835 research outputs found
Ages and Metallicities of Young Globular Clusters in the Merger Remnant NGC 7252
UV-to-visual spectra of eight young star clusters in the merger remnant and
protoelliptical galaxy NGC 7252, obtained with the Blanco 4-m telescope on
Cerro Tololo, are presented. These clusters lie at projected distances of 3-15
kpc from the center and move with a velocity dispersion of 140+/-35 km/s in the
line of sight. Seven of the clusters show strong Balmer absorption lines in
their spectra [EW(H-beta)= 6-13 Angstrom], while the eighth lies in a giant HII
region and shows no detectable absorption features.
Based on comparisons with model-cluster spectra by Bruzual & Charlot (1996)
and Bressan, Chiosi, & Tantalo (1996), six of the absorption-line clusters have
ages in the range of 400-600 Myr, indicating that they formed early on during
the recent merger. These clusters are globular clusters as judged by their
small effective radii and ages corresponding to ~100 core crossing times. The
one emission-line object is <10 Myr old and may be a nascent globular cluster
or an OB association.
The mean metallicities measured for three clusters are solar to within
+/-0.15 dex, suggesting that the merger of two likely Sc galaxies in NGC 7252
formed a globular-cluster system with a bimodal metallicity distribution. Since
NGC 7252 itself shows the characteristics of a 0.5-1 Gyr old protoelliptical,
its second-generation solar-metallicity globulars provide direct evidence that
giant ellipticals with bimodal globular-cluster systems can form through major
mergers of gas-rich disk galaxies.Comment: 34 pages, incl. 6 figures in EPS format, AAS LaTeX, to be published
in AJ, Vol. 116, Nov. 199
Fatal anaphylactic sting reaction in a patient with mastocytosis
We report on a 33-year-old female patient with indolent systemic mastocytosis and urticaria pigmentosa who died of an anaphylactic reaction after a yellow jacket sting. As she had no history of previous anaphylactic sting reaction, there was no testing performed in order to detect hymenoptera venom sensitization. But even if a sensitization had been diagnosed, no venom immunotherapy (VIT) would have been recommended. It is almost certain that VIT would have saved her life and it is most likely that VIT is indicated in some patients with mastocytosis with no history of anaphylactic sting reaction. However, no criteria have been established in order to allow a selection of mastocytosis patients eligible for such a `prophylactic' VIT. Copyright (C) 2008 S. Karger AG, Basel
HST observations of star clusters in NGC 1023: Evidence for three cluster populations?
Using HST images we have carried out a study of cluster populations in the
nearby S0 galaxy NGC 1023. In two WFPC2 pointings we have identified 221
cluster candidates. The small distance (~9 Mpc) combined with deep F555W and
F814W images allows us to reach about two magnitudes below the expected
turn-over of the globular cluster luminosity function. NGC 1023 appears to
contain at least three identifiable cluster populations: the brighter clusters
show a clearly bimodal color distribution with peaks at V-I = 0.92 and at V-I =
1.15 and in addition there are a number of fainter, more extended objects with
predominantly red colors. Among the brighter clusters, we find that the blue
clusters have somewhat larger sizes than the red ones with mean effective radii
of R(eff) ~ 2 and R(eff) ~ 1.7 pc, respectively. These clusters have luminosity
functions (LFs) and sizes consistent with what is observed for globular
clusters in other galaxies. Fitting Gaussians to the LFs of the blue and red
compact clusters we find turn-over magnitudes of M(TO,blue)=-7.58 and
M(TO,red)=-7.37 in V and dispersions of sigma(V,blue)=1.12 and
sigma(V,red)=0.97. The fainter, more extended clusters have effective radii up
to R(eff) ~ 10-15 pc and their LF appears to rise at least down to M(V) ~ -6,
few of them being brighter than M(V) = -7. We suggest that these fainter
objects may have a formation history distinct from that of the brighter GCs.Comment: 27 pages, 13 figures. Accepted for publication in the Astronomical
Journa
Stellar Populations and Star Cluster Formation in Interacting Galaxies with the Advanced Camera for Surveys
Pixel-by-pixel colour-magnitude and colour-colour diagrams - based on a
subset of the Hubble Space Telescope Advanced Camera for Surveys Early Release
Observations - provide a powerful technique to explore and deduce the star and
star cluster formation histories of the Mice and the Tadpole interacting
galaxies. In each interacting system we find some 40 bright young star clusters
(20 <= F606W (mag) <= 25, with a characteristic mass of ~3 x 10^6 Msun), which
are spatially coincident with blue regions of active star formation in their
tidal tails and spiral arms. We estimate that the main events triggering the
formation of these clusters occurred ~(1.5-2.0) x 10^8 yr ago. We show that
star cluster formation is a major mode of star formation in galaxy
interactions, with >= 35% of the active star formation in encounters occurring
in star clusters. This is the first time that young star clusters have been
detected along the tidal tails in interacting galaxies. The tidal tail of the
Tadpole system is dominated by blue star forming regions, which occupy some 60%
of the total area covered by the tail and contribute ~70% of the total flux in
the F475W filter (decreasing to ~40% in F814W). The remaining pixels in the
tail have colours consistent with those of the main disk. The tidally triggered
burst of star formation in the Mice is of similar strength in both interacting
galaxies, but it has affected only relatively small, spatially coherent areas.Comment: 23 pages in preprint form, 6 (encapsulated) postscript figures;
accepted for publication in New Astronomy; ALL figures (even the grey-scale
ones) need to be printed on a colour printer style files included; for
full-resolution paper, see http://www.ast.cam.ac.uk/STELLARPOPS/ACSpaper
Young Clusters in the Nuclear Starburst of M 83
We present a photometric catalog of 45 massive star clusters in the nuclear
starburst of M 83 (NGC 5236), observed with the Hubble Space Telescope WFPC2,
in both broad-band (F300W, F547M, and F814W) and narrow-band (F656N and F487N)
filters. By comparing the photometry to theoretical population synthesis
models, we estimate the age and mass of each cluster. We find that over 75% of
the star clusters more massive than 2*10^4 Msun in the central 300 pc of M 83
are less than 10 Myr old. Among the clusters younger than 10 Myr and more
massive than 5*10^3 Msun, 70% are between 5 and 7 Myr old. We list an
additional 330 clusters that are detected in our F300W images, but not in the
shallower F547M and F814W images. The clusters are distributed throughout a
semicircular annulus that identifies the active region in the galaxy core,
between 50 and 130 pc from the optical center of M 83. Clusters younger than 5
Myr are preferentially found along the perimeter of the semicircular annulus.
We suggest that the 5-7 Myr population has evacuated much of the interstellar
material from the active ringlet region, and that star formation is continuing
along the edges of the region.Comment: 40 pages, 13 figures, accepted to ApJ
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