Co-evolution of galaxies and supermassive black holes
How did galaxies form and evolve during the 13.8-billion-year history of the universe? This question has been the subject of intense observational and theoretical investigation. Recent studies have revealed that almost all massive galaxies harbor a supermassive black hole whose mass reaches up to a hundred million or even a billion times the mass of the Sun, and their masses are tightly correlated with those of their host galaxies. This correlation suggests that supermassive black holes and their host galaxies have evolved together, closely interacting as they grow.
The group of researchers, led by Yoshiki Toba from Ehime University, focused on the DOGs as a key population to tackle the mystery of the co-evolution of galaxies and black holes. DOGs are faint in visible light because of the large quantity of obscuring dust but are bright in the infrared. The brightest infrared DOGs in particular are expected to harbor the most actively growing black holes. In addition, most DOGs are seen in the epoch when the star formation activity of galaxies reached its peak, 8 to 10 billion years ago. Thus, both DOGs and their black holes are rapidly growing at an early phase of their co-evolution. However, since DOGs are rare and are hidden behind significant amounts of dust, previous visible-light surveys have found few such objects.
A search for dust-obscured galaxies with HSC
HSC is a new instrument installed on the 8.2-meter Subaru Telescope in 2012. It is a wide-field camera with a field of view nine times the apparent size of the Full Moon. An ambitious legacy survey with HSC started in March 2014 as a “Subaru strategic program”; a total of 300 nights have been allocated for a five-year period. The Subaru strategic program with HSC started to deliver large quantities of excellent imaging data.
The research team selected DOGs from early data from the HSC Subaru Strategic Program (SSP). DOGs are a thousand times brighter in the infrared than the optical, and the team selected their targets using the HSC and NASA’s Wide-field Infrared Survey Explorer (WISE). They also utilized the data from the VISTA Kilo-degree Infrared Galaxy survey (VIKING). The all-sky survey data with WISE are crucial to discover spatially rare DOGs, while the VIKING data are useful to identify the DOGs more precisely.
Consequently, 48 DOGs were discovered. Each of these is 10 trillion times more luminous in the infrared than the Sun. The number density of these luminous DOGs is about 300 per cubic gigaparsec. It is theoretically predicted that these DOGs harbor an actively evolving supermassive black hole. This result provides researchers new insights into the mysteries of the co-evolution of galaxies and supermassive black holes from the unique observational prospects.
Summary and future prospects
In this research, the team discovered 48 DOGS and revealed their statistical properties of infrared luminous DOGs, in particular, for the first time.
Toba, said: “There are no instruments on large telescopes with the sensitivity and field of view of HSC, and hence HSC is unique in its ability to search for DOGs. The HSC survey will cover more than 100 times as much area of the sky as the area used for this study when it is complete, allowing the identification of thousands of DOGs in the near future. We are planning to investigate the detailed properties of DOGs and their central black holes using observations from many telescopes.”
Tohru Nagao said: “The Subaru Strategic Program with HSC has just begun. In the near future, exciting results will be released not only from studies on galaxy evolution, but also from in-fields such as solar systems, stars, nearby galaxies, and cosmology.”