Star Formation History and Chemical Evolution of the Sextans Dwarf Spheroidal Galaxy

We present the star formation history and chemical evolution of the Sextans dSph dwarf galaxy as a function of galactocentric distance. We derive these from the $VI$ photometry of stars in the $42' \times 28'$ field using the SMART model developed by Yuk & Lee (2007, ApJ, 668, 876) and adopting a closed-box model for chemical evolution. For the adopted age of Sextans 15 Gyr, we find that $>$84% of the stars formed prior to 11 Gyr ago, significant star formation extends from 15 to 11 Gyr ago ($\sim$ 65% of the stars formed 13 to 15 Gyr ago while $\sim$ 25% formed 11 to 13 Gyr ago), detectable star formation continued to at least 8 Gyr ago, the star formation history is more extended in the central regions than the outskirts, and the difference in star formation rates between the central and outer regions is most marked 11 to 13 Gyr ago. Whether blue straggler stars are interpreted as intermediate age main sequence stars affects conclusions regarding the star formation history for times 4 to 8 Gyr ago, but this is at most only a trace population. We find that the metallicity of the stars increased rapidly up to [Fe/H]=--1.6 in the central region and to [Fe/H]=--1.8 in the outer region within the first Gyr, and has varied slowly since then. The abundance ratios of several elements derived in this study are in good agreement with the observational data based on the high resolution spectroscopy in the literature. We conclude that the primary driver for the radial gradient of the stellar population in this galaxy is the star formation history, which self-consistently drives the chemical enrichment history.Comment: 36 pages, 14 figures, To appear in the ApJ, 200

The positive soundscape project : a synthesis of results from many disciplines

This paper takes an overall view of ongoing findings from the Positive Soundscape Project, a large inter-disciplinary soundscapes study which is nearing completion. Qualitative fieldwork (soundwalks and focus groups) and lab-based listening tests have revealed that two key dimensions of the emotional response are calmness and vibrancy. In the lab these factors explain nearly 80% of the variance in listener response. Physiological validation is being sought using fMRI measurements, and these have so far shown significant differences in the response of the brain to affective and neutral soundscapes. A conceptual framework which links the key soundscape components and which could be used for future design is outlined. Metrics are suggested for some perceptual scales and possibilities for soundscape synthesis for design and user engagement are discussed, as are the applications of the results to future research and environmental noise policy

A wide-area view of the Phoenix dwarf galaxy from VLT/FORS imaging

We present results from a wide-area photometric survey of the Phoenix dwarf galaxy, one of the rare dwarf irregular/ dwarf spheroidal transition type galaxies (dTs) of the Local Group (LG). These objects offer the opportunity to study the existence of possible evolutionary links between the late- and early- type LG dwarf galaxies, since the properties of dTs suggest that they may be dwarf irregulars in the process of transforming into dwarf spheroidals. Using FORS at the VLT we have acquired VI photometry of Phoenix. The data reach a S/N~10 just below the horizontal branch of the system and consist of a mosaic of images that covers an area of 26' x 26' centered on the coordinates of the optical center of the galaxy. Examination of the colour-magnitude diagram and luminosity function revealed the presence of a bump above the red clump, consistent with being a red giant branch bump. The deep photometry combined with the large area covered allows us to put on a secure ground the determination of the overall structural properties of the galaxy and to derive the spatial distribution of stars in different evolutionary phases and age ranges, from 0.1 Gyr to the oldest stars. The best-fitting profile to the overall stellar population is a Sersic profile of Sersic radius R_S = 1.82'+-0.06' and m=0.83+-0.03. We confirm that the spatial distribution of stars is found to become more and more centrally concentrated the younger the stellar population, as reported in previous studies. This is similar to the stellar population gradients found for close-by Milky Way dwarf spheroidal galaxies. We quantify such spatial variations by analyzing the surface number density profiles of stellar populations in different age ranges; [Abridged]Comment: 21 pages; 11 figures. Accepted for publication in MNRA

The isolated elliptical NGC 4555 observed with Chandra

We present analysis of a Chandra observation of the elliptical galaxy NGC 4555. The galaxy lies in a very low density environment, either isolated from all galaxies of similar mass or on the outskirts of a group. Despite this, NGC 4555 has a large gaseous halo, extending to ~60 kpc. We find the mean gas temperature to be ~0.95 keV and the Iron abundance to be ~0.5 solar. We model the surface brightness, temperature and abundance distribution of the halo and use these results to estimate parameters such as the entropy and cooling time of the gas, and the total gravitational mass of the galaxy. In contrast to recent results showing that moderate luminosity ellipticals contain relatively small quantities of dark matter, our results show that NGC 4555 has a massive dark halo and large mass-to-light ratio (56.8 [+34.2,-35.8] solar at 50 kpc, 42.7 [+14.6,-21.2] solar at 5 effective radii, 1 sigma errors). We discuss this disparity and consider possible mechanisms by which galaxies might reduce their dark matter content.Comment: 10 pages, 7 postscript figures, accepted for publication in MNRA

A titanium-nitride near-infrared kinetic inductance photon-counting detector and its anomalous electrodynamics

We demonstrate single-photon counting at 1550 nm with titanium-nitride (TiN) microwave kinetic inductance detectors. Energy resolution of 0.4 eV and arrival-time resolution of 1.2 microseconds are achieved. 0-, 1-, 2-photon events are resolved and shown to follow Poisson statistics. We find that the temperature-dependent frequency shift deviates from the Mattis-Bardeen theory, and the dissipation response shows a shorter decay time than the frequency response at low temperatures. We suggest that the observed anomalous electrodynamics may be related to quasiparticle traps or subgap states in the disordered TiN films. Finally, the electron density-of-states is derived from the pulse response.Comment: 4 pages, 3 figure

The photometric properties of a vast stellar substructure in the outskirts of M33

We have surveyed $\sim40$sq.degrees surrounding M33 with CFHT MegaCam in the g and i filters, as part of the Pan-Andromeda Archaeological Survey. Our observations are deep enough to resolve the top 4mags of the red giant branch population in this galaxy. We have previously shown that the disk of M33 is surrounded by a large, irregular, low-surface brightness substructure. Here, we quantify the stellar populations and structure of this feature using the PAndAS data. We show that the stellar populations of this feature are consistent with an old population with $\sim-1.6$dex and an interquartile range in metallicity of $\sim0.5$dex. We construct a surface brightness map of M33 that traces this feature to $\mu_V\simeq33$mags\,arcsec$^{-2}$. At these low surface brightness levels, the structure extends to projected radii of $\sim40$kpc from the center of M33 in both the north-west and south-east quadrants of the galaxy. Overall, the structure has an "S-shaped" appearance that broadly aligns with the orientation of the HI disk warp. We calculate a lower limit to the integrated luminosity of the structure of $-12.7\pm0.5$mags, comparable to a bright dwarf galaxy such as Fornax or AndII and slightly less than $1\$ of the total luminosity of M33. Further, we show that there is tentative evidence for a distortion in the distribution of young stars near the edge of the HI disk that occurs at similar azimuth to the warp in HI. The data also hint at a low-level, extended stellar component at larger radius that may be a M33 halo component. We revisit studies of M33 and its stellar populations in light of these new results, and we discuss possible formation scenarios for the vast stellar structure. Our favored model is that of the tidal disruption of M33 in its orbit around M31.Comment: Accepted for publication in ApJ. 17 figures. ApJ preprint forma

A faint extended cluster in the outskirts of NGC 5128: evidence of a low mass accretion

We report the discovery of an extended globular cluster in a halo field in Centaurus A (NGC 5128), situated \sim 38\kpc from the centre of that galaxy, imaged with the Advanced Camera for Surveys on board the Hubble Space Telescope. At the distance of the galaxy, the half-light radius of the cluster is r_h ~ 17pc, placing it among the largest globular clusters known. The faint absolute magnitude of the star cluster, M_(V,o)=-5.2, and its large size render this object somewhat different from the population of extended globular clusters previously reported, making it the first firm detection in the outskirts of a giant galaxy of an analogue of the faint, diffuse globular clusters present in the outer halo of the Milky Way. The colour-magnitude diagram of the cluster, covering approximately the brightest four magnitudes of the red giant branch, is consistent with an ancient, i.e., older than ~8 Gyr, intermediate-metallicity, i.e., [M/H] ~-1.0 dex, stellar population. We also report the detection of a second, even fainter cluster candidate which would have r_h ~ 9pc, and M_(V,o)=-3.4 if it is at the distance of NGC 5128. The properties of the extended globular cluster and the diffuse stellar populations in its close vicinity suggest that they are part of a low mass accretion in the outer regions of NGC 5128.Comment: 9 pages, MNRAS, in pres

Preliminary Limits on the WIMP-Nucleon Cross Section from the Cryogenic Dark Matter Search (CDMS)

We are conducting an experiment to search for WIMPs, or weakly-interacting massive particles, in the galactic halo using terrestrial detectors. This generic class of hypothetical particles, whose properties are similar to those predicted by extensions of the standard model of particle physics, could comprise the cold component of non-baryonic dark matter. We describe our experiment, which is based on cooled germanium and silicon detectors in a shielded low-background cryostat. The detectors achieve a high degree of background rejection through the simultaneous measurement of the energy in phonons and ionization. Using exposures on the order of one kilogram-day from initial runs of our experiment, we have achieved (preliminary) upper limits on the WIMP-nucleon cross section that are comparable to much longer runs of other experiments.Comment: 5 LaTex pages, 5 eps figs, epsf.sty, espcrc2dsa2.sty. Proceedings of TAUP97, Gran Sasso, Italy, 7-11 Sep 1997, Nucl. Phys. Suppl., A. Bottino, A. di Credico and P. Monacelli (eds.). See also http://cfpa.berkeley.ed

The Nature of the Density Clump in the Fornax Dwarf Spheroidal Galaxy

We have imaged the recently discovered stellar overdensity located approximately one core radius from the center of the Fornax dwarf spheroidal galaxy using the Magellan Clay 6.5m telescope with the Magellan Instant Camera (MagIC). Superb seeing conditions allowed us to probe the stellar populations of this overdensity and of a control field within Fornax to a limiting magnitude of R=26. The color-magnitude diagram of the overdensity field is virtually identical to that of the control field with the exception of the presence of a population arising from a very short (less than 300 Myr in duration) burst of star formation 1.4 Gyr ago. Coleman et al. have argued that this overdensity might be related to a shell structure in Fornax that was created when Fornax captured a smaller galaxy. Our results are consistent with this model, but we argue that the metallicity of this young component favors a scenario in which the gas was part of Fornax itself.Comment: 24 pages including 8 figures and 3 tables. Accepted by Astronomical Journa

The Absence of Extra-Tidal Structure in the Sculptor Dwarf Spheroidal Galaxy

The results of a wide-field survey of the Sculptor dwarf spheroidal galaxy are presented. Our aims were to obtain an accurate map of the outer structure of Sculptor, and to determine the level of interaction between this system and the Galaxy. Photometry was obtained in two colours down to the magnitude limits of V=20 and I=19, covering a 3.1 times 3.1 square deg area centred on Sculptor. The resulting colour-magnitude data were used as a mask to select candidate horizontal branch and red giant branch stars for this system. Previous work has shown that the red horizontal branch (HB) stars are more concentrated than the blue HB stars. We have determined the radial distributions of these two populations and show that the overall Sculptor density profile is well described by a two component model based on a combination of these radial distributions. Additionally, spectra of the Ca ii triplet region were obtained for over 700 candidate red giant stars over the 10 square deg region using the 2dF instrument on the Anglo-Australian Telescope. These spectra were used to remove foreground Galactic stars based on radial velocity and Ca ii triplet strength. The final list of Sculptor members contained 148 stars, seven of which are located beyond the nominal tidal radius. Both the photometric and spectroscopic datasets indicate no significant extra-tidal structure. These results support at most a mild level of interaction between this system and the Galaxy, and we have measured an upper mass limit for extra-tidal material to be 2.3 +/- 0.6% of the Sculptor luminous mass. This lack of tidal interaction indicates that previous velocity dispersion measurements (and hence the amount of dark matter detected) in this system are not strongly influenced by the Galactic tidal field.Comment: 53 pages, 23 figures. Accepted for publication in the Astronomical Journal. Some figures are reduced in size, and a full version is available at: ftp://ftp.mso.anu.edu.au/pub/coleman/sculptor.pd