NANDINI SAHU
For decades astronomers are trying to understand how the central black hole may govern various properties of the host galaxy and vice-versa. Our work adds another step to this study. We have performed careful, multi-component, photometric-decompositions of the largest-to-date sample of galaxies with dynamically measured (central) SMBH masses. These decompositions enabled us to measure the bulge masses and reliably identify the galaxy morphologies. We explored the black hole mass scaling relations for various sub-morphological classes of the galaxies, including galaxies with and without a rotating stellar disk, early-type (E, ES, S0) versus late-type galaxies (all spirals), barred versus non-barred galaxies, and Sérsic versus core-Sérsic galaxies.
Consequently, we have discovered significantly modified correlations of black hole mass with galaxy properties, i.e., the spheroid/bulge stellar mass, the total galaxy stellar mass, the central stellar velocity dispersion, central luminosity/mass concentration (Sérsic index), effective half-light radius, and the internal/spatial stellar mass density. The final scaling relations are dependent on galaxy morphology, which is fundamentally-linked with the formation and evolutionary paths of galaxies. These new scaling relations more accurately predict the black hole masses in other galaxies, pose ramifications for the virial-mass f-factor, and offer insights into simulations and theories of black hole-galaxy co-evolution. Additionally, these scaling relations will improve the predictions for the ground-based and space-based detection of long-wavelength gravitational waves by the pulsar timing arrays and the upcoming space interferometer, respectively.
Black holes are incredibly dense regions of space with extreme gravity such that even the light with its unbeatable speed cannot escape from it; hence, they are invisible. Observations suggest that they exist in a continuum of mass, starting from stellar-mass black holes to supermassive black holes. Stellar-mass black holes have a mass less than a hundred times the mass of our Sun. Supermassive black holes have masses between about a million to billion solar masses. Intermediate-mass black holes have masses in between these two ranges. A galaxy may have millions of stellar-mass black holes, thousands of intermediate-mass black holes, but only one supermassive black hole at its centre.
The supermassive black hole feeds on the gas and dust at the centre of the galaxy. The friction between the inspiraling gas and dust releases an enormous amount of electromagnetic energy which illuminates the boundary of the invisible black hole and also powers the bipolar outflow of ionised materials, which are thought to regulate host galaxy properties. Our observational study investigates the correlations between black hole mass and various galaxy properties to understand the coevolution between the black hole and host galaxy. We used the largest sample of galaxies whose central supermassive black hole masses are measured directly using primary methods, for example, modelling the motion of stars and gases around the black hole.
Morphology indicates various structural components present in a galaxy, which depends on how and where the galaxy evolved over cosmic time. A galaxy may have an ellipsoidal bulge also called spheroid, a flat rotating stellar disk, and many other components like a bar, ring, and spiral arms. Two major morphological categories are early-type and late-type galaxies. The first category includes pure spheroidal galaxies and also galaxies with an additional disk as a significant component. The second category includes galaxies with a bulge, disk, and also spiral arms as major components.
As the bulge of a galaxy immediately surrounds the central supermassive black hole, astronomers envisioned a strong relationship between black hole mass and bulge mass. However, disassembling the bulge mass from the total galaxy mass is challenging. We performed two-dimensional modelling and multi-component decomposition technique on the high-resolution images of our galaxies to obtain the mass of bulge and the total galaxy mass. These decompositions also helped us to identify the detailed morphology of galaxies in our sample.
Here I am only discussing the scaling relations we observed between black hole mass and both the host bulge mass and total galaxy mass. We found that early-type galaxies and late-type galaxies follow two different relations between black hole mass and bulge mass, and also between black hole mass and total galaxy mass. Where, early-type galaxies with and without a disk further follow two different but parallel relations between black hole mass and bulge mass, which are offset from each other by more than ten times in the vertical direction.
Similarly, correlations of black hole mass with other properties of the host galaxy such as stellar velocity dispersion, central concentration, and size were also found to depend on galaxy morphology, which is fundamentally linked with the formation and evolutionary paths of galaxies. These new scaling relations can accurately predict the black hole masses in other galaxies, offer insights into simulations and theories of black hole-galaxy coevolution. Additionally, these relations will improve the predictions for the detection of long-wavelength gravitational waves by the pulsar timing arrays and the upcoming space interferometer.