111 research outputs found
Dust-lanes in elliptical and lenticular galaxies
A small proportion of elliptical and lenticular galaxies show
evidence of well-defined dust-lanes when examined on deep survey plates.
New near-infrared (JHK) observations of 46 dust-lane galaxies are presented
and compared with those for a sample of 24 morphologigally normal systems.
Infrared excesses are found preferentially in galaxies which exhibit at
least weak signs of a stellar disc ; the physics of the emission mechanism
is discussed in the light of additional 3.5 y observations and published
optical spectra. The most extreme properties are those of the heavily
obscured edge-on galaxy NGC7172. In this case, the infrared emission is
variable on a timescale ^0.5 yrs. New optical spectra are also discussed
and support the identification of the hard X-ray source H2158-321 with
this galaxy.A restricted sample of 'elliptical-like' galaxies with dust-lanes,
but containing no significant luminous disc, has been defined. It is
demonstrated that these galaxies cannot be distinguished from a comparable
sample of morphologically normal ellipticals on the basis of UBV colours
or luminosity. Four examples have been selected for a detailed study of
their structure and dynamics. Rotation curves and velocity dispersion
profiles have been derived from long-slit spectra using Fourier analysis
techniques ; surface photometry was obtained from large-scale photographic
plates. The data reduction and analysis techniques are discussed in some
detail. In three cases the angular momentum axis of the dust-lane is
different from that of the stellar component, favouring an external origin
for the dust and gas. Two of the galaxies rotate as rapidly as oblate
models with isotropic velocity dispersions ; this property is characteristic.
of the luminous bulges in disc systems. The orientation of the dust-lane
favours a prolate geometry for one of these bulges. It is demonstrated
that the kinematics are inconsistent with two recently published models
of prolate.galaxies
HST Observations of Gravitationally Lensed Features in the Rich Cluster Ac114
Deep Hubble Space Telescope images of superlative resolution obtained for the
distant rich cluster AC114 (z=0.31) reveal a variety of gravitational lensing
phenomena for which ground-based spectroscopy is available. We present a
luminous arc which is clearly resolved by HST and appears to be a lensed z=0.64
sub-L star spiral galaxy with a detected rotation curve. Of greatest interest
is a remarkably symmetrical pair of compact blue images separated by 10 arcsec
and lying close to the cluster cD. We propose that these images arise from a
single very faint background source gravitationally lensed by the cluster core.
Deep ground-based spectroscopy confirms the lensing hypothesis and suggests the
source is a compact star forming system at a redshift z=1.86. Taking advantage
of the resolved structure around each image and their very blue colours, we
have identified a candidate third image of the same source roughly 50 arcsec
away. The angular separation of the three images is much larger than previous
multiply-imaged systems and indicates a deep gravitational potential in the
cluster centre. Resolved multiply-imaged systems, readily recognised with HST,
promise to provide unique constraints on the mass distribution in the cores of
intermediate redshift clusters.Comment: submitted to ApJ, 6 pages (no figures), uuencoded Postscript,
compressed TAR of Postscript figures available via anonymous ftp in
users/irs/figs/ac114_figs.tar.gz on astro.caltech.edu. PAL-IRS-
Fibre spectroscopy and BV CCD photometry of the Southern open cluster NGC 3114
New spectroscopic and photometric observations are presented for a sample of faint stars which extends the color-magnitude diagram of the open cluster NGC 3114 down to V = 16 mag. The distance to the cluster is estimated as 940 ± 60 pc. The spectroscopic observations indicate the presence of six CaII emitters in a sample of 55 stars. Radial velocity measurements and spectral classification have been carried out for all the 55 stars. Both spectroscopic and photometric observations indicate the presence of a large number of field stars in the direction of the cluster. Isochrones fitted to the color-magnitude diagram as well as the position of the red giant concentration indicate that the age of the cluster is ~ 1.2 × 108 yr
HST Observations of Giant Arcs: High Resolution Imaging Of Distant Field Galaxies.
We present HST imaging of eight spectroscopically-confirmed giant arcs, pairs
and arclets. These objects have all been extensively studied from the ground
and we demonstrate the unique advantages of HST imaging in the study of such
features by a critical comparison of our data with the previous observations.
In particular we present new estimates of the core radii of two clusters
(Cl0024+16, A370) determined from lensed features which are identifiable in our
HST images. Although our HST observations include both pre- and
post-refurbishment images, the depth of the exposures guarantees that the
majority of the arcs are detected with diffraction-limited resolution. A number
of the objects in our sample are multiply-imaged and we illustrate the ease of
identification of such features when working at high resolution. We discuss the
morphological and scale information on these distant field galaxies in the
light of HST studies of lower redshift samples. We conclude that the dominant
population of star-forming galaxies at z=1 is a factor of 1.5-2 times smaller
than the similar group in the local field. This implies either a considerable
evolution in the sizes of star-forming galaxies within the last 10 Gyrs
or a shift in the relative space densities of massive and dwarf star-forming
systems over the same timescale.Comment: 9 pages (no figures), uuencoded, compressed Postscript. Postscript
text, tables and figures (803 Kb) available via anonymous ftp in at
ftp://ociw.edu//pub/irs/pub/hstarcs.tar.
Design of freeform diffraction gratings: performance, limitations and potential applications
Spectroscopy is a key technique in astronomy and nowadays most major telescopes include at least one spectrograph in their instrument suite. The dispersive element is one of the most important components and it defines the pupil size, spectral resolution and efficiency. Different types of dispersive elements have been developed including prisms, grisms, VPH and echelle gratings. In this paper, we investigate the design and optimization possibilities offered by metallic freeform gratings using diamond machining techniques. The incorporation of power in a diffraction grating enables several functionalities within the same optical component, such as the combination of dispersion, focusing and field reformat. The resulting benefit is a reduction in the number of surfaces and therefore, an improvement in the throughput. Freeform surfaces are also interesting for their enhanced optical performance by allowing extra degree of freedom in the optimization. These degrees of freedom include the shape of the substrate but also additional parameters such as the pitch or the number of blaze angle. Freeform gratings used as single optical component systems also present some limitations such as the trade-off between optical quality versus field of view or the spectral range versus spectral resolution. This paper discusses the possibility offered by the design of freeform gratings for low to medium spectral resolution, in the visible and near-infrared, for potential applications in ultra-compact integral field spectrographs
Evolution since z = 0.5 of the Morphology-Density relation for Clusters of Galaxies
Using traditional morphological classifications of galaxies in 10
intermediate-redshift (z~0.5) clusters observed with WFPC-2 on the Hubble Space
Telescope, we derive relations between morphology and local galaxy density
similar to that found by Dressler for low-redshift clusters. Taken
collectively, the `morphology-density' relationship, M-D, for these more
distant, presumably younger clusters is qualitatively similar to that found for
the local sample, but a detailed comparison shows two substantial differences:
(1) For the clusters in our sample, the M-D relation is strong in centrally
concentrated ``regular'' clusters, those with a strong correlation of radius
and surface density, but nearly absent for clusters that are less concentrated
and irregular, in contrast to the situation for low redshift clusters where a
strong relation has been found for both. (2) In every cluster the fraction of
elliptical galaxies is as large or larger than in low-redshift clusters, but
the S0 fraction is 2-3 times smaller, with a proportional increase of the
spiral fraction. Straightforward, though probably not unique, interpretations
of these observations are (1) morphological segregation proceeds
hierarchically, affecting richer, denser groups of galaxies earlier, and (2)
the formation of elliptical galaxies predates the formation of rich clusters,
and occurs instead in the loose-group phase or even earlier, but S0's are
generated in large numbers only after cluster virialization.Comment: 35 pages, 19 figures, uses psfig. Accepted for publication in Ap
The HR image slicer for GNIRS at Gemini North: optical design and performance
GNIRS (Gemini Near-InfraRed Spectrograph) is a multi-function spectrograph at Gemini North telescope offering four observational modes in the spectral range of 0.8 to 5.4 µm. It provides 2-pixel spectral resolutions from 1,200 up to 18,0000 and has single disperser and cross-disperser modes yielding simultaneous spectral bandwidths from 40 nm to 1,650 nm. GNIRS presented three existing modes: long-slit (50-100" slit), cross-dispersed (5-7" slit) and low resolution (LR) Integral Field Unit (IFU) (3.15" x 4.80") and it is now being upgraded with a fourth mode allowing high resolution (HR) IFU spectroscopy using an image slicer optimised for fully adaptively corrected images over a field of view of 2.25 arcsec2 (1.80" x 1.25") covered by 25 slices of 410 µm width offering a spatial sampling of 0.05 x 0.05 arscec2 with a diffraction limited optical quality. The proposed layout meets specifications and some challenging design constraints: it shall be contained within the same envelope defined by the LR image slicer (0.1 x 0.2 x 0.1 m3 ), the input and output focal-ratios of both image slicers shall be the same and at exact positions but providing different anamorphic magnifications and preserving the optical quality. The length of the generated slit will be similar to the length of the slit in long-slit mode to maximise detector use and avoid vignetting. This communication presents the optical design and performance of the high resolution image slicer compliant with all specifications and constraints and it shows some design adaptations adopted in order to facilitate its manufacturing in metal at Durham University
A Catalog of Morphological Types in 10 Distant Rich Clusters of Galaxies
We present catalogs of objects detected in deep images of 11 fields in 10
distant clusters obtained using WFPC-2 on board the Hubble Space Telescope. The
clusters span the redshift range z=0.37-0.56 and are the subject of a detailed
ground- and space-based study to investigate the evolution of galaxies as a
function of environment and epoch. The data presented here include positions,
photometry and basic morphological information on ~9000 objects in the fields
of the 10 clusters. For a brighter subset of 1857 objects in these areas we
provide more detailed morphological information.Comment: 24 pages, Latex (uses side.sty - included), 7 figures, accepted for
publication in ApJS. Full tables, plates and images are to be distributed on
AAS CD-ROM and are not included here. Catalogs now available at
http://www.ociw.edu/~ir
Gemini/GMOS spectroscopy of the spheroid and globular cluster system of NGC 3923
We present a technique to extract ultra-deep diffuse-light spectra from the standard multi-object spectroscopic observations used to investigate extragalactic globular cluster (GC) systems. This technique allows a clean extraction of the spectrum of the host galaxy diffuse light from the same slitlets as the GC targets. We show the utility of the method for investigating the kinematics and stellar populations of galaxies at radii much greater than usually probed in longslit studies, at no additional expense in terms of telescope time. To demonstrate this technique, we present Gemini South Multi-Object Spectrograph (GMOS) spectroscopy of 29 GCs associated with the elliptical galaxy NGC 3923. We compare the measured stellar population parameters of the GC system with those of the spheroid of NGC 3923 at the same projected radii, and find the GCs to have old ages >10 Gyr, [α/Fe] ∼ 0.3 and a range of metallicities running from [Z/H] = -1.8 to +0.35. The diffuse light of the galaxy is found to have ages, metallicities and [α/Fe] abundance ratios indistinguishable from those of the red GCs.Facultad de Ciencias Astronómicas y Geofísica
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