An international team of scientists led by the Center for Astrobiology (CAB, CSIC-INTA), with the participation of the Instituto de Astrofísica de Canarias (IAC), used the Gran Telescopio Canarias (GTC) to study a representative sample of galaxies, both disc and spheroidal, in a deep sky area of the constellation Ursa Major to characterize the properties of stellar populations of galactic bulges. The researchers were able to determine the mode of formation and development of these galactic structures. The results of this study were recently published in The Journal of Astrophysics.
The researchers focused their study on massive disks and spheroidal galaxies, using imaging data from the Hubble Space Telescope and spectroscopic data from the Survey for High-z Absorption Red and Dead Sources (SHARDS) project, a program of observations on the whole GOODS-N (Great Observatories Origins Deep Survey – North) region through 25 different filters taken with the OSIRIS instrument on the Gran Telescopio Canarias (GTC), the largest optical and infrared telescope in the world, at the Roque de los Muchachos Observatory (Garafía, La Palma, Canary Islands).
The analysis of the data allowed the researchers to discover something unexpected: the bulges of the galaxy discs formed in two waves. A third of the galaxy disc bulges formed at a redshift of 6.2, which corresponds to an early epoch of the Universe when it was only 5% of its current age, around 900 million years ago. “These bulges are the relics of the first structures formed in the Universe, which we found hidden in local disk galaxies”, explains Luca Costantin, researcher at CAB as part of an Attracting Talent program of the Community of Madrid, and the first author on paper.
But on the other hand, almost two-thirds of the swellings observed have an average redshift value of the order of 1.3, which means that they were formed much more recently, corresponding to an age of four billion years, or nearly 35% of the age of the Universe.
A special feature that distinguishes the two waves is that the central bulges of the first wave, the older bulges, are more compact and dense than those formed in the second, more recent wave. Additionally, data from the spheroidal galaxies in the sample shows an average redshift value of 1.1, suggesting that they formed at the same time as the second wave bulges.
For Jairo Méndez Abreu, researcher at the University of Granada (UGR) and co-author of the article, former postdoctoral researcher Severo Ochoa at the IAC, “the idea behind the technique used to observe the stars in the bulge center is quite simple, but it was not possible to apply it until the recent development of methods which made it possible to separate the light of the stars in the central bulb from that of the disc, in particular the algorithms GASP2D and C2D, which we have developed recently and that have allowed us to achieve unprecedented precision. “
Another important finding of the study is that the two bulge-forming waves differ not only in terms of the age of their stars, but also in terms of the rate of star formation. The data indicates that the stars in the first wave bulges formed rapidly, over time scales typically 200 million years. On the contrary, a significant fraction of the stars in the bulges of the second wave required formation times five times as long, a few billion years.
“We discovered that the Universe has two ways of forming the central areas of galaxies like ours: starting early and running very quickly, or taking a long time to start, but ultimately forming a large number of stars in what we call the bulge, “comments Pablo G. Pérez González, CAB researcher and principal investigator of the SHARDS project, who provided essential data for this study. In the words of Antonio Cabrera, head of scientific operations at GTC, “SHARDS is a perfect example of what is possible thanks to the combination of the enormous collecting capacity of the GTC and the extraordinary conditions of the Roque observatory of los Muchachos, to produce 180 hours of data with such excellent image quality, essential for the detection of the objects analyzed here. “
As described by Paola Dimauro, researcher at the National Observatory of Brazil and co-author of this article, “this study allowed us to explore the morphological evolution and the history of the assembly of structural components of galaxies, analogous to archaeological studies, by analyzing the information encoded in the millions of stars in each galaxy. The interesting point was that all the structures were not formed at the same time, or in the same way.
The results of this study allowed observers to draw a curious parallel between the formation and evolution over time of studies of disk galaxies and the creation and development of a large city over the centuries. Just as we find that some large cities have historic centers, which are older and house the oldest buildings in crowded narrow streets, the results of this work suggest that some of the centers of massive disk galaxies are home to some of the oldest. spheroids formed in the Universe. , which continued to acquire matter, forming discs more slowly, the new outskirts of the city in our analogy.