Photometry using the Infrared Array Camera on the Spitzer Space Telescope

We present several corrections for point source photometry to be applied to data from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. These corrections are necessary because of characteristics of the IRAC arrays and optics and the way the instrument is calibrated in-flight. When these corrections are applied, it is possible to achieve a ~2% relative photometric accuracy for sources of adequate signal to noise in an IRAC image.Comment: 16 pages, 13 figures. Accepted for publication in the Publications of the Astronomical Society of the Pacifi

Ten Simple Rules for Digital Data Storage

Data is the central currency of science, but the nature of scientific data has changed dramatically with the rapid pace of technology. This change has led to the development of a wide variety of data formats, dataset sizes, data complexity, data use cases, and data sharing practices. Improvements in high throughput DNA sequencing, sustained institutional support for large sensor networks, and sky surveys with large-format digital cameras have created massive quantities of data. At the same time, the combination of increasingly diverse research teams and data aggregation in portals (e.g. for biodiversity data, GBIF or iDigBio) necessitates increased coordination among data collectors and institutions. As a consequence, “data” can now mean anything from petabytes of information stored in professionally-maintained databases, through spreadsheets on a single computer, to hand-written tables in lab notebooks on shelves. All remain important, but data curation practices must continue to keep pace with the changes brought about by new forms and practices of data collection and storage.</jats:p

Lensing in the Hercules Supercluster

We report Keck LRIS observations of an arc-like background galaxy near the center of Abell 2152 (z=0.043), one of the three clusters comprising the Hercules supercluster. The background object has a redshift z=0.1423 and is situated 25 arcsec north of the primary component of the A2152 brightest cluster galaxy (BCG). The object is about 15 arcsec in total length and has a reddening-corrected R-band magnitude of $m_R = 18.55\pm0.03$. Its spectrum shows numerous strong emission lines, as well as absorption features. The strength of the H-alpha emission would imply a star formation rate \SFR \approx 3h^{-2} \msun yr$^{-1}$ in the absence of any lensing. However, the curved shaped of this object and its tangential orientation along the major axis of the BCG suggest lensing. We model the A2152 core mass distribution including the two BCG components and the cluster potential. We present velocity and velocity dispersion profile measurements for the two BCG components and use these to help constrain the potential. The lens modeling indicates a likely magnification factor of $\sim1.9$ for the lensed galaxy, making A2152 the nearest cluster in which such significant lensing of a background source has been observed. Finally, we see evidence for a concentration of early-type galaxies at $z=0.13$ near the centroid of the X-ray emission previously attributed to A2152. We suggest that emission from this background concentration is the cause of the offset of the X-ray center from the A2152 BCG. The background concentration and the dispersed mass of the Hercules supercluster could add further to the lensing strength of the A2152 cluster.Comment: Accepted for publication in AJ (January 2001). 9 pages; uses emulateapj.sty. The all-important "Figure 1" is included here in GIF format; for a version which includes Figure 1 as a high-resolution Postscript image, see: http://adcam.pha.jhu.edu/~jpb/a2152.ps.g

The ACS Fornax Cluster Survey VII. Half-Light Radii of Globular Clusters in Early-Type Galaxies

We measure the half-light radii of globular clusters (GCs) in 43 galaxies from the ACS Fornax Cluster Survey (ACSFCS). We use these data to extend previous work in which the environmental dependencies of the half-light radii of GCs in early type galaxies in the ACS Virgo Cluster Survey (ACSVCS) were studied, and a corrected mean half-light radius (corrected for the observed environmental trends) was suggested as a reliable distance indicator. This work both increases the sample size for the study of the environmental dependencies, and adds leverage to the study of the corrected half-light radius as a possible distance indicator (since Fornax lies at a larger distance than the Virgo cluster). We study the environmental dependencies of the size of GCs using both a Principal Component Analysis as well as 2D scaling relations. We largely confirm the environmental dependencies shown in Jordan et al. (2005), but find evidence that there is a residual correlation in the mean half-light radius of GC systems with galaxy magnitude, and subtle differences in the other correlations - so there may not be a universal correction for the half-light radii of lower luminosity galaxy GC systems. The main factor determining the size of a GC in an early type galaxy is the GC color. Red GCs have = 2.8+/-0.3 pc, while blue GCs have = 3.4+/-0.3 pc. We show that for bright early-type galaxies (M_B < -19 mag), the uncorrected mean half-light radius of the GC system is by itself an excellent distance indicator (with error ~11%), having the potential to reach cosmologically interesting distances in the era of high angular resolution adaptive optics on large optical telescopes.Comment: ApJ in press, 19 pages, 16 figures

Improving Teacher Recruitment and Retention: The Importance of workload and Pupil Behaviour

The shortage of teachers in England and Wales continues to be a high profile area of scrutiny. Particular subjects (including mathematics; science and English) are categorized by the Training and Development Agency (TDA)for schools as priority or shortage subjects, and London especially has experienced particular shortages in teacher numbers over recent years

Probing the Intermediate-Age Globular Clusters in NGC 5128 from Ultraviolet Observations

We explore the age distribution of the globular cluster (GC) system of the nearby elliptical galaxy NGC 5128 using ultraviolet (UV) photometry from Galaxy Evolution Explorer (GALEX) observations, with UV - optical colors used as the age indicator. Most GCs in NGC 5128 follow the general trends of GCs in M31 and Milky Way in UV - optical color-color diagram, which indicates that the majority of GCs in NGC 5128 are old similar to the age range of old GCs in M31 and Milky Way. A large fraction of spectroscopically identified intermediate-age GC (IAGC) candidates with ~ 3-8 Gyr are not detected in the FUV passband. Considering the nature of intermediate-age populations being faint in the far-UV (FUV) passband, we suggest that many of the spectroscopically identified IAGCs may be truly intermediate in age. This is in contrast to the case of M31 where a large fraction of spectroscopically suggested IAGCs are detected in FUV and therefore may not be genuine IAGCs but rather older GCs with developed blue horizontal branch stars. Our UV photometry strengthens the results previously suggesting the presence of GC and stellar subpopulation with intermediate age in NGC 5128. The existence of IAGCs strongly indicates the occurrence of at least one more major star formation episode after a starburst at high redshift.Comment: 8 pages, 3 figures, accepted for ApJ Lette

The remnants of galaxy formation from a panoramic survey of the region around M31

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach $10 - 100$ times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31's satellites brighter than $M_V < -6$ await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.Comment: Published in Nature. Supplementary movie available at https://www.astrosci.ca/users/alan/PANDAS/Latest%20news%3A%20movie%20of%20orbit.htm

Reconstructing Galaxy Histories from Globular Clusters

Nearly a century after the true nature of galaxies as distant "island universes" was established, their origin and evolution remain great unsolved problems of modern astrophysics. One of the most promising ways to investigate galaxy formation is to study the ubiquitous globular star clusters that surround most galaxies. Recent advances in our understanding of the globular cluster systems of the Milky Way and other galaxies point to a complex picture of galaxy genesis driven by cannibalism, collisions, bursts of star formation and other tumultuous events.Comment: Review Article published in Nature, 1 January 2004. 18 pages, 4 figures, pdf format onl