38 research outputs found
FCC046: a candidate gaseous polar ring dwarf elliptical galaxy in the Fornax Cluster
FCC046 is a Fornax Cluster dwarf elliptical galaxy. Optical observations have
shown that this galaxy, besides an old and metal-poor stellar population, also
contains a very young centrally concentrated population and is actively forming
stars, albeit at a very low level. Here, we report on 21cm observations of
FCC046 with the Australia Telescope Compact Array (ATCA) which we conducted in
the course of a small survey of Fornax Cluster early-type dwarf galaxies. We
have discovered a ~10^7 Mo HI cloud surrounding FCC046. We show that the
presence of this significant gas reservoir offers a concise explanation for
this galaxy's optical morphological and kinematical properties. Surprisingly,
the HI gas, as evidenced by its morphology and its rotational motion around the
galaxy's optical major axis, is kinematically decoupled from the galaxy's
stellar body. This is the first time such a ring of gaseous material in
minor-axis rotation is discovered around a dwarf galaxy.Comment: 5 pages, 4 figures, published in Astrophysical Journal Letter
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
EA01A/B : high-resolution HI imaging of an interacting pair of post-starburst (E+A) galaxies
We present high spatial resolution 21cm HI observations of EA01A and EA01B, a
pair of interacting post-starburst, or E+A, galaxies at z = 0.0746. Based on
optical HST/WFPC2 images, both galaxies are known to display disturbed
morphologies. They also appear to be linked by a bridge of stars. Previous HI
observations Chang et al. (2001) had already uncovered sizable quantities of
neutral gas in or near these galaxies but they lacked the spatial resolution to
locate the gas with any precision within this galactic binary system. We have
analysed deep, high resolution archival VLA observations of the couple. We find
evidence for three gaseous tidal tails; one connected to EA01A and two
emanating from EA01B. These findings confirm, independently from the optical
imaging, that (i) EA01A and EA01B are actively interacting, and that, as a
consequence, the starbursts that occurred in these galaxies were most likely
triggered by this interaction, and that (ii) 6.6+-0.9 10^9 Msun of neutral gas
are still present in the immediate vicinity of the optical bodies of both
galaxies. The HI column density is lowest at the optical positions of the
galaxies, suggesting that most of the neutral gas that is visible in our maps
is associated with the tidal arms and not with the galaxies themselves. This
might provide an explanation for the apparent lack of ongoing star formation in
these galaxies.Comment: 5 pages, 3 figures (lowered resolution), accepted for publication in
ApJ
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