ALBERT

Description: In many applications, independence of event occurrences is assumed, even if there is evidence for dependence. Capturing dependence leads to complex models, and even if the complex models were superior, they fail to beat the simplicity and scalability of the independence assumption. Therefore, many models assume independence and apply heuristics to improve results. Theoretical explanations of the heuristics are seldom given or generalizable. This paper reports that some of these heuristics can be explained as encoding dependence in an exponent based on the generalized harmonic sum . Unlike independence, where the probability of subsequent occurrences of an event is the product of the single event probability, harmony is based on a product with decaying exponent. For independence, the sequence probability is $p^{1+1+ \cdots +1}=p^n$ , whereas for harmony, it is $p^{1+1/2+ \cdots +1/n}$ . The generalized harmonic sum leads to a spectrum of harmony assumptions . This paper shows that harmony assumptions naturally extend probability theory. An experimental evaluation for information retrieval (IR; term occurrences) and social networks (SN's; user interactions) shows that assuming harmony is more suitable than assuming independence. The potential impact of harmony assumptions lies beyond IR and SN's, since many applications rely on probability theory and apply heuristics to compensate the independence assumption. Given the concept of harmony assumptions, the dependence between multiple occurrences of an event can be reflected in an intuitive and effective way.

Description: We update the treatment of chemical evolution in the Munich semi-analytic model, L-G alaxies . Our new implementation includes delayed enrichment from stellar winds, Type II supernovae (SNe-II) and Type Ia supernovae (SNe-Ia), as well as metallicity-dependent yields and a reformulation of the associated supernova feedback. Two different sets of SN-II yields and three different SN-Ia delay-time distributions (DTDs) are considered, and 11 heavy elements (including O, Mg and Fe) are self-consistently tracked. We compare the results of this new implementation with data on (a) local, star-forming galaxies, (b) Milky Way disc G dwarfs and (c) local, elliptical galaxies. We find that the z  = 0 gas-phase mass–metallicity relation is very well reproduced for all forms of DTD considered, as is the [Fe/H] distribution in the Milky Way disc. The [O/Fe] distribution in the Milky Way disc is best reproduced when using a DTD with ≤50 per cent of SNe-Ia exploding within ~400 Myr. Positive slopes in the mass–[α/Fe] relations of local ellipticals are also obtained when using a DTD with such a minor ‘prompt’ component. Alternatively, metal-rich winds that drive light α elements directly out into the circumgalactic medium also produce positive slopes for all forms of DTD and SN-II yields considered. Overall, we find that the best model for matching the wide range of observational data considered here should include a power-law SN-Ia DTD, SN-II yields that take account of prior mass-loss through stellar winds and some direct ejection of light α elements out of galaxies.

Description: We investigate whether dilution in some elliptical galaxies is the cause of a positive correlation between specific star formation rate (sSFR) and gas-phase metallicity ( Z g ) at high stellar mass in the local Universe. In the Munich semi-analytic model of galaxy formation, L-GALAXIES, massive, low-sSFR, elliptical galaxies are seen to undergo a gradual dilution of their interstellar medium, via accretion of metal-poor gas in cold-gas clumps and low-mass satellites. This occurs after a merger-induced starburst and the associated supernova feedback have quenched most of the original gas reservoir. Signatures of this evolution are present in these model galaxies at z  = 0, including low gas fractions, large central black holes, old ages, and importantly, low ( Z g - Z * ). Remarkably, all of these properties are also found in massive, low-sSFR, elliptical galaxies in the sloan digital sky survey data release 7 (SDSS-DR7). This provides strong, indirect evidence that gradual dilution is also occurring in nearby ellipticals in the real Universe. This scenario provides an explanation for the positive correlation between SFR and Z g measured in high- M * galaxies, and therefore has consequences for the local fundamental metallicity relation, which assumes a weak anticorrelation above ~10 10.5 M .

Description: We have updated our radially resolved semi-analytic models (SAMs) of galaxy formation, which track both the atomic and molecular gas phases of the interstellar medium. The models are adapted from those of Guo et al. using similar methodology as by Fu et al. and are run on halo merger trees from the Millennium and Millennium-II simulations with the following main changes. (1) We adopt a simple star formation law SFR H 2 . (2) We inject the heavy elements produced by supernovae directly into the halo hot gas, instead of first mixing them with the cold gas in the disc. (3) We include radial gas inflows in discs using a model of the form v inflow  = α r . The models are used to study the radial profiles of star formation rate and gas-phase metallicity in present-day galaxies. The surface density profiles of molecular gas in L * galaxies place strong constraints on inflow velocities, favouring models where v inflow  ~ 7 km s –1 at a galactocentric radius of 10 kpc. Radial gas inflow has little influence on gas-phase and stellar metallicity gradients, which are affected much more strongly by the fraction of metals that are directly injected into the halo gas, rather than mixed with the cold gas. Metals ejected out of the galaxy in early epochs result in late infall of pre-enriched gas and flatter present-day gas-phase metallicity gradients. A prescription in which 80 per cent of the metals are injected into the halo gas results in good fits to the flat observed metallicity gradients in galaxies with stellar masses greater than 10 10 M , as well as the relations between gas-phase metallicity and specific star formation rate in the outer parts of galactic discs. We examine the correlation between the gas-phase metallicity gradient and global galaxy properties, finding that it is most strongly correlated with the bulge-to-total ratio of the galaxy. This is because gas is consumed when the bulge forms during galaxy mergers, and the gas-phase metallicity gradient is then set by newly accreted gas.

Description: We present an analysis of the iron abundance in the hot gas surrounding galaxy groups and clusters. To do this, we first compile and homogenize a large data set of 79 low-redshift ( $\tilde{z} = 0.03$ ) systems (159 individual measurements) from the literature. Our analysis accounts for differences in aperture size, solar abundance, and cosmology, and scales all measurements using customized radial profiles for the temperature ( T ), gas density ( gas ), and iron abundance ( Z Fe ). We then compare this data set to groups and clusters in the L-G alaxies galaxy evolution model. Our homogenized data set reveals a tight T – Z Fe relation for clusters, with a scatter in Z Fe of only 0.10 dex and a slight negative gradient. After examining potential measurement biases, we conclude that some of this negative gradient has a physical origin. Our model suggests greater accretion of hydrogen in the hottest systems, via stripping from infalling satellites, as a cause. In groups, L-G alaxies over-estimates Z Fe , indicating that metal-rich gas removal (via e.g. AGN feedback) is required. L-G alaxies is consistent with the observed Z Fe in the intracluster medium (ICM) of the hottest clusters at z = 0, and shows a similar rate of ICM enrichment as that observed from at least z ~ 1.3 to the present day. This is achieved without needing to modify any of the galactic chemical evolution (GCE) model parameters. However, the Z Fe in intermediate- T clusters could be under-estimated in our model. We caution that modifications to the GCE modelling to correct this disrupt the agreement with observations of galaxies’ stellar components.

Description: We report on the detection of a bright, short, structured X-ray burst coming from the supernova remnant RCW 103 on 2016 June 22 caught by the Swift /Burst Alert Telescope (BAT) monitor, and on the follow-up campaign made with Swift /X-ray Telescope, Swift /UV/Optical Telescope, and the optical/near-infrared (NIR) Gamma-Ray burst Optical and Near-infrared Detector. The characteristics of this flash, such as duration and spectral shape, are consistent with typical short bursts observed from soft gamma repeaters. The BAT error circle at 68 per cent confidence range encloses the point-like X-ray source at the centre of the nebula, 1E 161348–5055. Its nature has been long debated due to a periodicity of 6.67 h in X-rays, which could indicate either an extremely slow pulsating neutron star, or the orbital period of a very compact X-ray binary system. We found that 20 min before the BAT trigger, the soft X-ray emission of 1E 161348–5055 was a factor of ~100 higher than measured 2 yr earlier, indicating that an outburst had already started. By comparing the spectral and timing characteristics of the source in the 2 yr before the outburst and after the BAT event, we find that, besides a change in luminosity and spectral shape, also the 6.67 h pulsed profile has significantly changed with a clear phase shift with respect to its low-flux profile. The UV/optical/NIR observations did not reveal any counterpart at the position of 1E 161348–5055. Based on these findings, we associate the BAT burst with 1E 161348–5055, we classify it as a magnetar, and pinpoint the 6.67 h periodicity as the magnetar spin period.