12-Jun-2022: Binary super massive black hole discovered in a system which could be site of future gravitational waves detection

An international collaboration of astronomers has discovered a binary super massive black hole in a system which will be a strong candidate for future detection of gravitational waves (GWs).

Blazars which are super massive black holes (SMBH) feeding on gas in the heart of a very distant galaxy, are among the most luminous and energetic objects in the Universe. When the jet, composed of ionized matter traveling at nearly the speed of light, is pointed towards an observer, it is called a blazar. The blazar AO 0235+164 is unique as it is gravitationally lensed by intervening galaxies (phenomenon by which light shining from far away to be bent and pulled by the gravity of an object between its source and the observer).

A group of astronomers from Argentina, Spain, Italy, USA and India has discovered a binary super massive black hole system in the gravitationally lensed blazar AO 0235+164 using extensive optical photometric observations carried out around the globe during last 4 decades (1982 - 2019). They discovered periodic double-peaked flaring events at an interval of around 8 years, and the separations between two peaks of these flares are around 2 years. Five such periodic patterns were detected, and it was predicted that the next such flaring event will occur between November 2022 and May 2025. To confirm the next periodic pattern, a global optical photometric monitoring campaign has been initiated under WEBT (Whole Earth Blazar Telescope) consortium. The observational campaign will be led by Dr. Alok C. Gupta.

Dr. Alok C. Gupta, Senior Scientist from Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, an autonomous institution of the Department of Science and Technology (DST), Government of India, has participated in this study which has been recently published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS). The study was led by Mr. Abhradeep Roy, a Ph.D. student of the Department of High Energy Physics (DHEP), Tata Institute of Fundamental Research (TIFR), Mumbai. The other members of the Indian team include Prof. V. R. Chitnis, Dr. Anshu Chatterjee and Dr. Arkadipta Sarkar from TIFR, Mumbai.

The team detected five sets of double-peaked flaring activities during time ranges ---  January 1982 - October 1984, March 1989 - July 1993, April 1996 - March 2001, June 2006 - June 2009 and May 2014 - May 2017.

They expect the next such 2 years long flaring episode to happen between November 2022 and May 2025. An intensive multi-wavelength WEBT campaign will be conducted during this period to test the persistence of this apparent nearly-periodic oscillation (QPO) in AO 0235+164.

The blazar AO 0235+164 is the first binary SMBH gravitationally lensed system, which will be a strong candidate of its kind for future detection of gravitational waves (GWs) using the pulsar timing array and future space-based GW detectors.

17-Mar-2021: Possible origin of winds from black hole accretion discs probed

As gas and dust fall toward a black hole, they form a disk around it. As material piles up in the disk, it heats up to temperatures in excess of millions of degrees. A fraction of this infalling matter is ejected in the form of winds.

Scientists have tracked the generation of this wind and how it is driven by the disc of diffused swirling materials around the black hole called an accretion disc.  Matter flowing out due to the wind should contaminate the environment play a major role in the evolution of the region harbouring these black holes. Therefore how such a process can be triggered need to be ascertained. Though these processes are still at the level of theoretical prediction, consensus has not been reached.

By blowing dense gas from the galactic nucleus and by halting inward flows from the galactic halo, the winds play a vital role in shaping the evolution of the black hole host galaxy. Hence the mechanism of generation of these winds and what drives them has intrigued scientists for a long as it helps them explore host galaxies.

Scientists from Aryabhatta Research Institute of observational sciences (ARIES), an autonomous institute under the Department of Science and Technology (DST), Govt. of India, in collaboration with scientists from other institutions Scientists, undertook a time-dependent study of the generation of wind and its subsequent driving by the radiation from the diffuse material in spiral motion around a massive central body called the black hole accretion disc using numerical simulation techniques developed indigenously.

The scientists tried to figure out whether driving of the wind by radiation flux can dominate the radiation drag effect--- a motion resisting effect which is similar to the resistance offered by air to a moving stone or to a descending parachute. This effect is produced when radiation penetrates a moving medium and is proportional to radiation energy density, various components of radiation pressure, and the velocity components of the wind.

The authors showed that luminous discs can produce winds up to speeds which is about ten percent of the speed of light, and also that these winds originated from regions close to the central black hole. Radiation drag plays a key role in reducing the speed of light. For less bright discs, radiation drag quenches the wind completely.

The research was led by Sananda Raychaudhuri of Bose Institute, Kolkata, in collaboration with Mukesh K. Vyas of Bar Ilan University, Israel, and Indranil Chattopadhyay of ARIES and has recently been accepted for publication in the scientific journal Monthly Notices of Royal Astronomical Society (MNRAS). The authors used a numerical simulation code developed earlier by Dr. Chattopadhyay to study the galaxy environment by modifying it suitably to perform the simulations of wind flow from black hole accretion disc.

1-Apr-2022: Hidden in Plain Sight: faint galaxy discovered in our local universe

Researchers have discovered a faint but star-forming galaxy, around 136 million light-years away which was so far undetected since it lies in front of a much brighter galaxy, The galaxy has a ‘ghost’ like appearance in the optical images because of its low disk density, but the inner disk shows star formation. The inner disk star formation helped its detection in UV and optical images. An accurate census of such faint galaxies is essential to measure the total mass of all objects made normal atomic matter (stars and gas) in the Universe.

As optical telescopes have become more and more powerful, they are sensitive enough to detect galaxies that are extremely faint. Such galaxies are called low surface brightness galaxies or ultra-diffuse galaxies and have a surface brightness that is at least ten times fainter than the surrounding night sky. Such faint galaxies may account for up to 15 % of the mass of the universe. However, they are difficult to detect because of their inherent low luminosities.

A team of researchers from the Indian Institute of Astrophysics, Bengaluru consisting of Jyoti Yadav, Mousumi Das and Sudhanshu Barway, along with Francoise Combes of College de France, Chaire Galaxies et Cosmologie, Paris, while studying a known interacting galaxy NGC6902A noticed that the colour image (Dark Energy Camera Legacy Survey (DECaLS) colour image) of the south-west outer region of the galaxy NGC 6902A in the shows diffuse blue emission. DECalS is a deep optical survey conducted on international telescopes that can be used to detect diffuse galaxies. This south-western region shows prominent star-forming regions in the Far Ultraviolet (FUV) image. Most of the FUV emission in galaxies is due to young stars of types O and B ---the most massive stars and also the most short-lived in the galaxies. But they emit FUV light for 100 million years which is comparatively long compared to the other star formation tracer. This excess FUV light prompted the researchers to investigate the peculiar feature in more detail to determine the cause of the interaction.

The researchers measured the distance of NGC6902A, a previously known galaxy and the faint star-forming regions using emission lines in the spectra. They found that these star-forming regions are at a distance of around 136 million light-years, whereas the distance of NGC 6902A is around 825 million light-years. This means that the diffuse blue emission was from a foreground galaxy, which they had discovered using FUV and MUSE data. They have named it UVIT J202258.73-441623.8 (or UVIT J2022 for short) based on the fact that data from Ultra-Violet Imaging Telescope (UVIT) on AstroSat helped them to discover the galaxy and determine its coordinates on the sky. They also used Multi-Unit Spectroscopic Explorer (MUSE) instrument on the Very Large Telescope (VLT) in Chile and images from IRSF in South Africa and from Dark Energy Camera Legacy Survey (DECaLS) in this study. This research has been published in the journal ‘Astronomy & Astrophysics’.

UVIT J2022 was previously mistakenly thought to be a part of the interacting tail of NGC 6902A. This study raises the possibilities that there could be similar diffuse galaxies that have been wrongly interpreted as interacting galaxies due to their superposition with foreground or background galaxies. Star formation traced by ultraviolet, optical emission could be a way to detect such diffuse star-forming galaxies in our local universe.

“The material that we see around us is known as baryonic matter. Cosmological studies suggest that baryonic matter should make 5% of the Universe's mass. The rest of the mass should be contributed by unknown forms, such as dark matter and dark energy. We still do not have a clear understanding about the 5% of the baryonic content present in the Universe; we do not know where all the baryons are present. These faint galaxies can act as a link for understanding the origin of missing baryons in the universe, as they may contribute significantly to the baryonic mass in the universe.”

The discovery of a new foreground galaxy that was mistaken as a tidal feature of a bright background galaxy using powerful instruments such as UVIT and MUSE opens a gateway to searching for similar cases, where blue diffuse tidal features in interacting galaxies may not be the remnant of a merger but instead a separate foreground and/or background galaxy. It also shows the power of using star formation tracers such as FUV and Hα emission to detect diffuse galaxies.