Hierarchical Structure Formation and Modes of Star Formation in Hickson Compact Group 31

The handful of low-mass, late-type galaxies that comprise Hickson Compact Group 31 is in the midst of complex, ongoing gravitational interactions, evocative of the process of hierarchical structure formation at higher redshifts. With sensitive, multicolor Hubble Space Telescope imaging, we characterize the large population of <10 Myr old star clusters that suffuse the system. From the colors and luminosities of the young star clusters, we find that the galaxies in HCG 31 follow the same universal scaling relations as actively star-forming galaxies in the local Universe despite the unusual compact group environment. Furthermore, the specific frequency of the globular cluster system is consistent with the low end of galaxies of comparable masses locally. This, combined with the large mass of neutral hydrogen and tight constraints on the amount of intragroup light, indicate that the group is undergoing its first epoch of interaction-induced star formation. In both the main galaxies and the tidal-dwarf candidate, F, stellar complexes, which are sensitive to the magnitude of disk turbulence, have both sizes and masses more characteristic of z=1-2 galaxies. After subtracting the light from compact sources, we find no evidence for an underlying old stellar population in F -- it appears to be a truly new structure. The low velocity dispersion of the system components, available reservoir of HI, and current star formation rate of ~10 solar masses per year, indicate that HCG31 is likely to both exhaust its cold gas supply and merge within ~1 Gyr. We conclude that the end product will be an isolated, X-ray-faint, low-mass elliptical.Comment: 24 pages, 14 figures (including low resolution versions of color images), latex file prepared with emulateapj. Accepted for publication by the Astronomical Journa

Germline DNA Repair Gene Mutations in Young-onset Prostate Cancer Cases in the UK: Evidence for a More Extensive Genetic Panel

Background Rare germline mutations in DNA repair genes are associated with prostate cancer (PCa) predisposition and prognosis. Objective To quantify the frequency of germline DNA repair gene mutations in UK PCa cases and controls, in order to more comprehensively evaluate the contribution of individual genes to overall PCa risk and likelihood of aggressive disease. Design, setting, and participants We sequenced 167 DNA repair and eight PCa candidate genes in a UK-based cohort of 1281 young-onset PCa cases (diagnosed at ≤60 yr) and 1160 selected controls. Outcome measurements and statistical analysis Gene-level SKAT-O and gene-set adaptive combination of p values (ADA) analyses were performed separately for cases versus controls, and aggressive (Gleason score ≥8, n = 201) versus nonaggressive (Gleason score ≤7, n = 1048) cases. Results and limitations We identified 233 unique protein truncating variants (PTVs) with minor allele frequency <0.5% in controls in 97 genes. The total proportion of PTV carriers was higher in cases than in controls (15% vs 12%, odds ratio [OR] = 1.29, 95% confidence interval [CI] 1.01–1.64, p = 0.036). Gene-level analyses selected NBN (pSKAT-O = 2.4 × 10−4) for overall risk and XPC (pSKAT-O = 1.6 × 10−4) for aggressive disease, both at candidate-level significance (p < 3.1 × 10−4 and p < 3.4 × 10−4, respectively). Gene-set analysis identified a subset of 20 genes associated with increased PCa risk (OR = 3.2, 95% CI 2.1–4.8, pADA = 4.1 × 10−3) and four genes that increased risk of aggressive disease (OR = 11.2, 95% CI 4.6–27.7, pADA = 5.6 × 10−3), three of which overlap the predisposition gene set. Conclusions The union of the gene-level and gene-set-level analyses identified 23 unique DNA repair genes associated with PCa predisposition or risk of aggressive disease. These findings will help facilitate the development of a PCa-specific sequencing panel with both predictive and prognostic potential. Patient summary This large sequencing study assessed the rate of inherited DNA repair gene mutations between prostate cancer patients and disease-free men. A panel of 23 genes was identified, which may improve risk prediction or treatment pathways in future clinical practice

The Extended Main-Sequence Turn-off Clusters of the Large Magellanic Cloud - Missing links in Globular Cluster Evolution

Recent observations of intermediate age (1 - 3 Gyr) massive star clusters in the Large Magellanic Cloud (LMC) have revealed that the majority possess bifurcated or extended main-sequence turn-off (EMSTO) morphologies. This effect can be understood to arise from subsequent star formation amongst the stellar population with age differences between constituent stars amounting to 50 - 300 Myr. Age spreads of this order are similarly invoked to explain the light element abundance variations witnessed in ancient globular clusters. In this paper we explore the proposition that the clusters exhibiting the EMSTO phenomenon are a general phase in the evolution of massive clusters, one that naturally leads to the particular chemical properties of the ancient globular cluster population. We show that the isolation of EMSTO clusters to intermediate ages is the consequence of observational selection effects. In our proposed scenario, the EMSTO phenomenon is identical to that which establishes the light element abundance variations that are ubiquitous in the ancient globular cluster population. Our scenario makes a strong prediction: EMSTO clusters will exhibit abundance variations in the light elements characteristic of the ancient GC population.Comment: ApJ accepted. 33 pages, 5 figure

Optimizing multifunctional agroecosystems in irrigated dryland agriculture to restore soil carbon – experiments and model

Advances in dryland irrigated agriculture can develop into sustainable multifunctional agroecosystems if land use and soil management are optimized to enhance soil organic carbon (SOC). We hypothesized that combining high inputs of plant-derived carbon with no-tillage will increase long-term SOC stocks in this ecosystem. We evaluated data from two field experiments under a mango orchard and melon crops, respectively. Both were managed with two tillage systems, no-till (NT) and conventional till (CT), and combined with three types of intercropping. Plant mixtures used 75% legumes + 25% grass and oilseed species (PM1), 25% legumes + 75% grass and oilseed species (PM2), and spontaneous vegetation (SV). The Caatinga shrubland was cleared in 1972 for use of arable (AC) and palm plantation (PP). The carbon turnover model Roth C was used to discriminate the effects of the cropping and tillage systems on SOC and to predict the impacts on the long-term dynamic of SOC. Plant-derived carbon, including roots, exudates, and aboveground residues, add on average 5.23 Mg ha-1yr-1, increasing SOC stocks between 0.23 and 0.42 Mg ha-1yr-1. If the current designs of agroecosystems will be maintained, the value of soil carbon stock equal from dry forest steady-state will be reached in about 20 years for annual crop and 30 years for the perennial crop. Given that agriculture in dryland areas is recognized as key mediators of soil carbon storage decline, our results warrant further efforts to increase carbon storage in irrigated multifunctional agroecosystems designs that include plant mixtures and no-till in semiarid regions

Star Cluster collisions - a formation scenario for the Extended Globular Cluster Scl-dE1 GC1

Recent observations of the dwarf elliptical galaxy Scl-dE1 (Sc22) in the Sculptor group of galaxies revealed an extended globular cluster (Scl-dE1 GC1), which exhibits an extremely large core radius of about 21.2 pc. The authors of the discovery paper speculated on whether this object could reside in its own dark matter halo and/or if it might have formed through the merging of two or more star clusters. In this paper, we present N-body simulations to explore thoroughly this particular formation scenario. We follow the merger of two star clusters within dark matter haloes of a range of masses (as well as in the absence of a dark matter halo). In order to obtain a remnant which resembles the observed extended star cluster, we find that the star formation efficiency has to be quite high (around 33 per cent) and the dark matter halo, if present at all, has to be of very low mass, i.e. raising the mass to light ratio of the object within the body of the stellar distribution by at most a factor of a few. We also find that expansion of a single star cluster following mass loss provides another viable formation path. Finally, we show that future measurements of the velocity dispersion of this system may be able to distinguish between the various scenarios we have explored.Comment: accepted by MNRAS, 9 pages, 2 figures, 9 table

A neutral hydrogen survey of polar ring galaxies IV. Parkes observations

A total of 33 polar ring galaxies and polar ring galaxy candidates were observed in the 21-cm HI line with the 64-m Parkes radio telescope. The objects, selected by their optical morphology, are all south of declination -39 degrees and in only 5 of them HI had been reported previously. HI line emission was detected towards 18 objects, though in 3 cases the detection may be confused by another galaxy in the telescope beam, and one is a marginal detection. Eight objects were detected for the first time in HI, of which 5 did not have previously known redshifts.Comment: Accepted by Astronomy and Astrophysics, 12/2/2002; new references added on 20/2/200

Age Determination of Six Intermediate-age SMC Star Clusters with HST/ACS

We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC339, NGC416, Lindsay 38, and NGC419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color magnitude diagrams (CMDs) extend ~3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC419 where the Padova isochrones provided the best fit. The CMD of NGC419 shows several main-sequence turn-offs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC419. Interestingly, our intermediate-age star clusters have a metallicity spread of ~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally concentrated blue straggler star candidates in NGC416, while for the other clusters these are not present. Using the red clump magnitudes, we find that the closest cluster, NGC419 (~50kpc), and the farthest cluster, Lindsay 38 (~67kpc), have a relative distance of ~17kpc, which confirms the large depth of the SMC.Comment: 25 pages, 45 Figure

The ACS Nearby Galaxy Survey Treasury. X. Quantifying the Star Cluster Formation Efficiency of Nearby Dwarf Galaxies

We study the relationship between the field star formation and cluster formation properties in a large sample of nearby dwarf galaxies. We use optical data from the Hubble Space Telescope and from ground-based telescopes to derive the ages and masses of the young (t_age < 100Myr) cluster sample. Our data provides the first constraints on two proposed relationships between the star formation rate of galaxies and the properties of their cluster systems in the low star formation rate regime. The data show broad agreement with these relationships, but significant galaxy-to-galaxy scatter exists. In part, this scatter can be accounted for by simulating the small number of clusters detected from stochastically sampling the cluster mass function. However, this stochasticity does not fully account for the observed scatter in our data suggesting there may be true variations in the fraction of stars formed in clusters in dwarf galaxies. Comparison of the cluster formation and the brightest cluster in our sample galaxies also provide constraints on cluster destruction models.Comment: 16 pages, 9 figures, Accepted to Ap

Star Cluster Formation and Disruption Time-Scales -- I. An empirical determination of the disruption time of star clusters in four galaxies

We present a new method to derive the cluster disruption time in selected regions of galaxies from the mass or age distribution of magnitude-limited cluster samples. If the disruption time of clusters in a region of a galaxy depends on their initial mass as t_4 x (M_cluster/10^4 M_sun)^gamma and if the cluster formation rate is constant, then the mass and age distributions of the observed clusters will each show two powerlaw relations. The values of t_4 and gamma can be derived from these relations. We used this method to derive the cluster disruption time in specific regions in four galaxies: the inner region of M51, a region of M33, the SMC and the solar neighbourhood. The values of gamma are the same in the four galaxies within the uncertainty and the mean value is gamma= 0.62 +- 0.06. However the disruption time t_4 of a cluster of 10^4 M_sun is very different in the different galaxies. The clusters in the SMC have the longest disruption time, t_4 = 8 Gyr, and the clusters at 1 to 3 kpc from the nucleus of M51 have the shortest disruption time of t_4 = 0.04 Gyr. The disruption time of clusters 1 to 5 kpc from the nucleus of M33 is t_4 = 0.13 Gyr and for clusters within 1 kpc from the Sun we find t_4 = 1.0 Gyr.Comment: 18 pages, 18 figures. Accepted for publication by Monthly Notice

The properties, origin and evolution of stellar clusters in galaxy simulations and observations

Published onlineWe investigate the properties and evolution of star particles in two simulations of isolated spiral galaxies, and two galaxies from cosmological simulations. Unlike previous numerical work, where typically each star particle represents one ‘cluster’, for the isolated galaxies we are able to model features we term ‘clusters’ with groups of particles. We compute the spatial distribution of stars with different ages, and cluster mass distributions, comparing our findings with observations including the recent LEGUS survey. We find that spiral structure tends to be present in older (100s Myr) stars and clusters in the simulations compared to the observations. This likely reflects differences in the numbers of stars or clusters, the strength of spiral arms, and whether the clusters are allowed to evolve. Where we model clusters with multiple particles, we are able to study their evolution. The evolution of simulated clusters tends to follow that of their natal gas clouds. Massive, dense, long-lived clouds host massive clusters, whilst short-lived clouds host smaller clusters which readily disperse. Most clusters appear to disperse fairly quickly, in basic agreement with observational findings. We note that embedded clusters may be less inclined to disperse in simulations in a galactic environment with continuous accretion of gas on to the clouds than isolated clouds and correspondingly, massive young clusters which are no longer associated with gas tend not to occur in the simulations. Caveats of our models include that the cluster densities are lower than realistic clusters, and the simplistic implementation of stellar feedback.We thank the referee for a useful report. The calculations for this paper were performed primarily on the DiRAC machine ‘Complexity’, as well as the supercomputer at Exeter, which is jointly funded by STFC, the Large Facilities Capital Fund of BIS, and the University of Exeter. We would like to thank Michele Fumagalli for work putting together the LEGUS cluster catalogues. CLD and CGF acknowledge funding from the European Research Council for the FP7 ERC starting grant project LOCALSTAR. CGF thanks Ben Thompson for performing data reduction. DG kindly acknowledges financial support by the German Research Foundation (DFG) through grant GO 1659/3-2. Figures in this paper were produced using splash (Price 2007)