Constraining galaxy and black hole mergers with pulsars

Lunch Colloquium
Dr. Siyuan Chen
University of Birmingham

We have strong evidence that super massive black holes reside in the centre of most galaxies. Galaxies have also been observed to merge, consequently the super massive black holes should also form a binary and eventually merge. These super massive black hole binaries (SMBHBs) emit gravitational waves (GWs) as they spiral into each other. The superposition of all the GWs emitted by the cosmic population of SMBHBs form a gravitational wave background (GWB). This GWB affect the time of arrival (TOA) series of the radio signals from pulsars in a characteristic and correlated fashion. Pulsar Timing Arrays (PTAs) make use of this fact and aim to detect gravitational waves by precisely timing an array ofpulsars. One of the prime targets for PTAs is the GWB from a population of SMBHBs. SMBHBs have a strong relation with their host galaxies, in fact, we believe that SMBHBs are created via the merger of the host galaxies. Thus, the rate at which galaxies merge is related to the population of merging SMBHBs. The GWB frequency spectrum can be computed by integrating the emission of one single binary over the whole population. I will present a parametric model to compute the strength of the GWB from the population of SMBHBs in terms of astrophysical observables. These include the galaxy stellar mass function, pair fraction and merger time scale as well as the galaxy mass - black hole mass relation with scattering. All of which are interesting quantities, that have already been measured and constained. Using a PTA upper limit on the GWB, we can directly compare and combine the constraints on these astrophysical observables from electromagnetic observations with those from PTAs. I will present preliminary results with our nested sampling algorithm, showing how much (or little) PTAs can tell us about galaxy and black hole mergers.


Main Colloquium
Prof. Samaya Nissanke
GRAPPA, University of Amsterdam


Terahertz Airborne Astronomy using a Four Detector High Resolution Spectrometer: 4GREAT

Carlos A. Durán

After the Herschel Space Telescope ceased operations in 2013, the astronomical community has been lacking access to those parts of the terahertz spectrum that are not visible from ground-based observatories. The atmosphere blocks most of the radiation at those frequency bands, even at high geographical altitude facilities like the Atacama desert (> 5000 m altitude), where APEX and ALMA operate. 4GREAT, an extension of the German Receiver at Terahertz frequencies (GREAT) operated aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA), has been developed in response to those needs. This works describes its design, test, commissioning and scientific capabilities. 4GREAT is a heterodyne receiver that comprises four different detector bands and their associated subsystems, which can be simultaneously operated fully independently in one system. All four detector beams are co-aligned on the sky. The four frequency bands of 4GREAT cover 492-627, 893-1073, 1239-1515 and 2495-2690 GHz respectively. Various astrophysically important spectral lines are observable in each band, and in some cases different transitions of the same species, for example CO, lie in two or more bands of 4GREAT. The very important ground state transitions of various molecules can be observed, including NH3 , H218O, CH, OH, OH+ , NH, NH2, and the deuterated isotopologues HDO, and OD, as well as fine structure lines from neutral atomic carbon, [CI], and ionized nitrogen, [NII]. The expanded capabilities of GREAT with 4GREAT are now being used for a variety of spectroscopic studies. Its potential has been demonstrated by an absorption study of two ground state transitions or the methylidyne radical (CH) in diffuse molecular gas. As CH traces the unobservable molecular hydrogen, such observations are of fundamental importance for diffuse cloud astrochemistry. [Referees: Prof. Dr. Karl M. Menten, Prof. Dr. Frank Bertoldi, Prof. Dr. Ian Brock, Prof. Dr. Hubert Schorle]

Relativistic redshift of the star S0-2 orbiting the Galactic center supermassive black hole

Special Colloquium
Dr. Tuan Do
UCLA, Los Angeles, USA

I will present a direct test of the Einstein equivalence principle around a supermassive black hole using the orbit of the Galactic center star, S0-2/S2. A key aspect of this experiment is the combination of our existing spectroscopic and astrometric measurements (1995-2017) that cover its 16-year orbit with new measurements in 2018 (March to September) that capture three key-observable events during its second measurable closest approach to the black hole. I will discuss the detection of the gravitational redshift on the orbit of S0-2 and the importance of systematic errors as measurements become increasingly more precise. I will also discuss the future of tests of gravity with stellar orbits at the Galactic center with the next generation of large ground-based telescopes such as the TMT and ELT.