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 são redes próprias para coletar dados em ambientes inóspitos, hostis, de difícil 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 também a importância da redundância de recursos para garantia de coleta de dados. A solução proposta é uma implementação factível e de baixo custo, como considera-se que deva ser uma rede para esses fins e com essas características

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