A color composite image of Centaurus A, showing blades and jets coming from the central black hole of an active galaxy.
With the help of integrated field spectroscopy IFS and modern modeling tools, scientists have managed to get closer to an important milestone in solving the puzzle of extragalactic astronomy. It is about the nature and formation of the central spherical part of spiral galaxies, like the Milky Way.
It is believed that the bulge is created in two different ways. Classical stars are represented by stars older than a disk, since they were formed more quickly more than 10 billion years ago. Pseudo-bulges are endowed with stars of the same age as the disk, because they were collected gradually by dynamic processes with continuous star formation, pushed by the flow of gas from the disk.
These two scenarios suggest that classical convexities and pseudo-convexities are endowed with strikingly different characteristics. However, numerous studies have not revealed a sharp contrast. To solve the riddle, the team conducted an unprecedented analysis of spectral modeling of more than half a million individual spectra. This was supposed to help understand the history of the stellar and disk components of 135 galaxies from the CALIFA IFS study.
New data indicate that the time frame of the formation of convexities is associated with the galactic mass: the creation of convexities is completed within the first 4 billion years in massive galaxies, but in less massive ones it continues at a lower rate.
Image of a large spiral galaxy NGC 1232. The colors of different areas are clearly visible here. In the center are more ancient reddish stars, and the young blue are observed in the spiral arms and areas of stellar birth
The study reveals a consistent new scenario for the formation of galactic bulges. The growth of the convexity is provided by the superposition of early slow processes and is governed by the mass and density of galaxies. The study also planned to evaluate the role of active galactic nuclei working on the accretion of matter in supermassive black holes. It turned out that the nuclei are the dominant source of gas ionization in massive convexes, but importance is lost in galaxies with a smaller mass.
The study showed that the contribution of stars under 9 billion years old is closely correlated with the stellar mass, surface density, age and level of chemical enrichment of galactic bulges. Therefore, we face a powerful new diagnostics of the physical and evolutionary properties of central protuberances.