214 research outputs found
A Faint Star-Forming System Viewed Through the Lensing Cluster Abell 2218: First Light at z~5.6?
We discuss the physical nature of a remarkably faint pair of Lyman
alpha-emitting images discovered close to the giant cD galaxy in the lensing
cluster Abell 2218 (z=0.18) during a systematic survey for highly-magnified
star-forming galaxies beyond z=5. A well-constrained mass model suggests the
pair arises via a gravitationally-lensed source viewed at high magnification.
Keck spectroscopy confirms the lensing hypothesis and implies the unlensed
source is a very faint (I~30) compact (<150 pc) and isolated object at z=5.576
whose optical emission is substantially contained within the Lyman alpha
emission line; no stellar continuum is detectable. The available data suggest
the source is a promising candidate for an isolated ~10^6 solar mass system
seen producing its first generation of stars close to the epoch of
reionization.Comment: 11 pages, 3 figures, to appear in Ap J Lett, minor revision following
referee's repor
The Abundance of Low-luminosity Lyman alpha Emitters at High Redshift
We derive the luminosity function of high-redshift Lyman alpha emitting
sources from a deep, blind, spectroscopic survey that utilized strong-lensing
magnification by intermediate-redshift clusters of galaxies. We observed
carefully selected regions near 9 clusters, consistent with magnification
factors generally greater than 10 for the redshift range 4.5<z<6.7. Eleven
emission-line candidates were located in the range 2.2<z<5.6 whose
identification we justify as Lyman alpha, in most cases via further
spectroscopic observations. The selection function we constructed for our
survey takes into account our varying intrinsic Lyman alpha line sensitivity as
a function of wavelength and sky position. By virtue of the strong
magnification factor, we provide constraints on the Lyman alpha luminosity
function to unprecedented limits of 10^40 erg/s, corresponding to a
star-formation rate of 0.01 Msun/yr. Our cumulative z=5 Lyman alpha luminosity
function is consistent with a power law form, n(>L) proportional to L^-1 over
10^41 to 10^42.5 erg/s. When combined with the results of other surveys,
limited at higher luminosities, our results suggest evidence for the
suppression of star formation in low-mass halos, as predicted in popular models
of galaxy formation.Comment: 22 pages, 14 figures, submitted to Ap
Dedicated On-Board Computer for Active Debris Removal Mission
This paper describes the first steps toward the implementation of a dedicated rendezvous payload on-board computer for the Active Debris Removal mission ClearSpace-1.
Challenges of ADR missions lay in their ability to first detect and track a target, then perform proximity operation and capture. It implies a variety of sensors which are needed for the Guidance, Navigation and Control of the spacecraft. Sensor outputs need to be processed to retrieve position and attitude estimation of the target, then results are transmitted to the GNC algorithms for precise navigation. To obtain accurate target information, the algorithms require a high input data rate and multiple sensor sources.
The EPFL Space Center and the start-up ClearSpace are working on a dedicated payload computer for their ADR mission. The mission will use a standard satellite bus developed for Earth observation and combine it with another on-board computer for all the tasks specific to the mission. The current testbench setup has the satellite bus physical processor board connected through SpaceWire and Ethernet to a simulator and a payload computer prototype. By implementing a Hardware-In-the-Loop setup, the team is able to assess various configurations for the satellite
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.
Collision-Free Coordination of Fiber Positioners in Multi-object Spectrographs
Many ber-fed spectroscopic survey projects, such as DESI, PFS and MOONS, will use thousands of fiber positioners packed at a focal plane. To maximize observation time, the positioners need to move simultaneously and reach their targets swiftly. We have previously presented a motion planning method based on a decentralized navigation function for the collision-free coordination of the fiber positioners in DESI. In MOONS, the end-effector of each positioner handling the ber can reach the centre of its neighbours. There is therefore a risk of collision with up to 18 surrounding positioners in the chosen dense hexagonal conguration. Moreover, the length of the second arm of the positioner is almost twice the length of the rst one. As a result, the geometry of the potential collision zone between two positioners is not limited to the extremity of their end-effector, but surrounds the second arm. In this paper, we modify the navigation function to take into account the larger collision zone resulting from the extended geometrical shape of the positioners. The proposed navigation function takes into account the conguration of the positioners as well as the constraints on the actuators, such as their maximal velocity and their mechanical clearance. Considering the fact that all the positioners' bases are xed to the focal plane, collisions can occur locally and the risk of collision is limited to the 18 surrounding positioners. The decentralizing motion planning and trajectory generation takes advantage of this limited number of positioners and the locality of collisions, hence signicantly reduces the complexity of the algorithm to a linear order. The linear complexity ensures short computation time. In addition, the time needed to move all the positioners to their targets is independent of the number of positioners. These two key advantages of the chosen decentralization approach turn this method to a promising solution for the collision-free motion-planning problem in the next-generation spectroscopic survey projects. A motion planning simulator, exploited as a software prototype, has been developed in Python. The pre-computed collision-free trajectories of the actuators of all the positioners are fed directly from the simulator to the electronics controlling the motors. A successful demonstration of the effectiveness of these trajectories on the real positioners as well as their simulated counterparts are put side by side in the following online video sequence (https://goo.gl/YuwwsE)
Separating baryons and dark matter in cluster cores: a full 2-D lensing and dynamic analysis of Abell 383 and MS2137-23
(abridged) We utilize existing imaging and spectroscopic data for the galaxy
clusters MS2137-23 and Abell 383 to present improved measures of the
distribution of dark and baryonic material in the clusters' central regions.
Our method, based on the combination of gravitational lensing and dynamical
data, is uniquely capable of separating the distribution of dark and baryonic
components at scales below 100 kpc. We find a variety of strong lensing models
fit the available data, including some with dark matter profiles as steep as
expected from recent simulations. However, when combined with stellar velocity
dispersion data for the brightest member, shallower inner slopes than predicted
by numerical simulations are preferred. For Abell 383, the preferred shallow
inner slopes are statistically a good fit only when the multiple image position
uncertainties associated with our lens model are assumed to be 0\farcs5, to
account for unknown substructure. No statistically satisfactory fit was
obtained matching both the multiple image lensing data and the velocity
dispersion profile of the brightest cluster galaxy in MS2137-23. This suggests
that the mass model we are using, which comprises a pseudo-elliptical
generalized NFW profile and a brightest cluster galaxy component may
inadequately represent the inner cluster regions. This may plausibly arise due
to halo triaxiality or by the gravitational interaction of baryons and dark
matter in cluster cores. However, the progress made via this detailed study
highlights the key role that complementary observations of lensed features and
stellar dynamics offer in understanding the interaction between dark and
baryonic matter on non-linear scales in the central regions of clusters.Comment: 18 pages, 9 figures; accepted for publication in the Astrophysical
Journa
Discovery of a possibly old galaxy at , multiply imaged by the massive cluster Abell 383
We report the discovery of a unique galaxy, multiply imaged by the
cluster Abell 383 and detected in new Hubble Space Telescope ACS and WFC3
imaging, as well as in Warm Spitzer observations. This galaxy was selected as a
pair of i-dropouts; its suspected high redshift was confirmed by the
measurement of a strong Lyman-alpha line in both images using Keck/DEIMOS.
Combining Hubble and Spitzer photometry after correcting for contamination by
line emission (estimated to be a small effect), we identify a strong Balmer
break of 1.5 magnitudes. Taking into account the magnification factor of
11.4+/-1.9 (2.65+/-0.17 mag) for the brightest image, the unlensed AB magnitude
for the source is 27.2+/-0.05 in the H band, corresponding to a 0.4 L* galaxy,
and 25.7+/-0.08 at 3.6 um. The UV slope is consistent with beta~2.0, and from
the rest-frame UV continuum we measure a current star formation rate of
2.4+/-1.1 Msol/yr. The unlensed half-light radius is measured to be 300 pc,
from which we deduce a star-forming surface density of ~10 Msol/yr/kpc2. The
Lyman-alpha emission is found to be extended over ~3" along the slit,
corresponding to ~5 kpc in the source plane. This can be explained by the
presence of a much larger envelope of neutral hydrogen around the star-forming
region. Finally, fitting the spectral energy distribution using 7 photometric
data points with simple SED models, we derive the following properties: very
little reddening, an inferred stellar mass of M*=6e9 Msol, and an inferred age
of ~800 Myrs (corresponding to a redshift of formation of ~18). The
star-formation rate of this object was likely much stronger in the past than at
the time of observation, suggesting that we may be missing a fraction of
galaxies at z~6 which have already faded in rest-frame UV wavelengths.Comment: 6 pages, 3 figures, MNRAS in press, replaced with accepted version
including minor comment
GALEX Observations of "Passive Spirals" in the Cluster Cl 0024+17: Clues to the Formation of S0 Galaxies
We present new results from deep GALEX UV imaging of the cluster Cl 0024+17
at z~0.4. Rest-frame far UV emission is detected from a large fraction of
so-called ``passive spiral galaxies'' -a significant population which exhibits
spiral morphology with little or no spectroscopic evidence for ongoing star
formation. This population is thought to represent infalling galaxies whose
star formation has been somehow truncated by environmental processes, possibly
in morphological transition to S0 galaxies. Compared to normal cluster spirals,
we find that passive spirals are redder in FUV-optical color, while exhibiting
much stronger UV emission than cluster E/S0s - as expected for
recently-truncated star formation. By modeling the different temporal
sensitivities of UV and spectroscopic data to recent activity, we show that
star formation in passive spirals decayed on timescales of less than 1 Gyr,
consistent with `gas starvation' - a process where the cluster environment
prevents cold gas from accreting onto the spiral disk. Intriguingly, the
fraction of spirals currently observed in the passive phase is consistent with
the longer period expected for the morphological transformation and the
subsequent build-up of cluster S0s observed since z=0.4.Comment: 5 pages, 3 figures, ApJL accepte
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