Follow up of GW170817 and its electromagnetic counterpart by Australian-led observing programmes

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

Jets blowing bubbles in the young radio galaxy 4C 31.04

International audienceWe report the discovery of shocked molecular and ionized gas resulting from jet-driven feedback in the low-redshift (z = 0.0602) compact radio galaxy 4C 31.04 using near-IR imaging spectroscopy. 4C 31.04 is a ˜100 pc double-lobed Compact Steep Spectrum source believed to be a very young active galactic nucleus (AGN). It is hosted by a giant elliptical with a ˜ 109 M_⊙ multiphase gaseous circumnuclear disc. We used high spatial resolution, adaptive optics-assisted H- and K-band integral field Gemini/NIFS observations to probe (1) the warm (˜103 K) molecular gas phase, traced by ro-vibrational transitions of H2, and (2), the warm ionized medium, traced by the [Fe II]_{1.644 μ m} line. The [Fe II] emission traces shocked gas ejected from the disc plane by a jet-blown bubble 300-400 pc in diameter, while the H2 emission traces shock-excited molecular gas in the interior ˜ 1 kpc of the circumnuclear disc. Hydrodynamical modelling shows that the apparent discrepancy between the extent of the shocked gas and the radio emission can occur when the brightest regions of the synchrotron-emitting plasma are temporarily halted by dense clumps, while less bright plasma can percolate through the porous ISM and form an energy-driven bubble that expands freely out of the disc plane. Simulations suggest that this bubble is filled with low surface brightness plasma not visible in existing VLBI observations of 4C 31.04 due to insufficient sensitivity. Additional radial flows of jet plasma may percolate to ˜ kpc radii in the circumnuclear disc, driving shocks and accelerating clouds of gas, giving rise to the H2 emission

Jet-driven AGN feedback on molecular gas and low star-formation efficiency in a massive local spiral galaxy with a bright X-ray halo

It has long been suspected that powerful radio sources may lower the efficiency with which stars form from the molecular gas in their host galaxy, however so far, alternative mechanisms, in particular related to the stellar mass distribution in the massive bulges of their host galaxies, have not been ruled out. We present new, arcsecond-resolution Atacama Large Millimeter Array (ALMA) CO(1−0) interferometry, which probes the spatially resolved, cold molecular gas in the nearby (z = 0.08), massive (Mstellar = 4 × 1011 M⊙), isolated, late-type spiral galaxy 2MASSX J23453269−044925, which is outstanding for having two pairs of powerful, giant radio jets, and a bright X-ray halo of hot circumgalactic gas. The molecular gas is in a massive (Mgas = 2.0 × 1010 M⊙), 24 kpc wide, rapidly rotating ring, which is associated with the inner stellar disk. Broad (FWHM = 70−180 km s−1) emission lines with complex profiles associated with the radio source are seen over large regions in the ring, indicating gas velocities that are high enough to keep the otherwise marginally Toomre-stable gas from fragmenting into gravitationally bound, star-forming clouds. About 1−2% of the jet kinetic energy is required to power these motions. Resolved star-formation rate surface densities derived from Galaxy Evolution Explorer and Wide-Field Infrared Survey Explorer fall by factors of 30−70 short of expectations from the standard Kennicutt–Schmidt law of star-forming galaxies, and near gas-rich early-type galaxies with signatures of star formation that are lowered by jet feedback. We argue that radio Active Galactic Nucleus (AGN) feedback is the only plausible mechanism to explain the low star-formation rates in this galaxy. Previous authors have already noted that the X-ray halo of J2345−0449 implies a baryon fraction that is close to the cosmic average, which is very high for a galaxy. We contrast this finding with other, equally massive, and equally baryon-rich spiral galaxies without prominent radio sources. Most of the baryons in these galaxies are in stars, not in the halos. We also discuss the implications of our results for our general understanding of AGN feedback in massive galaxies

Revisiting the giant radio galaxy ESO 422-G028 - I. Discovery of a neutral inflow and recent star formation in a restarted giant

International audienceGiant radio galaxies provide important clues into the life cycles and triggering mechanisms of radio jets. With large-scale jets spanning 1.8 Mpc, ESO 422-G028 (z = 0.038) is a giant radio galaxy that also exhibits signs of restarted jet activity in the form of pc-scale jets. We present a study of the spatially resolved stellar and gas properties of ESO 422-G028 using optical integral field spectroscopy from the Wide-Field Spectrograph (WiFeS). In addition to the majority ${\sim} 13\, \rm Gyr$ old stellar population, ESO 422-G028 exhibits a much younger (${\lesssim} 10\, \rm Myr$ old) component with an estimated mass of $10^{7.6}\, \rm M_\odot$ that is predominantly located in the north-west region of the galaxy. Unusually, the ionized gas kinematics reveal two distinct discs traced by narrow ($\sigma _{\rm H\alpha } 100 \, \rm km\, s^{-1}$) and broad ($\sigma _{\rm H\alpha } 150 \, \rm km\, s^{-1}$) Hα emission, respectively. Both ionized gas discs are misaligned with the axis of stellar rotation, suggesting an external origin. This is consistent with the prominent interstellar Na D absorption, which traces a $1 \!-\! 3 \, \rm M_\odot \, yr^{-1}$ inflow of neutral gas from the north. We posit that an inflow of gas - either from an accretion event or a gas-rich merger - has triggered both the starburst and the restarted jet activity, and that ESO 422-G028 is potentially on the brink of an epoch of powerful active galactic nucleus (AGN) activity

Star formation in a massive spiral galaxy with a radio-AGN

International audienceWe present an analysis of new VLT/MUSE optical imaging spectroscopic data of 2MASX J23453268–0449256 (J2345–0449), a nearby ( z = 0.0755) massive ( M stellar = 4 × 10 11 M ⊙ ) spiral galaxy. This is a particularly interesting source for a study of active galactic nucleus (AGN) feedback since it hosts two pairs of bright, giant radio jets and a massive, luminous X-ray halo, but it has no massive bulge. The galaxy has a 24 kpc wide ring of molecular gas, and a source-averaged star formation rate that is factors 30 to 70 lower than expected from the Kennicutt-Schmidt law. With MUSE, we have analyzed the stellar continuum and bright optical line emission and have constrained the spatially resolved past and present star formation on scales of approximately 1 kpc. More than 93% of the stellar mass formed ≥10 Gyrs ago including in the disk. Optical emission from the AGN is very faint and contributes 2% of the continuum around the nucleus at most. Most line emission is predominantly excited by shocks and old stellar populations except in 13 young star-forming regions that formed ≤11 Myrs ago, of which only seven are associated with the molecular ring (the others are at larger radii). They avoid a region of high electron densities aligned with the radio source, and form stars at efficiencies that are comparable to those in normal spiral galaxies. We discuss the implications of our findings for the regulation of star formation in galaxies through AGN feedback in the absence of competing mechanisms related to the presence of a massive stellar bulge, such as morphological quenching

Avalanche photo diodes in the observatory environment: Lucky imaging at 1-2.5 microns

The recent availability of large format near-infrared detectors with sub-election readout noise is revolutionizing our approach to wavefront sensing for adaptive optics. However, as with all near infrared detector technologies, challenges exist in moving from the comfort of the laboratory test bench into the harsh reality of the observatory environment. As part of the broader adaptive optics program for the GMT, we are developing a near-infrared Lucky Imaging camera for operational deployment at the ANU 2.3 m telescope at Siding Spring Observatory. The system provides an ideal test-bed for the rapidly evolving Selex/SAPHIRA eAPD technology while providing scientific imaging at angular resolution rivalling the Hubble Space Telescope at wavelengths λ = 1.3-2.5 μ