653 research outputs found
The compact group--fossil group connection: observations of a massive compact group at z=0.22
It has been suggested that fossil groups could be the cannibalized remains of
compact groups, that lost energy through tidal friction. However, in the nearby
universe, compact groups which are close to the merging phase and display a
wealth of interacting features (such as HCG 31 and HCG 79) have very low
velocity dispersions and poor neighborhoods, unlike the massive, cluster-like
fossil groups studied to date. In fact, known z=0 compact groups are very
seldom embedded in massive enough structures which may have resembled the
intergalactic medium of fossil groups. In this paper we study the dynamical
properties of CG6, a massive compact group at z=0.220 that has several
properties in common with known fossil groups. We report on new g' and i'
imaging and multi-slit spectroscopic performed with GMOS on Gemini South. The
system has 20 members, within a radius of 1 h_70^-1 Mpc, a velocity dispersion
of 700 km/s and has a mass of 1.8 x 10^14 h_70^-1 Msun, similar to that of the
most massive fossil groups known. The merging of the four central galaxies in
this group would form a galaxy with magnitude M_r' ~ -23.4, typical for
first-ranked galaxies of fossil groups. Although nearby compact groups with
similar properties to CG 6 are rare, we speculate that such systems occurred
more frequently in the past and they may have been the precursors of fossil
groups.Comment: 7 pages, 3 figures (one color, low resolution), uses emulateapj.sty.
Accepted for publication in ApJ Lette
Multi-conjugated adaptive optics imaging of distant galaxies -- A comparison of Gemini/GSAOI and VLT/HAWK-I data
Multi-conjugated adaptive optics (MCAO) yield nearly diffraction-limited
images at 2m wavelengths. Currently, GeMS/GSAOI at Gemini South is the
only MCAO facility instrument at an 8m telescope. Using real data and for the
first time, we investigate the gain in depth and S/N when MCAO is employed for
-band observations of distant galaxies. Our analysis is based on the
Frontier Fields cluster MACS J0416.1-2403, observed with GeMS/GSAOI (near
diffraction-limited) and compared against VLT/HAWK-I (natural seeing) data.
Using galaxy number counts, we show that the substantially increased thermal
background and lower optical throughput of the MCAO unit are fully compensated
for by the wavefront correction, because the galaxy images can be measured in
smaller apertures with less sky noise. We also performed a direct comparison of
the signal-to-noise ratios (S/N) of sources detected in both data sets. For
objects with intrinsic angular sizes corresponding to half the HAWK-I image
seeing, the gain in S/N is 40 per cent. Even smaller objects experience a boost
in S/N by a up to a factor of 2.5 despite our suboptimal natural guide star
configuration. The depth of the near diffraction limited images is more
difficult to quantify than that of seeing limited images, due to a strong
dependence on the intrinsic source profiles. Our results emphasize the
importance of cooled MCAO systems for -band observations with
future, extremely large telescopes.Comment: 7 pages, 7 figures. Accepted for publication in MNRA
Astrometric performance of the Gemini multi-conjugate adaptive optics system in crowded fields
The Gemini Multi-conjugate adaptive optics System (GeMS) is a facility
instrument for the Gemini-South telescope. It delivers uniform,
near-diffraction-limited image quality at near-infrared wavelengths over a 2
arcminute field of view. Together with the Gemini South Adaptive Optics Imager
(GSAOI), a near-infrared wide field camera, GeMS/GSAOI's combination of high
spatial resolution and a large field of view will make it a premier facility
for precision astrometry. Potential astrometric science cases cover a broad
range of topics including exo-planets, star formation, stellar evolution, star
clusters, nearby galaxies, black holes and neutron stars, and the Galactic
center. In this paper, we assess the astrometric performance and limitations of
GeMS/GSAOI. In particular, we analyze deep, mono-epoch images, multi-epoch data
and distortion calibration. We find that for single-epoch, un-dithered data, an
astrometric error below 0.2 mas can be achieved for exposure times exceeding
one minute, provided enough stars are available to remove high-order
distortions. We show however that such performance is not reproducible for
multi-epoch observations, and an additional systematic error of ~0.4 mas is
evidenced. This systematic multi-epoch error is the dominant error term in the
GeMS/GSAOI astrometric error budget, and it is thought to be due to
time-variable distortion induced by gravity flexure.Comment: 16 pages, 22 figures, accepted for publication in MNRA
First performance of the gems + gmos system. Part1. Imaging
During the commissioning of the Gemini MCAO System (GeMS), we had the
opportunity to obtain data with the Gemini Multi-Object Spectrograph (GMOS),
the most utilised instrument at Gemini South Observatory, in March and May
2012. Several globular clusters were observed in imaging mode that allowed us
to study the performance of this new and untested combination. GMOS is a
visible instrument, hence pushing MCAO toward the visible.We report here on the
results with the GMOS instruments, derive photometric performance in term of
Full Width Half Maximum (FWHM) and throughput. In most of the cases, we
obtained an improvement factor of at least 2 against the natural seeing. This
result also depends on the Natural Guide Star constellation selected for the
observations and we then study the impact of the guide star selection on the
FWHM performance.We also derive a first astrometric analysis showing that the
GeMS+GMOS system provide an absolute astrometric precision better than 8mas and
a relative astrometric precision lower than 50 mas.Comment: 13 pages, 11 figures, accepted for publication in MNRAS on March 23rd
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The stellar mass - size relation for cluster galaxies at z=1 with high angular resolution from the Gemini/GeMS multi-conjugate adaptive optics system
We present the stellar mass - size relation for 49 galaxies within the =
1.067 cluster SPT-CL J05465345, with FWHM 80-120 mas -band data from the Gemini multi-conjugate adaptive optics system
(GeMS/GSAOI). This is the first such measurement in a cluster environment,
performed at sub-kpc resolution at rest-frame wavelengths dominated by the
light of the underlying old stellar populations. The observed stellar mass -
size relation is offset from the local relation by 0.21 dex, corresponding to a
size evolution proportional to , consistent with the literature.
The slope of the stellar mass - size relation = 0.74 0.06,
consistent with the local relation. The absence of slope evolution indicates
that the amount of size growth is constant with stellar mass. This suggests
that galaxies in massive clusters such as SPT-CL J05465345 grow via
processes that increase the size without significant morphological
interference, such as minor mergers and/or adiabatic expansion. The slope of
the cluster stellar mass - size relation is significantly shallower if measured
in /ACS imaging at wavelengths blueward of the Balmer break, similar to
rest-frame UV relations at = 1 in the literature. The stellar mass - size
relation must be measured at redder wavelengths, which are more sensitive to
the old stellar population that dominates the stellar mass of the galaxies. The
slope is unchanged when GeMS -band imaging is degraded to the resolution
of -band HST/NICMOS resolution but dramatically affected when degraded to
-band Magellan/FourStar resolution. Such measurements must be made with AO
in order to accurately characterise the sizes of compact, = 1 galaxies.Comment: 24 pages, 13 figures, 3 tables. Accepted for publication in MNRAS.
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