The anatomy of an enigma: a team of astronomers using the Large Binocular Telescope Observatory (LBT) atop Mount Graham in southeastern Arizona collected 17 hours’ worth of light from the abnormally massive elliptical galaxy dubbed C1-23152 (image above). The young galaxy is 12 billion light years away and defies conventional models of its origins as it must have accumulated its enormous mass within 1.8 billion years after the big bang, less than 13% of the present age of the universe. Most elliptical galaxies, such as C1-23152 at the center of a galaxy cluster, take many billions of years to reach their massive sizes. Hence the enduring mystery of how and why these monster objects exist in the early universe.
The Enigma of C1-23152
The most massive galaxies that we observe in the universe reach masses of several hundred billion times that of our Sun. Although they comprise only one third of all galaxies, they contain more than 70% of the stars in the Universe. The current model of galaxy formation – the so-called hierarchical model – predicts that smaller galaxies formed earlier, while more massive systems formed later, through subsequent mergers of the pre-existing smaller galaxies. Hence the enigma of C1-23152 — how these galaxies formed so rapidly is among the most debated questions of modern astrophysics.
“The data show that the formation time of C1-23152, that is the time elapsed between the formation of the first stars from the pre-existing gas to the moment when the star formation has almost completely ceased, is less than 500 million of years” says Paolo Saracco, researcher at INAF in Milan and first author of the article published in The Astrophysical Journal. “Also, from the data collected with LBT we were able to establish that in this short time, corresponding to less than 4 hundredths of the age of the Universe, the galaxy formed a mass equal to about 200 billion stars like the Sun, that is about 450 suns per year.” or more than one per day, a star formation rate almost 300 times higher than the current rate in our galaxy, the Milky Way.”
“The Milky Way, now forms no more than two a year”, adds Danilo Marchesini, full professor at Tufts University and second author of the article. The large amount of information collected allowed the team to quantify for the first time in a galaxy so distant the abundance of chemical elements heavier than helium (the so-called metallicity). The stars of this ancient galaxy have a higher metallicity than that of our much younger Sun, similar to that observed in the most massive galaxies in the universe today.
Massive Galaxies in the Universe Can Occur Extremely Quickly
“These observations showed that the formation of the most massive galaxies in the universe can occur extremely quickly, through an extremely intense star formation process in the early Universe, as for C1-23152″, underscores Francesco La Barbera, researcher at INAF in Naples, in the team that conducted the study.
“Understanding whether the scenario that describes the formation of C1-23152 is a particular case or whether, on the contrary, it is what happens for most of the most massive galaxies in the universe is of fundamental importance since this would require a profound revision of the galaxy formation models”, adds Adriana Gargiulo, also a researcher at INAF in Milan and co-author of the study.
The image at the top of the page shows galaxy C1-23152 at a redshift when the Universe was 1.8 billion years old. The image is the sum of images at different wavelengths taken with the Hubble Space Telescope. Its light profile matches exactly those of typical elliptical galaxies in the local Universe but with a mass of about 200 billions sun-like stars that formed in less than 500 million years.
The image below from the LBT Observatory illustrates the formation scenario of massive elliptical galaxies like C1-23152. Massive primordial gas clouds, falling in the same region under the effect of gravitational force, collide triggering violent and massive star formation processes. The starburst phase is expected to last a few hundred million years during which hundreds to thousands of stars per year are formed, as for C1-23152. The resulting massive elliptical galaxy will then evolve with time, possibly experiencing different evolutionary phenomena.
The formation of stellar masses as high as for C1-23152 requires both high masses of gas to convert into stars and particular physical conditions, reports the LBT. A possible scenario hypothesized by the researchers is that massive primordial gas clouds, falling under the effect of gravitational force in the same region, collide, triggering violent and massive star formation processes. From the observational point of view, the precursors of the most massive galaxies could therefore be remote galaxies with a very high rate of star formation.
This image shows an example of starburst galaxies forming about a thousand stars per year at the time of observation. This phase is most likely the formation phase of massive galaxies in the early Universe, like C1-23152.
Galaxy Dark-Matter Halos 40 Million Years After Big Bang
Scientists have found clues elsewhere that may account for these ancient mega galaxies, reports Robin George Andrews for Scientific American: “Anastasia Fialkov, a cosmologist at the University of Cambridge, who was not involved with the latest work, says that, unlike full-blown simulations, analytic physics calculations can “take into account the whole volume of the universe.” And they suggest that a small number of dark matter halos capable of initiating star formation show up just 40 million years after the big bang.”
“To test our hypotheses, the observations that the next generation of instrumentations will allow us to carry out will be decisive, in particular the James Webb Space Telescope (JWST) which will be launched in orbit at the end of 2021, and the Extremely Large Telescope (ELT) the largest ground-based telescope ever built, with a main mirror of 39 meters in diameter, which will be operational in 2026”, concludes Saracco.