Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

A UV flare at the center of the elliptical galaxy NGC 4552

A self-consistent analysis of near-UV, HST/FOC images of the elliptical galaxy NGC 4552 is used to show that its central spike has brightened by a factor similar to 4.5 between 1991 and 1993, and has decreased its luminositv by a factor similar to 2.0 between 1993 and 1996. A strong UV continuum over the energy distribution of the underlying galaxy is concurrently revealed shortward of lambda similar to 3200 Angstrom by our FOS spectra extending from the near-UV to red wavelengths. Nuclear emission-line profiles of both permitted and forbidden lines are best modelled with a combination of broad and narrow components, with FWHM of similar to 3000 km s(-1) and similar to 700 km s(-1), respectively. Current diagnostics based on the emission line intensity ratios definitely places the spike among AGNs, just at the border between Seyferts and LINERs. This evidence argues for the variable central spike being produced by a modest accretion event onto a central massive black hole (BH), with the accreted material having possibly being stripped from a star in a close fly-by with the BH. In this regard, one has to look at NGC 4552 as the faintest known AGN

The variable UV central spike of NGC 4552

The nature of the UV-bright spike at the center of the otherwise normal galaxy NGC 4552 is investigated by means of both Hubble Space Telescope (HST) imaging (FOC) and spectroscopy (FOS). This central feature-which underwent a recent dramatic UV flare-provides a potential new tool for revealing the presence of massive black holes in normal galaxies. In particular the sub-arcsec FOS spectra of the nucleus suggest that NGC 4552 does harbor an object with mass M > 4 x 10(8) M. within the central 2 pc

The giant Ly alpha nebula associated with a z=2.5 radio galaxy

We present a kinematic and morphological study of the giant Ly alpha nebula associated with the radio galaxy MRC 2104-242 (z = 2.49) based on integral field spectroscopic VIMOS data from VLT. The galaxy appears to be embedded in a giant (greater than or similar to 120 kpc) gas reservoir that surrounds it completely. The kinematic properties of the nebula suggest that it is a rotating structure, which would imply a lower limit to the dynamical mass of similar to 3 x 10(11) M circle dot. An alternate scenario is that the gas is infalling. Such a process would be able to initiate and sustain significant central starburst activity, although it is likely to contribute with less than 10% of the total stellar mass of the galaxy

Dust in cluster dwarf elliptical galaxies

Based on single cross-scan data of the Herschel Virgo Cluster Survey, we report the first detections of dust in cluster early-type dwarf galaxies: VCC 209, VCC 781 and VCC 951. All three galaxies have dust masses M-d approximate to 10(5) - 10(6) M-circle dot and average dust temperatures approximate to 16-20 K. Since these three early-type dwarfs reside in densely crowded regions close to the center of the Virgo cluster, and several HI-detected dwarfs in the outskirts of Virgo were not detected by Herschel (implying a dust content < 10(4) M-circle dot), this might imply that dust in dwarfs is more closely related to the molecular gas, which is more centrally peaked in a galaxy's potential well and therefore, not easily removed by any stripping mechanism. We conclude that the removal of interstellar dust from these early-type dwarfs appears to be less efficient than the removal of the HI gas