Galaxy formation and evolution

Abstract:

Using the Horizon-AGN simulation data, we study the relative role of mergers and environmental effects in shaping the spin of early-type galaxies (ETGs) after $z \simeq 1$. We follow the spin evolution of 10,037 color-selected ETGs more massive than 10$^{10} \rm \, M_{\odot}$ that are divided into four groups: cluster centrals (3%), cluster satellites (33%), group centrals (5%), and field ETGs (59%). We find a strong mass dependence of the slow rotator fraction, $f_{\rm SR}$, and the mean spin of massive ETGs. Although we do not find a clear environmental dependence of $f_{\rm SR}$, a weak trend is seen in the mean value of spin parameter driven by the satellite ETGs as they gradually lose their spin as their environment becomes denser. Galaxy mergers appear to be the main cause of total spin changes in 94% of central ETGs of halos with $M_{vir} > 10^{12.5}\rm M_{\odot}$, but only 22% of satellite and field ETGs. We find that non-merger induced tidal perturbations better correlate with the galaxy spin-down in satellite ETGs than mergers. Given that the majority of ETGs are not central in dense environments, we conclude that non-merger tidal perturbation effects played a key role in the spin evolution of ETGs observed in the local ($z < 1$) universe.

Abstract:

The origin of ultra-high energy cosmic rays (UHECRs) is still unknown. It has recently been proposed that UHECR anisotropies can be attributed to starbust galaxies or active galactic nuclei. We suggest that the latter is more likely and that giant-lobed radio galaxies such as Centaurus A and Fornax A can explain the data.

Abstract:

Supermassive black hole binaries (SMBHBs) produced in galaxy mergers are thought to complete their coalescence, below separations of r_GW=10^{-3} (M_BH/10^8 M_sun)^{3/4} pc, as their orbit decays due to the emission of gravitational waves (GWs). It may be possible to identify such GW-driven inspirals statistically in an electromagnetic (EM) survey for variable sources. A GW-driven binary spends a characteristic time T_GW at each orbital separation r_orb < r_GW that scales with the corresponding orbital time t_orb as T_GW = (const) t_orb^{8/3}. If the coalescing binary produces variations in the EM emission on this timescale, then it could be identified as a variable source with a characteristic period t_var = t_orb. The incidence rate of sources with similar inferred BH masses, showing near-periodic variability on the time-scale t_var, would then be proportional to t_var^{8/3}. Luminosity variations corresponding to a fraction f_Edd<0.01 of the Eddington luminosity would have been missed in current surveys. However, if the binary inspirals are associated with quasars, we show that a dedicated survey could detect the population of SMBHBs with a range of periods around tens of weeks. The discovery of a population of periodic sources whose abundance obeys N_var = (const) t_var^{8/3} would confirm (i) that the orbital decay is indeed driven by GWs, and (ii) that circumbinary gas is present at small orbital radii and is being perturbed by the BHs. Deviations from the t_var^{8/3} power-law could constrain the structure of the circumbinary gas disk and viscosity-driven orbital decay. We discuss constraints from existing data, and quantify the sensitivity and sky coverage that could yield a detection in future surveys.