61 research outputs found
Revisiting the Impact of Atmospheric Dispersion and Differential Refraction on Widefield Multiobject Spectroscopic Observations. From VLT/VIMOS to Next Generation Instruments
(Abridged) Atmospheric dispersion and field differential refraction impose
severe constraints on widefield MOS observations. Flux reduction and spectral
distortions must be minimised by a careful planning of the observations --
which is especially true for instruments that use slits instead of fibres. This
is the case of VIMOS at the VLT, where MOS observations have been restricted,
since the start of operations, to a narrow two-hour range from the meridian to
minimise slit losses. We revisit in detail the impact of atmospheric effects on
the quality of VIMOS-MOS spectra. We model slit losses across the entire VIMOS
FOV as a function of target declination. We explore two different slit
orientations at the meridian: along the parallactic angle (North-South), and
perpendicular to it (East-West). We show that, for fields culminating at zenith
distances larger than 20 deg, slit losses are minimised with slits oriented
along the parallactic angle at the meridian. The two-hour angle rule holds for
these observations using N-S orientations. Conversely, for fields with zenith
angles smaller than 20 deg at culmination, losses are minimised with slits
oriented perpendicular to the parallactic angle at the meridian. MOS
observations can be effectively extended to plus/minus three hours from the
meridian in these cases. In general, night-long observations of a single field
will benefit from using the E-W orientation. All-sky or service mode
observations, however, require a more elaborate planning that depends on the
target declination, and the hour angle of the observations. We establish
general rules for the alignment of slits in MOS observations that will increase
target observability, enhance the efficiency of operations, and speed up the
completion of programmes -- a particularly relevant aspect for the forthcoming
spectroscopic public surveys with VIMOS.Comment: Accepted to A&A. 11 pages, 15 figures. This paper presents the new
recommendations for optimal slit alignment in VLT/VIMOS observation
The Pristine Dwarf-Galaxy survey -- V. The edges of the dwarf galaxy Hercules
We present a new spectroscopic study of the dwarf galaxy Hercules (d ~ 132
kpc) with data from the Anglo-Australian Telescope and its AAOmega spectrograph
together with the Two Degree Field multi-object system to solve the conundrum
that whether Hercules is tidally disrupting. We combine broadband photometry,
proper motions from Gaia, and our Pristine narrow-band and
metallicity-sensitive photometry to efficiently weed out the Milky Way
contamination. Such cleaning is particularly critical in this kinematic regime,
as both the transverse and heliocentric velocities of Milky Way populations
overlap with Hercules. Thanks to this method, three new member stars are
identified, including one at almost 10rh of the satellite. All three have
velocities and metallicities consistent with that of the main body. Combining
this new dataset with the entire literature cleaned out from contamination
shows that Hercules does not exhibit a velocity gradient (d/dX =
0.1+0.4/-0.2 km s-1 arcmin-1) and, as such, does not show evidence to undergo
tidal disruption
Stellar envelopes of globular clusters embedded in dark mini-haloes
We show that hard encounters in the central regions of globular clusters
embedded in dark matter (DM) haloes necessarily lead to the formation of
gravitationally-bound stellar envelopes that extend far beyond the nominal
tidal radius of the system. Using statistical arguments and numerical
techniques we derive the equilibrium distribution function of stars ejected
from the centre of a non-divergent spherical potential. Independently of the
velocity distribution with which stars are ejected, GC envelopes have density
profiles that approach asymptotically at large distances and
become isothermal towards the centre. Adding a DM halo component leaves two
clear-cut observational signatures: (i) a flattening, or slightly increase of
the projected velocity dispersion profile at large distances, and (ii) an outer
surface density profile that is systematically shallower than in models with no
dark matter.Comment: Accepted to MNRAS letter
The Most Massive Ultra-Compact Dwarf Galaxy in the Virgo Cluster
We report on the properties of the most massive ultra-compact dwarf galaxy
(UCD) in the nearby Virgo Cluster of galaxies using imaging from the Next
Generation Virgo Cluster Survey (NGVS) and spectroscopy from Keck/DEIMOS. This
object (M59-UCD3) appears to be associated with the massive Virgo galaxy M59
(NGC 4621), has an integrated velocity dispersion of 78 km/s, a dynamical mass
of , and an effective radius () of 25 pc. With an
effective surface mass density of , it is the
densest galaxy in the local Universe discovered to date, surpassing the density
of the luminous Virgo UCD, M60-UCD1. M59-UCD3 has a total luminosity of
mag, and a spectral energy distribution consistent with an old
(14 Gyr) stellar population with [Fe/H]=0.0 and [/Fe]=+0.2. We also
examine deep imaging around M59 and find a broad low surface brightness stream
pointing towards M59-UCD3, which may represent a tidal remnant of the UCD
progenitor. This UCD, along with similar objects like M60-UCD1 and M59cO,
likely represents an extreme population of tidally stripped galaxies more akin
to larger and more massive compact early-type galaxies than to nuclear star
clusters in present-day dwarf galaxies.Comment: 6 pages, 4 figures, 1 table, accepted for publication in ApJ Letter
The Pristine survey -- XII:Gemini-GRACES chemo-dynamical study of newly discovered extremely metal-poor stars in the Galaxy
The Pristine survey -- XII:Gemini-GRACES chemo-dynamical study of newly discovered extremely metal-poor stars in the Galaxy
The properties of bright globular clusters, ultra-compact dwarfs and dwarf nuclei in the Virgo core: hints on origin of ultra-compact dwarf galaxies (UCDs)
Based on the data from the Next Generation Virgo cluster Survey (NGVS), we statistically study the photometric properties of globular clusters (GCs), ultra-compact dwarfs (UCDs) and dwarf nuclei in the Virgo core (M87) region. We found an obvious negative color (g - z) gradient in GC system associate with M87, i.e. GCs in the outer regions are bluer. However, such color gradient does not exist in UCD system, neither in dwarf nuclei system around M87. In addition, we found that many UCDs are surrounded by extended, low surface brightness envelopes. The dwarf nuclei and UCDs show different spatial distributions from GCs, with dwarf nuclei and UCDs (especially for the UCDs with visible envelopes) lying at larger distances to the Virgo center. These results support the view that UCDs (at least for a fraction of UCDs) are more tied to dwarf nuclei than to GCs
The Pristine survey -- XXIII. Data Release 1 and an all-sky metallicity catalogue based on Gaia DR3 BP/RP spectro-photometry
We use the spectro-photometric information of ~219 million stars from Gaia's
DR3 to calculate synthetic, narrow-band, metallicity-sensitive CaHK magnitudes
that mimic the observations of the Pristine survey, a survey of photometric
metallicities of Milky Way stars that has been mapping more than 6,500 deg^2 of
the northern sky with the CFHT since 2015. These synthetic magnitudes are used
for an absolute re-calibration of the deeper Pristine photometry and, combined
with broadband Gaia information, synthetic and Pristine CaHK magnitudes are
used to estimate photometric metallicities over the whole sky. The resulting
metallicity catalogue is accurate down to [Fe/H]~-3.5 and is particularly
suited for the exploration of the metal-poor Milky Way ([Fe/H]<-1.0). We make
available here the catalogue of synthetic CaHK_syn magnitudes for all stars
with BP/RP information in Gaia DR3, as well as an associated catalogue of more
than ~30 million photometric metallicities for high S/N FGK stars. This paper
further provides the first public DR of the Pristine catalogue in the form of
higher quality recalibrated Pristine CaHK magnitudes and photometric
metallicities for all stars in common with the BP/RP information in Gaia DR3.
We demonstrate that, when available, the much deeper Pristine data greatly
enhances the quality of the derived metallicities, in particular at the faint
end of the catalogue (G_BP>16). Combined, both catalogues include more than 2
million metal-poor star candidates as well as more than 200,000 and ~8,000 very
and extremely metal-poor candidates. Finally, we show that these metallicity
catalogues can be used efficiently, among other applications, for Galactic
archaeology, to hunt for the most metal-poor stars, and to study how the
structure of the Milky Way varies with metallicity, from the flat distribution
of disk stars to the spheroid-shaped metal-poor halo. (Shortened)Comment: 30 pages, 24 figures, submitted to A&A. First two authors are
co-first author. The CaHK photometry catalogue and the two photometric
metallicity catalogues are available, before acceptance, as large compressed
csv files at: https://seafile.unistra.fr/d/ee0c0f05719d4368bcbb
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