240 research outputs found
The Star Formation History of NGC 1705: a Post-Starburst Galaxy on the Verge of Activity
We infer the star formation history in different regions of the blue compact
dwarf NGC 1705 by comparing synthetic color-magnitude diagrams with HST optical
and near-infrared photometry. We find that NGC 1705 is not a young galaxy
because its star formation commenced at least 5 Gyr ago. On the other hand, we
confirm the existence of a recent burst of star formation between 15 and 10 Myr
ago. We also find evidence for new strong activity, which started 3 Myr ago and
is still continuing. The old population is spread across the entire galaxy,
while the young and intermediate stars are more concentrated in the central
regions. We derive an almost continuous star formation with variable rate, and
exclude the presence of long quiescent phases between the episodes during the
last ~1 Gyr. The central regions experienced an episode of star formation of
\~0.07 Msun/yr (for a Salpeter initial mass function [IMF]) 15 to 10 Myr ago.
This coincides with the strong activity in the central super star cluster. We
find a rate of ~0.3 Msun/yr for the youngest ongoing burst which started ~3 Myr
ago. This is higher than in other dwarfs and comparable to the rate of NGC
1569. The star formation rate of earlier episodes is not especially high and
falls in the range 10^{-3}-10^{-1} Msun/yr. The IMF is close to the Salpeter
value or slightly steeper.Comment: 34 pages, including 6 tables and 14 .ps figures (9 in colour), AJ in
pres
The Resolved Stellar Populations in NGC 1705
We present HST photometry of the resolved stellar population in the dwarf
irregular galaxy NGC 1705. The galaxy has been observed with both WFPC2 and
NICMOS, and successful images have been obtained in the F555W, F814W, F110W and
F160W bands. The optical fields cover most of the galaxy, while the infrared
field (NIC2) maps only its central regions. The optical photometry provides
\~20000 objects down to m_F555W ~ 29 in the PC field of view and ~ 20000 in the
three WFCs. In the infrared we have been able to resolve ~ 2400 stars down to
m_{F110W}, m_{F160W} ~ 26. A subsample of 1834 stars have been unambiguously
measured in all the four bands. The corresponding color-magnitude diagrams
(CMDs) confirm the existence of an age gradient, showing that NGC 1705 hosts
both young (a few Myr old) and very old (up to 15 Gyr old) stars, with the
former strongly concentrated toward the galactic center and the latter present
everywhere, but much more easily visible in the external regions. The tip of
the red giant branch (TRGB) is clearly visible both in the optical and in the
infrared CMDs and allows us to derive the galaxy distance. Taking into account
the uncertainties related to both the photometry and the TRGB magnitude --
distance relation, we find that the distance modulus of NGC 1705 is
(m-M)_0=28.54 +- 0.26, corresponding to a distance D=5.1 +- 0.6 Mpc.Comment: aastex, 37 pages, 13 (reduced resolution) .ps figures. Accepted by
AJ. Full resolution figures are available upon request to the first autho
The evolution of H{\sc ii} galaxies: Testing the bursting scenario through the use of self-consistent models
We have computed a series of realistic and self-consistent models of the
emitted spectra of H{\sc ii} galaxies. Our models combine different codes of
chemical evolution, evolutionary population synthesis and photoionization. The
emitted spectrum of H{\sc ii} galaxies is reproduced by means of the
photoionization code CLOUDY, using as ionizing spectrum the spectral energy
distribution of the modelled H{\sc ii} galaxy, which in turn is calculated
according to a Star Formation History (SFH) and a metallicity evolution given
by a chemical evolution model that follows the abundances of 15 different
elements. The contribution of emission lines to the broad-band colours is
explicitly taken into account.
The results of our code are compared with photometric and spectroscopic data
of H{\sc ii} galaxies. Our technique reproduces observed diagnostic diagrams,
abundances, equivalent width-colour and equivalent width-metallicity relations
for local H{\sc ii} galaxies.Comment: 13 figures and 2 tables, accepted for publication in MNRAS Main
Journa
HST Studies of the WLM Galaxy. I. The Age and Metallicity of the Globular Cluster
We have obtained V and I images of the lone globular cluster that belongs to
the dwarf Local Group irregular galaxy known as WLM. The color-magnitude
diagram of the cluster shows that it is a normal old globular cluster with a
well-defined giant branch reaching to M_V=-2.5, a horizontal branch at
M_V=+0.5, and a sub-giant branch extending to our photometry limit of M_V=+2.0.
A best fit to theoretical isochrones indicates that this cluster has a
metallicity of [Fe/H]=-1.52\pm0.08 and an age of 14.8\pm0.6 Gyr, thus
indicating that it is similar to normal old halo globulars in our Galaxy. From
the fit we also find that the distance modulus of the cluster is 24.73\pm0.07
and the extinction is A_V=0.07\pm0.06, both values that agree within the errors
with data obtained for the galaxy itself by others. We conclude that this
normal massive cluster was able to form during the formation of WLM, despite
the parent galaxy's very small intrinsic mass and size.Comment: 14 pages, 5 figures, 1 tabl
Mitigation of Extreme Heat and Sustainable Cooling
The last 5 years have seen the hottest weather on record with many countries experiencing exceptionally warm spells. Extreme heat impacts on health, productivity and economics and the impact is greatest in cities and it dis-proportionally affects the urban poor. This paper initially gives data on global temperature change, as well as the prevalence and impact of extreme heat in cities.
To help cities mitigate the impacts of extreme heat the Adrienne Arsht-Rockefeller Foundation Resilience Center and the Extreme Heat Resilience Alliance in collaboration with the UN Environment Programme, the Cool Coalition, RMI, the Global Covenant of Mayors for Climate and Energy, Mission Innovation and the World Economic Forum’s Global Commission on BiodiverCities by 2030 are developing a toolkit for city officials. This paper describes the toolkit called the Heat Action Platform.
The Heat Action Platform is a living, engagement-oriented tool for city officials, practitioners, and financial institutions to find guidance, both existing resources and tailor-made solutions, on reducing the human and economic impacts of extreme heat at the regional or municipal level. The platform offers opportunities to engage with world-leading experts across a diversity of disciplines to plan, fund, implement, and measure heat resilience actions. The paper describes the rationale behind the heat action platform, its development and how it is being used to mitigate the impacts of extreme heat. Future opportunities to collaborate are identified
Halos of Spiral Galaxies. III. Metallicity Distributions
(Abriged) We report results of a campaign to image the stellar populations in
the halos of highly inclined spiral galaxies, with the fields roughly 10 kpc
(projected) from the nuclei. We use the F814W (I) and F606W (V) filters in the
Wide Field Planetary Camera 2, on board the Hubble Space telescope. Extended
halo populations are detected in all galaxies. The color-magnitude diagrams
appear to be completely dominated by giant-branch stars, with no evidence for
the presence of young stellar populations in any of the fields. We find that
the metallicity distribution functions are dominated by metal-rich populations,
with a tail extending toward the metal poor end. To first order, the overall
shapes of the metallicity distribution functions are similar to what is
predicted by simple, single-component model of chemical evolution with the
effective yields increasing with galaxy luminosity. However, metallicity
distributions significantly narrower than the simple model are observed for a
few of the most luminous galaxies in the sample. It appears clear that more
luminous spiral galaxies also have more metal-rich stellar halos. The
increasingly significant departures from the closed-box model for the more
luminous galaxies indicate that a parameter in addition to a single yield is
required to describe chemical evolution. This parameter, which could be related
to gas infall or outflow either in situ or in progenitor dwarf galaxies that
later merge to form the stellar halo, tends to act to make the metallicity
distributions narrower at high metallicity.Comment: 20 pages, 8 figures (ApJ, in press
Otimização e tolerância a falhas em uma rede de sensores sem fio.
Resumo. Este artigo apresenta uma proposta de solução para otimização e tolerância a falhas para uma Rede de Sensores Sem Fio, que sĂŁo redes prĂłprias para coletar dados em ambientes inĂłspitos, hostis, de difĂcil acesso ou de custo de infraestrutura elevados. Essas redes tem atualmente muitas restrições em seu tempo de vida, dada a pouca autonomia da carga de suas baterias, destaca-se, portanto, a importância de sua otimização. Discute-se tambĂ©m a importância da redundância de recursos para garantia de coleta de dados. A solução proposta Ă© uma implementação factĂvel e de baixo custo, como considera-se que deva ser uma rede para esses fins e com essas caracterĂsticas
Black Holes in Pseudobulges and Spheroidals: A Change in the Black Hole-Bulge Scaling Relations at Low Mass
We investigate the relationship between black hole mass and host galaxy
properties for active galaxies with the lowest black hole masses currently
known in galaxy nuclei. Hubble Space Telescope imaging confirms that the host
galaxies have correspondingly low luminosity; they are ~1 mag below L*. In
terms of morphology, ~60% of the members of the sample are disk-dominated, and
all of these are consistent with containing a bulge or (more likely)
pseudobulge, while the remainder are compact systems with no discernible disk
component. In general the compact components of the galaxies do not obey the
fundamental plane of giant elliptical galaxies and classical bulges, but rather
are less centrally concentrated at a given luminosity, much like spheroidal
galaxies. Our results strongly confirm that a classical bulge is not a
requirement for a nuclear black hole. At the same time, the observed ratio of
black hole to bulge mass is nearly an order of magnitude lower in this sample
than that seen for classical bulges. While the M-sigma relation appears to
continue to low mass, it seems that black hole-galaxy scaling relations do
depend on galaxy structure.Comment: to appear in ApJ; 22 pages; 8 figures; original version available at
http://www.astro.princeton.edu/~jgreene/paper.pd
The red stellar population in NGC 1569
We present HST NICMOS photometry of the resolved stellar population in NGC
1569. The CMD in the F110W and F160W photometric bands contains ~2400 stars
with a formal photometric error < 0.1 mag down to J~23.5 and H~22.5. We
describe the data processing which is required to calibrate the instrumental
peculiarities of NICMOS. Two different packages for PSF-fitting photometry are
used to strengthen the photometric results in the crowded stellar field of NGC
1569. The resulting CMD is discussed in terms of the major evolutionary
properties of the resolved stellar populations. For a distance modulus of 26.71
and a reddening E(B-V)=0.56, our CMD samples stars down to ~0.8 Mo,
corresponding to look-back times > 15 Gyr. This is clear indication of SF
activity spanning an entire Hubble time. The metallicity of the reddest RGB
stars is in better agreement with Z=0.004 as measured in HII regions, than with
Z=0.0004 as expected from the stellar ages. The presence of - yet undetected -
very metal-poor stars embedded in the stellar distribution around J=22.75 and
J-H=1.15 is, however, not ruled out. The youngest stars (< 50 Myr) are
preferentially found around the two central super star clusters, whereas the
oldest population has a more uniform spatial distribution. A SFR per unit area
of 1 Mo yr*(-1) kpc*(-2) and a mass formed in stars of ~ 1.4x10*6 Mo in the
last 50 Myr are derived from the CMD. The NIR CMD places strong constraints on
the lower limit of the onset of SF in NGC 1569. The exceptionally high crowding
in the NICMOS images of NGC 1569 is a challenge for the photometric analysis.
As a result, optical and NIR images of NGC 1569 sample different populations
and cannot be cross-correlated. Nevertheless, we demonstrate the consistency of
the SF histories derived from the optical and NIR CMDs.Comment: 41 pages including 1 table (Latex) and 14 figures (postscript).
Accepted for publication in the Astronomical Journal, March 2001 issu
β-Catenin controls cell sorting at the notochord–somite boundary independently of cadherin-mediated adhesion
In Xenopus laevis, patterning of the trunk mesoderm into the dorsal notochord and lateral somites depends on differential regulation of Wnt–β-catenin signaling. To study the cellular requirements for the physical separation of these tissues, we manipulated β-catenin activity in individual cells that were scattered within the trunk mesoderm. We found that high activity led to efficient cell sorting from the notochord to the somites, whereas reduced activity led to sorting in the opposite direction. Analysis of individual cells overexpressing β-catenin revealed that these cells were unable to establish stable contacts with notochord cells but could freely cross the boundary to integrate within the somitic tissue. Interference with cadherin-mediated adhesion disrupted tissue architecture, but it did not affect sorting and boundary formation. Based on these results, we propose that the boundary itself is the result of cell-autonomous changes in contact behavior that do not rely on differences in absolute levels of adhesion
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