24 research outputs found
On r-partition designs in hamming spaces
Résumé disponible dans le fichier PD
An Ancient Metal-Poor Population in M32, and Halo Satellite Accretion in M31, Identified by RR Lyrae Stars
We present time-series photometry of two fields near M32 using archival
observations from ACS/WFC onboard HST. One field is centered about 2 arcmin
from M32 while the other is located 15 arcmin to the southeast of M31. We
identify a total of 1139 RR Lyrae variables of which 821 are ab-type and 318
are c-type. In the field near M32, we find a radial gradient in the density of
RR Lyraes relative to the center of M32. This gradient is consistent with the
surface brightness profile of M32 suggesting that a significant number of the
RR Lyraes in this region belong to M32. This provides further confirmation that
M32 contains an ancient stellar population formed around the same time as the
oldest population in M31 and the Milky Way. The RR Lyrae stars in M32 exhibit a
mean metal abundance of [Fe/H] ~ -1.42 +/- 0.02, which is ~15 times lower than
the metal abundance of the overall M32 stellar population. Moreover, the
abundance of RR Lyrae stars normalized to the luminosity of M32 in the field
analyzed further indicates that the ancient metal-poor population in M32
represents only a very minor component of this galaxy, consistent with the 1%
to 4.5% in mass inferred from the CMD analysis of Monachesi et al. In the other
field, we find unprecedented evidence for two populations of RR Lyraes in M31
as shown by two distinct sequences among the ab-type variables in the Bailey
Diagram. When interpreted in terms of metal abundance, one population exhibits
a peak at [Fe/H] ~ -1.3 and the other is at [Fe/H] ~ -1.9. One possible
interpretation of this result is that the more metal-rich population represents
the dominant M31 halo, while the metal-poorer group could be a disrupted dwarf
satellite galaxy orbiting M31. If true, this represents a further indication
that the formation of the M31 spheroid has been significantly influenced by the
merger and accretion of dwarf galaxy satellites. [abridged]Comment: 22 pages, 17 figures, accepted for publication in the MNRA
The Star Formation History and Dust Content in the Far Outer Disc of M31
We present a detailed analysis of two fields located 26 kpc (~5 scalelengths)
from the centre of M31. One field samples the major axis populations--the Outer
Disc field--while the other is offset by ~18' and samples the Warp in the
stellar disc. The CMDs based on HST/ACS imaging reach old main-sequence
turn-offs (~12.5 Gyr). We apply the CMD-fitting technique to the Warp field to
reconstruct the star formation history (SFH). We find that after undergoing
roughly constant SF until about 4.5 Gyr ago, there was a rapid decline in
activity and then a ~1.5 Gyr lull, followed by a strong burst lasting 1.5 Gyr
and responsible for 25% of the total stellar mass in this field. This burst
appears to be accompanied by a decline in metallicity which could be a
signature of the inflow of metal-poor gas. The onset of the burst (~3 Gyr ago)
corresponds to the last close passage of M31 and M33 as predicted by detailed
N-body modelling, and may have been triggered by this event. We reprocess the
deep M33 outer disc field data of Barker et al. (2011) in order to compare
consistently-derived SFHs. This reveals a similar duration burst that is
exactly coeval with that seen in the M31 Warp field, lending further support to
the interaction hypothesis. The complex SFHs and the smoothly-varying
age-metallicity relations suggest that the stellar populations observed in the
far outer discs of both galaxies have largely formed in situ rather than
migrated from smaller galactocentric radii. The strong differential reddening
affecting the CMD of the Outer Disc field prevents derivation of the SFH.
Instead, we quantify this reddening and find that the fine-scale distribution
of dust precisely follows that of the HI gas. This indicates that the outer HI
disc of M31 contains a substantial amount of dust and therefore suggests
significant metal enrichment in these parts, consistent with inferences from
our CMD analysis.Comment: Abstract shortened. 17 pages, 12 figures (+ 6 pages & 5 figures in
Appendix). MNRAS, in pres
Identifying Local Group Field Galaxies which have interacted with the Milky Way
We distinguish between Local Group field galaxies which may have passed
through the virial volume of the Milky Way, and those which have not, via a
statistical compari- son against populations of dark matter haloes in the Via
Lactea II (VLII) simulation with known orbital histories. Analysis of VLII
provides expectations for this escaped population: they contribute 13 per cent
of the galactic population between 300 and 1500 kpc from the Milky Way, and
hence we anticipate that about 7 of the 54 known Local Group galaxies in that
distance range are likely to be Milky Way escapees. These objects can be of any
mass below that of the Milky Way, and they are expected to have positive radial
velocities with respect to the Milky Way. Comparison of the radius-velocity
distributions of VLII populations and measurements of Local Group galaxies
presents a strong likelihood that Tucana, Cetus, NGC3109, SextansA, SextansB,
Antlia, NGC6822, Phoenix, LeoT, and NGC185 have passed through the Milky Way.
Most of these dwarfs have a lower HI mass fraction than the majority of dwarfs
lying at similar distances to either the Milky Way or M31. Indeed, several of
these galaxies - especially those with lower masses - contain signatures in
their morphology, star formation history and/or gas content indicative of
evolution seen in simulations of satellite/parent galactic interactions. Our
results offer strong support for scenarios in which dwarfs of different types
form a sequence in morphology and gas content, with evolution along the
sequence being driven by interaction history.Comment: 12 pages, 7 figure
Sous la surface des lacs des Laurentides : des témoignages de la dernière période glaciaire
De remarquables formes rocheuses insolubles se dressent dans le fond de lacs creusés dans le marbre de Grenville de la région des Laurentides, au Québec (Canada). Elles atteignent une hauteur pouvant aller jusqu’à plusieurs décimètres. Selon les études sur les taux de dissolution du marbre du Bouclier canadien, il est improbable que celles-ci aient été mises en relief seulement pendant l’Holocène. Nous proposons qu’une érosion hydrique dans des poches d’eau alimentées par l’eau de fonte de la dernière calotte de glace des Laurentides soit à l’origine de la mise en relief de ces roches insolubles. Nous envisageons aussi la possibilité que les roches insolubles aient été dégagées de leur matrice de marbre par une eau courante bien avant la dernière déglaciation, et que celle-ci ait été précédée par un ou des lacs sous-glaciaires.Remarkable insoluble rock forms resulting from differential hydraulic erosion of Grenville marble have been discovered at the bottom of lakes in the Laurentian region (Québec, Canada). Some of these forms are several decimetres tall. Given the results of studies on the chemical dissolution rate of Canadian Shield marble, it is unlikely that these insoluble units were only released from the encasing marble during the Holocene. Instead, it is proposed that they likely result largely from differential hydraulic erosion caused by pockets of meltwater fed by the retreating Laurentide ice sheet. The authors also consider the possibility that the insoluble rocks were released from their marble matrix by running water well before the last deglaciation, and that this retreat of the ice would have been preceded by one or more subglacial lakes