning a resolution range from 6 arcseconds to 88
milliarcseconds are presented. Spectral index images, showing the evolution of
the jet component are shown. Polarimetry demonstrates the direction of the
magnetic fields responsible for the emission, and rotation measure maps show
the RM to be very small with no discernible trend along or across the jet. This
paper presents a small subset of these images to demonstrate the major
characteristics of the source emission. A library of all ~500 images has been
made available for open, free access by interested parties.Comment: 9 pages, 17 figure
Chandra detection of the radio and optical double hot spot of 3C 351
In this letter we report a Chandra X-ray detection of the double northern hot
spot of the radio quasar 3C 351. The hot spot has also been observed in the
optical with the Hubble Space Telescope (R-band) and with the 3.5m. Telescopio
Nazionale Galileo (B-band). The radio-to-optical and X-ray spectra are
interpreted as the results of the synchrotron and synchrotron-self-Compton
(SSC) mechanisms, respectively, with hot-spot magnetic field strengths ~3 times
smaller than the equipartition values. In the framework of shock acceleration
theory, we show that the requirement for such a relatively small field strength
is in agreement with the fitted synchrotron spectral models and with the sizes
of the hot spots. Finally, we show that the combination of a lower magnetic
field strength with the high frequencies of the synchrotron cut-off in the
fitted synchrotron spectra provides strong evidence for electron acceleration
in the hot spots.Comment: 16 pag. + 2 .PS figures (fig.2 color), ApJ Letter in pres
The radio-ultraviolet spectral energy distribution of the jet in 3C273
We present deep VLA and HST observations of the large-scale jet in 3C 273
matched to 0.3" resolution. The observed spectra show a significant flattening
in the infrared-ultraviolet wavelength range. The jet's emission cannot
therefore be assumed to arise from a single electron population and requires
the presence of an additional emission component. The observed smooth
variations of the spectral indices along the jet imply that the physical
conditions vary correspondingly smoothly. We determine the maximum particle
energy for the optical jet using synchrotron spectral fits. The slow decline of
the maximum energy along the jet implies particle reacceleration acting along
the entire jet. In addition to the already established global anti-correlation
between maximum particle energy and surface brightness, we find a weak positive
correlation between small-scale variations in maximum particle energy and
surface brightness. The origin of these conflicting global and local
correlations is unclear, but they provide tight constraints for reacceleration
models.Comment: 28 pages, lots of figures, accepted for publication in A&
X-ray Emission from the 3C 273 Jet
We present results from four recent Chandra monitoring observations of the
jet in 3C 273 using the ACIS detector, obtained between November 2003 and July
2004. We find that the X-ray emission comes in two components: unresolved knots
that are smaller than the corresponding optically emitting knots and a broad
channel that is about the same width as the optical interknot region. We
compute the jet speed under the assumption that the X-ray emission is due to
inverse Compton scattering of the cosmic microwave background, finding that the
dimming of the jet X-ray emission to the jet termination relative to the radio
emission may be due to bulk deceleration.Comment: 2 pages, 2 figures, to appear in the proceedings of "The X-ray
Universe 2005", San Lorenzo de El Escorial (Spain), 26-30 September 200
Revealing the large nuclear dust structures in NGC 1068 with MIDI/VLTI
To understand the relation between the small "obscuring torus" and dusty
structures at larger scales (5-10 pc) in NGC 1068, we use ESO's Mid-Infrared
Interferometer (MIDI) with the 1.8 m Auxiliary Telescopes to achieve the
necessary spatial resolution (~ 20-100 millarcsec). We use the chromatic phases
in the data to improve the spatial fidelity of the analysis. We present
interferometric data for NGC 1068 obtained in 2007 and 2012. We find no
evidence of source variability. Many (u,v) points show non-zero chromatic
phases indicating significant asymmetries. Gaussian model fitting of the
correlated fluxes and chromatic phases provides a 3-component best fit with
estimates of sizes, temperatures and positions of the components. A large,
warm, off-center component is required at a distance approximately 90 mas to
the north-west at a PA ~ -18 deg. The dust at 5-10 pc in the polar region
contributes 4 times more to the mid-infrared flux at 12 um than the dust
located at the center. This dust may represent the inner wall of a dusty cone.
If similar regions are heated by the direct radiation from the nucleus, then
they will contribute substantially to the classification of many Seyfert
galaxies as Type 2. Such a region is also consistent in other Seyfert galaxies
(the Circinus galaxy, NGC 3783 and NGC 424).Comment: 21 pages, 10 figures; Accepted for publication on A&
Morphology-dependent trends of galaxy age with environment in Abell 901/902 seen with COMBO-17
We investigate correlations between galaxy age and environment in the Abell
901/2 supercluster for separate morphologies. Using COMBO-17 data, we define a
sample of 530 galaxies, complete at on an area of (Mpc/). We explore several age indicators including an
extinction-corrected residual from the colour-magnitude relation (CMR). As a
result, we find a clear trend of age with density for galaxies of all
morphologies that include a spheroidal component, in the sense that galaxies in
denser environments are older. This trend is not seen among Scd/Irr galaxies
since they all have young ages. However, the trend among the other types is
stronger for fainter galaxies. While we also see an expected age-morphology
relation, we find no evidence for a morphology-density relation at fixed age.Comment: Accepted for publication in MNRAS (Letters
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