A Consistent Noncommutative Field Theory: the Wess-Zumino Model

We show that the noncommutative Wess-Zumino model is renormalizable to all orders of perturbation theory. The noncommutative scalar potential by itself is non-renormalizable but the Yukawa terms demanded by supersymmetry improve the situation turning the theory into a renormalizable one. As in the commutative case, there are neither quadratic nor linear divergences. Hence, the IR/UV mixing does not give rise to quadratic infrared poles.Comment: 22 pages, 7 figures. Added references. Typos correcte

The Sagittarius dwarf irregular galaxy: metallicity and stellar populations

We present deep $BVI$ observations of the dwarf irregular galaxy UKS1927-177 in Sagittarius. Statistically cleaned $V$, $(B-I)$ CMDs clearly display the key evolutionary features in this galaxy. Previously detected C stars are located in the CMDs and shown to be variable, thus confirming the presence of a significant upper-AGB intermediate age population. A group of likely red supergiants is also identified, whose magnitude and color is consistent with a 30 Myr old burst of star formation. The observed colors of both blue and red stars in SagDIG are best explained by introducing a differential reddening scenario in which internal dust extinction affects the star forming regions. Adopting a low reddening for the red giants, $E(B-V) = 0.07 \pm 0.02$, gives [Fe/H]=$-2.1 \pm 0.2$ for the mean stellar metallicity, a value consistent with the [O/H] abundance measured in the HII regions. This revised metallicity, which is in accord with the trend of metallicity against luminosity for dwarf irregular galaxies, is indicative of a ``normal'', although metal-poor, dIrr galaxy. A quantitative description is given of the spatial distribution of stars in different age intervals, in comparison with the distribution of the neutral hydrogen. We find that the youngest stars are located near the major peaks of emission on the HI shell, whereas the red giants and intermediate-age C stars define an extended halo or disk with scale length comparable to the size of the hydrogen cloud. The relationship between the distribution of ISM and star formation is briefly discussed.Comment: 10 pages, 7 figures, accepted A&

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 Extragalactic Distance Scale without Cepheids IV

The Cepheid period-luminosity relation is the primary distance indicator used in most determinations of the Hubble constant. The tip of the red giant branch (TRGB) is an alternative basis. Using the new ANU SkyMapper Telescope, we calibrate the Tully Fisher relation in the I band. We find that the TRGB and Cepheid distance scales are consistent.Comment: ApJ in press 201

Renormalization group approach to matrix models via noncommutative space

We develop a new renormalization group approach to the large-N limit of matrix models. It has been proposed that a procedure, in which a matrix model of size (N-1) \times (N-1) is obtained by integrating out one row and column of an N \times N matrix model, can be regarded as a renormalization group and that its fixed point reveals critical behavior in the large-N limit. We instead utilize the fuzzy sphere structure based on which we construct a new map (renormalization group) from N \times N matrix model to that of rank N-1. Our renormalization group has great advantage of being a nice analog of the standard renormalization group in field theory. It is naturally endowed with the concept of high/low energy, and consequently it is in a sense local and admits derivative expansions in the space of matrices. In construction we also find that our renormalization in general generates multi-trace operators, and that nonplanar diagrams yield a nonlocal operation on a matrix, whose action is to transport the matrix to the antipode on the sphere. Furthermore the noncommutativity of the fuzzy sphere is renormalized in our formalism. We then analyze our renormalization group equation, and Gaussian and nontrivial fixed points are found. We further clarify how to read off scaling dimensions from our renormalization group equation. Finally the critical exponent of the model of two-dimensional gravity based on our formalism is examined.Comment: 1+42 pages, 4 figure

Antlia Dwarf Galaxy: Distance, quantitative morphology and recent formation history via statistical field correction

We apply a statistical field correction technique originally designed to determine membership of high redshift galaxy clusters to Hubble Space Telescope imaging of the Antlia Dwarf Galaxy; a galaxy at the very edge of the Local Group. Using the tip of the red giant branch standard candle method coupled with a simple Sobel edge detection filter we find a new distance to Antlia of 1.31 +/- 0.03 Mpc. For the first time for a Local Group Member, we compute the concentration, asymmetry and clumpiness (CAS) quantitative morphology parameters for Antlia from the distribution of resolved stars in the HST/ACS field, corrected with a new method for contaminants and complement these parameters with the Gini coefficient (G) and the second order moment of the brightest 20 per cent of the flux (M_20). We show that it is a classic dwarf elliptical (C = 2.0, A = 0.063, S = 0.077, G = 0.39 and M_20 = -1.17 in the F814W band), but has an appreciable blue stellar population at its core, confirming on-going star-formation. The values of asymmetry and clumpiness, as well as Gini and M_20 are consistent with an undisturbed galaxy. Although our analysis suggests that Antlia may not be tidally influenced by NGC 3109 it does not necessarily preclude such interaction.Comment: Accepted for publication in MNRA

Determining the Location of the Tip of the Red Giant Branch in Old Stellar Populations: M33, Andromeda I & II

The absolute bolometric luminosity of the point of core Helium ignition in old, metal poor, red-giant stars is of roughly constant magnitude, varying only very slightly with mass or metallicity. It can thus be used as a standard candle. Here, we review the main difficulties in measuring this location in any real dataset and develop an empirical approach to optimise it for tip of the red giant branch (TRGB) analysis. We go on to present a new algorithm for the identification of the TRGB in nearby metal poor stellar systems. Our method uses a least-squares fit of a data-adaptive slope to the luminosity function in 1 magnitude windows. This finds the region of the luminosity function that shows the most significant decline in star counts as we go to brighter magnitudes; the base of this decline is attributed as the location of the tip. This technique then allows for the determination of realistic uncertainties which reflect the quality of the luminosity function used, but which are typically ~0.02 mags rms + ~ 0.03 mags systematic, a significant improvement upon previous methods that have used the tip as a standard candle. Finally, we apply our technique to the Local Group spiral galaxy M33 and the dwarf galaxies And I & II, and derive distance modulii of 24.50 +/- 0.06 (794 +/- 23 kpc), 24.33 +/- 0.07 (735 +/- 23 kpc) and 24.05 +/- 0.06 (645 +/- 19 kpc) respectively. The result for M33 is in excellent agreement with the Cepheid distances to this galaxy, and makes the possibility of a significant amount of reddening in this object unlikely.Comment: 12 pages, 10 figures (low resolution due to size constraints). Accepted for publication in MNRA

Anisotropy in the Hubble constant as observed in the HST Extragalactic Distance Scale Key Project results

Based on general relativity, it can be argued that deviations from a uniform Hubble flow should be thought of as variations in the Universe's expansion velocity field, rather than being thought of as peculiar velocities with respect to a uniformly expanding space. The aim of this paper is to use the observed motions of galaxies to map out variations in the Universe's expansion, and more importantly, to investigate whether real variations in the Hubble expansion are detectable given the observational uncertainties. All-sky maps of the observed variation in the expansion are produced using measurements obtained along specific lines-of-sight and smearing them across the sky using a Gaussian profile. A map is produced for the final results of the HST Extragalactic Distance Scale Key Project for the Hubble constant, a comparison map is produced from a set of essentially independent data, and Monte Carlo techniques are used to analyse the statistical significance of the variation in the maps. A statistically significant difference in expansion rate of 9 km/s/Mpc is found to occur across the sky. Comparing maps of the sky at different distances appears to indicate two distinct sets of extrema with even stronger statistically significant variations. Within our supercluster, variations tend to occur near the supergalactic plane, and beyond our supercluster, variations tend to occur away from the supergalactic plane. Comparison with bulk flow studies shows some concordance, yet also suggests the bulk flow studies may suffer confusion, failing to discern the influence of multiple perturbations.Comment: 23 pages, 5 figures, to be published in New Astronom

Distances and ages of globular clusters using Hipparcos parallaxes of local subdwarfs

We discuss the impact of Population II and Globular Cluster (GCs) stars on the derivation of the age of the Universe, and on the study of the formation and early evolution of galaxies, our own in particular. The long-standing problem of the actual distance scale to Population II stars and GCs is addressed, and a variety of different methods commonly used to derive distances to Population II stars are briefly reviewed. Emphasis is given to the discussion of distances and ages for GCs derived using Hipparcos parallaxes of local subdwarfs. Results obtained by different authors are slightly different, depending on different assumptions about metallicity scale, reddenings, and corrections for undetected binaries. These and other uncertainties present in the method are discussed. Finally, we outline progress expected in the near future.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 22 pages including 3 tables and 2 postscript figures, uses Kluwer's crckapb.sty LaTeX style file, enclose

Surface brightness fluctuation distances for nearby dwarf elliptical galaxies

We obtained B and R-band CCD images for the dwarf elliptical (dE) galaxies DDO44, UGC4998, KK98_77, DDO71, DDO113, and UGC7356 at the NOT. Using Fourier analysis technique we measure stellar R-band surface brightness fluctuations (SBFs) and magnitudes in 29 different fields of the galaxies. Independent tip of the red giant branch distances for DDO44, KK98_77, DDO71 are used to convert their set of apparent into absolute SBF magnitudes. The results are combined with the local (B-R) colours and compared with the (B-R)-\bar{M}_R relation for mainly old, metal-poor stellar populations as predicted by Worthey's population synthesis models using Padova isochrones. While the colour dependency of the theoretical relation is confirmed by the empirical data, we find a systematic zero point offset between observations and theory in the sense that models are too faint by 0.13+-0.02 mag. Based on these findings we establish a new semiempirical calibration of the SBF method as distance indicator for dE galaxies with an estimated uncertainty of ~10 percent. Taking first advantage of the improved calibration, we determine SBF distances for the other three early-type dwarfs. Although found in the M81 group region, previous velocity measurements suggested UGC4998 is in the background. This picture is confirmed by our SBF distance of 10.5+-0.9 Mpc. We can further identify DDO113 as a faint stellar system at the near side of the Canes Venatici I (CVn I) cloud at a distance of 3.1+-0.3 Mpc. The second CVn I member in our sample, UGC7356, lies at 6.7+-0.6 Mpc and spatially close to M106. We derive BR surface brightness profiles and colour gradients for all dwarfs and determine photometric and Sersic parameters. Finally, we discuss two objects in DDO71 and UGC7356 which may resemble GCs.Comment: 12 pages, 14 figures; Astronomy and Astrophysics in pres