1306.3218
Black hole variability and the star formation-AGN connection: do all star-forming galaxies host an AGN?
Hickox, et al
AGNs expected to vary significantly on a wide range of timescales (hours to Myr) that are far shorted than the typical timescale for SF (>~100 Myr). Present a simple model in which all SF galaxies host an AGN when averaged over ~100 Myr timescales, with long-term average AGN accretion rates that are perfectly correlated with the SFR. Show that reasonable prescriptions for AGN variability reproduce the observed weak correlations between SFR and L_AGN in typical AGN host galaxies, as well as the general trends in the observed AGN luminosity functions, merger fractions, and measurements of the average AGN luminosity as a function of SFR. These results imply there may be a tight connection between AGN activity and SFR over galaxy evolution timescales, and that the apparent similarities in rest-frame colors, merger rates, and clustering of AGNs compared to "inactive" galaxies may be due primarily to AGN variability. The results provide motivation for future deep, wide extragalactic surveys that can measure the distribution of AGN accretion rates a s function of SFR.
1306.3264
The intrinsic shape of galaxies in SDSS/Galaxy zoo
Rodriquez, Padilla
MOdel the axis ratio distributions of DR8 galaxies, find the intrinsic 3D shapes of spirals and ellipticals. Use morphological info from Galaxy Zoo project, assume a non-parameteric distribution intrinsic of shapes, while taking into account dust extinction. Measure the dust extinction of the full sample of spiral galaxies and find a smaller value than previous estimations, with an edge-on extinction of E_0=0.28pm0.02 in the SDSS r band. Also find that the distribution of minor to major axis ratio has a mean value of 0.267 pm 0.01, slightly larger than previous estimates mainly due to the lower extinctions used; the same affects the circularity of galactic disks, which are found to be less round in shape than in previous studies, with a mean ellipticity of 0.215 pm 0.013. For elliptical galaxies, find that the minor to major axis ratio, with a mean value of .584 pm 0.006 is larger than previous estimations due to the removal of spiral interlopers present in samples with morphological information from photometric profiles. These interlopers are removed when selecting ellipticals using GZ data. Find that the bright elliptical galaxies are more spherical than faint ones, a trend that is also present with galaxy size, and that there is no dependence of elliptical galaxy shape with color. For spiral galaxies find that the reddest ones have higher dust extinction as expected, due to the fact that this reddening is mainly due to dust. Also find that the thickness of disks increases with luminosity and size, and that brighter, smaller and redder galaxies have less round discs.
1306.3266
Water loss from terrestrial planets with CO2-rich atmospheres
Wordsworth, Pierrehumbert
Water photolysis and hydrogen loss from the upper atmospheres of terrestrial planets is of fundamental importance to climate evolution but remains poorly understood in general. Present a range of calculations to study the dependence of water loss rates from terrestiral planets on atmospheric composition (CO2 and N2 levels), planetary mass, and external parameters (stellar spectrum, orbital distance and impacts). From coupled 1d climate and escape modeling, show that CO2 can only cause significant water loss by increasing surface temperatures over a narrow range of conditions, with cooling of the middle and upper atmosphere acting as a bottleneck on escape in other circumstances. Around G-stars, efficient loss only occurs on planets with intermediate CO2 atmospheric partial pressures (0.1 to 1 bar) that receive a net flux close to the critical runaway greenhouse limit. Because G-star total luminosity increases with time but XUV/UV luminosity decreases, this places strong limits on moist stratosphere H2O photolysis for planets like Earth. In contrast, for a CO2-rich early Venus, diffusion limits on water loss are only important if clouds had a strong negative forcing (delta F < -70 W/m^2 vs. clear-sky case of albedo >0.8) [?], implying that scenarios where the planet never had surface liquid water are indeed plausible. Around M-stars, the lack of temporal luminosity variation means water loss is primarily a function of orbital distance, with planets that absorb less flux than ~270 W/m^2 (global mean) unlikely to lose more than one Earth ocean of H2O over their lifetimes. Because of the variability of H2O delivery to planetesimals during accretion, results suggest that many 'Earth-like' exoplanets in the habitable zone may have ocean-covered surfaces, stable CO2/H2O-rich atmospheres, and high mean surface temperatures.
1306.3387
Measuring the cooling of the neutron star in Cassiopeia A with all Chandra X-ray Observatory detectors
Elshamouty et al
The thermal evolution of young neutron stars (NSs) reflects the neutrino emission properties of their cores. 3.6% decay in the surface temperature of Cas A NS between 2000 and 2009 measured, using archival data from Chandra ACIS-S detector in Graded mode. This decay attributed to enhanced neutrino emission from a superfluid neutron transition in the core. Test this decline, combining analysis of the Cas A NS using all Chandra detectors and modes, using the most current calibrations. Measure the temperature changes from each detector separately and test for systematic effects due to the nearby filaments of the SNe remnant. Find a 1%-2% decay (HRC-S data) over 10 years in the effective temperature, depending on the choice of source and BG extraction regions, with a best-fit decay of 1.0pm0.7%. In comparison, the ACIS-S Graded data indicate temperature decay of 3-5% over 10 years, with best-fit decay of 3.5%. Shallower observations indicate ~2% decay (various instruments). Best estimate indicates decline of 3% over 10 years. The complexity of the bright and varying SNe remnant BG makes a definitive interpretation of archival Cas A Chandra observations difficult. A temperature decline of 1-3.5% over 10 years would indicate extraordinarily fast cooling of the NS that can be regulated by superfluidity of nucleons in the stellar core.
No comments:
Post a Comment