Galaxy formation and evolution

Abstract:

It has been known for more than thirty years that the distribution of molecular gas in the innermost 300 parsecs of the Milky Way, the Central Molecular Zone, is strongly asymmetric. Indeed, approximately three quarters of molecular emission comes from positive longitudes, and only one quarter from negative longitudes. However, despite much theoretical effort, the origin of this asymmetry has remained a mystery. Here we show that the asymmetry can be neatly explained by unsteady flow of gas in a barred potential. We use high-resolution 3D hydrodynamical simulations coupled to a state-of-the-art chemical network. Despite the initial conditions and the bar potential being point-symmetric with respect to the Galactic Centre, asymmetries develop spontaneously due to the combination of a hydrodynamical instability known as the “wiggle instability” and the thermal instability. The observed asymmetry must be transient: observations made tens of megayears in the past or in the future would often show an asymmetry in the opposite sense. Fluctuations of amplitude comparable to the observed asymmetry occur for a large fraction of the time in our simulations, and suggest that the present is not an exceptional moment in the life of our Galaxy.

Abstract:

We present the Herschel-SPIRE photometric atlas for a complete flux limited sample of 43 local Ultraluminous Infrared Galaxies (ULIRGs), selected at 60μm by IRAS, as part of the HERschel ULIRG Survey (HERUS). Photometry observations were obtained using the SPIRE instrument at 250, 350 and 500μm. We describe these observations, present the results, and combine the new observations with data from IRAS to examine the far-IR spectral energy distributions (SEDs) of these sources. We fit the observed SEDs of HERUS objects with a simple parameterised modified black body model where temperature and emissivity β are free parameters.We compare the fitted values to those of non-ULIRG local galaxies, and find, in agreement with earlier results, that HERUS ULIRGs have warmer dust (median temperature T = 37.9±4.7 K compared to 21.3±3.4 K) but a similar β distribution (median β = 1.7 compared to 1.8) to the Herschel reference sample (HRS, Cortese et al., 2014) galaxies. Dust masses are found to be in the range of 107.5 to 109 M⊙, significantly higher than that of Herschel Reference Sample (HRS) sources.We compare our results for local ULIRGs with higher redshift samples selected at 250 and 850μm. These latter sources generally have cooler dust and/or redder 100-to-250 μm colours than our 60μm-selected ULIRGs. We show that this difference may in part be the result of the sources being selected at different wavelengths rather than being a simple indication of rapid evolution in the properties of the population.