Halos Of Nearby Galaxies Research Articles

CALCIUM-RICH GAP TRANSIENTS: SOLVING THE CALCIUM CONUNDRUM IN THE INTRACLUSTER MEDIUM

X-ray measurements suggest the abundance of Calcium in the intracluster medium is higher than can be explained using favored models for core-collapse and Type Ia supernovae alone. We investigate whether the Calcium conundrum in the intracluster medium can be alleviated by including a contribution from the recently discovered subclass of supernovae known as Calcium-rich gap transients. Although the Calcium-rich gap transients make up only a small fraction of all supernovae events, we find that their high Calcium yields are sufficient to reproduce the X-ray measurements found for nearby rich clusters. We find the $\chi^{2}$ goodness-of-fit metric improves from 84 to 2 by including this new class. Moreover, Calcium-rich supernovae preferentially occur in the outskirts of galaxies making it easier for the nucleosynthesis products of these events to be incorporated in the intracluster medium via ram-pressure stripping. The discovery of a Calcium-rich gap transients in clusters and groups far from any individual galaxy suggests supernovae associated with intracluster stars may play an important role in enriching the intracluster medium. Calcium-rich gap transients may also help explain anomalous Calcium abundances in many other astrophysical systems including individual stars in the Milky Way, the halos of nearby galaxies and the circumgalactic medium. Our work highlights the importance of considering the diversity of supernovae types and corresponding yields when modeling the abundance of the intracluster medium and other gas reservoirs.

The WFPC2 Archival Pure Parallels Project

ABSTRACTWe introduce the WFPC2 Archival Pure Parallels Project, in which the methods and procedures described here are used to obtain a near‐automatic combination of WFPC2 images obtained as part of the WFPC2 Archival Parallels Program. Several techniques have been developed or refined to ensure proper alignment, registration, and combination of overlapping images that can be obtained at different times and with different orientations. We quantify the success rate and the accuracy of the registration of different types of images, and we develop techniques that are suitable to equalize the sky background without unduly affecting extended emission. About 600 combined images of the 1500 eventually planned have already been publicly released through the STScI Archive. The images released to date are especially suited to the study of star formation in the Magellanic Clouds, the stellar population in the halo of nearby galaxies, and the properties of star‐forming galaxies at z∼3.

Dark matter annihilation in the local group

Under the hypothesis of a dark matter composed by supersymmetric particles such as neutralinos, we investigate the possibility that their annihilation in the halos of nearby galaxies could produce detectable fluxes of $\ensuremath{\gamma}$ photons. Expected fluxes depend on several, poorly known quantities such as the density profiles of dark matter halos, the existence and prominence of central density cusps and the presence of a population of subhalos. We find that, for all reasonable choices of dark matter halo models, the intensity of the \ensuremath{\gamma}-ray flux from some of the nearest extragalactic objects, such as M31, is comparable to or higher than the diffuse galactic foreground. We show that next generation ground-based experiments could have the sensitivity to reveal such fluxes which could help us to unveil the nature of dark matter particles.

The analysis of red giant branch photometry in galaxies

We present an analysis of the many possible methods for simulating and analysing colour–magnitude diagrams, with application to studies of the field halo stellar populations of resolved galaxies. Special consideration is given to the analysis of stars on the red giant branch, and methods for obtaining metallicity distributions for old populations based on analysis of evolved sections of the colour–magnitude diagram. These tools are designed to provide a reliable and accurate method for the analysis of resolved Population II stars in the haloes of nearby galaxies. In particular, we introduce a perpendicular distance method for calculating most likely source isochrones for stars in our input data set. This method is shown to have several advantages over a more traditional approach considering the isochrone as a set of finely spaced Gaussian probability distributions. We also consider methods by which the obtained metallicity distribution might be most efficiently optimized and especially evaluate the difficulties involved in avoiding sub-optimal local maxima in the likelihood maximization procedure.

Properties of quasar-galaxy associations and gravitational mesolensing by halo objects

A new catalog of 8382 close quasar-galaxy pairs is presented. The catalog was composed using published catalogs of quasars and active galactic nuclei containing 11358 objects, as well as the LEDA catalog of galaxies, which contains on the order of 100 thousand objects. Based on these new data, the dependence of the number of pairs on a=zG/zQ is analyzed, where zG and zQ are the redshifts of the galaxy and quasar, respectively, revealing an excess of pairs with a 0.9. This means that the galaxies in pairs are preferably located close to either the observer or the quasar and avoid intermediate distances along the line of sight to the quasar. Computer simulations demonstrate that it is not possible to explain this number of pairs with the observed distribution in a as the result of chance positional coincidences with a uniform spatial distribution of galaxies. Data on globular clusters show that the excess of pairs with a 0.9 is consistent with the hypothesis that we are observing distant compact objects that are strongly gravitationally lensed by transparent lenses with a King mass distribution located in the halos of nearby galaxies. The Hubble diagram for galaxies and quasars is presented. Observational tests of the mesolensing hypothesis are formulated.