Cosmic Dawn
The First Stars and Galaxies
Massimo Stiavelli
Acting Mission Director
James Webb Space Telescope
Space Telescope Science Institute
About the Lecture
This talk will briefly review the early evolution of the Universe, from the epoch when ionized hydrogen recombined – and the cosmic background radiation was released – to the epoch when hydrogen reionized. This is a very important period in cosmic history. It was when the first stars formed from the gas generated by the Big Bang. These early stars were formed by processes quite different from that of subsequent star formation, because the cosmic gas from the Big Bang was extremely poor of metals. In addition, the galaxies formed from the first few generations of these early stars had very low masses because, during their formation, the predominance of neutral hydrogen in the intergalactic medium shielded them from energetic ultraviolet radiation that otherwise would have ionized their gas content. Subsequently, ultraviolet emissions from these first stars and galaxies built up a cosmological ultraviolet background radiation that reionized hydrogen. As a result, the shield protecting low mass proto-galaxies from energetic radiation disappeared. Thereafter the formation of these ultra-low mass galaxies was no longer possible and eventually gave to the formation of the type of galaxies that predominate now. The talk will describe theoretical considerations underpinning the model of cosmic evolution during this period and the experimental results that support it.
About the Speaker
MASSIMO STIAVELLI earned his PhD at the Scuola Normale Superiore of Pisa. He did postdoctoral work at Rutgers University and was a fellow at the European Southern Observatory in Garching. After a stint at the Scuola Normale Superiore of Pisa he joined the Space Telescope Science Institute in Baltimore, where he is currently an Astronomer and Acting Mission Head for the James Webb Space Telescope. His main scientific interest is the formation and evolution of galaxies both from the point of view of theory and observations. He was the team lead for the Hubble Ultra Deep Field, and he has authored or coauthored 109 research papers in professional journals, 161 technical reports and other publications, and three books. He has served on and chaired numerous NASA committees, and he is Interdisciplinary Scientist on the Science Working Group of the James Webb Space Telescope. He is a member of the American Astronomical Society, American Association for the Advancement of Science, American Institute of Aeronautics and Astronautics and International Astronomical Union.
Minutes
President John Ingersoll called the 2,300th meeting to order at 8:?? pm April 20, 2012 in the Powell Auditorium of the Cosmos Club. Mr. Ingersoll announced the order of business and introduced five new members of the Society, including the speaker of the evening.
The minutes of the 2,299th meeting were read by Corresponding Secretary Robin Taylor and approved.
Mr. Ingersoll then introduced the speaker, Mr. Massimo Stiavelli of the Space Telescope Science Institute. Mr. Stiavelli spoke on the "Cosmic Dawn: The First Stars and Galaxies."
Mr. Stiavelli began by explaining that the development of galaxies and the Universe is analogous to the development of a human being in that the rate of change is fastest during the first ten percent of its lifetime. The early Universe, about 300,000 years after the Big Bang, was generally homogeneous and isotropic, but contained tiny variations in density and temperature. The particles in question are hydrogen, protons, electrons, and molecular hydrogen, but molecular hydrogen was the most common molecule in this early time.
As the Universe cooled, it was not able to remain ionized since protons and electrons recombine to form neutral Hydrogen. Modeling this process shows a very small fraction of the Universe remain ionized, which is very important. Mr. Stiavelli explained that the Hubble Space Telescope (HST) can observe light from the distant Universe and we've determined that most hydrogen is ionized by observing and comparing the spectra of nearby and distant quasars. If Hydrogen was originally neutral, some process must have reionized the Hydrogen, Mr. Stiavelli said.
Mr. Stiavelli then explained how do the first objects in the Universe formed from the small but significant density perturbations. The overdense perturbations collapsed due to gravity, growing and becoming filamentary, as underdense areas become great voids. As an overdense perturbation collapses, it reaches an equilibrium point where its density is about 178 times the average density of the Universe at the time and it has acquired some kinetic energy to balance the gravitational attraction. However, the Sun is thirty orders of magnitude denser than the Universe so a perturbation 178 times denser forming at redshift 30, for example, would be only five million times denser today. Something else must bridge the twenty four order of magnitude gap between the initial collapse and the density of the Sun, Mr. Stiavelli said.
The additional density occurs due to cooling due to collisions, excitation, and photo emission. However, Hydrogen is inefficient at cooling objects below 10,000K so below this temperature we need molecular hydrogen's lower energy levels. The small fraction of the Universe still ionization acted as catalyzers to form molecular hydrogen, enough that the very first stars began to form. These "Population III" stars began in the "dark age" of the Universe but were the first sources of light and began the reionization of Hydrogen.
Mr. Stiavelli explained that a star will be begin forming when the Jeans mass of a gas cloud decreases below the total gas content, at about 200,000 solar masses, at which point the cloud collapses into a single very massive star, one per cloud. These stars were very bright in the ultraviolet and this radiation photo-dissociates the molecular hydrogen needed for their very formation. This process destroyed most of the other stars around them, Mr. Stiavelli said.
Mr. Stiavelli stated that modeling of this process have shown the first stars alone are not sufficient to completely reionize Hydrogen and it is suspected that the first galaxies are responsible. Observing distant quasars and comparing to simple models suggests the reionization process occurred during a very high redshift such as 10 and continued until redshift 6. However, our most powerful computers still cannot fully simulate even a dwarf galaxy, so we must find evidence of these first galaxies. Early galaxies are very faint objects and it is difficult to do spectroscopy above redshift 6, which takes at least a year of observation with the HST to see even the brightest objects, Mr. Stiavelli said.
The James Webb Space Telescope is designed to be a workhouse in this area of research, Mr. Stiavelli said. The JWST contains a much larger mirror than the HST, with cameras and spectrographs designed for both infrared and visible light, which should enable the detection of the first galaxies while effectively replicating the Hubble Deep Field collection at higher redshift values. The HST has collected data from galaxies at redshift 7 to 8, but we have observed an epoch of rapid change in the luminosity density close to the limit of the HST, Mr. Stiavelli explained. The JWST will be better able to study the nature of reionization sources and the luminosity function of the first galaxies, and may even be able to detect supernovae from Population III stars.
With that, he closed his talk and Mr. Ingersoll invited questions.
Someone wondered about the possibility of a defect similar to Hubble Space Telescope's mirror, Mr. Stiavelli stated that the reasons for Hubble's defect are well understood and won't be repeated. Further, he explained that James Webb's mirror is much more deformable, enabling the telescope to compensate as necessary after launch.
In response to a question about the current and future funding status of the James Webb Space Telescope, Mr. Stiavelli commented that, despite concern last year, the project is fully funded for the current fiscal year with an anticipated launch in 2018.
After the question and answer period, Mr. Ingersoll thanked the speaker, made the usual housekeeping announcements, and invited guests to apply for membership. At ?:?? pm, President John Ingersoll adjourned the 2,300th meeting to the social hour.
Attendance: ??
The weather: Scattered Clouds
The temperature: 20°C
Respectfully submitted,
Justin Stimatze,
Recording secretary