22 research outputs found
The Linear-Size Evolution of Classical Double Radio Sources
Recent investigations of how the median size of extragalactic radio sources
change with redshift have produced inconsistent results. Eales compared the
radio and optical properties of a bright 3C and faint 6C sample and concluded
that (), with being the median
size of the radio sources at a given epoch and z the redshift. Oort, Katgert,
and Windhorst, on the other hand, from a comparison of the properties of a
number of radio samples, found much stronger evolution, with
. In this paper we attempt to resolve the
difference. We have repeated the analysis of Eales using the virtually complete
redshift information that now exists for the 6C sample. Confining our analysis
to FR2 sources, which we argue is the best-understood class of radio sources
and the least likely to be affected by selection effects, we find
() and
(). Our complete redshift information allows us to gain insight
into our result by plotting a radio luminosity-size (P-D) diagram for the 6C
sample. The most obvious difference between the 3C and 6C P-D diagrams is the
clump of sources in the 6C diagram at . These clump sources have similar sizes to the emission-line
regions found around high-redshift radio galaxies, suggesting that the presence
of dense line-emitting gas around high-redshift radio galaxies is responsible
for the size evolution. We show that this explanation can quantitatively
explain the observed size evolution, as long as there is either little X-ray
emitting gas around these objects or, if there is, it is distributed in a
similar way to the emission-line gas: highly anisotropic and inhomogeneous.Comment: compressed and uuencoded postscript file. 33 pages including 5
figures (441951 bytes). Accepted for publication in September Ap
A panoramic VISTA of the stellar halo of NGC 253
Outskirts of large galaxies contain important information about the galaxy
formation and assembly process, and resolved star count studies can probe the
extremely low surface brightness of the outer halos. We use images obtained
with the VISTA telescope to construct spatially resolved J vs Z-J
colour-magnitude diagrams (CMDs) of NGC 253, a nearly edge-on disk galaxy in
the Sculptor group. The very deep photometry, down to J ~ 23.5, and the wide
area covered allows us to trace the red giant branch (RGB) and asymptotic giant
branch (AGB) stars that belong to the outer disk and the halo of NGC 253, out
to 50 kpc along the galaxy minor axis. We confirm the existence of an extra
planar stellar component of the disk, with a very prominent southern shelf and
a symmetrical feature on the north side. The only additional visible
sub-structure is an overdensity in the north-west part of the halo at about 28
kpc from the plane and extending over ~ 20 kpc parallel with the disk of the
galaxy. From the stellar count profile along the major axis we measure the
transition from the disk to the halo at a radial distance of about 25 kpc,
where a clear break appears in the number density profile. The isodensity
contours show that the inner halo is a flattened structure that blends with a
more extended, diffuse, rounder outer halo. Such external structure can be
traced to the very edge of our image out to 50 kpc from the disk plane. The
number density profile of the stars in the stellar halo follows a power law
with index -1.6, as function of radius. The CMD shows a very homogeneous
stellar population across the whole field; by comparison with theoretical
isochrones we conclude that the RGB stars are ~ 8 Gyr old or more, while the
AGB stars trace a population of about 2 x 10^8 Mo, formed from ~ 0.5 to a few
Gyr ago. Surprisingly, part of this latter population appears scattered over a
wide area.Comment: To appear on Astronomy and Astrophysic
MUSE library of stellar spectra
Context. Empirical stellar spectral libraries have applications in both extragalactic and stellar studies, and they confer an advantage over theoretical libraries because they naturally include all relevant chemical species and physical processes. In recent years we have seen a stream of new sets of high-quality spectra, but increasing the spectral resolution and widening the wavelength coverage means resorting to multi-order echelle spectrographs. Assembling the spectra from many pieces results in lower fidelity of their shapes. Aims: We aim to offer the community a library of high-signal-to-noise spectra with reliable continuum shapes. Furthermore, the use of an integral field unit (IFU) alleviates the issue of slit losses. Methods: Our library was built with the MUSE (Multi-Unit Spectroscopic Explorer) IFU instrument. We obtained spectra over nearly the entire visual band (lambda ~ 4800-9300 AA). Results: We assembled a library of 35 high-quality MUSE spectra for a subset of the stars from the X-shooter Spectral Library. We verified the continuum shape of these spectra with synthetic broadband colors derived from the spectra. We also report some spectral indices from the Lick system, derived from the new observations. Conclusions: We offer a high-fidelity set of stellar spectra covering the Hertzsprung-Russell diagram. These can be used for both extragalactic and stellar studies and demonstrate that the IFUs are excellent tools for building reliable spectral libraries
ERIS: revitalising an adaptive optics instrument for the VLT
ERIS is an instrument that will both extend and enhance the fundamental
diffraction limited imaging and spectroscopy capability for the VLT. It will
replace two instruments that are now being maintained beyond their operational
lifetimes, combine their functionality on a single focus, provide a new
wavefront sensing module that makes use of the facility Adaptive Optics System,
and considerably improve their performance. The instrument will be competitive
with respect to JWST in several regimes, and has outstanding potential for
studies of the Galactic Center, exoplanets, and high redshift galaxies. ERIS
had its final design review in 2017, and is expected to be on sky in 2020. This
contribution describes the instrument concept, outlines its expected
performance, and highlights where it will most excel.Comment: 12 pages, Proc SPIE 10702 "Ground-Based and Airborne Instrumentation
for Astronomy VII
Guarding the performance and data quality of OmegaCAM/VST at ESO
<p>OmegaCAM on the VST telescope at the Paranal Observatory has been supported by the ESO Quality Control group since the beginning of operations in 2011. We will present the schema that monitors the performance of the instrument and controls the data quality throughout the years. It has been evolving with time, being constantly improved and adapted to new demands, but the main goals remain unchanged. The system includes continuous checks of the availability of required calibrations, assessing the quality of the calibration data, and monitoring the health of the instrument. Continuous feedback is provided to the Observatory about the status of the instrument and its data.<br>
We will also discuss actual events from the life of OmegaCAM when our monitoring revealed problems with the instrument, and allowed for their quick repair. </p