10,772 research outputs found
The Formation of Supermassive Black Holes from Population III.1 Seeds. I. Cosmic Formation Histories and Clustering Properties
We calculate cosmic distributions in space and time of the formation sites of
the first, "Pop III.1" stars, exploring a model in which these are the
progenitors of all supermassive black holes (SMBHs), seen in the centers of
most large galaxies. Pop III.1 stars are defined to form from primordial
composition gas in dark matter minihalos with that are
isolated from neighboring astrophysical sources by a given isolation distance,
. We assume Pop III.1 sources are seeds of SMBHs, based on
protostellar support by dark matter annihilation heating that allows them to
accrete a large fraction of their minihalo gas, i.e., .
Exploring from (proper distances), we
predict the redshift evolution of Pop III.1 source and SMBH remnant number
densities. The local, density of SMBHs constrains (i.e., comoving distance at ). In our
simulated () comoving volume, Pop III.1 stars start
forming just after . Their formation is largely complete by
to for to . We follow source evolution to
, by which point most SMBHs reside in halos with .
Over this period, there is relatively limited merging of SMBHs for these values
of . We also predict SMBH clustering properties at :
feedback suppression of neighboring sources leads to relatively flat angular
correlation functions.Comment: 18 pages, 10 figures, MNRAS accepte
A spectroscopic study of the globular Cluster NGC 4147
Indexación: Web of ScienceWe present the abundance analysis for a sample of 18 red giant branch stars in the metal-poor globular cluster NGC 4147 based on medium- and high-resolution spectra. This is the first extensive spectroscopic study of this cluster. We derive abundances of C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Y, Ba, and Eu. We find a metallicity of [Fe/H] = -1.84 +/- 0.02 and an alpha-enhancement of +0.38 +/- 0.05 (errors on the mean), typical of halo globular clusters in this metallicity regime. A significant spread is observed in the abundances of light elements C, N, O, Na, and Al. In particular, we found an Na-O anticorrelation and Na-Al correlation. The cluster contains only similar to 15 per cent of stars that belong to the first generation (Na-poor and O-rich). This implies that it suffered a severe mass-loss during its lifetime. Its [Ca/Fe] and [Ti/Fe] mean values agree better with the Galactic halo trend than with the trend of extragalactic environments at the cluster metallicity. This possibly suggests that NGC 4147 is a genuine Galactic object at odd with what claimed by some author that proposed the cluster to be member of the Sagittarius dwarf galaxy. An antirelation between the light s-process element Y and Na may also be present.https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stw114
Distribution and sighting frequency of reef fishes in the Florida Keys National Marine Sanctuary
This study analyzed species richness, distribution, and sighting frequency of selected reef fishes to describe species assemblage composition, abundance, and spatial distribution patterns among sites and regions (Upper Keys, Middle Keys, Lower Keys, and Dry Tortugas) within the
Florida Keys National Marine Sanctuary (FKNMS) barrier reef ecosystem. Data were obtained from the Reef Environmental Education Foundation (REEF) Fish Survey Project, a volunteer
fish-monitoring program. A total of 4,324 visual fish surveys conducted at 112 sites throughout the FKNMS were used in these analyses. The data set contained sighting information on 341 fish species comprising 68 families. Species richness was generally highest in the Upper Keys sites (maximum was 220 species at Molasses Reef) and lowest in the Dry Tortugas sites. Encounter rates differed among regions, with the Dry Tortugas having the highest rate, potentially a result of differences in the evenness in fishes and the lower diversity of habitat types in the Dry
Tortugas region. Geographic coverage maps were developed for 29 frequently observed species. Fourteen of these species showed significant regional variation in mean sighting frequency (%SF). Six species had significantly lower mean %SF and eight species had significantly higher
mean %SF in the Dry Tortugas compared with other regions. Hierarchical clustering based on species composition (presence-absence) and species % SF revealed interesting patterns of similarities among sites that varied across spatial scales. Results presented here indicate that
phenomena affecting reef fish composition in the FKNMS operate at multiple spatial scales, including a biogeographic scale that defines the character of the region as a whole, a reef scale (~50-100 km) that include meso-scale physical oceanographic processes and regional variation in reef structure and associated reef habitats, and a local scale that includes level of protection,
cross-shelf location and a suite of physical characteristics of a given reef. It is likely that at both
regional and local scales, species habitat requirements strongly influence the patterns revealed in this study, and are particularly limiting for species that are less frequently observed in the Dry Tortugas. The results of this report serve as a benchmark for the current status of the reef fishes in the FKNMS. In addition, these data provide the basis for analyses on reserve effects and the
biogeographic coupling of benthic habitats and fish assemblages that are currently underway. (PDF contains 61 pages.
An Analytical Approach to Inhomogeneous Structure Formation
We develop an analytical formalism that is suitable for studying
inhomogeneous structure formation, by studying the joint statistics of dark
matter halos forming at two points. Extending the Bond et al. (1991) derivation
of the mass function of virialized halos, based on excursion sets, we derive an
approximate analytical expression for the ``bivariate'' mass function of halos
forming at two redshifts and separated by a fixed comoving Lagrangian distance.
Our approach also leads to a self-consistent expression for the nonlinear
biasing and correlation function of halos, generalizing a number of previous
results including those by Kaiser (1984) and Mo & White (1996). We compare our
approximate solutions to exact numerical results within the excursion-set
framework and find them to be consistent to within 2% over a wide range of
parameters. Our formalism can be used to study various feedback effects during
galaxy formation analytically, as well as to simply construct observable
quantities dependent on the spatial distribution of objects. A code that
implements our method is publicly available at
http://www.arcetri.astro.it/~evan/GeminiComment: 41 Pages, 11 figures, published in ApJ, 571, 585. Reference added,
Figure 2 axis relabele
Tachoastrometry: astrometry with radial velocities
Spectra of composite systems (e.g., spectroscopic binaries) contain spatial
information that can be retrieved by measuring the radial velocities (i.e.,
Doppler shifts) of the components in four observations with the slit rotated by
90 degrees in the sky. By using basic concepts of slit spectroscopy we show
that the geometry of composite systems can be reliably retrieved by measuring
only radial velocity differences taken with different slit angles. The spatial
resolution is determined by the precision with which differential radial
velocities can be measured. We use the UVES spectrograph at the VLT to observe
the known spectroscopic binary star HD 188088 (HIP 97944), which has a maximum
expected separation of 23 milli-arcseconds. We measure an astrometric signal in
radial velocity of 276 \ms, which corresponds to a separation between the two
components at the time of the observations of 18 milli-arcseconds. The
stars were aligned east-west. We describe a simple optical device to
simultaneously record pairs of spectra rotated by 180 degrees, thus reducing
systematic effects. We compute and provide the function expressing the shift of
the centroid of a seeing-limited image in the presence of a narrow slit.The
proposed technique is simple to use and our test shows that it is amenable for
deriving astrometry with milli-arcsecond accuracy or better, beyond the
diffraction limit of the telescope. The technique can be further improved by
using simple devices to simultaneously record the spectra with 180 degrees
angles.With tachoastrometry, radial velocities and astrometric positions can be
measured simultaneously for many double line system binaries in an easy way.
The method is not limited to binary stars, but can be applied to any
astrophysical configuration in which spectral lines are generated by separate
(non-rotational symmetric) regions.Comment: Accepted for publication in A&
Alachua Settlers and the Second Seminole War
The role of Florida\u27s interior settlements and relatively modest settler population has never been assigned much historical significance vis-a-vis the Second Seminole War (1835-1842). With few exceptions, the historiography has focused on the military aspects of this protracted conflict, with certain key battles, military commanders, Native American leaders, and the destruction of the east coast sugar plantations garnering the most interest.2 The ramifications of President Andrew Jackson\u27s Indian Removal Act (1830) which celebrated the prospect of placing a civilized population on large tracts of country now occupied by a few savage hunters 3 are well-known, along with the treaties of Moultrie Creek (1823), Payne\u27s Landing (1832) and Fort Gibson (1833), accords that further set the conflict on a direct course. African-Americans-free and enslaved-have also been acknowledged, albeit belatedly, as central players
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