200 research outputs found
Scanning electron microscopy of human islet cilia
Human islet primary cilia are vital glucose-regulating organelles whose structure remains uncharacterized. Scanning electron microscopy (SEM) is a useful technique for studying the surface morphology of membrane projections like cilia, but conventional sample preparation does not reveal the submembrane axonemal structure, which holds key implications for ciliary function. To overcome this challenge, we combined SEM with membrane-extraction techniques to examine primary cilia in native human islets. Our data show well-preserved cilia subdomains which demonstrate both expected and unexpected ultrastructural motifs. Morphometric features were quantified when possible, including axonemal length and diameter, microtubule conformations, and chirality. We further describe a ciliary ring, a structure that may be a specialization in human islets. Key findings are correlated with fluorescence microscopy and interpreted in the context of cilia function as a cellular sensor and communications locus in pancreatic islets
Disruption of Star Clusters in the Interacting Antennae Galaxies
We reexamine the age distribution of star clusters in the Antennae in the
context of N-body+hydrodynamical simulations of these interacting galaxies. All
of the simulations that account for the observed morphology and other
properties of the Antennae have star formation rates that vary relatively
slowly with time, by factors of only 1.3 - 2.5 in the past 10^8 yr. In
contrast, the observed age distribution of the clusters declines approximately
as a power law, dN/dt \propto t^{gamma} with gamma = -1.0, for ages 10^6 yr \la
t \la 10^9 yr. These two facts can only be reconciled if the clusters are
disrupted progressively for at least 10^8 yr and possibly 10^9 yr. When we
combine the simulated formation rates with a power-law model, f_surv \propto
t^{delta}, for the fraction of clusters that survive to each age t, we match
the observed age distribution with exponents in the range -0.9 \la delta \la
-0.6 (with a slightly different delta for each simulation). The similarity
between delta and gamma indicates that dN/dt is shaped mainly by the disruption
of clusters rather than variations in their formation rate. Thus, the situation
in the interacting Antennae resembles that in relatively quiescent galaxies
such as the Milky Way and the Magellanic Clouds.Comment: 9 pages, 5 figures, 1 table, accepted for publication in the
Astrophysical Journal, including revisions after referee repor
The Massive Star Content of NGC 3603
We investigate the massive star content of NGC 3603, the closest known giant
H II region. We have obtained spectra of 26 stars in the central cluster using
the Baade 6.5-m telescope (Magellan I). Of these 26 stars, 16 had no previous
spectroscopy. We also obtained photometry of all of the stars with previous or
new spectroscopy, primarily using archival HST ACS/HRC images. We use these
data to derive an improved distance to the cluster, and to construct an H-R
diagram for discussing the masses and ages of the massive star content of this
cluster.Comment: Accepted by the Astronomical Journal. This revision updates the
coordinates in Table 1 by (-0.18sec, +0.2") to place them on the UCAC2 syste
Probing the anomalous extinction of four young star clusters: the use of colour-excess, main sequence fitting and fractal analysis
Four young star clusters were studied in order to characterize their
anomalous extinction or variable reddening that could be due to a possible
contamination by dense clouds or circumstellar effects. The extinction law (Rv)
was evaluated by adopting two methods: (i) the use of theoretical expressions
based on the colour-excess of stars with known spectral type, and (ii) the
analysis of two-colour diagrams, where the slope of observed colours
distribution is compared to the normal distribution. An algorithm to reproduce
the zero age main sequence (ZAMS) reddened colours was developed in order to
derive the average visual extinction (Av) that provides the best fitting of the
observational data. The structure of the clouds was evaluated by means of
statistical fractal analysis, aiming to compare their geometric structure with
the spatial distribution of the cluster members. The cluster NGC 6530 is the
only object of our sample showing anomalous extinction. In average, the other
clusters are suffering normal extinction, but several of their members, mainly
in NGC 2264, seem to have high Rv, probably due to circumstellar effects. The
ZAMS fitting provides Av values that are in good agreement with those found in
the literature. The fractal analysis shows that NGC 6530 has a centrally
concentrated distribution of stars that is different of the sub-structures
found in the density distribution of the cloud projected in the Av map,
suggesting that the original cloud has been changed with the cluster formation.
On the other hand, the fractal dimension and the statistical parameters of
Berkeley 86, NGC 2244, and NGC 2264 indicate a good cloud-cluster correlation,
when compared to other works based on artificial distribution of points.Comment: 13 pages, 7 figure
Progressive star formation in the young galactic super star cluster NGC 3603
Early release science observations of the cluster NGC3603 with the WFC3 on
the refurbished HST allow us to study its recent star formation history. Our
analysis focuses on stars with Halpha excess emission, a robust indicator of
their pre-main sequence (PMS) accreting status. The comparison with theoretical
PMS isochrones shows that 2/3 of the objects with Halpha excess emission have
ages from 1 to 10 Myr, with a median value of 3 Myr, while a surprising 1/3 of
them are older than 10 Myr. The study of the spatial distribution of these PMS
stars allows us to confirm their cluster membership and to statistically
separate them from field stars. This result establishes unambiguously for the
first time that star formation in and around the cluster has been ongoing for
at least 10-20 Myr, at an apparently increasing rate.Comment: 10 pages, 8 figures, accepted for publication in The Astrophysical
Journa
Star Formation in the Starburst Cluster in NGC 3603
We have used new, deep, visible and near infrared observations of the compact
starburst cluster in the giant HII region NGC 3603 and its surroundings with
the WFC3 on HST and HAWK-I on the VLT to study in detail the physical
properties of its intermediate mass (~ 1 - 3 M_sun) stellar population. We show
that after correction for differential extinction and actively accreting stars,
and the study of field star contamination, strong evidence remains for a
continuous spread in the ages of pre-main sequence stars in the range ~ 2 to ~
30 Myr within the temporal resolution available. Existing differences among
presently available theoretical models account for the largest possible
variation in shape of the measured age histograms within these limits. We also
find that this isochronal age spread in the near infrared and visible
Colour-Magnitude Diagrams cannot be reproduced by any other presently known
source of astrophysical or instrumental scatter that could mimic the luminosity
spread seen in our observations except, possibly, episodic accretion. The
measured age spread and the stellar spatial distribution in the cluster are
consistent with the hypothesis that star formation started at least 20-30 Myrs
ago progressing slowly but continuously up to at least a few million years ago.
All the stars in the considered mass range are distributed in a flattened
oblate spheroidal pattern with the major axis oriented in an approximate
South-East - North-West direction, and with the length of the equatorial axis
decreasing with increasing age. This asymmetry is most likely due to the fact
that star formation occurred along a filament of gas and dust in the natal
molecular cloud oriented locally in this direction.Comment: 21 pages, 19 figures, accepted for publication in Astrophysics &
Space Scienc
Super star clusters in Haro 11: Properties of a very young starburst and evidence for a near-infrared flux excess
We have used multi-band imaging to investigate the nature of the extreme
starburst environment in Haro 11 galaxy. The central starburst region has been
observed in 8 HST wavebands and at 2.16 micron at the ESO-VLT. We constructed
integrated spectral energy distributions (SEDs) for about 200 star clusters and
compared them with single stellar population models in order to derive ages,
masses and extinctions of thestar clusters. The present starburst has lasted
for 40 Myr, and shows a peak of cluster formation only 3.5 Myr old. With such
an extremely young cluster population, Haro 11 represents a unique opportunity
to investigate the youngest phase of the cluster formation process and
evolution in starburst systems. Extinction tends to diminish as function of the
cluster age, but the spread is large and for clusters in partial embedded
phases ( 10^4 Msun), very young (1-3 Myr)
clusters is missing, either because they are embedded inthe parental molecular
cloud and heavily extinguished, or because of blending. Almost half of the
cluster sample is affected by flux excesses at wavelengths 8000 \AA which
cannot be explained by simple stellar evolutionary models. Fitting SED models
over all wavebands leads to systematic overestimates of cluster ages and
incorrect masses for the stellar population supplying the light in these
clusters. We show that the red excess affects also the HST F814W filter, which
is typically used to constrain cluster physical properties. The clusters which
show the red excess are younger than 40 Myr; we propose possible physical
explanations for the phenomenon. Finally, we estimate that Haro 11 hasproduced
bound clusters at a rate almost a factor of 10 higher than the massive and
regular spirals, like the Milky Way. (Abriged)Comment: Accepted for publication in MNRAS. 23 pages, 23 figure
Photon Dominated Regions in NGC 3603
Aims: We aim at deriving the excitation conditions of the interstellar gas as
well as the local FUV intensities in the molecular cloud surrounding NGC 3603
to get a coherent picture of how the gas is energized by the central stars.
Methods: The NANTEN2-4m submillimeter antenna is used to map the [CI] 1-0, 2-1
and CO 4-3, 7-6 lines in a 2' x 2' region around the young OB cluster NGC 3603
YC. These data are combined with C18O 2-1 data, HIRES-processed IRAS 60 and 100
micron maps of the FIR continuum, and Spitzer/IRAC maps. Results: The NANTEN2
observations show the presence of two molecular clumps located south-east and
south-west of the cluster and confirm the overall structure already found by
previous CS and C18O observations. We find a slight position offset of the peak
intensity of CO and [CI], and the atomic carbon appears to be further extended
compared to the molecular material. We used the HIRES far-infrared dust data to
derive a map of the FUV field heating the dust. We constrain the FUV field to
values of \chi = 3 - 6 \times 10^3 in units of the Draine field across the
clouds. Approximately 0.2 to 0.3 % of the total FUV energy is re-emitted in the
[CII] 158 {\mu}m cooling line observed by ISO. Applying LTE and escape
probability calculations, we derive temperatures (TMM1 = 43 K, TMM2 = 47 K),
column densities (N(MM1) = 0.9 \times 10^22 cm^-2, N(MM2) = 2.5 \times 10^22
cm^-2) and densities (n(MM1) = 3 \times 10^3 cm^-3, n(MM2) = 10^3 -10^4 cm^-3)
for the two observed molecular clumps MM1 and MM2. Conclusions: The cluster is
strongly interacting with the ambient molecular cloud, governing its structure
and physical conditions. A stability analysis shows the existence of
gravitationally collapsing gas clumps which should lead to star formation.
Embedded IR sources have already been observed in the outskirts of the
molecular cloud and seem to support our conclusions.Comment: 13 pages, 10 figures, accepted for publication by A&
A GALEX Ultraviolet Imaging Survey of Galaxies in the Local Volume
We present results from a GALEX ultraviolet (UV) survey of a complete sample
of 390 galaxies within ~11 Mpc of the Milky Way. The UV data are a key
component of the composite Local Volume Legacy (LVL), an
ultraviolet-to-infrared imaging program designed to provide an inventory of
dust and star formation in nearby spiral and irregular galaxies. The ensemble
dataset is an especially valuable resource for studying star formation in dwarf
galaxies, which comprise over 80% of the sample. We describe the GALEX survey
programs which obtained the data and provide a catalog of far-UV (~1500
Angstroms) and near-UV (~2200 Angstroms) integrated photometry. General UV
properties of the sample are briefly discussed. We compute two measures of the
global star formation efficiency, the SFR per unit HI gas mass and the SFR per
unit stellar mass, to illustrate the significant differences that can arise in
our understanding of dwarf galaxies when the FUV is used to measure the SFR
instead of H-alpha. We find that dwarf galaxies may not be as drastically
inefficient in coverting gas into stars as suggested by prior H-alpha studies.
In this context, we also examine the UV properties of late-type dwarf galaxies
that appear to be devoid of star formation because they were not detected in
previous H-alpha narrowband observations. Nearly all such galaxies in our
sample are detected in the FUV, and have FUV SFRs that fall below the limit
where the H-alpha flux is robust to Poisson fluctuations in the formation of
massive stars. The UV colors and star formation efficiencies of
H-alpha-undetected, UV-bright dwarf irregulars appear to be relatively
unremarkable with respect to those exhibited by the general population of
star-forming galaxies.Comment: submitted to ApJS, revised per referee's comments; accepted Oct. 30
w/o further revision; 37 pages; figure 6 omitted due to size; figure
available from http://users.obs.carnegiescience.edu/jlee/paper
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