415 research outputs found
BS196: an old star cluster far from the SMC main body
We present B and V photometry of the outlying SMC star cluster BS196 with the
4.1-m SOAR telescope. The photometry is deep (to V~25) showing ~3 mag below the
cluster turnoff point (TO) at Mv=2.5 (1.03 Msun). The cluster is located at the
SMC distance. The CMD and isochrone fittings provide a cluster age of 5.0+-0.5
Gyr, indicating that this is one of the 12 oldest clusters so far detected in
the SMC. The estimated metallicity is [Fe/H]=-1.68+-0.10. The structural
analysis gives by means of King profile fittings a core radius Rc=8.7+-1.1
arcsec (2.66+-0.14 pc) and a tidal radius Rt=69.4+-1.7 arcsec (21.2+-1.2 pc).
BS196 is rather loose with a concentration parameter c=0.90. With
Mv=-1.89+-0.39, BS196 belongs to the class of intrinsically fainter SMC
clusters, as compared to the well-known populous ones, which starts to be
explored.Comment: 8 pages, 10 figures; accepted by MNRA
Radar Measurements of High-Latitude Ion Composition between 140 and 300 km Altitude
The Chatanika radar has been used to measure the ratio of atomic (O+) ions to molecular (O2 +, NO+) ions in the high-latitude ionosphere. The radar results agreed well with simultaneous in situ rocket data, giving confidence in the radar method of deducing ion composition. Measurements made over long periods of time show seasonal variations, diurnal variations, and variations due to auroral processes. The transition altitude, where the number densities of atomic and molecular ions are equal, is a convenient parameter for describing the composition variation with altitude or ‘composition altitude profile.’ The transition altitude occurs at ∼190 km at night and ∼170 km during the day, in agreement with midlatitude results. During the winter the daytime transition altitude is 15 km lower than in summer, a seasonal variation similar to that at midlatitudes. Energetic particle precipitation results in the lowering of the transition altitude, by 10 km in one case when energetic particles deposited ∼20 ergs/cm² s in the atmosphere. The largest variations in ion composition were found during periods of large joule heat input resulting from electric fields on the order of 50 mV/m. The transition altitude increased by 50 km in a case where the joule heat input rate was 30 ergs/cm² s. These observations were compared to calculations from a simple steady state model involving the principal consituents and reactions. The results indicate that the transition altitude during particle precipitation is most influenced by the increased ion production. There do not appear to be significant effects from possible increases of N2 vibrational temperature. A number of interrelated effects contribute to the increase in transition altitude during joule heating. The most important effect is the electric field contribution in raising the effective ion temperature. In addition, it appears that increased N2 density is also required to account for the observed change
Parkes HI observations of galaxies behind the southern Milky Way. II. The Crux and Great Attractor regions (l ~ 289 degree to 338 degree)
As part of our programme to map the large-scale distribution of galaxies
behind the southern Milky Way, we observed 314 optically-selected,
partially-obscured galaxies in the Zone of Avoidance (ZOA) in the Crux and
Great Attractor (GA) regions. The observations were conducted with the Parkes
64m radio telescope, in a single-pixel pointed mode, reaching an rms noise
level of typically 2-6 mJy over the velocity search range of 400<v<10500km/s. A
total of 162 galaxies were detected. This can be explained by the prominence of
the GA overdensity in the survey regions, which leads to a relatively higher
fraction of nearby galaxies. It is also evident from the quite narrow velocity
distribution (largely confined to 3000-6000km/s) and deviates significantly
from the expectation of a uniform galaxy distribution for the given sensitivity
and velocity range. No systematic differences were found between detections and
non-detections, in terms of latitude, foreground extinction, or environment,
except for the very central part of the rich Norma cluster, where hardly any
galaxies were detected. A detailed investigation of the HI content of the
galaxies reveals strong HI deficiency at the core of the Norma cluster (within
about a 0.4 Abell radius), similar to what has been found in the Coma cluster.
The redshifts obtained by this observing technique result in a substantial
reduction of the so-called redshift ZOA. This is obvious when analysing the
large-scale structure of the new HI data in combination with data from optical
ZOA redshift surveys. The lower latitude detections provide further evidence of
the extension of the Norma Wall, across the ZOA, in particular its bending
towards the Cen-Crux clusters above the Galactic plane at slightly higher
redshift, rather than a straight continuation towards the Centaurus clusters.Comment: 30 pages, 11 figures, including aa.cls; accepted by A
The very local Hubble flow
We present Hubble Space Telescope/WFPC2 images of eighteen galaxies situated
in the vicinity of the Local Group (LG) as part of an ongoing snapshot survey
of nearby galaxies. Their distances derived from the magnitude of the tip of
the red giant branch are 1.92+-0.10 Mpc (ESO 294-010), 3.06+-0.37 (NGC404),
3.15+-0.32 (UGCA105), 1.36+-0.07 (SexB), 1.33+-0.08 (NGC3109), 2.64+-0.21
(UGC6817), 2.86+-0.15 (UGC8508), 3.01+-0.29 (UGC8651), 2.61+-0.16 (KKH86), 2.79+-0.18 (IC5152),
2.23+-0.15 (UGCA438), and 2.45+-0.13 (KKH98).
Based on the velocity-distance data for 36 nearest galaxies around the LG, we
find the radius of the zero-velocity surface of the LG to be R_0 = (0.94+-0.10)
Mpc, which yields a total mass M_{LG} = (1.3+-0.3)*10^{12} M_{\sun}. The galaxy
distribution around the LG reveals a Local Minivoid which does not contain any
galaxy brighter than M_V=-10 mag within a volume of ~100 Mpc^3. The local
Hubble flow seems to be very cold, having a one-dimensional mean random motion
of ~30 km/s. The best-fit value of the local Hubble parameter is 73+-15 km/s
Mpc^{-1}.
The luminosity function for the nearby field galaxies is far less steep than
one for members of the nearest groups.Comment: 13 pages, 6 figures, accepted for publication in the A&A. Version has
low resolution figures and does not include Figure 2. All high resolution
figures (8366k) are available at http://luna.sao.ru/~sme/figures.tar.g
A close look at the Centaurus A group of galaxies IV. Recent star formation histories of late-type dwarfs around CenA
We study a sample of 5 dwarf irregular galaxies in the CenA/M83 group, which
are companions to the giant elliptical CenA. We aim at deriving their physical
properties over their lifetime and compare them to those of dwarfs located in
different environments. We use archival HST/ACS data and apply synthetic
color-magnitude diagram fitting in order to reconstruct the past star formation
activity of the target galaxies. The average star formation rate for the
studied galaxies ranges from 10^{-3} up to \sim 7x10^{-2} M_odot/yr, and their
mean metallicities correlate with their luminosities (from [Fe/H]\sim -1.4 up
to \sim -1.0). The form of the star formation histories varies across the
sample, with quiescent periods alternating with intermittent enhancements in
the star formation (from a few up to several times the average lifetime value).
The dwarfs in this sample formed ~35% to ~60% of their stellar content prior to
~5 Gyr ago. The resulting star formation histories for the CenA companions are
similar to those found for comparable Local Group and M81 group dwarfs. We
consider this sample of dwarfs together with 5 previously studied M83 dwarf
irregular companions. We find no trend of the average star formation rate with
tidal index or distance from the main galaxy of the group. However, dwarfs with
higher baryonic masses do show higher average star formation rates, underlining
the importance of intrinsic properties in governing the evolution of these
galaxies. On the other hand, there is also a clear trend when looking at the
recent (~0.5-1 Gyr) level of activity. Namely, dwarfs within a denser region of
the group appear to have had their star formation quenched while dwarfs located
in the group outskirts show a wide range of possible star formation rates, thus
indicating that external processes play a fundamental role, complementary to
mass, in shaping the star formation histories of dwarf galaxies.Comment: 16 pages, 10 figures; A&A accepte
Star Formation in Dwarf Galaxies of the Nearby Centaurus A Group
We present Halpha narrow-band imaging of 17 dwarf irregular galaxies (dIs) in
the nearby Centaurus A Group. Although all large galaxies of the group have a
current or recent enhanced star formation episode, the dIs have normal star
formation rates and do not contain a larger fraction of dwarf starbursts than
other nearby groups. Relative distances between dIs and larger galaxies of the
group can be computed in 3D since most of them have now fairly accurately known
distances. We find that the dI star formation rates do not depend on local
environment, and in particular they do not show any correlation with the
distance of the dI to the nearest large galaxy of the group. There is a clear
morphology-density relation in the Centaurus A Group, similarly to the Sculptor
and Local Groups, in the sense that dEs/dSphs tend to be at small distances
from the more massive galaxies of the group, while dIs are on average at larger
distances. We find four transition dwarfs in the Group, dwarfs that show
characteristics of both dE/dSphs and dIs, and which contain cold gas but no
current star formation. Interestingly the transition dwarfs have an average
distance to the more massive galaxies which is intermediate between those of
the dEs/dSphs and dIs, and which is quite large: 0.54 +- 0.31 Mpc. This large
distance poses some difficulty for the most popular scenarios proposed for
transforming a dI into a dE/dSph (ram-pressure with tidal stripping or galaxy
harassment). If the observed transition dwarfs are indeed missing links between
dIs and dE/dSphs, their relative isolation makes it less likely to have been
produced by these mechanisms. We propose that an inhomogeneous IGM containing
higher density clumps would be able to ram-pressure stripped the dIs at such
large distances.Comment: 57 pages, 10 fi5gure
A close look at the Centaurus A group of galaxies III. Recent star formation histories of late-type dwarfs around M83
We study the resolved stellar populations of dwarf galaxies in the nearby
Centaurus A/M83 group of galaxies. Our goal is to characterize their
evolutionary history and to investigate eventual similarities or differences
with the dwarf population in other group environments. This work presents the
analysis of five late-type (irregular) dwarfs found in the vicinity of the
giant spiral M83. Using archival HST/ACS data, we perform synthetic
color-magnitude diagram modeling to derive the star formation histories of
these late-type dwarfs. The target objects show heterogeneous star formation
histories, with average star formation rates of 0.08 to 0.70x10^{-2} M_odot/yr.
Some of them present prolonged, global bursts of star formation (~300-500 Myr).
The studied galaxies are all metal-poor ([Fe/H] ~-1.4). We further investigate
the spatial extent of different stellar populations, finding that the young
stars show a clumpy distribution, as opposed to the smooth, broad extent of the
old ones. The actively star forming regions have sizes of ~100 pc and lifetimes
of >~100 Myr, thus suggesting a stochastic star formation mode for the target
dwarf irregular galaxies. The galaxies formed ~20% to 70% of their stars more
than ~7 Gyr ago. The studied dwarfs have average star formation rates slightly
higher than their analogues in the Local Group, but comparable to those in the
M81 group. Our preliminary sample indicates that the neutral gas content of the
target dwarfs does seem to be affected by the group environment: galaxies
within a denser region have a much lower M_HI/ than the isolated ones,
meaning that they will exhaust their gas reservoir more quickly.Comment: 20 pages, 12 figures; accepted for publication in A&
Characterization of the expression of cytokeratins 5, 8, and 14 in mouse thymic epithelial cells during thymus regeneration following acute thymic involution
The thymus is a central lymphoid organ for T cell development. Thymic epithelial cells (TECs) constitute a major component of the thymic stroma, which provides a specialized microenvironment for survival, proliferation, and differentiation of immature T cells. In this study, subsets of TECs were examined immunohistochemically to investigate their cytokeratin (CK) expression patterns during thymus regeneration following thymic involution induced by cyclophosphamide treatment. The results demonstrated that both normal and regenerating mouse thymuses showed a similar CK expression pattern. The major medullary TECs (mTEC) subset, which is stellate in appearance, exhibited CK5 and CK14 staining, and the minor mTEC subset, which is globular in appearance, exhibited CK8 staining, whereas the vast majority of cortical TECs (cTECs) expressed CK8 during thymus regeneration. Remarkably, the levels of CK5 and CK14 expression were enhanced in mTECs, and CK8 expression was upregulated in cTECs during mouse thymus regeneration after cyclophosphamide-induced acute thymic involution. Of special interest, a relatively high number of CK5+CK8+ TEC progenitors occurred in the thymic cortex during thymus regeneration. Taken together, these findings shed more light on the role of CK5, CK8, and CK14 in the physiology of TECs during mouse thymus regeneration, and on the characterization of TEC progenitors for restoration of the epithelial network and for concomitant regeneration of the adult thymus
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