Galaxy formation and evolution

Abstract:

The discovery of the first electromagnetic counterpart to a gravitational wave signal has generated follow-up observations by over 50 facilities world-wide, ushering in the new era of multi-messenger astronomy. In this paper, we present follow-up observations of the gravitational wave event GW170817 and its electromagnetic counterpart SSS17a/DLT17ck (IAU label AT2017gfo) by 14 Australian telescopes and partner observatories as part of Australian-based and Australian-led research programs. We report early- to late-time multi-wavelength observations, including optical imaging and spectroscopy, mid-infrared imaging, radio imaging, and searches for fast radio bursts. Our optical spectra reveal that the transient source emission cooled from approximately 6 400 K to 2 100 K over a 7-d period and produced no significant optical emission lines. The spectral profiles, cooling rate, and photometric light curves are consistent with the expected outburst and subsequent processes of a binary neutron star merger. Star formation in the host galaxy probably ceased at least a Gyr ago, although there is evidence for a galaxy merger. Binary pulsars with short (100 Myr) decay times are therefore unlikely progenitors, but pulsars like PSR B1534+12 with its 2.7 Gyr coalescence time could produce such a merger. The displacement (~2.2 kpc) of the binary star system from the centre of the main galaxy is not unusual for stars in the host galaxy or stars originating in the merging galaxy, and therefore any constraints on the kick velocity imparted to the progenitor are poor.

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.