15 research outputs found
N-body/SPH study of the evolution of dwarf galaxies in a cluster environment
Using an N-body/SPH code, we explore the scenario in which a dwarf elliptical
galaxy (dE) is subjected to ram-pressure stripping due to the intracluster
medium (ICM). Our simulations show that while (i) smaller dEs lose their ISM
almost immediately after entering the cluster, (ii) more massive dEs are able
to retain their gas for considerable timespans.Comment: 1 page, no figures, poster contribution to the Splinter Meeting
"Galaxies in interaction" at the joint meeting of the Czech Astronomical
Society and the Astronomische Gesellschaft (20-25 Sept. 2004, Prague, Czech
Republic
The CO content of the Local Group dwarf irregular galaxies IC5152, UGCA438, and the Phoenix dwarf
We present a search for CO(1->0) emission in three Local Group dwarf
irregular galaxies: IC5152, the Phoenix dwarf, and UGCA438, using the ATNF
Mopra radio telescope. Our scans largely cover the optical extent of the
galaxies and the stripped HI cloud West of the Phoenix dwarf. Apart from a
tentative but non-significant emission peak at one position in the Phoenix
dwarf, no significant emission was detected in the CO spectra of these
galaxies. For a velocity width of 6 km/s, we derive 4sigma upper limits of 0.03
K km/s, 0.04 K km/s and 0.06 K km/s for IC5152, the Phoenix dwarf and UGCA438,
respectively. This is an improvement of over a factor of 10 compared with
previous observations of IC5152; the other two galaxies had not yet been
observed at millimeter wavelengths. Assuming a Galactic CO-to-H_2 conversion
factor, we derive upper limits on the molecular gas mass of 6.2 x 10^4 M_sun,
3.7 x 10^3 M_sun and 1.4 x 10^5 M_sun for IC5152, the Phoenix dwarf and
UGCA438, respectively. We investigate two possible causes for the lack of CO
emission in these galaxies. On the one hand, there may be a genuine lack of
molecular gas in these systems, in spite of the presence of large amounts of
neutral gas. However, in the case of IC5152 which is actively forming stars,
molecular gas is at least expected to be present in the star forming regions.
On the other hand, there may be a large increase in the CO-to-H_2 conversion
factor in very low-metallicity dwarfs (-2 <= [Fe/H] <= -1), making CO a poor
tracer of the molecular gas content in dwarf galaxies.Comment: 7 pages, 5 figures, 2 tables, accepted for publication in MNRA
Towards a Solution for the Ca II Triplet Puzzle : Results from Dwarf Elliptical Galaxies
We present new estimates of ages and metallicities, based on FORS/VLT optical
(4400-5500A) spectroscopy, of 16 dwarf elliptical galaxies (dE's) in the Fornax
Cluster and in Southern Groups. These dE's are more metal-rich and younger than
previous estimates based on narrow-band photometry and low-resolution
spectroscopy. For our sample we find a mean metallicity [Z/H] = -0.33 dex and
mean age 3.5 Gyr, consistent with similar samples of dE's in other environments
(Local Group, Virgo). Three dE's in our sample show emission lines and very
young ages. This suggests that some dE's formed stars until a very recent epoch
and were self-enriched by a long star formation history. Previous observations
of large near-infrared (~8500A) Ca II absorption strengths in these dE's are in
good agreement with the new metallicity estimates, solving part of the
so-called Calcium puzzle.Comment: ApJ Letters accepted, 5 pages emulateapj, 2 figure
Typing Clostridium difficile strains based on tandem repeat sequences
Background: Genotyping of epidemic Clostridium difficile strains is necessary to track their emergence and spread. Portability of genotyping data is desirable to facilitate inter-laboratory comparisons and epidemiological studies.
Results: This report presents results from a systematic screen for variation in repetitive DNA in the genome of C. difficile. We describe two tandem repeat loci, designated \u27TR6\u27 and \u27TR10\u27, which display extensive sequence variation that may be useful for sequence-based strain typing. Based on an investigation of 154 C. difficile isolates comprising 75 ribotypes, tandem repeat sequencing demonstrated excellent concordance with widely used PCR ribotyping and equal discriminatory power. Moreover, tandem repeat sequences enabled the reconstruction of the isolates\u27 largely clonal population structure and evolutionary history.
Conclusion: We conclude that sequence analysis of the two repetitive loci introduced here may be highly useful for routine typing of C. difficile. Tandem repeat sequence typing resolves phylogenetic diversity to a level equivalent to PCR ribotypes. DNA sequences may be stored in databases accessible over the internet, obviating the need for the exchange of reference strains
Formation and Evolution of Dwarf Elliptical Galaxies - II. Spatially resolved star-formation histories
We present optical VLT spectroscopy of 16 dwarf elliptical galaxies (or dEs)
comparable in mass to NGC 205, and belonging to the Fornax cluster and to
nearby groups of galaxies. Using ULySS and STECKMAP, we derive radial profiles
of the SSP-equivalent ages, metallicities and star-formation histories. The old
stellar population of the dEs, which dominates their mass, is likely coeval
with that of massive ellipticals or bulges, but the star formation efficiency
is lower. Important intermediate age (1-5 Gyr) populations, and frequently
tails of star formation until recent times are detected. These histories are
reminiscent of their lower mass dSph counterparts of the Local Group. Most
galaxies (10/16) show significant metallicity gradients, with metallicity
declining by 0.5 dex over one half-light radius on average. These gradients are
already present in the old population. The flattened (or discy), rotating
objects (6/16) have flat metallicity profiles. This may be consistent with a
distinct origin for these galaxies or it may be due to their geometry. The
central SSP-equivalent age varies between 1 and 6 Gyr, with the age slowly
increasing with radius in the vast majority of objects. The group and cluster
galaxies have similar radial gradients and star-formation histories. The strong
and old metallicity gradients place important constraints on the possible
formation scenarios of dEs. Numerical simulations of the formation of spherical
low-mass galaxies reproduce these gradients, but they require a longer time for
them to build up. A gentle depletion of the gas, by ram-pressure stripping or
starvation, could drive the gas-rich, star-forming progenitors to the present
dEs.Comment: 21 pages, 9 figures. Accepted in MNRA
E+A Galaxies: Did They Lose The A to Become E?
We present deep HI observations of a sample of 9 nearby E+A galaxies (0.05 < z < 0.1). In 7 of them, we detected up to a few times 109 M of neutral gas, making the link between E+As and early-types less direct than previously thought