209 research outputs found
Globular cluster systems II: On the formation of old globular clusters and their sites of formation
We studied the metal-poor globular cluster (GC) populations of a large
variety of galaxies (47 galaxies spanning about 10mag in absolute brightness)
and compared their mean [Fe/H] with the properties of the host galaxies. The
mean [Fe/H] of the systems lie in the -1.65<[Fe/H]<-1.20 range (74% of the
population). Using only GC systems with more than 6 objects detected, 85% of
the population lie within -1.65<[Fe/H]<-1.20. The relation between the mean
[Fe/H] of the metal-poor GC systems and the Mv of their host galaxies presents
a very low slope which includes zero. An analysis of the correlation of the
mean [Fe/H] with other galaxy properties also leads to the conclusion that no
strong correlation exists. The lack of correlation suggests a formation of all
metal-poor GC in similar gas fragments. A weak correlation might exist between
mean [Fe/H] of the metal-poor GC and host galaxy metallicity. This would imply
that some fragments in which metal-poor GC formed were already embedded in the
larger dark matter halo of the final galaxy (as oppose to being independent
satellites that were accreted later). Our result suggests a homogeneous
formation of metal-poor GC in all galaxies, in typical fragments of masses
around 10^9-10^10 solar masses with very similar metallicities, compatible with
hierarchical formation scenarios for galaxies. We compared the mean [Fe/H] of
the metal-poor GC populations with the typical metallicities of high-z objects.
If we add the constraint that GC need a high column density of gas to form,
DLAs are the most likely sites for the formation of metal-poor GC populations.Comment: accepted for publication in AJ, scheduled for the May 2001 issu
Globular Cluster Population of Hickson Compact Group 22a and 90c
We present the first measurement of the globular cluster populations of
galaxies in Hickson compact groups, in order to investigate the effect of these
high density environments on the formation and evolution of globular cluster
systems. Based on V and R band images that we obtained of HCG 22a and HCG 90c
with the ESO New Technology Telescope (NTT), we find a total globular cluster
population of for HCG 22a and for 90c. The specific
frequency for HCG 22a was found to be and for HCG 90c. A power-law fit to the globular cluster radial profile of HCG
22a yields and for HCG 90c we found . A comparison of the globular cluster radial profiles with
the surface brightness of the parent galaxy shows that the globular cluster
systems are at least as extended as the halo light. The measured values for the
specific frequency are consistent with a scenario in which the host galaxies
were in a low density ``field-like'' environment when they formed their
globular cluster systems.Comment: 28 pages, 13 figures, 3 tables, accepted for publication in the
Astrophysical Journa
Evidence for the Hierarchical Formation of the Galactic Spheroid
The possibility that the Galactic spheroid was assembled from numerous
chemically-distinct, proto-Galactic fragments is investigated using a
Monte-Carlo technique designed to simulate the chemical evolution of the Galaxy
in hierarchical formation scenarios which involve no gas dissipation. By
comparing the observed and simulated metallicity distributions of Galactic
globular clusters and halo field stars, we estimate the level of fragmentation
in the collapsing proto-Galaxy. Although the formation process is highly
stochastic, the simulations often show good agreement with the observed
metallicity distributions, provided the luminosity function of proto-Galactic
fragments had a power-law form with exponent ~ -2. While this steep slope is
strongly at odds with the presently observed luminosity function of the Local
Group, it is in close agreement with the predictions of semi-analytic and
numerical models of hierarchical galaxy formation. We discuss a number of
possible explanations for this discrepancy. These simulations suggest that the
Galactic halo and its globular cluster system were assembled via the accretion
and disruption of approximately 1000 metal-poor, proto-Galactic fragments by
the dominant Galactic building block: a proto-bulge whose own metal-rich
globular clusters system has been preferentially eroded by dynamical processes.
We argue that the same process (ie, hierarchical growth involving little gas
dissipation) is responsible for the formation of both giant elliptical galaxies
and the bulge-halo components of spiral galaxies. (ABRIDGED).Comment: 20 pages, 9 postscript figures. Accepted for publication in the ApJ,
April 10 2000 issu
Horizontal-Branch Morphology and the Photometric Evolution of Old Stellar Populations
Theoretical integrated broad-band colors ranging from far-UV to near-IR have
been computed for old stellar systems from our evolutionary population
synthesis code. These models take into account, for the first time, the
detailed systematic variation of horizontal-branch (HB) morphology with age and
metallicity. Our models show that some temperature-sensitive color indices are
significantly affected by the presence of `blue HB stars'. Our models are
calibrated in the B-V, V-I, C-T1, and M-T1 vs. [Fe/H] planes, using
low-reddened Galactic globular clusters (GCs) [E(B-V) < 0.2] and the relative
age difference between the older inner halo Galactic GCs and younger outer halo
counterparts is well reproduced. Several empirical linear color-metallicity
transformation relations are assessed with our models and it is noted that they
may not be safely used to estimate metallicity if there are sizable age
differences amongst GCs within and between galaxies. It is anticipated that the
detailed population models presented here coupled with further precise
spectrophotometric observations of globular cluster systems in external
galaxies from the large ground-based telescopes and space UV facilities will
enable us to accurately estimate their ages and metallicities. (abridged)Comment: 40 pages, 14 figures, Tentatively scheduled for the November 2002
issue of The Astronomical Journa
The Efficiency of Globular Cluster Formation
(Abridged): The total populations of globular cluster systems (GCSs) are
discussed in terms of their connection to the efficiency of globular cluster
formation---the mass fraction of star-forming gas that was able to form bound
stellar clusters rather than isolated stars or unbound associations---in galaxy
halos. Observed variations in GCS specific frequencies (S_N=N_gc/L_gal), both
as a function of galactocentric radius in individual systems and globally
between entire galaxies, are reviewed in this light. It is argued that trends
in S_N do not reflect any real variation in the underlying efficiency of
cluster formation; rather, they result from ignoring the hot gas in many large
ellipticals. This claim is checked and confirmed in each of M87, M49, and NGC
1399, for which existing data are combined to show that the volume density
profile of globular clusters, rho_cl, is directly proportional to the sum of
(rho_gas+rho_stars) at large radii. The constant of proportionality is the same
in each case: epsilon=0.0026 +/- 0.0005 in the mean. This is identified with
the globular cluster formation efficiency. The implication that epsilon might
have had a universal value is supported by data on the GCSs of 97 early-type
galaxies, on the GCS of the Milky Way, and on the ongoing formation of open
clusters. These results have specific implications for some issues in GCS and
galaxy formation, and they should serve as a strong constraint on more general
theories of star and cluster formation.Comment: 36 pages with 11 figures; accepted for publication in The
Astronomical Journa
An HST/WFPC2 survey of bright young clusters in M31. I. VdB0, a massive star cluster seen at t â 25 Myr
Aims. We introduce our imaging survey of possible young massive globular clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). We obtained shallow (to B ~ 25) photometry of individual stars in 20 candidate clusters. We present here details of the data reduction pipeline that is being applied to all the survey data and describe its application to the brightest among our targets, van den Bergh 0 (VdB0), taken as a test case.
Methods. Point spread function fitting photometry of individual stars was obtained for all the WFPC2 images of VdB0 and the completeness of the final samples was estimated using an extensive set of artificial stars experiments. The reddening, the age and the metallicity of the cluster were estimated by comparing the observed color magnitude diagram (CMD) with theoretical isochrones. Structural parameters were obtained from model-fitting to the intensity profiles measured within circular apertures on the WFPC2 images.
Results. Under the most conservative assumptions, the stellar mass of VdB0 is M> 2.4 x 10^4 M_â , but our best estimates lie in the range â4-9 x 10^4 M_â. The CMD of VdB0 is best reproduced by models having solar metallicity and age â25 Myr. Ages less than â12 Myr and greater than â60 Myr are clearly ruled out by the available data. The cluster has a remarkable number of red super giants (âł18) and a CMD very similar to Large Magellanic Cloud clusters usually classified as young globulars such as NGC 1850, for example.
Conclusions. VdB0 is significantly brighter (âł1 mag) than Galactic open clusters of similar age. Its present-day mass and half-light radius ((r_h = 7.4 pc) are more typical of faint globular clusters than of open clusters. However, given its position within the disk of M31, it is expected to be destroyed by dynamical effects, in particular by encounters with giant molecular clouds, within the next ~4 Gyr
Normal Globular Cluster Systems in Massive Low Surface Brightness Galaxies
We present the results of a study of the globular cluster systems of 6
massive spiral galaxies, originally cataloged as low surface brightness
galaxies but here shown to span a wide range of central surface brightness
values, including two intermediate to low surface brightness galaxies. We used
the Advanced Camera for Surveys on board HST to obtain photometry in the F475W
and F775W bands and select sources with photometric and morphological
properties consistent with those of globular clusters. A total of 206
candidates were identified in our target galaxies. From a direct comparison
with the Galactic globular cluster system we derive specific frequency values
for each galaxy that are in the expected range for late-type galaxies. We show
that the globular cluster candidates in all galaxies have properties consistent
with globular cluster systems of previously studied galaxies in terms of
luminosity, sizes and color. We establish the presence of globular clusters in
the two intermediate to low surface brightness galaxies in our sample and show
that their properties do not have any significant deviation from the behavior
observed in the other sample galaxies. Our results are broadly consistent with
a scenario in which low surface brightness galaxies follow roughly the same
evolutionary history as normal (i.e. high surface) brightness galaxies except
at a much lower rate, but require the presence of an initial period of star
formation intense enough to allow the formation of massive star clusters.Comment: 14 pages, 6 figures. AJ accepte
Viral Kinetics of Sequential SARS-CoV-2 Infections
The impact of a prior SARS-CoV-2 infection on the progression of subsequent infections has been unclear. Using a convenience sample of 94,812 longitudinal RT-qPCR measurements from anterior nares and oropharyngeal swabs, we identified 71 individuals with two well-sampled SARS-CoV-2 infections between March 11th, 2020, and July 28th, 2022. We compared the SARS-CoV-2 viral kinetics of first vs. second infections in this group, adjusting for viral variant, vaccination status, and age. Relative to first infections, second infections usually featured a faster clearance time. Furthermore, a person\u27s relative (rank-order) viral clearance time, compared to others infected with the same variant, was roughly conserved across first and second infections, so that individuals who had a relatively fast clearance time in their first infection also tended to have a relatively fast clearance time in their second infection (Spearman correlation coefficient: 0.30, 95% credible interval (0.12, 0.46)). These findings provide evidence that, like vaccination, immunity from a prior SARS-CoV-2 infection shortens the duration of subsequent acute SARS-CoV-2 infections principally by reducing viral clearance time. Additionally, there appears to be an inherent element of the immune response, or some other host factor, that shapes a person\u27s relative ability to clear SARS-CoV-2 infection that persists across sequential infections
Chemo-dynamical evolution of Globular Cluster Systems
We studied the relation between the ratio of rotational velocity to velocity
dispersion and the metallicity (/\sigma_{v}-metallicity relation) of
globular cluster systems (GCS) of disk galaxies by comparing the relation
predicted from simple chemo-dynamical models for the formation and evolution of
disk galaxies with the observed kinematical and chemical properties of their
GCSs. We conclude that proto disk galaxies underwent a slow initial collapse
that was followed by a rapid contraction and derive that the ratio of the
initial collapse time scale to the active star formation time scale is \sim 6
for our Galaxy and \sim 15 for M31. The fundamental formation process of disk
galaxies was simulated based on simple chemo-dynamical models assuming the
conservation of their angular momentum. We suggest that there is a typical
universal pattern in the /\sigma_{v}-metallicity relation of the GCS
of disk galaxies. This picture is supported by the observed properties of GCSs
in the Galaxy and in M31. This relation would deviate from the universal
pattern, however, if large-scale merging events took major role in
chemo-dynamical evolution of galaxies and will reflect the epoch of such
merging events. We discuss the properties of the GCS of M81 and suggest the
presence of past major merging event.Comment: 25 pages, 8 figures, Accepted for publication in the Astrophysical
Journa
SINFONI in the Galactic Center: young stars and IR flares in the central light month
We report 75 milli-arcsec resolution, near-IR imaging spectroscopy within the
central 30 light days of the Galactic Center [...]. To a limiting magnitude of
K~16, 9 of 10 stars in the central 0.4 arcsec, and 13 of 17 stars out to 0.7
arcsec from the central black hole have spectral properties of B0-B9, main
sequence stars. [...] all brighter early type stars have normal rotation
velocities, similar to solar neighborhood stars. We [...] derive improved 3d
stellar orbits for six of these S-stars in the central 0.5 arcsec. Their
orientations in space appear random. Their orbital planes are not co-aligned
with those of the two disks of massive young stars 1-10 arcsec from SgrA*. We
can thus exclude [...] that the S-stars as a group inhabit the inner regions of
these disks. They also cannot have been located/formed in these disks [...].
[...] we conclude that the S-stars were most likely brought into the central
light month by strong individual scattering events. The updated estimate of
distance to the Galactic center from the S2 orbit fit is Ro = 7.62 +/- 0.32
kpc, resulting in a central mass value of 3.61 +/- 0.32 x 10^6 Msun. We
happened to catch two smaller flaring events from SgrA* [...]. The 1.7-2.45 mum
spectral energy distributions of these flares are fit by a featureless, red
power law [...]. The observed spectral slope is in good agreement with
synchrotron models in which the infrared emission comes from [...] radiative
inefficient accretion flow in the central R~10 Rs region.Comment: 50 pages, 10 figures, 2 tables, submitted to ApJ, February 6th, 2005,
abstract abridge
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