Clues to the Origin of the Mass-Metallicity Relation: Dependence on Star Formation Rate and Galaxy Size

We use a sample of 43,690 galaxies selected from the Sloan Digital Sky Survey Data Release 4 to study the systematic effects of specific star formation rate (SSFR) and galaxy size (as measured by the half light radius, r_h) on the mass-metallicity relation. We find that galaxies with high SSFR or large r_h for their stellar mass have systematically lower gas phase-metallicities (by up to 0.2 dex) than galaxies with low SSFR or small r_h. We discuss possible origins for these dependencies, including galactic winds/outflows, abundance gradients, environment and star formation rate efficiencies.Comment: Accepted by ApJ Letter

Galaxy pairs in the Sloan Digital Sky Survey - IX: Merger-induced AGN activity as traced by the Wide-field Infrared Survey Explorer

Interactions between galaxies are predicted to cause gas inflows that can potentially trigger nuclear activity. Since the inflowing material can obscure the central regions of interacting galaxies, a potential limitation of previous optical studies is that obscured Active Galactic Nuclei (AGNs) can be missed at various stages along the merger sequence. We present the first large mid-infrared study of AGNs in mergers and galaxy pairs, in order to quantify the incidence of obscured AGNs triggered by interactions. The sample consists of galaxy pairs and post-mergers drawn from the Sloan Digital Sky Survey that are matched to detections by the Wide Field Infrared Sky Explorer (WISE). We find that the fraction of AGN in the pairs, relative to a mass-, redshift- and environment-matched control sample, increases as a function of decreasing projected separation. This enhancement is most dramatic in the post-merger sample, where we find a factor of 10-20 excess in the AGN fraction compared with the control. Although this trend is in qualitative agreement with results based on optical AGN selection, the mid-infrared selected AGN excess increases much more dramatically in the post-mergers than is seen for optical AGN. Our results suggest that energetically dominant optically obscured AGNs become more prevalent in the most advanced mergers, consistent with theoretical predictions.Comment: 8 pages, 7 figures accepted to MNRAS (with minor revisions

Charles Dickens and Intellectual Disability

Throughout civilization, the power of the word has significantly influenced and shaped societies. The contributions of writers has been substantial and this is certainly true in the field of intellectual disability. The renowned author, Charles Dickens, spoke of the need for appropriate education and treatment for people with these disabilities. He is notable for his early and prophetic vision of their potential for growth. This paper reviews important examples that were included in his novels as well as in other writings on people with intellectual disabilities. The manuscript places his work in an historical perspective, highlights his contributions to the literature of disability advocacy, and references his relevance to the field of intellectual disability

The Millennium Galaxy Catalogue: The connection between close pairs and asymmetry; implications for the galaxy merger rate

We compare the use of galaxy asymmetry and pair proximity for measuring galaxy merger fractions and rates for a volume limited sample of 3184 galaxies with -21 < M(B) -5 log h < -18 mag. and 0.010 < z < 0.123 drawn from the Millennium Galaxy Catalogue. Our findings are that: (i) Galaxies in close pairs are generally more asymmetric than isolated galaxies and the degree of asymmetry increases for closer pairs. At least 35% of close pairs (with projected separation of less than 20 h^{-1} kpc and velocity difference of less than 500 km s^{-1}) show significant asymmetry and are therefore likely to be physically bound. (ii) Among asymmetric galaxies, we find that at least 80% are either interacting systems or merger remnants. However, a significant fraction of galaxies initially identified as asymmetric are contaminated by nearby stars or are fragmented by the source extraction algorithm. Merger rates calculated via asymmetry indices need careful attention in order to remove the above sources of contamination, but are very reliable once this is carried out. (iii) Close pairs and asymmetries represent two complementary methods of measuring the merger rate. Galaxies in close pairs identify future mergers, occurring within the dynamical friction timescale, while asymmetries are sensitive to the immediate pre-merger phase and identify remnants. (iv) The merger fraction derived via the close pair fraction and asymmetries is about 2% for a merger rate of (5.2 +- 1.0) 10^{-4} h^3 Mpc^{-3} Gyr^{-1}. These results are marginally consistent with theoretical simulations (depending on the merger time-scale), but imply a flat evolution of the merger rate with redshift up to z ~1.Comment: 10 pages, 10 figures, emulateapj format. ApJ, accepte

Bulge mass is king: The dominant role of the bulge in determining the fraction of passive galaxies in the Sloan Digital Sky Survey

We investigate the origin of galaxy bimodality by quantifying the relative role of intrinsic and environmental drivers to the cessation (or `quenching') of star formation in over half a million local Sloan Digital Sky Survey (SDSS) galaxies. Our sample contains a wide variety of galaxies at z=0.02-0.2, with stellar masses of 8 < log(M*/M_sun) < 12, spanning the entire morphological range from pure disks to spheroids, and over four orders of magnitude in local galaxy density and halo mass. We utilise published star formation rates and add to this recent GIM2D photometric and stellar mass bulge + disk decompositions from our group. We find that the passive fraction of galaxies increases steeply with stellar mass, halo mass, and bulge mass, with a less steep dependence on local galaxy density and bulge-to-total stellar mass ratio (B/T). At fixed internal properties, we find that central and satellite galaxies have different passive fraction relationships. For centrals, we conclude that there is less variation in the passive fraction at a fixed bulge mass, than for any other variable, including total stellar mass, halo mass, and B/T. This implies that the quenching mechanism must be most tightly coupled to the bulge. We argue that radio-mode AGN feedback offers the most plausible explanation of the observed trends.Comment: Accepted to MNRAS. 32 pages, 27 figures. [This version is virtually identical to v1

Monitoring groundwater temperatures in a shallow urban aquifer before, during and after installation of a Ground Source Heat System in Cardiff, U.K.

Exploitation of shallow urban aquifers, warmed by the Urban Heat Island Effect, is a relatively new concept in the U.K. An extensive groundwater temperature baseline monitoring network has been established for a shallow superficial aquifer in the city of Cardiff, U.K., to characterise groundwater temperatures and monitor the impacts of the first open-loop ground source heat pump (GSHP) installed in the city. In Spring 2014, temperature profiling was carried out at 1m depth intervals at 168 groundwater monitoring boreholes across Cardiff, establishing baseline groundwater temperatures within the shallow (<20m) superficial aquifer during the groundwater’s forecast coldest time of year. Data was contoured to form the first U.K. 2D city heat map. During the warmest time of year, Autumn 2014, a subset of boreholes were re-profiled to ascertain seasonal temperature variation, defining the Zone of Seasonal Fluctuation. Re-profiling was again carried out at these boreholes in Autumn 2015 to confirm these temperatures as normal for that time of year. By comparing Spring and Autumn profiles, the average depth to the base of the Zone of Seasonal Fluctuation was found to be 9.5mbgl. Two >100m boreholes showed the urban warming effect may extend to 80mbgl, before temperatures follow the predicted geothermal gradient. We term this the Zone of Anthropogenic Influence. After initial baseline temperatures were established, a site was selected for the installation of a shallow GSHP. Before installation work began, a monitoring network was set up to establish a temperature baseline for future GSHPs and identify any impacts on the thermal resource caused by removing ~2°C from the abstracted groundwater prior to reinjection into the aquifer. This comprised of 97 temperature loggers in 60 boreholes, including the abstraction and recharge boreholes and boreholes up and down gradient of the site. Some of these boreholes have multiple loggers at different depths, including the near-surface, but the majority of loggers were placed within the boreholes’ slotted sections, below the base of the Zone of Seasonal Fluctuation. In addition, six boreholes, including those used for the GSHP, have been telemetered, providing real-time temperature data. The aim of the monitoring network was to establish a baseline for groundwater temperatures in the shallow aquifer and to monitor local changes in temperatures close to the GSHP system. This study aimed to provide understanding of how GSHPs interact with the groundwater in order to confirm the sustainability of groundwater temperatures as a long-term thermal resource and provide planners with knowledge needed to develop sustainable wide-scale GSHP systems/networks. We present temperature data taken before and after installation