74 research outputs found
On The Nature of ultra-faint Dwarf Galaxy Candidates II: The case of Cetus II
We obtained deep Gemini GMOS-S photometry of the ultra-faint dwarf
galaxy candidate Cetus II with the aim of providing stronger constraints on its
size, luminosity and stellar population. Cetus II is an important object in the
size-luminosity plane as it occupies the transition zone between dwarf galaxies
and star clusters. All known objects smaller than Cetus II ( pc)
are reported to be star clusters, while most larger objects are likely dwarf
galaxies. We found a prominent excess of main-sequence stars in the
colour-magnitude diagram of Cetus II, best described by a single stellar
population with an age of 11.2 Gyr, metallicity of [Fe/H] = dex, an
[/Fe] = 0.0 dex at a heliocentric distance of 26.31.2 kpc. As well
as being spatially located within the Sagittarius dwarf tidal stream, these
properties are well matched to the Sagittarius galaxy's Population B stars.
Interestingly, like our recent findings on the ultra-faint dwarf galaxy
candidate Tucana V, the stellar field in the direction of Cetus II shows no
evidence of a concentrated overdensity despite tracing the main sequence for
over six magnitudes. These results strongly support the picture that Cetus II
is not an ultra-faint stellar system in the Milky Way halo, but made up of
stars from the Sagittarius tidal stream.Comment: 12 pages, 12 figures, Accepted for publication in Ap
Correcting the influence of an asymmetric line spread function in 2-degree Field spectrograph data
We investigate the role of asymmetries in the line spread function of the
2-degree field spectrograph and the variations in these asymmetries with the
CCD, the plate, the time of observation and the fibre. A data-reduction
pipeline is developed that takes these deformations into account for the
calibration and cross-correlation of the spectra. We show that, using the
emission lines of calibration lamp observations, we can fit the line spread
function with the sum of two Gaussian functions representing the theoretical
signal and a perturbation of the system. This model is then used to calibrate
the spectra and generate templates by downgrading high resolution spectra.
Thus, we can cross-correlate the observed spectra with templates degraded in
the same way. Our reduction pipeline is tested on real observations and
provides a significant improvement in the accuracy of the radial velocities
obtained. In particular, the systematic errors that were as high as ~20 km/s
when applying the AAO reduction package 2dfDR are now reduced to ~5 km/s. Even
though the 2-degree Field spectrograph is to be decommissioned at the end of
2005, the analysis of archival data and previous studies could be improved by
the reduction procedure we propose here.Comment: 9 pages, 9 figures, accepted to PASA, minor change
Gemini and Lowell observations of 67P/Churyumov−Gerasimenko during the <i>Rosetta</i> mission
We present observations of comet 67P/Churyumov−Gerasimenko acquired in support of the Rosetta mission. We obtained usable data on 68 nights from 2014 September until 2016 May, with data acquired regularly whenever the comet was observable. We collected an extensive set of near-IR J, H and Ks data throughout the apparition plus visible-light images in g', r', i' and z' when the comet was fainter. We also obtained broad-band R and narrow-band CN filter observations when the comet was brightest using telescopes at Lowell Observatory. The
appearance was dominated by a central condensation and the tail until 2015 June. From 2015 August onwards, there were clear asymmetries in the coma, which enhancements revealed to be due to the presence of up to three features (i.e. jets). The features were similar in all broad-band filters; CN images did not show these features but were instead broadly enhanced in the southeastern hemisphere. Modelling using the parameters from Vincent et al. replicated the dust morphology reasonably well, indicating that the pole orientation and locations of active areas have been relatively unchanged over at least the last three apparitions. The dust production, as measured by A(0°)fρ peaked ∼30 d after perihelion and was consistent with
predictions from previous apparitions. A(0°)fρ as a function of heliocentric distance was well fitted by a power law with slope −4.2 from 35 to 120 d post-perihelion. We detected photometric evidence of apparent outbursts on 2015 August 22 and 2015 September 19, although neither was discernible morphologically in this data set
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